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Title: Coral Reefs, Volcanic Islands, South American Geology
also:
Title: The Structure and Distribution of Coral-Reefs, Geological
Observations on Volcanic Islands, and Geological Observations on
South America.

Author: Charles Darwin

Release Date: May, 2003 [Etext #4022]
[Most recently updated: May 23, 2020]

Language: English

Character set encoding: UTF-8

*** START OF THIS PROJECT GUTENBERG EBOOK CORAL REEFS, VOLCANIC ISLANDS, SOUTH AMERICAN GEOLOGY ***



Produced by Sue Asscher




Coral Reefs, Volcanic Islands, South American Geology

by Charles Darwin




EDITORIAL NOTE


Although in some respects more technical in their subjects and style
than Darwin’s “Journal,” the books here reprinted will never lose their
value and interest for the originality of the observations they
contain. Many parts of them are admirably adapted for giving an insight
into problems regarding the structure and changes of the earth’s
surface, and in fact they form a charming introduction to physical
geology and physiography in their application to special domains. The
books themselves cannot be obtained for many times the price of the
present volume, and both the general reader, who desires to know more
of Darwin’s work, and the student of geology, who naturally wishes to
know how a master mind reasoned on most important geological subjects,
will be glad of the opportunity of possessing them in a convenient and
cheap form.

The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.

G.T.B.




CONTENTS

THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.

CRITICAL INTRODUCTION

INTRODUCTION

Chapter I—ATOLLS OR LAGOON-ISLANDS.

_Section I_—DESCRIPTION OF KEELING ATOLL.
Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.

_Section II_—GENERAL DESCRIPTION OF ATOLLS. General form and size of
atolls, their reefs and islets.—External slope.—Zone of
Nulliporæ.—Conglomerate.—Depth of lagoons.—Sediment.—Reefs submerged
wholly or in part.—Breaches in the reef.—Ledge-formed shores round
certain lagoons.—Conversion of lagoons into land.

_Section III_—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.
Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
depths in the lagoons of the southern atolls.—Reefs in the lagoons all
rising to the surface.—Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the
waves.—Destruction of islets.—Connection in the position and submarine
foundation of distinct atolls.—The apparent disseverment of large
atolls.—The Great Chagos Bank.—Its submerged condition and
extraordinary structure.

Chapter II—BARRIER REEFS.

Closely resemble in general form and structure atoll-reefs.—Width and
depth of the lagoon-channels.—Breaches through the reef in front of
valleys, and generally on the leeward side.—Checks to the filling up of
the lagoon-channels.—Size and constitution of the encircled
islands.—Number of islands within the same reef.—Barrier-reefs of New
Caledonia and Australia.—Position of the reef relative to the slope of
the adjoining land.—Probable great thickness of barrier-reefs.

Chapter III—FRINGING OR SHORE-REEFS.

Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
up.—Currents of water formed within it.—Upraised reefs.—Narrow
fringing-reefs in deep seas.—Reefs on the coast of E. Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
and on worn-down islands.—Fringing-reefs affected by currents of the
sea.—Coral coating the bottom of the sea, but not forming reefs.

Chapter IV—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.

_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.

_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS.

_Section III_—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.

Chapter V—THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS.

The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.—Recent changes in their state.—The origin of barrier-reefs and
of atolls.—Their relative forms.—The step-formed ledges and walls round
the shores of some lagoons.—The ring-formed reefs of the Maldiva
atolls.—The submerged condition of parts or of the whole of some
annular reefs.—The disseverment of large atolls.—The union of atolls by
linear reefs.—The Great Chagos Bank.—Objections, from the area and
amount of subsidence required by the theory, considered.—The probable
composition of the lower parts of atolls.

Chapter VI—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.

Description of the coloured map.—Proximity of atolls and barrier-
reefs.—Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.

Appendix

Containing a detailed description of the reefs and islands in Plate
III.

GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.

CRITICAL INTRODUCTION

Chapter I—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.

Rocks of the lowest series.—A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent.—Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriæ.—Ancient and
obliterated orifices of eruption of small size.—Difficulty of tracing
over a bare plain recent streams of lava.—Inland hills of more ancient
volcanic rock.—Decomposed olivine in large masses.—Feldspathic rocks
beneath the upper crystalline basaltic strata.—Uniform structure and
form of the more ancient volcanic hills.—Form of the valleys near the
coast.—Conglomerate now forming on the sea beach.

Chapter II—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.

FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
rocks: their singular decomposition by steam of high temperature.
TAHITI.—Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
elevation.—Structure of its more ancient mountains; similarity with St.
Jago. ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular
mineralogical composition.

Chapter III—ASCENSION.

Basaltic lavas.—Numerous craters truncated on the same side.—Singular
structure of volcanic bombs.—Aeriform explosions.—Ejected granite
fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
formation.—Concretions in pumiceous tuff.—Calcareous deposits and
frondescent incrustations on the coast.—Remarkable laminated beds,
alternating with, and passing into obsidian.—Origin of
obsidian.—Lamination of volcanic rocks.

Chapter IV—ST. HELENA.

Lavas of the feldspathic, basaltic, and submarine series.—Section of
Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
and a parapet.—Cones of phonolite.—Superficial beds of calcareous
sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
land.—Denudation.—Craters of elevation.

Chapter V—GALAPAGOS ARCHIPELAGO.

Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
basaltic craters, with hollows at their bases.—Albemarle Island; fluid
lavas, their composition.—Craters of tuff; inclination of their
exterior diverging strata, and structure of their interior converging
strata.—James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected
fragments.—Concluding remarks on the craters of tuff, and on the
breached condition of their southern sides.—Mineralogical composition
of the rocks of the archipelago.—Elevation of the land.—Direction of
the fissures of eruption.

Chapter VI—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.

The sinking of crystals in fluid lava.—Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation.—Obsidian.—Apparent non-separation of the elements of
plutonic rocks.—Origin of trap-dikes in the plutonic
series.—Distribution of volcanic islands; their prevalence in the great
oceans.—They are generally arranged in lines.—The central volcanoes of
Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
volcanic islands, and their elevation in mass.—Eruptions on parallel
lines of fissure within the same geological period.

Chapter VII—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.

New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
Land.—Palæozoic formation.—Newer formation with volcanic
rocks.—Travertin with leaves of extinct plants.—Elevation of the
land.—New Zealand.—King George’s Sound.—Superficial ferruginous
beds.—Superficial calcareous deposits, with casts of branches; its
origin from drifted particles of shells and corals.—Their extent.—Cape
of Good Hope.—Junction of the granite and clay-slate.—Sandstone
formation.

GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.

CRITICAL INTRODUCTION

Chapter I—ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.

Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
shells.—Terrace-bounded valley of Santa Cruz, formerly a
sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.—Great shingle formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.

Chapter II—ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.

Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth or
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea-beach on San Lorenzo.—Human
remains, fossil earthenware, earthquake debacle, recent subsidence.—On
the decay of upraised shells.—General summary.

Chapter III—ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL
DEPOSITS.

Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake-waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.—Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.

Chapter IV—ON THE FORMATIONS OF THE PAMPAS.

Mineralogical constitution.—Microscopical structure.—Buenos Ayres,
shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S.
Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones, and infusoria of; co-existence of the recent shells and
extinct mammifers.—Buenos Ayres to St. Fe.—Skeletons of
Mastodon.—Infusoria.—Inferior marine tertiary strata, their
age.—Horse’s tooth. BANDA ORIENTAL.—Superficial Pampean
formation.—Inferior tertiary strata, variation of, connected with
volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia,
age of, subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—Contemporaneous with existing
mollusca.—Relations to underlying tertiary strata. Ancient deposit of
estuary origin.—Elevation and successive deposition of the Pampean
formation.—Number and state of the remains of mammifers; their
habitation, food, extinction, and range.—Conclusion.—Supplement on the
thickness of the Pampean formation.—Localities in Pampas at which
mammiferous remains have been found.

Chapter V—ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.

Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.—Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary
formations of the Western Coast.—Chonos and Chiloe groups, volcanic
rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.—Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coasts of South America.—On the
contemporaneous deposition and preservation of sedimentary formations.

Chapter VI—PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.

Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, palæozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.

Chapter VII—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.

Central Chile.—Basal formations of the Cordillera.—Origin of the
porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section
of the Cordillera by the Peuquenes or Portillo Pass.—Great gypseous
formation.—Peuquenes line; thickness of strata, fossils of.—Portillo
line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks
of.—Concluding remarks on the denudation and elevation of the Portillo
line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous
strata.—Section near the Puente del Inca; fossils of.—Great
subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the
Uspallata chain.—Structure and nature of the strata.—Silicified
vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding
remarks on the Uspallata range; origin subsequent to that of the main
Cordillera; two periods of subsidence; comparison with the Portillo
chain.

Chapter VIII—NORTHERN CHILE.—CONCLUSION.

Section from Illapel to Combarbala; gypseous formation with silicified
wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley;
fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas,
silicified wood.—Conglomerates, nature of former land, fossils,
thickness of strata, great subsidence.—Valley of Despoblado, fossils,
tufaceous deposit, complicated dislocations of.—Relations between
ancient orifices of eruption and subsequent axes of injection.—Iquique,
Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the
porphyritic conglomerate and gypseous formations.—Great subsidence with
partial elevations during the cretaceo-oolitic period.—On the elevation
and structure of the Cordillera.—Recapitulation on the tertiary
series.—Relation between movements of subsidence and volcanic
action.—Pampean formation.—Recent elevatory movements.—Long-continued
volcanic action in the Cordillera.—Conclusion.

Index to “Coral-Reefs”

Index to “Volcanic Islands”

Index to “South American Observations”




THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.




CRITICAL INTRODUCTION


A scientific discovery is the outcome of an interesting process of
evolution in the mind of its author. When we are able to detect the
germs of thought in which such a discovery has originated, and to trace
the successive stages of the reasoning by which the crude idea has
developed into an epoch-making book, we have the materials for
reconstructing an important chapter of scientific history. Such a
contribution to the story of the “making of science” may be furnished
in respect to Darwin’s famous theory of coral-reefs, and the clearly
reasoned treatise in which it was first fully set forth.

The subject of corals and coral-reefs is one concerning which much
popular misconception has always prevailed. The misleading comparison
of coral-rock with the combs of bees and the nests of wasps is perhaps
responsible for much of this misunderstanding; one writer has indeed
described a coral-reef as being “built by fishes by means of their
teeth.” Scarcely less misleading, however, are the references we so
frequently meet with, both in prose and verse, to the “skill,”
“industry,” and “perseverance” of the “coral-insect” in “building” his
“home.” As well might we praise men for their cleverness in making
their own skeletons, and laud their assiduity in filling churchyards
with the same. The polyps and other organisms, whose remains accumulate
to form a coral-reef, simply live and perform their natural functions,
and then die, leaving behind them, in the natural course of events, the
hard calcareous portions of their structures to add to the growing
reef.

While the forms of coral-reefs and coral-islands are sometimes very
remarkable and worthy of attentive study, there is no ground, it need
scarcely be added, for the suggestion that they afford proofs of design
on the part of the living builders, or that, in the
words of Flinders, they constitute breastworks, defending the workshops
from whence “infant colonies might be safely sent forth.”

It was not till the beginning of the present century that travellers
like Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others,
began to collect accurate details concerning the forms and structure of
coral-masses, and to make such observations on the habits of
reef-forming polyps, as might serve as a basis for safe reasoning
concerning the origin of coral-reefs and islands. In the second volume
of Lyell’s “Principles of Geology,” published in 1832, the final
chapter gives an admirable summary of all that was then known on the
subject. At that time, the ring-form of the atolls was almost
universally regarded as a proof that they had grown up on submerged
volcanic craters; and Lyell gave his powerful support to that theory.

Charles Darwin was never tired of acknowledging his indebtedness to
Lyell. In dedicating to his friend the second edition of his
“Naturalist’s Voyage Round the World,” Darwin writes that he does so
“with grateful pleasure, as an acknowledgment that the chief part of
whatever scientific merit this journal and the other works of the
author may possess, has been derived from studying the well-known and
admirable ‘Principles of Geology.’”

The second volume of Lyell’s “Principles” appeared after Darwin had
left England; but it was doubtless sent on to him without delay by his
faithful friend and correspondent, Professor Henslow. It appears to
have reached Darwin at a most opportune moment, while, in fact, he was
studying the striking evidences of slow and long-continued, but often
interrupted movement on the west coast of South America. Darwin’s acute
mind could not fail to detect the weakness of the then prevalent theory
concerning the origin of the ring-shaped atolls—and the difficulty
which he found in accepting the volcanic theory, as an explanation of
the phenomena of coral-reefs, is well set forth in his book.

In an interesting fragment of autobiography, Darwin has given us a very
clear account of the way in which the leading idea of the theory of
coral-reefs originated in his mind; he writes, “No other work of mine
was begun in so deductive a spirit as this, for the whole theory was
thought out on the west coast of South America, before I had seen a
true coral-reef. I had therefore only to verify and extend my views by
a careful examination of living reefs. But it should be observed that I
had during the two previous years been incessantly attending to the
effects on the
shores of South America of the intermittent elevation of the land,
together with the denudation and deposition of sediment. This
necessarily led me to reflect much on the effects of subsidence, and it
was easy to replace in imagination the continued deposition of sediment
by the upward growth of corals. To do this was to form my theory of the
formation of barrier-reefs and atolls.”

On her homeward voyage, the _Beagle_ visited Tahiti, Australia, and
some of the coral-islands in the Indian Ocean, and Darwin had an
opportunity of testing and verifying the conclusion at which he had
arrived by studying the statements of other observers.

I well recollect a remarkable conversation I had with Darwin, shortly
after the death of Lyell. With characteristic modesty, he told me that
he never fully realised the importance of his theory of coral-reefs
till he had an opportunity of discussing it with Lyell, shortly after
the return of the _Beagle_. Lyell, on receiving from the lips of its
author a sketch of the new theory, was so overcome with delight that he
danced about and threw himself into the wildest contortions, as was his
manner when excessively pleased. He wrote shortly afterwards to Darwin
as follows:—“I could think of nothing for days after your lesson on
coral-reefs, but of the tops of submerged continents. It is all true,
but do not flatter yourself that you will be believed till you are
growing bald like me, with hard work and vexation at the incredulity of
the world.” On May 24th, 1837, Lyell wrote to Sir John Herschel as
follows:—“I am very full of Darwin’s new theory of coral-islands, and
have urged Whewell to make him read it at our next meeting. I must give
up my volcanic crater forever, though it cost me a pang at first, for
it accounted for so much.” Dr. Whewell was president of the Geological
Society at the time, and on May 31st, 1837, Darwin read a paper
entitled “On Certain Areas of Elevation and Subsidence in the Pacific
and Indian oceans, as deduced from the Study of Coral Formations,” an
abstract of which appeared in the second volume of the Society’s
proceedings.

It was about this time that Darwin, having settled himself in lodgings
at Great Marlborough Street, commenced the writing of his book on
“Coral-Reefs.” Many delays from ill-health and the interruption of
other work, caused the progress to be slow, and his journal speaks of
“recommencing” the subject in February 1839, shortly after his
marriage, and again in October of the same year. In July 1841, he
states that he began once more “after more than thirteen month’s
interval,” and the last proof-sheet of
the book was not corrected till May 6th, 1842. Darwin writes in his
autobiography, “This book, though a small one, cost me twenty months of
hard work, as I had to read every work on the islands of the Pacific,
and to consult many charts.” The task of elaborating and writing out
his books was, with Darwin, always a very slow and laborious one; but
it is clear that in accomplishing the work now under consideration,
there was a long and constant struggle with the lethargy and weakness
resulting from the sad condition of his health at that time.

Lyell’s anticipation that the theory of coral-reefs would be slow in
meeting with general acceptance was certainly not justified by the
actual facts. On the contrary the new book was at once received with
general assent among both geologists and zoologists, and even attracted
a considerable amount of attention from the general public.

It was not long before the coral-reef theory of Darwin found an able
exponent and sturdy champion in the person of the great American
naturalist, Professor James D. Dana. Two years after the return of the
_Beagle_ to England, the ships of the United States Exploring
Expedition set sail upon their four years’ cruise, under the command of
Captain Wilkes, and Dana was a member of the scientific staff. When, in
1839, the expedition arrived at Sydney, a newspaper paragraph was found
which gave the American naturalist the first intimation of Darwin’s new
theory of the origin of atolls and barrier-reefs. Writing in 1872, Dana
describes the effect produced on his mind by reading this passage:—“The
paragraph threw a flood of light over the subject, and called forth
feelings of peculiar satisfaction, and of gratefulness to Mr. Darwin,
which still come up afresh whenever the subject of coral islands is
mentioned. The Gambier Islands in the Paumotus, which gave him the key
to the theory, I had not seen; but on reaching the Feejees, six months
later, in 1840, I found there similar facts on a still grander scale
and of a more diversified character, so that I was afterward enabled to
speak of his theory as established with more positiveness than he
himself, in his philosophic caution, had been ready to adopt. His work
on coral-reefs appeared in 1842, when my report on the subject was
already in manuscript. It showed that the conclusions on other points,
which we had independently reached, were for the most part the same.
The principal points of difference relate to the reason for the absence
of corals from some coasts, and the evidence therefrom as to changes of
level, and the distribution of the oceanic regions of
elevation and subsidence—topics which a wide range of travel over the
Pacific brought directly and constantly to my attention.”

Among the Reports of the United States Exploring Expedition, two
important works from the pen of Professor Dana made their
appearance;—one on “Zoophytes,” which treats at length on “Corals and
Coral-Animals,” and the other on “Coral-Reefs and Islands.” In 1872,
Dana prepared a work of a more popular character in which some of the
chief results of his studies are described; it bore the title of
“Corals and Coral-Islands.” Of this work, new and enlarged editions
appeared in 1874 and 1890 in America, while two editions were published
in this country in 1872 and 1875. In all these works their author,
while maintaining an independent judgment on certain matters of detail,
warmly defends the views of Darwin on all points essential to the
theory.

Another able exponent and illustrator of the theory of coral-reefs was
found in Professor J. B. Jukes, who accompanied H.M.S. _Fly_, as
naturalist, during the survey of the Great Barrier-Reef—in the years
1842 to 1846. Jukes, who was a man of great acuteness as well as
independence of mind, concludes his account of the great Australian
reefs with the following words:—“After seeing much of the Great
Barrier-Reefs, and reflecting much upon them, and trying if it were
possible by any means to evade the conclusions to which Mr. Darwin has
come, I cannot help adding that his hypothesis is perfectly
satisfactory to my mind, and rises beyond a mere hypothesis into the
true theory of coral-reefs.”

As the result of the clear exposition of the subject by Darwin, Lyell,
Dana, and Jukes, the theory of coral-reefs had, by the middle of the
present century, commanded the almost universal assent of both
biologists and geologists. In 1859 Baron von Richthofen brought forward
new facts in its support, by showing that the existence of the thick
masses of dolomitic limestone in the Tyrol could be best accounted for
if they were regarded as of coralline origin and as being formed during
a period of long continued subsidence. The same views were maintained
by Professor Mojsisovics in his “Dolomit-riffe von Südtirol und
Venetien,” which appeared in 1879.

The first serious note of dissent to the generally accepted theory was
heard in 1863, when a distinguished German naturalist, Dr. Karl Semper,
declared that his study of the Pelew Islands showed that uninterrupted
subsidence could not have been going on in that region. Dr. Semper’s
objections were very carefully
considered by Mr. Darwin, and a reply to them appeared in the second
and revised edition of his “Coral-Reefs,” which was published in 1874.
With characteristic frankness and freedom from prejudice, Darwin
admitted that the facts brought forward by Dr. Semper proved that in
certain specified cases, subsidence could not have played the chief
part in originating the peculiar forms of the coral-islands. But while
making this admission, he firmly maintained that exceptional cases,
like those described in the Pelew Islands, were not sufficient to
invalidate the theory of subsidence as applied to the widely spread
atolls, encircling reefs, and barrier-reefs of the Pacific and Indian
Oceans. It is worthy of note that to the end of his life Darwin
maintained a friendly correspondence with Semper concerning the points
on which they were at issue.

After the appearance of Semper’s work, Dr. J. J. Rein published an
account of the Bermudas, in which he opposed the interpretation of the
structure of the islands given by Nelson and other authors, and
maintained that the facts observed in them are opposed to the views of
Darwin. Although, so far as I am aware, Darwin had no opportunity of
studying and considering these particular objections, it may be
mentioned that two American geologists have since carefully re-examined
the district—Professor W. N. Rice in 1884 and Professor A. Heilprin in
1889—and they have independently arrived at the conclusion that Dr.
Rein’s objections cannot be maintained.

The most serious opposition to Darwin’s coral-reef theory, however, was
that which developed itself after the return of H.M.S. _Challenger_
from her famous voyage. Mr. John Murray, one of the staff of
naturalists on board that vessel, propounded a new theory of
coral-reefs, and maintained that the view that they were formed by
subsidence was one that was no longer tenable; these objections have
been supported by Professor Alexander Agassiz in the United States, and
by Dr. A. Geikie, and Dr. H. B. Guppy in this country.

Although Mr. Darwin did not live to bring out a third edition of his
“Coral-Reefs,” I know from several conversations with him that he had
given the most patient and thoughtful consideration to Mr. Murray’s
paper on the subject. He admitted to me that had he known, when he
wrote his work, of the abundant deposition of the remains of calcareous
organisms on the sea floor, he might have regarded this cause as
sufficient in a few cases to raise the summits of submerged volcanoes
or other mountains
to a level at which reef-forming corals can commence to flourish. But
he did not think that the admission that under certain favourable
conditions, atolls might be thus formed without subsidence,
necessitated an abandonment of his theory in the case of the
innumerable examples of the kind which stud the Indian and Pacific
Oceans.

A letter written by Darwin to Professor Alexander Agassiz in May 1881
shows exactly the attitude which careful consideration of the subject
led him to maintain towards the theory propounded by Mr. Murray:—“You
will have seen,” he writes, “Mr. Murray’s views on the formation of
atolls and barrier-reefs. Before publishing my book, I thought long
over the same view, but only as far as ordinary marine organisms are
concerned, for at that time little was known of the multitude of minute
oceanic organisms. I rejected this view, as from the few dredgings made
in the _Beagle_, in the south temperate regions, I concluded that
shells, the smaller corals, etc., decayed and were dissolved when not
protected by the deposition of sediment, and sediment could not
accumulate in the open ocean. Certainly, shells, etc., were in several
cases completely rotten, and crumbled into mud between my fingers; but
you will know whether this is in any degree common. I have expressly
said that a bank at the proper depth would give rise to an atoll, which
could not be distinguished from one formed during subsidence. I can,
however, hardly believe in the existence of as many banks (there having
been no subsidence) as there are atolls in the great oceans, within a
reasonable depth, on which minute oceanic organisms could have
accumulated to the depth of many hundred feet.”

Darwin’s concluding words in the same letter written within a year of
his death, are a striking proof of the candour and openness of mind
which he preserved so well to the end, in this as in other
controversies.

“If I am wrong, the sooner I am knocked on the head and annihilated so
much the better. It still seems to me a marvellous thing that there
should not have been much, and long-continued, subsidence in the beds
of the great oceans. I wish some doubly rich millionaire would take it
into his head to have borings made in some of the Pacific and Indian
atolls, and bring home cores for slicing from a depth of 500 or 600
feet.”

It is noteworthy that the objections to Darwin’s theory have for the
most part proceeded from zoologists, while those who have fully
appreciated the geological aspect of the question, have been
the staunchest supporters of the theory of subsidence. The desirability
of such boring operations in atolls has been insisted upon by several
geologists, and it may be hoped that before many years have passed
away, Darwin’s hopes may be realised, either with or without the
intervention of the “doubly rich millionaire.”

Three years after the death of Darwin, the veteran Professor Dana
re-entered the lists and contributed a powerful defence of the theory
of subsidence in the form of a reply to an essay written by the ablest
exponent of the anti-Darwinian views on this subject, Dr. A. Geikie.
While pointing out that the Darwinian position had been to a great
extent misunderstood by its opponents, he showed that the rival theory
presented even greater difficulties than those which it professed to
remove.

During the last five years, the whole question of the origin of
coral-reefs and islands has been re-opened, and a controversy has
arisen, into which, unfortunately, acrimonious elements have been very
unnecessarily introduced. Those who desire it, will find clear and
impartial statements of the varied and often mutually destructive views
put forward by different authors, in three works which have made their
appearance within the last year,—“The Bermuda Islands,” by Professor
Angelo Heilprin; “Corals and Coral-Islands,” new edition by Professor
J. D. Dana; and the third edition of Darwin’s “Coral-Reefs,” with Notes
and Appendix by Professor T. G. Bonney.

Most readers will, I think, rise from the perusal of these works with
the conviction that, while on certain points of detail it is clear
that, through the want of knowledge concerning the action of marine
organisms in the open ocean, Darwin was betrayed into some grave
errors, yet the main foundations of his argument have not been
seriously impaired by the new facts observed in the deep-sea
researches, or by the severe criticism to which his theory has been
subjected during the last ten years. On the other hand, I think it will
appear that much misapprehension has been exhibited by some of Darwin’s
critics, as to what his views and arguments really were; so that the
reprint and wide circulation of the book in its original form is
greatly to be desired, and cannot but be attended with advantage to all
those who will have the fairness to acquaint themselves with Darwin’s
views at first hand, before attempting to reply to them.

JOHN W. JUDD.


CORAL-REEFS


INTRODUCTION

The object of this volume is to describe from my own observation and
the works of others, the principal kinds of coral-reefs, more
especially those occurring in the open ocean, and to explain the origin
of their peculiar forms. I do not here treat of the polypifers, which
construct these vast works, except so far as relates to their
distribution, and to the conditions favourable to their vigorous
growth. Without any distinct intention to classify coral-reefs, most
voyagers have spoken of them under the following heads:
“lagoon-islands,” or “atolls,” “barrier” or “encircling reefs,” and
“fringing” or “shore-reefs.” The lagoon-islands have received much the
most attention; and it is not surprising, for every one must be struck
with astonishment, when he first beholds one of these vast rings of
coral-rock, often many leagues in diameter, here and there surmounted
by a low verdant island with dazzling white shores, bathed on the
outside by the foaming breakers of the ocean, and on the inside
surrounding a calm expanse of water, which from reflection, is of a
bright but pale green colour. The naturalist will feel this
astonishment more deeply after having examined the soft and almost
gelatinous bodies of these apparently insignificant creatures, and when
he knows that the solid reef increases only on the outer edge, which
day and night is lashed by the breakers of an ocean never at rest. Well
did François Pyrard de Laval, in the year 1605, exclaim, “C’est une
mérueille de voir chacun de ces atollons, enuironné d’un grand banc de
pierre tout autour, n’y ayant point d’artifice humain.” The
accompanying sketch of Whitsunday island, in the South Pacific, taken
from Captain Beechey’s admirable “Voyage,” although excellent of its
kind, gives but a faint idea of the singular aspect of one of these
lagoon-islands.

Whitsunday Island is of small size, and the whole circle has been
converted into land, which is a comparatively rare circumstance. As the
reef of a lagoon-island generally supports many separate small islands,
the word “island,” applied to the whole, is often the cause of
confusion; hence I have invariably used in this volume the term
“atoll,” which is the name given to these circular groups of
coral-islets by their
inhabitants in the Indian Ocean, and is synonymous with “lagoon-
island.”

[Illustration: Whitsunday Island]

Barrier-reefs, when encircling small islands, have been comparatively
little noticed by voyagers; but they well deserve attention. In their
structure they are little less marvellous than atolls, and they give a
singular and most picturesque character to the scenery of the islands
they surround. In the accompanying sketch, taken from the “Voyage of
the _Coquille_,” the reef is seen from within, from one of the high
peaks of the island of Bolabola.[1] Here, as in Whitsunday Island, the
whole of that part of the reef which is visible is converted into land.
This is a circumstance of rare occurrence; more usually a snow-white
line of great breakers, with here and there an islet crowned by
cocoa-nut trees, separates the smooth waters of the lagoon-like channel
from the waves of the open sea. The barrier-reefs of Australia and of
New Caledonia, owing to their enormous dimensions, have excited much
attention: in structure and form they resemble those encircling many of
the smaller islands in the Pacific Ocean.

 [1] I have taken the liberty of simplifying the foreground, and
 leaving out a mountainous island in the far distance.


[Illustration: Island of Bolabola]

With respect to fringing, or shore-reefs, there is little in their
structure which needs explanation; and their name expresses their
comparatively
small extension. They differ from barrier-reefs in not lying so far
from the shore, and in not having within a broad channel of deep water.
Reefs also occur around submerged banks of sediment and of worn-down
rock; and others are scattered quite irregularly where the sea is very
shallow; these in most respects are allied to those of the fringing
class, but they are of comparatively little interest.

I have given a separate chapter to each of the above classes, and have
described some one reef or island, on which I possessed most
information, as typical; and have afterwards compared it with others of
a like kind. Although this classification is useful from being obvious,
and from including most of the coral-reefs existing in the open sea, it
admits of a more fundamental division into barrier and atoll-formed
reefs on the one hand, where there is a great apparent difficulty with
respect to the foundation on which they must first have grown; and into
fringing-reefs on the other, where, owing to the nature of the slope of
the adjoining land, there is no such difficulty. The two blue tints and
the red colour[2] on the map (Plate III), represent this main division,
as explained in the beginning of the last chapter. In the Appendix,
every existing coral-reef, except some on the coast of Brazil not
included in the map, is briefly described in geographical order, as far
as I possessed information; and any particular spot may be found by
consulting the Index.

Several theories have been advanced to explain the origin of atolls or
lagoon-islands, but scarcely one to account for barrier-reefs. From the
limited depths at which reef-building polypifers can flourish, taken
into consideration with certain other circumstances, we are compelled
to conclude, as it will be seen, that both in atolls and barrier-reefs,
the foundation on which the coral was primarily attached, has subsided;
and that during this downward movement, the reefs have grown upwards.
This conclusion, it will be further seen, explains most satisfactorily
the outline and general form of atolls and barrier-reefs, and likewise
certain peculiarities in their structure. The distribution, also, of
the different kinds of coral-reefs, and their position with relation to
the areas of recent elevation, and to the points subject to volcanic
eruptions, fully accord with this theory of their origin.[3]

 [2] Replaced by numbers in this edition.


 [3] A brief account of my views on coral formations, now published in
 my Journal of Researches, was read May 31st, 1837, before the
 Geological Society, and an abstract has appeared in the Proceedings.


In the several original surveys, from which the small plans on this
plate have been reduced, the coral-reefs are engraved in very different
styles. For the sake of uniformity, I have adopted the style used in
the charts of the Chagos Archipelago, published by the East Indian
Company, from the survey by Captain Moresby and Lieutenant Powell. The
surface of the reef, which dries at low water, is represented by a
surface with small crosses: the coral-islets on the reef are marked by
small linear spaces, on which a few cocoa-nut trees, out of all
proportion too large, have been introduced for the sake of clearness.
The entire _annular reef_, which when surrounding an open expanse of
water, forms an “atoll,” and when surrounding one or more high islands,
forms an encircling “barrier-reef,” has a nearly uniform structure. The
reefs in some of the original surveys are represented merely by a
single line with crosses, so that their breadth is not given; I have
had such reefs engraved of the width usually attained by coral-reefs. I
have not thought it worth while to introduce all those small and very
numerous reefs, which occur within the lagoons of most atolls and
within the lagoon-channels of most barrier-reefs, and which stand
either isolated, or are attached to the shores of the reef or land. At
Peros Banhos none of the lagoon-reefs rise to the surface of the water;
a few of them have been introduced, and are marked by plain dotted
circles. A few of the deepest soundings are laid down within each reef;
they are in fathoms, of six English feet.

_Plate I_—Map showing the resemblance in form between barrier
coral-reefs surrounding mountainous islands, and atolls or lagoon
islands.


[Illustration: Map showing the resemblance in form.]

Fig. 1—VANIKORO, situated in the western part of the South Pacific;
taken from the survey by Captain D’Urville in the _Astrolabe_; the
soundings on the southern side of the island, namely, from thirty to
forty fathoms, are given from the voyage of the Chev. Dillon; the other
soundings are laid down from the survey by D’Urville; height of the
summit of the island is 3,032 feet. The principal small detached reefs
within the lagoon-channel have in this instance been represented. The
southern shore of the island is narrowly fringed by a reef: if the
engraver had carried this reef entirely round both islands, this figure
would have served (by leaving out in imagination the barrier-reef) as a
good specimen of an abruptly-sided island, surrounded by a reef of the
fringing class.

Fig. 2—HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
“Atlas of the Voyage of the _Astrolabe,_” compiled from the surveys of
Captains Duperrey and D’Urville; the depth of the immense lagoon-like
space within the reef is not known.

Fig. 3—RAIATEA, in the Society Archipelago; from the map given in the
quarto edition of “Cook’s First Voyage;” it is probably not accurate.

Fig. 4—BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago,
from the survey by Captain Beechey, R.N.; the lagoon is choked up with
reefs, but the average greatest depth of about twenty fathoms, is given
from the published account of the voyage.

Fig. 5—BOLABOLA, in the Society Archipelago, from the survey of Captain
Duperrey in the _ Coquille_: the soundings in this and the following
figures have been altered from French feet to English fathoms; height
of highest point of the island 4,026 feet.

[Illustration: Map showing the resemblance in form.]

Fig. 6.—MAURUA, in the Society Archipelago; from the survey by Captain
Duperrey in the _ Coquille_: height of land about eight hundred feet.

Fig. 7—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from the
survey by Admiral Lutké.

Fig. 8—GAMBIER ISLANDS, in the southern part of the Low Archipelago;
from the survey by Captain Beechey; height of highest island, 1,246
feet; the islands are surrounded by extensive and irregular reefs; the
reef on the southern side is submerged.

Fig. 9—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
the Indian Ocean; from the survey by Captain Moresby and Lieutenant
Powell; not nearly all the small submerged reefs in the lagoon are
represented; the annular reef on the southern side is submerged.

Fig. 10—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
Ocean; from the survey by Captain Fitzroy; the lagoon south of the
dotted line is very shallow, and is left almost bare at low water; the
part north of the line is choked up with irregular reefs. The annular
reef on the north-west side is broken, and blends into a shoal
sandbank, on which the sea breaks.




Chapter I ATOLLS OR LAGOON-ISLANDS

_Section I_—KEELING ATOLL

Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.

KEELING or COCOS atoll is situated in the Indian Ocean, in 12° 5′ S.,
and longitude 90° 55′ E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. _Beagle_, is given
in Plate I, Fig. 10. The greatest width of this atoll is nine miles and
a half. Its structure is in most respects characteristic of the class
to which it belongs, with the exception of the shallowness of the
lagoon. The accompanying woodcut represents a vertical section,
supposed to be drawn at low water from the outer coast across one of
the low islets (one being taken of average dimensions) to within the
lagoon.

[Illustration: Vertical section of one of the low islets.]

A.—Level of the sea at low water: where the letter A is placed, the
depth is twenty-five fathoms, and the distance rather more than one
hundred and fifty yards from the edge of the reef.
B.—Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of
rugged points, like those a little farther seaward, beneath the water.
 C.—A flat of coral-rock, covered at high water.
 D.—A low projecting ledge of brecciated coral-rock washed by the waves
 at high water.
 E.—A slope of loose fragments, reached by the sea only during gales:
 the upper part, which is from six to twelve feet high, is clothed with
 vegetation. The surface of the islet gently slopes to the lagoon.
 F.—Level of the lagoon at low water.


The section is true to the scale in a horizontal line, but it could not
be made so in a vertical one, as the average greatest height of the
land is only between six and twelve feet above high-water mark.
I will describe the section, commencing with the outer margin. I must
first observe that the reef-building polypifers, not being tidal
animals, require to be constantly submerged or washed by the breakers.
I was assured by Mr. Liesk, a very intelligent resident on these
islands, as well as by some chiefs at Tahiti (Otaheite), that an
exposure to the rays of the sun for a very short time invariably causes
their destruction. Hence it is possible only under the most favourable
circumstances, afforded by an unusually low tide and smooth water, to
reach the outer margin, where the coral is alive. I succeeded only
twice in gaining this part, and found it almost entirely composed of a
living Porites, which forms great irregularly rounded masses (like
those of an Astræa, but larger) from four to eight feet broad, and
little less in thickness. These mounds are separated from each other by
narrow crooked channels, about six feet deep, most of which intersect
the line of reef at right angles. On the furthest mound, which I was
able to reach by the aid of a leaping-pole, and over which the sea
broke with some violence, although the day was quite calm and the tide
low, the polypifers in the uppermost cells were all dead, but between
three and four inches lower down on its side they were living, and
formed a projecting border round the upper and dead surface. The coral
being thus checked in its upward growth, extends laterally, and hence
most of the masses, especially those a little further inwards, had
broad flat dead summits. On the other hand I could see, during the
recoil of the breakers, that a few yards further seaward, the whole
convex surface of the Porites was alive; so that the point where we
were standing was almost on the exact upward and shoreward limit of
existence of those corals which form the outer margin of the reef. We
shall presently see that there are other organic productions, fitted to
bear a somewhat longer exposure to the air and sun.

Next, but much inferior in importance to the Porites, is the _
Millepora complanata._[1]

 [1] This Millepora (Palmipora of Blainville), as well as the _M.
 alcicornis_, possesses the singular property of stinging the skin
 where it is delicate, as on the face and arm.

It grows in thick vertical plates, intersecting each other at various
angles, and forms an exceedingly strong honeycombed mass, which
generally affects a circular form, the marginal plates alone being
alive. Between these plates and in the protected crevices on the reef,
a multitude of branching zoophytes and other productions flourish, but
the Porites and Millepora alone seem able to resist the fury of the
breakers on its upper and outer edge: at the depth of a few fathoms
other kinds of stony corals live. Mr. Liesk, who was intimately
acquainted with every part of this reef, and likewise with that of
North Keeling atoll, assured me that these corals invariably compose
the outer margin. The lagoon is inhabited by quite a distinct set of
corals, generally brittle and thinly branched; but a Porites,
apparently of the same species with that on the outside, is found
there, although it does not seem to thrive, and certainly does not
attain the thousandth part in bulk of the masses opposed to the
breakers.


The woodcut shows the form of the bottom off the reef: the water
deepens for a space between one and two hundred yards wide, very
gradually to twenty-five fathoms (A in section), beyond which the sides
plunge into the unfathomable ocean at an angle of 45°.[2] To the depth
of ten or twelve fathoms the bottom is exceedingly rugged, and seems
formed of great masses of living coral, similar to those on the margin.
The arming of the lead here invariably came up quite clean, but deeply
indented, and chains and anchors which were lowered, in the hopes of
tearing up the coral, were broken. Many small fragments, however, of _
Millepora alcicornis_ were brought up; and on the arming from an
eight-fathom cast, there was a perfect impression of an Astræa,
apparently alive. I examined the rolled fragments cast on the beach
during gales, in order further to ascertain what corals grew outside
the reef. The fragments consisted of many kinds, of which the Porites
already mentioned and a Madrepora, apparently the _M. corymbosa_, were
the most abundant. As I searched in vain in the hollows on the reef and
in the lagoon, for a living specimen of this Madrepore, I conclude that
it is confined to a zone outside, and beneath the surface, where it
must be very abundant. Fragments of the _Millepora alcicornis_ and of
an Astræa were also numerous; the former is found, but not in
proportionate numbers, in the hollows on the reef; but the Astræa I did
not see living. Hence we may infer, that these are the kinds of coral
which form the rugged sloping surface (represented in the woodcut by an
uneven line), round and beneath the external margin. Between twelve and
twenty fathoms the arming came up an equal number of times smoothed
with sand, and indented with coral: an anchor and lead were lost at the
respective depths of thirteen and sixteen fathoms. Out of twenty-five
soundings taken at a greater depth than twenty fathoms, every one
showed that the bottom was covered with sand; whereas, at a less depth
than twelve fathoms, every sounding showed that it was exceedingly
rugged, and free from all extraneous particles. Two soundings were
obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms. The sand brought up from these depths
consisted of finely triturated fragments of stony zoophytes, but not,
as far as I could distinguish, of a particle of any lamelliform genus:
fragments of shells were rare.

 [2] The soundings from which this section is laid down were taken with
 great care by Captain Fitzroy himself. He used a bell-shaped lead,
 having a diameter of four inches, and the armings each time were cut
 off and brought on board for me to examine. The arming is a
 preparation of tallow, placed in the concavity at the bottom of the
 lead. Sand, and even small fragments of rock, will adhere to it; and
 if the bottom be of rock it brings up an exact impression of its
 surface.

At a distance of 2,200 yards from the breakers, Captain Fitzroy found
no bottom with a line of 7,200 feet in length; hence the submarine
slope of this coral formation is steeper than that of any volcanic
cone. Off the mouth of the lagoon, and likewise off the northern point
of the atoll, where the currents act violently, the inclination, owing
to the accumulation of sediment, is less. As the arming of the lead
from
all the greater depths showed a smooth sandy bottom, I at first
concluded that the whole consisted of a vast conical pile of calcareous
sand, but the sudden increase of depth at some points, and the
circumstance of the line having been cut, as if rubbed, when between
five hundred and six hundred fathoms were out, indicate the probable
existence of submarine cliffs.

On the margin of the reef, close within the line where the upper
surface of the Porites and of the Millepora is dead, three species of
Nullipora flourish. One grows in thin sheets, like a lichen on old
trees; the second in stony knobs, as thick as a man’s finger, radiating
from a common centre; and the third, which is less common, in a
moss-like reticulation of thin, but perfectly rigid branches.[3] The
three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour. These Nulliporæ, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each
tide by breaking water, for they are not found in any abundance in the
protected hollows on the back part of the reef, where they might be
immersed either during the whole or an equal proportional time of each
tide. It is remarkable that organic productions of such extreme
simplicity, for the Nulliporæ undoubtedly belong to one of the lowest
classes of the vegetable kingdom, should be limited to a zone so
peculiarly circumstanced. Hence the layer composed by their growth
merely fringes the reef for a space of about twenty yards in width,
either under the form of separate mammillated projections, where the
outer masses of coral are separate, or, more commonly, where the corals
are united into a solid margin, as a continuous smooth convex mound (B
in woodcut), like an artificial breakwater. Both the mound and
mammillated projections stand about three feet higher than any other
part of the reef, by which term I do not include the islets, formed by
the accumulation of rolled fragments. We shall hereafter see that other
coral reefs are protected by a similar thick growth of Nulliporæ on the
outer margin, the part most exposed to the breakers, and this must
effectually aid in preserving it from being worn down.

 [3] This last species is of a beautiful bright peach-blossom colour.
 Its branches are about as thick as crow-quills; they are slightly
 flattened and knobbed at the extremities. The extremities only are
 alive and brightly coloured. The two other species are of a dirty
 purplish-white. The second species is extremely hard; its short
 knob-like branches are cylindrical, and do not grow thicker at their
 extremities.

The woodcut represents a section across one of the islets on the reef,
but if all that part which is above the level of C were removed, the
section would be that of a simple reef, as it occurs where no islet has
been formed. It is this reef which essentially forms the atoll. It is a
ring, enclosing the lagoon on all sides except at the northern end,
where there are two open spaces, through one of which ships can enter.
The reef varies in width from two hundred and fifty to five
hundred yards, its surface is level, or very slightly inclined towards
the lagoon, and at high tide the sea breaks entirely over it: the water
at low tide thrown by the breakers on the reef, is carried by the many
narrow and shoal gullies or channels on its surface, into the lagoon: a
return stream sets out of the lagoon through the main entrance. The
most frequent coral in the hollows on the reef is _Pocillopora
verrucosa_, which grows in short sinuous plates, or branches, and when
alive is of a beautiful pale lake-red: a Madrepora, closely allied or
identical with _M. pocillifera_, is also common. As soon as an islet is
formed, and the waves are prevented breaking entirely over the reef,
the channels and hollows in it become filled up with cemented
fragments, and its surface is converted into a hard smooth floor (C of
woodcut), like an artificial one of freestone. This flat surface varies
in width from one hundred to two hundred, or even three hundred yards,
and is strewed with a few large fragments of coral torn up during
gales: it is uncovered only at low water. I could with difficulty, and
only by the aid of a chisel, procure chips of rock from its surface,
and therefore could not ascertain how much of it is formed by the
aggregation of detritus, and how much by the outward growth of mounds
of corals, similar to those now living on the margin. Nothing can be
more singular than the appearance at low tide of this “flat” of naked
stone, especially where it is externally bounded by the smooth convex
mound of Nulliporæ, appearing like a breakwater built to resist the
waves, which are constantly throwing over it sheets of foaming water.
The characteristic appearance of this “flat” is shown in the foregoing
woodcut of Whitsunday atoll.

The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile
of fragments, thrown together by some unusually strong gale. Their
ordinary width is under a quarter of a mile, and their length varies
from a few yards to several miles. Those on the south-east and windward
side of the atoll, increase solely by the addition of fragments on
their outer side; hence the loose blocks of coral, of which their
surface is composed, as well as the shells mingled with them, almost
exclusively consist of those kinds which live on the outer coast. The
highest part of the islets (excepting hillocks of blown sand, some of
which are thirty feet high), is close to the outer beach (E of the
woodcut), and averages from six to ten feet above ordinary high-water
mark. From the outer beach the surface slopes gently to the shores of
the lagoon, which no doubt has been caused by the breakers the further
they have rolled over the reef, having had less power to throw up
fragments. The little waves of the lagoon heap up sand and fragments of
thinly-branched corals on the inner side of the islets on the leeward
side of the atoll; and these islets are broader than those to windward,
some being even eight hundred yards in width; but the land thus added
is very low. The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (D of the woodcut), projecting
some yards in front of the outer shore and from two to four feet high.
This ledge is just reached by the waves at ordinary high-water:
it extends in front of all the islets, and everywhere has a water-worn
and scooped appearance. The fragments of coral which are occasionally
cast on the “flat” are during gales of unusual violence swept together
on the beach, where the waves each day at high-water tend to remove and
gradually wear them down; but the lower fragments having become firmly
cemented together by the percolation of calcareous matter, resist the
daily tides longer, and hence project as a ledge. The cemented mass is
generally of a white colour, but in some few parts reddish from
ferruginous matter; it is very hard, and is sonorous under the hammer;
it is obscurely divided by seams, dipping at a small angle seaward; it
consists of fragments of the corals which grow on the outer margin,
some quite and others partially rounded, some small and others between
two and three feet across; and of masses of previously formed
conglomerate, torn up, rounded, and re-cemented; or it consists of a
calcareous sandstone, entirely composed of rounded particles, generally
almost blended together, of shells, corals, the spines of echini, and
other such organic bodies; rocks, of this latter kind, occur on many
shores, where there are no coral reefs. The structure of the coral in
the conglomerate has generally been much obscured by the infiltration
of spathose calcareous matter; and I collected a very interesting
series, beginning with fragments of unaltered coral, and ending with
others, where it was impossible to discover with the naked eye any
trace of organic structure. In some specimens I was unable, even with
the aid of a lens, and by wetting them, to distinguish the boundaries
of the altered coral and spathose limestone. Many even of the blocks of
coral lying loose on the beach, had their central parts altered and
infiltrated.

The lagoon alone remains to be described; it is much shallower than
that of most atolls of considerable size. The southern part is almost
filled up with banks of mud and fields of coral, both dead and alive,
but there are considerable spaces, between three and four fathoms, and
smaller basins, from eight to ten fathoms deep. Probably about half its
area consists of sediment, and half of coral-reefs. The corals
composing these reefs have a very different aspect from those on the
outside; they are very numerous in kind, and most of them are thinly
branched. Meandrina, however, lives in the lagoon, and great rounded
masses of this coral are numerous, lying quite or almost loose on the
bottom. The other commonest kinds consist of three closely allied
species of true Madrepora in thin branches; of _Seriatapora subulata_;
two species of Porites[4] with cylindrical branches, one of which forms
circular clumps, with the exterior branches only alive; and lastly, a
coral something like an Explanaria, but with stars on both surfaces,
growing in thin, brittle, stony, foliaceous expansions, especially in
the deeper basins of the lagoon. The reefs on which
these corals grow are very irregular in form, are full of cavities, and
have not a solid flat surface of dead rock, like that surrounding the
lagoon; nor can they be nearly so hard, for the inhabitants made with
crowbars a channel of considerable length through these reefs, in which
a schooner, built on the S.E. islet, was floated out. It is a very
interesting circumstance, pointed out to us by Mr. Liesk, that this
channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh
excavations would be absolutely necessary to allow another vessel to
pass through it.

 [4] This Porites has somewhat the habit of _P. clavaria_, but the
 branches are not knobbed at their ends. When alive it is of a yellow
 colour, but after having been washed in fresh water and placed to dry,
 a jet-black slimy substance exuded from the entire surface, so that
 the specimen now appears as if it had been dipped in ink.

The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand. Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its entire
solution in acids to be purely calcareous. I have seen in the Museum of
the Geological Society, a similar but more remarkable substance,
brought by Lieutenant Nelson from the reefs of Bermuda, which, when
shown to several experienced geologists, was mistaken by them for true
chalk. On the outside of the reef much sediment must be formed by the
action of the surf on the rolled fragments of coral; but in the calm
waters of the lagoon, this can take place only in a small degree. There
are, however, other and unexpected agents at work here: large shoals of
two species of Scarus, one inhabiting the surf outside the reef and the
other the lagoon, subsist entirely, as I was assured by Mr. Liesk, the
intelligent resident before referred to, by browsing on the living
polypifers. I opened several of these fish, which are very numerous and
of considerable size, and I found their intestines distended by small
pieces of coral, and finely ground calcareous matter. This must daily
pass from them as the finest sediment; much also must be produced by
the infinitely numerous vermiform and molluscous animals, which make
cavities in almost every block of coral. Dr. J. Allan, of Forres, who
has enjoyed the best means of observation, informs me in a letter that
the Holothuriæ (a family of Radiata) subsist on living coral; and the
singular structure of bone within the anterior extremity of their
bodies, certainly appears well adapted for this purpose. The number of
the species of Holothuria, and of the individuals which swarm on every
part of these coral-reefs, is extraordinarily great; and many shiploads
are annually freighted, as is well-known, for China with the trepang,
which is a species of this genus. The amount of coral yearly consumed,
and ground down into the finest mud, by these several creatures, and
probably by many other kinds, must be immense. These facts are,
however, of more importance in another point of view, as showing us
that there are living checks to the growth of coral-reefs, and that the
almost universal law of “consumed and be consumed,” holds good even
with the polypifers forming those massive bulwarks, which are able to
withstand the force of the open ocean.

Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation
of their detritus, one is naturally led to inquire how long it has
continued, and how long it is likely to continue, in its present state.
Mr. Liesk informed me that he had seen an old chart in which the
present long island on the S.E. side was divided by several channels
into as many islets; and he assures me that the channels can still be
distinguished by the smaller size of the trees on them. On several
islets, also, I observed that only young cocoa-nut trees were growing
on the extremities; and that older and taller trees rose in regular
succession behind them; which shows that these islets have very lately
increased in length. In the upper and south-eastern part of the lagoon,
I was much surprised by finding an irregular field of at least a mile
square of branching corals, still upright, but entirely dead. They
consisted of the species already mentioned; they were of a brown
colour, and so rotten, that in trying to stand on them I sank halfway
up the leg, as if through decayed brushwood. The tops of the branches
were barely covered by water at the time of lowest tide. Several facts
having led me to disbelieve in any elevation of the whole atoll, I was
at first unable to imagine what cause could have killed so large a
field of coral. Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause;
for before this, a strong breeze by forcing water through them into the
head of the lagoon, would tend to raise its level. But now this cannot
happen, and the inhabitants observe that the tide rises to a less
height, during a high S.E. wind, at the head than at the mouth of the
lagoon. The corals, which, under the former condition of things, had
attained the utmost possible limit of upward growth, would thus
occasionally be exposed for a short time to the sun, and be killed.

Besides the increase of dry land, indicated by the foregoing facts, the
exterior solid reef appears to have grown outwards. On the western side
of the atoll, the “flat” lying between the margin of the reef and the
beach, is very wide; and in front of the regular beach with its
conglomerate basis, there is, in most parts, a bed of sand and loose
fragments with trees growing out of it, which apparently is not reached
even by the spray at high water. It is evident some change has taken
place since the waves formed the inner beach; that they formerly beat
against it with violence was evident, from a remarkably thick and
water-worn point of conglomerate at one spot, now protected by
vegetation and a bank of sand; that they beat against it in the same
peculiar manner in which the swell from windward now obliquely curls
round the margin of the reef, was evident from the conglomerate having
been worn into a point projecting from the beach in a similarly oblique
manner. This retreat in the line of action of the breakers might
result, either from the surface of the reef in front of the islets
having been submerged at one time, and afterward having grown upwards,
or from the mounds of coral on the margin having continued to grow
outwards. That an outward growth of this part is in process, can hardly
be doubted from the fact already mentioned of the mounds of Porites
with their summits apparently lately killed, and their sides only three
or four inches lower down thickened by a fresh layer of living coral.
But there is a difficulty on this supposition which I must not pass
over. If the
whole, or a large part of the “flat,” had been formed by the outward
growth of the margin, each successive margin would naturally have been
coated by the Nulliporæ, and so much of the surface would have been of
equal height with the existing zone of living Nulliporæ: this is not
the case, as may be seen in the woodcut. It is, however, evident from
the abraded state of the “flat,” with its original inequalities filled
up, that its surface has been much modified; and it is possible that
the hinder portions of the zone of Nulliporæ, perishing as the reef
grows outwards, might be worn down by the surf. If this has not taken
place, the reef can in no part have increased outwards in breadth since
its formation, or at least since the Nulliporæ formed the convex mound
on its margin; for the zone thus formed, and which stands between two
and three feet above the other parts of the reef, is nowhere much above
twenty yards in width.

Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there
are others having an opposite tendency. On the south-east side,
Lieutenant Sulivan, to whose kindness I am indebted for many
interesting observations, found the conglomerate projecting on the reef
nearly fifty yards in front of the beach: we may infer from what we see
in all other parts of the atoll, that the conglomerate was not
originally so much exposed, but formed the base of an islet, the front
and upper part of which has since been swept away. The degree to which
the conglomerate, round nearly the whole atoll, has been scooped,
broken up, and the fragments cast on the beach, is certainly very
surprising, even on the view that it is the office of occasional gales
to pile up fragments, and of the daily tides to wear them away. On the
western side, also, of the atoll, where I have described a bed of sand
and fragments with trees growing out of it, in front of an old beach,
it struck both Lieutenant Sulivan and myself, from the manner in which
the trees were being washed down, that the surf had lately recommenced
an attack on this line of coast. Appearances indicating a slight
encroachment of the water on the land, are plainer within the lagoon: I
noticed in several places, both on its windward and leeward shores, old
cocoa-nut trees falling with their roots undermined, and the rotten
stumps of others on the beach, where the inhabitants assured us the
cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near
the settlement, the foundation posts of a shed, now washed by every
tide, but which the inhabitants stated, had seven years before stood
above high watermark. In the calm waters of the lagoon, directly
connected with a great, and therefore stable ocean, it seems very
improbable that a change in the currents, sufficiently great to cause
the water to eat into the land on all sides, should have taken place
within a limited period. From these considerations I inferred, that
probably the atoll had lately subsided to a small amount; and this
inference was strengthened by the circumstance, that in 1834, two years
before our visit, the island had been shaken by a severe earthquake,
and by two slighter ones during the ten previous years. If, during
these subterranean disturbances, the atoll did subside, the downward
movement must have been very small, as we must
conclude from the fields of dead coral still lipping the surface of the
lagoon, and from the breakers on the western shore not having yet
regained the line of their former action. The subsidence must, also,
have been preceded by a long period of rest, during which the islets
extended to their present size, and the living margin of the reef grew
either upwards, or as I believe outwards, to its present distance from
the beach.

Whether this view be correct or not, the above facts are worthy of
attention, as showing how severe a struggle is in progress on these low
coral formations between the two nicely balanced powers of land and
water. With respect to the future state of Keeling atoll, if left
undisturbed, we can see that the islets may still extend in length; but
as they cannot resist the surf until broken by rolling over a wide
space, their increase in breadth must depend on the increasing breadth
of the reef; and this must be limited by the steepness of the submarine
flanks, which can be added to only by sediment derived from the wear
and tear of the coral. From the rapid growth of the coral in the
channel cut for the schooner, and from the several agents at work in
producing fine sediment, it might be thought that the lagoon would
necessarily become quickly filled up. Some of this sediment, however,
is transported into the open sea, as appears from the soundings off the
mouth of the lagoon, instead of being deposited within it. The
deposition, moreover, of sediment, checks the growth of coral-reefs, so
that these two agencies cannot act together with full effect in filling
it up. We know so little of the habits of the many different species of
corals, which form the lagoon-reefs, that we have no more reasons for
supposing that their whole surface would grow up as quickly as the
coral did in the schooner-channel, than for supposing that the whole
surface of a peat-moss would increase as quickly as parts are known to
do in holes, where the peat has been cut away. These agencies,
nevertheless, tend to fill up the lagoon; but in proportion as it
becomes shallower, so must the polypifers be subject to many injurious
agencies, such as impure water and loss of food. For instance, Mr.
Liesk informed me, that some years before our visit unusually heavy
rain killed nearly all the fish in the lagoon, and probably the same
cause would likewise injure the corals. The reefs also, it must be
remembered, cannot possibly rise above the level of the lowest
spring-tide, so that the final conversion of the lagoon into land must
be due to the accumulation of sediment; and in the midst of the clear
water of the ocean, and with no surrounding high land, this process
must be exceedingly slow.

_Section II_—GENERAL DESCRIPTION OF ATOLLS.

General form and size of atolls, their reefs and islets.—External
slope.—Zone of Nulliporæ.—Conglomerate.—Depth of
lagoons.—Sediment.—Reefs submerged wholly or in part.—Breaches in the
reef.—Ledge-formed shores round certain lagoons.—Conversion of lagoons
into land.

I will here give a sketch of the general form and structure of the many
atolls and atoll-formed reefs which occur in the Pacific and Indian
Oceans, comparing them with Keeling atoll. The Maldiva atolls
and the Great Chagos Bank differ in so many respects, that I shall
devote to them, besides occasional references, a third section of this
chapter. Keeling atoll may be considered as of moderate dimensions and
of regular form. Of the thirty-two islands surveyed by Captain Beechey
in the Low Archipelago, the longest was found to be thirty miles, and
the shortest less than a mile; but Vliegen atoll, situated in another
part of the same group, appears to be sixty miles long and twenty
broad. Most of the atolls in this group are of an elongated form; thus
Bow Island is thirty miles in length, and on an average only six in
width (See Fig. 4, Plate I), and Clermont Tonnere has nearly the same
proportions. In the Marshall Archipelago (the Ralick and Radack group
of Kotzebue) several of the atolls are more than thirty miles in
length, and Rimsky Korsacoff is fifty-four long, and twenty wide, at
the broadest part of its irregular outline. Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, however,
bears a double name) measured in a medial and slightly curved line, is
no less than eighty-eight geographical miles long, its greatest width
being under twenty, and its least only nine and a half miles. Some
atolls have spurs projecting from them; and in the Marshall group there
are atolls united together by linear reefs, for instance Menchikoff
Island (See Fig. 3, Plate II), which is sixty miles in length, and
consists of three loops tied together. In far the greater number of
cases an atoll consists of a simple elongated ring, with its outline
moderately regular.

The average width of the annular wreath may be taken as about a quarter
of a mile. Captain Beechey[5] says that in the atolls of the Low
Archipelago it exceeded in no instance half a mile. The description
given of the structure and proportional dimensions of the reef and
islets of Keeling atoll, appears to apply perfectly to nearly all the
atolls in the Pacific and Indian Oceans. The islets are first formed
some way back either on the projecting points of the reef, especially
if its form be angular, or on the sides of the main entrances into the
lagoon—that is in both cases, on points where the breakers can act
during gales of wind in somewhat different directions, so that the
matter thrown up from one side may accumulate against that before
thrown up from another. In Lutké’s chart of the Caroline atolls, we see
many instances of the former case; and the occurrence of islets, as if
placed for beacons, on the points where there is a gateway or breach
through the reef, has been noticed by several authors. There are some
atoll-formed reefs, rising to the surface of the sea and partly dry at
low water, on which from some cause islets have never been formed; and
there are others on which they have been formed, but have subsequently
been worn away. In atolls of small dimensions the islets frequently
become united into a single horse-shoe or ring-formed strip; but Diego
Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in
width. To show how small the total area of the annular reef and the
land is in islands of this class,
I may quote a remark from the voyage of Lutké, namely, that if the
forty-three rings, or atolls, in the Caroline Archipelago, were put one
within another, and over a steeple in the centre of St. Petersburg, the
whole world would not cover that city and its suburbs.

 [5] Beechey’s “Voyage to the Pacific and Beering’s Straits,” chapter
 viii.


The form of the bottom off Keeling atoll, which gradually slopes to
about twenty fathoms at the distance of between one and two hundred
yards from the edge of the reef, and then plunges at an angle of 45°
into unfathomable depths, is exactly the same[6] with that of the
sections of the atolls in the Low Archipelago given by Captain Beechey.
The nature, however, of the bottom seems to differ, for this officer[7]
informs me that all the soundings, even the deepest, were on coral, but
he does not know whether dead or alive. The slope round Christmas atoll
(Lat. 1° 4′ N., 157° 45′ W.), described by Cook,[8] is considerably
less, at about half a mile from the edge of the reef, the average depth
was about fourteen fathoms on a fine sandy bottom, and at a mile, only
between twenty and forty fathoms. It has no doubt been owing to this
gentle slope, that the strip of land surrounding its lagoon, has
increased in one part to the extraordinary width of three miles; it is
formed of successive ridges of broken shells and corals, like those on
the beach. I know of no other instance of such width in the reef of an
atoll; but Mr. F. D. Bennett informs me that the inclination of the
bottom round Caroline atoll in the Pacific, is like that off Christmas
Island, very gentle. Off the Maldiva and Chagos atolls, the inclination
is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell[9]
found fifty and sixty fathoms close to the edge of the reef, and at 300
yards distance there was no bottom with a 300-yard line. Captain
Moresby informs me, that at 100 fathoms from the mouth of the lagoon of
Diego Garcia, he found no bottom with 150 fathoms; this is the more
remarkable, as the slope is generally less abrupt in front of channels
through a reef, owing to the accumulation of sediment. At Egmont
Island, also, at 150 fathoms from the reef, soundings were struck with
150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef,
no bottom was obtained, as I am informed by Captain Moresby, with a
line of 200 fathoms! The currents run with great force round these
atolls, and where they are strongest, the inclination appears to be
most abrupt. I am informed by the same authority, that wherever
soundings were obtained off these islands, the bottom was invariably
sandy: nor was there any reason to suspect the existence of submarine
cliffs, as there was at Keeling Island.[10] Here then occurs a
difficulty; can sand accumulate on a slope, which, in some cases,
appears to exceed fifty-five degrees? It must be observed, that I speak
of slopes where soundings were obtained, and not of such cases, as that
of Cardoo, where the nature of the bottom is unknown, and where its
inclination must be nearly vertical. M. Elie de Beaumont[11] has
argued, and there is no higher authority on this subject, from the
inclination at which snow slides down in avalanches, that a bed of sand
or mud cannot be formed at a greater angle than thirty degrees.
Considering the number of soundings on sand, obtained round the Maldiva
and Chagos atolls, which appears to indicate a greater angle, and the
extreme abruptness of the sand-banks in the West Indies, as will be
mentioned in the Appendix, I must conclude that the adhesive property
of wet sand counteracts its gravity, in a much greater ratio than has
been allowed for by M. Elie de Beaumont. From the facility with which
calcareous sand becomes agglutinated, it is not necessary to suppose
that the bed of loose sand is thick.

 [6] The form of the bottom round the Marshall atolls in the Northern
 Pacific is probably similar: Kotzebue (“First Voyage,” vol. ii, p. 16)
 says: “We had at a small distance from the reef, forty fathoms depth,
 which increased a little further so much that we could find no
 bottom.”


 [7] I must be permitted to express my obligation to Captain Beechey,
 for the very kind manner in which he has given me information on
 several points, and to own the great assistance I have derived from
 his excellent published work.


 [8] Cook’s “Third Voyage,” vol. ii, chap. 10.


 [9] This fact is taken from a MS. account of these groups lent me by
 Captain Moresby. See also Captain Moresby’s paper on the Maldiva
 atolls in the _Geographical Journal_, vol. v, p. 401.


 [10] Off some of the islands in the Low Archipelago the bottom appears
 to descend by ledges. Off Elizabeth Island, which, however, consists
 of raised coral, Captain Beechey (page 45, 4to ed.) describes three
 ledges: the first had an easy slope from the beach to a distance of
 about fifty yards: the second extended two hundred yards with
 twenty-five fathoms on it, and then ended abruptly, like the first;
 and immediately beyond this there was no bottom with two hundred
 fathoms.


 [11] “Memoires pour servir à une description Geolog. de France,” tome
 iv, p. 216.


Captain Beechey has observed, that the submarine slope is much less at
the extremities of the more elongated atolls in the Low Archipelago,
than at their sides; in speaking of Ducie’s Island he says[12] the
buttress, as it may be called, which “has the most powerful enemy (the
S.W. swell) to oppose, is carried out much further, and with less
abruptness than the other.” In some cases, the less inclination of a
certain part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a prevailing
current which there accumulates a bed of sand. Where the water is
perfectly tranquil, as within a lagoon, the reefs generally grow up
perpendicularly, and sometimes even overhang their bases; on the other
hand, on the leeward side of Mauritius, where the water is generally
tranquil, although not invariably so, the reef is very gently inclined.
Hence it appears that the exterior angle varies much; nevertheless in
the close similarity in form between the sections of Keeling atoll and
of the atolls in the Low Archipelago, in the general steepness of the
reefs of the Maldiva and Chagos atolls, and in the perpendicularity of
those rising out of water always tranquil, we may discern the effects
of uniform laws; but from the complex action of the surf and currents,
on the growing powers of the coral and on the deposition of sediment,
we can by no means follow out all the results.

 [12] Beechey’s “Voyage,” 4to ed., p. 44.


Where islets have been formed on the reef, that part which I have
sometimes called the “flat” and which is partly dry at low water,
appears similar in every atoll. In the Marshall group in the North
Pacific, it may be inferred from Chamisso’s description, that the reef,
where islets have not been formed on it, slopes gently from the
external margin to the shores of the lagoon; Flinders states that the
Australian barrier has a similar inclination inwards, and I have no
doubt it is of general occurrence, although, according to Ehrenberg,
the reefs of the Red Sea offer an exception. Chamisso observes that
“the red colour of the reef (at the Marshall atolls) under the breakers
is caused by a Nullipora, which covers the stone _wherever the waves
beat_; and, under favourable circumstances, assumes a stalactical
form,”—a description perfectly applicable to the margin of Keeling
atoll.[13] Although Chamisso does not state that the masses of
Nulliporæ form points or a mound, higher than the flat, yet I believe
that this is the case; for Kotzebue,[14] in another part, speaks of the
rocks on the edge of the reef “as visible for about two feet at low
water,” and these rocks we may feel quite certain are not formed of
true coral,[15] Whether a smooth convex mound of Nulliporæ, like that
which appears as if artificially constructed to protect the margin of
Keeling Island, is of frequent occurrence round atolls, I know not; but
we shall presently meet with it, under precisely the same form, on the
outer edge of the “barrier-reefs” which encircle the Society Islands.

 [13] Kotzebue’s “First Voyage,” vol. iii, p. 142. Near Porto Praya, in
 the Cape de Verde Islands, some basaltic rocks, lashed by no
 inconsiderable surf, were completely enveloped with a layer of
 Nulliporæ. The entire surface over many square inches, was coloured of
 a peach-blossomed red; the layer, however, was of no greater thickness
 than paper. Another kind, in the form of projecting knobs, grew in the
 same situation. These Nulliporæ are closely related to those described
 on the coral-reefs, but I believe are of different species.


 [14] Kotzebue’s “First Voyage,” vol. ii, p. 16. Lieutenant Nelson, in
 his excellent memoir in the Geological Transactions (vol. ii, p. 105),
 alludes to the rocky points mentioned by Kotzebue, and infers that
 they consist of Serpulæ, which compose incrusting masses on the reefs
 of Bermudas, as they likewise do on a sandstone bar off the coast of
 Brazil (which I have described in _London Phil. Journal,_ October
 1841). These masses of Serpulæ hold the same position, relatively to
 the action of the sea, with the Nulliporæ on the coral-reefs in the
 Indian and Pacific Oceans.


 [15] Captain Moresby, in his valuable paper “on the Northern atolls of
 Maldivas” (_Geographical Journal_, vol. v), says that the edges of the
 reefs there stand above water at low spring-tides.


There appears to be scarcely a feature in the structure of Keeling
reef, which is not of common, if not of universal occurrence, in other
atolls. Thus Chamisso describes[16] a layer of coarse conglomerate,
outside the islets round the Marshall atolls which “appears on its
upper surface uneven and eaten away.” From drawings, with appended
remarks, of Diego Garcia in the Chagos group and of several of the
Maldiva atolls, shown me by Captain Moresby,[17] it is evident that
their outer coasts are subject to the same round of decay and
renovation as those of Keeling atoll. From the description of the
atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it
is not apparent that any conglomerate coral-rock was there observed.

 [16] Kotzebue’s “First Voyage,” vol. iii, p. 144.


 [17] See also Moresby on the Northern atolls of the Maldivas,
 _Geographical Journal_, vol v, p. 400.


The lagoon in Keeling atoll is shallow; in the atolls of the Low
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
Group, according to Chamisso, from 30 to 35; in the Caroline atolls it
is only a little less. Within the Maldiva atolls there are large spaces
with 45 fathoms, and some soundings are laid down of 49 fathoms. The
greater part of the bottom in most lagoons, is formed of sediment;
large spaces have exactly the same depth, or the depth varies so
insensibly, that it is evident that no other means, excepting aqueous
deposition, could have leveled the surface so equally. In the Maldiva
atolls this is very conspicuous, and likewise in some of the Caroline
and Marshall Islands. In the former large spaces consist of sand and
_soft clay_; and Kotzebue speaks of clay having been found within one
of the Marshall atolls. No doubt this clay is calcareous mud, similar
to that at Keeling Island, and to that at Bermuda already referred to,
as undistinguishable from disintegrated chalk, and which Lieutenant
Nelson says is called there pipe-clay.[18]

 [18] I may here observe that on the coast of Brazil, where there is
 much coral, the soundings near the land are described by Admiral
 Roussin, in the _Pilote du Brésil_, as siliceous sand, mingled with
 much finely comminuted particles of shells and coral. Further in the
 offing, for a space of 1,300 miles along the coast, from the Abrolhos
 Islands to Maranham, the bottom in many places is composed of “tuf
 blanc, mêlé ou formé de madrépores broyés.” This white substance,
 probably, is analogous to that which occurs within the above-mentioned
 lagoons; it is sometimes, according to Roussin, firm, and he compares
 it to mortar.


Where the waves act with unequal force on the two sides of an atoll,
the islets appear to be first formed, and are generally of greater
continuity on the more exposed shore. The islets, also, which are
placed to leeward, are in most parts of the Pacific liable to be
occasionally swept entirely away by gales, equalling hurricanes in
violence, which blow in an opposite direction to the ordinary
trade-wind. The absence of the islets on the leeward side of atolls, or
when present their lesser dimensions compared with those to windward,
is a comparatively unimportant fact; but in several instances the reef
itself on the leeward side, retaining its usual defined outline, does
not rise to the surface by several fathoms. This is the case with the
southern side of Peros Banhos (Plate 1, Fig. 9) in the Chagos group,
with Mourileu atoll,[19] in the Caroline Archipelago, and with the
barrier-reef (Plate I, Fig. 8) of the Gambier Islands. I allude to the
latter reef, although belonging to
another class, because Captain Beechey was first led by it to observe
the peculiarity in the question. At Peros Banhos the submerged part is
nine miles in length, and lies at an average depth of about five
fathoms; its surface is nearly level, and consists of hard stone, with
a thin covering of loose sand. There is scarcely any living coral on
it, even on the outer margin, as I have been particularly assured by
Captain Moresby; it is, in fact, a wall of dead coral-rock, having the
same width and transverse section with the reef in its ordinary state,
of which it is a continuous portion. The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same
manner as on the sides of an ordinary passage through the reef. The
reef to leeward in other cases is nearly or quite obliterated, and one
side of the lagoon is left open; for instance, at Oulleay (Caroline
Archipelago), where a crescent-formed reef is fronted by an irregular
bank, on which the other half of the annular reef probably once stood.
At Namonouito, in the same Archipelago, both these modifications of the
reef concur; it consists of a great flat bank, with from twenty to
twenty-five fathoms water on it; for a length of more than forty miles
on its southern side it is open and without any reef, whilst on the
other sides it is bounded by a reef, in parts rising to the surface and
perfectly characterised, in parts lying some fathoms submerged. In the
Chagos group there are annular reefs, entirely submerged, which have
the same structure as the submerged and defined portions just
described. The Speaker’s Bank offers an excellent example of this
structure; its central expanse, which is about twenty-two fathoms deep,
is twenty-four miles across; the external rim is of the usual width of
annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon. Captain Moresby believes the rim consists of dead
rock, thinly covered with sand, and he is certain this is the case with
the external rim of the Great Chagos Bank, which is also essentially a
submerged atoll. In both these cases, as in the submerged portion of
the reef at Peros Banhos, Captain Moresby feels sure that the quantity
of living coral, even on the outer edge overhanging the deep-sea water,
is quite insignificant. Lastly, in several parts of the Pacific and
Indian Oceans there are banks, lying at greater depths than in the
cases just mentioned, of the same form and size with the neighbouring
atolls, but with their atoll-like structure wholly obliterated. It
appears from the survey of Freycinet, that there are banks of this kind
in the Caroline Archipelago, and, as is reported, in the Low
Archipelago. When we discuss the origin of the different classes of
coral formations, we shall see that the submerged state of the whole of
some atoll-formed reefs, and of portions of others, generally but not
invariably on the leeward side, and the existence of more deeply
submerged banks now possessing little or no signs of their original
atoll-like structure, are probably the effects of a uniform
cause,—namely, the death of the coral, during the subsidence of the
area, in which the atolls or banks are situated.

 [19] Frederick Lutké’s “Voyage autour du Monde,” vol. ii, p. 291. See
 also his account of Namonouito, at pp. 97 and 105, and the chart of
 Oulleay in the Atlas.


There is seldom, with the exception of the Maldiva atolls, more than
two or three channels, and generally only one leading into the lagoon,
of sufficient depth for a ship to enter. in small atolls, there is
usually not even one. Where there is deep water, for instance above
twenty fathoms, in the middle of the lagoon, the channels through the
reef are seldom as deep as the centre,—it may be said that the rim only
of the saucer-shaped hollow forming the lagoon is notched. Mr.
Lyell[20] has observed that the growth of the coral would tend to
obstruct all the channels through a reef, except those kept open by
discharging the water, which during high tide and the greater part of
each ebb is thrown over its circumference. Several facts indicate that
a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed,
during the change of the monsoon, the sea discoloured to a distance off
the entrances into the Maldiva and Chagos atolls. This, probably, would
check the growth of the coral in them, far more effectually than a mere
current of water. In the many small atolls without any channel, these
causes have not prevented the entire ring attaining the surface. The
channels, like the submerged and effaced parts of the reef, very
generally though not invariably occur on the leeward side of the atoll,
or on that side, according to Beechey,[21] which, from running in the
same direction with the prevalent wind, is not fully exposed to it.
Passages between the islets on the reef, through which boats can pass
at high water, must not be confounded with ship-channels, by which the
annular reef itself is breached. The passages between the islets occur,
of course, on the windward as well as on the leeward side; but they are
more frequent and broader to leeward, owing to the lesser dimensions of
the islets on that side.

 [20] “Principles of Geology,” vol. iii, p. 289.


 [21] Beechey’s “Voyage,” 4to ed., vol. i, p. 189.


At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other
atolls. Chamisso,[22] speaking in general terms of the lagoons in the
Marshall atolls, says the lead generally sinks “from a depth of two or
three fathoms to twenty or twenty-four, and you may pursue a line in
which on one side of the boat you may see the bottom, and on the other
the azure-blue deep water.” The shores of the lagoon-like channel
within the barrier-reef at Vanikoro have a similar structure. Captain
Beechey has described a modification of this structure (and he believes
it is not uncommon) in two atolls in the Low Archipelago, in which the
shores of the lagoon descend by a few, broad, slightly inclined ledges
or steps: thus at Matilda atoll,[23] the great exterior reef, the
surface of which is gently inclined towards and beneath the surface of
the lagoon, ends abruptly in a little cliff three fathoms deep; at its
foot, a ledge forty yards wide extends, shelving gently inwards
like the surface-reef, and terminated by a second little cliff five
fathoms deep; beyond this, the bottom of the lagoon slopes to twenty
fathoms, which is the average depth of its centre. These ledges seem to
be formed of coral-rock; and Captain Beechey says that the lead often
descended several fathoms through holes in them. In some atolls, all
the coral reefs or knolls in the lagoon come to the surface at low
water; in other cases of rarer occurrence, all lie at nearly the same
depth beneath it, but most frequently they are quite irregular,—some
with perpendicular, some with sloping sides,—some rising to the
surface, and others lying at all intermediate depths from the bottom
upwards. I cannot, therefore, suppose that the union of such reefs
could produce even one uniformly sloping ledge, and much less two or
three, one beneath the other, and each terminated by an abrupt wall. At
Matilda Island, which offers the best example of the step-like
structure, Captain Beechey observes that the coral-knolls within the
lagoon are quite irregular in their height. We shall hereafter see that
the theory which accounts for the ordinary form of atolls, apparently
includes this occasional peculiarity in their structure.

 [22] Kotzebue’s “First Voyage,” vol. iii, p. 142.


 [23] Beechey’s “Voyage,” 4to ed., vol. i, p. 160. At Whitsunday Island
 the bottom of the lagoon slopes gradually towards the centre, and then
 deepens suddenly, the edge of the bank being nearly perpendicular.
 This bank is formed of coral and dead shells.


In the midst of a group of atolls, there sometimes occur small, flat,
very low islands of coral formation, which probably once included a
lagoon, since filled up with sediment and coral-reefs. Captain Beechey
entertains no doubt that this has been the case with the two small
islands, which alone of thirty-one surveyed by him in the Low
Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15 deg
S.) is described by Chamisso[24] as formed by a dam of madreporitic
rock inclosing a flat space, thinly covered with trees, into which the
sea on the leeward side occasionally breaks. North Keeling atoll
appears to be in a rather less forward stage of conversion into land;
it consists of a horse-shoe shaped strip of land surrounding a muddy
flat, one mile in its longest axis, which is covered by the sea only at
high water. When describing South Keeling atoll, I endeavoured to show
how slow the final process of filling up a lagoon must be;
nevertheless, as all causes do tend to produce this effect, it is very
remarkable that not one instance, as I believe, is known of a
moderately sized lagoon being filled up even to the low water-line at
spring-tides, much less of such a one being converted into land. It is,
likewise, in some degree remarkable, how few atolls, except small ones,
are surrounded by a single linear strip of land, formed by the union of
separate islets. We cannot suppose that the many atolls in the Pacific
and Indian Oceans all have had a late origin, and yet should they
remain at their present level, subjected only to the action of the sea
and to the growing powers of the coral, during as many centuries as
must have elapsed since any of the earlier tertiary epochs, it cannot,
I think, be doubted that their lagoons and the islets on their reef,
would present a totally different appearance from what they now do.
This consideration leads to the suspicion that some renovating agency
(namely subsidence) comes into play at intervals, and perpetuates their
original structure.

 [24] Kotzebue’s “First Voyage,” vol. iii, p. 221.

_Section III_—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK

Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
depths in the lagoons of the southern atolls.—Reefs in the lagoons all
rising to the surface.—Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the
waves.—Destruction of islets.—Connection in the position and submarine
foundation of distinct atolls.—The apparent disseverment of large
atolls.—The Great Chagos Bank.—Its submerged condition and
extraordinary structure.

Although occasional references have been made to the Maldiva atolls,
and to the banks in the Chagos group, some points of their structure
deserve further consideration. My description is derived from an
examination of the admirable charts lately published from the survey of
Captain Moresby and Lieutenant Powell, and more especially from
information which Captain Moresby has communicated to me in the kindest
manner.

The Maldiva Archipelago is 470 miles in length, with an average breadth
of about 50 miles. The form and dimensions of the atolls, and their
singular position in a double line, may be seen, but not well, in the
greatly reduced chart (Fig. 6) in  Plate II. The dimensions of the
longest atoll in the group (called by the double name of
Milla-dou-Madou and Tilla-dou-Matte) have already been given; it is 88
miles in a medial and slightly curved line, and is less than 20 miles
in its broadest part. Suadiva, also, is a noble atoll, being 44 miles
across in one direction, and 34 in another, and the great included
expanse of water has a depth of between 250 and 300 feet. The smaller
atolls in this group differ in no respect from ordinary ones; but the
larger ones are remarkable from being breached by numerous deep-water
channels leading into the lagoon; for instance, there are 42 channels,
through which a ship could enter the lagoon of Suadiva. In the three
southern large atolls, the separate portions of reef between these
channels have the ordinary structure, and are linear; but in the other
atolls, especially the more northern ones, these portions are
ring-formed, like miniature atolls. Other ring-formed reefs rise out of
the lagoons, in the place of those irregular ones which ordinarily
occur there. In the reduction of the chart of Mahlos Mahdoo (Plate II,
Fig. 4), it was not found easy to define the islets and the little
lagoons within each reef, so that the ring-formed structure is very
imperfectly shown; in the large published charts of Tilla-dou-Matte,
the appearance of these rings, from standing further apart from each
other, is very remarkable. The rings on the margin are generally
elongated; many of them are three, and some even five miles, in
diameter; those within the lagoon are usually smaller, few being more
than two miles across, and the greater number rather less than one. The
depth of the little lagoon within these small annular reefs is
generally from five to seven fathoms, but occasionally more; and in Ari
atoll many of the central ones are twelve, and some even more than
twelve fathoms deep. These rings rise abruptly from the platform or
bank, on which they are placed; their outer margin is
invariably bordered by living coral[25] within which there is a flat
surface of coral rock; of this flat, sand and fragments have in many
cases accumulated and been converted into islets, clothed with
vegetation. I can, in fact, point out no essential difference between
these little ring-formed reefs (which, however, are larger, and contain
deeper lagoons than many true atolls that stand in the open sea), and
the most perfectly characterised atolls, excepting that the ring-formed
reefs are based on a shallow foundation, instead of on the floor of the
open sea, and that instead of being scattered irregularly, they are
grouped closely together on one large platform, with the marginal rings
arranged in a rudely formed circle.

 [25] Captain Moresby informs me that _Millepora complanata_ is one of
 the commonest kinds on the outer margin, as it is at Keeling atoll.

The perfect series which can be traced from portions of simple linear
reef, to others including long linear lagoons, and from these again to
oval or almost circular rings, renders it probable that the latter are
merely modifications of the linear or normal state. It is conformable
with this view, that the ring-formed reefs on the margin, even where
most perfect and standing furthest apart, generally have their longest
axes directed in the line which the reef would have held, if the atoll
had been bounded by an ordinary wall. We may also infer that the
central ring-formed reefs are modifications of those irregular ones,
which are found in the lagoons of all common atolls. It appears from
the charts on a large scale, that the ring-like structure is contingent
on the marginal channels or breaches being wide; and, consequently, on
the whole interior of the atoll being freely exposed to the waters of
the open sea. When the channels are narrow or few in number, although
the lagoon be of great size and depth (as in Suadiva), there are no
ring-formed reefs; where the channels are somewhat broader, the
marginal portions of reef, and especially those close to the larger
channels, are ring-formed, but the central ones are not so; where they
are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on
the openness of the marginal channels, the theory of their formation,
as we shall hereafter see, is included in that of the parent atolls, of
which they form the separate portions.

The lagoons of all the atolls in the southern part of the Archipelago
are from ten to twenty fathoms deeper than those in the northern part.
This is well exemplified in the case of Addoo, the southernmost atoll
in the group, for although only nine miles in its longest diameter, it
has a depth of thirty-nine fathoms, whereas all the other small atolls
have comparatively shallow lagoons; I can assign no adequate cause for
this difference in depth. In the central and deepest part of the
lagoons, the bottom consists, as I am informed by Captain Moresby, of
stiff clay (probably a calcareous mud); nearer the border it consists
of sand, and in the channels through the reef, of hard sand-banks,
sandstone, conglomerate rubble, and a little live coral. Close outside
the reef and the line joining its detached portions (where intersected
by many channels), the bottom is sandy, and it slopes abruptly into
unfathomable depths.
In most lagoons the depth is considerably greater in the centre than in
the channels; but in Tilla-dou-Matte, where the marginal ring-formed
reefs stand far apart, the same depth is carried across the entire
atoll, from the deep-water line on one side to that on the other. I
cannot refrain from once again remarking on the singularity of these
atolls,—a great sandy and generally concave disc rises abruptly from
the unfathomable ocean, with its central expanse studded and its border
symmetrically fringed with oval basins of coral-rock, just lipping the
surface of the sea, sometimes clothed with vegetation, and each
containing a little lake of clear water!

In the southern Maldiva atolls, of which there are nine large ones, all
the small reefs within the lagoons come to the surface, and are dry at
low water spring-tides; hence in navigating them, there is no danger
from submarine banks. This circumstance is very remarkable, as within
some atolls, for instance those of the neighbouring Chagos group, not a
single reef comes to the surface, and in most other cases a few only
do, and the rest lie at all intermediate depths from the bottom
upwards. When treating of the growth of coral I shall again refer to
this subject.

Although in the neighbourhood of the Maldiva Archipelago the winds,
during the monsoons, blow during nearly an equal time from opposite
quarters, and although, as I am informed by Captain Moresby, the
westerly winds are the strongest, yet the islets are almost all placed
on the eastern side of the northern atolls, and on the south-eastern
side of the southern atolls. That the formation of the islets is due to
detritus thrown up from the outside, as in the ordinary manner, and not
from the interior of the lagoons, may, I think be safely inferred from
several considerations, which it is hardly worth while to detail. As
the easterly winds are not the strongest, their action probably is
aided by some prevailing swell or current.

In groups of atolls, exposed to a trade-wind, the ship-channels into
the lagoons are almost invariably situated on the leeward or less
exposed side of the reef, and the reef itself is sometimes either
wanting there, or is submerged. A strictly analogous, but different
fact, may be observed at the Maldiva atolls—namely, that where two
atolls stand in front of each other, the breaches in the reef are the
most numerous on their near, and therefore less exposed, sides. Thus on
the near sides of Ari and the two Nillandoo atolls, which face S. Mãle,
Phaleedoo, and Moloque atolls, there are seventy-three deep-water
channels, and only twenty-five on their outer sides; on the near side
of the three latter named atolls there are fifty-six openings, and only
thirty-seven on their outsides. It is scarcely possible to attribute
this difference to any other cause than the somewhat different action
of the sea on the two sides, which would ensue from the protection
afforded by the two rows of atolls to each other. I may here remark
that in most cases, the conditions favourable to the greater
accumulation of fragments on the reef and to its more perfect
continuity on one side of the atoll than on the other, have concurred,
but this has not been the case with the Maldivas; for we have seen that
the islets are placed on the eastern or south-eastern
sides, whilst the breaches in the reef occur indifferently on any side,
where protected by an opposite atoll. The reef being more continuous on
the outer and more exposed sides of those atolls which stand near each
other, accords with the fact, that the reef of the southern atolls is
more continuous than that of the northern ones; for the former, as I am
informed by Captain Moresby, are more constantly exposed than the
northern atolls to a heavy surf.

The date of the first formation of some of the islets in this
Archipelago is known to the inhabitants; on the other hand, several
islets, and even some of those which are believed to be very old, are
now fast wearing away. The work of destruction has, in some instances,
been completed in ten years. Captain Moresby found on one water-washed
reef the marks of wells and graves, which were excavated when it
supported an islet. In South Nillandoo atoll, the natives say that
three of the islets were formerly larger: in North Nillandoo there is
one now being washed away; and in this latter atoll Lieutenant Prentice
found a reef, about six hundred yards in diameter, which the natives
positively affirmed was lately an island covered with cocoa-nut trees.
It is now only partially dry at low water spring-tides, and is (in
Lieutenant Prentice’s words) “entirely covered with live coral and
madrepore.” In the northern part, also, of the Maldiva Archipelago and
in the Chagos group, it is known that some of the islets are
disappearing. The natives attribute these effects to variations in the
currents of the sea. For my own part I cannot avoid suspecting that
there must be some further cause, which gives rise to such a cycle of
change in the action of the currents of the great and open ocean.

Several of the atolls in this Archipelago are so related to each other
in form and position, that at the first glance one is led to suspect
that they have originated in the disseverment of a single one. Mãle
consists of three perfectly characterised atolls, of which the shape
and relative position are such, that a line drawn closely round all
three, gives a symmetrical figure; to see this clearly, a larger chart
is required than that of the Archipelago in Plate II; the channel
separating the two northern Male atolls is only little more than a mile
wide, and no bottom was found in it with 100 fathoms. Powell’s Island
is situated at the distance of two miles and a half off the northern
end of Mahlos Mahdoo (see Fig. 4, Plate II), at the exact point where
the two sides of the latter, if prolonged, would meet; no bottom,
however, was found in the channel with 200 fathoms; in the wider
channel between Horsburgh atoll and the southern end of Mahlos Mahdoo,
no bottom was found with 250 fathoms. In these and similar cases, the
relation consists only in the form and position of the atolls. But in
the channel between the two Nillandoo atolls, although three miles and
a quarter wide, soundings were struck at the depth of 200 fathoms; the
channel between Ross and Ari atolls is four miles wide, and only 150
fathoms deep. Here then we have, besides the relation of form, a
submarine connection. The fact of soundings having been obtained
between two separate and perfectly characterised atolls is in itself
interesting, as it has never, I believe, been effected in any of the
many
other groups of atolls in the Pacific and Indian seas. In continuing to
trace the connection of adjoining atolls, if a hasty glance be taken at
the chart (Fig. 4, Plate II) of Mahlos Mahdoo, and the line of
unfathomable water be followed, no one will hesitate to consider it as
one atoll. But a second look will show that it is divided by a
bifurcating channel, of which the northern arm is about one mile and
three-quarters in width, with an average depth of 125 fathoms, and the
southern one three-quarters of a mile wide, and rather less deep. These
channels resemble in the slope of their sides and general form, those
which separate atolls in every respect distinct; and the northern arm
is wider than that dividing two of the Mãle atolls. The ring-formed
reefs on the sides of this bifurcating channel are elongated, so that
the northern and southern portions of Mahlos Mahdoo may claim, as far
as their external outline is concerned, to be considered as distinct
and perfect atolls. But the intermediate portion, lying in the fork of
the channel, is bordered by reefs less perfect than those which
surround any other atoll in the group of equally small dimensions.
Mahlos Mahdoo, therefore, is in every respect in so intermediate a
condition, that it may be considered either as a single atoll nearly
dissevered into three portions, or as three atolls almost perfect and
intimately connected. This is an instance of a very early stage of the
apparent disseverment of an atoll, but a still earlier one in many
respects is exhibited at Tilla-dou-Matte. In one part of this atoll,
the ring-formed reefs stand so far apart from each other, that the
inhabitants have given different names to the northern and southern
halves; nearly all the rings, moreover, are so perfect and stand so
separate, and the space from which they rise is so level and unlike a
true lagoon, that we can easily imagine the conversion of this one
great atoll, not into two or three portions, but into a whole group of
miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter
be seen, that the theory of subsidence, with the upward growth of the
coral, modified by accidents of probable occurrence, will account for
the occasional disseverment of large atolls.

_Plate II_—Great Chagos Bank, New Caledonia, Menchikoff Atoll, etc.


[Illustration: Great Chagos Bank]

Fig. 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey
by Captain Moresby and Lieutenant Powell; the parts which are shaded,
with the exception of two or three islets on the western and northern
sides, do not rise to the surface, but are submerged from four to ten
fathoms; the banks bounded by the dotted lines lie from fifteen to
twenty fathoms beneath the surface, and are formed of sand; the central
space is of mud, and from thirty to fifty fathoms deep.


Fig. 2.—A vertical section, on the same scale, in an eastern and
western line across the Great Chagos Bank, given for the sake of
exhibiting more clearly its structure.


[Illustration: New Caledonia, Menchikoff Atoll, etc.]

Fig. 3.—Menchikoff Atoll (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the
Pacific;” originally surveyed by Captain Hagemeister; the depth within
the lagoons is unknown.


Fig. 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs,
both on the margin and in the middle part, are meant to represent
little lagoons; but it was found not possible to distinguish them
clearly from the small islets, which have been formed on these same
small reefs; many of the smaller reefs could not be introduced; the
nautical mark (dot over a dash) over the figures 250 and 200, between
Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that
soundings were not obtained at these depths.


Fig. 5.—NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern’s “Atlas,” compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the “Atlas
of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef
is represented by a single line with crosses; I have for the sake of
uniformity added an interior line.


Fig. 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.


The Great Chagos bank alone remains to be described. In the Chagos
group there are some ordinary atolls, some annular reefs rising to the
surface but without any islets on them, and some atoll-formed banks,
either quite submerged, or nearly so. Of the latter, the Great Chagos
Bank is much the largest, and differs in its structure from the others:
a plan of it is given in Plate II, Fig. 1, in which, for the sake of
clearness, I have had the parts under ten fathoms deep finely shaded:
an east and west vertical section is given in Fig. 2, in which the
vertical scale has been necessarily exaggerated. Its longest axis is
ninety nautical miles, and another line drawn at right angles to the
first, across the broadest part, is seventy. The central part consists
of a level muddy flat, between forty and fifty fathoms deep, which is
surrounded on all sides, with the exception of some breaches, by the
steep edges of a set of banks, rudely arranged in a circle. These banks
consist of sand, with a very little live coral; they vary in breadth
from five to twelve miles, and on an average lie about sixteen fathoms
beneath the surface;
they are bordered by the steep edges of a third narrow and upper bank,
which forms the rim to the whole. This rim is about a mile in width,
and with the exception of two or three spots where islets have been
formed, is submerged between five and ten fathoms. It consists of
smooth hard rock, covered with a thin layer of sand, but with scarcely
any live coral; it is steep on both sides, and outwards slopes abruptly
into unfathomable depths. At the distance of less than half a mile from
one part, no bottom was found with 190 fathoms; and off another point,
at a somewhat greater distance, there was none with 210 fathoms. Small
steep-sided banks or knolls, covered with luxuriantly growing coral,
rise from the interior expanse to the same level with the external rim,
which, as we have seen, is formed only of dead rock. It is impossible
to look at the plan (Fig. 1, Plate II), although reduced to so small a
scale, without at once perceiving that the Great Chagos Bank is, in the
words of Captain Moresby,[26] “nothing more than a half-drowned atoll.”
But of what great dimensions, and of how extraordinary an internal
structure? We shall hereafter have to consider both the cause of its
submerged condition, a state common to other banks in the group, and
the origin of the singular submarine terraces, which bound the central
expanse: these, I think, it can be shown, have resulted from a cause
analogous to that which has produced the bifurcating channel across
Mahlos Mahdoo.

 [26] This officer has had the kindness to lend me an excellent MS.
 account of the Chagos Islands; from this paper, from the published
 charts, and from verbal information communicated to me by Captain
 Moresby, the above account of the Great Chagos Bank is taken.




Chapter II BARRIER REEFS


Closely resemble in general form and structure atoll-reefs.—Width and
depth of the lagoon-channels.—Breaches through the reef in front of
valleys, and generally on the leeward side.—Checks to the filling up of
the lagoon-channels.—Size and constitution of the encircled
islands.—Number of islands within the same reef.—Barrier-reefs of New
Caledonia and Australia.—Position of the reef relative to the slope of
the adjoining land.—Probable great thickness of barrier-reefs.

The term “barrier” has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to
that on the western coast of New Caledonia. At one time I thought it
convenient thus to restrict the term, but as these reefs are similar in
structure, and in position relatively to the land, to those, which,
like a wall with a deep moat within, encircle many smaller islands, I
have classed them together. The reef, also, on the west coast of New
Caledonia, circling round the extremities of the island, is an
intermediate form between a small encircling reef and the Australian
barrier, which stretches for a thousand miles in nearly a straight
line.

The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high
land rising from within their central expanse. The general resemblance
between the reefs of the barrier and atoll classes may be seen in the
small, but accurately reduced charts on Plate I,[1] and this
resemblance can be further shown to extend to every part of the
structure. Beginning with the outside of the reef; many scattered
soundings off Gambier, Oualan, and some other encircled islands, show
that close to the breakers there exists a narrow shelving margin,
beyond which the ocean becomes suddenly unfathomable; but off the west
coast of New Caledonia, Captain Kent[2] found no bottom with 150
fathoms, at two ships’ length from the reef; so that the slope here
must be nearly as precipitous as off the Maldiva atolls.

 [1] The authorities from which these charts have been reduced,
 together with some remarks on them are given in a separately appended
 page, descriptive of the Plates.


 [2] Dalrymple, “Hydrog. Mem.” vol. iii.

I can give little information regarding the kinds of corals which live
on the outer margin. When I visited the reef at Tahiti, although it was
low water, the surf was too violent for me to see the living masses;
but, according to what I heard from some intelligent native chiefs,
they resemble in their rounded and branchless forms, those on the
margin of Keeling atoll. The extreme verge of the reef, which was
visible between the breaking waves at low water, consisted of a
rounded, convex, artificial-like breakwater, entirely coated with
Nulliporæ, and absolutely similar to that which I have described at
Keeling atoll. From what I heard when at Tahiti, and from the writings
of the Revs. W. Ellis and J. Williams, I conclude that this peculiar
structure is common to most of the encircled islands of the Society
Archipelago. The reef within this mound or breakwater, has an extremely
irregular surface, even more so than between the islets on the reef of
Keeling atoll, with which alone (as there are no islets on the reef of
Tahiti) it can properly be compared. At Tahiti, the reef is very
irregular in width; but round many other encircled islands, for
instance, Vanikoro or Gambier Islands (Figs 1 and 8, Plate I), it is
quite as regular, and of the same average width, as in true atolls.
Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for
a short distance, and then terminates abruptly in a submarine wall,
forty feet high,—a structure absolutely similar to that described by
Chamisso in the Marshall atolls.

In the Society Archipelago, Ellis[3] states, that the reefs generally
lie at the distance of from one to one and a half miles, and,
occasionally, even at more than three miles, from the shore. The
central mountains are generally bordered by a fringe of flat, and often
marshy, alluvial
land, from one to four miles in width. This fringe consists of
coral-sand and detritus thrown up from the lagoon-channel, and of soil
washed down from the hills; it is an encroachment on the channel,
analogous to that low and inner part of the islets in many atolls which
is formed by the accumulation of matter from the lagoon. At Hogoleu
(Fig. 2, Plate I), in the Caroline Archipelago,[4] the reef on the
south side is no less than twenty miles; on the east side, five; and on
the north side, fourteen miles from the encircled high islands.

 [3] Consult, on this and other points, the “Polynesian Researches,” by
 the Rev. W. Ellis, an admirable work, full of curious information.


 [4] See “Hydrographical Mem.” and the “Atlas of the Voyage of the
 _Astrolabe_,” by Captain Dumont D’Urville, p. 428.


The lagoon channels may be compared in every respect with true lagoons.
In some cases they are open, with a level bottom of fine sand; in
others they are choked up with reefs of delicately branched corals,
which have the same general character as those within the Keeling
atoll. These internal reefs either stand separately, or more commonly
skirt the shores of the included high islands. The depth of the
lagoon-channel round the Society Islands varies from two or three to
thirty fathoms; in Cook’s[5] chart of Ulieta, however, there is one
sounding laid down of forty-eight fathoms; at Vanikoro there are
several of fifty-four and one of fifty-six and a half fathoms
(English), a depth which even exceeds by a little that of the interior
of the great Maldiva atolls. Some barrier-reefs have very few islets on
them; whilst others are surmounted by numerous ones; and those round
part of Bolabola (Plate I, Fig. 5) form a single linear strip. The
islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the
windward side. The reef to leeward retaining its usual width, sometimes
lies submerged several fathoms beneath the surface; I have already
mentioned Gambier Island as an instance of this structure. Submerged
reefs, having a less defined outline, dead, and covered with sand, have
been observed (see Appendix) off some parts of Huaheine and Tahiti. The
reef is more frequently breached to leeward than to windward; thus I
find in Krusenstern’s “Memoir on the Pacific,” that there are passages
through the encircling reef on the leeward side of each of the seven
Society Islands, which possess ship-harbours; but that there are
openings to windward through the reef of only three of them. The
breaches in the reef are seldom as deep as the interior lagoon-like
channel; they generally occur in front of the main valleys, a
circumstance which can be accounted for, as will be seen in the fourth
chapter, without much difficulty. The breaches being situated in front
of the valleys, which descend indifferently on all sides, explains
their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,—for in atolls
there is no included land to influence the position of the breaches.

 [5] See the chart in vol. i of Hawkesworth’s 4to ed. of “Cook’s First
 Voyage.”

It is remarkable, that the lagoon-channels round mountainous islands
have not in every instance been long ago filled up with coral and
sediment; but it is more easily accounted for than appears at first
sight. In cases like that of Hogoleu and the Gambier Islands, where a
few
small peaks rise out of a great lagoon, the conditions scarcely differ
from those of an atoll, and I have already shown, at some length, that
the filling up of a true lagoon must be an extremely slow process.
Where the channel is narrow, the agency, which on unprotected coasts is
most productive of sediment, namely the force of the breakers, is here
entirely excluded, and the reef being breached in the front of the main
valleys, much of the finer mud from the rivers must be transported into
the open sea. As a current is formed by the water thrown over the edge
of atoll-formed reefs, which carries sediment with it through the
deep-water breaches, the same thing probably takes place in
barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up. The low alluvial border, however,
at the foot of the encircled mountains, shows that the work of filling
up is in progress; and at Maura (Plate I, Fig. 6), in the Society
group, it has been almost effected, so that there remains only one
harbour for small craft.

If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many
points already noticed of resemblance, or rather of identity in
structure with atolls, there is a close general agreement in form,
average dimensions, and grouping. Encircling barrier-reefs, like
atolls, are generally elongated, with an irregularly rounded, though
sometimes angular outline. There are atolls of all sizes, from less
than two miles in diameter to sixty miles (excluding Tilla-dou-Matte,
as it consists of a number of almost independent atoll-formed reefs);
and there are encircling barrier-reefs from three miles and a half to
forty-six miles in diameter,—Turtle Island being an instance of the
former, and Hogoleu of the latter. At Tahiti the encircled island is
thirty-six miles in its longest axis, whilst at Maurua it is only a
little more than two miles. It will be shown, in the last chapter in
this volume, that there is the strictest resemblance in the grouping of
atolls and of common islands, and consequently there must be the same
resemblance in the grouping of atolls and of encircling barrier-reefs.

The islands lying within reefs of this class, are of very various
heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and
Manouai only 50. The geological nature of the included land varies: in
most cases it is of ancient volcanic origin, owing apparently to the
fact that islands of this nature are most frequent within all great
seas; some, however, are of madreporitic limestone, and others of
primary formation, of which latter kind New Caledonia offers the best
example. The central land consists either of one island, or of several:
thus, in the Society group, Eimeo stands by itself; while Taha and
Raiatea (Fig. 3, Plate I), both moderately large islands of nearly
equal size, are included in one reef. Within the reef of the Gambier
group there are four large and some smaller islands (Fig. 8, Plate I);
within that of
Hogoleu (Fig. 2, Plate I) nearly a dozen small islands are scattered
over the expanse of one vast lagoon.

 [6] The height of Tahiti is given from Captain Beechey; Maurua from
 Mr. F. D. Bennett (_Geograph. Journ._ vol. viii, p. 220); Aitutaki
 from measurements made on board the _Beagle_; and Manouai or Harvey
 Island, from an estimate by the Rev. J. Williams. The two latter
 islands, however, are not in some respects well characterised examples
 of the encircled class.

After the details now given, it may be asserted that there is not one
point of essential difference between encircling barrier-reefs and
atolls: the latter enclose a simple sheet of water, the former encircle
an expanse with one or more islands rising from it. I was much struck
with this fact, when viewing, from the heights of Tahiti, the distant
island of Eimeo standing within smooth water, and encircled by a ring
of snow-white breakers. Remove the central land, and an annular reef
like that of an atoll in an early stage of its formation is left;
remove it from Bolabola, and there remains a circle of linear
coral-islets, crowned with tall cocoa-nut trees, like one of the many
atolls scattered over the Pacific and Indian Oceans.

The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions. The reef on the west coast of New
Caledonia (Fig. 5, Plate II) is 400 miles in length; and for a length
of many leagues it seldom approaches within eight miles of the shore;
and near the southern end of the island, the space between the reef and
the land is sixteen miles in width. The Australian barrier extends,
with a few interruptions, for nearly a thousand miles; its average
distance from the land is between twenty and thirty miles; and in some
parts from fifty to seventy. The great arm of the sea thus included, is
from ten to twenty-five fathoms deep, with a sandy bottom; but towards
the southern end, where the reef is further from the shore, the depth
gradually increases to forty, and in some parts to more than sixty
fathoms. Flinders[7] has described the surface of this reef as
consisting of a hard white agglomerate of different kinds of coral,
with rough projecting points. The outer edge is the highest part; it is
traversed by narrow gullies, and at rare intervals is breached by
ship-channels. The sea close outside is profoundly deep; but, in front
of the main breaches, soundings can sometimes be obtained. Some low
islets have been formed on the reef.

 [7] Flinders’ “Voyage to Terra Australis,” vol. ii, p. 88.


There is one important point in the structure of barrier-reefs which
must here be considered. The accompanying diagrams represent north and
south vertical sections, taken through the highest points of Vanikoro,
Gambier, and Maurua Islands, and through their encircling reefs. The
scale both in the horizontal and vertical direction is the same,
namely, a quarter of an inch to a nautical mile. The height and width
of these islands is known; and I have attempted to represent the form
of the land from the shading of the hills in the large published
charts. It has long been remarked, even from the time of Dampier, that
considerable degree of relation subsists between the inclination of
that part of the land which is beneath water and that above it; hence
the dotted line in the three sections, probably, does not widely differ
in inclination from the actual submarine prolongation of the land. If
we now look at the outer edge of the reef (AA), and bear in mind that
the plummet on the right hand represents a depth of 1,200 feet, we must
conclude that the vertical thickness of these barrier coral-reefs is
very great.


[Illustration: Vertical thickness of Vanikoro, Gambier and Maurua.]

1. VANIKORO, from the “Atlas of the Voyage of the _Astrolabe_,” by D.
D’Urville.
2. GAMBIER ISLAND, from Beechey.
3. MAURUA, from the “Atlas of the Voyage of the _ Coquille_,” by
Duperrey.

The horizontal line is the level of the sea, from which on the right
hand a plummet descends, representing a depth of 200 fathoms, or 1,200
feet. The vertical shading shows the section of the land, and the
horizontal shading that of the encircling barrier-reef: from the
smallness of the scale, the lagoon-channel could not be represented.
AA.—Outer edge of the coral-reefs, where the sea breaks.
BB.—The shore of the encircled islands.


I must observe that if the sections had been taken in any other
direction across these islands, or across other encircled islands,[8]
the result would have been the same. In the succeeding chapter it will
be shown that reef-building polypifers cannot flourish at great
depths,—for instance, it is highly improbable that they could exist at
a quarter of the depth represented by the plummet on the right hand of
the woodcut. Here there is a great _apparent_ difficulty—how were the
basal parts of these barrier-reef formed? It will, perhaps, occur to
some, that the actual reefs formed of coral are not of great thickness,
but that before their first growth, the coasts of these encircled
islands were deeply eaten into, and a broad but shallow submarine ledge
thus left, on the edge of which the coral grew; but if this had been
the case, the shore would have been invariably bounded by lofty cliffs,
and not have sloped down to the lagoon-channel, as it does in many
instances. On this view,[9] moreover, the cause of the reef springing
up at such a great distance from the land, leaving a deep and broad
moat within, remains altogether unexplained. A supposition of the same
nature,
and appearing at first more probable is, that the reefs sprung up from
banks of sediment, which had accumulated round the shore previously to
the growth of the coral; but the extension of a bank to the same
distance round an unbroken coast, and in front of those deep arms of
the sea (as in Raiatea, see Plate II, Fig. 3) which penetrate nearly to
the heart of some encircled islands, is exceedingly improbable. And
why, again, should the reef spring up, in some cases steep on both
sides like a wall, at a distance of two, three or more miles from the
shore, leaving a channel often between two hundred and three hundred
feet deep, and rising from a depth which we have reason to believe is
destructive to the growth of coral? An admission of this nature cannot
possibly be made. The existence, also, of the deep channel, utterly
precludes the idea of the reef having grown outwards, on a foundation
slowly formed on its outside, by the accumulation of sediment and coral
detritus. Nor, again, can it be asserted, that the reef-building corals
will not grow, excepting at a great distance from the land; for, as we
shall soon see, there is a whole class of reefs, which take their name
from growing closely attached (especially where the sea is deep) to the
beach. At New Caledonia (see Plate II, Fig. 5) the reefs which run in
front of the west coast are prolonged in the same line 150 miles beyond
the northern extremity of the island, and this shows that some
explanation, quite different from any of those just suggested, is
required. The continuation of the reefs on each side of the submarine
prolongation of New Caledonia, is an exceedingly interesting fact, if
this part formerly existed as the northern extremity of the island, and
before the attachment of the coral had been worn down by the action of
the sea, or if it originally existed at its present height, with or
without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but
fronting its sides, in the same line with the reefs which front the
shores of the lofty island? We shall hereafter see, that there is one,
and I believe only one, solution of this difficulty.

 [8] In the fifth chapter an east and west section across the Island of
 Bolabola and its barrier-reefs is given, for the sake of illustrating
 another point. The unbroken line in it (woodcut No. 5) is the section
 referred to. The scale is .57 of an inch to a mile; it is taken from
 the “Atlas of the Voyage of the _Coquille_,” by Duperrey. The depth of
 the lagoon-channel is exaggerated.


 [9] The Rev. D. Tyerman and Mr. Bennett (“Journal of Voyage and
 Travels,” vol. i, p. 215) have briefly suggested this explanation of
 the origin of the encircling reefs of the Society Islands.


One other supposition to account for the position of encircling
barrier-reefs remains, but it is almost too preposterous to be
mentioned; namely, that they rest on enormous submarine craters,
surrounding the included islands. When the size, height, and form of
the islands in the Society group are considered, together with the fact
that all are thus encircled, such a notion will be rejected by almost
every one. New Caledonia, moreover, besides its size, is composed of
primitive formations, as are some of the Comoro Islands;[10] and
Aitutaki consists of calcareous rock. We must, therefore, reject these
several explanations, and conclude that the vertical thickness of
barrier-reefs, from their outer edges to the foundation on which they
rest (from AA in the section to the dotted lines) is really great; but
in this, there is no difficulty, for it is not necessary to suppose
that the coral has sprung up from an immense depth, as will be evident
when the theory of the upward growth of coral-reefs, during the slow
subsidence of their foundation, is discussed.

 [10] I have been informed that this is the case by Dr. Allan of
 Forres, who has visited this group.




Chapter III FRINGING OR SHORE-REEFS


Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
up.—Currents of water formed within it.—Upraised reefs.—Narrow
fringing-reefs in deep seas.—Reefs on the coast of East Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
and on worn-down islands.—Fringing-reefs affected by currents of the
sea.—Coral coating the bottom of the sea, but not forming reefs.

Fringing-reefs, or, as they have been called by some voyagers,
shore-reefs, whether skirting an island or part of a continent, might
at first be thought to differ little, except in generally being of less
breadth, from barrier-reefs. As far as the superficies of the actual
reef is concerned this is the case; but the absence of an interior
deep-water channel, and the close relation in their horizontal
extension with the probable slope beneath the sea of the adjoining
land, present essential points of difference.

The reefs which fringe the island of Mauritius offer a good example of
this class. They extend round its whole circumference, with the
exception of two or three parts,[1] where the coast is almost
precipitous, and where, if as is probable the bottom of the sea has a
similar inclination, the coral would have no foundation on which to
become attached. A similar fact may sometimes be observed even in reefs
of the barrier class, which follow much less closely the outline of the
adjoining land; as, for instance, on the south-east and precipitous
side of Tahiti, where the encircling reef is interrupted. On the
western side of the Mauritius, which was the only part I visited, the
reef generally lies at the distance of about half a mile from the
shore; but in some parts it is distant from one to two, and even three
miles. But even in this last case, as the coast-land is gently inclined
from the foot of the mountains to the sea-beach, and as the soundings
outside the reef indicate an equally gentle slope beneath the water,
there is no reason for supposing that the basis of the reef, formed by
the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef.
Some allowance, however, must be made for the outward extension of the
corals on a foundation of sand and detritus, formed from their own
wear, which would give to the reef a somewhat greater vertical
thickness, than would otherwise be possible.

 [1] This fact is stated on the authority of the Officier du Roi, in
 his extremely interesting “Voyage à l’Isle de France,” undertaken in
 1768. According to Captain Carmichael (Hooker’s _Bot. Misc._ vol. ii,
 p. 316) on one part of the coast there is a space for sixteen miles
 without a reef.

The outer edge of the reef on the western or leeward side of the island
is tolerably well defined, and is a little higher than any other part.
It chiefly consists of large strongly branched corals, of the genus
Madrepora, which also form a sloping bed some way out to sea: the
kinds of coral growing in this part will be described in the ensuing
chapter. Between the outer margin and the beach, there is a flat space
with a sandy bottom and a few tufts of living coral; in some parts it
is so shallow, that people, by avoiding the deeper holes and gullies,
can wade across it at low water; in other parts it is deeper, seldom
however exceeding ten or twelve feet, so that it offers a safe coasting
channel for boats. On the eastern and windward side of the island,
which is exposed to a heavy surf, the reef was described to me as
having a hard smooth surface, very slightly inclined inwards, just
covered at low-water, and traversed by gullies; it appears to be quite
similar in structure to the reefs of the barrier and atoll classes.

The reef of Mauritius, in front of every river and streamlet, is
breached by a straight passage: at Grand Port, however, there is a
channel like that within a barrier-reef; it extends parallel to the
shore for four miles, and has an average depth of ten or twelve
fathoms; its presence may probably be accounted for by two rivers which
enter at each end of the channel, and bend towards each other. The fact
of reefs of the fringing class being always breached in front of
streams, even of those which are dry during the greater part of the
year, will be explained, when the conditions unfavourable to the growth
of coral are considered. Low coral-islets, like those on barrier-reefs
and atolls, are seldom formed on reefs of this class, owing apparently
in some cases to their narrowness, and in others to the gentle slope of
the reef outside not yielding many fragments to the breakers. On the
windward side, however, of the Mauritius, two or three small islets
have been formed.

It appears, as will be shown in the ensuing chapter, that the action of
the surf is favourable to the vigorous growth of the stronger corals,
and that sand or sediment, if agitated by the waves, is injurious to
them. Hence it is probable that a reef on a shelving shore, like that
of Mauritius, would at first grow up, not attached to the actual beach,
but at some little distance from it; and the corals on the outer margin
would be the most vigorous. A shallow channel would thus be formed
within the reef, and as the breakers are prevented acting on the shores
of the island, and as they do not ordinarily tear up many fragments
from the outside, and as every streamlet has its bed prolonged in a
straight line through the reef, this channel could be filled up only
very slowly with sediment. But a beach of sand and of fragments of the
smaller kinds of coral seems, in the case of Mauritius, to be slowly
encroaching on the shallow channel. On many shelving and sandy coasts,
the breakers tend to form a bar of sand a little way from the beach,
with a slight increase of depth within it; for instance, Captain
Grey[2] states that the west coast of Australia, in latitude 24°, is
fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away
of the
water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone,[3] which has the
same external form and height as a coral-reef, extends nearly parallel
to the coast; within this bar currents, apparently caused by the water
thrown over it during the greater part of each tide, run strongly, and
are wearing away its inner wall. From these facts it can hardly be
doubted, that within most fringing-reefs, especially within those lying
some distance from the land, a return stream must carry away the water
thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even
deepen it under certain circumstances. To this latter belief I am led,
by finding that channels are almost universally present within the
fringing-reefs of those islands which have undergone recent elevatory
movements; and this could hardly have been the case, if the conversion
of the very shallow channel into land had not been counteracted to a
certain extent.

 [2] Captain Grey’s “Journal of Two Expeditions,” vol. i, p. 369.


 [3] I have described this singular structure in the _Lond. and Edin.
 Phil. Mag.,_ October 1841.


A fringing-reef, if elevated in a perfect condition above the level of
the sea, ought to present the singular appearance of a broad dry moat
within a low mound. The author[4] of an interesting pedestrian tour
round the Mauritius, seems to have met with a structure of this kind:
he says “J’observai que là, où la mer étale, indépendamment des rescifs
du large, il y à terre _une espèce d’effoncement_ ou chemin couvert
naturel. On y pourrait mettre du canon,” etc. In another place he adds,
“Avant de passer le Cap, on remarque un gros banc de corail élevé de
plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné,
il regne au pied une longue flaque d’eau, dont on pourrait faire un
bassin pour de petits vaisseaux.” But the margin of the reef, although
the highest and most perfect part, from being most exposed to the surf,
would generally during a slow rise of the land be either partially or
entirely worn down to that level, at which corals could renew their
growth on its upper edge. On some parts of the coast-land of Mauritius
there are little hillocks of coral-rock, which are either the last
remnants of a continuous reef, or of low islets formed on it. I
observed that two such hillocks between Tamarin Bay and the Great Black
River; they were nearly twenty feet high, about two hundred yards from
the present beach, and about thirty feet above its level. They rose
abruptly from a smooth surface, strewed with worn fragments of coral.
They consisted in their lower part of hard calcareous sandstone, and in
their upper of great blocks of several species of Astræa and Madrepora,
loosely aggregated; they were divided into irregular beds, dipping
seaward, in one hillock at an angle of 8°, and in the other at 18°. I
suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been
much more modified by the wearing action of the sea, than those of
Mauritius.

 [4] “Voyage à l’Isle de France, par un Officier du Roi,” part i, pp.
 192, 200.


Many islands[5] are fringed by reefs quite similar to those of
Mauritius; but on coasts where the sea deepens very suddenly the reefs
are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation,
which, as we have seen, does not exist in reefs of the barrier class.
The fringing-reefs on steep coasts are frequently not more than from
fifty to one hundred yards in width; they have a nearly smooth, hard
surface, scarcely uncovered at low water, and without any interior
shoal channel, like that within those fringing-reefs, which lie at a
greater distance from the land. The fragments torn up during gales from
the outer margin are thrown over the reef on the shores of the island.
I may give as instances, Wateeo, where the reef is described by Cook as
being a hundred yards wide; and Mauti and Elizabeth[6] Islands, where
it is only fifty yards in width: the sea round these islands is very
deep.

 [5] I may give Cuba, as another instance; Mr. Taylor (_Loudon’s Mag.
 of Nat. Hist.,_ vol. ix, p. 449) has described a reef several miles in
 length between Gibara and Vjaro, which extends parallel to the shore
 at the distance of between half and the third part of a mile, and
 encloses a space of shallow water, with a sandy bottom and tufts of
 coral. Outside the edge of the reef, which is formed of great
 branching corals, the depth is six and seven fathoms. This coast has
 been upheaved at no very distant geological period.


 [6] Mauti is described by Lord Byron in the voyage of H.M.S. _
 Blonde_, and Elizabeth Island by Captain Beechey.


Fringing-reefs, like barrier-reefs, both surround islands, and front
the shores of continents. In the charts of the eastern coast of Africa,
by Captain Owen, many extensive fringing-reefs are laid down; thus, for
a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a
reef fringes the shore at an average distance of rather more than one
mile, and therefore at a greater distance than is usual in reefs of
this class; but as the coast-land is not lofty, and as the bottom
shoals very gradually (the depth being only from eight to fourteen
fathoms at a mile and a half outside the reef), its extension thus far
from the land offers no difficulty. The external margin of this reef is
described, as formed of projecting points, within which there is a
space, from six to twelve feet deep, with patches of living coral on
it. At Mukdeesha (lat. 2° 1′ N.) “the port is formed,” it is said,[7]
“by a long reef extending eastward, four or five miles, within which
there is a narrow channel, with ten to twelve feet of water at low
spring-tides;” it lies at the distance of a quarter of a mile from the
shore. Again, in the plan of Mombas (lat. 4° S.), a reef extends for
thirty-six miles, at the distance of from half a mile to one mile and a
quarter from the shore; within it, there is a channel navigable “for
canoes and small craft,” between six and fifteen feet deep: outside the
reef the depth is about thirty fathoms at the distance of nearly half a
mile. Part of this reef is very symmetrical, and has a uniform breadth
of two hundred yards.

 [7] Owen’s “Africa,” vol. i, p. 357, from which work the foregoing
 facts are likewise taken.


The coast of Brazil is in many parts fringed by reefs. Of these, some
are not of coral formation; for instance, those near Bahia and in front
of Pernambuco; but a few miles south of this latter city, the reef
follows[8] so closely every turn of the shore, that I can hardly doubt
it is of coral; it runs at the distance of three-quarters of a mile
from the land, and within it the depth is from ten to fifteen feet. I
was assured by an intelligent pilot that at Ports Frances and Maceio,
the outer part of the reef consists of living coral, and the inner of a
white stone, full of large irregular cavities, communicating with the
sea. The bottom of the sea off the coast of Brazil shoals gradually to
between thirty and forty fathoms, at the distance of between nine and
ten leagues from the land.

 [8] See Baron Roussin’s “Pilote du Brésil,” and accompanying
 hydrographical memoir.

From the description now given, we must conclude that the dimensions
and structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows
from this, that where the sea is very shallow, as in the Persian Gulf
and in parts of the East Indian Archipelago, the reefs lose their
fringing character, and appear as separate and irregularly scattered
patches, often of considerable area. From the more vigorous growth of
the coral on the outside, and from the conditions being less favourable
in several respects within, such reefs are generally higher and more
perfect in their marginal than in their central parts; hence these
reefs sometimes assume (and this circumstance ought not to be
overlooked) the appearance of atolls; but they differ from atolls in
their central expanse being much less deep, in their form being less
defined, and in being based on a shallow foundation. But when in a deep
sea reefs fringe banks of sediment, which have accumulated beneath the
surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling
barrier-reefs, and on the other from atolls. In the West Indies there
are reefs, which I should probably have arranged under both these
classes, had not the existence of large and level banks, lying a little
beneath the surface, ready to serve as the basis for the attachment of
coral, been occasionally brought into view by the entire or partial
absence of reefs on them, and had not the formation of such banks,
through the accumulation of sediment now in progress, been sufficiently
evident. Fringing-reefs sometimes coat, and thus protect the
foundations of islands, which have been worn down by the surf to the
level of the sea. According to Ehrenberg, this has been extensively the
case with the islands in the Red Sea, which formerly ranged parallel to
the shores of the mainland, with deep water within them: hence the
reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as
I believe, in the Red Sea some true barrier-reefs. The reefs of this
sea and of the West Indies will be described in the Appendix. In some
cases, fringing-reefs appear to be considerably modified in outline by
the course of the prevailing currents. Dr. J. Allan informs me that on
the east coast of Madagascar almost every headland and low point of
sand has a coral-reef extending from it in a S.W. and N.E. line,
parallel
to the currents on that shore. I should think the influence of the
currents chiefly consisted in causing an extension, in a certain
direction, of a proper foundation for the attachment of the coral.
Round many intertropical islands, for instance the Abrolhos on the
coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by
Mr. Cuming, round the Philippines, the bottom of the sea is entirely
coated by irregular masses of coral, which although often of large
size, do not reach the surface and form proper reefs. This must be
owing, either to insufficient growth, or to the absence of those kinds
of corals which can withstand the breaking of the waves.

The three classes, atoll-formed, barrier, and fringing-reefs, together
with the modifications just described of the latter, include all the
most remarkable coral formations anywhere existing. At the commencement
of the last chapter in the volume, where I detail the principles on
which the map (Plate III) is coloured, the exceptional cases will be
enumerated.




Chapter IV ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS


In this chapter I will give all the facts which I have collected,
relating to the distribution of coral-reefs,—to the conditions
favourable to their increase,—to the rate of their growth,—and to the
depth at which they are formed.

These subjects have an important bearing on the theory of the origin of
the different classes of coral-reefs.

_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE

With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32° 15′ N., is the
point furthest removed from the equator, in which they appear to exist;
and it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27° N., have reefs on their shores, and
there is an atoll in 28° 30′, situated N.W. of the Sandwich
Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In
the southern hemisphere coral-reefs do not extend so far from the
equatorial sea. In the Southern Pacific there are only a few reefs
beyond the line of the tropics, but Houtmans Abrolhos, on the western
shores of Australia in latitude 29° S., are of coral formation.

The proximity of volcanic land, owing to the lime generally evolved
from it, has been thought to be favourable to the increase of
coral-reefs.
There is, however, not much foundation for this view; for nowhere are
coral-reefs more extensive than on the shores of New Caledonia, and of
north-eastern Australia, which consist of primary formations; and in
the largest groups of atolls, namely the Maldiva, Chagos, Marshall,
Gilbert, and Low Archipelagoes, there is no volcanic or other kind of
rock, excepting that formed of coral.

The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the _Beagle_ and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none[1] north of the equator;
Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that
although he had seen corals living in the Bay of Panama, yet he had
never observed any reefs formed by them. I at first attributed this
absence of reefs on the coasts of Peru and of the Galapagos Islands,[2]
to the coldness of the currents from the south, but the Gulf of Panama
is one of the hottest pelagic districts in the world.[3] In the central
parts of the Pacific there are islands entirely free from reefs; in
some few of these cases I have thought that this was owing to recent
volcanic action; but the existence of reefs round the greater part of
Hawaii, one of the Sandwich Islands, shows that recent volcanic action
does not necessarily prevent their growth.

 [1] I have been informed that this is the case, by Lieutenant Ryder,
 R.N., and others who have had ample opportunities for observation.


 [2] The mean temperature of the surface sea from observations made by
 the direction of Captain Fitzroy on the shores of the Galapagos
 Islands, between the 16th of September and the 20th of October, 1835,
 was 68° Fahr. The lowest temperature observed was 58.5° at the
 south-west end of Albemarle Island; and on the west coast of this
 island, it was several times 62° and 63°. The mean temperature of the
 sea in the Low Archipelago of atolls, and near Tahiti, from similar
 observations made on board the _Beagle_, was (although further from
 the equator) 77.5°, the lowest any day being 76.5°. Therefore we have
 here a difference of 9.5° in mean temperature, and 18° in extremes; a
 difference doubtless quite sufficient to affect the distribution of
 organic beings in the two areas.


 [3] Humboldt’s “Personal Narrative,” vol. vii, p. 434.

In the last chapter I stated that the bottom of the sea round some
islands is thickly coated with living corals, which nevertheless do not
form reefs, either from insufficient growth, or from the species not
being adapted to contend with the breaking waves.

I have been assured by several people, that there are no coral-reefs on
the west coast of Africa,[4] or round the islands in the Gulf of
Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks,
which line great part of it. But the islands of St. Helena, Ascension,
the Cape Verdes, St. Paul’s, and Fernando Noronha, are, also, entirely
without reefs, although they lie far out at sea, are composed of the
same ancient volcanic rocks, and have the same general form, with those
islands in the Pacific, the shores of which are surrounded by gigantic
walls of coral-rock. With the exception of Bermuda, there is not a
single coral-reef in the central expanse of the Atlantic Ocean. It
will, perhaps, be suggested that the quantity of carbonate of lime in
different parts of the sea, may regulate the presence of reefs. But
this cannot be the case, for at Ascension, the waves charged to excess
precipitate a thick layer of calcareous matter on the tidal rocks; and
at St. Jago, in the Cape Verdes, carbonate of lime not only is abundant
on the shores, but it forms the chief part of some upraised
post-tertiary strata. The apparently capricious distribution,
therefore, of coral-reefs, cannot be explained by any of these obvious
causes; but as the study of the terrestrial and better known half of
the world must convince every one that no station capable of supporting
life is lost,—nay more, that there is a struggle for each station,
between the different orders of nature,—we may conclude that in those
parts of the intertropical sea, in which there are no coral-reefs,
there are other organic bodies supplying the place of the reef-building
polypifers. It has been shown in the chapter on Keeling atoll that
there are some species of large fish, and the whole tribe of Holothuriæ
which prey on the tenderer parts of the corals. On the other hand, the
polypifers in their turn must prey on some other organic beings; the
decrease of which from any cause would cause a proportionate
destruction of the living coral. The relations, therefore, which
determine the formation of reefs on any shore, by the vigorous growth
of the efficient kinds of coral, must be very complex, and with our
imperfect knowledge quite inexplicable. From these considerations, we
may infer that changes in the condition of the sea, not obvious to our
senses, might destroy all the coral-reefs in one area, and cause them
to appear in another: thus, the Pacific or Indian Ocean might become as
barren of coral-reefs as the Atlantic now is, without our being able to
assign any adequate cause for such a change.

 [4] It might be concluded, from a paper by Captain Owen (_Geograph.
 Journ._, vol. ii, p. 89), that the reefs off Cape St. Anne and the
 Sherboro’ Islands were of coral, although the author states that they
 are not purely coralline. But I have been assured by Lieutenant
 Holland, R.N., that these reefs are not of coral, or at least that
 they do not at all resemble those in the West Indies.


It has been a question with some naturalists, which part of a reef is
most favourable to the growth of coral. The great mounds of living
Porites and of Millepora round Keeling atoll occur exclusively on the
extreme verge of the reef, which is washed by a constant succession of
breakers; and living coral nowhere else forms solid masses. At the
Marshall islands the larger kinds of coral (chiefly species of Astræa,
a genus closely allied to Porites) “which form rocks measuring several
fathoms in thickness,” prefer, according to Chamisso,[5] the most
violent surf. I have stated that the outer margin of the Maldiva atolls
consists of living corals (some of which, if not all, are of the same
species with those at Keeling atoll), and here the surf is so
tremendous, that even large ships have been thrown, by a single heave
of the sea, high and dry on the reef, all on board thus escaping with
their lives.


Ehrenberg[6] remarks, that in the Red Sea the strongest corals live on
the outer reefs, and appear to love the surf; he adds, that the more
branched kinds abound a little way within, but that even these in still
more protected places, become smaller. Many other facts having a
similar tendency might be adduced.[7] It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand,
much less flourish in, the breakers of an open sea:[8] they affirm that
the saxigenous lithophytes flourish only where the water is tranquil,
and the heat intense. This statement has passed from one geological
work to another; nevertheless, the protection of the whole reef
undoubtedly is due to those kinds of coral, which cannot exist in the
situations thought by these naturalists to be most favourable to them.
For should the outer and living margin perish, of any one of the many
low coral-islands, round which a line of great breakers is incessantly
foaming, the whole, it is scarcely possible to doubt, would be washed
away and destroyed, in less than half a century. But the vital energies
of the corals conquer the mechanical power of the waves; and the large
fragments of reef torn up by every storm, are replaced by the slow but
steady growth of the innumerable polypifers, which form the living zone
on its outer edge.

 [5] Kotzebue’s “First Voyage” (Eng. Trans.), vol. iii, pp. 142, 143,
 331.


 [6] Ehrenberg, “Über die Natur und Bildung der Corallen Bänke im
 rothen Meere,” p. 49.


 [7] In the West Indies, as I am informed by Captain Bird Allen, R.N.,
 it is the common belief of those, who are best acquainted with the
 reefs, that the coral flourishes most, where freely exposed to the
 swell of the open sea.


 [8] “Annales des Sciences Naturelles,” tome vi, pp. 276, 278.—“Là où
 les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
 qu’elles détruiraient leurs fragiles édifices,” etc.


From these facts, it is certain, that the strongest and most massive
corals flourish, where most exposed. The less perfect state of the reef
of most atolls on the leeward and less exposed side, compared with its
state to windward; and the analogous case of the greater number of
breaches on the near sides of those atolls in the Maldiva Archipelago,
which afford some protection to each other, are obviously explained by
this circumstance. If the question had been, under what conditions the
greater number of species of coral, not regarding their bulk and
strength, were developed, I should answer,—probably in the situations
described by MM. Quoy and Gaimard, where the water is tranquil and the
heat intense. The total number of species of coral in the
circumtropical seas must be very great: in the Red Sea alone, 120
kinds, according to Ehrenberg,[9] have been observed.

 [9] Ehrenberg, “Über die Natur,” etc., p. 46.


The same author has observed that the recoil of the sea from a steep
shore is injurious to the growth of coral, although waves breaking over
a bank are not so. Ehrenberg also states, that where there is much
sediment, placed so as to be liable to be moved by the waves there is
little or no coral; and a collection of living specimens placed by him
on a sandy shore died in the course of a few days.[10] An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on
a sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in
sounding off Keeling atoll, and (as will hereafter be shown) off
Mauritius, the arming of the lead invariably came up clean, where the
coral was growing vigorously. This same circumstance has probably given
rise to a strange belief, which, according to Captain Owen,[11] is
general amongst the inhabitants of the Maldiva atolls, namely that
corals have roots, and therefore that if merely broken down to the
surface, they grow up again; but, if rooted out, they are permanently
destroyed. By this means the inhabitants keep their harbours clear; and
thus the French Governor of St. Mary’s in Madagascar, “cleared out and
made a beautiful little port at that place.” For it is probable that
sand would accumulate in the hollows formed by tearing out the corals,
but not on the broken and projecting stumps, and therefore, in the
former case, the fresh growth of the coral might be thus prevented.

 [10] _Ibid_., p. 49.


 [11] Captain Owen on the Geography of the Maldiva Islands, _Geograph.
 Journal_, vol. ii, p. 88.


In the last chapter I remarked that fringing-reefs are almost
universally breached, where streams enter the sea.[12] Most authors
have attributed this fact to the injurious effects of the fresh water,
even where it enters the sea only in small quantity, and during a part
of the year. No doubt brackish water would prevent or retard the growth
of coral; but I believe that the mud and sand which is deposited, even
by rivulets when flooded, is a much more efficient check. The reef on
each side of the channel leading into Port Louis at Mauritius, ends
abruptly in a wall, at the foot of which I sounded and found a bed of
thick mud. This steepness of the sides appears to be a general
character in such breaches. Cook,[13] speaking of one at Raiatea, says,
“like all the rest, it is very steep on both sides.” Now, if it were
the fresh water mingling with the salt which prevented the growth of
coral, the reef certainly would not terminate abruptly, but as the
polypifers nearest the impure stream would grow less vigorously than
those farther off, so would the reef gradually thin away. On the other
hand, the sediment brought down from the land would only prevent the
growth of the coral in the line of its deposition, but would not check
it on the side, so that the reefs might increase till they overhung the
bed of the channel. The breaches are much fewer in number, and front
only the larger valleys in reefs of the encircling barrier class. They
probably are kept open in the same manner as those into the lagoon of
an atoll, namely, by the
force of the currents and the drifting outwards of fine sediment. Their
position in front of valleys, although often separated from the land by
deep water lagoon-channels, which it might be thought would entirely
remove the injurious effects both of the fresh water and the sediment,
will receive a simple explanation when we discuss the origin of
barrier-reefs.

 [12] Lieutenant Wellstead and others have remarked that this is the
 case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i, p. 142)
 says that there are pear-shaped harbours in the upraised coral-coast,
 into which periodical streams enter. From this circumstance, I
 presume, we must infer that before the upheaval of the strata now
 forming the coast-land, fresh water and sediment entered the sea at
 these points; and the coral being thus prevented growing, the
 pear-shaped harbours were produced.


 [13] Cook’s “First Voyage,” vol. ii, p. 271 (Hawkesworth’s edit.)


In the vegetable kingdom every different station has its peculiar group
of plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the
lagoon of an atoll and those on its outer margin. The corals, also, on
the margin of Keeling Island occurred in zones; thus the _Porites_ and
_Millepora complanata_ grow to a large size only where they are washed
by a heavy sea, and are killed by a short exposure to the air; whereas,
three species of Nullipora also live amidst the breakers, but are able
to survive uncovered for a part of each tide; at greater depths, a
strong Madrepora and _Millepora alcicornis_ are the commonest kinds,
the former appearing to be confined to this part, beneath the zone of
massive corals, minute encrusting corallines and other organic bodies
live. If we compare the external margin of the reef at Keeling atoll
with that on the leeward side of Mauritius, which are very differently
circumstanced, we shall find a corresponding difference in the
appearance of the corals. At the latter place, the genus Madrepora is
preponderant over every other kind, and beneath the zone of massive
corals there are large beds of Seriatopora. There is also a marked
difference, according to Captain Moresby,[14]between the great
branching corals of the Red Sea, and those on the reefs of the Maldiva
atolls.

 [14] Captain Moresby on the Northern Maldiva atolls, _Geograph.
 Journal_, vol. v, p. 401.


These facts, which in themselves are deserving of notice, bear,
perhaps, not very remotely, on a remarkable circumstance which has been
pointed out to me by Captain Moresby, namely, that with very few
exceptions, none of the coral-knolls within the lagoons of Peros
Banhos, Diego Garcia, and the Great Chagos Bank (all situated in the
Chagos group), rise to the surface of the water; whereas all those,
with equally few exceptions, within Solomon and Egmont atolls in the
same group, and likewise within the large southern Maldiva atolls,
reach the surface. I make these statements, after having examined the
charts of each atoll. In the lagoon of Peros Banhos, which is nearly
twenty miles across, there is only one single reef which rises to the
surface; in Diego Garcia there are seven, but several of these lie
close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand,
in the lagoons of some of the great southern Maldiva atolls, although
thickly studded with reefs, every one without exception rises to the
surface; and on an average there are less than two submerged reefs in
each atoll; in the northern atolls, however, the submerged lagoon-reefs
are not quite so rare. The submerged reefs in the Chagos atolls
generally have from one to seven fathoms water on them, but some have
from seven to ten. Most of them are small with
very steep sides;[15] at Peros Banhos they rise from a depth of about
thirty fathoms, and some of them in the Great Chagos Bank from above
forty fathoms; they are covered, Captain Moresby informs me, with
living and healthy coral, two and three feet high, consisting of
several species. Why then have not these lagoon-reefs reached the
surface, like the innumerable ones in the atolls above named? If we
attempt to assign any difference in their external conditions, as the
cause of this diversity, we are at once baffled. The lagoon of Diego
Garcia is not deep, and is almost wholly surrounded by its reef; Peros
Banhos is very deep, much larger, with many wide passages communicating
with the open sea. On the other hand, of those atolls, in which all or
nearly all the lagoon-reefs have reached the surface, some are small,
others large, some shallow, others deep, some well-enclosed, and others
open.

 [15] Some of these statements were not communicated to me verbally by
 Captain Moresby, but are taken from the MS. account before alluded to,
 of the Chagos Group.

Captain Moresby informs me that he has seen a French chart of Diego
Garcia made eighty years before his survey, and apparently very
accurate; and from it he infers, that during this interval there has
not been the smallest change in the depth on any of the knolls within
the lagoon. It is also known that during the last fifty-one years, the
eastern channel into the lagoon has neither become narrower, nor
decreased in depth; and as there are numerous small knolls of living
coral within it, some change might have been anticipated. Moreover, as
the whole reef round the lagoon of this atoll has been converted into
land—an unparalleled case, I believe, in an atoll of such large
size,—and as the strip of land is for considerable spaces more than
half a mile wide—also a very unusual circumstance,—we have the best
possible evidence, that Diego Garcia has remained at its present level
for a very long period. With this fact, and with the knowledge that no
sensible change has taken place during eighty years in the
coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance
of being older than Diego Garcia and Peros Banhos, and which are placed
under the same external conditions with them, one is led to conclude
that these submerged reefs, although covered with luxuriant coral, have
no tendency to grow upwards, and that they would remain at their
present levels for an almost indefinite period.

From the number of these knolls, from their position, size, and form,
many of them being only one or two hundred yards across, with a rounded
outline, and precipitous sides,—it is indisputable that they have been
formed by the growth of coral; and this makes the case much more
remarkable. In Peros Banhos and in the Great Chagos Bank, some of these
almost columnar masses are 200 feet high, and their summits lie only
from two to eight fathoms beneath the surface; therefore, a small
proportional amount more of growth would cause them to attain the
surface, like those numerous knolls, which rise from an equally great
depth within the Maldiva atolls. We can hardly suppose that time has
been wanting for the upward growth of
the coral, whilst in Diego Garcia, the broad annular strip of land,
formed by the continued accumulation of detritus, shows how long this
atoll has remained at its present level. We must look to some other
cause than the rate of growth; and I suspect it will be found in the
reefs being formed of different species of corals, adapted to live at
different depths. The Great Chagos Bank is situated in the centre of
the Chagos Group, and the Pitt and Speaker Banks at its two extreme
points. These banks resemble atolls, except in their external rim being
about eight fathoms submerged, and in being formed of dead rock, with
very little living coral on it: a portion nine miles long of the
annular reef of Peros Banhos atoll is in the same condition. These
facts, as will hereafter be shown, render it very probable that the
whole group at some former period subsided seven or eight fathoms; and
that the coral perished on the outer margin of those atolls which are
now submerged, but that it continued alive, and grew up to the surface
on those which are now perfect. If these atolls did subside, and if
from the suddenness of the movement or from any other cause, those
corals which are better adapted to live at a certain depth than at the
surface, once got possession of the knolls, supplanting the former
occupants, they would exert little or no tendency to grow upwards. To
illustrate this, I may observe, that if the corals of the upper zone on
the outer edge of Keeling atoll were to perish, it is improbable that
those of the lower zone would grow to the surface, and thus become
exposed to conditions for which they do not appear to be adapted. The
conjecture, that the corals on the submerged knolls within the Chagos
atolls have analogous habits with those of the lower zone outside
Keeling atoll, receives some support from a remark by Captain Moresby,
namely, that they have a different appearance from those on the reefs
in the Maldiva atolls, which, as we have seen, all rise to the surface:
he compares the kind of difference to that of the vegetation under
different climates. I have entered at considerable length into this
case, although unable to throw much light on it, in order to show that
an equal tendency to upward growth ought not to be attributed to all
coral-reefs,—to those situated at different depths,—to those forming
the ring of an atoll or those on the knolls within a lagoon,—to those
in one area and those in another. The inference, therefore, that one
reef could not grow up to the surface within a given time, because
another, not known to be covered with the same species of corals, and
not known to be placed under conditions exactly the same, has not
within the same time reached the surface, is unsound.

_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS

The remark made at the close of the last section, naturally leads to
this division of our subject, which has not, I think, hitherto been
considered under a right point of view. Ehrenberg[16] has stated, that
in
the Red Sea, the corals only coat other rocks in a layer from one to
two feet in thickness, or at most to a fathom and a half; and he
disbelieves that, in any case, they form, by their own proper growth,
great masses, stratum over stratum. A nearly similar observation has
been made by MM. Quoy and Gaimard,[17] with respect to the thickness of
some upraised beds of coral, which they examined at Timor and some
other places. Ehrenberg[18] saw certain large massive corals in the Red
Sea, which he imagines to be of such vast antiquity, that they might
have been beheld by Pharaoh; and according to Mr. Lyell[19] there are
certain corals at Bermuda, which are known by tradition, to have been
living for centuries. To show how slowly coral-reefs grow upwards,
Captain Beechey[20] has adduced the case of the Dolphin Reef off
Tahiti, which has remained at the same depth beneath the surface,
namely about two fathoms and a half, for a period of sixty-seven years.
There are reefs in the Red Sea, which certainly do not appear[21] to
have increased in dimensions during the last half-century, and from the
comparison of old charts with recent surveys, probably not during the
last two hundred years. These, and other similar facts, have so
strongly impressed many with the belief of the extreme slowness of the
growth of corals, that they have even doubted the possibility of
islands in the great oceans having been formed by their agency. Others,
again, who have not been overwhelmed by this difficulty, have admitted
that it would require thousands, and tens of thousands of years, to
form a mass, even of inconsiderable thickness; but the subject has not,
I believe, been viewed in the proper light.

 [16] Ehrenberg, as before cited, pp. 39, 46, and 50.


 [17] “Annales des Sciences Nat.” tom. vi, p. 28.


 [18] Ehrenberg, _ut sup._, p. 42.


 [19] Lyell’s “Principles of Geology,” book iii, ch. xviii.


 [20] Beechey’s “Voyage to the Pacific,” ch. viii.


 [21] Ehrenberg, _ut sup._, p. 43.

That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are,
as already described, small steep-sided knolls covered with living
coral. There are similar knolls in the southern Maldiva atolls, some of
which, as Captain Moresby assures me, are less than a hundred yards in
diameter, and rise to the surface from a depth of between two hundred
and fifty and three hundred feet. Considering their number, form, and
position, it would be preposterous to suppose that they are based on
pinnacles of any rock, not of coral formation; or that sediment could
have been heaped up into such small and steep isolated cones. As no
kind of living coral grows above the height of a few feet, we are
compelled to suppose that these knolls have been formed by the
successive growth and death of many individuals,—first one being broken
off or killed by some accident, and then another, and one set of
species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The
spaces between the corals would become filled up with fragments and
sand, and such matter would probably soon be consolidated, for we learn
from
Lieutenant Nelson,[22] that at Bermuda a process of this kind takes
place beneath water, without the aid of evaporation. In reefs, also, of
the barrier class, we may feel sure, as I have shown, that masses of
great thickness have been formed by the growth of the coral; in the
case of Vanikoro, judging only from the depth of the moat between the
land and the reef, the wall of coral-rock must be at least three
hundred feet in vertical thickness.

 [22] “Geological Transactions,” vol. v, p. 113.


It is unfortunate that the upraised coral-islands in the Pacific have
not been examined by a geologist. The cliffs of Elizabeth Island, in
the Low Archipelago, are eighty feet high, and appear, from Captain
Beechey’s description, to consist of a homogeneous coral-rock. From the
isolated position of this island, we may safely infer that it is an
upraised atoll, and therefore that it has been formed by masses of
coral, grown together. Savage Island seems, from the description of the
younger Forster,[23] to have a similar structure, and its shores are
about forty feet high: some of the Cook Islands also appear[24] to be
similarly composed. Captain Belcher, R.N., in a letter which Captain
Beaufort showed me at the admiralty, speaking of Bow atoll, says, “I
have succeeded in boring forty-five feet through coral-sand, when the
auger became jammed by the falling in of the surrounding _ creamy_
matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth
of twenty-six feet, when his auger also broke: he has had the kindness
to give me the matter brought up; it is perfectly white, and like
finely triturated coral-rock.

 [23] Forster’s “Voyage round the World with Cook,” vol. ii, pp. 163,
 167.


 [24] Williams’s “Narrative of Missionary Enterprise,” p. 30.


In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just
within the outer margin, the great mounds of Porites and of Millepora,
with their summits lately killed, and their sides subsequently
thickened by the growth of the coral: a layer, also, of Nullipora had
already coated the dead surface. As the external slope of the reef is
the same round the whole of this atoll, and round many other atolls,
the angle of inclination must result from an adaption between the
growing powers of the coral, and the force of the breakers, and their
action on the loose sediment. The reef, therefore, could not increase
outwards, without a nearly equal addition to every part of the slope,
so that the original inclination might be preserved, and this would
require a large amount of sediment, all derived from the wear of corals
and shells, to be added to the lower part. Moreover, at Keeling atoll,
and probably in many other cases, the different kinds of corals would
have to encroach on each other; thus the Nulliporæ cannot increase
outwards without encroaching on the Porites and _ Millepora
complanata,_ as is now taking place; nor these latter without
encroaching on the strongly branched Madreporet, the _ Millepora
alcicornis,_ and some Astræas; nor these again without a foundation
being formed for them within the requisite depth, by the accumulation
of sediment. How slow, then, must be the ordinary lateral or outward
growth of such reefs. But off
Christmas atoll, where the sea is much more shallow than is usual, we
have good reason to believe that, within a period not very remote, the
reef has increased considerably in width. The land has the
extraordinary breadth of three miles; it consists of parallel ridges of
shells and broken corals, which furnish “an incontestable proof,” as
observed by Cook,[25] “that the island has been produced by accessions
from the sea, and is in a state of increase.” The land is fronted by a
coral-reef, and from the manner in which islets are known to be formed,
we may feel confident that the reef was not three miles wide, when the
first, or most backward ridge, was thrown up; and, therefore, we must
conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the
living margin, within a period during which ridges of shells and
corals, lying on the bare surface, have not decayed. There can be
little doubt, from the account given by Captain Beechey, that Matilda
atoll, in the Low Archipelago, has been converted in the space of
thirty-four years, from being, as described by the crew of a wrecked
whaling vessel, a “reef of rocks” into a lagoon-island, fourteen miles
in length, with “one of its sides covered nearly the whole way with
high trees.”[26] The islets, also, on Keeling atoll, it has been shown,
have increased in length, and since the construction of an old chart,
several of them have become united into one long islet; but in this
case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets,
has increased as well as the islets themselves.

 [25] Cook’s “Third Voyage,” book III, ch. x.


 [26] Beechey’s “Voyage to the Pacific,” ch. vii and viii.


After these considerations, I attach little importance, as indicating
the ordinary and still less the possible rate of _ outward_ growth of
coral-reefs, to the fact that certain reefs in the Red Sea have not
increased during a long interval of time; or to other such cases, as
that of Ouluthy atoll in the Caroline group, where every islet,
described a thousand years before by Cantova was found in the same
state by Lutké,[27]—without it could be shown that, in these cases, the
conditions were favourable to the vigorous and unopposed growth of the
corals living in the different zones of depth, and that a proper basis
for the extent of the reef was present. The former conditions must
depend on many contingencies, and in the deep oceans where coral
formations most abound, a basis within the requisite depth can rarely
be present.

 [27] F. Lutké’s “Voyage autour du Monde.” In the group Elato, however,
 it appears that what is now the islet Falipi, is called in Cantova’s
 Chart, the Banc de Falipi. It is not stated whether this has been
 caused by the growth of coral, or by the accumulation of sand.

Nor do I attach any importance to the fact of certain submerged reefs,
as those off Tahiti, or those within Diego Garcia not now being nearer
the surface than they were many years ago, as an indication of the rate
under favourable circumstances of the _upward_ growth of reefs; after
it has been shown, that all the reefs have grown to the surface in some
of the Chagos atolls, but that in neighbouring atolls which appear to
be of equal antiquity and to be exposed to the same external
conditions, every reef remains submerged; for we are almost driven to
attribute this to a difference, not in the rate of growth, but in the
habits of the corals in the two cases.

In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a struggle
one with another, and with external nature; hence we may infer that
their growth would generally be slow, except under peculiarly
favourable circumstances. Almost the only natural condition, allowing a
quick upward growth of the whole surface of a reef, would be a slow
subsidence of the area in which it stood; if, for instance, Keeling
atoll were to subside two or three feet, can we doubt that the
projecting margin of live coral, about half an inch in thickness, which
surrounds the dead upper surfaces of the mounds of Porites, would in
this case form a concentric layer over them, and the reef thus increase
upwards, instead of, as at present, outwards? The Nulliporæ are now
encroaching on the Porites and Millepora, but in this case might we not
confidently expect that the latter would, in their turn, encroach on
the Nulliporæ? After a subsidence of this kind, the sea would gain on
the islets, and the great fields of dead but upright corals in the
lagoon, would be covered by a sheet of clear water; and might we not
then expect that these reefs would rise to the surface, as they
anciently did when the lagoon was less confined by islets, and as they
did within a period of ten years in the schooner-channel, cut by the
inhabitants? In one of the Maldiva atolls, a reef, which within a very
few years existed as an islet bearing cocoa-nut trees, was found by
Lieutenant Prentice “_entirely covered with live coral and Madrepore._”
The natives believe that the islet was washed away by a change in the
currents, but if, instead of this, it had quietly subsided, surely
every part of the island which offered a solid foundation, would in a
like manner have become coated with living coral.

Through steps such as these, any thickness of rock, composed of a
singular intermixture of various kinds of corals, shells, and
calcareous sediment, might be formed; but without subsidence, the
thickness would necessarily be determined by the depth at which the
reef-building polypifers can exist. If it be asked, at what rate in
years I suppose a reef of coral favourably circumstanced could grow up
from a given depth; I should answer, that we have no precise evidence
on this point, and comparatively little concern with it. We see, in
innumerable points over wide areas, that the rate has been sufficient,
either to bring up the reefs from various depths to the surface, or, as
is more probable, to keep them at the surface, during progressive
subsidences; and this is a much more important standard of comparison
than any cycle of years.

It may, however, be inferred from the following facts, that the rate
in years under favourable circumstances would be very far from slow.
Dr. Allan, of Forres, has, in his MS. Thesis deposited in the library
of the Edinburgh University (extracts from which I owe to the kindness
of Dr. Malcolmson), the following account of some experiments, which he
tried during his travels in the years 1830 to 1832 on the east coast of
Madagascar. “To ascertain the rise and progress of the coral-family,
and fix the number of species met with at Foul Point (latitude 17° 40′)
twenty species of coral were taken off the reef and planted apart on a
sand-bank _three feet deep at low water._ Each portion weighed ten
pounds, and was kept in its place by stakes. Similar quantities were
placed in a clump and secured as the rest. This was done in December
1830. In July following, each detached mass was nearly level with the
sea at low water, quite immovable, and several feet long, stretching as
the parent reef, with the coast current from north to south. The masses
accumulated in a clump were found equally increased, but some of the
species in such unequal ratios, as to be growing over each other.” The
loss of Dr. Allan’s magnificent collection by shipwreck, unfortunately
prevents its being known to what genera these corals belonged; but from
the numbers experimented on, it is certain that all the more
conspicuous kinds must have been included. Dr. Allan informs me, in a
letter, that he believes it was a Madrepora, which grew most
vigorously. One may be permitted to suspect that the level of the sea
might possibly have been somewhat different at the two stated periods;
nevertheless, it is quite evident that the growth of the ten-pound
masses, during the six or seven months, at the end of which they were
found immovably fixed[28] and several feet in length, must have been
very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very
interesting, as it shows the manner in which a reef, supporting many
species of coral, would probably be affected by a change in the
external conditions favouring one kind more than another. The growth of
the masses of coral in N. and S. lines parallel to the prevailing
currents, whether due to the drifting of sediment or to the simple
movement of the water, is, also, a very interesting circumstance.

 [28] It is stated by De la Beche (“Geological Manual,” p. 143), on the
 authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that some
 specimens of Polypifers, placed by him in a sheltered pool of water,
 were found in the course of a few days firmly fixed by the secretion
 of a stony matter, to the bottom.


A fact, communicated to me by Lieutenant Wellstead, I.N., in some
degree corroborates the result of Dr. Allan’s experiments: it is, that
in the Persian Gulf a ship had her copper bottom encrusted in the
course of twenty months with a layer of coral, _two feet_ in thickness,
which it required great force to remove, when the vessel was docked: it
was not ascertained to what order this coral belonged. The case of the
schooner-channel choked up with coral in an interval of less than ten
years, in the lagoon of Keeling atoll, should be here borne
in mind. We may also infer, from the trouble which the inhabitants of
the Maldiva atolls take to root out, as they express it, the
coral-knolls from their harbours, that their growth can hardly be very
slow.[29]

 [29] Mr. Stutchbury (_West of England Journal_, No. I, p. 50.) has
 described a specimen of Agaricia, “weighing 2 lbs. 9 oz., which
 surrounds a species of oyster, whose age could not be more than two
 years, and yet is completely enveloped by this dense coral.” I presume
 that the oyster was living when the specimen was procured; otherwise
 the fact tells nothing. Mr. Stutchbury also mentions an anchor, which
 had become entirely encrusted with coral in fifty years; other cases,
 however, are recorded of anchors which have long remained amidst
 coral-reefs without having become coated. The anchor of the _Beagle_,
 in 1832, after having been down exactly one month at Rio de Janeiro,
 was so thickly coated by two species of Tubularia, that large spaces
 of the iron were entirely concealed; the tufts of this horny zoophyte
 were between two and three inches in length. It has been attempted to
 compute, but I believe erroneously, the rate of growth of a reef, from
 the fact mentioned by Captain Beechey, of the _ Chama gigas_ being
 embedded in coral-rock. But it should be remembered, that some species
 of this genus invariably live, both whilst young and old, in cavities,
 which the animal has the power of enlarging with its growth. I saw
 many of these shells thus embedded in the outer “flat” of Keeling
 atoll, which is composed of dead rock; and therefore the cavities in
 this case had no relation whatever with the growth of coral. M.
 Lesson, also, speaking of this shell (Partie Zoolog. “Voyage de la
 _Coquille_”), has remarked, “que constamment ses valves étaient
 engagés complétement dans la masse des Madrepores.”

From the facts given in this section, it may be concluded, first, that
considerable thicknesses of rock have certainly been formed within the
present geological area by the growth of coral and the accumulation of
its detritus; and, secondly, that the increase of individual corals and
of reefs, both outwards or horizontally and upwards or vertically,
under the peculiar conditions favourable to such increase, is not slow,
when referred either to the standard of the average oscillations of
level in the earth’s crust, or to the more precise but less important
one of a cycle of years.

_Section III_—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE

I have already described in detail, which might have appeared trivial,
the nature of the bottom of the sea immediately surrounding Keeling
atoll; and I will now describe with almost equal care the soundings off
the fringing-reefs of Mauritius. I have preferred this arrangement, for
the sake of grouping together facts of a similar nature. I sounded with
the wide bell-shaped lead which Captain Fitzroy used at Keeling Island,
but my examination of the bottom was confined to a few miles of coast
(between Port Louis and Tomb Bay) on the leeward side of the island.
The edge of the reef is formed of great shapeless masses
of branching Madrepores, which chiefly consist of two
species,—apparently _M. corymbosa_ and _ pocillifera_,—mingled with a
few other kinds of coral. These masses are separated from each other by
the most irregular gullies and cavities, into which the lead sinks many
feet. Outside this irregular border of Madrepores, the water deepens
gradually to twenty fathoms, which depth generally is found at the
distance of from half to three-quarters of a mile from the reef. A
little further out the depth is thirty fathoms, and thence the bank
slopes rapidly into the depths of the ocean. This inclination is very
gentle compared with that outside Keeling and other atolls, but
compared with most coasts it is steep. The water was so clear outside
the reef, that I could distinguish every object forming the rugged
bottom. In this part, and to a depth of eight fathoms, I sounded
repeatedly, and at each cast pounded the bottom with the broad lead,
nevertheless the arming invariably came up perfectly clean, but deeply
indented. From eight to fifteen fathoms a little calcareous sand was
occasionally brought up, but more frequently the arming was simply
indented. In all this space the two Madrepores above mentioned, and two
species of Astræa, with rather large[30] stars, seemed the commonest
kinds; and it must be noticed that twice at the depth of fifteen
fathoms, the arming was marked with a clean impression of an Astræa.
Besides these lithophytes, some fragments of the _Millepora
alcicornis,_ which occurs in the same relative position at Keeling
Island, were brought up; and in the deeper parts there were large beds
of a Seriatopora, different from _S. subulata_, but closely allied to
it. On the beach within the reef, the rolled fragments consisted
chiefly of the corals just mentioned, and of a massive Porites, like
that at Keeling atoll, of a Meandrina, _ Pocillopora verrucosa_, and of
numerous fragments of Nullipora. From fifteen to twenty fathoms the
bottom was, with few exceptions, either formed of sand, or thickly
covered with Seriatopora: this delicate coral seems to form at these
depths extensive beds unmingled with any other kind. At twenty fathoms,
one sounding brought up a fragment of Madrepora apparently _M.
pocillifera_, and I believe it is the same species (for I neglected to
bring specimens from both stations) which mainly forms the upper margin
of the reef; if so, it grows in depths varying from
0 to 20 fathoms. Between 20 and 23 fathoms I obtained several
soundings, and they all showed a sandy bottom, with one exception at 30
fathoms, when the arming came up scooped out, as if by the margin of a
large Caryophyllia. Beyond 33 fathoms I sounded only once; and from 86
fathoms, at the distance of one mile and a third from the edge of the
reef, the arming brought up calcareous sand with a pebble of volcanic
rock. The circumstance of the arming having invariably come up quite
clean, when sounding within a certain number of fathoms off the reefs
of Mauritius and Keeling atoll (eight fathoms in the former case, and
twelve in the latter) and of its having always come up (with one
exception) smoothed and covered with sand, when the depth exceeded
twenty fathoms, probably indicates a criterion, by which the limits of
the vigorous growth of coral might in all cases be readily ascertained.
I do not, however, suppose that if a vast number of soundings were
obtained round these islands, the limit above assigned would be found
never to vary, but I conceive the facts are sufficient to show, that
the exceptions would be few. The circumstance of a _gradual_ change, in
the two cases, from a field of clean coral to a smooth sandy bottom, is
far more important in indicating the depth at which the larger kinds of
coral flourish than almost any number of separate observations on the
depth, at which certain species have been dredged up. For we can
understand the gradation, only as a prolonged struggle against
unfavourable conditions. If a person were to find the soil clothed with
turf on the banks of a stream of water, but on going to some distance
on one side of it, he observed the blades of grass growing thinner and
thinner, with intervening patches of sand, until he entered a desert of
sand, he would safely conclude, especially if changes of the same kind
were noticed in other places, that the presence of the water was
absolutely necessary to the formation of a thick bed of turf: so may we
conclude, with the same feeling of certainty, that thick beds of coral
are formed only at small depths beneath the surface of the sea.

 [30] Since the preceding pages were printed off, I have received from
 Mr. Lyell a very interesting pamphlet, entitled “Remarks upon Coral
 Formations,” etc., by J. Couthouy, Boston, United States, 1842. There
 is a statement (p. 6), on the authority of the Rev. J. Williams,
 corroborating the remarks made by Ehrenberg and Lyell (p. 71 of this
 volume), on the antiquity of certain individual corals in the Red Sea
 and at Bermuda; namely, that at Upolu, one of the Navigator Islands,
 “particular clumps of coral are known to the fishermen by name,
 derived from either some particular configuration or tradition
 attached to them, and handed down from time immemorial.” With respect
 to the thickness of masses of coral-rock, it clearly appears, from the
 descriptions given by Mr. Couthouy (pp. 34, 58) that Mangaia and
 Aurora Islands are upraised atolls, composed of coral rock: the level
 summit of the former is about three hundred feet, and that of Aurora
 Island is two hundred feet above the sea-level.


I have endeavoured to collect every fact, which might either invalidate
or corroborate this conclusion. Captain Moresby, whose opportunities
for observation during his survey of the Maldiva and Chagos
Archipelagoes have been unrivalled, informs me, that the upper part or
zone of the steep-sided reefs, on the inner and outer coasts of the
atolls in both groups, invariably consists of coral, and the lower
parts of sand. At seven or eight fathoms depth, the bottom is formed,
as could be seen through the clear water, of great living masses of
coral, which at about ten fathoms generally stand some way apart from
each other, with patches of white sand between them, and at a little
greater depth these patches become united into a smooth steep slope,
without any coral. Captain Moresby, also, informs me in support of his
statement, that he found only decayed coral on the Padua Bank (northern
part of the Laccadive group) which has an average depth between
twenty-five and thirty-five fathoms, but that on some other banks in
the same group with only ten or twelve fathoms water on them (for
instance, the Tillacapeni bank), the coral was living.
With regard to the coral-reefs in the Red Sea, Ehrenberg has the
following passage:—“The living corals do not descend there into great
depths. On the edges of islets and near reefs, where the depth was
small, very many lived; but we found no more even at six fathoms. The
pearl-fishers at Yemen and Massaua asserted that there was no coral
near the pearl-banks at nine fathoms depth, but only sand. We were not
able to institute any more special researches.”[31] I am, however,
assured both by Captain Moresby and Lieutenant Wellstead, that in the
more northern parts of the Red Sea, there are extensive beds of living
coral at a depth of twenty-five fathoms, in which the anchors of their
vessels were frequently entangled. Captain Moresby attributes the less
depth, at which the corals are able to live in the places mentioned by
Ehrenberg, to the greater quantity of sediment there; and the
situations, where they were flourishing at the depth of twenty-five
fathoms, were protected, and the water was extraordinarily limpid. On
the leeward side of Mauritius where I found the coral growing at a
somewhat greater depth than at Keeling atoll, the sea, owing apparently
to its tranquil state, was likewise very clear. Within the lagoons of
some of the Marshall atolls, where the water can be but little
agitated, there are, according to Kotzebue, living beds of coral in
twenty-five fathoms. From these facts, and considering the manner in
which the beds of clean coral off Mauritius, Keeling Island, the
Maldiva and Chagos atolls, graduated into a sandy slope, it appears
very probable that the depth, at which reef-building polypifers can
exist, is partly determined by the extent of inclined surface, which
the currents of the sea and the recoiling waves have the power to keep
free from sediment.

 [31] Ehrenberg, “Über die Natur,” etc., p. 50.


MM. Quoy and Gaimard[32] believe that the growth of coral is confined
within very limited depths; and they state that they never found any
fragment of an Astræa (the genus they consider most efficient in
forming reefs) at a depth above twenty-five or thirty feet. But we have
seen that in several places the bottom of the sea is paved with massive
corals at more than twice this depth; and at fifteen fathoms (or twice
this depth) off the reefs of Mauritius, the arming was marked with the
distinct impression of a living Astræa. _Millepora alcicornis_ lives in
from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0
to 20 fathoms. Captain Moresby has given me a specimen of _Sideropora
scabra_ (Porites of Lamarck) brought up alive from 17 fathoms. Mr.
Couthouy[33] states that he has dredged up on the Bahama banks
considerable masses of Meandrina from 16 fathoms, and he has seen this
coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter,
was dredged up alive from 80 fathoms off Juan Fernandez (latitude 33°
S.) by Captain P. P. King:[34] this is the most remarkable fact with
which I am acquainted, showing the depth at which a genus of
corals often found on reefs, can exist.[35] We ought, however, to feel
less
surprise at this fact, as Caryophyllia alone of the lamelliform genera,
ranges far beyond the tropics; it is found in Zetland[36] in Lat. 60°
N. in deep water, and I procured a small species from Tierra del Fuego
in Lat. 53° S. Captain Beechey informs me, that branches of pink and
yellow coral were frequently brought up from between twenty and
twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing
to me from the N.W. coast of Australia, says that a strongly branched
coral was procured there from thirty fathoms; unfortunately it is not
known to what genera these corals belong.

 [32] “Annales des Sci. Nat.” tom. vi.


 [33] “Remarks on Coral Formations,” p. 12.


 [34] I am indebted to Mr. Stokes for having kindly communicated this
 fact to me, together with much other valuable information.


 [35] I will record in the form of a note all the facts that I have
 been able to collect on the depths, both within and without the
 tropics, at which those corals and corallines can live, which there is
 no reason to suppose ever materially aid in the construction of a
 reef.
    Ellis (“Nat. Hist. of Coralline,” p. 96) states that Ombellularia
    was procured in latitude 79° N. _ sticking_ to a _line_ from the
    depth of 236 fathoms; hence this coral either must have been
    floating loose, or was entangled in stray line at the bottom. Off
    Keeling atoll a compound Ascidia (Sigillina) was brought up from 39
    fathoms, and a piece of sponge, apparently living, from 70, and a
    fragment of Nullipora also apparently living from 92 fathoms. At a
    greater depth than 90 fathoms off this coral island, the bottom was
    thickly strewed with joints of Halimeda and small fragments of
    other Nulliporæ, but all dead. Captain B. Allen, R.N., informs me
    that in the survey of the West Indies it was noticed that between
    the depth of 10 and 200 fathoms, the sounding lead very generally
    came up coated with the dead joints of a Halimeda, of which he
    showed me specimens. Off Pernambuco, in Brazil, in about twelve
    fathoms, the bottom was covered with fragments dead and alive of a
    dull red Nullipora, and I infer from Roussin’s chart, that a bottom
    of this kind extends over a wide area. On the beach, within the
    coral-reefs of Mauritius, vast quantities of fragments of Nulliporæ
    were piled up. From these facts it appears, that these simply
    organized bodies are amongst the most abundant productions of the
    sea.


 [36] Fleming’s “British Animals,” genus Caryophyllia.

Name of Zoophyte	Depth in
Fathoms	Country and
S. Latitude	Authority Sertularia	40	Cape Horn 66°	(Where
none is given, the observation is my own.) Cellaria	Ditto	Ditto
Cellaria. A minute scarlet encrusted species, found
living	190	Keeling Atoll 12° Cellaria. An allied, small stony
sub-generic form	48	S. Cruz River 50° A coral allied to
Vincularia, with eight rows of cells	40	Cape Horn Tubulipora,
near to T. patima	Ditto	Ditto Ditto	94	East Chiloe 43°
Cellepora, several species, and allied sub-generic forms	40	Cape
Horn Ditto	40 and 57	Chonos Arch. 45° Ditto	48	S. Cruz 50°
Eschara	30	Tierra del Fuego 53° Ditto	48	S. Cruz R. 50°
Retepora	40	Cape Horn Ditto	100	C. Good Hope 34°	Quoy
and Gaimard, _Ann. Scien. Nat.,_ t. vi, p. 284. Millepora, a strong
coral with cylindrical branches, of a pink colour, abut two inches
high, resembling in the form of its orifices M. aspera of Lamarck	94
and 30	E. Chiloe 43°
Tierra del Fuego 53° Coralium	120	Barbary 33° N.	Peyssonel in
paper read to Royal society May 1752. Antipathes	16	Chonos 45°
Gorgonia (or an allied form)	160	Abrolhos on the coast of Brazil
18°	Capt. Beechey informed me of this fact in a letter.

Although the limit of depth, at which each particular kind of coral
ceases to exist, is far from being accurately known; yet when we bear
in mind the manner in which the clumps of coral gradually became
infrequent at about the same depth, and wholly disappeared at a greater
depth than twenty fathoms, on the slope round Keeling atoll, on the
leeward side of the Mauritius, and at rather less depth, both without
and within the atolls of the Maldiva and Chagos Archipelagoes; and when
we know that the reefs round these islands do not differ from other
coral formations in their form and structure, we may, I think, conclude
that in ordinary cases, reef- building polypifers do not flourish at
greater depths than between twenty and thirty fathoms.

It has been argued[37] that reefs may possibly rise from very great
depths through the means of small corals, first making a platform for
the growth of the stronger kinds. This, however, is an arbitrary
supposition: it is not always remembered, that in such cases there is
an antagonist power in action, namely, the decay of organic bodies,
when not protected by a covering of sediment, or by their own rapid
growth. We have, moreover, no right to calculate on unlimited time for
the accumulation of small organic bodies into great masses. Every fact
in geology proclaims that neither the land, nor the bed of the sea
retain for indefinite periods the same level. As well might it be
imagined that the British Seas would in time become choked up with beds
of oysters, or that the numerous small corallines off the inhospitable
shores of Tierra del Fuego would in time form a solid and extensive
coral-reef.

 [37] _Journal of the Royal Geographical Society,_ 1831, p. 218.




Chapter V THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS


The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.—Their subsidence.—The effects of storms and earthquakes on
atolls.—Recent changes in their state.—The origin of barrier-reefs and
of atolls.—Their relative forms.—The step-formed ledges and walls round
the shores of some lagoons.—The ring-formed reefs of the Maldiva
atolls.—The submerged condition of parts or of the whole of some
annular reefs.—The disseverment of large atolls.—The union of atolls by
linear reefs.—The Great Chagos Bank.—Objections from the area and
amount of subsidence required by the theory, considered.—The probable
composition of the lower parts of atolls.

The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,—those singular
rings of coral-land which rise abruptly out of the unfathomable
ocean—and have passed over, almost unnoticed, the scarcely less
wonderful encircling barrier-reefs. The theory most generally received
on the formation of atolls, is that they are based on submarine
craters; but where can we find a crater of the shape of Bow atoll,
which is five times as long as it is broad (Plate I, Fig. 4); or like
that of Menchikoff Island (Plate II, Fig. 3), with its three loops,
together sixty miles in length; or like Rimsky Korsacoff, narrow,
crooked, and fifty-four miles long; or like the northern Maldiva
atolls, made up of numerous ring-formed reefs, placed on the margin of
a disc,—one of which discs is eighty-eight miles in length, and only
from ten to twenty in breadth? It is, also, not a little improbable,
that there should have existed as many craters of immense size crowded
together beneath the sea, as there are now in some parts atolls. But
this theory lies under a greater difficulty, as will be evident, when
we consider on what foundations the atolls of the larger archipelagoes
rest: nevertheless, if the rim of a crater afforded a basis at the
proper depth, I am far from denying that a reef like a perfectly
characterised atoll might not be formed; some such, perhaps, now exist;
but I cannot believe in the possibility of the greater number having
thus originated.

An earlier and better theory was proposed by Chamisso;[1] he supposes
that as the more massive kinds of corals prefer the surf, the outer
portions, in a reef rising from a submarine basis, would first reach
the surface and consequently form a ring. But on this view it must be
assumed, that in every case the basis consists of a flat bank; for if
it were conically formed, like a mountainous mass, we can see no reason
why the coral should spring up from the flanks, instead of from the
central and highest parts: considering the number of the atolls in the
Pacific and Indian Oceans, this assumption is very improbable. As the
lagoons of atolls are sometimes even more than forty fathoms deep, it
must, also, be assumed on this view, that at a depth at which the waves
do not break, the coral grows more vigorously on the edges of a bank
than on its central part; and this is an assumption without any
evidence in support of it. I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a
level surface some fathoms submerged, having steep sides and being
situated in a deep sea, a reef not to be distinguished from an atoll,
might be formed: I believe some such exist in the West Indies. But a
difficulty of the same kind with that affecting the crater theory,
runners, as we shall presently see, this view inapplicable to the
greater number of atolls.

 [1] Kotzebue’s “First Voyage,” vol. iii, p. 331.

No theory worthy of notice has been advanced to account for those
barrier-reefs, which encircle islands of moderate dimensions. The great
reef which fronts the coast of Australia has been supposed, but without
any special facts, to rest on the edge of a submarine precipice,
extending parallel to the shore. The origin of the third class or of
fringing-reefs presents, I believe, scarcely any difficulty, and is
simply consequent on the polypifers not growing up from great depths,
and their not flourishing close to gently shelving beaches where the
water is often turbid.

What cause, then, has given to atolls and barrier-reefs their
characteristic forms? Let us see whether an important deduction will
not follow from the consideration of these two circumstances, first,
the reef-building corals flourishing only at limited depths; and
secondly, the vastness of the areas interspersed with coral-reefs and
coral-islets, none of which rise to a greater height above the level of
the sea, than that attained by matter thrown up by the waves and winds.
I do not make this latter statement vaguely; I have carefully sought
for descriptions of every island in the intertropical seas; and my task
has been in some degree abridged by a map of the Pacific, corrected in
1834 by MM. D’Urville and Lottin, in which the low islands are
distinguished from the high ones (even from those much less than a
hundred feet in height) by being written without a capital letter; I
have detected a few errors in this map, respecting the height of some
of the islands, which will be noticed in the Appendix, where I treat of
coral formations in geographical order. To the Appendix, also, I must
refer for a more particular account of the data on which the statements
on the next page are grounded. I have ascertained, and chiefly from the
writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and
Lutké, regarding the Pacific; and from Moresby[2] with respect to the
Indian Ocean, that in the following cases the term “low island”
strictly means land of the height commonly attained by matter thrown up
by the winds and the
waves of an open sea. If we draw a line (the plan I have always
adopted) joining the external atolls of that part of the Low
Archipelago in which the islands are numerous, the figure will be a
pointed ellipse (reaching from Hood to Lazaref Island), of which the
longer axis is 840 geographical miles, and the shorter 420 miles; in
this space[3] none of the innumerable islets united into great rings
rise above the stated level. The Gilbert group is very narrow, and 300
miles in length. In a prolonged line from this group, at the distance
of 240 miles, is the Marshall Archipelago, the figure of which is an
irregular square, one end being broader than the other; its length is
520 miles, with an average width of 240; these two groups together are
1,040 miles in length, and all their islets are low. Between the
southern end of the Gilbert and the northern end of Low Archipelago,
the ocean is thinly strewed with islands, all of which, as far as I
have been able to ascertain, are low; so that from nearly the southern
end of the Low Archipelago, to the northern end of the Marshall
Archipelago, there is a narrow band of ocean, more than 4,000 miles in
length, containing a great number of islands, all of which are low. In
the western part of the Caroline Archipelago, there is a space of 480
miles in length, and about 100 broad, thinly interspersed with low
islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas
is 470 miles in length, and 60 in breadth; that of the Laccadives is
150 by 100 miles; as there is a low island between these two groups,
they may be considered as one group of 1,000 miles in length. To this
may be added the Chagos group of low islands, situated 280 miles
distant, in a line prolonged from the southern extremity of the
Maldivas. This group, including the submerged banks, is 170 miles in
length and 80 in breadth. So striking is the uniformity in direction of
these three archipelagoes, all the islands of which are low, that
Captain Moresby, in one of his papers, speaks of them as parts of one
great chain, nearly 1,500 miles long. I am, then, fully justified in
repeating, that enormous spaces, both in the Pacific and Indian Oceans,
are interspersed with islands, of which not one rises above that
height, to which the waves and winds in an open sea can heap up matter.
On what foundations, then, have these reefs and islets of coral been
constructed? A foundation must originally have been present beneath
each atoll at that limited depth, which is indispensable for the first
growth of the reef-building polypifers. A conjecture will perhaps be
hazarded, that the requisite bases might have been afforded by the
accumulation of great banks of sediment, which owing to the action of
superficial currents (aided possibly by the undulatory movement of the
sea) did not quite reach the surface,—as actually appears to have been
the case in some parts of the West Indian Sea. But in the form and
disposition of the groups of atolls, there is nothing to countenance
this notion; and the assumption without any proof, that a number of
immense piles of sediment have been heaped on the floor of the great
Pacific and Indian Oceans, in their central parts far remote from land,
and where the dark blue colour of the limpid water bespeaks its purity,
cannot for one moment be admitted.

 [2] See also Captain Owen’s and Lieutenant Wood’s papers in the
 _Geographical Journal_, on the Maldiva and Laccadive Archipelagoes.
 These officers particularly refer to the lowness of the islets; but I
 chiefly ground my assertion respecting these two groups, and the
 Chagos group, from information communicated to me by Captain Moresby.


 [3] I find from Mr. Couthouy’s pamphlet (p. 58) that Aurora Island is
 about two hundred feet in height; it consists of coral-rock, and seems
 to have been formed by the elevation of an atoll. It lies north-east
 of Tahiti, close without the line bounding the space coloured dark
 blue in the map appended to this volume. Honden Island, which is
 situated in the extreme north-west part of the Low Archipelago,
 according to measurements made on board the _Beagle_, whilst sailing
 by, is 114 feet from the _summit of the trees_ to the water’s edge.
 This island appeared to resemble the other atolls of the group.


The many widely-scattered atolls must, therefore, rest on rocky bases.
But we cannot believe that the broad summit of a mountain lies buried
at the depth of a few fathoms beneath every atoll, and nevertheless
throughout the immense areas above-named, with not one point of rock
projecting above the level of the sea; for we may judge with some
accuracy of mountains beneath the sea, by those on the land; and where
can we find a single chain several hundred miles in length and of
considerable breadth, much less several such chains, with their many
broad summits attaining the same height, within from 120 to 180 feet?
If the data be thought insufficient, on which I have grounded my
belief, respecting the depth at which the reef-building polypifers can
exist, and it be assumed that they can flourish at a depth of even one
hundred fathoms, yet the weight of the above argument is but little
diminished, for it is almost equally improbable, that as many submarine
mountains, as there are low islands in the several great and widely
separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they
should be of the same height within the smaller limit of one or two
hundred feet. So highly improbable is this supposition, that we are
compelled to believe, that the bases of the many atolls did never at
any one period all lie submerged within the depth of a few fathoms
beneath the surface, but that they were brought into the requisite
position or level, some at one period and some at another, through
movements in the earth’s crust. But this could not have been effected
by elevation, for the belief that points so numerous and so widely
separated were successively uplifted to a certain level, but that not
one point was raised above that level, is quite as improbable as the
former supposition, and indeed differs little from it. It will probably
occur to those who have read Ehrenberg’s account of the Reefs of the
Red Sea, that many points in these great areas may have been elevated,
but that as soon as raised, the protuberant parts were cut off by the
destroying action of the waves: a moment’s reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for
the upheaval and subsequent abrasion of an island would leave a flat
disc, which might become coated with coral, but not a deeply concave
surface; moreover, we should expect to see, in some parts at least, the
rock of the foundation brought to the surface. If, then, the
foundations of the many atolls were not uplifted into the requisite
position, they must of necessity have subsided into it; and this at
once solves every difficulty,[4]
for we may safely infer, from the facts given in the last chapter, that
during a gradual subsidence the corals would be favourably
circumstanced for building up their solid frame works and reaching the
surface, as island after island slowly disappeared. Thus areas of
immense extent in the central and most profound parts of the great
oceans, might become interspersed with coral-islets, none of which
would rise to a greater height than that attained by detritus heaped up
by the sea, and nevertheless they might all have been formed by corals,
which absolutely required for their growth a solid foundation within a
few fathoms of the surface.

 [4] The additional difficulty on the crater hypothesis before alluded
 to, will now be evident; for on this view the volcanic action must be
 supposed to have formed within the areas specified a vast number of
 craters, all rising within a few fathoms of the surface, and not one
 above it. The supposition that the craters were at different times
 upraised above the surface, and were there abraded by the surf and
 subsequently coated by corals, is subject to nearly the same
 objections with those given above in this paragraph; but I consider it
 superfluous to detail all the arguments opposed to such a notion.
 Chamisso’s theory, from assuming the existence of so many banks, all
 lying at the proper depth beneath the water, is also vitally
 defective. The same observation applies to an hypothesis of Lieutenant
 Nelson’s (“Geolog. Trans.” vol. v, p. 122), who supposes that the
 ring-formed structure is caused by a greater number of germs of corals
 becoming attached to the declivity, than to the central plateau of a
 submarine bank: it likewise applies to the notion formerly entertained
 (Forster’s “Observ.,” p. 151), that lagoon-islands owe their peculiar
 form to the instinctive tendencies of the polypifers. According to
 this latter view, the corals on the outer margin of the reef
 instinctively expose themselves to the surf in order to afford
 protection to corals living in the lagoon, which belong to other
 genera, and to other families!


It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas
is by no means improbable. We have the clearest proof that a movement
of this kind is possible, in the upright trees buried under the strata
many thousand feet in thickness; we have also every reason for
believing that there are now large areas gradually sinking, in the same
manner as others are rising. And when we consider how many parts of the
surface of the globe have been elevated within recent geological
periods, we must admit that there have been subsidences on a
corresponding scale, for otherwise the whole globe would have swollen.
It is very remarkable that Mr. Lyell,[5] even in the first edition of
his “Principles of Geology,” inferred that the amount of subsidence in
the Pacific must have exceeded that of elevation, from the area of land
being very small relatively to the agents there tending to form it,
namely, the growth of coral and volcanic action. But it will be asked,
are there any direct proofs of a subsiding movement in those areas, in
which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most
difficult, excepting in countries long civilised, to detect a movement,
the tendency of which is to conceal the part affected. In barbarous and
semi-civilised
nations how long might not a slow movement, even of elevation such as
that now affecting Scandinavia, have escaped attention!

 [5] “Principles of Geology,” sixth edition, vol. iii, p. 386.


Mr. Williams[6] insists strongly that the traditions of the natives,
which he has taken much pains in collecting, do not indicate the
appearance of any new islands: but on the theory of a gradual
subsidence, all that would be apparent would be, the water sometimes
encroaching slowly on the land, and the land again recovering by the
accumulation of detritus its former extent, and perhaps sometimes the
conversion of an atoll with coral islets on it, into a bare or into a
sunken annular reef. Such changes would naturally take place at the
periods when the sea rose above its usual limits, during a gale of more
than ordinary strength; and the effects of the two causes would be
hardly distinguishable. In Kotzebue’s “Voyage” there are accounts of
islands, both in the Caroline and Marshall Archipelagoes, which have
been partly washed away during hurricanes; and Kadu, the native who was
on board one of the Russian vessels, said “he saw the sea at Radack
rise to the feet of the cocoa-nut trees; but it was conjured in
time.”[7] A storm lately entirely swept away two of the Caroline
islands, and converted them into shoals; it partly, also, destroyed two
other islands.[8] According to a tradition which was communicated to
Captain Fitzroy, it is believed in the Low Archipelago, that the
arrival of the first ship caused a great inundation, which destroyed
many lives. Mr. Stutchbury relates, that in 1825, the western side of
Chain Atoll, in the same group, was completely devastated by a
hurricane, and not less than 300 lives lost: “in this instance it was
evident, even to the natives, that the hurricane alone was not
sufficient to account for the violent agitation of the ocean.”[9] That
considerable changes have taken place recently in some of the atolls in
the Low Archipelago, appears certain from the case already given of
Matilda Island: with respect to Whitsunday and Gloucester Islands in
this same group, we must either attribute great inaccuracy to their
discoverer, the famous circumnavigator Wallis, or believe that they
have undergone a considerable change in the period of fifty-nine years,
between his voyage and that of Captain Beechey’s. Whitsunday Island is
described by Wallis as “about four miles long, and three wide,” now it
is only one mile and a half long. The appearance of Gloucester Island,
in Captain Beechey’s words,[10] “has been accurately described by its
discoverer, but its present form and extent differ materially.”
Blenheim reef, in the Chagos group, consists of a water-washed annular
reef, thirteen miles in circumference, surrounding a lagoon ten fathoms
deep: on its surface there were a few worn patches of conglomerate
coral-rock, of about the size of hovels; and these Captain Moresby
considered as being, without doubt, the last remnants of islets; so
that here an atoll has been converted into an atoll-formed reef. The
inhabitants of the Maldiva Archipelago, as long ago as 1605, declared,
“that the high tides and violent currents were diminishing the number
of the islands:”[11] and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to
the present inhabitants. In such cases, it would be exceedingly
difficult to detect a gradual subsidence of the foundation, on which
these mutable structures rest.

 [6] Williams’s “Narrative of Missionary Enterprise,” p. 31.


 [7] Kotzebue’s “First Voyage,” vol. iii, p. 168.


 [8] M. Desmoulins in “Comptes Rendus,” 1840, p. 837.


 [9] _West of England Journal_, No. I, p. 35.


 [10] Beechey’s “Voyage to the Pacific,” chap. vii, and Wallis’s
 “Voyage in the _Dolphin_,” chap. iv.


 [11] See an extract from Pyrard’s Voyage in Captain Owen’s paper on
 the Maldiva Archipelago, in the _Geographical Journal_, vol. ii, p.
 84.


Some of the archipelagoes of low coral-islands are subject to
earthquakes: Captain Moresby informs me that they are frequent, though
not very strong, in the Chagos group, which occupies a very central
position in the Indian Ocean, and is far from any land not of coral
formation. One of the islands in this group was formerly covered by a
bed of mould, which, after an earthquake, disappeared, and was believed
by the residents to have been washed by the rain through the broken
masses of underlying rock; the island was thus rendered unproductive.
Chamisso[12] states, that earthquakes are felt in the Marshall atolls,
which are far from any high land, and likewise in the islands of the
Caroline Archipelago. On one of the latter, namely Oulleay atoll,
Admiral Lutké, as he had the kindness to inform me, observed several
straight fissures about a foot in width, running for some hundred yards
obliquely across the whole width of the reef. Fissures indicate a
stretching of the earth’s crust, and, therefore, probably changes in
its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably
they have subsided. In the chapter on Keeling atoll, I attempted to
show by direct evidence, that the island underwent a movement of
subsidence, during the earthquakes lately felt there.

 [12] See Chamisso, in Kotzebue’s “First Voyage,” vol. iii, p. 182 and
 136.

The facts stand thus;—there are many large tracts of ocean, without any
high land, interspersed with reefs and islets, formed by the growth of
those kinds of corals, which cannot live at great depths; and the
existence of these reefs and low islets, in such numbers and at such
distant points, is quite inexplicable, excepting on the theory, that
the bases on which the reefs first became attached, slowly and
successively sank beneath the level of the sea, whilst the corals
continued to grow upwards. No positive facts are opposed to this view,
and some general considerations render it probable. There is evidence
of change in form, whether or not from subsidence, on some of these
coral-islands; and there is evidence of subterranean disturbances
beneath them. Will then the theory, to which we have thus been led,
solve the curious problem,—what has given to each class of reef its
peculiar form?

Let us in imagination place within one of the subsiding areas, an
island surrounded by a “fringing-reef,”—that kind, which alone offers
no difficulty in the explanation of its origin. Let the unbroken lines,
and the oblique shading in the woodcut (No. 4) represent a vertical
section through such an island; and the horizontal shading will
represent the section of the reef. Now, as the island sinks down,
either a few feet at a time or quite insensibly, we may safely infer
from what we know of the conditions favourable to the growth of coral,
that the living masses bathed by the surf on the margin of the reef,
will soon regain the surface. The water, however, will encroach, little
by little, on the shore, the island becoming lower and smaller, and the
space between the edge of the reef and the beach proportionately
broader. A section of the reef and island in this state, after a
subsidence of several hundred feet, is given by the dotted lines:
coral-islets are supposed to have been formed on the new reef, and a
ship is anchored in the lagoon-channel. This section is in every
respect that of an encircling barrier-reef; it is, in fact, a section
taken[13] east and west through the highest point of the encircled
island of Bolabola; of which a plan is given in Plate I, Fig. 5. The
same section is more clearly shown in the following woodcut (No. 5) by
the unbroken lines. The width of the reef, and its slope, both on the
outer and inner side, will have been determined by the growing powers
of the coral, under the conditions (for instance the force of the
breakers and of the currents) to which it has been exposed; and the
lagoon-channel will be deeper or shallower, in proportion to the growth
of the delicately branched corals within the reef, and to the
accumulation of sediment, relatively, also, to the rate of subsidence
and the length of the intervening stationary periods.

 [13] The section has been made from the chart given in the “Atlas of
 the Voyage of the _Coquille_.” The scale is .57 of an inch to a mile.
 The height of the island, according to M. Lesson, is 4,026 feet. The
 deepest part of the lagoon-channel is 162 feet; its depth is
 exaggerated in the woodcut for the sake of clearness.


[Illustration: Vertical section of an island of Bolabola.]

AA—Outer edge of the reef at the level of the sea.
BB—Shores of the island.
A′A′—Outer edge of the reef, after its upward growth during a period of
subsidence.
CC—The lagoon-channel between the reef and the shores of the now
encircled land.
B′B′—The shores of the encircled island.

N.B.—In this, and the following woodcut, the subsidence of the land
could only be represented by an apparent rise in the level of the sea.

It is evident in this section, that a line drawn perpendicularly down
from the outer edge of the new reef to the foundation of solid rock,
exceeds by as many feet as there have been feet of subsidence, that
small limit of depth at which the effective polypifers can live—the
corals having grown up, as the whole sank down, from a basis formed of
other corals and their consolidated fragments. Thus the difficulty on
this head, which before seemed so great, disappears.

As the space between the reef and the subsiding shore continued to
increase in breadth and depth, and as the injurious effects of the
sediment and fresh water borne down from the land were consequently
lessened, the greater number of the channels, with which the reef in
its fringing state must have been breached, especially those which
fronted the smaller streams, will have become choked up with the growth
of coral: on the windward side of the reef, where the coral grows most
vigorously, the breaches will probably have first been closed. In
barrier-reefs, therefore, the breaches kept open by draining the tidal
waters of the lagoon-channel, will generally be placed on the leeward
side, and they will still face the mouths of the larger streams,
although removed beyond the influence of their sediment and fresh
water;—and this, it has been shown, is commonly the case.

[Illustration: Vertical section of an island of Bolabola.]

A′A′—Outer edges of the barrier-reef at the level of the sea. The
cocoa-nut trees represent coral-islets formed on the reef.
CC—The lagoon-channel.
B′B′—The shores of the island, generally formed of low alluvial land
and of coral detritus from the lagoon-channel.
A″A″—The outer edges of the reef now forming an atoll.
C′—The lagoon of the newly formed atoll. According to the scale, the
depth of the lagoon and of the lagoon-channel is exaggerated.


Referring to the diagram shown above, in which the newly formed
barrier-reef is represented by unbroken lines, instead of by dots as in
the former woodcut, let the work of subsidence go on, and the doubly
pointed hill will form two small islands (or more, according to the
number of the hills) included within one annular reef. Let the island
continue subsiding, and the coral-reef will continue growing up on its
own foundation, whilst the water gains inch by inch on the land, until
the last and highest pinnacle is covered, and there remains a perfect
atoll. A vertical section of this atoll is shown in the woodcut by the
dotted lines;—a ship is anchored in its lagoon, but islets are not
supposed yet to have been formed on the reef. The depth of the lagoon
and the
width and slope of the reef, will depend on the circumstances just
referred to under barrier-reefs. Any further subsidence will produce no
change in the atoll, except perhaps a diminution in its size, from the
reef not growing vertically upwards; but should the currents of the sea
act violently upon it, and should the corals perish on part or on the
whole of its margin, changes would result during subsidence which will
be presently noticed. I may here observe, that a bank either of rock or
of hardened sediment, level with the surface of the sea, and fringed
with living coral, would (if not so small as to allow the central space
to be quickly filled up with detritus) by subsidence be converted
immediately into an atoll, without passing, as in the case of a reef
fringing the shore of an island, through the intermediate form of a
barrier-reef. If such a bank lay a few fathoms submerged, the simple
growth of the coral (as remarked in the third chapter) without the aid
of subsidence, would produce a structure scarcely to be distinguished
from a true atoll; for in all cases the corals on the outer margin of a
reef, from having space and being freely exposed to the open sea, will
grow vigorously and tend to form a continuous ring whilst the growth of
the less massive kinds on the central expanse, will be checked by the
sediment formed there, and by that washed inwards by the breakers; and
as the space becomes shallower, their growth will, also, be checked by
the impurities of the water, and probably by the small amount of food
brought by the enfeebled currents, in proportion to the surface of
living reefs studded with innumerable craving mouths: the subsidence of
a reef based on a bank of this kind, would give depth to its central
expanse or lagoon, steepness to its flanks, and through the free growth
of the coral, symmetry to its outline:—I may here repeat that the
larger groups of atolls in the Pacific and Indian Oceans cannot be
supposed to be founded on banks of this nature.

If, instead of the island in the diagram, the shore of a continent
fringed by a reef had subsided, a great barrier-reef, like that on the
north-east coast of Australia, would have necessarily resulted; and it
would have been separated from the main land by a deep-water channel,
broad in proportion to the amount of subsidence, and to the less or
greater inclination of the neighbouring coast-line. The effect of the
continued subsidence of a great barrier-reef of this kind, and its
probable conversion into a chain of separate atolls, will be noticed,
when we discuss the apparent progressive disseverment of the larger
Maldiva atolls.

We now are able to perceive that the close similarity in form,
dimensions, structure, and relative position (which latter point will
hereafter be more fully noticed) between fringing and encircling
barrier-reefs, and between these latter and atolls, is the necessary
result of the transformation, during subsidence of the one class into
the other. On this view, the three classes of reefs ought to graduate
into each other. Reefs having intermediate character between those of
the fringing and barrier classes do exist; for instance, on the
south-west coast of Madagascar, a reef extends for several miles,
within which there is a broad channel from seven to eight fathoms deep,
but the sea does not deepen abruptly outside the reef. Such cases,
however, are open to
some doubts, for an old fringing-reef, which had extended itself a
little on a basis of its own formation, would hardly be distinguishable
from a barrier-reef, produced by a small amount of subsidence, and with
its lagoon-channel nearly filled up with sediment during a long
stationary period. Between barrier-reefs, encircling either one lofty
island or several small low ones, and atolls including a mere expanse
of water, a striking series can be shown: in proof of this, I need only
refer to the first plate in this volume, which speaks more plainly to
the eye, than any description could to the ear. The authorities from
which the charts have been engraved, together with some remarks on them
and descriptive of the plates, are given above. At New Caledonia (Plate
II, Fig. 5.) the barrier-reefs extend for 150 miles on each side of the
submarine prolongation of the island; and at their northern extremity
they appear broken up and converted into a vast atoll-formed reef,
supporting a few low coral-islets: we may imagine that we here see the
effects of subsidence actually in progress, the water always
encroaching on the northern end of the island, towards which the
mountains slope down, and the reefs steadily building up their massive
fabrics in the lines of their ancient growth.

We have as yet only considered the origin of barrier-reefs and atolls
in their simplest form; but there remain some peculiarities in
structure and some special cases, described in the two first chapters,
to be accounted for by our theory. These consist—in the inclined ledge
terminated by a wall, and sometimes succeeded by a second ledge with a
wall, round the shores of certain lagoons and lagoon-channels; a
structure which cannot, as I endeavoured to show, be explained by the
simple growing powers of the corals,—in the ring or basin-like forms of
the central reefs, as well as of the separate marginal portions of the
northern Maldiva atolls,—in the submerged condition of the whole, or of
parts of certain barrier and atoll-formed reefs; where only a part is
submerged, this being generally to leeward,—in the apparent progressive
disseverment of some of the Maldiva atolls,—in the existence of
irregularly formed atolls, some being tied together by linear reefs,
and others with spurs projecting from them,—and, lastly, in the
structure and origin of the Great Chagos Bank.

_Step-formed ledges round certain lagoons._—If we suppose an atoll to
subside at an extremely slow rate, it is difficult to follow out the
complex results. The living corals would grow up on the outer margin;
and likewise probably in the gullies and deeper parts of the bare
surface of the annular reef; the water would encroach on the islets,
but the accumulation of fresh detritus might possibly prevent their
entire submergence. After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would
probably become united with the irregular reefs and banks of sand,
which line the shores of most lagoons. Should, however, the atoll be
carried down by a more rapid movement, the whole surface of the annular
reef, where there was a foundation of solid matter, would be favourably
circumstanced for the fresh growth of coral; but as the corals grew
upwards on its exterior margin, and the waves broke heavily on this
part, the increase of the massive polypifers on the inner side would be
checked from the want of water. Consequently, the exterior parts would
first reach the surface, and the new annular reef thus formed on the
old one, would have its summit inclined inwards, and be terminated by a
subaqueous wall, formed by the upward growth of the coral (before being
much checked), from the inner edge of the solid parts of the old reef.
The inner portion of the new reef, from not having grown to the
surface, would be covered by the waters of the lagoon. Should a
subsidence of the same kind be repeated, the corals would again grow up
in a wall, from all the solid parts of the resunken reef, and,
therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked
in its upward growth, be of less height than the exterior parts, and
therefore would not reach the surface of the lagoon. In this case the
shores of the lagoon would be surrounded by two inclined ledges, one
beneath the other, and both abruptly terminated by subaqueous
cliffs.[14]

 [14] According to Mr. Couthouy (p. 26) the external reef round many
 atolls descends by a succession of ledges or terraces. He attempts, I
 doubt whether successfully, to explain this structure somewhat in the
 same manner as I have attempted, with respect to the internal ledges
 round the lagoons of some atolls. More facts are wanted regarding the
 nature both of the interior and exterior step-like ledges: are all the
 ledges, or only the upper ones, covered with living coral? If they are
 all covered, are the kinds different on the ledges according to the
 depth? Do the interior and exterior ledges occur together in the same
 atolls; if so, what is their total width, and is the intervening
 surface-reef narrow, etc.?

_The ring or basin-formed reefs of the northern Maldiva atolls._—I may
first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards
during subsidence in the same manner as the annular rim; and,
therefore, we might expect that such lagoon- reefs, when not surrounded
and buried by an accumulation of sediment more rapid than the rate of
subsidence, would rise abruptly from a greater depth than that at which
the efficient polypifers can flourish: we see this well exemplified in
the small abruptly-sided reefs, with which the deep lagoons of the
Chagos and Southern Maldiva atolls are studded. With respect to the
ring or basin-formed reefs of the Northern Maldiva atolls, it is
evident, from the perfectly continuous series which exists that the
marginal rings, although wider than the exterior or bounding reef of
ordinary atolls, are only modified portions of such a reef; it is also
evident that the central rings, although wider than the knolls or reefs
which commonly occur in lagoons, occupy their place. The ring-like
structure has been shown to be contingent on the breaches into the
lagoon being broad and numerous, so that all the reefs which are bathed
by the waters of the lagoon are placed under nearly the same conditions
with the outer coast of an atoll standing in the open sea. Hence the
exterior and living margins of these reefs must have been favourably
circumstanced for growing outwards, and increasing beyond the usual
breadth; and they must likewise have been favourably circumstanced for
growing
vigorously upwards, during the subsiding movements, to which by our
theory the whole archipelago has been subjected; and subsidence with
this upward growth of the margins would convert the central space of
each little reef into a small lagoon. This, however, could only take
place with those reefs, which had increased to a breadth sufficient to
prevent their central spaces from being almost immediately filled up
with the sand and detritus driven inwards from all sides: hence it is
that few reefs, which are less than half a mile in diameter, even in
the atolls where the basin-like structure is most strikingly exhibited,
include lagoons. This remark, I may add, applies to all coral-reefs
wherever found. The basin-formed reefs of the Maldiva Archipelago may,
in fact, be briefly described, as small atolls formed during subsidence
over the separate portions of large and broken atolls, in the same
manner as these latter were formed over the barrier-reefs, which
encircled the islands of a large archipelago now wholly submerged.

_Submerged and dead reefs._—In the second section of the first chapter,
I have shown that there are in the neighbourhood of atolls, some deeply
submerged banks, with level surfaces; that there are others, less
deeply but yet wholly submerged, having all the characters of perfect
atolls, but consisting merely of dead coral-rock; that there are
barrier-reefs and atolls with merely a portion of their reef, generally
on the leeward side, submerged; and that such portions either retain
their perfect outline, or they appear to be quite effaced, their former
place being marked only by a bank, conforming in outline with that part
of the reef which remains perfect. These several cases are, I believe,
intimately related together, and can be explained by the same means.
There, perhaps, exist some submerged reefs, covered with living coral
and growing upwards, but to these I do not here refer. As we see that
in those parts of the ocean, where coral-reefs are most abundant, one
island is fringed and another neighbouring one is not fringed; as we
see in the same archipelago, that all the reefs are more perfect in one
part of it than in another, for instance, in the southern half compared
with the northern half of the Maldiva Archipelago, and likewise on the
outer coasts compared with the inner coasts of the atolls in this same
group, which are placed in a double row; as we know that the existence
of the innumerable polypifers forming a reef, depends on their
sustenance, and that they are preyed on by other organic beings; and,
lastly, as we know that some inorganic causes are highly injurious to
the growth of coral, it cannot be expected that during the round of
change to which earth, air, and water are exposed, the reef-building
polypifers should keep alive for perpetuity in any one place; and still
less can this be expected, during the progressive subsidences, perhaps
at some periods more rapid than at others, to which by our theory these
reefs and islands have been subjected and are liable. It is, then, not
improbable that the corals should sometimes perish either on the whole
or on part of a reef; if on part, the dead portion, after a small
amount of subsidence, would still retain its proper outline and
position beneath the water. After a more prolonged
subsidence, it would probably form, owing to the accumulation of
sediment, only the margin of a flat bank, marking the limits of the
former lagoon. Such dead portions of reef would generally lie on the
leeward side,[15] for the impure water and fine sediment would more
easily flow out from the lagoon over this side of the reef, where the
force of the breakers is less than to windward; and therefore the
corals would be less vigorous on this side, and be less able to resist
any destroying agent. It is likewise owing to this same cause, that
reefs are more frequently breached to leeward by narrow channels,
serving as by ship-channels, than to windward. If the corals perished
entirely, or on the greater part of the circumference of an atoll, an
atoll-shaped bank of dead rock, more or less entirely submerged, would
be produced; and further subsidence, together with the accumulation of
sediment, would often obliterate its atoll-like structure, and leave
only a bank with a level surface.

 [15] Mr. Lyell, in the first edition of his “Principles of Geology,”
 offered a somewhat different explanation of this structure. He
 supposes that there has been subsidence; but he was not aware that the
 submerged portions of reef were in most cases, if not in all, dead;
 and he attributes the difference in height in the two sides of most
 atolls, chiefly to the greater accumulation of detritus to windward
 than to leeward. But as matter is accumulated only on the backward
 part of the reef, the front part would remain of the same height on
 both sides. I may here observe that in most cases (for instance, at
 Peros Banhos, the Gambier group and the Great Chagos Bank), and I
 suspect in all cases, the dead and submerged portions do not blend or
 slope into the living and perfect parts, but are separated from them
 by an abrupt line. In some instances small patches of living reef rise
 to the surface from the middle of the submerged and dead parts.

In the Chagos group of atolls, within an area of 160 miles by 60, there
are two atoll-formed banks of dead rock (besides another very imperfect
one), entirely submerged; a third, with merely two or three very small
pieces of living reef rising to the surface; and a fourth, namely,
Peros Banhos (Plate I, Fig. 9), with a portion nine miles in length
dead and submerged. As by our theory this area has subsided, and as
there is nothing improbable in the death, either from changes in the
state of the surrounding sea or from the subsidence being great or
sudden, of the corals on the whole, or on portions of some of the
atolls, the case of the Chagos group presents no difficulty. So far
indeed are any of the above-mentioned cases of submerged reefs from
being inexplicable, that their occurrence might have been anticipated
on our theory, and as fresh atolls are supposed to be in progressive
formation by the subsidence of encircling barrier-reefs, a weighty
objection, namely that the number of atolls must be increasing
infinitely, might even have been raised, if proofs of the occasional
destruction and loss of atolls could not have been adduced.

_The disseverment of the larger Maldiva atolls._—The apparent
progressive disseverment in the Maldiva Archipelago of large atolls
into smaller ones, is, in many respects, an important consideration,
and requires an explanation. The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting
of detached basin-formed reefs. The currents of the sea flow across
these atolls, as I am informed by Captain Moresby, with considerable
force, and drift the sediment from side to side during the monsoons,
transporting much of it seaward; yet the currents sweep with greater
force round their flanks. It is historically known that these atolls
have long existed in their present state; and we can believe, that even
during a very slow subsidence they might thus remain, the central
expanse being kept at nearly its original depth by the accumulation of
sediment. But in the action of such nicely balanced forces during a
progressive subsidence (like that, to which by our theory this
archipelago has been subjected), it would be strange if the currents of
the sea should never make a direct passage across some one of the
atolls, through the many wide breaches in their margins. If this were
once effected, a deep-water channel would soon be formed by the removal
of the finer sediment, and the check to its further accumulation; and
the sides of the channel would be worn into a slope like that on the
outer coasts, which are exposed to the same force of the currents. In
fact, a channel precisely like that bifurcating one which divides
Mahlos Mahdoo (Plate II, Fig. 4), would almost necessarily be formed.
The scattered reefs situated near the borders of the new ocean-channel,
from being favourably placed for the growth of coral, would, by their
extension, tend to produce fresh margins to the dissevered portions;
such a tendency is very evident (as may be seen in the large published
chart) in the elongated reefs on the borders of the two channels
intersecting Mahlos Mahdoo. Such channels would become deeper with
continued subsidence, and probably from the reefs not growing up
perpendicularly, somewhat broader. In this case, and more especially if
the channels had been formed originally of considerable breadth, the
dissevered portions would become perfect and distinct atolls, like Ari
and Ross atolls (Plate II, Fig. 6), or like the two Nillandoo atolls,
which must be considered as distinct, although related in form and
position, and separated from each other by channels, which though deep
have been sounded. Further subsidence would render such channels
unfathomable, and the dissevered portions would then resemble Phaleedoo
and Moluque atolls, or Mahlos Mahdoo and Horsburgh atolls (Plate II,
Fig. 4), which are related to each other in no respect except in
proximity and position. Hence, on the theory of subsidence, the
disseverment of large atolls, which have imperfect margins (for
otherwise their disseverment would be scarcely possible), and which are
exposed to strong currents, is far from being an improbable event; and
the several stages, from close relation to entire isolation in the
atolls of the Maldiva Archipelago, are readily explicable.

We might go even further, and assert as not improbable, that the first
formation of the Maldiva Archipelago was due to a barrier-reef, of
nearly the same dimensions with that of New Caledonia (Plate II, Fig.
5), for if, in imagination, we complete the subsidence of that great
island, we might anticipate from the present broken condition of the
northern portion of the reef, and from the almost entire absence of
reefs on the eastern coast, that the barrier-reef after repeated
subsidences, would become during its upward growth separated into
distinct portions; and these portions would tend to assume an
atoll-like structure, from the coral growing with vigour round their
entire circumferences, when freely exposed to an open sea. As we have
some large islands partly submerged with barrier-reefs marking their
former limits, such as New Caledonia, so our theory makes it probable
that there should be other large islands wholly submerged; and these,
we may now infer, would be surmounted, not by one enormous atoll, but
by several large elongated ones, like the atolls in the Maldiva group;
and these again, during long periods of subsidence, would sometimes
become dissevered into smaller atolls. I may add, that both in the
Marshall and Caroline Archipelagoes, there are atolls standing close
together, which have an evident relationship in form: we may suppose,
in such cases, either that two or more encircled islands originally
stood close together, and afforded bases for two or more atolls, or
that one atoll has been dissevered. From the position, as well as form,
of three atolls in the Caroline Archipelago (the Namourrek and Elato
group), which are placed in an irregular circle, I am strongly tempted
to believe that they have originated by the process of
disseverment.[16]

 [16] The same remark is, perhaps, applicable to the islands of Ollap,
 Fanadik, and Tamatam in the Caroline Archipelago, of which charts are
 given in the atlas of Duperrey’s voyage: a line drawn through the
 linear reefs and lagoons of these three islands forms a semicircle.
 Consult also, the atlas of Lutké’s voyage; and for the Marshall group
 that of Kotzebue; for the Gilbert group consult the atlas of
 Duperrey’s voyage. Most of the points here referred to may, however,
 be seen in Krusenstern’s general Atlas of the Pacific.

_Irregularly formed atolls._—In the Marshall group, Musquillo atoll
consists of two loops united in one point; and Menchikoff atoll is
formed of three loops, two of which (as may be seen in Fig. 3, Plate
II) are connected by a mere ribbon-shaped reef, and the three together
are sixty miles in length. In the Gilbert group some of the atolls have
narrow strips of reef, like spurs, projecting from them. There occur
also in parts of the open sea, a few linear and straight reefs,
standing by themselves; and likewise some few reefs in the form of
crescents, with their extremities more or less curled inwards. Now, the
upward growth of a barrier-reef which fronted only one side of an
island, or one side of an elongated island with its extremities (of
which cases exist), would produce after the complete subsidence of the
land, mere strips or crescent or hook-formed reefs: if the island thus
partially fronted became divided during subsidence into two or more
islands, these islands would be united together by linear reefs; and
from the further growth of the coral along their shores together with
subsidence, reefs of various forms might ultimately be produced, either
atolls united together by linear reefs, or atolls with spurs projecting
from them. Some, however, of the more simple forms above specified,
might, as we have seen, be equally well produced by the coral perishing
during
subsidence on part of the circumference of an atoll, whilst on the
other parts it continued to grow up till it reached the surface.

_The Great Chagos Bank._—I have already shown that the submerged
condition of the Great Chagos Bank (Plate II, Fig. 1, with its section
Fig. 2), and of some other banks in the Chagos group, may in all
probability be attributed to the coral having perished before or during
the movements of subsidence, to which this whole area by our theory has
been subjected. The external rim or upper ledge (shaded in the chart),
consists of dead coral-rock thinly covered with sand; it lies at an
average depth of between five and eight fathoms, and perfectly
resembles in form the annular reef of an atoll. The banks of the second
level, the boundaries of which are marked by dotted lines in the chart,
lie from about fifteen to twenty fathoms beneath the surface; they are
several miles broad, and terminate in a very steep slope round the
central expanse. This central expanse I have already described, as
consisting of a level muddy flat between thirty and forty fathoms deep.
The banks of the second level, might at first sight be thought
analogous to the internal step-like ledge of coral-rock which borders
the lagoons of some atolls, but their much greater width, and their
being formed of sand, are points of essential difference. On the
eastern side of the atoll some of the banks are linear and parallel,
resembling islets in a great river, and pointed directly towards a
great breach on the opposite side of the atoll; these are best seen in
the large published chart. I inferred from this circumstance, that
strong currents sometimes set directly across this vast bank; and I
have since heard from Captain Moresby that this is the case. I
observed, also, that the channels or breaches through the rim, were all
of the same depth as the central lagoon-like space into which they
lead; whereas the channels into the other atolls of the Chagos group,
and as I believe into most other large atolls, are not nearly as deep
as their lagoons: for instance at Peros Banhos, the channels are only
of the same depth, namely between ten and twenty fathoms, as the bottom
of the lagoon for a space about a mile and a half in width round its
shores, whilst the central expanse of the lagoon is from thirty-five to
forty fathoms deep. Now, if an atoll during a gradual subsidence once
became entirely submerged, like the Great Chagos Bank, and therefore no
longer exposed to the surf, very little sediment could be formed from
it; and consequently the channels leading into the lagoon from not
being filled up with drifted sand and coral detritus, would continue
increasing in depth, as the whole sank down. In this case, we might
expect that the currents of the open sea, instead of any longer
sweeping round the submarine flanks, would flow directly through the
breaches across the lagoon, removing in their course the finer
sediment, and preventing its further accumulation. We should then have
the submerged reef forming an external and upper rim of rock, and
beneath this portion of the sandy bottom of the old lagoon, intersected
by deep-water channels or breaches, and thus formed into separate
marginal banks; and these would be cut off by steep slopes, overhanging
the central space, worn down by the passage of the oceanic currents.


By these means, I have scarcely any doubt that the Great Chagos Bank
has originated,—a structure which at first appeared to me far more
anomalous than any I had met with. The process of formation is nearly
the same with that, by which Mahlos Mahdoo had been trisected; but in
the Chagos Bank the channels of the oceanic currents entering at
several different quarters, have united in a central space.

This great atoll-formed bank appears to be in an early stage of
disseverment; should the work of subsidence go on, from the submerged
and dead condition of the whole reef, and the imperfection of the
south-east quarter a mere wreck would probably be left. The Pitt’s
Bank, situated not far southward, appears to be precisely in this
state; it consists of a moderately level, oblong bank of sand, lying
from 10 to 20 fathoms beneath the surface, with two sides protected by
a narrow ledge of rock which is submerged between 5 and 8 fathoms. A
little further south, at about the same distance as the southern rim of
the Great Chagos Bank is from the northern rim, there are two other
small banks with from 10 to 20 fathoms on them; and not far eastward
soundings were struck on a sandy bottom, with between 110 and 145
fathoms. The northern portion with its ledge-like margin, closely
resembles any one segment of the Great Chagos Bank, between two of the
deep-water channels, and the scattered banks, southward appear to be
the last wrecks of less perfect portions.

I have examined with care the charts of the Indian and Pacific Oceans,
and have now brought before the reader all the examples, which I have
met with, of reefs differing from the type of the class to which they
belong; and I think it has been satisfactorily shown, that they are all
included in our theory, modified by occasional accidents which might
have been anticipated as probable. In this course we have seen, that in
the lapse of ages encircling barrier-reefs are occasionally converted
into atolls, the name of atoll being properly applicable, at the moment
when the last pinnacle of encircled land sinks beneath the surface of
the sea. We have, also, seen that large atolls during the progressive
subsidence of the areas in which they stand, sometimes become
dissevered into smaller ones; at other times, the reef-building
polypifers having entirely perished, atolls are converted into
atoll-formed banks of dead rock; and these again through further
subsidence and the accumulation of sediment modified by the force of
the oceanic currents, pass into level banks with scarcely any
distinguishing character. Thus may the history of an atoll be followed
from its first origin, through the occasional accidents of its
existence, to its destruction and final obliteration.

_Objections to the theory of the formation of atolls and
barrier-reefs._—The vast amount of subsidence, both horizontally or in
area, and vertically or in depth, necessary to have submerged every
mountain, even the highest, throughout the immense spaces of ocean
interspersed with atolls, will probably strike most people as a
formidable objection to my theory. But as continents, as large as the
spaces supposed to have subsided, have been raised above the level of
the sea,—as whole regions are now rising, for instance, in Scandinavia
and South America,—and as
no reason can be assigned, why subsidences should not have occurred in
some parts of the earth’s crust on as great a scale both in extent and
amount as those of elevation, objections of this nature strike me as of
little force. The remarkable point is that movements to such an extent
should have taken place within a period, during which the polypifers
have continued adding matter on and above the same reefs. Another and
less obvious objection to the theory will perhaps be advanced from the
circumstance, of the lagoons within atolls and within barrier-reefs
never having become in any one instance during prolonged subsidences of
a greater depth than sixty fathoms, and seldom more than forty fathoms;
but we already admit, if the theory be worth considering, that the rate
of subsidence has not exceeded that of the upward growth of the coral
on the exterior margin; we are, therefore, only further required to
admit, that the subsidence has not exceeded in rate the filling up of
the interior spaces by the growth of the corals living there, and by
the accumulation of sediment. As this filling up must take place very
slowly within barrier-reefs lying far from the land, and within atolls
which are of large dimensions and which have open lagoons with very few
reefs, we are led to conclude that the subsidence thus
counter-balanced, must have been slow in an extraordinary degree; a
conclusion which accords with our only means, namely, with what is
known of the rate and manner of recent elevatory movements, of judging
by analogy what is the probable rate of subsidence.

In this chapter it has, I think, been shown, that the theory of
subsidence, which we were compelled to receive from the necessity of
giving to the corals, in certain large areas, foundations at the
requisite depth, explains both the normal structure and the less
regular forms of those two great classes of reefs, which have justly
excited the astonishment of all persons who have sailed through the
Pacific and Indian Oceans. But further to test the truth of the theory,
a crowd of questions will occur to the reader: Do the different kinds
of reefs, which have been produced by the same kind of movement,
generally lie within the same areas? What is their relation of form and
position,—for instance, do adjoining groups of atolls, and the separate
atolls in these groups, bear the same relation to each other which
islands do in common archipelagoes? Have we reason to believe, that
where there are fringing-reefs, there has not lately been subsidence;
or, for it is almost our only way of ascertaining this point, are there
frequently proofs of recent elevation? Can we by this means account for
the presence of certain classes of reefs in some large areas, and their
entire absence in others? Do the areas which have subsided, as
indicated by the presence of atolls and barrier-reefs, and the areas
which have remained stationary or have been upraised, as shown by
fringing-reefs, bear any determinate relation to each other; and are
the dimensions of these areas such as harmonise with the greatness of
the subterranean changes, which, it must be supposed, have lately taken
place beneath them? Is there any connection between the movements thus
indicated, and recent volcanic action? All these questions ought to
receive answers in accordance with the theory; and if this can be
satisfactorily shown, not only is the theory
confirmed, but as deductions, the answers are in themselves important.
Under this latter point of view, these questions will be chiefly
considered in the following chapter.[17]

 [17] I may take this opportunity of briefly considering the
 appearances, which would probably be presented by a vertical and deep
 section across a coral formation (referring chiefly to an atoll),
 formed by the upward growth of coral during successive subsidences.
 This is a subject worthy of attention, as a means of comparison with
 ancient coral-strata. The circumferential parts would consist of
 massive species, in a vertical position, with their interstices filled
 up with detritus; but this would be the part most subject to
 subsequent denudation and removal. It is useless to speculate how
 large a portion of the exterior annular reef would consist of upright
 coral, and how much of fragmentary rock, for this would depend on many
 contingencies,—such as on the rate of subsidence, occasionally
 allowing a fresh growth of coral to cover the whole surface, and on
 the breakers having force sufficient to throw fragments over this same
 space. The conglomerate which composes the base of the islets, would
 (if not removed by denudation together with the exterior reef on which
 it rests) be conspicuous from the size of the fragments,—the different
 degrees in which they have been rounded,—the presence of fragments of
 conglomerate torn up, rounded, and recemented,—and from the oblique
 stratification. The corals which lived in the lagoon-reefs at each
 successive level, would be preserved upright, and they would consist
 of many kinds, generally much branched. In this part, however, a very
 large proportion of the rock (and in some cases nearly all of it)
 would be formed of sedimentary matter, either in an excessively fine,
 or in a moderately coarse state, and with the particles almost blended
 together. The conglomerate which was formed of rounded pieces of the
 branched corals, on the shores of the lagoon, would differ from that
 formed on the islets and derived from the outer coast; yet both might
 have accumulated very near each other. I have seen a conglomerate
 limestone from Devonshire like a conglomerate now forming on the
 shores of the Maldiva atolls. The stratification taken as a whole,
 would be horizontal; but the conglomerate beds resting on the exterior
 reef, and the beds of sandstone on the shores of the lagoon (and no
 doubt on the external flanks) would probably be divided (as at Keeling
 atoll and at Mauritius) by numerous layers dipping at considerable
 angles in different directions. The calcareous sandstone and
 coral-rock would almost necessarily contain innumerable shells,
 echini, and the bones of fish, turtle, and perhaps of birds; possibly,
 also, the bones of small saurians, as these animals find their way to
 the islands far remote from any continent. The large shells of some
 species of Tridacna would be found vertically imbedded in the solid
 rock, in the position in which they lived. We might expect also to
 find a mixture of the remains of pelagic and littoral animals in the
 strata formed in the lagoon, for pumice and the seeds of plants are
 floated from distant countries into the lagoons of many atolls: on the
 outer coast of Keeling atoll, near the mouth of the lagoon, the case
 of a pelagic Pteropodous animal was brought up on the arming of the
 sounding lead. All the loose blocks of coral on Keeling atoll were
 burrowed by vermiform animals; and as every cavity, no doubt,
 ultimately becomes filled with spathose limestone, slabs of the rock
 taken from a considerable depth, would, if polished, probably exhibit
 the excavations of such burrowing animals. The conglomerate and
 fine-grained beds of coral-rock would be hard, sonorous, white and
 composed of nearly pure calcareous matter; in some few parts, judging
 from the specimens at Keeling atoll, they would probably contain a
 small quantity of iron. Floating pumice and scoriæ, and occasionally
 stones transported in the root of trees (see my “Journal of
 Researches,” page 549) appear the only sources, through which foreign
 matter is brought to coral-formations standing in the open ocean. The
 area over which sediment is transported from coral-reefs must be
 considerable: Captain Moresby informs me that during the change of
 monsoons the sea is discoloured to a considerable distance off the
 Maldiva and Chagos atolls. The sediment of fringing and barrier
 coral-reefs must be mingled with the mud, which is brought down from
 the land, and is transported seaward through the breaches, which occur
 in front of almost every valley. If the atolls of the larger
 archipelagoes were upraised, the bed of the ocean being converted into
 land, they would form flat-topped mountains, varying in diameter from
 a few miles (the smallest atolls being worn away) to sixty miles; and
 from being horizontally stratified and of similar composition, they
 would, as Mr. Lyell has remarked, falsely appear as if they had
 originally been united into one vast continuous mass. Such great
 strata of coral-rock would rarely be associated with erupted volcanic
 matter, for this could only take place, as may be inferred from what
 follows in the next chapter, when the area, in which they were
 situated, commenced to rise, or at least ceased to subside. During the
 enormous period necessary to effect an elevation of the kind just
 alluded to, the surface would necessarily be denuded to a great
 thickness; hence it is highly improbable that any fringing-reef, or
 even any barrier-reef, at least of those encircling small islands,
 would be preserved. From this same cause, the strata which were formed
 within the lagoons of atolls and lagoon-channels of barrier-reefs, and
 which must consist in a large part of sedimentary matter, would more
 often be preserved to future ages, than the exterior solid reef,
 composed of massive corals in an upright position; although it is on
 this exterior part that the present existence and further growth of
 atolls and barrier-reefs entirely depend.


_ Plate III_—Map showing the distribution of coral-reefs and active
volcanoes.


[Illustration: Map showing distribution of coral-reefs and active
volcanoes.]

[Illustration: Map showing distribution of coral-reefs and active
volcanoes.]

[Illustration: Map showing distribution of coral-reefs and active
volcanoes.]

The principles, on which this map was coloured, are explained in the
beginning of Chapter VI; and the authorities for each particular spot
are detailed in the Appendix to _Coral Reefs._ The names not printed in
upper case in the Index refer to the Appendix.




Chapter VI ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION


Description of the coloured map.—Proximity of atolls and
barrier-reefs.—Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.


It will be convenient to give here a short account of the appended map
(Plate III):[1] a fuller one, with the data for colouring each spot, is
reserved
for the Appendix; and every place there referred to may be found in the
Index. A larger chart would have been desirable; but, small as the
adjoined one is, it is the result of many months’ labour. I have
consulted, as far as I was able, every original voyage and map; and the
colours were first laid down on charts on a larger scale. The same blue
colour, with merely a difference in the depth of tint, is used for
atolls or lagoon-islands, and barrier-reefs, for we have seen, that as
far as the actual coral-formation is concerned, they have no
distinguishing character. Fringing-reefs have been coloured red, for
between them on the one hand, and barrier-reefs and atolls on the
other, there is an important distinction with respect to the depth
beneath the surface, at which we are compelled to believe their
foundations lie. The two distinct colours, therefore, mark two great
types of structure.

 [1] Inasmuch as the coloured map would have proved too costly to be
 given in this series, the indications of colour have been replaced by
 numbers referring to the dotted groups of reefs, etc. The author’s
 original wording, however, is retained in full, as it will be easy to
 refer to the map by the numbers, and thus the flow of the narrative is
 undisturbed.

The _dark blue colour_ [represented by (3) in our plate] represents
atolls and submerged annular reefs, with deep water in their centres. I
have coloured as atolls, a few low and small coral-islands, without
lagoons; but this has been done only when it clearly appeared that they
originally contained lagoons, since filled up with sediment: when there
were not good grounds for this belief, they have been left uncoloured.

The _pale blue colour_ [represented by (2)] represents barrier-reefs.
The most obvious character of reefs of this class is the broad and
deep-water moat within the reef: but this, like the lagoons of small
atolls, is liable to become filled up with detritus and with reefs of
delicately branched corals: when, therefore, a reef round the entire
circumference of an island extends very far into a profoundly deep sea,
so that it can hardly be confounded with a fringing-reef which must
rest on a foundation of rock within a small depth, it has been coloured
pale blue, although it does not include a deep-water moat: but this has
only been done rarely, and each case is distinctly mentioned in the
Appendix.

The _red colour_ (4) represents reefs fringing the land quite closely
where the sea is deep, and where the bottom is gently inclined
extending to a moderate distance from it, but not having a deep-water
moat or lagoon-like space parallel to the shore. It must be remembered
that fringing-reefs are frequently _breached_ in front of rivers and
valleys by deepish channels, where mud has been deposited. A space of
thirty miles in width has been coloured round or in front of the reefs
of each class, in order that the colours might be conspicuous on the
appended map, which is reduced to so small a scale.

The _vermillion spots_, and streaks (1) represent volcanoes now in
action, or historically known to have been so. They are chiefly laid
down from Von Buch’s work on the Canary Islands; and my reasons for
making a few alterations are given in the note below.[2]

 [2] I have also made considerable use of the geological part of
 Berghaus’ “Physical Atlas.” Beginning at the eastern side of the
 Pacific, I have added to the number of the volcanoes in the southern
 part of the Cordillera, and have coloured Juan Fernandez according to
 observations collected during the voyage of the _Beagle_ (“Geol.
 Trans.,” vol. v, p. 601). I have added a volcano to Albemarle Island,
 one of the Galapagos Archipelago (the author’s “Journal of
 Researches,” p. 457). In the Sandwich group there are no active
 volcanoes, except at Hawaii; but the Rev. W. Ellis informs me, there
 are streams of lava apparently modern on Maui, having a very recent
 appearance, which can be traced to the craters whence they flowed. The
 same gentleman informs me, that there is no reason to believe that any
 active volcano exists in the Society Archipelago; nor are there any
 known in the Samoa or Navigator group, although some of the streams of
 lava and craters there appear recent. In the Friendly group, the Rev.
 J. Williams says (“Narrative of Missionary Enterprise,” p. 29) that
 Toofoa and Proby Islands are active volcanoes. I infer from Hamilton’s
 “Voyage in the _Pandora_” (p. 95), that Proby Island is synonymous
 with Onouafou, but I have not ventured to colour it. There can be no
 doubt respecting Toofoa, and Captain Edwards (Von Buch, p. 386) found
 the lava of recent eruption at Amargura still smoking. Berghaus marks
 four active volcanoes actually within the Friendly group; but I do not
 know on what authority: I may mention that Maurelle describes Latte as
 having a burnt-up appearance: I have marked only Toofoa and Amargura.
 South of the New Hebrides lies Matthews Rock, which is drawn and
 described as an active crater in the “Voyage of the _Astrolabe_.”
 Between it and the volcano on the eastern side of New Zealand, lies
 Brimstone Island, which from the high temperature of the water in the
 crater, may be ranked as active (Berghaus “Vorbemerk,” II Lief. S.
 56). Malte Brun, vol. xii, p. 231, says that there is a volcano near
 port St. Vincent in New Caledonia. I believe this to be an error,
 arising from a smoke seen on the _opposite_ coast by Cook (“Second
 Voyage,” vol. ii, p. 23) which smoke went out at night. The Mariana
 Islands, especially the northern ones, contain many craters (see
 Freycinet’s “Hydrog. Descript.”) which are not active. Von Buch,
 however, states (p. 462) on the authority of La Peyrouse, that there
 are no less than seven volcanoes between these islands and Japan.
 Gemelli Creri (Churchill’s “Collect.” vol. iv, p. 458), says there are
 two active volcanoes in latitude 23° 30′, and in latitude 24°: but I
 have not coloured them. From the statements in Beechey’s “Voyage” (p.
 518, 4to edit.) I have coloured one in the northern part of the Bonin
 group. M. S. Julien has clearly made out from Chinese manuscripts not
 very ancient (“Comptes Rendus,” 1840, p. 832), that there are two
 active volcanoes on the eastern side of Formosa. In Torres Straits, on
 Cap Island (9° 48′ S., 142° 39′ E.) a volcano was seen burning with
 great violence in 1793 by Captain Bampton (see Introduction to
 Flinders’ “Voyage,” p. 41). Mr. M’Clelland (Report of Committee for
 investigating Coal in India, p. 39) has shown that the volcanic band
 passing through Barren Island must be extended northwards. It appears
 by an old chart, that Cheduba was once an active volcano (see also _
 Silliman’s North American Journal_, vol. xxxviii, p. 385). In
 Berghaus’ “Phys. Atlas,” 1840, No. 7 of Geological Part, a volcano on
 the coast of Pondicherry is said to have burst forth in 1757.
 Ordinaire (“Hist. Nat. des Volcans,” p. 218) says that there is one at
 the mouth of the Persian Gulf, but I have not coloured it, as he gives
 no particulars. A volcano in Amsterdam, or St. Paul’s, in the southern
 part of the Indian Ocean, has been seen (_Naut. Mag._ 1838, p. 842) in
 action. Dr. J. Allan, of Forres, informs me in a letter, that when he
 was at Joanna, he saw at night flames apparently volcanic, issuing
 from the chief Comoro Island, and that the Arabs assured him that they
 were volcanic, adding that the volcano burned more during the wet
 season. I have marked this as a volcano, though with some hesitation,
 on account of the possibility of the flame arising from gaseous
 sources.


The uncoloured coasts consist, first and chiefly, of those, where there
are no coral-reefs, or such small portions as to be quite
insignificant. Secondly, of those coasts where there are reefs, but
where the sea is very shallow, for in this case the reefs generally lie
far from the land, and become very irregular, in their forms: where
they have not become irregular, they have been coloured. thirdly, if I
had the means of ascertaining the fact, I should not colour a reef
merely coating the edges of a submarine crater, or of a level submerged
bank; for such superficial
formations differ essentially, even when not in external appearance,
from reefs whose foundations as well as superficies have been wholly
formed by the growth of coral. Fourthly, in the Red Sea, and within
some parts of the East Indian Archipelago (if the imperfect charts of
the latter can be trusted), there are many scattered reefs, of small
size, represented in the chart by mere dots, which rise out of deep
water: these cannot be arranged under either of the three classes: in
the Red Sea, however, some of these little reefs, from their position,
seem once to have formed parts of a continuous barrier. There exist,
also, scattered in the open ocean, some linear and irregularly formed
strips of coral-reef, which, as shown in the last chapter, are probably
allied in their origin to atolls; but as they do not belong to that
class, they have not been coloured; they are very few in number and of
insignificant dimensions. Lastly, some reefs are left uncoloured from
the want of information respecting them, and some because they are of
an intermediate structure between the barrier and fringing classes. The
value of the map is lessened, in proportion to the number of reefs
which I have been obliged to leave uncoloured, although, in a
theoretical point of view, few of them present any great difficulty:
but their number is not very great, as will be found by comparing the
map with the statements in the Appendix. I have experienced more
difficulty in colouring fringing-reefs than in colouring barrier-reefs,
as the former, from their much less dimensions, have less attracted the
attention of navigators. As I have had to seek my information from all
kinds of sources, and often from indirect ones, I do not venture to
hope that the map is free from many errors. Nevertheless, I trust it
will give an approximately correct view of the general distribution of
the coral-reefs over the whole world (with the exception of some
fringing-reefs on the coast of Brazil, not included within the limits
of the map), and of their arrangement into the three great classes,
which, though necessarily very imperfect from the nature of the objects
classified, have been adopted by most voyagers. I may further remark,
that the dark blue colour represents land entirely composed of
coral-rock; the pale blue, land with a wide and thick border of
coral-rock; and the red, a mere narrow fringe of coral-rock.

Looking now at the map under the theoretical point of view indicated in
the last chapter, the two blue tints signify that the foundations of
the reefs thus coloured have subsided to a considerable amount, at a
slower rate than that of the upward growth of the corals, and that
probably in many cases they are still subsiding. The red signifies that
the shores which support fringing-reefs have not subsided (at least to
any considerable
amount, for the effects of a subsidence on a small scale would in no
case be distinguishable); but that they have remained nearly stationary
since the period when they first became fringed by reefs; or that they
are now rising or have been upraised, with new lines of reefs
successively formed on them: these latter alternatives are obviously
implied, as newly formed lines of shore, after elevations of the land,
would be in the same state with respect to the growth of
fringing-reefs, as stationary coasts. If during the prolonged
subsidence of a shore, coral-reefs grew for the first time on it, or if
an old barrier-reef were destroyed and submerged, and new reefs became
attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of
error, any coast has been coloured wrongly with respect to movement
indicated. Well characterised atolls and encircling barrier-reefs,
where several occur in a group, or a single barrier-reef if of large
dimensions, leave scarcely any doubt on the mind respecting the
movement by which they have been produced; and even a small amount of
subsequent elevation is soon betrayed. The evidence from a single atoll
or a single encircling barrier-reef, must be received with some
caution, for the former may possibly be based upon a submerged crater
or bank, and the latter on a submerged margin of sediment, or of
worn-down rock. From these remarks we may with greater certainty infer
that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or
have been upraised.

_On the grouping of the different classes of reefs._—Having made these
preliminary remarks, I will consider first how far the grouping of the
different kinds of coral-islands and reefs is corroborative of the
truth of the theory. A glance at the map shows that the reefs, coloured
blue and red, produced under widely different conditions, are not
indiscriminately mixed together. Atolls and barrier-reefs, on the other
hand, as may be seen by the two blue tints, generally lie near each
other; and this would be the natural result of both having been
produced during the subsidence of the areas in which they stand. Thus,
the largest group of encircled islands is that of the Society
Archipelago; and these islands are surrounded by atolls, and only
separated by a narrow space from the large group of Low atolls. In the
midst of the Caroline atolls, there are three fine encircled islands.
The northern point of the barrier-reef of New Caledonia seems itself,
as before remarked, to form a complete large atoll. The great
Australian barrier is described as including both atolls and small
encircled islands. Captain King[3] mentions many atoll-formed and
encircling coral-reefs, some of which lie within the barrier, and
others may be said (for instance between lat. 16° and 13°) to form part
of it. Flinders[4] has described an atoll-formed reef in lat. 10°,
seven miles long and from one to three broad, resembling a boot in
shape, with apparently very deep water
within. Eight miles westward of this, and forming part of the barrier,
lie the Murray Islands, which are high and are encircled. In the
Corallian Sea, between the two great barriers of Australia and New
Caledonia, there are many low islets and coral-reefs, some of which are
annular, or horse-shoe shaped. Observing the smallness of the scale of
the map, the parallels of latitude being nine hundred miles apart, we
see that none of the large groups of reefs and islands supposed to have
been produced by long-continued subsidence, lie near extensive lines of
coast coloured red, which are supposed to have remained stationary
since the growth of their reefs, or to have been upraised and new lines
of reefs formed on them. Where the red and blue circles do occur near
each other, I am able, in several instances, to show that there have
been oscillations of level, subsidence having preceded the elevation of
the red spots; and elevation having preceded the subsidence of the blue
spots: and in this case the juxtaposition of reefs belonging to the two
great types of structure is little surprising. We may, therefore,
conclude that the proximity in the same areas of the two classes of
reefs, which owe their origin to the subsidence of the earth’s crust,
and their separation from those formed during its stationary or
uprising condition, holds good to the full extent, which might have
been anticipated by our theory.

 [3] Sailing directions, appended to vol. ii of his “Surveying Voyage
 to Australia.”


 [4] “Voyage to Terra Australis,” vol. ii, p. 336.

As groups of atolls have originated in the upward growth, at each fresh
sinking of the land, of those reefs which primarily fringed the shores
of one great island, or of several smaller ones; so we might expect
that these rings of coral-rock, like so many rude outline charts, will
still retain some traces of the general form, or at least general
range, of the land, round which they were first modelled. That this is
the case with the atolls in the Southern Pacific as far as their range
is concerned, seems highly probable, when we observe that the three
principal groups are directed in north-west and south-east lines, and
that nearly all the land in the S. Pacific ranges in this same
direction; namely, N. Western Australia, New Caledonia, the northern
half of New Zealand, the New Hebrides, Saloman, Navigator, Society,
Marquesas, and Austral archipelagoes: in the Northern Pacific, the
Caroline atolls abut against the north-west line of the Marshall
atolls, much in the same manner as the east and west line of islands
from Ceram to New Britain do on New Ireland: in the Indian Ocean the
Laccadive and Maldiva atolls extend nearly parallel to the western and
mountainous coast of India. In most respects, there is a perfect
resemblance with ordinary islands in the grouping of atolls and in
their form: thus the outline of all the larger groups is elongated; and
the greater number of the individual atolls are elongated in the same
direction with the group, in which they stand. The Chagos group is less
elongated than is usual with other groups, and the individual atolls in
it are likewise but little elongated; this is strikingly seen by
comparing them with the neighbouring Maldiva atolls. In the Marshall
and Maldiva archipelagoes, the atolls are ranged in two parallel lines,
like the mountains in a great double mountain-chain. Some of the
atolls, in the larger archipelagoes, stand so near to each other, and
have such an evident relationship in
form, that they compose little sub-groups: in the Caroline Archipelago,
one such sub-group consists of Pouynipete, a lofty island encircled by
a barrier-reef, and separated by a channel only four miles and a half
wide from Andeema atoll, with a second atoll a little further off. In
all these respects an examination of a series of charts will show how
perfectly groups of atolls resemble groups of common islands.

_On the direct evidence of the blue spaces in the map having subsided
during the upward growth of the reefs so coloured, and of the red
spaces having remained stationary, or having been upraised._—With
respect to subsidence, I have shown in the last chapter, that we cannot
expect to obtain in countries inhabited only by semi-civilised races,
demonstrative proofs of a movement, which invariably tends to conceal
its own evidence. But on the coral-islands supposed to have been
produced by subsidence, we have proofs of changes in their external
appearance—of a round of decay and renovation—of the last vestiges of
land on some—of its first commencement on others: we hear of storms
desolating them to the astonishment of their inhabitants: we know by
the great fissures with which some of them are traversed, and by the
earthquakes felt under others, that subterranean disturbances of some
kind are in progress. These facts, if not directly connected with
subsidence, as I believe they are, at least show how difficult it would
be to discover proofs of such movement by ordinary means. At Keeling
atoll, however, I have described some appearances, which seem directly
to show that subsidence did take place there during the late
earthquakes. Vanikoro, according to Chevalier Dillon,[5] is often
violently shaken by earthquakes, and there, the unusual depth of the
channel between the shore and the reef,—the almost entire absence of
islets on the reef,—its wall-like structure on the inner side, and the
small quantity of low alluvial land at the foot of the mountains, all
seem to show that this island has not remained long at its present
level, with the lagoon-channel subjected to the accumulation of
sediment, and the reef to the wear and tear of the breakers. At the
Society Archipelago, on the other hand, where a slight tremor is only
rarely felt, the shoaliness of the lagoon-channels round some of the
islands, the number of islets formed on the reefs of others, and the
broad belt of low land at the foot of the mountains, indicate that,
although there must have been great subsidence to have produced the
barrier-reefs, there has since elapsed a long stationary period.[6]

 [5] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier
 in his “Report on the Voyage of the ‘Astrolabe’” (p. cxi, vol. i),
 speaking of Vanikoro, says the shores are surrounded by reefs of
 madrepore, “qu’on assure etre de formation tout-a-fait moderne.” I
 have in vain endeavoured to learn some further particulars about this
 remarkable passage. I may here add, that according to our theory, the
 island of Pouynipete (Plate I, Fig. 7), in the Caroline Archipelago,
 being encircled by a barrier-reef, must have subsided. In the _ New S.
 Wales Lit. Advert._ February 1835 (which I have seen through the
 favour of Dr. Lloghtsky), there is an account of this island
 (subsequently confirmed by Mr. Campbell), in which it is said, “At the
 N.E. end, at a place called Tamen, there are ruins of a town, _now
 only_ accessible by boats, the waves _reaching to the steps of the
 houses._” Judging from this passage, one would be tempted to conclude
 that the island must have subsided, since these houses were built. I
 may, also, here append a statement in Malte Brun (vol. ix, p. 775,
 given without any authority), that the sea gains in an extraordinary
 manner on the coast of Cochin China, which lies in front and near the
 subsiding coral-reefs in the China Sea: as the coast is granitic, and
 not alluvial, it is scarcely possible that the encroachment of the sea
 can be owing to the washing away of the land; and if so, it must be
 due to subsidence.


 [6] Mr. Couthouy states (“Remarks,” p. 44) that at Tahiti and Eimeo
 the space between the reef and the shore has been nearly filled up by
 the extension of those coral-reefs, which within most barrier-reefs
 merely fringe the land. From this circumstance, he arrives at the same
 conclusion as I have done, that the Society Islands since their
 subsidence, have remained stationary during a long period; but he
 further believes that they have recently commenced rising, as well as
 the whole area of the Low Archipelago. He does not give any detailed
 proofs regarding the elevation of the Society Islands, but I shall
 refer to this subject in another part of this chapter. Before making
 some further comments, I may observe how satisfactory it is to me, to
 find Mr. Couthouy affirming, that “having personally examined a large
 number of coral-islands, and also residing eight months among the
 volcanic class, having shore and partially encircling reefs, I may be
 permitted to state that my own observations have impressed a
 conviction of the correctness of the theory of Mr. Darwin.”
    This gentleman believes, that subsequently to the subsidence by
    which the atolls in the Low Archipelago were produced, the whole
    area has been elevated to the amount of a few feet; this would
    indeed be a remarkable fact; but as far as I am able to judge, the
    grounds of his conclusion are not sufficiently strong. He states
    that he found in almost every atoll which he visited, the shores of
    the lagoon raised from eighteen to thirty inches above the
    sea-level, and containing imbedded Tridacnæ and corals standing as
    they grew; some of the corals were dead in their upper parts, but
    below a certain line they continued to flourish. In the lagoons,
    also, he frequently met with clusters of Madrepore, with their
    extremities standing from one inch to a foot above the surface of
    the water. Now, these appearances are exactly what I should have
    expected, without any subsequent elevation having taken place; and
    I think Mr. Couthouy has not borne in mind the indisputable fact,
    that corals, when constantly bathed by the surf, can exist at a
    higher level than in quite tranquil water, as in a lagoon. As long,
    therefore, as the waves continued at low water to break entirely
    over parts of the annular reef of an atoll, submerged to a small
    depth, the corals and shells attached on these parts might continue
    living at a level above the smooth surface of the lagoon, into
    which the waves rolled; but as soon as the outer edge of the reef
    grew up to its utmost possible height, or if the reef were very
    broad nearly to that height, the force of the breakers would be
    checked, and the corals and shells on the inner parts near the
    lagoon would occasionally be left dry, and thus be partially or
    wholly destroyed. Even in atolls, which have not lately subsided,
    if the outer margin of the reef continued to increase in breadth
    seaward (each fresh zone of corals rising to the same vertical
    height as at Keeling atoll), the line where the waves broke most
    heavily would advance outwards, and therefore the corals, which
    when living near the margin, were washed by the breaking waves
    during the whole of each tide, would cease being so, and would
    therefore be left on the backward part of the reef standing exposed
    and dead. The case of the madrepores in the lagoons with the tops
    of their branches exposed, seems to be an analogous fact, to the
    great fields of dead but upright corals in the lagoon of Keeling
    atoll; a condition of things which I have endeavoured to show, has
    resulted from the lagoon having become more and more enclosed and
    choked up with reefs, so that during high winds, the rising of the
    tide (as observed by the inhabitants) is checked, and the corals,
    which had formerly grown to the greatest possible height, are
    occasionally exposed, and thus are killed: and this is a condition
    of things, towards which almost every atoll in the intervals of its
    subsidence must be tending. Or if we look to the state of an atoll
    directly after a subsidence of some fathoms, the waves would roll
    heavily over the entire circumference of the reef, and the surface
    of the lagoon would, like the ocean, never be quite at rest, and
    therefore the corals in the lagoon, from being constantly laved by
    the rippling water, might extend their branches to a little greater
    height than they could, when the lagoon became enclosed and
    protected. Christmas atoll (2° N. lat.) which has a very shallow
    lagoon, and differs in several respects from most atolls, possibly
    may have been elevated recently; but its highest part appears
    (Couthouy, p. 46) to be only ten feet above the sea-level. The
    facts of a second class, adduced by Mr. Couthouy, in support of the
    alleged recent elevation of the Low Archipelago, are not all
    (especially those referring to a shelf of rock) quite intelligible
    to me; he believes that certain enormous fragments of rock on the
    reef, must have been moved into their present position, when the
    reef was at a lower level; but here again the force of the breakers
    on any inner point of the reef being diminished by its outward
    growth without any change in its level, has not, I think, been
    borne in mind. We should, also, not overlook the occasional agency
    of waves caused by earthquakes and hurricanes. Mr. Couthouy further
    argues, that since these great fragments were deposited and fixed
    on the reef, they have been elevated; he infers this from the
    greatest amount of erosion not being near their bases, where they
    are unceasingly washed by the reflux of the tides, but at some
    height on their sides, near the line of high-water mark, as shown
    in an accompanying diagram. My former remark again applies here,
    with this further observation, that as the waves have to roll over
    a wide space of reef before they reach the fragments, their force
    must be greatly increased with the increasing depth of water as the
    tide rises, and therefore I should have expected that the chief
    line of present erosion would have coincided with the line of
    high-water mark; and if the reef had grown outwards, that there
    would have been lines of erosion at greater heights. The
    conclusion, to which I am finally led by the interesting
    observations of Mr. Couthouy is, that the atolls in the Low
    Archipelago have, like the Society Islands, remained at a
    stationary level for a long period: and this probably is the
    ordinary course of events, subsidence supervening after long
    intervals of rest.


Turning now to the red colour; as on our map, the areas which have sunk
slowly downwards to great depths are many and large, we might naturally
have been led to conjecture, that with such great changes of level in
progress, the coasts which have been fringed probably for ages (for we
have no reason to believe that coral-reefs are of short duration),
would not have remained all this time stationary, but would frequently
have undergone movements of elevation. This supposition, we shall
immediately see, holds good to a remarkable extent; and although a
stationary condition of the land can hardly ever be open to proof, from
the evidence being only negative, we are, in some degree, enabled to
ascertain the correctness of the parts coloured red on the map, by the
direct testimony of upraised organic remains of a modern date. Before
going into the details on this head (printed in small type), I may
mention, that when reading a memoir on coral formations by MM. Quoy and
Gaimard[7] I was astonished to find, for I knew that they had crossed
both the Pacific and Indian Oceans, that their descriptions were
applicable only to reefs of the fringing class; but my astonishment
ended satisfactorily, when I discovered that, by a strange chance, all
the islands which these eminent naturalists had visited, though several
in number, namely, the Mauritius, Timor, New Guinea, the Mariana, and
Sandwich Archipelagoes, could be shown by their own statements to have
been elevated within a recent geological era.

 [7] “Annales des Sciences Nat.” tom. vi, p. 279, etc.


In the eastern half of the Pacific, the _ Sandwich Islands_ are all
fringed, and almost every naturalist who has visited them, has remarked
on the abundance of elevated corals and shells, apparently identical
with living species. The Rev. W. Ellis informs me, that he has noticed
round several parts of Hawaii, beds of coral-detritus, about twenty
feet above the level of the sea, and where the coast is low they extend
far inland. Upraised coral-rock forms a considerable part of the
borders of Oahu; and at Elizabeth Island[8] it composes three strata,
each about ten feet thick. Nihau, which forms the northern, as Hawaii
does the southern end of the group (350 miles in length), likewise
seems to consist of coral and volcanic rocks. Mr. Couthouy[9] has
lately described with interesting details, several upraised beaches,
ancient reefs with their surfaces perfectly preserved, and beds of
recent shells and corals, at the islands of Maui, Morokai, Oahu, and
Tauai (or Kauai) in this group. Mr. Pierce, an intelligent resident at
Oahu, is convinced, from changes which have taken place within his
memory, during the last sixteen years, “that the elevation is at
present going forward at a very perceptible rate.” The natives at Kauai
state that the land is there gaining rapidly on the sea, and Mr.
Couthouy has no doubt, from the nature of the strata, that this has
been effected by an elevation of the land.

 [8] “Zoology of Captain Beechey’s Voyage,” p. 176. See also MM. Quoy
 and Gaimard in “Annales de Scien. Nat.” tom. vi.


 [9] “Remarks on Coral Formations,” p. 51.


In the southern part of the Low Archipelago, Elizabeth Island is
described by Captain Beechey,[10] as being quite flat, and about eighty
feet in height; it is entirely composed of dead corals, forming a
honeycombed, but compact rock. In cases like this, of an island having
exactly the appearance, which the elevation of any one of the smaller
surrounding atolls with a shallow lagoon would present, one is led to
conclude (with little better reason, however, than the improbability of
such small and low fabrics lasting, for an immense period, exposed to
the many destroying agents of nature), that the elevation has taken
place at an epoch not geologically remote. When merely the surface of
an island of ordinary formation is strewed with marine
bodies, and that continuously, or nearly so, from the beach to a
certain height, and not above that height, it is exceedingly improbable
that such organic remains, although they may not have been specially
examined, should belong to any ancient period. It is necessary to bear
these remarks in mind, in considering the evidence of the elevatory
movements in the Pacific and Indian Oceans, as it does not often rest
on specific determinations, and therefore should be received with
caution. Six of the _Cook and Austral Islands_ (S.W. of the Society
group), are fringed; of these, five were described to me by the Rev. J.
Williams, as formed of coral-rock, associated with some basalt in
Mangaia), and the sixth as lofty and basaltic. Mangaia is nearly three
hundred feet high, with a level summit; and according to Mr. S.
Wilson[11] it is an upraised reef; “and there are in the central
hollow, formerly the bed of the lagoon, many scattered patches of
coral-rock, some of them raised to a height of forty feet.” These
knolls of coral-rock were evidently once separate reefs in the lagoon
of an atoll. Mr. Martens, at Sydney, informed me that this island is
surrounded by a terrace-like plain at about the height of a hundred
feet, which probably marks a pause in its elevation. From these facts
we may infer, perhaps, that the Cook and Austral Islands have been
upheaved at a period probably not very remote.

 [10] Beechey’s “Voyage in the Pacific,” p. 46, 4to ed.


 [11] Couthouy’s “Remarks,” p. 34.


_Savage Island_ (S.E. of the Friendly group), is about forty feet in
height. Forster[12] describes the plants as already growing out of the
dead, but still upright and spreading trees of coral; and the younger
Forster[13] believes that an ancient lagoon is now represented by a
central plain; here we cannot doubt that the elevatory forces have
recently acted. The same conclusion may be extended, though with
somewhat less certainty, to the islands of the _friendly group_, which
have been well described in the second and third voyages of Cook. The
surface of Tongatabou is low and level, but with some parts a hundred
feet high; the whole consists of coral-rock, “which yet shows the
cavities and irregularities worn into it by the action of the
tides.”[14] On Eoua the same appearances were noticed at an elevation
of between two hundred and three hundred feet. Vavao, also, at the
opposite or northern end of the group, consists, according to the Rev.
J. Williams, of coral-rock. Tongatabou, with its northern extensive
reefs, resembles either an upraised atoll with one half originally
imperfect, or one unequally elevated; and Anamouka, an atoll equally
elevated. This latter island contains[15] in its centre a salt-water
lake, about a mile-and-a-half in diameter, without any communication
with the sea, and around it the land rises gradually like a bank; the
highest part is only between twenty and thirty feet; but on this part,
as well as on the rest of the land (which, as Cook observes, rises
above the height of true lagoon-islands), coral-rock, like that on the
beach, was found. In the _Navigator Archipelago_, Mr. Couthouy[16]
found on Manua many and very large fragments of coral at the height of
eighty feet, “on a steep hill-side, rising half a mile inland from a
low sandy plain abounding in marine remains.” The fragments were
embedded in a mixture of decomposed lava and sand. It is not stated
whether they were accompanied by shells, or whether the corals
resembled recent species; as these
remains were embedded they possibly may belong to a remote epoch; but I
presume this was not the opinion of Mr. Couthouy. Earthquakes are very
frequent in this archipelago.

 [12] “Observations made during Voyage round the World,” p. 147.


 [13] “Voyage,” vol. ii, p. 163.


 [14] Cook’s “Third Voyage” (4to ed.), vol. i, p. 314.


 [15] _Ibid_., vol. i, p. 235.


 [16] “Remarks on Coral-Formations,” p. 50.


Still proceeding westward we come to the _New Hebrides_; on these
islands, Mr. G. Bennett (author of “Wanderings in New South Wales”),
informs me he found much coral at a great altitude, which he considered
of recent origin. Respecting _Santa Cruz_, and the _Solomon
Archipelago_, I have no information; but at New Ireland, which forms
the northern point of the latter chain, both Labillardiere and Lesson
have described large beds of an apparently very modern madreporitic
rock, with the form of the corals little altered. The latter author[17]
states that this formation composes a newer line of coast, modelled
round an ancient one. There only remains to be described in the
Pacific, that curved line of fringed islands, of which the _ Marianas_
form the main part. Of these Guam, Rota, Tiniam, Saypan, and some
islets farther north, are described by Quoy and Gaimard,[18] and
Chamisso,[19] as chiefly composed of madreporitic limestone, which
attains a considerable elevation, and is in several cases worn into
successively rising cliffs: the two former naturalists seem to have
compared the corals and shells with the existing ones, and state that
they are of recent species. _Fais_, which lies in the prolonged line of
the Marianas, is the only island in this part of the sea which is
fringed; it is ninety feet high, and consists entirely of madreporitic
rock.[20]

 [17] “Voyage de la _Coquille_,” Part. Zoolog.


 [18] Freycinet’s “Voyage autour du Monde.” See also the
 “Hydrographical Memoir,” p. 215.


 [19] Kotzebue’s “First Voyage.”


 [20] Lutké’s “Voyage,” vol. ii, p. 304.


In the _East Indian Archipelago_, many authors have recorded proofs of
recent elevation. M. Lesson[21] states, that near Port Dory, on the
north coast of New Guinea, the shores are flanked, to the height of 150
feet, by madreporitic strata of modern date. He mentions similar
formations at Waigiou, Amboina, Bourou, Ceram, Sonda, and Timor: at
this latter place, MM. Quoy and Gaimard[22] have likewise described the
primitive rocks, as coated to a considerable height with coral. Some
small islets eastward of Timor are said in Kolff’s “Voyage,”[23] to
resemble small coral islets upraised some feet above the sea. Dr.
Malcolmson informs me that Dr. Hardie found in JAVA an extensive
formation, containing an abundance of shells, of which the greater part
appear to be of existing species. Dr. Jack[24] has described some
upraised shells and corals, apparently recent, on Pulo Nias off
_Sumatra_; and Marsden relates in his history of this great island,
that the names of many promontories, show that they were originally
islands. On part of the west
coast of _Borneo_ and at the _Sooloo Islands_, the form of the land,
the nature of the soil, and the water-washed rocks, present
appearances[25] (although it is doubtful whether such vague evidence is
worthy of mention), of having recently been covered by the sea; and the
inhabitants of the Sooloo Islands believe that this has been the case.
Mr. Cuming, who has lately investigated, with so much success, the
natural history of the _Philippines_, found near Cabagan, in Luzon,
about fifty feet above the level of the R. Cagayan, and seventy miles
from its mouth, a large bed of fossil shells: these, he informs me, are
of the same species with those now existing on the shores of the
neighbouring islands. From the accounts given us by Captain Basil Hall
and Captain Beechey[26] of the lines of inland reefs, and walls of
coral-rock worn into caves, above the present reach of the waves, at
the _Loo Choo_ Islands, there can be little doubt that they have been
upraised at no very remote period.

 [21] Partie Zoolog., “Voyage de la _Coquille_.”


 [22] “Ann. des Scien. Nat.” tom. vi, p. 281.


 [23] Translated by Windsor Earl, chapters vi, vii.


 [24] “Geolog. Transact.” 2nd series, vol. i, p. 403. On the Peninsula
 of Malacca, in front of Pinang, 5° 30′ N., Dr. Ward collected some
 shells, which Dr. Malcolmson informs me, although not compared with
 existing species, had a recent appearance. Dr. Ward describes in this
 neighbourhood (“Trans. Asiat. Soc.” vol. xviii, part ii, p. 166) a
 single water-worn rock, with a conglomerate of sea-shells at its base,
 situated six miles inland, which, according to the traditions of the
 natives, was once surrounded by the sea. Captain Low has also
 described (_Ibid_., part i, p. 131) mounds of shells lying two miles
 inland on this line of coast.


 [25] “Notices of the East Indian Arch.” Singapore, 1828, p. 6, and
 Append., p. 43.


 [26] Captain B. Hall, “Voyage to Loo Choo,” Append., pp. xxi and xxv.
 Captain Beechey’s “Voyage,” p. 496.


Dr. Davy[27] describes the northern province of _Ceylon_ as being very
low, and consisting of a limestone with shells and corals of very
recent origin; he adds, that it does not admit of a doubt that the sea
has retired from this district even within the memory of man. There is
also some reason for believing that the western shores of India, north
of Ceylon, have been upraised within the recent period.[28] _Mauritius_
has certainly been upraised within the recent period, as I have stated
in the chapter on fringing-reefs. The northern extremity of
_Madagascar_ is described by Captain Owen[29] as formed of madreporitic
rock, as likewise are the shores and outlying islands along an immense
space of _ Eastern Africa_, from a little north of the equator for nine
hundred miles southward. Nothing can be more vague than the expression
“madreporitic rock;” but at the same time it is, I think, scarcely
possible to look at the chart of the linear islets, which rise to a
greater height than can be accounted for by the growth of coral, in
front of the coast, from the equator to 2° S., without feeling
convinced that a line of fringing-reefs has been elevated at a period
so recent, that no great changes have since taken place on the surface
of this part of the globe. Some, also, of the
higher islands of madreporitic rock on this coast, for instance Pemba,
have very singular forms, which seem to show the combined effect of the
growth of coral round submerged banks, and their subsequent upheaval.
Dr. Allan informs me that he never observed any elevated organic
remains on the _Seychelles_, which come under our fringed class.

 [27] “Travels in Ceylon,” p. 13. This madreporitic formation is
 mentioned by M. Cordier in his report to the Institute (May 4th,
 1839), on the voyage of the _Chevrette_, as one of immense extent, and
 belonging to the latest tertiary period.


 [28] Dr. Benza, in his “Journey through the N. Circars” (the _ Madras
 Lit. and Scient. Journ._ vol. v.) has described a formation with
 recent fresh-water and marine shells, occurring at the distance of
 three or four miles from the present shore. Dr. Benza, in conversation
 with me, attributed their position to a rise of the land. Dr.
 Malcolmson, however (and there cannot be a higher authority on the
 geology of India) informs me that he suspects that these beds may have
 been formed by the mere action of the waves and currents accumulating
 sediment. From analogy I should much incline to Dr. Benza’s opinion.


 [29] Owen’s “Africa,” vol. ii, p. 37, for Madagascar; and for S.
 Africa, vol. i, pp. 412 and 426. Lieutenant Boteler’s narrative
 contains fuller particulars regarding the coral-rock, vol. i, p. 174,
 and vol. ii, pp. 41 and 54. See also Ruschenberger’s “Voyage round the
 World,” vol. i, p. 60.


The nature of the formations round the shores of the _Red Sea_, as
described by several authors, shows that the whole of this large area
has been elevated within a very recent tertiary epoch. A part of this
space in the appended map, is coloured blue, indicating the presence of
barrier-reefs: on which circumstance I shall presently make some
remarks. Rüppell[30] states that the tertiary formation, of which he
has examined the organic remains, forms a fringe along the shores with
a uniform height of from thirty and forty feet from the mouth of the
Gulf of Suez to about latitude 26°; but that south of 26°, the beds
attain only the height of from twelve to fifteen feet. This, however,
can hardly be quite accurate; although possibly there may be a decrease
in the elevation of the shores in the middle parts of the Red Sea, for
Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran
Island (lat. 15° 30′ S.) shells and corals, apparently recent, at a
height between thirty and forty feet; and Mr. Salt (“Travels in
Abyssinia”) describes a similar formation a little southward on the
opposite shore at Amphila. Moreover, near the mouth of the Gulf of
Suez, although on the coast opposite to that on which Dr. Rüppell says
that the modern beds attain a height of only thirty to forty feet, Mr.
Burton[31] found a deposit replete with existing species of shells, at
the height of 200 feet. In an admirable series of drawings by Captain
Moresby, I could see how continuously the cliff-bounded low plains of
this formation extended with a nearly equable height, both on the
eastern and western shores. The southern coast of Arabia seems to have
been subjected to the same elevatory movement, for Dr. Malcolmson found
at Sahar low cliffs containing shells and corals, apparently of recent
species.

 [30] Rüppell, “Reise in Abyssinien,” Band i., s. 141.


 [31] Lyell’s “Principles of Geology,” 5th ed., vol. iv, p. 25.


The _Persian Gulf_ abounds with coral-reefs; but as it is difficult to
distinguish them from sand-banks in this shallow sea, I have coloured
only some near the mouth; towards the head of the gulf Mr.
Ainsworth[32] says that the land is worn into terraces, and that the
beds contain organic remains of existing forms. The _West Indian
Archipelago_ of “fringed” islands, alone remains to be mentioned;
evidence of an elevation within a late tertiary epoch of nearly the
whole of this great area, may be found in the works of almost all the
naturalists who have visited it. I will give some of the principal
references in a note.[33]

 [32] Ainsworth’s “Assyria and Babylon,” p. 217.


 [33] On Florida and the north shores of the Gulf of Mexico, Rogers’
 “Report to Brit. Assoc.” vol. iii, p. 14.—On the shores of Mexico,
 Humboldt, “Polit. Essay on New Spain,” vol. i, p. 62. (I have also
 some corroborative facts with respect to the shores of
 Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th ed.,
 vol. iv, p. 22.—Santa Cruz and Barbadoes, Prof. Hovey, “Silliman’s
 Journal”, vol. xxxv, p. 74.—St. Domingo, Courrojolles, “Journ de
 Phys.” tom. liv., p. 106.—Bahamas, “United Service Journal”, No. lxxi,
 pp. 218 and 224. Jamaica, De la Beche, “Geol. Man.” p. 142.—Cuba,
 Taylor in “Lond. and Edin. Mag.” vol. xi, p. 17. Dr. Daubeny also, at
 a meeting of the Geolog. Soc., orally described some very modern beds
 lying on the N.W. parts of Cuba. I might have added many other less
 important references.


It is very remarkable on reviewing these details, to observe in how
many instances fringing-reefs round the shores, have coincided with the
existence on the land of upraised organic remains, which seem, from
evidence more or less satisfactory, to belong to a late tertiary
period. It may, however, be objected, that similar proofs of elevation,
perhaps, occur on the coasts coloured blue in our map: but this
certainly is not the case with the few following and doubtful
exceptions.

The entire area of the Red Sea appears to have been upraised within a
modern period; nevertheless I have been compelled (though on
unsatisfactory evidence, as given in the Appendix) to class the reefs
in the middle part, as barrier-reefs; should, however, the statements
prove accurate to the less height of the tertiary bed in this middle
part, compared with the northern and southern districts, we might well
suspect that it had subsided subsequently to the general elevation by
which the whole area has been upraised. Several authors[34] have stated
that they have observed shells and corals high up on the mountains of
the Society Islands,—a group encircled by barrier-reefs, and,
therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found on
the apex of one of the highest mountains, between 5,000 and 7,000 feet
above the level of the sea, “a distinct and regular stratum of
semi-fossil coral.” At Tahiti, however, other naturalists, as well as
myself, have searched in vain at a low level near the coast, for
upraised shells or masses of coral-reef, where if present they could
hardly have been overlooked. From this fact, I concluded that probably
the organic remains strewed high up on the surface of the land, had
originally been embedded in the volcanic strata, and had subsequently
been washed out by the rain. I have since heard from the Rev. W. Ellis,
that the remains which he met with, were (as he believes)
interstratified with an argillaceous tuff; this likewise was the case
with the shells observed by the Rev. D. Tyerman at Huaheine. These
remains have not been specifically examined; they may, therefore, and
especially the stratum observed by Mr. Stutchbury at an immense height,
be contemporaneous with the first formation of the Society Islands, and
be of any degree of antiquity; or they may have been deposited at some
subsequent, but probably not very recent, period of elevation; for if
the period had been recent, the entire surface of the coast land of
these islands, where the reefs are so extensive, would have been coated
with upraised coral,
which certainly is not the case. Two of the Harvey, or Cook Islands,
namely, Aitutaki and Manouai, are encircled by reefs, which extend so
far from the land, that I have coloured them blue, although with much
hesitation, as the space within the reef is shallow, and the outline of
the land is not abrupt. These two islands consist of coral-rock; but I
have no evidence of their recent elevation, besides, the improbability
of Mangaia, a fringed island in the same group (but distant 170 miles),
having retained its nearly perfect atoll-like structure, during any
immense lapse of time after its upheaval. The Red Sea, therefore, is
the only area in which we have clear proofs of the recent elevation of
a district, which, by our theory (although the barrier-reefs are there
not well characterised), has lately subsided. But we have no reason to
be surprised at oscillation, of level of this kind having occasionally
taken place. There can be scarcely any doubt that Savage, Aurora,[35]
and Mangaia Islands, and several of the islands in the Friendly group,
existed originally as atolls, and these have undoubtedly since been
upraised to some height above the level of the sea; so that by our
theory, there has here, also, been an oscillation of level,—elevation
having succeeded subsidence, instead of, as in the middle part of the
Red Sea and at the Harvey Islands, subsidence having probably succeeded
recent elevation.

 [34] Ellis, in his “Polynesian Researches,” was the first to call
 attention to these remains (vol. i, p. 38), and the tradition of the
 natives concerning them. See also Williams, “Nar. of Missionary
 Enterprise,” p. 21; also Tyerman and G. Bennett, “Journal of Voyage,”
 vol. i, p. 213; also Mr. Couthouy’s “Remarks,” p. 51; but this
 principal fact, namely, that there is a mass of upraised coral on the
 narrow peninsula of Tiarubu, is from hearsay evidence; also Mr.
 Stutchbury, _West of England Journal_, No. i, p. 54. There is a
 passage in Von Zach, “Corres. Astronom.” vol. x, p. 266, inferring an
 uprising at Tahiti, from a footpath now used, which was formerly
 impassable; but I particularly inquired from several native chiefs,
 whether they knew of any change of this kind, and they were unanimous
 in giving me an answer in the negative.


 [35] Aurora Island is described by Mr. Couthouy (“Remarks,” p. 58); it
 lies 120 miles north-east of Tahiti; it is not coloured in the
 appended map, because it does not appear to be fringed by living
 reefs. Mr. Couthouy describes its summit as “presenting a broad
 table-land which declines a few feet towards the centre, where we may
 suppose the lagoon to have been placed.” It is about two hundred feet
 in height, and consists of reef-rock and conglomerate, with existing
 species of coral embedded in it. The island has been elevated at two
 successive periods; the cliffs being marked halfway up with a
 horizontal water-worn line of deep excavations. Aurora Island seems
 closely to resemble in structure Elizabeth Island, at the southern end
 of the Low Archipelago.

It is an interesting fact, that Fais, which, from its composition,
form, height, and situation at the western end of the Caroline
Archipelago, one is strongly induced to believe existed before its
upheaval as an atoll, lies exactly in the prolongation of the curved
line of the Mariana group, which we know to be a line of recent
elevation. I may add, that Elizabeth Island, in the southern part of
the Low Archipelago, which seems to have had the same kind of origin as
Fais, lies near Pitcairn Island, the only one in this part of the ocean
which is high, and at the same time not surrounded by an encircling
barrier-reef.

_On the absence of active volcanoes in the areas of subsidence, and on
their frequent presence in the areas of elevation._—Before making some
concluding remarks on the relations of the spaces coloured blue and
red, it will be convenient to consider the position on our map of the
volcanoes historically known to have been in action. It is impossible
not to be struck, first with the absence of volcanoes in the great
areas of subsidence tinted pale and dark blue,—namely, in the central
parts of the Indian Ocean, in the China Sea, in the sea between the
barriers
of Australia and New Caledonia, in the Caroline, Marshall, Gilbert, and
Low Archipelagoes; and, secondly, with the coincidence of the principal
volcanic chains with the parts coloured red, which indicates the
presence of fringing-reefs; and, as we have just seen, the presence in
most cases of upraised organic remains of a modern date. I may here
remark that the reefs were all coloured before the volcanoes were added
to the map, or indeed before I knew of the existence of several of
them.

The volcano in Torres Strait, at the northern point of Australia, is
that which lies nearest to a large subsiding area, although situated
125 miles within the outer margin of the actual barrier-reef. The Great
Comoro Island, which probably contains a volcano, is only twenty miles
distant from the barrier-reef of Mohila; Ambil volcano, in the
Philippines, is distant only a little more than sixty miles from the
atoll-formed Appoo reef: and there are two other volcanoes in the map
within ninety miles of circles coloured blue. These few cases, which
thus offer partial exceptions to the rule, of volcanoes being placed
remote from the areas of subsidence, lie either near single and
isolated atolls, or near small groups of encircled islands; and these
by our theory can have, in few instances, subsided to the same amount
in depth or area, as groups of atolls. There is not one active volcano
within several hundred miles of an archipelago, or even a small group
of atolls. It is, therefore, a striking fact that in the Friendly
Archipelago, which owes its origin to the elevation of a group of
atolls, two volcanoes, and, perhaps, others are known to be in action:
on the other hand, on several of the encircled islands in the Pacific,
supposed by our theory to have subsided, there are old craters and
streams of lava, which show the effects of past and ancient eruptions.
In these cases, it would appear as if the volcanoes had come into
action, and had become extinguished on the same spots, according as the
elevating or subsiding movements prevailed.

There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured
red from being fringed by coral-reefs. Thus I hope to show in a future
volume, that nearly the whole line of the west coast of South America,
which forms the greatest volcanic chain in the world, from near the
equator for a space of between 2,000 and 3,000 miles southward, has
undergone an upward movement during a late geological period. The
islands on the north-western shores of the Pacific, which form the
second greatest volcanic chain, are very imperfectly known; but Luzon,
in the Philippines, and the Loo Choo Islands, have been recently
elevated; and at Kamtschatka[36] there are extensive tertiary beds of
modern date. Evidence of the same nature, but not very satisfactory,
may be detected in Northern New Zealand where there are two volcanoes.
The co-existence in other parts of the world of active volcanoes, with
upraised beds of a modern tertiary origin, will occur to
every geologist.[37] Nevertheless, until it could be shown that
volcanoes were inactive, or did not exist in subsiding areas, the
conclusion that their distribution depended on the nature of the
subterranean movements in progress, would have been hazardous. But now,
viewing the appended map, it may, I think, be considered as almost
established, that volcanoes are often (not necessarily always) present
in those areas where the subterranean motive power has lately forced,
or is now forcing outwards, the crust of the earth, but that they are
invariably absent in those, where the surface has lately subsided or is
still subsiding.[38]

 [36] At Sedanka, in latitude 58° N. (Von Buch’s “Descrip. des Isles
 Canaries,” p. 455). In a forthcoming part, I shall give the evidence
 referred to with respect to the elevation of New Zealand.


 [37] During the subterranean disturbances which took place in Chile,
 in 1835, I have shown (“Geolog. Trans.” 2nd Ser., vol. v, p. 606) that
 at the same moment that a large district was upraised, volcanic matter
 burst forth at widely separated points, through both new and old
 vents.


 [38] We may infer from this rule, that in any old deposit, which
 contains interstratified beds of erupted matter, there was at the
 period, and in the area of its formation, a _tendency_ to an upward
 movement in the earth’s surface, and certainly no movement of
 subsidence.


_On the relations of the areas of subsidence and elevation._—The
immense surfaces on the map, which, both by our theory and by the plain
evidence of upraised marine remains, have undergone a change of level
either downwards or upwards during a late period, is a most remarkable
fact. The existence of continents shows that the areas have been
immense which at some period have been upraised; in South America we
may feel sure, and on the north-western shores of the Indian Ocean we
may suspect, that this rising is either now actually in progress, or
has taken place quite recently. By our theory, we may conclude that the
areas are likewise immense which have lately subsided, or, judging from
the earthquakes occasionally felt and from other appearances, are now
subsiding. The smallness of the scale of our map should not be
overlooked: each of the squares on it contains (not allowing for the
curvature of the earth) 810,000 square miles. Look at the space of
ocean from near the southern end of the Low Archipelago to the northern
end of the Marshall Archipelago, a length of 4,500 miles, in which, as
far as is known, every island, except Aurora which lies just without
the Low Archipelago, is atoll-formed. The eastern and western
boundaries of our map are continents, and they are rising areas: the
central spaces of the great Indian and Pacific Oceans, are mostly
subsiding; between them, north of Australia, lies the most broken land
on the globe, and there the rising parts are surrounded and penetrated
by areas of subsidence,[39] so that the prevailing movements now in
progress, seem to accord with the actual states of surface of the great
divisions of the world.

 [39] I suspect that the Arru and Timor-laut Islands present an
 included small area of subsidence, like that of the China Sea, but I
 have not ventured to colour them from my imperfect information, as
 given in the Appendix.

The blue spaces on the map are nearly all elongated; but it does not
necessarily follow from this (a caution, for which I am indebted to Mr.
Lyell), that the areas of subsidence were likewise elongated; for
the subsidence of a long, narrow space of the bed of the ocean,
including in it a transverse chain of mountains, surmounted by atolls,
would only be marked on the map by a transverse blue band. But where a
chain of atolls and barrier-reefs lies in an elongated area, between
spaces coloured red, which therefore have remained stationary or have
been upraised, this must have resulted either from the area of
subsidence having originally been elongated (owing to some tendency in
the earth’s crust thus to subside), or from the subsiding area having
originally been of an irregular figure, or as broad as long, and having
since been narrowed by the elevation of neighbouring districts. Thus
the areas, which subsided during the formation of the great north and
south lines of atolls in the Indian Ocean,—of the east and west line of
the Caroline atolls,—and of the north-west and south-east line of the
barrier-reefs of New Caledonia and Louisiade, must have originally been
elongated, or if not so, they must have since been made elongated by
elevations, which we know to belong to a recent period.

I infer from Mr. Hopkins’ researches,[40] that for the formation of a
long chain of mountains, with few lateral spurs, an area elongated in
the same direction with the chain, must have been subjected to an
elevatory movement. Mountain-chains, however, when already formed,
although running in very different directions, it seems[41] may be
raised together by a widely-acting force: so, perhaps, mountain-chains
may subside together. Hence, we cannot tell, whether the Caroline and
Marshall Archipelagoes, two groups of atolls running in different
directions and meeting each other, have been formed by the subsidence
of two areas, or of one large area, including two distinct lines of
mountains. We have, however, in the southern prolongation of the
Mariana Islands, probable evidence of a line of recent elevation having
intersected one of recent subsidence. A view of the map will show that,
generally, there is a tendency to alternation in the parallel areas
undergoing opposite kinds of movement; as if the sinking of one area
balanced the rising of another.

 [40] “Researches in Physical Geology,” Transact. Cambridge Phil. Soc.,
 vol. vi, part i.


 [41] For instance in S. America from lat. 34°, for very many degrees
 southward there are upraised beds containing recent species of shells,
 on both the Atlantic and Pacific side of the continent, and from the
 gradual ascent of the land, although with very unequal slopes, on both
 sides towards the Cordillera, I think it can hardly be doubted that
 the entire width has been upraised in mass within the recent period.
 In this case the two W.N.W. and E.S.E. mountain-lines, namely the
 Sierra Ventana and the S. Tapalguen, and the great north and south
 line of the Cordillera have been together raised. In the West Indies
 the N. and S. line of the Eastern Antilles, and the E. and W. line of
 Jamaica, appear both to have been upraised within the latest
 geological period.

The existence in many parts of the world of high table-land, proves
that large surfaces have been upraised in mass to considerable heights
above the level of the ocean; although the highest points in almost
every country consist of upturned strata, or erupted matter: and from
the immense spaces scattered with atolls, which indicate that land
originally existed there, although not one pinnacle now remains above
the level of the sea, we may conclude that wide areas have subsided to
an amount, sufficient to bury not only any formerly existing
table-land, but even the heights formed by fractured strata, and
erupted matter. The effects produced on the land by the later elevatory
movements, namely, successively rising cliffs, lines of erosion, and
beds of literal shells and pebbles, all requiring time for their
production, prove that these movements have been very slow; we can,
however, infer this with safety, only with respect to the few last
hundred feet of rise. But with reference to the whole vast amount of
subsidence, necessary to have produced the many atolls widely scattered
over immense spaces, it has already been shown (and it is, perhaps, the
most interesting conclusion in this volume), that the movements must
either have been uniform and exceedingly slow, or have been effected by
small steps, separated from each other by long intervals of time,
during which the reef-constructing polypifers were able to bring up
their solid frameworks to the surface. We have little means of judging
whether many considerable oscillations of level have generally occurred
during the elevation of large tracts; but we know, from clear
geological evidence, that this has frequently taken place; and we have
seen on our map, that some of the same islands have both subsided and
been upraised. I conclude, however, that most of the large blue spaces,
have subsided without many and great elevatory oscillations, because
only a few upraised atolls have been observed: the supposition that
such elevations have taken place, but that the upraised parts have been
worn down by the surf, and thus have escaped observation, is overruled
by the very considerable depth of the lagoons of all the larger atolls;
for this could not have been the case, if they had suffered repeated
elevations and abrasion. From the comparative observations made in
these latter pages, we may finally conclude, that the subterranean
changes which have caused some large areas to rise, and others to
subside, have acted in a very similar manner.

_Recapitulation._—In the three first chapters, the principal kinds of
coral-reefs were described in detail, and they were found to differ
little, as far as relates to the actual surface of the reef. An atoll
differs from an encircling barrier-reef only in the absence of land
within its central expanse; and a barrier-reef differs from a
fringing-reef, in being placed at a much greater distance from the land
with reference to the probable inclination of its submarine foundation,
and in the presence of a deep-water lagoon-like space or moat within
the reef. In the fourth chapter the growing powers of the
reef-constructing polypifers were discussed; and it was shown, that
they cannot flourish beneath a very limited depth. In accordance with
this limit, there is no difficulty respecting the foundations on which
fringing-reefs are based; whereas, with barrier-reefs and atolls, there
is a great apparent difficulty on this head; in barrier-reefs from the
improbability of the rock of the coast or of banks of sediment
extending, in every instance, so far seaward within the required
depth;—and in atolls, from the immensity of the
spaces over which they are interspersed, and the apparent necessity for
believing that they are all supported on mountain-summits, which
although rising very near to the surface-level of the sea, in no one
instance emerge above it. To escape this latter most improbable
admission, which implies the existence of submarine chains of mountains
of almost the same height, extending over areas of many thousand square
miles, there is but one alternative; namely, the prolonged subsidence
of the foundations, on which the atolls were primarily based, together
with the upward growth of the reef-constructing corals. On this view
every difficulty vanishes; fringing reefs are thus converted into
barrier-reefs; and barrier-reefs, when encircling islands, are thus
converted into atolls, the instant the last pinnacle of land sinks
beneath the surface of the ocean.

Thus the ordinary forms and certain peculiarities in the structure of
atolls and barrier-reefs can be explained;—namely, the wall-like
structure on their inner sides, the basin or ring-like shape both of
the marginal and central reefs in the Maldiva atolls—the union of some
atolls as if by a ribbon—the apparent disseverment of others—and the
occurrence, in atolls as well as in barrier-reefs, of portions of reef,
and of the whole of some reefs, in a dead and submerged state, but
retaining the outline of living reefs. Thus can be explained the
existence of breaches through barrier-reefs in front of valleys, though
separated from them by a wide space of deep water; thus, also, the
ordinary outline of groups of atolls and the relative forms of the
separate atolls one to another; thus can be explained the proximity of
the two kinds of reefs formed during subsidence, and their separation
from the spaces where fringing-reefs abound. On searching for other
evidence of the movements supposed by our theory, we find marks of
change in atolls and in barrier-reefs, and of subterranean disturbances
under them; but from the nature of things, it is scarcely possible to
detect any direct proofs of subsidence, although some appearances are
strongly in favour of it. On the fringed coasts, however, the presence
of upraised marine bodies of a recent epoch, plainly show, that these
coasts, instead of having remained stationary, which is all that can be
directly inferred from our theory, have generally been elevated.

Finally, when the two great types of structure, namely barrier-reefs
and atolls on the one hand, and fringing-reefs on the other, were laid
down in colours on our map, a magnificent and harmonious picture of the
movements, which the crust of the earth has within a late period
undergone, is presented to us. We there see vast areas rising, with
volcanic matter every now and then bursting forth through the vents or
fissures with which they are traversed. We see other wide spaces slowly
sinking without any volcanic outburst, and we may feel sure, that this
sinking must have been immense in amount as well as in area, thus to
have buried over the broad face of the ocean every one of those
mountains, above which atolls now stand like monuments, marking the
place of their former existence. Reflecting how powerful an agent with
respect to denudation, and consequently to the nature and thickness of
the deposits in accumulation, the sea must ever be, when acting
for prolonged periods on the land, during either its slow emergence or
subsidence; reflecting, also, on the final effects of these movements
in the interchange of land and ocean-water on the climate of the earth,
and on the distribution of organic beings, I may be permitted to hope,
that the conclusions derived from the study of coral-formations,
originally attempted merely to explain their peculiar forms, may be
thought worthy of the attention of geologists.




APPENDIX.
CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE
III.


In the beginning of the last chapter I stated the principles on which
the map is coloured. There only remains to be said, that it is an exact
copy of one by M. C. Gressier, published by the Dépôt Général de la
Marine, in 1835. The names have been altered into English, and the
longitude has been reduced to that of Greenwich. The colours were first
laid down on accurate charts, on a large scale. The data, on which the
volcanoes historically known to have been in action, have been marked
with vermillion, were given in a note to the last chapter. I will
commence my description on the eastern side of the map, and will
describe each group of islands consecutively, proceeding westward
across the Pacific and Indian Oceans, but ending with the West Indies.

The WESTERN SHORES OF AMERICA appear to be entirely without
coral-reefs; south of the equator the survey of the _Beagle_, and north
of it, the published charts show that this is the case. Even in the Bay
of _Panama_, where corals flourish, there are no true coral-reefs, as I
have been informed by Mr. Lloyd. There are no coral-reefs in the
_Galapagos_ Archipelago, as I know from personal inspection; and I
believe there are none on the _ Cocos, Revilla-gigedo_, and other
neighbouring islands. _ Clipperton_ rock, 10° N., 109° W., has lately
been surveyed by Captain Belcher; in form it is like the crater of a
volcano. From a drawing appended to the MS. plan in the Admiralty, it
evidently is not an atoll. The eastern parts of the Pacific present an
enormous area, without any islands, except _E_, and _Sala_, and _Gomez_
Islands, which do not appear to be surrounded by reefs.

The LOW ARCHIPELAGO.—This group consists of about eighty atolls: it
will be quite superfluous to refer to descriptions of each. In
D’Urville and Lottin’s chart, one island (_Wolchonsky_) is written with
a capital letter, signifying, as explained in a former chapter, that it
is a high island; but this must be a mistake, as the original chart by
Bellinghausen shows that it is a true atoll. Captain Beechey says of
the thirty-two groups which he examined (of the greater number of which
I have seen beautiful MS. charts in the Admiralty), that twenty-nine
now contain lagoons, and he believes the other three originally did.
Bellinghausen (see an account of his Russian voyage, in the “Biblioth.
des Voyages,” 1834, p. 443) says, that the seventeen islands which he
discovered resembled each other in structure, and he has given charts
on a large scale of all of them. Kotzebue has given plans of several;
Cook and Bligh mention others; a few were seen during the voyage of the
_Beagle_; and notices of other atolls are scattered through several
publications. The _ Actæon_ group in this archipelago has lately been
discovered (_Geograph. Journ._, vol. vii, p. 454); it consists of three
small and low islets, one of which has a lagoon. Another lagoon-island
has been discovered (_Naut. Mag._, 1839, p. 770), in 22° 4′ S., and
136° 20′ W. Towards the S.E. part of the group, there are some islands
of different formation: _ Elizabeth_ Island is described by Beechey (p.
46, 4to ed.) as fringed by reefs, at the distance of between two and
three hundred yards; coloured red. _Pitcairn_ Island, in the immediate
neighbourhood, according to the same authority, has no reefs of any
kind, although numerous pieces of coral are thrown up on the beach; the
sea close to its shore is very deep (see “Zool. of Beechey’s Voyage,”
p. 164); it is left uncoloured. _Gambier_ Islands (see Plate I Fig. 8),
are encircled by a barrier-reef; the greatest depth within is
thirty-eight fathoms; coloured pale blue. _Aurora_ Island, which lies
N.E. of Tahiti close to the large space coloured dark blue in the map,
has been already described in a note (), on the authority of Mr.
Couthouy; it is an upraised atoll, but as it does not appear to be
fringed by living reefs, it is left uncoloured.

The SOCIETY Arch. is separated by a narrow space from the Low
Archipelago; and in their parallel direction they manifest some
relation to each other. I have already described the general character
of the reefs of these fine encircled islands. In the “Atlas of the
_Coquille’s_ Voyage” there is a good general chart of the group, and
separate plans of some of the islands. _ Tahiti_, the largest island in
the group, is almost surrounded, as seen in Cook’s chart, by a reef
from half a mile to a mile and a half from the shore, with from ten to
thirty fathoms within it. Some considerable submerged reefs lying
parallel to the shore, with a broad and deep space within, have lately
been discovered (_Naut. Mag._, 1836, p. 264) on the N.E. coast of the
island, where none are laid down by Cook. At _Eimeo_ the reef “which
like a ring surrounds it, is in some places one or two miles distant
from the shore, in others united to the beach” (Ellis, “Polynesian
Researches,” vol. i, p. 18, 12mo edition). Cook found deep water
(twenty fathoms) in some of the harbours within the reef. Mr. Couthouy,
however, states (“Remarks,” p. 45) that both at Tahiti and Eimeo, the
space between the barrier-reef and the shore, has been almost filled
up,—“a nearly continuous fringing-reef surrounding the island, and
varying from a few yards to rather more than a mile in width, the
lagoons merely forming canals between this and the sea-reef,” that is
the barrier-reef. _Tapamanoa_ is surrounded by a reef at a considerable
distance from the shore; from the island being small it is breached, as
I am informed by the Rev. W.
Ellis, only by a narrow and crooked boat channel. This is the lowest
island in the group, its height probably not exceeding 500 feet. A
little way north of Tahiti, the low coral-islets of _ Teturoa_ are
situated; from the description of them given me by the Rev. J. Williams
(the author of the “Narrative of Missionary Enterprise”), I should have
thought they had formed a small atoll, and likewise from the
description given by the Rev. D. Tyerman and G. Bennett (“Journal of
Voyage and Travels,” vol. i, p. 183), who say that ten low coral-islets
“are comprehended within one general reef, and separated from each
other by interjacent lagoons;” but as Mr. Stutchbury (_West of England
Journal_, vol. i, p. 54) describes it as consisting of a mere narrow
ridge, I have left it uncoloured. _Maitea_, eastward of the group, is
classed by Forster as a high encircled island; but from the account
given by the Rev. D. Tyerman and G. Bennett (vol. i, p. 57) it appears
to be an exceedingly abrupt cone, rising from the sea without any reef;
I have left it uncoloured. It would be superfluous to describe the
northern islands in this group, as they may be well seen in the chart
accompanying the 4to edition of Cook’s “Voyages,” and in the “Atlas of
the _Coquille’s_ Voyage.” _Maurua_ is the only one of the northern
islands, in which the water within the reef is not deep, being only
four and a half fathoms; but the great width of the reef, stretching
three miles and a half southward of the land (which is represented in
the drawing in the “Atlas of the _ Coquille’s_ Voyage” as descending
abruptly to the water) shows, on the principle explained in the
beginning of the last chapter, that it belongs to the barrier class. I
may here mention, from information communicated to me by the Rev. W.
Ellis, that on the N.E. side of _Huaheine_ there is a bank of sand,
about a quarter of a mile wide, extending parallel to the shore, and
separated from it by an extensive and deep lagoon; this bank of sand
rests on coral-rock, and undoubtedly was originally a living reef.
North of Bolabola lies the atoll of _Toubai_ (Motou-iti of the
“_Coquille’s_ Atlas”) which is coloured dark blue; the other islands,
surrounded by barrier-reefs, are pale blue; three of them are
represented in Figs 3, 4, and 5, in Plate I. There are three low
coral-groups lying a little E. of the Society Archipelago, and almost
forming part of it, namely _ Bellinghausen_, which is said by Kotzebue
(“Second Voyage,” vol. ii, p. 255), to be a lagoon-island; _Mopeha_,
which, from Cook’s description (“Second Voyage,” book iii, chap. i), no
doubt is an atoll; and the _Scilly_ Islands, which are said by Wallis
(“Voyage,” chap. ix) to form a _group_ of _low_ islets and shoals, and,
therefore, probably, they compose an atoll: the two former have been
coloured blue, but not the latter.

MENDANA OR MARQUESAS Group.—These islands are entirely without reefs,
as may be seen in Krusenstern’s Atlas, making a remarkable contrast
with the adjacent group of the Society Islands. Mr. F. D. Bennett has
given some account of this group, in the seventh volume of the
_Geograph. Journ._ He informs me that all the islands have the same
general character, and that the water is very deep close to their
shores. He visited three of them, namely, _Dominicana, Christiana,_ and
_Roapoa_; their beaches are strewed with rounded masses of coral, and
although no regular reefs exist, yet the shore is in many places lined
by coral-rock, so that a boat grounds on this formation. Hence these
islands ought probably to come within the class of fringed islands and
be coloured red; but as I am determined to err on the cautious side, I
have left them uncoloured.

COOK or HARVEY and AUSTRAL ISLAND.—_Palmerston_ Island is minutely
described as an atoll by Captain Cook during his voyage in 1774;
coloured blue. _Aitutaki_ was partially surveyed by the _ Beagle_ (see
map accompanying “Voyages of _Adventure_ and _Beagle_”); the land is
hilly, sloping gently to the beach; the highest point is 360 feet; on
the southern side the reef projects five miles from the land: off this
point the _Beagle_ found no bottom with 270 fathoms: the reef is
surmounted by many low coral-islets. Although within the reef the water
is exceedingly shallow, not being more than a few feet deep, as I am
informed by the Rev. J. Williams, nevertheless, from the great
extension of this reef into a profoundly deep ocean, this island
probably belongs, on the principle lately adverted to, to the barrier
class, and I have coloured it pale blue; although with much
hesitation.—_Manouai_ or _Harvey_ Island. The highest point is about
fifty feet: the Rev. J. Williams informs me that the reef here,
although it lies far from the shore, is less distant than at Aitutaki,
but the water within the reef is rather deeper: I have also coloured
this pale blue with many doubts.—Round _Mitiaro_ Island, as I am
informed by Mr. Williams, the reef is attached to the shore; coloured
red.—_Mauki_ or Maouti; the reef round this island (under the name of
Parry Island, in the “Voyage of H.M.S. _ Blonde_,” p. 209) is described
as a coral-flat, only fifty yards wide, and two feet under water. This
statement has been corroborated by Mr. Williams, who calls the reef
attached; coloured red.—_Aitu_, or Wateeo; a moderately elevated hilly
island, like the others of this group. The reef is described in Cook’s
“Voyage,” as attached to the shore, and about one hundred yards wide;
coloured red.—_Fenoua-iti_; Cook describes this island as very low, not
more than six or seven feet high (vol. i, book ii, chap. iii, 1777); in
the chart published in the “_Coquille’s_ Atlas,” a reef is engraved
close to the shore: this island is not mentioned in the list given by
Mr. Williams (page 16) in the “Narrative of Missionary Enterprise;”
nature doubtful. As it is so near Atiu, it has been unavoidably
coloured red.—_Rarotonga_; Mr. Williams informs me that it is a lofty
basaltic island with an attached reef; coloured red.—There are three
islands, _Rourouti, Roxburgh_, and _Hull_, of which I have not been
able to obtain any account, and have left them uncoloured. Hull Island,
in the French chart, is written with small letters as being
low.—_Mangaia_; height about three hundred feet; “the surrounding reef
joins the shore” (Williams, “Narrative,” p. 18); coloured
red.—_Rimetara_; Mr. Williams informs me that the reef is rather close
to the shore; but, from information given me by Mr. Ellis, the reef
does not appear to be quite so closely attached to it as in the
foregoing cases: the island is about three hundred feet high (_Naut.
Mag._, 1839, p. 738); coloured red.—_Rurutu_; Mr. Williams and Mr.
Ellis inform me that this island has an attached reef; coloured red. It
is described by Cook under the name of Oheteroa: he says it is not
surrounded, like the neighbouring islands by a reef; he must have meant
a distant reef.—_Toubouai_; in Cook’s chart (“Second Voyage,” vol. ii,
p. 2) the reef is laid down in part one mile, and in part two miles
from the shore. Mr. Ellis (“Polynes. Res.” vol. iii, p. 381) says the
low land round the base of the island is very extensive; and this
gentleman informs me that the water within the reef appears deep;
coloured blue.—_Raivaivai_, or Vivitao; Mr. Williams informs me that
the reef is here distant: Mr. Ellis, however, says that this is
certainly not the case on one side of the island; and he believes that
the water within the reef is not deep; hence I have left it
uncoloured.—_Lancaster_ Reef, described in _ Naut. Mag._, 1833 (p.
693), as an extensive crescent-formed coral-reef. I have not coloured
it.—_Rapa_, or Oparree; from the accounts given of it by Ellis and
Vancouver, there does not appear to be any reef.—_I. de Bass_ is an
adjoining island, of which I cannot find any account.—_Kemin_ Island;
Krusenstern seems hardly to know its position, and gives no further
particulars.

ISLANDS BETWEEN _the Low and Gilbert Archipelagoes._

_Caroline_ Island (10° S., 150 deg W.) is described by Mr. F. D.
Bennett (_Geograph. Journ._, vol. vii, p. 225) as containing a fine
lagoon; coloured blue.—_Flint_ Island (11° S., 151° W.); Krusenstern
believes that it is the same with Peregrino, which is described by
Quiros (Burney’s “Chron. Hist.,” vol. ii, p. 283) as “a cluster of
small islands connected by a reef, and forming a lagoon in the middle;”
coloured blue.—_Wostock_ is an island a little more than half a mile in
diameter, and apparently quite flat and low, and was discovered by
Bellinghausen; it is situated a little west of Caroline Island, but it
is not placed on the French charts; I have not coloured it, although I
entertain little doubt from the chart of Bellinghausen, that it
originally contained a small lagoon.—_Penrhyn_ Island (9° S., 158° W.);
a plan of it in the “Atlas of the First Voyage” of Kotzebue, shows that
it is an atoll; blue.—_Slarbuck_ Island (5° S., 156° W.) is described
in Byron’s “Voyage in the _Blonde_” (p. 206) as formed of a flat
coral-rock, with no trees; the height not given; not coloured.—_Malden_
Island (4° S., 154° W.); in the same voyage (p. 205) this island is
said to be of coral formation, and no part above forty feet high; I
have not ventured to colour it, although, from being of
coral-formation, it is probably fringed; in which case it should be
red.—_Jarvis_, or _Bunker_ Island (0° 20′ S., 160° W.) is described by
Mr. F. D. Bennett (_Geograph. Journ._, vol. vii, p. 227) as a narrow,
low strip of coral-formation; not coloured.—_Brook_, is a small low
island between the two latter; the position, and perhaps even the
existence of it is doubtful; not coloured.—_Pescado_ and _Humphrey_
Islands; I can find out nothing about these islands, except that the
latter appears to be small and low; not coloured.—_Rearson_, or Grand
Duke Alexander’s (10° S., 161° W.); an atoll, of which a plan is given
by Bellinghausen; blue.—_Souvoroff_ Islands (13° S., 163° W.); Admiral
Krusenstern, in the most obliging manner, obtained for me an account of
these islands from Admiral Lazareff, who discovered them. They consist
of five very low
islands of coral-formation, two of which are connected by a reef, with
deep water close to it. They do not surround a lagoon, but are so
placed that a line drawn through them includes an oval space, part of
which is shallow; these islets, therefore, probably once (as is the
case with some of the islands in the Caroline Archipelago) formed a
single atoll; but I have not coloured them.—_Danger_ Island (10° S.,
166° W.); described as low by Commodore Byron, and more lately surveyed
by Bellinghausen; it is a small atoll with three islets on it;
blue.—_Clarence_ Island (9° S., 172° W.); discovered in the _Pandora_
(G. Hamilton’s “Voyage,” p. 75): it is said, “in running along the
land, we saw several canoes crossing the _lagoons_;” as this island is
in the close vicinity of other low islands, and as it is said, that the
natives make reservoirs of water in old cocoa-nut trees (which shows
the nature of the land), I have no doubt it is an atoll, and have
coloured it blue. _York_ Island (8° S., 172° W.) is described by
Commodore Byron (chap. x of his “Voyage”) as an atoll; blue.—_Sydney_
Island (4° S., 172° W.) is about three miles in diameter, with its
interior occupied by a lagoon (Captain Tromelin, “Annal. Marit.” 1829,
p. 297); blue.—_Phoenix_ Island (4° S., 171° W.) is nearly circular,
low, sandy, not more than two miles in diameter, and very steep outside
(Tromelin, “Annal. Marit.” 1829, p. 297); it may be inferred that this
island originally contained a lagoon, but I have not coloured it.—_New
Nantucket_ (0° 15′ N., 174° W.). From the French chart it must be a low
island; I can find nothing more about it or about _Mary_ Island; both
uncoloured.—_Gardner_ Island (5° S., 174° W.) from its position is
certainly the same as _Kemin_ Island described (Krusenstern, p. 435,
Appen. to Mem., published 1827) as having a lagoon in its centre; blue.

ISLANDS SOUTH _of the Sandwich Archipelago._

_Christmas_ Island (2° N., 157° W.). Captain Cook, in his “Third
Voyage” (vol. ii, chap. x), has given a detailed account of this atoll.
The breadth of the islets on the reef is unusually great, and the sea
near it does not deepen so suddenly as is generally the case. It has
more lately been visited by Mr. F. D. Bennett (_Geograph. Journ._, vol.
vii, p. 226); and he assures me that it is low and of coral-formation:
I particularly mention this, because it is engraved with a capital
letter, signifying a high island, in D’Urville and Lottin’s chart. Mr.
Couthouy, also, has given some account of it (“Remarks,” p. 46) from
the Hawaiian “Spectator”; he believes it has lately undergone a small
elevation, but his evidence does not appear to me satisfactory; the
deepest part of the lagoon is said to be only ten feet; nevertheless, I
have coloured it blue.—_Fanning_ Island (4° N., 158° W.) according to
Captain Tromelin (“Ann. Maritim.,” 1829, p. 283), is an atoll: his
account as observed by Krusenstern, differs from that given in
Fanning’s “Voyage” (p. 224), which, however, is far from clear;
coloured blue.—_Washington_ Island (4° N., 159° W.) is engraved as a
low island in D’Urville’s chart, but is described by Fanning (p. 226)
as having a much greater elevation than Fanning Island, and hence I
presume it is not an atoll; not coloured.—_Palmyra_ Island (6° N., 162°
W.) is an atoll divided into two parts (Krusenstern’s “Mem. Suppl.,” p.
50, also Fanning’s “Voyage,” p. 233); blue.—_Smyth’s_ or Johnston’s
Islands (17° N., 170° W.). Captain Smyth, R.N., has had the kindness to
inform me that they consist of two very low, small islands, with a
dangerous reef off the east end of them. Captain Smyth does not
recollect whether these islets, together with the reef, surrounded a
lagoon; uncoloured.

SANDWICH ARCHIPELAGO.—_Hawaii_; in the chart in Freycinet’s “Atlas,”
small portions of the coast are fringed by reefs; and in the
accompanying “Hydrog. Memoir,” reefs are mentioned in several places,
and the coral is said to injure the cables. On one side of the islet of
Kohaihai there is a bank of sand and coral with five feet water on it,
running parallel to the shore, and leaving a channel of about fifteen
feet deep within. I have coloured this island red, but it is very much
less perfectly fringed than others of the group.—_Maui_; in Freycinet’s
chart of the anchorage of Raheina, two or three miles of coast are seen
to be fringed; and in the “Hydrog. Memoir,” “banks of coral along
shore” are spoken of. Mr. F. D. Bennett informs me that the reefs, on
an average, extend about a quarter of a mile from the beach; the land
is not very steep, and outside the reefs the sea does not become deep
very suddenly; coloured red.—_Morotoi_, I presume, is fringed:
Freycinet speaks of the breakers extending along the shore at a little
distance from it. From the chart, I believe it is fringed; coloured
red.—_Oahu_; Freycinet, in his “Hydrog. Memoir,” mentions some of the
reefs. Mr. F. D. Bennett informs me that the shore is skirted for forty
or fifty miles in length. There is even a harbour for ships formed by
the reefs, but it is at the mouth of a valley; red.—_Atooi_, in La
Peyrouse’s charts, is represented as fringed by a reef, in the same
manner as Oahu and Morotoi; and this, as I have been informed by Mr.
Ellis, on part at least of the shore, is of coral-formation: the reef
does not leave a deep channel within; red.—_Oneehow_; Mr. Ellis
believes that this island is also fringed by a coral-reef: considering
its close proximity to the other islands, I have ventured to colour it
red. I have in vain consulted the works of Cook, Vancouver, La
Peyrouse, and Lisiansky, for any satisfactory account of the small
islands and reefs, which lie scattered in a N.W. line prolonged from
the Sandwich group, and hence have left them uncoloured, with one
exception; for I am indebted to Mr. F. D. Bennett for informing me of
an atoll-formed reef, in latitude 28° 22′, longitude 178° 30′ W., on
which the _ Gledstanes_ was wrecked in 1837. It is apparently of large
size, and extends in a N.W. and S.E. line: very few islets have been
formed on it. The lagoon seems to be shallow; at least, the deepest
part which was surveyed was only three fathoms. Mr. Couthouy
(“Remarks,” p. 38) describes this island under the name of _ Ocean_
island. Considerable doubts should be entertained regarding the nature
of a reef of this kind, with a very shallow lagoon, and standing far
from any other atoll, on account of the possibility of a crater or flat
bank of rock lying at the proper depth beneath the surface of the
water, thus affording a foundation for a ring-formed coral-reef. I
have, however, thought myself compelled, from its large size and
symmetrical outline, to colour it blue.

SAMOA or NAVIGATOR GROUP.—Kotzebue, in his “Second Voyage,”
contrasts the structure of these islands with many others in the
Pacific, in not being furnished with harbours for ships, formed by
distant coral-reefs. The Rev. J. Williams, however, informs me, that
coral-reefs do occur in irregular patches on the shores of these
islands; but that they do not form a continuous band, as round Mangaia,
and other such perfect cases of fringed islands. From the charts
accompanying La Peyrouse’s “Voyage,” it appears that the north shore of
_Savaii, Maouna, Orosenga_, and _ Manua_, are fringed by reefs. La
Peyrouse, speaking of Maouna (p. 126), says that the coral-reef
surrounding its shores, almost touches the beach; and is breached in
front of the little coves and streams, forming passages for canoes, and
probably even for boats. Further on (p. 159), he extends the same
observation to all the islands which he visited. Mr. Williams in his
“Narrative,” speaks of a reef going round a small island attached to
_Oyolava_, and returning again to it: all these islands have been
coloured red.—A chart of _Rose_ Island, at the extreme west end of the
group, is given by Freycinet, from which I should have thought that it
had been an atoll; but according to Mr. Couthouy (“Remarks,” p. 43), it
consists of a reef, only a league in circuit, surmounted by a very few
low islets; the lagoon is very shallow, and is strewed with numerous
large boulders of volcanic rock. This island, therefore, probably
consists of a bank of rock, a few feet submerged, with the outer margin
of its upper surface fringed with reefs; hence it cannot be properly
classed with atolls, in which the foundations are always supposed to
lie at a depth, greater than that at which the reef-constructing
polypifers can live; not coloured.

_Beveridge_ Reef, 20° S., 167° W., is described in the _Naut. Mag._
(May 1833, p. 442) as ten miles long in a N. and S. line, and eight
wide; “in the inside of the reef there appears deep water;” there is a
passage near the S.W. corner: this therefore seems to be a submerged
atoll, and is coloured blue.

_Savage_ Island, 19° S., 170° W., has been described by Cook and
Forster. The younger Forster (vol. ii, p. 163) says it is about forty
feet high: he suspects that it contains a low plain, which formerly was
the lagoon. The Rev. J. Williams informs me that the reef fringing its
shores, resembles that round Mangaia; coloured red.

FRIENDLY ARCHIPELAGO.—_Pylstaart_ Island. Judging from the chart in
Freycinet’s “Atlas,” I should have supposed that it had been regularly
fringed; but as nothing is said in the “Hydrog. Memoir” (or in the
“Voyage” of Tasman, the discoverer) about coral-reefs, I have left it
uncoloured.—_Tongatabou_: In the “Atlas of the Voyage of the
_Astrolabe_,” the whole south side of the island is represented as
narrowly fringed by the same reef which forms an extensive platform on
the northern side. The origin of this latter reef, which might have
been mistaken for a barrier-reef, has already been attempted to be
explained, when giving the proofs of the recent elevation of this
island.—In Cook’s charts the little outlying island also of _Eoaigee_,
is represented as fringed; coloured red.—_Eoua._ I cannot make out from
Captain Cook’s charts and descriptions, that this island has any reef,
although the bottom of the neighbouring sea seems to be corally, and
the island itself is formed of coral-rock.


Forster, however, distinctly (“Observations,” p. 14) classes it with
high islands having reefs, but it certainly is not encircled by a
barrier-reef and the younger Forster (“Voyage,” vol. i, p. 426) says,
that “a bed of coral-rocks surrounded the coast towards the
landing-place.” I have therefore classed it with the fringed islands
and coloured it red. The several islands lying N.W. of Tongatabou,
namely _Anamouka, Komango, Kotou, Lefouga, Foa_, etc., are seen in
Captain Cook’s chart to be fringed by reefs, in several of them are
connected together. From the various statements in the first volume of
Cook’s “Third Voyage,” and especially in the fourth and sixth chapters,
it appears that these reefs are of coral-formation, and certainly do
not belong to the barrier class; coloured red.—_Toufoa and Kao_,
forming the western part of the group, according to Forster have no
reefs; the former is an active volcano.—_Vavao._ There is a chart of
this singularly formed island, by Espinoza: according to Mr. Williams
it consists of coral-rock: the Chevalier Dillon informs me that it is
not fringed; not coloured. Nor are the islands of _Latte_ and
_Amargura_, for I have not seen plans on a large scale of them, and do
not know whether they are fringed.

_Niouha_, 16° S., 174° W., or _Keppel_ Island of Wallis, or _Cocos_
Island. From a view and chart of this island given in Wallis’s “Voyage”
(4to ed.) it is evidently encircled by a reef; coloured blue: it is
however remarkable that _Boscawen_ Island, immediately adjoining, has
no reef of any kind; uncoloured.

_Wallis_ Island, 13° S., 176° W., a chart and view of this island in
Wallis’s “Voyage” (4to ed.) shows that it is encircled. A view of it in
the _Naut. Mag._, July 1833, p. 376, shows the same fact; blue.

_Alloufatou_, or _Horn_ Island, _Onouafu_, or _ Proby_ Island, and
_Hunter_ Islands, lie between the Navigator and Fidji groups. I can
find no distinct accounts of them.

FIDJI or VITI GROUP.—The best chart of the numerous islands of this
group, will be found in the “Atlas of the _ Astrolabe’s_ Voyage.” From
this, and from the description given in the “Hydrog. Memoir,”
accompanying it, it appears that many of these islands are bold and
mountainous, rising to the height of between 3,000 and 4,000 feet. Most
of the islands are surrounded by reefs, lying far from the land, and
outside of which the ocean appears very deep. The _Astrolabe_ sounded
with ninety fathoms in several places about a mile from the reefs, and
found no bottom. Although the depth within the reef is not laid down,
it is evident from several expressions, that Captain D’Urville believes
that ships could anchor within, if passages existed through the outer
barriers. The Chevallier Dillon informs me that this is the case: hence
I have coloured this group blue. In the S.E. part lies _ Batoa_, or
_Turtle_ Island of Cook (“Second Voyage,” vol. ii, p. 23, and chart,
4to ed.) surrounded by a coral-reef, “which in some places extends two
miles from the shore;” within the reef the water appears to be deep,
and outside it is unfathomable; coloured pale blue. At the distance of
a few miles, Captain Cook (_Ibid_., p. 24) found a circular coral-reef,
four or five leagues in circuit, with deep water within; “in short, the
bank wants only a few little islets to make it exactly like one of the
half-drowned isles so
often mentioned,”—namely, atolls. South of Batoa, lies the high island
of _Ono_, which appears in Bellinghausen’s “Atlas” to be encircled; as
do some other small islands to the south; coloured pale blue; near Ono,
there is an annular reef, quite similar to the one just described in
the words of Captain Cook; coloured dark blue.

_Rotoumah_, 13° S., 179° E.—From the chart in Duperrey’s “Atlas,” I
thought this island was encircled, and had coloured it blue, but the
Chevallier Dillon assures me that the reef is only a shore or fringing
one; red.

_Independence_ Island, 10° S., 179° E., is described by Mr. G. Bennett,
(_United Service Journ._, 1831, part ii, p. 197) as a low island of
coral-formation, it is small, and does not appear to contain a lagoon,
although an opening through the reef is referred to. A lagoon probably
once existed, and has since been filled up; left uncoloured.

ELLICE GROUP.—_Oscar, Peyster_, and _Ellice_ Islands are figured in
Arrowsmith’s “Chart of the Pacific” (corrected to 1832) as atolls, and
are said to be very low; blue.—_Nederlandisch_ Island. I am greatly
indebted to the kindness of Admiral Krusenstern, for sending me the
original documents concerning this island. From the plans given by
Captains Eeg and Khremtshenko, and from the detailed account given by
the former, it appears that it is a narrow coral-island, about two
miles long, containing a small lagoon. The sea is very deep close to
the shore, which is fronted by sharp coral-rocks. Captain Eeg compares
the lagoon with that of other coral-islands; and he distinctly says,
the land is “very low.” I have therefore coloured it blue. Admiral
Krusenstern (“Memoir on the Pacific,” Append., 1835) states that its
shores are eighty feet high; this probably arose from the height of the
cocoa-nut trees, with which it is covered, being mistaken for
land.—_Gran Cocal_ is said in Krusenstern’s “Memoir,” to be low, and to
be surrounded by a reef; it is small, and therefore probably once
contained a lagoon; uncoloured.—_St. Augustin._ From a chart and view
of it, given in the “Atlas of the _Coquille’s_ Voyage,” it appears to
be a small atoll, with its lagoon partly filled up; coloured blue.

GILBERT GROUP.—The chart of this group, given in the “Atlas of the
_Coquille’s_ Voyage,” at once shows that it is composed of ten well
characterised atolls. In D’Urville and Lottin’s chart, _Sydenham_ is
written with a capital letter, signifying that it is high; but this
certainly is not the case, for it is a perfectly characterised atoll,
and a sketch, showing how low it is, is given in the “_Coquille’s_
Atlas.” Some narrow strip-like reefs project from the southern side of
_Drummond_ atoll, and render it irregular. The southern island of the
group is called _Chase_ (in some charts, _ Rotches_); of this I can
find no account, but Mr. F. D. Bennett discovered (_Geograph. Journ._,
vol. vii, p. 229), a low extensive island in nearly the same latitude,
about three degrees westward of the longitude assigned to Rotches, but
very probably it is the same island. Mr. Bennett informs me that the
man at the masthead reported an appearance of lagoon-water in the
centre; and, therefore, considering its position, I have coloured it
blue.—_Pitt_ Island, at the extreme northern point of the group, is
left uncoloured, as its exact position and nature
is not known.—_Byron_ Island, which lies a little to the eastward, does
not appear to have been visited since Commodore Byron’s voyage, and it
was then seen only from a distance of eighteen miles; it is said to be
low; uncoloured.

_Ocean, Pleasant_, and _Atlantic_ Islands all lie considerably to the
west of the Gilbert group: I have been unable to find any distinct
account of them. Ocean Island is written with small letters in the
French chart, but in Krusenstern’s “Memoir” it is said to be high.

MARSHALL GROUP.—We are well acquainted with this group from the
excellent charts of the separate islands, made during the two voyages
of Kotzebue: a reduced one of the whole group may be easily seen in
Krusenstern’s “Atlas,” and in Kotzebue’s “Second Voyage.” The group
consists (with the exception of two _little_ islands which probably
have had their lagoon filled up) of a double row of twenty-three large
and well-characterised atolls, from the examination of which Chamisso
has given us his well-known account of coral-formations. I include
_Gaspar Rico_, or _Cornwallis_ Island in this group, which is described
by Chamisso (Kotzebue’s “First Voyage,” vol. iii, p. 179) “as a low
sickle-formed group, with mould only on the windward side.” Gaspard
Island is considered by some geographers as a distinct island lying
N.E. of the group, but it is not entered in the chart by Krusenstern;
left uncoloured. In the S.W. part of this group lies _Baring_ Island,
of which little is known (see Krusenstern’s “Appendix,” 1835, p. 149).
I have left it uncoloured; but _Boston_ Island I have coloured blue, as
it is described (_Ibid_.) as consisting of fourteen small islands,
which, no doubt, enclose a lagoon, as represented in a chart in the
“‘Coquille’s’ Atlas.”—Two islands, _Aur Kawen_ and _Gaspar Rico_, are
written in the French chart with capital letters; but this is an error,
for from the account given by Chamisso in Kotzebue’s “First Voyage,”
they are certainly low. The nature, position, and even existence, of
the shoals and small islands north of the Marshall group, are doubtful.

NEW HEBRIDES.—Any chart, on even a small scale, of these islands, will
show that their shores are almost without reefs, presenting a
remarkable contrast with those of New Caledonia on the one hand, and
the Fidji group on the other. Nevertheless, I have been assured by Mr.
G. Bennett, that coral grows vigorously on their shores; as indeed,
will be further shown in some of the following notices. As, therefore,
these islands are not encircled, and as coral grows vigorously on their
shores, we might almost conclude, without further evidence, that they
were fringed, and hence I have applied the red colour with rather
greater freedom than in other instances.—_Matthew’s Rock_, an active
volcano, some way south of the group (of which a plan is given in the
“Atlas of the _Astrolabe’s_ Voyage”) does not appear to have reefs of
any kind about it.—_Annatom_, the southernmost of the Hebrides; from a
rough woodcut given in the _United Service Journal_ (1831, part iii, p.
190), accompanying a paper by Mr. Bennett, it appears that the shore is
fringed; coloured red.—_Tanna._ Forster, in his “Observations” (p. 22),
says Tanna has on its shores coral-rock and madrepores; and the younger
Forster, in his account (vol. ii, p. 269) speaking of the harbour
says, the whole S.E. side consists of coral-reefs, which are overflowed
at high-water; part of the southern shore in Cook’s chart is
represented as fringed; coloured red.—_Immer_ is described (_United
Service Journal,_ 1831, part iii, p. 192) by Mr. Bennett as being of
moderate elevation, with cliffs appearing like sandstone: coral grows
in patches on its shore, but I have not coloured it; and I mention
these facts, because Immer might have been thought from Forster’s
classification (“Observations,” p. 14), to have been a low island or
even an atoll.—_Erromango_ Island; Cook (“Second Voyage,” vol. ii, p.
45, 4to ed.) speaks of rocks everywhere _lining_ the coast, and the
natives offered to haul his boat over the breakers to the sandy beach:
Mr. Bennett, in a letter to the Editor of the _Singapore Chron._,
alludes to the _reefs_ on its shores. It may, I think, be safely
inferred from these passages that the shore is fringed in parts by
coral-reefs; coloured red.—_Sandwich_ Island. The east coast is said
(Cook’s “Second Voyage,” vol. ii, p. 41) to be low, and to be guarded
by a chain of breakers. In the accompanying chart it is seen to be
fringed by a reef; coloured red.—_Mallicollo._ Forster speaks of the
reef-bounded shore: the reef is about thirty yards wide, and so shallow
that a boat cannot pass over it. Forster also (“Observations,” p. 23)
says, that the rocks of the sea-shore consist of madrepore. In the plan
of Sandwich harbour, the headlands are represented as fringed; coloured
red.—_Aurora_ and _Pentecost_ Islands, according to Bougainville,
apparently have no reefs; nor has the large island of _S. Espiritu_,
nor _Bligh_ Island or _Banks’_ Islands, which latter lie to the N.E. of
the Hebrides. But in none of these cases, have I met with any detailed
account of their shores, or seen plans on a large scale; and it will be
evident, that a fringing-reef of only thirty or even a few hundred
yards in width, is of so little importance to navigation, that it will
seldom be noticed, excepting by chance; and hence I do not doubt that
several of these islands, now left uncoloured, ought to be red.

SANTA CRUZ GROUP.—_Vanikoro_ (Fig. 1, Plate I) offers a striking
example of a barrier- reef: it was first described by the Chevalier
Dillon, in his voyage, and was surveyed in the _Astrolabe_; coloured
pale blue.—_Tikopia_ and _Fataka_ Islands appear, from the descriptions
of Dillon and D’Urville, to have no reefs; _ Anouda_ is a low, flat
island, surrounded by cliffs (“_Astrolabe_ Hydrog.” and Krusenstern,
“Mem.” vol. ii, p. 432); these are uncoloured. _Toupoua_ (_Otooboa_ of
Dillon) is stated by Captain Tromelin (“Annales Marit.” 1829, p. 289)
to be almost entirely included in a reef, lying at the distance of two
miles from the shore. There is a space of three miles without any reef,
which, although indented with bays, offers no anchorage from the
extreme depth of the water close to the shore: Captain Dillon also
speaks of the reefs fronting this island; coloured blue.—_Santa-Cruz._
I have carefully examined the works of Carteret, D’Entrecasteaux,
Wilson, and Tromelin, and I cannot discover any mention of reefs on its
shores; left uncoloured.—_Tinakoro_ is a constantly active volcano
without reefs.—_Mendana Isles_ (mentioned by Dillon under the name of
_Mammee_, etc.); said by Krusenstern to be low, and intertwined with
reefs. I do not believe they include a lagoon; I have left them
uncoloured.—_Duff’s_ Islands compose a small group
directed in a N.W. and S.E. band; they are described by Wilson (p. 296,
“Miss. Voy.” 4to ed.), as formed by bold-peaked land, with the islands
surrounded by coral-reefs, extending about half a mile from the shore;
at a distance of a mile from the reefs he found only seven fathoms. As
I have no reason for supposing there is deep water within these reefs,
I have coloured them red. _Kennedy_ Island, N.E. of Duff’s. I have been
unable to find any account of it.

NEW CALEDONIA.—The great barrier-reefs on the shores of this island
have already been described (Fig. 5, Plate II). They have been visited
by Labillardiere, Cook, and the northern point by D’Urville; this
latter part so closely resembles an atoll that I have coloured it dark
blue. The _Loyalty_ group is situated eastward of this island; from the
chart and description given in the “Voyage of the _Astrolabe_,” they do
not appear to have any reefs; north of this group, there are some
extensive low reefs (called _Astrolabe_ and _Beaupré_,) which do not
seem to be atoll-formed; these are left uncoloured.

AUSTRALIAN BARRIER-REEF.—The limits of this great reef, which has
already been described, have been coloured from the charts of Flinders
and King. In the northern parts, an atoll-formed reef, lying outside
the barrier, has been described by Bligh, and is coloured dark blue. In
the space between Australia and New Caledonia, called by Flinders the
Corallian Sea, there are numerous reefs. Of these, some are represented
in Krusenstern’s “Atlas” as having an atoll-like structure; namely,
_Bampton_ shoal, _Frederic, Vine_ or Horse-shoe, and _ Alert_ reefs;
these have been coloured dark blue.

LOUISIADE.—The dangerous reefs which front and surround the western,
southern, and northern coasts of this so-called peninsula and
archipelago, seem evidently to belong to the barrier class. The land is
lofty, with a low fringe on the coast; the reefs are distant, and the
sea outside them profoundly deep. Nearly all that is known of this
group is derived from the labours of D’Entrecasteaux and Bougainville:
the latter has represented one continuous reef ninety miles long,
parallel to the shore, and in places as much as ten miles from it;
coloured pale blue. A little distance northward we have the _Laughlan_
Islands, the reefs round which are engraved in the “Atlas of the Voyage
of the _Astrolabe_,” in the same manner as in the encircled islands of
the Caroline Archipelago, the reef is, in parts, a mile and a half from
the shore, to which it does not appear to be attached; coloured blue.
At some little distance from the extremity of the Louisiade lies the
_Wells_ reef, described in G. Hamilton’s “Voyage in H.M.S. _Pandora_”
(p. 100): it is said, “We found we had got embayed in a double reef,
which will soon be an island.” As this statement is only intelligible
on the supposition of the reef being crescent or horse-shoe formed,
like so many other submerged annular reefs, I have ventured to colour
it blue.

SOLOMON ARCHIPELAGO.—The chart in Krusenstern’s “Atlas” shows that
these islands are not encircled, and as coral appears from the works of
Surville, Bougainville, and Labillardiere, to grow on their shores,
this circumstance, as in the case of the New Hebrides, is a presumption
that they are fringed. I cannot find out anything from
D’Entrecasteaux’s
“Voyage,” regarding the southern islands of the group, so have left
them uncoloured.—_Malayta_ Island in a rough MS. chart in the Admiralty
has its northern shore fringed.—_Ysabel_ Island, the N.E. part of this
island, in the same chart, is also fringed: Mendana, speaking (Burney,
vol. i, p. 280) of an islet adjoining the northern coast, says it is
surrounded by reefs; the shores, also of Port Praslin appear regularly
fringed.—_Choiseul_ Island. In Bougainville’s “Chart of Choiseul Bay,”
parts of the shores are fringed by coral-reefs.—_Bougainville_ Island.
According to D’Entrecasteaux the western shore abounds with
coral-reefs, and the smaller islands are said to be attached to the
larger ones by reefs; all the before-mentioned islands have been
coloured red.—_Bouka_ Islands. Captain Duperrey has kindly informed me
in a letter that he passed close round the northern side of this island
(of which a plan is given in his “Atlas of the _Coquille’s_ Voyage”),
and that it was “garnie d’une bande de récifs à fleur d’eau adherentes
au rivage;” and he infers, from the abundance of coral on the islands
north and south of Bouka, that the reef probably is of coral; coloured
red.

Off the north coast of the Solomon Archipelago there are several small
groups which are little known; they appear to be low, and of
coral-formation; and some of them probably have an atoll-like
structure; the Chevallier Dillon, however, informs me that this is not
the case with the B. de _Candelaria.—Outong Java_, according to the
Spanish navigator, Maurelle, is thus characterised; but this is the
only one which I have ventured to colour blue.

NEW IRELAND.—The shores of the S.W. point of this island and some
adjoining islets, are fringed by reefs, as may be seen in the “Atlases
of the Voyages of the _Coquille_ and _Astrolabe_.” M. Lesson observes
that the reefs are open in front of each streamlet. The _Duke of
York’s_ Island is also fringed; but with regard to the other parts of
_New Ireland, New Hanover_, and the small islands lying northward, I
have been unable to obtain any information. I will only add that no
part of New Ireland appears to be fronted by distant reefs. I have
coloured red only the above specified portions.

NEW BRITAIN AND THE NORTHERN SHORE OF NEW GUINEA.—From the charts in
the “Voyage of the _Astrolabe_,” and from the “Hydrog. Memoir,” it
appears that these coasts are entirely without reefs, as are the
_Schouten_ Islands, lying close to the northern shore of New Guinea.
The western and south-western parts of New Guinea, will be treated of
when we come to the islands of the East Indian Archipelago.

ADMIRALTY GROUP.—From the accounts by Bougainville, Maurelle,
D’Entrecasteaux, and the scattered notices collected by Horsburgh, it
appears, that some of the many islands composing it, are high, with a
bold outline; and others are very low, small and interlaced with reefs.
All the high islands appear to be fronted by distant reefs rising
abruptly from the sea, and within some of which there is reason to
believe that the water is deep. I have therefore little doubt they are
of the barrier class.—In the southern part of the group we have _
Elizabeth_ Island, which is surrounded by a reef at the distance of a
mile; and two miles eastward of it (Krusenstern, “Append.” 1835, p. 42)
there is a little island
containing a lagoon.—Near here, also lies _ Circular-reef_ (Horsburgh,
“Direct.,” vol. i, p. 691, 4th ed.), “three or four miles in diameter
having deep water inside with an opening at the N.N.W. part, and on the
outside steep to.” I have from these data, coloured the group pale
blue, and _ Circular-reef_ dark blue.—The _Anachorites, Echequier_, and
_Hermites_, consist of innumerable low islands of coral-formation,
which probably have atoll-like forms; but not being able to ascertain
this, I have not coloured them, nor _Durour_ Island, which is described
by Carteret as low.

The CAROLINE ARCHIPELAGO is now well-known, chiefly from the
hydrographical labours of Lutké; it contains about forty groups of
atolls, and three encircled islands, two of which are engraved in Figs
2 and 7, Plate I. Commencing with the eastern part; the encircling reef
round _Ualen_ appears to be only about half a mile from the shore; but
as the land is low and covered with mangroves (“Voyage autour du
Monde,” par F. Lutké, vol. i, p. 339), the real margin has not probably
been ascertained. The extreme depth in one of the harbours within the
reef is thirty-three fathoms (see charts in “Atlas of _Coquille’s_
Voyage”), and outside at half a mile distant from the reef, no bottom
was obtained with two hundred and fifty fathoms. The reef is surmounted
by many islets, and the lagoon-like channel within is mostly shallow,
and appears to have been much encroached on by the low land surrounding
the central mountains; these facts show that time has allowed much
detritus to accumulate; coloured pale blue.—_Pouynipète_, or Seniavine.
In the greater part of the circumference of this island, the reef is
about one mile and three quarters distant; on the north side it is five
miles off the included high islets. The reef is broken in several
places; and just within it, the depth in one place is thirty fathoms,
and in another, twenty-eight, beyond which, to all appearance, there
was “un porte vaste et sur” (Lutké, vol. ii, p. 4); coloured pale
blue.—_Hogoleu_ or _Roug_. This wonderful group contains at least
sixty-two islands, and its reef is one hundred and thirty-five miles in
circuit. Of the islands, only a few, about six or eight (see “Hydrog.
Descrip.” p. 428, of the “Voyage of the _Astrolabe_,” and the large
accompanying chart taken chiefly from that given by Duperrey) are high,
and the rest are all small, low, and formed on the reef. The depth of
the great interior lake has not been ascertained; but Captain D’Urville
appears to have entertained no doubt about the possibility of taking in
a frigate. The reef lies no less than fourteen miles distant from the
northern coasts of the interior high islands, seven from their western
sides, and twenty from the southern; the sea is deep outside. This
island is a likeness on a grand scale to the Gambier group in the Low
Archipelago. Of the groups of low[1] islands forming the chief part of
the Caroline Archipelago, all those of larger size, have the true
atoll-structure (as may be seen in the “Atlas” by Captain Lutké), and
some even of the very small ones, as _ Macaskill_ and _Duperrey_, of
which plans are given in the
“Atlas of the _Coquille’s_ Voyage.” There are, however, some low small
islands of coral-formation, namely _Ollap, Tamatam, Bigali, Satahoual_,
which do not contain lagoons; but it is probable that lagoons
originally existed, but have since filled up: Lutké (vol. ii, p. 304)
seems to have thought that all the low islands, with only one
exception, contained lagoons. From the sketches, and from the manner in
which the margins of these islands are engraved in the “Atlas of the
Voyage of the _Coquille_,” it might have been thought that they were
not low; but by a comparison with the remarks of Lutké (vol. ii, p.
107, regarding Bigali) and of Freycinet (“Hydrog. Memoir _ L’Uranie_
Voyage,” p. 188, regarding Tamatam, Ollap, etc.), it will be seen that
the artist must have represented the land incorrectly. The most
southern island in the group, namely _ Piguiram_, is not coloured,
because I have found no account of it. _Nougouor_, or _Monte Verdison_,
which was not visited by Lutké, is described and figured by Mr. Bennett
(_United Service Journal_, January 1832) as an atoll. All the
above-mentioned islands have been coloured blue.

 [1] In D’Urville and Lottin’s chart, Peserare is written with capital
 letters; but this evidently is an error, for it is one of the low
 islets on the reef of Namonouyto (see Lutké’s charts)—a regular atoll.

WESTERN PART OF THE CAROLINE ARCHIPELAGO.—_Fais_ Island is ninety feet
high, and is surrounded, as I have been informed by Admiral Lutké, by a
narrow reef of living coral, of which the broadest part, as represented
in the charts, is only 150 yards; coloured red.—_Philip_ Island., I
believe, is low; but Hunter, in his “Historical Journal,” gives no
clear account of it; uncoloured.—_Elivi_; from the manner in which the
islets on the reefs are engraved, in the “Atlas of the _Astrolabe’s_
Voyage,” I should have thought they were above the ordinary height, but
Admiral Lutké assures me this is not the case: they form a regular
atoll; coloured blue.—_Gouap_ (_Eap_ of Chamisso), is a high island
with a reef (see chart in “Voyage of the _Astrolabe_”), more than a
mile distant in most parts from the shore, and two miles in one part.
Captain D’Urville thinks that there would be anchorage (“Hydrog.
Descript. _Astrolabe_ Voyage,” p. 436) for ships within the reef, if a
passage could be found; coloured pale blue.—_Goulou_, from the chart in
the “_Astrolabe’s_ Atlas,” appears to be an atoll. D’Urville (“Hydrog.
Descript.” p. 437) speaks of the low islets on the reef; coloured dark
blue.

PELEW ISLANDS.—Krusenstern speaks of some of the islands being
mountainous; the reefs are distant from the shore, and there are spaces
within them, and not opposite valleys, with from ten to fifteen
fathoms. According to a MS. chart of the group by Lieutenant Elmer in
the Admiralty, there is a large space within the reef with deepish
water; although the high land does not hold a central position with
respect to the reefs, as is generally the case, I have little doubt
that the reefs of the Pelew Islands ought to be ranked with the barrier
class, and I have coloured them pale blue. In Lieutenant Elmer’s chart
there is a horseshoe-formed shoal, laid down thirteen miles N.W. of
Pelew, with fifteen fathoms within the reef, and some dry banks on it;
coloured dark blue.—_Spanish, Martires, Sanserot, Pulo Anna_ and
_Mariere_ Islands are not coloured, because I know nothing about them,
excepting that according to Krusenstern, the second, third, and fourth
mentioned, are
low, placed on coral-reefs, and therefore, perhaps, contain lagoons;
but Pulo Mariere is a little higher.

MARIANA ARCHIPELAGO, or LADRONES.—_Guahan._ Almost the whole of this
island is fringed by reefs, which extend in most parts about a third of
a mile from the land. Even where the reefs are most extensive, the
water within them is shallow. In several parts there is a navigable
channel for boats and canoes within the reefs. In Freycinet’s “Hydrog.
Mem.” there is an account of these reefs, and in the “Atlas,” a map on
a large scale; coloured red.—_Rota_. “L’ile est presque entièrement
entourée des récifs” (p. 212, Freycinet’s “Hydrog. Mem.”). These reefs
project about a quarter of a mile from the shore; coloured
red.—_Tinian. The eastern_ coast is precipitous, and is without reefs;
but the western side is fringed like the last island; coloured
red.—_Saypan_. The N.E. coast, and likewise the western shores appear
to be fringed; but there is a great, irregular, horn-like reef
projecting far from this side; coloured red.—_Farallon de Medinilla_,
appears so regularly and closely fringed in Freycinet’s charts, that I
have ventured to colour it red, although nothing is said about reefs in
the “Hydrographical Memoir.” The several islands which form the
northern part of the group are volcanic (with the exception perhaps of
Torres, which resembles in form the madreporitic island of Medinilla),
and appear to be without reefs.—_Mangs_, however, is described (by
Freycinet, p. 219, “Hydrog.”) from some Spanish charts, as formed of
small islands placed “au milieu des nombreux récifs;” and as these
reefs in the general chart of the group do not project so much as a
mile; and as there is no appearance from a double line, of the
existence of deep water within, I have ventured, although with much
hesitation, to colour them red. Respecting _Folger_ and _ Marshall_
Islands which lie some way east of the Marianas, I can find out
nothing, excepting that they are probably low. Krusenstern says this of
Marshall Island; and Folger Island is written with small letters in
D’Urville’s chart; uncoloured.

BONIN OR ARZOBISPO GROUP.—_Peel_ Island has been examined by Captain
Beechey, to whose kindness I am much indebted for giving me information
regarding it: “At Port Lloyd there is a great deal of coral; and the
inner harbour is entirely formed by coral-reefs, which extend outside
the port along the coast.” Captain Beechey, in another part of his
letter to me, alludes to the reefs fringing the island in all
directions; but at the same time it must be observed that the surf
washes the volcanic rocks of the coast in the greater part of its
circumference. I do not know whether the other islands of the
Archipelago are fringed; I have coloured Peel Island red.—_Grampus_
Island to the eastward, does not appear (Meare’s “Voyage,” p. 95) to
have any reefs, nor does _ Rosario_ Island (from Lutké’s chart), which
lies to the westward. Respecting the few other islands in this part of
the sea, namely the _Sulphur_ Islands, with an active volcano, and
those lying between Bonin and Japan (which are situated near the
extreme limit in latitude, at which reefs are formed), I have not been
able to find any clear account.

WEST END OF NEW GUINEA.—_Port Dory._ From the charts in the “Voyage of
the _Coquille_,” it would appear that the coast in this part
is fringed by coral-reefs; M. Lesson, however, remarks that the coral
is sickly; coloured red.—_Waigiou._ A considerable portion of the
northern shores of these islands is seen in the charts (on a large
scale) in Freycinet’s “Atlas” to be fringed by coral-reefs. Forrest (p.
21, “Voyage to New Guinea”) alludes to the coral-reefs lining the heads
of Piapis Bay; and Horsburgh (vol. ii, p. 599, 4th edit.), speaking of
the islands in Dampier Strait, says “sharp coral-rocks line their
shores;” coloured red.—In the sea north of these islands, we have
_Guedes_ (or _ Freewill_, or _St. David’s_), which from the chart given
in the 4to edit. of Carteret’s “Voyage,” must be an atoll. Krusenstern
says the islets are very low; coloured blue.—_Carteret’s Shoals_, in 2°
53′ N., are described as circular, with stony points showing all round,
with deeper water in the middle; coloured blue.—_Aiou_; the plan of
this group, given in the “Atlas of the Voyage of the _Astrolabe_,”
shows that it is an atoll; and, from a chart in Forrest’s “Voyage,” it
appears that there is twelve fathoms within the circular reef; coloured
blue.—The S.W. coast of New Guinea appears to be low, muddy, and devoid
of reefs. The _Arru, Timor-laut_, and _ Tenimber_ groups have lately
been examined by Captain Kolff, the MS. translation of which, by Mr. W.
Earl, I have been permitted to read, through the kindness of Captain
Washington, R.N. These islands are mostly rather low, and are
surrounded by distant reefs (the Ki Islands, however, are lofty, and,
from Mr. Stanley’s survey, appear without reefs); the sea in some parts
is shallow, in others profoundly deep (as near Larrat). From the
imperfection of the published charts, I have been unable to decide to
which class these reefs belong. From the distance to which they extend
from the land, where the sea is very deep, I am strongly inclined to
believe they ought to come within the barrier class, and be coloured
blue; but I have been forced to leave them uncoloured.—The
last-mentioned groups are connected with the east end of Ceram by a
chain of small islands, of which the small groups of _Ceram-laut,
Goram_ and _Keffing_ are surrounded by very extensive reefs, projecting
into deep water, which, as in the last case, I strongly suspect belong
to the barrier class; but I have not coloured them. From the south side
of Keffing, the reefs project five miles (Windsor Earl’s “Sailing
Direct. for the Arafura Sea,” p. 9).

CERAM.—In various charts which I have examined, several parts of the
coast are represented as fringed by reefs.—_Manipa_ Island, between
Ceram and Bourou, in an old MS. chart in the Admiralty, is fringed by a
very irregular reef, partly dry at low water, which I do not doubt is
of coral-formation; both islands coloured red.—_Bourou_; parts of this
island appear fringed by coral-reefs, namely, the eastern coast, as
seen in Freycinet’s chart; and _Cajeli Bay_, which is said by Horsburgh
(vol. ii, p. 630) to be lined by coral-reefs, that stretch out a little
way, and have only a few feet water on them. In several charts,
portions of the islands forming the AMBOINA GROUP are fringed by reefs;
for instance, _Noessa, Harenca_, and _Ucaster_, in Freycinet’s charts.
The above-mentioned islands have been coloured red, although the
evidence is not very satisfactory.—North of Bourou the parallel line of
the _ Xulla_ Isles extends: I have not been able to find out anything
about them, excepting
that Horsburgh (vol. ii, p. 543) says that the northern shore is
surrounded by a reef at the distance of two or three miles;
uncoloured.—_Mysol Group_; the Kanary Islands are said by Forrest
(“Voyage,” p. 130) to be divided from each other by deep straits, and
are lined with coral-rocks; coloured red.—_Guebe_, lying between
Waigiou and Gilolo, is engraved as if fringed; and it is said by
Freycinet, that all the soundings under five fathoms were on coral;
coloured red.—_Gilolo_. In a chart published by Dalrymple, the numerous
islands on the western, southern (_Batchian_ and the _Strait of
Patientia_), and eastern sides appear fringed by narrow reefs; these
reefs, I suppose, are of coral, for it is said in “Malte Brun” (vol.
xii, p. 156), “Sur les côtes (of Batchian) comme _dans les plupart_ des
iles de cet archipel, il y a de rocs de médrepores d’une beauté et
d’une variété infimies.” Forrest, also (p. 50), says Seland, near
Batchian, is a little island with reefs of coral; coloured red.—_Morty_
Island (north of Gilolo). Horsburgh (vol. ii, p. 506) says the northern
coast is lined by reefs, projecting one or two miles, and having no
soundings close to them; I have left it uncoloured, although, as in
some former cases, it ought probably to be pale blue.—_Celebes._ The
western and northern coasts appear in the charts to be bold and without
reefs. Near the extreme northern point, however, an islet in the
_Straits of Limbe_, and parts of the adjoining shore, appear to be
fringed: the east side of the bay of _Manado_, has deep water, and is
fringed by sand and coral (“_Astrol._ Voyage,” Hydrog. Part, pp.
453-4); this extreme point, therefore, I have coloured red.—Of the
islands leading from this point to Magindanao, I have not been able to
find any account, except of _ Serangani_, which appears surrounded by
narrow reefs; and Forrest (“Voyage,” p. 164) speaks of coral on its
shores; I have, therefore, coloured this island red. To the eastward of
this chain lie several islands; of which I cannot find any account,
except of _Karkalang_, which is said by Horsburgh (vol. ii, p. 504) to
be lined by a dangerous reef, projecting several miles from the
northern shore; not coloured.

ISLANDS NEAR TIMOR.—The account of the following islands is taken from
Captain D. Kolff’s “Voyage,” in 1825, translated by Mr. W. Earl, from
the Dutch.—_Lette_ has “reefs extending along shore at the distance of
half a mile from the land.”—_Moa_ has reefs on the S.W. part.—_Lakor_
has a reef lining its shore; these islands are coloured red.—Still more
eastward, _ Luan_ has, differently from the last-mentioned islands, an
extensive reef; it is steep outside, and within there is a depth of
twelve feet; from these facts, it is impossible to decide to which
class this island belongs.—_Kissa_, off the point of Timor, has its
“shore fronted by a reef, steep too on the outer side, over which small
proahs can go at the time of high water;” coloured red.—_Timor_; most
of the points, and some considerable spaces of the northern shore, are
seen in Freycinet’s chart to be fringed by coral-reefs; and mention is
made of them in the accompanying “Hydrog. Memoir;” coloured
red.—_Savu_, S.E. of Timor, appears in Flinders’ chart to be fringed;
but I have not coloured it, as I do not know that the reefs are of
coral.—_Sandalwood_ Island has, according to Horsburgh (vol. ii, p.
607), a reef on its southern shore, four miles distant from the land;
as the neighbouring sea is deep,
and generally bold, this probably is a barrier-reef, but I have not
ventured to colour it.

N.W. COAST OF AUSTRALIA.—It appears, in Captain King’s Sailing
Directions (“Narrative of Survey,” vol. ii, pp. 325-369), that there
are many extensive coral-reefs skirting, often at considerable
distances, the N.W. shores, and encompassing the small adjoining
islets. Deep water, in no instance, is represented in the charts
between these reefs and the land; and, therefore, they probably belong
to the fringing class. But as they extend far into the sea, which is
generally shallow, even in places where the land seems to be somewhat
precipitous; I have not coloured them. Houtman’s Abrolhos (lat. 28° S.
on west coast) have lately been surveyed by Captain Wickham (as
described in _Naut. Mag._ 1841, p. 511): they lie on the edge of a
steeply shelving bank, which extends about thirty miles seaward, along
the whole line of coast. The two southern reefs, or islands, enclose a
lagoon-like space of water, varying in depth from five to fifteen
fathoms, and in one spot with twenty-three fathoms. The greater part of
the island has been formed on their inland sides, by the accumulation
of fragments of coral; the seaward face consisting of nearly bare
ledges of rock. Some of the specimens, brought home by Captain Wickham,
contained fragments of marine shells, but others did not; and these
closely resembled a formation at King George’s Sound, principally due
to the action of the wind on calcareous dust, which I shall describe in
a forthcoming part. From the extreme irregularity of these reefs with
their lagoons, and from their position on a bank, the usual depth of
which is only thirty fathoms, I have not ventured to class them with
atolls, and hence have left them uncoloured.—_Rowley Shoals._ These lie
some way from the N.W. coast of Australia: according to Captain King
(“Narrative of Survey,” vol. i, p. 60), they are of coral-formation.
They rise abruptly from the sea, and Captain King had no bottom with
170 fathoms close to them. Three of them are crescent-shaped; they are
mentioned by Mr. Lyell, on the authority of Captain King, with
reference to the direction of their open sides. “A third oval reef of
the same group is entirely submerged” (“Principles of Geology,” book
iii, chap. xviii); coloured blue.—_Scott’s Reefs_, lying north of
Rowley Shoals, are briefly described by Captain Wickham (_Naut. Mag._
1841, p. 440): they appear to be of great size, of a circular form, and
“with smooth water within, forming probably a lagoon of great extent.”
There is a break on the western side, where there probably is an
entrance: the water is very deep off these reefs; coloured blue.

Proceeding westward along the great volcanic chain of the East Indian
Archipelago, _Solor Strait_ is represented in a chart published by
Dalrymple from a Dutch MS., as fringed; as are parts of _Flores_, of
_Adenara_, and of _Solor._ Horsburgh speaks of coral growing on these
shores; and therefore I have no doubt that the reefs are of coral, and
accordingly have coloured them red. We hear from Horsburgh (vol. ii, p.
602) that a coral-flat bounds the shores of _Sapy_ Bay. From the same
authority it appears (p. 610) that reefs fringe the island of _
Timor-Young_, on the N. shore of Sumbawa; and, likewise (p. 600),
that _Bally_ town in _Lombock_, is fronted by a reef, stretching along
the shore at a distance of a hundred fathoms, with channels through it
for boats; these places, therefore, have been coloured red.—_Bally_
Island. In a Dutch MS. chart on a large scale of Java, which was
brought from that island by Dr. Horsfield, who had the kindness to show
it me at the India House, its western, northern, and southern shores
appear very regularly fringed by a reef (see also Horsburgh, vol. ii,
p. 593); and as coral is found abundantly there, I have not the least
doubt that the reef is of coral, and therefore have coloured it red.

JAVA.—My information regarding the reefs of this great island is
derived from the chart just mentioned. The greater part of _Maduara_ is
represented in it as regularly fringed, and likewise portions of the
coast of Java immediately south of it. Dr. Horsfield informs me that
coral is very abundant near _Sourabaya._ The islets and parts of the N.
coast of Java, west of _Point Buang_, or _Japara_, are fringed by
reefs, said to be of coral. _Lubeck_, or _Bavian_ Islands, lying at
some distance from the shore of Java, are regularly fringed by
coral-reefs. _Carimon Java_ appears equally so, though it is not
directly said that the reefs are of coral; there is a depth between
thirty and forty fathoms round these islands. Parts of the shores of
_Sunda Strait_, where the water is from forty to eighty fathoms deep,
and the islets near _Batavia_ appear in several charts to be fringed.
In the Dutch chart the southern shore, in the narrowest part of the
island, is in two places fringed by reefs of coral. West of _
Segorrowodee_ Bay, and the extreme S.E. and E. portions are likewise
fringed by coral-reefs; all the above-mentioned places coloured red.

_Macassar Strait_; the east coast of Borneo appears, in most parts,
free from reefs, and where they occur, as on the east coast of
_Pamaroong_, the sea is very shallow; hence no part is coloured. In
_Macassar_ Strait itself, in about lat. 2° S., there are many small
islands with coral-shoals projecting far from them. There are also (old
charts by Dalrymple) numerous little flats of coral, not rising to the
surface of the water, and shelving suddenly from five fathoms to no
bottom with fifty fathoms; they do not appear to have a lagoon-like
structure. There are similar coral-shoals a little farther south; and
in lat. 4° 55′ there are two, which are engraved from modern surveys,
in a manner which might represent an annular reef with deep water
inside: Captain Moresby, however, who was formerly in this sea, doubts
this fact, so that I have left them uncoloured: at the same time I may
remark, that these two shoals make a nearer approach to the atoll-like
structure than any other within the E. Indian Archipelago. Southward of
these shoals there are other low islands and irregular coral-reefs; and
in the space of sea, north of the great volcanic chain, from Timor to
Java, we have also other islands, such as the _Postillions, Kalatoa,
Tokan-Bessees_, etc., which are chiefly low, and are surrounded by very
irregular and distant reefs. From the imperfect charts I have seen, I
have not been able to decide whether they belong to the atoll or
barrier-classes, or whether they merely fringe submarine banks, and
gently sloping land. In the Bay of _Bonin_, between the two southern
arms of Celebes, there are numerous coral-reefs;
but none of them seem to have an atoll-like structure. I have,
therefore, not coloured any of the islands in this part of the sea; I
think it, however, exceedingly probable that some of them ought to be
blue. I may add that there is a harbour on the S.E. coast of _Bouton_
which, according to an old chart, is formed by a reef, parallel to the
shore, with deep water within; and in the “Voyage of the _Coquille_,”
some neighbouring islands are represented with reefs a good way
distant, but I do not know whether with deep water within. I have not
thought the evidence sufficient to permit me to colour them.

SUMATRA.—Commencing with the west coast and outlying islands, _Engano
Island_ is represented in the published chart as surrounded by a narrow
reef, and Napier, in his “Sailing Directions,” speaks of the reef being
of coral (also Horsburgh, vol. ii, p. 115); coloured red.—_Rat Island_
(3° 51′ S.) is surrounded by reefs of coral, partly dry at low water,
(Horsburgh, vol. ii, p. 96).—_Trieste Island_ (4° 2′ S.). The shore is
represented in a chart which I saw at the India House, as fringed in
such a manner, that I feel sure the fringe consists of coral; but as
the island is so low, that the sea sometimes flows quite over it
(Dampier, “Voyage,” vol. i, p. 474), I have not coloured it.—_Pulo
Dooa_ (lat. 3°). In an old chart it is said there are chasms in the
reefs round the island, admitting boats to the watering-place, and that
the southern islet consists of a mass of sand and coral.—_Pulo Pisang_;
Horsburgh (vol. ii, p. 86) says that the rocky coral-bank, which
stretches about forty yards from the shore, is steep to all round: in a
chart, also, which I have seen, the island is represented as regularly
fringed.—_Pulo Mintao_ is lined with reefs on its west side (Horsburgh,
vol. ii, p. 107).—_Pulo Baniak_; the same authority (vol. ii, p. 105),
speaking of a part, says it is faced with coral-rocks.—_Minguin_ (3°
36′ N.). A coral-reef fronts this place, and projects into the sea
nearly a quarter of a mile (“Notices of the Indian Arch.” published at
Singapore, p. 105).—_Pulo Brassa_ (5° 46′ N.). A reef surrounds it at a
cable’s length (Horsburgh, vol. ii, p. 60). I have coloured all the
above-specified points red. I may here add, that both Horsburgh and Mr.
Moor (in the “Notices” just alluded to) frequently speak of the
numerous reefs and banks of coral on the west coast of Sumatra; but
these nowhere have the structure of a barrier-reef, and Marsden
(“History of Sumatra”) states, that where the coast is flat, the
fringing-reefs extend furthest from it. The northern and southern
points, and the greater part of the east coast, are low, and faced with
mud banks, and therefore without coral.

NICOBAR ISLANDS.—The chart represents the islands of this group as
fringed by reefs. With regard to _Great Nicobar_, Captain Moresby
informs me, that it is fringed by reefs of coral, extending between two
and three hundred yards from the shore. The _Northern Nicobars_ appear
so regularly fringed in the published charts, that I have no doubt the
reefs are of coral. This group, therefore, is coloured red.

ANDAMAN ISLANDS.—From an examination of the MS. chart, on a large
scale, of this island, by Captain Arch. Blair, in the Admiralty,
several portions of the coast appear fringed; and as Horsburgh speaks
of coral-reefs being numerous in the vicinity of these islands, I
should have
coloured them red, had not some expressions in a paper in the “Asiatic
Researches” (vol. iv, p. 402) led me to doubt the existence of reefs;
uncoloured.

The coast of _Malacca, Tenasserim_ and the coasts northward, appear in
the greater part to be low and muddy: where reefs occur, as in parts of
_Malacca Straits_, and near _ Singapore_, they are of the fringing
kind; but the water is so shoal, that I have not coloured them. In the
sea, however, between Malacca and the west coast of Borneo, where there
is a greater depth from forty to fifty fathoms, I have coloured red
some of the groups, which are regularly fringed. The northern _Natunas_
and the _Anambas_ Islands are represented in the charts on a large
scale, published in the “Atlas of the Voyage of the _ Favourite_,” as
fringed by reefs of coral, with very shoal water within
them.—_Tumbelan_ and _Bunoa_ Islands (1° N.) are represented in the
English charts as surrounded by a very regular fringe.—_St. Barbes_ (0°
15′ N.) is said by Horsburgh (vol. ii, p. 279) to be fronted by a reef,
over which boats can land only at high water.—The shore of _Borneo_ at
_Tunjong Apee_ is also fronted by a reef, extending not far from the
land (Horsburgh, vol. ii, p. 468). These places I have coloured red;
although with some hesitation, as the water is shallow. I might perhaps
have added _Pulo Leat_, in Gaspar Strait, _Lucepara_, and _Carimata_;
but as the sea is confined and shallow, and the reefs not very regular,
I have left them uncoloured.

The water shoals gradually towards the whole west coast of _ Borneo_: I
cannot make out that it has any reefs of coral. The islands, however,
off the northern extremity, and near the S.W. end of _Palawan_, are
fringed by very distant coral-reefs; thus the reefs in the case of
_Balabac_ are no less than five miles from the land; but the sea, in
the whole of this district, is so shallow, that the reefs might be
expected to extend very far from the land. I have not, therefore,
thought myself authorised to colour them. The N.E. point of Borneo,
where the water is very shoal, is connected with Magindanao by a chain
of islands called the _Sooloo Archipelago_, about which I have been
able to obtain very little information; _Pangootaran_, although ten
miles long, entirely consists of a bed of coral-rock (“Notices of E.
Indian Arch.” p. 58): I believe from Horsburgh that the island is low;
not coloured.—_Tahow Bank_, in some old charts, appears like a
submerged atoll; not coloured. Forrest (“Voyage,” p. 21) states that
one of the islands near Sooloo is surrounded by coral-rocks; but there
is no distant reef. Near the S. end of _ Basselan_, some of the islets
in the chart accompanying Forrest’s “Voyage,” appear fringed with
reefs; hence I have coloured, though unwillingly, parts of the Sooloo
group red. The sea between Sooloo and Palawan, near the shoal coast of
Borneo, is interspersed with irregular reefs and shoal patches; not
coloured: but in the northern part of this sea, there are two low
islets, _ Cagayanes_ and _Cavilli_, surrounded by extensive
coral-reefs; the breakers round the latter (Horsburgh, vol. ii, p. 513)
extend five or six miles from a sandbank, which forms the only dry
part; these breakers are steep to outside; there appears to be an
opening through them on one side, with four or five fathoms within:
from this description, I strongly suspect that Cavilli
ought to be considered an atoll; but, as I have not seen any chart of
it, on even a moderately large scale, I have not coloured it. The
islets off the northern end of _Palawan_, are in the same case as those
off the southern end, namely they are fringed by reefs, some way
distant from the shore, but the water is exceedingly shallow;
uncoloured. The western shore of Palawan will be treated of under the
head of China Sea.

PHILIPPINE ARCHIPELAGO.—A chart on a large scale of _Appoo Shoal_,
which lies near the S.E. coast of Mindoro, has been executed by Captain
D. Ross: it appears atoll-formed, but with rather an irregular outline;
its diameter is about ten miles; there are two well-defined passages
leading into the interior lagoon, which appears open; close outside the
reef all round, there is no bottom with seventy fathoms; coloured
blue.—_Mindoro_: the N.W. coast is represented in several charts, as
fringed by a reef, and _Luban Island_ is said, by Horsburgh (vol. ii,
p. 436), to be “lined by a reef.”—_Luzon_: Mr. Cuming, who has lately
investigated with so much success the Natural History of the
Philippines, informs me, that about three miles of the shore north of
Point St. Jago, is fringed by a reef; as are (Horsburgh, vol. ii, p.
437) the Three Friars off Silanguin Bay. Between Point Capones and
Playa Honda, the coast is “lined by a coral-reef, stretching out nearly
a mile in some places,” (Horsburgh); and Mr. Cuming visited some
fringing-reefs on parts of this coast, namely, near Puebla, Iba, and
Mansinglor. In the neighbourhood of Solon-solon Bay, the shore is lined
(Horsburgh, ii, p. 439) by coral-reefs, stretching out a great way:
there are also reefs about the islets off Solamague; and as I am
informed by Mr. Cuming, near St. Catalina, and a little north of it.
The same gentleman informs me there are reefs on the S.E. point of this
island in front of Samar, extending from Malalabon to Bulusan. These
appear to be the principal fringing-reefs on the coasts of Luzon; and
they have all been coloured red. Mr. Cuming informs me that none of
them have deep water within; although it appears from Horsburgh that
some few extend to a considerable distance from the shore. Within the
Philippine Archipelago, the shores of the islands do not appear to be
commonly fringed, with the exception of the S. shore of _ Masbate_, and
nearly the whole of _Bohol_; which are both coloured red. On the S.
shore of _Magindanao_, Bunwoot Island is surrounded (according to
Forrest, “Voyage,” p. 253), by a coral-reef, which in the chart appears
one of the fringing class. With respect to the eastern coasts of the
whole Archipelago, I have not been able to obtain any account.

BABUYAN ISLANDS.—Horsburgh says (vol. ii, p. 442), coral-reefs line the
shores of the harbour in Fuga; and the charts show there are other
reefs about these islands. Camiguin has its shore in parts lined by
coral-rock (Horsburgh, p. 443); about a mile off shore there is between
thirty and thirty-five fathoms. The plan of Port San Pio Quinto shows
that its shores are fringed with coral; coloured red.—BASHEE ISLANDS:
Horsburgh, speaking of the southern part of the group (vol. ii, p. 445)
says the shores of both islands are fortified by a reef, and through
some of the gaps in it, the natives can pass in their
boats in fine weather; the bottom near the land is coral-rock. From the
published charts, it is evident that several of these islands are most
regularly fringed; coloured red. The northern islands are left
uncoloured, as I have been unable to find any account of them.—FORMOSA.
The shores, especially the western one, seem chiefly composed of mud
and sand, and I cannot make out that they are anywhere lined by reefs;
except in a harbour (Horsburgh, vol. ii, p. 449) at the extreme
northern point: hence, of course, the whole of this island is left
uncoloured. The small adjoining islands are in the same case.—PATCHOW,
OR MADJIKO-SIMA GROUPS. _Patchuson_ has been described by Captain
Broughton (“Voy. to the N. Pacific,” p. 191); he says, the boats, with
some difficulty, found a passage through the coral-reefs, which extend
along the coast, nearly half a mile off it. The boats were well
sheltered within the reef; but it does not appear that the water is
deep there. Outside the reef the depth is very irregular, varying from
five to fifty fathoms; the form of the land is not very abrupt;
coloured red.—_Taypin-san_; from the description given (p. 195) by the
same author, it appears that a very irregular reef extends, to the
distance of several miles, from the southern island; but whether it
encircles a space of deep water is not evident; nor, indeed, whether
these outlying reefs are connected with those more immediately
adjoining the land; left uncoloured. I may here just add that the shore
of _Kumi_ (lying west of Patchow), has a narrow reef attached to it in
the plan of it, in La Peyrouse’s “Atlas;” but it does not appear in the
account of the voyage that it is of coral; uncoloured.—LOO CHOO. The
greater part of the coast of this moderately hilly island, is skirted
by reefs, which do not extend far from the shore, and which do not
leave a channel of deep water within them, as may be seen in the charts
accompanying Captain B. Hall’s voyage to Loo Choo (see also remarks in
Appendix, pp. xxi. and xxv.). There are, however, some ports with deep
water, formed by reefs in front of valleys, in the same manner as
happens at Mauritius. Captain Beechey, in a letter to me, compares
these reefs with those encircling the Society Islands; but there
appears to me a marked difference between them, in the less distance at
which the Loo Choo reefs lie from the land with relation to the
probable submarine inclination, and in the absence of an interior deep
water-moat or channel, parallel to the land. Hence, I have classed
these reefs with fringing-reefs, and coloured them red.—PESCADORES
(west of Formosa). Dampier (vol. i, p. 416), has compared the
appearance of the land to the southern parts of England. The islands
are interlaced with coral-reefs; but as the water is very shoal, and as
spits of sand and gravel (Horsburgh, vol. ii, p. 450) extend far out
from them, it is impossible to draw any inferences regarding the nature
of the reefs.

CHINA SEA.—Proceeding from north to south, we first meet the _Pratas
Shoal_ (lat. 20° N.) which, according to Horsburgh (vol. ii, p. 335),
is composed of coral, is of a circular form, and has a low islet on it.
The reef is on a level with the water’s edge, and when the sea runs
high, there are breakers mostly all round, “but the water within seems
pretty deep in some places; although steep-to in most parts outside,
there appear to be several parts where a ship might find anchorage
outside the breakers;” coloured blue.—The _ Paracells_ have been
accurately surveyed by Captain D. Ross, and charts on a large scale
published: but few low islets have been formed on these shoals, and
this seems to be a general circumstance in the China Sea; the sea close
outside the reefs is very deep; several of them have a lagoon-like
structure; or separate islets (_Prattle, Robert, Drummond_, etc.) are
so arranged round a moderately shallow space, as to appear as if they
had once formed one large atoll.—_Bombay Shoal_ (one of the Paracells)
has the form of an annular reef, and is “apparently deep within;” it
seems to have an entrance (Horsburgh, vol. ii, p. 332) on its west
side; it is very steep outside.—_Discovery Shoal_, also is of an oval
form, with a lagoon-like space within, and three openings leading into
it, in which there is a depth from two to twenty fathoms. Outside, at
the distance (Horsburgh, vol. ii, p. 333) of only twenty yards from the
reef, soundings could not be obtained. The Paracells are coloured
blue.—_Macclesfield Bank_: this is a coral-bank of great size, lying
east of the Paracells; some parts of the bank are level, with a sandy
bottom, but, generally, the depth is very irregular. It is intersected
by deep cuts or channels. I am not able to perceive in the published
charts (its limits, however, are not very accurately known) whether the
central part is deeper, which I suspect is the case, as in the Great
Chagos Bank, in the Indian Ocean; not coloured.—_Scarborough Shoal_:
this coral-shoal is engraved with a double row of crosses, forming a
circle, as if there was deep water within the reef: close outside there
was no bottom, with a hundred fathoms; coloured blue.—The sea off the
west coast of Palawan and the northern part of Borneo is strewed with
shoals: _Swallow Shoal_, according to Horsburgh (vol. ii, p. 431) “is
formed, _like most_ of the shoals hereabouts, of a belt of coral-rocks,
“with a basin of deep water within.”—_Half-Moon Shoal_ has a similar
structure; Captain D. Ross describes it, as a narrow belt of
coral-rock, with a basin of deep water in the centre,” and deep sea
close outside.—_Bombay Shoal_ appears (Horsburgh, vol. ii, p. 432) “to
be a basin of smooth water surrounded by breakers.” These three shoals
I have coloured blue.—The _Paraquas Shoals_ are of a circular form,
with deep gaps running through them; not coloured.—A bank gradually
shoaling to the depth of thirty fathoms, extends to a distance of about
twenty miles from the northern part of _Borneo_, and to thirty miles
from the northern part of _Palawan._ Near the land this bank appears
tolerably free from danger, but a little further out it is thickly
studded with coral-shoals, which do not generally rise quite to the
surface; some of them are very steep-to, and others have a fringe of
shoal-water round them. I should have thought that these shoals had
level surfaces, had it not been for the statement made by Horsburgh
“that most of the shoals hereabouts are formed of a belt of coral.”
But, perhaps that expression was more particularly applied to the
shoals further in the offing. If these reefs of coral have a
lagoon-like structure, they should have been coloured blue, and they
would have formed an imperfect barrier in front of Palawan and the
northern part of Borneo. But, as the water
is not very deep, these reefs may have grown up from inequalities on
the bank: I have not coloured them.—The coast of _China, Tonquin_, and
_Cochin-China_, forming the western boundary of the China Sea, appear
to be without reefs: with regard to the two last-mentioned coasts, I
speak after examining the charts on a large scale in the “Atlas of the
Voyage of the _ Favourite_.”

INDIAN OCEAN.—_South Keeling_ atoll has been specially described. Nine
miles north of it lies North Keeling, a very small atoll, surveyed by
the _ Beagle_, the lagoon of which is dry at low water.—_Christmas
Island_, lying to the east, is a high island, without, as I have been
informed by a person who passed it, any reefs at all.—CEYLON: a space
about eighty miles in length of the south-western and southern shores
of these islands has been described by Mr. Twynam (_Naut. Mag._ 1836,
pp. 365 and 518); parts of this space appear to be very regularly
fringed by coral-reefs, which extend from a quarter to half a mile from
the shore. These reefs are in places breached, and afford safe
anchorage for the small trading craft. Outside, the sea gradually
deepens; there is forty fathoms about six miles off shore: this part I
have coloured red. In the published charts of Ceylon there appear to be
fringing-reefs in several parts of the south-eastern shores, which I
have also coloured red.—At Venloos Bay the shore is likewise fringed.
North of Trincomalee there are also reefs of the same kind. The sea off
the northern part of Ceylon is exceedingly shallow; and therefore I
have not coloured the reefs which fringe portions of its shores, and
the adjoining islets, as well as the Indian promontory of _Madura._

CHAGOS, MALDIVA, AND LACCADIVE ARCHIPELAGOES.—These three great groups
which have already been often noticed, are now well-known from the
admirable surveys of Captain Moresby and Lieutenant Powell. The
published charts, which are worthy of the most attentive examination,
at once show that the _Chagos_ and _Maldiva_ groups are entirely formed
of great atolls, or lagoon-formed reefs, surmounted by islets. In the
_Laccadive_ group, this structure is less evident; the islets are low,
not exceeding the usual height of coral-formations (see Lieutenant
Wood’s account, _Geograph. Journ._, vol. vi, p. 29), and most of the
reefs are circular, as may be seen in the published charts; and within
several of them, as I am informed by Captain Moresby, there is deepish
water; these, therefore, have been coloured blue. Directly north, and
almost forming part of this group, there is a long, narrow, slightly
curved bank, rising out of the depths of the ocean, composed of sand,
shells, and decayed coral, with from twenty-three to thirty fathoms on
it. I have no doubt that it has had the same origin with the other
Laccadive banks; but as it does not deepen towards the centre I have
not coloured it. I might have referred to other authorities regarding
these three archipelagoes; but after the publication of the charts by
Captain Moresby, to whose personal kindness in giving me much
information I am exceedingly indebted, it would have been superfluous.

_Sahia de Malha_ bank consists of a series of narrow banks, with from
eight to sixteen fathoms on them; they are arranged in a semicircular
manner, round a space about forty fathoms deep, which slopes on the
S.E. quarter to unfathomable depths; they are steep-to on both sides,
but more especially on the ocean-side. Hence this bank closely
resembles in structure, and I may add from Captain Moresby’s
information in composition, the Pitt’s Bank in the Chagos group; and
the Pitt’s Bank, must, after what has been shown of the Great Chagos
Bank, be considered as a sunken, half-destroyed atoll; hence coloured
blue.—_Cargados Carajos Bank._ Its southern portion consists of a
large, curved, coral-shoal, with some low islets on its eastern edge,
and likewise some on the western side, between which there is a depth
of about twelve fathoms. Northward, a great bank extends. I cannot
(probably owing to the want of perfect charts) refer this reef and bank
to any class;—therefore not coloured.—_Ile de Sable_ is a little
island, lying west of C. Carajos, only some toises in height (“Voyage
of the _Favourite_,” vol. i, p. 130); it is surrounded by reefs; but
its structure is unintelligible to me. There are some small banks north
of it, of which I can find no clear account.—_Mauritius._ The reefs
round this island have been described in the chapter on fringing-reefs;
coloured red.—_Rodriguez._ The coral-reefs here are exceedingly
extensive; in one part they project even five miles from the shore. As
far as I can make out, there is no deep-water moat within them; and the
sea outside does not deepen very suddenly. The outline, however, of the
land appears to be (“Life of Sir J. Makintosh,” vol. ii, p. 165) hilly
and rugged. I am unable to decide whether these reefs belong to the
barrier class; as seems probable from their great extension, or to the
fringing class; uncoloured.—_Bourbon._ The greater part of the shores
of this island are without reefs; but Captain Carmichael (Hooker’s
“Bot. Misc.”) states that a portion, fifteen miles in length, on the
S.E. side, is imperfectly fringed with coral reefs: I have not thought
this sufficient to colour the island.

SEYCHELLES.—The rocky islands of primary formation, composing this
group, rise from a very extensive and tolerably level bank, having a
depth between twenty and forty fathoms. In Captain Owen’s chart, and in
that in the “Atlas of the Voyage of the _Favourite_,” it appears that
the east side of _Mahe_ and the adjoining islands of _St. Anne_ and _
Cerf_, are regularly fringed by coral-reefs. A portion of the S.E. part
of _Curieuse Island_, the N., and part of the S.W. shore of _Praslin
Island_, and the whole west side of _Digue Island_, appear fringed.
From a MS. account of these islands by Captain F. Moresby, in the
Admiralty, it appears that _ Silhouette_ is also fringed; he states
that all these islands are formed of granite and quartz, that they rise
abruptly from the sea, and that “coral-reefs have grown round them, and
project for some distance.” Dr. Allan, of Forres, who visited these
islands, informs me that there is no deep water between the reefs and
the shore. The above specified points have been coloured red. _
Amirantes Islands_: The small islands of this neighbouring group,
according to the MS. account of them by Captain F. Moresby, are
situated on an extensive bank; they consist of the debris of corals and
shells; are only about twenty feet in height, and are environed by
reefs, some attached to the shore, and some rather distant from it.—I
have taken great pains to procure plans and
information regarding the several islands lying between S.E. and S.W.
of the Amirantes, and the Seychelles; relying chiefly on Captain F.
Moresby and Dr. Allan, it appears that the greater number,
namely—_Platte, Alphonse, Coetivi, Galega, Providence, St. Pierre,
Astova, Assomption_, and _ Glorioso_, are low, formed of sand or
coral-rock, and irregularly shaped; they are situated on very extensive
banks, and are connected with great coral-reefs. Galega is said by Dr.
Allan, to be rather higher than the other islands; and St. Pierre is
described by Captain F. Moresby, as being cavernous throughout, and as
not consisting of either limestone or granite. These islands, as well
as the Amirantes, certainly are not atoll-formed, and they differ as a
group from every other group with which I am acquainted; I have not
coloured them; but probably the reefs belong to the fringing class.
Their formation is attributed, both by Dr. Allan and Captain F.
Moresby, to the action of the currents, here exceedingly violent, on
banks, which no doubt have had an independent geological origin. They
resemble in many respects some islands and banks in the West Indies,
which owe their origin to a similar agency, in conjunction with an
elevation of the entire area. In close vicinity to the several islands,
there are three others of an apparently different nature: first, _Juan
de Nova_, which appears from some plans and accounts to be an atoll;
but from others does not appear to be so; not coloured. Secondly
_Cosmoledo_; “this group consists of a ring of coral, ten leagues in
circumference, and a quarter of a mile broad in some places, enclosing
a magnificent lagoon, into which there did not appear a single opening”
(Horsburgh, vol. i, p. 151); coloured blue. Thirdly, _Aldabra_; it
consists of three islets, about twenty-five feet in height, with red
cliffs (Horsburgh, vol. i, p. 176) surrounding a very shallow basin or
lagoon. The sea is profoundly deep close to the shore. Viewing this
island in a chart, it would be thought an atoll; but the foregoing
description shows that there is something different in its nature; Dr.
Allan also states that it is cavernous, and that the coral-rock has a
vitrified appearance. Is it an upheaved atoll, or the crater of a
volcano?—uncoloured.

COMORO GROUP.—_Mayotta_, according to Horsburgh (vol. i, p. 216, 4th
ed.), is completely surrounded by a reef, which runs at the distance of
three, four, and in some places even five miles from the land; in an
old chart, published by Dalrymple, a depth in many places of thirty-six
and thirty-eight fathoms is laid down within the reef. In the same
chart, the space of open water within the reef in some parts is even
more than three miles wide: the land is bold and peaked; this island,
therefore, is encircled by a well-characterised barrier-reef, and is
coloured pale blue.—_Johanna_; Horsburgh says (vol. i, p. 217) this
island from the N.W. to the S.W. point, is bounded by a reef, at the
distance of two miles from the shore; in some parts, however, the reef
must be attached, since Lieutenant Boteler (“Narr.” vol. i, p. 161)
describes a passage through it, within which there is room only for a
few boats. Its height, as I am informed by Dr. Allan, is about 3,500
feet; it is very precipitous, and is composed of granite, greenstone,
and quartz; coloured blue.—_Mohilla_; on the S. side of this island
there is
anchorage, in from thirty to forty-five fathoms, between a reef and the
shore (Horsburgh, vol. i, p. 214); in Captain Owen’s chart of
Madagascar, this island is represented as encircled; coloured
blue.—_Great Comoro Island_ is, as I am informed by Dr. Allan, about
8,000 feet high, and apparently volcanic; it is not regularly
encircled; but reefs of various shapes and dimensions, jut out from
every headland on the W., S., and S.E. coasts, inside of which reefs
there are channels, often parallel with the shore, with deep water. On
the north-western coasts the reefs appear attached to the shores. The
land near the coast is in some places bold, but generally speaking it
is flat; Horsburgh says (vol. i, p. 214) the water is profoundly deep
close to the _shore_, from which expression I presume some parts are
without reefs. From this description I apprehend the reef belongs to
the barrier class; but I have not coloured it, as most of the charts
which I have seen, represent the reefs round it as very much less
extensive than round the other islands in the group.

MADAGASCAR.—My information is chiefly derived from the published charts
by Captain Owen, and the accounts given by him and by Lieutenant
Boteler. Commencing at the S.W. extremity of the island; towards the
northern part of the _Star Bank_ (in lat. 25° S.) the coast for ten
miles is fringed by a reef; coloured red. The shore immediately S. of
_St. Augustine’s Bay_ appears fringed; but _Tullear_ Harbour, directly
N. of it, is formed by a narrow reef ten miles long, extending parallel
to the shore, with from four to ten fathoms within it. If this reef had
been more extensive, it must have been classed as a barrier-reef; but
as the line of coast falls inwards here, a submarine bank perhaps
extends parallel to the shore, which has offered a foundation for the
growth of the coral; I have left this part uncoloured. From lat. 22°
16′ to 21° 37′, the shore is fringed by coral-reefs (see Lieutenant
Boteler’s “Narrative,” vol. ii, p. 106), less than a mile in width, and
with shallow water within. There are outlying coral-shoals in several
parts of the offing, with about ten fathoms between them and the shore,
and the depth of the sea one mile and a half seaward, is about thirty
fathoms. The part above specified is engraved on a large scale; and as
in the charts on rather a smaller scale the same fringe of reef extends
as far as lat. 33° 15′; I have coloured the whole of this part of the
coast red. The islands of _Juan de Nova_ (in lat. 17° S.) appear in the
charts on a large scale to be fringed, but I have not been able to
ascertain whether the reefs are of coral; uncoloured. The main part of
the west coast appears to be low, with outlying sandbanks, which,
Lieutenant Boteler (vol. ii, p. 106) says, “are faced on the edge of
deep water by a line of sharp-pointed coral-rocks.” Nevertheless I have
not coloured this part, as I cannot make out by the charts that the
coast itself is fringed. The headlands of _Narrenda_ and _Passandava_
Bays (14° 40′) and the islands in front of _Radama Harbour_ are
represented in the plans as regularly fringed, and have accordingly
been coloured red. With respect to the _East coast of Madagascar_, Dr.
Allan informs me in a letter, that the whole line of coast, from
_Tamatave_, in 18° 12′, to _C. Amber_, at the extreme northern point of
the island, is bordered by coral-reefs. The land is low, uneven,
and gradually rising from the coast. From Captain Owen’s charts, also,
the existence of these reefs, which evidently belong to the fringing
class, on some parts, namely N. of _British Sound_, and near _Ngoncy_,
of the above line of coast might have been inferred. Lieutenant Boteler
(vol. i, p. 155) speaks of “the reef surrounding the island of _St.
Mary’s_ at a small distance from the shore.” In a previous chapter I
have described, from the information of Dr. Allan, the manner in which
the reefs extend in N.E. lines from the headlands on this coast, thus
sometimes forming rather deep channels within them, this seems caused
by the action of the currents, and the reefs spring up from the
submarine prolongations of the sandy headlands. The above specified
portion of the coast is coloured red. The remaining S.E. portions do
not appear in any published chart to possess reefs of any kind; and the
Rev. W. Ellis, whose means of information regarding this side of
Madagascar have been extensive, informs me he believes there are none.

EAST COAST OF AFRICA.—Proceeding from the northern part, the coast
appears, for a considerable space, without reefs. My information, I may
here observe, is derived from the survey by Captain Owen, together with
his narrative; and that by Lieutenant Boteler. At _Mukdeesha_ (10° 1′
N.) there is a coral-reef extending four or five miles along the shore
(Owen’s “Narr.” vol. i, p. 357) which in the chart lies at the distance
of a quarter of a mile from the shore, and has within it from six to
ten feet water: this then is a fringing-reef, and is coloured red. From
_ Juba_, a little S. of the equator, to _Lamoo_ (in 2° 20′ S.) “the
coast and islands are formed of madrepore” (Owen’s “Narrative,” vol. i,
p. 363). The chart of this part (entitled _ Dundas Islands_), presents
an extraordinary appearance; the coast of the mainland is quite
straight and it is fronted at the average distance of two miles, by
exceedingly narrow, straight islets, fringed with reefs. Within the
chain of islets, there are extensive tidal flats and muddy bays, into
which many rivers enter; the depths of these spaces varies from one to
four fathoms—the latter depth not being common, and about twelve feet
the average. Outside the chain of islets, the sea, at the distance of a
mile, varies in depth from eight to fifteen fathoms. Lieutenant Boteler
(“Narr.,” vol. i, p. 369) describes the muddy bay of _Patta_, which
seems to resemble other parts of this coast, as fronted by small,
narrow, level islets formed of decomposing coral, the margin of which
is seldom of greater height than twelve feet, overhanging the rocky
surface from which the islets rise. Knowing that the islets are formed
of coral, it is, I think, scarcely possible to view the coast, and not
at once conclude that we here see a fringing-reef, which has been
upraised a few feet: the unusual depth of from two to four fathoms
within some of these islets, is probably due to muddy rivers having
prevented the growth of coral near the shore. There is, however, one
difficulty on this view, namely, that before the elevation took place,
which converted the reef into a chain of islets, the water must
apparently have been still deeper; on the other hand it may be supposed
that the formation of a nearly perfect barrier in front, of so large an
extent of coast, would cause the currents (especially in front of the
rivers), to deepen their muddy beds. When describing in
the chapter on fringing-reefs, those of Mauritius, I have given my
reasons for believing that the shoal spaces within reefs of this kind,
must, in many instances, have been deepened. However this may be, as
several parts of this line of coast are undoubtedly fringed by living
reefs, I have coloured it red.—_Maleenda_ (3° 20′ S.). In the plan of
the harbour, the south headland appears fringed; and in Owen’s chart on
a larger scale, the reefs are seen to extend nearly thirty miles
southward; coloured red.—_Mombas_ (4° 5′ S.). The island which forms
the harbour, “is surrounded by cliffs of madrepore, capable of being
rendered almost impregnable” (Owen’s “Narr.,” vol. i, p. 412). The
shore of the mainland N. and S. of the harbour, is most regularly
fringed by a coral-reef at a distance from half a mile to one mile and
a quarter from the land; within the reef the depth is from nine to
fifteen feet; outside the reef the depth at rather less than half a
mile is thirty fathoms. From the charts it appears that a space about
thirty-six miles in length, is here fringed; coloured red.—_Pemba_ (5°
S.) is an island of coral-formation, level, and about two hundred feet
in height (Owen’s “Narr.,” vol. i, p. 425); it is thirty-five miles
long, and is separated from the mainland by a deep sea. The outer coast
is represented in the chart as regularly fringed; coloured red. The
mainland in front of Pemba is likewise fringed; but there also appear
to be some outlying reefs with deep water between them and the shore. I
do not understand their structure, either from the charts or the
description, therefore have not coloured them.—_Zanzibar_ resembles
Pemba in most respects; its southern half on the western side and the
neighbouring islets are fringed; coloured red. On the mainland, a
little S. of Zanzibar, there are some banks parallel to the coast,
which I should have thought had been formed of coral, had it not been
said (Boteler’s “Narr.,” vol. ii, p. 39) that they were composed of
sand; not coloured.—_Latham’s Bank_ is a small island, fringed by
coral-reefs; but being only ten feet high, it has not been
coloured.—_Monfeea_ is an island of the same character as Pemba; its
outer shore is fringed, and its southern extremity is connected with
Keelwa Point on the mainland by a chain of islands fringed by reefs;
coloured red. The four last-mentioned islands resemble in many respects
some of the islands in the Red Sea, which will presently be
described.—_Keelwa._ In a plan of the shore, a space of twenty miles N.
and S. of this place is fringed by reefs, apparently of coral: these
reefs are prolonged still further southward in Owen’s general chart.
The coast in the plans of the rivers _Lindy_ and _Monghow_ (9° 59′ and
10° 7′ S.) has the same structure; coloured red.—_Querimba Islands_
(from 10° 40′ to 13° S.). A chart on a large scale is given of these
islands; they are low, and of coral-formation (Boteler’s “Narr.,” vol.
ii, p. 54); and generally have extensive reefs projecting from them
which are dry at low water, and which on the outside rise abruptly from
a deep sea: on their insides they are separated from the continent by a
channel, or rather a succession of bays, with an average depth of ten
fathoms. The small headlands on the continent also have coral-banks
attached to them; and the Querimba islands and banks are placed on the
lines of prolongation of these headlands, and are
separated from them by very shallow channels. It is evident that
whatever cause, whether the drifting of sediment or subterranean
movements, produced the headlands, likewise produced, as might have
been expected, submarine prolongations to them; and these towards their
outer extremities, have since afforded a favourable basis for the
growth of coral-reefs, and subsequently for the formation of islets. As
these reefs clearly belong to the fringing class, the Querimba islands
have been coloured red.—_Monabila_ (13° 32′ S.). In the plan of this
harbour, the headlands outside are fringed by reefs apparently of
coral; coloured red.—_Mozambique_ (150° S.) The outer part of the
island on which the city is built, and the neighbouring islands, are
fringed by coral-reefs; coloured red. From the description given in
Owen’s “Narr.” (vol. i, p. 162), the shore from _ Mozambique_ to
_Delagoa Bay_ appears to be low and sandy; many of the shoals and
islets off this line of coast are of coral-formation; but from their
small size and lowness, it is not possible, from the charts, to know
whether they are truly fringed. Hence this portion of coast is left
uncoloured, as are likewise those parts more northward, of which no
mention has been made in the foregoing pages from the want of
information.

PERSIAN GULF.—From the charts lately published on a large scale by the
East India Company, it appears that several parts, especially the
southern shores of this gulf, are fringed by coral-reefs; but as the
water is very shallow, and as there are numerous sandbanks, which are
difficult to distinguish on the chart from reefs, I have not coloured
the upper part red. Towards the mouth, however, where the water is
rather deeper, the islands of _Ormuz_ and _Larrack_, appear so
regularly fringed, that I have coloured them red. There are certainly
no atolls in the Persian Gulf. The shores of _ Immaum_, and of the
promontory forming the southern headland of the Persian Gulf, seem to
be without reefs. The whole S.W. part (except one or two small patches)
of _Arabia Felix_, and the shores of _Socotra_ appear from the charts
and memoir of Captain Haines (_Geograph. Journ._, 1839, p. 125) to be
without any reefs. I believe there are no extensive coral-reefs on any
part of the coasts of _India_, except on the low promontory of _Madura_
(as already mentioned) in front of Ceylon.

RED SEA.—My information is chiefly derived from the admirable charts
published by the East India Company in 1836, from personal
communication with Captain Moresby, one of the surveyors, and from the
excellent memoir, “Über die Natur der Corallen-Bänken des Rothen
Meeres,” by Ehrenberg. The plains immediately bordering the Red Sea
seem chiefly to consist of a sedimentary formation of the newer
tertiary period. The shore is, with the exception of a few parts,
fringed by coral-reefs. The water is generally profoundly deep close to
the shore; but this fact, which has attracted the attention of most
voyagers, seems to have no necessary connection with the presence of
reefs; for Captain Moresby particularly observed to me, that, in lat.
24° 10′ on the eastern side, there is a piece of coast, with very deep
water close to it, without any reefs, but not differing in other
respects from the usual nature of the coast-line. The most remarkable
feature in the Red Sea
is the chain of submerged banks, reefs, and islands, lying some way
from the shore, chiefly on the eastern side; the space within being
deep enough to admit a safe navigation in small vessels. The banks are
generally of an oval form, and some miles in width; but some of them
are very long in proportion to their width. Captain Moresby informs me
that any one, who had not made actual plans of them, would be apt to
think that they were much more elongated than they really are. Many of
them rise to the surface, but the greater number lie from five to
thirty fathoms beneath it, with irregular soundings on them. They
consist of sand and living coral; coral on most of them, according to
Captain Moresby, covering the greater part of their surface. They
extend parallel to the shore, and they are not unfrequently connected
in their middle parts by short transverse banks with the mainland. The
sea is generally profoundly deep quite close to them, as it is near
most parts of the coast of the mainland; but this is not universally
the case, for between lat. 15° and 17° the water deepens quite
gradually from the banks, both on the eastern and western shores,
towards the middle of the sea. Islands in many parts arise from these
banks; they are low, flat-topped, and consist of the same horizontally
stratified formation with that forming the plain-like margin of the
mainland. Some of the smaller and lower islands consist of mere sand.
Captain Moresby informs me, that small masses of rock, the remnants of
islands, are left on many banks where there is now no dry land.
Ehrenberg also asserts that most of the islets, even the lowest, have a
flat abraded basis, composed of the same tertiary formation: he
believes that as soon as the surf wears down the protuberant parts of a
bank, just beneath the level of the sea, the surface becomes protected
from further abrasion by the growth of coral, and he thus accounts for
the existence of so many banks standing on a level with the surface of
this sea. It appears that most of the islands are certainly decreasing
in size.

The form of the banks and islands is most singular in the part just
referred to, namely, from lat. 15° to 17°, where the sea deepens quite
gradually: the _Dhalac_ group, on the western coast, is surrounded by
an intricate archipelago of islets and shoals; the main island is very
irregularly shaped, and it includes a bay seven miles long, by four
across, in which no bottom was found with 252 feet: there is only one
entrance into this bay, half a mile wide, and with an island in front
of it. The submerged banks on the eastern coast, within the same
latitudes, round _ Farsan Island_, are, likewise, penetrated by many
narrow creeks of deep water; one is twelve miles long, in the form of a
hatchet, in which, close to its broad upper end, soundings were not
struck with 360 feet, and its entrance is only half a mile wide: in
another creek of the same nature, but even with a more irregular
outline, there was no bottom with 480 feet. The island of Farsan,
itself, has as singular a form as any of its surrounding banks. The
bottom of the sea round the Dhalac and Farsan Islands consists chiefly
of sand and agglutinated fragments, but, in the deep and narrow creeks,
it consists of mud; the islands themselves consist of thin,
horizontally stratified, modern tertiary
beds, containing but little broken coral,[2] their shores are fringed
by living coral-reefs.

 [2] Rüppell, “Reise in Abyssinie,” Band. i, S. 247.


From the account given by Rüppell[3] of the manner in which Dhalac has
been rent by fissures, the opposite sides of which have been unequally
elevated (in one instance to the amount of fifty feet), it seems
probable that its irregular form, as well as probably that of Farsan,
may have been partly caused by unequal elevations; but, considering the
general form of the banks, and of the deep-water creeks, together with
the composition of the land, I think their configuration is more
probably due in great part to strong currents having drifted sediment
over an uneven bottom: it is almost certain that their form cannot be
attributed to the growth of coral. Whatever may have been the precise
origin of the Dhalac and Farsan Archipelagoes, the greater number of
the banks on the eastern side of the Red Sea seem to have originated
through nearly similar means. I judge of this from their similarity in
configuration (in proof of which I may instance a bank on the east
coast in lat. 22°; and although it is true that the northern banks
generally have a less complicated outline), and from their similarity
in composition, as may be observed in their upraised portions. The
depth within the banks northward of lat. 17°, is usually greater, and
their outer sides shelve more abruptly (circumstances which seem to go
together) than in the Dhalac and Farsan Archipelagoes; but this might
easily have been caused by a difference in the action of the currents
during their formation: moreover, the greater quantity of living coral,
which, according to Captain Moresby, exists on the northern banks,
would tend to give them steeper margins.

 [3] _Ibid_., S. 245.

From this account, brief and imperfect as it is, we can see that the
great chain of banks on the eastern coast, and on the western side in
the southern portion, differ greatly from true barrier-reefs wholly
formed by the growth of coral. It is indeed the direct conclusion of
Ehrenberg (“Über die,” etc., pp. 45 and 51), that they are connected in
their origin quite secondarily with the growth of coral; and he remarks
that the islands off the coast of Norway, if worn down level with the
sea, and merely coated with living coral, would present a nearly
similar appearance. I cannot, however, avoid suspecting, from
information given me by Dr. Malcolmson and Captain Moresby, that
Ehrenberg has rather under-rated the influence of corals, in some
places at least, on the formation of the tertiary deposits of the Red
Sea.

_The west coast of the Red Sea between lat. 19° and 22°._—There are, in
this space, reefs, which, if I had known nothing of those in other
parts of the Red Sea, I should unhesitatingly have considered as
barrier-reefs; and, after deliberation, I have come to the same
conclusion. One of these reefs, in 20° 15′, is twenty miles long, less
than a mile in width (but expanding at the northern end into a disc),
slightly sinuous, and extending parallel to the mainland at the
distance of five miles from it, with very deep water within; in one
spot soundings were not obtained with 205 fathoms. Some leagues further
south, there is another linear reef, very narrow, ten miles long, with
other small portions
of reef, north and south, almost connected with it; and within this
line of reefs (as well as outside) the water is profoundly deep. There
are also some small linear and sickle-formed reefs, lying a little way
out at sea. All these reefs are covered, as I am informed by Captain
Moresby, by living corals. Here, then, we have all the characters of
reefs of the barrier class; and in some outlying reefs we have an
approach to the structure of atolls. The source of my doubts about the
classification of these reefs, arises from having observed in the
Dhalac and Farsan groups the narrowness and straightness of several
spits of sand and rock: one of these spits in the Dhalac group is
nearly fifteen miles long, only two broad, and it is bordered on each
side with deep water; so that, if worn down by the surf, and coated
with living corals, it would form a reef nearly similar to those within
the space under consideration. There is, also, in this space (lat. 21°)
a peninsula, bordered by cliffs, with its extremity worn down to the
level of the sea, and its basis fringed with reefs: in the line of
prolongation of this peninsula, there lies the island of _ Macowa_
(formed, according to Captain Moresby, of the usual tertiary deposit),
and some smaller islands, large parts of which likewise appear to have
been worn down, and are now coated with living corals. If the removal
of the strata in these several cases had been more complete, the reefs
thus formed would have nearly resembled those barrier-like ones now
under discussion. Notwithstanding these facts, I cannot persuade myself
that the many very small, isolated, and sickle-formed reefs and others,
long, nearly straight, and very narrow, with the water unfathomably
deep close round them, could possibly have been formed by corals merely
coating banks of sediment, or the abraded surfaces of irregularly
shaped islands. I feel compelled to believe that the foundations of
these reefs have subsided, and that the corals, during their upward
growth, have given to these reefs their present forms: I may remark
that the subsidence of narrow and irregularly-shaped peninsulas and
islands, such as those existing on the coasts of the Red Sea, would
afford the requisite foundations for the reefs in question.

_The west coast from lat. 22° to 24°._—This part of the coast (north of
the space coloured blue on the map) is fronted by an irregularly
shelving bank, from about ten to thirty fathoms deep; numerous little
reefs, some of which have the most singular shapes, rise from this
bank. It may be observed, respecting one of them, in lat. 23° 10′, that
if the promontory in lat. 24° were worn down to the level of the sea,
and coated with corals, a very similar and grotesquely formed reef
would be produced. Many of the reefs on this part of the coast may thus
have originated; but there are some sickle, and almost atoll-formed
reefs lying in deep water off the promontory in lat. 24°, which lead me
to suppose that all these reefs are more probably allied to the barrier
or atoll classes. I have not, however, ventured to colour this portion
of coast. _On the west coast from lat. 19° to 17°_ (south of space
coloured blue on the map), there are many low islets of very small
dimensions, not much elongated, and rising out of great depths at a
distance from the coast; these cannot be classed either with atolls, or
barrier- or fringing-reefs. I may here remark that the outlying reefs
on the west
coast, between lat. 19° and 24°, are the only ones in the Red Sea,
which approach in structure to the true atolls of the Indian and
Pacific Oceans, but they present only imperfect miniature likenesses of
them.

_Eastern coast._—I have felt the greatest doubt about colouring any
portion of this coast, north of the fringing-reefs round the Farsan
Islands in 16° 10′. There are many small outlying coral-reefs along the
whole line of coast; but as the greater number rise from banks not very
deeply submerged (the formation of which has been shown to be only
secondarily connected with the growth of coral), their origin may be
due simply to the growth of knolls of corals, from an irregular
foundation situated within a limited depth. But between lat. 18° and
20°, there are so many linear, elliptic, and extremely small reefs,
rising abruptly out of profound depths, that the same reasons, which
led me to colour blue a portion of the west coast, have induced me to
do the same in this part. There exist some small outlying reefs rising
from deep water, north of lat. 20° (the northern limit coloured blue),
on the east coast; but as they are not very numerous and scarcely any
of them linear, I have thought it right to leave them uncoloured.

In the _southern parts_ of the Red Sea, considerable spaces of the
mainland, and of some of the Dhalac islands, are skirted by reefs,
which, as I am informed by Captain Moresby, are of living coral, and
have all the characters of the fringing class. As in these latitudes,
there are no outlying linear or sickle-formed reefs, rising out of
unfathomable depths, I have coloured these parts of the coast red. On
similar grounds, I have coloured red the _northern parts of the western
coast_ (north of lat. 24° 30′), and likewise the shores of the chief
part of the _Gulf of Suez._ In the _Gulf of Acaba_, as I am informed by
Captain Moresby there are no coral-reefs, and the water is profoundly
deep.

WEST INDIES.—My information regarding the reefs of this area, is
derived from various sources, and from an examination of numerous
charts; especially of those lately executed during the survey under
Captain Owen, R.N. I lay under particular obligation to Captain Bird
Allen, R.N., one of the members of the late survey, for many personal
communications on this subject. As in the case of the Red Sea, it is
necessary to make some preliminary remarks on the submerged banks of
the West Indies, which are in some degree connected with coral-reefs,
and cause considerable doubts in their classification. That large
accumulations of sediment are in progress on the West Indian shores,
will be evident to any one who examines the charts of that sea,
especially of the portion north of a line joining Yucutan and Florida.
The area of deposition seems less intimately connected with the
debouchement of the great rivers, than with the course of the
sea-currents; as is evident from the vast extension of the banks from
the promontories of Yucutan and Mosquito.

Besides the coast-banks, there are many of various dimensions which
stand quite isolated; these closely resemble each other, they lie from
two or three to twenty or thirty fathoms under water, and are composed
of sand, sometimes firmly agglutinated, with little or no coral; their
surfaces are smooth and nearly level, shelving only to the amount of a
few fathoms, very gradually all round towards their edges, where they
plunge abruptly into the unfathomable sea. This steep inclination of
their sides, which is likewise characteristic of the coast-banks, is
very remarkable: I may give as an instance, the Misteriosa Bank, on the
edges of which the soundings change in 250 fathoms horizontal distance,
from 11 to 210 fathoms; off the northern point of the bank of Old
Providence, in 200 fathoms horizontal distance, the change is from 19
to 152 fathoms; off the Great Bahama Bank, in 160 fathoms horizontal
distance, the inclination is in many places from 10 fathoms to no
bottom with 190 fathoms. On coasts in all parts of the world, where
sediment is accumulating, something of this kind may be observed; the
banks shelve very gently far out to sea, and then terminate abruptly.
The form and composition of the banks standing in the middle parts of
the W. Indian Sea, clearly show that their origin must be chiefly
attributed to the accumulation of sediment; and the only obvious
explanation of their isolated position is the presence of a nucleus,
round which the currents have collected fine drift matter. Any one who
will compare the character of the bank surrounding the hilly island of
Old Providence, with those banks in its neighbourhood which stand
isolated, will scarcely doubt that they surround submerged mountains.
We are led to the same conclusion by examining the bank called Thunder
Knoll, which is separated from the Great Mosquito Bank by a channel
only seven miles wide, and 145 fathoms deep. There cannot be any doubt
that the Mosquito Bank has been formed by the accumulation of sediment
round the promontory of the same name; and Thunder Knoll resembles the
Mosquito Bank, in the state of its surface submerged twenty fathoms, in
the inclinations of its sides, in composition, and in every other
respect. I may observe, although the remark is here irrelevant, that
geologists should be cautious in concluding that all the outlyers of
any formation have once been connected together, for we here see that
deposits, doubtless of exactly the same nature, may be deposited with
large valley-like spaces between them.

Linear strips of coral-reefs and small knolls project from many of the
isolated, as well as coast-banks; sometimes they occur quite
irregularly placed, as on the Mosquito Bank, but more generally they
form crescents on the windward side, situated some little distance
within the outer edge of the banks:—thus on the Serranilla Bank they
form an interrupted chain which ranges between two and three miles
within the windward margin: generally they occur, as on Roncador,
Courtown, and Anegada Banks, nearer the line of deep water. Their
occurrence on the windward side is conformable to the general rule, of
the efficient kinds of corals flourishing best where most exposed; but
their position some way within the line of deep water I cannot explain,
without it be, that a depth somewhat less than that close to the outer
margin of the banks, is most favourable to their growth. Where the
corals have formed a nearly continuous rim, close to the windward edge
of a bank some fathoms submerged, the reef closely resembles an atoll;
but if the bank surrounds an island (as in the case of Old Providence),
the reef resembles an encircling barrier-reef. I should undoubtedly
have classed some of these fringed banks as imperfect atolls, or
barrier-reefs, if the sedimentary nature of their foundations had not
been evident from the presence of other neighbouring banks, of similar
forms and of similar composition, but without the crescent-like
marginal reef: in the third chapter, I observed that probably some
atoll-like reefs did exist, which had originated in the manner here
supposed.

Proofs of elevation within recent tertiary periods abound, as referred
to in the sixth chapter, over nearly the whole area of the West Indies.
Hence it is easy to understand the origin of the low land on the
coasts, where sediment is now accumulating; for instance on the
northern part of Yucutan, and on the N.E. part of Mosquito, where the
land is low, and where extensive banks appear to be in progressive
formation. Hence, also, the origin of the Great Bahama Banks, which are
bordered on their western and southern edges by very narrow, long,
singularly shaped islands, formed of sand, shells, and coral-rock, and
some of them about a hundred feet in height, is easily explained by the
elevation of banks fringed on their windward (western and southern)
sides by coral-reefs. On this view, however, we must suppose either
that the chief part of the surfaces of the great Bahama sandbanks were
all originally deeply submerged, and were brought up to their present
level by the same elevatory action, which formed the linear islands; or
that during the elevation of the banks, the superficial currents and
swell of the waves continued wearing them down and keeping them at a
nearly uniform level: the level is not quite uniform; for, in
proceeding from the N.W. end of the Bahama group towards the S.E. end,
the depth of the banks increases, and the area of land decreases, in a
very gradual and remarkable manner. The latter view, namely, that these
banks have been worn down by the currents and swell during their
elevation, seems to me the most probable one. It is, also, I believe,
applicable to many banks, situated in widely distant parts of the West
Indian Sea, which are wholly submerged; for, on any other view, we must
suppose, that the elevatory forces have acted with astonishing
uniformity.

The shores of the Gulf of Mexico, for the space of many hundred miles,
is formed by a chain of lagoons, from one to twenty miles in breadth
(“Columbian Navigator,” p. 178, etc.), containing either fresh or salt
water, and separated from the sea by linear strips of sand. Great
spaces of the shores of Southern Brazil,[4] and of the United States
from Long Island (as observed by Professor Rogers) to Florida have the
same character. Professor Rogers, in his “Report to the British
Association” (vol. iii, p. 13), speculates on the origin of these low,
sandy, linear islets; he states that the layers of which they are
composed are too homogeneous, and contain too large a proportion of
shells, to permit the common supposition of their formation being
simply due to matter thrown up, where it now lies, by the surf: he
considers these islands as upheaved bars or shoals, which were
deposited in lines where opposed currents met. It is evident that these
islands and spits of sand parallel to the coast, and separated from it
by shallow lagoons, have no necessary connection with coral-formations.
But in Southern Florida, from the accounts I have received from persons
who have resided there, the upraised islands seem to be formed of
strata, containing a good deal of coral, and they are extensively
fringed by living reefs; the channels within these islands are in some
places between two and three miles wide, and five or six fathoms deep,
though generally[5] they are less in depth than width. After having
seen how frequently banks of sediment in the West Indian Sea are
fringed by reefs, we can readily conceive that bars of sediment might
be greatly aided in their formation along a line of coast, by the
growth of corals; and such bars would, in that case, have a deceptive
resemblance with true barrier-reefs.

 [4] In the _London and Edinburgh Philosophical Journal,_ 1841, p. 257,
 I have described a singular bar of sandstone lying parallel to the
 coast off Pernambuco in Brazil, which probably is an analogous
 formation.


 [5] In the ordinary sea-charts, no lagoons appear on the coast of
 Florida, north of 26°; but Major Whiting (_Silliman’s Journal_, vol.
 xxxv, p. 54) says that many are formed by sand thrown up along the
 whole line of coast from St. Augustine’s to Jupiter Inlet.

Having now endeavoured to remove some sources of doubt in classifying
the reefs of the West Indies, I will give my authorities for colouring
such portions of the coast as I have thought myself warranted in doing.
Captain Bird Allen informs me, that most of the islands on the _Bahama
Banks_ are fringed, especially on their windward sides, with living
reefs; and hence I have coloured those, which are thus represented in
Captain Owen’s late chart, red. The same officer informs me, that the
islands along the southern part of _Florida_ are similarly fringed;
coloured red. CUBA: Proceeding along the northern coast, at the
distance of forty miles from the extreme S.E. point, the shores are
fringed by reefs, which extend westward for a space of 160 miles, with
only a few breaks. Parts of these reefs are represented in the plans of
the harbours on this coast by Captain Owen; and an excellent
description is given of them by Mr. Taylor (Loudon’s “Mag. of Nat.
Hist.,” vol. ix, p. 449); he states that they enclosed a space called
the “_baxo_,” from half to three-quarters of a mile in width, with a
sandy bottom, and a little coral. In most parts people can wade, at low
water, to the reef; but in some parts the depth is between two and
three fathoms. Close outside the reef, the depth is between six and
seven fathoms; these well-characterised fringing-reefs are coloured
red. Westward of longitude 77° 30′, on the northern side of Cuba, a
great bank commences, which extends along the coast for nearly four
degrees of longitude. In the place of its commencement, in its
structure, and in the “_cays_,” or low islands on its edge, there is a
marked correspondence (as observed by Humboldt, “Pers. Narr.,” vol.
vii, p. 88) between it and the Great Bahama and Sal Banks, which lie
directly in front. Hence one is led to attribute the same origin to
both these sets of banks; namely, the accumulation of sediment,
conjoined with an elevatory movement, and the growth of
coral on their outward edges; those parts which appear fringed by
living reefs are coloured red. Westward of these banks, there is a
portion of coast apparently without reefs, except in the harbours, the
shores of which seem in the published plans to be fringed. The
_Colorado Shoals_ (see Captain Owen’s charts), and the low land at the
western end of Cuba, correspond as closely in relative position and
structure to the banks at the extreme point of Florida, as the banks
above described on the north side of Cuba, do to the Bahamas, the depth
within the islets and reefs on the outer edge of the _Colorados_, is
generally between two and three fathoms, increasing to twelve fathoms
in the southern part, where the bank becomes nearly open, without
islets or coral-reefs; the portions which are fringed are coloured red.
The southern shore of Cuba is deeply concave, and the included space is
filled up with mud and sandbanks, low islands and coral-reefs. Between
the mountainous _Isle of Pines_ and the southern shore of Cuba, the
general depth is only between two and three fathoms; and in this part
small islands, formed of fragmentary rock and broken madrepores
(Humboldt, “Pers. Narr.,” vol. vii, pp. 51, 86 to 90, 291, 309, 320),
rise abruptly, and just reach the surface of the sea. From some
expressions used in the “Columbian Navigator” (vol. i, pt ii, p. 94),
it appears that considerable spaces along the outer coast of Southern
Cuba are bounded by cliffs of coral-rock, formed probably by the
upheaval of coral-reefs and sandbanks. The charts represent the
southern part of the Isle of Pines as fringed by reefs, which the
“Columb. Navig.” says extend some way from the coast, but have only
from nine to twelve feet water on them; these are coloured red.—I have
not been able to procure any detailed description of the large groups
of banks and “cays” further eastward on the southern side of Cuba;
within them there is a large expanse, with a muddy bottom, from eight
to twelve fathoms deep; although some parts of this line of coast are
represented in the general charts of the West Indies, as fringed, I
have not thought it prudent to colour them. The remaining portion of
the south coast of Cuba appears to be without coral-reefs.

YUCUTAN.—The N.E. part of the promontory appears in Captain Owen’s
charts to be fringed; coloured red. The eastern coast, from 20° to 18°
is fringed. South of lat. 18°, there commences the most remarkable reef
in the West Indies: it is about one hundred and thirty miles in length,
ranging in a N. and S. line, at an average distance of fifteen miles
from the coast. The islets on it are all low, as I have been informed
by Captain B. Allen; the water deepens suddenly on the outside of the
reef, but not more abruptly than off many of the sedimentary banks:
within its southern extremity (off _Honduras_) the depth is twenty-five
fathoms; but in the more northern parts, the depth soon increases to
ten fathoms, and within the northernmost part, for a space of twenty
miles, the depth is only from one to two fathoms. In most of these
respects we have the characteristics of a barrier-reef; nevertheless,
from observing, first, that the channel within the reef is a
continuation of a great irregular bay, which penetrates the mainland to
the depth of fifty miles; and secondly, that considerable spaces of
this barrier-like reef are
described in the charts (for instance, in lat. 16° 45′ and 16° 12′) as
formed of pure sand; and thirdly, from knowing that sediment is
accumulating in many parts of the West Indies in banks parallel to the
shore; I have not ventured to colour this reef as a barrier, without
further evidence that it has really been formed by the growth of
corals, and that it is not merely in parts a spit of sand, and in other
parts a worn down promontory, partially coated and fringed by reefs; I
lean, however, to the probability of its being a barrier-reef, produced
by subsidence. To add to my doubts, immediately on the outside of this
barrier-like reef, _Turneffe, Lighthouse_, and _Glover_ reefs are
situated, and these reefs have so completely the form of atolls, that
if they had occurred in the Pacific, I should not have hesitated about
colouring them blue. _Turneffe Reef_ seems almost entirely filled up
with low mud islets; and the depth within the other two reefs is only
from one to three fathoms. From this circumstance and from their
similarity in form, structure, and relative position, both to the bank
called _Northern Triangles_, on which there is an islet between seventy
and eighty feet, and to _Cozumel_ Island, the level surface of which is
likewise between seventy and eighty feet in height, I consider it more
probable that the three foregoing banks are the worn down bases of
upheaved shoals, fringed with corals, than that they are true atolls,
wholly produced by the growth of coral during subsidence; left
uncoloured.

In front of the eastern _Mosquito_ coast, there are between lat. 12°
and 16° some extensive banks (already mentioned, p. 148), with high
islands rising from their centres; and there are other banks wholly
submerged, both of which kinds of banks are bordered, near their
windward margins, by crescent-shaped coral-reefs. But it can hardly be
doubted, as was observed in the preliminary remarks, that these banks
owe their origin, like the great bank extending from the Mosquito
promontory, almost entirely to the accumulation of sediment, and not to
the growth of corals; hence I have not coloured them.

_Cayman Island:_ this island appears in the charts to be fringed; and
Captain B. Allen informs me that the reefs extend about a mile from the
shore, and have only from five to twelve feet water within them;
coloured red.—_Jamaica:_ judging from the charts, about fifteen miles
of the S.E. extremity, and about twice that length on the S.W.
extremity, and some portions on the S. side near Kingston and Port
Royal, are regularly fringed, and therefore are coloured red. From the
plans of some harbours on the N. side of Jamaica, parts of the coast
appear to be fringed; but as these are not represented in the charts of
the whole island, I have not coloured them.—_St. Domingo:_ I have not
been able to obtain sufficient information, either from plans of the
harbours, or from general charts, to enable me to colour any part of
the coast, except sixty miles from Port de Plata westward, which seems
very regularly fringed; many other parts, however, of the coast are
probably fringed, especially towards the eastern end of the
island.—_Puerto Rico:_ considerable portions of the southern, western,
and eastern coasts, and some parts of the northern coast, appear in the
charts to be fringed; coloured red.—Some miles in length of the
southern side of the Island of _St. Thomas_ is fringed; most of the
_Virgin Gorda_ Islands, as I am informed by Mr. Schomburgk, are
fringed; the shores of _Anegada_, as well as the bank on which it
stands, are likewise fringed; these islands have been coloured red. The
greater part of the southern side of _Santa Cruz_ appears in the Danish
survey to be fringed (see also Prof. Hovey’s account of this island, in
_Silliman’s Journal_, vol. xxxv, p. 74); the reefs extend along the
shore for a considerable space, and project rather more than a mile;
the depth within the reef is three fathoms; coloured red.—The _
Antilles_, as remarked by Von Buch (“Descrip. Iles Canaries,” p. 494),
may be divided into two linear groups, the western row being volcanic,
and the eastern of modern calcareous origin; my information is very
defective on the whole group. Of the eastern islands, _Barbuda_ and the
western coasts of _Antigua_ and _Mariagalante_ appear to be fringed:
this is also the case with _Barbadoes_, as I have been informed by a
resident; these islands are coloured red. On the shores of the Western
Antilles, of volcanic origin, very few coral-reefs appear to exist. The
island of _Martinique_, of which there are beautifully executed French
charts, on a very large scale, alone presents any appearance worthy of
special notice. The south-western, southern, and eastern coasts,
together forming about half the circumference of the island, are
skirted by very irregular banks, projecting generally rather less than
a mile from the shore, and lying from two to five fathoms submerged. In
front of almost every valley, they are breached by narrow, crooked,
steep-sided passages. The French engineers ascertained by boring, that
these submerged banks consisted of madreporitic rocks, which were
covered in many parts by thin layers of mud or sand. From this fact,
and especially from the structure of the narrow breaches, I think there
can be little doubt that these banks once formed living reefs, which
fringed the shores of the island, and like other reefs probably reached
the surface. From some of these submerged banks reefs of living coral
rise abruptly, either in small detached patches, or in lines parallel
to, but some way within the outer edges of the banks on which they are
based. Besides the above banks which skirt the shores of the island,
there is on the eastern side a range of linear banks, similarly
constituted, twenty miles in length, extending parallel to the coast
line, and separated from it by a space between two and four miles in
width, and from five to fifteen fathoms in depth. From this range of
detached banks, some linear reefs of living coral likewise rise
abruptly; and if they had been of greater length (for they do not front
more than a sixth part of the circumference of the island), they would
necessarily from their position have been coloured as barrier-reefs; as
the case stands they are left uncoloured. I suspect that after a small
amount of subsidence, the corals were killed by sand and mud being
deposited on them, and the reefs being thus prevented from growing
upwards, the banks of madreporitic rock were left in their present
submerged condition.

THE BERMUDA ISLANDS have been carefully described by Lieutenant Nelson,
in an excellent Memoir in the “Geological Transactions” (vol. v, part
i, p. 103). In the form of the bank or reef, on one side of which
the islands stand, there is a close general resemblance to an atoll;
but in the following respects there is a considerable
difference,—first, in the margin of the reef not forming (as I have
been informed by Mr. Chaffers, R.N.) a flat, solid surface, laid bare
at low water, and regularly bounding the internal space of shallow
water or lagoon; secondly, in the border of gradually shoaling water,
nearly a mile and a half in width, which surrounds the entire outside
of the reef (as is laid down in Captain Hurd’s chart); and thirdly, in
the size, height, and extraordinary form of the islands, which present
little resemblance to the long, narrow, simple islets, seldom exceeding
half a mile in breadth, which surmount the annular reefs of almost all
the atolls in the Indian and Pacific Oceans. Moreover, there are
evident proofs (Nelson, _ Ibid_., p. 118), that islands similar to the
existing ones, formerly extended over other parts of the reef. It
would, I believe, be difficult to find a true atoll with land exceeding
thirty feet in height; whereas, Mr. Nelson estimates the highest point
of the Bermuda Islands to be 260 feet; if, however, Mr. Nelson’s view,
that the whole of the land consists of sand drifted by the winds, and
agglutinated together, were proved correct, this difference would be
immaterial; but, from his own account (p. 118), there occur in one
place, five or six layers of red earth, interstratified with the
ordinary calcareous rock, and including stones too heavy for the wind
to have moved, without having at the same time utterly dispersed every
grain of the accompanying drifted matter. Mr. Nelson attributes the
origin of these several layers, with their embedded stones, to as many
violent catastrophes; but further investigation in such cases has
generally succeeded in explaining phenomena of this kind by ordinary
and simpler means. Finally, I may remark, that these islands have a
considerable resemblance in shape to Barbuda in the West Indies, and to
Pemba on the eastern coast of Africa, which latter island is about two
hundred feet in height, and consists of coral-rock. I believe that the
Bermuda Islands, from being fringed by living reefs, ought to have been
coloured red; but I have left them uncoloured, on account of their
general resemblance in external form to a lagoon-island or atoll.




GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.

CRITICAL INTRODUCTION.


The preparation of the series of works published under the general
title “Geology of the Voyage of the _Beagle_” occupied a great part of
Darwin’s time during the ten years that followed his return to England.
The second volume of the series, entitled “Geological Observations on
Volcanic Islands, with Brief Notices on the Geology of Australia and
the Cape of Good Hope,” made its appearance in 1844. The materials for
this volume were collected in part during the outward voyage, when the
_Beagle_ called at St. Jago in the Cape de Verde Islands, and St.
Paul’s Rocks, and at Fernando Noronha, but mainly during the homeward
cruise; then it was that the Galapagos Islands were surveyed, the Low
Archipelago passed through, and Tahiti visited; after making calls at
the Bay of Islands, in New Zealand, and also at Sydney, Hobart Town and
King George’s Sound in Australia, the _Beagle_ sailed across the Indian
Ocean to the little group of the Keeling or Cocos Islands, which Darwin
has rendered famous by his observations, and thence to Mauritius;
calling at the Cape of Good Hope on her way, the ship then proceeded
successively to St. Helena and Ascension, and revisited the Cape de
Verde Islands before finally reaching England.

Although Darwin was thus able to gratify his curiosity by visits to a
great number of very interesting volcanic districts, the voyage opened
for him with a bitter disappointment. He had been reading Humboldt’s
“Personal Narrative” during his last year’s residence in Cambridge, and
had copied out from it long passages about Teneriffe. He was actually
making inquiries as to the best means of visiting that island, when the
offer was made to him to accompany Captain Fitzroy in the _Beagle._ His
friend Henslow too, on parting with him, had given him the advice to
procure and read the recently published first volume of the
“Principles of Geology,” though he warned him against accepting the
views advocated by its author. During the time the _ Beagle_ was
beating backwards and forwards when the voyage commenced, Darwin,
although hardly ever able to leave his berth, was employing all the
opportunities which the terrible sea-sickness left him, in studying
Humboldt and Lyell. We may therefore form an idea of his feelings when,
on the ship reaching Santa Cruz, and the Peak of Teneriffe making its
appearance among the clouds, they were suddenly informed that an
outbreak of cholera would prevent any landing!

Ample compensation for this disappointment was found, however, when the
ship reached Porta Praya in St. Jago, the largest of the Cape de Verde
Islands. Here he spent three most delightful weeks, and really
commenced his work as a geologist and naturalist. Writing to his father
he says, “Geologising in a volcanic country is most delightful; besides
the interest attached to itself, it leads you into most beautiful and
retired spots. Nobody but a person fond of Natural History can imagine
the pleasure of strolling under cocoa-nuts in a thicket of bananas and
coffee-plants, and an endless number of wild flowers. And this island,
that has given me so much instruction and delight, is reckoned the most
uninteresting place that we perhaps shall touch at during our voyage.
It certainly is generally very barren, but the valleys are more
exquisitely beautiful, from the very contrast. It is utterly useless to
say anything about the scenery; it would be as profitable to explain to
a blind man colours, as to a person who has not been out of Europe, the
total dissimilarity of a tropical view. Whenever I enjoy anything, I
always look forward to writing it down, either in my log-book (which
increases in bulk), or in a letter; so you must excuse raptures, and
those raptures badly expressed. I find my collections are increasing
wonderfully, and from Rio I think I shall be obliged to send a cargo
home.”

The indelible impression made on Darwin’s mind by this first visit to a
volcanic island, is borne witness to by a remarkable passage in the
“Autobiography” written by him in 1876. “The geology of St. Jago is
very striking, yet simple; a stream of lava formerly flowed over the
bed of the sea, formed of triturated recent shells and corals, which it
has baked into a hard white rock. Since then the whole island has been
upheaved. But the line of white rock revealed to me a new and important
fact, namely that there had been afterwards subsidence round the
craters which had
since been in action, and had poured forth lava. It then first dawned
on me that I might perhaps write a book on the geology of the various
countries visited, and this made me thrill with delight. That was a
memorable hour to me, and how distinctly I can call to mind the low
cliff of lava beneath which I rested, with the sun glaring hot, a few
strange desert plants growing near and with living corals in the tidal
pools at my feet.”

Only five years before, when listening to poor Professor Jameson’s
lectures on the effete Wernerianism, which at that time did duty for
geological teaching, Darwin had found them “incredibly dull,” and he
declared that “the sole effect they produced on me was a determination
never so long as I lived to read a book on Geology, or in any way to
study the science.”

What a contrast we find in the expressions which he makes use of in
referring to Geological Science, in his letters written home from the
_Beagle_! After alluding to the delight of collecting and studying
marine animals, he exclaims, “But Geology carries the day!” Writing to
Henslow he says, “I am quite charmed with Geology, but, like the wise
animal between two bundles of hay, I do not know which to like best;
the old crystalline group of rocks, or the softer and more
fossiliferous beds.” And just as the long voyage is about to come to a
close he again writes, “I find in Geology a never-failing interest; as
it has been remarked, it creates the same grand ideas respecting this
world which Astronomy does for the Universe.” In this passage Darwin
doubtless refers to a remark of Sir John Herschel’s in his admirable
“Preliminary Discourse on the Study of Natural Philosophy,”—a book
which exercised a most remarkable and beneficial influence on the mind
of the young naturalist.

If there cannot be any doubt as to the strong predilection in Darwin’s
mind for geological studies, both during and after the memorable
voyage, there is equally little difficulty in perceiving the school of
geological thought which, in spite of the warnings of Sedgwick and
Henslow, had obtained complete ascendancy over his mind. He writes in
1876: “The very first place which I examined, namely St. Jago in the
Cape de Verde Islands, showed me clearly the wonderful superiority of
Lyell’s manner of treating Geology, compared with that of any other
author, whose works I had with me, or ever afterwards read.” And again,
“The science of Geology is enormously indebted to Lyell—more so, as I
believe, than to any other man who ever lived . . . I am proud to
remember that the first place, namely, St. Jago, in the Cape de
Verde Archipelago, in which I geologised, convinced me of the infinite
superiority of Lyell’s views over those advocated in any other work
known to me.”

The passages I have cited will serve to show the spirit in which Darwin
entered upon his geological studies, and the perusal of the following
pages will furnish abundant proofs of the enthusiasm, acumen, and
caution with which his researches were pursued.

Large collections of rocks and minerals were made by Darwin during his
researches, and sent home to Cambridge, to be kept under the care of
his faithful friend Henslow. After visiting his relations and friends,
Darwin’s first care on his return to England was to unpack and examine
these collections. He accordingly, at the end of 1836, took lodgings
for three months in Fitzwilliam Street, Cambridge, so as to be near
Henslow; and in studying and determining his geological specimens
received much valuable aid from the eminent crystallographer and
mineralogist, Professor William Hallows Miller.

The actual writing of the volume upon volcanic islands was not
commenced till 1843, when Darwin had settled in the spot which became
his home for the rest of his life—the famous house at Down, in Kent.
Writing to his friend Mr. Fox, on March 28th, 1843, he says, “I am very
slowly progressing with a volume, or rather pamphlet, on the volcanic
islands which we visited: I manage only a couple of hours per day, and
that not very regularly. It is uphill work writing books, which cost
money in publishing, and which are not read even by geologists.”

The work occupied Darwin during the whole of the year 1843, and was
issued in the spring of the following year, the actual time engaged in
preparing it being recorded in his diary as “from the summer of 1842 to
January 1844;” but the author does not appear to have been by any means
satisfied with the result when the book was finished. He wrote to
Lyell, “You have pleased me much by saying that you intend looking
through my ‘Volcanic Islands;’ it cost me eighteen months!!! and I have
heard of very few who have read it. Now I shall feel, whatever little
(and little it is) there is confirmatory of old work, or new, will work
its effect and not be lost.” To Sir Joseph Hooker he wrote, “I have
just finished a little volume on the volcanic islands which we visited.
I do not know how far you care for dry simple geology, but I hope you
will let me send you a copy.”

Every geologist knows how full of interest and suggestiveness is
this book of Darwin’s on volcanic islands. Probably the scant
satisfaction which its author seemed to find in it may be traced to the
effect of a contrast which he felt between the memory of glowing
delights he had experienced when, hammer in hand, he roamed over new
and interesting scenes, and the slow, laborious, and less congenial
task of re-writing and arranging his notes in book-form.

In 1874, in writing an account of the ancient volcanoes of the
Hebrides, I had frequent occasion to quote Mr. Darwin’s observations on
the Atlantic volcanoes, in illustration of the phenomena exhibited by
the relics of still older volcanoes in our own islands. Darwin, in
writing to his old friend Sir Charles Lyell upon the subject, says, “I
was not a little pleased to see my volcanic book quoted, for I thought
it was completely dead and forgotten.”

Two years later the original publishers of this book and of that on
South America proposed to re-issue them. Darwin at first hesitated, for
he seemed to think there could be little of abiding interest in them;
he consulted me upon the subject in one of the conversations which I
used to have with him at that time, and I strongly urged upon him the
reprint of the works. I was much gratified when he gave way upon the
point, and consented to their appearing just as originally issued. In
his preface he says, “Owing to the great progress which Geology has
made in recent times, my views on some few points may be somewhat
antiquated, but I have thought it best to leave them as they originally
appeared.”

It may be interesting to indicate, as briefly as possible, the chief
geological problem upon which the publication of Darwin’s “Volcanic
Islands” threw new and important light. The merit of the work consisted
in supplying interesting observations, which in some cases have proved
of crucial value in exploding prevalent fallacies; in calling attention
to phenomena and considerations that had been quite overlooked by
geologists, but have since exercised an important influence in moulding
geological speculation; and lastly in showing the importance which
attaches to small and seemingly insignificant causes, some of which
afford a key to the explanation of very curious geological problems.

Visiting as he did the districts in which Von Buch and others had found
what they thought to be evidence of the truth of “Elevation-craters,”
Darwin was able to show that the facts were capable of a totally
different interpretation. The views originally put forward by the old
German geologist and traveller, and almost
universally accepted by his countrymen, had met with much support from
Elie de Beaumont and Dufrenoy, the leaders of geological thought in
France. They were, however, stoutly opposed by Scrope and Lyell in this
country, and by Constant Prevost and Virlet on the other side of the
channel. Darwin, in the work before us, shows how little ground there
is for the assumption that the great ring-craters of the Atlantic
islands have originated in gigantic blisters of the earth’s surface
which, opening at the top, have given origin to the craters. Admitting
the influence of the injection of lava into the structure of the
volcanic cones, in increasing their bulk and elevation, he shows that,
in the main, the volcanoes are built up by repeated ejections causing
an accumulation of materials around the vent.

While, however, agreeing on the whole with Scrope and Lyell, as to the
explosive origin of ordinary volcanic craters, Darwin clearly saw that,
in some cases, great craters might be formed or enlarged, by the
subsidence of the floors after eruptions. The importance of this
agency, to which too little attention has been directed by geologists,
has recently been shown by Professor Dana, in his admirable work on
Kilauea and the other great volcanoes of the Hawaiian Archipelago.

The effects of subsidence at a volcanic centre in producing a downward
dip of the strata around it, was first pointed out by Darwin, as the
result of his earliest work in the Cape de Verde Islands. Striking
illustrations of the same principle have since been pointed out by M.
Robert and others in Iceland, by Mr. Heaphy in New Zealand, and by
myself in the Western Isles of Scotland.

Darwin again and again called attention to the evidence that volcanic
vents exhibit relations to one another which can only be explained by
assuming the existence of lines of fissure in the earth’s crust, along
which the lavas have made their way to the surface. But he, at the same
time, clearly saw that there was no evidence of the occurrence of great
deluges of lava along such fissures; he showed how the most remarkable
plateaux, composed of successive lava sheets, might be built up by
repeated and moderate ejections from numerous isolated vents; and he
expressly insists upon the rapidity with which the cinder-cones around
the orifices of ejection and the evidences of successive outflows of
lava would be obliterated by denudation.


One of the most striking parts of the book is that in which he deals
with the effects of denudation in producing “basal wrecks”
or worn down stumps of volcanoes. He was enabled to examine a series of
cases in which could be traced every gradation, from perfect volcanic
cones down to the solidified plugs which had consolidated in the vents
from which ejections had taken place. Darwin’s observations on these
points have been of the greatest value and assistance to all who have
essayed to study the effects of volcanic action during earlier periods
of the earth’s history. Like Lyell, he was firmly persuaded of the
continuity of geological history, and ever delighted in finding
indications, in the present order of nature, that the phenomena of the
past could be accounted for by means of causes which are still in
operation. Lyell’s last work in the field was carried on about his home
in Forfarshire, and only a few months before his death he wrote to
Darwin: “All the work which I have done has confirmed me in the belief
that the only difference between Palæozoic and recent volcanic rocks is
no more than we must allow for, by the enormous time to which the
products of the oldest volcanoes have been subjected to chemical
changes.”

Darwin was greatly impressed, as the result of his studies of volcanic
phenomena, followed by an examination of the great granite-masses of
the Andes, with the relations between the so-called Plutonic rocks and
those of undoubtedly volcanic origin. It was indeed a fortunate
circumstance, that after studying some excellent examples of recent
volcanic rocks, he proceeded to examine in South America many fine
illustrations of the older igneous rock-masses, and especially of the
most highly crystalline types of the same, and then on his way home had
opportunities of reviving the impression made upon him by the fresh and
unaltered volcanic rocks. Some of the general considerations suggested
by these observations were discussed in a paper read by him before the
Geological Society, on March 7th, 1838, under the title “On the
Connection of Certain Volcanic Phenomena, and On the Formation of
Mountain-chains, and the Effect of Continental Elevations.” The exact
bearing of these two classes of facts upon one another are more fully
discussed in his book on South American geology.

The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in
which upward movements were taking place, and he was naturally led to
contrast them with the areas in which, as he showed, the occurrence of
atolls, encircling reefs, and barrier-reefs afford indication of
subsidence. In this way he was able to
map out the oceanic areas in different zones, along which opposite
kinds of movement were taking place. His conclusions on this subject
were full of novelty and suggestiveness.

Very clearly did Darwin recognise the importance of the fact that most
of the oceanic islands appear to be of volcanic origin, though he was
careful to point out the remarkable exceptions which somewhat
invalidate the generalisation. In his “Origin of Species” he has
elaborated the idea and suggested the theory of the permanence of
ocean-basins, a suggestion which has been adopted and pushed farther by
subsequent authors, than we think its originator would have approved.
His caution and fairness of mind on this and similar speculative
questions was well-known to all who were in the habit of discussing
them with him.


Some years before the voyage of the _Beagle,_ Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the
foliated crystalline schists. It does not appear that Darwin was
acquainted with this remarkable memoir, but quite independently he
called attention to the same phenomena when he came to study some very
similar rocks which occur in the island of Ascension. Coming fresh from
the study of the great masses of crystalline schist in the South
American continent, he was struck by the circumstance that in the
undoubtedly igneous rocks of Ascension we find a similar separation of
the constituent minerals along parallel “folia.” These observations led
Darwin to the same conclusion as that arrived at some time before by
Scrope—namely that when crystallisation takes place in rock masses
under the influence of great deforming stresses, a separation and
parallel arrangement of the constituent minerals will result. This is a
process which is now fully recognised as having been a potent factor in
the production of the metamorphic rock, and has been called by more
recent writers “dynamo-metamorphism.”

In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was in
making out the true facts with regard to the rocks he studied by the
simple aid of a penknife and pocket-lens, supplemented by a few
chemical tests and the constant use of the blowpipe. Since his day, the
method of study of rocks by thin sections under the microscope has been
devised, and has become a most efficient aid in all petrographical
inquiries. During the voyage of H.M.S. _Challenger,_ many of the
islands studied by
Darwin have been revisited and their rocks collected. The results of
their study by one of the greatest masters of the science of
micropetrography—Professor Renard of Brussels—have been recently
published in one of the volumes of “Reports on the _Challenger_
Expedition.” While much that is new and valuable has been contributed
to geological science by these more recent investigations, and many
changes have been made in nomenclature and other points of detail, it
is interesting to find that all the chief facts described by Darwin and
his friend Professor Miller have stood the test of time and further
study, and remain as a monument of the acumen and accuracy in minute
observation of these pioneers in geological research.

JOHN W. JUDD.




Chapter I ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.


Rocks of the lowest series.—A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent.—Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriæ.—Ancient and
obliterated orifices of eruption of small size. Difficulty of tracing
over a bare plain recent streams of lava.—Inland hills of more ancient
volcanic rock.—Decomposed olivine in large masses. Feldspathic rocks
beneath the upper crystalline basaltic strata. Uniform structure and
form of the more ancient volcanic hills.—Form of the valleys near the
coast. Conglomerate now forming on the sea beach.


The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty
miles in length by about twelve in breadth. My observations, made
during two visits, were confined to the southern portion within the
distance of a few leagues from Porto Praya. The country, viewed from
the sea, presents a varied outline: smooth conical hills of a reddish
colour (like Red Hill in Fig. 1[1]) and others less regular,
flat-topped, and of a blackish colour (like A, B, C,) rise from
successive, step-formed plains of lava. At a distance, a chain of
mountains, many thousand feet in height, traverses the interior of the
island. There is no active volcano in St. Jago, and only one in the
group, namely at Fogo. The island since being inhabited has not
suffered from destructive earthquakes.

 [1] The outline of the coast, the position of the villages,
 streamlets, and of most of the hills in this woodcut, are copied from
 the chart made on board H.M.S. _Leven._ The square-topped hills (A, B,
 C, etc.) are put in merely by eye, to illustrate my description.

The lowest rocks exposed on the coast near Porto Praya, are highly
crystalline and compact; they appear to be of ancient, submarine,
volcanic origin; they are unconformably covered by a thin, irregular,
calcareous deposit, abounding with shells of a late tertiary period;
and this again is capped by a wide sheet of basaltic lava, which has
flowed in successive streams from the interior of the island, between
the square-topped hills marked A, B, C, etc. Still more recent streams
of lava have been erupted from the scattered cones, such as Red and
Signal Post Hills. The upper strata of the square-topped hills are
intimately related in mineralogical composition, and in other respects,
with the lowest series of the coast-rocks, with which they seem to be
continuous.

[Illustration: Part of St. Jago, one of the Cape de Verde islands.]

Part of St. Jago, one of the Cape de Verde islands.

_Mineralogical description of the rocks of the lowest series._—These
rocks possess an extremely varying character; they consist of black,
brown, and grey, compact, basaltic bases, with numerous crystals of
augite, hornblende, olivine, mica, and sometimes glassy feldspar. A
common variety is almost entirely composed of crystals of augite with
olivine. Mica, it is known, seldom occurs where augite abounds; nor
probably does the present case offer a real exception, for the mica (at
least in my best characterised specimen, in which one nodule of this
mineral is nearly half an inch in length) is as perfectly rounded as a
pebble in a conglomerate, and evidently has not been crystallised in
the base, in which it is now enclosed, but has proceeded from the
fusion of some pre-existing rock. These compact lavas alternate with
tuffs, amygdaloids, and wacke, and in some places with coarse
conglomerate. Some of the argillaceous wackes are of a dark green
colour, others, pale yellowish-green, and others nearly white; I was
surprised to find that some of the latter varieties, even where
whitest, fused into a jet black enamel, whilst some of the green
varieties afforded only a pale gray bead. Numerous dikes, consisting
chiefly of highly compact augitic rocks, and of gray amygdaloidal
varieties, intersect the strata, which have in several places been
dislocated with considerable violence, and thrown into highly inclined
positions. One line of disturbance crosses the northern end of Quail
Island (an islet in the Bay of Porto Praya), and can be followed to the
mainland. These disturbances took place before the deposition of the
recent sedimentary bed; and the
surface, also, had previously been denuded to a great extent, as is
shown by many truncated dikes.

_Description of the calcareous deposit overlying the foregoing volcanic
rocks._—This stratum is very conspicuous from its white colour, and
from the extreme regularity with which it ranges in a horizontal line
for some miles along the coast. Its average height above the sea,
measured from the upper line of junction with the superincumbent
basaltic lava, is about sixty feet; and its thickness, although varying
much from the inequalities of the underlying formation, may be
estimated at about twenty feet. It consists of quite white calcareous
matter, partly composed of organic _débris_, and partly of a substance
which may be aptly compared in appearance with mortar. Fragments of
rock and pebbles are scattered throughout this bed, often forming,
especially in the lower part, a conglomerate. Many of the fragments of
rock are whitewashed with a thin coating of calcareous matter. At Quail
Island, the calcareous deposit is replaced in its lowest part by a
soft, brown, earthy tuff, full of Turritellæ; this is covered by a bed
of pebbles, passing into sandstone, and mixed with fragments of echini,
claws of crabs, and shells; the oyster-shells still adhering to the
rock on which they grew. Numerous white balls appearing like pisolitic
concretions, from the size of a walnut to that of an apple, are
embedded in this deposit; they usually have a small pebble in their
centres. Although so like concretions, a close examination convinced me
that they were Nulliporæ, retaining their proper forms, but with their
surfaces slightly abraded: these bodies (plants as they are now
generally considered to be) exhibit under a microscope of ordinary
power, no traces of organisation in their internal structure. Mr.
George R. Sowerby has been so good as to examine the shells which I
collected: there are fourteen species in a sufficiently perfect
condition for their characters to be made out with some degree of
certainty, and four which can be referred only to their genera. Of the
fourteen shells, of which a list is given in the Appendix, eleven are
recent species; one, though undescribed, is perhaps identical with a
species which I found living in the harbour of Porto Praya; the two
remaining species are unknown, and have been described by Mr. Sowerby.
Until the shells of this Archipelago and of the neighbouring coasts are
better known, it would be rash to assert that even these two latter
shells are extinct. The number of species which certainly belong to
existing kinds, although few in number, are sufficient to show that the
deposit belongs to a late tertiary period. From its mineralogical
character, from the number and size of the embedded fragments, and from
the abundance of Patellæ, and other littoral shells, it is evident that
the whole was accumulated in a shallow sea, near an ancient coast-line.

_Effects produced by the flowing of the superincumbent basaltic lava
over the calcareous deposit._—These effects are very curious. The
calcareous matter is altered to the depth of about a foot beneath the
line of junction; and a most perfect gradation can be traced, from
loosely aggregated, small, particles of shells, corallines, and
Nulliporæ, into a rock, in which not a trace of mechanical origin can
be discovered,
even with a microscope. Where the metamorphic change has been greatest,
two varieties occur. The first is a hard, compact, white, fine-grained
rock, striped with a few parallel lines of black volcanic particles,
and resembling a sandstone, but which, upon close examination, is seen
to be crystallised throughout, with the cleavages so perfect that they
can be readily measured by the reflecting goniometer. In specimens,
where the change has been less complete, when moistened and examined
under a strong lens, the most interesting gradation can be traced, some
of the rounded particles retaining their proper forms, and others
insensibly melting into the granulo-crystalline paste. The weathered
surface of this stone, as is so frequently the case with ordinary
limestones, assumes a brick-red colour.

The second metamorphosed variety is likewise a hard rock, but without
any crystalline structure. It consists of a white, opaque, compact,
calcareous stone, thickly mottled with rounded, though regular, spots
of a soft, earthy, ochraceous substance. This earthy matter is of a
pale yellowish-brown colour, and appears to be a mixture of carbonate
of lime with iron; it effervesces with acids, is infusible, but
blackens under the blowpipe, and becomes magnetic. The rounded form of
the minute patches of earthy substance, and the steps in the progress
of their perfect formation, which can be followed in a suit of
specimens, clearly show that they are due either to some power of
aggregation in the earthy particles amongst themselves, or more
probably to a strong attraction between the atoms of the carbonate of
line, and consequently to the segregation of the earthy extraneous
matter. I was much interested by this fact, because I have often seen
quartz rocks (for instance, in the Falkland Islands, and in the lower
Silurian strata of the Stiper-stones in Shropshire), mottled in a
precisely analogous manner, with little spots of a white, earthy
substance (earthy feldspar?); and these rocks, there was good reason to
suppose, had undergone the action of heat,—a view which thus receives
confirmation. This spotted structure may possibly afford some
indication in distinguishing those formations of quartz, which owe
their present structure to igneous action, from those produced by the
agency of water alone; a source of doubt, which I should think from my
own experience, that most geologists, when examining arenaceo-quartzose
districts must have experienced.

The lowest and most scoriaceous part of the lava, in rolling over the
sedimentary deposit at the bottom of the sea, has caught up large
quantities of calcareous matter, which now forms a snow-white, highly
crystalline basis to a breccia, including small pieces of black, glossy
scoriæ. A little above this, where the lime is less abundant, and the
lava more compact, numerous little balls, composed of spicula of
calcareous spar, radiating from common centres, occupy the interstices.
In one part of Quail Island, the lime has thus been crystallised by the
heat of the superincumbent lava, where it is only thirteen feet in
thickness; nor had the lava been originally thicker, and since reduced
by degradation, as could be told from the degree of cellularity of its
surface. I have already observed that the sea must have been shallow
in which the calcareous deposit was accumulated. In this case,
therefore, the carbonic acid gas has been retained under a pressure,
insignificant compared with that (a column of water, 1,708 feet in
height) originally supposed by Sir James Hall to be requisite for this
end: but since his experiments, it has been discovered that pressure
has less to do with the retention of carbonic acid gas, than the nature
of the circumjacent atmosphere; and hence, as is stated to be the case
by Mr. Faraday,[2] masses of limestone are sometimes fused and
crystallised even in common limekilns. Carbonate of lime can be heated
to almost any degree, according to Faraday, in an atmosphere of
carbonic acid gas, without being decomposed; and Gay-Lussac found that
fragments of limestone, placed in a tube and heated to a degree, not
sufficient by itself to cause their decomposition, yet immediately
evolved their carbonic acid, when a stream of common air or steam was
passed over them: Gay-Lussac attributes this to the mechanical
displacement of the nascent carbonic acid gas. The calcareous matter
beneath the lava, and especially that forming the crystalline spicula
between the interstices of the scoriæ, although heated in an atmosphere
probably composed chiefly of steam, could not have been subjected to
the effects of a passing stream; and hence it is, perhaps, that they
have retained their carbonic acid, under a small amount of pressure.

 [2] I am much indebted to Mr. E. W. Brayley in having given me the
 following references to papers on this subject: Faraday in the
 _Edinburgh New Philosophical Journal_, vol. xv, p. 398; Gay-Lussac in
 _Annales de Chem. et Phys.,_ tome lxiii, p. 219, translated in the
 _London and Edinburgh Philosophical Magazine,_ vol. x, p. 496.

The fragments of scoriæ, embedded in the crystalline calcareous basis,
are of a jet black colour, with a glossy fracture like pitchstone.
Their surfaces, however, are coated with a layer of a reddish-orange,
translucent substance, which can easily be scratched with a knife;
hence they appear as if overlaid by a thin layer of rosin. Some of the
smaller fragments are partially changed throughout into this substance:
a change which appears quite different from ordinary decomposition. At
the Galapagos Archipelago (as will be described in a future chapter),
great beds are formed of volcanic ashes and particles of scoriæ, which
have undergone a closely similar change.

_The extent and horizontality of the calcareous stratum._—The upper
line of surface of the calcareous stratum, which is so conspicuous from
being quite white and so nearly horizontal, ranges for miles along the
coast, at the height of about sixty feet above the sea. The sheet of
basalt, by which it is capped, is on an average eighty feet in
thickness. Westward of Porto Praya beyond Red Hill, the white stratum
with the superincumbent basalt is covered up by more recent streams.
Northward of Signal Post Hill, I could follow it with my eye, trending
away for several miles along the sea cliffs. The distance thus observed
is about seven miles; but I cannot doubt from its regularity that it
extends much farther. In some ravines at right angles to the coast, it
is seen gently dipping towards the sea, probably with the same
inclination
as when deposited round the ancient shores of the island. I found only
one inland section, namely, at the base of the hill marked A, where, at
the height of some hundred feet, this bed was exposed; it here rested
on the usual compact augitic rock associated with wacke, and was
covered by the widespread sheet of modern basaltic lava. Some
exceptions occur to the horizontality of the white stratum: at Quail
Island, its upper surface is only forty feet above the level of the
sea; here also the capping of lava is only between twelve and fifteen
feet in thickness; on the other hand, at the north-east side of Porto
Praya harbour, the calcareous stratum, as well as the rock on which it
rests, attain a height above the average level: the inequality of level
in these two cases is not, as I believe, owing to unequal elevation,
but to original irregularities at the bottom of the sea. Of this fact,
at Quail Island, there was clear evidence in the calcareous deposit
being in one part of much greater than the average thickness, and in
another part being entirely absent; in this latter case, the modern
basaltic lavas rested directly on those of more ancient origin.

Fig. 2


[Illustration: Signal Post Hill]

SIGNAL POST HILL
A—Ancient volcanic rocks.  B—Calcareous stratum. C—Upper balastic lava.

Under Signal Post Hill, the white stratum dips into the sea in a
remarkable manner. This hill is conical, 450 feet in height, and
retains some traces of having had a crateriform structure; it is
composed chiefly of matter erupted posteriorly to the elevation of the
great basaltic plain, but partly of lava of apparently submarine origin
and of considerable antiquity. The surrounding plain, as well as the
eastern flank of this hill, has been worn into steep precipices,
overhanging the sea. In these precipices, the white calcareous stratum
may be seen, at the height of about seventy feet above the beach,
running for some miles both northward and southward of the hill, in a
line appearing to be perfectly horizontal; but for a space of a quarter
of a mile directly under the hill, it dips into the sea and disappears.
On the south side the dip is gradual, on the north side it is more
abrupt, as is shown in Fig. 2. As neither the calcareous stratum, nor
the superincumbent basaltic lava (as far as the latter can be
distinguished from the more modern ejections), appears to thicken as it
dips, I infer that these strata were not originally accumulated in a
trough, the centre of which afterwards became a point of eruption; but
that they have subsequently been disturbed and bent. We may suppose
either that Signal Post Hill subsided after its elevation with the
surrounding country, or that it never was uplifted to the same height
with it. This latter seems to me the most probable alternative, for
during the slow and equable elevation of this portion of the island,
the subterranean motive
power, from expending part of its force in repeatedly erupting volcanic
matter from beneath this point, would, it is likely, have less force to
uplift it. Something of the same kind seems to have occurred near Red
Hill, for when tracing upwards the naked streams of lava from near
Porto Praya towards the interior of the island, I was strongly induced
to suspect, that since the lava had flowed, the slope of the land had
been slightly modified, either by a small subsidence near Red Hill, or
by that portion of the plain having been uplifted to a less height
during the elevation of the whole area.

_The basaltic lava, superincumbent on the calcareous deposit._—This
lava is of a pale grey colour, fusing into a black enamel; its fracture
is rather earthy and concretionary; it contains olivine in small
grains. The central parts of the mass are compact, or at most
crenulated with a few minute cavities, and are often columnar. At Quail
Island this structure was assumed in a striking manner; the lava in one
part being divided into horizontal laminæ, which became in another part
split by vertical fissures into five-sided plates; and these again,
being piled on each other, insensibly became soldered together, forming
fine symmetrical columns. The lower surface of the lava is vesicular,
but sometimes only to the thickness of a few inches; the upper surface,
which is likewise vesicular, is divided into balls, frequently as much
as three feet in diameter, made up of concentric layers. The mass is
composed of more than one stream; its total thickness being, on an
average, about eighty feet: the lower portion has certainly flowed
beneath the sea, and probably likewise the upper portion. The chief
part of this lava has flowed from the central districts, between the
hills marked A, B, C, etc., in the woodcut-map. The surface of the
country, near the coast, is level and barren; towards the interior, the
land rises by successive terraces, of which four, when viewed from a
distance, could be distinctly counted.

_Volcanic eruptions subsequent to the elevation of the coastland; the
ejected matter associated with earthy lime._—These recent lavas have
proceeded from those scattered, conical, reddish-coloured hills, which
rise abruptly from the plain-country near the coast. I ascended some of
them, but will describe only one, namely, _Red Hill_, which may serve
as a type of its class, and is remarkable in some especial respects.
Its height is about six hundred feet; it is composed of bright red,
highly scoriaceous rock of a basaltic nature; on one side of its summit
there is a hollow, probably the last remnant of a crater. Several of
the other hills of this class, judging from their external forms, are
surmounted by much more perfect craters. When sailing along the coast,
it was evident that a considerable body of lava had flowed from Red
Hill, over a line of cliff about one hundred and twenty feet in height,
into the sea: this line of cliff is continuous with that forming the
coast, and bounding the plain on both sides of this hill; these
streams, therefore, were erupted, after the formation of the
coast-cliffs, from Red Hill, when it must have stood, as it now does,
above the level of the sea. This conclusion accords with the highly
scoriaceous condition of all the rock on it, appearing to be of
subaerial formation: and this is
important, as there are some beds of calcareous matter near its summit,
which might, at a hasty glance, have been mistaken for a submarine
deposit. These beds consist of white, earthy, carbonate of lime,
extremely friable so as to be crushed with the least pressure; the most
compact specimens not resisting the strength of the fingers. Some of
the masses are as white as quicklime, and appear absolutely pure; but
on examining them with a lens, minute particles of scoriæ can always be
seen, and I could find none which, when dissolved in acids, did not
leave a residue of this nature. It is, moreover, difficult to find a
particle of the lime which does not change colour under the blowpipe,
most of them even becoming glazed. The scoriaceous fragments and the
calcareous matter are associated in the most irregular manner,
sometimes in obscure beds, but more generally as a confused breccia,
the lime in some parts and the scoriæ in others being most abundant.
Sir H. De la Beche has been so kind as to have some of the purest
specimens analysed, with a view to discover, considering their volcanic
origin, whether they contained much magnesia; but only a small portion
was found, such as is present in most limestones.

Fragments of the scoriæ embedded in the calcareous mass, when broken,
exhibit many of their cells lined and partly filled with a white,
delicate, excessively fragile, moss-like, or rather conferva-like,
reticulation of carbonate of lime. These fibres, examined under a lens
of one-tenth of an inch focal distance, appear cylindrical; they are
rather above one-thousandth of an inch in diameter; they are either
simply branched, or more commonly united into an irregular mass of
network, with the meshes of very unequal sizes and of unequal numbers
of sides. Some of the fibres are thickly covered with extremely minute
spicula, occasionally aggregated into little tuffs; and hence they have
a hairy appearance. These spicula are of the same diameter throughout
their length; they are easily detached, so that the object-glass of the
microscope soon becomes scattered over with them. Within the cells of
many fragments of the scoria, the lime exhibits this fibrous structure,
but generally in a less perfect degree. These cells do not appear to be
connected with one another. There can be no doubt, as will presently be
shown, that the lime was erupted, mingled with the lava in its fluid
state, and therefore I have thought it worth while to describe minutely
this curious fibrous structure, of which I know nothing analogous. From
the earthy condition of the fibres, this structure does not appear to
be related to crystallisation.

Other fragments of the scoriaceous rock from this hill, when broken,
are often seen marked with short and irregular white streaks, which are
owing to a row of separate cells being partly, or quite, filled with
white calcareous powder. This structure immediately reminded me of the
appearance in badly kneaded dough, of balls and drawn-out streaks of
flour, which have remained unmixed with the paste; and I cannot doubt
that small masses of the lime, in the same manner remaining unmixed
with the fluid lava, have been drawn out when the whole was in motion.
I carefully examined, by trituration and solution in acids, pieces of
the scoriæ, taken from within half-an-inch of those
cells which were filled with the calcareous powder, and they did not
contain an atom of free lime. It is obvious that the lava and lime have
on a large scale been very imperfectly mingled; and where small
portions of the lime have been entangled within a piece of the viscid
lava, the cause of their now occupying, in the form of a powder or of a
fibrous reticulation, the vesicular cavities, is, I think, evidently
due to the confined gases having most readily expanded at the points
where the incoherent lime rendered the lava less adhesive.

A mile eastward of the town of Praya, there is a steep-sided gorge,
about one hundred and fifty yards in width, cutting through the
basaltic plain and underlying beds, but since filled up by a stream of
more modern lava. This lava is dark grey, and in most parts compact and
rudely columnar; but at a little distance from the coast, it includes
in an irregular manner a brecciated mass of red scoriæ mingled with a
considerable quantity of white, friable, and in some parts, nearly pure
earthy lime, like that on the summit of Red Hill. This lava, with its
entangled lime, has certainly flowed in the form of a regular stream;
and, judging from the shape of the gorge, towards which the drainage of
the country (feeble though it now be) still is directed, and from the
appearance of the bed of loose water-worn blocks with their interstices
unfilled, like those in the bed of a torrent, on which the lava rests,
we may conclude that the stream was of subaerial origin. I was unable
to trace it to its source, but, from its direction, it seemed to have
come from Signal Post Hill, distant one mile and a quarter, which, like
Red Hill, has been a point of eruption subsequent to the elevation of
the great basaltic plain. It accords with this view, that I found on
Signal Post Hill, a mass of earthy, calcareous matter of the same
nature, mingled with scoriæ. I may here observe that part of the
calcareous matter forming the horizontal sedimentary bed, especially
the finer matter with which the embedded fragments of rock are
whitewashed, has probably been derived from similar volcanic eruptions,
as well as from triturated organic remains: the underlying, ancient,
crystalline rocks, also, are associated with much carbonate of lime,
filling amygdaloidal cavities, and forming irregular masses, the nature
of which latter I was unable to understand.

Considering the abundance of earthy lime near the summit of Red Hill, a
volcanic cone six hundred feet in height, of subaerial
growth,—considering the intimate manner in which minute particles and
large masses of scoriæ are embedded in the masses of nearly pure lime,
and on the other hand, the manner in which small kernels and streaks of
the calcareous powder are included in solid pieces of the
scoriæ,—considering, also, the similar occurrence of lime and scoriæ
within a stream of lava, also supposed, with good reason, to have been
of modern subaerial origin, and to have flowed from a hill, where
earthy lime also occurs: I think, considering these facts, there can be
no doubt that the lime has been erupted, mingled with the molten lava.
I am not aware that any similar case has been described: it appears to
me an interesting one, inasmuch as most geologists must have speculated
on the probable effects of a volcanic focus, bursting through
deep-seated beds
of different mineralogical composition. The great abundance of free
silex in the trachytes of some countries (as described by Beudant in
Hungary, and by P. Scrope in the Panza Islands), perhaps solves the
inquiry with respect to deep-seated beds of quartz; and we probably
here see it answered, where the volcanic action has invaded subjacent
masses of limestone. One is naturally led to conjecture in what state
the now earthy carbonate of lime existed, when ejected with the
intensely heated lava: from the extreme cellularity of the scoriæ on
Red Hill, the pressure cannot have been great, and as most volcanic
eruptions are accompanied by the emission of large quantities of steam
and other gases, we here have the most favourable conditions, according
to the views at present entertained by chemists, for the expulsion of
the carbonic acid.[3] Has the slow re-absorption of this gas, it may be
asked, given to the lime in the cells of the lava, that peculiar
fibrous structure, like that of an efflorescing salt? Finally, I may
remark on the great contrast in appearance between this earthy lime,
which must have been heated in a free atmosphere of steam and other
gases, while the white, crystalline, calcareous spar, produced by a
single thin sheet of lava (as at Quail Island) rolling over similar
earthy lime and the _débris_ of organic remains, at the bottom of a
shallow sea.

 [3] Whilst deep beneath the surface, the carbonate of lime was, I
 presume, in a fluid state. Hutton, it is known, thought that all
 amygdaloids were produced by drops of molten limestone floating in the
 trap, like oil in water: this no doubt is erroneous, but if the matter
 forming the summit of Red Hill had been cooled under the pressure of a
 moderately deep sea, or within the walls of a dike, we should, in all
 probability, have had a trap rock associated with large masses of
 compact, crystalline, calcareous spar, which, according to the views
 entertained by many geologists, would have been wrongly attributed to
 subsequent infiltration.


_Signal Post Hill._—This hill has already been several times mentioned,
especially with reference to the remarkable manner in which the white
calcareous stratum, in other parts so horizontal (Fig. 2), dips under
it into the sea. It has a broad summit, with obscure traces of a
crateriform structure, and is composed of basaltic rocks,[4] some
compact, others highly cellular with inclined beds of loose scoriæ, of
which some are associated with earthy lime. Like Red Hill, it has been
the source of eruptions, subsequently to the elevation of the
surrounding basaltic plain; but unlike that hill, it has undergone
considerable denudation, and has been the seat of volcanic action at a
remote period, when beneath the sea. I judge of this latter
circumstance from finding on its inland flank the last remnants of
three small
points of eruption. These points are composed of glossy scoriæ,
cemented by crystalline calcareous spar, exactly like the great
submarine calcareous deposit, where the heated lava has rolled over it:
their demolished state can, I think, be explained only by the denuding
action of the waves of the sea. I was guided to the first orifice by
observing a sheet of lava, about two hundred yards square, with
steepish sides, superimposed on the basaltic plain with no adjoining
hillock, whence it could have been erupted; and the only trace of a
crater which I was able to discover, consisted of some inclined beds of
scoriæ at one of its corners. At the distance of fifty yards from a
second level-topped patch of lava, but of much smaller size, I found an
irregular circular group of masses of cemented, scoriaceous breccia,
about six feet in height, which doubtless had once formed the point of
eruption. The third orifice is now marked only by an irregular circle
of cemented scoriæ, about four yards in diameter, and rising in its
highest point scarcely three feet above the level of the plain, the
surface of which, close all round, exhibits its usual appearance: here
we have a horizontal basal section of a volcanic spiracle, which,
together with all its ejected matter, has been almost totally
obliterated.

 [4] Of these, one common variety is remarkable for being full of small
 fragments of a dark jasper-red earthy mineral, which, when examined
 carefully, shows an indistinct cleavage; the little fragments are
 elongated in form, are soft, are magnetic before and after being
 heated, and fuse with difficulty into a dull enamel. This mineral is
 evidently closely related to the oxides of iron, but I cannot
 ascertain what it exactly is. The rock containing this mineral is
 crenulated with small angular cavities, which are lined and filled
 with yellowish crystals of carbonate of lime.


The stream of lava, which fills the narrow gorge[5] eastward of the
town of Praya, judging from its course, seems, as before remarked, to
have come from Signal Post Hill, and to have flowed over the plain,
after its elevation: the same observation applies to a stream (possibly
part of the same one) capping the sea cliffs, a little eastward of the
gorge. When I endeavoured to follow these streams over the stony level
plain, which is almost destitute of soil and vegetation, I was much
surprised to find, that although composed of hard basaltic matter, and
not having been exposed to marine denudation, all distant traces of
them soon became utterly lost. But I have since observed at the
Galapagos Archipelago, that it is often impossible to follow even great
deluges of quite recent lava across older streams, except by the size
of the bushes growing on them, or by the comparative states of
glossiness of their surfaces,—characters which a short lapse of time
would be sufficient quite to obscure. I may remark, that in a level
country, with a dry climate, and with the wind blowing always in one
direction (as at the Cape de Verde Archipelago), the effects of
atmospheric degradation are probably much greater than would at first
be expected; for soil in this case accumulates only in a few protected
hollows, and being blown in one direction, it is always travelling
towards the sea in the form of the finest dust, leaving the surface of
the rocks bare, and exposed to the full effects of renewed meteoric
action.

 [5] The sides of this gorge, where the upper basaltic stratum is
 intersected, are almost perpendicular. The lava, which has since
 filled it up, is attached to these sides, almost as firmly as a dike
 is to its walls. In most cases, where a stream of lava has flowed down
 a valley, it is bounded on each side by loose scoriaceous masses.

_Inland hills of more ancient volcanic rocks._—These hills are laid
down by eye, and marked as A, B, C, etc., in Map 1. They are related in
mineralogical composition, and are probably directly
continuous with the lowest rocks exposed on the coast. These hills,
viewed from a distance, appear as if they had once formed part of an
irregular tableland, and from their corresponding structure and
composition this probably has been the case. They have flat, slightly
inclined summits, and are, on an average, about six hundred feet in
height; they present their steepest slope towards the interior of the
island, from which point they radiate outwards, and are separated from
each other by broad and deep valleys, through which the great streams
of lava, forming the coast-plains, have descended. Their inner and
steeper escarpments are ranged in an irregular curve, which rudely
follows the line of the shore, two or three miles inland from it. I
ascended a few of these hills, and from others, which I was able to
examine with a telescope, I obtained specimens, through the kindness of
Mr. Kent, the assistant-surgeon of the _Beagle_; although by these
means I am acquainted with only a part of the range, five or six miles
in length, yet I scarcely hesitate, from their uniform structure, to
affirm that they are parts of one great formation, stretching round
much of the circumference of the island.

The upper and lower strata of these hills differ greatly in
composition. The upper are basaltic, generally compact, but sometimes
scoriaceous and amygdaloidal, with associated masses of wacke: where
the basalt is compact, it is either fine-grained or very coarsely
crystallised; in the latter case it passes into an augitic rock,
containing much olivine; the olivine is either colourless, or of the
usual yellow and dull reddish shades. On some of the hills, beds of
calcareous matter, both in an earthy and in a crystalline form,
including fragments of glossy scoriæ, are associated with the basaltic
strata. These strata differ from the streams of basaltic lava forming
the coast-plains, only in being more compact, and in the crystals of
augite, and in the grains of olivine being of much greater
size;—characters which, together with the appearance of the associated
calcareous beds, induce me to believe that they are of submarine
formation.

Some considerable masses of wacke, which are associated with these
basaltic strata, and which likewise occur in the basal series on the
coast, especially at Quail Island, are curious. They consist of a pale
yellowish-green argillaceous substance, of a crumbling texture when
dry, but unctuous when moist: in its purest form, it is of a beautiful
green tint, with translucent edges, and occasionally with obscure
traces of an original cleavage. Under the blowpipe it fuses very
readily into a dark grey, and sometimes even black bead, which is
slightly magnetic. From these characters, I naturally thought that it
was one of the pale species, decomposed, of the genus augite;—a
conclusion supported by the unaltered rock being full of large separate
crystals of black augite, and of balls and irregular streaks of dark
grey augitic rock. As the basalt ordinarily consists of augite, and of
olivine often tarnished and of a dull red colour, I was led to examine
the stages of decomposition of this latter mineral, and I found, to my
surprise, that I could trace a nearly perfect gradation from unaltered
olivine to the green wacke. Part of the same grain under the blowpipe
would in some instances
behave like olivine, its colour being only slightly changed, and part
would give a black magnetic bead. Hence I can have no doubt that the
greenish wacke originally existed as olivine; but great chemical
changes must have been effected during the act of decomposition thus to
have altered a very hard, transparent, infusible mineral, into a soft,
unctuous, easily melted, argillaceous substance.[6]

 [6] D’Aubuisson “Traité de Géognosie” (tome ii, p. 569) mentions, on
 the authority of M. Marcel de Serres, masses of green earth near
 Montpellier, which are supposed to be due to the decomposition of
 olivine. I do not, however, find, that the action of this mineral
 under the blowpipe being entirely altered, as it becomes decomposed,
 has been noticed; and the knowledge of this fact is important, as at
 first it appears highly improbable that a hard, transparent,
 refractory mineral should be changed into a soft, easily fused clay,
 like this of St. Jago. I shall hereafter describe a green substance,
 forming threads within the cells of some vesicular basaltic rocks in
 Van Diemen’s Land, which behave under the blowpipe like the green
 wacke of St. Jago; but its occurrence in cylindrical threads, shows it
 cannot have resulted from the decomposition of olivine, a mineral
 always existing in the form of grains or crystals.

The basal strata of these hills, as well as some neighbouring,
separate, bare, rounded hillocks, consist of compact, fine-grained,
non-crystalline (or so slightly as scarcely to be perceptible),
ferruginous, feldspathic rocks, and generally in a state of
semi-decomposition. Their fracture is exceedingly irregular, and
splintery; yet small fragments are often very tough. They contain much
ferruginous matter, either in the form of minute grains with a metallic
lustre, or of brown hair-like threads: the rock in this latter case
assuming a pseudo-brecciated structure. These rocks sometimes contain
mica and veins of agate. Their rusty brown or yellowish colour is
partly due to the oxides of iron, but chiefly to innumerable,
microscopically minute, black specks, which, when a fragment is heated,
are easily fused, and evidently are either hornblende or augite. These
rocks, therefore, although at first appearing like baked clay or some
altered sedimentary deposit, contain all the essential ingredients of
trachyte; from which they differ only in not being harsh, and in not
containing crystals of glassy feldspar. As is so often the case with
trachytic formation, no stratification is here apparent. A person would
not readily believe that these rocks could have flowed as lava; yet at
St. Helena there are well-characterised streams (as will be described
in an ensuing chapter) of nearly similar composition. Amidst the
hillocks composed of these rocks, I found in three places, smooth
conical hills of phonolite, abounding with fine crystals of glassy
feldspar, and with needles of hornblende. These cones of phonolite, I
believe, bear the same relation to the surrounding feldspathic strata
which some masses of coarsely crystallised augitic rock, in another
part of the island, bear to the surrounding basalt, namely, that both
have been injected. The rocks of a feldspathic nature being anterior in
origin to the basaltic strata, which cap them, as well as to the
basaltic streams of the coast-plains, accords with the usual order of
succession of these two grand divisions of the volcanic series.


The strata of most of these hills in the upper part, where alone the
planes of division are distinguishable, are inclined at a small angle
from the interior of the island towards the sea-coast. The inclination
is not the same in each hill; in that marked A it is less than in B, D,
or E; in C the strata are scarcely deflected from a horizontal plane,
and in F (as far as I could judge without ascending it) they are
slightly inclined in a reverse direction, that is, inwards and towards
the centre of the island. Notwithstanding these differences of
inclination, their correspondence in external form, and in the
composition both of their upper and lower parts,—their relative
position in one curved line, with their steepest sides turned
inwards,—all seem to show that they originally formed parts of one
platform; which platform, as before remarked, probably extended round a
considerable portion of the circumference of the island. The upper
strata certainly flowed as lava, and probably beneath the sea, as
perhaps did the lower feldspathic masses: how then come these strata to
hold their present position, and whence were they erupted?

In the centre of the island[7] there are lofty mountains, but they are
separated from the steep inland flanks of these hills by a wide space
of lower country: the interior mountains, moreover, seem to have been
the source of those great streams of basaltic lava which, contracting
as they pass between the bases of the hills in question, expand into
the coast-plains. Round the shores of St. Helena there is a rudely
formed ring of basaltic rocks, and at Mauritius there are remnants of
another such a ring round part, if not round the whole, of the island;
here again the same question immediately occurs, how came these masses
to hold their present position, and whence were they erupted? The same
answer, whatever it may be, probably applies in these three cases; and
in a future chapter we shall recur to this subject.

 [7] I saw very little of the inland parts of the island. Near the
 village of St. Domingo, there are magnificent cliffs of rather
 coarsely crystallised basaltic lava. Following the little stream in
 this valley, about a mile above the village, the base of the great
 cliff was formed of a compact fine-grained basalt, conformably covered
 by a bed of pebbles. Near Fuentes, I met with pap-formed hills of the
 compact feldspathic series of rocks.

_Valleys near the coast._—These are broad, very flat, and generally
bounded by low cliff-formed sides. Portions of the basaltic plain are
sometimes nearly or quite isolated by them; of which fact, the space on
which the town of Praya stands offers an instance. The great valley
west of the town has its bottom filled up to a depth of more than
twenty feet by well-rounded pebbles, which in some parts are firmly
cemented together by white calcareous matter. There can be no doubt,
from the form of these valleys, that they were scooped out by the waves
of the sea, during that equable elevation of the land, of which the
horizontal calcareous deposit, with its existing species of marine
remains, gives evidence. Considering how well shells have been
preserved in this stratum, it is singular that I could not find even a
single small fragment of shell in the conglomerate at the bottom of the
valleys. The bed of pebbles in the valley west of the town is
intersected by a second valley joining it as a tributary, but even this
valley appears much too wide and flat-bottomed to have been formed by
the small quantity of water, which falls only during one short wet
season; for at other times of the year these valleys are absolutely
dry.

_Recent conglomerate._—On the shores of Quail Island, I found fragments
of brick, bolts of iron, pebbles, and large fragments of basalt, united
by a scanty base of impure calcareous matter into a firm conglomerate.
To show how exceedingly firm this recent conglomerate is, I may
mention, that I endeavoured with a heavy geological hammer to knock out
a thick bolt of iron, which was embedded a little above low-water mark,
but was quite unable to succeed.




Chapter II FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.


FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
rocks: their singular decomposition by steam of high temperature.
TAHITI.—Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
elevation. Structure of its more ancient mountains; similarity with St.
Jago. ST. PAUL’S ROCKS.—Not of volcanic origin. Their singular
mineralogical composition.

_Fernando Noronha._—During our short visit at this and the four
following islands, I observed very little worthy of description.
Fernando Noronha is situated in the Atlantic Ocean, in lat. 3° 50′ S.,
and 230 miles distant from the coast of South America. It consists of
several islets, together nine miles in length by three in breadth. The
whole seems to be of volcanic origin; although there is no appearance
of any crater, or of any one central eminence. The most remarkable
feature is a hill 1,000 feet high, of which the upper 400 feet consist
of a precipitous, singularly shaped pinnacle, formed of columnar
phonolite, containing numerous crystals of glassy feldspar, and a few
needles of hornblende. From the highest accessible point of this hill,
I could distinguish in different parts of the group several other
conical hills, apparently of the same nature. At St. Helena there are
similar, great, conical, protuberant masses of phonolite, nearly one
thousand feet in height, which have been formed by the injection of
fluid feldspathic lava into yielding strata. If this hill has had, as
is probable, a similar origin, denudation has been here effected on an
enormous scale. Near the base of this hill, I observed beds of white
tuff, intersected by numerous dikes, some of amygdaloidal basalt and
others of trachyte; and beds of slaty phonolite with the planes of
cleavage directed N.W. and S.E. Parts of this rock, where the crystals
were scanty, closely resembled common clay-slate, altered by the
contact of a trap-dike. The lamination of rocks, which undoubtedly have
once been fluid, appears to me a subject well deserving attention. On
the beach there were numerous
fragments of compact basalt, of which rock a distant façade of columns
seemed to be formed.

_Terceira in the Azores._—The central parts of this island consist of
irregularly rounded mountains of no great elevation, composed of
trachyte, which closely resembles in general character the trachyte of
Ascension, presently to be described. This formation is in many parts
overlaid, in the usual order of superposition, by streams of basaltic
lava, which near the coast compose nearly the whole surface. The course
which these streams have followed from their craters, can often be
followed by the eye. The town of Angra is overlooked by a crateriform
hill (Mount Brazil), entirely built of thin strata of fine-grained,
harsh, brown-coloured tuff. The upper beds are seen to overlap the
basaltic streams on which the town stands. This hill is almost
identical in structure and composition with numerous crateriformed
hills in the Galapagos Archipelago.

_Effects of steam on the trachytic rocks._—In the central part of the
island there is a spot, where steam is constantly issuing in jets from
the bottom of a small ravine-like hollow, which has no exit, and which
abuts against a range of trachytic mountains. The steam is emitted from
several irregular fissures: it is scentless, soon blackens iron, and is
of much too high temperature to be endured by the hand. The manner in
which the solid trachyte is changed on the borders of these orifices is
curious: first, the base becomes earthy, with red freckles evidently
due to the oxidation of particles of iron; then it becomes soft; and
lastly, even the crystals of glassy feldspar yield to the dissolving
agent. After the mass is converted into clay, the oxide of iron seems
to be entirely removed from some parts, which are left perfectly white,
whilst in other neighbouring parts, which are of the brightest red
colour, it seems to be deposited in greater quantity; some other masses
are marbled with two distinct colours. Portions of the white clay, now
that they are dry, cannot be distinguished by the eye from the finest
prepared chalk; and when placed between the teeth they feel equally
soft-grained; the inhabitants use this substance for white-washing
their houses. The cause of the iron being dissolved in one part, and
close by being again deposited, is obscure; but the fact has been
observed in several other places.[1] In some half-decayed specimens, I
found small, globular aggregations of yellow hyalite, resembling
gum-arabic, which no doubt had been deposited by the steam.

 [1] Spallanzani, Dolomieu, and Hoffman have described similar cases in
 the Italian volcanic islands. Dolomieu says the iron at the Panza
 Islands is redeposited in the form of veins (p. 86 “Mémoire sur les
 Isles Ponces”). These authors likewise believe that the steam deposits
 silica: it is now experimentally known that vapour of a high
 temperature is able to dissolve silica.

As there is no escape for the rain-water, which trickles down the sides
of the ravine-like hollow, whence the steam issues, it must all
percolate downwards through the fissures at its bottom. Some of the
inhabitants informed me that it was on record that flames (some
luminous appearance?) had originally proceeded from these cracks,
and that the flames had been succeeded by the steam; but I was not able
to ascertain how long this was ago, or anything certain on the subject.
When viewing the spot, I imagined that the injection of a large mass of
rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid
state, by arching the surface might have caused a wedge-shaped hollow
with cracks at the bottom, and that the rain-water percolating to the
neighbourhood of the heated mass, would during many succeeding years be
driven back in the form of steam.

_Tahiti (Otaheite)._—I visited only a part of the north-western side of
this island, and this part is entirely composed of volcanic rocks. Near
the coast there are several varieties of basalt, some abounding with
large crystals of augite and tarnished olivine, others compact and
earthy,—some slightly vesicular, and others occasionally amygdaloidal.
These rocks are generally much decomposed, and to my surprise, I found
in several sections that it was impossible to distinguish, even
approximately, the line of separation between the decayed lava and the
alternating beds of tuff. Since the specimens have become dry, it is
rather more easy to distinguish the decomposed igneous rocks from the
sedimentary tuffs. This gradation in character between rocks having
such widely different origins, may I think be explained by the yielding
under pressure of the softened sides of the vesicular cavities, which
in many volcanic rocks occupy a large proportion of their bulk. As the
vesicles generally increase in size and number in the upper parts of a
stream of lava, so would the effects of their compression increase; the
yielding, moreover, of each lower vesicle must tend to disturb all the
softened matter above it. Hence we might expect to trace a perfect
gradation from an unaltered crystalline rock to one in which all the
particles (although originally forming part of the same solid mass) had
undergone mechanical displacement; and such particles could hardly be
distinguished from others of similar composition, which had been
deposited as sediment. As lavas are sometimes laminated in their upper
parts even horizontal lines, appearing like those of aqueous
deposition, could not in all cases be relied on as a criterion of
sedimentary origin. From these considerations it is not surprising that
formerly many geologists believed in real transitions from aqueous
deposits, through wacke, into igneous traps.

In the valley of Tia-auru, the commonest rocks are basalts with much
olivine, and in some cases almost composed of large crystals of augite.
I picked up some specimens, with much glassy feldspar, approaching in
character to trachyte. There were also many large blocks of vesicular
basalt, with the cavities beautifully lined with chabasie (?), and
radiating bundles of mesotype. Some of these specimens presented a
curious appearance, owing to a number of the vesicles being half filled
up with a white, soft, earthy mesotypic mineral, which intumesced under
the blowpipe in a remarkable manner. As the upper surfaces in all the
half-filled cells are exactly parallel, it is evident that this
substance has sunk to the bottom of each cell from its weight.
Sometimes, however, it entirely fills the cells. Other cells are either
quite
filled, or lined, with small crystals, apparently of chabasie; these
crystals, also, frequently line the upper half of the cells partly
filled with the earthy mineral, as well as the upper surface of this
substance itself, in which case the two minerals appear to blend into
each other. I have never seen any other amygdaloid[2] with the cells
half filled in the manner here described; and it is difficult to
imagine the causes which determined the earthy mineral to sink from its
gravity to the bottom of the cells, and the crystalline mineral to
adhere in a coating of equal thickness round the sides of the cells.

 [2] MacCulloch, however, has described and given a plate of (“Geolog.
 Trans.” 1st series, vol. iv, p. 225) a trap rock, with cavities filled
 up horizontally with quartz and chalcedony. The upper halves of these
 cavities are often filled by layers, which follow each irregularity of
 the surface, and by little depending stalactites of the same siliceous
 substances.

The basic strata on the sides of the valley are gently inclined
seaward, and I nowhere observed any sign of disturbance; the strata are
separated from each other by thick, compact beds of conglomerate, in
which the fragments are large, some being rounded, but most angular.
From the character of these beds, from the compact and crystalline
condition of most of the lavas, and from the nature of the infiltrated
minerals, I was led to conjecture that they had originally flowed
beneath the sea. This conclusion agrees with the fact that the Rev. W.
Ellis found marine remains at a considerable height, which he believes
were interstratified with volcanic matter; as is likewise described to
be the case by Messrs. Tyerman and Bennett at Huaheine, an island in
this same archipelago. Mr. Stutchbury also discovered near the summit
of one of the loftiest mountains of Tahiti, at the height of several
thousand feet, a stratum of semi-fossil coral. None of these remains
have been specifically examined. On the coast, where masses of
coral-rock would have afforded the clearest evidence, I looked in vain
for any signs of recent elevation. For references to the above
authorities, and for more detailed reasons for not believing that
Tahiti has been recently elevated, I must refer to the “Structure and
Distribution of Coral-Reefs.”

_Mauritius._—Approaching this island on the northern or north-western
side, a curved chain of bold mountains, surmounted by rugged pinnacles,
is seen to rise from a smooth border of cultivated land, which gently
slopes down to the coast. At the first glance, one is tempted to
believe that the sea lately reached the base of these mountains, and
upon examination, this view, at least with respect to the inferior
parts of the border, is found to be perfectly correct. Several
authors[3] have described masses of upraised coral-rock round the
greater part of the circumference of the island. Between Tamarin Bay
and the Great Black River I observed, in company with Captain Lloyd,
two hillocks of coral-rock, formed in their lower part of hard
calcareous sandstone, and in their upper of great blocks, slightly
aggregated, of Astræa and Madrepora, and of fragments of basalt; they
were divided into beds dipping seaward, in one case at an angle of 8°,
and in the other at 18°; they had a water-worn appearance, and they
rose abruptly from a smooth surface, strewed with rolled débris of
organic remains, to a height of about twenty feet. The Officier du Roi,
in his most interesting tour in 1768 round the island, has described
masses of upraised coral-rocks, still retaining that moat-like
structure (see my “Coral Reefs”) which is characteristic of the living
reefs. On the coast northward of Port Louis, I found the lava concealed
for a considerable space inland by a conglomerate of corals and shells,
like those on the beach, but in parts consolidated by red ferruginous
matter. M. Bory St. Vincent has described similar calcareous beds over
nearly the whole of the plain of Pamplemousses. Near Port Louis, when
turning over some large stones, which lay in the bed of a stream at the
head of a protected creek, and at the height of some yards above the
level of spring tides, I found several shells of serpula still adhering
to their under sides.

 [3] Captain Carmichael, in Hooker’s “Bot. Misc.,” vol. ii, p. 301.
 Captain Lloyd has lately, in the “Proceedings of the Geological
 Society” (vol. iii, p. 317), described carefully some of these masses.
 In the “Voyage à l’Isle de France, par un Officier du Roi,” many
 interesting facts are given on this subject. Consult also “Voyage aux
 Quatre Isles d’Afrique, par M. Bory St. Vincent.”

The jagged mountains near Port Louis rise to a height of between two
and three thousand feet; they consist of strata of basalt, obscurely
separated from each other by firmly aggregated beds of fragmentary
matter; and they are intersected by a few vertical dikes. The basalt in
some parts abounds with large crystals of augite and olivine, and is
generally compact. The interior of the island forms a plain, raised
probably about a thousand feet above the level of the sea, and composed
of streams of lava which have flowed round and between the rugged
basaltic mountains. These more recent lavas are also basaltic, but less
compact, and some of them abound with feldspar, so that they even fuse
into a pale coloured glass. On the banks of the Great River, a section
is exposed nearly five hundred feet deep, worn through numerous thin
sheets of the lava of this series, which are separated from each other
by beds of scoriæ. They seem to have been of subaerial formation, and
to have flowed from several points of eruption on the central platform,
of which the Piton du Milieu is said to be the principal one. There are
also several volcanic cones, apparently of this modern period, round
the circumference of the island, especially at the northern end, where
they form separate islets.

The mountains composed of the more compact and crystalline basalt, form
the main skeleton of the island. M. Bailly[4] states that they all “se
développent autour d’elle comme une ceinture d’immenses remparts,
toutes affectant une pente plus ou moins enclinée vers le rivage de la
mer; tandis, au contraire, que vers le centre de l’ile elles presentent
une coupe abrupte, et souvent taillée à pic. Toutes ces montagnes sont
formées de couches parallèles inclinées du centre de l’ile vers la
mer.” These statements have been disputed, though not in detail, by M.
Quoy, in the voyage of Freycinet. As far as my limited means of
observation went, I found
them perfectly correct.[5] The mountains on the N.W. side of the
island, which I examined, namely, La Pouce, Peter Botts, Corps de
Garde, Les Mamelles, and apparently another farther southward, have
precisely the external shape and stratification described by M. Bailly.
They form about a quarter of his girdle of ramparts. Although these
mountains now stand quite detached, being separated from each other by
breaches, even several miles in width, through which deluges of lava
have flowed from the interior of the island; nevertheless, seeing their
close general similarity, one must feel convinced that they originally
formed parts of one continuous mass. Judging from the beautiful map of
the Mauritius, published by the Admiralty from a French MS., there is a
range of mountains (M. Bamboo) on the opposite side of the island,
which correspond in height, relative position, and external form, with
those just described. Whether the girdle was ever complete may well be
doubted; but from M. Bailly’s statements, and my own observations, it
may be safely concluded that mountains with precipitous inland flanks,
and composed of strata dipping outwards, once extended round a
considerable portion of the circumference of the island. The ring
appears to have been oval and of vast size; its shorter axis, measured
across from the inner sides of the mountains near Port Louis and those
near Grand Port, being no less than thirteen geographical miles in
length. M. Bailly boldly supposes that this enormous gulf, which has
since been filled up to a great extent by streams of modern lava, was
formed by the sinking in of the whole upper part of one great volcano.

 [4] “Voyage aux Terres Australes,” tome i, p. 54.


 [5] M. Lesson, in his account of this island, in the “Voyage of the
 _Coquille_,” seems to follow M. Bailly’s views.

It is singular in how many respects those portions of St. Jago and of
Mauritius which I visited agree in their geological history. At both
islands, mountains of similar external form, stratification, and (at
least in their upper beds) composition, follow in a curved chain the
coast-line. These mountains in each case appear originally to have
formed parts of one continuous mass. The basaltic strata of which they
are composed, from their compact and crystalline structure, seem, when
contrasted with the neighbouring basaltic streams of subaerial
formation, to have flowed beneath the pressure of the sea, and to have
been subsequently elevated. We may suppose that the wide breaches
between the mountains were in both cases worn by the waves, during
their gradual elevation—of which process, within recent times, there is
abundant evidence on the coast-land of both islands. At both, vast
streams of more recent basaltic lavas have flowed from the interior of
the island, round and between the ancient basaltic hills; at both,
moreover, recent cones of eruption are scattered around the
circumference of the island; but at neither have eruptions taken place
within the period of history. As remarked in the last chapter, it is
probable that these ancient basaltic mountains, which resemble (at
least in many respects) the basal and disturbed remnants of two
gigantic volcanoes, owe their present form, structure, and position, to
the action of similar causes.


_St. Paul’s Rocks._—This small island is situated in the Atlantic
Ocean, nearly one degree north of the equator, and 540 miles distant
from South America, in 29° 15′ west longitude. Its highest point is
scarcely fifty feet above the level of the sea; its outline is
irregular, and its entire circumference barely three-quarters of a
mile. This little point of rock rises abruptly out of the ocean; and,
except on its western side, soundings were not obtained, even at the
short distance of a quarter of a mile from its shore. It is not of
volcanic origin; and this circumstance, which is the most remarkable
point in its history (as will hereafter be referred to), properly ought
to exclude it from the present volume. It is composed of rocks, unlike
any which I have met with, and which I cannot characterise by any name,
and must therefore describe.

The simplest, and one of the most abundant kinds, is a very compact,
heavy, greenish-black rock, having an angular, irregular fracture, with
some points just hard enough to scratch glass, and infusible. This
variety passes into others of paler green tints, less hard, but with a
more crystalline fracture, and translucent on their edges; and these
are fusible into a green enamel. Several other varieties are chiefly
characterised by containing innumerable threads of dark-green
serpentine, and by having calcareous matter in their interstices. These
rocks have an obscure, concretionary structure, and are full of
variously coloured angular pseudo fragments. These angular pseudo
fragments consist of the first-described dark green rock, of a brown
softer kind, of serpentine, and of a yellowish harsh stone, which,
perhaps, is related to serpentine rock. There are other vesicular,
calcareo-ferruginous, soft stones. There is no distinct stratification,
but parts are imperfectly laminated; and the whole abounds with
innumerable veins, and vein-like masses, both small and large. Of these
vein-like masses, some calcareous ones, which contain minute fragments
of shells, are clearly of subsequent origin to the others.

_A glossy incrustation._—Extensive portions of these rocks are coated
by a layer of a glossy polished substance, with a pearly lustre and of
a greyish white colour; it follows all the inequalities of the surface,
to which it is firmly attached. When examined with a lens, it is found
to consist of numerous exceedingly thin layers, their aggregate
thickness being about the tenth of an inch. It is considerably harder
than calcareous spar, but can be scratched with a knife; under the
blowpipe it scales off, decrepitates, slightly blackens, emits a fetid
odour, and becomes strongly alkaline: it does not effervesce in
acids.[6] I presume this substance has been deposited by water draining
from the birds’ dung, with which the rocks are covered. At Ascension,
near a cavity in the rocks which was filled with a laminated mass of
infiltrated birds’ dung, I found some irregularly formed, stalactitical
masses of apparently the same nature. These masses, when broken, had an
earthy texture; but on their outsides, and especially at their
extremities, they were formed of a pearly substance, generally in
little globules, like the
enamel of teeth, but more translucent, and so hard as just to scratch
plate-glass. This substance slightly blackens under the blowpipe, emits
a bad smell, then becomes quite white, swelling a little, and fuses
into a dull white enamel; it does not become alkaline; nor does it
effervesce in acids. The whole mass had a collapsed appearance, as if
in the formation of the hard glossy crust the whole had shrunk much. At
the Abrolhos Islands on the coast of Brazil, where also there is much
birds’ dung, I found a great quantity of a brown, arborescent substance
adhering to some trap-rock. In its arborescent form, this substance
singularly resembles some of the branched species of Nullipora. Under
the blowpipe, it behaves like the specimens from Ascension; but it is
less hard and glossy, and the surface has not the shrunk appearance.

 [6] In my “Journal” I have described this substance; I then believed
 that it was an impure phosphate of lime.




Chapter III ASCENSION.


Basaltic lavas.—Numerous craters truncated on the same side.—Singular
structure of volcanic bombs.—Aeriform explosions.—Ejected granitic
fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
formation.—Concretions in pumiceous tuff.—Calcareous deposits and
frondescent incrustations on the coast.—Remarkable laminated beds,
alternating with, and passing into, obsidian.—Origin of
obsidian.—Lamination of volcanic rocks.


This island is situated in the Atlantic Ocean, in lat. 8° S., long. 14°
W. It has the form of an irregular triangle (see map below), each side
being about six miles in length. Its highest point is 2,870 feet[1]
above the level of the sea. The whole is volcanic, and, from the
absence of proofs to the contrary, I believe of subaerial origin. The
fundamental rock is everywhere of a pale colour, generally compact, and
of a feldspathic nature. In the S.E. portion of the island, where the
highest land is situated, well characterised trachyte, and other
congenerous rocks of that varying here and there a hill or single point
of rock (one of which near the sea-coast, north of the Fort, is only
two or three yards across) of the trachyte still remaining exposed.

 [1] _Geographical Journal_, vol. v, p. 243.


Illustration: Island of Ascension PLATE IV.

_Basaltic rocks._—The overlying basaltic lava is in some parts
extremely vesicular, in others little so; it is of a black colour, but
sometimes contains crystals of glassy feldspar, and seldom much
olivine. These streams appear to have possessed singularly little
fluidity; their side walls and lower ends being very steep, and even as
much as between twenty and thirty feet in height. Their surface is
extraordinarily rugged,
and from a short distance appears as if studded with small craters.
These projections consist of broad, irregularly conical, hillocks,
traversed by fissures, and composed of the same unequally scoriaceous
basalt with the surrounding streams, but having an obscure tendency to
a columnar structure; they rise to a height between ten and thirty feet
above the general surface, and have been formed, as I presume, by the
heaping up of the viscid lava at points of greater resistance. At the
base of several of these hillocks, and occasionally likewise on more
level parts, solid ribs, composed of angulo-globular masses of basalt,
resembling in size and outline arched sewers or gutters of brickwork,
but not being hollow, project between two or three feet above the
surface of the streams; what their origin may have been, I do not know.
Many of the superficial fragments from these basaltic streams present
singularly convoluted forms; and some specimens could hardly be
distinguished from logs of dark-coloured wood without their bark.

Many of the basaltic streams can be traced, either to points of
eruption at the base of the great central mass of trachyte, or to
separate, conical, red-coloured hills, which are scattered over the
northern and western borders of the island. Standing on the central
eminence, I counted between twenty and thirty of these cones of
eruption. The greater number of them had their truncated summits cut
off obliquely, and they all sloped towards the S.E., whence the
trade-wind blows.[2] This structure no doubt has been caused by the
ejected fragments and ashes being always blown, during eruptions, in
greater quantity towards one side than towards the other. M. Moreau de
Jonnes has made a similar observation with respect to the volcanic
orifices in the West Indian Islands.

 [2] M. Lesson in the “Zoology of the Voyage of the _Coquille_,” p. 490
 has observed this fact. Mr. Hennah (“Geolog. Proceedings,” 1835, p.
 189) further remarks that the most extensive beds of ashes at
 Ascension invariably occur on the leeward side of the island.


_Volcanic bombs._—These occur in great numbers strewed on the ground,
and some of them lie at considerable distances from any points of
eruption. They vary in size from that of an apple to that of a man’s
body; they are either spherical or pear-shaped, or with the hinder part
(corresponding to the tail of a comet) irregular, studded with
projecting points, and even concave. Their surfaces are rough, and
fissured with branching cracks; their internal structure is either
irregularly scoriaceous and compact, or it presents a symmetrical and
very curious appearance. An irregular segment of a bomb of this latter
kind, of which I found several, is accurately represented in figure No.
3. Its size was about that of a man’s head. The whole interior is
coarsely cellular; the cells averaging in diameter about the tenth of
an inch; but nearer the outside they gradually decrease in size. This
part is succeeded by a well-defined shell of compact lava, having a
nearly uniform thickness of about the third of an inch; and the shell
is overlaid by a somewhat thicker coating of finely cellular lava (the
cells varying from the fiftieth to the hundredth of an inch in
diameter), which forms the external surface: the line separating the
shell of compact
lava from the outer scoriaceous crust is distinctly defined. This
structure is very simply explained, if we suppose a mass of viscid,
scoriaceous matter, to be projected with a rapid, rotatory motion
through the air; for whilst the external crust, from cooling, became
solidified (in the state we now see it), the centrifugal force, by
relieving the pressure in the interior parts of the bomb, would allow
the heated vapours to expand their cells; but these being driven by the
same force against the already-hardened crust, would become, the nearer
they were to this part, smaller and smaller or less expanded, until
they became packed into a solid, concentric shell. As we know that
chips from a grindstone[3] can be flirted off, when made to revolve
with sufficient velocity, we need not doubt that the centrifugal force
would have power to modify the structure of a softened bomb, in the
manner here supposed. Geologists have remarked, that the external form
of a bomb at once bespeaks the history of its aerial course, and few
now see that the internal structure can speak, with almost equal
plainness, of its rotatory movement.

 [3] Nichol’s “Architecture of the Heavens.”


No. 3


[Illustration: Fragment of a spherical volcanic bomb.]

Fragment of a spherical volcanic bomb, with the inferior parts coarsely
cellular, coated by a concentric layer of compact lava, and this again
by a crust of finely cellular rock.


No. 4


[Illustration: Volcanic bomb of obsidian from Australia.]

Volcanic bomb of obsidian from Australia. The figure at left gives a
front view; the figure at right a side view of the same object.


M. Bory St. Vincent[4] has described some balls of lava from the Isle
of Bourbon, which have a closely similar structure. His explanation,
however (if I understand it rightly), is very different from that which
I have given; for he supposes that they have rolled, like snowballs,
down the sides of the crater. M. Beudant,[5] also, has described some
singular little balls of obsidian, never more than six or eight inches
in diameter, which he found strewed on the surface of the ground: their
form is always oval; sometimes they are much swollen in the middle, and
even spindle-shaped: their surface is regularly marked with concentric
ridges and furrows, all of which on the same ball are at right angles
to one axis: their interior is compact and glassy. M. Beudant supposes
that masses of lava, when soft, were shot into the air, with a rotatory
movement round the same axis, and that the form and superficial ridges
of the bombs were thus produced. Sir Thomas Mitchell has given me what
at first appears to be the half of a much flattened oval ball of
obsidian; it has a singular artificial-like appearance, which is well
represented (of the natural size) in figure No. 4. It was found in its
present state, on a great sandy plain between the rivers Darling and
Murray, in Australia, and at the distance of several hundred miles from
any known volcanic region. It seems to have been embedded in some
reddish tufaceous matter; and may have been transported either by the
aborigines or by natural means. The external saucer consists of compact
obsidian, of a bottle-green colour, and is filled with finely cellular
black lava, much less transparent and glassy than the obsidian. The
external surface is marked with four or five not quite perfect ridges,
which are represented rather too distinctly in figure No. 4. Here,
then, we have the external structure described by M. Beudant, and the
internal cellular condition of the bombs from Ascension. The lip of the
saucer is slightly concave, exactly like the margin of a soup-plate,
and its inner edge overlaps a little the central cellular lava. This
structure is so symmetrical round the entire circumference, that one is
forced to suppose that the bomb burst during its rotatory course,
before being quite solidified, and that the lip and edges were thus
slightly modified and turned inwards. It may be remarked that the
superficial ridges are in planes, at right angles to an axis,
transverse to the longer axis of the flattened oval: to explain this
circumstance, we may suppose that when the bomb burst, the axis of
rotation changed.

 [4] “Voyage aux Quatre Isles d’Afrique” tome i, p. 222.


 [5] “Voyage en Hongrie,” tome ii, p. 214.


_Aeriform explosions._—The flanks of Green Mountain and the surrounding
country are covered by a great mass, some hundred feet in thickness, of
loose fragments. The lower beds generally consist of fine-grained,
slightly consolidated tuffs,[6] and the upper beds of great
loose fragments, with alternating finer beds.[7] One white ribbon-like
layer of decomposed, pumiceous breccia, was curiously bent into deep
unbroken curves, beneath each of the large fragments in the
superincumbent stratum. From the relative position of these beds, I
presume that a narrow-mouthed crater, standing nearly in the position
of Green Mountain, like a great air-gun, shot forth, before its final
extinction, this vast accumulation of loose matter. Subsequently to
this event, considerable dislocations have taken place, and an oval
circus has been formed by subsidence. This sunken space lies at the
north-eastern foot of Green Mountain, and is well represented in Map 2.
Its longer axis, which is connected with a N.E. and S.W. line of
fissure, is three-fifths of a nautical mile in length; its sides are
nearly perpendicular, except in one spot, and about four hundred feet
in height; they consist, in the lower part, of a pale basalt with
feldspar, and in the upper part, of the tuff and loose ejected
fragments; the bottom is smooth and level, and under almost any other
climate a deep lake would have been formed here. From the thickness of
the bed of loose fragments, with which the surrounding country is
covered, the amount of aeriform matter necessary for their projection
must have been enormous; hence we may suppose it probable that after
the explosions vast subterranean caverns were left, and that the
falling in of the roof of one of these produced the hollow here
described. At the Galapagos Archipelago, pits of a similar character,
but of a much smaller size, frequently occur at the bases of small
cones of eruption.

 [6] Some of this peperino, or tuff, is sufficiently hard not to be
 broken by the greatest force of the fingers.


 [7] On the northern side of the Green Mountain a thin seam, about an
 inch in thickness, of compact oxide of iron, extends over a
 considerable area; it lies conformably in the lower part of the
 stratified mass of ashes and fragments. This substance is of a
 reddish-brown colour, with an almost metallic lustre; it is not
 magnetic, but becomes so after having been heated under the blowpipe,
 by which it is blackened and partly fused. This seam of compact stone,
 by intercepting the little rain-water which falls on the island, gives
 rise to a small dripping spring, first discovered by Dampier. It is
 the only fresh water on the island, so that the possibility of its
 being inhabited has entirely depended on the occurrence of this
 ferruginous layer.


_Ejected granitic fragments._—In the neighbourhood of Green Mountain,
fragments of extraneous rock are not unfrequently found embedded in the
midst of masses of scoriæ. Lieutenant Evans, to whose kindness I am
indebted for much information, gave me several specimens, and I found
others myself. They nearly all have a granitic structure, are brittle,
harsh to the touch, and apparently of altered colours. _First_, a white
syenite, streaked and mottled with red; it consists of
well-crystallised feldspar, numerous grains of quartz, and brilliant,
though small, crystals of hornblende. The feldspar and hornblende in
this and the succeeding cases have been determined by the reflecting
goniometer, and the quartz by its action under the blowpipe. The
feldspar in these ejected fragments, like the glassy kind in the
trachyte, is from its cleavage a potash-feldspar. _Secondly_, a
brick-red mass of feldspar, quartz, and small dark patches of a decayed
mineral; one minute particle of which I was able to ascertain, by its
cleavage, to be hornblende.
_Thirdly_, a mass of confusedly crystallised white feldspar, with
little nests of a dark-coloured mineral, often carious, externally
rounded, having a glossy fracture, but no distinct cleavage: from
comparison with the second specimen, I have no doubt that it is fused
hornblende. _Fourthly_, a rock, which at first appears a simple
aggregation of distinct and large-sized crystals of dusty-coloured
Labrador feldspar;[8] but in their interstices there is some white
granular feldspar, abundant scales of mica, a little altered
hornblende, and, as I believe, no quartz. I have described these
fragments in detail, because it is rare[9] to find granitic rocks
ejected from volcanoes with their _minerals unchanged_, as is the case
with the first specimen, and partially with the second. One other large
fragment, found in another spot, is deserving of notice; it is a
conglomerate, containing small fragments of granitic, cellular, and
jaspery rocks, and of hornstone porphyries, embedded in a base of
wacke, threaded by numerous thin layers of a concretionary pitchstone
passing into obsidian. These layers are parallel, slightly tortuous,
and short; they thin out at their ends, and resemble in form the layers
of quartz in gneiss. It is probable that these small embedded fragments
were not separately ejected, but were entangled in a fluid volcanic
rock, allied to obsidian; and we shall presently see that several
varieties of this latter series of rock assume a laminated structure.

 [8] Professor Miller has been so kind as to examine this mineral. He
 obtained two good cleavages of 86° 30′ and 86° 50′. The mean of
 several, which I made, was 86° 30′. Professor Miller states that these
 crystals, when reduced to a fine powder, are soluble in hydrochloric
 acid, leaving some undissolved silex behind; the addition of oxalate
 of ammonia gives a copious precipitate of lime. He further remarks,
 that according to Von Kobell, anorthite (a mineral occurring in the
 ejected fragments at Mount Somma) is always white and transparent, so
 that if this be the case, these crystals from Ascension must be
 considered as Labrador feldspar. Professor Miller adds, that he has
 seen an account, in Erdmann’s “Journal für tecnische Chemie,” of a
 mineral ejected from a volcano which had the external characters of
 Labrador feldspar, but differed in the analysis from that given by
 mineralogists of this mineral: the author attributed this difference
 to an error in the analysis of Labrador feldspar, which is very old.


 [9] Daubeny, in his work on Volcanoes (p. 386), remarks that this is
 the case; and Humboldt, in his “Personal Narrative,” vol. i, p. 236,
 says “In general, the masses of known primitive rocks, I mean those
 which perfectly resemble our granites, gneiss, and mica-slate, are
 very rare in lavas: the substances we generally denote by the name of
 granite, thrown out by Vesuvius, are mixtures of nepheline, mica, and
 pyroxene.”


_Trachytic series of rocks._—Those occupy the more elevated and
central, and likewise the south-eastern, parts of the island. The
trachyte is generally of a pale brown colour, stained with small darker
patches; it contains broken and bent crystals of glassy feldspar,
grains of specular iron, and black microscopical points, which latter,
from being easily fused, and then becoming magnetic, I presume are
hornblende. The greater number of the hills, however, are composed of a
quite white, friable stone, appearing like a trachytic tuff. Obsidian,
hornstone, and several kinds of laminated feldspathic rocks, are
associated with the trachyte. There is no distinct stratification; nor
could I distinguish a crateriform structure in any of the hills of this
series. Considerable dislocations have taken place; and many fissures
in these rocks are yet left open, or are only partially filled with
loose fragments. Within the space,[10] mainly formed of trachyte, some
basaltic streams have burst forth; and not far from the summit of Green
Mountain, there is one stream of quite black, vesicular basalt,
containing minute crystals of glassy feldspar, which have a rounded
appearance.

 [10] This space is nearly included by a line sweeping round Green
 Mountain, and joining the hills, called the Weather Port Signal,
 Holyhead, and that denominated (improperly in a geological sense) “the
 Crater of an old volcano.”


The soft white stone above mentioned is remarkable from its singular
resemblance, when viewed in mass, to a sedimentary tuff: it was long
before I could persuade myself that such was not its origin; and other
geologists have been perplexed by closely similar formations in
trachytic regions. In two cases, this white earthy stone formed
isolated hills; in a third, it was associated with columnar and
laminated trachyte; but I was unable to trace an actual junction. It
contains numerous crystals of glassy feldspar and black microscopical
specks, and is marked with small darker patches, exactly as in the
surrounding trachyte. Its basis, however, when viewed under the
microscope, is generally quite earthy; but sometimes it exhibits a
decidedly crystalline structure. On the hill marked “Crater of an old
volcano,” it passes into a pale greenish-grey variety, differing only
in its colour, and in not being so earthy; the passage was in one case
effected insensibly; in another, it was formed by numerous, rounded and
angular, masses of the greenish variety, being embedded in the white
variety;—in this latter case, the appearance was very much like that of
a sedimentary deposit, torn up and abraded during the deposition of a
subsequent stratum. Both these varieties are traversed by innumerable
tortuous veins (presently to be described), which are totally unlike
injected dikes, or indeed any other veins which I have ever seen. Both
varieties include a few scattered fragments, large and small, of
dark-coloured scoriaceous rocks, the cells of some of which are
partially filled with the white earthy stone; they likewise include
some huge blocks of a cellular porphyry.[11] These fragments project
from the weathered surface, and perfectly resemble fragments embedded
in a true sedimentary tuff. But as it is known that extraneous
fragments of cellular rock are sometimes included in columnar trachyte,
in phonolite,[12] and in other compact lavas, this circumstance is not
any real argument for the sedimentary origin of the white earthy
stone.[13] The insensible passage of the greenish variety
into the white one, and likewise the more abrupt passage by fragments
of the former being embedded in the latter, might result from slight
differences in the composition of the same mass of molten stone, and
from the abrading action of one such part still fluid on another part
already solidified. The curiously formed veins have, I believe, been
formed by siliceous matter being subsequently segregated. But my chief
reason for believing that these soft earthy stones, with their
extraneous fragments, are not of sedimentary origin, is the extreme
improbability of crystals of feldspar, black microscopical specks, and
small stains of a darker colour occurring in the same proportional
numbers in an aqueous deposit, and in masses of solid trachyte.
Moreover, as I have remarked, the microscope occasionally reveals a
crystalline structure in the apparently earthy basis. On the other
hand, the partial decomposition of such great masses of trachyte,
forming whole mountains, is undoubtedly a circumstance of not easy
explanation.

 [11] The porphyry is dark coloured; it contains numerous, often
 fractured, crystals of white opaque feldspar, also decomposing
 crystals of oxide of iron; its vesicles include masses of delicate,
 hair-like, crystals, apparently of analcime.


 [12] D’Aubuisson, “Traité de Géognosie,” tome ii, p. 548.


 [13] Dr. Daubeny (on Volcanoes, p. 180) seems to have been led to
 believe that certain trachytic formations of Ischia and of the Puy de
 Dôme, which closely resemble these of Ascension, were of sedimentary
 origin, chiefly from the frequent presence in them “of scoriform
 portions, different in colour from the matrix.” Dr. Daubeny adds, that
 on the other hand, Brocchi, and other eminent geologists, have
 considered these beds as earthy varieties of trachyte; he considers
 the subject deserving of further attention.

_Veins in the earthy trachytic masses._—These veins are extraordinarily
numerous, intersecting in the most complicated manner both coloured
varieties of the earthy trachyte: they are best seen on the flanks of
the “Crater of the old volcano.” They contain crystals of glassy
feldspar, black microscopical specks and little dark stains, precisely
as in the surrounding rock; but the basis is very different, being
exceedingly hard, compact, somewhat brittle, and of rather less easy
fusibility. The veins vary much, and suddenly, from the tenth of an
inch to one inch in thickness; they often thin out, not only on their
edges, but in their central parts, thus leaving round, irregular
apertures; their surfaces are rugged. They are inclined at every
possible angle with the horizon, or are horizontal; they are generally
curvilinear, and often interbranch one with another. From their
hardness they withstand weathering, and projecting two or three feet
above the ground, they occasionally extend some yards in length; these
plate-like veins, when struck, emit a sound, almost like that of a
drum, and they may be distinctly seen to vibrate; their fragments,
which are strewed on the ground, clatter like pieces of iron when
knocked against each other. They often assume the most singular forms;
I saw a pedestal of the earthy trachyte, covered by a hemispherical
portion of a vein, like a great umbrella, sufficiently large to shelter
two persons. I have never met with, or seen described, any veins like
these; but in form they resemble the ferruginous seams, due to some
process of segregation, occurring not uncommonly in sandstones,—for
instance, in the New Red sandstone of England. Numerous veins of jasper
and of siliceous sinter, occurring on the summit of this same hill,
show that there has been some abundant source of silica, and as these
plate-like veins differ from the trachyte
only in their greater hardness, brittleness, and less easy fusibility,
it appears probable that their origin is due to the segregation or
infiltration of siliceous matter, in the same manner as happens with
the oxides of iron in many sedimentary rocks.

_Siliceous sinter and jasper._—The siliceous sinter is either quite
white, of little specific gravity, and with a somewhat pearly fracture,
passing into pinkish pearl quartz; or it is yellowish white, with a
harsh fracture, and it then contains an earthy powder in small
cavities. Both varieties occur, either in large irregular masses in the
altered trachyte, or in seams included in broad, vertical, tortuous,
irregular veins of a compact, harsh stone of a dull red colour,
appearing like a sandstone. This stone, however, is only altered
trachyte; and a nearly similar variety, but often honeycombed,
sometimes adheres to the projecting plate-like veins, described in the
last paragraph. The jasper is of an ochre yellow or red colour; it
occurs in large irregular masses, and sometimes in veins, both in the
altered trachyte and in an associated mass of scoriaceous basalt. The
cells of the scoriaceous basalt are lined or filled with fine,
concentric layers of chalcedony, coated and studded with bright-red
oxide of iron. In this rock, especially in the rather more compact
parts, irregular angular patches of the red jasper are included, the
edges of which insensibly blend into the surrounding mass; other
patches occur having an intermediate character between perfect jasper
and the ferruginous, decomposed, basaltic base. In these patches, and
likewise in the large vein-like masses of jasper, there occur little
rounded cavities, of exactly the same size and form with the air-cells,
which in the scoriaceous basalt are filled and lined with layers of
chalcedony. Small fragments of the jasper, examined under the
microscope, seem to resemble the chalcedony with its colouring matter
not separated into layers, but mingled in the siliceous paste, together
with some impurities. I can understand these facts,—namely, the
blending of the jasper into the semi-decomposed basalt,—its occurrence
in angular patches, which clearly do not occupy pre-existing hollows in
the rock,—and its containing little vesicles filled with chalcedony,
like those in the scoriaceous lava,—only on the supposition that a
fluid, probably the same fluid which deposited the chalcedony in the
air-cells, removed in those parts where there were no cavities, the
ingredients of the basaltic rock, and left in their place silica and
iron, and thus produced the jasper. In some specimens of silicified
wood, I have observed, that in the same manner as in the basalt, the
solid parts were converted into a dark-coloured homogeneous stone,
whereas the cavities formed by the larger sap-vessels (which may be
compared with the air-vesicles in the basaltic lava) and other
irregular hollows, apparently produced by decay, were filled with
concentric layers of chalcedony; in this case, there can be little
doubt that the same fluid deposited the homogeneous base and the
chalcedonic layers. After these considerations, I cannot doubt but that
the jasper of Ascension may be viewed as a volcanic rock silicified, in
precisely the same sense as this term is applied to wood, when
silicified; we are equally ignorant of the means by which every atom of
wood, whilst in a perfect state, is
removed and replaced by atoms of silica, as we are of the means by
which the constituent parts of a volcanic rock could be thus acted
on.[14] I was led to the careful examination of these rocks, and to the
conclusion here given, from having heard the Rev. Professor Henslow
express a similar opinion, regarding the origin in trap-rocks of many
chalcedonies and agates. Siliceous deposits seem to be very general, if
not of universal occurrence, in partially decomposed trachytic
tuffs;[15] and as these hills, according to the view above given,
consist of trachyte softened and altered in situ, the presence of free
silica in this case may be added as one more instance to the list.

 [14] Beudant (“Voyage en Hongrie,” tome iii, pp. 502, 504) describes
 kidney-shaped masses of jasper-opal, which either blend into the
 surrounding trachytic conglomerate, or are embedded in it like
 chalk-flints; and he compares them with the fragments of opalised
 wood, which are abundant in this same formation. Beudant, however,
 appears to have viewed the process of their formation rather as one of
 simple infiltration than of molecular exchange; but the presence of a
 concretion, wholly different from the surrounding matter, if not
 formed in a pre-existing hollow, clearly seems to me to require,
 either a molecular or mechanical displacement of the atoms, which
 occupied the space afterwards filled by it. The jasper-opal of Hungary
 passes into chalcedony, and therefore in this case, as in that of
 Ascension, jasper seems to be intimately related in origin with
 chalcedony.


 [15] Beudant (“Voyage Min.,” tome iii, p. 507) enumerates cases in
 Hungary, Germany, Central France, Italy, Greece, and Mexico.

_Concretions in pumiceous tuff._—The hill, marked in Map 2 “Crater of
an old volcano,” has no claims to this appellation, which I could
discover, except in being surmounted by a circular, very shallow,
saucer-like summit, nearly half a mile in diameter. This hollow has
been nearly filled up with many successive sheets of ashes and scoriæ,
of different colours, and slightly consolidated. Each successive
saucer-shaped layer crops out all round the margin, forming so many
rings of various colours, and giving to the hill a fantastic
appearance. The outer ring is broad, and of a white colour; hence it
resembles a course round which horses have been exercised, and has
received the name of the Devil’s Riding School, by which it is most
generally known. These successive layers of ashes must have fallen over
the whole surrounding country, but they have all been blown away except
in this one hollow, in which probably moisture accumulated, either
during an extraordinary year when rain fell, or during the storms often
accompanying volcanic eruptions. One of the layers of a pinkish colour,
and chiefly derived from small, decomposed fragments of pumice, is
remarkable, from containing numerous concretions. These are generally
spherical, from half an inch to three inches in diameter; but they are
occasionally cylindrical, like those of iron-pyrites in the chalk of
Europe. They consist of a very tough, compact, pale-brown stone, with a
smooth and even fracture. They are divided into concentric layers by
thin white partitions, resembling the external superficies; six or
eight of such layers are distinctly defined near the outside; but those
towards the inside generally become indistinct, and blend into a
homogeneous
mass. I presume that these concentric layers were formed by the
shrinking of the concretion, as it became compact. The interior part is
generally fissured by minute cracks or septaria, which are lined, both
by black, metallic, and by other white and crystalline specks, the
nature of which I was unable to ascertain. Some of the larger
concretions consist of a mere spherical shell, filled with slightly
consolidated ashes. The concretions contain a small proportion of
carbonate of lime: a fragment placed under the blowpipe decrepitates,
then whitens and fuses into a blebby enamel, but does not become
caustic. The surrounding ashes do not contain any carbonate of lime;
hence the concretions have probably been formed, as is so often the
case, by the aggregation of this substance. I have not met with any
account of similar concretions; and considering their great toughness
and compactness, their occurrence in a bed, which probably has been
subjected only to atmospheric moisture, is remarkable.

_Formation of calcareous rocks on the sea-coast._—On several of the
sea-beaches, there are immense accumulations of small, well-rounded
particles of shells and corals, of white, yellowish, and pink colours,
interspersed with a few volcanic particles. At the depth of a few feet,
these are found cemented together into stone, of which the softer
varieties are used for building; there are other varieties, both coarse
and fine-grained, too hard for this purpose: and I saw one mass divided
into even layers half an inch in thickness, which were so compact that
when struck with a hammer they rang like flint. It is believed by the
inhabitants, that the particles become united in the course of a single
year. The union is effected by calcareous matter; and in the most
compact varieties, each rounded particle of shell and volcanic rock can
be distinctly seen to be enveloped in a husk of pellucid carbonate of
lime. Extremely few perfect shells are embedded in these agglutinated
masses; and I have examined even a large fragment under a microscope,
without being able to discover the least vestige of striæ or other
marks of external form: this shows how long each particle must have
been rolled about, before its turn came to be embedded and
cemented.[16] One of the most compact varieties, when placed in acid,
was entirely dissolved, with the exception of some flocculent animal
matter; its specific gravity was 2·63. The specific gravity of ordinary
limestone varies from 2·6 to 2·75; pure Carrara marble was found by Sir
H. De la Beche[17] to be 2·7. It is remarkable that these rocks of
Ascension, formed close to the surface, should be nearly as compact as
marble, which has undergone the action of heat and pressure in the
plutonic regions.

 [16] The eggs of the turtle being buried by the parent, sometimes
 become enclosed in the solid rock. Mr. Lyell has given a figure
 (“Principles of Geology,” book iii, ch. 17) of some eggs, containing
 the bones of young turtles, found thus entombed.


 [17] “Researches in Theoretical Geology,” p. 12.

The great accumulation of loose calcareous particles, lying on the
beach near the Settlement, commences in the month of October, moving
towards the S.W., which, as I was informed by Lieutenant
Evans, is caused by a change in the prevailing direction of the
currents. At this period the tidal rocks, at the S.W. end of the beach,
where the calcareous sand is accumulating, and round which the currents
sweep, become gradually coated with a calcareous incrustation, half an
inch in thickness. It is quite white, compact, with some parts slightly
spathose, and is firmly attached to the rock. After a short time it
gradually disappears, being either redissolved, when the water is less
charged with lime, or more probably is mechanically abraded. Lieutenant
Evans has observed these facts, during the six years he has resided at
Ascension. The incrustation varies in thickness in different years: in
1831 it was unusually thick. When I was there in July, there was no
remnant of the incrustation; but on a point of basalt, from which the
quarrymen had lately removed a mass of the calcareous freestone, the
incrustation was perfectly preserved. Considering the position of the
tidal-rocks, and the period at which they become coated, there can be
no doubt that the movement and disturbance of the vast accumulation of
calcareous particles, many of them being partially agglutinated
together, cause the waves of the sea to be so highly charged with
carbonate of lime, that they deposit it on the first objects against
which they impinge. I have been informed by Lieutenant Holland, R.N.,
that this incrustation is formed on many parts of the coast, on most of
which, I believe, there are likewise great masses of comminuted shells.

_A frondescent calcareous incrustation._—In many respects this is a
singular deposit; it coats throughout the year the tidal volcanic
rocks, that project from the beaches composed of broken shells. Its
general appearance is well represented in Figure 5; but the fronds or
discs, of which it is composed, are generally so closely crowded
together as to touch. These fronds have their sinuous edges finely
crenulated, and they project over their pedestals or supports; their
upper surfaces are either slightly concave, or slightly convex; they
are highly polished, and of a dark grey or jet black colour; their form
is irregular, generally circular, and from the tenth of an inch to one
inch and a half in diameter; their thickness, or amount of their
projection from the rock on which they stand, varies much, about a
quarter of an inch being perhaps most usual. The fronds occasionally
become more and more convex, until they pass into botryoidal masses
with their summits fissured; when in this state, they are glossy and of
an intense black, so as to resemble some fused metallic substance. I
have shown the incrustation, both in this latter and in its ordinary
state to several geologists, but not one could conjecture its origin,
except that perhaps it was of volcanic nature!

No. 5


[Illustration: An incrustration of calcareous and animal matter.]

An incrustation of calcareous and animal matter, coating the
tidal-rocks at Ascension.


The substance forming the fronds has a very compact and often almost
crystalline fracture; the edges being translucent, and hard enough
easily to scratch calcareous spar. Under the blowpipe it immediately
becomes white, and emits a strong animal odour, like that from fresh
shells. It is chiefly composed of carbonate of lime; when placed in
muriatic acid it froths much, leaving a residue of sulphate of lime,
and of an oxide of iron, together with a black powder, which is not
soluble in heated acids. This latter substance seems to be
carbonaceous,
and is evidently the colouring matter. The sulphate of lime is
extraneous, and occurs in distinct, excessively minute, lamellar
plates, studded on the surface of the fronds, and embedded between the
fine layers of which they are composed; when a fragment is heated in
the blowpipe, these lamellæ are immediately rendered visible. The
original outline of the fronds may often be traced, either to a minute
particle of shell fixed in a crevice of the rock, or to several
cemented together; these first become deeply corroded, by the
dissolving power of the waves, into sharp ridges, and then are coated
with successive layers of the glossy, grey, calcareous incrustation.
The inequalities of the primary support affect the outline of every
successive layer, in the same manner as may often be seen in
bezoar-stones, when an object like a nail forms the centre of
aggregation. The crenulated edges, however, of the frond appear to be
due to the corroding power of the surf on its own deposit, alternating
with fresh depositions. On some smooth basaltic rocks on the coast of
St. Jago, I found an exceedingly thin layer of brown calcareous matter,
which under a lens presented a miniature likeness of the crenulated and
polished fronds of Ascension; in this case a basis was not afforded by
any projecting extraneous particles. Although the incrustation at
Ascension is persistent throughout the year; yet from the abraded
appearance of some parts, and from the fresh appearance of other parts,
the whole seems to undergo a round of decay and renovation, due
probably to changes in the form of the shifting beach, and consequently
in the action of the breakers: hence probably it is, that the
incrustation never acquires a great thickness. Considering the position
of the encrusted rocks in the midst of the calcareous beach, together
with its composition, I think there can be no doubt that its origin is
due to the dissolution and subsequent deposition of the matter
composing the rounded particles of shells and corals.[18] From this
source
it derives its animal matter, which is evidently the colouring
principle. The nature of the deposit, in its incipient stage, can often
be well seen upon a fragment of white shell, when jammed between two of
the fronds; it then appears exactly like the thinnest wash of a pale
grey varnish. Its darkness varies a little, but the jet blackness of
some of the fronds and of the botryoidal masses seems due to the
translucency of the successive grey layers. There is, however, this
singular circumstance, that when deposited on the under side of ledges
of rock or in fissures, it appears always to be of a pale, pearly grey
colour, even when of considerable thickness: hence one is led to
suppose, that an abundance of light is necessary to the development of
the dark colour, in the same manner as seems to be the case with the
upper and exposed surfaces of the shells of living mollusca, which are
always dark, compared with their under surfaces and with the parts
habitually covered by the mantle of the animal. In this
circumstance,—in the immediate loss of colour and in the odour emitted
under the blowpipe,—in the degree of hardness and translucency of the
edges,—and in the beautiful polish of the surface,[19] rivalling when
in a fresh state that of the finest Oliva, there is a striking analogy
between this inorganic incrustation and the shells of living molluscous
animals.[20] This appears to me to be an interesting physiological
fact.[21]

 [18] The selenite, as I have remarked is extraneous, and must have
 been derived from the sea-water. It is an interesting circumstance
 thus to find the waves of the ocean, sufficiently charged with
 sulphate of lime, to deposit it on the rocks, against which they dash
 every tide. Dr. Webster has described (“Voyage of the _Chanticleer,_”
 vol. ii, p. 319) beds of gypsum and salt, as much as two feet in
 thickness, left by the evaporation of the spray on the rocks on the
 windward coast. Beautiful stalactites of selenite, resembling in form
 those of carbonate of lime, are formed near these beds. Amorphous
 masses of gypsum, also, occur in caverns in the interior of the
 island; and at Cross Hill (an old crater) I saw a considerable
 quantity of salt oozing from a pile of scoriæ. In these latter cases,
 the salt and gypsum appear to be volcanic products.)


 [19] From the fact described in my “Journal of Researches” of a
 coating of oxide of iron, deposited by a streamlet on the rocks in its
 bed (like a nearly similar coating at the great cataracts of the
 Orinoco and Nile), becoming finely polished where the surf acts, I
 presume that the surf in this instance, also, is the polishing agent.)


 [20] In the section descriptive of St. Paul’s Rocks, I have described
 a glossy, pearly substance, which coats the rocks, and an allied
 stalactitical incrustation from Ascension, the crust of which
 resembles the enamel of teeth, but is hard enough to scratch
 plate-glass. Both these substances contain animal matter, and seem to
 have been derived from water in filtering through birds’ dung.


 [21] Mr. Horner and Sir David Brewster have described (“Philosophical
 Transactions,” 1836, p. 65) a singular “artificial substance,
 resembling shell.” It is deposited in fine, transparent, highly
 polished, brown-coloured laminæ, possessing peculiar optical
 properties, on the inside of a vessel, in which cloth, first prepared
 with glue and then with lime, is made to revolve rapidly in water. It
 is much softer, more transparent, and contains more animal matter,
 than the natural incrustation at Ascension; but we here again see the
 strong tendency which carbonate of lime and animal matter evince to
 form a solid substance allied to shell.


_Singular laminated beds alternating with and passing into
obsidian._—These beds occur within the trachytic district, at the
western base of Green Mountain, under which they dip at a high
inclination. They are only partially exposed, being covered up by
modern ejections; from this cause, I was unable to trace their junction
with the trachyte, or to discover whether they had flowed as a stream
of lava, or had been injected amidst the overlying strata. There are
three principal beds of obsidian, of which the thickest forms the base
of the section. The alternating stony layers appear to me eminently
curious, and shall be first described, and afterwards their passage
into the obsidian. They have an extremely diversified appearance; five
principal varieties may be noticed, but these insensibly blend into
each other by endless gradations.

First.—A pale grey, irregularly and coarsely laminated,[22]
harsh-feeling rock, resembling clay-slate which has been in contact
with a trap-dike, and with a fracture of about the same degree of
crystalline structure. This rock, as well as the following varieties,
easily fuses into a pale glass. The greater part is honeycombed with
irregular, angular, cavities, so that the whole has a curious
appearance, and some fragments resemble in a remarkable manner
silicified logs of decayed wood. This variety, especially where more
compact, is often marked with thin whitish streaks, which are either
straight or wrap round, one behind the other, the elongated carious
hollows.

 [22] This term is open to some misinterpretation, as it may be applied
 both to rocks divided into laminæ of exactly the same composition, and
 to layers firmly attached to each other, with no fissile tendency, but
 composed of different minerals, or of different shades of colour. The
 term “laminated,” in this chapter, is applied in these latter senses;
 where a homogeneous rock splits, as in the former sense, in a given
 direction, like clay-slate, I have used the term “fissile.”

Secondly.—A bluish grey or pale brown, compact, heavy, homogeneous
stone, with an angular, uneven, earthy fracture; viewed, however, under
a lens of high power, the fracture is seen to be distinctly
crystalline, and even separate minerals can be distinguished.

Thirdly.—A stone of the same kind with the last, but streaked with
numerous, parallel, slightly tortuous, white lines of the thickness of
hairs. These white lines are more crystalline than the parts between
them; and the stone splits along them: they frequently expand into
exceedingly thin cavities, which are often only just perceptible with a
lens. The matter forming the white lines becomes better crystallised in
these cavities, and Professor Miller was fortunate enough, after
several trials, to ascertain that the white crystals, which are the
largest, were of quartz,[23] and that the minute green transparent
needles were augite, or, as they would more generally be called,
diopside: besides these crystals, there are some minute, dark specks
without a trace of
crystalline, and some fine, white, granular, crystalline matter which
is probably feldspar. Minute fragments of this rock are easily fusible.

 [23] Professor Miller informs me that the crystals which he measured
 had the faces _P, z, m_ of the figure (147) given by Haidinger in his
 Translation of Mohs; and he adds, that it is remarkable, that none of
 them had the slightest trace of faces _r_ of the regular six-sided
 prism.

Fourthly.—A compact crystalline rock, banded in straight lines with
innumerable layers of white and grey shades of colour, varying in width
from the thirtieth to the two-hundredth of an inch; these layers seem
to be composed chiefly of feldspar, and they contain numerous perfect
crystals of glassy feldspar, which are placed lengthways; they are also
thickly studded with microscopically minute, amorphous, black specks,
which are placed in rows, either standing separately, or more
frequently united, two or three or several together, into black lines,
thinner than a hair. When a small fragment is heated in the blowpipe,
the black specks are easily fused into black brilliant beads, which
become magnetic,—characters that apply to no common mineral except
hornblende or augite. With the black specks there are mingled some
others of a red colour, which are magnetic before being heated, and no
doubt are oxide of iron. Round two little cavities, in a specimen of
this variety, I found the black specks aggregated into minute crystals,
appearing like those of augite or hornblende, but too dull and small to
be measured by the goniometer; in the specimen, also, I could
distinguish amidst the crystalline feldspar, grains, which had the
aspect of quartz. By trying with a parallel ruler, I found that the
thin grey layers and the black hair-like lines were absolutely straight
and parallel to each other. It is impossible to trace the gradation
from the homogeneous grey rocks to these striped varieties, or indeed
the character of the different layers in the same specimen, without
feeling convinced that the more or less perfect whiteness of the
crystalline feldspathic matter depends on the more or less perfect
aggregation of diffused matter, into the black and red specks of
hornblende and oxide of iron.

Fifthly.—A compact heavy rock, not laminated, with an irregular,
angular, highly crystalline, fracture; it abounds with distinct
crystals of glassy feldspar, and the crystalline feldspathic base is
mottled with a black mineral, which on the weathered surface is seen to
be aggregated into small crystals, some perfect, but the greater number
imperfect. I showed this specimen to an experienced geologist, and
asked him what it was; he answered, as I think every one else would
have done, that it was a primitive greenstone. The weathered surface,
also, of the banded variety in figure No. 4, strikingly resembles a
worn fragment of finely laminated gneiss.

These five varieties, with many intermediate ones, pass and repass into
each other. As the compact varieties are quite subordinate to the
others, the whole may be considered as laminated or striped. The
laminæ, to sum up their characteristics, are either quite straight, or
slightly tortuous, or convoluted; they are all parallel to each other,
and to the intercalating strata of obsidian; they are generally of
extreme thinness; they consist either of an apparently homogeneous,
compact rock, striped with different shades of grey and brown colours,
or of crystalline feldspathic layers in a more or less perfect state of
purity, and of different thicknesses, with distinct crystals of glassy
feldspar
placed lengthways, or of very thin layers chiefly composed of minute
crystals of quartz and augite, or composed of black and red specks of
an augitic mineral and of an oxide of iron, either not crystallised or
imperfectly so. After having fully described the obsidian, I shall
return to the subject of the lamination of rocks of the trachytic
series.

The passage of the foregoing beds into the strata of glassy obsidian is
effected in several ways: first, angulo-modular masses of obsidian,
both large and small, abruptly appear disseminated in a slaty, or in an
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
fracture. Secondly, small irregular nodules of the obsidian, either
standing separately, or united into thin layers, seldom more than the
tenth of an inch in thickness, alternate repeatedly with very thin
layers of a feldspathic rock, which is striped with the finest parallel
zones of colour, like an agate, and which sometimes passes into the
nature of pitchstone; the interstices between the nodules of obsidian
are generally filled by soft white matter, resembling pumiceous ashes.
Thirdly, the whole substance of the bounding rock suddenly passes into
an angulo-concretionary mass of obsidian. Such masses (as well as the
small nodules) of obsidian are of a pale green colour, and are
generally streaked with different shades of colour, parallel to the
laminæ of the surrounding rock; they likewise generally contain minute
white sphærulites, of which half is sometimes embedded in a zone of one
shade of colour, and half in a zone of another shade. The obsidian
assumes its jet black colour and perfectly conchoidal fracture, only
when in large masses; but even in these, on careful examination and on
holding the specimens in different lights, I could generally
distinguish parallel streaks of different shades of darkness.

No. 6


[Illustration: Opaque brown sphærulites, drawn on an enlarged scale.]

Opaque brown sphærulites, drawn on an enlarged scale. The upper ones
are externally marked with parallel ridges. The internal radiating
structure of the lower ones, is much too plainly represented.


No. 7


[Illustration: A layer, formed by the union of minute brown
sphærulites.

A layer, formed by the union of minute brown sphærulites, intersecting
two other similar layers: the whole represented of nearly the natural
size.

One of the commonest transitional rocks deserves in several respects a
further description. It is of a very complicated nature, and consists
of numerous thin, slightly tortuous layers of a pale-coloured
feldspathic stone, often passing into an imperfect pitchstone,
alternating with layers formed of numberless little globules of two
varieties of obsidian, and of two kinds of sphærulites, embedded in a
soft or in a hard pearly base. The sphærulites are either white and
translucent, or dark brown and opaque; the former are quite spherical,
of small size, and distinctly radiated from their centre. The dark
brown sphærulites are less perfectly round, and vary in diameter from
the twentieth to the thirtieth of an inch; when broken they exhibit
towards their centres, which are whitish, an obscure radiating
structure; two of them when united sometimes have only one central
point of radiation; there is occasionally a trace of or a hollow
crevice in their centres. They stand either separately, or are united
two or three or many together into irregular groups, or more commonly
into layers, parallel to the stratification of the mass. This union in
many cases is so perfect, that the two sides of the layer thus formed,
are quite even; and these layers, as they become less brown and opaque,
cannot be distinguished from the alternating layers of the
pale-coloured feldspathic stone. The sphærulites, when not united, are
generally compressed in the plane of the lamination of the mass; and in
this same plane, they are often marked internally, by zones of
different
shades of colour, and externally by small ridges and furrows. In the
upper part of figure No. 6, the sphærulites with the parallel ridges
and furrows are represented on an enlarged scale, but they are not well
executed; and in the lower part, their usual manner of grouping is
shown. In another specimen, a thin layer formed of the brown
sphærulites closely united together, intersects, as represented in
figure No. 7, a layer of similar composition; and after running for a
short space in a slightly curved line, again intersects it, and
likewise a second layer lying a little way beneath that first
intersected. The small nodules also of obsidian are sometimes
externally marked with ridges and furrows, parallel to the lamination
of the mass, but always less plainly than the sphærulites. These
obsidian nodules are generally angular, with their edges blunted: they
are often impressed with the form of the adjoining sphærulites, than
which they are always larger; the separate nodules seldom appear to
have drawn each other out by exerting a mutually attractive force. Had
I not found in some cases, a distinct centre of attraction in these
nodules of obsidian, I should have
been led to have considered them as residuary matter, left during the
formation of the pearlstone, in which they are embedded, and of the
sphærulitic globules.

The sphærulites and the little nodules of obsidian in these rocks so
closely resemble, in general form and structure, concretions in
sedimentary deposits, that one is at once tempted to attribute to them
an analogous origin. They resemble ordinary concretions in the
following respects: in their external form,—in the union of two or
three, or of several, into an irregular mass, or into an even-sided
layer,—in the occasional intersection of one such layer by another, as
in the case of chalk-flints,—in the presence of two or three kinds of
nodules, often close together, in the same basis,—in their fibrous,
radiating structure, with occasional hollows in their centres,—in the
co-existence of a laminary, concretionary, and radiating structure, as
is so well developed in the concretions of magnesian limestone,
described by Professor Sedgwick.[24] Concretions in sedimentary
deposits, it is known, are due to the separation from the surrounding
mass of the whole or part of some mineral substance, and its
aggregation round certain points of attraction. Guided by this fact, I
have endeavoured to discover whether obsidian and the sphærulites (to
which may be added marekanite and pearlstone, both of them occurring in
nodular concretions in the trachytic series) differ in their
constituent parts, from the minerals generally composing trachytic
rocks. It appears from three analyses, that obsidian contains on an
average 76 per cent of silica; from one analysis, that sphærulites
contain 79·12; from two, that marekanite contains 79·25; and from two
other analyses, that pearlstone contains 75·62 of silica.[25] Now, the
constituent parts of trachyte, as far as they can be distinguished
consist of feldspar, containing 65·21 of silica; or of albite,
containing 69·09; of hornblende, containing 55·27,[26] and of oxide of
iron: so that the foregoing glassy concretionary substances all contain
a larger proportion of silica than that occurring in ordinary
feldspathic or trachytic rocks. D’Aubuisson,[27] also, has remarked on
the large proportion of silica compared with alumina, in six analyses
of obsidian and pearlstone given in Brongniart’s “Mineralogy.” Hence I
conclude, that the foregoing concretions have been formed by a process
of aggregation, strictly analogous to that which takes place in aqueous
deposits, acting chiefly on the silica, but likewise on some of the
other elements of the surrounding mass, and thus producing the
different concretionary varieties. From the well-known effects of rapid
cooling[28] in giving glassiness of
texture, it is probably necessary that the entire mass, in cases like
that of Ascension, should have cooled at a certain rate; but
considering the repeated and complicated alterations of nodules and
thin layers of a glassy texture with other layers quite stony or
crystalline, all within the space of a few feet or even inches, it is
hardly possible that they could have cooled at different rates, and
thus have acquired their different textures.

 [24] “Geological Transactions,” vol. 3, part i, p. 37.


 [25] The foregoing analyses are taken from Beudant “Traité de
 Minéralogie,” tome ii, p. 113; and one analysis of obsidian from
 Phillips’s “Mineralogy.”


 [26] These analyses are taken from Von Kobell’s “Grundzüge der
 Mineralogie,” 1838.


 [27] “Traité de Géogn.,” tome ii, p. 535.


 [28] This is seen in the manufacture of common glass, and in Gregory
 Watts’s experiments on molten trap; also on the natural surfaces of
 lava-streams, and on the side-walls of dikes.


The natural sphærulites in these rocks[29] very closely resemble those
produced in glass, when slowly cooled. In some fine specimens of
partially devitrified glass, in the possession of Mr. Stokes, the
sphærulites are united into straight layers with even sides, parallel
to each other, and to one of the outer surfaces, exactly as in the
obsidian. These layers sometimes interbranch and form loops; but I did
not see any case of actual intersection. They form the passage from the
perfectly glassy portions, to those nearly homogeneous and stony, with
only an obscure concretionary structure. In the same specimen, also,
sphærulites differing slightly in colour and in structure, occur
embedded close together. Considering these facts, it is some
confirmation of the view above given of the concretionary origin of the
obsidian and natural sphærulites, to find that M. Dartigues,[30] in his
curious paper on this subject, attributes the production of sphærulites
in glass, to the different ingredients obeying their own laws of
attraction and becoming aggregated. He is led to believe that this
takes place, from the difficulty in remelting sphærulitic glass,
without the whole be first thoroughly pounded and mixed together; and
likewise from the fact, that the change takes place most readily in
glass composed of many ingredients. In confirmation of M. Dartigues’
view, I may remark, that M. Fleuriau de Bellevue[31] found that the
sphærulitic portions of devitrified glass were acted on both by nitric
acid and under the blowpipe, in a different manner from the compact
paste in which they were embedded.

 [29] I do not know whether it is generally known, that bodies having
 exactly the same appearance as sphærulites, sometimes occur in agates.
 Mr. Robert Brown showed me in an agate, formed within a cavity in a
 piece of silicified wood, some little specks, which were only just
 visible to the naked eye: these specks, when placed by him under a
 lens of high power, presented a beautiful appearance: they were
 perfectly circular, and consisted of the finest fibres of a brown
 colour, radiating with great exactness from a common centre. These
 little radiating stars are occasionally intersected, and portions are
 quite cut off by the fine, ribbon-like zones of colour in the agate.
 In the obsidian of Ascension, the halves of a sphærulite often lie in
 different zones of colour, but they are not cut off by them, as in the
 agate.


 [30] _Journal de Physique,_ tome 59 (1804), pp. 10, 12.


 [31] _Idem,_ tome 60 (1805), p. 418.


_Comparison of the obsidian beds and alternating strata of ascension,
with those of other countries._—I have been struck with much surprise,
how closely the excellent description of the obsidian rocks of Hungary,
given by Beudant,[32] and that by Humboldt, of the same formation in
Mexico and Peru,[33] and likewise the descriptions given by several
authors[34] of the trachytic regions in the Italian islands, agree with
my observations at Ascension. Many passages might have been transferred
without alteration from the works of the above authors, and would have
been applicable to this island. They all agree in the laminated and
stratified character of the whole series; and Humboldt speaks of some
of the beds of obsidian being ribboned like jasper.[35] They all agree
in the nodular or concretionary character of the obsidian, and of the
passage of these nodules into layers. They all refer to the repeated
alterations, often in undulatory planes, of glassy, pearly, stony, and
crystalline layers: the crystalline layers, however, seem to be much
more perfectly developed at Ascension, than in the above-named
countries. Humboldt compares some of the stony beds, when viewed from a
distance, to strata of a schistose sandstone. Sphærulites are described
as occurring abundantly in all cases; and they everywhere seem to mark
the passage, from the perfectly glassy to the stony and crystalline
beds. Beudant’s account[36] of his “perlite lithoide globulaire” in
every, even the most trifling particular, might have been written for
the little brown sphærulitic globules of the rocks of Ascension.

 [32] “Voyage en Hongrie,” tome i, p. 330; tome ii, pp. 221 and 315;
 tome iii, pp. 369, 371, 377, 381.


 [33] “Essai Géognostique,” pp. 176, 326, 328.


 [34] P. Scrope, in “Geological Transactions,” vol. ii (second series)
 p. 195. Consult, also, Dolomieu’s “Voyage aux Isles Lipari,” and
 D’Aubuisson, “Traité de Géogn.,” tome ii, p. 534.


 [35] In Mr. Stokes’ fine collection of obsidians from Mexico, I
 observe that the sphærulites are generally much larger than those of
 Ascension; they are generally white, opaque, and are united into
 distinct layers: there are many singular varieties, different from any
 at Ascension. The obsidians are finely zoned, in quite straight or
 curved lines, with exceedingly slight differences of tint, of
 cellularity, and of more or less perfect degrees of glassiness.
 Tracing some of the less perfectly glassy zones, they are seen to
 become studded with minute white sphærulites, which become more and
 more numerous, until at last they unite and form a distinct layer: on
 the other hand, at Ascension, only the brown sphærulites unite and
 form layers; the white ones always being irregularly disseminated.
 Some specimens at the Geological Society, said to belong to an
 obsidian formation from Mexico, have an earthy fracture, and are
 divided in the finest parallel laminæ, by specks of a black mineral,
 like the augitic or hornblendic specks in the rocks at Ascension.


 [36] Beudant’s “Voyage,” tome iii, p. 373.


From the close similarity in so many respects, between the obsidian
formations of Hungary, Mexico, Peru, and of some of the Italian
islands, with that of Ascension, I can hardly doubt that in all these
cases, the obsidian and the sphærulites owe their origin to a
concretionary aggregation of the silica, and of some of the other
constituent elements, taking place whilst the liquified mass cooled at
a certain required rate. It is, however, well-known, that in several
places, obsidian has flowed in streams like lava; for instance, at
Teneriffe, at the Lipari Islands, and at Iceland.[37] In these cases,
the superficial parts are the most
perfectly glassy, the obsidian passing at the depth of a few feet into
an opaque stone. In an analysis by Vauquelin of a specimen of obsidian
from Hecla, which probably flowed as lava, the proportion of silica is
nearly the same as in the nodular or concretionary obsidian from
Mexico. It would be interesting to ascertain, whether the opaque
interior portions and the superficial glassy coating contained the same
proportional constituent parts: we know from M. Dufrénoy[38] that the
exterior and interior parts of the same stream of lava sometimes differ
considerably in their composition. Even should the whole body of the
stream of obsidian turn out to be similarly composed with nodular
obsidian, it would only be necessary, in accordance with the foregoing
facts, to suppose that lava in these instances had been erupted with
its ingredients mixed in the same proportion, as in the concretionary
obsidian.

 [37] For Teneriffe, see von Buch, “Descript. des Isles Canaries,” pp.
 184 and 190; for the Lipari Islands, see Dolomieu’s “Voyage,” p. 34;
 for Iceland, see Mackenzie’s “Travels,” p. 369.


 [38] “Mémoires pour servir a une Descript. Géolog. de la France,” tome
 iv, p. 371.

_Lamination of volcanic rocks of the trachytic series._

We have seen that, in several and widely distant countries, the strata
alternating with beds of obsidian, are highly laminated. The nodules,
also, both large and small, of the obsidian, are zoned with different
shades of colour; and I have seen a specimen from Mexico in Mr. Stokes’
collection, with its external surface weathered[39] into ridges and
furrows, corresponding with the zones of different degrees of
glassiness: Humboldt,[40] moreover, found on the Peak of Teneriffe, a
stream of obsidian divided by very thin, alternating, layers of pumice.
Many other lavas of the feldspathic series are laminated; thus, masses
of common trachyte at Ascension are divided by fine earthy lines, along
which the rock splits, separating thin layers of slightly different
shades of colour; the greater number, also, of the embedded crystals of
glassy feldspar are placed lengthways in the same direction. Mr. P.
Scrope[41] has described a remarkable columnar trachyte in the Panza
Islands, which seems to have been injected into an overlying mass of
trachytic conglomerate: it is striped with zones, often of extreme
tenuity, of different textures and colours; the harder and darker zones
appearing to contain a larger proportion of silica. In another part of
the island, there are layers of pearlstone and pitchstone, which in
many respects resemble those of Ascension. The zones in the columnar
trachyte are generally contorted; they extend uninterruptedly for a
great length in a vertical direction, and apparently parallel to the
walls of the dike-like mass. Von Buch[42] has described at Teneriffe, a
stream of lava
containing innumerable thin, plate-like crystals of feldspar, which are
arranged like white threads, one behind the other, and which mostly
follow the same direction. Dolomieu[43] also states, that the grey
lavas of the modern cone of Vulcano, which have a vitreous texture, are
streaked with parallel white lines: he further describes a solid
pumice-stone which possesses a fissile structure, like that of certain
micaceous schists. Phonolite, which I may observe is often, if not
always, an injected rock, also, often has a fissile structure; this is
generally due to the parallel position of the embedded crystals of
feldspar, but sometimes, as at Fernando Noronha, seems to be nearly
independent of their presence.[44] From these facts we see, that
various rocks of the feldspathic series have either a laminated or
fissile structure, and that it occurs both in masses which have
injected into overlying strata, and in others which have flowed as
streams of lava.

 [39] MacCulloch states (“Classification of Rocks,” p. 531), that the
 exposed surfaces of the pitchstone dikes in Arran are furrowed “with
 undulating lines, resembling certain varieties of marbled paper, and
 which evidently result from some corresponding difference of laminar
 structure.”


 [40] “Personal Narrative,” vol. i, p. 222.


 [41] “Geological Transactions,” vol. ii (second series), p. 195.


 [42] “Description des Iles Canaries,” p. 184.


 [43] “Voyage aux Isles de Lipari,” pp. 35 and 85.


 [44] In this case, and in that of the fissile pumice-stone, the
 structure is very different from that in the foregoing cases, where
 the laminæ consist of alternate layers of different composition or
 texture. In some sedimentary formations, however, which apparently are
 homogeneous and fissile, as in glossy clay-slate, there is reason to
 believe, according to D’Aubuisson, that the laminæ are really due to
 excessively thin, alternating, layers of mica.


The laminæ of the beds, alternating with the obsidian at Ascension, dip
at a high angle under the mountain, at the base of which they are
situated; and they do not appear as if they had been inclined by
violence. A high inclination is common to these beds in Mexico, Peru,
and in some of the Italian islands:[45] on the other hand, in Hungary,
the layers are horizontal; the laminæ, also, of some of the
lava-streams above referred to, as far as I can understand the
descriptions given of them, appear to be highly inclined or vertical. I
doubt whether in any of these cases, the laminæ have been tilted into
their present position; and in some instances, as in that of the
trachyte described by Mr. Scrope, it is almost certain that they have
been originally formed with a high inclination. In many of these cases,
there is evidence that the mass of liquified rock has moved in the
direction of the laminæ. At Ascension, many of the air-cells have a
drawn out appearance, and are crossed by coarse semi-glassy fibres, in
the direction of the laminæ; and some of the layers, separating the
sphærulitic globules, have a scored appearance, as if produced by the
grating of the globules. I have seen a specimen of zoned obsidian from
Mexico, in Mr. Stokes’ collection, with the surfaces of the
best-defined layers streaked or furrowed with parallel lines; and these
lines or streaks precisely resembled those, produced on the surface of
a mass of artificial glass by its having been poured out of a vessel.
Humboldt, also, has described little cavities, which he compares to the
tails of comets, behind sphærulites in laminated obsidian rocks from
Mexico, and Mr. Scrope has
described other cavities behind fragments embedded in his laminated
trachyte, and which he supposes to have been produced during the
movement of the mass.[46] From such facts, most authors have attributed
the lamination of these volcanic rocks to their movement whilst
liquified. Although it is easy to perceive, why each separate air-cell,
or each fibre in pumice-stone,[47] should be drawn out in the direction
of the moving mass; it is by no means at first obvious why such
air-cells and fibres should be arranged by the movement, in the same
planes, in laminæ absolutely straight and parallel to each other, and
often of extreme tenuity; and still less obvious is it, why such layers
should come to be of slightly different composition and of different
textures.

 [45] See Phillips’ “Mineralogy,” for the Italian Islands, p. 136. For
 Mexico and Peru, see Humboldt’s “Essai Géognostique.” Mr. Edwards also
 describes the high inclination of the obsidian rocks of the Cerro del
 Navaja in Mexico in the _Proc. of the Geolog. Soc._ June 1838.


 [46] “Geological Transactions,” vol. ii (second series), p. 200 etc.
 These embedded fragments, in some instances, consist of the laminated
 trachyte broken off and “enveloped in those parts, which still
 remained liquid.” Beudant, also, frequently refers in his great work
 on “Hungary” (tome iii, p. 386), to trachytic rocks, irregularly
 spotted with fragments of the same varieties, which in other parts
 form the parallel ribbons. In these cases, we must suppose, that after
 part of the molten mass had assumed a laminated structure, a fresh
 irruption of lava broke up the mass, and involved fragments, and that
 subsequently the whole became relaminated.


 [47] Dolomieu’s “Voyage,” p. 64.


In endeavouring to make out the cause of the lamination of these
igneous feldspathic rocks, let us return to the facts so minutely
described at Ascension. We there see, that some of the thinnest layers
are chiefly formed by numerous, exceedingly minute, though perfect,
crystals of different minerals; that other layers are formed by the
union of different kinds of concretionary globules, and that the layers
thus formed, often cannot be distinguished from the ordinary
feldspathic and pitchstone layers, composing a large portion of the
entire mass. The fibrous radiating structure of the sphærulites seems,
judging from many analogous cases, to connect the concretionary and
crystalline forces: the separate crystals, also, of feldspar all lie in
the same parallel planes.[48] These allied forces, therefore, have
played an important part in the lamination of the mass, but they cannot
be considered the primary force; for the several kinds of nodules, both
the smallest and largest, are internally zoned with excessively fine
shades of colour, parallel to the lamination of the whole; and many of
them are, also, externally marked in the same direction with parallel
ridges and furrows, which have not been produced by weathering.

 [48] The formation, indeed, of a large crystal of any mineral in a
 rock of mixed composition implies an aggregation of the requisite
 atoms, allied to concretionary action. The cause of the crystals of
 feldspar in these rocks of Ascension, being all placed lengthways, is
 probably the same with that which elongates and flattens all the brown
 sphærulitic globules (which behave like feldspar under the blowpipe)
 in this same direction.

Some of the finest streaks of colour in the stony layers, alternating
with the obsidian, can be distinctly seen to be due to an incipient
crystallisation of the constituent minerals. The extent to which the
minerals have crystallised can, also, be distinctly seen to be
connected
with the greater or less size, and with the number, of the minute,
flattened, crenulated air-cavities or fissures. Numerous facts, as in
the case of geodes, and of cavities in silicified wood, in primary
rocks, and in veins, show that crystallisation is much favoured by
space. Hence, I conclude, that, if in a mass of cooling volcanic rock,
any cause produced in parallel planes a number of minute fissures or
zones of less tension (which from the pent-up vapours would often be
expanded into crenulated air-cavities), the crystallisation of the
constituent parts, and probably the formation of concretions, would be
superinduced or much favoured in such planes; and thus, a laminated
structure of the kind we are here considering would be generated.

That some cause does produce parallel zones of less tension in volcanic
rocks, during their consolidation, we must admit in the case of the
thin alternate layers of obsidian and pumice described by Humboldt, and
of the small, flattened, crenulated air-cells in the laminated rocks of
Ascension; for on no other principle can we conceive why the confined
vapours should through their expansion form air-cells or fibres in
separate, parallel planes, instead of irregularly throughout the mass.
In Mr. Stokes’ collection, I have seen a beautiful example of this
structure, in a specimen of obsidian from Mexico, which is shaded and
zoned, like the finest agate, with numerous, straight, parallel layers,
more or less opaque and white, or almost perfectly glassy; the degree
of opacity and glassiness depending on the number of microscopically
minute, flattened air-cells; in this case, it is scarcely possible to
doubt but that the mass, to which the fragment belonged, must have been
subjected to some, probably prolonged, action, causing the tension
slightly to vary in the successive planes.

Several causes appear capable of producing zones of different tension,
in masses semi-liquified by heat. In a fragment of devitrified glass, I
have observed layers of sphærulites which appeared, from the manner in
which they were abruptly bent, to have been produced by the simple
contraction of the mass in the vessel, in which it cooled. In certain
dikes on Mount Etna, described by M. Elie de Beaumont,[49] as bordered
by alternating bands of scoriaceous and compact rock, one is led to
suppose that the stretching movement of the surrounding strata, which
originally produced the fissures, continued whilst the injected rock
remained fluid. Guided, however, by Professor Forbes’[50] clear
description of the zoned structure of glacier-ice, far the most
probable explanation of the laminated structure of these feldspathic
rocks appears to be, that they have been stretched whilst slowly
flowing onwards in a pasty condition,[51] in precisely the same manner
as Professor Forbes believes, that the ice of moving glaciers is
stretched and fissured. In both cases,
the zones may be compared to those in the finest agates; in both, they
extend in the direction in which the mass has flowed, and those exposed
on the surface are generally vertical: in the ice, the porous laminæ
are rendered distinct by the subsequent congelation of infiltrated
water, in the stony feldspathic lavas, by subsequent crystalline and
concretionary action. The fragment of glassy obsidian in Mr. Stokes’
collection, which is zoned with minute air-cells must strikingly
resemble, judging from Professor Forbes’ descriptions, a fragment of
the zoned ice; and if the rate of cooling and nature of the mass had
been favourable to its crystallisation or to concretionary action, we
should here have had the finest parallel zones of different composition
and texture. In glaciers, the lines of porous ice and of minute
crevices seem to be due to an incipient stretching, caused by the
central parts of the frozen stream moving faster than the sides and
bottom, which are retarded by friction: hence in glaciers of certain
forms and towards the lower end of most glaciers, the zones become
horizontal. May we venture to suppose that in the feldspathic lavas
with horizontal laminæ, we see an analogous case? All geologists, who
have examined trachytic regions, have come to the conclusion, that the
lavas of this series have possessed an exceedingly imperfect fluidity;
and as it is evident that only matter thus characterised would be
subject to become fissured and to be formed into zones of different
tensions, in the manner here supposed, we probably see the reason why
augitic lavas, which appear generally to have possessed a high degree
of fluidity, are not,[52] like the feldspathic lavas, divided into
laminæ of different composition and texture. Moreover, in the augitic
series, there never appears to be any tendency to concretionary action,
which we have seen plays an important part in the lamination of rocks,
of the trachytic series, or at least in rendering that structure
apparent.

 [49] “Mém. pour servir,” etc., tome iv, p. 131.


 [50] _Edinburgh New Phil. Journal,_ 1842, p. 350.


 [51] I presume that this is nearly the same explanation which Mr.
 Scrope had in his mind, when he speaks (“Geolog. Transact.,” vol. ii,
 second series, p. 228) of the ribboned structure of his trachytic
 rocks, having arisen, from “a linear extension of the mass, while in a
 state of imperfect liquidity, coupled with a concretionary process.”


 [52] Basaltic lavas, and many other rocks, are not unfrequently
 divided into thick laminæ or plates, of the same composition, which
 are either straight or curved; these being crossed by vertical lines
 of fissure, sometimes become united into columns. This structure seems
 related, in its origin, to that by which many rocks, both igneous and
 sedimentary, become traversed by parallel systems of fissures.

Whatever may be thought of the explanation here advanced of the
laminated structure of the rocks of the trachytic series, I venture to
call the attention of geologists to the simple fact, that in a body of
rock at Ascension, undoubtedly of volcanic origin, layers often of
extreme tenuity, quite straight, and parallel to each other, have been
produced;—some composed of distinct crystals of quartz and diopside,
mingled with amorphous augitic specks and granular feldspar,—others
entirely composed of these black augitic specks, with granules of oxide
of iron,—and lastly, others formed of crystalline feldspar, in a more
or less perfect state of purity, together with numerous crystals of
feldspar, placed lengthways. At this island, there is reason to
believe, and in some analogous cases, it is certainly known, that the
laminæ have originally been formed with their present high inclination.
Facts of this nature are manifestly of importance, with relation to the
structural
origin of that grand series of plutonic rocks, which like the volcanic
have undergone the action of heat, and which consist of alternate
layers of quartz, feldspar, mica and other minerals.




Chapter IV ST. HELENA


Lavas of the feldspathic, basaltic, and submarine series.—Section of
Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
and a parapet. Cones of phonolite. Superficial beds of calcareous
sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
land.—Denudation.—Craters of elevation.


The whole island is of volcanic origin; its circumference, according to
Beatson,[1] is about twenty-eight miles. The central and largest part
consists of rocks of a feldspathic nature, generally decomposed to an
extraordinary degree; and when in this state, presenting a singular
assemblage of alternating, red, purple, brown, yellow, and white, soft,
argillaceous beds. From the shortness of our visit, I did not examine
these beds with care; some of them, especially those of the white,
yellow, and brown shades, originally existed as streams of lava, but
the greater number were probably ejected in the form of scoriæ and
ashes: other beds of a purple tint, porphyritic with crystal-shaped
patches of a white, soft substance, which are now unctuous, and yield,
like wax, a polished streak to the nail, seem once to have existed as
solid claystone-porphyries: the red argillaceous beds generally have a
brecciated structure, and no doubt have been formed by the
decomposition of scoriæ. Several extensive streams, however, belonging
to this series, retain their stony character; these are either of a
blackish-green colour, with minute acicular crystals of feldspar, or of
a very pale tint, and almost composed of minute, often scaly, crystals
of feldspar, abounding with microscopical black specks; they are
generally compact and laminated; others, however, of similar
composition, are cellular and somewhat decomposed. None of these rocks
contain large crystals of feldspar, or have the harsh fracture peculiar
to trachyte. These feldspathic lavas and tuffs are the uppermost or
those last erupted; innumerable dikes, however, and great masses of
molten rock, have subsequently been injected into them. They converge,
as they rise, towards the central curved ridge, of which one point
attains the elevation of 2,700 feet. This ridge is the highest land in
the island; and it once formed the northern rim of a great crater,
whence the lavas of this series flowed: from its ruined condition, from
the southern half having been removed, and from the violent dislocation
which the whole island has undergone, its structure is rendered very
obscure.

 [1] Governor Beatson’s “Account of St. Helena.”


_Basaltic series._—The margin of the island is formed by a rude circle
of great, black, stratified, ramparts of basalt, dipping seaward, and
worn into cliffs, which are often nearly perpendicular, and vary in
height from a few hundred feet to two thousand. This circle, or rather
horse-shoe shaped ring, is open to the south, and is breached by
several other wide spaces. Its rim or summit generally projects little
above the level of the adjoining inland country; and the more recent
feldspathic lavas, sloping down from the central heights, generally
abut against and overlap its inner margin; on the north-western side of
the island, however, they appear (judging from a distance) to have
flowed over and concealed portions of it. In some parts, where the
basaltic ring has been breached, and the black ramparts stand detached,
the feldspathic lavas have passed between them, and now overhang the
sea-coast in lofty cliffs. The basaltic rocks are of a black colour and
thinly stratified; they are generally highly vesicular, but
occasionally compact; some of them contain numerous crystals of glassy
feldspar and octahedrons of titaniferous iron; others abound with
crystals of augite and grains of olivine. The vesicles are frequently
lined with minute crystals (of chabasie?) and even become amygdaloidal
with them. The streams are separated from each other by cindery matter,
or by a bright red, friable, saliferous tuff, which is marked by
successive lines like those of aqueous deposition; and sometimes it has
an obscure, concretionary structure. The rocks of this basaltic series
occur nowhere except near the coast. In most volcanic districts the
trachytic lavas are of anterior origin to the basaltic; but here we
see, that a great pile of rock, closely related in composition to the
trachytic family, has been erupted subsequently to the basaltic strata:
the number, however, of dikes, abounding with large crystals of augite,
with which the feldspathic lavas have been injected, shows perhaps some
tendency to a return to the more usual order of superposition.

_Basal submarine lavas._—The lavas of this basal series lie immediately
beneath both the basaltic and feldspathic rocks. According to Mr.
Seale,[2] they may be seen at intervals on the sea-beach round the
entire island. In the sections which I examined, their nature varied
much; some of the strata abound with crystals of augite; others are of
a brown colour, either laminated or in a rubbly condition; and many
parts are highly amygdaloidal with calcareous matter. The successive
sheets are either closely united together, or are separated from each
other by beds of scoriaceous rock and of laminated tuff, frequently
containing well-rounded fragments. The interstices of these beds are
filled with gypsum and salt; the gypsum also sometimes occurring in
thin layers. From the large quantity of these two substances, from the
presence of rounded pebbles in the tuffs, and from the abundant
amygdaloids, I cannot doubt that these basal volcanic strata flowed
beneath the sea. This remark ought perhaps to be extended to a part of
the superincumbent basaltic rocks; but on this point, I was not able to
obtain clear evidence. The
strata of the basal series, whenever I examined them, were intersected
by an extraordinary number of dikes.

 [2] “Geognosy of the Island of St. Helena.” Mr. Seale has constructed
 a gigantic model of St. Helena, well worth visiting, which is now
 deposited at Addiscombe College, in Surrey.

_Flagstaff Hill and the Barn._—I will now describe some of the more
remarkable sections, and will commence with these two hills, which form
the principal external feature on the north-eastern side of the island.
The square, angular outline, and black colour of the Barn, at once show
that it belongs to the basaltic series; whilst the smooth, conical
figure, and the varied bright tints of Flagstaff Hill, render it
equally clear, that it is composed of the softened, feldspathic rocks.
These two lofty hills are connected (as is shown in figure No. 8) by a
sharp ridge, which is composed of the rubbly lavas of the basal series.
The strata of this ridge dip westward, the inclination becoming less
and less towards the Flagstaff; and the upper feldspathic strata of
this hill can be seen, though with some difficulty, to dip conformably
to the W.S.W. Close to the Barn, the strata of the ridge are nearly
vertical, but are much obscured by innumerable dikes; under this hill,
they probably change from being vertical into being inclined into an
opposite direction; for the upper or basaltic strata, which are about
eight hundred or one thousand feet in thickness, are inclined
north-eastward, at an angle between thirty and forty degrees.

No. 8


[Illustration: Flagstaff Hill and the Barn.]

The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata; the dikes
are shaded transversely.


This ridge, and likewise the Barn and Flagstaff Hills, are interlaced
by dikes, many of which preserve a remarkable parallelism in a N.N.W.
and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic
with large crystals of augite; others are formed of a fine-grained and
brown-coloured trap. Most of these dikes are coated by a glossy
layer,[3] from one to two-tenths of an inch in thickness, which, unlike
true pitchstone, fuses into a black enamel; this layer is evidently
analogous to the glossy superficial coating of many lava streams. The
dikes can often be followed for great lengths both horizontally and
vertically, and
they seem to preserve a nearly uniform thickness:[4] Mr. Seale states,
that one near the Barn, in a height of 1,260 feet, decreases in width
only four inches,—from nine feet at the bottom, to eight feet and eight
inches at the top. On the ridge, the dikes appear to have been guided
in their course, to a considerable degree, by the alternating soft and
hard strata: they are often firmly united to the harder strata, and
they preserve their parallelism for such great lengths, that in very
many instances it was impossible to conjecture, which of the beds were
dikes, and which streams of lava. The dikes, though so numerous on this
ridge, are even more numerous in the valleys a little south of it, and
to a degree I never saw equalled anywhere else: in these valleys they
extend in less regular lines, covering the ground with a network, like
a spider’s web, and with some parts of the surface even appearing to
consist wholly of dikes, interlaced by other dikes.

 [3] This circumstance has been observed (Lyell, “Principles of
 Geology,” vol. iv, chap. x, p. 9) in the dikes of the Atrio del
 Cavallo, but apparently it is not of very common occurrence. Sir G.
 Mackenzie, however, states (p. 372, “Travels in Iceland”) that all the
 veins in Iceland have a “black vitreous coating on their sides.”
 Captain Carmichael, speaking of the dikes in Tristan d’Acunha, a
 volcanic island in the Southern Atlantic, says (“Linnæan
 Transactions,” vol. xii, p. 485) that their sides, “where they come in
 contact with the rocks, are invariably in a semi-vitrified state.”


 [4] “Geognosy of the Island of St. Helena,” plate 5.


From the complexity produced by the dikes, from the high inclination
and anticlinal dip of the strata of the basal series, which are
overlaid, at the opposite ends of the short ridge, by two great masses
of different ages and of different composition, I am not surprised that
this singular section has been misunderstood. It has even been supposed
to form part of a crater; but so far is this from having been the case,
that the summit of Flagstaff Hill once formed the lower extremity of a
sheet of lava and ashes, which were erupted from the central,
crateriform ridge. Judging from the slope of the contemporaneous
streams in an adjoining and undisturbed part of the island, the strata
of the Flagstaff Hill must have been upturned at least twelve hundred
feet, and probably much more, for the great truncated dikes on its
summit show that it has been largely denuded. The summit of this hill
now nearly equals in height the crateriform ridge; and before having
been denuded, it was probably higher than this ridge, from which it is
separated by a broad and much lower tract of country; we here,
therefore, see that the lower extremities of a set of lava-streams have
been tilted up to as great a height as, or perhaps greater height than,
the crater, down the flanks of which they originally flowed. I believe
that dislocations on so grand a scale are extremely rare[5] in volcanic
districts. The formation of such numbers of dikes in this part of the
island shows that the surface must here have been stretched to a quite
extraordinary degree: this stretching, on the ridge between Flagstaff
and Barn Hills, probably took place subsequently (though perhaps
immediately so) to the strata being tilted; for had the strata at that
time extended horizontally, they would in all probability have been
fissured and injected transversely, instead of in the planes of their
stratification. Although the space between the Barn and Flagstaff Hill
presents a distinct anticlinal line extending north and south, and
though most of the dikes range with much regularity in the same line,
nevertheless, at only a mile due south of the ridge the strata lie
undisturbed. Hence the disturbing force seems to have acted under
a point, rather than along a line. The manner in which it has acted, is
probably explained by the structure of Little Stony-top, a mountain
2,000 feet high, situated a few miles southward of the Barn; we there
see, even from a distance, a dark-coloured, sharp, wedge of compact
columnar rock, with the bright-coloured feldspathic strata, sloping
away on each side from its uncovered apex. This wedge, from which it
derives its name of Stony-top, consists of a body of rock, which has
been injected whilst liquified into the overlying strata; and if we may
suppose that a similar body of rock lies injected, beneath the ridge
connecting the Barn and Flagstaff, the structure there exhibited would
be explained.

 [5] M. Constant Prevost (“Mém. de la Soc. Géolog.,” tome ii) observes
 that “les produits volcaniques n’ont que localement et rarement même
 dérangé le sol, à travers lequel ils se sont fait jour.”


No. 9


[Illustration: Prosperous Hill and The Barn.]

The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata.

_Turk’s Cap and Prosperous Bays._—Prosperous Hill is a great, black,
precipitous mountain, situated two miles and a half south of the Barn,
and composed, like it, of basaltic strata. These rest, in one part, on
the brown-coloured, porphyritic beds of the basal series, and in
another part, on a fissured mass of highly scoriaceous and amygdaloidal
rock, which seems to have formed a small point of eruption beneath the
sea, contemporaneously with the basal series. Prosperous Hill, like the
Barn, is traversed by many dikes, of which the greater number range
north and south, and its strata dip, at an angle of about 20 degrees,
rather obliquely from the island towards the sea. The space between
Prosperous Hill and the Barn, as represented in figure No. 9, consists
of lofty cliffs, composed of the lavas of the upper or feldspathic
series, which rest, though unconformably, on the basal submarine
strata, as we have seen that they do at Flagstaff Hill. Differently,
however, from in that hill, these upper strata are nearly horizontal,
gently rising towards the interior of the island; and they are composed
of greenish-black, or more commonly, pale brown, compact lavas, instead
of softened and highly coloured matter. These brown-coloured, compact
lavas, consist almost entirely of small glimmering scales, or of minute
acicular crystals, of feldspar, placed close by the side of each other,
and abounding with minute black specks, apparently of hornblende. The
basaltic strata of Prosperous Hill project only a little above the
level of the gently-sloping, feldspathic streams, which wind round and
abut against their upturned edges. The inclination of the basaltic
strata seems to be too great to have been caused by their having flowed
down a slope, and they must have been tilted into their present
position before the eruption of the feldspathic streams.

_Basaltic ring._—Proceeding round the Island, the lavas of the upper
series, southward of Prosperous Hill, overhang the sea in lofty
precipices. Further on, the headland, called Great Stony-top, is
composed, as I
believe, of basalt; as is Long Range Point, on the inland side of which
the coloured beds abut. On the southern side of the island, we see the
basaltic strata of the South Barn, dipping obliquely seaward at a
considerable angle; this headland, also, stands a little above the
level of the more modern, feldspathic lavas. Further on, a large space
of coast, on each side of Sandy Bay, has been much denuded, and there
seems to be left only the basal wreck of the great, central crater. The
basaltic strata reappear, with their seaward dip, at the foot of the
hill, called Man-and-Horse; and thence they are continued along the
whole north-western coast to Sugar-Loaf Hill, situated near to the
Flagstaff; and they everywhere have the same seaward inclination, and
rest, in some parts at least, on the lavas of the basal series. We thus
see that the circumference of the island is formed by a much-broken
ring, or rather, a horse-shoe, of basalt, open to the south, and
interrupted on the eastern side by many wide breaches. The breadth of
this marginal fringe on the north-western side, where alone it is at
all perfect, appears to vary from a mile to a mile and a half. The
basaltic strata, as well as those of the subjacent basal series, dip,
with a moderate inclination, where they have not been subsequently
disturbed, towards the sea. The more broken state of the basaltic ring
round the eastern half, compared with the western half of the island,
is evidently due to the much greater denuding power of the waves on the
eastern or windward side, as is shown by the greater height of the
cliffs on that side, than to leeward. Whether the margin of basalt was
breached, before or after the eruption of the lavas of the upper
series, is doubtful; but as separate portions of the basaltic ring
appear to have been tilted before that event, and from other reasons,
it is more probable, that some at least of the breaches were first
formed. Reconstructing in imagination, as far as is possible, the ring
of basalt, the internal space or hollow, which has since been filled up
with the matter erupted from the great central crater, appears to have
been of an oval figure, eight or nine miles in length by about four
miles in breadth, and with its axis directed in a N.E. and S.W. line,
coincident with the present longest axis of the island.

_The central curved ridge._—This ridge consists, as before remarked, of
grey feldspathic lavas, and of red, brecciated, argillaceous tuffs,
like the beds of the upper coloured series. The grey lavas contain
numerous, minute, black, easily fusible specks; and but very few large
crystals of feldspar. They are generally much softened; with the
exception of this character, and of being in many parts highly
cellular, they are quite similar to those great sheets of lava which
overhang the coast at Prosperous Bay. Considerable intervals of time
appear to have elapsed, judging from the marks of denudation, between
the formation of the successive beds, of which this ridge is composed.
On the steep northern slope, I observed in several sections a much worn
undulating surface of red tuff, covered by grey, decomposed,
feldspathic lavas, with only a thin earthy layer interposed between
them. In an adjoining part, I noticed a trap-dike, four feet wide, cut
off and covered up by the feldspathic lava, as is represented in figure
No. 9. The ridge ends on the eastern side in a hook, which is not
represented clearly enough in any
map which I have seen; towards the western end, it gradually slopes
down and divides into several subordinate ridges. The best defined
portion between Diana’s Peak and Nest Lodge, which supports the highest
pinnacles in the island varying from 2,000 to 2,700 feet, is rather
less than three miles long in a straight line. Throughout this space
the ridge has a uniform appearance and structure; its curvature
resembles that of the coast-line of a great bay, being made up of many
smaller curves, all open to the south. The northern and outer side is
supported by narrow ridges or buttresses, which slope down to the
adjoining country. The inside is much steeper, and is almost
precipitous; it is formed of the basset edges of the strata, which
gently decline outwards. Along some parts of the inner side, a little
way beneath the summit, a flat ledge extends, which imitates in outline
the smaller curvatures of the crest. Ledges of this kind occur not
unfrequently within volcanic craters, and their formation seems to be
due to the sinking down of a level sheet of hardened lava, the edges of
which remain (like the ice round a pool, from which the water has been
drained) adhering to the sides.[6]

 [6] A most remarkable instance of this structure is described in Ellis
 “Polynesian Researches” (second edition), where an admirable drawing
 is given of the successive ledges or terraces, on the borders of the
 immense crater at Hawaii, in the Sandwich Islands.


No. 10


[Illustration: Dike]

1—Grey feldspathic lava.
2—A layer, one inch in thickness, of a reddish earthy matter.
3—Brecciated, red, argillaceous tuff.


In some parts, the ridge is surmounted by a wall or parapet,
perpendicular on both sides. Near Diana’s Peak this wall is extremely
narrow. At the Galapagos Archipelago I observed parapets, having a
quite similar structure and appearance, surmounting several of the
craters; one, which I more particularly examined, was composed of
glossy, red scoriæ firmly cemented together; being externally
perpendicular, and extending round nearly the whole circumference of
the crater, it rendered it almost inaccessible. The Peak of Teneriffe
and Cotopaxi, according to Humboldt, are similarly constructed; he
states[7] that “at their summits a circular wall surrounds the crater,
which wall, at a distance, has the appearance of a small cylinder
placed on a truncated cone. On Cotopaxi[8] this peculiar structure is
visible to the naked eye at more than two thousand toises’ distance;
and no person has ever reached its crater. On the Peak of Teneriffe,
the parapet is so high, that it would be impossible to reach the
caldera, if on the eastern side there did not exist a breach.” The
origin of these circular parapets is probably due to the heat or
vapours from the crater, penetrating and hardening the sides to a
nearly equal depth, and afterwards to the mountain being slowly acted
on by the weather, which would leave the hardened part, projecting in
the form of a cylinder or circular parapet.

 [7] “Personal Narrative,” vol. i, p. 171.


 [8] Humboldt’s “Picturesque Atlas,” folio, pl. 10.


From the points of structure in the central ridge, now
enumerated,—namely, from the convergence towards it of the beds of the
upper series,—from the lavas there becoming highly cellular,—from the
flat ledge, extending along its inner and precipitous side, like that
within some still active craters,—from the parapet-like wall on its
summit,—and lastly, from its peculiar curvature, unlike that of any
common line of elevation, I cannot doubt that this curved ridge forms
the last remnant of a great crater. In endeavouring, however, to trace
its former outline, one is soon baffled; its western extremity
gradually slopes down, and, branching into other ridges, extends to the
sea-coast; the eastern end is more curved, but it is only a little
better defined. Some appearances lead me to suppose that the southern
wall of the crater joined the present ridge near Nest Lodge; in this
case the crater must have been nearly three miles long, and about a
mile and a half in breadth. Had the denudation of the ridge and the
decomposition of its constituent rocks proceeded a few steps further,
and had this ridge, like several other parts of the island, been broken
up by great dikes and masses of injected matter, we should in vain have
endeavoured to discover its true nature. Even now we have seen that at
Flagstaff Hill the lower extremity and most distant portion of one
sheet of the erupted matter has been upheaved to as great a height as
the crater down which it flowed, and probably even to a greater height.
It is interesting thus to trace the steps by which the structure of a
volcanic district becomes obscured, and finally obliterated: so near to
this last stage is St. Helena, that I believe no one has hitherto
suspected that the central ridge or axis of the island is the last
wreck of the crater, whence the most modern volcanic streams were
poured forth.

The great hollow space or valley southward of the central curved ridge,
across which the half of the crater must once have extended, is formed
of bare, water-worn hillocks and ridges of red, yellow, and brown
rocks, mingled together in chaos-like confusion, interlaced by dikes,
and without any regular stratification. The chief part consists of red
decomposing scoriæ, associated with various kinds of tuff and yellow
argillaceous beds, full of broken crystals, those of augite being
particularly large. Here and there masses of highly cellular and
amygdaloidal lavas protrude. From one of the ridges in the midst of the
valley, a conical precipitous hill, called Lot, boldly stands up, and
forms a most singular and conspicuous object. It is composed of
phonolite, divided in one part into great curved laminæ, in another,
into angular concretionary balls, and in a third part into outwardly
radiating columns. At its base the strata of lava, tuff, and scoriæ,
dip away on all sides;[9] the uncovered portion is 197 feet[10] in
height, and its horizontal section gives an oval figure. The phonolite
is of a greenish-grey
colour, and is full of minute acicular crystals of feldspar; in most
parts it has a conchoidal fracture, and is sonorous, yet it is
crenulated with minute air-cavities. In a S.W. direction from Lot,
there are some other remarkable columnar pinnacles, but of a less
regular shape, namely, Lot’s Wife, and the Asses’ Ears, composed of
allied kinds of rock. From their flattened shape, and their relative
position to each other, they are evidently connected on the same line
of fissure. It is, moreover, remarkable that this same N.E. and S.W.
line, joining Lot and Lot’s Wife, if prolonged would intersect
Flagstaff Hill, which, as before stated, is crossed by numerous dikes
running in this direction, and which has a disturbed structure,
rendering it probable that a great body of once fluid rock lies
injected beneath it.

 [9] Abich in his “Views of Vesuvius” (plate vi), has shown the manner
 in which beds, under nearly similar circumstances, are tilted up. The
 upper beds are more turned up than the lower; and he accounts for
 this, by showing that the lava insinuates itself horizontally between
 the lower beds.


 [10] This height is given by Mr. Seale in his Geognosy of the island.
 The height of the summit above the level of the sea is said to be
 1,444 feet.


In this same great valley there are several other conical masses of
injected rock (one, I observed, was composed of compact greenstone),
some of which are not connected, as far as is apparent, with any line
of dike; whilst others are obviously thus connected. Of these dikes,
three or four great lines stretch across the valley in a N.E. and S.W.
direction, parallel to that one connecting the Asses’ Ears, Lot’s Wife,
and probably Lot. The number of these masses of injected rock is a
remarkable feature in the geology of St. Helena. Besides those just
mentioned, and the hypothetical one beneath Flagstaff Hill, there is
Little Stony-top and others, as I have reason to believe, at the
Man-and-Horse, and at High Hill. Most of these masses, if not all of
them, have been injected subsequently to the last volcanic eruptions
from the central crater. The formation of conical bosses of rock on
lines of fissure, the walls of which are in most cases parallel, may
probably be attributed to inequalities in the tension, causing small
transverse fissures, and at these points of intersection the edges of
the strata would naturally yield, and be easily turned upwards.
Finally, I may remark, that hills of phonolite everywhere are apt[11]
to assume singular and even grotesque shapes, like that of Lot: the
peak at Fernando Noronha offers an instance; at St. Jago, however, the
cones of phonolite, though tapering, have a regular form. Supposing, as
seems probable, that all such hillocks or obelisks have originally been
injected, whilst liquified, into a mould formed by yielding strata, as
certainly has been the case with Lot, how are we to account for the
frequent abruptness and singularity of their outlines, compared with
similarly injected masses of greenstone and basalt? Can it be due to a
less perfect degree of fluidity, which is generally supposed to be
characteristic of the allied trachytic lavas?

 [11] D’Aubuisson, in his “Traité de Géognosie” (tome ii, p. 540)
 particularly remarks that this is the case.

_Superficial deposits._—Soft calcareous sandstone occurs in extensive,
though thin, superficial beds, both on the northern and southern shores
of the island. It consists of very minute, equal-sized, rounded
particles of shells, and other organic bodies, which partially retain
their yellow, brown, and pink colours, and occasionally, though very
rarely, present an obscure trace of their original external forms. I in
vain endeavoured to find a single unrolled fragment of a shell. The
colour of the particles
is the most obvious character by which their origin can be recognised,
the tints being affected (and an odour produced) by a moderate heat, in
the same manner as in fresh shells. The particles are cemented
together, and are mingled with some earthy matter: the purest masses,
according to Beatson, contain 70 per cent of carbonate of lime. The
beds, varying in thickness from two or three feet to fifteen feet, coat
the surface of the ground; they generally lie on that side of the
valley which is protected from the wind, and they occur at the height
of several hundred feet above the level of the sea. Their position is
the same which sand, if now drifted by the trade-wind, would occupy;
and no doubt they thus originated, which explains the equal size and
minuteness of the particles, and likewise the entire absence of whole
shells, or even of moderately-sized fragments. It is remarkable that at
the present day there are no shelly beaches on any part of the coast,
whence calcareous dust could be drifted and winnowed; we must,
therefore, look back to a former period when before the land was worn
into the present great precipices, a shelving coast, like that of
Ascension, was favourable to the accumulation of shelly detritus. Some
of the beds of this limestone are between six hundred and seven hundred
feet above the sea; but part of this height may possibly be due to an
elevation of the land, subsequent to the accumulation of the calcareous
sand.

The percolation of rain-water has consolidated parts of these beds into
a solid rock, and has formed masses of dark brown, stalagmitic
limestone. At the Sugar-Loaf quarry, fragments of rock on the adjoining
slopes[12] have been thickly coated by successive fine layers of
calcareous matter. It is singular, that many of these pebbles have
their entire surfaces coated, without any point of contact having been
left uncovered; hence, these pebbles must have been lifted up by the
slow deposition between them of the successive films of carbonate of
lime. Masses of white, finely oolitic rock are attached to the outside
of some of these coated pebbles. Von Buch has described a compact
limestone at Lanzarote, which seems perfectly to resemble the
stalagmitic deposition just mentioned: it coats pebbles, and in parts
is finely oolitic: it forms a far-extended layer, from one inch to two
or three feet in thickness, and it occurs at the height of 800 feet
above the sea, but only on that side of the island exposed to the
violent north-western winds. Von Buch remarks,[13] that it is not found
in hollows, but only on the unbroken and inclined surfaces of the
mountain. He believes, that it has been deposited by the spray which is
borne over the whole island by these violent winds. It appears,
however, to me much more probable that it has been formed, as at St.
Helena, by the percolation of water through finely comminuted shells:
for when sand is blown on
a much-exposed coast, it always tends to accumulate on broad, even
surfaces, which offer a uniform resistance to the winds. At the
neighbouring island, moreover, of Feurteventura,[14] there is an earthy
limestone, which, according to Von Buch, is quite similar to specimens
which he has seen from St. Helena, and which he believes to have been
formed by the drifting of shelly detritus.

 [12] In the earthy detritus on several parts of this hill, irregular
 masses of very impure, crystallised sulphate of lime occur. As this
 substance is now being abundantly deposited by the surf at Ascension,
 it is possible that these masses may thus have originated; but if so,
 it must have been at a period when the land stood at a much lower
 level. This earthy selenite is now found at a height of between six
 hundred and seven hundred feet.


 [13] “Description des Isles Canaries,” p. 293.


 [14] _Idem,_ pp. 314 and 374.


The upper beds of the limestone, at the above-mentioned quarry on the
Sugar-Loaf Hill, are softer, finer-grained and less pure, than the
lower beds. They abound with fragments of land-shells, and with some
perfect ones; they contain, also, the bones of birds, and the large
eggs,[15] apparently of water-fowl. It is probable that these upper
beds remained long in an unconsolidated form, during which time, these
terrestrial productions were embedded. Mr. G. R. Sowerby has kindly
examined three species of land-shells, which I procured from this bed,
and has described them in detail. One of them is a Succinea, identical
with a species now living abundantly on the island; the two others,
namely, _ Cochlogena fossilis_ and _Helix biplicata_, are not known in
a recent state: the latter species was also found in another and
different locality, associated with a species of Cochlogena which is
undoubtedly extinct.

 [15] Colonel Wilkes, in a catalogue presented with some specimens to
 the Geological Society, states that as many as ten eggs were found by
 one person. Dr. Buckland has remarked (“Geolog. Trans.,” vol. v, p.
 474) on these eggs.


_Beds of extinct land-shells._—Land-shells, all of which appear to be
species now extinct, occur embedded in earth, in several parts of the
island. The greater number have been found at a considerable height on
Flagstaff Hill. On the N.W. side of this hill, a rain-channel exposes a
section of about twenty feet in thickness, of which the upper part
consists of black vegetable mould, evidently washed down from the
heights above, and the lower part of less black earth, abounding with
young and old shells, and with their fragments: part of this earth is
slightly consolidated by calcareous matter, apparently due to the
partial decomposition of some of the shells. Mr. Seale, an intelligent
resident, who first called attention to these shells, gave me a large
collection from another locality, where the shells appear to have been
embedded in very black earth. Mr. G. R. Sowerby has examined these
shells, and has described them. There are seven species, namely, one
Cochlogena, two species of the genus Cochlicopa, and four of Helix;
none of these are known in a recent state, or have been found in any
other country. The smaller species were picked out of the inside of the
large shells of the _Cochlogena aurisvulpina._ This last-mentioned
species is in many respects a very singular one; it was classed, even
by Lamarck, in a marine genus, and having thus been mistaken for a
sea-shell, and the smaller accompanying species having been overlooked,
the exact localities where it was found have been measured, and the
elevation of this island thus deduced! It is very remarkable that all
the shells of this species found by me in one spot, form a distinct
variety, as described by Mr. Sowerby, from those
procured from another locality by Mr. Seale. As this Cochlogena is a
large and conspicuous shell, I particularly inquired from several
intelligent countrymen whether they had ever seen it alive; they all
assured me that they had not, and they would not even believe that it
was a land animal: Mr. Seale, moreover, who was a collector of shells
all his life at St. Helena, never met with it alive. Possibly some of
the smaller species may turn out to be yet living kinds; but, on the
other hand, the two land-shells which are now living on the island in
great numbers, do not occur embedded, as far as is yet known, with the
extinct species. I have shown in my “Journal,”[16] that the extinction
of these land-shells possibly may not be an ancient event; as a great
change took place in the state of the island about one hundred and
twenty years ago, when the old trees died, and were not replaced by
young ones, these being destroyed by the goats and hogs, which had run
wild in numbers, from the year 1502. Mr. Seale states, that on
Flagstaff Hill, where we have seen that the embedded land-shells are
especially numerous, traces are everywhere discoverable, which plainly
indicate that it was once thickly clothed with trees; at present not
even a bush grows there. The thick bed of black vegetable mould which
covers the shell-bed, on the flanks of this hill, was probably washed
down from the upper part, as soon as the trees perished, and the
shelter afforded by them was lost.

 [16] “Journal of Researches,” p. 582.


_Elevation of the land._—Seeing that the lavas of the basal series,
which are of submarine origin, are raised above the level of the sea,
and at some places to the height of many hundred feet, I looked out for
superficial signs of the elevation of the land. The bottoms of some of
the gorges, which descend to the coast, are filled up to the depth of
about a hundred feet, by rudely divided layers of sand, muddy clay, and
fragmentary masses; in these beds, Mr. Seale has found the bones of the
tropic-bird and of the albatross; the former now rarely, and the latter
never visiting the island. From the difference between these layers,
and the sloping piles of detritus which rest on them, I suspect that
they were deposited, when the gorges stood beneath the sea. Mr. Seale,
moreover, has shown that some of the fissure-like gorges[17] become,
with a concave outline, gradually rather wider at the bottom than at
the top; and this peculiar structure was probably caused by the wearing
action of the sea, when it entered the lower part of these gorges. At
greater heights, the evidence of the rise of the land is even less
clear: nevertheless, in a bay-like depression on the table-land behind
Prosperous Bay, at the height of about a thousand feet, there are
flat-topped masses of rock, which it is scarcely conceivable, could
have been insulated from the surrounding and similar strata, by any
other agency than the denuding action of a sea-beach. Much denudation,
indeed, has been effected at great elevations, which it would not be
easy to explain by any other means: thus, the flat summit of the Barn,
which is 2,000 feet high, presents, according to Mr. Seale, a perfect
network of truncated dikes; on hills like the Flagstaff, formed of soft
rock, we might suppose that the dikes had been worn down and cut off by
meteoric agency, but we can hardly suppose this possible with the hard,
basaltic strata of the Barn.

 [17] A fissure-like gorge, near Stony-top, is said by Mr. Seale to be
 840 feet deep, and only 115 feet in width.


_Coast denudation._—The enormous cliffs, in many parts between one and
two thousand feet in height, with which this prison-like island is
surrounded, with the exception of only a few places, where narrow
valleys descend to the coast, is the most striking feature in its
scenery. We have seen that portions of the basaltic ring, two or three
miles in length by one or two miles in breadth, and from one to two
thousand feet in height, have been wholly removed. There are, also,
ledges and banks of rock, rising out of profoundly deep water, and
distant from the present coast between three and four miles, which,
according to Mr. Seale, can be traced to the shore, and are found to be
the continuations of certain well-known great dikes. The swell of the
Atlantic Ocean has obviously been the active power in forming these
cliffs; and it is interesting to observe that the lesser, though still
great, height of the cliffs on the leeward and partially protected side
of the island (extending from the Sugar-Loaf Hill to South West Point),
corresponds with the lesser degree of exposure. When reflecting on the
comparatively low coasts of many volcanic islands, which also stand
exposed in the open ocean, and are apparently of considerable
antiquity, the mind recoils from an attempt to grasp the number of
centuries of exposure, necessary to have ground into mud and to have
dispersed the enormous cubic mass of hard rock which has been pared off
the circumference of this island. The contrast in the superficial state
of St. Helena, compared with the nearest island, namely, Ascension, is
very striking. At Ascension, the surfaces of the lava-streams are
glossy, as if just poured forth, their boundaries are well defined, and
they can often be traced to perfect craters, whence they were erupted;
in the course of many long walks, I did not observe a single dike; and
the coast round nearly the entire circumference is low, and has been
eaten back (though too much stress must not be placed on this fact, as
the island may have been subsiding) into a little wall only from ten to
thirty feet high. Yet during the 340 years, since Ascension has been
known, not even the feeblest signs of volcanic action have been
recorded.[18] On the other hand, at St. Helena, the course of no one
stream of lava can be traced, either by the state of its boundaries or
of its superficies; the mere wreck of one great crater is left; not the
valleys only, but the surfaces of some of the highest hills, are
interlaced by worn-down dikes, and, in many
places, the denuded summits of great cones of injected rock stand
exposed and naked; lastly, as we have seen, the entire circuit of the
island has been deeply worn back into the grandest precipices.

 [18] In the _Nautical Magazine_ for 1835, p. 642, and for 1838, p.
 361, and in the “Comptes Rendus,” April 1838, accounts are given of a
 series of volcanic phenomena—earthquakes—troubled water—floating
 scoriæ and columns of smoke—which have been observed at intervals
 since the middle of the last century, in a space of open sea between
 longitudes 20° and 22° west, about half a degree south of the equator.
 These facts seem to show, that an island or an archipelago is in
 process of formation in the middle of the Atlantic: a line joining St.
 Helena and Ascension, prolonged, intersects this slowly nascent focus
 of volcanic action.

_Craters of Elevation._

There is much resemblance in structure and in geological history
between St. Helena, St. Jago, and Mauritius. All three islands are
bounded (at least in the parts which I was able to examine) by a ring
of basaltic mountains, now much broken, but evidently once continuous.
These mountains have, or apparently once had, their escarpments steep
towards the interior of the island, and their strata dip outwards. I
was able to ascertain, only in a few cases, the inclination of the
beds; nor was this easy, for the stratification was generally obscure,
except when viewed from a distance. I feel, however, little doubt that,
according to the researches of M. Elie de Beaumont, their average
inclination is greater than that which they could have acquired,
considering their thickness and compactness, by flowing down a sloping
surface. At St. Helena, and at St. Jago, the basaltic strata rest on
older and probably submarine beds of different composition. At all
three islands, deluges of more recent lavas have flowed from the centre
of the island, towards and between the basaltic mountains; and at St.
Helena the central platform has been filled up by them. All three
islands have been raised in mass. At Mauritius the sea, within a late
geological period, must have reached to the foot of the basaltic
mountains, as it now does at St. Helena; and at St. Jago it is cutting
back the intermediate plain towards them. In these three islands, but
especially at St. Jago and at Mauritius, when, standing on the summit
of one of the old basaltic mountains, one looks in vain towards the
centre of the island,—the point towards which the strata beneath one’s
feet, and of the mountains on each side, rudely converge,—for a source
whence these strata could have been erupted; but one sees only a vast
hollow platform stretched beneath, or piles of matter of more recent
origin.

These basaltic mountains come, I presume, into the class of Craters of
elevation: it is immaterial whether the rings were ever completely
formed, for the portions which now exist have so uniform a structure,
that, if they do not form fragments of true craters, they cannot be
classed with ordinary lines of elevation. With respect to their origin,
after having read the works of Mr. Lyell,[19] and of MM. C. Prevost and
Virlet, I cannot believe that the great central hollows have been
formed by a simple dome-shaped elevation, and the consequent arching of
the strata. On the other hand, I have very great difficulty in
admitting that these basaltic mountains are merely the basal fragments
of great volcanoes, of which the summits have either been blown off, or
more probably swallowed up by subsidence. These rings are, in some
instances, so immense, as at St. Jago and at Mauritius, and their
occurrence is so frequent, that I can hardly persuade myself to adopt
this explanation. Moreover, I suspect that the following circumstances,
from their frequent concurrence, are someway connected together,—a
connection not implied in either of the above views: namely, first, the
broken state of the ring; showing that the now detached portions have
been exposed to great denudation, and in some cases, perhaps, rendering
it probable that the ring never was entire; secondly, the great amount
of matter erupted from the central area after or during the formation
of the ring; and thirdly, the elevation of the district in mass. As far
as relates to the inclination of the strata being greater than that
which the basal fragments of ordinary volcanoes would naturally
possess, I can readily believe that this inclination might have been
slowly acquired by that amount of elevation, of which, according to M.
Elie de Beaumont, the numerous upfilled fissures or dikes are the
evidence and the measure,—a view equally novel and important, which we
owe to the researches of that geologist on Mount Etna.

 [19] “Principles of Geology” (fifth edit.), vol. ii, p. 171.


A conjecture, including the above circumstances, occurred to me, when,—
with my mind fully convinced, from the phenomena of 1835 in South
America,[20] that the forces which eject matter from volcanic orifices
and raise continents in mass are identical,—I viewed that part of the
coast of St. Jago, where the horizontally upraised, calcareous stratum
dips into the sea, directly beneath a cone of subsequently erupted
lava. The conjecture is that, during the slow elevation of a volcanic
district or island, in the centre of which one or more orifices
continue open, and thus relieve the subterranean forces, the borders
are elevated more than the central area; and that the portions thus
upraised do not slope gently into the central, less elevated area, as
does the calcareous stratum under the cone at St. Jago, and as does a
large part of the circumference of Iceland,[21] but that they are
separated from it by curved faults.
We might expect, from what we see along ordinary faults, that the
strata on the upraised side, already dipping outwards from their
original formation as lava-streams, would be tilted from the line of
fault, and thus have their inclination increased. According to this
hypothesis, which I am tempted to extend only to some few cases, it is
not probable that the ring would ever be formed quite perfect; and from
the elevation being slow, the upraised portions would generally be
exposed to much denudation, and hence the ring become broken; we might
also expect to find occasional inequalities in the dip of the upraised
masses, as is the case at St. Jago. By this hypothesis the elevation of
the districts in mass, and the flowing of deluges of lava from the
central platforms, are likewise connected together. On this view the
marginal basaltic mountains of the three foregoing islands might still
be considered as forming “Craters of elevation;” the kind of elevation
implied having been slow, and the central hollow or platform having
been formed, not by the arching of the surface, but simply by that part
having been upraised to a less height.

 [20] I have given a detailed account of these phenomena, in a paper
 read before the Geological Society in March 1838. At the instant of
 time, when an immense area was convulsed and a large tract elevated,
 the districts immediately surrounding several of the great vents in
 the Cordillera remained quiescent; the subterranean forces being
 apparently relieved by the eruptions, which then recommenced with
 great violence. An event of somewhat the same kind, but on an
 infinitely smaller scale, appears to have taken place, according to
 Abich (“Views of Vesuvius,” plates i and ix), within the great crater
 of Vesuvius, where a platform on one side of a fissure was raised in
 mass twenty feet, whilst on the other side, a train of small volcanoes
 burst forth in eruption.


 [21] It appears, from information communicated to me in the most
 obliging manner by M. E. Robert, that the circumferential parts of
 Iceland, which are composed of ancient basaltic strata alternating
 with tuff, dip inland, thus forming a gigantic saucer. M. Robert found
 that this was the case, with a few and quite local exceptions, for a
 space of coast several hundred miles in length. I find this statement
 corroborated, as far as regards one place, by Mackenzie in his
 “Travels” (p. 377), and in another place by some MS. notes kindly lent
 me by Dr. Holland. The coast is deeply indented by creeks, at the head
 of which the land is generally low. M. Robert informs me, that the
 inwardly dipping strata appear to extend as far as this line, and that
 their inclination usually corresponds with the slope of the surface,
 from the high coast-mountains to the low land at the head of these
 creeks. In the section described by Sir G. Mackenzie, the dip is 120.
 The interior parts of the island chiefly consist, as far as is known,
 of recently erupted matter. The great size, however, of Iceland,
 equalling the bulkiest part of England, ought perhaps to exclude it
 from the class of islands we have been considering; but I cannot avoid
 suspecting that if the coast-mountains, instead of gently sloping into
 the less elevated central area, had been separated from it by
 irregularly curved faults, the strata would have been tilted seaward,
 and a “Crater of elevation,” like that of St. Jago or that of
 Mauritius, but of much vaster dimensions, would have been formed. I
 will only further remark, that the frequent occurrence of extensive
 lakes at the foot of large volcanoes, and the frequent association of
 volcanic and fresh-water strata, seem to indicate that the areas
 around volcanoes are apt to be depressed beneath the level of the
 adjoining country, either from having been less elevated, or from the
 effects of subsidence.




Chapter V GALAPAGOS ARCHIPELAGO.


Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
basaltic craters, with hollows at their bases.—Albemarle Island, fluid
lavas, their composition.—Craters of tuff, inclination of their
exterior diverging strata, and structure of their interior converging
strata.—James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected
fragments.—Concluding remarks on the craters of tuff, and on the
breached condition of their southern sides.—Mineralogical composition
of the rocks of the archipelago.—Elevation of the land. Direction of
the fissures of eruption.


This archipelago is situated under the equator, at a distance of
between five and six hundred miles from the west coast of South
America. It consists of five principal islands, and of several small
ones, which together are equal in area,[1] but not in extent of land,
to Sicily, conjointly with the Ionian Islands. They are all volcanic:
on two, craters have been seen in eruption, and on several of the other
islands, streams of lava have a recent appearance. The larger islands
are chiefly composed of solid rock, and they rise with a tame outline
to a height of between one and four thousand feet. They are sometimes,
but not generally, surmounted by one principal orifice. The craters
vary in size from mere spiracles to huge caldrons several miles in
circumference; they are extraordinarily numerous, so that I should
think, if enumerated, they would be found to exceed two thousand; they
are formed either of scoriæ and lava, or of a brown-coloured tuff; and
these latter craters are in several respects remarkable. The whole
group was surveyed by the officers of the _Beagle._ I visited myself
four of the principal islands, and received specimens from all the
others. Under the head of the different islands I will describe only
that which appears to me deserving of attention.

 [1] I exclude from this measurement, the small volcanic islands of
 Culpepper and Wenman, lying seventy miles northward of the group.
 Craters were visible on all the islands of the group, except on Towers
 Island, which is one of the lowest; this island is, however, formed of
 volcanic rocks.


No. 11


[Illustration: Galapagos Archipelago.]

Galapagos Archipelago


CHATHAM ISLAND. _Craters composed of a singular kind of tuff._—Towards
the eastern end of this island there occur two craters composed of two
kinds of tuff; one kind being friable, like slightly consolidated
ashes; and the other compact, and of a different nature from anything
which I have met with described. This latter substance, where it is
best characterised, is of a yellowish-brown colour, translucent, and
with a lustre somewhat resembling resin; it is brittle, with an
angular, rough, and very irregular fracture, sometimes, however, being
slightly granular, and even obscurely crystalline: it can readily be
scratched with a knife, yet some points are hard enough just to mark
common glass; it fuses with ease into a blackish-green glass. The mass
contains numerous broken crystals of olivine and augite, and small
particles of black and brown scoriæ; it is often traversed by thin
seams of calcareous matter. It generally affects a nodular or
concretionary structure. In a hand specimen, this substance would
certainly be mistaken for a pale and peculiar variety of pitchstone;
but when seen in mass its stratification, and the numerous layers of
fragments of basalt, both angular and rounded, at once render its
subaqueous origin evident. An examination of a series of specimens
shows that this resin-like substance results from a chemical change on
small particles of pale and dark-coloured scoriaceous rocks; and this
change could be distinctly traced in different stages round the edges
of even the same particle. The position near the coast of all the
craters composed of this kind of tuff or peperino, and their breached
condition, renders it probable that they were all formed when standing
immersed in the sea; considering this circumstance, together with the
remarkable absence of large beds of ashes in the whole archipelago, I
think it highly probable that much the greater part of the tuff has
originated from the trituration of fragments of the grey, basaltic
lavas in the mouths of craters standing in the sea. It may be asked
whether the heated water within these craters has produced this
singular change in the small scoriaceous particles and given to them
their translucent, resin-like fracture. Or has the associated lime
played any part in this change? I ask these questions from having found
at St. Jago, in the Cape de Verde Islands, that where a great stream of
molten lava has flowed over a calcareous bottom into the sea, the
outermost film, which in other parts resembles pitchstone, is changed,
apparently by its contact with the carbonate of lime, into a resin-like
substance, precisely like the best characterised specimens of the tuff
from this archipelago.[2]

 [2] The concretions containing lime, which I have described at
 Ascension, as formed in a bed of ashes, present some degree of
 resemblance to this substance, but they have not a resinous fracture.
 At St. Helena, also, I found veins of a somewhat similar, compact, but
 non-resinous substance, occurring in a bed of pumiceous ashes,
 apparently free from calcareous matter: in neither of these cases
 could heat have acted.


To return to the two craters: one of them stands at the distance of a
league from the coast, the intervening tract consisting of a calcareous
tuff, apparently of submarine origin. This crater consists of a circle
of hills some of which stand quite detached, but all have a very
regular,
quâ-quâ versal dip, at an inclination of between thirty and forty
degrees. The lower beds, to the thickness of several hundred feet,
consist of the resin-like stone, with embedded fragments of lava. The
upper beds, which are between thirty and forty feet in thickness, are
composed of a thinly stratified, fine-grained, harsh, friable,
brown-coloured tuff, or peperino.[3] A central mass without any
stratification, which must formerly have occupied the hollow of the
crater, but is now attached only to a few of the circumferential hills,
consists of a tuff, intermediate in character between that with a
resin-like, and that with an earthy fracture. This mass contains white
calcareous matter in small patches. The second crater (520 feet in
height) must have existed until the eruption of a recent, great stream
of lava, as a separate islet; a fine section, worn by the sea, shows a
grand funnel-shaped mass of basalt, surrounded by steep, sloping flanks
of tuff, having in parts an earthy, and in others a semi-resinous
fracture. The tuff is traversed by several broad, vertical dikes, with
smooth and parallel sides, which I did not doubt were formed of basalt,
until I actually broke off fragments. These dikes, however, consist of
tuff like that of the surrounding strata, but more compact, and with a
smoother fracture; hence we must conclude, that fissures were formed
and filled up with the finer mud or tuff from the crater, before its
interior was occupied, as it now is, by a solidified pool of basalt.
Other fissures have been subsequently formed, parallel to these
singular dikes, and are merely filled with loose rubbish. The change
from ordinary scoriaceous particles to the substance with a
semi-resinous fracture, could be clearly followed in portions of the
compact tuff of these dikes.

 [3] Those geologists who restrict the term of “tuff” to ashes of a
 white colour, resulting from the attrition of feldspathic lavas, would
 call these brown-coloured strata “peperino.”


No. 12


[Illustration: The Kicker Rock.]

The Kicker Rock, 400 feet high.

At the distance of a few miles from these two craters, stands the
Kicker Rock, or islet, remarkable from its singular form. It is
unstratified, and is composed of compact tuff, in parts having the
resin-like fracture. It is probable that this amorphous mass, like that
similar mass in the case first described, once filled up the central
hollow of a crater, and that its flanks, or sloping walls, have since
been worn quite away by the sea, in which it stands exposed.

_Small basaltic craters._—A bare, undulating tract, at the eastern end
of Chatham Island, is remarkable from the number, proximity, and form
of the small basaltic craters with which it is studded. They consist,
either
of a mere conical pile, or, but less commonly, of a circle, of black
and red, glossy scoriæ, partially cemented together. They vary in
diameter from thirty to one hundred and fifty yards, and rise from
about fifty to one hundred feet above the level of the surrounding
plain. From one small eminence, I counted sixty of these craters, all
of which were within a third of a mile from each other, and many were
much closer. I measured the distance between two very small craters,
and found that it was only thirty yards from the summit-rim of one to
the rim of the other. Small streams of black, basaltic lava, containing
olivine and much glassy feldspar, have flowed from many, but not from
all of these craters. The surfaces of the more recent streams were
exceedingly rugged, and were crossed by great fissures; the older
streams were only a little less rugged; and they were all blended and
mingled together in complete confusion. The different growth, however,
of the trees on the streams, often plainly marked their different ages.
Had it not been for this latter character, the streams could in few
cases have been distinguished; and, consequently, this wide undulatory
tract might have (as probably many tracts have) been erroneously
considered as formed by one great deluge of lava, instead of by a
multitude of small streams, erupted from many small orifices.

In several parts of this tract, and especially at the base of the small
craters, there are circular pits, with perpendicular sides, from twenty
to forty feet deep. At the foot of one small crater, there were three
of these pits. They have probably been formed, by the falling in of the
roofs of small caverns.[4] In other parts, there are mammiform
hillocks, which resemble great bubbles of lava, with their summits
fissured by irregular cracks, which appeared, upon entering them, to be
very deep; lava has not flowed from these hillocks. There are, also,
other very regular, mammiform hillocks, composed of stratified lava,
and surmounted by circular, steep-sided hollows, which, I suppose have
been formed by a body of gas, first, arching the strata into one of the
bubble-like hillocks, and then, blowing off its summit. These several
kinds of hillocks and pits, as well as the numerous, small, scoriaceous
craters, all show that this tract has been penetrated, almost like a
sieve, by the passage of heated vapours. The more regular hillocks
could only have been heaved up, whilst the lava was in a softened
state.[5]

 [4] (M. Elie de Beaumont has described (“Mém. pour servir,” etc., tome
 iv, p. 113) many “petits cirques d’eboulement” on Etna, of some of
 which the origin is historically known.


 [5] Sir G. Mackenzie (“Travels in Iceland,” pp. 389 to 392) has
 described a plain of lava at the foot of Hecla, everywhere heaved up
 into great bubbles or blisters. Sir George states that this cavernous
 lava composes the uppermost stratum; and the same fact is affirmed by
 Von Buch (“Descript. des Isles Canaries,” p. 159), with respect to the
 basaltic stream near Rialejo, in Teneriffe. It appears singular that
 it should be the upper streams that are chiefly cavernous, for one
 sees no reason why the upper and lower should not have been equally
 affected at different times;—have the inferior streams flowed beneath
 the pressure of the sea, and thus been flattened, after the passage
 through them, of bodies of gas?


ALBEMARLE ISLAND.—This island consists of five, great, flat-topped
craters, which, together with the one on the adjoining island of
Narborough, singularly resemble each other, in form and height. The
southern one is 4,700 feet high, two others are 3,720 feet, a third
only 50 feet higher, and the remaining ones apparently of nearly the
same height. Three of these are situated on one line, and their craters
appear elongated in nearly the same direction. The northern crater,
which is not the largest, was found by the triangulation to measure,
externally, no less than three miles and one-eighth of a mile in
diameter. Over the lips of these great, broad caldrons, and from little
orifices near their summits, deluges of black lava have flowed down
their naked sides.

_Fluidity of different lavas._—Near Tagus or Banks’ Cove, I examined
one of these great streams of lava, which is remarkable from the
evidence of its former high degree of fluidity, especially when its
composition is considered. Near the sea-coast this stream is several
miles in width. It consists of a black, compact base, easily fusible
into a black bead, with angular and not very numerous air-cells, and
thickly studded with large, fractured crystals of glassy albite,[6]
varying from the tenth of an inch to half an inch in diameter. This
lava, although at first sight appearing eminently porphyritic, cannot
properly be considered so, for the crystals have evidently been
enveloped, rounded, and penetrated by the lava, like fragments of
foreign rock in a trap-dike. This was very clear in some specimens of a
similar lava, from Abingdon Island, in which the only difference was,
that the vesicles were spherical and more numerous. The albite in these
lavas is in a similar condition with the leucite of Vesuvius, and with
the olivine, described by Von Buch,[7] as projecting in great balls
from the basalt of Lanzarote. Besides the albite, this lava contains
scattered grains of a green mineral, with no distinct cleavage, and
closely resembling olivine;[8] but as it fuses easily into a green
glass, it belongs probably to the augitic family: at James Island,
however, a similar lava contained true olivine. I obtained specimens
from the actual
surface, and from a depth of four feet, but they differed in no
respect. The high degree of fluidity of this lava-stream was at once
evident, from its smooth and gently sloping surface, from the manner in
which the main stream was divided by small inequalities into little
rills, and especially from the manner in which its edges, far below its
source, and where it must have been in some degree cooled, thinned out
to almost nothing; the actual margin consisting of loose fragments, few
of which were larger than a man’s head. The contrast between this
margin, and the steep walls, above twenty feet high, bounding many of
the basaltic streams at Ascension, is very remarkable. It has generally
been supposed that lavas abounding with large crystals, and including
angular vesicles,[9] have possessed little fluidity; but we see that
the case has been very different at Albemarle Island. The degree of
fluidity in different lavas, does not seem to correspond with any
_apparent_ corresponding amount of difference in their composition: at
Chatham Island, some streams, containing much glassy albite and some
olivine, are so rugged, that they may be compared to a sea frozen
during a storm; whilst the great stream at Albemarle Island is almost
as smooth as a lake when ruffled by a breeze. At James Island, black
basaltic lava, abounding with small grains of olivine, presents an
intermediate degree of roughness; its surface being glossy, and the
detached fragments resembling, in a very singular manner, folds of
drapery, cables, and pieces of the bark of trees.[10]

 [6] In the Cordillera of Chile, I have seen lava very closely
 resembling this variety at the Galapagos Archipelago. It contained,
 however, besides the albite, well-formed crystals of augite, and the
 base (perhaps in consequence of the aggregation of the augitic
 particles) was a shade lighter in colour. I may here remark, that in
 all these cases, I call the feldspathic crystals, _albite_, from their
 cleavage-planes (as measured by the reflecting goniometer)
 corresponding with those of that mineral. As, however, other species
 of this genus have lately been discovered to cleave in nearly the same
 planes with albite, this determination must be considered as only
 provisional. I examined the crystals in the lavas of many different
 parts of the Galapagos group, and I found that none of them, with the
 exception of some crystals from one part of James Island, cleaved in
 the direction of orthite or potash-feldspar.


 [7] “Description des Isles Canaries,” p. 295.


 [8] Humboldt mentions that he mistook a green augitic mineral,
 occurring in the volcanic rocks of the Cordillera of Quito, for
 olivine.


 [9] The irregular and angular form of the vesicles is probably caused
 by the unequal yielding of a mass composed, in almost equal
 proportion, of solid crystals and of a viscid base. It certainly seems
 a general circumstance, as might have been expected, that in lava,
 which has possessed a high degree of fluidity, _as well as an
 even-sized grain_, the vesicles are internally smooth and spherical.


 [10] A specimen of basaltic lava, with a few small broken crystals of
 albite, given me by one of the officers, is perhaps worthy of
 description. It consists of cylindrical ramifications, some of which
 are only the twentieth of an inch in diameter, and are drawn out into
 the sharpest points. The mass has not been formed like a stalactite,
 for the points terminate both upwards and downwards. Globules, only
 the fortieth of an inch in diameter, have dropped from some of the
 points, and adhere to the adjoining branches. The lava is vesicular,
 but the vesicles never reach the surface of the branches, which are
 smooth and glossy. As it is generally supposed that vesicles are
 always elongated in the direction of the movement of the fluid mass, I
 may observe, that in these cylindrical branches, which vary from a
 quarter to only the twentieth of an inch in diameter, every air-cell
 is spherical.


_Craters of tuff._—About a mile southward of Banks’ Cove, there is a
fine elliptic crater, about five hundred feet in depth, and
three-quarters of a mile in diameter. Its bottom is occupied by a lake
of brine, out of which some little crateriform hills of tuff rise. The
lower beds are formed of compact tuff, appearing like a subaqueous
deposit; whilst the upper beds, round the entire circumference, consist
of a harsh, friable tuff, of little specific gravity, but often
containing fragments of rock in layers. This upper tuff contains
numerous pisolitic balls, about the size of small bullets, which differ
from the surrounding matter, only in being slightly
harder and finer grained. The beds dip away very regularly on all
sides, at angles varying, as I found by measurement, from twenty-five
to thirty degrees. The external surface of the crater slopes at a
nearly similar inclination, and is formed by slightly convex ribs, like
those on the shell of a pecten or scallop, which become broader as they
extend from the mouth of the crater to its base. These ribs are
generally from eight to twenty feet in breadth, but sometimes they are
as much as forty feet broad; and they resemble old, plastered, much
flattened vaults, with the plaster scaling off in plates: they are
separated from each other by gullies, deepened by alluvial action. At
their upper and narrow ends, near the mouth of the crater, these ribs
often consist of real hollow passages, like, but rather smaller than,
those often formed by the cooling of the crust of a lava-stream, whilst
the inner parts have flowed onward;—of which structure I saw many
examples at Chatham Island. There can be no doubt but that these hollow
ribs or vaults have been formed in a similar manner, namely, by the
setting or hardening of a superficial crust on streams of mud, which
have flowed down from the upper part of the crater. In another part of
this same crater, I saw open concave gutters between one and two feet
wide, which appear to have been formed by the hardening of the lower
surface of a mud stream, instead of, as in the former case, of the
upper surface. From these facts I think it is certain that the tuff
must have flowed as mud.[11] This mud may have been formed either
within the crater, or from ashes deposited on its upper parts, and
afterwards washed down by torrents of rain. The former method, in most
of the cases, appears the more probable one; at James Island, however,
some beds of the friable kind of tuff extend so continuously over an
uneven surface, that probably they were formed by the falling of
showers of ashes.

 [11] This conclusion is of some interest, because M. Dufrenoy (“Mém.
 pour servir,” tome iv, p. 274) has argued from strata of tuff,
 apparently of similar composition with that here described, being
 inclined at angles between 18° and 20°, that Monte Nuevo and some
 other craters of Southern Italy have been formed by upheaval. From the
 facts given above, of the vaulted character of the separate rills, and
 from the tuff not extending in horizontal sheets round these
 crateriform hills, no one will suppose that the strata have here been
 produced by elevation; and yet we see that their inclination is above
 20°, and often as much as 30°. The consolidated strata also, of the
 internal talus, as will be immediately seen, dips at an angle of above
 30°.

Within this same crater, strata of coarse tuff, chiefly composed of
fragments of lava, abut, like a consolidated talus, against the inside
walls. They rise to a height of between one hundred and one hundred and
fifty feet above the surface of the internal brine-lake; they dip
inwards, and are inclined at an angle varying from thirty to thirty-six
degrees. They appear to have been formed beneath water, probably at a
period when the sea occupied the hollow of the crater. I was surprised
to observe that beds having this great inclination did not, as far as
they could be followed, thicken towards their lower extremities.

_Banks’ Cove._—This harbour occupies part of the interior of a
shattered crater of tuff larger than that last described. All the tuff
is
compact, and includes numerous fragments of lava; it appears like a
subaqueous deposit. The most remarkable feature in this crater is the
great development of strata converging inwards, as in the last case, at
a considerable inclination, and often deposited in irregular curved
layers. These interior converging beds, as well as the proper,
diverging crateriform strata, are represented in figure No. 13, a rude,
sectional sketch of the headlands, forming this Cove. The internal and
external strata differ little in composition, and the former have
evidently resulted from the wear and tear, and redeposition of the
matter forming the external crateriform strata. From the great
development of these inner beds, a person walking round the rim of this
crater might fancy himself on a circular anticlinal ridge of stratified
sandstone and conglomerate. The sea is wearing away the inner and outer
strata, and especially the latter; so that the inwardly converging
strata will, perhaps, in some future age, be left standing alone—a case
which might at first perplex a geologist.[12]

 [12] I believe that this case actually occurs in the Azores, where Dr.
 Webster (“Description,” p. 185) has described a basin-formed, little
 island, composed of _strata of tuff_, dipping inwards and bounded
 externally by steep sea-worn cliffs. Dr. Daubeny supposes (on
 Volcanoes, p. 266), that this cavity must have been formed by a
 circular subsidence. It appears to me far more probable, that we here
 have strata which were originally deposited within the hollow of a
 crater, of which the exterior walls have since been removed by the
 sea.


No. 13


[Illustration: Sectional sketch of headlands forming Banks’ Cove.]

A sectional sketch of the headlands forming Banks’ Cove, showing the
diverging craterform strata, and the converging stratified talus. The
highest point of these hills is 817 feet above the sea.

JAMES ISLAND.—Two craters of tuff on this island are the only remaining
ones which require any notice. One of them lies a mile and a half
inland from Puerto Grande: it is circular, about the third of a mile in
diameter, and 400 feet in depth. It differs from all the other
tuff-craters which I examined, in having the lower part of its cavity,
to the height of between one hundred and one hundred and fifty feet,
formed by a precipitous wall of basalt, giving to the crater the
appearance of having burst through a solid sheet of rock. The upper
part of this crater consists of strata of the altered tuff, with a
semi-resinous fracture. Its bottom is
occupied by a shallow lake of brine, covering layers of salt, which
rest on deep black mud. The other crater lies at the distance of a few
miles, and is only remarkable from its size and perfect condition. Its
summit is 1,200 feet above the level of the sea, and the interior
hollow is 600 feet deep. Its external sloping surface presented a
curious appearance from the smoothness of the wide layers of tuff,
which resembled a vast plastered floor. Brattle Island is, I believe,
the largest crater in the Archipelago composed of tuff; its interior
diameter is nearly a nautical mile. At present it is in a ruined
condition, consisting of little more than half a circle open to the
south; its great size is probably due, in part, to internal
degradation, from the action of the sea.

No. 14


[Illustration: Segment of very small orifice of eruption.]

Segment of a very small orifice of eruption, on the beach of
Fresh-water Bay.

_Segment of a basaltic crater._—One side of Fresh-water Bay, in James
Island, is bounded by a promontory, which forms the last wreck of a
great crater. On the beach of this promontory, a quadrant-shaped
segment of a small subordinate point of eruption stands exposed. It
consists of nine separate little streams of lava piled upon each other;
and of an irregular pinnacle, about fifteen feet high, of
reddish-brown, vesicular basalt, abounding with large crystals of
glassy albite, and with fused augite. This pinnacle, and some adjoining
paps of rock on the beach, represent the axis of the crater. The
streams of lava can be followed up a little ravine, at right angles to
the coast, for between ten and fifteen yards, where they are hidden by
detritus: along the beach they are visible for nearly eighty yards, and
I do not believe that they extend much further. The three lower streams
are united to the pinnacle; and at the point of junction (as shown in
figure No. 14, a rude sketch made on the spot), they are slightly
arched, as if in the act of flowing over the lip of the crater. The six
upper streams no doubt were originally united to this same column
before it was worn down by the sea. The lava of these streams is of
similar composition with that of the pinnacle, excepting that the
crystals of albite appear to be more comminuted, and the grains of
fused augite are absent. Each stream is separated from the one above it
by a few inches, or at most by one or two feet in thickness, of loose
fragmentary scoriæ,
apparently derived from the abrasion of the streams in passing over
each other. All these streams are very remarkable from their thinness.
I carefully measured several of them; one was eight inches thick, but
was firmly coated with three inches above, and three inches below, of
red scoriaceous rock (which is the case with all the streams), making
altogether a thickness of fourteen inches: this thickness was preserved
quite uniformly along the entire length of the section. A second stream
was only eight inches thick, including both the upper and lower
scoriaceous surfaces. Until examining this section, I had not thought
it possible that lava could have flowed in such uniformly thin sheets
over a surface far from smooth. These little streams closely resemble
in composition that great deluge of lava at Albemarle Island, which
likewise must have possessed a high degree of fluidity.

_Pseudo-extraneous, ejected fragments._—In the lava and in the scoriæ
of this little crater, I found several fragments, which, from their
angular form, their granular structure, their freedom from air-cells,
their brittle and burnt condition, closely resembled those fragments of
primary rocks which are occasionally ejected, as at Ascension, from
volcanoes. These fragments consist of glassy albite, much mackled, and
with very imperfect cleavages, mingled with semi-rounded grains, having
tarnished, glossy surfaces, of a steel-blue mineral. The crystals of
albite are coated by a red oxide of iron, appearing like a residual
substance; and their cleavage-planes also are sometimes separated by
excessively fine layers of this oxide, giving to the crystals the
appearance of being ruled like a glass micrometer. There was no quartz.
The steel-blue mineral, which is abundant in the pinnacle, but which
disappears in the streams derived from the pinnacle, has a fused
appearance, and rarely presents even a trace of cleavage; I obtained,
however, one measurement, which proved that it was augite; and in one
other fragment, which differed from the others, in being slightly
cellular, and in gradually blending into the surrounding matrix the
small grains of this mineral were tolerably well crystallised. Although
there is so wide a difference in appearance between the lava of the
little streams, and especially of their red scoriaceous crusts, and one
of these angular ejected fragments, which at first sight might readily
be mistaken for syenite, yet I believe that the lava has originated
from the melting and movement of a mass of rock of absolutely similar
composition with the fragments. Besides the specimen above alluded to,
in which we see a fragment becoming slightly cellular, and blending
into the surrounding matrix, some of the grains of the steel-blue
augite also have their surfaces becoming very finely vesicular, and
passing into the nature of the surrounding paste; other grains are
throughout, in an intermediate condition. The paste seems to consist of
the augite more perfectly fused, or, more probably, merely disturbed in
its softened state by the movement of the mass, and mingled with the
oxide of iron and with finely comminuted, glassy albite. Hence probably
it is that the fused albite, which is abundant in the pinnacle,
disappears in the streams. The albite is in exactly the same state,
with the exception of most of the crystals being smaller in the lava
and in the embedded fragments; but
in the fragments they appear to be less abundant: this, however, would
naturally happen from the intumescence of the augitic base, and its
consequent apparent increase in bulk. It is interesting thus to trace
the steps by which a compact granular rock becomes converted into a
vesicular, pseudo-porphyritic lava, and finally into red scoriæ. The
structure and composition of the embedded fragments show that they are
parts either of a mass of primary rock which has undergone considerable
change from volcanic action, or more probably of the crust of a body of
cooled and crystallised lava, which has afterwards been broken up and
re-liquified; the crust being less acted on by the renewed heat and
movement.

_Concluding remarks on the tuff-craters._—These craters, from the
peculiarity of the resin-like substance which enters largely into their
composition, from their structure, their size and number, present the
most striking feature in the geology of this Archipelago. The majority
of them form either separate islets, or promontories attached to the
larger islands; and those which now stand at some little distance from
the coast are worn and breached, as if by the action of the sea. From
this general circumstance of their position, and from the small
quantity of ejected ashes in any part of the Archipelago, I am led to
conclude, that the tuff has been chiefly produced, by the grinding
together of fragments of lava within active craters, communicating with
the sea. In the origin and composition of the tuff, and in the frequent
presence of a central lake of brine and of layers of salt, these
craters resemble, though on a gigantic scale, the “salses,” or hillocks
of mud, which are common in some parts of Italy and in other
countries.[13] Their closer connection, however, in this Archipelago,
with ordinary volcanic action, is shown by the pools of solidified
basalt, with which they are sometimes filled up.

 [13] D’Aubuisson’s “Traité de Géognosie,” tome i, p. 189. I may
 remark, that I saw at Terceira, in the Azores, a crater of tuff or
 peperino, very similar to these of the Galapagos Archipelago. From the
 description given in Freycinet “Voyage,” similar ones occur at the
 Sandwich Islands; and probably they are present in many other places.


It at first appears very singular, that all the craters formed of tuff
have their southern sides, either quite broken down and wholly removed,
or much lower than the other sides. I saw and received accounts of
twenty-eight of these craters; of these, twelve form separate
islets,[14] and now exist as mere crescents quite open to the south,
with occasionally a few points of rock marking their former
circumference: of the remaining sixteen, some form promontories, and
others stand at a little distance inland from the shore; but all have
their southern sides either the lowest, or quite broken down. Two,
however, of the sixteen had
their northern sides also low, whilst their eastern and western sides
were perfect. I did not see, or hear of, a single exception to the
rule, of these craters being broken down or low on the side, which
faces a point of the horizon between S.E. and S.W. This rule does not
apply to craters composed of lava and scoriæ. The explanation is
simple: at this Archipelago, the waves from the trade-wind, and the
swell propagated from the distant parts of the open ocean, coincide in
direction (which is not the case in many parts of the Pacific), and
with their united forces attack the southern sides of all the islands;
and consequently the southern slope, even when entirely formed of hard
basaltic rock, is invariably steeper than the northern slope. As the
tuff-craters are composed of a soft material, and as probably all, or
nearly all, have at some period stood immersed in the sea, we need not
wonder that they should invariably exhibit on their exposed sides the
effects of this great denuding power. Judging from the worn condition
of many of these craters, it is probable that some have been entirely
washed away. As there is no reason to suppose, that the craters formed
of scoriæ and lava were erupted whilst standing in the sea, we can see
why the rule does not apply to them. At Ascension, it was shown that
the mouths of the craters, which are there all of terrestrial origin,
have been affected by the trade-wind; and this same power might here,
also, aid in making the windward and exposed sides of some of the
craters originally the lowest.

 [14] These consist of the three Crossman Islets, the largest of which
 is 600 feet in height; Enchanted Island; Gardner Island (760 feet
 high); Champion Island (331 feet high); Enderby Island; Brattle
 Island; two islets near Indefatigable Island; and one near James
 Island. A second crater near James Island (with a salt lake in its
 centre) has its southern side only about twenty feet high, whilst the
 other parts of the circumference are about three hundred feet in
 height.


_Mineralogical composition of the rocks._—In the northern islands, the
basaltic lavas seem generally to contain more albite than they do in
the southern half of the Archipelago; but almost all the streams
contain some. The albite is not unfrequently associated with olivine. I
did not observe in any specimen distinguishable crystals of hornblende
or augite; I except the fused grains in the ejected fragments, and in
the pinnacle of the little crater, above described. I did not meet with
a single specimen of true trachyte; though some of the paler lavas,
when abounding with large crystals of the harsh and glassy albite,
resemble in some degree this rock; but in every case the basis fuses
into a black enamel. Beds of ashes and far-ejected scoriæ, as
previously stated, are almost absent; nor did I see a fragment of
obsidian or of pumice. Von Buch[15] believes that the absence of pumice
on Mount Etna is consequent on the feldspar being of the Labrador
variety; if the presence of pumice depends on the constitution of the
feldspar, it is remarkable, that it should be absent in this
archipelago, and abundant in the Cordillera of South America, in both
of which regions the feldspar is of the albitic variety. Owing to the
absence of ashes, and the general indecomposable character of the lava
in this Archipelago, the islands are slowly clothed with a poor
vegetation, and the scenery has a desolate and frightful aspect.

 [15] “Description des Isles Canaries,” p. 328.

_Elevation of the land._—Proofs of the rising of the land are scanty
and imperfect. At Chatham Island, I noticed some great blocks of lava,
cemented by calcareous matter, containing recent shells; but they
occurred at the height of only a few feet above high-water mark. One
of the officers gave me some fragments of shells, which he found
embedded several hundred feet above the sea, in the tuff of two
craters, distant from each other. It is possible, that these fragments
may have been carried up to their present height in an eruption of mud;
but as, in one instance, they were associated with broken
oyster-shells, almost forming a layer, it is more probable that the
tuff was uplifted with the shells in mass. The specimens are so
imperfect that they can be recognised only as belonging to recent
marine genera. On Charles Island, I observed a line of great rounded
blocks, piled on the summit of a vertical cliff, at the height of
fifteen feet above the line, where the sea now acts during the heaviest
gales. This appeared, at first, good evidence in favour of the
elevation of the land; but it was quite deceptive, for I afterwards saw
on an adjoining part of this same coast, and heard from eye-witnesses,
that wherever a recent stream of lava forms a smooth inclined plane,
entering the sea, the waves during gales have the power of _rolling up
rounded_ blocks to a great height, above the line of their ordinary
action. As the little cliff in the foregoing case is formed by a stream
of lava, which, before being worn back, must have entered the sea with
a gently sloping surface, it is possible or rather it is probable, that
the rounded boulders, now lying on its summit, are merely the remnants
of those which had been _rolled up_ during storms to their present
height.

_Direction of the fissures of eruption._—The volcanic orifices in this
group cannot be considered as indiscriminately scattered. Three great
craters on Albermarle Island form a well-marked line, extending N.W. by
N. and S.E. by S. Narborough Island, and the great crater on the
rectangular projection of Albemarle Island, form a second parallel
line. To the east, Hood’s Island, and the islands and rocks between it
and James Island, form another nearly parallel line, which, when
prolonged, includes Culpepper and Wenman Islands, lying seventy miles
to the north. The other islands lying further eastward, form a less
regular fourth line. Several of these islands, and the vents on
Albemarle Island, are so placed, that they likewise fall on a set of
rudely parallel lines, intersecting the former lines at right angles;
so that the principal craters appear to lie on the points where two
sets of fissures cross each other. The islands themselves, with the
exception of Albemarle Island, are not elongated in the same direction
with the lines on which they stand. The direction of these islands is
nearly the same with that which prevails in so remarkable a manner in
the numerous archipelagoes of the great Pacific Ocean. Finally, I may
remark, that amongst the Galapagos Islands there is no one dominant
vent much higher than all the others, as may be observed in many
volcanic archipelagoes: the highest is the great mound on the
south-western extremity of Albemarle Island, which exceeds by barely a
thousand feet several other neighbouring craters.




Chapter VI TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.


The sinking of crystals in fluid lava.—Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation.—Obsidian.—Apparent non-separation of the elements of
plutonic rocks.—Origin of trap-dikes in the plutonic
series.—Distribution of volcanic islands; their prevalence in the great
oceans.—They are generally arranged in lines.—The central volcanoes of
Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
volcanic islands, and their elevation in mass.—Eruptions on parallel
lines of fissure within the same geological period.


_On the separation of the constituent minerals of lava, according to
their specific gravities._—One side of Fresh-water Bay, in James
Island, is formed by the wreck of a large crater, mentioned in the last
chapter, of which the interior has been filled up by a pool of basalt,
about two hundred feet in thickness. This basalt is of a grey colour,
and contains many crystals of glassy albite, which become much more
numerous in the lower, scoriaceous part. This is contrary to what might
have been expected, for if the crystals had been originally
disseminated in equal numbers, the greater intumescence of this lower
scoriaceous part would have made them appear fewer in number. Von
Buch[1] has described a stream of obsidian on the Peak of Teneriffe, in
which the crystals of feldspar become more and more numerous, as the
depth or thickness increases, so that near the lower surface of the
stream the lava even resembles a primary rock. Von Buch further states,
that M. Dree, in his experiments in melting lava, found that the
crystals of feldspar always tended to precipitate themselves to the
bottom of the crucible. In these cases, I presume there can be no
doubt[2] that the crystals sink from their weight. The specific gravity
of feldspar varies[3] from 2·4 to 2·58, whilst obsidian seems commonly
to be from 2·3 to 2·4; and in a fluidified state its specific gravity
would probably be less, which would
facilitate the sinking of the crystals of feldspar. At James Island,
the crystals of albite, though no doubt of less weight than the grey
basalt, in the parts where compact, might easily be of greater specific
gravity than the scoriaceous mass, formed of melted lava and bubbles of
heated gas.

 [1] “Description des Isles Canaries,” pp. 190 and 191.


 [2] In a mass of molten iron, it is found (_Edinburgh New
 Philosophical Journal_, vol. xxiv, p. 66) that the substances, which
 have a closer affinity for oxygen than iron has, rise from the
 interior of the mass to the surface. But a similar cause can hardly
 apply to the separation of the crystals of these lava-streams. The
 cooling of the surface of lava seems, in some cases, to have affected
 its composition; for Dufrenoy (“Mém. pour servir,” tome iv, p. 271)
 found that the interior parts of a stream near Naples contained
 two-thirds of a mineral which was acted on by acids, whilst the
 surface consisted chiefly of a mineral unattackable by acids.


 [3] I have taken the specific gravities of the simple minerals from
 Von Kobell, one of the latest and best authorities, and of the rocks
 from various authorities. Obsidian, according to Phillips, is 2·35;
 and Jameson says it never exceeds 2·4; but a specimen from Ascension,
 weighed by myself, was 2·42.


The sinking of crystals through a viscid substance like molten rock, as
is unequivocally shown to have been the case in the experiments of M.
Drée, is worthy of further consideration, as throwing light on the
separation of the trachytic and basaltic series of lavas. Mr. P. Scrope
has speculated on this subject; but he does not seem to have been aware
of any positive facts, such as those above given; and he has overlooked
one very necessary element, as it appears to me, in the
phenomenon—namely, the existence of either the lighter or heavier
mineral in globules or in crystals. In a substance of imperfect
fluidity, like molten rock, it is hardly credible, that the separate,
infinitely small atoms, whether of feldspar, augite, or of any other
mineral, would have power from their slightly different gravities to
overcome the friction caused by their movement; but if the atoms of any
one of these minerals became, whilst the others remained fluid, united
into crystals or granules, it is easy to perceive that from the
lessened friction, their sinking or floating power would be greatly
increased. On the other hand, if all the minerals became granulated at
the same time, it is scarcely possible, from their mutual resistance,
that any separation could take place. A valuable, practical discovery,
illustrating the effect of the granulation of one element in a fluid
mass, in aiding its separation, has lately been made: when lead
containing a small proportion of silver, is constantly stirred whilst
cooling, it becomes granulated, and the grains of imperfect crystals of
nearly pure lead sink to the bottom, leaving a residue of melted metal
much richer in silver; whereas if the mixture be left undisturbed,
although kept fluid for a length of time, the two metals show no signs
of separating.[4] The sole use of the stirring seems to be, the
formation of detached granules. The specific gravity of silver is 10·4,
and of lead 11·35: the granulated lead, which sinks, is never
absolutely pure, and the residual fluid metal contains, when richest,
only 1/119 part of silver. As the difference in specific gravity,
caused by the different proportions of the two metals, is so
exceedingly small, the separation is probably aided in a great degree
by the difference in gravity between the lead, when granular though
still hot, and when fluid.

 [4] A full and interesting account of this discovery, by Mr.
 Pattinson, was read before the British Association in September 1838.
 In some alloys, according to Turner (“Chemistry,” p. 210), the
 heaviest metal sinks, and it appears that this takes place whilst both
 metals are fluid. Where there is a considerable difference in gravity,
 as between iron and the slag formed during the fusion of the ore, we
 need not be surprised at the atoms separating, without either
 substance being granulated.


In a body of liquified volcanic rock, left for some time without any
violent disturbance, we might expect, in accordance with the above
facts, that if one of the constituent minerals became aggregated into
crystals or granules, or had been enveloped in this state from some
previously existing mass, such crystals or granules would rise or sink,
according to their specific gravity. Now we have plain evidence of
crystals being embedded in many lavas, whilst the paste or basis has
continued fluid. I need only refer, as instances, to the several,
great, pseudo-porphyritic streams at the Galapagos Islands, and to the
trachytic streams in many parts of the world, in which we find crystals
of feldspar bent and broken by the movement of the surrounding,
semi-fluid matter. Lavas are chiefly composed of three varieties of
feldspar, varying in specific gravity from 2·4 to 2·74; of hornblende
and augite, varying from 3·0 to 3·4; of olivine, varying from 3·3 to
3·4; and lastly, of oxides of iron, with specific gravities from 4·8 to
5·2. Hence crystals of feldspar, enveloped in a mass of liquified, but
not highly vesicular lava, would tend to rise to the upper parts; and
crystals or granules of the other minerals, thus enveloped, would tend
to sink. We ought not, however, to expect any perfect degree of
separation in such viscid materials. Trachyte, which consists chiefly
of feldspar, with some hornblende and oxide of iron, has a specific
gravity of about 2·45;[5] whilst basalt, composed chiefly of augite and
feldspar, often with much iron and olivine, has a gravity of about 3·0.
Accordingly we find, that where both trachytic and basaltic streams
have proceeded from the same orifice, the trachytic streams have
generally been first erupted owing, as we must suppose, to the molten
lava of this series having accumulated in the upper parts of the
volcanic focus. This order of eruption has been observed by Beudant,
Scrope, and by other authors; three instances, also, have been given in
this volume. As the later eruptions, however, from most volcanic
mountains, burst through their basal parts, owing to the increased
height and weight of the internal column of molten rock, we see why, in
most cases, only the lower flanks of the central, trachytic masses, are
enveloped by basaltic streams. The separation of the ingredients of a
mass of lava, would, perhaps, sometimes take place within the body of a
volcanic mountain, if lofty and of great dimensions, instead of within
the underground focus; in which case, trachytic streams might be poured
forth, almost contemporaneously, or at short recurrent intervals, from
its summit, and basaltic streams from its base: this seems to have
taken place at Teneriffe.[6] I need only further remark, that from
violent disturbances the separation of the two series, even under
otherwise favourable conditions, would naturally often be prevented,
and likewise their usual order of eruption be inverted. From the high
degree of fluidity of most basaltic lavas, these perhaps, alone, would
in many cases reach the surface.

 [5] Trachyte from Java was found by Von Buch to be 2·47; from
 Auvergne, by De la Beche, it was 2·42; from Ascension, by myself, it
 was 2·42. Jameson and other authors give to basalt a specific gravity
 of 3·0; but specimens from Auvergne were found, by De la Beche, to be
 only 2·78; and from the Giant’s Causeway, to be 2·91.


 [6] Consult Von Buch’s well-known and admirable “Description Physique”
 of this island, which might serve as a model of descriptive geology.


As we have seen that crystals of feldspar, in the instance described by
Von Buch, sink in obsidian, in accordance with their known greater
specific gravity, we might expect to find in every trachytic district,
where obsidian has flowed as lava, that it had proceeded from the upper
or highest orifices. This, according to Von Buch, holds good in a
remarkable manner both at the Lipari Islands and on the Peak of
Teneriffe; at this latter place obsidian has never flowed from a less
height than 9,200 feet. Obsidian, also, appears to have been erupted
from the loftiest peaks of the Peruvian Cordillera. I will only further
observe, that the specific gravity of quartz varies from 2·6 to 2·8;
and therefore, that when present in a volcanic focus, it would not tend
to sink with the basaltic bases; and this, perhaps, explains the
frequent presence, and the abundance of this mineral, in the lavas of
the trachytic series, as observed in previous parts of this volume.

An objection to the foregoing theory will, perhaps, be drawn from the
plutonic rocks not being separated into two evidently distinct series,
of different specific gravities; although, like the volcanic, they have
been liquified. In answer, it may first be remarked, that we have no
evidence of the atoms of any one of the constituent minerals in the
plutonic series having been aggregated, whilst the others remained
fluid, which we have endeavoured to show is an almost necessary
condition of their separation; on the contrary, the crystals have
generally impressed each other with their forms.[7]

 [7] The crystalline paste of phonolite is frequently penetrated by
 long needles of hornblende; from which it appears that the hornblende,
 though the more fusible mineral, has crystallised before, or at the
 same time with a more refractory substance. Phonolite, as far as my
 observations serve, in every instance appears to be an injected rock,
 like those of the plutonic series; hence probably, like these latter,
 it has generally been cooled without repeated and violent
 disturbances. Those geologists who have doubted whether granite could
 have been formed by igneous liquefaction, because minerals of
 different degrees of fusibility impress each other with their forms,
 could not have been aware of the fact of crystallised hornblende
 penetrating phonolite, a rock undoubtedly of igneous origin. The
 viscidity, which it is now known, that both feldspar and quartz retain
 at a temperature much below their points of fusion, easily explains
 their mutual impressment. Consult on this subject Mr. Horner’s paper
 on Bonn, “Geolog. Transact.,” vol. iv, p. 439; and “L’Institut,” with
 respect to quartz, 1839, p. 161.

In the second place, the perfect tranquillity, under which it is
probable that the plutonic masses, buried at profound depths, have
cooled, would, most likely, be highly unfavourable to the separation of
their constituent minerals; for, if the attractive force, which during
the progressive cooling draws together the molecules of the different
minerals, has power sufficient to keep them together, the friction
between such half-formed crystals or pasty globules would effectually
prevent the heavier ones from sinking, or the lighter ones from rising.
On the other hand, a small amount of disturbance, which would probably
occur in most volcanic foci, and which we have seen does not prevent
the separation of granules of lead from a mixture of molten lead and
silver, or crystals of feldspar from streams of lava, by breaking
and dissolving the less perfectly formed globules, would permit the
more perfect and therefore unbroken crystals, to sink or rise,
according to their specific gravity.

Although in plutonic rocks two distinct species, corresponding to the
trachytic and basaltic series, do not exist, I much suspect that a
certain amount of separation of their constituent parts has often taken
place. I suspect this from having observed how frequently dikes of
greenstone and basalt intersect widely extended formations of granite
and the allied metamorphic rocks. I have never examined a district in
an extensive granitic region without discovering dikes; I may instance
the numerous trap-dikes, in several districts of Brazil, Chile, and
Australia, and at the Cape of Good Hope: many dikes likewise occur in
the great granitic tracts of India, in the north of Europe, and in
other countries. Whence, then, has the greenstone and basalt, forming
these dikes, come? Are we to suppose, like some of the elder
geologists, that a zone of trap is uniformly spread out beneath the
granitic series, which composes, as far as we know, the foundations of
the earth’s crust? Is it not more probable, that these dikes have been
formed by fissures penetrating into partially cooled rocks of the
granitic and metamorphic series, and by their more fluid parts,
consisting chiefly of hornblende, oozing out, and being sucked into
such fissures? At Bahia, in Brazil, in a district composed of gneiss
and primitive greenstone, I saw many dikes, of a dark augitic (for one
crystal certainly was of this mineral) or hornblendic rock, which, as
several appearances clearly proved, either had been formed before the
surrounding mass had become solid, or had together with it been
afterwards thoroughly softened.[8] On both sides of one of these dikes,
the gneiss was penetrated, to the distance of several yards, by
numerous, curvilinear threads or streaks of dark matter, which
resembled in form clouds of the class called cirrhi-comæ; some few of
these threads could be traced to their junction with the dike. When
examining them, I doubted whether such hair-like and curvilinear veins
could have been injected, and I now suspect, that instead of having
been injected from the dike, they were its feeders. If the foregoing
views of the origin of trap-dikes in widely extended granitic regions
far from rocks of any other formation, be admitted as probable, we may
further admit, in the case of a great body of plutonic rock, being
impelled by repeated movements into the axis of a mountain-chain, that
its more liquid constituent parts might drain into deep and unseen
abysses; afterwards, perhaps, to be brought to the surface under the
form, either of injected masses of greenstone and augitic porphyry,[9]
or of basaltic eruptions. Much of
the difficulty which geologists have experienced when they have
compared the composition of volcanic with plutonic formations, will, I
think, be removed, if we may believe that most plutonic masses have
been, to a certain extent, drained of those comparatively weighty and
easily liquified elements, which compose the trappean and basaltic
series of rocks.

 [8] Portions of these dikes have been broken off, and are now
 surrounded by the primary rocks, with their laminæ conformably winding
 round them. Dr. Hubbard also (_Silliman’s Journal,_ vol. xxxiv, p.
 119), has described an interlacement of trap-veins in the granite of
 the White Mountains, which he thinks must have been formed when both
 rocks were soft.


 [9] Mr. Phillips (“Lardner’s Encyclop.,” vol. ii, p. 115) quotes Von
 Buch’s statement, that augitic porphyry ranges parallel to, and is
 found constantly at the base of, great chains of mountains. Humboldt,
 also, has remarked the frequent occurrence of trap-rock, in a similar
 position; of which fact I have observed many examples at the foot of
 the Chilian Cordillera. The existence of granite in the axes of great
 mountain chains is always probable, and I am tempted to suppose, that
 the laterally injected masses of augitic porphyry and of trap, bear
 nearly the same relation to the granitic axes which basaltic lavas
 bear to the central trachytic masses, round the flanks of which they
 have so frequently been erupted.


_On the distribution of volcanic islands._—During my investigations on
coral-reefs, I had occasion to consult the works of many voyagers, and
I was invariably struck with the fact, that with rare exceptions, the
innumerable islands scattered throughout the Pacific, Indian, and
Atlantic Oceans, were composed either of volcanic, or of modern
coral-rocks. It would be tedious to give a long catalogue of all the
volcanic islands; but the exceptions which I have found are easily
enumerated: in the Atlantic, we have St. Paul’s Rock, described in this
volume, and the Falkland Islands, composed of quartz and clay-slate;
but these latter islands are of considerable size, and lie not very far
from the South American coast:[10] in the Indian Ocean, the Seychelles
(situated in a line prolonged from Madagascar) consist of granite and
quartz: in the Pacific Ocean, New Caledonia, an island of large size,
belongs (as far as is known) to the primary class. New Zealand, which
contains much volcanic rock and some active volcanoes, from its size
cannot be classed with the small islands, which we are now considering.
The presence of a small quantity of non-volcanic rock, as of clay-slate
on three of the Azores,[11] or of tertiary limestone at Madeira, or of
clay-slate at Chatham Island in the Pacific, or of lignite at Kerguelen
Land, ought not to exclude such islands or archipelagoes, if formed
chiefly of erupted matter, from the volcanic class.

 [10] Judging from Forster’s imperfect observation, perhaps Georgia is
 not volcanic. Dr. Allan is my informant with regard to the Seychelles.
 I do not know of what formation Rodriguez, in the Indian Ocean, is
 composed.


 [11] This is stated on the authority of Count V. de Bedemar, with
 respect to Flores and Graciosa (Charlsworth, “Magazine of Nat. Hist.,”
 vol. i, p. 557). St. Maria has no volcanic rock, according to Captain
 Boyd (Von Buch “Descript.,” p. 365). Chatham Island has been described
 by Dr. Dieffenbach in the “Geographical Journal,” 1841, p. 201. As yet
 we have received only imperfect notices on Kerguelen Land, from the
 Antarctic Expedition.

The composition of the numerous islands scattered through the great
oceans being with such rare exceptions volcanic, is evidently an
extension of that law, and the effect of those same causes, whether
chemical or mechanical, from which it results, that a vast majority of
the volcanoes now in action stand either as islands in the sea, or near
its shores. This fact of the ocean-islands being so generally volcanic
is also interesting in relation to the nature of the mountain-chains on
our
continents, which are comparatively seldom volcanic; and yet we are led
to suppose that where our continents now stand an ocean once extended.
Do volcanic eruptions, we may ask, reach the surface more readily
through fissures formed during the first stages of the conversion of
the bed of the ocean into a tract of land?

Looking at the charts of the numerous volcanic archipelagoes, we see
that the islands are generally arranged either in single, double, or
triple rows, in lines which are frequently curved in a slight
degree.[12] Each separate island is either rounded, or more generally
elongated in the same direction with the group in which it stands, but
sometimes transversely to it. Some of the groups which are not much
elongated present little symmetry in their forms; M. Virlet[13] states
that this is the case with the Grecian Archipelago: in such groups I
suspect (for I am aware how easy it is to deceive oneself on these
points), that the vents are generally arranged on one line, or on a set
of short parallel lines, intersecting at nearly right angles another
line, or set of lines. The Galapagos Archipelago offers an example of
this structure, for most of the islands and the chief orifices on the
largest island are so grouped as to fall on a set of lines ranging
about N.W. by N., and on another set ranging about W.S.W.: in the
Canary Archipelago we have a simpler structure of the same kind: in the
Cape de Verde group, which appears to be the least symmetrical of any
oceanic volcanic archipelago, a N.W. and S.E. line formed by several
islands, if prolonged, would intersect at right angles a curved line,
on which the remaining islands are placed. Von Buch[14] has classed all
volcanoes under two heads, namely, _central volcanoes_, round which
numerous eruptions have taken place on all sides, in a manner almost
regular, and _volcanic chains._ In the examples given of the first
class, as far as position is concerned, I can see no grounds for their
being called “central;” and the evidence of any difference in
mineralogical nature between _central volcanoes_ and _volcanic chains_
appears slight. No doubt some one island in most small volcanic
archipelagoes is apt to be considerably higher than the others; and in
a similar manner, whatever the cause may be, that on the same island
one vent is generally higher than all the others. Von Buch does not
include in his class of volcanic chains small archipelagoes, in which
the islands are admitted by him, as at the Azores, to be arranged in
lines; but when viewing on a map of the world how perfect a series
exists from a few volcanic islands placed in a row to a train of linear
archipelagoes following each other in a straight line, and so on to a
great wall like the Cordillera of America, it is difficult to believe
that there exists any essential difference between short and long
volcanic chains. Von Buch[15] states that his volcanic chains surmount,
or are closely connected with, mountain-ranges of primary formation:
but if trains of linear archipelagoes are, in the course of time, by
the long-continued action of the elevatory and volcanic forces,
converted into mountain-ranges, it would naturally result that the
inferior primary rocks would often be uplifted and brought into view.

 [12] Professors William and Henry Darwin Rogers have lately insisted
 much, in a memoir read before the American Association, on the
 regularly curved lines of elevation in parts of the Appalachian range.


 [13] “Bulletin de la Soc. Géolog.,” tome iii, p. 110.


 [14] “Description des Isles Canaries,” p. 324.


 [15] _Idem,_ p. 393.

Some authors have remarked that volcanic islands occur scattered,
though at very unequal distances, along the shores of the great
continents, as if in some measure connected with them. In the case of
Juan Fernandez, situated 330 miles from the coast of Chile, there was
undoubtedly a connection between the volcanic forces acting under this
island and under the continent, as was shown during the earthquake of
1835. The islands, moreover, of some of the small volcanic groups which
thus border continents, are placed in lines, related to those along
which the adjoining shores of the continents trend; I may instance the
lines of intersection at the Galapagos, and at the Cape de Verde
Archipelagoes, and the best marked line of the Canary Islands. If these
facts be not merely accidental, we see that many scattered volcanic
islands and small groups are related not only by proximity, but in the
direction of the fissures of eruption to the neighbouring continents—a
relation, which Von Buch considers, characteristic of his great
volcanic chains.

In volcanic archipelagoes, the orifices are seldom in activity on more
than one island at a time; and the greater eruptions usually recur only
after long intervals. Observing the number of craters, that are usually
found on each island of a group, and the vast amount of matter which
has been erupted from them, one is led to attribute a high antiquity
even to those groups, which appear, like the Galapagos, to be of
comparatively recent origin. This conclusion accords with the
prodigious amount of degradation, by the slow action of the sea, which
their originally sloping coasts must have suffered, when they are worn
back, as is so often the case, into grand precipices. We ought not,
however, to suppose, in hardly any instance, that the whole body of
matter, forming a volcanic island, has been erupted at the level, on
which it now stands: the number of dikes, which seem invariably to
intersect the interior parts of every volcano, show, on the principles
explained by M. Elie de Beaumont, that the whole mass has been uplifted
and fissured. A connection, moreover, between volcanic eruptions and
contemporaneous elevations in mass[16] has, I think, been shown to
exist in my work on Coral-Reefs, both from the frequent presence of
upraised organic remains, and from the structure of the accompanying
coral-reefs. Finally, I may remark, that in the same Archipelago,
eruptions have taken place within the historical period on more than
one of the parallel lines of fissure: thus, at the Galapagos
Archipelago, eruptions have taken place from a vent on Narborough
Island, and from one on Albemarle Island, which vents do not fall on
the same line; at the Canary Islands, eruptions have taken place in
Teneriffe and Lanzarote; and at
the Azores, on the three parallel lines of Pico, St. Jorge, and
Terceira. Believing that a mountain-axis differs essentially from a
volcano, only in plutonic rocks having been injected, instead of
volcanic matter having been ejected, this appears to me an interesting
circumstance; for we may infer from it as probable, that in the
elevation of a mountain-chain, two or more of the parallel lines
forming it may be upraised and injected within the same geological
period.

 [16] A similar conclusion is forced on us, by the phenomena, which
 accompanied the earthquake of 1835, at Concepcion, and which are
 detailed in my paper (vol. v, p. 601) in the “Geological
 Transactions.”




Chapter VII AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.


New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
Land.—Palæozoic formation.—Newer formation with volcanic
rocks.—Travertin with leaves of extinct plants.—Elevation of the
land.—New Zealand.—King George’s Sound.—Superficial ferruginous
beds.—Superficial calcareous deposits, with casts of branches.—Their
origin from drifted particles of shells and corals.—Their extent.—Cape
of Good Hope.—Junction of the granite and clay-slate.—Sandstone
formation.

The _Beagle_, in her homeward voyage, touched at New Zealand,
Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to
confine the Third Part of these Geological Observations to South
America, I will here briefly describe all that I observed at these
places worthy of the attention of geologists.

_New South Wales._—My opportunities of observation consisted of a ride
of ninety geographical miles to Bathurst, in a W.N.W. direction from
Sydney. The first thirty miles from the coast passes over a sandstone
country, broken up in many places by trap-rocks, and separated by a
bold escarpment overhanging the river Nepean, from the great sandstone
platform of the Blue Mountains. This upper platform is 1,000 feet high
at the edge of the escarpment, and rises in a distance of twenty-five
miles to between three and four thousand feet above the level of the
sea. At this distance the road descends to a country rather less
elevated, and composed in chief part of primary rocks. There is much
granite, in one part passing into a red porphyry with octagonal
crystals of quartz, and intersected in some places by trap-dikes. Near
the Downs of Bathurst I passed over much pale-brown, glossy clay-slate,
with the shattered laminæ running north and south; I mention this fact,
because Captain King informs me that, in the country a hundred miles
southward, near Lake George, the mica-slate ranges so invariably north
and south that the inhabitants take advantage of it in finding their
way through the forests.

The sandstone of the Blue Mountains is at least 1,200 feet thick, and
in some parts is apparently of greater thickness; it consists of
small grains of quartz, cemented by white earthy matter, and it abounds
with ferruginous veins. The lower beds sometimes alternate with shales
and coal: at Wolgan I found in carbonaceous shale leaves of the
_Glossopteris Brownii_, a fern which so frequently accompanies the coal
of Australia. The sandstone contains pebbles of quartz; and these
generally increase in number and size (seldom, however, exceeding an
inch or two in diameter) in the upper beds: I observed a similar
circumstance in the grand sandstone formation at the Cape of Good Hope.
On the South American coast, where tertiary and supra-tertiary beds
have been extensively elevated, I repeatedly noticed that the uppermost
beds were formed of coarser materials than the lower: this appears to
indicate that, as the sea became shallower, the force of the waves or
currents increased. On the lower platform, however, between the Blue
Mountains and the coast, I observed that the upper beds of the
sandstone frequently passed into argillaceous shale,—the effect,
probably, of this lower space having been protected from strong
currents during its elevation. The sandstone of the Blue Mountains
evidently having been of mechanical origin, and not having suffered any
metamorphic action, I was surprised at observing that, in some
specimens, nearly all the grains of quartz were so perfectly
crystallised with brilliant facets that they evidently had not in their
_present_ form been aggregated in any previously existing rock.[1] It
is difficult to imagine how these crystals could have been formed; one
can hardly believe that they were separately precipitated in their
present crystallised state. Is it possible that rounded grains of
quartz may have been acted on by a fluid corroding their surfaces, and
depositing on them fresh silica? I may remark that, in the sandstone
formation of the Cape of Good Hope, it is evident that silica has been
profusely deposited from aqueous solution.

 [1] I have lately seen, in a paper by Smith (the father of English
 geologists), in the _Magazine of Natural History_, that the grains of
 quartz in the millstone grit of England are often crystallised. Sir
 David Brewster, in a paper read before the British Association, 1840,
 states, that in old decomposed glass, the silex and metals separate
 into concentric rings, and that the silex regains its crystalline
 structure, as is shown by its action on light.

In several parts of the sandstone I noticed patches of shale which
might at the first glance have been mistaken for extraneous fragments;
their horizontal laminæ, however, being parallel with those of the
sandstone, showed that they were the remnants of thin, continuous beds.
One such fragment (probably the section of a long narrow strip) seen in
the face of a cliff, was of greater vertical thickness than breadth,
which proves that this bed of shale must have been in some slight
degree consolidated, after having been deposited, and before being worn
away by the currents. Each patch of the shale shows, also, how slowly
many of the successive layers of sandstone were deposited. These
pseudo-fragments of shale will perhaps explain, in some cases, the
origin of apparently extraneous fragments in crystalline metamorphic
rocks. I mention this, because I found near Rio de Janeiro a
well-defined angular fragment, seven yards long by two yards in
breadth, of gneiss
containing garnets and mica in layers, enclosed in the ordinary,
stratified, porphyritic gneiss of the country. The laminæ of the
fragment and of the surrounding matrix ran in exactly the same
direction, but they dipped at different angles. I do not wish to affirm
that this singular fragment (a solitary case, as far as I know) was
originally deposited in a layer, like the shale in the Blue Mountains,
between the strata of the porphyritic gneiss, before they were
metamorphosed; but there is sufficient analogy between the two cases to
render such an explanation possible.

_Stratification of the escarpment._—The strata of the Blue Mountains
appear to the eye horizontal; but they probably have a similar
inclination with the surface of the platform, which slopes from the
west towards the escarpment over the Nepean, at an angle of one degree,
or of one hundred feet in a mile.[2] The strata of the escarpment dip
almost conformably with its steeply inclined face, and with so much
regularity, that they appear as if thrown into their present position;
but on a more careful examination, they are seen to thicken and to thin
out, and in the upper part to be succeeded and almost capped by
horizontal beds. These appearances render it probable, that we here see
an original escarpment, not formed by the sea having eaten back into
the strata, but by the strata having originally extended only thus far.
Those who have been in the habit of examining accurate charts of
sea-coasts, where sediment is accumulating, will be aware, that the
surfaces of the banks thus formed, generally slope from the coast very
gently towards a certain line in the offing, beyond which the depth in
most cases suddenly becomes great. I may instance the great banks of
sediment within the West Indian Archipelago,[3] which terminate in
submarine slopes, inclined at angles of between thirty and forty
degrees, and sometimes even at more than forty degrees: every one knows
how steep such a slope would appear on the land. Banks of this nature,
if uplifted, would probably have nearly the same external form as the
platform of the Blue Mountains, where it abruptly terminates over the
Nepean.

 [2] This is stated on the authority of Sir T. Mitchell, in his
 “Travels,” vol. ii, p. 357.


 [3] I have described these very curious banks in the Appendix to my
 volume on the structure of Coral-Reefs. I have ascertained the
 inclination of the edges of the banks, from information given me by
 Captain B. Allen, one of the surveyors, and by carefully measuring the
 horizontal distances between the last sounding on the bank and the
 first in the deep water. Widely extended banks in all parts of the
 West Indies have the same general form of surface.


_Current-cleavage._—The strata of sandstone in the low coast country,
and likewise on the Blue Mountains, are often divided by cross or
current laminæ, which dip in different directions, and frequently at an
angle of forty-five degrees. Most authors have attributed these cross
layers to successive small accumulations on an inclined surface; but
from a careful examination in some parts of the New Red Sandstone of
England, I believe that such layers generally form parts of a series of
curves, like gigantic tidal ripples, the tops of which have since been
cut off, either by nearly horizontal layers, or by another set of great
ripples, the folds of which do not exactly coincide with those below
them. It is well-known to surveyors that mud and sand are disturbed
during storms at considerable depths, at least from three hundred to
four hundred and fifty feet,[4] so that the nature of the bottom even
becomes temporarily changed; the bottom, also, at a depth between sixty
and seventy feet, has been observed[5] to be broadly rippled. One may,
therefore, be allowed to suspect, from the appearance just mentioned in
the New Red Sandstone, that at greater depths, the bed of the ocean is
heaped up during gales into great ripple-like furrows and depressions,
which are afterwards cut off by the currents during more tranquil
weather, and again furrowed during gales.

 [4] See Martin White, on “Soundings in the British Channel,” pp. 4 and
 166.


 [5] M. Siau on the “Action of Waves,” _Edin. New Phil. Journ.,_ vol.
 xxxi, p. 245.


_Valleys in the sandstone platforms._—The grand valleys, by which the
Blue Mountains and the other sandstone platforms of this part of
Australia are penetrated, and which long offered an insuperable
obstacle to the attempts of the most enterprising colonist to reach the
interior country, form the most striking feature in the geology of New
South Wales. They are of grand dimensions, and are bordered by
continuous links of lofty cliffs. It is not easy to conceive a more
magnificent spectacle, than is presented to a person walking on the
summit-plains, when without any notice he arrives at the brink of one
of these cliffs, which are so perpendicular, that he can strike with a
stone (as I have tried) the trees growing, at the depth of between one
thousand and one thousand five hundred feet below him; on both hands he
sees headland beyond headland of the receding line of cliff, and on the
opposite side of the valley, often at the distance of several miles, he
beholds another line rising up to the same height with that on which he
stands, and formed of the same horizontal strata of pale sandstone. The
bottoms of these valleys are moderately level, and the fall of the
rivers flowing in them, according to Sir T. Mitchell, is gentle. The
main valleys often send into the platform great baylike arms, which
expand at their upper ends; and on the other hand, the platform often
sends promontories into the valley, and even leaves in them great,
almost insulated, masses. So continuous are the bounding lines of
cliff, that to descend into some of these valleys, it is necessary to
go round twenty miles; and into others, the surveyors have only lately
penetrated, and the colonists have not yet been able to drive in their
cattle. But the most remarkable point of structure in these valleys,
is, that although several miles wide in their upper parts, they
generally contract towards their mouths to such a degree as to become
impassable. The Surveyor-General, Sir T. Mitchell,[6] in vain
endeavoured, first on foot and then by crawling between the great
fallen
fragments of sandstone, to ascend through the gorge by which the river
Grose joins the Nepean; yet the valley of the Grose in its upper part,
as I saw, forms a magnificent basin some miles in width, and is on all
sides surrounded by cliffs, the summits of which are believed to be
nowhere less than 3,000 feet above the level of the sea. When cattle
are driven into the valley of the Wolgan by a path (which I descended)
partly cut by the colonists, they cannot escape; for this valley is in
every other part surrounded by perpendicular cliffs, and eight miles
lower down, it contracts, from an average width of half a mile, to a
mere chasm impassable to man or beast. Sir T. Mitchell[7] states, that
the great valley of the Cox river with all its branches contracts,
where it unites with the Nepean, into a gorge 2,200 yards wide, and
about one thousand feet in depth. Other similar cases might have been
added.

 [6] “Travels in Australia,” vol. i, p. 154.—I must express my
 obligation to Sir T. Mitchell for several interesting personal
 communications on the subject of these great valleys of New South
 Wales.


 [7] _Idem_, vol. ii, p. 358.


The first impression, from seeing the correspondence of the horizontal
strata, on each side of these valleys and great amphitheatre-like
depressions, is that they have been in chief part hollowed out, like
other valleys, by aqueous erosion; but when one reflects on the
enormous amount of stone, which on this view must have been removed, in
most of the above cases through mere gorges or chasms, one is led to
ask whether these spaces may not have subsided. But considering the
form of the irregularly branching valleys, and of the narrow
promontories, projecting into them from the platforms, we are compelled
to abandon this notion. To attribute these hollows to alluvial action,
would be preposterous; nor does the drainage from the summit-level
always fall, as I remarked near the Weatherboard, into the head of
these valleys, but into one side of their bay-like recesses. Some of
the inhabitants remarked to me, that they never viewed one of these
baylike recesses, with the headlands receding on both hands, without
being struck with their resemblance to a bold sea-coast. This is
certainly the case; moreover, the numerous fine harbours, with their
widely branching arms, on the present coast of New South Wales, which
are generally connected with the sea by a narrow mouth, from one mile
to a quarter of a mile in width, passing through the sandstone
coast-cliffs, present a likeness, though on a miniature scale, to the
great valleys of the interior. But then immediately occurs the
startling difficulty, why has the sea worn out these great, though
circumscribed, depressions on a wide platform, and left mere gorges,
through which the whole vast amount of triturated matter must have been
carried away? The only light I can throw on this enigma, is by showing
that banks appear to be forming in some seas of the most irregular
forms, and that the sides of such banks are so steep (as before stated)
that a comparatively small amount of subsequent erosion would form them
into cliffs: that the waves have power to form high and precipitous
cliffs, even in landlocked harbours, I have observed in many parts of
South America. In the Red Sea, banks with an extremely irregular
outline and composed of sediment, are penetrated by the most singularly
shaped creeks with narrow mouths: this is likewise the case, though on
a larger scale,
with the Bahama Banks. Such banks, I have been led to suppose,[8] have
been formed by currents heaping sediment on an irregular bottom. That
in some cases, the sea, instead of spreading out sediment in a uniform
sheet, heaps it round submarine rocks and islands, it is hardly
possible to doubt, after having examined the charts of the West Indies.
To apply these ideas to the sandstone platforms of New South Wales, I
imagine that the strata might have been heaped on an irregular bottom
by the action of strong currents, and of the undulations of an open
sea; and that the valley-like spaces thus left unfilled might, during a
slow elevation of the land, have had their steeply sloping flanks worn
into cliffs; the worn-down sandstone being removed, either at the time
when the narrow gorges were cut by the retreating sea, or subsequently
by alluvial action.

 [8] See the “Appendix” to the Part on Coral-Reefs. The fact of the sea
 heaping up mud round a submarine nucleus, is worthy of the notice of
 geologists: for outlyers of the same composition with the coast banks
 are thus formed; and these, if upheaved and worn into cliffs, would
 naturally be thought to have been once connected together.

_Van Diemen’s Land._

The southern part of this island is mainly formed of mountains of
greenstone, which often assumes a syenitic character, and contains much
hypersthene. These mountains, in their lower half, are generally
encased by strata containing numerous small corals and some shells.
These shells have been examined by Mr. G. B. Sowerby, and have been
described by him: they consist of two species of Producta, and of six
of Spirifera; two of these, namely, _P. rugata_ and _S. rotundata_,
resemble, as far as their imperfect condition allows of comparison,
British mountain-limestone shells. Mr. Lonsdale has had the kindness to
examine the corals; they consist of six undescribed species, belonging
to three genera. Species of these genera occur in the Silurian,
Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks,
that all these fossils have undoubtedly a Palæozoic character, and that
probably they correspond in age to a division of the system above the
Silurian formations.

The strata containing these remains are singular from the extreme
variability of their mineralogical composition. Every intermediate form
is present, between flinty-slate, clay-slate passing into grey wacke,
pure limestone, sandstone, and porcellanic rock; and some of the beds
can only be described as composed of a siliceo-calcareo-clay-slate. The
formation, as far as I could judge, is at least a thousand feet in
thickness: the upper few hundred feet usually consist of a siliceous
sandstone, containing pebbles and no organic remains; the inferior
strata, of which a pale flinty slate is perhaps the most abundant, are
the most variable; and these chiefly abound with the remains. Between
two beds of hard crystalline limestone, near Newtown, a layer of white
soft calcareous matter is quarried, and is used for whitewashing
houses. From information given to me by Mr. Frankland, the
Surveyor-General,
it appears that this Palæozoic formation is found in different parts of
the whole island; from the same authority, I may add, that on the
north-eastern coast and in Bass’ Straits primary rocks extensively
occur.

The shores of Storm Bay are skirted, to the height of a few hundred
feet, by strata of sandstone, containing pebbles of the formation just
described, with its characteristic fossils, and therefore belonging to
a subsequent age. These strata of sandstone often pass into shale, and
alternate with layers of impure coal; they have in many places been
violently disturbed. Near Hobart Town, I observed one dike, nearly a
hundred yards in width, on one side of which the strata were tilted at
an angle of 60 degrees, and on the other they were in some parts
vertical, and had been altered by the effects of the heat. On the west
side of Storm Bay, I found these strata capped by streams of basaltic
lava with olivine; and close by there was a mass of brecciated scoriæ,
containing pebbles of lava, which probably marks the place of an
ancient submarine crater. Two of these streams of basalt were separated
from each other by a layer of argillaceous wacke, which could be traced
passing into partially altered scoriæ. The wacke contained numerous
rounded grains of a soft, grass-green mineral, with a waxy lustre, and
translucent on its edges: under the blowpipe it instantly blackened,
and the points fused into a strongly magnetic, black enamel. In these
characters, it resembles those masses of decomposed olivine, described
at St. Jago in the Cape de Verde group; and I should have thought that
it had thus originated, had I not found a similar substance, in
cylindrical threads, within the cells of the vesicular basalt,—a state
under which olivine never appears; this substance,[9] I believe, would
be classed as bole by mineralogists.

 [9] Chlorophæite, described by Dr. MacCulloch (“Western Islands,” vol.
 i, p. 504) as occurring in a basaltic amygdaloid, differs from this
 substance, in remaining unchanged before the blowpipe, and in
 blackening from exposure to the air. May we suppose that olivine, in
 undergoing the remarkable change described at St. Jago, passes through
 several states?

_Travertin with extinct plants._—Behind Hobart Town there is a small
quarry of a hard travertin, the lower strata of which abound with
distinct impressions of leaves. Mr. Robert Brown has had the kindness
to look at my specimens, and he informed me that there are four or five
kinds, none of which he recognises as belonging to existing species.
The most remarkable leaf is palmate, like that of a fan-palm, and no
plant having leaves of this structure has hitherto been discovered in
Van Diemen’s Land. The other leaves do not resemble the most usual form
of the Eucalyptus (of which tribe the existing forests are chiefly
composed), nor do they resemble that class of exceptions to the common
form of the leaves of the Eucalyptus, which occur in this island. The
travertin containing this remnant of a lost vegetation, is of a pale
yellow colour, hard, and in parts even crystalline; but not compact,
and is everywhere penetrated by minute, tortuous, cylindrical pores. It
contains a very few pebbles of quartz, and occasionally layers of
chalcedonic nodules, like those of chert in our Greensand. From
the pureness of this calcareous rock, it has been searched for in other
places, but has never been found. From this circumstance, and from the
character of the deposit, it was probably formed by a calcareous spring
entering a small pool or narrow creek. The strata have subsequently
been tilted and fissured; and the surface has been covered by a
singular mass, with which, also, a large fissure has been filled up,
formed of balls of trap embedded in a mixture of wacke and a white,
earthy, alumino-calcareous substance. Hence it would appear, as if a
volcanic eruption had taken place on the borders of the pool, in which
the calcareous matter was depositing, and had broken it up and drained
it.

_Elevation of the land._—Both the eastern and western shores of the
bay, in the neighbourhood of Hobart Town, are in most parts covered to
the height of thirty feet above the level of high-water mark, with
broken shells, mingled with pebbles. The colonists attribute these
shells to the aborigines having carried them up for food: undoubtedly,
there are many large mounds, as was pointed out to me by Mr. Frankland,
which have been thus formed; but I think from the numbers of the
shells, from their frequent small size, from the manner in which they
are thinly scattered, and from some appearances in the form of the
land, that we must attribute the presence of the greater number to a
small elevation of the land. On the shore of Ralph Bay (opening into
Storm Bay) I observed a continuous beach about fifteen feet above
high-water mark, clothed with vegetation, and by digging into it,
pebbles encrusted with Serpulæ were found: along the banks, also, of
the river Derwent, I found a bed of broken sea-shells above the surface
of the river, and at a point where the water is now much too fresh for
sea-shells to live; but in both these cases, it is just possible, that
before certain spits of sand and banks of mud in Storm Bay were
accumulated, the tides might have risen to the height where we now find
the shells.[10]

 [10] It would appear that some changes are now in progress in Ralph
 Bay, for I was assured by an intelligent farmer, that oysters were
 formerly abundant in it, but that about the year 1834 they had,
 without any apparent cause, disappeared. In the “Transactions of the
 Maryland Academy” (vol. i, part i, p. 28) there is an account by Mr.
 Ducatel of vast beds of oysters and clams having been destroyed by the
 gradual filling up of the shallow lagoons and channels, on the shores
 of the southern United States. At Chiloe, in South America, I heard of
 a similar loss, sustained by the inhabitants, in the disappearance
 from one part of the coast of an edible species of Ascidia.


Evidence more or less distinct of a change of level between the land
and water, has been detected on almost all the land on this side of the
globe. Captain Grey, and other travellers, have found in Southern
Australia upraised shells, belonging either to the recent, or to a late
tertiary period. The French naturalists in Baudin’s expedition, found
shells similarly circumstanced on the S.W. coast of Australia. The Rev.
W. B. Clarke[11] finds proofs of the elevation of the land, to the
amount of 400 feet, at the Cape of Good Hope. In the neighbourhood
of the Bay of Islands in New Zealand,[12] I observed that the shores
were scattered to some height, as at Van Diemen’s Land, with
sea-shells, which the colonists attribute to the natives. Whatever may
have been the origin of these shells, I cannot doubt, after having seen
a section of the valley of the Thames River (37 degrees S.), drawn by
the Rev. W. Williams, that the land has been there elevated: on the
opposite sides of this great valley, three step-like terraces, composed
of an enormous accumulation of rounded pebbles, exactly correspond with
each other: the escarpment of each terrace is about fifty feet in
height. No one after having examined the terraces in the valleys on the
western shores of South America, which are strewed with sea-shells, and
have been formed during intervals of rest in the slow elevation of the
land, could doubt that the New Zealand terraces have been similarly
formed. I may add, that Dr. Dieffenbach, in his description of the
Chatham Islands[13] (S.W. of New Zealand), states that it is manifest
“that the sea has left many places bare which were once covered by its
waters.”

 [11] “Proceedings of the Geological Society,” vol. iii, p. 420.


 [12] I will here give a catalogue of the rocks which I met with near
 the Bay of Islands, in New Zealand:—1st, Much basaltic lava, and
 scoriform rocks, forming distinct craters;—2nd, A castellated hill of
 horizontal strata of flesh-coloured limestone, showing when fractured
 distinct crystalline facets: the rain has acted on this rock in a
 remarkable manner, corroding its surface into a miniature model of an
 Alpine country: I observed here layers of chert and clay ironstone;
 and in the bed of a stream, pebbles of clay-slate;—3rd, The shores of
 the Bay of Islands are formed of a feldspathic rock, of a bluish-grey
 colour, often much decomposed, with an angular fracture, and crossed
 by numerous ferruginous seams, but without any distinct stratification
 or cleavage. Some varieties are highly crystalline, and would at once
 be pronounced to be trap; others strikingly resembled clay-slate,
 slightly altered by heat: I was unable to form any decided opinion on
 this formation.


 [13] _Geographical Journal_, vol. xi, pp. 202, 205.

_King George’s Sound._

This settlement is situated at the south-western angle of the
Australian continent: the whole country is granitic, with the
constituent minerals sometimes obscurely arranged in straight or curved
laminæ. In these cases, the rock would be called by Humboldt,
gneiss-granite, and it is remarkable that the form of the bare conical
hills, appearing to be composed of great folding layers, strikingly
resembles, on a small scale, those composed of gneiss-granite at Rio de
Janeiro, and those described by Humboldt at Venezuela. These plutonic
rocks are, in many places, intersected by trappean-dikes; in one place,
I found ten parallel dikes ranging in an E. and W. line; and not far
off another set of eight dikes, composed of a different variety of
trap, ranging at right angles to the former ones. I have observed in
several primary districts, the occurrence of systems of dikes parallel
and close to each other.

_Superficial ferruginous beds._—The lower parts of the country are
everywhere covered by a bed, following the inequalities of the surface,
of a honeycombed sandstone, abounding with oxides of iron. Beds of
nearly similar composition are common, I believe, along the whole
western coast of Australia, and on many of the East Indian islands. At
the Cape of Good Hope, at the base of the mountains formed of granite
and capped with sandstone, the ground is everywhere coated either by a
fine-grained, rubbly, ochraceous mass, like that at King George’s
Sound, or by a coarser sandstone with fragments of quartz, and rendered
hard and heavy by an abundance of the hydrate of iron, which presents,
when freshly broken, a metallic lustre. Both these varieties have a
very irregular texture, including spaces either rounded or angular,
full of loose sand: from this cause the surface is always honeycombed.
The oxide of iron is most abundant on the edges of the cavities, where
alone it affords a metallic fracture. In these formations, as well as
in many true sedimentary deposits, it is evident that iron tends to
become aggregated, either in the form of a shell, or of a network. The
origin of these superficial beds, though sufficiently obscure, seems to
be due to alluvial action on detritus abounding with iron.

_ Superficial calcareous deposit._—A calcareous deposit on the summit
of Bald Head, containing branched bodies, supposed by some authors to
have been corals, has been celebrated by the descriptions of many
distinguished voyagers.[14] It folds round and conceals irregular
hummocks of granite, at the height of 600 feet above the level of the
sea. It varies much in thickness; where stratified, the beds are often
inclined at high angles, even as much as at thirty degrees, and they
dip in all directions. These beds are sometimes crossed by oblique and
even-sided laminæ. The deposit consists either of a fine, white
calcareous powder, in which not a trace of structure can be discovered,
or of exceedingly minute, rounded grains, of brown, yellowish, and
purplish colours; both varieties being generally, but not always, mixed
with small particles of quartz, and being cemented into a more or less
perfect stone. The rounded calcareous grains, when heated in a slight
degree, instantly lose their colours; in this and in every other
respect, closely resembling those minute, equal-sized particles of
shells and corals, which at St. Helena have been drifted up the side of
the mountains, and have thus been winnowed of all coarser fragments. I
cannot doubt that the coloured calcareous particles here have had a
similar origin. The impalpable powder has probably been derived from
the decay of the rounded particles; this certainly is possible, for on
the coast of Peru, I have traced _large unbroken_ shells gradually
falling into a substance as fine as powdered chalk. Both of the
above-mentioned varieties of calcareous sandstone frequently alternate
with, and blend into, thin layers of a hard substalagmitic[15] rock,
which, even
when the stone on each side contains particles of quartz, is entirely
free from them: hence we must suppose that these layers, as well as
certain vein-like masses, have been formed by rain dissolving the
calcareous matter and re-precipitating it, as has happened at St.
Helena. Each layer probably marks a fresh surface, when the, now firmly
cemented, particles existed as loose sand. These layers are sometimes
brecciated and re-cemented, as if they had been broken by the slipping
of the sand when soft. I did not find a single fragment of a sea-shell;
but bleached shells of the _Helix melo_, an existing land species,
abound in all the strata; and I likewise found another Helix, and the
case of an Oniscus.

 [14] I visited this hill, in company with Captain Fitzroy, and we came
 to a similar conclusion regarding these branching bodies.


 [15] I adopt this term from Lieutenant Nelson’s excellent paper on the
 Bermuda Islands (“Geolog. Trans.,” vol. v, p. 106), for the hard,
 compact, cream- or brown-coloured stone, without any crystalline
 structure, which so often accompanies superficial calcareous
 accumulations. I have observed such superficial beds, coated with
 substalagmitic rock, at the Cape of Good Hope, in several parts of
 Chile, and over wide spaces in La Plata and Patagonia. Some of these
 beds have been formed from decayed shells, but the origin of the
 greater number is sufficiently obscure. The causes which determine
 water to dissolve lime, and then soon to redeposit it, are not, I
 think, known. The surface of the substalagmitic layers appears always
 to be corroded by the rain-water. As all the above-mentioned countries
 have a long dry season, compared with the rainy one, I should have
 thought that the presence of the substalagmitic was connected with the
 climate, had not Lieutenant Nelson found this substance forming under
 sea-water. Disintegrated shell seems to be extremely soluble; of which
 I found good evidence, in a curious rock at Coquimbo in Chile, which
 consisted of small, pellucid, empty husks, cemented together. A series
 of specimens clearly showed that these husks had originally contained
 small rounded particles of shells, which had been enveloped and
 cemented together by calcareous matter (as often happens on
 sea-beaches), and which subsequently had decayed, and been dissolved
 by water, that must have penetrated through the calcareous husks,
 without corroding them,—of which processes every stage could be seen.


The branches are absolutely undistinguishable in shape from the broken
and upright stumps of a thicket; their roots are often uncovered, and
are seen to diverge on all sides; here and there a branch lies
prostrate. The branches generally consist of the sandstone, rather
firmer than the surrounding matter, with the central parts filled,
either with friable, calcareous matter, or with a substalagmitic
variety; this central part is also frequently penetrated by linear
crevices, sometimes, though rarely, containing a trace of woody matter.
These calcareous, branching bodies, appear to have been formed by fine
calcareous matter being washed into the casts or cavities, left by the
decay of branches and roots of thickets, buried under drifted sand. The
whole surface of the hill is now undergoing disintegration, and hence
the casts, which are compact and hard, are left projecting. In
calcareous sand at the Cape of Good Hope, I found the casts, described
by Abel, quite similar to these at Bald Head; but their centres are
often filled with black carbonaceous matter not yet removed. It is not
surprising, that the woody matter should have been almost entirely
removed from the casts on Bald Head; for it is certain, that many
centuries must have elapsed since the thickets were buried; at present,
owing to the form and height of the narrow promontory, no sand is
drifted up, and the whole surface, as I have remarked, is wearing away.
We must, therefore,
look back to a period when the land stood lower, of which the French
naturalists[16] found evidence in upraised shells of recent species,
for the drifting on Bald Head of the calcareous and quartzose sand, and
the consequent embedment of the vegetable remains. There was only one
appearance which at first made me doubt concerning the origin of the
cast,—namely, that the finer roots from different stems sometimes
became united together into upright plates or veins; but when the
manner is borne in mind in which fine roots often fill up cracks in
hard earth, and that these roots would decay and leave hollows, as well
as the stems, there is no real difficulty in this case. Besides the
calcareous branches from the Cape of Good Hope, I have seen casts, of
exactly the same forms, from Madeira[17] and from Bermuda; at this
latter place, the surrounding calcareous rocks, judging from the
specimens collected by Lieutenant Nelson, are likewise similar, as is
their subaerial formation. Reflecting on the stratification of the
deposit on Bald Head,—on the irregularly alternating layers of
substalagmitic rock,—on the uniformly sized, and rounded particles,
apparently of sea-shells and corals,—on the abundance of land-shells
throughout the mass,—and finally, on the absolute resemblance of the
calcareous casts, to the stumps, roots, and branches of that kind of
vegetation, which would grow on sand-hillocks, I think there can be no
reasonable doubt, notwithstanding the different opinion of some
authors, that a true view of their origin has been here given.

 [16] See M. Péron’s “Voyage,” tome i, p. 204.


 [17] Dr. J. Macaulay has fully described (_Edinb. New Phil. Journ.,_
 vol. xxix, p. 350) the casts from Madeira. He considers (differently
 from Mr. Smith of Jordan Hill) these bodies to be corals, and the
 calcareous deposit to be of subaqueous origin. His arguments chiefly
 rest (for his remarks on their structure are vague) on the great
 quantity of the calcareous matter, and on the casts containing animal
 matter, as shown by their evolving ammonia. Had Dr. Macaulay seen the
 enormous masses of rolled particles of shells and corals on the beach
 of Ascension, and especially on coral-reefs; and had he reflected on
 the effects of long-continued, gentle winds, in drifting up the finer
 particles, he would hardly have advanced the argument of quantity,
 which is seldom trustworthy in geology. If the calcareous matter has
 originated from disintegrated shells and corals, the presence of
 animal matter is what might have been expected. Mr. Anderson analysed
 for Dr. Macaulay part of a cast, and he found it composed of—

Carbonate of lime	73·15 Silica	11·90 Phosphate of lime	8·81
Animal matter	4·25 Sulphate of lime	a trace ——— 98·11

Calcareous deposits, like these of King George’s Sound, are of vast
extent on the Australian shores. Dr. Fitton remarks, that “recent
calcareous breccia (by which term all these deposits are included) was
found during Baudin’s voyage, over a space of no less than twenty-five
degrees
of latitude and an equal extent of longitude, on the southern, western,
and north-western coasts.”[18] It appears also from M. Peron, with
whose observations and opinions on the origin of the calcareous matter
and branching casts mine entirely accord, that the deposit is generally
much more continuous than near King George’s Sound. At Swan River,
Archdeacon Scott[19] states that in one part it extends ten miles
inland. Captain Wickham, moreover, informs me that during his late
survey of the western coast, the bottom of the sea, wherever the vessel
anchored, was ascertained, by crowbars being let down, to consist of
white calcareous matter. Hence it seems that along this coast, as at
Bermuda and at Keeling Atoll, submarine and subaerial deposits are
contemporaneously in process of formation, from the disintegration of
marine organic bodies. The extent of these deposits, considering their
origin, is very striking; and they can be compared in this respect only
with the great coral-reefs of the Indian and Pacific Oceans. In other
parts of the world, for instance in South America, there are
_superficial_ calcareous deposits of great extent, in which not a trace
of organic structure is discoverable; these observations would lead to
the inquiry, whether such deposits may not, also, have been formed from
disintegrated shells and corals.

 [18] For ample details on this formation consult Dr. Fitton’s
 “Appendix to Captain King’s Voyage.” Dr. Fitton is inclined to
 attribute a concretionary origin to the branching bodies: I may
 remark, that I have seen in beds of sand in La Plata cylindrical stems
 which no doubt thus originated; but they differed much in appearance
 from these at Bald Head, and the other places above specified.


 [19] “Proceedings of the Geolog. Soc.,” vol. i, p. 320.

_Cape of Good Hope._

After the accounts given by Barrow, Carmichael, Basil Hall, and W. B.
Clarke of the geology of this district, I shall confine myself to a few
observations on the junction of the three principal formations. The
fundamental rock is granite,[20] overlaid by clay-slate: the latter is
generally hard, and glossy from containing minute scales of mica; it
alternates with, and passes into, beds of slightly crystalline,
feldspathic, slaty rock. This clay-slate is remarkable from being in
some places (as on the Lion’s Rump) decomposed, even to the depth of
twenty feet, into a pale-coloured, sandstone-like rock, which has been
mistaken, I believe, by some observers, for a separate formation. I was
guided by Dr. Andrew Smith to a fine junction at Green Point between
the granite and clay-slate: the latter at the distance of a quarter of
a mile
from the spot, where the granite appears on the beach (though,
probably, the granite is much nearer underground), becomes slightly
more compact and crystalline. At a less distance, some of the beds of
clay-slate are of a homogeneous texture, and obscurely striped with
different zones of colour, whilst others are obscurely spotted. Within
a hundred yards of the first vein of granite, the clay-slate consists
of several varieties; some compact with a tinge of purple, others
glistening with numerous minute scales of mica and imperfectly
crystallised feldspar; some obscurely granular, others porphyritic with
small, elongated spots of a soft white mineral, which being easily
corroded, gives to this variety a vesicular appearance. Close to the
granite, the clay-slate is changed into a dark-coloured, laminated
rock, having a granular fracture, which is due to imperfect crystals of
feldspar, coated by minute, brilliant scales of mica.

 [20] In several places I observed in the granite, small dark-coloured
 balls, composed of minute scales of black mica in a tough basis. In
 another place, I found crystals of black schorl radiating from a
 common centre. Dr. Andrew Smith found, in the interior parts of the
 country, some beautiful specimens of granite, with silvery mica
 radiating or rather branching, like moss, from central points. At the
 Geological Society, there are specimens of granite with crystallised
 feldspar branching and radiating in like manner.


The actual junction between the granitic and clay-slate districts
extends over a width of about two hundred yards, and consists of
irregular masses and of numerous dikes of granite, entangled and
surrounded by the clay-slate: most of the dikes range in a N.W. and
S.E. line, parallel to the cleavage of the slate. As we leave the
junction, thin beds, and lastly, mere films of the altered clay-slate
are seen, quite isolated, as if floating, in the coarsely crystallised
granite; but although completely detached, they all retain traces of
the uniform N.W. and S.E. cleavage. This fact has been observed in
other similar cases, and has been advanced by some eminent
geologists,[21] as a great difficulty on the ordinary theory, of
granite having been injected whilst liquified; but if we reflect on the
probable state of the lower surface of a laminated mass, like
clay-slate, after having been violently arched by a body of molten
granite, we may conclude that it would be full of fissures parallel to
the planes of cleavage; and that these would be filled with granite, so
that wherever the fissures were close to each other, mere parting
layers or wedges of the slate would depend into the granite. Should,
therefore, the whole body of rock afterwards become worn down and
denuded, the lower ends of these dependent masses or wedges of slate
would be left quite isolated in the granite; yet they would retain
their proper lines of cleavage, from having been united, whilst the
granite was fluid, with a continuous covering of clay-slate.

 [21] See M. Keilhau’s “Theory on Granite” translated in the _Edinburgh
 New Philosophical Journal,_ vol.xxiv, p. 402.

Following, in company with Dr. A. Smith, the line of junction between
the granite and the slate, as it stretched inland, in a S.E. direction,
we came to a place, where the slate was converted into a fine-grained,
perfectly characterised gneiss, composed of yellow-brown granular
feldspar, of abundant black brilliant mica, and of few and thin laminæ
of quartz. From the abundance of the mica in this gneiss, compared with
the small quantity and excessively minute scales, in which it exists in
the glossy clay-slate, we must conclude, that it has been here formed
by the metamorphic action—a circumstance doubted, under nearly similar
circumstances, by some authors. The laminæ of the clay-slate are
straight; and it was interesting to observe, that as they assumed
the character of gneiss, they became undulatory with some of the
smaller flexures angular, like the laminæ of many true metamorphic
schists.

_Sandstone formation._—This formation makes the most imposing feature
in the geology of Southern Africa. The strata are in many parts
horizontal, and attain a thickness of about two thousand feet. The
sandstone varies in character; it contains little earthy matter, but is
often stained with iron; some of the beds are very fine-grained and
quite white; others are as compact and homogeneous as quartz rock. In
some places I observed a breccia of quartz, with the fragments almost
dissolved in a siliceous paste. Broad veins of quartz, often including
large and perfect crystals, are very numerous; and it is evident in
nearly all the strata, that silica has been deposited from solution in
remarkable quantity. Many of the varieties of quartzite appeared quite
like metamorphic rocks; but from the upper strata being as siliceous as
the lower, and from the undisturbed junctions with the granite, which
in many places can be examined, I can hardly believe that these
sandstone-strata have been exposed to heat.[22] On the lines of
junction between these two great formations, I found in several places
the granite decayed to the depth of a few inches, and succeeded, either
by a thin layer of ferruginous shale, or by four or five inches in
thickness of the re-cemented crystals of the granite, on which the
great pile of sandstone immediately rested.

 [22] The Rev. W. B. Clarke, however, states, to my surprise (“Geolog.
 Proceedings,” vol. iii, p. 422), that the sandstone in some parts is
 penetrated by granitic dikes: such dikes must belong to an epoch
 altogether subsequent to that when the molten granite acted on the
 clay-slate.


Mr. Schomburgk has described[23] a great sandstone formation in
Northern Brazil, resting on granite, and resembling to a remarkable
degree, in composition and in the external form of the land, this
formation of the Cape of Good Hope. The sandstones of the great
platforms of Eastern Australia, which also rest on granite, differ in
containing more earthy and less siliceous matter. No fossil remains
have been discovered in these three vast deposits. Finally, I may add
that I did not see any boulders of far-transported rocks at the Cape of
Good Hope, or on the eastern and western shores of Australia, or at Van
Diemen’s Land. In the northern island of New Zealand, I noticed some
large blocks of greenstone, but whether their parent rock was far
distant, I had no opportunity of determining.

 [23] _Geographical Journal,_ vol. x, p. 246.




GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.

CRITICAL INTRODUCTION.


Of the remarkable “trilogy” constituted by Darwin’s writings which deal
with the geology of the _Beagle_, the member which has perhaps
attracted least attention, up to the present time is that which treats
of the geology of South America. The actual writing of this book
appears to have occupied Darwin a shorter period than either of the
other volumes of the series; his diary records that the work was
accomplished within ten months, namely, between July 1844 and April
1845; but the book was not actually issued till late in the year
following, the preface bearing the date “September 1846.” Altogether,
as Darwin informs us in his “Autobiography,” the geological books
“consumed four and a half years’ steady work,” most of the remainder of
the ten years that elapsed between the return of the _Beagle_, and the
completion of his geological books being, it is sad to relate, “lost
through illness!”

Concerning the “Geological Observations on South America,” Darwin wrote
to his friend Lyell, as follows:—“My volume will be about 240 pages,
dreadfully dull, yet much condensed. I think whenever you have time to
look through it, you will think the collection of facts on the
elevation of the land and on the formation of terraces pretty good.”

“Much condensed” is the verdict that everyone must endorse, on rising
from the perusal of this remarkable book; but by no means “dull.” The
three and a half years from April 1832 to September 1835, were spent by
Darwin in South America, and were devoted to continuous scientific
work; the problems he dealt with were either purely geological or those
which constitute the borderland between the geological and biological
sciences. It is impossible to read the journal which he kept during
this time without being impressed by the conviction
that it contains all the germs of thought which afterwards developed
into the “Origin of Species.” But it is equally evident that after his
return to England, biological speculations gradually began to exercise
a more exclusive sway over Darwin’s mind, and tended to dispossess
geology, which during the actual period of the voyage certainly
engrossed most of his time and attention. The wonderful series of
observations made during those three and a half years in South America
could scarcely be done justice to, in the 240 pages devoted to their
exposition. That he executed the work of preparing the book on South
America in somewhat the manner of a task, is shown by many references
in his letters. Writing to Sir Joseph Hooker in 1845, he says, “I hope
this next summer to finish my South American Geology, then to get out a
little Zoology, and _hurrah for my species work!_”

It would seem that the feeling of disappointment, which Darwin so often
experienced in comparing a book when completed, with the observations
and speculations which had inspired it, was more keenly felt in the
case of his volume on South America than any other. To one friend he
writes, “I have of late been slaving extra hard, to the great
discomfiture of wretched digestive organs, at South America, and thank
all the fates, I have done three-fourths of it. Writing plain English
grows with me more and more difficult, and never attainable. As for
your pretending that you will read anything so dull as my pure
geological descriptions, lay not such a flattering unction on my soul,
for it is incredible.” To another friend he writes, “You do not know
what you threaten when you propose to read it—it is purely geological.
I said to my brother, ‘You will of course read it,’ and his answer was,
‘Upon my life, I would sooner even buy it.’”

In spite of these disparaging remarks, however, we are strongly
inclined to believe that this book, despised by its author, and
neglected by his contemporaries, will in the end be admitted to be one
of Darwin’s chief titles to fame. It is, perhaps, an unfortunate
circumstance that the great success which he attained in biology by the
publication of the “Origin of Species” has, to some extent,
overshadowed the fact that Darwin’s claims as a geologist, are of the
very highest order. It is not too much to say that, had Darwin not been
a geologist, the “Origin of Species” could never have been written by
him. But apart from those geological questions, which have an important
bearing on biological thought and speculation, such as the proofs of
imperfection in the geological record, the relations of the later
tertiary faunas to the recent ones in the same areas, and the apparent
intermingling of types belonging to distant geological epochs, when we
study the palæontology of remote districts,—there are other purely
geological problems, upon which the contributions made by Darwin are of
the very highest value. I believe that the verdict of the historians of
science will be that if Darwin had not taken a foremost place among the
biologists of this century, his position as a geologist would have been
an almost equally commanding one.

But in the case of Darwin’s principal geological work—that relating to
the origin of the crystalline schists,—geologists were not at the time
prepared to receive his revolutionary teachings. The influence of
powerful authority was long exercised, indeed, to stifle his teaching,
and only now, when this unfortunate opposition has disappeared, is the
true nature and importance of Darwin’s purely geological work beginning
to be recognised.

The two first chapters of the “Geological Observations on South
America,” deal with the proofs which exist of great, but frequently
interrupted, movements of elevation during very recent geological
times. In connection with this subject, Darwin’s particular attention
was directed to the relations between the great earthquakes of South
America—of some of which he had impressive experience—and the permanent
changes of elevation which were taking place. He was much struck by the
rapidity with which the evidence of such great earth movements is
frequently obliterated; and especially with the remarkable way in which
the action of rain-water, percolating through deposits on the earth’s
surface, removes all traces of shells and other calcareous organisms.
It was these considerations which were the parents of the
generalisation that a palæontological record can only be preserved
during those periods in which long-continued slow subsidence is going
on. This in turn, led to the still wider and more suggestive conclusion
that the geological record as a whole is, and never can be more than, a
series of more or less isolated fragments. The recognition of this
important fact constitutes the keystone to any theory of evolution
which seeks to find a basis in the actual study of the types of life
that have formerly inhabited our globe.

In his third chapter, Darwin gives a number of interesting facts,
collected during his visits to the plains and valleys of Chili, which
bear on the question of the origin of saliferous deposits—the
accumulation of salt, gypsum, and nitrate of soda. This is a
problem that has excited much discussion among geologists, and which,
in spite of many valuable observations, still remains to a great extent
very obscure. Among the important considerations insisted upon by
Darwin is that relating to the absence of marine shells in beds
associated with such deposits. He justly argues that if the strata were
formed in shallow waters, and then exposed by upheaval to subaerial
action, all shells and other calcareous organisms would be removed by
solution.

Following Lyell’s method, Darwin proceeds from the study of deposits
now being accumulated on the earth’s surface, to those which have been
formed during the more recent periods of the geological history.

His account of the great Pampean formation, with its wonderful
mammalian remains—_Mastodon, Toxodon, Scelidotherium, Macrauchenia,
Megatherium, Megalonyx, Mylodon,_ and _ Glyptodon_—this full of
interest. His discovery of the remains of a true _Equus_ afforded a
remarkable confirmation of the fact—already made out in North
America—that species of horse had existed and become extinct in the New
World, before their introduction by the Spaniards in the sixteenth
century. Fully perceiving the importance of the microscope in studying
the nature and origin of such deposits as those of the Pampas, Darwin
submitted many of his specimens both to Dr. Carpenter in this country,
and to Professor Ehrenberg in Berlin. Many very important notes on the
microscopic organisms contained in the formation will be found
scattered through the chapter.

Darwin’s study of the older tertiary formations, with their abundant
shells, and their relics of vegetable life buried under great sheets of
basalt, led him to consider carefully the question of climate during
these earlier periods. In opposition to prevalent views on this
subject, Darwin points out that his observations are opposed to the
conclusion that a higher temperature prevailed universally over the
globe during early geological periods. He argues that “the causes which
gave to the older tertiary productions of the quite temperate zones of
Europe a tropical character, _were of a local character and did not
affect the whole globe._” In this, as in many similar instances, we see
the beneficial influence of extensive travel in freeing Darwin’s mind
from prevailing prejudices. It was this widening of experience which
rendered him so especially qualified to deal with the great problem of
the origin of species, and in doing so to emancipate himself from ideas
which were received with unquestioning faith by
geologists whose studies had been circumscribed within the limits of
Western Europe.

In the Cordilleras of Northern and Central Chili, Darwin, when studying
still older formations, clearly recognised that they contain an
admixture of the forms of life, which in Europe are distinctive of the
Cretaceous and Jurassic periods respectively. He was thus led to
conclude that the classification of geological periods, which fairly
well expresses the facts that had been discovered in the areas where
the science was first studied, is no longer capable of being applied
when we come to the study of widely distant regions. This important
conclusion led up to the further generalisation that each great
geological period has exhibited a geographical distribution of the
forms of animal and vegetable life, comparable to that which prevails
in the existing fauna and flora. To those who are familiar with the
extent to which the doctrine of universal formations has affected
geological thought and speculation, both long before and since the time
that Darwin wrote, the importance of this new standpoint to which he
was able to attain will be sufficiently apparent. Like the idea of the
extreme imperfection of the Geological Record, the doctrine of _ local_
geological formations is found permeating and moulding all the
palæontological reasonings of his great work.

In one of Darwin’s letters, written while he was in South America,
there is a passage we have already quoted, in which he expresses his
inability to decide between the rival claims upon his attention of “the
old crystalline group of rocks,” and “the softer fossiliferous beds”
respectively. The sixth chapter of the work before us, entitled
“Plutonic and Metamorphic Rocks—Cleavage and Foliation,” contains a
brief summary of a series of observations and reasonings upon these
crystalline rocks, which are, we believe, calculated to effect a
revolution in geological science, and—though their value and importance
have long been overlooked—are likely to entitle Darwin in the future to
a position among geologists, scarcely, if at all, inferior to that
which he already occupies among biologists.

Darwin’s studies of the great rock-masses of the Andes convinced him of
the close relations between the granitic or Plutonic rocks, and those
which were undoubtedly poured forth as lavas. Upon his return, he set
to work, with the aid of Professor Miller, to make a careful study of
the minerals composing the granites and those which occur in the lavas,
and he was able to show that in all essential respects they are
identical. He was further able to
prove that there is a complete gradation between the highly crystalline
or granitic rock-masses, and those containing more or less glassy
matter between their crystals, which constitute ordinary lavas. The
importance of this conclusion will be realised when we remember that it
was then the common creed of geologists—and still continues to be so on
the Continent—that all highly crystalline rocks are of great geological
antiquity, and that the igneous ejections which have taken place since
the beginning of the tertiary periods differ essentially, in their
composition, their structure, and their mode of occurrence, from those
which have made their appearance at earlier periods of the world’s
history.

Very completely have the conclusions of Darwin upon these subjects been
justified by recent researches. In England, the United States, and
Italy, examples of the gradual passage of rocks of truly granitic
structure into ordinary lavas have been described, and the reality of
the transition has been demonstrated by the most careful studies with
the microscope. Recent researches carried on in South America by
Professor Stelzner, have also shown the existence of a class of highly
crystalline rocks—the “Andengranites”—which combine in themselves many
of the characteristics which were once thought to be distinctive of the
so-called Plutonic and volcanic rocks. No one familiar with recent
geological literature—even in Germany and France, where the old views
concerning the distinction of igneous products of different ages have
been most stoutly maintained—can fail to recognise the fact that the
principles contended for by Darwin bid fair at no distant period to win
universal acceptance among geologists all over the globe.

Still more important are the conclusions at which Darwin arrived with
respect to the origin of the schists and gneisses which cover so large
an area in South America.

Carefully noting, by the aid of his compass and clinometer, at every
point which he visited, the direction and amount of inclination of the
parallel divisions in these rocks, he was led to a very important
generalisation—namely, that over very wide areas the direction (strike)
of the planes of cleavage in slates, and of foliation in schists and
gneisses, remained constant, though the amount of their inclination
(dip) often varied within wide limits. Further than this it appeared
that there was always a close correspondence between the strike of the
cleavage and foliation and the direction of the great axes along which
elevation had taken place in the district.


In Tierra del Fuego, Darwin found striking evidence that the cleavage
intersecting great masses of slate-rocks was quite independent of their
original stratification, and could often, indeed, be seen cutting
across it at right angles. He was also able to verify Sedgwick’s
observation that, in some slates, glossy surfaces on the planes of
cleavage arise from the development of new minerals, chlorite, epidote
or mica, and that in this way a complete graduation from slates to true
schists may be traced.

Darwin further showed that in highly schistose rocks, the folia bend
around and encircle any foreign bodies in the mass, and that in some
cases they exhibit the most tortuous forms and complicated puckerings.
He clearly saw that in all cases the forces by which these striking
phenomena must have been produced were persistent over wide areas, and
were connected with the great movements by which the rocks had been
upheaved and folded.

That the distinct folia of quartz, feldspar, mica, and other minerals
composing the metamorphic schists could not have been separately
deposited as sediment was strongly insisted upon by Darwin; and in
doing so he opposed the view generally prevalent among geologists at
that time. He was thus driven to the conclusion that foliation, like
cleavage, is not an original, but a superinduced structure in
rock-masses, and that it is the result of re-crystallisation, under the
controlling influence of great pressure, of the materials of which the
rock was composed.

In studying the lavas of Ascension, as we have already seen, Darwin was
led to recognise the circumstance that, when igneous rocks are
subjected to great differential movements during the period of their
consolidation, they acquire a foliated structure, closely analogous to
that of the crystalline schists. Like his predecessor in this field of
inquiry, Mr. Poulett Scrope, Charles Darwin seems to have been greatly
impressed by these facts, and he argued from them that the rocks
exhibiting the foliated structure must have been in a state of
plasticity, like that of a cooling mass of lava. At that time the
suggestive experiments of Tresca, Daubree, and others, showing that
solid masses under the influence of enormous pressure become actually
plastic, had not been published. Had Darwin been aware of these facts
he would have seen that it was not necessary to assume a state of
imperfect solidity in rock-masses in order to account for their having
yielded to pressure and tension, and, in doing so, acquiring the new
characters which distinguish the crystalline schists.


The views put forward by Darwin on the origin of the crystalline
schists found an able advocate in Mr. Daniel Sharpe, who in 1852 and
1854 published two papers, dealing with the geology of the Scottish
Highlands and of the Alps respectively, in which he showed that the
principles arrived at by Darwin when studying the South American rocks
afford a complete explanation of the structure of the two districts in
question.

But, on the other hand, the conclusions of Darwin and Sharpe were met
with the strongest opposition by Sir Roderick Murchison and Dr. A.
Geikie, who in 1861 read a paper before the Geological Society “On the
Coincidence between Stratification and Foliation in the Crystalline
Rocks of the Scottish Highlands,” in which they insisted that their
observations in Scotland tended to entirely disprove the conclusions of
Darwin that foliation in rocks is a secondary structure, and entirely
independent of the original stratification of the rock-masses.

Now it is a most significant circumstance that, no sooner did the
officers of the Geological Survey commence the careful and detailed
study of the Scottish Highlands than they found themselves compelled to
make a formal retraction of the views which had been put forward by
Murchison and Geikie in opposition to the conclusions of Darwin. The
officers of the Geological Survey have completely abandoned the view
that the foliation of the Highland rocks has been determined by their
original stratification, and admit that the structure is the result of
the profound movements to which the rocks have been subjected. The same
conclusions have recently been supported by observations made in many
different districts—among which we may especially refer to those of Dr.
H. Reusch in Norway, and those of Dr. J. Lehmann in Saxony. At the
present time the arguments so clearly stated by Darwin in the work
before us, have, after enduring opposition or neglect for a whole
generation, begun to “triumph all along the line,” and we may look
forward confidently to the near future, when his claim to be regarded
as one of the greatest of geological discoverers shall be fully
vindicated.

JOHN W. JUDD.




Chapter I ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.


Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
Shells.—Terrace-bounded Valley of Santa Cruz, formerly a
Sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.—Great Shingle Formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.

In the following Volume, which treats of the geology of South America,
and almost exclusively of the parts southward of the Tropic of
Capricorn, I have arranged the chapters according to the age of the
deposits, occasionally departing from this order, for the sake of
geographical simplicity.

The elevation of the land within the recent period, and the
modifications of its surface through the action of the sea (to which
subjects I paid particular attention) will be first discussed; I will
then pass on to the tertiary deposits, and afterwards to the older
rocks. Only those districts and sections will be described in detail
which appear to me to deserve some particular attention; and I will, at
the end of each chapter, give a summary of the results. We will
commence with the proofs of the upheaval of the eastern coast of the
continent, from the Rio Plata southward; and, in the Second Chapter,
follow up the same subject along the shores of Chile and Peru.

On the northern bank of the great estuary of the Rio Plata, near
Maldonado, I found at the head of a lake, sometimes brackish but
generally containing fresh water, a bed of muddy clay, six feet in
thickness, with numerous shells of species still existing in the Plata,
namely, the _Azara labiata_, d’Orbigny, fragments of _Mytilus
eduliformis_, d’Orbigny, _Paludestrina Isabellei_, d’Orbigny, and the
_Solen Caribæus_, Lam., which last was embedded vertically in the
position in which it had lived. These shells lie at the height of only
two feet above the lake, nor would they have been worth mentioning,
except in connection with analogous facts.


At Monte Video, I noticed near the town, and along the base of the
mount, beds of a living Mytilus, raised some feet above the surface of
the Plata: in a similar bed, at a height from thirteen to sixteen feet,
M. Isabelle collected eight species, which,[1] according to M.
d’Orbigny, now live at the mouth of the estuary. At Colonia del
Sacramiento, further westward, I observed at the height of about
fifteen feet above the river, there of quite fresh water, a small bed
of the same Mytilus, which lives in brackish water at Monte Video. Near
the mouth of Uruguay, and for at least thirty-five miles northward,
there are at intervals large sandy tracts, extending several miles from
the banks of the river, but not raised much above its level, abounding
with small bivalves, which occur in such numbers that at the Agraciado
they are sifted and burnt for lime. Those which I examined near the A.
S. Juan were much worn: they consisted of _Mactra Isabellei_,
d’Orbigny, mingled with few of _Venus sinuosa_, Lam., both inhabiting,
as I am informed by M. d’Orbigny, brackish water at the mouth of the
Plata, nearly or quite as salt as the open sea. The loose sand, in
which these shells are packed, is heaped into low, straight, long lines
of dunes, like those left by the sea at the head of many bays. M.
d’Orbigny has described[2] an analogous phenomenon on a greater scale,
near San Pedro on the river Parana, where he found widely extended beds
and hillocks of sand, with vast numbers of the _Azara labiata_, at the
height of nearly 100 feet (English) above the surface of that river.
The Azara inhabits brackish water, and is not known to be found nearer
to San Pedro than Buenos Ayres, distant above a hundred miles in a
straight line. Nearer Buenos Ayres, on the road from that place to San
Isidro, there are extensive beds, as I am informed by Sir Woodbine
Parish,[3] of the _Azara labiata_, lying at about forty feet above the
level of the river, and distant between two and three miles from it.
These shells are always found on the highest banks in the district:
they are embedded in a stratified earthy mass, precisely like that of
the great Pampean deposit hereafter to be described. In one collection
of these shells, there were some valves of the _Venus sinuosa_, Lam.,
the same species found with the Mactra on the banks of the Uruguay.
South of Buenos Ayres, near Ensenada, there are other beds of the
Azara, some of which seem to have been embedded in yellowish,
calcareous, semi-crystalline matter; and Sir W. Parish has given me
from the banks of the Arroyo del Tristan, situated in this same
neighbourhood, at the distance of about a league from the Plata, a
specimen of a pale-reddish, calcereo-argillaceous stone (precisely like
parts of the Pampean deposit the importance of which fact will be
referred to in a succeeding chapter), abounding with shells of an
Azara, much worn, but which in general form and appearance closely
resemble, and are probably identical with, the _A. labiata._ Besides
these shells, cellular, highly crystalline rock, formed of the casts of
small bivalves, is found near Ensenada; and likewise beds of
sea-shells, which from their appearance
appear to have lain on the surface. Sir W. Parish has given me some of
these shells, and M. d’Orbigny pronounces them to be:—

Buccinanops globulosum, d’Orbigny.

Olivancillaria auricularia, d’Orbigny.

Venus flexuosa, Lam.

Cytheræa (imperfect).

Mactra Isabellei, d’Orbigny.

Ostrea pulchella, d’Orbigny.


Besides these, Sir W. Parish procured[4] (as named by Mr. G. B.
Sowerby) the following shells:—

Voluta colocynthis.

Voluta angulata.

Buccinum (not spec.?).


 [1] “Voyage dans l’Amérique Mérid.: Part. Géolog.,” p. 21.


 [2] _Ibid._, p. 43.


 [3] “Buenos Ayres,” etc., by Sir Woodbine Parish, p. 168.


 [4] “Buenos Ayres,” etc., by Sir W. Parish, p. 168.

All these species (with, perhaps, the exception of the last) are
recent, and live on the South American coast. These shell-beds extend
from one league to six leagues from the Plata, and must lie many feet
above its level. I heard, also, of beds of shells on the Somborombon,
and on the Rio Salado, at which latter place, as M. d’Orbigny informs
me, the _Mactra Isabellei_ and _Venus sinuosa_ are found.

During the elevation of the Provinces of La Plata, the waters of the
ancient estuary have but little affected (with the exception of the
sand-hills on the banks of the Parana and Uruguay) the outline of the
land. M. Parchappe,[5] however, has described groups of sand dunes
scattered over the wide extent of the Pampas southward of Buenos Ayres,
which M. d’Orbigny attributes with much probability to the action of
the sea, before the plains were raised above its level.[6]

 [5] D’Orbigny’s “Voyage Géolog.,” p. 44.


 [6] Before proceeding to the districts southward of La Plata, it may
 be worth while just to state, that there is some evidence that the
 coast of Brazil has participated in a small amount of elevation. Mr.
 Burchell informs me, that he collected at Santos (lat. 24° S.)
 oyster-shells, apparently recent, some miles from the shore, and quite
 above the tidal action. Westward of Rio de Janeiro, Captain Elliot is
 asserted (see Harlan, “Med. and Phys. Res.,” p. 35, and Dr. Meigs, in
 “Trans. Amer. Phil. Soc.”), to have found human bones, encrusted with
 sea-shells, between fifteen and twenty feet above the level of the
 sea. Between Rio de Janeiro and Cape Frio I crossed sandy tracts
 abounding with sea-shells, at a distance of a league from the coast;
 but whether these tracts have been formed by upheaval, or through the
 mere accumulation of drift sand, I am not prepared to assert. At Bahia
 (lat. 13° S.), in some parts near the coast, there are traces of
 sea-action at the height of about twenty feet above its present level;
 there are also, in many parts, remnants of beds of sandstone and
 conglomerate with numerous recent shells, raised a little above the
 sea-level. I may add, that at the head of Bahia Bay there is a
 formation, about forty feet in thickness, containing tertiary shells
 apparently of fresh-water origin, now washed by the sea and encrusted
 with Balini; this appears to indicate a small amount of subsidence
 subsequent to its deposition. At Pernambuco (lat. 8° S.), in the
 alluvial or tertiary cliffs, surrounding the low land on which the
 city stands, I looked in vain for organic remains, or other evidence
 of changes in level.


_Southward of the Plata._—The coast as far as Bahia Blanca (in lat. 39°
S.) is formed either of a horizontal range of cliffs, or of immense
accumulations of sand-dunes. Within Bahia Blanca, a small piece of
tableland, about twenty feet above high-water mark, called Punta Alta,
is formed of strata of cemented gravel and of red earthy mud, abounding
with shells (with others lying loose on the surface), and the bones of
extinct mammifers. These shells, twenty in number, together with a
Balanus and two corals, are all recent species, still inhabiting the
neighbouring seas. They will be enumerated in the Fourth Chapter, when
describing the Pampean formation; five of them are identical with the
upraised ones from near Buenos Ayres. The northern shore of Bahia
Blanca is, in main part, formed of immense sand-dunes, resting on
gravel with recent shells, and ranging in lines parallel to the shore.
These ranges are separated from each other by flat spaces, composed of
stiff impure red clay, in which, at the distance of about two miles
from the coast, I found by digging a few minute fragments of
sea-shells. The sand-dunes extend several miles inland, and stand on a
plain, which slopes up to a height of between one hundred and two
hundred feet. Numerous, small, well-rounded pebbles of pumice lie
scattered both on the plain and sand-hillocks: at Monte Hermoso, on the
flat summit of a cliff, I found many of them at a height of 120 feet
(angular measurement) above the level of the sea. These pumice pebbles,
no doubt, were originally brought down from the Cordillera by the
rivers which cross the continent, in the same way as the river Negro
anciently brought down, and still brings down, pumice, and as the river
Chupat brings down scoriæ: when once delivered at the mouth of a river,
they would naturally have travelled along the coasts, and been cast up
during the elevation of the land, at different heights. The origin of
the argillaceous flats, which separate the parallel ranges of
sand-dunes, seems due to the tides here having a tendency (as I believe
they have on most shoal, protected coasts) to throw up a bar parallel
to the shore, and at some distance from it; this bar gradually becomes
larger, affording a base for the accumulation of sand-dunes, and the
shallow space within then becomes silted up with mud. The repetition of
this process, without any elevation of the land, would form a level
plain traversed by parallel lines of sand-hillocks; during a slow
elevation of the land, the hillocks would rest on a gently inclined
surface, like that on the northern shore of Bahia Blanca. I did not
observe any shells in this neighbourhood at a greater height than
twenty feet; and therefore the age of the sea-drifted pebbles of
pumice, now standing at the height of 120 feet, must remain uncertain.

The main plain surrounding Bahia Blanca I estimated at from two hundred
to three hundred feet; it insensibly rises towards the distant Sierra
Ventana. There are in this neighbourhood some other and lower plains,
but they do not abut one at the foot of the other, in the manner
hereafter to be described, so characteristic of Patagonia. The plain on
which the settlement stands is crossed by many low sand-dunes,
abounding with the minute shells of the _ Paludestrina australis_,
d’Orbigny, which now lives in the bay. This low plain is bounded to the
south, at
the Cabeza del Buey, by the cliff-formed margin of a wide plain of the
Pampean formation, which I estimated at sixty feet in height. On the
summit of this cliff there is a range of high sand-dunes extending
several miles in an east and west line.

Southward of Bahia Blanca, the river Colorado flows between two plains,
apparently from thirty to forty feet in height. Of these plains, the
southern one slopes up to the foot of the great sandstone plateau of
the Rio Negro; and the northern one against an escarpment of the
Pampean deposit; so that the Colorado flows in a valley fifty miles in
width, between the upper escarpments. I state this, because on the low
plain at the foot of the northern escarpment, I crossed an immense
accumulation of high sand-dunes, estimated by the Gauchos at no less
than eight miles in breadth. These dunes range westward from the coast,
which is twenty miles distant, to far inland, in lines parallel to the
valley; they are separated from each other by argillaceous flats,
precisely like those on the northern shore of Bahia Blanca. At present
there is no source whence this immense accumulation of sand could
proceed; but if, as I believe, the upper escarpments once formed the
shores of an estuary, in that case the sandstone formation of the river
Negro would have afforded an inexhaustible supply of sand, which would
naturally have accumulated on the northern shore, as on every part of
the coast open to the south winds between Bahia Blanca and Buenos
Ayres.

At San Blas (40° 40′ S.) a little south of the mouth of the Colorado,
M. d’Orbigny[7] found fourteen species of existing shells (six of them
identical with those from Bahia Blanca), embedded in their natural
positions. From the zone of depth which these shells are known to
inhabit, they must have been uplifted thirty-two feet. He also found,
at from fifteen to twenty feet above this bed, the remains of an
ancient beach.

 [7] “Voyage,” etc., p. 54.

Ten miles southward, but 120 miles to the west, at Port S. Antonio, the
Officers employed on the Survey assured me that they saw many old
sea-shells strewed on the surface of the ground, similar to those found
on other parts of the coast of Patagonia. At San Josef, ninety miles
south in nearly the same longitude, I found, above the gravel, which
caps an old tertiary formation, an irregular bed and hillock of sand,
several feet in thickness, abounding with shells of _Patella deaurita,
Mytilus Magellanicus,_ the latter retaining much of its colour; _Fusus
Magellanicus_ (and a variety of the same), and a large Balanus
(probably _B. Tulipa_), all now found on this coast: I estimated this
bed at from eighty to one hundred feet above the level of the sea. To
the westward of this bay, there is a plain estimated at between two
hundred and three hundred feet in height: this plain seems, from many
measurements, to be a continuation of the sandstone platform of the
river Negro. The next place southward, where I landed, was at Port
Desire, 340 miles distant; but from the intermediate districts I
received, through the kindness of the Officers of the Survey,
especially from Lieutenant Stokes and Mr. King, many specimens and
sketches, quite
sufficient to show the general uniformity of the whole line of coast. I
may here state, that the whole of Patagonia consists of a tertiary
formation, resting on and sometimes surrounding hills of porphyry and
quartz: the surface is worn into many wide valleys and into level
step-formed plains, rising one above another, all capped by irregular
beds of gravel, chiefly composed of porphyritic rocks. This gravel
formation will be separately described at the end of the chapter.

In the following diagrams:
Baseline is Level of sea.
Scale is 1/20 of inch to 100 feet vertical.

Height is shown in feet thus:
An. M. always stands for angular or trigonometrical measurement.
Ba. M. always stands for barometrical measurement.
Est. always stands for estimation by the Officers of the Survey.

No. 1
Section of step-formed plains south of Nuevo Gulf.


[Illustration: Section of step-formed plains south of Nuevo Gulf.]

My object in giving the following measurements of the plains, as taken
by the Officers of the Survey, is, as will hereafter be seen, to show
the remarkable equability of the recent elevatory movements. Round the
southern parts of Nuevo Gulf, as far as the River Chupat (seventy miles
southward of San Josef), there appear to be several plains, of which
the best defined are here represented.

The upper plain is here well defined (called Table Hills); its edge
forms a cliff or line of escarpment many miles in length, projecting
over a lower plain. The lowest plain corresponds with that at San Josef
with the recent shells on its surface. Between this lowest and the
uppermost plain, there is probably more than one step-formed terrace:
several measurements show the existence of the intermediate one of the
height given in diagram No. 1.

No. 2
Section of plains in the Bay of St. George.


[Illustration: Section of plains in the Bay of St. George.]

Near the north headland of the great Bay of St. George (100 miles south
of the Chupat), two well-marked plains of 250 and 330 feet were
measured: these are said to sweep round a great part of the Bay. At its
south headland, 120 miles distant from the north headland, the 250 feet
plain was again measured. In the middle of the bay, a higher plain was
found at two neighbouring places (Tilli Roads and C. Marques) to be 580
feet in height. Above this plain, towards the interior, Mr. Stokes
informs me that there were several other step-formed plains, the
highest of which was estimated at 1,200 feet, and was seen ranging at
apparently the same height for 150 miles northward. All these plains
have been worn into great valleys and much denuded. The section in
diagram No. 3 is illustrative of the general structure of the great Bay
of St. George. At the south headland of the Bay of St. George (near C.
Three Points) the 250 plain is very extensive. At Port Desire (forty
miles southward) I made several measurements with the barometer of a
plain, which extends along the north side of the port and along the
open coast, and which varies from 245 to 255 feet in height: this plain
abuts against the foot of a higher plain of 330 feet, which extends
also far northward along the coast, and likewise into the interior. In
the distance a higher inland platform was seen, of which I do not know
the height. In three separate places, I observed the cliff of the
245-255 feet plain, fringed by a terrace or narrow plain estimated at
about one hundred feet in height. These plains are represented in the
following section:—

No. 3
Section of plains at Port Desire.


[Illustration: Section of plains at Port Desire.]

In many places, even at the distance of three and four miles from the
coast, I found on the gravel-capped surface of the 245-255 feet, and of
the 330 feet plain, shells of _Mytilus Magellanicus, M. edulis, Patella
deaurita_, and another Patella, too much worn to be identified, but
apparently similar to one found abundantly adhering to the leaves of
the kelp. These species are the commonest now living on this coast. The
shells all appeared very old; the blue of the mussels was much faded;
and only traces of colour could be perceived in the Patellas, of which
the outer surfaces were scaling off. They lay scattered on the smooth
surface of the gravel, but abounded most in certain patches, especially
at the heads of the smaller valleys: they generally contained sand in
their insides; and I presume that they have been washed by alluvial
action out of thin sandy layers, traces of which may sometimes be seen
covering the gravel. The several plains have very level surfaces; but
all are scooped out by numerous broad, winding, flat-bottomed valleys,
in which, judging from the bushes, streams never flow. These remarks on
the state of the shells, and on the
nature of the plains, apply to the following cases, so need not be
repeated.

Southward of Port Desire, the plains have been greatly denuded, with
only small pieces of tableland marking their former extension. But
opposite Bird Island, two considerable step-formed plains were
measured, and found respectively to be 350 and 590 feet in height. This
latter plain extends along the coast close to Port St. Julian (110
miles south of Port Desire); where we have the following section:—

No. 4
Section of plains at Port St. Julian.


[Illustration: Section of plains at Port St. Julian.]

The lowest plain was estimated at ninety feet: it is remarkable from
the usual gravel-bed being deeply worn into hollows, which are filled
up with, as well as the general surface covered by, sandy and reddish
earthy matter: in one of the hollows thus filled up, the skeleton of
the Macrauchenia Patachonica, as will hereafter be described, was
embedded. On the surface and in the upper parts of this earthy mass,
there were numerous shells of Mytilus Magellanicus and M. edulis,
Patella deaurita, and fragments of other species. This plain is
tolerably level, but not extensive; it forms a promontory seven or
eight miles long, and three or four wide. The upper plains in Diagram 4
were measured by the Officers of the Survey; they were all capped by
thick beds of gravel, and were all more or less denuded; the 950 plain
consists merely of separate, truncated, gravel-capped hills, two of
which, by measurement, were found to differ only three feet. The 430
feet plain extends, apparently with hardly a break, to near the
northern entrance of the Rio Santa Cruz (fifty miles to the south); but
it was there found to be only 330 feet in height.

On the southern side of the mouth of the Santa Cruz we have Diagram 5,
which I am able to give with more detail than in the foregoing cases:—

No. 5
Section of plains at the mouth of the Rio Santa Cruz.


[Illustration: Section of plains at the mouth of the Rio Santa Cruz.]

The plain marked 355 feet (as ascertained by the barometer and by
angular measurement) is a continuation of the above-mentioned 330
feet plain: it extends in a N.W. direction along the southern shores of
the estuary. It is capped by gravel, which in most parts is covered by
a thin bed of sandy earth, and is scooped out by many flat-bottomed
valleys. It appears to the eye quite level, but in proceeding in a
S.S.W. course, towards an escarpment distant about six miles, and
likewise ranging across the country in a N.W. line, it was found to
rise at first insensibly, and then for the last half-mile, sensibly,
close up to the base of the escarpment: at this point it was 463 feet
in height, showing a rise of 108 feet in the six miles. On this 355-463
feet plain, I found several shells of _Mytilus Magellanicus_ and of a
Mytilus, which Mr. Sowerby informs me is yet unnamed, though well-known
as recent on this coast; _Patella deaurita_; _Fusus_, I believe, _
Magellanicus_, but the specimen has been lost; and at the distance of
four miles from the coast, at the height of about four hundred feet,
there were fragments of the same Patella and of a Voluta (apparently
_V. ancilla_) partially embedded in the superficial sandy earth. All
these shells had the same ancient appearance with those from the
foregoing localities. As the tides along this part of the coast rise at
the Syzygal period forty feet, and therefore form a well-marked
beach-line, I particularly looked out for ridges in crossing this
plain, which, as we have seen, rises 108 feet in about six miles, but I
could not see any traces of such. The next highest plain is 710 feet
above the sea; it is very narrow, but level, and is capped with gravel;
it abuts to the foot of the 840 feet plain. This summit-plain extends
as far as the eye can range, both inland along the southern side of the
valley of the Santa Cruz, and southward along the Atlantic.

_The Valley of the R. Santa Cruz._—This valley runs in an east and west
direction to the Cordillera, a distance of about one hundred and sixty
miles. It cuts through the great Patagonian tertiary formation,
including, in the upper half of the valley, immense streams of basaltic
lava, which as well as the softer beds, are capped by gravel; and this
gravel, high up the river, is associated with a vast boulder
formation.[8] In ascending the valley, the plain which at the mouth on
the southern side is 355 feet high, is seen to trend towards the
corresponding plain on the northern side, so that their escarpments
appear like the shores of a former estuary, larger than the existing
one: the escarpments, also, of the 840 feet summit-plain (with a
corresponding northern one, which is met with some way up the valley),
appear like the shores of a still larger estuary. Farther up the
valley, the sides are bounded throughout its entire length by level,
gravel-capped terraces, rising above each other in steps. The width
between the upper escarpments is on an average between seven and ten
miles; in one spot, however, where cutting through the basaltic lava,
it was only one mile and a half. Between the escarpments of the second
highest terrace the average width is about four or five miles. The
bottom of the valley, at the distance of 110 miles from its mouth,
begins sensibly
to expand, and soon forms a considerable plain, 440 feet above the
level of the sea, through which the river flows in a gut from twenty to
forty feet in depth. I here found, at a point 140 miles from the
Atlantic, and seventy miles from the nearest creek of the Pacific, at
the height of 410 feet, a very old and worn shell of _Patella
deaurita._ Lower down the valley, 105 miles from the Atlantic (long.
71° W.), and at an elevation of about 300 feet, I also found, in the
bed of the river, two much worn and broken shells of the _Voluta
ancilla_, still retaining traces of their colours; and one of the
_Patella deaurita._ It appeared that these shells had been washed from
the banks into the river; considering the distance from the sea, the
desert and absolutely unfrequented character of the country, and the
very ancient appearance of the shells (exactly like those found on the
plains nearer the coast), there is, I think, no cause to suspect that
they could have been brought here by Indians.

 [8] I have described this formation in a paper in the “Geological
 Transactions,” vol. vi, p. 415.

The plain at the head of the valley is tolerably level, but water-worn,
and with many sand-dunes on it like those on a sea-coast. At the
highest point to which we ascended, it was sixteen miles wide in a
north and south line; and forty-five miles in length in an east and
west line. It is bordered by the escarpments, one above the other, of
two plains, which diverge as they approach the Cordillera, and
consequently resemble, at two levels, the shores of great bays facing
the mountains; and these mountains are breached in front of the lower
plain by a remarkable gap. The valley, therefore, of the Santa Cruz
consists of a straight broad cut, about ninety miles in length,
bordered by gravel-capped terraces and plains, the escarpments of which
at both ends diverge or expand, one over the other, after the manner of
the shores of great bays. Bearing in mind this peculiar form of the
land—the sand-dunes on the plain at the head of the valley—the gap in
the Cordillera, in front of it—the presence in two places of very
ancient shells of existing species—and lastly, the circumstance of the
355-453 feet plain, with the numerous marine remains on its surface,
sweeping from the Atlantic coast, far up the valley, I think we must
admit, that within the recent period, the course of the Santa Cruz
formed a sea-strait intersecting the continent. At this period, the
southern part of South America consisted of an archipelago of islands
360 miles in a north and south line. We shall presently see, that two
other straits also, since closed, then cut through Tierra del Fuego; I
may add, that one of them must at that time have expanded at the foot
of the Cordillera into a great bay (now Otway Water) like that which
formerly covered the 440 feet plain at the head of the Santa Cruz.

I have said that the valley in its whole course is bordered by
gravel-capped plains. The section (diagram No. 6), supposed to be drawn
in a north and south line across the valley, can scarcely be considered
as more than illustrative; for during our hurried ascent it was
impossible to measure all the plains at any one place. At a point
nearly midway between the Cordillera and the Atlantic, I found the
plain (A north) 1,122 feet above the river; all the lower plains on
this side were here united into one great broken cliff: at a point
sixteen miles lower down
the stream, I found by measurement and estimation that B (_n_) was 869
above the river: very near to where A (_n_) was measured, C (_n_) was
639 above the same level: the terrace D (_n_) was nowhere measured: the
lowest E (_n_) was in many places about twenty feet above the river.
These plains or terraces were best developed where the valley was
widest; the whole five, like gigantic steps, occurred together only at
a few points. The lower terraces are less continuous than the higher
ones, and appear to be entirely lost in the upper third of the valley.
Terrace C (_s_), however was traced continuously for a great distance.
The terrace B (_n_), at a point fifty-five miles from the mouth of the
river, was four miles in width; higher up the valley this terrace (or
at least the second highest one, for I could not always trace it
continuously) was about eight miles wide. This second plain was
generally wider than the lower ones—as indeed follows from the valley
from A (_n_) to A (_s_) being generally nearly double the width of from
B (_n_) to B (_s_).

No. 6
North and South Section across the terraces bounding the valley of the
River Santa Cruz, high up its course.


[Illustration: North and South Section across the terraces bounding the
River Santa Cruz.]

Low down the valley, the summit-plain A (_s_) is continuous with the
840 feet plain on the coast, but it is soon lost or unites with the
escarpment of B (_s_). The corresponding plain A (_n_), on the north
side of the valley, appears to range continuously from the Cordillera
to the head of the present estuary of the Santa Cruz, where it trends
northward towards Port St. Julian. Near the Cordillera the summit-plain
on both sides of the valley is between 3,200 and 3,300 feet in height;
at 100 miles from the Atlantic, it is 1,416 feet, and on the coast 840
feet, all above the sea-beach; so that in a distance of 100 miles the
plain rises 576 feet, and much more rapidly near to the Cordillera. The
lower terraces B and C also appear to rise as they run up the valley;
thus D (_n_), measured at two points twenty-four miles apart, was found
to have risen 185 feet. From several reasons I suspect, that this
gradual inclination of the plains up the valley, has been chiefly
caused by the elevation of the continent in mass, having been the
greater the nearer to the Cordillera.

All the terraces are capped with well-rounded gravel, which rests
either on the denuded and sometimes furrowed surface of the soft
tertiary deposits, or on the basaltic lava. The difference in height
between some of the lower steps or terraces seems to be entirely owing
to a difference in the thickness of the capping gravel. Furrows and
inequalities in the gravel, where such occur, are filled up and
smoothed over with sandy earth. The pebbles, especially on the higher
plains, are often whitewashed, and even cemented together by a white
aluminous substance, and I occasionally found this to be the case with
the gravel on the terrace D. I could not perceive any trace of a
similar deposition on the pebbles now thrown up by the river, and
therefore I do not think that terrace D was river-formed. As the
terrace E generally stands about twenty feet above the bed of the
river, my first impression was to doubt whether even this lowest one
could have been so formed; but it should always be borne in mind, that
the horizontal upheaval of a district, by increasing the total descent
of the streams, will always tend to increase, first near the sea-coast
and then further and further up the valley, their corroding and
deepening powers: so that an alluvial plain, formed almost on a level
with a stream, will, after an elevation of this kind, in time be cut
through, and left standing at a height never again to be reached by the
water. With respect to the three upper terraces of the Santa Cruz, I
think there can be no doubt, that they were modelled by the sea, when
the valley was occupied by a strait, in the same manner (hereafter to
be discussed) as the greater step-formed, shell-strewed plains along
the coast of Patagonia.

To return to the shores of the Atlantic: the 840 feet plain, at the
mouth of the Santa Cruz, is seen extending horizontally far to the
south; and I am informed by the Officers of the Survey, that bending
round the head of Coy Inlet (sixty-five miles southward), it trends
inland. Outliers of apparently the same height are seen forty miles
farther south, inland of the river Gallegos; and a plain comes down to
Cape Gregory (thirty-five miles southward), in the Strait of Magellan,
which was estimated at between eight hundred and one thousand feet in
height, and which, rising towards the interior, is capped by the
boulder formation. South of the Strait of Magellan, there are large
outlying masses of apparently the same great tableland, extending at
intervals along the eastern coast of Tierra del Fuego: at two places
here, 110 miles a part, this plain was found to be 950 and 970 feet in
height.

From Coy Inlet, where the high summit-plain trends inland, a plain
estimated at 350 feet in height, extends for forty miles to the river
Gallegos. From this point to the Strait of Magellan, and on each side
of that Strait, the country has been much denuded and is less level. It
consists chiefly of the boulder formation, which rises to a height of
between one hundred and fifty and two hundred and fifty feet, and is
often capped by beds of gravel. At N.S. Gracia, on the north side of
the Inner Narrows of the Strait of Magellan, I found on the summit of a
cliff, 160 feet in height, shells of existing Patellæ and Mytili,
scattered on the surface and partially embedded in earth. On the
eastern coast, also, of Tierra del Fuego, in latitude 53° 20′ south, I
found many Mytili on some level land, estimated at 200 feet in height.
Anterior to the elevation attested by these shells, it is evident by
the present form of the land, and by the distribution of the great
erratic boulders[9] on the surface, that two sea-channels connected the
Strait of Magellan both with Sebastian Bay and with Otway Water.

 [9] “Geolog. Transactions,” vol. vi, p. 419.


_Concluding remarks on the recent elevation of the south-eastern coasts
of America, and on the action of the sea on the land._—Upraised shells
of species, still existing as the commonest kinds in the adjoining sea,
occur, as we have seen, at heights of between a few feet and 410 feet,
at intervals from latitude 33° 40′ to 53° 20′ south. This is a distance
of 1,180 geographical miles—about equal from London to the North Cape
of Sweden. As the boulder formation extends with nearly the same height
150 miles south of 53° 20′, the most southern point where I landed and
found upraised shells; and as the level Pampas ranges many hundred
miles northward of the point, where M. d’Orbigny found at the height of
100 feet beds of the Azara, the space in a north and south line, which
has been uplifted within the recent period, must have been much above
the 1,180 miles. By the term “recent,” I refer only to that period
within which the now living mollusca were called into existence; for it
will be seen in the Fourth Chapter, that both at Bahia Blanca and P. S.
Julian, the mammiferous quadrupeds which co-existed with these shells
belong to extinct species. I have said that the upraised shells were
found only at intervals on this line of coast, but this in all
probability may be attributed to my not having landed at the
intermediate points; for wherever I did land, with the exception of the
river Negro, shells were found: moreover, the shells are strewed on
plains or terraces, which, as we shall immediately see, extend for
great distances with a uniform height. I ascended the higher plains
only in a few places, owing to the distance at which their escarpments
generally range from the coast, so that I am far from knowing that 410
feet is the maximum of elevation of these upraised remains. The shells
are those now most abundant in a living state in the adjoining sea.[10]
All of them have an ancient appearance; but some, especially the
mussels, although lying fully exposed to the weather, retain to a
considerable extent their colours: this circumstance appears at first
surprising, but it is now known that the colouring principle of the
Mytilus is so enduring, that it is preserved when the shell itself is
completely disintegrated.[11] Most of the shells are broken; I nowhere
found two valves united; the fragments are not rounded, at least in
none of the specimens which I brought home.

 [10] Captain King, “Voyages of _Adventure_ and _Beagle_,” vol. i, 1
 pp. 6 and 133.


 [11] See Mr. Lyell “Proofs of a Gradual Rising in Sweden,” in the
 “Philosoph. Transact.,” 1835, p. 1. See also Mr. Smith of Jordan Hill
 in the _Edin. New Phil. Journal_, vol. xxv, p. 393.

With respect to the breadth of the upraised area in an east and west
line, we know from the shells found at the Inner Narrows of the
Strait of Magellan, that the entire width of the plain, although there
very narrow, has been elevated. It is probable that in this
southernmost part of the continent, the movement has extended under the
sea far eastward; for at the Falkland Islands, though I could not find
any shells, the bones of whales have been noticed by several competent
observers, lying on the land at a considerable distance from the sea,
and at the height of some hundred feet above it.[12] Moreover, we know
that in Tierra del Fuego the boulder formation has been uplifted within
the recent period, and a similar formation occurs[13] on the
north-western shores (Byron Sound) of these islands. The distance from
this point to the Cordillera of Tierra del Fuego, is 360 miles, which
we may take as the probable width of the recently upraised area. In the
latitude of the R. Santa Cruz, we know from the shells found at the
mouth and head, and in the middle of the valley, that the entire width
(about 160 miles) of the surface eastward of the Cordillera has been
upraised. From the slope of the plains, as shown by the course of the
rivers, for several degrees northward of the Santa Cruz, it is probable
that the elevation attested by the shells on the coast has likewise
extended to the Cordillera. When, however, we look as far northward as
the provinces of La Plata, this conclusion would be very hazardous; not
only is the distance from Maldonado (where I found upraised shells) to
the Cordillera great, namely, 760 miles, but at the head of the estuary
of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic rocks has
been observed,[14] which may well indicate an axis of elevation quite
distinct from that of the Andes. Moreover, in the centre of the Pampas
in the chain of Cordova, severe earthquakes have been felt;[15] whereas
at Mendoza, at the eastern foot of the Cordillera, only gentle
oscillations, transmitted from the shores of the Pacific, have ever
been experienced. Hence the elevation of the Pampas may be due to
several distinct axes of movement; and we cannot judge, from the
upraised shells round the estuary of the Plata, of the breadth of the
area uplifted within the recent period.

 [12] “Voyages of the _Adventure_ and _ Beagle_,” vol. ii, p. 227. And
 Bougainville’s “Voyage,” tome i, p. 112.


 [13] I owe this fact to the kindness of Captain Sulivan, R.N., a
 highly competent observer. I mention it more especially, as in my
 Paper (p. 427) on the Boulder Formation, I have, after having examined
 the northern and middle parts of the eastern island, said that the
 formation was here wholly absent.


 [14] This volcanic formation will be described in Chapter IV. It is
 not improbable that the height of the upraised shells at the head of
 the estuary of the Plata, being greater than at Bahia Blanca or at San
 Blas, may be owing to the upheaval of these latter places having been
 connected with the distant line of the Cordillera, whilst that of the
 provinces of La Plata was in connection with the adjoining tertiary
 volcanic axis.


 [15] See Sir W. Parish’s work on “La Plata,” p. 242. For a notice of
 an earthquake which drained a lake near Cordova, see also Temple’s
 “Travels in Peru.” Sir W. Parish informs me, that a town between Salta
 and Tucuman (north of Cordova) was formerly utterly overthrown by an
 earthquake.


Not only has the above specified long range of coast been elevated
within the recent period, but I think it may be safely inferred from
the similarity in height of the gravel-capped plains at distant points,
that there has been a remarkable degree of equability in the elevatory
process. I may premise, that when I measured the plains, it was simply
to ascertain the heights at which shells occurred; afterwards,
comparing these measurements with some of those made during the Survey,
I was struck with their uniformity, and accordingly tabulated all those
which represented the summit-edges of plains. The extension of the 330
to 355 feet plain is very striking, being found over a space of 500
geographical miles in a north and south line. A table (Table 1) of the
measurements is given below. The angular measurements and all the
estimations (in feet) are by the Officers of the Survey; the
barometrical ones by myself:—

 	Feet Gallegos River to Coy Inlet (partly angular partly
 	estimation)	350 South Side of Santa Cruz (angular and
 	barometric)	355 North Side of Santa Cruz (angular and
 	barometric)	330 Bird Island, plain opposite to (angular)	350
 	Port Desire, plain extending far along coast (barometric)	330
 	St. George’s Bay, north promontory (angular)	330 Table Land,
 	south of New Bay (angular)	350

A plain, varying from 245 to 255 feet, seems to extend with much
uniformity from Port Desire to the north of St. George’s Bay, a
distance of 170 miles; and some approximate measurements (in feet),
also given in the table below, indicate the much greater extension of
780 miles:—

 	Feet Coy Inlet, south of (partly angular and partly
 	estimation)	200 to 300 Port Desire (barometric)	245 to 255
 	C. Blanco (angular)	250 North Promontory of St. George’s Bay
 	(angular)	250 South of New Bay (angular)	200 to 220 North of
 	S. Josef (estimation)	200 to 300 Plain of Rio Negro
 	(angular)	200 to 220 Bahia Blanca (estimation)	200 to 300

The extension, moreover, of the 560 to 580, and of the 80 to 100 feet,
plains is remarkable, though somewhat less obvious than in the former
cases. Bearing in mind that I have not picked these measurements out of
a series, but have used all those which represented the edges of
plains, I think it scarcely possible that these coincidences in height
should be accidental. We must therefore conclude that the action,
whatever it may have been, by which these plains have been modelled
into their present forms, has been singularly uniform.

These plains or great terraces, of which three and four often rise like
steps one behind the other, are formed by the denudation of the old
Patagonian tertiary beds, and by the deposition on their surfaces of a
mass of well-rounded gravel, varying, near the coast, from ten to
thirty-five
feet in thickness, but increasing in thickness towards the interior.
The gravel is often capped by a thin irregular bed of sandy earth. The
plains slope up, though seldom sensibly to the eye, from the summit
edge of one escarpment to the foot of the next highest one. Within a
distance of 150 miles, between Santa Cruz to Port Desire, where the
plains are particularly well developed, there are at least seven stages
or steps, one above the other. On the three lower ones, namely, those
of 100 feet, 250 feet, and 350 feet in height, existing littoral shells
are abundantly strewed, either on the surface, or partially embedded in
the superficial sandy earth. By whatever action these three lower
plains have been modelled, so undoubtedly have all the higher ones, up
to a height of 950 feet at S. Julian, and of 1,200 feet (by estimation)
along St. George’s Bay. I think it will not be disputed, considering
the presence of the upraised marine shells, that the sea has been the
active power during stages of some kind in the elevatory process.

We will now briefly consider this subject: if we look at the existing
coast-line, the evidence of the great denuding power of the sea is very
distinct; for, from Cape St. Diego, in lat. 54° 30′ to the mouth of the
Rio Negro, in lat. 31° (a length of more than eight hundred miles), the
shore is formed, with singularly few exceptions, of bold and naked
cliffs: in many places the cliffs are high; thus, south of the Santa
Cruz, they are between eight and nine hundred feet in height, with
their horizontal strata abruptly cut off, showing the immense mass of
matter which has been removed. Nearly this whole line of coast consists
of a series of greater or lesser curves, the horns of which, and
likewise certain straight projecting portions, are formed of hard
rocks; hence the concave parts are evidently the effect and the measure
of the denuding action on the softer strata. At the foot of all the
cliffs, the sea shoals very gradually far outwards; and the bottom, for
a space of some miles, everywhere consists of gravel. I carefully
examined the bed of the sea off the Santa Cruz, and found that its
inclination was exactly the same, both in amount and in its peculiar
curvature, with that of the 355 feet plain at this same place. If,
therefore, the coast, with the bed of the adjoining sea, were now
suddenly elevated one or two hundred feet, an inland line of cliffs,
that is an escarpment, would be formed, with a gravel-capped plain at
its foot gently sloping to the sea, and having an inclination like that
of the existing 355 feet plain. From the denuding tendency of the sea,
this newly formed plain would in time be eaten back into a cliff: and
repetitions of this elevatory and denuding process would produce a
series of gravel-capped sloping terraces, rising one above another,
like those fronting the shores of Patagonia.

The chief difficulty (for there are other inconsiderable ones) on this
view, is the fact,—as far as I can trust two continuous lines of
soundings carefully taken between Santa Cruz and the Falkland Islands,
and several scattered observations on this and other coasts,—that the
pebbles at the bottom of the sea _quickly_ and _regularly_ decrease in
size with the increasing depth and distance from the shore, whereas in
the gravel on the sloping plains, no such decrease in size was
perceptible.
The following table gives the average result of many soundings off the
Santa Cruz:—

Under two miles from the shore, many of the pebbles were of large size,
mingled with some small ones.


Distance in miles from shore	Depth in fathoms	Size of Pebbles 3
to 4	11 to 12	As large as walnuts; mingled in every case with
some smaller ones. 6 to 7	17 to 19	As large as hazel-nuts. 10 to
11	23 to 25	From three- to four-tenths of an inch in diameter.
12	30 to 40	Two-tenths of an inch. 22 to 150	45 to
65	One-tenth of an inch, to the finest sand.

I particularly attended to the size of the pebbles on the 355 feet
Santa Cruz plain, and I noticed that on the summit-edge of the present
sea cliffs many were as large as half a man’s head; and in crossing
from these cliffs to the foot of the next highest escarpment, a
distance of six miles, I could not observe any increase in their size.
We shall presently see that the theory of a slow and almost insensible
rise of the land, will explain all the facts connected with the
gravel-capped terraces, better than the theory of sudden elevations of
from one to two hundred feet.

M. d’Orbigny has argued, from the upraised shells at San Blas being
embedded in the positions in which they lived, and from the valves of
the _Azara labiata_ high on the banks of the Parana being united and
unrolled, that the elevation of Northern Patagonia and of La Plata must
have been sudden; for he thinks, if it had been gradual, these shells
would all have been rolled on successive beach-lines. But in
_protected_ bays, such as in that of Bahia Blanca, wherever the sea is
accumulating extensive mud-banks, or where the winds quietly heap up
sand-dunes, beds of shells might assuredly be preserved buried in the
positions in which they had lived, even whilst the land retained the
same level; any, the smallest, amount of elevation would directly aid
in their preservation. I saw a multitude of spots in Bahia Blanca where
this might have been effected; and at Maldonado it almost certainly has
been effected. In speaking of the elevation of the land having been
slow, I do not wish to exclude the small starts which accompany
earthquakes, as on the coast of Chile; and by such movements beds of
shells might easily be uplifted, even in positions exposed to a heavy
surf, without undergoing any attrition: for instance, in 1835, a rocky
flat off the island of Santa Maria was at one blow upheaved above
high-water mark, and was left covered with gaping and putrefying
mussel-shells, still attached to the bed on which they had lived. If M.
d’Orbigny had been aware of the many long parallel lines of
sand-hillocks, with infinitely numerous shells of the Mactra and Venus,
at
a low level near the Uruguay; if he had seen at Bahia Blanca the
immense sand-dunes, with water-worn pebbles of pumice, ranging in
parallel lines, one behind the other, up a height of at least 120 feet;
if he had seen the sand-dunes, with the countless Paludestrinas, on the
low plain near the Fort at this place, and that long line on the edge
of the cliff, sixty feet higher up; if he had crossed that long and
great belt of parallel sand-dunes, eight miles in width, standing at
the height of from forty to fifty feet above the Colorado, where sand
could not now collect,—I cannot believe he would have thought that the
elevation of this great district had been sudden. Certainly the
sand-dunes (especially when abounding with shells), which stand in
ranges at so many different levels, must all have required long time
for their accumulation; and hence I do not doubt that the last 100 feet
of elevation of La Plata and Northern Patagonia has been exceedingly
slow.

If we extend this conclusion to Central and Southern Patagonia, the
inclination of the successively rising gravel-capped plains can be
explained quite as well, as by the more obvious view already given of a
few comparatively great and sudden elevations; in either case we must
admit long periods of rest, during which the sea ate deeply into the
land. Let us suppose the present coast to rise at a nearly equable,
slow rate, yet sufficiently quick to prevent the waves quite removing
each part as soon as brought up; in this case every portion of the
present bed of the sea will successively form a beach-line, and from
being exposed to a like action will be similarly affected. It cannot
matter to what height the tides rise, even if to forty feet as at Santa
Cruz, for they will act with equal force and in like manner on each
successive line. Hence there is no difficulty in the fact of the 355
feet plain at Santa Cruz sloping up 108 feet to the foot of the next
highest escarpment, and yet having no marks of any one particular
beach-line on it; for the whole surface on this view has been a beach.
I cannot pretend to follow out the precise action of the tidal-waves
during a rise of the land, slow, yet sufficiently quick to prevent or
check denudation: but if it be analogous to what takes place on
protected parts of the present coast, where gravel is now accumulating
in large quantities,[16] an inclined surface, thickly capped by
well-rounded pebbles of about the same size, would be ultimately left.
On the gravel now accumulating, the waves, aided by the wind, sometimes
throw up a thin covering of sand, together with the common
coast-shells. Shells thus cast up by gales, would, during an elevatory
period, never again be touched by the sea. Hence, on this view of a
slow and gradual rising of the land, interrupted by periods of rest and
denudation, we can understand the pebbles being of about the same size
over the entire width of the step-like plains,—the occasional thin
covering of sandy earth,—and the presence of broken, unrolled fragments
of those shells, which now live exclusively near the coast.

 [16] On the eastern side of Chiloe, which island we shall see in the
 next chapter is now rising, I observed that all the beaches and
 extensive tidal-flats were formed of shingle.


_Summary of results._—It may be concluded that the coast on this side
of the continent, for a space of at least 1,180 miles, has been
elevated to a height of 100 feet in La Plata, and of 400 feet in
Southern Patagonia, within the period of existing shells, but not of
existing mammifers. That in La Plata the elevation has been very slowly
effected: that in Patagonia the movement may have been by considerable
starts, but much more probably slow and quiet. In either case, there
have been long intervening periods of comparative rest,[17] during
which the sea corroded deeply, as it is still corroding, into the land.
That the periods of denudation and elevation were contemporaneous and
equable over great spaces of coast, as shown by the equable heights of
the plains; that there have been at least eight periods of denudation,
and that the land, up to a height of from 950 to 1,200 feet, has been
similarly modelled and affected: that the area elevated, in the
southernmost part of the continent, extended in breadth to the
Cordillera, and probably seaward to the Falkland Islands; that
northward, in La Plata, the breadth is unknown, there having been
probably more than one axis of elevation; and finally, that, anterior
to the elevation attested by these upraised shells, the land was
divided by a Strait where the River Santa Cruz now flows, and that
further southward there were other sea-straits, since closed. I may
add, that at Santa Cruz, in lat. 50° S., the plains have been uplifted
at least 1,400 feet, since the period when gigantic boulders were
transported between sixty and seventy miles from their parent rock, on
floating icebergs.

 [17] I say _comparative_ and not _ absolute_ rest, because the sea
 acts, as we have seen, with great denuding power on this whole line of
 coast; and therefore, during an elevation of the land, if excessively
 slow (and of course during a subsidence of the land), it is quite
 possible that lines of cliff might be formed.)

Lastly, considering the great upward movements which this long line of
coast has undergone, and the proximity of its southern half to the
volcanic axis of the Cordillera, it is highly remarkable that in the
many fine sections exposed in the Pampean, Patagonian tertiary, and
Boulder formations, I nowhere observed the smallest fault or abrupt
curvature in the strata.

_Gravel Formation of Patagonia._

I will here describe in more detail than has been as yet incidentally
done, the nature, origin, and extent of the great shingle covering of
Patagonia: but I do not mean to affirm that all of this shingle,
especially that on the higher plains, belongs to the recent period. A
thin bed of sandy earth, with small pebbles of various porphyries and
of quartz, covering a low plain on the north side of the Rio Colorado,
is the extreme northern limit of this formation. These little pebbles
have probably been derived from the denudation of a more regular bed of
gravel, capping the old tertiary sandstone plateau of the Rio Negro.
The gravel-bed near the Rio Negro is, on an average, about ten or
twelve feet in thickness; and the pebbles are larger than on the
northern side of the Colorado, being from one or two inches in
diameter, and composed chiefly of rather dark-tinted porphyries.
Amongst them I here first noticed a variety often to be referred to,
namely, a peculiar gallstone-yellow siliceous porphyry, frequently, but
not invariably, containing grains of quartz. The pebbles are embedded
in a white, gritty, calcareous matrix, very like mortar, sometimes
merely coating with a whitewash the separate stones, and sometimes
forming the greater part of the mass. In one place I saw in the gravel
concretionary nodules (not rounded) of crystallised gypsum, some as
large as a man’s head. I traced this bed for forty-five miles inland,
and was assured that it extended far into the interior. As the surface
of the calcareo-argillaceous plain of Pampean formation, on the
northern side of the wide valley of the Colorado, stands at about the
same height with the mortar-like cemented gravel capping the sandstone
on the southern side, it is probable, considering the apparent
equability of the subterranean movements along this side of America,
that this gravel of the Rio Negro and the upper beds of the Pampean
formation northward of the Colorado, are of nearly contemporaneous
origin, and that the calcareous matter has been derived from the same
source.

Southward of the Rio Negro, the cliffs along the great bay of S.
Antonio are capped with gravel: at San Josef, I found that the pebbles
closely resembled those on the plain of the Rio Negro, but that they
were not cemented by calcareous matter. Between San Josef and Port
Desire, I was assured by the Officers of the Survey that the whole face
of the country is coated with gravel. At Port Desire and over a space
of twenty-five miles inland, on the three step-formed plains and in the
valleys, I everywhere passed over gravel which, where thickest, was
between thirty and forty feet. Here, as in other parts of Patagonia,
the gravel, or its sandy covering, was, as we have seen, often strewed
with recent marine shells. The sandy covering sometimes fills up
furrows in the gravel, as does the gravel in the underlying tertiary
formations. The pebbles are frequently whitewashed and even cemented
together by a peculiar, white, friable, aluminous, fusible substance,
which I believe is decomposed feldspar. At Port Desire, the gravel
rested sometimes on the basal formation of porphyry, and sometimes on
the upper or the lower denuded tertiary strata. It is remarkable that
most of the porphyritic pebbles differ from those varieties of porphyry
which occur here abundantly _in situ._ The peculiar gallstone-yellow
variety was common, but less numerous than at Port S. Julian, where it
formed nearly one-third of the mass of the gravel; the remaining part
there consisting of pale grey and greenish porphyries with many
crystals of feldspar. At Port S. Julian, I ascended one of the
flat-topped hills, the denuded remnant of the highest plain, and found
it, at the height of 950 feet, capped with the usual bed of gravel.

Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet
plain is from twenty to about thirty-five feet in thickness. The
pebbles vary from minute ones to the size of a hen’s egg, and even to
that of half a man’s head; they consist of paler varieties of porphyry
than those found further northward, and there are fewer of the
gallstone-yellow kind; pebbles of compact black clay-slate were here
first observed. The gravel, as we have seen, covers the step-formed
plains at the mouth, head, and on the sides of the great valley of the
Santa Cruz. At a distance of 110 miles from the coast, the plain has
risen to the height of 1,416 feet above the sea; and the gravel, with
the associated great boulder formation, has attained a thickness of 212
feet. The plain, apparently with its usual gravel covering, slopes up
to the foot of the Cordillera to the height of between 3,200 and 3,300
feet. In ascending the valley, the gravel gradually becomes entirely
altered in character: high up, we have pebbles of crystalline
feldspathic rocks, compact clay-slate, quartzose schists, and
pale-coloured porphyries; these rocks, judging both from the gigantic
boulders in the surface and from some small pebbles embedded beneath
700 feet in thickness of the old tertiary strata, are the prevailing
kinds in this part of the Cordillera; pebbles of basalt from the
neighbouring streams of basaltic lava are also numerous; there are few
or none of the reddish or of the gallstone-yellow porphyries so common
near the coast. Hence the pebbles on the 350 feet plain at the mouth of
the Santa Cruz cannot have been derived (with the exception of those of
compact clay-slate, which, however, may equally well have come from the
south) from the Cordillera in this latitude; but probably, in chief
part, from farther north.

Southward of the Santa Cruz, the gravel may be seen continuously
capping the great 840 feet plain: at the Rio Gallegos, where this plain
is succeeded by a lower one, there is, as I am informed by Captain
Sulivan, an irregular covering of gravel from ten to twelve feet in
thickness over the whole country. The district on each side of the
Strait of Magellan is covered up either with gravel or the boulder
formation: it was interesting to observe the marked difference between
the perfectly rounded state of the pebbles in the great shingle
formation of Patagonia, and the more or less angular fragments in the
boulder formation. The pebbles and fragments near the Strait of
Magellan nearly all belong to rocks known to occur in Fuegia. I was
therefore much surprised in dredging south of the Strait to find, in
lat. 54° 10′ south, many pebbles of the gallstone-yellow siliceous
porphyry; I procured others from a great depth off Staten Island, and
others were brought me from the western extremity of the Falkland
Islands.[18] The distribution of the pebbles of this peculiar porphyry,
which I venture to affirm is not found _in situ_ either in Fuegia, the
Falkland Islands, or on the coast of Patagonia, is very remarkable, for
they are
found over a space of 840 miles in a north and south line, and at the
Falklands, 300 miles eastward of the coast of Patagonia. Their
occurrence in Fuegia and the Falklands may, however, perhaps be due to
the same ice-agency by which the boulders have been there transported.

 [18] At my request, Mr. Kent collected for me a bag of pebbles from
 the beach of White Rock harbour, in the northern part of the sound,
 between the two Falkland Islands. Out of these well-rounded pebbles,
 varying in size from a walnut to a hen’s egg, with some larger,
 thirty-eight evidently belonged to the rocks of these islands;
 twenty-six were similar to the pebbles of porphyry found on the
 Patagonian plains, which rocks do not exist _in situ_ in the
 Falklands; one pebble belonged to the peculiar yellow siliceous
 porphyry; thirty were of doubtful origin.

We have seen that porphyritic pebbles of a small size are first met
with on the northern side of the Rio Colorado, the bed becoming well
developed near the Rio Negro: from this latter point I have every
reason to believe that the gravel extends uninterruptedly over the
plains and valleys of Patagonia for at least 630 nautical miles
southward to the Rio Gallegos. From the slope of the plains, from the
nature of the pebbles, from their extension at the Rio Negro far into
the interior, and at the Santa Cruz close up to the Cordillera, I think
it highly probable that the whole breadth of Patagonia is thus covered.
If so, the average width of the bed must be about two hundred miles.
Near the coast the gravel is generally from ten to thirty feet in
thickness; and as in the valley of Santa Cruz it attains, at some
distance from the Cordillera, a thickness of 214 feet, we may, I think,
safely assume its average thickness over the whole area of 630 by 200
miles, at fifty feet!

The transportal and origin of this vast bed of pebbles is an
interesting problem. From the manner in which they cap the step-formed
plains, worn by the sea within the period of existing shells, their
deposition, at least on the plains up to a height of 400 feet, must
have been a recent geological event. From the form of the continent, we
may feel sure that they have come from the westward, probably, in chief
part from the Cordillera, but, perhaps, partly from unknown rocky
ridges in the central districts of Patagonia. That the pebbles have not
been transported by rivers, from the interior towards the coast, we may
conclude from the fewness and smallness of the streams of Patagonia:
moreover, in the case of the one great and rapid river of Santa Cruz,
we have good evidence that its transporting power is very trifling.
This river is from two to three hundred yards in width, about seventeen
feet deep in its middle, and runs with a singular degree of uniformity
five knots an hour, with no lakes and scarcely any still reaches:
nevertheless, to give one instance of its small transporting power,
upon careful examination, pebbles of compact basalt could not be found
in the bed of the river at a greater distance than ten miles below the
point where the stream rushes over the debris of the great basaltic
cliffs forming its shore: fragments of the _ cellular_ varieties have
been washed down twice or thrice as far. That the pebbles in Central
and Northern Patagonia have not been transported by ice-agency, as
seems to have been the case to a considerable extent farther south, and
likewise in the northern hemisphere, we may conclude, from the absence
of all angular fragments in the gravel, and from the complete contrast
in many other respects between the shingle and neighbouring boulder
formation.

Looking to the gravel on any one of the step-formed plains, I cannot
doubt, from the several reasons assigned in this chapter, that it has
been spread out and leveled by the long-continued action of the sea,
probably during the slow rise of the land. The smooth and perfectly
rounded
condition of the innumerable pebbles alone would prove long-continued
action. But how the whole mass of shingle on the coast-plains has been
transported from the mountains of the interior, is another and more
difficult question. The following considerations, however, show that
the sea by its ordinary action has considerable power in distributing
pebbles. A table has already been given, showing how very uniformly and
gradually[19] the pebbles decrease in size with the gradually seaward
increasing depth and distance. A series of this kind irresistibly leads
to the conclusion, that the sea has the power of sifting and
distributing the loose matter on its bottom. According to Martin
White,[20] the bed of the British Channel is disturbed during gales at
depths of sixty-three and sixty-seven fathoms, and at thirty fathoms,
shingle and fragments of shells are often deposited, afterwards to be
carried away again. Groundswells, which are believed to be caused by
distant gales, seem especially to affect the bottom: at such times,
according to Sir R. Schomburgk,[21] the sea to a great distance round
the West Indian Islands, at depths from five to fifteen fathoms,
becomes discoloured, and even the anchors of vessels have been moved.
There are, however, some difficulties in understanding how the sea can
transport pebbles lying at the bottom, for, from experiments instituted
on the power of running water, it would appear that the currents of the
sea have not sufficient velocity to move stones of even moderate size:
moreover, I have repeatedly found in the most exposed situations that
the pebbles which lie at the bottom are encrusted with full-grown
living corallines, furnished with the most delicate, yet unbroken
spines: for instance, in ten fathoms water off the mouth of the Santa
Cruz, many pebbles, under half an inch in diameter, were thus coated
with Flustracean zoophytes.[22] Hence we must conclude
that these pebbles are not often violently disturbed: it should,
however, be borne in mind that the growth of corallines is rapid. The
view, propounded by Professor Playfair, will, I believe, explain this
apparent difficulty,—namely, that from the undulations of the sea
_tending_ to lift up and down pebbles or other loose bodies at the
bottom, such are liable, when thus quite or partially raised, to be
moved even by a very small force, a little onwards. We can thus
understand how oceanic or tidal currents of no great strength, or that
recoil movement of the bottom-water near the land, called by sailors
the “undertow” (which I presume must extend out seaward as far as the
_breaking_ waves impel the surface-water towards the beach), may gain
the power during storms of sifting and distributing pebbles even of
considerable size, and yet without so violently disturbing them as to
injure the encrusting corallines.[23]

 [19] I may mention, that at the distance of 150 miles from the
 Patagonian shore I carefully examined the minute rounded particles in
 the sand, and found them to be fusible like the porphyries of the
 great shingle bed. I could even distinguish particles of the
 gallstone-yellow porphyry. It was interesting to notice how gradually
 the particles of white quartz increased, as we approached the Falkland
 Islands, which are thus constituted. In the whole line of soundings
 between these islands and the coast of Patagonia dead or living
 organic remains were most rare. On the relations between the depth of
 water and the nature of the bottom, see Martin White on “Soundings in
 the Channel,” pp. 4, 6, 175; also Captain Beechey’s “Voyage to the
 Pacific,” chap. xviii.


 [20] “Soundings in the Channel,” pp. 4, 166. M. Siau states (_Edin.
 New Phil. Jour._, vol. xxxi, p. 246), that he found the sediment, at a
 depth of 188 metres, arranged in ripples of different degrees of
 fineness. There are some excellent discussions on this and allied
 subjects in Sir H. De la Beche’s “Theoretical Researches.”


 [21] _Journal of Royal Geograph. Soc.,_ vol. v, p. 25. It appears from
 Mr. Scott Russell’s investigations (see Mr. Murchison’s “Anniver.
 Address Geolog. Soc.,” 1843, p. 40), that in waves of translation the
 motion of the particles of water is nearly as great at the bottom as
 at the top.


 [22] (A pebble, one and a half inch square and half an inch thick, was
 given me, dredged up from twenty-seven fathoms depth off the western
 end of the Falkland Islands, where the sea is remarkably stormy, and
 subject to violent tides. This pebble was encrusted on all sides by a
 delicate living coralline. I have seen many pebbles from depths
 between forty and seventy fathoms thus encrusted; one from the latter
 depth off Cape Horn.


 [23] I may take this opportunity of remarking on a singular, but very
 common character in the form of the bottom, in the creeks which deeply
 penetrate the western shores of Tierra del Fuego; namely, that they
 are almost invariably much shallower close to the open sea at their
 mouths than inland. Thus, Cook, in entering Christmas Sound, first had
 soundings in thirty-seven fathoms, then in fifty, then in sixty, and a
 little farther in no bottom with 170 fathoms. The sealers are so
 familiar with this fact, that they always look out for anchorage near
 the entrances of the creeks. See, also, on this subject, the “Voyages
 of the _ Adventure_ and _Beagle_,” vol. i, p. 375 and “Appendix,” p.
 313. This Shoalness of the sea-channels near their entrances probably
 results from the quantity of sediment formed by the wear and tear of
 the outer rocks exposed to the full force of the open sea. I have no
 doubt that many lakes, for instance in Scotland, which are very deep
 within, and are separated from the sea apparently only by a tract of
 detritus, were originally sea-channels with banks of this nature near
 their mouths, which have since been upheaved.


The sea acts in another and distinct manner in the distribution of
pebbles, namely by the waves on the beach. Mr. Palmer,[24] in his
excellent memoir on this subject, has shown that vast masses of shingle
travel with surprising quickness along lines of coast, according to the
direction with which the waves break on the beach and that this is
determined by the prevailing direction of the winds. This agency must
be powerful in mingling together and disseminating pebbles derived from
different sources: we may, perhaps, thus understand the wide
distribution of the gallstone-yellow porphyry; and likewise, perhaps,
the great difference in the nature of the pebbles at the mouth of the
Santa Cruz from those in the same latitude at the head of the valley.

 [24] “Philosophical Transactions,” 1834, p. 576.

I will not pretend to assign to these several and complicated agencies
their shares in the distribution of the Patagonian shingle: but from
the several considerations given in this chapter, and I may add, from
the frequency of a capping of gravel on tertiary deposits in all parts
of the world, as I have myself observed and seen stated in the works of
various authors, I cannot doubt that the power of widely dispersing
gravel is an ordinary contingent on the action of the sea; and that
even in the case of the great Patagonian shingle-bed we have no
occasion to call in the aid of debacles. I at one time imagined that
perhaps an immense accumulation of shingle had originally been
collected at the foot of the Cordillera; and that this accumulation,
when upraised above the level of the sea, had been eaten into and
partially spread out (as off the present line of coast); and that the
newly-spread out bed had in its turn been upraised, eaten into, and
re-spread out; and so onwards, until the shingle, which was first
accumulated in great thickness at the foot of the Cordillera, had
reached in thinner beds its present extension. By whatever means the
gravel formation of Patagonia may have been distributed, the vastness
of its area, its thickness, its superficial position, its recent
origin, and the great degree of similarity in the nature of its
pebbles, all appear to me well deserving the attention of geologists,
in relation to the origin of the widely-spread beds of conglomerate
belonging to past epochs.

No. 7
Section of coast-cliffs and bottom of sea, off the island of St.
Helena.


[Illustration: Section of clast-cliffs and bottom of sea, off the
island of St. Helena.]

_Formation of Cliffs._—When viewing the sea-worn cliffs of Patagonia,
in some parts between eight hundred and nine hundred feet in height,
and formed of horizontal tertiary strata, which must once have extended
far seaward—or again, when viewing the lofty cliffs round many volcanic
islands, in which the gentle inclination of the lava-streams indicates
the former extension of the land, a difficulty often occurred to me,
namely, how the strata could possibly have been removed by the action
of the sea at a considerable depth beneath its surface. The section in
diagram No. 7, which represents the general form of the land on the
northern and leeward side of St. Helena (taken from Mr. Seale’s large
model and various measurements), and of the bottom of the adjoining sea
(taken chiefly from Captain Austin’s survey and some old charts), will
show the nature of this difficulty.

If, as seems probable, the basaltic streams were originally prolonged
with nearly their present inclination, they must, as shown by the
dotted line in the section, once have extended at least to a point, now
covered
by the sea to a depth of nearly thirty fathoms: but I have every reason
to believe they extended considerably further, for the inclination of
the streams is less near the coast than further inland. It should also
be observed, that other sections on the coast of this island would have
given far more striking results, but I had not the exact measurements;
thus, on the windward side, the cliffs are about two thousand feet in
height and the cut-off lava streams very gently inclined, and the
bottom of the sea has nearly a similar slope all round the island. How,
then, has all the hard basaltic rock, which once extended beneath the
surface of the sea, been worn away? According to Captain Austin, the
bottom is uneven and rocky only to that very small distance from the
beach within which the depth is from five to six fathoms; outside this
line, to a depth of about one hundred fathoms, the bottom is smooth,
gently inclined, and formed of mud and sand; outside the one hundred
fathoms, it plunges suddenly into unfathomable depths, as is so very
commonly the case on all coasts where sediment is accumulating. At
greater depths than the five or six fathoms, it seems impossible, under
existing circumstances, that the sea can both have worn away hard rock,
in parts to a thickness of at least 150 feet, and have deposited a
smooth bed of fine sediment. Now, if we had any reason to suppose that
St. Helena had, during a long period, gone on slowly subsiding, every
difficulty would be removed: for looking at the diagram, and imagining
a fresh amount of subsidence, we can see that the waves would then act
on the coast-cliffs with fresh and unimpaired vigour, whilst the rocky
ledge near the beach would be carried down to that depth, at which sand
and mud would be deposited on its bare and uneven surface: after the
formation near the shore of a new rocky shoal, fresh subsidence would
carry it down and allow it to be smoothly covered up. But in the case
of the many cliff-bounded islands, for instance in some of the Canary
Islands and of Madeira, round which the inclination of the strata shows
that the land once extended far into the depths of the sea, where there
is no apparent means of hard rock being worn away—are we to suppose
that all these islands have slowly subsided? Madeira, I may remark,
has, according to Mr. Smith of Jordan Hill, subsided. Are we to extend
this conclusion to the high, cliff-bound, horizontally stratified
shores of Patagonia, off which, though the water is not deep even at
the distance of several miles, yet the smooth bottom of pebbles
gradually decreasing in size with the increasing depth, and derived
from a foreign source, seem to declare that the sea is now a depositing
and not a corroding agent? I am much inclined to suspect, that we shall
hereafter find in all such cases, that the land with the adjoining bed
of the sea has in truth subsided: the time will, I believe, come, when
geologists will consider it as improbable, that the land should have
retained the same level during a whole geological period, as that the
atmosphere should have remained absolutely calm during an entire
season.




Chapter II ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.


Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth of
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea-beach on San Lorenzo, human
remains, fossil earthenware, earthquake debacle, recent subsidence. On
the decay of upraised shells.—General summary.


Commencing at the south and proceeding northward, the first place at
which I landed, was at Cape Tres Montes, in lat. 46° 35′. Here, on the
shores of Christmas Cove, I observed in several places a beach of
pebbles with recent shells, about twenty feet above high-water mark.
Southward of Tres Montes (between lat. 47° and 48°), Byron[1] remarks,
“We thought it very strange, that upon the summits of the highest hills
were found beds of shells, a foot or two thick.” In the Chonos
Archipelago, the island of Lemus (lat. 44° 30′) was, according to M.
Coste,2 suddenly elevated eight feet, during the earthquake of 1829: he
adds, “Des roches jadis toujours couvertes par la mer, restant
aujourd’hui constamment decouvertes.” In other parts of this
archipelago, I observed two terraces of gravel, abutting to the foot of
each other: at Lowe’s Harbour (43° 48′), under a great mass of the
boulder formation, about three hundred feet in thickness, I found a
layer of sand, with numerous comminuted fragments of sea-shells, having
a fresh aspect, but too small to be identified.

 [1] “Narrative of the Loss of the _Wager_.”


 [2] “Comptes Rendus,” October 1838, p. 706.

_The Island of Chiloe._—The evidence of recent elevation is here more
satisfactory. The bay of San Carlos is in most parts bounded by
precipitous cliffs from about ten to forty feet in height, their bases
being separated from the present line of tidal action by a talus, a few
feet in height, covered with vegetation. In one sheltered creek (west
of P. Arena), instead of a loose talus, there was a bare sloping bank
of tertiary mudstone, perforated, above the line of the highest tides,
by numerous shells of a Pholas now common in the harbour. The upper
extremities of these shells, standing upright in their holes with grass
growing out of them, were abraded about a quarter of an inch, to the
same level with the surrounding worn strata. In other parts, I observed
(as at Pudeto) a great beach, formed of comminuted shells, twenty feet
above the present shore. In other parts again, there were small caves
worn into the foot of the low cliffs, and protected from the waves by
the talus with its vegetation: one such cave, which I examined, had its
mouth about twenty feet, and its bottom, which was filled with sand
containing fragments of shells and legs of crabs, from eight to ten
feet above high-water mark. From these several facts, and from the
appearance of the upraised shells, I inferred that the elevation had
been quite recent; and on inquiring from Mr. Williams, the Portmaster,
he told me he was convinced that the land had risen, or the sea fallen,
four feet within the last four years. During this period, there had
been one severe earthquake, but no particular change of level was then
observed; from the habits of the people who all keep boats in the
protected creeks, it is absolutely impossible that a rise of four feet
could have taken place suddenly and been unperceived. Mr. Williams
believes that the change has been quite gradual. Without the elevatory
movement continues at a quick rate, there can be no doubt that the sea
will soon destroy the talus of earth at the foot of the cliffs round
the bay, and will then reach its former lateral extension, but not of
course its former level: some of the inhabitants assured me that one
such talus, with a footpath on it, was even already sensibly decreasing
in width. I received several accounts of beds of shells, existing at
considerable heights in the inland parts of Chiloe; and to one of
these, near Catiman, I was guided by a countryman. Here, on the south
side of the peninsula of Lacuy, there was an immense bed of the _Venus
costellata_ and of an oyster, lying on the summit-edge of a piece of
tableland, 350 feet (by the barometer) above the level of the sea. The
shells were closely packed together, embedded in and covered by a very
black, damp, peaty mould, two or three feet in thickness, out of which
a forest of great trees was growing. Considering the nature and
dampness of this peaty soil, it is surprising that the fine ridges on
the outside of the Venus are perfectly preserved, though all the shells
have a blackened appearance. I did not doubt that the black soil, which
when dry, cakes hard, was entirely of terrestrial origin, but on
examining it under the microscope, I found many very minute rounded
fragments of shells, amongst which I could distinguish bits of Serpulæ
and mussels. The _Venus costellata_, and the Ostrea (_O. edulis_,
according to Captain King) are now the commonest shells in the
adjoining bays. In a bed of shells, a few feet below the 350 feet bed,
I found a horn of the little _Cervus humilis_, which now inhabits
Chiloe.

The eastern or inland side of Chiloe, with its many adjacent islets,
consists of tertiary and boulder deposits, worn into irregular plains
capped by gravel. Near Castro, and for ten miles southward, and on the
islet of Lemuy, I found the surface of the ground to a height of
between twenty and thirty feet above high-water mark, and in several
places apparently up to fifty feet, thickly coated by much comminuted
shells, chiefly of the _Venus costellata_ and _Mytilus Chiloensis_; the
species now most abundant on this line of coast. As the inhabitants
carry immense numbers of these shells inland, the continuity of the bed
at the same height was often the only means of recognising its natural
origin. Near Castro, on each side of the creek and rivulet of the
Gamboa, three distinct terraces are seen: the lowest was estimated at
about one hundred and fifty feet in height, and the highest at about
five hundred feet, with the country irregularly rising behind it;
obscure
traces, also, of these same terraces could be seen along other parts of
the coast. There can be no doubt that their three escarpments record
pauses in the elevation of the island. I may remark that several
promontories have the word Huapi, which signifies in the Indian tongue,
island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi,
etc.; and these, according to Indian traditions, once existed as
islands. In the same manner the term Pulo in Sumatra is appended[3] to
the names of promontories, traditionally said to have been islands; in
Sumatra, as in Chiloe, there are upraised recent shells. The Bay of
Carelmapu, on the mainland north of Chiloe, according to Agüerros,[4]
was in 1643 a good harbour; it is now quite useless, except for boats.

 [3] Marsden’s “Sumatra,” p. 31.


 [4] “Descripcion Hist. de la Provincia de Chiloé,” p. 78. From the
 account given by the old Spanish writers, it would appear that several
 other harbours, between this point and Concepcion, were formerly much
 deeper than they now are.


_Valdivia._—I did not observe here any distinct proofs of recent
elevation; but in a bed of very soft sandstone, forming a fringe-like
plain, about sixty feet in height, round the hills of mica-slate, there
are shells of Mytilus, Crepidula, Solen, Novaculina, and Cytheræa, too
imperfect to be specifically recognised. At Imperial, seventy miles
north of Valdivia, Agüerros[5] states that there are large beds of
shells, at a considerable distance from the coast, which are burnt for
lime. The island of Mocha, lying a little north of Imperial, was
uplifted two feet,[6] during the earthquake of 1835.

 [5] _Ibid.,_ p. 25.


 [6] “Voyages of _Adventure_ and _Beagle_,” vol. ii, p. 415.


_Concepcion._—I cannot add anything to the excellent account by Captain
Fitzroy[7] of the elevation of the land at this place, which
accompanied the earthquake of 1835. I will only recall to the
recollection of geologists, that the southern end of the island of St.
Mary was uplifted eight feet, the central part nine, and the northern
end ten feet; and the whole island more than the surrounding districts.
Great beds of mussels, patellæ, and chitons still adhering to the rocks
were upraised above high-water mark; and some acres of a rocky flat,
which was formerly always covered by the sea, was left standing dry,
and exhaled an offensive smell, from the many attached and putrefying
shells. It appears from the researches of Captain Fitzroy that both the
island of St. Mary and Concepcion (which was uplifted only four or five
feet) in the course of some weeks subsided, and lost part of their
first elevation. I will only add as a lesson of caution, that round the
sandy shores of the great Bay of Concepcion, it was most difficult,
owing to the obliterating effects of the great accompanying wave, to
recognise any distinct
evidence of this considerable upheaval; one spot must be excepted,
where there was a detached rock which before the earthquake had always
been covered by the sea, but afterwards was left uncovered.

 [7] _Ibid.,_ vol. ii, p. 412, _et seq._ In vol. v, p. 601 of the
 “Geological Transactions” I have given an account of the remarkable
 volcanic phenomena, which accompanied this earthquake. These phenomena
 appear to me to prove that the action, by which large tracts of land
 are uplifted, and by which volcanic eruptions are produced, is in
 every respect identical.


On the island of Quiriquina (in the Bay of Concepcion), I found, at an
estimated height of four hundred feet, extensive layers of shells,
mostly comminuted, but some perfectly preserved and closely packed in
black vegetable mould; they consisted of Concholepas, Fissurella,
Mytilus, Trochus, and Balanus. Some of these layers of shells rested on
a thick bed of bright-red, dry, friable earth, capping the surface of
the tertiary sandstone, and extending, as I observed whilst sailing
along the coast, for 150 miles southward: at Valparaiso, we shall
presently see that a similar red earthy mass, though quite like
terrestrial mould, is really in chief part of recent marine origin. On
the flanks of this island of Quiriquina, at a less height than the 400
feet, there were spaces several feet square, thickly strewed with
fragments of similar shells. During a subsequent visit of the _Beagle_
to Concepcion, Mr. Kent, the assistant-surgeon, was so kind as to make
for me some measurements with the barometer: he found many marine
remains along the shores of the whole bay, at a height of about twenty
feet; and from the hill of Sentinella behind Talcahuano, at the height
of 160 feet, he collected numerous shells, packed together close
beneath the surface in black earth, consisting of two species of
Mytilus, two of Crepidula, one of Concholepas, of Fissurella, Venus,
Mactra, Turbo, Monoceros, and the _Balanus psittacus._ These shells
were bleached, and within some of the Balani other Balani were growing,
showing that they must have long lain dead in the sea. The above
species I compared with living ones from the bay, and found them
identical; but having since lost the specimens, I cannot give their
names: this is of little importance, as Mr. Broderip has examined a
similar collection, made during Captain Beechey’s expedition, and
ascertained that they consisted of ten recent species, associated with
fragments of Echini, crabs, and Flustræ; some of these remains were
estimated by Lieutenant Belcher to lie at the height of nearly a
thousand feet above the level of the sea.[8] In some places round the
bay, Mr. Kent observed that there were beds formed exclusively of the
_Mytilus Chiloensis_: this species now lives in parts never uncovered
by the tides. At considerable heights, Mr. Kent found only a few
shells; but from the summit of one hill, 625 feet high, he brought me
specimens of the Concholepas, _Mytilus Chiloensis_, and a Turbo. These
shells were softer and more brittle than those from the height of 164
feet; and these latter had obviously a much more ancient appearance
than the same species from the height of only twenty feet.

 [8] “Zoology of Captain Beechey’s Voyage,” p. 162.


_Coast north of Concepcion._—The first point examined was at the mouth
of the Rapel (160 miles north of Concepcion and sixty miles south of
Valparaiso), where I observed a few shells at the height of 100 feet,
and some barnacles adhering to the rocks three or four feet above the
highest tides: M. Gay[9] found here recent shells at the distance
of two leagues from the shore. Inland there are some wide,
gravel-capped plains, intersected by many broad, flat-bottomed valleys
(now carrying insignificant streamlets), with their sides cut into
successive wall-like escarpments, rising one above another, and in many
places, according to M. Gay, worn into caves. The one cave (C. del
Obispo) which I examined, resembled those formed on many sea-coasts,
with its bottom filled with shingle. These inland plains, instead of
sloping towards the coast, are inclined in an opposite direction
towards the Cordillera, like the successively rising terraces on the
inland or eastern side of Chiloe: some points of granite, which project
through the plains near the coast, no doubt once formed a chain of
outlying islands, on the inland shores of which the plains were
accumulated. At Bucalemu, a few miles northward of the Rapel, I
observed at the foot, and on the summit-edge of a plain, ten miles from
the coast, many recent shells, mostly comminuted, but some perfect.
There were, also, many at the bottom of the great valley of the Maypu.
At San Antonio, shells are said to be collected and burnt for lime. At
the bottom of a great ravine (Quebrada Onda, on the road to Casa
Blanca), at the distance of several miles from the coast, I noticed a
considerable bed, composed exclusively of _Mesodesma donaciforme_,
Desh., lying on a bed of muddy sand: this shell now lives associated
together in great numbers, on tidal-flats on the coast of Chile.

 [9] “Annales des Scienc. Nat.,” Avril 1833.

_Valparaiso._

During two successive years I carefully examined, part of the time in
company with Mr. Alison, into all the facts connected with the recent
elevation of this neighbourhood. In very many parts a beach of broken
shells, about fourteen or fifteen feet above high-water mark, may be
observed; and at this level the coast-rocks, where precipitous, are
corroded in a band. At one spot, Mr. Alison, by removing some birds’
dung, found at this same level barnacles adhering to the rocks. For
several miles southward of the bay, almost every flat little headland,
between the heights of 60 and 230 feet (measured by the barometer), is
smoothly coated by a thick mass of comminuted shells, of the same
species, and apparently in the same proportional numbers with those
existing in the adjoining sea. The Concholepas is much the most
abundant, and the best preserved shell; but I extracted perfectly
preserved specimens of the _Fissurella biradiata_, a Trochus and
Balanus (both well-known, but according to Mr. Sowerby yet unnamed) and
parts of the _Mytilus Chiloensis._ Most of these shells, as well as an
encrusting Nullipora, partially retain their colour; but they are
brittle, and often stained red from the underlying brecciated mass of
primary rocks; some are packed together, either in black or reddish
moulds; some lie loose on the bare rocky surfaces. The total number of
these shells is immense; they are less numerous, though still far from
rare, up a height of 1,000 feet above the sea. On the summit of a hill,
measured 557 feet, there was a small horizontal band of comminuted
shells, of which _many_ consisted (and likewise from lesser heights) of
very young and small
specimens of the still living Concholepas, Trochus, Patellæ, Crepidulæ,
and of _Mytilus Magellanicus_ (?):[10] several of these shells were
under a quarter of an inch in their greatest diameter. My attention was
called to this circumstance by a native fisherman, whom I took to look
at these shell-beds; and he ridiculed the notion of such small shells
having been brought up for food; nor could some of the species have
adhered when alive to other larger shells. On another hill, some miles
distant, and 648 feet high, I found shells of the Concholepas and
Trochus, perfect, though very old, with fragments of _Mytilus
Chiloensis_, all embedded in reddish-brown mould: I also found these
same species, with fragments of an Echinus and of _Balanus psittacus_,
on a hill 1,000 feet high. Above this height, shells became very rare,
though on a hill 1,300 feet high,[11] I collected the Concholepas,
Trochus, Fissurella, and a Patella. At these greater heights the shells
are almost invariably embedded in mould, and sometimes are exposed only
by tearing up bushes. These shells obviously had a very much more
ancient appearance than those from the lesser heights; the apices of
the Trochi were often worn down; the little holes made by burrowing
animals were greatly enlarged; and the Concholepas was often perforated
quite through, owing to the inner plates of shell having scaled off.

 [10] Mr. Cuming informs me that he does not think this species
 identical with, though closely resembling, the true _M. Magellanicus_
 of the southern and eastern coast of South America; it lives
 abundantly on the coast of Chile.


 [11] Measured by the barometer: the highest point in the range behind
 Valparaiso I found to be 1,626 feet above the level of the sea.

Many of these shells, as I have said, were packed in, and were quite
filled with, blackish or reddish-brown earth, resting on the granitic
detritus. I did not doubt until lately that this mould was of purely
terrestrial origin, when with a microscope examining some of it from
the inside of a Concholepas from the height of about one hundred feet,
I found that it was in considerable part composed of minute fragments
of the spines, mouth-bones, and shells of Echini, and of minute
fragments, of chiefly very young Patellæ, Mytili, and other species. I
found similar microscopical fragments in earth filling up the central
orifices of some large Fissurellæ. This earth when crushed emits a
sickly smell, precisely like that from garden-mould mixed with guano.
The earth accidentally preserved within the shells, from the greater
heights, has the same general appearance, but it is a little redder; it
emits the same smell when rubbed, but I was unable to detect with
certainty any marine remains in it. This earth resembles in general
appearance, as before remarked, that capping the rocks of Quiriquina in
the Bay of Concepcion, on which beds of sea-shells lay. I have, also,
shown that the black, peaty soil, in which the shells at the height of
350 feet at Chiloe were packed, contained many minute fragments of
marine animals. These facts appear to me interesting, as they show that
soils, which would naturally be considered of purely terrestrial
nature, may owe their origin in chief part to the sea.

Being well aware from what I have seen at Chiloe and in Tierra del
Fuego, that vast quantities of shells are carried, during successive
ages, far inland, where the inhabitants chiefly subsist on these
productions, I am bound to state that at greater heights than 557 feet,
where the number of very young and small shells proved that they had
not been carried up for food, the only evidence of the shells having
been naturally left by the sea, consists in their invariable and
uniform appearance of extreme antiquity—in the distance of some of the
places from the coast, in others being inaccessible from the nearest
part of the beach, and in the absence of fresh water for men to
drink—in the shells _not lying in heaps_,—and, lastly, in the close
similarity of the soil in which they are embedded, to that which lower
down can be unequivocally shown to be in great part formed from the
debris of the sea animals.[12]

 [12] In the “Proceedings of the Geolog. Soc.,” vol. ii, p. 446, I have
 given a brief account of the upraised shells on the coast of Chile,
 and have there stated that the proofs of elevation are not
 satisfactory above the height of 230 feet. I had at that time
 unfortunately overlooked a separate page written during my second
 visit to Valparaiso, describing the shells now in my possession from
 the 557 feet hill; I had not then unpacked my collections, and had not
 reconsidered the obvious appearance of greater antiquity of the shells
 from the greater heights, nor had I at that time discovered the marine
 origin of the earth in which many of the shells are packed.
 Considering these facts, I do not now feel a shadow of doubt that the
 shells, at the height of 1,300 feet, have been upraised by natural
 causes into their present position.

With respect to the position in which the shells lie, I was repeatedly
struck here, at Concepcion, and at other places, with the frequency of
their occurrence on the summits and edges either of separate hills, or
of little flat headlands often terminating precipitously over the sea.
The several above-enumerated species of mollusca, which are found
strewed on the surface of the land from a few feet above the level of
the sea up to the height of 1,300 feet, all now live either on the
beach, or at only a few fathoms’ depth: Mr. Edmondston, in a letter to
Professor E. Forbes, states that in dredging in the Bay of Valparaiso,
he found the common species of Concholepas, Fissurella, Trochus,
Monoceros, Chitons, etc., living in abundance from the beach to a depth
of seven fathoms; and dead shells occurred only a few fathoms deeper.
The common _Turritella cingulata_ was dredged up living at even from
ten to fifteen fathoms; but this is a species which I did not find here
amongst the upraised shells. Considering this fact of the species being
all littoral or sub-littoral, considering their occurrence at various
heights, their vast numbers, and their generally comminuted state,
there can be little doubt that they were left on successive beach-lines
during a gradual elevation of the land. The presence, however, of so
many whole and perfectly preserved shells appears at first a difficulty
on this view, considering that the coast is exposed to the full force
of an open ocean: but we may suppose, either that these shells were
thrown during gales on flat ledges of rock just above the level of
high-water mark, and that during the elevation of the land they are
never again touched by the waves, or, that during earthquakes, such as
those of
1822, 1835, and 1837, rocky reefs covered with marine-animals were it
one blow uplifted above the future reach of the sea. This latter
explanation is, perhaps, the most probable one with respect to the beds
at Concepcion entirely composed of the _Mytilus Chiloensis_, a species
which lives below the lowest tides; and likewise with respect to the
great beds occurring both north and south of Valparaiso, of the
_Mesodesma donaciforme_,—a shell which, as I am informed by Mr. Cuming,
inhabits sandbanks at the level of the lowest tides. But even in the
case of shells having the habits of this Mytilus and Mesodesma, beds of
them, wherever the sea gently throws up sand or mud, and thus protects
its own accumulations, might be upraised by the slowest movement, and
yet remain undisturbed by the waves of each new beach-line.

It is worthy of remark, that nowhere near Valparaiso above the height
of twenty feet, or rarely of fifty feet, I saw any lines of erosion on
the solid rocks, or any beds of pebbles; this, I believe, may be
accounted for by the disintegrating tendency of most of the rocks in
this neighbourhood. Nor is the land here modelled into terraces: Mr.
Alison, however, informs me, that on both sides of one narrow ravine,
at the height of 300 feet above the sea, he found a succession of
rather indistinct step-formed beaches, composed of broken shells, which
together covered a space of about eighty feet vertical.

I can add nothing to the accounts already published of the elevation of
the land at Valparaiso,[13] which accompanied the earthquake of 1822:
but I heard it confidently asserted, that a sentinel on duty,
immediately after the shock, saw a part of a fort, which previously was
not within the line of his vision, and this would indicate that the
uplifting was not horizontal: it would even appear from some facts
collected by Mr. Alison, that only the eastern half of the bay was then
elevated. Through the kindness of this same gentleman, I am able to
give an interesting account of the changes of level, which have
supervened here within historical periods: about the year 1680 a long
sea-wall (or Prefil) was built, of which only a few fragments now
remain; up to the year 1817, the sea often broke over it, and washed
the houses on the opposite side of the road (where the prison now
stands); and even in 1819, Mr. J. Martin remembers walking at the foot
of this wall, and being often obliged to climb over it to escape the
waves. There now stands (1834) on the seaward side of this wall, and
between it and the beach, in one part a single row of houses, and in
another part two rows with a street between them. This great extension
of the beach in so short a time cannot be attributed simply to the
accumulation of detritus; for a resident engineer measured for me the
height between the lowest part of the wall visible, and the present
beach-line at spring-tides, and the difference was eleven feet six
inches. The church of S. Augustin is believed to have been built in
1614, and there is a tradition that the sea formerly flowed very near
it; by levelling, its foundations were found
to stand nineteen feet six inches above the highest beach-line; so that
we see in a period of 220 years, the elevation cannot have been as much
as nineteen feet six inches. From the facts given with respect to the
sea-wall, and from the testimony of the elder inhabitants, it appears
certain that the change in level began to be manifest about the year
1817. The only sudden elevation of which there is any record occurred
in 1822, and this seems to have been less than three feet. Since that
year, I was assured by several competent observers, that part of an old
wreck, which is firmly embedded near the beach, has sensibly emerged;
hence here, as at Chiloe, a slow rise of the land appears to be now in
progress. It seems highly probable that the rocks which are corroded in
a band at the height of fourteen feet above the sea were acted on
during the period, when by tradition the base of S. Augustin church,
now nineteen feet six inches above the highest water-mark, was
occasionally washed by the waves.

 [13] Dr. Meyen (“Reise um Erde,” Th. I, s. 221) found in 1831 seaweed
 and other bodies still adhering to some rocks which during the shock
 of 1822 were lifted above the sea.


_Valparaiso to Coquimbo._—For the first seventy-five miles north of
Valparaiso I followed the coast-road, and throughout this space I
observed innumerable masses of upraised shells. About Quintero there
are immense accumulations (worked for lime) of the _Mesodesma
donaciforme_, packed in sandy earth; they abound chiefly about fifteen
feet above high-water, but shells are here found, according to Mr.
Miers,[14] to a height of 500 feet, and at a distance of three leagues
from the coast: I here noticed barnacles adhering to the rocks three or
four feet above the highest tides. In the neighbourhood of Plazilla and
Catapilco, at heights of between two hundred and three hundred feet,
the number of comminuted shells, with some perfect ones, especially of
the Mesodesma, packed in layers, was truly immense: the land at
Plazilla had evidently existed as a bay, with abrupt rocky masses
rising out of it, precisely like the islets in the broken bays now
indenting this coast. On both sides of the rivers Ligua, Longotomo,
Guachen, and Quilimari, there are plains of gravel about two hundred
feet in height, in many parts absolutely covered with shells. Close to
Conchalee, a gravel-plain is fronted by a lower and similar plain about
sixty feet in height, and this again is separated from the beach by a
wide tract of low land: the surfaces of all three plains or terraces
were strewed with vast numbers of the Concholepas, Mesodesma, an
existing Venus, and other still existing littoral shells. The two upper
terraces closely resemble in miniature the plains of Patagonia; and
like them are furrowed by dry, flat-bottomed, winding valleys.
Northward of this place I turned inward; and therefore found no more
shells: but the valleys of Chuapa, Illapel, and Limari, are bounded by
gravel-capped plains, often including a lower terrace within. These
plains send bay-like arms between and into the surrounding hills; and
they are continuously united with other extensive gravel-capped plains,
separating the coast mountain-ranges from the Cordillera.

 [14] “Travels in Chile,” vol. i, pp. 395, 458. I received several
 similar accounts from the inhabitants, and was assured that there are
 many shells on the plain of Casa Blanca, between Valparaiso and
 Santiago, at the height of 800 feet.

_Coquimbo._

A narrow fringe-like plain, gently inclined towards the sea, here
extends for eleven miles along the coast, with arms stretching up
between the coast-mountains, and likewise up the valley of Coquimbo: at
its southern extremity it is directly connected with the plain of
Limari, out of which hills abruptly rise like islets, and other hills
project like headlands on a coast. The surface of the fringe-like plain
appears level, but differs insensibly in height, and greatly in
composition, in different parts.

At the mouth of the valley of Coquimbo, the surface consists wholly of
gravel, and stands from 300 to 350 feet above the level of the sea,
being about one hundred feet higher than in other parts. In these other
and lower parts the superficial beds consist of calcareous matter, and
rest on ancient tertiary deposits hereafter to be described. The
uppermost calcareous layer is cream-coloured, compact,
smooth-fractured, sub-stalactiform, and contains some sand, earthy
matter, and recent shells. It lies on, and sends wedge-like veins
into,[15] a much more friable, calcareous, tuff-like variety; and both
rest on a mass about twenty feet in thickness, formed of fragments of
recent shells, with a few whole ones, and with small pebbles firmly
cemented together. This latter rock is called by the inhabitants
_losa_, and is used for building: in many parts it is divided into
strata, which dip at an angle of ten degrees seaward, and appear as if
they had originally been heaped in successive layers (as may be seen on
coral-reefs) on a steep beach. This stone is remarkable from being in
parts entirely formed of empty, pellucid capsules or cells of
calcareous matter, of the size of small seeds: a series of specimens
unequivocally showed that all these capsules once contained minute
rounded fragments of shells which have since been gradually dissolved
by water percolating through the mass.[16]

 [15] In many respects this upper hard, and the underlying more
 friable, varieties, resemble the great superficial beds at King
 George’s Sound in Australia, which I have described in my “Geological
 Observations on Volcanic Islands.” There could be little doubt that
 the upper layers there have been hardened by the action of rain on the
 friable, calcareous matter, and that the whole mass has originated in
 the decay of minutely comminuted sea-shells and corals.


 [16] I have incidentally described this rock in the above work on
 Volcanic Islands.

The shells embedded in the calcareous beds forming the surface of this
fringe-like plain, at the height of from 200 to 250 feet above the sea,
consist of:—

Venus opaca.

Mulinia Byronensis.

Pecten purpuratus.

Mesodesma donaciforme.

Turritella cingulata.

Monoceros costatum.

Concholepas Peruviana.

Trochus (common Valparaiso species).

Calyptræa Byronensis.

Although these species are all recent, and are all found in the
neighbouring sea, yet I was particularly struck with the difference in
the
proportional numbers of the several species, and of those now cast up
on the present beach. I found only one specimen of the Concholepas, and
the Pecten was very rare, though both these shells are now the
commonest kinds, with the exception, perhaps, of the _Calyptræa
radians_, of which I did not find one in the calcareous beds. I will
not pretend to determine how far this difference in the proportional
numbers depends on the age of the deposit, and how far on the
difference in nature between the present sandy beaches and the
calcareous bottom, on which the embedded shells must have lived.

No. 8
Section of plain of Coquimbo.


[Illustration: Section of plain of Coquimbo.]

On the bare surface of the calcareous plain, or in a thin covering of
sand, there were lying, at a height from 200 to 252 feet, many recent
shells, which had a much fresher appearance than the embedded ones:
fragments of the Concholepas, and of the common Mytilus, still
retaining a tinge of its colour, were numerous, and altogether there
was manifestly a closer approach in proportional numbers to those now
lying on the beach. In a mass of stratified, slightly agglutinated
sand, which in some places covers up the lower half of the seaward
escarpment of the plain, the included shells appeared to be in exactly
the same proportional numbers with those on the beach. On one side of a
steep-sided ravine, cutting through the plain behind Herradura Bay, I
observed a narrow strip of stratified sand, containing similar shells
in similar proportional numbers; a section of the ravine is represented
in Diagram 8, which serves also to show the general composition of the
plain. I mention this case of the ravine chiefly because without the
evidence of the marine shells in the sand, any one would have supposed
that it had been hollowed out by simple alluvial action.

The escarpment of the fringe-like plain, which stretches for eleven
miles along the coast, is in some parts fronted by two or three narrow,
step-formed terraces, one of which at Herradura Bay expands into a
small plain. Its surface was there formed of gravel, cemented together
by calcareous matter; and out of it I extracted the following recent
shells, which are in a more perfect condition than those from the upper
plain:—

Calyptræa radians.

Turritella cingulata.

Oliva Peruviana.

Murex labiosus, var.

Nassa (identical with a living species).

Solen Dombeiana.

Pecten purpuratus.

Venus Chilensis.

Amphidesma rugulosum. The small irregular wrinkles of the posterior
part of this shell are rather stronger than in the recent specimens of
this species from Coquimbo. (G. B. Sowerby.)

Balanus (identical with living species).

On the syenitic ridge, which forms the southern boundary of Herradura
Bay and Plain, I found the Concholepas and _Turritella cingulata_
(mostly in fragments), at the height of 242 feet above the sea. I could
not have told that these shells had not formerly been brought up by
man, if I had not found one very small mass of them cemented together
in a friable calcareous tuff. I mention this fact more particularly,
because I carefully looked, in many apparently favourable spots, at
lesser heights on the side of this ridge, and could not find even the
smallest fragment of a shell. This is only one instance out of many,
proving that the absence of sea-shells on the surface, though in many
respects inexplicable, is an argument of very little weight in
opposition to other evidence on the recent elevation of the land. The
highest point in this neighbourhood at which I found upraised shells of
existing species was on an inland calcareous plain, at the height of
252 feet above the sea.

It would appear from Mr. Caldcleugh’s researches,[17] that a rise has
taken place here within the last century and a half; and as no sudden
change of level has been observed during the not very severe
earthquakes, which have occasionally occurred here, the rising has
probably been slow, like that now, or quite lately, in progress at
Chiloe and at Valparaiso: there are three well-known rocks, called the
Pelicans, which in 1710, according to Feuillèe, were _à fleur d’eau_,
but now are said to stand twelve feet above low-water mark: the
spring-tides rise here only five feet. There is another rock, now nine
feet above high-water mark, which in the time of Frezier and Feuillèe
rose only five or six feet out of water. Mr. Caldcleugh, I may add,
also shows (and I received similar accounts) that there has been a
considerable decrease in the soundings during the last twelve years in
the Bays of Coquimbo, Concepcion, Valparaiso, and Guasco; but as in
these cases it is nearly impossible to distinguish between the
accumulation of sediment and the upheavement of the bottom, I have not
entered into any details.

 [17] “Proceedings of the Geological Society,” vol. ii, p. 446.


_Valley of Coquimbo._—The narrow coast-plain sends, as before stated,
an arm, or more correctly a fringe, on both sides, but chiefly on the
southern side, several miles up the valley. These fringes are worn into
steps or terraces, which present a most remarkable appearance, and have
been compared (though not very correctly) by Captain Basil
Hall, to the parallel roads of Glen Roy in Scotland: their origin has
been ably discussed by Mr. Lyell.[18] The first section which I will
give (Figure 9), is not drawn across the valley, but in an east and
west line at its mouth, where the step-formed terraces debouch and
present their very gently inclined surfaces towards the Pacific.

 [18] “Principles of Geology” (1st edit.), vol. iii, p. 131.


No. 9
East and west section through the terraces at Coquimbo, where they
debouch from the valley, and front the sea.


[Illustration: East and west section through terraces at Coquimbo.]

The bottom plain (A) is about a mile in width, and rises quite
insensibly from the beach to a height of twenty-five feet at the foot
of the next plain; it is sandy, and abundantly strewed with shells.

Plain or terrace B is of small extent, and is almost concealed by the
houses of the town, as is likewise the escarpment of terrace C. On both
sides of a ravine, two miles south of the town, there are two little
terraces, one above the other, evidently corresponding with B and C;
and on them marine remains of the species already enumerated were
plentiful. Terrace E is very narrow, but quite distinct and level; a
little southward of the town there were traces of a terrace D
intermediate between E and C. Terrace F is part of the fringe-like
plain, which stretches for the eleven miles along the coast; it is here
composed of shingle, and is 100 feet higher than where composed of
calcareous matter. This greater height is obviously due to the quantity
of shingle, which at some former period has been brought down the great
valley of Coquimbo.

Considering the many shells strewed over the terraces A, B, and C, and
a few miles southward on the calcareous plain, which is continuously
united with the upper step-like plain F, there cannot, I apprehend, be
any doubt, that these six terraces have been formed by the action of
the sea; and that their five escarpments mark so many periods of
comparative rest in the elevatory movement, during which the sea wore
into the land. The elevation between these periods may have been sudden
and on _an average_ not more than seventy-two feet each time, or it may
have been gradual and insensibly slow. From the shells on the three
lower terraces, and on the upper one, and I may add on the three
gravel-capped terraces at Conchalee, being all littoral and
sub-littoral species, and from the analogical facts given at
Valparaiso, and lastly from the evidence of a slow rising lately or
still in progress here, it appears to me far more probable that the
movement has been slow. The existence of these successive escarpments,
or old cliff-lines, is in another respect highly instructive, for they
show periods of comparative rest in the elevatory movement, and of
denudation, which would never even have been suspected from a close
examination of many miles of coast southward of Coquimbo.

We come now to the terraces on the opposite sides of the east and west
valley of Coquimbo: the section in figure No. 10 is taken in a north
and south line across the valley at a point about three miles from the
sea. The valley measured from the edges of the escarpments of the upper
plain FF is about a mile in width; but from the bases of the bounding
mountains it is from three to four miles wide. The terraces marked with
an interrogative do not exist on that side of the valley, but are
introduced merely to render the diagram more intelligible.

No. 10
North and south section across the valley of Coquimbo.


[Illustration: North and south section across the valley of Coquimbo.]

Terraces marked with ? do not occur on that side of the valley, and are
introduced only to make the diagram more intelligible. A river and
bottom-plain of valley C, E, and F, on the south side of valley, are
respectively, 197, 377, and 420 feet above the level of the sea.

 AA. The bottom of the valley, believed to be 100 feet above the sea:
 it is continuously united with the lowest plain A of figure No. 9.
 B. This terrace higher up the valley expands considerably; seaward it
 is soon lost, its escarpment being united with that of C: it is not
 developed at all on the south side of the valley.
 C. This terrace, like the last, is considerably expanded higher up the
 valley. These two terraces apparently correspond with B and C of
 figure No. 9.
 D is not well developed in the line of this section; but seaward it
 expands into a plain: it is not present on the south side of the
 valley; but it is met with, as stated under the former section, a
 little south of the town.
 E is well developed on the south side, but absent on the north side of
 the valley: though not continuously united with E of figure No. 9, it
 apparently corresponds with it.
 F. This is the surface-plain, and is continuously united with that
 which stretches like a fringe along the coast. In ascending the valley
 it gradually becomes narrower, and is at last, at the distance of
 about ten miles from the sea, reduced to a row of flat-topped patches
 on the sides of the mountains. None of the lower terraces extend so
 far up the valley.

These five terraces are formed of shingle and sand; three of them, as
marked by Captain B. Hall (namely, B, C, and F), are much more
conspicuous than the others. From the marine remains copiously strewed
at the mouth of the valley on the lower terraces, and southward of the
town on the upper one, they are, as before remarked, undoubtedly of
marine origin; but within the valley, and this fact well deserves
notice, at a distance of from only a mile and a half to three or four
miles from the sea, I could not find even a fragment of a shell.


_On the inclination of the terraces of Coquimbo, and on the upper and
basal edges of their escarpments not being horizontal._—The surfaces of
these terraces slope in a slight degree, as shown by the two last
sections taken conjointly, both towards the centre of the valley, and
seawards towards its mouth. This double or diagonal inclination, which
is not the same in the several terraces, is, as we shall immediately
see, of simple explanation. There are, however, some other points which
at first appear by no means obvious,—namely, first, that each terrace,
taken in its whole breadth from the summit-edge of one escarpment to
the base of that above it, and followed up the valley, is not
horizontal; nor have the several terraces, when followed up the valley,
all the same inclination; thus I found the terraces C, E, and F,
measured at a point about two miles from the mouth of the valley, stood
severally between fifty-six to seventy-seven feet higher than at the
mouth. Again, if we look to any one line of cliff or escarpment,
neither its summit-edge nor its base is horizontal. On the theory of
the terraces having been formed during a slow and equable rise of the
land, with as many intervals of rest as there are escarpments, it
appears at first very surprising that horizontal lines of some kind
should not have been left on the land.

The direction of the diagonal inclination in the different terraces
being different,—in some being directed more towards the middle of the
valley, in others more towards its mouth,—naturally follows on the view
of each terrace, being an accumulation of successive beach-lines round
bays, which must have been of different forms and sizes when the land
stood at different levels: for if we look to the actual beach of a
narrow creek, its slope is directed towards the middle; whereas, in an
open bay, or slight concavity on a coast, the slope is towards the
mouth, that is, almost directly seaward; hence as a bay alters in form
and size, so will the direction of the inclination of its successive
beaches become changed.

[Illustration: A bay in the district which has begun slowly rising.]

If it were possible to trace any one of the many beach-lines, composing
each sloping terrace, it would of course be horizontal; but the only
lines of demarcation are the summit and basal edges of the escarpments.
Now the summit-edge of one of these escarpments marks the furthest line
or point to which the sea has cut into a mass of gravel sloping
seaward; and as the sea will generally have greater power at the mouth
than at the protected head of the bay, so will the escarpment at the
mouth be cut deeper into the land, and its summit-edge be higher;
consequently it will not be horizontal. With respect to the basal or
lower edges of the escarpments, from picturing in one’s mind ancient
bays _ entirely_ surrounded at successive periods by cliff-formed
shores, one’s first impression is that they at least necessarily must
be horizontal, if the elevation has been horizontal. But here is a
fallacy: for after the sea has, during a cessation of the elevation,
worn cliffs all round the shores of a bay, when the movement
recommences, and especially if it recommences slowly, it might well
happen that, at the exposed mouth of the bay, the waves might continue
for some time wearing into the land, whilst in the protected and upper
parts
successive beach-lines might be accumulating in a sloping surface or
terrace at the foot of the cliffs which had been lately reached: hence,
supposing the whole line of escarpment to be finally uplifted above the
reach of the sea, its basal line or foot near the mouth will run at a
lower level than in the upper and protected parts of the bay;
consequently this basal line will not be horizontal. And it has already
been shown that the summit-edges of each escarpment will generally be
higher near the mouth (from the seaward sloping land being there most
exposed and cut into) than near the head of the bay; therefore the
total height of the escarpments will be greatest near the mouth; and
further up the old bay or valley they will on both sides generally thin
out and die away: I have observed this thinning out of the successive
escarpment at other places besides Coquimbo; and for a long time I was
quite unable to understand its meaning. The rude diagram in Figure 11
will perhaps render what I mean more intelligible; it represents a bay
in a district which has begun slowly rising. Before the movement
commenced, it is supposed that the waves had been enabled to eat into
the land and form cliffs, as far up, but with gradually diminishing
power, as the points AA: after the movement had commenced and gone on
for a little time, the sea is supposed still to have retained the
power, at the exposed mouth of the bay, of cutting down and into the
land as it slowly emerged; but in the upper parts of the bay it is
supposed soon to have lost this power, owing to the more protected
situation and to the quantity of detritus brought down by the river;
consequently low land was there accumulated. As this low land was
formed during a slow elevatory movement, its surface will gently slope
upwards from the beach on all sides. Now, let us imagine the bay, not
to make the diagram more complicated, suddenly converted into a valley:
the basal line of the cliffs will of course be horizontal, as far as
the beach is now seen extending in the diagram; but in the upper part
of the valley, this line will be higher, the level of the district
having been raised whilst the low land was accumulating at the foot of
the inland cliffs. If, instead of the bay in the diagram being suddenly
converted into a valley, we suppose with much more probability it to be
upraised slowly, then the waves in the upper parts of the bay will
continue very gradually to fail to reach the cliffs, which are now in
the diagram represented as washed by the sea, and which, consequently,
will be left standing higher and higher above its level; whilst at the
still exposed mouth, it might well happen that the waves might be
enabled to cut deeper and deeper, both down and into the cliffs, as the
land slowly rose.

The greater or lesser destroying power of the waves at the mouths of
successive bays, comparatively with this same power in their upper and
protected parts, will vary as the bays become changed in form and size,
and therefore at different levels, at their mouths and heads, more or
less of the surfaces between the escarpments (that is, the accumulated
beach-lines or terraces) will be left undestroyed: from what has gone
before we can see that, according as the elevatory movements after each
cessation recommence more or less slowly, according to the amount of
detritus delivered by the river at the heads of the successive bays,
and according to the degree of protection afforded by their altered
forms, so will a greater or less extent of terrace be accumulated in
the upper part, to which there will be no surface at a corresponding
level at the mouth: hence we can perceive why no one terrace, taken in
its whole breadth and followed up the valley, is horizontal, though
each separate beach-line must have been so; and why the inclination of
the several terraces, both transversely, and longitudinally up the
valley, is not alike.

I have entered into this case in some detail, for I was long perplexed
(and others have felt the same difficulty) in understanding how, on the
idea of an equable elevation with the sea at intervals eating into the
land, it came that neither the terraces nor the upper nor lower edges
of the escarpments were horizontal. Along lines of coast, even of great
lengths, such as that of Patagonia, if they are nearly uniformly
exposed, the corroding power of the waves will be checked and conquered
by the elevatory movement, as often as it recommences, at about the
same period; and hence the terraces, or accumulated beach-lines, will
commence being formed at nearly the same levels: at each succeeding
period of rest, they will, also, be eaten into at nearly the same rate,
and consequently there will be a much closer coincidence in their
levels and inclinations, than in the terraces and escarpments formed
round bays with their different parts very differently exposed to the
action of the sea. It is only where the waves are enabled, after a long
lapse of time, slowly to corrode hard rocks, or to throw up, owing to
the supply of sediment being small and to the surface being steeply
inclined, a narrow beach or mound, that we can expect, as at Glen Roy
in Scotland,[19] a distinct line marking an old sea-level, and which
will be strictly horizontal, if the subsequent elevatory movements have
been so: for in these cases no discernible effects will be produced,
except during the long intervening periods of rest; whereas in the case
of step-formed coasts, such as those described in this and the
preceding chapter, the terraces themselves are accumulated during the
slow elevatory process, the accumulation commencing sooner in protected
than in exposed situations, and sooner where there is copious supply of
detritus than where there is little; on the other hand, the steps or
escarpments are formed during the stationary periods, and are more
deeply cut down and into the coast-land in exposed than in protected
situations;—the cutting action, moreover, being prolonged in the most
exposed parts, both during the beginning and ending, if slow, of the
upward movement.

 [19] “Philosophical Transactions,” 1839, p. 39.


Although in the foregoing discussion I have assumed the elevation to
have been horizontal, it may be suspected, from the considerable
seaward slope of the terraces, both up the valley of S. Cruz and up
that of Coquimbo, that the rising has been greater inland than nearer
the coast. There is reason to believe,[20] from the effects produced on
the water-course of a mill during the earthquake of 1822 in Chile, that
the upheaval one mile inland was nearly double, namely, between five
and seven feet, to what it was on the Pacific. We know, also, from the
admirable researches of M. Bravais,[21] that in Scandinavia the ancient
sea-beaches gently slope from the interior mountain-ranges towards the
coast, and that they are not parallel one to the other showing that the
proportional difference in the amount of elevation on the coast and in
the interior, varied at different periods.

 [20] Mr. Place in the _Quarterly Journal of Science,_ 1824, vol. xvii,
 p. 42.


 [21] “Voyages de la Comm. du Nord,” etc., also “Comptes Rendus,” Oct.
 1842.

_Coquimbo to Guasco._—In this distance of ninety miles, I found in
almost every part marine shells up to a height of apparently from two
hundred to three hundred feet. The desert plain near Choros is thus
covered; it is bounded by the escarpment of a higher plain, consisting
of pale-coloured, earthy, calcareous stone, like that of Coquimbo, with
the same recent shells embedded in it. In the valley of Chaneral, a
similar bed occurs in which, differently from that of Coquimbo, I
observed many shells of the Concholepas: near Guasco the same
calcareous bed is likewise met with.

In the valley of Guasco, the step-formed terraces of gravel are
displaced in a more striking manner than at any other point. I followed
the valley for thirty-seven miles (as reckoned by the inhabitants) from
the coast to Ballenar; in nearly the whole of this distance, five grand
terraces, running at corresponding heights on both sides of the broad
valley, are more conspicuous than the three best-developed ones at
Coquimbo. They give to the landscape the most singular and formal
aspect; and when the clouds hung low, hiding the neighbouring
mountains, the valley resembled in the most striking manner that of
Santa Cruz. The whole thickness of these terraces or plains seems
composed of gravel, rather firmly aggregated together, with occasional
parting seams of clay: the pebbles on the upper plain are often
whitewashed with an aluminous substance, as in Patagonia. Near the
coast I observed many sea-shells on the lower plains. At Freyrina
(twelve miles up the valley), there are six terraces beside the
bottom-surface of the valley: the two lower ones are here only from two
hundred to three hundred yards in width, but higher up the valley they
expand into plains; the third terrace is generally narrow; the fourth I
saw only in one place, but there it was distinct for the length of a
mile; the fifth is very broad; the sixth is the summit-plain, which
expands inland into a great basin. Not having a barometer with me, I
did not ascertain the height of these plains, but they appeared
considerably higher than those at Coquimbo. Their width varies much,
sometimes being very broad, and sometimes contracting into mere fringes
of separate flat-topped projections, and then quite disappearing: at
the one spot, where the fourth terrace was visible, the whole six
terraces were cut off for a short space by one single bold escarpment.
Near Ballenar (thirty-seven miles from the mouth of the river), the
valley between the summit-edges of the highest escarpments is several
miles in width, and the five terraces on both sides are broadly
developed: the highest cannot be less than six hundred feet above the
bed of the river, which itself must, I conceive, be some hundred feet
above the sea.

No. 12
North and south section across the valley of Guasco, and of a plain
north of it.


[Illustration: North and south section across the valley of Guasco.]

On the northern side of the valley the summit-plain of gravel (A) has
two escarpments, one facing the valley, and the other a great
basin-like plain (B), which stretches for several leagues northward.
This narrow plain (A) with the double escarpment, evidently once formed
a spit or promontory of gravel, projecting into and dividing two great
bays, and subsequently was worn on both sides into steep cliffs.
Whether the several escarpments in this valley were formed during the
same stationary periods with those of Coquimbo, I will not pretend to
conjecture; but if so the intervening and subsequent elevatory
movements must have been here much more energetic, for these plains
certainly stand at a much higher level than do those of Coquimbo.

_Copiapo._—From Guasco to Copiapo, I followed the road near the foot of
the Cordillera, and therefore saw no upraised remains. At the mouth,
however, of the valley of Copiapo there is a plain, estimated by
Meyen[22] between fifty and seventy feet in height, of which the upper
part consists chiefly of gravel, abounding with recent shells, chiefly
of the Concholepas, _Venus Dombeyi_, and _Calyptræa trochiformis._ A
little
inland, on a plain estimated by myself at nearly three hundred feet,
the upper stratum was formed of broken shells and sand cemented by
white calcareous matter, and abounding with embedded recent shells, of
which the _Mulinia Byronensis_ and _Pecten purpuratus_ were the most
numerous. The lower plain stretches for some miles southward, and for
an unknown distance northward, but not far up the valley; its seaward
face, according to Meyen, is worn into caves above the level of the
present beach. The valley of Copiapo is much less steeply inclined and
less direct in its course than any other valley which I saw in Chile;
and its bottom does not generally consist of gravel: there are no
step-formed terraces in it, except at one spot near the mouth of the
great lateral valley of the Despoblado where there are only two, one
above the other: lower down the valley, in one place I observed that
the solid rock had been cut into the shape of a beach, and was smoothed
over with shingle.

 [22] “Reise um die Erde,” Th. I, s. 372, _et seq._


Northward of Copiapo, in lat. 26° S., the old voyager Wafer[23] found
immense numbers of sea-shells some miles from the coast. At Cobija
(lat. 22° 34′) M. d’Orbigny observed beds of gravel and broken shells,
containing ten species of recent shells; he also found, on projecting
points of porphyry, at a height of 300 feet, shells of Concholepas,
Chiton, Calyptræa, Fissurella, and Patella, still attached to the spots
on which they had lived. M. d’Orbigny argues from this fact, that the
elevation must have been great and sudden:[24] to me it appears far
more probable that the movement was gradual, with small starts as
during the earthquakes of 1822 and 1835, by which whole beds of shells
attached to the rocks were lifted above the subsequent reach of the
waves. M. d’Orbigny also found rolled pebbles extending up the mountain
to a height of at least six hundred feet. At Iquique (lat. 20° 12′ S.),
in a great accumulation of sand, at a height estimated between one
hundred and fifty and two hundred feet, I observed many large
sea-shells which I thought could not have been blown up by the wind to
that height. Mr. J. H. Blake has lately[25] described these
shells: he states that “inland toward the mountains they form a compact
uniform bed, scarcely a trace of the original shells being discernible;
but as we approach the shore, the forms become gradually more distinct
till we meet with the living shells on the coast.” This interesting
observation, showing by the gradual decay of the shells how slowly and
gradually the coast must have been uplifted, we shall presently see
fully confirmed at Lima. At Arica (lat. 18° 28′), M. d’Orbigny[26]
found a great range of sand-dunes, fourteen leagues in length,
stretching towards Tacna, including recent shells and bones of Cetacea,
and reaching up to a height of 300 feet above the sea. Lieutenant
Freyer has given some more precise facts: he states[27] that the Morro
of Arica is about four hundred feet high; it is worn into obscure
terraces, on the bare rock of which he found Balini and Milleporæ
adhering. At the height of between twenty and thirty feet the shells
and corals were in a quite fresh state, but at fifty feet they were
much abraded; there were, however, traces of organic remains at greater
heights. On the road from Tacna to Arequipa, between Loquimbo and
Moquegua, Mr. M. Hamilton[28] found numerous recent sea shells in sand,
at a considerable distance from the sea.

 [23] Burnett’s “Collection of Voyages,” vol. iv, p. 193.


 [24] “Voyage, Part Géolog.,” p. 94. M. d’Orbigny (p. 98), in summing
 up, says: “S’il est certain (as he believes) que tous les terrains en
 pente, compris entre la mer et les montagnes sont l’ancien rivage de
 la mer, on doit supposer, pour l’ensemble, un exhaussement que ce ne
 serait pas moindre de deux cent mètres; il faudrait supposer encore
 que ce soulèvement n’a point été graduel; . . . mais qu’il résulterait
 d’une seule et même cause fortuite,” etc. Now, on this view, when the
 sea was forming the beach at the foot of the mountains, many shells of
 Concholepas, Chiton, Calyptræa, Fissurella, and Patella (which are
 known to live close to the beach), were attached to rocks at a depth
 of 300 feet, and at a depth of 600 feet several of these same shells
 were accumulating in great numbers in horizontal beds. From what I
 have myself seen in dredging, I believe this to be improbable in the
 highest degree, if not impossible; and I think everyone who has read
 Professor E. Forbes’s excellent researches on the subject, will
 without hesitation agree in this conclusion.


 [25] _Silliman’s Amer. Journ. of Science,_ vol. xliv, p. 2.


 [26] “Voyage,” etc., p. 101.


 [27] In a letter to Mr. Lyell, “Geolog. Proc.,” vol. ii, p. 179.


 [28] _Edin. New Phil. Journ.,_ vol. xxx, p. 155.

_Lima._

Northward of Arica, I know nothing of the coast for about a space of
five degrees of latitude; but near Callao, the port of Lima, there is
abundant and very curious evidence of the elevation of the land. The
island of San Lorenzo is upwards of one thousand feet high; the basset
edges of the strata composing the lower part are worn into three
obscure, narrow, sloping steps or ledges, which can be seen only when
standing on them: they probably resemble those described by Lieutenant
Freyer at Arica. The surface of the lower ledge, which extends from a
low cliff overhanging the sea to the foot of the next upper escarpment,
is covered by an enormous accumulation of recent shells.[29] The bed is
level, and in some parts more than two feet in thickness; I traced it
over a space of one mile in length, and heard of it in other places:
the uppermost part is eighty-five feet by the barometer above
high-water mark. The shells are packed together, but not stratified:
they are mingled with earth and stones, and are generally covered by a
few inches of detritus; they rest on a mass of nearly angular fragments
of the underlying sandstone, sometimes cemented together by common
salt. I collected eighteen species of shells of all ages and sizes.
Several of the univalves had evidently long lain dead at the bottom of
the sea, for their _ insides_ were incrusted with Balani and Serpulæ.
All, according to Mr. G.B. Sowerby, are recent species: they consist
of:—

Mytilus Magellanicus: same as that found at Valparaiso, and there
stated to be probably distinct from the true _M. Magellanicus_ of the
east coast.

Venus costellata, Sowerby “Zoological Proceedings.”

Pecten purpuratus, Lam.

Chama, probably echinulata, Brod.

Calyptræa Byronensis, Gray.

Calyptræa radians (Trochus, Lam.)

Fissurella affinis, Gray.

Fissurella biradiata, Trembly.

Purpura chocolatta, Duclos.

Purpura Peruviana, Gray.

Purpura labiata, Gray.

Purpura buxea (Murex, Brod.).

Concholepas Peruviana.

Nassa, related to reticulata.

Triton rudis, Brod.

Trochus, not yet described, but well-known and very common.

and 18. Balanus, two species, both common on the coast.


 [29] M. Chevalier, in the “Voyage of the _Bonite_,” observed these
 shells; but his specimens were lost.—“L’Institut,” 1838, p. 151.


These upraised shells appear to be nearly in the same proportional
numbers—with the exception of the Crepidulæ being more numerous—with
those on the existing beach. The state of preservation of the different
species differed much; but most of them were much corroded, brittle,
and bleached: the upper and lower surfaces of the Concholepas had
generally quite scaled off: some of the Trochi and Fissurellæ still
partially retain their colours. It is remarkable that these shells,
taken all together, have fully as ancient an appearance, although the
extremely arid climate appears highly favourable for their
preservation, as those from 1,300 feet at Valparaiso, and certainly a
more ancient appearance than those from five to six hundred feet from
Valparaiso and Concepcion; at which places I have seen grass and other
vegetables actually growing out of the shells. Many of the univalves
here at San Lorenzo were filled with, and united together by, pure
salt, probably left by the evaporation of the sea-spray, as the land
slowly emerged.[30] On the highest parts of the ledge, small fragments
of the shells were mingled with, and evidently in process of reduction
into, a yellowish-white, soft, calcareous powder, tasting strongly of
salt, and in some places as fine as prepared medicinal chalk.

 [30] The underlying sandstone contains true layers of salt; so that
 the salt may possibly have come from the beds in the higher parts of
 the island; but I think more probably from the sea-spray. It is
 generally asserted that rain never falls on the coast of Peru; but
 this is not quite accurate; for, on several days, during our visit,
 the so-called Peruvian dew fell in sufficient quantity to make the
 streets muddy, and it would certainly have washed so deliquescent a
 substance as salt into the soil. I state this because M. d’Orbigny, in
 discussing an analogous subject, supposes that I had forgotten that it
 never rains on this whole line of coast. See Ulloa’s “Voyage” (vol.
 ii, Eng. Trans., p. 67) for an account of the muddy streets of Lima,
 and on the continuance of the mists during the whole winter. Rain,
 also, falls at rare intervals even in the driest districts, as, for
 instance, during forty days, in 1726, at Chocope (7° 46′); this rain
 entirely ruined (“Ulloa,” etc., p. 18) the mud houses of the
 inhabitants.


_Fossil-remains of human art._—In the midst of these shells on San
Lorenzo, I found light corallines, the horny ovule-cases of Mollusca,
roots of seaweed,[31] bones of birds, the heads of Indian corn and
other vegetable matter, a piece of woven rushes, and another of nearly
decayed _cotton_ string. I extracted these remains by digging a hole,
on a level spot; and they had all indisputably been embedded with the
shells. I compared the plaited rush, the _cotton_ string, and Indian
corn, at the house of an antiquary, with similar objects, taken from
the Huacas or burial-grounds of the ancient Peruvians, and they were
undistinguishable; it should be observed that the Peruvians used string
only of cotton. The small quantity of sand or gravel with the shells,
the absence of large stones, the width and thickness of the bed, and
the time requisite for a ledge to be cut into the sandstone, all show
that these remains were not thrown high up by an earthquake-wave: on
the other hand, these facts, together with the number of dead shells,
and of floating objects, both marine and terrestrial, both natural and
human, render it almost certain that they were accumulated on a true
beach, since upraised eighty-five feet, and upraised this much since
_Indian man inhabited Peru._ The elevation may have been, either by
several small sudden starts, or quite gradual; in this latter case the
unrolled shells having been thrown up during gales beyond the reach of
the waves which afterwards broke on the slowly emerging land. I have
made these remarks, chiefly because I was at first surprised at the
complete difference in nature, between this broad, smooth, upraised bed
of shells, and the present shingle-beach at the foot of the low
sandstone-cliffs; but a beach formed, when the sea is cutting into the
land, as is shown now to be the case by the low bare sandstone-cliffs,
ought not to be compared with a beach accumulated on a gently inclined
rocky surface, at a period when the sea (probably owing to the
elevatory movement in process) was not able to eat into the land. With
respect to the mass of nearly angular, salt-cemented fragments of
sandstone, which lie under the shells, and which are so unlike the
materials of an ordinary sea-beach; I think it probable after having
seen the remarkable effects[32] of the earthquake of 1835, in
absolutely shattering as if by gunpowder the _surface_ of the primary
rocks near Concepcion, that a smooth bare surface of stone was left by
the sea covered by the shelly mass, and that afterwards when upraised,
it was superficially shattered by the severe shocks so often
experienced here.

 [31] Mr. Smith of Jordan Hill found pieces of seaweed in an upraised
 pleistocene deposit in Scotland. See his admirable Paper in the _Edin.
 New Phil. Journal,_ vol. xxv, p. 384.


 [32] I have described this in my “Journal of Researches,” p. 303, 2nd
 edit.

The very low land surrounding the town of Callao, is to the south
joined by an obscure escarpment to a higher plain (south of Bella
Vista), which stretches along the coast for a length of about eight
miles. This plain appears to the eye quite level; but the sea-cliffs
show that its height varies (as far as I could estimate) from seventy
to one hundred and twenty feet. It is composed of thin, sometimes
waving, beds of clay, often of bright red and yellow colours, of layers
of impure sand, and in one part with a great stratified mass of
granitic pebbles. These
beds are capped by a remarkable mass, varying from two to six feet in
thickness, of reddish loam or mud, containing many scattered and broken
fragments of recent marine shells, sometimes though rarely single large
round pebble, more frequently short irregular layers of fine gravel,
and very many pieces of red coarse earthenware, which from their
curvatures must once have formed parts of large vessels. The
earthenware is of Indian manufacture; and I found exactly similar
pieces accidentally included within the bricks, of which the
neighbouring ancient Peruvian burial-mounds are built. These fragments
abounded in such numbers in certain spots, that it appeared as if
waggon-loads of earthenware had been smashed to pieces. The broken
sea-shells and pottery are strewed both on the surface, and throughout
the whole thickness of this upper loamy mass. I found them wherever I
examined the cliffs, for a space of between two and three miles, and
for half a mile inland; and there can be little doubt that this same
bed extends with a smooth surface several miles further over the entire
plain. Besides the little included irregular layers of small pebbles,
there are occasionally very obscure traces of stratification.

At one of the highest parts of the cliff, estimated 120 feet above the
sea, where a little ravine came down, there were two sections, at right
angles to each other, of the floor of a shed or building. In both
sections or faces, two rows, one over the other, of large round stones
could be distinctly seen; they were packed close together on an
artificial layer of sand two inches thick, which had been placed on the
natural clay-beds; the round stones were covered by three feet in
thickness of the loam with broken sea-shells and pottery. Hence, before
this widely spread-out bed of loam was deposited, it is certain that
the plain was inhabited; and it is probable, from the broken vessels
being so much more abundant in certain spots than in others, and from
the underlying clay being fitted for their manufacture, that the kilns
stood here.

The smoothness and wide extent of the plain, the bulk of matter
deposited, and the obscure traces of stratification seem to indicate
that the loam was deposited under water; on the other hand, the
presence of sea-shells, their broken state, the pebbles of various
sizes, and the artificial floor of round stones, almost prove that it
must have originated in a rush of water from the sea over the land. The
height of the plain, namely, 120 feet, renders it improbable that an
earthquake-wave, vast as some have here been, could have broken over
the surface at its present level; but when the land stood eighty-five
feet lower, at the period when the shells were thrown up on the ledge
at S. Lorenzo, and when as we know man inhabited this district, such an
event might well have occurred; and if we may further suppose, that the
plain was at that time converted into a temporary lake, as actually
occurred, during the earthquakes of 1713 and 1746, in the case of the
low land round Callao owing to its being encircled by a high
shingle-beach, all the appearances above described will be perfectly
explained. I must add, that at a lower level near the point where the
present low land round Callao joins the higher plain, there are
appearances of two
distinct deposits both apparently formed by debacles: in the upper one,
a horse’s tooth and a dog’s jaw were embedded; so that both must have
been formed after the settlement of the Spaniards: according to Acosta,
the earthquake-wave of 1586 rose eighty-four feet.

The inhabitants of Callao do not believe, as far as I could ascertain,
that any change in level is now in progress. The great fragments of
brickwork, which it is asserted can be seen at the bottom of the sea,
and which have been adduced as a proof of a late subsidence, are, as I
am informed by Mr. Gill, a resident engineer, loose fragments; this is
probable, for I found on the beach, and not near the remains of any
building, masses of brickwork, three and four feet square, which had
been washed into their present places, and smoothed over with shingle
during the earthquake of 1746. The spit of land, on which the ruins of
_Old_ Callao stand, is so extremely low and narrow, that it is
improbable in the highest degree that a town should have been founded
on it in its present state; and I have lately heard[33] that M. Tschudi
has come to the conclusion, from a comparison of old with modern
charts, that the coast both south and north of Callao has subsided. I
have shown that the island of San Lorenzo has been upraised eighty-five
feet since the Peruvians inhabited this country; and whatever may have
been the amount of recent subsidence, by so much more must the
elevation have exceeded the eighty-five feet. In several places[34] in
this neighbourhood, marks of sea-action have been observed: Ulloa gives
a detailed account of such appearances at a point five leagues
northward of Callao: Mr. Cruikshank found near Lima successive lines of
sea-cliffs, with rounded blocks at their bases, at a height of 700 feet
above the present level of the sea.

 [33] I am indebted for this fact to Dr. E. Dieffenbach. I may add that
 there is a tradition, that the islands of San Lorenzo and Fronton were
 once joined, and that the channel between San Lorenzo and the
 mainland, now above two miles in width, was so narrow that cattle used
 to swim over.


 [34] “Observaciones sobre el Clima del Lima” par Dr. H. Unanùe, p.
 4.—Ulloa’s “Voyage,” vol. ii, Eng. Trans., p. 97.—For Mr. Cruikshank’s
 observations, see Mr. Lyell’s “Principles of Geology” (1st edition)
 vol. iii, p. 130.


_On the decay of upraised sea-shells._—I have stated that many of the
shells on the lower inclined ledge or terrace of San Lorenzo are
corroded in a peculiar manner, and that they have a much more ancient
appearance than the same species at considerably greater heights on the
coast of Chile. I have, also, stated that these shells in the upper
part of the ledge, at the height of eighty-five feet above the sea, are
falling, and in some parts are quite changed into a fine, soft, saline,
calcareous powder. The finest part of this powder has been analysed for
me, at the request of Sir H. De la Beche, by the kindness of Mr.
Trenham Reeks of the Museum of Economic Geology; it consists of
carbonate of lime in abundance, of sulphate and muriate of lime, and of
muriate and sulphate of soda. The carbonate of lime is obviously
derived from the shells; and common salt is so abundant in parts of
the bed, that, as before remarked, the univalves are often filled with
it. The sulphate of lime may have been derived, as has probably the
common salt, from the evaporation of the sea-spray, during the
emergence of the land; for sulphate of lime is now copiously deposited
from the spray on the shores of Ascension.[35] The other saline bodies
may perhaps have been partially thus derived, but chiefly, as I
conclude from the following facts, through a different means.

 [35] See “Volcanic Islands,” etc., by the Author.


On most parts of the second ledge or old sea-beach, at a height of 170
feet, there is a layer of white powder of variable thickness, as much
in some parts as two inches, lying on the angular, salt-cemented
fragments of sandstone and under about four inches of earth, which
powder, from its close resemblance in nature to the upper and most
decayed parts of the shelly mass, I can hardly doubt originally existed
as a bed of shells, now much collapsed and quite disintegrated. I could
not discover with the microscope a trace of organic structure in it;
but its chemical constituents, according to Mr. Reeks, are the same as
in the powder extracted from amongst the decaying shells on the lower
ledge, with the marked exception that the carbonate of lime is present
in only very small quantity. On the third and highest ledge, I observed
some of this powder in a similar position, and likewise occasionally in
small patches at considerably greater heights near the summit of the
island. At Iquique, where the whole face of the country is covered by a
highly saliferous alluvium, and where the climate is extremely dry, we
have seen that, according to Mr. Blake, the shells which are perfect
near the beach become, in ascending, gradually less and less perfect,
until scarcely a trace of their original structure can be discovered.
It is known that carbonate of lime and common salt left in a mass
together,[36] and slightly moistened, partially decompose each other:
now we have at San Lorenzo and at Iquique, in the shells and salt
packed together, and occasionally moistened by the so-called Peruvian
dew, the proper elements for this action. We can thus understand the
peculiar corroded appearance of the shells on San Lorenzo, and the
great decrease of quantity in the carbonate of lime in the powder on
the upper ledge. There is, however, a great difficulty on this view,
for the resultant salts should be carbonate of soda and muriate of
lime; the latter is present, but not the carbonate of soda. Hence I am
led to the perhaps unauthorised conjecture (which I shall hereafter
have to refer to) that the carbonate of soda, by some unexplained
means, becomes converted into a sulphate. If the above remarks be just,
we are led to the very unexpected conclusion, that a dry climate, by
leaving the salt from the sea-spray
undissolved, is much less favourable to the preservation of upraised
shells than a humid climate. However this may be, it is interesting to
know the manner in which masses of shells, gradually upraised above the
sea-level, decay and finally disappear.

 [36] I am informed by Dr. Kane, through Mr. Reeks, that a manufactory
 was established on this principle in France, but failed from the small
 quantity of carbonate of soda produced. Sprengel (_Gardeners’ Chron.,_
 1845, p. 157) states, that salt and carbonate of lime are liable to
 mutual decomposition in the soil. Sir H. De la Beche informs me, that
 calcareous rocks washed by the spray of the sea, are often corroded in
 a peculiar manner; see also on this latter subject _Gardeners’
 Chron.,_ p. 675, 1844.


_Summary on the recent elevation of the west coast of South
America._—We have seen that upraised marine remains occur at intervals,
and in some parts almost continuously, from lat. 45° 35′ to 12° S.,
along the shores of the Pacific. This is a distance, in a north and
south line, of 2,075 geographical miles. From Byron’s observations, the
elevation has no doubt extended sixty miles further south; and from the
similarity in the form of the country near Lima, it has probably
extended many leagues further north.[37] Along this great line of
coast, besides the organic remains, there are in very many parts, marks
of erosion, caves, ancient beaches, sand-dunes, and successive terraces
of gravel, all above the present level of the sea. From the steepness
of the land on this side of the continent, shells have rarely been
found at greater distances inland than from two to three leagues; but
the marks of sea-action are evident farther from the coast; for
instance, in the valley of Guasco, at a distance of between thirty and
forty miles. Judging from the upraised shells alone, the elevation in
Chiloe has been 350 feet, at Concepcion certainly 625 feet; and by
estimation 1,000 feet; at Valparaiso 1,300 feet; at Coquimbo 252 feet;
northward of this place, sea-shells have not, I believe, been found
above 300 feet; and at Lima they were falling into decay (hastened
probably by the salt) at 85 feet. Not only has this amount of elevation
taken place within the period of existing Mollusca and Cirripedes; but
their proportional numbers in the neighbouring sea have in most cases
remained the same. Near Lima, however, a small change in this respect
between the living and the upraised was observed: at Coquimbo this was
more evident, all the shells being existing species, but with those
embedded in the uppermost calcareous plain not approximating so closely
in proportional numbers, as do those that lie loose on its surface at
the height of 252 feet, and still less closely than those which are
strewed on the lower plains, which latter are identical in proportional
numbers with those now cast up on the beach. From this circumstance,
and from not finding, upon careful examination, near Coquimbo any
shells at a greater height than 252 feet, I believe that the recent
elevation there has been much less than at Valparaiso, where it has
been 1,300 feet, and I may add, than at Concepcion. This considerable
inequality in the amount of elevation at Coquimbo and Valparaiso,
places only 200 miles apart, is not improbable, considering, first, the
difference in the force and number of the shocks now yearly affecting
different parts of this coast; and, secondly, the fact of single areas,
such as that of the province of Concepcion, having been uplifted very
unequally during the same earthquake. It would, in most cases, be very
hazardous to infer an inequality
of elevation, from shells being found on the surface or in superficial
beds at different heights; for we do not know on what their rate of
decay depends; and at Coquimbo one instance out of many has been given,
of a promontory, which, from the occurrence of one very small
collection of lime-cemented shells, has indisputably been elevated 242
feet, and yet on which, not even a fragment of shell could be found on
careful examination between this height and the beach, although many
sites appeared very favourable for the preservation of organic remains:
the absence, also, of shells on the gravel-terraces a short distance up
the valley of Coquimbo, though abundant on the corresponding terraces
at its mouth, should be borne in mind.

 [37] I may take this opportunity of stating that in a MS. in the
 Geological Society by Mr. Weaver, it is stated that beds of oysters
 and other recent shells are found thirty feet above the level of the
 sea, in many parts of Tampico, in the Gulf of Mexico.

There are other epochs, besides that of the existence of recent
Mollusca, by which to judge of the changes of level on this coast. At
Lima, as we have just seen, the elevation has been at least eighty-five
feet, within the Indo-human period; and since the arrival of the
Spaniards in 1530, there has apparently been a sinking of the surface.
At Valparaiso, in the course of 220 years, the rise must have been less
than nineteen feet; but it has been as much as from ten to eleven feet
in the seventeen years subsequently to 1817, and of this rise only a
part can be attributed to the earthquake of 1822, the remainder having
been insensible and apparently still, in 1834, in progress. At Chiloe
the elevation has been gradual, and about four feet during four years.
At Coquimbo, also, it has been gradual, and in the course of 150 years
has amounted to several feet. The sudden small upheavals, accompanied
by earthquakes, as in 1822 at Valparaiso, in 1835 at Concepcion, and in
1837 in the Chonos Archipelago, are familiar to most geologists, but
the gradual rising of the coast of Chile has been hardly noticed; it
is, however, very important, as connecting together these two orders of
events.

The rise of Lima, having been eighty-five feet within the period of
man, is the more surprising if we refer to the eastern coast of the
continent, for at Port S. Julian, in Patagonia, there is good evidence
(as we shall hereafter see) that when the land stood ninety feet lower,
the Macrauchenia, a mammiferous beast, was alive; and at Bahia Blanca,
when it stood only a few feet lower than it now does, many gigantic
quadrupeds ranged over the adjoining country. But the coast of
Patagonia is some way distant from the Cordillera, and the movement at
Bahia Blanca is perhaps noways connected with this great range, but
rather with the tertiary volcanic rocks of Banda Oriental, and
therefore the elevation at these places may have been infinitely slower
than on the coast of Peru. All such speculations, however, must be
vague, for as we know with certainty that the elevation of the whole
coast of Patagonia has been interrupted by many and long pauses, who
will pretend to say that, in such cases, many and long periods of
subsidence may not also have been intercalated?

In many parts of the coast of Chile and Peru there are marks of the
action of the sea at successive heights on the land, showing that the
elevation has been interrupted by periods of comparative rest in the
upward movement, and of denudation in the action of the sea. These
are plainest at Chiloe, where, in a height of about five hundred feet,
there are three escarpments,—at Coquimbo, where in a height of 364
feet, there are five,—at Guasco, where there are six, of which five may
perhaps correspond with those at Coquimbo, but if so, the subsequent
and intervening elevatory movements have been here much more
energetic,—at Lima, where, in a height of about 250 feet there are
three terraces, and others, as it is asserted, at considerably greater
heights. The almost entire absence of ancient marks of sea-action at
defined levels along considerable spaces of coast, as near Valparaiso
and Concepcion, is highly instructive, for as it is improbable that the
elevation at these places alone should have been continuous, we must
attribute the absence of such marks to the nature and form of the
coast-rocks. Seeing over how many hundred miles of the coast of
Patagonia, and on how many places on the shores of the Pacific, the
elevatory process has been interrupted by periods of comparative rest,
we may conclude, conjointly with the evidence drawn from other quarters
of the world, that the elevation of the land is generally an
intermittent action. From the quantity of matter removed in the
formation of the escarpments, especially of those of Patagonia, it
appears that the periods of rest in the movement, and of denudation of
the land, have generally been very long. In Patagonia, we have seen
that the elevation has been equable, and the periods of denudation
synchronous over very wide spaces of coast; on the shores of the
Pacific, owing to the terraces chiefly occurring in the valleys, we
have not equal means of judging on this point; and the very different
heights of the upraised shells at Coquimbo, Valparaiso, and Concepcion
seem directly opposed to such a conclusion.

Whether on this side of the continent the elevation, between the
periods of comparative rest when the escarpments were formed, has been
by small sudden starts, such as those accompanying recent earthquakes,
or, as is most probable, by such starts conjointly with a gradual
upward movement, or by great and sudden upheavals, I have no direct
evidence. But as on the eastern coast, I was led to think, from the
analogy of the last hundred feet of elevation in La Plata, and from the
nearly equal size of the pebbles over the entire width of the terraces,
and from the upraised shells being all littoral species, that the
elevation had been gradual; so do I on this western coast, from the
analogy of the movements now in progress, and from the vast numbers of
shells now living exclusively on or close to the beach, which are
strewed over the whole surface of the land up to very considerable
heights, conclude, that the movement here also has been slow and
gradual, aided probably by small occasional starts. We know at least
that at Coquimbo, where five escarpments occur in a height of 364 feet,
the successive elevations, if they have been sudden, cannot have been
very great. It has, I think, been shown that the occasional
preservation of shells, unrolled and unbroken, is not improbable even
during a quite gradual rising of the land; and their preservation, if
the movement has been aided by small starts, is quite conformable with
what actually takes place during recent earthquakes.


Judging from the present action of the sea, along the shores of the
Pacific, on the deposits of its own accumulation, the present time
seems in most places to be one of comparative rest in the elevatory
movement, and of denudation of the land. Undoubtedly this is the case
along the whole great length of Patagonia. At Chiloe, however, we have
seen that a narrow sloping fringe, covered with vegetation, separates
the present sea-beach from a line of low cliffs, which the waves lately
reached; here, then, the land is gaining in breadth and height, and the
present period is not one of rest in the elevation and of contingent
denudation; but if the rising be not prolonged at a quick rate, there
is every probability that the sea will soon regain its former
horizontal limits. I observed similar low sloping fringes on several
parts of the coast, both northward of Valparaiso and near Coquimbo; but
at this latter place, from the change in form which the coast has
undergone since the old escarpments were worn, it may be doubted
whether the sea, acting for any length of time at its present level,
would eat into the land; for it now rather tends to throw up great
masses of sand. It is from facts such as these that I have generally
used the term _comparative rest_, as applied to the elevation of the
land; the rest or cessation in the movement being comparative both with
what has preceded it and followed it, and with the sea’s power of
corrosion at each spot and at each level. Near Lima, the cliff-formed
shores of San Lorenzo, and on the mainland south of Callao, show that
the sea is gaining on the land; and as we have here some evidence that
its surface has lately subsided or is still sinking, the periods of
comparative rest in the elevation and of contingent denudation, may
probably in many cases include periods of subsidence. It is only, as
was shown in detail when discussing the terraces of Coquimbo, when the
sea with difficulty and after a long lapse of time has either corroded
a narrow ledge into solid rock, or has heaped up on a steep surface a
_narrow_ mound of detritus, that we can confidently assert that the
land at that level and at that period long remained absolutely
stationary. In the case of terraces formed of gravel or sand, although
the elevation may have been strictly horizontal, it may well happen
that no one level beach-line may be traceable, and that neither the
terraces themselves nor the summit nor basal edges of their escarpments
may be horizontal.

Finally, comparing the extent of the elevated area, as deduced from the
upraised recent organic remains, on the two sides of the continent, we
have seen that on the Atlantic, shells have been found at intervals
from Eastern Tierra del Fuego for 1,180 miles northward, and on the
Pacific for a space of 2,075 miles. For a length of 775 miles, they
occur in the same latitudes on both sides of the continent. Without
taking this circumstance into consideration, it is probable from the
reasons assigned in the last chapter, that the entire breadth of the
continent in Central Patagonia has been uplifted in mass; but from
other reasons there given, it would be hazardous to extend this
conclusion to La Plata. From the continent being narrow in the
southern-most parts of Patagonia, and from the shells found at the
Inner Narrows of the Strait of Magellan, and likewise far up the valley
of the Santa Cruz,
it is probable that the southern part of the western coast, which was
not visited by me, has been elevated within the period of recent
Mollusca: if so, the shores of the Pacific have been continuously,
recently, and in a geological sense synchronously upraised, from Lima
for a length of 2,480 nautical miles southward,—a distance equal to
that from the Red Sea to the North Cape of Scandinavia!




Chapter III ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL
DEPOSITS.


Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake-waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.—Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.

The space between the Cordillera and the coast of Chile is on a rude
average from eighty to above one hundred miles in width; it is formed,
either of an almost continuous mass of mountains, or more commonly of
several nearly parallel ranges, separated by plains; in the more
southern parts of this province the mountains are quite subordinate to
the plains; in the northern part the mountains predominate.

The basin-like plains at the foot of the Cordillera are in several
respects remarkable; that on which the capital of Chile stands is
fifteen miles in width, in an east and west line, and of much greater
length in a north and south line; it stands 1,750 feet above the sea;
its surface appears smooth, but really falls and rises in wide gentle
undulations, the hollows corresponding with the main valleys of the
Cordillera: the striking manner in which it abruptly comes up to the
foot of this great range has been remarked by every author[1] since the
time of Molina. Near the Cordillera it is composed of a stratified mass
of pebbles of all sizes, occasionally including rounded boulders: near
its western boundary, it consists of reddish sandy clay, containing
some pebbles and numerous fragments of pumice, and sometimes passes
into pure sand or into volcanic ashes. At Podaguel, on this western
side of the plain, beds of sand are capped by a calcareous tuff, the
uppermost layers being generally hard and substalagmitic, and the lower
ones white and friable, both together precisely resembling the beds at
Coquimbo, which
contain recent marine shells. Abrupt, but rounded, hummocks of rock
rise out of this plain: those of Sta. Lucia and S. Cristoval are formed
of greenstone-porphyry almost entirely denuded of its original covering
of porphyritic claystone breccia; on their summits, many fragments of
rock (some of them kinds not found in situ) are coated and united
together by a white, friable, calcareous tuff, like that found at
Podaguel. When this matter was deposited on the summit of S. Cristoval,
the water must have stood 946 feet[2] above the surface of the
surrounding plain.

 [1] This plain is partially separated into two basins by a range of
 hills; the southern half, according to Meyen (“Reise um Erde,” Th. i,
 s. 274), falls in height, by an abrupt step, of between fifteen and
 twenty feet.


 [2] Or 2,690 feet above the sea, as measured barometrically by Mr.
 Eck. This tuff appears to the eye nearly pure; but when placed in acid
 it leaves a considerable residue of sand and broken crystals,
 apparently of feldspar. Dr. Meyen (“Reise,” Th. i, s. 269) says he
 found a similar substance on the neighbouring hill of Dominico (and I
 found it also on the Cerro Blanco), and he attributes it to the
 weathering of the stone. In some places which I examined, its bulk put
 this view of its origin quite out of the question; and I should much
 doubt whether the decomposition of a porphyry would, in any case,
 leave a crust chiefly composed of carbonate of lime. The white crust,
 which is commonly seen on weathered feldspathic rocks, does not appear
 to contain any free carbonate of lime.


To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable level
gap in the mountains, forming a true land-strait, and called the
Angostura. It then immediately expands into a second basin-formed
plain: this again to the south contracts into another land-strait, and
expands into a third basin, which, however, falls suddenly in level
about forty feet. This third basin, to the south, likewise contracts
into a strait, and then again opens into the great plain of San
Fernando, stretching so far south that the snowy peaks of the distant
Cordillera are seen rising above its horizon as above the sea. These
plains, near the Cordillera, are generally formed of a thick stratified
mass of shingle;[3] in other parts, of a red sandy clay, often with an
admixture of pumiceous matter. Although these basins are connected
together like a necklace, in a north and south line, by smooth
land-straits, the streams which drain them do not all flow north and
south, but mostly westward, through breaches worn in the bounding
mountains; and in the case of the second basin, or that of Rancagua,
there are two distinct breaches. Each basin, moreover, is not drained
singly; thus, to give the most striking instance, but not the only one,
in proceeding southward over the plain of Rancagua, we first find the
water flowing northward to and through the northern land-strait; then,
without crossing any marked ridge or watershed, we see it flowing
south-westward towards the northern one of the two breaches in the
western mountainous boundary; and lastly, again without any ridge, it
flows towards the southern breach in these same
mountains. Hence the surface of this one basin-like plain, appearing to
the eye so level, has been modelled with great nicety, so that the
drainage, without any conspicuous watersheds, is directed towards three
openings in the encircling mountains.[4] The streams flowing from the
southern basin-like plains, after passing through the breaches to the
west, unite and form the river Rapel, which enters the Pacific near
Navidad. I followed the southernmost branch of this river, and found
that the basin or plain of San Fernando is continuously and smoothly
united with those plains, which were described in the Second Chapter,
as being worn near the coast into successive cave-eaten escarpments,
and still nearer to the coast, as being strewed with upraised recent
marine remains. I might have given descriptions of numerous other
plains of the same general form, some at the foot of the Cordillera,
some near the coast, and some halfway between these points. I will
allude only to one other, namely, the plain of Uspallata, lying on the
eastern or opposite side of the Cordillera, between that great range
and the parallel lower range of Uspallata. According to Miers, its
surface is 6,000 feet above the level of the sea: it is from ten to
fifteen miles in width, and is said to extend with an unbroken surface
for 180 miles northwards: it is drained by two rivers passing through
breaches in the mountains to the east. On the banks of the River
Mendoza it is seen to be composed of a great accumulation of stratified
shingle, estimated at 400 feet in thickness. In general appearance, and
in numerous points of structure, this plain closely resembles those of
Chile.

 [3] The plain of San Fernando has, according to MM. Meyen and Gay
 “Reise,” etc., Th. i, ss. 295 and 298, near the Cordillera, an upper
 step-formed plain of clay, on the surface of which they found numerous
 blocks of rocks, from two to three feet long, either lying single or
 piled in heaps, but all arranged in nearly straight lines.


 [4] It appears from Captain Herbert’s account of the Diluvium of the
 Himalaya, (“Gleanings of Science,” Calcutta, vol. ii, p. 164), that
 precisely similar remarks apply to the drainage of the plains or
 valleys between those great mountains.

The origin and manner of formation of the thick beds of gravel, sandy
clay, volcanic detritus, and calcareous tuff, composing these
basin-like plains, is very important; because, as we shall presently
show, they send arms or fringes far up the main valleys of the
Cordillera. Many of the inhabitants believe that these plains were once
occupied by lakes, suddenly drained; but I conceive that the number of
the separate breaches at nearly the same level in the mountains
surrounding them quite precludes this idea. Had not such distinguished
naturalists as MM. Meyen and Gay stated their belief that these
deposits were left by great debacles rushing down from the Cordillera,
I should not have noticed a view, which appears to me from many reasons
improbable in the highest degree—namely, from the vast accumulation of
_well-rounded pebbles_—their frequent stratification with layers of
sand—the overlying beds of calcareous tuff—this same substance coating
and uniting the fragments of rock on the hummocks in the plain of
Santiago—and lastly even from the worn, rounded, and much denuded state
of these hummocks, and of the headlands which project from the
surrounding mountains. On the other hand, these several circumstances,
as well as the continuous union of the basins at the foot of the
Cordillera, with the great plain of the Rio Rapel which still retains
the marks of sea-action
at various levels, and their general similarity in form and composition
with the many plains near the coast, which are either similarly marked
or are strewed with upraised marine remains, fully convince me that the
mountains bounding these basin-plains were breached, their islet-like
projecting rocks worn, and the loose stratified detritus forming their
now level surfaces deposited, by the sea, as the land slowly emerged.
It is hardly possible to state too strongly the perfect resemblance in
outline between these basin-like, long, and narrow plains of Chile
(especially when in the early morning the mists hanging low represented
water), and the creeks and fiords now intersecting the southern and
western shores of the continent. We can on this view of the sea, when
the land stood lower, having long and tranquilly occupied the spaces
between the mountain-ranges, understand how the boundaries of the
separate basins were breached in more than one place; for we see that
this is the general character of the inland bays and channels of Tierra
del Fuego; we there, also, see in the sawing action of the tides, which
flow with great force in the cross channels, a power sufficient to keep
the breaches open as the land emerged. We can further see that the
waves would naturally leave the smooth bottom of each great bay or
channel, as it became slowly converted into land, gently inclined to as
many points as there were mouths, through which the sea finally
retreated, thus forming so many watersheds, without any marked ridges,
on a nearly level surface. The absence of marine remains in these high
inland plains cannot be properly adduced as an objection to their
marine origin: for we may conclude, from shells not being found in the
great shingle beds of Patagonia, though copiously strewed on their
surfaces, and from many other analogous facts, that such deposits are
eminently unfavourable for the embedment of such remains; and with
respect to shells not being found strewed on the surface of these
basin-like plains, it was shown in the last chapter that remains thus
exposed in time decay and disappear.

No. 13
Section of the plain at the eastern foot of the Chilian Cordillera.


[Illustration: Section of the plain at the eastern foot of the Chilian
Cordillera.]

I observed some appearances on the plains at the eastern and opposite
foot of the Cordillera which are worth notice, as showing that the sea
there long acted at nearly the same level as on the basin-plains of
Chile. The mountains on this eastern side are exceedingly abrupt; they
rise out of a smooth, talus-like, very gentle, slope, from five to ten
miles in width (as represented in Figure 13), entirely composed of
perfectly rounded pebbles, often white-washed with an aluminous
substance like decomposed feldspar. This sloping plain or talus blends
into a perfectly flat space a few miles in width, composed
of reddish impure clay, with small calcareous concretions as in the
Pampean deposit,—of fine white sand with small pebbles in layers,—and
of the above-mentioned white aluminous earth, all interstratified
together. This flat space runs as far as Mendoza, thirty miles
northward, and stands probably at about the same height, namely, 2,700
feet (Pentland and Miers) above the sea. To the east it is bounded by
an escarpment, eighty feet in height, running for many miles north and
south, and composed of perfectly round pebbles, and loose,
white-washed, or embedded in the aluminous earth: behind this
escarpment there is a second and similar one of gravel. Northward of
Mendoza, these escarpments become broken and quite obliterated; and it
does not appear that they ever enclosed a lake-like area: I conclude,
therefore, that they were formed by the sea, when it reached the foot
of the Cordillera, like the similar escarpments occurring at so many
points on the coasts of Chile and Patagonia.

The talus-like plain slopes up with a smooth surface into the great dry
valleys of the Cordillera. On each hand of the Portillo valley, the
mountains are formed of red granite, mica-slate, and basalt, which all
have suffered a truly astonishing amount of denudation; the gravel in
the valley, as well as on the talus-like plain in front of it, is
composed of these rocks; but at the mouth of the valley, in the middle
(height probably about three thousand five hundred feet above the sea),
a few small isolated hillocks of several varieties of porphyry project,
round which, on all sides, smooth and often white-washed pebbles of
these same porphyries, to the exclusion of all others, extend to a
circumscribed distance. Now, it is difficult to conceive any other
agency, except the quiet and long-continued action of the sea on these
hillocks, which could have rounded and whitewashed the fragments of
porphyry, and caused them to radiate from such small and quite
insignificant centres, in the midst of that vast stream of stones which
has descended from the main Cordillera.

_Sloping terraces of gravel in the valleys of the Cordillera._—All the
main valleys on both flanks of the Chilean Cordillera have formerly
had, or still have, their bottoms filled up to a considerable thickness
by a mass of rudely stratified shingle. In Central Chile the greater
part of this mass has been removed by the torrents; cliff-bounded
fringes, more or less continuous, being left at corresponding heights
on both sides of the valleys. These fringes, or as they may be called
terraces, have a smooth surface, and as the valleys rise, they gently
rise with them: hence they are easily irrigated, and afford great
facilities for the construction of the roads. From their uniformity,
they give a remarkable character to the scenery of these grand, wild,
broken valleys. In width, the fringes vary much, sometimes being only
broad enough for the roads, and sometimes expanding into narrow plains.
Their surfaces, besides gently rising up the valley, are slightly
inclined towards its centre in such a manner as to show that the whole
bottom must once have been filled up with a smooth and slightly concave
mass, as still are the dry unfurrowed valleys of Northern Chile. Where
two valleys unite into one, these terraces are particularly well
exhibited, as is represented in Figure 14. The thickness of the gravel
forming these fringes, on a rude average, may be said to vary from
thirty to sixty or eighty feet; but near the mouths of the valleys it
was in several places from two to three hundred feet. The amount of
matter removed by the torrents has been immense; yet in the lower parts
of the valleys the terraces have seldom been entirely worn away on
either side, nor has the solid underlying rock been reached: higher up
the valleys, the terraces have frequently been removed on one or the
other side, and sometimes on both sides; but in this latter case they
reappear after a short interval on the line, which they would have held
had they been unbroken. Where the solid rock has been reached, it has
been cut into deep and narrow gorges. Still higher up the valleys, the
terraces gradually become more and more broken, narrower, and less
thick, until, at a height of from seven to nine thousand feet, they
become lost, and blended with the piles of fallen detritus.

[Illustration: Ground plan of a bifurcating valley in the Cordillera.]

I carefully examined in many places the state of the gravel, and almost
everywhere found the pebbles equally and perfectly rounded,
occasionally with great blocks of rock, and generally distinctly
stratified, often with parting seams of sand. The pebbles were
sometimes coated with a white aluminous, and less frequently with a
calcareous, crust. At great heights up the valleys the pebbles become
less rounded; and as the terraces become obliterated, the whole mass
passes into the nature of ordinary detritus. I was repeatedly struck
with the great difference between this detritus high up the valleys,
and the gravel of the terraces low down, namely, in the greater number
of the quite angular fragments in the detritus,—in the unequal degree
to which the other fragments have been rounded,—in the quantity of
associated earth,—in the absence of stratification,—and in the
irregularity of the upper surfaces. This difference was likewise well
shown at points low down the valleys, where precipitous ravines,
cutting through mountains of highly coloured rock, have thrown down
wide, fan-shaped accumulations of detritus on the terraces: in such
cases, the line of separation between the detritus and the terrace
could be pointed out to within an inch or two; the detritus consisting
entirely of angular and only partially rounded fragments of the
adjoining coloured rocks; the stratified shingle (as I ascertained by
close inspection, especially in one case, in the valley of the River
Mendoza) containing only a small proportion of these fragments, and
those few well rounded.


I particularly attended to the appearance of the terraces where the
valleys made abrupt and considerable bends, but I could perceive no
difference in their structure: they followed the bends with their usual
nearly equable inclination. I observed, also, in several valleys, that
wherever large blocks of any rock became numerous, either on the
surface of the terrace or embedded in it, this rock soon appeared
higher up _in situ_: thus I have noticed blocks of porphyry, of
andesitic syenite, of porphyry and of syenite, alternately becoming
numerous, and in each case succeeded by mountains thus constituted.
There is, however, one remarkable exception to this rule; for along the
valley of the Cachapual, M. Gay found numerous large blocks of white
granite, which does not occur in the neighbourhood. I observed these
blocks, as well as others of andesitic syenite (not occurring here _in
situ_), near the baths of Cauquenes at a height of between two and
three hundred feet above the river, and therefore quite above the
terrace or fringe which borders that river; some miles up the valleys
there were other blocks at about the same height. I also noticed, at a
less height, just above the terrace, blocks of porphyries (apparently
not found in the immediately impending mountains), arranged in rude
lines, as on a sea-beach. All these blocks were rounded, and though
large, not gigantic, like the true erratic boulders of Patagonia and
Fuegia. M. Gay[5] states that the granite does not occur in situ within
a distance of twenty leagues; I suspect, for several reasons, that it
will ultimately be found at a much less distance, though certainly not
in the immediate neighbourhood. The boulders found by MM. Meyen and Gay
on the upper plain of San Fernando (mentioned in a previous note)
probably belong to this same class of phenomena.

 [5] “Annales des Science Nat.” (I. séries, tome 28). M. Gay, as I was
 informed, penetrated the Cordillera by the great oblique valley of Los
 Cupressos, and not by the most direct line.


These fringes of stratified gravel occur along all the great valleys of
the Cordillera, as well as along their main branches; they are
strikingly developed in the valleys of the Maypu, Mendoza, Aconcagua,
Cachapual, and according to Meyen,[6] in the Tinguirica. XIn the
valleys, however, of Northern Chile, and in some on the eastern flank
of the Cordillera, as in the Portillo Valley, where streams have never
flowed, or are quite insignificant in volume, the presence of a mass of
stratified gravel can be inferred only from the smooth slightly concave
form of the bottom. One naturally seeks for some explanation of so
general and striking a phenomenon; that the matter forming the fringes
along the valleys, or still filling up their entire beds, has not
fallen from the adjoining mountains like common detritus, is evident
from the complete contrast in every respect between the gravel and the
piles of detritus, whether seen high up the valleys on their sides, or
low down in front of the more precipitous ravines; that the matter has
not been deposited by debacles, even if we could believe in debacles
having rushed down _every_ valley, and all their branches, eastward and
westward from the central pinnacles of the Cordillera, we must admit
from the following
reasons,—from the distinct stratification of the mass,—its smooth upper
surface,—the well-rounded and sometimes encrusted state of the pebbles,
so different from the loose debris on the mountains,—and especially
from the terraces preserving their uniform inclination round the most
abrupt bends. To suppose that as the land now stands, the rivers
deposited the shingle along the course of every valley, and all their
main branches, appears to me preposterous, seeing that these same
rivers not only are now removing and have removed much of this deposit,
but are everywhere tending to cut deep and narrow gorges in the hard
underlying rocks.

 [6] “Reise,” etc., Th. I, s. 302.


I have stated that these fringes of gravel, the origin of which are
inexplicable on the notion of debacles or of ordinary alluvial action,
are directly continuous with the similarly-composed basin-like plains
at the foot of the Cordillera, which, from the several reasons before
assigned, I cannot doubt were modelled by the agency of the sea. Now if
we suppose that the sea formerly occupied the valleys of the Chilean
Cordillera, in precisely the same manner as it now does in the more
southern parts of the continent, where deep winding creeks penetrate
into the very heart of, and in the case of Obstruction Sound quite
through, this great range; and if we suppose that the mountains were
upraised in the same slow manner as the eastern and western coasts have
been upraised within the recent period, then the origin and formation
of these sloping, terrace-like fringes of gravel can be simply
explained. For every part of the bottom of each valley will, on this
view, have long stood at the head of a sea creek, into which the then
existing torrents will have delivered fragments of rocks, where, by the
action of the tides, they will have been rolled, sometimes encrusted,
rudely stratified, and the whole surface levelled by the blending
together of the successive beach lines.[7] As the land rose, the
torrents in every valley will have tended to have removed the matter
which just before had been arrested on, or near, the beach-lines; the
torrents, also, having continued to gain in force by the continued
elevation increasing their total descent from their sources to the sea.
This slow rising of the Cordillera, which explains so well the
otherwise inexplicable origin and structure of the terraces, judging
from all known analogies, will probably have been interrupted by many
periods of rest; but we ought not to expect to find any evidence of
these periods in the structure of the gravel-terraces: for, as the
waves at the heads of deep creeks have little erosive power, so the
only effect of the sea having long remained at the same level will be
that the upper parts of the creeks will have
become filled up at such periods to the level of the water with gravel
and sand; and that afterwards the rivers will have thrown down on the
filled-up parts a talus of similar matter, of which the inclination (as
at the head of a partially filled-up lake) will have been determined by
the supply of detritus, and the force of the stream.[8] Hence, after
the final conversion of the creeks into valleys, almost the only
difference in the terraces at those points at which the sea stood long,
will be a somewhat more gentle inclination, with river-worn instead of
sea-worn detritus on the surface.

 [7] Sloping terraces of precisely similar structure have been
 described by me (“Philosoph. Transactions,” 1839, p. 58) in the
 valleys of Lochaber in Scotland, where, at higher levels, the parallel
 roads of Glen Roy show the marks of the long and quiet residence of
 the sea. I have no doubt that these sloping terraces would have been
 present in the valleys of most of the European ranges, had not every
 trace of them, and all wrecks of sea-action, been swept away by the
 glaciers which have since occupied them. I have shown that this is the
 case with the mountains (_London and Edin. Phil. Journal,_ vol. xxi,
 p. 187) of North Wales.


 [8] I have attempted to explain this process in a more detailed
 manner, in a letter to Mr. Maclaren, published in the _Edinburgh New
 Phil. Journal,_ vol. xxxv, p. 288.


I know of only one difficulty on the foregoing view, namely, the
far-transported blocks of rock high on the sides of the valley of the
Cachapual: I will not attempt any explanation of this phenomenon, but I
may state my belief that a mountain-ridge near the Baths of Cauquenes
has been upraised long subsequently to all the other ranges in the
neighbourhood, and that when this was effected the whole face of the
country must have been greatly altered. In the course of ages,
moreover, in this and other valleys, events may have occurred like, but
even on a grander scale than, that described by Molina,[9] when a slip
during the earthquake of 1762 banked up for ten days the great River
Lontue, which then bursting its barrier “inundated the whole country,”
and doubtless transported many great fragments of rock. Finally,
notwithstanding
this one case of difficulty, I cannot entertain any doubt, that these
terrace-like fringes, which are continuously united with the
basin-shaped plains at the foot of the Cordillera, have been formed by
the arrestment of river-borne detritus at successive levels, in the
same manner as we see now taking place at the heads of all those many,
deep, winding fiords intersecting the southern coasts. To my mind, this
has been one of the most important conclusions to which my observations
on the geology of South America have led me; for we thus learn that one
of the grandest and most symmetrical mountain-chains in the world, with
its several parallel lines,[10] has been together uplifted in mass
between seven and nine thousand feet, in the same gradual manner as
have the eastern and western coasts within the recent period.

 [9] “Compendio de la Hist.,” etc., tome i, p. 30. M. Brongniart, in
 his report on M. Gay’s labours (“Annales des Sciences” 1833),
 considers that the boulders in the Cachapual belong to the same class
 with the erratic boulders of Europe. As the blocks which I saw are not
 gigantic, and especially as they are not angular, and as they have not
 been transported fairly across low spaces or wide valleys, I am
 unwilling to class them with those which, both in the northern and
 southern hemisphere (“Geolog. Transac.,” vol. vi, p. 415), have been
 transported by ice. It is to be hoped that when M. Gay’s
 long-continued and admirable labours in Chile are published, more
 light will be thrown on this subject. However, the boulders may have
 been primarily transported; the final position of those of porphyry,
 which have been described as arranged at the foot of the mountain in
 rude lines, I cannot doubt, has been due to the action of waves on a
 beach. The valley of the Cachapual, in the part where the boulders
 occur, bursts through the high ridge of Cauquenes, which runs parallel
 to, but at some distance from, the Cordillera. This ridge has been
 subjected to excessive violence; trachytic lava has burst from it, and
 hot springs yet flow at its base. Seeing the enormous amount of
 denudation of solid rock in the upper and much broader parts of this
 valley where it enters the Cordillera, and seeing to what extent the
 ridge of Cauquenes now protects the great range, I could not help
 believing (as alluded to in the text) that this ridge with its
 trachytic eruptions had been thrown up at a much later period than the
 Cordillera. If this has been the case, the boulders, after having been
 transported to a low level by the torrents (which exhibit in every
 valley proofs of their power of moving great fragments), may have been
 raised up to their present height, with the land on which they rested.


 [10] I do not wish to affirm that all the lines have been uplifted
 quite equally; slight differences in the elevation would leave no
 perceptible effect on the terraces. It may, however, be inferred,
 perhaps with one exception, that since the period when the sea
 occupied these valleys, the several ranges have not been dislocated by
 _great_ and _abrupt_ faults or upheavals; for if such had occurred,
 the terraces of gravel at these points would not have been continuous.
 The one exception is at the lower end of a plain in the Valle del Yeso
 (a branch of the Maypu), where, at a great height, the terraces and
 valley appear to have been broken through by a line of upheaval, of
 which the evidence is plain in the adjoining mountains; this
 dislocation, perhaps, occurred _after the elevation_ of this part of
 the valley above the level of the sea. The valley here is almost
 blocked up by a pile about one thousand feet in thickness, formed, as
 far as I could judge, from three sides, entirely, or at least in chief
 part, of gravel and detritus. On the south side, the river has cut
 quite through this mass; on the northern side, and on the very summit,
 deep ravines, parallel to the line of the valley, are worn, as if the
 drainage from the valley above had passed by these two lines before
 following its present course.

_Formation of Valleys._

The bulk of solid rock which has been removed in the lower parts of the
valleys of the Cordillera has been enormous. It is only by reflecting
on such cases as that of the gravel beds of Patagonia, covering so many
thousand square leagues of surface, and which, if heaped into a ridge,
would form a mountain-range almost equal to the Cordillera, that the
amount of denudation becomes credible. The valleys within this range
often follow anticlinal but rarely synclinal lines; that is, the strata
on the two sides more often dip from the line of valley than towards
it. On the flanks of the range, the valleys most frequently run neither
along anticlinal nor synclinal axes, but along lines of flexure or
faults: that is, the strata on both sides dip in the same direction,
but with different, though often only slightly different, inclinations.
As most of the nearly parallel ridges which together form the
Cordillera run approximately north and south, the east and west valleys
cross them in zig-zag lines, bursting through the points where the
strata have been
least inclined. No doubt the greater part of the denudation was
affected at the periods when tidal-creeks occupied the valleys, and
when the outer flanks of the mountains were exposed to the full force
of an open ocean. I have already alluded to the power of the tidal
action in the channels connecting great bays; and I may here mention
that one of the surveying vessels in a channel of this kind, though
under sail, was whirled round and round by the force of the current. We
shall hereafter see, that of the two main ridges forming the Chilean
Cordillera, the eastern and loftiest one owes the greater part of its
_angular_ upheaval to a period subsequent to the elevation of the
western ridge; and it is likewise probable that many of the other
parallel ridges have been angularly upheaved at different periods;
consequently many parts of the surfaces of these mountains must
formerly have been exposed to the full force of the waves, which, if
the Cordillera were now sunk into the sea, would be protected by
parallel chains of islands. The torrents in the valleys certainly have
great power in wearing the rocks; as could be told by the dull rattling
sound of the many fragments night and day hurrying downwards; and as
was attested by the vast size of certain fragments, which I was assured
had been carried onwards during floods; yet we have seen in the lower
parts of the valleys, that the torrents have seldom removed all the
sea-checked shingle forming the terraces, and have had time since the
last elevation in mass only to cut in the underlying rocks, gorges,
deep and narrow, but quite insignificant in dimensions compared with
the entire width and depth of the valleys.

Along the shores of the Pacific, I never ceased during my many and long
excursions to feel astonished at seeing every valley, ravine, and even
little inequality of surface, both in the hard granitic and soft
tertiary districts, retaining the exact outline, which they had when
the sea left their surfaces coated with organic remains. When these
remains shall have decayed, there will be scarcely any difference in
appearance between this line of coast-land and most other countries,
which we are accustomed to believe have assumed their present features
chiefly through the agency of the weather and fresh-water streams. In
the old granitic districts, no doubt it would be rash to attribute all
the modifications of outline exclusively to the sea-action; for who can
say how often this lately submerged coast may not previously have
existed as land, worn by running streams and washed by rain? This
source of doubt, however, does not apply to the districts superficially
formed of the modern tertiary deposits. The valleys worn by the sea,
through the softer formations, both on the Atlantic and Pacific sides
of the continent, are generally broad, winding, and flat-bottomed: the
only district of this nature now penetrated by arms of the sea, is the
island of Chiloe.

Finally, the conclusion at which I have arrived, with respect to the
relative powers of rain and sea water on the land, is, that the latter
is far the most efficient agent, and that its chief tendency is to
widen the valleys; whilst torrents and rivers tend to deepen them, and
to remove the wreck of the sea’s destroying action. As the waves have
more
power, the more open and exposed the space may be, so will they always
tend to widen more and more the mouths of valleys compared with their
upper parts: hence, doubtless, it is, that most valleys expand at their
mouths,—that part, at which the rivers flowing in them, generally have
the least wearing power.

When reflecting on the action of the sea on the land at former levels,
the effect of the great waves, which generally accompany earthquakes,
must not be overlooked: few years pass without a severe earthquake
occurring on some part of the west coast of South America; and the
waves thus caused have great power. At Concepcion, after the shock of
1835, I saw large slabs of sandstone, one of which was six feet long,
three in breadth, and two in thickness, thrown high up on the beach;
and from the nature of the marine animals still adhering to it, it must
have been torn up from a considerable depth. On the other hand, at
Callao, the recoil-wave of the earthquake of 1746 carried great masses
of brickwork, between three and four feet square, some way out seaward.
During the course of ages, the effect thus produced at each successive
level, cannot have been small; and in some of the tertiary deposits on
this line of coast, I observed great boulders of granite and other
neighbouring rocks, embedded in fine sedimentary layers, the
transportal of which, except by the means of earthquake-waves, always
appeared to me inexplicable.

_Superficial Saline Deposits._

This subject may be here conveniently treated of: I will begin with the
most interesting case, namely, the superficial saline beds near Iquique
in Peru. The porphyritic mountains on the coast rise abruptly to a
height of between one thousand nine hundred and three thousand feet:
between their summits and an inland plain, on which the celebrated
deposit of nitrate of soda lies, there is a high undulatory district,
covered by a remarkable superficial saliferous crust, chiefly composed
of common salt, either in white, hard, opaque nodules, or mingled with
sand, in this latter case forming a compact sandstone. This saliferous
superficial crust extends from the edge of the coast-escarpment, over
the whole face of the country; but never attains, as I am assured by
Mr. Bollaert (long resident here) any great thickness. Although a very
slight shower falls only at intervals of many years, yet small
funnel-shaped cavities show that the salt has been in some parts
dissolved.[11] In several places I saw large patches of sand, quite
moist, owing to the quantity of muriate of lime (as ascertained by Mr.
T. Reeks) contained in them. From the compact salt-cemented sand being
either red, purplish, or yellow, according to the colour of the rocky
strata on which
it rested, I imagined that this[12] substance had probably been derived
through common alluvial action from the layers of salt which occur
interstratified in the surrounding mountains: but from the interesting
details given by M. d’Orbigny, and from finding on a fresh examination
of this agglomerated sand, that it is not irregularly cemented, but
consists of thin layers of sand of different tints of colour,
alternating with excessively fine parallel layers of salt, I conclude
that it is not of alluvial origin. M. d’Orbigny[13] observed analogous
saline beds extending from Cobija for five degrees of latitude
northward, and at heights varying from six hundred to nine hundred
feet: from finding recent sea-shells strewed on these saliferous beds,
and under them, great well-rounded blocks, exactly like those on the
existing beach, he believes that the salt, which is invariably
superficial, has been left by the evaporation of the sea-water. This
same conclusion must, I now believe, be extended to the superficial
saliferous beds of Iquique, though they stand about three thousand feet
above the level of the sea.

 [11] It is singular how slowly, according to the observations of M.
 Cordier on the salt-mountain of Cardona in Spain (“Ann. des Mines,
 Transl. of Geolog. Mem.” by De la Beche, p. 60), salt is dissolved,
 where the amount of rain is supposed to be as much as 31·4 of an inch
 in the year. It is calculated that only five feet in thickness is
 dissolved in the course of a century.


 [12] “Journal of Researches,” p. 444, first edit.


 [13] “Voyage,” etc., p. 102. M. d’Orbigny found this deposit
 intersected, in many places, by deep ravines, in which there was no
 salt. Streams must once, though historically unknown, have flowed in
 them; and M. d’Orbigny argues from the presence of undissolved salt
 over the whole surrounding country, that the streams must have arisen
 from rain or snow having fallen, not in the adjoining country, but on
 the now arid Cordillera. I may remark, that from having observed ruins
 of Indian buildings in absolutely sterile parts of the Chilian
 Cordillera (“Journal,” 2nd edit., p. 357), I am led to believe that
 the climate, at a time when Indian man inhabited this part of the
 continent, was in some slight degree more humid than it is at present.


Associated with the salt in the superficial beds, there are numerous,
thin, horizontal layers of impure, dirty-white, friable, gypseous and
calcareous tuffs. The gypseous beds are very remarkable, from abounding
with, so as sometimes to be almost composed of, irregular concretions,
from the size of an egg to that of a man’s head, of very hard, compact,
heavy gypsum, in the form of anhydrite. This gypsum contains some
foreign particles of stone; it is stained, judging from its action with
borax, with iron, and it exhales a strong aluminous odour. The surfaces
of the concretions are marked by sharp, radiating, or bifurcating
ridges, as if they had been (but not really) corroded: internally they
are penetrated by branching veins (like those of calcareous spar in the
septaria of the London clay) of pure white anhydrite. These veins might
naturally have been thought to have been formed by subsequent
infiltration, had not each little embedded fragment of rock been
likewise edged in a very remarkable manner by a narrow border of the
same white anhydrite: this shows that the veins must have been formed
by a process of segregation, and not of infiltration. Some of the
little included and _cracked_ fragments of foreign rock are penetrated
by the anhydrite, and portions have evidently been thus mechanically
displaced: at St. Helena, I observed that calcareous matter, deposited
by rain water, also had the power to
separate small fragments of rock from the larger masses.[14] I believe
the superficial gypseous deposit is widely extended: I received
specimens of it from Pisagua, forty miles north of Iquique, and
likewise from Arica, where it coats a layer of pure salt. M.
d’Orbigny[15] found at Cobija a bed of clay, lying above a mass of
upraised recent shells, which was saturated with sulphate of soda, and
included thin layers of fibrous gypsum. These widely extended,
superficial, beds of salt and gypsum, appear to me an interesting
geological phenomenon, which could be presented only under a very dry
climate.

 [14] “Volcanic Islands,” etc., p. 87.


 [15] “Voyage Géolog.,” etc., p. 95.


The plain or basin, on the borders of which the famous bed of nitrate
of soda lies, is situated at the distance of about thirty miles from
the sea, being separated from it by the saliferous district just
described. It stands at a height of 3,300 feet; its surface is level,
and some leagues in width; it extends forty miles northward, and has a
total length (as I was informed by Mr. Belford Wilson, the
Consul-General at Lima) of 420 miles. In a well near the works,
thirty-six yards in depth, sand, earth, and a little gravel were found:
in another well, near Almonte, fifty yards deep, the whole consisted,
according to Mr. Blake,[16] of clay, including a layer of sand two feet
thick, which rested on fine gravel, and this on coarse gravel, with
large rounded fragments of rock. In many parts of this now utterly
desert plain, rushes and large prostrate trees in a hardened state,
apparently Mimosas, are found buried, at a depth from three to six
feet; according to Mr. Blake, they have all fallen to the south-west.
The bed of nitrate of soda is said to extend for forty to fifty leagues
along the western margin of the plain, but is not found in its central
parts: it is from two to three feet in thickness, and is so hard that
it is generally blasted with gunpowder; it slopes gently upwards from
the edge of the plain to between ten and thirty feet above its level.
It rests on sand in which, it is said, vegetable remains and broken
shells have been found; shells have also been found, according to Mr.
Blake, both on and in the nitrate of soda. It is covered by a
superficial mass of sand, containing nodules of common salt, and, as I
was assured by a miner, much soft gypseous matter, precisely like that
in the superficial crust already described: certainly this crust, with
its characteristic concretions of anhydrite, comes close down to the
edge of the plain.

 [16] See an admirable paper “Geolog. and Miscell. Notices of
 Tarapaca,” in _Silliman’s American Journal_, vol. xliv, p. 1.

The nitrate of soda varies in purity in different parts, and often
contains nodules of common salt. According to Mr. Blake, the proportion
of nitrate of soda varies from 20 to 75 per cent. An analysis by Mr. A.
Hayes, of an average specimen, gave:—

Nitrate of Soda		64·98 Sulphate of Soda		3·00 Chloride of
Soda		28·69 Iodic Salts		0·63 Shells and Marl		2·60
———
99.90


The “mother-water” at some of the refineries is very rich in iodic
salts, and is supposed[17] to contain much muriate of lime. In an
unrefined specimen brought home by myself, Mr. T. Reeks has ascertained
that the muriate of lime is very abundant. With respect to the origin
of this saline mass, from the manner in which the gently inclined,
compact bed follows for so many miles the sinuous margin of the plain,
there can be no doubt that it was deposited from a sheet of water: from
the fragments of embedded shells, from the abundant iodic salts, from
the superficial saliferous crust occurring at a higher level and being
probably of marine origin, and from the plain resembling in form those
of Chile and that of Uspallata, there can be little doubt that this
sheet of water was, at least originally, connected with the sea.[18]

 [17] _Literary Gazette_, 1841, p. 475.


 [18] (From an official document, shown me by Mr. Belford Wilson, it
 appears that the first export of nitrate of soda to Europe was in July
 1830, on French account, in a British ship:—

Entire export in		Quintals 1830		17,300 1831		40,885
1832		51,400 1833		91,335 1834		149,538

The Spanish quintal nearly equals 100 English pounds.


_Thin, superficial, saline incrustations._—These saline incrustations
are common in many parts of America: Humboldt met with them on the
tableland of Mexico, and the Jesuit Falkner and other authors[19] state
that they occur at intervals over the vast plains extending from the
mouth of the Plata to Rioja and Catamarca. Hence it is that during
droughts, most of the streams in the Pampas are saline. I nowhere met
with these incrustations so abundantly as near Bahia Blanca: square
miles of the mud-flats, which near that place are raised only a few
feet above the sea, just enough to protect them from being overflowed,
appear, after dry weather, whiter than the ground after the thickest
hoar-frost. After rain the salts disappear, and every puddle of water
becomes highly saline; as the surface dries, the capillary action draws
the moisture up pieces of broken earth, dead sticks, and tufts of
grass, where the salt effloresces. The incrustation, where thickest,
does not exceed a quarter of an inch. M. Parchappe[20] has analysed it;
and finds that the specimens collected at the extreme head of the low
plain, near the River Manuello, consist of 93 per cent of sulphate of
soda, and 7 of common salt; whilst the specimens taken close to the
coast contain only 63 per cent of the sulphate, and 37 of the muriate
of soda. This remarkable fact, together with our knowledge that the
whole of this low muddy plain has been covered by the sea within the
recent period, must lead to the suspicion that
the common salt, by some unknown process, becomes in time changed into
the sulphate. Friable, calcareous matter is here abundant, and the case
of the apparent double decomposition of the shells and salt on San
Lorenzo, should not be forgotten.

 [19] Azara (“Travels,” vol. i, p. 55) considers that the Parana is the
 eastern boundary of the saliferous region; but I heard of “salitrales”
 in the Province of Entre Rios.


 [20] M. d’Orbigny’s “Voyage,” etc., Part. Hist., tome i, p. 664.

The saline incrustations, near Bahia Blanca, are not confined to,
though most abundant on, the low muddy flats; for I noticed some on a
calcareous plain between thirty and forty feet above the sea, and even
a little occurs in still higher valleys. Low alluvial tracts in the
valleys of the Rivers Negro and Colorado are also encrusted, and in the
latter valley such spaces appeared to be occasionally overflowed by the
river. I observed saline incrustations in some of the valleys of
Southern Patagonia. At Port Desire a low, flat, muddy valley was
thickly incrusted by salts, which on analysis by Mr. T. Reeks, are
found to consist of a mixture of sulphate and muriate of soda, with
carbonate of lime and earthy matter. On the western side of the
continent, the southern coasts are much too humid for this phenomenon;
but in Northern Chile I again met with similar incrustations. On the
hardened mud, in parts of the broad, flat-bottomed valley of Copiapo,
the saline matter encrusts the ground to the thickness of some inches:
specimens, sent by Mr. Bingley to Apothecaries’ Hall for analysis, were
said to consist of carbonate and sulphate of soda. Much sulphate of
soda is found in the desert of Atacama. In all parts of South America,
the saline incrustations occur most frequently on low damp surfaces of
mud, where the climate is rather dry; and these low surfaces have, in
almost every case, been upraised above the level of the sea, within the
recent period.

_Salt-lakes of Patagonia and La Plata._—Salinas, or natural salt-lakes,
occur in various formations on the eastern side of the continent,—in
the argillaceo-calcareous deposit of the Pampas, in the sandstone of
the Rio Negro, where they are very numerous, in the pumiceous and other
beds of the Patagonian tertiary formation, and in small primary
districts in the midst of this latter formation. Port S. Julian is the
most southerly point (lat. 49° to 50°) at which salinas are known to
occur.[21] The depressions, in which these salt-lakes lie, are from a
few feet to sixty metres, as asserted by M. d’Orbigny,[22] below the
surface of the surrounding plains; and, according to this same author,
near the Rio Negro they all trend, either in the N.E. and S.W. or in E.
and W. lines, coincident with the general slope of the plain. These
depressions in the plain generally have one side lower than the others,
but there are no outlets for drainage. Under a less dry climate, an
outlet would soon have been formed, and the salt washed away. The
salinas occur at different elevations above the sea; they are often
several leagues in diameter; they are generally very shallow, but there
is a deep one in a quartz-rock formation near C. Blanco. In the wet
season, the whole, or
a part, of the salt is dissolved, being redeposited during the
succeeding dry season. At this period the appearance of the snow-white
expanse of salt crystallised in great cubes, is very striking. In a
large salina, northward of the Rio Negro, the salt at the bottom,
during the whole year, is between two and three feet in thickness.

 [21] According to Azara (“Travels,” vol. i, p. 56) there are
 salt-lakes as far north as Chaco (lat. 25°), on the banks of the
 Vermejo. The salt-lakes of Siberia appear (Pallas’s “Travels,” English
 Trans., vol. i, p. 284) to occur in very similar depressions to those
 of Patagonia.


 [22] “Voyage Géolog.,” p. 63.


The salt rests almost always on a thick bed of black muddy sand, which
is fetid, probably from the decay of the burrowing worms inhabiting
it.[23] In a salina, situated about fifteen miles above the town of El
Carmen on the Rio Negro, and three or four miles from the banks of that
river, I observed that this black mud rested on gravel with a
calcareous matrix, similar to that spread over the whole surrounding
plains: at Port S. Julian the mud, also, rested on the gravel: hence
the depressions must have been formed anteriorly to, or
contemporaneously with, the spreading out of the gravel. I was informed
that one small salina occurs in an alluvial plain within the valley of
the Rio Negro, and therefore its origin must be subsequent to the
excavation of that valley. When I visited the salina, fifteen miles
above the town, the salt was beginning to crystallise, and on the muddy
bottom there were lying many crystals, generally placed crossways of
sulphate of soda (as ascertained by Mr. Reeks), and embedded in the
mud, numerous crystals of sulphate of lime, from one to three inches in
length: M. d’Orbigny[24] states that some of these crystals are
acicular and more than even nine inches in length; others are macled
and of great purity: those I found all contained some sand in their
centres. As the black and fetid sand overlies the gravel, and that
overlies the regular tertiary strata, I think there can be no doubt
that these remarkable crystals of sulphate of lime have been deposited
from the waters of the lake. The inhabitants call the crystals of
selenite, the _padre del sal_, and those of the sulphate of soda, the
_madre del sal_; they assured me that both are found under the same
circumstances in several of the neighbouring salinas; and that the
sulphate of soda is annually dissolved, and is always crystallised
before the common salt on the muddy bottom.[25] The association of
gypsum and salt in this case, as well as in the superficial deposits of
Iquique, appears to me interesting, considering how generally these
substances are associated in the older stratified formations.

 [23] Professor Ehrenberg examined some of this muddy sand, but was
 unable to find in it any infusoria.


 [24] “Voyage Géolog.,” p. 64.


 [25] This is what might have been expected; for M. Ballard asserts
 (_Acad. des Sciences_, Oct. 7, 1844, that sulphate of soda is
 precipitated from solution more readily from water containing muriate
 of soda in excess, than from pure water.


Mr. Reeks has analysed for me some of the salt from the salina near the
Rio Negro; he finds it composed entirely of chloride of sodium, with
the exception of 0·26 of sulphate of lime and of 0·22 of earthy matter:
there are no traces of iodic salts. Some salt from the salina
Chiquitos, in the Pampean formation, is equally pure. It is a singular
fact, that the salt from these salinas does not serve so well for
preserving meat,  as sea-salt from the Cape de Verde Islands; and a
merchant at Buenos Ayres told me that he considered it as 50 per cent
less valuable. The purity of the Patagonian salt, or absence from it of
those other saline bodies found in all sea-water, is the only
assignable cause for this inferiority; a conclusion which is supported
by the fact lately ascertained,[26] that those salts answer best for
preserving cheese which contain most of the deliquescent chlorides.[27]

 [26] _Hort. and Agricult. Gazette_, 1845, p. 93.


 [27] It would probably well answer for the merchants of Buenos Ayres
 (considering the great consumption there of salt for preserving meat)
 to import the deliquescent chlorides to mix with the salt from the
 salinas: I may call attention to the fact, that at Iquique, a large
 quantity of muriate of lime, left in the _ mother-water_ during the
 refinement of the nitrate of soda, is annually thrown away.


With respect to the origin of the salt in the salinas, the foregoing
analysis seems opposed to the view entertained by M. d’Orbigny and
others, and which seems so probable considering the recent elevation of
this line of coast, namely, that it is due to the evaporation of
sea-water and to the drainage from the surrounding strata impregnated
with sea-salt. I was informed (I know not whether accurately) that on
the northern side of the salina on the Rio Negro, there is a small
brine spring which flows at all times of the year: if this be so, the
salt in this case at least, probably is of subterranean origin. It at
first appears very singular that fresh water can often be procured in
wells,[28] and is sometimes found in small lakes, quite close to these
salinas. I am not aware that this fact bears particularly on the origin
of the salt; but perhaps it is rather opposed to the view of the salt
having been washed out of the surrounding superficial strata, but not
to its having been the residue of sea-water, left in depressions as the
land was slowly elevated.

 [28] Sir W. Parish states (“Buenos Ayres,” etc., pp. 122 and 170) that
 this is the case near the great salinas westward of the S. Ventana. I
 have seen similar statements in an ancient MS. Journal lately
 published by S. Angelis. At Iquique, where the surface is so thickly
 encrusted with saline matter, I tasted water only slightly brackish,
 procured in a well thirty-six yards deep; but here one feels less
 surprise at its presence, as pure water might percolate under ground
 from the not very distant Cordillera.




Chapter IV ON THE FORMATIONS OF THE PAMPAS.


Mineralogical constitution.—Microscopical structure.—Buenos Ayres,
shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—San
Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones, and infusoria of; co-existence of the recent shells and
extinct mammifers.—Buenos Ayres to Santa Fé.—Skeletons of
Mastodon.—Infusoria.—Inferior marine tertiary strata, their
age.—Horse’s tooth. BANDA ORIENTAL.—Superficial Pampean
formation.—Inferior tertiary strata, variation of, connected with
volcanic action; Macrauchenia Patachonica at San Julian in Patagonia,
age of, subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—
Contemporaneous with existing mollusca.—Relations to underlying
tertiary strata.—Ancient deposit of estuary origin.—Elevation and
successive deposition of the Pampean formation.—Number and state of the
remains of mammifers; their habitation, food, extinction, and
range.—Conclusion.—Localities in Pampas at which mammiferous remains
have been found.

The Pampean formation is highly interesting from its vast extent, its
disputed origin, and from the number of extinct gigantic mammifers
embedded in it. It has upon the whole a very uniform character:
consisting of a more or less dull reddish, slightly indurated,
argillaceous earth or mud, often, but not always, including in
horizontal lines concretions of marl, and frequently passing into a
compact marly rock. The mud, wherever I examined it, even close to the
concretions, did not contain any carbonate of lime. The concretions are
generally nodular, sometimes rough externally, sometimes
stalactiformed; they are of a compact structure, but often penetrated
(as well as the mud) by hair-like serpentine cavities, and occasionally
with irregular fissures in their centres, lined with minute crystals of
carbonate of lime; they are of white, brown, or pale pinkish tints,
often marked by black dendritic manganese or iron; they are either
darker or lighter tinted than the surrounding mass; they contain much
carbonate of lime, but exhale a strong aluminous odour, and leave, when
dissolved in acids, a large but varying residue, of which the greater
part consists of sand. These concretions often unite into irregular
strata; and over very large tracts of country, the entire mass consists
of a hard, but generally cavernous marly rock: some of the varieties
might be called calcareous tuffs.

Dr. Carpenter has kindly examined under the microscope, sliced and
polished specimens of these concretions, and of the solid marl-rock,
collected in various places between the Colorado and Santa Fe Bajada.
In the greater number, Dr. Carpenter finds that the whole substance
presents a tolerably uniform amorphous character, but with traces of
incipient crystalline metamorphosis; in other specimens he finds
microscopically minute rounded concretions of an amorphous substance
(resembling in size those in oolitic rocks, but not having a concentric
structure), united by a cement which is often crystalline. In some, Dr.
Carpenter can perceive distinct traces of shells, corals, Polythalamia,
and rarely of spongoid bodies. For the sake of comparison, I sent Dr.
Carpenter specimens of the calcareous rock, formed chiefly of fragments
of recent shells, from Coquimbo in Chile: in one of these specimens,
Dr. Carpenter finds, besides the larger fragments, microscopical
particles of shells, and a varying quantity of opaque amorphous matter;
in another specimen from the same bed, he finds the whole composed of
the amorphous matter, with layers showing indications of
an incipient crystalline metamorphosis: hence these latter specimens,
both in external appearance and in microscopical structure, closely
resemble those of the Pampas. Dr. Carpenter informs me that it is well
known that chemical precipitation throws down carbonate of lime in the
opaque amorphous state; and he is inclined to believe that the
long-continued attrition of a calcareous body in a state of crystalline
or semi-crystalline aggregation (as, for instance, in the ordinary
shells of Mollusca, which, when sliced, are transparent) may yield the
same result. From the intimate relations between all the Coquimbo
specimens, I can hardly doubt that the amorphous carbonate of lime in
them has resulted from the attrition and decay of the larger fragments
of shell: whether the amorphous matter in the marly rocks of the Pampas
has likewise thus originated, it would be hazardous to conjecture.

For convenience’ sake, I will call the marly rock by the name given to
it by the inhabitants, namely, Tosca-rock; and the reddish argillaceous
earth, Pampean mud. This latter substance, I may mention, has been
examined for me by Professor Ehrenberg, and the result of his
examination will be given under the proper localities.

I will commence my descriptions at a central spot, namely, at Buenos
Ayres, and thence proceed first southward to the extreme limit of the
deposit, and afterwards northward. The plain on which Buenos Ayres
stands is from thirty to forty feet in height. The Pampean mud is here
of a rather pale colour, and includes small nearly white nodules, and
other irregular strata of an unusually arenaceous variety of
tosca-rock. In a well at the depth of seventy feet, according to
Ignatio Nunez, much tosca-rock was met with, and at several points, at
one hundred feet deep, beds of sand have been found. I have already
given a list of the recent marine and estuary shells found in many
parts on the surface near Buenos Ayres, as far as three or four leagues
from the Plata. Specimens from near Ensenada, given me by Sir W.
Parish, where the rock is quarried just beneath the surface of the
plain, consist of broken bivalves, cemented by and converted into white
crystalline carbonate of lime. I have already alluded, in the first
chapter, to a specimen (also given me by Sir W. Parish) from the A. del
Tristan, in which shells, resembling in every respect the _Azara
labiata_, d’Orbigny, as far as their worn condition permits of
comparison, are embedded in a reddish, softish, somewhat arenaceous
marly rock: after careful comparison, with the aid of a microscope and
acids, I can perceive no difference between the basis of this rock and
the specimens collected by me in many parts of the Pampas. I have also
stated, on the authority of Sir W. Parish, that northward of Buenos
Ayres, on the highest parts of the plain, about forty feet above the
Plata, and two or three miles from it, numerous shells of the _Azara
labiata_ (and I believe of _Venus sinuosa_) occur embedded in a
stratified earthy mass, including small marly concretions, and said to
be precisely like the great Pampean deposit. Hence we may conclude that
the mud of the Pampas continued to be deposited to within the period of
this existing estuary shell. Although this formation is of such immense
extent, I know of no other instance of the presence of shells in it.


_Buenos Ayres to the Rio Colorado._—With the exception of a few
metamorphic ridges, the country between these two points, a distance of
400 geographical miles, belongs to the Pampean formation, and in the
southern part is generally formed of the harder and more calcareous
varieties. I will briefly describe my route: about twenty-five miles
S.S.W. of the capital, in a well forty yards in depth, the upper part,
and, as I was assured, the entire thickness, was formed of dark red
Pampean mud without concretions. North of the River Salado, there are
many lakes; and on the banks of one (near the Guardia) there was a
little cliff similarly composed, but including many nodular and
stalactiform concretions: I found here a large piece of tessellated
armour, like that of the Glyptodon, and many fragments of bones. The
cliffs on the Salado consist of pale-coloured Pampean mud, including
and passing into great masses of tosca-rock: here a skeleton of the
Megatherium and the bones of other extinct quadrupeds (see the list at
the end of this chapter) were found. Large quantities of crystallised
gypsum (of which specimens were given me) occur in the cliffs of this
river; and likewise (as I was assured by Mr. Lumb) in the Pampean mud
on the River Chuelo, seven leagues from Buenos Ayres: I mention this
because M. d’Orbigny lays some stress on the supposed absence of this
mineral in the Pampean formation.

Southward of the Salado the country is low and swampy, with tosca-rock
appearing at long intervals at the surface. On the banks, however, of
the Tapalguen (sixty miles south of the Salado) there is a large extent
of tosca-rock, some highly compact and even semi-crystalline, overlying
pale Pampean mud with the usual concretions. Thirty miles further
south, the small quartz-ridge of Tapalguen is fringed on its northern
and southern flank, by little, narrow, flat-topped hills of tosca-rock,
which stand higher than the surrounding plain. Between this ridge and
the Sierra of Guitru-gueyu, a distance of sixty miles, the country is
swampy, with the tosca-rock appearing only in four or five spots: this
sierra, precisely like that of Tapalguen, is bordered by horizontal,
often cliff-bounded, little hills of tosca-rock, higher than the
surrounding plain. Here, also, a new appearance was presented in some
extensive and level banks of alluvium or detritus of the neighbouring
metamorphic rocks; but I neglected to observe whether it was stratified
or not. Between Guitru-gueyu and the Sierra Ventana, I crossed a dry
plain of tosca-rock higher than the country hitherto passed over, and
with small pieces of denuded tableland of the same formation, standing
still higher.

The marly or calcareous beds not only come up nearly horizontally to
the northern and southern foot of the great quartzose mountains of the
Sierra Ventana, but interfold between the parallel ranges. The
superficial beds (for I nowhere obtained sections more than twenty feet
deep) retain, even close to the mountains, their usual character: the
uppermost layer, however, in one place included pebbles of quartz, and
rested on a mass of detritus of the same rock. At the very foot of the
mountains, there were some few piles of quartz and tosca-rock detritus,
including land-shells; but at the distance of only half a mile
from these lofty, jagged, and battered mountains, I could not, to my
great surprise, find on the boundless surface of the calcareous plain
even a single pebble. Quartz-pebbles, however, of considerable size
have at some period been transported to a distance of between forty and
fifty miles to the shores of Bahia Blanca.[1]

 [1] Schmidtmeyer (“Travels in Chile,” p. 150) states that he first
 noticed on the Pampas, very small bits of red granite, when fifty
 miles distant from the southern extremity of the mountains of Cordova,
 which project on the plain, like a reef into the sea.


The highest peak of the St. Ventana is, by Captain Fitzroy’s
measurement, 3,340 feet, and the calcareous plain at its foot (from
observations taken by some Spanish officers[2]) 840 feet above the
sea-level. On the flanks of the mountains, at a height of three hundred
or four hundred feet above the plain, there were a few small patches of
conglomerate and breccia, firmly cemented by ferruginous matter to the
abrupt and battered face of the quartz—traces being thus exhibited of
ancient sea-action. The high plain round this range sinks quite
insensibly to the eye on all sides, except to the north, where its
surface is broken into low cliffs. Round the Sierras Tapalguen,
Guitru-gueyu, and between the latter and the Ventana we have seen (and
shall hereafter see round some hills in Banda Oriental), that the
tosca-rock forms low, flat-topped, cliff-bounded hills, higher than the
surrounding plains of similar composition. From the horizontal
stratification and from the appearance of the broken cliffs, the
greater height of the Pampean formation round these primary hills ought
not to be altogether or in chief part attributed to these several
points having been uplifted more energetically than the surrounding
country, but to the argillaceo-calcareous mud having collected round
them, when they existed as islets or submarine rocks, at a greater
height, than at the bottom of the adjoining open sea;—the cliffs having
been subsequently worn during the elevation of the whole country in
mass.

 [2] “La Plata,” etc., by Sir W. Parish, p. 146.


Southward of the Ventana, the plain extends farther than the eye can
range; its surface is not very level, having slight depressions with no
drainage exits; it is generally covered by a few feet in thickness of
sandy earth; and in some places, according to M. Parchappe,[3] beds of
clay two yards thick. On the banks of the Sauce, four leagues S.E. of
the Ventana, there is an imperfect section about two hundred feet in
height, displaying in the upper part tosca-rock and in the lower part
red Pampean mud. At the settlement of Bahia Blanca, the uppermost plain
is composed of very compact, stratified tosca-rock, containing rounded
grains of quartz distinguishable by the naked eye: the lower plain, on
which the fortress stands, is described by M. Parchappe[4] as composed
of solid tosca-rock; but the sections which I examined appeared more
like a redeposited mass of this rock, with small pebbles and fragments
of quartz. I shall immediately return to the important sections on the
shores of Bahia Blanca. Twenty miles southward of
this place, there is a remarkable ridge extending W. by N. and E. by
S., formed of small, separate, flat-topped, steep-sided hills, rising
between one hundred and two hundred feet above the Pampean plain at its
southern base, which plain is a little lower than that to the north.
The uppermost stratum in this ridge consists of pale, highly
calcareous, compact tosca-rock, resting (as seen in one place) on
reddish Pampean mud, and this again on a paler kind: at the foot of the
ridge, there is a well in reddish clay or mud. I have seen no other
instance of a chain of hills belonging to the Pampean formation; and as
the strata show no signs of disturbance, and as the direction of the
ridge is the same with that common to all the metamorphic lines in this
whole area, I suspect that the Pampean sediment has in this instance
been accumulated on and over a ridge of hard rocks, instead of, as in
the case of the above-mentioned Sierras, round their submarine flanks.
South of this little chain of tosca-rock, a plain of Pampean mud
declines towards the banks of the Colorado: in the middle a well has
been dug in red Pampean mud, covered by two feet of white, softish,
highly calcareous tosca-rock, over which lies sand with small pebbles
three feet in thickness—the first appearance of that vast shingle
formation described in the First Chapter. In the first section after
crossing the Colorado, an old tertiary formation, namely, the Rio Negro
sandstone (to be described in the next chapter), is met with: but from
the accounts given me by the Gauchos, I believe that at the mouth of
the Colorado the Pampean formation extends a little further southwards.

 [3] M. d’Orbigny, “Voyage,” Part. Géolog., pp. 47, 48.


 [4] _Ibid._


_Bahia Blanca._—To return to the shores of this bay. At Monte Hermoso
there is a good section, about one hundred feet in height, of four
distinct strata, appearing to the eye horizontal, but thickening a
little towards the N.W. The uppermost bed, about twenty feet in
thickness, consists of obliquely laminated, soft sandstone, including
many pebbles of quartz, and falling at the surface into loose sand. The
second bed, only six inches thick, is a hard, dark-coloured sandstone.
The third bed is pale-coloured Pampean mud; and the fourth is of the
same nature, but darker coloured, including in its lower part
horizontal layers and lines of concretions of not very compact pinkish
tosca-rock. The bottom of the sea, I may remark, to a distance of
several miles from the shore, and to a depth of between sixty and one
hundred feet, was found by the anchors to be composed of tosca-rock and
reddish Pampean mud. Professor Ehrenberg has examined for me specimens
of the two lower beds, and finds in them three Polygastrica and six
Phytolitharia.[5] Of these, only one (_Spongolithis
Fustis?_) is a marine form; five of them are identical with
microscopical structures of brackish-water origin, hereafter to be
mentioned, which form a central point in the Pampean formation. In
these two beds, especially in the lower one, bones of extinct
mammifers, some embedded in their proper relative positions and others
single, are very numerous in a small extent of the cliffs. These
remains consist of, first, the head of Ctenomys antiquus, allied to the
living Ctenomys Braziliensis; secondly, a fragment of the remains of a
rodent; thirdly, molar teeth and other bones of a large rodent, closely
allied to, but distinct from, the existing species of Hydrochoerus, and
therefore probably an inhabitant of fresh water; fourth and fifthly,
portions of vertebræ, limbs, ribs, and other bones of two rodents;
sixthly, bones of the extremities of some great megatheroid
quadruped.[6] The number of the remains of rodents gives to this
collection a peculiar character, compared with those found in any other
locality. All these bones are compact and heavy; many of them are
stained red, with their surfaces polished; some of the smaller ones are
as black as jet.

 [5] The following list is given in the “Monatsberichten der könig.
 Akad. zu Berlin,” April 1845:—

POLYGASTRICA.
    Fragilaria rhabdosoma.
    Gallionella distans.
    Pinnularia?
PHYTOLITHARIA.
    Lithodontium Bursa.
    Lithodontium furcatum.
    Lithostylidium exesum.
    Lithostylidium rude.
    Lithostylidium Serra.
    Spongolithis Fustis?


 [6] See “Fossil Mammalia” (p. 109) by Professor Owen, in the “Zoology
 of the Voyage of the _Beagle_;” and Catalogue (p. 36) of Fossil
 Remains in Museum of Royal College of Surgeons.

Monte Hermoso is between fifty and sixty miles distant in a S.E. line
from the Ventana, with the intermediate country gently rising towards
it, and all consisting of the Pampean formation. What relation, then,
do these beds, at the level of the sea and under it, bear to those on
the flanks of the Ventana, at the height of 840 feet, and on the flanks
of the other neighbouring sierras, which, from the reasons already
assigned, do not appear to owe their greater height to unequal
elevation? When the tosca-rock was accumulating round the Ventana, and
when, with the exception of a few small rugged primary islands, the
whole wide surrounding plains must have been under water, were the
strata at Monte Hermoso depositing at the bottom of a great open sea,
between eight hundred and one thousand feet in depth? I much doubt
this; for if so, the almost perfect carcasses of the several small
rodents, the remains of which are so very numerous in so limited a
space, must have been drifted to this spot from the distance of many
hundred miles. It appears to me far more probable, that during the
Pampean period this whole area had commenced slowly rising (and in the
cliffs, at several different heights we have proofs of the land having
been exposed to sea-action at several levels), and that tracts of land
had thus been formed of Pampean sediment round the Ventana and the
other primary ranges, on which the several rodents and other quadrupeds
lived, and that a stream (in which perhaps the extinct aquatic
Hydrochoerus lived) drifted their bodies into the adjoining sea, into
which the Pampean mud continued to be poured from the north. As the
land continued to rise, it appears that this source of sediment was cut
off; and in its place sand and pebbles were borne down by stronger
currents, and conformably deposited over the Pampean strata.

Punta Alta is situated about thirty miles higher up on the northern
side of this same bay: it consists of a small plain, between twenty and
thirty feet in height, cut off on the shore by a line of low cliffs
about a mile in length, represented in figure No. 15 with its vertical
scale necessarily exaggerated. The lower bed (A) is more extensive than
the upper ones; it consists of stratified gravel or conglomerate,
cemented by calcareo-arenaceous matter, and is divided by curvilinear
layers of pinkish marl, of which some are precisely like tosca-rock,
and some more sandy. The beds are curvilinear, owing to the action of
currents, and dip in different directions; they include an
extraordinary number of bones of gigantic mammifers and many shells.
The pebbles are of considerable size, and are of hard sandstone, and of
quartz, like that of the Ventana: there are also a few well-rounded
masses of tosca-rock.

No. 15
Section of beds with recent shells and extinct mammifers, at Punta Alta
in Bahia Blanca.


[Illustration: Section of beds at Punta Alta in Bahia Blanca.]

The second bed (B) is about fifteen feet in thickness, but towards both
extremities of the cliff (not included in the diagram) it either thins
out and dies away, or passes insensibly into an overlying bed of
gravel. It consists of red, tough clayey mud, with minute linear
cavities; it is marked with faint horizontal shades of colour; it
includes a few pebbles, and rarely a minute particle of shell: in one
spot, the dermal armour and a few bones of a Dasypoid quadruped were
embedded in it: it fills up furrows in the underlying gravel. With the
exception of the few pebbles and particles of shells, this bed
resembles the true Pampean mud; but it still more closely resembles the
clayey flats (mentioned in the First Chapter) separating the
successively rising parallel ranges of sand-dunes.

The bed (C) is of stratified gravel, like the lowest one; it fills up
furrows in the underlying red mud, and is sometimes interstratified
with it, and sometimes insensibly passes into it; as the red mud thins
out, this upper gravel thickens. Shells are more numerous in it than in
the lower gravel; but the bones, though some are still present, are
less numerous. In one part, however, where this gravel and the red mud
passed into each other, I found several bones and a tolerably perfect
head of the Megatherium. Some of the large Volutas, though embedded in
the gravel-bed (C), were filled with the red mud, including great
numbers of the little recent _Paludestrina australis._ These three
lower beds are covered by an unconformable mantle (D) of stratified
sandy earth, including many pebbles of quartz, pumice and phonolite,
land and sea-shells.

M. d’Orbigny has been so obliging as to name for me the twenty species
of Mollusca embedded in the two gravel beds: they consist of:—


Volutella angulata, d’Orbigny, “Voyage” Mollusq. and Pal.

Voluta Braziliana, Sol.

Olicancilleria Braziliensis, d’Orbigny.

Olicancilleria auricularia, d’Orbigny.

Olivina puelchana, d’Orbigny.

Buccinanops cochlidium, d’Orbigny.

Buccinanops globulosum, d’Orbigny.

Colombella sertulariarum, d’Orbigny.

Trochus Patagonicus, and var. of ditto, d’Orbigny.

Paludestrina Australis, d’Orbigny.

Fissurella Patagonica, d’Orbigny.

Crepidula muricata, Lam.

Venus purpurata, Lam.

Venus rostrata, Phillippi.

Mytilus Darwinianus, d’Orbigny.

Nucula semiornata, d’Orbigny.

Cardita Patagonica, d’Orbigny.

Corbula Patagonica (?), d’Orbigny.

Pecten tethuelchus, d’Orbigny.

Ostrea puelchana, d’Orbigny.

A living species of Balanus.

and 23. An Astræ and encrusting Flustra, apparently identical with
species now living in the bay.


All these shells now live on this coast, and most of them in this same
bay. I was also struck with the fact, that the proportional numbers of
the different kinds appeared to be the same with those now cast up on
the beach: in both cases specimens of Voluta, Crepidula, Venus, and
Trochus are the most abundant. Four or five of the species are the same
with the upraised shells on the Pampas near Buenos Ayres. All the
specimens have a very ancient and bleached appearance,[7] and do not
emit, when heated, an animal odour: some of them are changed throughout
into a white, soft, fibrous substance; others have the space between
the external walls, either hollow, or filled up with crystalline
carbonate of lime.

 [7] A Bulinus, mentioned in the Introduction to the “Fossil Mammalia”
 in the “Zoology of the Voyage of the _ Beagle_” has so much fresher an
 appearance, than the marine species, that I suspect it must have
 fallen amongst the others, and been collected by mistake.


The remains of the extinct mammiferous animals, from the two gravel
beds have been described by Professor Owen in the “Zoology of the
Voyage of the _Beagle_:” they consist of, 1st, one nearly perfect head
and three fragments of heads of the _ Megatherium Cuvierii_; 2nd, a
lower jaw of _Megalonyx Jeffersonii_; 3rd, lower jaw of _Mylodon
Darwinii_; 4th, fragments of a head of some gigantic Edental quadruped;
5th, an almost entire skeleton of the great _Scelidotherium
leptocephalum_, with most of the bones, including the head, vertebræ,
ribs, some of the extremities to the claw-bone, and even, as remarked
by Professor Owen, the knee-cap, all nearly in their proper relative
positions; 6th, fragments of the jaw and a separate tooth of a Toxodon,
belonging either to _T. Platensis_, or to a second species lately
discovered near Buenos Ayres; 7th, a tooth of _Equus curvidens_; 8th,
tooth of a Pachyderm, closely allied to Palæotherium, of which parts of
the head have been lately sent from Buenos Ayres to the British Museum;
in all probability this pachyderm is identical with the _ Macrauchenia
Patagonica_ from Port S. Julian, hereafter to be referred to. Lastly,
and 9thly, in a cliff of the red clayey bed (B), there was a double
piece, about three feet long and two wide, of the bony armour of a
large Dasypoid quadruped, with the two sides pressed nearly close
together: as the
cliff is now rapidly washing away, this fossil probably was lately much
more perfect; from between its doubled-up sides, I extracted the middle
and ungual phalanges, united together, of one of the feet, and likewise
a separate phalanx: hence one or more of the limbs must have been
attached to the dermal case, when it was embedded. Besides these
several remains in a distinguishable condition, there were very many
single bones: the greater number were embedded in a space 200 yards
square. The preponderance of the Edental quadrupeds is remarkable; as
is, in contrast with the beds of Monte Hermoso, the absence of Rodents.
Most of the bones are now in a soft and friable condition, and, like
the shells, do not emit when burnt an animal odour. The decayed state
of the bones may be partly owing to their late exposure to the air and
tidal-waves. Barnacles, Serpulæ, and corallines are attached to many of
the bones, but I neglected to observe[8] whether these might not have
grown on them since being exposed to the present tidal action; but I
believe that some of the barnacles must have grown on the
Scelidotherium, soon after being deposited, and before being _wholly_
covered up by the gravel. Besides the remains in the condition here
described, I found one single fragment of bone very much rolled, and as
black as jet, so as perfectly to resemble some of the remains from
Monte Hermoso.

 [8] After having packed up my specimens at Bahia Blanca, this point
 occurred to me, and I noted it; but forgot it on my return, until the
 remains had been cleaned and oiled: my attention has been lately
 called to the subject by some remarks by M. d’Orbigny.

Very many of the bones had been broken, abraded, and rolled, before
being embedded. Others, even some of those included in the coarsest
parts of the the now hard conglomerate, still retain all their minutest
prominences perfectly preserved; so that I conclude that they probably
were protected by skin, flesh, or ligaments, whilst being covered up.
In the case of the Scelidotherium, it is quite certain that the whole
skeleton was held together by its ligaments, when deposited in the
gravel in which I found it. Some cervical vertebræ and a humerus of
corresponding size lay so close together, as did some ribs and the
bones of a leg, that I thought that they must originally have belonged
to two skeletons, and not have been washed in single; but as remains
were here very numerous, I will not lay much stress on these two cases.
We have just seen that the armour of the Dasypoid quadruped was
certainly embedded together with some of the bones of the feet.

Professor Ehrenberg[9] has examined for me specimens of the finer
matter from in contact with these mammiferous remains: he finds in them
two Polygastrica, decidedly marine forms; and six Phytolitharia, of
which one is probably marine, and the others either of fresh-water or
terrestrial origin. Only one of these eight microscopical bodies is
common to the nine from Monte Hermoso: but five of them are in common
with those from the Pampean mud on the banks of the Parana. The
presence of any fresh-water infusoria, considering the aridity of the
surrounding country, is here remarkable: the most probable explanation
appears to be, that these microscopical organisms were washed out of
the adjoining great Pampean formation during its denudation, and
afterwards redeposited.

 [9] “Monatsberichten der Akad. zu Berlin,” April 1845. The list
 consists of:—

POLYGASTRICA.
    Gallionella sulcata.
    Stauroptera aspera? fragm.
 PHYTOLITHARIA.
    Lithasteriscus tuberculatus.
    Lithostylidium Clepsammidium.
    Lithostylidium quadratum.
    Lithostylidium rude.
    Lithostylidium unidentatum.
    Spongolithis acicularis.


We will now see what conclusions may be drawn from the facts above
detailed. It is certain that the gravel-beds and intermediate red mud
were deposited within the period, when existing species of Mollusca
held to each other nearly the same relative proportions as they do on
the present coast. These beds, from the number of littoral species,
must have been accumulated in shallow water; but not, judging from the
stratification of the gravel and the layers of marl, on a beach. From
the manner in which the red clay fills up furrows in the underlying
gravel, and is in some parts itself furrowed by the overlying gravel,
whilst in other parts it either insensibly passes into, or alternates
with, this upper gravel, we may infer several local changes in the
currents, perhaps caused by slight changes, up or down, in the level of
the land. By the elevation of these beds, to which period the alluvial
mantle with pumice-pebbles, land and sea-shells belongs, the plain of
Punta Alta, from twenty to thirty feet in height, was formed. In this
neighbourhood there are other and higher sea-formed plains and lines of
cliffs in the Pampean formation worn by the denuding action of the
waves at different levels. Hence we can easily understand the presence
of rounded masses of tosca-rock in this lowest plain; and likewise, as
the cliffs at Monte Hermoso with their mammiferous remains stand at a
higher level, the presence of the one much-rolled fragment of bone
which was as black as jet: possibly some few of the other much-rolled
bones may have been similarly derived, though I saw only the one
fragment, in the same condition with those from Monte Hermoso. M.
d’Orbigny has suggested[10] that all these mammiferous remains may have
been washed out of the Pampean formation, and afterwards redeposited
together with the recent shells. Undoubtedly it is a marvellous fact
that these numerous gigantic quadrupeds, belonging, with the exception
of the _Equus curvidens_, to seven extinct genera, and one, namely, the
Toxodon, not falling into any existing family, should have co-existed
with Mollusca, all of which are still living species; but analogous
facts have been observed in North America and in Europe. In the first
place, it should not be overlooked, that most of the co-embedded shells
have a more ancient and altered appearance than the bones. In the
second place, is it probable that numerous bones not hardened by silex
or any other mineral, could have retained their delicate prominences
and surfaces perfect if they
had been washed out of one deposit, and re-embedded in another:—this
later deposit being formed of large, hard pebbles, arranged by the
action of currents or breakers in shallow water into variously curved
and inclined layers? The bones which are now in so perfect a state of
preservation, must, I conceive, have been fresh and sound when
embedded, and probably were protected by skin, flesh, or ligaments. The
skeleton of the Scelidotherium indisputably was deposited entire: shall
we say that when held together by its matrix it was washed out of an
old gravel-bed (totally unlike in character to the Pampean formation),
and re-embedded in another gravel-bed, composed (I speak after careful
comparison) of exactly the same kind of pebbles, in the same kind of
cement? I will lay no stress on the two cases of several ribs and bones
of the extremities having _apparently_ been embedded in their proper
relative position: but will any one be so bold as to affirm that it is
possible, that a piece of the thin tessellated armour of a Dasypoid
quadruped, at least three feet long and two in width, and now so tender
that I was unable with the utmost care to extract a fragment more than
two or three inches square, could have been washed out of one bed, and
re-embedded in another, together with some of the small bones of the
feet, without having been dashed into atoms? We must then wholly reject
M. d’Orbigny’s supposition, and admit as certain, that the
Scelidotherium and the large Dasypoid quadruped, and as highly
probable, that the Toxodon, Megatherium, etc., some of the bones of
which are perfectly preserved, were embedded for the first time, and in
a fresh condition, in the strata in which they were found entombed.
These gigantic quadrupeds, therefore, though belonging to extinct
genera and families, coexisted with the twenty above-enumerated
Mollusca, the barnacle and two corals, still living on this coast. From
the rolled fragment of black bone, and from the plain of Punta Alta
being lower than that of Monte Hermoso, I conclude that the coarse
sub-littoral deposits of Punta Alta, are of subsequent origin to the
Pampean mud of Monte Hermoso; and the beds at this latter place, as we
have seen, are probably of subsequent origin to the high tosca-plain
round the Sierra Ventana: we shall, however, return, at the end of this
chapter, to the consideration of these several stages in the great
Pampean formation.

 [10] “Voyage,” Part. Géolog., p. 49.


_Buenos Ayres to St. Fé Bajada, in Entre Rios._—For some distance
northward of Buenos Ayres, the escarpment of the Pampean formation does
not approach very near to the Plata, and it is concealed by vegetation:
but in sections on the banks of the Rios Luxan, Areco, and Arrecifes, I
observed both pale and dark reddish Pampean mud, with small, whitish
concretions of tosca; at all these places mammiferous remains have been
found. In the cliffs on the Parana, at San Nicolas, the Pampean mud
contains but little tosca; here M. d’Orbigny found the remains of two
rodents (_Ctenomys Bonariensis_ and _Kerodon antiquus_) and the jaw of
a Canis: when on the river I could clearly distinguish in this fine
line of cliffs, “horizontal lines of variation both in tint and
compactness.”[11] The plain northward of this point is very
level, but with some depressions and lakes; I estimated its height at
from forty to sixty feet above the Parana. At the A. Medio the bright
red Pampean mud contains scarcely any tosca-rock; whilst at a short
distance the stream of the Pabon, forms a cascade, about twenty feet in
height, over a cavernous mass of two varieties of tosca-rock; of which
one is very compact and semi-crystalline, with seams of crystallised
carbonate of lime: similar compact varieties are met with on the
Salidillo and Seco. The absolute identity (I speak after a comparison
of my specimens) between some of these varieties, and those from
Tapalguen, and from the ridge south of Bahia Blanca, a distance of 400
miles of latitude, is very striking.

 [11] I quote these words from my note-book, as written down on the
 spot, on account of the general absence of stratification in the
 Pampean formation having been insisted on by M. d’Orbigny as a proof
 of the diluvial origin of this great deposit.

At Rosario there is but little tosca-rock: near this place I first
noticed at the edge of the river traces of an underlying formation,
which, twenty-five miles higher up in the estancia of Gorodona,
consists of a pale yellowish clay, abounding with concretionary
cylinders of a ferruginous sandstone. This bed, which is probably the
equivalent of the older tertiary marine strata, immediately to be
described in Entre Rios, only just rises above the level of the Parana
when low. The rest of the cliff at Gorodona, is formed of red Pampean
mud, with, in the lower part, many concretions of tosca, some
stalacti-formed, and with only a few in the upper part: at the height
of six feet above the river, two gigantic skeletons of the _Mastodon
Andium_ were here embedded; their bones were scattered a few feet
apart, but many of them still held their proper relative positions:
they were much decayed and as soft as cheese, so that even one of the
great molar teeth fell into pieces in my hand. We here see that the
Pampean deposit contains mammiferous remains close to its base. On the
banks of the Carcarana, a few miles distant, the lowest bed visible was
pale Pampean mud, with masses of tosca-rock, in one of which I found a
much decayed tooth of the Mastodon: above this bed, there was a thin
layer almost composed of small concretions of white tosca, out of which
I extracted a well preserved, but slightly broken tooth of _Toxodon
Platensis_: above this there was an unusual bed of very soft impure
sandstone. In this neighbourhood I noticed many single embedded bones,
and I heard of others having been found in so perfect a state that they
were long used as gate-posts: the Jesuit Falkner found here the dermal
armour of some gigantic Edental quadruped.

In some of the red mud scraped from a tooth of one of the Mastodons at
Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen
Phytolitharia,[12] all of them, I believe, with two exceptions, already
known species. Of these twenty, the preponderating number are of
fresh-water origin; only two species of Coscinodiscus and a
Spongolithis show the direct influence of the sea; therefore Professor
Ehrenberg arrives at the important conclusion that the deposit must
have been of brackish-water origin. Of the thirteen Phytolitharia, nine
are met with in the two deposits in Bahia Blanca, where there is
evidence from two other species of Polygastrica that the beds were
accumulated in brackish water. The traces of coral, sponges, and
Polythalamia, found by Dr. Carpenter in the tosca-rock (of which I must
observe the greater number of specimens were from the upper beds in the
southern parts of the formation), apparently show a more purely marine
origin.

 [12] “Monatsberichten der könig. Akad. zu Berlin,” April 1845. The
 list consists of:—

 POLYGASTRICA.
    Campylodiscus clypeus.
    Coscinodiscus subtilis.
    Coscinodiscus al. sp.
    Eunotia.
    Gallionella granulata.
    Himantidium gracile.
    Pinnularia borealis.


At _St. Fé Bajada_, in Entre Rios, the cliffs, estimated at between
sixty and seventy feet in height, expose an interesting section: the
lower half consists of tertiary strata with marine shells, and the
upper half of the Pampean formation. The lowest bed is an obliquely
laminated, blackish, indurated mud, with distinct traces of vegetable
remains.[13] Above this there is a thick bed of yellowish sandy clay,
with much crystallised gypsum and many shells of Ostreæ, Pectens, and
Arcæ: above this there generally comes an arenaceous crystalline
limestone, but there is sometimes interposed a bed, about twelve feet
thick, of dark green, soapy clay, weathering into small angular
fragments. The limestone, where purest, is white, highly crystalline,
and full of cavities: it includes small pebbles of quartz, broken
shells, teeth of sharks, and sometimes, as I was informed, large bones:
it often contains so much sand as to pass into a calcareous sandstone,
and in such parts the great _Ostrea Patagonica_[14] chiefly abounds. In
the upper part, the limestone alternates with layers of fine white
sand. The shells included in these beds have been named for me by M.
d’Orbigny: they consist of:—

Ostrea Patagonica, d’Orbigny, “Voyage,” Part. Pal.

Ostrea Alvarezii, d’Orbigny, “Voyage,” Part. Pal.

Pecten Paranensis, d’Orbigny, “Voyage,” Part. Pal.

Pecten Darwinianus, d’Orbigny, “Voyage,” Part. Pal.

Venus Munsterii, d’Orbigny, “Voyage,” Pal.

Arca Bonplandiana, d’Orbigny, “Voyage,” Pal.

Cardium Platense, d’Orbigny, “Voyage,” Pal.

Tellina, probably nov. spec., but too imperfect for description.

PHYTOLITHARIA.
    Lithasteriscus tuberculatus.
    Lithodontium bursa.
    Lithodontium furcatum.
    Lithodontium rostratum.
    Lithostylidium Amphiodon.
    Lithostylidium Clepsammidium.
    Lithostylidium Hamus.
    Lithostylidium polyedrum.
    Lithostylidium quadratum.
    Lithostylidium rude.
    Lithostylidium Serra.
    Lithostylidium unidentatum.
    Spongolithis Fustis.

 [13] M. d’Orbigny has given (“Voyage,” Part. Géolog., p. 37) a
 detailed description of this section, but as he does not mention this
 lowest bed, it may have been concealed when he was there by the river.
 There is a considerable discrepancy between his description and mine,
 which I can only account for by the beds themselves varying
 considerably in short distances.


 [14] Captain Sulivan, R.N., has given me a specimen of this shell,
 which he found in the cliffs at Point Cerrito, between twenty and
 thirty miles above the Bajada.


These species are all extinct: the six first were found by M. d’Orbigny
and myself in the formations of the Rio Negro, S. Josef, and other
parts of Patagonia; and therefore, as first observed by M. d’Orbigny,
these beds certainly belong to the great Patagonian formation, which
will be described in the ensuing chapter, and which we shall see must
be considered as a very ancient tertiary one. North of the Bajada, M.
d’Orbigny found, in beds which he considers as lying beneath the strata
here described, remains of a Toxodon, which he has named as a distinct
species from the _T. Platensis_ of the Pampean formation. Much
silicified wood is found on the banks of the Parana (and likewise on
the Uruguay), and I was informed that they come out of these lower
beds; four specimens collected by myself are dicotyledonous.

The upper half of the cliff, to a thickness of about thirty feet,
consists of Pampean mud, of which the lower part is pale-coloured, and
the upper part of a brighter red, with some irregular layers of an
arenaceous variety of tosca, and a few small concretions of the
ordinary kind. Close above the marine limestone, there is a thin
stratum with a concretionary outline of white hard tosca-rock or marl,
which may be considered either as the uppermost bed of the inferior
deposits, or the lowest of the Pampean formation; at one time I
considered this bed as marking a passage between the two formations:
but I have since become convinced that I was deceived on this point. In
the section on the Parana, I did not find any mammiferous remains; but
at two miles distance on the A. Tapas (a tributary of the Conchitas),
they were extremely numerous in a low cliff of red Pampean mud with
small concretions, precisely like the upper bed on the Parana. Most of
the bones were solitary and much decayed; but I saw the dermal armour
of a gigantic Edental quadruped, forming a caldron-like hollow, four or
five feet in diameter, out of which, as I was informed, the almost
entire skeleton had been lately removed. I found single teeth of the
_Mastodon Andium, Toxodon Platensis_, and _Equus curvidens_, near to
each other. As this latter tooth approaches closely to that of the
common horse, I paid particular attention to its true embedment, for I
did not at that time know that there was a similar tooth hidden in the
matrix with the other mammiferous remains from Punta Alta. It is an
interesting circumstance, that Professor Owen finds that the teeth of
this horse approach more closely in their peculiar curvature to a
fossil specimen brought by Mr. Lyell[15] from North America, than to
those of any other species of Equus.

 [15] Lyell’s “Travels in North America,” vol. i, p. 164 and “Proc. of
 Geolog. Soc.,” vol. iv, p. 39.

The underlying marine tertiary strata extend over a wide area: I was
assured that they can be traced in ravines in an east and west line
across Entre Rios to the Uruguay, a distance of about 135 miles. In a
S.E. direction I heard of their existence at the head of the R. Nankay;
and at P. Gorda in Banda Oriental, a distance of
170 miles, I found the same limestone, containing the same fossil
shells, lying at about the same level above the river as at St. Fe. In
a southerly direction, these beds sink in height, for at another P.
Gorda in Entre Rios, the limestone is seen at a much less height; and
there can be little doubt that the yellowish sandy clay, on a level
with the river, between the Carcarana and S. Nicholas, belongs to this
same formation; as perhaps do the beds of sand at Buenos Ayres, which
lie at the bottom of the Pampean formation, about sixty feet beneath
the surface of the Plata. The southerly declination of these beds may
perhaps be due, not to unequal elevation, but to the original form of
the bottom of the sea, sloping from land situated to the north; for
that land existed at no great distance, we have evidence in the
vegetable remains in the lowest bed at St. Fe; and in the silicified
wood and in the bones of _Toxodon Paranensis_, found (according to M.
d’Orbigny) in still lower strata.

_Banda Oriental._—This province lies on the northern side of the Plata,
and eastward of the Uruguay: it has a gentle undulatory surface, with a
basis of primary rocks; and is in most parts covered up with an
unstratified mass, of no great thickness, of reddish Pampean mud. In
the eastern half, near Maldonado, this deposit is more arenaceous than
in the Pampas, it contains many though small concretions of marl or
tosca-rock, and others of highly ferruginous sandstone; in one section,
only a few yards in depth, it rested on stratified sand. Near Monte
Video this deposit in some spots appears to be of greater thickness;
and the remains of the Glyptodon and other extinct mammifers have been
found in it. In the long line of cliffs, between fifty and sixty feet
in height, called the Barrancas de S. Gregorio, which extend westward
of the Rio S. Lucia, the lower half is formed of coarse sand of quartz
and feldspar without mica, like that now cast up on the beach near
Maldonado; and the upper half of Pampean mud, varying in colour and
containing honeycombed veins of soft calcareous matter and small
concretions of tosca-rock arranged in lines, and likewise a few pebbles
of quartz. This deposit fills up hollows and furrows in the underlying
sand; appearing as if water charged with mud had invaded a sandy beach.
These cliffs extend far westward, and at a distance of sixty miles,
near Colonia del Sacramiento, I found the Pampean deposit resting in
some places on this sand, and in others on the primary rocks: between
the sand and the reddish mud, there appeared to be interposed, but the
section was not a very good one, a thin bed of shells of an existing
Mytilus, still partially retaining their colour. The Pampean formation
in Banda Oriental might readily be mistaken for an alluvial deposit:
compared with that of the Pampas, it is often more sandy, and contains
small fragments of quartz; the concretions are much smaller, and there
are no extensive masses of tosca-rock.

In the extreme western parts of this province, between the Uruguay and
a line drawn from Colonia to the R. Perdido (a tributary of the R.
Negro), the formations are far more complicated. Besides primary rocks,
we meet with extensive tracts and many flat-topped, horizontally
stratified, cliff-bounded, isolated hills of tertiary strata, varying
extraordinarily in mineralogical nature, some identical with the old
marine beds of St. Fé Bajada, and some with those of the much more
recent Pampean formation. There are, also, extensive _low_ tracts of
country covered with a deposit containing mammiferous remains,
precisely like that just described in the more eastern parts of the
province. Although from the smooth and unbroken state of the country, I
never obtained a section of this latter deposit close to the foot of
the higher tertiary hills, yet I have not the least doubt that it is of
quite subsequent origin; having been deposited after the sea had worn
the tertiary strata into the cliff-bounded hills. This later formation,
which is certainly the equivalent of that of the Pampas, is well seen
in the valleys in the estancia of Berquelo, near Mercedes; it here
consists of reddish earth, full of rounded grains of quartz, and with
some small concretions of tosca-rock arranged in horizontal lines, so
as perfectly to resemble, except in containing a little calcareous
matter, the formation in the eastern parts of Banda Oriental, in Entre
Rios, and at other places: in this estancia the skeleton of a great
Edental quadruped was found. In the valley of the Sarandis, at the
distance of only a few miles, this deposit has a somewhat different
character, being whiter, softer, finer-grained, and full of little
cavities, and consequently of little specific gravity; nor does it
contain any concretions or calcareous matter: I here procured a head,
which when first discovered must have been quite perfect, of the
_Toxodon Platensis_, another of a Mylodon,[16] perhaps _M. Darwinii_,
and a large piece of dermal armour, differing from that of the
_Glyptodon clavipes._ These bones are remarkable from their
extraordinarily fresh appearance; when held over a lamp of spirits of
wine, they give out a strong odour and burn with a small flame; Mr. T.
Reeks has been so kind as to analyse some of the fragments, and he
finds that they contain about 7 per cent of animal matter, and 8 per
cent of water.[17]

 [16] This head was at first considered by Professor Owen (in the
 “Zoology of the _Beagle_’s Voyage”) as belonging to a distinct genus,
 namely, Glossotherium.


 [17] Liebig (“Chemistry of Agriculture,” p. 194) states that fresh dry
 bones contain from 32 to 33 per cent of dry gelatine. See also Dr.
 Daubeny, in _Edin. New Phil. Journ._, vol. xxxvii, p. 293.


The older tertiary strata, forming the higher isolated hills and
extensive tracts of country, vary, as I have said, extraordinarily in
composition: within the distance of a few miles, I sometimes passed
over crystalline limestone with agate, calcareous tuffs, and marly
rocks, all passing into each other,—red and pale mud with concretions
of tosca-rock, quite like the Pampean formation,—calcareous
conglomerates and sandstones,—bright red sandstones passing either into
red conglomerate, or into white sandstone,—hard siliceous sandstones,
jaspery and chalcedonic rocks, and numerous other subordinate
varieties. I was unable to mark out the relations of all these strata,
and will describe only a few distinct sections:—in the cliffs between
P. Gorda on the Uruguay and the A. de Vivoras, the upper bed is
crystalline
cellular limestone often passing into calcareous sandstone, with
impressions of some of the same shells as at St. Fé Bajada; at P.
Gorda,[18] this limestone is interstratified with and rests on, white
sand, which covers a bed about thirty feet thick of pale-coloured clay,
with many shells of the great _Ostrea Patagonica_: beneath this, in the
vertical cliff, nearly on a level with the river, there is a bed of red
mud absolutely like the Pampean deposit, with numerous often large
concretions of perfectly characterised white, compact tosca-rock. At
the mouth of the Vivoras, the river flows over a pale cavernous
tosca-rock, quite like that in the Pampas, and this _appeared_ to
underlie the crystalline limestone; but the section was not unequivocal
like that at P. Gorda. These beds now form only a narrow and much
denuded strip of land; but they must once have extended much further;
for on the next stream, south of the S. Juan, Captain Sulivan, R.N.,
found a little cliff, only just above the surface of the river, with
numerous shells of the _Venus Munsterii_, D’Orbigny,—one of the species
occurring at St. Fé, and of which there are casts at P. Gorda: the line
of cliffs of the subsequently deposited true Pampean mud, extend from
Colonia to within half a mile of this spot, and no doubt once covered
up this denuded marine stratum. Again at Colonia, a Frenchman found, in
digging the foundations of a house, a great mass of the _Ostrea
Patagonica_ (of which I saw many fragments), packed together just
beneath the surface, and directly superimposed on the gneiss. These
sections are important: M. d’Orbigny is unwilling to believe that beds
of the same nature with the Pampean formation ever underlie the ancient
marine tertiary strata; and I was as much surprised at it as he could
have been; but the vertical cliff at P. Gorda allowed of no mistake,
and I must be permitted to affirm, that after having examined the
country from the Colorado to St. Fé Bajada, I could not be deceived in
the mineralogical character of the Pampean deposit.

 [18] In my “Journal” (p. 171, 1st edit.), I have hastily and
 inaccurately stated that the Pampean mud, which is found over the
 eastern part of B. Oriental, lies _over_ the limestone at P. Gorda; I
 should have said that there was reason to infer that it was a
 subsequent or superior deposit.

Moreover, in a precipitous part of the ravine of Las Bocas, a red
sandstone is distinctly seen to overlie a thick bed of pale mud, also
quite like the Pampean formation, abounding with concretions of true
tosca-rock. This sandstone extends over many miles of country: it is as
red as the brightest volcanic scoriæ; it sometimes passes into a coarse
red conglomerate composed of the underlying primary rocks; and often
passes into a soft white sandstone with red streaks. At the Calera de
los Huerfanos, only a quarter of a mile south of where I first met with
the red sandstone, the crystalline white limestone is quarried: as this
bed is the uppermost, and as it often passes into calcareous sandstone,
interstratified with pure sand; and as the red sandstone likewise
passes into soft white sandstone, and is also the uppermost bed, I
believe that these two beds, though so different, are equivalents. A
few leagues southward of these two places, on each side of the low
primary range of S. Juan, there are some flat-topped, cliff-bounded,
separate little hills,
very similar to those fringing the primary ranges in the great plain
south of Buenos Ayres: they are composed—1st, of calcareous tuff with
many particles of quartz, sometimes passing into a coarse conglomerate;
2nd, of a stone undistinguishable on the closest inspection from the
compacter varieties of tosca-rock; and 3rd, of semi-crystalline
limestone, including nodules of agate: these three varieties pass
insensibly into each other, and as they form the uppermost stratum in
this district, I believe that they, also, are the equivalents of the
pure crystalline limestone, and of the red and white sandstones and
conglomerates.

Between these points and Mercedes on the Rio Negro, there are scarcely
any good sections, the road passing over limestone, tosca-rock,
calcareous and bright red sandstones, and near the source of the San
Salvador over a wide extent of jaspery rocks, with much milky agate,
like that in the limestone near San Juan. In the estancia of Berquelo,
the separate, flat-topped, cliff-bounded hills are rather higher than
in the other parts of the country; they range in a N.E. and S.W.
direction; their uppermost beds consist of the same bright red
sandstone, passing sometimes into a conglomerate, and in the lower part
into soft white sandstone, and even into loose sand: beneath this
sandstone, I saw in two places layers of calcareous and marly rocks,
and in one place red Pampean-like earth; at the base of these sections,
there was a hard, stratified, white sandstone, with chalcedonic layers.
Near Mercedes, beds of the same nature and apparently of the same age,
are associated with compact, white, crystalline limestone, including
much botryoidal agate, and singular masses, like porcelain, but really
composed of a calcareo-siliceous paste. In sinking wells in this
district the chalcedonic strata seem to be the lowest. Beds, such as
there described, occur over the whole of this neighbourhood; but twenty
miles further up the R. Negro, in the cliffs of Perika, which are about
fifty feet in height, the upper bed is a prettily variegated
chalcedony, mingled with a pure white tallowy limestone; beneath this
there is a conglomerate of quartz and granite; beneath this many
sandstones, some highly calcareous; and the whole lower two-thirds of
the cliff consists of earthy calcareous beds of various degrees of
purity, with one layer of reddish Pampean-like mud.

When examining the agates, the chalcedonic and jaspery rocks, some of
the limestones, and even the bright red sandstones, I was forcibly
struck with their resemblance to deposits formed in the neighbourhood
of volcanic action. I now find that M. Isabelle, in his “Voyage a
Buenos Ayres,” has described closely similar beds on Itaquy and Ibicuy
(which enter the Uruguay some way north of the R. Negro) and these beds
include fragments of red decomposed true scoriæ hardened by zeolite,
and of black retinite: we have then here good evidence of volcanic
action during our tertiary period. Still further north, near S.
Anna,[19] where the Parana makes a remarkable bend, M. Bonpland found
some singular amygdaloidal rocks, which perhaps may belong to this same
epoch. I may remark that, judging from the size and well-rounded
condition of the blocks of rock in the above-described conglomerates,
masses of primary formation probably existed at this tertiary period
above water: there is, also, according to M. Isabelle, much
conglomerate further north, at Salto.

 [19] M. d’Orbigny, “Voyage,” Part. Géolog., p. 29.

From whatever source and through whatever means the great Pampean
formation originated, we here have, I must repeat, unequivocal evidence
of a similar action at a period before that of the deposition of the
marine tertiary strata with extinct shells, at Santa Fé and P. Gorda.
During also the deposition of these strata, we have in the intercalated
layers of red Pampean-like mud and tosca-rock, and in the passage near
S. Juan of the semi-crystalline limestones with agate into tosca
undistinguishable from that of the Pampas, evidence of the same action,
though continued only at intervals and in a feeble manner. We have
further seen that in this district, at a period not only subsequent to
the deposition of the tertiary strata, but to their upheavement and
most extensive denudation, true Pampean mud with its usual characters
and including mammiferous remains, was deposited round and between the
hills or islets formed of these tertiary strata, and over the whole
eastern and low primary districts of Banda Oriental.

No. 16
Section of the lowest plain at Port S. Julian.


[Illustration: Section of the lowest plain at Port S. Julian.]

_Earthy mass, with extinct mammiferous remains, over the porphyritic
gravel at S. Julian, lat. 49° 14′ S., in Patagonia._—This case, though
not coming strictly under the Pampean formation, may be conveniently
given here. On the south side of the harbour, there is a nearly level
plain (mentioned in the First Chapter) about seven miles long, and
three or four miles wide, estimated at ninety feet in height, and
bordered by perpendicular cliffs, of which a section is represented
above.

The lower old tertiary strata (to be described in the next chapter) are
covered by the usual gravel bed; and this by an irregular earthy,
sometimes sandy mass, seldom more than two or three feet in thickness,
except where it fills up furrows or gullies worn not only through the
underlying gravel, but even through the upper tertiary beds. This
earthy mass is of a pale reddish colour, like the less pure varieties
of Pampean mud in Banda Oriental; it includes small calcareous
concretions, like those of tosca-rock but more arenaceous, and other
concretions of a greenish, indurated argillaceous substance: a few
pebbles, also, from the underlying gravel-bed are also included in it,
and these being occasionally arranged in horizontal lines, show that
the mass is of sub-aqueous origin. On the surface and embedded in the
superficial parts, there are numerous shells, partially retaining their
colours, of three or four of the now commonest littoral species. Near
the bottom of one deep furrow (represented in figure No. 16), filled up
with this earthy deposit, I found a large part of the skeleton of the
_Macrauchenia Patachonica_—a gigantic and most extraordinary pachyderm,
allied, according to Professor Owen, to the Palæotherium, but with
affinities to the Ruminants, especially to the American division of the
Camelidæ. Several of the vertebræ in a chain, and nearly all the bones
of one of the limbs, even to the smallest bones of the foot, were
embedded in their proper relative positions: hence the skeleton was
certainly united by its flesh or ligaments, when enveloped in the mud.
This earthy mass, with its concretions and mammiferous remains, filling
up furrows in the underlying gravel, certainly presents a very striking
resemblance to some of the sections (for instance, at P. Alta in B.
Blanca, or at the Barrancas de S. Gregorio) in the Pampean formation;
but I must believe that this resemblance is only accidental. I suspect
that the mud which at the present day is accumulating in deep and
narrow gullies at the head of the harbour, would, after elevation,
present a very similar appearance. The southernmost part of the true
Pampean formation, namely, on the Colorado, lies 560 miles of latitude
north of this point.[20]

 [20] In the succeeding chapter I shall have to refer to a great
 deposit of extinct mammiferous remains, lately discovered by Captain
 Sulivan, R.N., at a point still further south, namely, at the R.
 Gallegos; their age must at present remain doubtful.

With respect to the age of the Macrauchenia, the shells on the surface
prove that the mass in which the skeleton was enveloped has been
elevated above the sea within the recent period: I did not see any of
the shells embedded at a sufficient depth to assure me (though it be
highly probable) that the whole thickness of the mass was
contemporaneous with these _individual specimens._ That the
Macrauchenia lived subsequently to the spreading out of the gravel on
this plain is certain; and that this gravel, at the height of ninety
feet, was spread out long after the existence of recent shells, is
scarcely less certain. For, it was shown in the First Chapter, that
this line of coast has been upheaved with remarkable equability, and
that over a vast space both north and south of S. Julian, recent
species of shells are strewed on (or embedded in) the surface of the
250 feet plain, and of the 350 feet plain up to a height of 400 feet.
These wide step-formed plains have been formed by the denuding action
of the coast-waves on the old tertiary strata; and therefore, when the
surface of the 350 feet plain, with the shells on it, first rose above
the level of the sea, the 250 feet plain did not exist, and its
formation, as well as the spreading out of the gravel on its summit,
must have taken place subsequently. So also the denudation and the
gravel-covering of the 90 feet plain must have taken
place subsequently to the elevation of the 250 feet plain, on which
recent shells are also strewed. Hence there cannot be any doubt that
the Macrauchenia, which certainly was entombed in a fresh state, and
which must have been alive after the spreading out of the gravel on the
90 feet plain, existed, not only subsequently to the upraised shells on
the surface of the 250 feet plain, but also to those on the 350 to 400
feet plain: these shells, eight in number (namely, three species of
Mytilus, two of Patella, one Fusus, Voluta, and Balanus), are
undoubtedly recent species, and are the commonest kinds now living on
this coast. At Punta Alta in B. Blanca, I remarked how marvellous it
was, that the Toxodon, a mammifer so unlike to all known genera, should
have co-existed with twenty-three still living marine animals; and now
we find that the Macrauchenia, a quadruped only a little less anomalous
than the Toxodon, also co-existed with eight other still existing
Mollusca: it should, moreover, be borne in mind, that a tooth of a
pachydermatous animal was found with the other remains at Punta Alta,
which Professor Owen thinks almost certainly belonged to the
Macrauchenia.

Mr. Lyell[21] has arrived at a highly important conclusion with respect
to the age of the North American extinct mammifers (many of which are
closely allied to, and even identical with, those of the Pampean
formation), namely, that they lived subsequently to the period when
erratic boulders were transported by the agency of floating ice in
temperate latitudes. Now in the valley of the Santa Cruz, only fifty
miles of latitude south of the spot where the Macrauchenia was
entombed, vast numbers of gigantic, angular boulders, which must have
been transported from the Cordillera on icebergs, lie strewed on the
plain, at the height of 1,400 feet above the level of the sea. In
ascending to this level, several step-formed plains must be crossed,
all of which have necessarily required long time for their formation;
hence the lowest or ninety feet plain, with its superficial bed
containing the remains of the Macrauchenia, must have been formed very
long subsequently to the period when the 1,400 feet plain was beneath
the sea, and boulders were dropped on it from floating masses of
ice.[22] Mr. Lyell’s conclusion, therefore, is thus far confirmed in
the southern hemisphere; and it is the more important, as one is
naturally tempted to admit so simple an explanation, that it was the
ice-period that caused the extinction of the numerous great mammifers
which so lately swarmed over the two Americas.

 [21] “Geological Proceedings,” vol. iv, p. 36.


 [22] It must not be inferred from these remarks, that the ice-action
 ceased in South America at this comparatively ancient period; for in
 Tierra del Fuego boulders were probably transported contemporaneously
 with, if not subsequently to, the formation of the ninety feet plain
 at S. Julian, and at other parts of the coast of Patagonia.

_Summary and concluding remarks on the Pampean formation._—One of its
most striking features is its great extent; I passed continuously over
it from the Colorado to St. Fe Bajada, a distance of 500 geographical
miles; and M. d’Orbigny traced it for 250 miles further north. In
the latitude of the Plata, I examined this formation at intervals over
an east and west line of 300 miles from Maldonado to the R. Carcarana;
and M. d’Orbigny believes it extends 100 miles further inland: from Mr.
Caldcleugh’s travels, however, I should have thought that it had
extended, south of the Cordovese range, to near Mendoza, and I may add
that I heard of great bones having been found high up the R. Quinto.
Hence the area of the Pampean formation, as remarked by M. d’Orbigny,
is probably at least equal to that of France, and perhaps twice or
thrice as great. In a basin, surrounded by gravel-cliff (at a height of
nearly three thousand feet), south of Mendoza, there is, as described
in the Third Chapter, a deposit very like the Pampean, interstratified
with other matter; and again at S. Julian’s, in Patagonia, 560 miles
south of the Colorado, a small irregular bed of a nearly similar nature
contains, as we have just seen, mammiferous remains. In the provinces
of Moxos and Chiquitos (1,000 miles northward of the Pampas), and in
Bolivia, at a height of 4,000 metres, M. d’Orbigny has described
similar deposits, which he believes to have been formed by the same
agency contemporaneously with the Pampean formation. Considering the
immense distances between these several points, and their different
heights, it appears to me infinitely more probable, that this
similarity has resulted not from contemporaneousness of origin, but
from the similarity of the rocky framework of the continent: it is
known that in Brazil an immense area consists of gneissic rocks, and we
shall hereafter see, over how great a length the plutonic rocks of the
Cordillera, the overlying purple porphyries, and the trachytic
ejections, are almost identical in nature.

Three theories on the origin of the Pampean formation have been
propounded:—First, that of a great debacle by M. d’Orbigny; this seems
founded chiefly on the absence of stratification, and on the number of
embedded remains of terrestrial quadrupeds. Although the Pampean
formation (like so many argillaceous deposits) is not divided into
distinct and separate strata, yet we have seen that in one good section
it was striped with horizontal zones of colour, and that in several
specified places the upper and lower parts differed, not only
considerably in colour, but greatly in constitution. In the southern
part of the Pampas the upper mass (to a certain extent stratified)
generally consists of hard tosca-rock, and the lower part of red
Pampean mud, often itself divided into two or more masses, varying in
colour and in the quantity of included calcareous matter. In Western
Banda Oriental, beds of a similar nature, but of a greater age,
conformably underlie and are intercalated with the regularly stratified
tertiary formation. As a general rule, the marly concretions are
arranged in horizontal lines, sometimes united into irregular strata:
surely, if the mud had been tumultuously deposited in mass, the
included calcareous matter would have segregated itself irregularly,
and not into nodules arranged in horizontal lines, one above the other
and often far apart: this arrangement appears to me to prove that mud,
differing slightly in composition, was successively and quietly
deposited. On the theory of a debacle, a prodigious amount of mud,
without a single pebble, is supposed to have been borne over the wide
surface of the Pampas, when under water: on the other hand, over the
whole of Patagonia, the same or another debacle is supposed to have
borne nothing but gravel,—the gravel and the fine mud in the
neighbourhood of the Rios Negro and Colorado having been borne to an
equal distance from the Cordillera, or imagined line of disturbance:
assuredly directly opposite effects ought not to be attributed to the
same agency. Where, again, could a mass of fine sediment, charged with
calcareous matter in a fit state for chemical segregation, and in
quantity sufficient to cover an area at least 750 miles long, and 400
miles broad, to a depth of from twenty to thirty feet to a hundred
feet, have been accumulated, ready to be transported by the supposed
debacle? To my mind it is little short of demonstration, that a great
lapse of time was necessary for the production and deposition of the
enormous amount of mudlike matter forming the Pampas; nor should I have
noticed the theory of a debacle, had it not been adduced by a
naturalist so eminent as M. d’Orbigny.

A second theory, first suggested, I believe, by Sir W. Parish, is that
the Pampean formation was thrown down on low and marshy plains by the
rivers of this country before they assumed their present courses. The
appearance and composition of the deposit, the manner in which it
slopes up and round the primary ranges, the nature of the underlying
marine beds, the estuary and sea-shells on the surface, the overlying
sandstone beds at M. Hermoso, are all quite opposed to this view. Nor
do I believe that there is a single instance of a skeleton of one of
the extinct mammifers having been found in an upright position, as if
it had been mired.

The third theory, of the truth of which I cannot entertain the smallest
doubt, is that the Pampean formation was slowly accumulated at the
mouth of the former estuary of the Plata and in the sea adjoining it. I
have come to this conclusion from the reasons assigned against the two
foregoing theories, and from simple geographical considerations. From
the numerous shells of the _Azara labiata_ lying loose on the surface
of the plains, and near Buenos Ayres embedded in the tosca-rock, we
know that this formation not only was formerly covered by, but that the
uppermost parts were deposited in, the brackish water of the ancient La
Plata. Southward and seaward of Buenos Ayres, the plains were upheaved
from under water inhabited by true marine shells. We further know from
Professor Ehrenberg’s examination of the twenty microscopical organisms
in the mud round the tooth of the Mastodon high up the course of the
Parana, that the bottom-most part of this formation was of
brackish-water origin. A similar conclusion must be extended to the
beds of like composition, at the level of the sea and under it, at M.
Hermoso in Bahia Blanca. Dr. Carpenter finds that the harder varieties
of tosca-rock, collected chiefly to the south, contain marine spongoid
bodies, minute fragments of shells, corals, and Polythalamia; these
perhaps may have been drifted inwards by the tides, from the more open
parts of the sea. The absence of shells, throughout this deposit, with
the exception of the uppermost layers near Buenos Ayres, is a
remarkable fact: can it be explained by the brackish condition of the
water, or by the deep mud at the bottom? I have stated that both the
reddish mud and the concretions of tosca-rock are
often penetrated by minute, linear cavities, such as frequently may be
observed in fresh-water calcareous deposits:—were they produced by the
burrowing of small worms? Only on this view of the Pampean formation
having been of estuary origin, can the extraordinary numbers (presently
to be alluded to) of the embedded mammiferous remains be explained.[23]

 [23] It is almost superfluous to give the numerous cases (for
 instance, in Sumatra; Lyell’s “Principles,” vol. iii, p. 325, sixth
 edit.), of the carcasses of animals having been washed out to sea by
 swollen rivers; but I may refer to a recent account by Mr. Bettington
 (“Asiatic Soc.,” 1845, June 21st), of oxen, deer, and bears being
 carried into the Gulf of Cambray; see also the account in my “Journal”
 (2nd edit., p. 133), of the numbers of animals drowned in the Plata
 during the great, often recurrent, droughts.


With respect to the first origin of the reddish mud, I will only
remark, that the enormous area of Brazil consists in chief part of
gneissic and other granitic rocks, which have suffered decomposition,
and been converted into a red, gritty, argillaceous mass, to a greater
depth than in any other country which I have seen. The mixture of
rounded grains, and even of small fragments and pebbles of quartz, in
the Pampean mud of Banda Oriental, is evidently due to the neighbouring
and underlying primary rocks. The estuary mud was drifted during the
Pampean period in a much more southerly course, owing probably to the
east and west primary ridges south of the Plata not having been then
elevated, than the mud of the Plata at present is; for it was formerly
deposited as far south as the Colorado. The quantity of calcareous
matter in this formation, especially in those large districts where the
whole mass passes into tosca-rock, is very great: I have already
remarked on the close resemblance in external and microscopical
appearance, between this tosca-rock and the strata at Coquimbo, which
have certainly resulted from the decay and attrition of recent
shells:[24] I dare not, however, extend this conclusion to the
calcareous rocks of the Pampas, more especially as the underlying
tertiary strata in western Banda Oriental show that at that period
there was a copious emission of carbonate of lime, in connection with
volcanic action.

 [24] I may add, that there are nearly similar superficial calcareous
 beds at King George’s Sound in Australia; and these undoubtedly have
 been formed by the disintegration of marine remains (see “Volcanic
 Islands,” etc., p. 144). There is, however, something very remarkable
 in the frequency of superficial, thin beds of earthy calcareous
 matter, in districts where the surrounding rocks are not calcareous.
 Major Charters, in a Paper read before the Geographical Society (April
 13th, 1840, and abstracted in the _ Athenæum_, p. 317), states that
 this is the case in parts of Mexico, and that he has observed similar
 appearances in many parts of South Africa. The circumstance of the
 uppermost stratum round the ragged Sierra Ventana, consisting of
 calcareous or marly matter, without any covering of alluvial matter,
 strikes me as very singular, in whatever manner we view the deposition
 and elevation of the Pampean formation.


The Pampean formation, judging from its similar composition, and from
the apparent absolute specific identity of some of its mammiferous
remains, and from the generic resemblance of others, belongs over its
vast area—throughout Banda Oriental, Entre Rios, and the wide extent of
the Pampas as far south as the Colorado,—to the same geological epoch.
The mammiferous remains occur at all depths from the top to the bottom
of the deposit; and I may add that nowhere in the Pampas is there any
appearance of much superficial denudation: some bones which I found
near the Guardia del Monte were embedded close to the surface; and this
appears to have been the case with many of those discovered in Banda
Oriental: on the Matanzas, twenty miles south of Buenos Ayres, a
Glyptodon was embedded five feet beneath the surface; numerous remains
were found by S. Muniz, near Luxan, at an average depth of eighteen
feet; in Buenos Ayres a skeleton was disinterred at sixty feet depth,
and on the Parana I have described two skeletons of the Mastodon only
five or six feet above the very base of the deposit. With respect to
the age of this formation, as judged of by the ordinary standard of the
existence of Mollusca, the only evidence within the limits of the true
Pampas which is at all trustworthy, is afforded by the still living
_Azara labiata_ being embedded in tosca-rock near Buenos Ayres. At
Punta Alta, however, we have seen that several of the extinct
mammifers, most characteristic of the Pampean formation, co-existed
with twenty species of Mollusca, a barnacle and two corals, all still
living on this same coast;—for when we remember that the shells have a
more ancient appearance than the bones; that many of the bones, though
embedded in a coarse conglomerate, are perfectly preserved; that almost
all the parts of the skeleton of the Scelidotherium, even to the
knee-cap, were lying in their proper relative positions; and that a
large piece of the fragile dermal armour of a Dasypoid quadruped,
connected with some of the bones of the foot, had been entombed in a
condition allowing the two sides to be doubled together, it must
assuredly be admitted that these mammiferous remains were embedded in a
fresh state, and therefore that the living animals co-existed with the
co-embedded shells. Moreover, the _Macrauchenia Patachonica_ (of which,
according to Professor Owen, remains also occur in the Pampas of Buenos
Ayres, and at Punta Alta) has been shown by satisfactory evidence of
another kind, to have lived on the plains of Patagonia long after the
period when the adjoining sea was first tenanted by its present
commonest molluscous animals. We must, therefore, conclude that the
Pampean formation belongs, in the ordinary geological sense of the
word, to the Recent Period.[25]

 [25] M. d’Orbigny believes (“Voyage,” Part. Géolog., p. 81) that this
 formation, though “très voisine de la nôtre, est néanmoins de beaucoup
 antérieure à notre création.”

At St. Fé Bajada, the Pampean estuary formation, with its mammiferous
remains, conformably overlies the marine tertiary strata, which (as
first shown by M. d’Orbigny) are contemporaneous with those of
Patagonia, and which, as we shall hereafter see, belong to a very
ancient tertiary stage. When examining the junction between these two
formations, I thought that the concretionary layer of marl marked a
passage between the marine and estuary stages. M. d’Orbigny
disputes this view (as given in my “Journal”), and I admit that it is
erroneous, though in some degree excusable, from their conformability
and from both abounding with calcareous matter. It would, indeed, have
been a great anomaly if there had been a true passage between a deposit
contemporaneous with existing species of mollusca, and one in which all
the mollusca appear to be extinct. Northward of Santa Fe, M. d’Orbigny
met with ferruginous sandstones, marly rocks, and other beds, which he
considers as a distinct and lower formation; but the evidence that they
are not parts of the same with an altered mineralogical character, does
not appear to me quite satisfactory.

In Western Banda Oriental, while the marine tertiary strata were
accumulating, there were volcanic eruptions, much silex and lime were
precipitated from solution, coarse conglomerates were formed, being
derived probably from adjoining land, and layers of red mud and marly
rocks, like those of the Pampean formation, were occasionally
deposited. The true Pampean deposit, with mammiferous remains, instead
of as at Santa Fe overlying conformably the tertiary strata, is here
seen at a lower level folding round and between the flat-topped,
cliff-bounded hills, formed by a upheaval and denudation of these same
tertiary strata. The upheaval, having occurred here earlier than at
Santa Fe, may be naturally accounted for by the contemporaneous
volcanic action. At the Barrancas de S. Gregorio, the Pampean deposit,
as we have seen, overlies and fills up furrows in coarse sand,
precisely like that now accumulating on the shores near the mouth of
the Plata. I can hardly believe that this loose and coarse sand is
contemporaneous with the old tertiary and often crystalline strata of
the more western parts of the province; and am induced to suspect that
it is of subsequent origin. If that section near Colonia could be
implicitly trusted, in which, at a height of only fifteen feet above
the Plata, a bed of fresh-looking mussels, of an existing _littoral_
species, appeared to lie between the sand and the Pampean mud, I should
conclude that Banda Oriental must have stood, when the coarse sand was
accumulating, at only a little below its present level, and had then
subsided, allowing the estuary Pampean mud to cover far and wide its
surface up to a height of some hundred feet; and that after this
subsidence the province had been uplifted to its present level.

In Western Banda Oriental, we know, from two unequivocal sections that
there is a mass, absolutely undistinguishable from the true Pampean
deposit, beneath the old tertiary strata. This inferior mass must be
very much more ancient than the upper deposit with its mammiferous
remains, for it lies beneath the tertiary strata in which all the
shells are extinct. Nevertheless, the lower and upper masses, as well
as some intermediate layers, are so similar in mineralogical character,
that I cannot doubt that they are all of estuary origin, and have been
derived from the same great source. At first it appeared to me
extremely improbable, that mud of the same nature should have been
deposited on nearly the same spot, during an immense lapse of time,
namely, from a period equivalent perhaps to the Eocene of Europe to
that of the Pampean formation. But as, at the very commencement of the
Pampean
period, if not at a still earlier period, the Sierra Ventana formed a
boundary to the south,—the Cordillera or the plains in front of them to
the west,—the whole province of Corrientes probably to the north, for,
according to M. d’Orbigny, it is not covered by the Pampean
deposit,—and Brazil, as known by the remains in the caves, to the
north-east; and as again, during the older tertiary period, land
already existed in Western Banda Oriental and near St. Fé Bajada, as
may be inferred from the vegetable debris, from the quantities of
silicified wood, and from the remains of a Toxodon found, according to
M. d’Orbigny, in still lower strata, we may conclude, that at this
ancient period a great expanse of water was surrounded by the same
rocky framework which now bounds the plains of Pampean formation. This
having been the case, the circumstance of sediment of the same nature
having been deposited in the same area during an immense lapse of time,
though highly remarkable, does not appear incredible.

The elevation of the Pampas, at least of the southern parts, has been
slow and interrupted by several periods of rest, as may be inferred
from the plains, cliffs, and lines of sand-dunes (with shells and
pumice-pebbles) standing at different heights. I believe, also, that
the Pampean mud continued to be deposited, after parts of this
formation had already been elevated, in the same manner as mud would
continue to be deposited in the estuary of the Plata, if the mud-banks
on its shores were now uplifted and changed into plains: I believe in
this from the improbability of so many skeletons and bones having been
accumulated at one spot, where M. Hermoso now stands, at a depth of
between eight hundred and one thousand feet, and at a vast distance
from any land except small rocky islets,—as must have been the case, if
the high tosca-plain round the Ventana and adjoining Sierras, had not
been already uplifted and converted into land, supporting mammiferous
animals. At Punta Alta we have good evidence that the gravel-strata,
which certainly belong to the true Pampean period, were accumulated
after the elevation in that neighbourhood of the main part of the
Pampean deposit, whence the rounded masses of tosca-rock were derived,
and that rolled fragment of black bone in the same peculiar condition
with the remains at Monte Hermoso.

The number of the mammiferous remains embedded in the Pampas is, as I
have remarked, wonderful: it should be borne in mind that they have
almost exclusively been found in the cliffs and steep banks of rivers,
and that, until lately, they excited no attention amongst the
inhabitants: I am firmly convinced that a deep trench could not be cut
in any line across the Pampas, without intersecting the remains of some
quadruped. It is difficult to form an opinion in what part of the
Pampas they are most numerous; in a limited spot they could not well
have been more numerous than they were at P. Alta; the number, however,
lately found by Senor F. Muniz, near Luxan, in a central spot in the
Pampas, is extraordinarily great: at the end of this chapter I will
give a list of all the localities at which I have heard of remains
having been discovered. Very frequently the remains consist of almost
perfect
skeletons; but there are, also, numerous single bones, as for instance
at St. Fé. Their state of preservation varies much, even when embedded
near each other: I saw none others so perfectly preserved as the heads
of the Toxodon and Mylodon from the white soft earthy bed on the
Sarandis in Banda Oriental. It is remarkable that in two limited
sections I found no less than five teeth separately embedded, and I
heard of teeth having been similarly found in other parts: may we
suppose that the skeletons or heads were for a long time gently drifted
by currents over the soft muddy bottom, and that the teeth
occasionally, here and there, dropped out?

It may be naturally asked, where did these numerous animals live? From
the remarkable discoveries of MM. Lund and Clausen, it appears that
some of the species found in the Pampas inhabited the highlands of
Brazil: the _Mastodon Andium_ is embedded at great heights in the
Cordillera from north of the equator[26] to at least as far south as
Tarija; and as there is no higher land, there can be little doubt that
this Mastodon must have lived on the plains and valleys of that great
range. These countries, however, appear too far distant for the
habitation of the individuals entombed in the Pampas: we must probably
look to nearer points, for instance to the province of Corrientes,
which, as already remarked, is said not to be covered by the Pampean
formation, and may therefore, at the period of its deposition, have
existed as dry land. I have already given my reasons for believing that
the animals embedded at M. Hermoso and at P. Alta in Bahia Blanca,
lived on adjoining land, formed of parts of the already elevated
Pampean deposit. With respect to the food of these many great extinct
quadrupeds, I will not repeat the facts given in my “Journal” (second
edit., p. 85), showing that there is no correlation between the
luxuriance of the vegetation of a country and the size of its
mammiferous inhabitants. I do not doubt that large animals could now
exist, as far as the amount, not kind, of vegetation is concerned, on
the sterile plains of Bahia Blanca and of the R. Negro, as well as on
the equally, if not more sterile plains of Southern Africa. The
climate, however, may perhaps have somewhat deteriorated since the
mammifers embedded at Bahia Blanca lived there; for we must not infer,
from the continued existence of the same shells on the present coasts,
that there has been no change in climate; for several of these shells
now range northward along the shores of Brazil, where the most
luxuriant vegetation flourishes under a tropical temperature. With
respect to the extinction, which at first fills the mind with
astonishment, of the many great and small mammifers of this period, I
may also refer to the work above cited (second edit., p. 173), in which
I have endeavoured to show, that however unable we may be to explain
the precise cause, we ought not properly to feel more surprised at a
species becoming extinct than at one being rare; and yet we are
accustomed to
view the rarity of any particular species as an ordinary event, not
requiring any extraordinary agency.

 [26] Humboldt states that the Mastodon has been discovered in New
 Granada: it has been found in Quito. When at Lima, I saw a tooth of a
 Mastodon in the possession of Don M. Rivero, found at Playa Chica on
 the Maranon, near the Guallaga. Every one has heard of the numerous
 remains of Mastodon in Bolivia.


The several mammifers embedded in the Pampean formation, which mostly
belong to extinct genera, and some even to extinct families or orders,
and which differ nearly, if not quite, as much as do the Eocene
mammifers of Europe from living quadrupeds having existed
contemporaneously with mollusca, all still inhabiting the adjoining
sea, is certainly a most striking fact. It is, however, far from being
an isolated one; for, during the late tertiary deposits of Britain, an
elephant, rhinoceros, and hippopotamus co-existed with many recent land
and fresh-water shells; and in North America, we have the best evidence
that a mastodon, elephant, megatherium, megalonyx, mylodon, an extinct
horse and ox, likewise co-existed with numerous land, fresh-water, and
marine recent shells.[27] The enumeration of these extinct North
American animals naturally leads me to refer to the former closer
relation of the mammiferous inhabitants of the two Americas, which I
have discussed in my “Journal,” and likewise to the vast extent of
country over which some of them ranged: thus the same species of the
_Megatherium, Megalonyx, Equus_ (as far as the state of their remains
permits of identification), extended from the Southern United States of
North America to Bahia Blanca, in lat. 39° S., on the coast of
Patagonia. The fact of these animals having inhabited tropical and
temperate regions, does not appear to me any great difficulty, seeing
that at the Cape of Good Hope several quadrupeds, such as the elephant
and hippopotamus, range from the equator to lat. 35° south. The case of
the Mastodon Andium is one of more difficulty, for it is found from
lat. 36° S., over, as I have reason to believe, nearly the whole of
Brazil, and up the Cordillera to regions which, according to M.
d’Orbigny, border on perpetual snow, and which are almost destitute of
vegetation: undoubtedly the climate of the Cordillera must have been
different when the mastodon inhabited it; but we should not forget the
case of the Siberian mammoth and rhinoceros, as showing how severe a
climate the larger pachydermata can endure; nor overlook the fact of
the guanaco ranging at the present day over the hot low deserts of
Peru, the lofty pinnacles of the Cordillera, and the damp forest-clad
land of Southern Tierra del Fuego; the puma, also, is found from the
equator to the Strait of Magellan, and I have seen its footsteps only a
little below the limits of perpetual snow in the Cordillera of Chile.

 [27] Many original observations, and a summary on this subject, are
 given in Mr. Lyell’s paper in the “Geolog. Proc.,” vol. iv, p. 3 and
 in his “Travels in North America,” vol. i, p. 164 and vol. ii, p. 60.
 For the European analogous cases see Mr. Lyell’s “Principles of
 Geology” (6th edit.), vol. i, p. 37.

At the period, so recent in a geological sense, when these extinct
mammifers existed, the two Americas must have swarmed with quadrupeds,
many of them of gigantic size; for, besides those more particularly
referred to in this chapter, we must include in this same period those
wonderfully numerous remains, some few of them specifically, and others
generically related to those of the Pampas, discovered by
MM. Lund and Clausen in the caves of Brazil. Finally, the facts here
given show how cautious we ought to be in judging of the antiquity of a
formation from even a great amount of difference between the extinct
and living species in any one class of animals;—we ought even to be
cautious in accepting the general proposition, that change in organic
forms and lapse of time are at all, necessarily, correlatives.


_Localities within the region of the Pampas where great bones have been
found._

The following list, which includes every account which I have hitherto
met with of the discovery of fossil mammiferous remains in the Pampas,
may be hereafter useful to a geologist investigating this region, and
it tends to show their extraordinary abundance. I heard of and saw many
fossils, the original position of which I could not ascertain; and I
received many statements too vague to be here inserted. Beginning to
the south:—we have the two stations in Bahia Blanca, described in this
chapter, where at P. Alta, the Megatherium, Megalonyx, Scelidotherium,
Mylodon, Holophractus (or an allied genus), Toxodon, Macrauchenia, and
an Equus were collected; and at M. Hermoso a Ctenomys, Hydrochærus,
some other rodents and the bones of a great megatheroid quadruped.
Close north-east of the S. Tapalguen, we have the Rios ‘Huesos’ (i.e.
_bones_), which probably takes its name from large fossil bones. Near
Villa Nuevo, and at Las Averias, not far from the Salado, three nearly
perfect skeletons, one of the Megatherium, one of the _Glyptodon
clavipes_, and one of some great Dasypoid quadruped, were found by the
agent of Sir W. Parish (see his work “Buenos Ayres,” etc., p. 171). I
have seen the tooth of a Mastodon from the Salado; a little northward
of this river, on the borders of a lake near the G. del Monte, I saw
many bones, and one large piece of dermal armour; higher up the Salado,
there is a place called Monte “Huesos.” On the Matanzas, about twenty
miles south of Buenos Ayres, the skeleton (_vide_ p. 178 of “Buenos
Ayres,” etc., by Sir W. Parish) of a Glyptodon was found about five
feet beneath the surface; here also (see Catalogue of Royal College of
Surgeons) remains of _Glyptodon clavipes, G. ornatus_, and _G.
reticulatus_ were found. Signor Angelis, in a letter which I have seen,
refers to some great remains found in Buenos Ayres, at a depth of
twenty varas from the surface. Seven leagues north of this city the
same author found the skeletons of _Mylodon robustus_ and _Glyptodon
ornatus._ From this neighbourhood he has lately sent to the British
Museum the following fossils:—Remains of three or four individuals of
Megatherium; of three species of Glyptodon; of three individuals of the
_Mastodon Andium_; of Macrauchenia; of a second species of Toxodon,
different from _T. Platensis_; and lastly, of the Machairodus, a
wonderful large carnivorous animal. M. d’Orbigny has lately received
from the Recolate (“Voyage,” Pal., p. 144), near Buenos Ayres, a tooth
of _Toxodon Platensis._

Proceeding northward, along the west bank of the Parana, we come to the
Rio Luxan, where two skeletons of the Megatherium have been found; and
lately, within eight leagues of the town of Luxan, Dr. F. X. Muniz has
collected (_British Packet_, Buenos Ayres, September 25, 1841), from an
average depth of eighteen feet, very numerous remains, of no less than,
as he believes, nine distinct species of mammifers. At Areco, large
bones have been found, which are believed, by the inhabitants, to have
been changed
from small bones, by the water of the river! At Arrecifes, the
Glyptodon, sent to the College of Surgeons, was found; and I have seen
two teeth of a Mastodon from this quarter. At S. Nicolas, M. d’Orbigny
found remains of a Canis, Ctenomys, and Kerodon; and M. Isabelle
(“Voyage,” p. 332) refers to a gigantic Armadillo found there. At S.
Carlos, I heard of great bones. A little below the mouth of the
Carcarana, the two skeletons of Mastodon were found; on the banks of
this river, near S. Miguel, I found teeth of the Mastodon and Toxodon;
and “Falkner” (p. 55) describes the osseous armour of some great
animal; I heard of many other bones in this neighbourhood. I have seen,
I may add, in the possession of Mr. Caldcleugh, the tooth of a
_Mastodon Andium_, said to have been found in Paraguay; I may here also
refer to a statement in this gentleman’s travels (vol. i, p. 48), of a
great skeleton having been found in the province of Bolivia in Brazil,
on the R. de las Contas. The furthest point westward in the Pampas, at
which I have _heard_ of fossil bones, was high up on the banks of R.
Quinto.

In Entre Rios, besides the remains of the Mastodon, Toxodon, Equus, and
a great Dasypoid quadruped near St. Fe Bajada, I received an account of
bones having been found a little S.E. of P. Gorda (on the Parana), and
of an entire skeleton at Matanzas, on the Arroyo del Animal.

In Banda Oriental, besides the remains of the Toxodon, Mylodon, and two
skeletons of great animals with osseous armour (distinct from that of
the Glyptodon), found on the Arroyos Sarandis and Berquelo, M. Isabelle
(“Voyage,” p. 322) says, many bones have been found near the R. Negro,
and on the R. Arapey, an affluent of the Paraguay, in lat. 30° 40′
south. I heard of bones near the source of the A. Vivoras. I saw the
remains of a Dasypoid quadruped from the Arroyo Seco, close to M.
Video; and M. d’Orbigny refers (“Voyage,” Géolog., p. 24), to another
found on the Pedernal, an affluent of the St. Lucia; and Signor
Angelis, in a letter, states that a third skeleton of this family has
been found, near Canelones. I saw a tooth of the Mastodon from Talas,
another affluent of the St. Lucia. The most eastern point at which I
heard of great bones having been found, was at Solis Grande, between M.
Video and Maldonado.




Chapter V ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.


Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.—Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary
formations of the Western Coast.—Chonos and Chiloe groups, volcanic
rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.—Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coast of S. America.—On the
contemporaneous deposition and preservation of sedimentary formations.


_Rio Negro._—I can add little to the details given by M. d’Orbigny[1]
on the sandstone formation of this district. The cliffs to the south of
the
river are about two hundred feet in height, and are composed of
sandstone of various tints and degrees of hardness. One layer, which
thinned out at both ends, consisted of earthy matter, of a pale reddish
colour, with some gypsum, and very like (I speak after comparison of
the specimens brought home) Pampean mud: above this was a layer of
compact marly rock with dendritic manganese. Many blocks of a
conglomerate of pumice-pebbles embedded in hard sandstone were strewed
at the foot of the cliff, and had evidently fallen from above. A few
miles N.E. of the town, I found, low down in the sandstone, a bed, a
few inches in thickness, of a white, friable, harsh-feeling sediment,
which adheres to the tongue, is of easy fusibility, and of little
specific gravity; examined under the microscope, it is seen to be
pumiceous tuff, formed of broken transparent crystals. In the cliffs
south of the river, there is, also, a thin layer of nearly similar
nature, but finer grained, and not so white; it might easily have been
mistaken for a calcareous tuff, but it contains no lime: this substance
precisely resembles a most widely extended and thick formation in
Southern Patagonia, hereafter to be described, and which is remarkable
for being partially formed of infusoria. These beds, conjointly with
the conglomerate of pumice, are interesting, as showing the nature of
the volcanic action in the Cordillera during this old tertiary period.

 [1] “Voyage,” Part. Géolog., pp. 57-65.

In a bed at the base of the southern cliffs, M. d’Orbigny found two
extinct fresh-water shells, namely, a Unio and Chilina. This bed rested
on one with bones of an extinct rodent, namely, the _ Megamys
Patagoniensis_; and this again on another with extinct marine shells.
The species found by M. d’Orbigny in different parts of this formation
consist of:—

Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole
coast of Patagonia).

Ostrea Ferrarisi, d’Orbigny, “Voyage, Pal.”

Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and S.
Josef).

Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.”

Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. Fé).

Arca Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fé).

According to M. d’Orbigny, the sandstone extends westward along the
coast as far as Port S. Antonio, and up the R. Negro far into the
interior: northward I traced it to the southern side of the Rio
Colorado, where it forms a low denuded plain. This formation, though
contemporaneous with that of the rest of Patagonia, is quite different
in mineralogical composition, being connected with it only by the one
thin white layer: this difference may be reasonably attributed to the
sediment brought down in ancient times by the Rio Negro; by which
agency, also, we can understand the presence of the fresh-water shells,
and of the bones of land animals. Judging from the identity of four of
the above shells, this formation is contemporaneous (as remarked by M.
d’Orbigny) with that under the Pampean deposit in Entre Rios and in
Banda Oriental. The gravel capping the sandstone plain, with its
calcareous cement and nodules of gypsum, is probably, from the reasons
given in the First Chapter, contemporaneous with the uppermost beds of
the Pampean formation on the upper plain north of the Colorado.

_San Josef._—My examination here was very short: the cliffs are about a
hundred feet high; the lower third consists of yellowish-brown, soft,
slightly calcareous, muddy sandstone, parts of which when struck emit a
fetid smell. In this bed the great Ostræa Patagonica, often marked with
dendritic manganese and small coral-lines, were extraordinarily
numerous. I found here the following shells:—

Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole
coast of Patagonia).

Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé and R.
Negro).

Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S.
Julian, and Port Desire).

Pecten Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. Fé).

Pecten actinodes, G. B. Sowerby.

Terebratula Patagonica, G. B. Sowerby (also S. Julian).

Casts of a Turritella.

The four first of these species occur at St. Fé in Entre Rios, and the
two first in the sandstone of the Rio Negro. Above this fossiliferous
mass, there is a stratum of very fine-grained, pale brown mudstone,
including numerous laminæ of selenite. All the strata appear
horizontal, but when followed by the eye for a long distance, they are
seen to have a small easterly dip. On the surface we have the
porphyritic gravel, and on it sand with recent shells.

_Nuevo Gulf._—From specimens and notes given me by Lieutenant Stokes,
it appears that the lower bed consists of soft muddy sandstone, like
that of S. Josef, with many imperfect shells, including the _Pecten
Paranensis_, d’Orbigny, casts of a Turritella and Scutella. On this
there are two strata of the pale brown mudstone, also like that of _S.
Josef_, separated by a darker-coloured, more argillaceous variety,
including the _Ostrea Patagonica._ Professor Ehrenberg has examined
this mudstone for me: he finds in it three already known microscopic
organisms, enveloped in a fine-grained pumiceous tuff, which I shall
have immediately to describe in detail. Specimens brought to me from
the uppermost bed, north of the Rio Chupat, consist of this same
substance, but of a whiter colour.

Tertiary strata, such as here described, appear to extend along the
whole coast between Rio Chupat and Port Desire, except where
interrupted by the underlying claystone porphyry, and by some
metamorphic rocks; these hard rocks, I may add, are found at intervals
over a space of about five degrees of latitude, from Point Union to a
point between Port S. Julian and S. Cruz, and will be described in the
ensuing chapter. Many gigantic specimens of the _Ostrea Patagonica_
were collected in the Gulf of St. George.

_Port Desire._—A good section of the lowest fossiliferous mass, about
forty feet in thickness, resting on claystone porphyry, is exhibited a
few miles south of the harbour. The shells sufficiently perfect to be
recognised consist of:—


Ostrea Patagonica, d’Orbigny, (also at St. Fé, and whole coast of
Patagonia).

Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S. Josef,
S. Julian).

Pecten centralis, G. B. Sowerby (also at S. Julian and S. Cruz).

Cucullæa alta, G. B. Sowerby (also at S. Cruz).

Nucula ornata, G. B. Sowerby.

Turritella Patagonica, G. B. Sowerby.


The fossiliferous strata, when not denuded, are conformably covered by
a considerable thickness of the fine-grained pumiceous mudstone,
divided into two masses: the lower half is very fine-grained, slightly
unctuous, and so compact as to break with a semi-conchoidal fracture,
though yielding to the nail; it includes laminæ of selenite: the upper
half precisely resembles the one layer at the Rio Negro, and with the
exception of being whiter, the upper beds at San Josef and Nuevo Gulf.
In neither mass is there any trace to the naked eye of organic forms.
Taking the entire deposit, it is generally quite white, or yellowish,
or feebly tinted with green; it is either almost friable under the
finger, or as hard as chalk; it is of easy fusibility, of little
specific gravity, is not harsh to the touch, adheres to the tongue, and
when breathed on exhales a strong aluminous odour; it sometimes
contains a very little calcareous matter, and traces (besides the
included laminæ) of gypsum. Under the microscope, according to
Professor Ehrenberg,[2] it consists of minute, triturated, cellular,
glassy fragments of pumice, with some broken crystals. In the minute
glassy fragments, Professor Ehrenberg recognises organic structures,
which have been affected by volcanic heat: in the specimens from this
place, and from Port S. Julian, he finds sixteen Polygastrica and
twelve Phytolitharia. Of these organisms, seven are new forms, the
others being previously known: all are of marine, and chiefly of
oceanic, origin. This deposit to the naked eye resembles the crust
which often appears on weathered surfaces of feldspathic rocks; it
likewise resembles those beds of earthy feldspathic matter, sometimes
interstratified with porphyritic rocks, as is the case in this very
district with the underlying purple claystone porphyry. From examining
specimens under a common microscope, and comparing them with other
specimens undoubtedly of volcanic origin, I had come to the same
conclusion with Professor Ehrenberg, namely, that this great deposit,
in its first origin, is of volcanic nature.

 [2] “Monatsberichten de könig. Akad. zu Berlin,” vom April 1845.

_Port S. Julian._—On the south side of the harbour, the following
section (figure No. 17) gives the nature of the beds seen in the cliffs
of the ninety feet plain. Beginning at the top:—first, the earthy mass
(AA), including the remains of the Macrauchenia, with recent shells on
the surface; second, the porphyritic shingle (B), which in its lower
part is interstratified (owing, I believe, to redisposition during
denudation) with the white pumiceous mudstone; third, this white
mudstone, about twenty feet in thickness, and divided into two
varieties (C and D), both closely resembling the lower, fine-grained,
more unctuous
and compact kind at Port Desire; and, as at that place, including much
selenite; fourth, a fossiliferous mass, divided into three main beds,
of which the uppermost is thin, and consists of ferruginous sandstone,
with many shells of the great oyster and _ Pecten Paranensis_; the
middle bed (E) is a yellowish earthy sandstone abounding with Scutellæ;
and the lowest bed (F) is an indurated, greenish, sandy clay, including
large concretions of calcareous sandstone, many shells of the great
oyster, and in parts almost made up of fragments of Balanidæ. Out of
these three beds, I procured the following twelve species, of which the
two first were exceedingly numerous in individuals, as were the
Terebratulæ and Turritellæ in certain layers:—

Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole
coast of Patagonia).

Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (St. Fé, S. Josef, Port
Desire).

Pecten centralis, G. B. Sowerby (also at Port Desire and S. Cruz).

Pecten geminatus, G. B. Sowerby.

Terebratula Patagonica, G. B. Sowerby (also S. Josef).

Struthiolaria ornata, G. B. Sowerby (also S. Cruz).

Fusus Patagonicus, G. B. Sowerby.

Fusus Noachinus, G. B. Sowerby.

Scalaria rugulosa, G. B. Sowerby.

Turritella ambulacrum, G. B. Sowerby (also S. Cruz).

Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat.

Balanus varians, G. B. Sowerby.

Scutella, differing from the species from Nuevo Gulf.


No. 17
Section of the strata exhibited in the cliffs of the ninety feet plain
at Port S. Julian.


[Illustration: Section of the strata exhibited in the cliffs of the
ninety feet plain at Port S. Julian.]

At the head of the inner harbour of Port S. Julian, the fossiliferous
mass is not displayed, and the sea-cliffs from the water’s edge to a
height of between one and two hundred feet are formed of the white
pumiceous mudstone, which here includes innumerable, far-extended,
sometimes horizontal, sometimes inclined or vertical laminæ of
transparent gypsum, often about an inch in thickness. Further inland,
with the exception of the superficial gravel, the whole thickness of
the truncated hills, which represent a formerly continuous plain 950
feet in height, appears to be formed of this white mudstone: here and
there, however, at various heights, thin earthy layers, containing the
great oyster, _Pecten Paranensis_ and _Turritella ambulacrum_, are
interstratified;
thus showing that the whole mass belongs to the same epoch. I nowhere
found even a fragment of a shell actually in the white deposit, and
only a single cast of a Turritella. Out of the eighteen microscopic
organisms discovered by Ehrenberg in the specimens from this place, ten
are common to the same deposit at Port Desire. I may add that specimens
of this white mudstone, with the same identical characters were brought
me from two points,—one twenty miles north of S. Julian, where a wide
gravel-capped plain, 350 feet in height, is thus composed; and the
other forty miles south of S. Julian, where, on the old charts, the
cliffs are marked as “_Chalk Hills._”

_Santa Cruz._—The gravel-capped cliffs at the mouth of the river are
355 feet in height: the lower part, to a thickness of fifty or sixty
feet, consists of a more or less hardened, darkish, muddy, or
argillaceous sandstone (like the lowest bed of Port Desire), containing
very many shells, some silicified and some converted into yellow
calcareous spar. The great oyster is here numerous in layers; the
Trigonocelia and Turritella are also very numerous: it is remarkable
that the _Pecten Paranensis_, so common in all other parts of the
coast, is here absent: the shells consist of:—

Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole
coast of Patagonia).

Pecten centralis, G. B. Sowerby (also P. Desire and S. Julian).

Venus meridionalis of G. B. Sowerby.

Crassatella Lyellii, G. B. Sowerby.

Cardium puelchum, G. B. Sowerby.

Cardita Patagonica, G. B. Sowerby.

Mactra rugata, G. B. Sowerby.

Mactra Darwinii, G. B. Sowerby.

Cucullæa alta, G. B. Sowerby (also P. Desire).

Trigonocelia insolita, G. B. Sowerby.

Nucula (?) glabra, G. B. Sowerby.

Crepidula gregaria, G. B. Sowerby.

Voluta alta, G. B. Sowerby.

Trochus collaris, G. B. Sowerby.

Natica solida (?), G. B. Sowerby.

Struthiolaria ornata, G. B. Sowerby (also P. Desire).

Turritella ambulacrum, G. B. Sowerby (also P. S. Julian).
Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.

The upper part of the cliff is generally divided into three great
strata, differing slightly in composition, but essentially resembling
the pumiceous mudstone of the places farther north; the deposit,
however, here is more arenaceous, of greater specific gravity, and not
so white: it is interlaced with numerous thin veins, partially or quite
filled with transverse fibres of gypsum; these fibres were too short to
reach across the vein, have their extremities curved or bent: in the
same veins with the gypsum, and likewise in separate veins as well as
in little nests, there is much powdery sulphate of magnesia (as
ascertained by Mr. Reeks) in an uncompressed form: I believe that this
salt has not heretofore
been found in veins. Of the three beds, the central one is the most
compact, and more like ordinary sandstone: it includes numerous
flattened spherical concretions, often united like a necklace, composed
of hard calcareous sandstone, containing a few shells: some of these
concretions were four feet in diameter, and in a horizontal line nine
feet apart, showing that the calcareous matter must have been drawn to
the centres of attraction, from a distance of four feet and a half on
both sides. In the upper and lower finer-grained strata, there were
other concretions of a grey colour, containing calcareous matter, and
so fine-grained and compact, as almost to resemble porcelain-rock: I
have seen exactly similar concretions in a volcanic tufaceous bed in
Chiloe. Although in this upper fine-grained strata, organic remains
were very rare, yet I noticed a few of the great oyster; and in one
included soft ferruginous layer, there were some specimens of the _
Cucullæa alta_ (found at Port Desire in the lower fossiliferous mass)
and of the _Mactra rugata_, which latter shell has been partially
converted into gypsum.

No. 18
Section of the plain at Patagonia, on the banks of the S. Cruz.


[Illustration: Section of the plain at Patagonia, on the banks of the
S. Cruz.]

In ascending the valley of the S. Cruz, the upper strata of the
coast-cliffs are prolonged, with nearly the same characters, for fifty
miles: at about this point, they begin in the most gradual and scarcely
perceptible manner, to be banded with white lines; and after ascending
ten miles farther, we meet with distinct thin layers of whitish,
greenish, and yellowish fine-grained, fusible sediments. At eighty
miles from the coast,[3] in a cliff thus composed, there were a few
layers of ferruginous
sandstone, and of an argillaceous sandstone with concretions of marl
like those in the Pampas. At one hundred miles from the coast, that is
at a central point between the Atlantic and the Cordillera, we have the
section in figure No. 18.

 [3] At this spot, for a space of three-quarters of a mile along the
 north side of the river, and for a width of half a mile, there has
 been a great slip, which has formed hills between sixty and seventy
 feet in height, and has tilted the strata into highly inclined and
 even vertical positions. The strata generally dipped at an angle of
 45° towards the cliff from which they had slided. I have observed in
 slips, both on a small and large scale, that this inward dip is very
 general. Is it due to the hydrostatic pressure of water percolating
 with difficulty through the strata acting with greater force at the
 base of the mass than against the upper part?

The upper half of the sedimentary mass, under the basaltic lava,
consists of innumerable zones of perfectly white bright green,
yellowish and brownish, fine-grained, sometimes incoherent, sedimentary
matter. The white, pumiceous, trachytic tuff-like varieties are of
rather greater specific gravity than the pumiceous mudstone on the
coast to the north; some of the layers, especially the browner ones,
are coarser, so that the broken crystals are distinguishable with a
weak lens. The layers vary in character in short distances. With the
exception of a few of the _Ostrea Patagonica_, which appeared to have
rolled down from the cliff above, no organic remains were found. The
chief difference between these layers taken as a whole, and the upper
beds both at the mouth of the river and on the coast northward, seems
to lie in the occasional presence of more colouring matter, and in the
supply having been intermittent; these characters, as we have seen,
very gradually disappear in descending the valley, and this fact may
perhaps be accounted for by the currents of a more open sea having
blended together the sediment from a distant and intermittent source.

The coloured layers in the foregoing section rest on a mass, apparently
of great thickness (but much hidden by the talus), of soft sandstone,
almost composed of minute pebbles, from one-tenth to two-tenths of an
inch in diameter, of the rocks (with the entire exception of the
basaltic lava) composing the great boulders on the surface of the
plain, and probably composing the neighbouring Cordillera. Five miles
higher up the valley, and again thirty miles higher up[4] (that is
twenty miles from the nearest range of the Cordillera), the lower plain
included within the upper escarpments, is formed, as seen on the banks
of the river, of a nearly similar but finer-grained, more earthy,
laminated sandstone, alternating with argillaceous beds, and containing
numerous moderately sized pebbles of the same rocks, and some shells of
the great _Ostrea Patagonica._ As most of these shells had been rolled
before being here embedded, their presence does not prove that the
sandstone belongs to the great Patagonian tertiary formation, for they
might have been redeposited in it, when the valley existed as a
sea-strait;
but as amongst the pebbles there were none of basalt, although the
cliffs on both sides of the valley are composed of this rock, I believe
that the sandstone does belong to this formation. At the highest point
to which we ascended, twenty miles distant from the nearest slope of
the Cordillera, I could see the horizontally zoned white beds,
stretching under the black basaltic lava, close up to the mountains; so
that the valley of the S. Cruz gives a fair idea of the constitution of
the whole width of Patagonia.

 [4] I found at both places, but not _in situ_, quantities of
 coniferous and ordinary dicotyledonous silicified wood, which was
 examined for me by Mr. R. Brown.

_Basaltic lava of the S. Cruz._—This formation is first met with
sixty-seven miles from the mouth of the river; thence it extends
uninterruptedly, generally but not exclusively on the northern side of
the valley, close up to the Cordillera. The basalt is generally black
and fine-grained, but sometimes grey and laminated; it contains some
olivine, and high up the valley much glassy feldspar, where, also, it
is often amygdaloidal; it is never highly vesicular, except on the
sides of rents and on the upper and lower, spherically laminated
surfaces. It is often columnar; and in one place I saw magnificent
columns, each face twelve feet in width, with their interstices filled
up with calcareous tuff. The streams rest conformably on the white
sedimentary beds, but I nowhere saw the actual junction; nor did I
anywhere see the white beds actually superimposed on the lava; but some
way up the valley at the foot of the uppermost escarpments, they must
be thus superimposed. Moreover, at the lowest point down the valley,
where the streams thin out and terminate in irregular projections, the
spaces or intervals between these projections are filled up to the
level of the now denuded and gravel-capped surfaces of the plains, with
the white-zoned sedimentary beds; proving that this matter continued to
be deposited after the streams had flowed. Hence we may conclude that
the basalt is contemporaneous with the upper parts of the great
tertiary formation.

The lava where first met with is 130 feet in thickness: it there
consists of two, three, or perhaps more streams, divided from each
other by vesicular spheroids like those on the surface. From the
streams having, as it appears, extended to different distances, the
terminal points are of unequal heights. Generally the surface of the
basalt is smooth them in one part high up the valley, it was so uneven
and hummocky, that until I afterwards saw the streams extending
continuously on both sides of the valley up to a height of about three
thousand feet close to the Cordillera, I thought that the craters of
eruption were probably close at hand. This hummocky surface I believe
to have been caused by the crossing and heaping up of different
streams. In one place, there were several rounded ridges about twenty
feet in height, some of them as broad as high, and some broader, which
certainly had been formed whilst the lava was fluid, for in transverse
sections each ridge was seen to be concentrically laminated, and to be
composed of imperfect columns radiating from common centres, like the
spokes of wheels.

The basaltic mass where first met with is, as I have said, 130 feet in
thickness, and, thirty-five miles higher up the valley, it increases to
322 feet. In the first fourteen and a half miles of this distance, the
upper surface of the lava, judging from three measurements taken above
the level of the river (of which the apparently very uniform
inclination has been calculated from its total height at a point 135
miles from the mouth), slopes towards the Atlantic at an angle of only
0° 7′ 20″: this must be considered only as an approximate measurement,
but it cannot be far wrong. Taking the whole thirty-five miles, the
upper surface slopes at an angle of 0° 10′ 53″; but this result is of
no value in showing the inclination of any one stream, for halfway
between the two points of measurement, the surface suddenly rises
between one hundred and two hundred feet, apparently caused by some of
the uppermost streams having extended thus far and no farther. From the
measurement made at these two points, thirty-five miles apart, the mean
inclination of the sedimentary beds, over which the lava has flowed, is
_now_ (after elevation from under the sea) only 0° 7′ 52″: for the sake
of comparison, it may be mentioned that the bottom of the present sea
in a line from the mouth of the S. Cruz to the Falkland Islands, from a
depth of seventeen fathoms to a depth of eighty-five fathoms, declines
at an angle of 0° 1′ 22″; between the beach and the depth of seventeen
fathoms, the slope is greater. From a point about half-way up the
valley, the basaltic mass rises more abruptly towards the foot of the
Cordillera, namely, from a height of 1,204 feet, to about 3,000 feet
above the sea.

This great deluge of lava is worthy, in its dimensions, of the great
continent to which it belongs. The aggregate streams have flowed from
the Cordillera to a distance (unparalleled, I believe, in any case yet
known) of about one hundred geographical miles. Near their furthest
extremity their total thickness is 130 feet, which increase thirty-five
miles farther inland, as we have just seen, to 322 feet. The least
inclination given by M. E. de Beaumont of the upper surface of a
lava-stream, namely 0° 30′, is that of the great subaerial eruption in
1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows[5] that
it must have flowed down a mean inclination of less than 0° 20′. But we
now see that under the pressure of the sea, successive streams have
flowed over a smooth bottom with a mean inclination of not more than 0°
7′ 52″; and that the upper surface of the terminal portion (over a
space of fourteen and a half miles) has an inclination of not more than
0° 7′ 20″. If the elevation of Patagonia has been greater nearer the
Cordillera than near the Atlantic (as is probable), then these angles
are now all too large. I must repeat, that although the foregoing
measurements, which were all carefully taken with the barometer, may
not be absolutely correct, they cannot be widely erroneous.

 [5] “Mémoires pour servir,” etc., pp. 178 and 217.

Southward of the S. Cruz, the cliffs of the 840 feet plain extend to
Coy Inlet, and owing to the naked patches of the white sediment, they
are said on the charts to be “like the coast of Kent.” At Coy Inlet the
high plain trends inland, leaving flat-topped outliers. At Port
Gallegos (lat. 51° 35′, and ninety miles south of S. Cruz), I am
informed by Captain Sulivan, R.N., that there is a gravel-capped plain
from two to
three hundred feet in height, formed of numerous strata, some
fine-grained and pale-coloured, like the upper beds at the mouth of the
S. Cruz, others rather dark and coarser, so as to resemble gritstones
or tuffs; these latter include rather large fragments of apparently
decomposed volcanic rocks; there are, also, included layers of gravel.
This formation is highly remarkable, from abounding with mammiferous
remains, which have not as yet been examined by Professor Owen, but
which include some large, but mostly small, species of Pachydermata,
Edentata, and Rodentia. From the appearance of the pale-coloured,
fine-grained beds, I was inclined to believe that they corresponded
with the upper beds of the S. Cruz; but Professor Ehrenberg, who has
examined some of the specimens, informs me that the included
microscopical organisms are wholly different, being fresh and
brackish-water forms. Hence the two to three hundred feet plain at Port
Gallegos is of unknown age, but probably of subsequent origin to the
great Patagonian tertiary formation.

_Eastern Tierra del Fuego._—Judging from the height, the general
appearance, and the white colour of the patches visible on the hill
sides, the uppermost plain, both on the north and western side of the
Strait of Magellan, and along the eastern coast of Tierra del Fuego as
far south as near Port St. Polycarp, probably belongs to the great
Patagonian tertiary formation. These higher table-ranges are fringed by
low, irregular, extensive plains, belonging to the boulder
formation,[6] and composed of coarse unstratified masses, sometimes
associated (as north of C. Virgin’s) with fine, laminated, muddy
sandstones. The cliffs in Sebastian Bay are 200 feet in height, and are
composed of fine sandstones, often in curvilinear layers, including
hard concretions of calcareous sandstone, and layers of gravel. In
these beds there are fragments of wood, legs of crabs, barnacles
encrusted with corallines still partially retaining their colour,
imperfect fragments of a Pholas distinct from any known species, and of
a Venus, approaching very closely to, but slightly different in form
from, the _V. lenticularis_, a species living on the coast of Chile.
Leaves of trees are numerous between the laminæ of the muddy sandstone;
they belong, as I am informed by Dr. J. D. Hooker,[7] to three species
of deciduous beech, different from the two species which compose the
great proportion of trees in this forest-clad land. From these facts it
is difficult to conjecture, whether we here see the basal part of the
great Patagonian formation, or some later deposit.

 [6] Described in the “Geological Transactions,” vol. vi, p. 415.


 [7] “Botany of the Antarctic Voyage,” p. 212.

_Summary on the Patagonian tertiary formation._—Four out of the seven
fossil shells, from St. Fé in Entre Rios, were found by M. d’Orbigny in
the sandstone of the Rio Negro, and by me at San Josef. Three out of
the six from San Josef are identical with those from Port Desire and S.
Julian, which two places have together fifteen species, out of which
three are common to both. Santa Cruz has seventeen species, out of
which five are common to Port Desire and S. Julian. Considering the
difference in latitude between these several places, and
the small number of species altogether collected, namely thirty-six, I
conceive the above proportional number of species in common, is
sufficient to show that the lower fossiliferous mass belongs nearly, I
do not say absolutely, to the same epoch. What this epoch may be,
compared with the European tertiary stages, M. d’Orbigny will not
pretend to determine. The thirty-six species (including those collected
by myself and by M. d’Orbigny) are all extinct, or at least unknown;
but it should be borne in mind, that the present coast consists of
shingle, and that no one, I believe, has dredged here for shells; hence
it is not improbable that some of the species may hereafter be found
living. Some few of the species are closely related with existing ones;
this is especially the case, according to M. d’Orbigny and Mr. Sowerby,
with the _ Fusus Patagonicus_; and, according to Mr. Sowerby, with the
_ Pyrula_, the _Venus meridionalis_, the _Crepidula gregaria_, and the
_Turritella ambulacrum_, and _T. Patagonica._ At least three of the
genera, namely, Cucullæa, Crassatella, and (as determined by Mr.
Sowerby) Struthiolaria, are not found in this quarter of the world; and
Trigonocelia is extinct. The evidence taken altogether indicates that
this great tertiary formation is of considerable antiquity; but when
treating of the Chilean beds, I shall have to refer again to this
subject.

The white pumiceous mudstone, with its abundant gypsum, belongs to the
same general epoch with the underlying fossiliferous mass, as may be
inferred from the shells included in the intercalated layers at Nuevo
Gulf, S. Julian, and S. Cruz. Out of the twenty-seven marine
microscopic structures found by Professor Ehrenberg in the specimens
from S. Julian and Port Desire, ten are common to these two places: the
three found at Nuevo Gulf are distinct. I have minutely described this
deposit, from its remarkable characters and its wide extension. From
Coy Inlet to Port Desire, a distance of 230 miles, it is certainly
continuous; and I have reason to believe that it likewise extends to
the Rio Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we
have, also, seen that a single layer occurs at the Rio Negro. At Port
S. Julian it is from eight to nine hundred feet in thickness; and at S.
Cruz it extends, with a slightly altered character, up to the
Cordillera. From its microscopic structure, and from its analogy with
other formations in volcanic districts, it must be considered as
originally of volcanic origin: it may have been formed by the
long-continued attrition of vast quantities of pumice, or judging from
the manner in which the mass becomes, in ascending the valley of S.
Cruz, divided into variously coloured layers, from the long-continued
eruption of clouds of fine ashes. In either case, we must conclude,
that the southern volcanic orifices of the Cordillera, now in a dormant
state, were at about this period over a wide space, and for a great
length of time, in action. We have evidence of this fact, in the
latitude of the Rio Negro, in the sandstone-conglomerate with pumice,
and demonstrative proof of it, at S. Cruz, in the vast deluges of
basaltic lava: at this same tertiary period, also, there is distinct
evidence of volcanic action in Western Banda Oriental.

The Patagonian tertiary formation extends continuously, judging
from fossils alone, from S. Cruz to near the Rio Colorado, a distance
of above six hundred miles, and reappears over a wide area in Entre
Rios and Banda Oriental, making a total distance of 1,100 miles; but
this formation undoubtedly extends (though no fossils were collected)
far south of the S. Cruz, and, according to M. d’Orbigny, 120 miles
north of St. Fé. At S. Cruz we have seen that it extends across the
continent; being on the coast about eight hundred feet in thickness
(and rather more at S. Julian), and rising with the contemporaneous
lava-streams to a height of about three thousand feet at the base of
the Cordillera. It rests, wherever any underlying formation can be
seen, on plutonic and metamorphic rocks. Including the newer Pampean
deposit, and those strata in Eastern Tierra del Fuego of doubtful age,
as well as the boulder formation, we have a line of more than
twenty-seven degrees of latitude, equal to that from the Straits of
Gibraltar to the south of Iceland, continuously composed of tertiary
formations. Throughout this great space the land has been upraised,
without the strata having been in a single instance, as far as my means
of observation went, unequally tilted or dislocated by a fault.

_Tertiary Formations on the West Coast._

_Chonos Archipelago._—The numerous islands of this group, with the
exception of Lemus, Ypun, consist of metamorphic schists; these two
islands are formed of softish grey and brown, fusible, often laminated
sandstones, containing a few pebbles, fragments of black lignite, and
numerous mammillated concretions of hard calcareous sandstone. Out of
these concretions at Ypun (lat. 40° 30′ S.), I extracted the four
following extinct species of shells:—

Turritella suturalis, G. B. Sowerby (also Navidad).

Sigaretus subglobosus, G. B. Sowerby (also Navidad).

Cytheræa (?) sulculosa (?), G. B. Sowerby (also Chiloe and Huafo?).

Voluta, fragments of.

In the northern parts of this group there are some cliffs of gravel and
of the boulder formation. In the southern part (at P. Andres in Tres
Montes), there is a volcanic formation, probably of tertiary origin.
The lavas attain a thickness of from two to three hundred feet; they
are extremely variable in colour and nature, being compact, or
brecciated, or cellular, or amygdaloidal with zeolite, agate and bole,
or porphyritic with glassy albitic feldspar. There is also much
imperfect rubbly pitchstone, with the interstices charged with powdery
carbonate of lime apparently of contemporaneous origin. These lavas are
conformably associated with strata of breccia and of brown tuff
containing lignite. The whole mass has been broken up and tilted at an
angle of 45°, by a series of great volcanic dikes, one of which was
thirty yards in breadth. This volcanic formation resembles one,
presently to be described, in Chiloe.

_Huafo._—This island lies between the Chonos and Chiloe groups: it is
about eight hundred feet high, and perhaps has a nucleus of metamorphic
rocks. The strata which I examined consisted of fine-grained
muddy sandstones, with fragments of lignite and concretions of
calcareous sandstone. I collected the following extinct shells, of
which the Turritella was in great numbers:—

Bulla cosmophila, G. B. Sowerby.

Pleurotoma subæqualis, G. B. Sowerby.

Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at Coquimbo).

Triton leucostomoides, G. B. Sowerby.

Turritella Chilensis, G. B. Sowerby (also Mocha).

Venus, probably a distinct species, but very imperfect.

Cytheræa (?) sulculosa (?), probably a distinct species, but very
imperfect.

Dentalium majus, G. B. Sowerby.

_Chiloe._—This fine island is about one hundred miles in length. The
entire southern part, and the whole western coast, consists of
mica-schist, which likewise is seen in the ravines of the interior. The
central mountains rise to a height of 3,000 feet, and are said to be
partly formed of granite and greenstone: there are two small volcanic
districts. The eastern coast, and large parts of the northern extremity
of the island are composed of gravel, the boulder formation, and
underlying horizontal strata. The latter are well displayed for twenty
miles north and south of Castro; they vary in character from common
sandstone to fine-grained, laminated mudstones: all the specimens which
I examined are easily fusible, and some of the beds might be called
volcanic grit-stones. These latter strata are perhaps related to a mass
of columnar trachyte which occurs behind Castro. The sandstone
occasionally includes pebbles, and many fragments and layers of
lignite; of the latter, some are apparently formed of wood and others
of leaves: one layer on the N.W. side of Lemuy is nearly two feet in
thickness. There is also much silicified wood, both common
dicotyledonous and coniferous: a section of one specimen in the
direction of the medullary rays has, as I am informed by Mr. R. Brown,
the discs in a double row placed alternately, and not opposite as in
the true Araucaria. I found marine remains only in one spot, in some
concretions of hard calcareous sandstone: in several other districts I
have observed that organic remains were exclusively confined to such
concretions; are we to account for this fact, by the supposition that
the shells lived only at these points, or is it not more probable that
their remains were preserved only where concretions were formed? The
shells here are in a bad state, they consist of:—

Tellinides (?) oblonga, G. B. Sowerby (a solenella in M. d’Orbigny’s
opinion).

Natica striolata, G. B. Sowerby.

Natica (?) pumila, G. B. Sowerby.

Cytheræa (?) sulculosa, G. B. Sowerby (also Ypun and Huafo?).


At the northern extremity of the island, near S. Carlos, there is a
large volcanic formation, between five and seven hundred feet in
thickness. The commonest lava is blackish-grey or brown, either
vesicular, or amygdaloidal with calcareous spar and bole: most even of
the darkest varieties fuse into a pale-coloured glass. The next
commonest variety is a rubbly, rarely well characterised pitchstone
(fusing into a white glass) which passes in the most irregular manner
into stony grey lavas. This pitchstone, as well as some purple
claystone porphyry, certainly flowed in the form of streams. These
various lavas often pass, at a considerable depth from the surface, in
the most abrupt and singular manner into wacke. Great masses of the
solid rock are brecciated, and it was generally impossible to discover
whether the recementing process had been an igneous or aqueous
action.[8] The beds are obscurely separated from each other; they are
sometimes parted by seams of tuff and layers of pebbles. In one place
they rested on, and in another place were capped by, tuffs and
girt-stones, apparently of submarine origin.

 [8] In a cliff of the hardest fragmentary mass, I found several
 tortuous, vertical veins, varying in thickness from a few tenths of an
 inch to one inch and a half, of a substance which I have not seen
 described. It is glossy, and of a brown colour; it is thinly
 laminated, with the laminæ transparent and elastic; it is a little
 harder than calcareous spar; it is infusible under the blowpipe,
 sometimes decrepitates, gives out water, curls up, blackens, and
 becomes magnetic. Borax easily dissolves a considerable quantity of
 it, and gives a glass tinged with green. I have no idea what its true
 nature is. On first seeing it, I mistook it for lignite!

The neighbouring peninsula of Lacuy is almost wholly formed of
tufaceous deposits, connected probably in their origin with the
volcanic hills just described. The tuffs are pale-coloured, alternating
with laminated mudstones and sandstones (all easily fusible), and
passing sometimes into fine-grained white beds strikingly resembling
the great upper infusorial deposit of Patagonia, and sometimes into
brecciolas with pieces of pumice in the last stage of decay; these
again pass into ordinary coarse breccias and conglomerates of hard
rocks. Within very short distances, some of the finer tuffs often
passed into each other in a peculiar manner, namely, by irregular
polygonal concretions of one variety increasing so much and so suddenly
in size, that the second variety, instead of any longer forming the
entire mass, was left merely in thin veins between the concretions. In
a straight line of cliffs, at Point Tenuy, I examined the following
remarkable section (figure No. 19):—

No. 19


[Illustration: Section at Point Tenuy]

On the left hand, the lower part (AA) consists of regular, alternating
strata of brown tuffs and greenish laminated mudstone, gently inclined
to the right, and conformably covered by a mass (B _left_) of a white,
tufaceous and brecciolated deposit. On the right hand, the whole cliff
(BB _right_) consists of the same white tufaceous matter, which on this
side presents scarcely a trace of stratification, but to the left
becomes very gradually and rather indistinctly divided into strata
quite conformable with the underlying beds (AA): moreover, a few
hundred yards further to the left, where the surface has been less
denuded, the tufaceous strata (B _left_) are conformably covered by
another set of strata, like the underlying ones (AA) of this section.
In the middle of the diagram, the beds (AA) are seen to be abruptly cut
off, and to abut against the tufaceous non-stratified mass; but the
line of junction has
been accidentally not represented steep enough, for I particularly
noticed that before the beds had been tilted to the right, this line
must have been nearly vertical. It appears that a current of water cut
for itself a deep and steep submarine channel, and at the same time or
afterwards filled it up with the tufaceous and brecciolated matter, and
spread the same over the surrounding submarine beds; the matter
becoming stratified in these more distant and less troubled parts, and
being moreover subsequently covered up by other strata (like AA) not
shown in the diagram. It is singular that three of the beds (of AA) are
prolonged in their proper direction, as represented, beyond the line of
junction into the white tufaceous matter: the prolonged portions of two
of the beds are rounded; in the third, the terminal fragment has been
pushed upwards: how these beds could have been left thus prolonged, I
will not pretend to explain. In another section on the opposite side of
a promontory, there was at the foot of this same line of junction, that
is at the bottom of the old submarine channel, a pile of fragments of
the strata (AA), with their interstices filled up with white tufaceous
matter: this is exactly what might have been anticipated under such
circumstances.

No. 20
Ground plan showing the relation between veins and concretionary zones
in a mass of tuff.


[Illustration: Ground plan showing the relation between veins and
concretionary zones in a mass of tuff.]

The various tufaceous and other beds at this northern end of Chiloe
probably belong to about the same age with those near Castro, and they
contain, as there, many fragments of black lignite and of silicified
and pyritous wood, often embedded close together. They also contain
many and singular concretions: some are of hard calcareous sandstone,
in which it would appear that broken volcanic crystals and scales of
mica have been better preserved (as in the case of the
organic remains near Castro) than in the surrounding mass. Other
concretions in the white brecciola are of a hard, ferruginous, yet
fusible, nature; they are as round as cannon-balls, and vary from two
or three inches to two feet in diameter; their insides generally
consist either of fine, scarcely coherent volcanic sand,[9] or of an
argillaceous tuff; in this latter case, the external crust was quite
thin and hard. Some of these spherical balls were encircled in the line
of their equators, by a necklace-like row of smaller concretions. Again
there were other concretions, irregularly formed, and composed of a
hard, compact, ash-coloured stone, with an almost porcelainous
fracture, adhesive to the tongue, and without any calcareous matter.
These beds are, also, interlaced by many veins, containing gypsum,
ferruginous matter, calcareous spar, and agate. It was here seen with
remarkable distinctness, how intimately concretionary action and the
production of fissures and veins are related together. Figure 20 is an
accurate representation of a horizontal space of tuff, about four feet
long by two and a half in width: the double lines represent the
fissures partially filled with oxide of iron and agate: the curvilinear
lines show the course of the innumerable, concentric, concretionary
zones of different shades of colour and of coarseness in the particles
of tuff. The symmetry and complexity of the arrangement gave the
surface an elegant appearance.
It may be seen how obviously the fissures determine (or have been
determined by) the shape, sometimes of the whole concretion, and
sometimes only of its central parts. The fissures also determine the
curvatures of the long undulating zones of concretionary action. From
the varying composition of the veins and concretions, the amount of
chemical action which the mass has undergone is surprisingly great; and
it would likewise appear from the difference in size in the particles
of the concretionary zones, that the mass, also, has been subjected to
internal mechanical movements.

 [9] The frequent tendency in iron to form hollow concretions or shell
 containing incoherent matter is singular; D’Aubuisson (“Traité de
 Géogn.” tome i, p. 318) remarks on this circumstance.

In the peninsula of Lacuy, the strata over a width of four miles have
been upheaved by three distinct, and some other indistinct, lines of
elevation, ranging within a point of north and south. One line, about
two hundred feet in height, is regularly anticlinal, with the strata
dipping away on both sides, at an angle of 15°, from a central “valley
of elevation,” about three hundred yards in width. A second narrow
steep ridge, only sixty feet high, is uniclinal, the strata throughout
dipping westward; those on both flanks being inclined at an angle of
from ten to fifteen degrees; whilst those on the ridge dip in the same
direction at an angle of between thirty and forty degrees. This ridge,
traced northwards, dies away; and the beds at its terminal point,
instead of dipping westward, are inclined 12° to the north. This case
interested me, as being the first in which I found in South America,
formations perhaps of tertiary origin, broken by lines of elevation.

_Valdivia: Island of Mocha._—The formations of Chiloe seem to extend
with nearly the same character to Valdivia, and for some leagues
northward of it: the underlying rocks are micaceous schists, and are
covered up with sandstone and other sedimentary beds, including, as I
was assured, in many places layers of lignite. I did not land on Mocha
(lat. 38° 20′), but Mr. Stokes brought me specimens of the grey,
fine-grained, slightly calcareous sandstone, precisely like that of
Huafo, containing lignite and numerous Turritellæ. The island is flat
topped, 1,240 feet in height, and appears like an outlier of the
sedimentary beds on the mainland. The few shells collected consist of:—

Turritella Chilensis, G. B. Sowerby (also at Huafo).

Fusus, very imperfect, somewhat resembling F. subreflexus of Navidad,
but probably different.

Venus, fragments of.

_Concepcion._—Sailing northward from Valdivia, the coast-cliffs are
seen, first to assume near the R. Tolten, and thence for 150 miles
northward, to be continued with the same mineralogical characters,
immediately to be described at Concepcion. I heard in many places of
beds of lignite, some of it fine and glossy, and likewise of silicified
wood; near the Tolten the cliffs are low, but they soon rise in height;
and the horizontal strata are prolonged, with a nearly level surface,
until coming to a more lofty tract between points Rumena and Lavapie.
Here the beds have been broken up by at least eight or nine parallel
lines of elevation, ranging E. or E.N.E. and W. or W.S.W. These lines
can be followed with the eye many miles into the interior; they
are all uniclinal, the strata in each dipping to a point between S. and
S.S.E. with an inclination in the central lines of about forty degrees,
and in the outer ones of under twenty degrees. This band of
symmetrically troubled country is about eight miles in width.

The island of Quiriquina, in the Bay of Concepcion, is formed of
various soft and often ferruginous sandstones, with bands of pebbles,
and with the lower strata sometimes passing into a conglomerate resting
on the underlying metamorphic schists. These beds include subordinate
layers of greenish impure clay, soft micaceous and calcareous
sandstones, and reddish friable earthy matter with white specks like
decomposed crystals of feldspar; they include, also, hard concretions,
fragments of shells, lignite, and silicified wood. In the upper part
they pass into white, soft sediments and brecciolas, very like those
described at Chiloe; as indeed is the whole formation. At Lirguen and
other places on the eastern side of the bay, there are good sections of
the lower sandstones, which are generally ferruginous, but which vary
in character, and even pass into an argillaceous nature; they contain
hard concretions, fragments of lignite, silicified wood, and pebbles
(of the same rocks with the pebbles in the sandstones of Quiriquina),
and they alternate with numerous, often very thin layers of imperfect
coal, generally of little specific gravity. The main bed here is three
feet thick; and only the coal of this one bed has a glossy fracture.
Another irregular, curvilinear bed of brown, compact lignite, is
remarkable for being included in a mass of coarse gravel. These
imperfect coals, when placed in a heap, ignite spontaneously. The
cliffs on this side of the bay, as well as on the island of Quiriquina,
are capped with red friable earth, which, as stated in the Second
Chapter, is of recent formation. The stratification in this
neighbourhood is generally horizontal; but near Lirguen the beds dip
N.W. at an angle of 23°; near Concepcion they are also inclined: at the
northern end of Quiriquina they have been tilted at an angle of 30°,
and at the southern end at angles varying from 15° to 40°: these
dislocations must have taken place under the sea.

A collection of shells, from the island of Quiriquina, has been
described by M. d’Orbigny: they are all extinct, and from their generic
character, M. d’Orbigny inferred that they were of tertiary origin:
they consist of:—

Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.”

Natica Araucana, d’Orbigny, “Voyage, Part Pal.”

Natica australis, d’Orbigny, “Voyage, Part Pal.”

Fusus difficilis, d’Orbigny, “Voyage, Part Pal.”

Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.”

Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.”

Cardium auca, d’Orbigny, “Voyage, Part Pal.”

Cardium acuticostatum, d’Orbigny, “Voyage, Part Pal.”

Venus auca, d’Orbigny, “Voyage, Part Pal.”

Mactra cecileana, d’Orbigny, “Voyage, Part Pal.”

Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”

Arca Araucana, d’Orbigny, “Voyage, Part Pal.”

Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.”

Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”

During a second visit of the _Beagle_ to Concepcion, Mr. Kent collected
for me some silicified wood and shells out of the concretions in the
sandstone from Tome, situated a short distance north of Lirguen. They
consist of:—


Natica australis, d’Orbigny, “Voyage, Part Pal.”

Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”

Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”

Pecten, fragments of, probably two species, but too imperfect for
description.

Baculites vagina, E. Forbes.

Nautilus d’Orbignyanus, E. Forbes.


Besides these shells, Captain Belcher[10] found here an _Ammonite_,
nearly three feet in diameter, and so heavy that he could not bring it
away; fragments are deposited at Haslar Hospital: he also found the
silicified vertebræ of some very large animal. From the identity in
mineralogical nature of the rocks, and from Captain Belcher’s minute
description of the coast between Lirguen and Tome, the fossiliferous
concretions at this latter place certainly belong to the same formation
with the beds examined by myself at Lirguen; and these again are
undoubtedly the same with the strata of Quiriquina; moreover; the three
first of the shells from Tome, though associated in the same
concretions with the Baculite, are identical with the species from
Quiriquina. Hence all the sandstone and lignitiferous beds in this
neighbourhood certainly belong to the same formation. Although the
generic character of the Quiriquina fossils naturally led M. d’Orbigny
to conceive that they were of tertiary origin, yet as we now find them
associated with the _Baculites vagina_ and with an Ammonite, we must,
in the opinion of M. d’Orbigny, and if we are guided by the analogy of
the northern hemisphere, rank them in the Cretaceous system. Moreover,
the _Baculites vagina_, which is in a tolerable state of preservation,
appears to Professor E. Forbes certainly to be identical with a
species, so named by him, from Pondicherry in India; where it is
associated with numerous decidedly cretaceous species, which approach
most nearly to Lower Greensand or Neocomian forms: this fact,
considering the vast distance between Chile and India, is truly
surprising. Again, the _Nautilus d’Orbignyanus_, as far as its
imperfect state allows of comparison, resembles, as I am informed by
Professor Forbes, both in its general form and in that of its chambers,
two species from the Upper Greensand. It may be added that every one of
the above-named genera from Quiriquina, which have an apparently
tertiary character, are found in the Pondicherry strata. There are,
however, some difficulties on this view of the formations at Concepcion
being cretaceous, which I shall afterwards allude to; and I will here
only state that the _Cardium auca_ is found also at Coquimbo, the beds
at which place, there can be no doubt, are tertiary.

 [10] “Zoology of Captain Beechey’s Voyage,” p. 163.


_Navidad._[11]—The Concepcion formation extends some distance
northward, but how far I know not; for the next point at which I landed
was at Navidad, 160 miles north of Concepcion, and 60 miles south of
Valparaiso. The cliffs here are about eight hundred feet in height:
they consist, wherever I could examine them, of fine-grained,
yellowish, earthy sandstones, with ferruginous veins, and with
concretions of
hard calcareous sandstone. In one part, there were many pebbles of the
common metamorphic porphyries of the Cordillera: and near the base of
the cliff, I observed a single rounded boulder of greenstone, nearly a
yard in diameter. I traced this sandstone formation beneath the
superficial covering of gravel, for some distance inland: the strata
are slightly inclined from the sea towards the Cordillera, which
apparently has been caused by their having been accumulated against or
round outlying masses of granite, of which some points project near the
coast. The sandstone contains fragments of wood, either in the state of
lignite or partially silicified, sharks’ teeth, and shells in great
abundance, both high up and low down the sea-cliffs. Pectunculus and
Oliva were most numerous in individuals, and next to them Turritella
and Fusus. I collected in a short time, though suffering from illness,
the following thirty-one species, all of which are extinct, and several
of the genera do not now range (as we shall hereafter show) nearly so
far south:—

Gastridium cepa, G. B. Sowerby.

Monoceros, fragments of, considered by M. d’Orbigny as a new species.

Voluta alta, G. B. Sowerby (considered by M. d’Orbigny as distinct from
the V. alta of Santa Cruz).

Voluta triplicata, G. B. Sowerby.

Oliva dimidiata, G. B. Sowerby.

Pleurotoma discors, G. B. Sowerby.

Pleurotoma turbinelloides, G. B. Sowerby.

Fusus subreflexus, G. B. Sowerby.

Fusus pyruliformis, G. B. Sowerby.

Fusus, allied to F. regularis (considered by M. d’Orbigny as a distinct
species).

Turritella suturalis, G. B. Sowerby.

Turritella Patagonica, G. B. Sowerby (fragments of).

Trochus lævis, G. B. Sowerby.

Trochus collaris, G. B. Sowerby (considered by M. d’Orbigny as the
young of the T. lævis).

Cassis monilifer, G. B. Sowerby.

Pyrula distans, G. B. Sowerby.

Triton verruculosus, G. B. Sowerby.

Sigaretus subglobosus, G. B. Sowerby.

Natica solida, G. B. Sowerby. (It is doubtful whether the Natica solida
of S. Cruz is the same species with this.)

Terebra undulifera, G. B. Sowerby.

Terebra costellata, G. B. Sowerby.

Bulla (fragments of).

Dentalium giganteum, do.

Dentalium sulcosum, do.

Corbis (?) lævigata, do.

Cardium multiradiatum, do.

Venus meridionalis, do.

Pectunculus dispar, (?) Desh. (considered by M. d’Orbigny as a distinct
species).

and 30. Cytheræa and Mactra, fragments of (considered by M. d’Orbigny
as new species).

Pecten, fragments of.

 [11] I was guided to this locality by the Report on M. Gay’s
 “Geological Researches,” in the “Annales des Scienc. Nat.” (1st
 series, tome 28.

_Coquimbo._—For more than two hundred miles northward of Navidad, the
coast consists of plutonic and metamorphic rocks, with the exception of
some quite insignificant superficial beds of recent origin. At Tonguay,
twenty-five miles south of Coquimbo, tertiary beds recommence. I have
already minutely described in the Second Chapter, the step-formed
plains of Coquimbo, and the upper calcareous beds (from twenty to
thirty feet in thickness) containing shells of recent species, but in
different proportions from those on the beach. There remains to be
described only the underlying ancient tertiary beds, represented in
Figure 21 by the letters F and E:—

No. 21
Section of the tertiary formation at Coquimbo.


[Illustration: Section of the tertiary formation at Coquimbo.]

I obtained good sections of bed (F) only in Herradura Bay: it consists
of soft whitish sandstone, with ferruginous veins, some pebbles of
granite, and concretionary layers of hard calcareous sandstone. These
concretions are remarkable from the great number of large silicified
bones, apparently of cetaceous animals, which they contain; and
likewise of a shark’s teeth, closely resembling those of the
_Carcharias megalodon._ Shells of the following species, of which the
gigantic Oyster and Perna are the most conspicuous, are numerously
embedded in the concretions:—

Bulla ambigua, d’Orbigny, “Voyage,” Pal.

Monoceros Blainvillii, d’Orbigny, “Voyage,” Pal.

Cardium auca, d’Orbigny, “Voyage,” Pal.

Panopæa Coquimbensis, d’Orbigny, “Voyage,” Pal.

Perna Gaudichaudi, d’Orbigny, “Voyage,” Pal.

Artemis ponderosa; Mr. Sowerby can find no distinguishing character
between this fossil and the recent A. ponderosa; it is certainly an
Artemis, as shown by the pallial impression.

Ostrea Patagonica (?); Mr. Sowerby can point out no distinguishing
character between this species and that so eminently characteristic of
the great Patagonian formation; but he will not pretend to affirm that
they are identical.

Fragments of a Venus and Natica.


The cliffs on one side of Herradura Bay are capped by a mass of
stratified shingle, containing a little calcareous matter, and I did
not doubt that it belonged to the same recent formation with the gravel
on the surrounding plains, also cemented by calcareous matter, until to
my surprise, I found in the midst of it, a single thin layer almost
entirely composed of the above gigantic oyster.

At a little distance inland, I obtained several sections of the bed
(E), which, though different in appearance from the lower bed (F),
belongs to the same formation: it consists of a highly ferruginous
sandy mass, almost composed, like the lowest bed at Port S. Julian, of
fragments of Balanidæ; it includes some pebbles, and layers of
yellowish-brown mudstone. The embedded shells consist of:—

Monoceros Blainvillii, d’Orbigny, “Voyage” Pal.

Monoceros ambiguus, G. B. Sowerby.

Anomia alternans, G. B. Sowerby.

Pecten rudis, G. B. Sowerby.

Perna Gaudichaudi, d’Orbigny, “Voyage” Pal.

Ostrea Patagonica (?), d’Orbigny, “Voyage” Pal.

Ostrea, small species, in imperfect state; it appeared to me like a
small kind now living in, but very rare in the bay.

Mytilus Chiloensis; Mr. Sowerby can find no distinguishing character
between this fossil, as far as its not very perfect condition allows of
comparison, and the recent species.

Balanus Coquimbensis, G. B. Sowerby.

Balanus psittacus? King. This appears to Mr. Sowerby and myself
identical with a very large and common species now living on the coast.

The uppermost layers of this ferrugino-sandy mass are conformably
covered by, and impregnated to the depth of several inches with, the
calcareous matter of the bed (D) called _ losa_: hence I at one time
imagined that there was a gradual passage between them; but as all the
species are recent in the bed (D), whilst the most characteristic
shells of the uppermost layers of (E) are the extinct Perna, Pecten,
and Monoceros, I agree with M. d’Orbigny, that this view is erroneous,
and that there is only a mineralogical passage between them, and no
gradual transition in the nature of their organic remains. Besides the
fourteen species enumerated from these two lower beds, M. d’Orbigny has
described ten other species given to him from this locality; namely:—

Fusus Cleryanus, d’Orbigny, “Voyage” Pal.

Fusus petitianus, d’Orbigny, “Voyage” Pal.

Venus hanetiana, d’Orbigny, “Voyage” Pal.

Venus incerta (?) d’Orbigny, “Voyage” Pal.

Venus Cleryana, d’Orbigny, “Voyage” Pal.

Venus petitiana, d’Orbigny, “Voyage” Pal.

Venus Chilensis, d’Orbigny, “Voyage” Pal.

Solecurtus hanetianus, d’Orbigny, “Voyage” Pal.

Mactra auca, d’Orbigny, “Voyage” Pal.

Oliva serena, d’Orbigny, “Voyage” Pal.

Of these twenty-four shells, all are extinct, except, according to Mr.
Sowerby, the _Artemis ponderosa, Mytilus Chiloensis,_ and probably the
great Balanus.

_Coquimbo to Copiapo._—A few miles north of Coquimbo, I met with
the ferruginous, balaniferous mass (E) with many silicified bones; I
was informed that these silicified bones occur also at Tonguay, south
of Coquimbo: their number is certainly remarkable, and they seem to
take the place of the silicified wood, so common on the
coast-formations of Southern Chile. In the valley of Chañeral, I again
saw this same formation, capped with the recent calcareous beds. I here
left the coast, and did not see any more of the tertiary formations,
until descending to the sea at Copiapo: here in one place I found
variously coloured layers of sand and soft sandstone, with seams of
gypsum, and in another place, a comminuted shelly mass, with layers of
rotten-stone and seams of gypsum, including many of the extinct
gigantic oyster: beds with these oysters are said to occur at English
Harbour, a few miles north of Copiapo.

_Coast of Peru._—With the exception of deposits containing recent
shells and of quite insignificant dimensions, no tertiary formations
have been observed on this coast, for a space of twenty-two degrees of
latitude north of Copiapo, until coming to Payta, where there is said
to be a considerable calcareous deposit: a few fossils have been
described by M. d’Orbigny from this place, namely:—

Rostellaria Gaudichaudi, d’Orbigny, “Voyage” Pal.

Pectunculus Paytensis, d’Orbigny, “Voyage” Pal.

Venus petitiana, d’Orbigny, “Voyage” Pal.

Ostrea Patagonica? This great oyster (of which specimens have been
given me) cannot be distinguished by Mr. Sowerby from some of the
varieties from Patagonia; though it would be hazardous to assert it is
the same with that species, or with that from Coquimbo.

_Concluding Remarks._—The formations described in this chapter, have,
in the case of Chiloe and probably in that of Concepcion and Navidad,
apparently been accumulated in troughs formed by submarine ridges
extending parallel to the ancient shores of the continent; in the case
of the islands of Mocha and Huafo it is highly probable, and in that of
Ypun and Lemus almost certain, that they were accumulated round
isolated rocky centres or nuclei, in the same manner as mud and sand
are now collecting round the outlying islets and reefs in the West
Indian Archipelago. Hence, I may remark, it does not follow that the
outlying tertiary masses of Mocha and Huafo were ever continuously
united at the same level with the formations on the mainland, though
they may have been of contemporaneous origin, and been subsequently
upraised to the same height. In the more northern parts of Chile, the
tertiary strata seem to have been separately accumulated in bays, now
forming the mouths of valleys.

The relation between these several deposits on the shores of the
Pacific, is not nearly so clear as in the case of the tertiary
formations on the Atlantic. Judging from the form and height of the
land (evidence which I feel sure is here much more trustworthy than it
can ever be in such broken continents as that of Europe), from the
identity of mineralogical composition, from the presence of fragments
of lignite and of silicified wood, and from the intercalated layers of
imperfect coal, I must believe
that the coast-formations from Central Chiloe to Concepcion, a distance
of 400 miles, are of the same age: from nearly similar reasons, I
suspect that the beds of Mocha, Huafo, and Ypun, belong also to the
same period. The commonest shell in Mocha and Huafo is the same species
of Turritella; and I believe the same Cytheræa is found on the islands
of Huafo, Chiloe, and Ypun; but with these trifling exceptions, the few
organic remains found at these places are distinct. The numerous shells
from Navidad, with the exception of two, namely, the Sigaretus and
Turritella found at Ypun, are likewise distinct from those found in any
other part of this coast. Coquimbo has _Cardium auca_ in common with
Concepcion, and _Fusus Cleryanus_ with Huafo; I may add, that Coquimbo
has _Venus petitiana_, and a gigantic oyster (said by M. d’Orbigny also
to be found a little south of Concepcion) in common with Payta, though
this latter place is situated twenty-two degrees northward of lat. 27°,
to which point the Coquimbo formation extends.

From these facts, and from the generic resemblance of the fossils from
the different localities, I cannot avoid the suspicion that they all
belong to nearly the same epoch, which epoch, as we shall immediately
see, must be a very ancient tertiary one. But as the Baculite,
especially considering its apparent identity with the Cretaceous
Pondicherry species, and the presence of an Ammonite, and the
resemblance of the Nautilus to two upper greensand species, together
afford very strong evidence that the formation of Concepcion is a
Secondary one; I will, in my remarks on the fossils from the other
localities, put on one side those from Concepcion and from Eastern
Chiloe, which, whatever their age may be, appear to me to belong to one
group. I must, however, again call attention to the fact that the
_Cardium auca_ is found both at Concepcion and in the undoubtedly
tertiary strata of Coquimbo: nor should the possibility be overlooked,
that as Trigonia, though known in the northern hemisphere only as a
Secondary genus, has living representatives in the Australian seas, so
a Baculite, Ammonite, and Trigonia may have survived in this remote
part of the southern ocean to a somewhat later period than to the north
of the equator.

Before passing in review the fossils from the other localities, there
are two points, with respect to the formations between Concepcion and
Chiloe, which deserve some notice. First, that though the strata are
generally horizontal, they have been upheaved in Chiloe in a set of
parallel anticlinal and uniclinal lines ranging north and south,—in the
district near P. Rumena by eight or nine far-extended, most
symmetrical, uniclinal lines ranging nearly east and west,—and in the
neighbourhood of Concepcion by less regular single lines, directed both
N.E. and S.W., and N.W. and S.E. This fact is of some interest, as
showing that within a period which cannot be considered as very ancient
in relation to the history of the continent, the strata between the
Cordillera and the Pacific have been broken up in the same variously
directed manner as have the old plutonic and metamorphic rocks in this
same district. The second point is, that the sandstone between
Concepcion and Southern Chiloe is everywhere lignitiferous, and
includes much silicified wood; whereas the formations in Northern Chile
do not
include beds of lignite or coal, and in place of the fragments of
silicified wood there are silicified bones. Now, at the present day,
from Cape Horn to near Concepcion, the land is entirely concealed by
forests, which thin out at Concepcion, and in Central and Northern
Chile entirely disappear. This coincidence in the distribution of the
fossil wood and the living forests may be quite accidental; but I
incline to take a different view of it; for, as the difference in
climate, on which the presence of forests depends, is here obviously in
chief part due to the form of the land, and as the Cordillera
undoubtedly existed when the lignitiferous beds were accumulating, I
conceive it is not improbable that the climate, during the
lignitiferous period, varied on different parts of the coast in a
somewhat similar manner as it now does. Looking to an earlier epoch,
when the strata of the Cordillera were depositing, there were islands
which even in the latitude of Northern Chile, where now all is
irreclaimably desert, supported large coniferous forests.

Seventy-nine species of fossil shells, in a tolerably recognisable
condition, from the coast of Chile and Peru, are described in this
volume, and in the Palæontological part of M. d’Orbigny’s “Voyage”: if
we put on one side the twenty species exclusively found at Concepcion
and Chiloe, fifty-nine species from Navidad and the other specified
localities remain. Of these fifty-nine species only an Artemis, a
Mytilus and Balanus, all from Coquimbo, are (in the opinion of Mr.
Sowerby, but not in that of M. d’Orbigny) identical with living shells;
and it would certainly require a better series of specimens to render
this conclusion certain. Only the _Turritella Chilensis_ from Huafo and
Mocha, the _T. Patagonica_ and _Venus meridionalis_ from Navidad, come
very near to recent South American shells, namely, the two Turritellas
to _T. cingulata_, and the Venus to _V. exalbida_: some few other
species come rather less near; and some few resemble forms in the older
European tertiary deposits: none of the species resemble secondary
forms. Hence I conceive there can be no doubt that these formations are
tertiary,—a point necessary to consider, after the case of Concepcion.
The fifty-nine species belong to thirty-two genera; of these,
Gastridium is extinct, and three or four of the genera (viz. Panopæa,
Rostellaria, Corbis (?), and I believe Solecurtus) are not now found on
the west coast of South America. Fifteen of the genera have on this
coast living representatives in about the same latitudes with the
fossil species; but twelve genera now range very differently to what
they formerly did. The idea of the table on the following page, in
which the difference between the extension in latitude of the fossil
and existing species is shown, is taken from M. d’Orbigny’s work; but
the range of the living shells is given on the authority of Mr. Cuming,
whose long-continued researches on the conchology of South America are
well-known.

When we consider that very few, if any, of the fifty-nine fossil shells
are identical with, or make any close approach to, living species; when
we consider that some of the genera do not now exist on the west coast
of South America, and that no less than twelve genera out of the
thirty-two formerly ranged very differently from the existing species
of the
same genera, we must admit that these deposits are of considerable
antiquity, and that they probably verge on the commencement of the
tertiary era. May we not venture to believe, that they are of nearly
contemporaneous origin with the Eocene formations of the northern
hemisphere?

Genera, with living and tertiary species on the west coast of S.
America.[12]	Latitudes, in which found fossil on the coasts of Chile
and Peru.	Southernmost latitude, in which found living on the west
coast of
S. America. Bulla	30° to 43° 30′	12° near Lima.
Cassis	34°	1° 37′ Pyrula	34° (and 36° 30′ at Concepcion)	5°
Payta Fusus	30° to 43° 30′	23° Mexillones; reappears at the St.
of Magellan Pleurotoma	34° to 43° 30′	2° 18′ St. Elena
Terebra	34°	5° Payta Sigaretus	34° to 44° 30′	12° Lima
Anomia	30°	7° 48′ Perna	30°	1° 23′ Xixappa Cardium	30°
to 34° (and 36° 30′ at Concepcion)	5° Payta Artemis	30°	5°
Payta Voluta	34° to 44° 30′	Mr. Cuming does not know of any
species living on the west coast, between the equator and lat. 43°
south; from this latitude a species is found as far south as Tierra del
Fuego.

 [12] M. d’Orbigny states that the genus Natica is not found on the
 coast of Chile; but Mr. Cuming found it at Valparaiso. Scalaria was
 found at Valparaiso; Arca, at Iquique, in lat. 20°, by Mr. Cuming;
 Arca, also, was found by Captain King, at Juan Fernandez, in lat. 33°
 30′.

Comparing the fossil remains from the coast of Chile (leaving out, as
before, Concepcion and Chiloe) with those from Patagonia, we may
conclude, from their generic resemblance, and from the small number of
the species which from either coast approach closely to living forms,
that the formations of both belong to nearly the same epoch; and this
is the opinion of M. D’Orbigny. Had not a single fossil shell been
common to the two coasts, it could not have been argued that the
formations belonged to different ages; for Messrs. Cuming and Hinds
have found, on the comparison of nearly two thousand living species
from the opposite sides of South America, only one in common, namely,
the _Purpura lapillus_ from both sides of the Isthmus of Panama: even
the shells collected by myself amongst the Chonos Islands and on the
coast of Patagonia, are dissimilar, and we must descend to the apex of
the
continent, to Tierra del Fuego, to find these two great conchological
provinces united into one. Hence it is remarkable that four or five of
the fossil shells from Navidad, namely, _ Voluta alta, Turritella
Patagonica, Trochus collaris, Venus meridionalis,_ perhaps (Natica
solida), and perhaps the large oyster from Coquimbo, are considered by
Mr. Sowerby as identical with species from Santa Cruz and P. Desire. M.
d’Orbigny, however, admits the perfect identity only of the Trochus.

_On the temperature of the Tertiary period._—As the number of the
fossil species and genera from the western and eastern coasts is
considerable, it will be interesting to consider the probable nature of
the climate under which they lived. We will first take the case of
Navidad, in lat. 34°, where thirty-one species were collected, and
which, as we shall presently see, must have inhabited shallow water,
and therefore will necessarily well exhibit the effects of temperature.
Referring to the table given in the previous page, we find that the
existing species of the genera Cassis, Pyrula, Pleurotoma, Terebra, and
Sigaretus, which are generally (though by no means invariably)
characteristic of warmer latitudes, do not at the present day range
nearly so far south on this line of coast as the fossil species
formerly did. Including Coquimbo, we have Perna in the same
predicament. The first impression from this fact is, that the climate
must formerly have been warmer than it now is; but we must be very
cautious in admitting this, for Cardium, Bulla, and Fusus (and, if we
include Coquimbo, Anomia and Artemis) likewise formerly ranged farther
south than they now do; and as these genera are far from being
characteristic of hot climates, their former greater southern range may
well have been owing to causes quite distinct from climate: Voluta,
again, though generally so tropical a genus, is at present confined on
the west coast to colder or more southern latitudes than it was during
the tertiary period. The _Trochus collaris_, moreover, and, as we have
just seen according to Mr. Sowerby, two or three other species,
formerly ranged from Navidad as far south as Santa Cruz in latitude 50
degrees. If, instead of comparing the fossils of Navidad, as we have
hitherto done, with the shells now living on the west coast of South
America, we compare them with those found in other parts of the world,
under nearly similar latitudes; for instance, in the southern parts of
the Mediterranean or of Australia, there is no evidence that the sea
off Navidad was formerly hotter than what might have been expected from
its latitude, even if it was somewhat warmer than it now is when cooled
by the great southern polar current. Several of the most tropical
genera have no representative fossils at Navidad; and there are only
single species of Cassis, Pyrula, and Sigaretus, two of Pleurotoma and
two of Terebra, but none of these species are of conspicuous size. In
Patagonia, there is even still less evidence in the character of the
fossils, of the climate having been formerly warmer.[13]
As from the various reasons already assigned, there can be little doubt
that the formations of Patagonia and at least of Navidad and Coquimbo
in Chile, are the equivalents of an ancient stage in the tertiary
formations of the northern hemisphere, the conclusion that the climate
of the southern seas at this period was not hotter than what might have
been expected from the latitude of each place, appears to me highly
important; for we must believe, in accordance with the views of Mr.
Lyell, that the causes which gave to the older tertiary productions of
the quite temperate zones of Europe a tropical character, _were of a
local character and did not affect the entire globe._ On the other
hand, I have endeavoured to show, in the “Geological Transactions,”
that, at a much later period, Europe and North and South America were
nearly contemporaneously subjected to ice-action, and consequently to a
colder, or at least more equable, climate than that now characteristic
of the same latitudes.

 [13] It may be worth while to mention that the shells living at the
 present day on this eastern side of South America, in lat. 40°, have
 perhaps a more tropical character than those in corresponding
 latitudes on the shores of Europe: for at Bahia Blanca and S. Blas,
 there are two fine species of Voluta and four of Oliva.

_On the absence of extensive modern conchiferous deposits in South
America; and on the contemporaneousness of the older Tertiary deposits
at distant points being due to contemporaneous movements of
subsidence._—Knowing from the researches of Professor E. Forbes, that
molluscous animals chiefly abound within a depth of 100 fathoms and
under, and bearing in mind how many thousand miles of both coasts of
South America have been upraised within the recent period by a slow,
long-continued, intermittent movement,—seeing the diversity in nature
of the shores and the number of shells now living on them,—seeing also
that the sea off Patagonia and off many parts of Chile, was during the
tertiary period highly favourable to the accumulation of sediment,—the
absence of extensive deposits including recent shells over these vast
spaces of coast is highly remarkable. The conchiferous calcareous beds
at Coquimbo, and at a few isolated points northward, offer the most
marked exception to this statement; for these beds are from twenty to
thirty feet in thickness, and they stretch for some miles along shore,
attaining, however, only a very trifling breadth. At Valdivia there is
some sandstone with imperfect casts of shells, which _possibly_ may
belong to the recent period: parts of the boulder formation and the
shingle-beds on the lower plains of Patagonia probably belong to this
same period, but neither are fossiliferous: it also so happens that the
great Pampean formation does not include, with the exception of the
Azara, any mollusca. There cannot be the smallest doubt that the
upraised shells along the shores of the Atlantic and Pacific, whether
lying on the bare surface, or embedded in mould or in sand-hillocks,
will in the course of ages be destroyed by alluvial action: this
probably will be the case even with the calcareous beds of Coquimbo, so
liable to dissolution by rain-water. If we take into consideration the
probability of oscillations of level and the consequent action of the
tidal-waves at different heights, their destruction will appear almost
certain. Looking to an epoch as far distant in futurity as we now are
from the past Miocene period, there seems to me scarcely a chance,
under existing conditions, of the numerous shells now living in those
zones of depths most fertile in life, and found exclusively on the
western and south-eastern coasts of S. America, being preserved to this
imaginary distant epoch. A whole conchological series will in time be
swept away, with no memorials of their existence preserved in the
earth’s crust.

Can any light be thrown on this remarkable absence of recent
conchiferous deposits on these coasts, on which, at an ancient tertiary
epoch, strata abounding with organic remains were extensively
accumulated? I think there can, namely, by considering the conditions
necessary for the preservation of a formation to a distant age. Looking
to the enormous amount of denudation which on all sides of us has been
effected,—as evidenced by the lofty cliffs cutting off on so many
coasts horizontal and once far-extended strata of no great antiquity
(as in the case of Patagonia),—as evidenced by the level surface of the
ground on both sides of great faults and dislocations,—by inland lines
of escarpments, by outliers, and numberless other facts, and by that
argument of high generality advanced by Mr. Lyell, namely, that every
_sedimentary_ formation, whatever its thickness may be, and over
however many hundred square miles it may extend, is the result and the
measure of an equal amount of wear and tear of pre-existing formations;
considering these facts, we must conclude that, as an ordinary rule, a
formation to resist such vast destroying powers, and to last to a
distant epoch, must be of wide extent, and either in itself, or
together with superincumbent strata, be of great thickness. In this
discussion, we are considering only formations containing the remains
of marine animals, which, as before mentioned, live, with some
exceptions within (most of them much within) depths of 100 fathoms.
How, then, can a thick and widely extended formation be accumulated,
which shall include such organic remains? First, let us take the case
of the bed of the sea long remaining at a stationary level: under these
circumstances it is evident that _conchiferous_ strata can accumulate
only to the same thickness with the depth at which the shells can live;
on gently inclined coasts alone can they accumulate to any considerable
width; and from the want of superincumbent pressure, it is probable
that the sedimentary matter will seldom be much consolidated: such
formations have no very good chance, when in the course of time they
are upraised, of long resisting the powers of denudation. The chance
will be less if the submarine surface, instead of having remained
stationary, shall have gone on slowly rising during the deposition of
the strata, for in this case their total thickness must be less, and
each part, before being consolidated or thickly covered up by
superincumbent matter, will have had successively to pass through the
ordeal of the beach; and on most coasts, the waves on the beach tend to
wear down and disperse every object exposed to their action. Now, both
on the south-eastern and western shores of S. America, we have had
clear proofs that the land has been slowly rising, and in the long
lines of lofty cliffs, we have seen that the tendency of the sea is
almost everywhere to eat into the land. Considering these facts, it
ceases, I think, to be surprising, that extensive recent conchiferous
deposits are entirely absent on the southern and western shores of
America.


Let us take the one remaining case, of the bed of the sea slowly
subsiding during a length of time, whilst sediment has gone on being
deposited. It is evident that strata might thus accumulate to any
thickness, each stratum being deposited in shallow water, and
consequently abounding with those shells which cannot live at great
depths: the pressure, also, I may observe, of each fresh bed would aid
in consolidating all the lower ones. Even on a rather steep coast,
though such must ever be unfavourable to widely extended deposits, the
formations would always tend to increase in breadth from the water
encroaching on the land. Hence we may admit that periods of slow
subsidence will commonly be most favourable to the accumulation of
_conchiferous_ deposits, of sufficient thickness, extension, and
hardness, to resist the average powers of denudation.

We have seen that at an ancient tertiary epoch, fossiliferous deposits
were extensively deposited on the coasts of S. America; and it is a
very interesting fact, that there is evidence that these ancient
tertiary beds were deposited during a period of subsidence. Thus, at
Navidad, the strata are about eight hundred feet in thickness, and the
fossil shells are abundant both at the level of the sea and some way up
the cliffs; having sent a list of these fossils to Professor E. Forbes,
he thinks they must have lived in water between one and ten fathoms in
depth: hence the bottom of the sea on which these shells once lived
must have subsided at least 700 feet to allow of the superincumbent
matter being deposited. I must here remark, that, as all these and the
following fossil shells are extinct species, Professor Forbes
necessarily judges of the depths at which they lived only from their
generic character, and from the analogical distribution of shells in
the northern hemisphere; but there is no just cause from this to doubt
the general results. At Huafo the strata are about the same thickness,
namely, 800 feet, and Professor Forbes thinks the fossils found there
cannot have lived at a greater depth than fifty fathoms, or 300 feet.
These two points, namely, Navidad and Huafo, are 570 miles apart, but
nearly halfway between them lies Mocha, an island 1,200 feet in height,
apparently formed of tertiary strata up to its level summit, and with
many shells, including the same Turritella with that found at Huafo,
embedded close to the level of the sea. In Patagonia, shells are
numerous at Santa Cruz, at the foot of the 350 feet plain, which has
certainly been formed by the denudation of the 840 feet plain, and
therefore was originally covered by strata that number of feet in
thickness, and these shells, according to Professor Forbes, probably
lived at a depth of between seven and fifteen fathoms: at Port S.
Julian, sixty miles to the north, shells are numerous at the foot of
the ninety feet plain (formed by the denudation of the 950 feet plain),
and likewise occasionally at the height of several hundred feet in the
upper strata; these shells must have lived in water somewhere between
five and fifty fathoms in depth. Although in other parts of Patagonia I
have no direct evidence of shoal-water shells having been buried under
a great thickness of superincumbent submarine strata, yet it should be
borne in mind that the lower fossiliferous strata with several of the
same species of Mollusca, the
upper tufaceous beds, and the high summit-plain, stretch for a
considerable distance southward, and for hundreds of miles northward;
seeing this uniformity of structure, I conceive it may be fairly
concluded that the subsidence by which the shells at Santa Cruz and S.
Julian were carried down and covered up, was not confined to these two
points, but was co-extensive with a considerable portion of the
Patagonian tertiary formation. In a succeeding chapter it will be seen,
that we are led to a similar conclusion with respect to the secondary
fossiliferous strata of the Cordillera, namely, that they also were
deposited during a long-continued and great period of subsidence.

From the foregoing reasoning, and from the facts just given, I think we
must admit the probability of the following proposition: namely, that
when the bed of the sea is either stationary or rising, circumstances
are far less favourable, than when the level is sinking, to the
accumulation of _conchiferous_ deposits of sufficient thickness and
extension to resist, when upheaved, the average vast amount of
denudation. This result appears to me, in several respects, very
interesting: every one is at first inclined to believe that at
innumerable points, wherever there is a supply of sediment,
fossiliferous strata are now forming, which at some future distant
epoch will be upheaved and preserved; but on the views above given, we
must conclude that this is far from being the case; on the contrary, we
require (1st), a long-continued supply of sediment; (2nd), an extensive
shallow area; and (3rd), that this area shall slowly subside to a great
depth, so as to admit the accumulation of a widely extended thick mass
of superincumbent strata. In how few parts of the world, probably, do
these conditions at the present day concur! We can thus, also,
understand the general want of that close sequence in fossiliferous
formations which we might theoretically have anticipated; for, without
we suppose a subsiding movement to go on at the same spot during an
enormous period, from one geological era to another, and during the
whole of this period sediment to accumulate at the proper rate, so that
the depth should not become too great for the continued existence of
molluscous animals, it is scarcely possible that there should be a
perfect sequence at the same spot in the fossil shells of the two
geological formations.[14] So far from a very long-continued subsidence
being probable, many facts lead to the belief that the earth’s surface
oscillates up and down; and we have seen that during the elevatory
movements there is but a small chance of _durable_ fossiliferous
deposits accumulating.

 [14] Professor H. D. Rogers, in his excellent address to the
 Association of American Geologists (_Silliman’s Journal,_ vol. xlvii,
 p. 277) makes the following remark: “I question if we are at all aware
 how _completely_ the whole history of all departed time lies indelibly
 recorded with the amplest minuteness of detail in the successive
 sediments of the globe, how effectually, in other words, every period
 of time _has written its own history_, carefully preserving every
 created form and every trace of action.” I think the correctness of
 such remarks is more than doubtful, even if we except (as I suppose he
 would) all those numerous organic forms which contain no hard parts.)


Lastly, these same considerations appear to throw some light on the
fact that certain periods appear to have been favourable to the
deposition, or at least to the preservation, of contemporaneous
formations at very distant points. We have seen that in S. America an
enormous area has been rising within the recent period; and in other
quarters of the globe immense spaces appear to have risen
contemporaneously. From my examination of the coral-reefs of the great
oceans, I have been led to conclude that the bed of the sea has gone on
slowly sinking within the present era, over truly vast areas: this,
indeed, is in itself probable, from the simple fact of the rising areas
having been so large. In South America we have distinct evidence that
at nearly the same tertiary period, the bed of the sea off parts of the
coast of Chile and off Patagonia was sinking, though these regions are
very remote from each other. If, then, it holds good, as a general
rule, that in the same quarter of the globe the earth’s crust tends to
sink and rise contemporaneously over vast spaces, we can at once see,
that we have at distant points, at the same period, those very
conditions which appear to be requisite for the accumulation of
fossiliferous masses of sufficient extension, thickness, and hardness,
to resist denudation, and consequently to last unto an epoch distant in
futurity.[15]

 [15] Professor Forbes has some admirable remarks on this subject, in
 his “Report on the Shells of the Ægean Sea.” In a letter to Mr.
 Maclaren (_Edinburgh New Phil. Journal,_ January 1843), I partially
 entered into this discussion, and endeavoured to show that it was
 highly improbable, that upraised atolls or barrier-reefs, though of
 great thickness, should, owing to their small extension or breadth, be
 preserved to a distant future period.




Chapter VI PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.


Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, Palæozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar origin.
—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.

The metamorphic and plutonic formations of the several districts
visited by the _Beagle_ will be here chiefly treated of, but only such
cases as appear to me new, or of some special interest, will be
described in detail; at the end of the chapter I will sum up all the
facts on cleavage and foliation,—to which I particularly attended.

_Bahia, Brazil: lat. 13° south._—The prevailing rock is gneiss, often
passing, by the disappearance of the quartz and mica, and by the
feldspar losing its red colour, into a brilliantly grey primitive
greenstone. Not unfrequently quartz and hornblende are arranged in
layers in almost amorphous feldspar. There is some fine-grained
syenitic granite, orbicularly marked by ferruginous lines, and
weathering into vertical, cylindrical holes, almost touching each
other. In the gneiss, concretions of granular feldspar and others of
garnets with mica occur. The gneiss is traversed by numerous dikes
composed of black, finely crystallised, hornblendic rock, containing a
little glassy feldspar and sometimes mica, and varying in thickness
from mere threads to ten feet: these threads, which are often
curvilinear, could sometimes be traced running into the larger dikes.
One of these dikes was remarkable from having been in two or three
places laterally disjointed, with unbroken gneiss interposed between
the broken ends, and in one part with a portion of the gneiss driven,
apparently whilst in a softened state, into its side or wall. In
several neighbouring places, the gneiss included angular, well-defined,
sometimes bent, masses of hornblende rock, quite like, except in being
more perfectly crystallised, that forming the dikes, and, at least in
one instance, containing (as determined by Professor Miller) augite as
well as hornblende. In one or two cases these angular masses, though
now quite separate from each other by the solid gneiss, had, from their
exact correspondence in size and shape, evidently once been united;
hence I cannot doubt that most or all of the fragments have been
derived from the breaking up of the dikes, of which we see the first
stage in the above-mentioned laterally disjointed one. The gneiss close
to the fragments generally contained many large crystals of hornblende,
which are entirely absent or rare in other parts: its folia or laminæ
were gently bent round the fragments, in the same manner as they
sometimes are round concretions. Hence the gneiss has certainly been
softened, its composition modified, and its folia arranged,
subsequently to the breaking up of the dikes,[1] these latter also
having been at the same time bent and softened.

 [1] Professor Hitchcock (“Geology of Massachusetts,” vol. ii, p. 673,
 gives a closely similar case of a greenstone dike in syenite.

I must here take the opportunity of premising, that by the term
_cleavage_ I imply those planes of division which render a rock,
appearing to the eye quite or nearly homogeneous, fissile. By the term
_foliation_, I refer to the layers or plates of different mineralogical
nature of which most metamorphic schists are composed; there are, also,
often included in such masses, alternating, homogeneous, fissile layers
or folia, and in this case the rock is both foliated and has a
cleavage. By _ stratification_, as applied to these formations, I mean
those alternate, parallel, large masses of different composition, which
are themselves frequently either foliated or fissile,—such as the
alternating so-called strata of mica-slate, gneiss, glossy clay-slate,
and marble.

The folia of the gneiss within a few miles round Bahia generally
strike irregularly, and are often curvilinear, dipping in all
directions at various angles: but where best defined, they extended
most frequently in a N.E. by N. (or East 50° N.) and S.W. by S. line,
corresponding nearly with the coast-line northwards of the bay. I may
add that Mr. Gardner[2] found in several parts of the province of
Ceara, which lies between four and five hundred miles north of Bahia,
gneiss with the folia extending E. 45° N.; and in Guyana according to
Sir R. Schomburgk, the same rock strikes E. 57° N. Again, Humboldt
describes the gneiss-granite over an immense area in Venezuela and even
in Colombia, as striking E. 50° N., and dipping to the N.W. at an angle
of fifty degrees. Hence all the observations hitherto made tend to show
that the gneissic rocks over the whole of this part of the continent
have their folia extending generally within almost a point of the
compass of the same direction.[3]

 [2] “Geological Section of the Brit. Assoc.,” 1840. For Sir R.
 Schomburgk’s observations see _Geograph. Journal,_ 1842, p. 190. See
 also Humboldt’s discussion on Loxodrism in the “Personal Narrative.”


 [3] I landed at only one place north of Bahia, namely, at Pernambuco.
 I found there only soft, horizontally stratified matter, formed from
 disintegrated granitic rocks, and some yellowish impure limestone,
 probably of a tertiary epoch. I have described a most singular natural
 bar of hard sandstone, which protects the harbour, in the 19th vol.
 (1841) p. 258 of the _ London and Edin. Phil. Magazine._
    ABROLHOS ISLETS, _lat. 18° S. off the coast of Brazil._—Although
    not strictly in place, I do not know where I can more conveniently
    describe this little group of small islands. The lowest bed is a
    sandstone with ferruginous veins; it weathers into an extraordinary
    honeycombed mass; above it there is a dark-coloured argillaceous
    shale; above this a coarser sandstone—making a total thickness of
    about sixty feet; and lastly, above these sedimentary beds, there
    is a fine conformable mass of greenstone, in some parts having a
    columnar structure. All the strata, as well as the surface of the
    land, dip at an angle of about 12° to N. by W. Some of the islets
    are composed entirely of the sedimentary, others of the trappean
    rocks, generally, however, with the sandstone, cropping out on the
    southern shores.

_Rio de Janeiro._—This whole district is almost exclusively formed of
gneiss, abounding with garnets, and porphyritic with large crystals,
even three and four inches in length, of orthoclase feldspar: in these
crystals mica and garnets are often enclosed. At the western base of
the Corcovado, there is some ferruginous carious quartz-rock; and in
the Tijeuka range, much fine-grained granite. I observed boulders of
greenstone in several places; and on the islet of Villegagnon, and
likewise on the coast some miles northward, two large trappean dikes.
The porphyritic gneiss, or gneiss-granite as it has been called by
Humboldt, is only so far foliated that the constituent minerals are
arranged with a certain degree of regularity, and may be said to have a
“_grain_,” but they are not separated into distinct folia or laminæ.
There are, however, several other varieties of gneiss regularly
foliated, and alternating with each other in so-called strata. The
stratification and foliation of the ordinary gneisses, and the
foliation or “grain” of the gneiss-granite, are parallel to each other,
and generally strike within
a point of N.E. and S.W. dipping at a high angle (between 50° and 60°)
generally to S.E.: so that here again we meet with the strike so
prevalent over the more northern parts of this continent. The mountains
of gneiss-granite are to a remarkable degree abruptly conical, which
seems caused by the rock tending to exfoliate in thick, conically
concentric layers: these peaks resemble in shape those of phonolite and
other injected rocks on volcanic islands; nor is the grain or foliation
(as we shall afterwards see) any difficulty on the idea of the
gneiss-granite having been an intrusive rather than a metamorphic
formation. The lines of mountains, but not always each separate hill,
range nearly in the same direction with the foliation and so-called
stratification, but rather more easterly.

No. 22
Fragment of gneiss embedded in another variety of the same rock.


[Illustration: Fragment of gneiss embedded in another variety of the
same rock.]

On a bare gently inclined surface of the porphyritic gneiss in Botofogo
Bay, I observed the appearance here represented.

A fragment seven yards long and two in width, with angular and
distinctly defined edges, composed of a peculiar variety of gneiss with
dark layers of mica and garnets, is surrounded on all sides by the
ordinary gneiss-granite; both having been dislocated by a granitic
vein. The folia in the fragment and in the surrounding rock strike in
the same N.N.E. and S.S.W. line; but in the fragment they are vertical,
whereas in the gneiss-granite they dip at a small angle, as shown by
the arrows, to S.S.E. This fragment, considering its great size, its
solitary position, and its foliated structure parallel to that of the
surrounding rock, is, as far as I know, a unique case: and I will not
attempt any explanation of its origin.


The numerous travellers[4] in this country, have all been greatly
surprised at the depth to which the gneiss and other granitic rocks, as
well as the talcose slates of the interior, have been decomposed. Near
Rio, every mineral except the quartz has been completely softened, in
some places to a depth little less than one hundred feet.[5] The
minerals retain their positions in folia ranging in the usual
direction; and fractured quartz veins may be traced from the solid
rock, running for some distance into the softened, mottled, highly
coloured, argillaceous mass. It is said that these decomposed rocks
abound with gems of various kinds, often in a fractured state, owing,
as some have supposed, to the collapse of geodes, and that they contain
gold and diamonds. At Rio, it appeared to me that the gneiss had been
softened before the excavation (no doubt by the sea) of the existing,
broad, flat-bottomed valleys; for the depth of decomposition did not
appear at all conformable with the present undulations of the surface.
The porphyritic gneiss, where now exposed to the air, seems to
withstand decomposition remarkably well; and I could see no signs of
any tendency to the production of argillaceous masses like those here
described. I was also struck with the fact, that where a bare surface
of this rock sloped into one of the quiet bays, there were no marks of
erosion at the level of the water, and the parts both beneath and above
it preserved a uniform curve. At Bahia, the gneiss rocks are similarly
decomposed, with the upper parts insensibly losing their foliation, and
passing, without any distinct line of separation, into a bright red
argillaceous earth, including partially rounded fragments of quartz and
granite. From this circumstance, and from the rocks appearing to have
suffered decomposition before the excavation of the valleys, I suspect
that here, as at Rio, the decomposition took place under the sea. The
subject appeared to me a curious one, and would probably well repay
careful examination by an able mineralogist.

 [4] Spix and Martius have collected in an Appendix to their “Travels,”
 the largest body of facts on this subject. See also some remarks by M.
 Lund in his communications to the Academy at Copenhagen; and others by
 M. Gaudichaud in Freycinet’s “Voyage.”


 [5] Dr. Benza describes granitic rock (_Madras Journal of Lit.,_ etc.,
 Oct. 183? p. 246), in the Neelgherries, decomposed to a depth of forty
 feet.


_The Northern Provinces of La Plata._—According to some observations
communicated to me by Mr. Fox, the coast from Rio de Janeiro to the
mouth of the Plata seems everywhere to be granitic, with a few trappean
dikes. At Port Alegre, near the boundary of Brazil, there are
porphyries and diorites.[6] At the mouth of the Plata, I examined the
country for twenty-five miles west, and for about seventy miles north
of Maldonado: near this town, there is some common gneiss, and much, in
all parts of the country, of a coarse-grained mixture of quartz and
reddish feldspar, often, however, assuming a little dark-green
imperfect hornblende, and then immediately becoming foliated. The
abrupt hillocks thus composed, as well as the highly inclined folia of
the
common varieties of gneiss, strike N.N.E. or a little more easterly,
and S.S.W. Clay-slate is occasionally met with, and near the L. del
Potrero, there is white marble, rendered fissile from the presence of
hornblende, mica, and asbestus; the cleavage of these rocks and their
stratification, that is the alternating masses thus composed, strike
N.N.E. and S.S.W. like the foliated gneisses, and have an almost
vertical dip. The Sierra Larga, a low range five miles west of
Maldonado, consists of quartzite, often ferruginous, having an
arenaceous feel, and divided into excessively thin, almost vertical
laminæ or folia by microscopically minute scales, apparently of mica,
and striking in the usual N.N.E. and S.S.W. direction. The range itself
is formed of one principal line with some subordinate ones; and it
extends with remarkable uniformity far northward (it is said even to
the confines of Brazil), in the same line with the vertically ribboned
quartz rock of which it is composed. The S. de Las Animas is the
highest range in the country; I estimated it at 1,000 feet; it runs
north and south, and is formed of feldspathic porphyry; near its base
there is a N.N.W. and S.S.E. ridge of a conglomerate in a highly
porphyritic basis.

 [6] M. Isabelle, “Voyage à Buenos Ayres,” p. 479.

Northward of Maldonado, and south of Las Minas, there is an E. and W.
hilly band of country, some miles in width, formed of siliceous
clay-slate, with some quartz, rock, and limestone, having a tortuous
irregular cleavage, generally ranging east and west. E. and S.E. of Las
Minas there is a confused district of imperfect gneiss and laminated
quartz, with the hills ranging in various directions, but with each
separate hill generally running in the same line with the folia of the
rocks of which it is composed: this confusion appears to have been
caused by the intersection of the [E. and W.] and [N.N.E. and S.S.W.]
strikes. Northward of Las Minas, the more regular northerly ranges
predominate: from this place to near Polanco, we meet with the
coarse-grained mixture of quartz and feldspar, often with the imperfect
hornblende, and then becoming foliated in a N. and S. line—with
imperfect clay-slate, including laminæ of red crystallised
feldspar—with white or black marble, sometimes containing asbestus and
crystals of gypsum—with quartz-rock—with syenite—and lastly, with much
granite. The marble and granite alternate repeatedly in apparently
vertical masses: some miles northward of the Polanco, a wide district
is said to be entirely composed of marble. It is remarkable, how rare
mica is in the whole range of country north and westward of Maldonado.
Throughout this district, the cleavage of the clay-slate and marble—the
foliation of the gneiss and the quartz—the stratification or
alternating masses of these several rocks—and the range of the hills,
all coincide in direction; and although the country is only hilly, the
planes of division are almost everywhere very highly inclined or
vertical.

Some ancient submarine volcanic rocks are worth mentioning, from their
rarity on this eastern side of the continent. In the valley of the
Tapas (fifty or sixty miles N. of Maldonado) there is a tract three or
four miles in length, composed of various trappean rocks with glassy
feldspar—of apparently metamorphosed grit-stones—of purplish
amygdaloids with large kernels of carbonate of lime[7]—and much of a
harshish rock with glassy feldspar intermediate in character between
claystone porphyry and trachyte. This latter rock was in one spot
remarkable from being full of drusy cavities, lined with quartz
crystals, and arranged in planes, dipping at an angle of 50° to the
east, and striking parallel to the foliation of an adjoining hill
composed of the common mixture of quartz, feldspar, and imperfect
hornblende: this fact perhaps indicates that these volcanic rocks have
been metamorphosed, and their constituent parts rearranged, at the same
time and according to the same laws, with the granitic and metamorphic
formations of this whole region. In the valley of the Marmaraya, a few
miles south of the Tapas, a band of trappean and amygdaloidal rock is
interposed between a hill of granite and an extensive surrounding
formation of red conglomerate, which (like that at the foot of the S.
Animas) has its basis porphyritic with crystals of feldspar, and which
hence has certainly suffered metamorphosis.

 [7] Near the Pan de Azucar there is some greenish porphyry, in one
 place amygdaloidal with agate.


_Monte Video._—The rocks here consist of several varieties of gneiss,
with the feldspar often yellowish, granular and imperfectly
crystallised, alternating with, and passing insensibly into, beds, from
a few yards to nearly a mile in thickness, of fine or coarse grained,
dark-green hornblendic slate; this again often passing into chloritic
schist. These passages seem chiefly due to changes in the mica, and its
replacement by other minerals. At Rat Island I examined a mass of
chloritic schist, only a few yards square, irregularly surrounded on
all sides by the gneiss, and intricately penetrated by many curvilinear
veins of quartz, which gradually _blend_ into the gneiss: the cleavage
of the chloritic schist and the foliation of the gneiss were exactly
parallel. Eastward of the city there is much fine-grained,
dark-coloured gneiss, almost assuming the character of
hornblende-slate, which alternates in thin laminæ with laminæ of
quartz, the whole mass being transversely intersected by numerous large
veins of quartz: I particularly observed that these veins were
absolutely continuous with the alternating laminæ of quartz. In this
case and at Rat Island, the passage of the gneiss into imperfect
hornblendic or into chloritic slate, seemed to be connected with the
segregation of the veins of quartz.[8]

 [8] Mr. Greenough (p. 78, “Critical Examination,” etc.) observes that
 quartz in mica-slate sometimes appears in beds and sometimes in veins.
 Von Buch also in his “Travels in Norway” (p. 236), remarks on
 alternating laminæ of quartz and hornblende-slate replacing
 mica-schist.

The Mount, a hill believed to be 450 feet in height, from which the
place takes its name, is much the highest land in this neighbourhood:
it consists of hornblendic slate, which (except on the eastern and
disturbed base) has an east and west nearly vertical cleavage; the
longer axis of the hill also ranges in this same line. Near the summit
the hornblende-slate gradually becomes more and more coarsely
crystallised, and less plainly laminated, until it passes into a heavy,
sonorous greenstone, with a slaty conchoidal fracture; the laminæ on
the north
and south sides near the summit dip inwards, as if this upper part had
expanded or bulged outwards. This greenstone must, I conceive, be
considered as metamorphosed hornblende-slate. The Cerrito, the next
highest, but much less elevated point, is almost similarly composed. In
the more western parts of the province, besides gneiss, there is
quartz-rock, syenite, and granite; and at Colla, I heard of marble.

Near M. Video, the space which I more accurately examined was about
fifteen miles in an east and west line, and here I found the foliation
of the gneiss and the cleavage of the slates generally well developed,
and extending parallel to the alternating strata composed of the
gneiss, hornblendic and chloritic schists. These planes of division all
range within one point of east and west, frequently east by south and
west by north; their dip is generally almost vertical, and scarcely
anywhere under 45°: this fact, considering how slightly undulatory the
surface of the country is, deserves attention. Westward of M. Video,
towards the Uruguay, wherever the gneiss is exposed, the highly
inclined folia are seen striking in the same direction; I must except
one spot where the strike was N.W. by W. The little Sierra de S. Juan,
formed of gneiss and laminated quartz, must also be excepted, for it
ranges between [N. to N.E.] and [S. to S.W.] and seems to belong to the
same system with the hills in the Maldonado district. Finally, we have
seen that, for many miles northward of Maldonado and for twenty-five
miles westward of it, as far as the S. de las Animas, the foliation,
cleavage, so-called stratification and lines of hills, all range N.N.E.
and S.S.W., which is nearly coincident with the adjoining coast of the
Atlantic. Westward of the S. de las Animas, as far as even the Uruguay,
the foliation, cleavage, and stratification (but not lines of hills,
for there are no defined ones) all range about E. by S. and W. by N.,
which is nearly coincident with the direction of the northern shore of
the Plata; in the confused country near Las Minas, where these two
great systems appear to intersect each other, the cleavage, foliation,
and stratification run in various directions, but generally coincide
with the line of each separate hill.

_Southern La Plata._—The first ridge, south of the Plata, which
projects through the Pampean formation, is the Sierra Tapalguen and
Vulcan, situated 200 miles southward of the district just described.
This ridge is only a few hundred feet in height, and runs from C.
Corrientes in a W.N.W. line for at least 150 miles into the interior:
at Tapalguen, it is composed of unstratified granular quartz,
remarkable from forming tabular masses and small plains, surrounded by
precipitous cliffs: other parts of the range are said to consist of
granite: and marble is found at the S. Tinta. It appears from M.
Parchappe’s[9] observations, that at Tandil there is a range of
quartzose gneiss, very like the rocks of the S. Larga near Maldonado,
running in the same N.N.E. and S.S.W. direction; so that the framework
of the country here is very similar to that on the northern shore of
the Plata.

 [9] M. d’Orbigny’s “Voyage,” Part. Géolog., p. 46. I have given a
 short account of the peculiar forms of the quartz hills of Tapalguen,
 so unusual in a metamorphic formation, in my “Journal of Researches”
 (2nd edit.), p. 116.


The Sierra Guitru-gueyu is situated sixty miles south of the S.
Tapalguen: it consists of numerous parallel, sometimes blended together
ridges, about twenty-three miles in width, and five hundred feet in
height above the plain, and extending in a N.W. and S.E. direction.
Skirting round the extreme S.E. termination, I ascended only a few
points, which were composed of a fine-grained gneiss, almost composed
of feldspar with a little mica, and passing in the upper parts of the
hills into a rather compact purplish clay-slate. The cleavage was
nearly vertical, striking in a N.W. by W. and S.E. by E. line, nearly,
though not quite, coincident with the direction of the parallel ridges.

The Sierra Ventana lies close south of that of Guitru-gueyu; it is
remarkable from attaining a height, very unusual on this side of the
continent, of 3,340 feet. It consists up to its summit, of quartz,
generally pure and white, but sometimes reddish, and divided into thick
laminæ or strata: in one part there is a little glossy clay-slate with
a tortuous cleavage. The thick layers of quartz strike in a W. 30° N.
line, dipping southerly at an angle of 45° and upwards. The principal
line of mountains, with some quite subordinate parallel ridges, range
about W. 45° N.: but at their S.E. termination, only W. 25° N. This
Sierra is said to extend between twenty and thirty leagues into the
interior.

_Patagonia._—With the exception perhaps of the hill of S. Antonio (600
feet high) in the Gulf of S. Matias, which has never been visited by a
geologist, crystalline rocks are not met with on the coast of Patagonia
for a space of 380 miles south of the S. Ventana. At this point (lat.
43° 50′), at Points Union and Tombo, plutonic rocks are said to appear,
and are found, at rather wide intervals, beneath the Patagonian
tertiary formation for a space of about three hundred miles southward,
to near Bird Island, in latitude 48° 56′. Judging from specimens kindly
collected for me by Mr. Stokes, the prevailing rock at Ports St. Elena,
Camerones, Malaspina, and as far south as the Paps of Pineda, is a
purplish-pink or brownish claystone porphyry, sometimes laminated,
sometimes slightly vesicular, with crystals of opaque feldspar and with
a few grains of quartz; hence these porphyries resemble those
immediately to be described at Port Desire, and likewise a series which
I have seen from P. Alegre on the southern confines of Brazil. This
porphyritic formation further resembles in a singularly close manner
the lowest stratified formation of the Cordillera of Chile, which, as
we shall hereafter see, has a vast range, and attains a great
thickness. At the bottom of the Gulf of St. George, only tertiary
deposits appear to be present. At Cape Blanco, there is quartz rock,
very like that of the Falkland Islands, and some hard, blue siliceous
clay-slate.

At Port Desire there is an extensive formation of the claystone
porphyry, stretching at least twenty-five miles into the interior: it
has been denuded and deeply worn into gullies before being covered up
by the tertiary deposits, through which it here and there projects in
hills; those north of the bay being 440 feet in height. The strata have
in several places been tilted at small angles, generally either to
N.N.W. or S.S.E. By gradual passages and alternations, the porphyries
change incessantly in nature. I will describe only some of the
principal
mineralogical changes, which are highly instructive, and which I
carefully examined. The prevailing rock has a compact purplish base,
with crystals of earthy or opaque feldspar, and often with grains of
quartz. There are other varieties, with an almost truly trachytic base,
full of little angular vesicles and crystals of glassy feldspar; and
there are beds of black perfect pitchstone, as well as of a
concretionary imperfect variety. On a casual inspection, the whole
series would be thought to be of the same plutonic or volcanic nature
with the trachytic varieties and pitchstone; but this is far from being
the case, as much of the porphyry is certainly of metamorphic origin.
Besides the true porphyries, there are many beds of earthy, quite white
or yellowish, friable, easily fusible matter, resembling chalk, which
under the microscope is seen to consist of minute broken crystals, and
which, as remarked in a former chapter, singularly resembles the upper
tufaceous beds of the Patagonian tertiary formation. This earthy
substance often becomes coarser, and contains minute rounded fragments
of porphyries and rounded grains of quartz, and in one case so many of
the latter as to resemble a common sandstone. These beds are sometimes
marked with true lines of aqueous deposition, separating particles of
different degrees of coarseness; in other cases there are parallel
ferruginous lines not of true deposition, as shown by the arrangement
of the particles, though singularly resembling them. The more indurated
varieties often include many small and some larger angular cavities,
which appear due to the removal of earthy matter: some varieties
contain mica. All these earthy and generally white stones insensibly
pass into more indurated sonorous varieties, breaking with a conchoidal
fracture, yet of small specific gravity; many of these latter varieties
assume a pale purple tint, being singularly banded and veined with
different shades, and often become plainly porphyritic with crystals of
feldspar. The formation of these crystals could be most clearly traced
by minute angular and often partially hollow patches of earthy matter,
first assuming a _fibrous structure_, then passing into opaque
imperfectly shaped crystals, and lastly, into perfect glassy crystals.
When these crystals have appeared, and when the basis has become
compact, the rock in many places could not be distinguished from a true
claystone porphyry without a trace of mechanical structure.

In some parts, these earthy or tufaceous beds pass into jaspery and
into beautifully mottled and banded porcelain rocks, which break into
splinters, translucent at their edges, hard enough to scratch glass,
and fusible into white transparent beads: grains of quartz included in
the porcelainous varieties can be seen melting into the surrounding
paste. In other parts, the earthy or tufaceous beds either insensibly
pass into, or alternate with, breccias composed of large and small
fragments of various purplish porphyries, with the matrix generally
porphyritic: these breccias, though their subaqueous origin is in many
places shown both by the arrangement of their smaller particles and by
an oblique or current lamination, also pass into porphyries, in which
every trace of mechanical origin and stratification has been
obliterated.

Some highly porphyritic though coarse-grained masses, evidently
of sedimentary origin, and divided into thin layers, differing from
each other chiefly in the number of embedded grains of quartz,
interested me much from the peculiar manner in which here and there
some of the layers terminated in abrupt points, quite unlike those
produced by a layer of sediment naturally thinning out, and apparently
the result of a subsequent process of metamorphic aggregation. In
another common variety of a finer texture, the aggregating process had
gone further, for the whole mass consisted of quite short, parallel,
often slightly curved layers or patches, of whitish or reddish finely
granulo-crystalline feldspathic matter, generally terminating at both
ends in blunt points; these layers or patches further tended to pass
into wedge or almond-shaped little masses, and these finally into true
crystals of feldspar, with their centres often slightly drusy. The
series was so perfect that I could not doubt that these large crystals,
which had their longer axes placed parallel to each other, had
primarily originated in the metamorphosis and aggregation of
alternating layers of tuff; and hence their parallel position must be
attributed (unexpected though the conclusion may be), not to laws of
chemical action, but to the original planes of deposition. I am tempted
briefly to describe three other singular allied varieties of rock; the
first without examination would have passed for a stratified
porphyritic breccia, but all the included angular fragments consisted
of a border of pinkish crystalline feldspathic matter, surrounding a
dark translucent siliceous centre, in which grains of quartz not quite
blended into the paste could be distinguished: this uniformity in the
nature of the fragments shows that they are not of mechanical, but of
concretionary origin, having resulted perhaps from the self-breaking up
and aggregation of layers of indurated tuff containing numerous grains
of quartz,—into which, indeed, the whole mass in one part passed. The
second variety is a reddish non-porphyritic claystone, quite full of
spherical cavities, about half an inch in diameter, each lined with a
collapsed crust formed of crystals of quartz. The third variety also
consists of a pale purple non-porphyritic claystone, almost wholly
formed of concretionary balls, obscurely arranged in layers, of a less
compact and paler coloured claystone; each ball being on one side
partly hollow and lined with crystals of quartz.

_Pseudo-dikes._—Some miles up the harbour, in a line of cliffs formed
of slightly metamorphosed tufaceous and porphyritic claystone beds, I
observed three vertical dikes, so closely resembling in general
appearance ordinary volcanic dikes, that I did not doubt, until closely
examining their composition, that they had been injected from below.
The first is straight, with parallel sides, and about four feet wide;
it consists of whitish, indurated tufaceous matter, precisely like some
of the beds intersected by it. The second dike is more remarkable; it
is slightly tortuous, about eighteen inches thick, and can be traced
for a considerable distance along the beach; it is of a purplish-red or
brown colour, and is formed chiefly of _ rounded_ grains of quartz,
with broken crystals of earthy feldspar, scales of black mica, and
minute fragments of claystone porphyry, all firmly united together in a
hard sparing base. The structure of this dike shows obviously that it
is of mechanical and
sedimentary origin; yet it thinned out upwards, and did not cut through
the uppermost strata in the cliffs. This fact at first appears to
indicate that the matter could not have been washed in from above;[10]
but if we reflect on the suction which would result from a deep-seated
fissure being formed, we may admit that if the fissure were in any part
open to the surface, mud and water might well be drawn into it along
its whole course. The third dike consisted of a hard, rough, white
rock, almost composed of broken crystals of glassy feldspar, with
numerous scales of black mica, cemented in a scanty base; there was
little in the appearance of this rock, to preclude the idea of its
having been a true injected feldspathic dike. The matter composing
these three pseudo-dikes, especially the second one, appears to have
suffered, like the surrounding strata, a certain degree of metamorphic
action; and this has much aided the deceptive appearance. At Bahia, in
Brazil, we have seen that a true injected hornblendic dike, not only
has suffered metamorphosis, but has been dislocated and even diffused
in the surrounding gneiss, under the form of separate crystals and of
fragments.

 [10] Upfilled fissures are known to occur both in volcanic and in
 ordinary sedimentary formations. At the Galapagos Archipelago
 (“Volcanic Islands” etc.), there are some striking examples of
 pseudo-dikes composed of hard tuff.

_Falkland Islands._—I have described these islands in a paper published
in the third volume of the _Geological Journal._ The mountain-ridges
consist of quartz, and the lower country of clay-slate and sandstone,
the latter containing Palæozoic fossils. These fossils have been
separately described by Messrs. Morris and Sharpe: some of them
resemble Silurian, and others Devonian forms. In the eastern part of
the group the several parallel ridges of quartz extend in a west and
east line; but further westward the line becomes W.N.W. and E.S.E., and
even still more northerly. The cleavage-planes of the clay-slate are
highly inclined, generally at an angle of above 50°, and often
vertical; they strike almost invariably in the same direction with the
quartz ranges. The outline of the indented shores of the two main
islands, and the relative positions of the smaller islets, accord with
the strike both of the main axes of elevation and of the cleavage of
the clay-slate.

_Tierra del Fuego._—My notes on the geology of this country are
copious, but as they are unimportant, and as fossils were found only in
one district, a brief sketch will be here sufficient. The east coast
from the S. of Magellan (where the boulder formation is largely
developed) to St. Polycarp’s Bay is formed of horizontal tertiary
strata, bounded some way towards the interior by a broad mountainous
band of clay-slate. This great clay-slate formation extends from St. Le
Maire westward for 140 miles, along both sides of the Beagle Channel to
near its bifurcation. South of this channel, it forms all Navarin
Island, and the eastern half of Hoste Island and of Hardy Peninsula;
north of the Beagle Channel it extends in a north-west line on both
sides of Admiralty Sound to Brunswick Peninsula in the St. of Magellan,
and I have reason to believe, stretches far up the eastern
side of the Cordillera. The western and broken side of Tierra del Fuego
towards the Pacific is formed of metamorphic schists, granite and
various trappean rocks: the line of separation between the crystalline
and clay-slate formations can generally be distinguished, as remarked
by Captain King,[11] by the parallelism in the clay-slate districts of
the shores and channels, ranging in a line between [W. 20° to 40° N.]
and [E. 20° to 40° S.].

 [11] _Geographical Journal,_ vol. i, p. 155.

The clay-slate is generally fissile, sometimes siliceous or
ferruginous, with veins of quartz and calcareous spar; it often
assumes, especially on the loftier mountains, an altered feldspathic
character, passing into feldspathic porphyry: occasionally it is
associated with breccia and grauwacke. At Good Success Bay, there is a
little intercalated black crystalline limestone. At Port Famine much of
the clay-slate is calcareous, and passes either into a mudstone or into
grauwacke, including odd-shaped concretions of dark argillaceous
limestone. Here alone, on the shore a few miles north of Port Famine,
and on the summit of Mount Tarn (2,600 feet high), I found organic
remains; they consist of:—

Ancyloceras simplex, d’Orbigny, “Pal Franc,” Mount Tarn.

Fusus (in imperfect state), d’Orbigny, “Pal Franc,” Mount Tarn.

Natica, d’Orbigny, “Pal Franc,” Mount Tarn.

Pentacrimus, d’Orbigny, “Pal Franc,” Mount Tarn.

Lucina excentrica, G. B. Sowerby, Port Famine.

Venus (in imperfect state), G. B. Sowerby, Port Famine.

Turbinolia (?), G. B. Sowerby, Port Famine.

Hamites elatior, G. B. Sowerby, Port Famine.


M. d’Orbigny states[12] that MM. Hombron and Grange found in this
neighbourhood an Ancyloceras, perhaps _A. simplex_, an Ammonite, a
Plicatula and Modiola. M. d’Orbigny believes from the general character
of these fossils, and from the Ancyloceras being identical (as far as
its imperfect condition allows of comparison) with the _A. simplex_ of
Europe, that the formation belongs to an early stage of the Cretaceous
system. Professor E. Forbes, judging only from my specimens, concurs in
the probability of this conclusion. The _Hamites elatior_ of the above
list, of which a description has been given by Mr. Sowerby, and which
is remarkable from its large size, has not been seen either by M.
d’Orbigny or Professor E. Forbes, as, since my return to England, the
specimens have been lost. The great clay-slate formation of Tierra del
Fuego being cretaceous, is certainly a very interesting fact,—whether
we consider the appearance of the country, which, without the evidence
afforded by the fossils, would form the analogy of most known
districts, probably have been considered as belonging to the Palæozoic
series,—or whether we view it as showing that the age of this terminal
portion of the great axis of South America, is the same (as will
hereafter be seen) with the Cordillera of Chile and Peru.

 [12] “Voyage,” Part Géolog., p. 242.


The clay-slate in many parts of Tierra del Fuego, is broken by
dikes[13] and by great masses of greenstone, often highly hornblendic:
almost all the small islets within the clay-slate districts are thus
composed. The slate near the dikes generally becomes paler-coloured,
harder, less fissile, of a feldspathic nature, and passes into a
porphyry or greenstone: in one case, however, it became more fissile,
of a red colour, and contained minute scales of mica, which were absent
in the unaltered rock. On the east side of Ponsonby Sound some dikes
composed of a pale sonorous feldspathic rock, porphyritic with a little
feldspar, were remarkable from their number,—there being within the
space of a mile at least one hundred,—from their nearly equalling in
bulk the intermediate slate,—and more especially from the excessive
fineness (like the finest inlaid carpentry) and perfect parallelism of
their junctions with the almost vertical laminæ of clay-slate. I was
unable to persuade myself that these great parallel masses had been
injected, until I found one dike which abruptly thinned out to half its
thickness, and had one of its walls jagged, with fragments of the slate
embedded in it.

 [13] In a greenstone-dike in the Magdalen Channel, the feldspar
 cleaved with the angle of albite. This dike was crossed, as well as
 the surrounding slate, by a large vein of quartz, a circumstance of
 unusual occurrence.


In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary,
becomes much altered and feldspathic. Thus on Wollaston Island slate
and grauwacke can be distinctly traced passing into feldspathic rocks
and greenstones, including iron pyrites and epidote, but still
retaining traces of cleavage with the usual strike and dip. One such
metamorphosed mass was traversed by large vein-like masses of a
beautiful mixture (as ascertained by Professor Miller) of green
epidote, garnets, and white calcareous spar. On the northern point of
this same island, there were various ancient submarine volcanic rocks,
consisting of amygdaloids with dark bole and agate,—of basalt with
decomposed olivine—of compact lava with glassy feldspar,—and of a
coarse conglomerate of red scoriæ, parts being amygdaloidal with
carbonate of lime. The southern part of Wollaston Island and the whole
of Hermite and Horn Islands, seem formed of cones of greenstone; the
outlying islets of Il Defenso and D. Raminez are said[14] to consist of
porphyritic lava. In crossing Hardy Peninsula, the slate still
retaining traces of its usual cleavage, passes into columnar
feldspathic rocks, which are succeeded by an irregular tract of
trappean and basaltic rocks, containing glassy feldspar and much iron
pyrites: there is, also, some harsh red claystone porphyry, and an
almost true trachyte, with needles of hornblende, and in one spot a
curious slaty rock divided into quadrangular columns, having a base
almost like trachyte, with drusy cavities lined by crystals, too
imperfect, according to Professor Miller, to be measured, but
resembling Zeagonite.[15] In the midst of these singular rocks, no
doubt of ancient submarine volcanic origin, a high hill of feldspathic
clay-slate projected, retaining
its usual cleavage. Near this point, there was a small hillock, having
the aspect of granite, but formed of white albite, brilliant crystals
of hornblende (both ascertained by the reflecting goniometer) and mica;
but with no quartz. No recent volcanic district has been observed in
any part of Tierra del Fuego.

 [14] Determined by Professor Jameson. Weddell’s “Voyage,” p. 169.


 [15] See Mr. Brooke’s Paper in the _London Phil. Mag.,_ vol. x. This
 mineral occurs in an ancient volcanic rock near Rome.


Five miles west of the bifurcation of the Beagle Channel, the
slate-formation, instead of becoming, as in the more southern parts of
Tierra del Fuego, feldspathic, and associated with trappean or old
volcanic rocks, passes by alternations into a great underlying mass of
fine gneiss and glossy clay-slate, which at no great distance is
succeeded by a grand formation of mica-slate containing garnets. The
folia of these metamorphic schists strike parallel to the
cleavage-planes of the clay-slate, which have a very uniform direction
over the whole of this part of the country: the folia, however, are
undulatory and tortuous, whilst the cleavage-laminæ of the slate are
straight. These schists compose the chief mountain-chain of Southern
Tierra del Fuego, ranging along the north side of the northern arm of
the Beagle Channel, in a short W.N.W. and E.S.E. line, with two points
(Mounts Sarmiento and Darwin) rising to heights of 6,800 and 6,900
feet. On the south-western side of this northern arm of the Beagle
Channel, the clay-slate is seen with its _strata_ dipping from the
great chain, so that the metamorphic schists here form a ridge bordered
on each side by clay-slate. Further north, however, to the west of this
great range, there is no clay-slate, but only gneiss, mica, and
hornblendic slates, resting on great barren hills of true granite, and
forming a tract about sixty miles in width. Again, westward of these
rocks, the outermost islands are of trappean formation, which, from
information obtained during the voyages of the _Adventure_ and
_Beagle,_[16] seem, together with granite, chiefly to prevail along the
western coast as far north as the entrance of the St. of Magellan: a
little more inland, on the eastern side of Clarence Island and S.
Desolation, granite, greenstone, mica-slate, and gneiss appear to
predominate. I am tempted to believe, that where the clay-slate has
been metamorphosed at great depths beneath the surface, gneiss,
mica-slate, and other allied rocks have been formed, but where the
action has taken place nearer the surface, feldspathic porphyries,
greenstones, etc., have resulted, often accompanied by submarine
volcanic eruptions.

 [16] See the Paper by Captain King in the _ Geograph. Journal_; also a
 Letter to Dr. Fitton in “Geolog. Proc.,” vol. i, p. 29; also some
 observations by Captain Fitzroy, “Voyages,” vol. i, p. 375. I am
 indebted also to Mr. Lyell for a series of specimens collected by
 Lieutenant Graves.

Only one other rock, met with in both arms of the Beagle Channel,
deserves any notice, namely a granulo-crystalline mixture of white
albite, black hornblende (ascertained by measurement of the crystals,
and confirmed by Professor Miller), and more or less of brown mica, but
without any quartz. This rock occurs in large masses, closely
resembling in external form granite or syenite: in the southern arm of
the Channel, one such mass underlies the mica-slate, on which
clay-slate was superimposed: this peculiar plutonic rock which, as we
have
seen, occurs also in Hardy Peninsula, is interesting, from its perfect
similarity with that (hereafter often to be referred to under the name
of andesite) forming the great injected axes of the Cordillera of
Chile.

The stratification of the clay-slate is generally very obscure, whereas
the cleavage is remarkably well defined: to begin with the extreme
eastern parts of Tierra del Fuego; the cleavage-planes near the St. of
Le Maire strike either W. and E. or W.S.W. and E.N.E., and are highly
inclined; the form of the land, including Staten Island, indicates that
the axes of elevation have run in this same line, though I was unable
to distinguish the planes of stratification. Proceeding westward, I
accurately examined the cleavage of the clay-slate on the northern,
eastern, and western sides (thirty-five miles apart) of Navarin Island,
and everywhere found the laminæ ranging with extreme regularity, W.N.W.
and E.S.E., seldom varying more than one point of the compass from this
direction.[17] Both on the east and west coasts, I crossed at right
angles the cleavage-planes for a space of about eight miles, and found
them dipping at an angle of between 45° and 90°, generally to S.S.W.,
sometimes to N.N.E., and often quite vertically. The S.S.W. dip was
occasionally succeeded abruptly by a N.N.E. dip, and this by a vertical
cleavage, or again by the S.S.W. dip; as in a lofty cliff on the
eastern end of the island the laminæ of slate were seen to be folded
into very large steep curves, ranging in the usual W.N.W. line, I
suspect that the varying and opposite dips may possibly be accounted
for by the cleavage-laminæ, though to the eye appearing straight, being
parts of large abrupt curves, with their summits cut off and worn down.

 [17] The clay-slate in this island was in many places crossed by
 parallel smooth joints. Out of five cases, the angle of intersection
 between the strike of these joints and that of the cleavage-laminæ was
 in two cases 45° and in two others 79°.

In several places I was particularly struck with the fact, that the
fine laminæ of the clay-slate, where cutting straight through the bands
of stratification, and therefore indisputably true cleavage-planes,
differed slightly in their greyish and greenish tints of colour, in
compactness, and in some of the laminæ having a rather more jaspery
appearance than others. I have not seen this fact recorded, and it
appears to me important, for it shows that the same cause which has
produced the highly fissile structure, has altered in a slight degree
the mineralogical character of the rock in the same planes. The bands
of stratification, just alluded to, can be distinguished in many
places, especially in Navarin Island, but only on the weathered
surfaces of the slate; they consist of slightly undulatory zones of
different shades of colour and of thicknesses, and resemble the marks
(more closely than anything else to which I can compare them) left on
the inside of a vessel by the draining away of some dirty slightly
agitated liquid: no difference in composition, corresponding with these
zones, could be seen in freshly fractured surfaces. In the more level
parts of Navarin Island, these bands of stratification were nearly
horizontal; but on the flanks of the mountains they were inclined from
them, but in no instance that I saw at a very high angle. There can, I
think, be no doubt that these zones,
which appear only on the weathered surfaces, are the last vestiges of
the original planes of stratification, now almost obliterated by the
highly fissile and altered structure which the mass has assumed.

The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on
Navarin Island, on both sides of the Beagle Channel, on the eastern
side of Hoste Island, on the N.E. side of Hardy Peninsula, and on the
northern point of Wollaston Island; although in these two latter
localities the cleavage has been much obscured by the metamorphosed and
feldspathic condition of the slate. Within the area of these several
islands, including Navarin Island, the direction of the stratification
and of the mountain-chains is very obscure; though the mountains in
several places appeared to range in the same W.N.W. line with the
cleavage: the outline of the coast, however, does not correspond with
this line. Near the bifurcation of the Beagle Channel, where the
underlying metamorphic schists are first seen, they are foliated (with
some irregularities), in this same W.N.W. line, and parallel, as before
stated, to the main mountain-axis of this part of the country. Westward
of this main range, the metamorphic schists are foliated, though less
plainly, in the same direction, which is likewise common to the zone of
old erupted trappean rocks, forming the outermost islets. Hence the
area, over which the cleavage of the slate and the foliation of the
metamorphic schists extends with an average W.N.W. and E.S.E. strike,
is about forty miles in a north and south line, and ninety miles in an
east and west line.

Further northward, near Port Famine, the stratification of the
clay-slate and of the associated rocks, is well defined, and there
alone the cleavage and strata-planes are parallel. A little north of
this port there is an anticlinal axis ranging N.W. (or a little more
westerly) and S.E.: south of the port, as far as Admiralty Sound and
Gabriel Channel, the outline of the land clearly indicates the
existence of several lines of elevation in this same N.W. direction,
which, I may add, is so uniform in the western half of the St. of
Magellan, that, as Captain King[18] has remarked, “a parallel ruler
placed on the map upon the projecting points of the south shore, and
extended across the strait, will also touch the headlands on the
opposite coast.” It would appear, from Captain King’s observations,
that over all this area the cleavage extends in the same line.
Deep-water channels, however, in all parts of Tierra del Fuego have
burst through the trammels both of stratification and cleavage; most of
them may have been formed during the elevation of the land by
long-continued erosion, but others, for instance the Beagle Channel,
which stretches like a narrow canal for 120 miles obliquely through the
mountains, can hardly have thus originated.

 [18] _Geograph. Journal,_ vol. i, p. 170.

Finally, we have seen that in the extreme eastern point of Tierra del
Fuego, the cleavage and coast-lines extend W. and E. and even W.S.W.
and E.N.E.: over a large area westward, the cleavage, the main range of
mountains, and some subordinate ranges, but not the outlines of the
coast, strike W.N.W., and E.S.E.: in the central and western parts of
the St. of Magellan, the stratification, the mountain-ranges, the
outlines of the coast, and the cleavage all strike nearly N.W. and S.E.
North of the strait, the outline of the coast, and the mountains on the
mainland, run nearly north and south. Hence we see, at this southern
point of the continent, how gradually the Cordillera bend, from their
north and south course of so many thousand miles in length, into an E.
and even E.N.E. direction.

_West coast, from the Southern Chonos Islands to Northern Chile._—The
first place at which we landed north of the St. of Magellan was near
Cape Tres Montes, in lat. 47° S. Between this point and the Northern
Chonos Islands, a distance of 200 miles, the _Beagle_ visited several
points, and specimens were collected for me from the intermediate
spaces by Lieutenant Stokes. The predominant rock is mica-slate, with
thick folia of quartz, very frequently alternating with and passing
into a chloritic, or into a black, glossy, often striated, slightly
anthracitic schist, which soils paper, and becomes white under a great
heat, and then fuses. Thin layers of feldspar, swelling at intervals
into well crystallised kernels, are sometimes included in these black
schists; and I observed one mass of the ordinary black variety
insensibly lose its fissile structure, and pass into a singular mixture
of chlorite, epidote, feldspar, and mica. Great veins of quartz are
numerous in the mica-schists; wherever these occur the folia are much
convoluted. In the southern part of the Peninsula of Tres Montes, a
compact altered feldspathic rock with crystals of feldspar and grains
of quartz is the commonest variety; this rock[19] exhibits occasionally
traces of an original brecciated structure, and often presents (like
the altered state of Tierra del Fuego) traces of cleavage-planes, which
strike in the same direction with the folia of mica-schist further
northward. At Inchemo Island, a similar rock gradually becomes
granulo-crystalline and acquires scales of mica; and this variety at S.
Estevan becomes highly laminated, and though still exhibiting some
rounded grains of quartz, passes into the black, glossy, slightly
anthracitic schist, which, as we have seen, repeatedly alternates with
and passes into the micaceous and chloritic schists. Hence all the
rocks on this line of coast belong to one series, and insensibly vary
from an altered feldspathic clay-slate into largely foliated, true
mica-schist.

 [19] The peculiar, abruptly conical form of the hills in this
 neighbourhood, would have led any one at first to have supposed that
 they had been formed of injected or intrusive rocks.

The cleavage of the homogeneous schists, the foliation of those
composed of more or less distinct minerals in layers, and the planes of
alternation of the different varieties or so-called stratification, are
all parallel, and preserve over this 200 miles of coast a remarkable
degree of uniformity in direction. At the northern end of the group, at
Low’s Harbour, the well-defined folia of mica-schist everywhere ranged
within eight degrees (or less than one point of the compass) of N. 19°
W. and S. 19° E.; and even the point of dip varied very little, being
always directed to the west and generally at an angle of forty degrees;
I should mention that I had here good opportunities of observation, for
I followed the naked rock on the beach, transversely to the strike, for
a distance of four miles and a half, and all the way attended to the
dip. Along the outer islands for 100 miles south of Low’s Harbour,
Lieutenant Stokes, during his boat-survey, kindly observed for me the
strike of the foliation, and he assures me that it was invariably
northerly, and the dip with one single exception to the west. Further
south at Vallenar Bay, the strike was almost universally N. 25° W. and
the dip, generally at an angle of about 40° to W. 25° S., but in some
places almost vertical. Still farther south, in the neighbourhood of
the harbours of Anna Pink, S. Estevan and S. Andres, and (judging from
a distance) along the southern part of Tres Montes, the foliation and
cleavage extended in a line between [N. 11° to 22° W.] and [S. 11° to
22° E.]; and the planes dipped generally westerly, but often easterly,
at angles varying from a gentle inclination to vertical. At A. Pink’s
Harbour, where the schists generally dipped easterly, wherever the
angle became very high, the strike changed from N. 11° W. to even as
much as N. 45° W.: in an analogous manner at Vallenar Bay, where the
dip was westerly (viz. on an average directed to W. 25° S.), as soon as
the angle became very high, the planes struck in a line more than 25°
west of north. The average result from all the observations on this 200
miles of coast, is a strike of N. 19° W. and S. 19° E.: considering
that in each specified place my examination extended over an area of
several miles, and that Lieutenant Stokes’ observations apply to a
length of 100 miles, I think this remarkable uniformity is pretty well
established. The prevalence, throughout the northern half of this line
of coast, of a dip in one direction, that is to the west, instead of
being sometimes west and sometimes east, is, judging from what I have
elsewhere seen, an unusual circumstance. In Brazil, La Plata, the
Falkland Islands, and Tierra del Fuego, there is generally an obvious
relation between the axis of elevation, the outline of the coast, and
the strike of the cleavage or foliation: in the Chonos Archipelago,
however, neither the minor details of the coast-line, nor the chain of
the Cordillera, nor the subordinate transverse mountain-axes, accord
with the strike of the foliation and cleavage: the seaward face of the
numerous islands composing this Archipelago, and apparently the line of
the Cordillera, range N. 11° E., whereas, as we have just seen, the
average strike of the foliation is N. 19° W.

There is one interesting exception to the uniformity in the strike of
the foliation. At the northern point of Tres Montes (lat. 45° 52′) a
bold chain of granite, between two and three thousand feet in height,
runs from the coast far into the interior,[20] in an E.S.E. line, or
more strictly E. 28° S. and W. 28° N. In a bay, at the northern foot of
this range, there are a few islets of mica-slate, with the folia in
some parts horizontal, but mostly inclined at an average angle of 20°
to the north. On the northern steep flank of the range, there are a few
patches (some quite isolated, and not larger than half-a-crown!) of the
mica-schist, foliated with the same northerly dip. On the broad summit,
as far as the
southern crest, there is much mica-slate, in some places even 400 feet
in thickness, with the folia all dipping north, at angles varying from
5° to 20°, but sometimes mounting up to 30°. The southern flank
consists of bare granite. The mica-slate is penetrated by small
veins[21] of granite, branching from the main body. Leaving out of view
the prevalent strike of the folia in other parts of this Archipelago,
it might have been expected that they would have dipped N. 28° E., that
is directly from the ridge, and, considering its abruptness, at a high
inclination; but the real dip, as we have just seen, both at the foot
and on the northern flank, and over the entire summit, is at a small
angle, and directed nearly due north. From these considerations it
occurred to me, that perhaps we here had the novel and curious case of
already inclined laminæ obliquely tilted at a subsequent period by the
granitic axis. Mr. Hopkins, so well known from his mathematical
investigations, has most kindly calculated the problem: the proposition
sent was,—Take a district composed of laminæ, dipping at an angle of 40
degrees to W. 19° S., and let an axis of elevation traverse it in an E.
28° S. line, what will the position of the laminæ be on the northern
flank after a tilt, we will first suppose, of 45°? Mr. Hopkins informs
me, that the angle of the dip will be 28° 31′, and its direction to
north 30° 33′ west.[22] By varying the supposed angle of the tilt, our
previously inclined folia can be thrown into any angle between 26°,
which is the least possible angle, and 90°; but if a small inclination
be thus given to them, their point of dip will depart far from the
north, and therefore not accord with the actual position of the folia
of mica-schist on our granitic range. Hence it appears very difficult,
without varying considerably the elements of the problem, thus to
explain the anomalous strike and dip of the foliated mica-schist,
especially in those parts, namely, at the base of the range, where the
folia are almost horizontal. Mr. Hopkins, however, adds, that great
irregularities and lateral thrusts might be expected in every great
line of elevation, and that these would account for considerable
deviations from the calculated results: considering that the granitic
axis, as shown by the veins, has indisputably been injected after the
perfect formation of the mica-slate, and considering the uniformity of
the strike of the folia throughout the rest of the Archipelago, I
cannot but still think that their anomalous position at this one point
is someway directly and mechanically related to the intrusion of this
W.N.W. and E.S.E. mountain-chain of granite.

 [20] In the distance, other mountains could be seen apparently ranging
 N.N.E. and S.S.W. at right angles to this one. I may add, that not far
 from Vallenar Bay there is a fine range, apparently of granite, which
 has burst through the mica-slate in a N.E. by E. and S.W. by S. line.


 [21] The granite within these veins, as well as generally at the
 junction with the mica-slate, is more quartzose than elsewhere. The
 granite, I may add, is traversed by dikes running for a very great
 length in the line of the mountains; they are composed of a somewhat
 laminated eurite, containing crystals of feldspar, hornblende, and
 octagons of quartz.


 [22] On the south side of the axis (where, however, I did not see any
 mica-slate) the dip of the folia would be at an angle of 77° 55′,
 directed to west 35° 33′ south. Hence the two points of dip on the
 opposite sides of the range, instead of being as in ordinary cases
 directly opposed to each other at an angle of 180°, would here be only
 86° 50′ apart.

Dikes are frequent in the metamorphic schists of the Chonos Islands,
and seem feebly to represent that great band of trappean and ancient
volcanic rocks on the south-western coast of Tierra del Fuego. At S.
Andres I observed in the space of half-a-mile, seven broad, parallel
dikes, composed of three varieties of trap, running in a N.W. and S.E.
line, parallel to the neighbouring mountain-ranges of altered
clay-slate; but they must be of long subsequent origin to these
mountains; for they intersected the volcanic formation described in the
last chapter. North of Tres Montes, I noticed three dikes differing
from each other in composition, one of them having a euritic base
including large octagons of quartz; these dikes, as well as several of
porphyritic greenstone at Vallenar Bay, extended N.E. and S.W., nearly
at right angles to the foliation of the schists, but in the line of
their joints. At Low’s Harbour, however, a set of great parallel dikes,
one ninety yards and another sixty yards in width, have been guided by
the foliation of the mica-schist, and hence are inclined westward at an
angle of 45°: these dikes are formed of various porphyritic traps, some
of which are remarkable from containing numerous rounded grains of
quartz. A porphyritic trap of this latter kind, passed in one of the
dikes into a most curious hornstone, perfectly white, with a waxy
fracture and pellucid edges, fusible, and containing many grains of
quartz and specks of iron pyrites. In the ninety-yard dike several
large, apparently now quite isolated, fragments of mica-slate were
embedded: but as their foliation was exactly parallel to that of the
surrounding solid rock, no doubt these new separate fragments
originally formed wedge-shaped depending portions of a continuous vault
or crust, once extending over the dike, but since worn down and
denuded.

_Chiloe, Valdivia, Concepcion._—In Chiloe, a great formation of
mica-schist strikingly resembles that of the Chonos Islands. For a
space of eleven miles on the S.E. coast, the folia were very distinct,
though slightly convoluted, and ranged within a point of N.N.W. and
S.S.E., dipping either E.N.E. or more commonly W.S.W., at an average
angle of 22° (in one spot, however, at 60°), and therefore decidedly at
a lesser inclination than amongst the Chonos Islands. On the west and
north-western shores, the foliation was often obscure, though, where
best defined, it ranged within a point of N. by W. and S. by E.,
dipping either easterly or westerly, at varying and generally very
small angles. Hence, from the southern part of Tres Montes to the
northern end of Chiloe, a distance of 300 miles, we have closely allied
rocks with their folia striking on an average in the same direction,
namely between N. 11° and 22° W. Again, at Valdivia, we meet with the
same mica-schist, exhibiting nearly the same mineralogical passages as
in the Chonos Archipelago, often, however, becoming more ferruginous,
and containing so much feldspar as to pass into gneiss. The folia were
generally well defined; but nowhere else in South America did I see
them varying so much in direction: this seemed chiefly caused by their
forming parts, as I could sometimes distinctly trace, of large flat
curves: nevertheless, both near the settlement and towards the
interior, a N.W. and S.E. strike seemed more frequent than any other
direction; the angle of the dip was generally small. At Concepcion, a
highly
glossy clay-slate had its cleavage often slightly curvilinear, and
inclined, seldom at a high angle, towards various points of the
compass:[23] but here, as at Valdivia, a N.W. and S.E. strike seemed to
be the most frequent one. In certain spots large quartz veins were
numerous, and near them, the cleavage, as was the case with the
foliation of the schists in the Chonos Archipelago, became extremely
tortuous.

 [23] I observed in some parts that the tops of the laminæ of the
 clay-slate (_b_ of the diagram) under the superficial detritus and
 soil (_a_) were bent, sometimes without being broken, as represented
 in the accompanying diagram, which is copied from one given by Sir H.
 De la Beche (p. 42 “Geological Manual”) of an exactly similar
 phenomenon in Devonshire. Mr. R. A. C. Austen, also, in his excellent
 paper on S.E. Devon (“Geolog. Transact.,” vol. vi, p. 437), has
 described this phenomenon; he attributes it to the action of frosts,
 but at the same time doubts whether the frosts of the present day
 penetrate to a sufficient depth. As it is known that earthquakes
 particularly affect the surface of the ground, it occurred to me that
 this appearance might perhaps be due, at least at Concepcion, to their
 frequent occurrence; the superficial layers of detritus being either
 jerked in one direction, or, where the surface was inclined, pushed a
 little downwards during each strong vibration. In North Wales I have
 seen a somewhat analogous but less regular appearance, though on a
 greater scale (_London Phil. Mag.,_ vol. xxi, p. 184), and produced by
 a quite different cause, namely, by the stranding of great icebergs;
 this latter appearance has also been observed in N. America.


[Illustration: Diagram described in note 23.]

At the northern end of Quiriquina Island, in the Bay of Concepcion, at
least eight rudely parallel dikes, which have been guided to a certain
extent by the cleavage of the slate, occur within the space of a
quarter of a mile. They vary much in composition, resembling in many
respects the dikes at Low’s Harbour: the greater number consist of
feldspathic porphyries, sometimes containing grains of quartz: one,
however, was black and brilliant, like an augitic rock, but really
formed of feldspar; others of a feldspathic nature were perfectly
white, with either an earthy or crystalline fracture, and including
grains and regular octagons of quartz; these white varieties passed
into ordinary greenstones. Although, both here and at Low’s Harbour,
the nature of the rock varied considerably in the same dike, yet I
cannot but think that at these two places and in other parts of the
Chonos group, where the dikes, though close to each other and running
parallel, are of different composition, that they must have been formed
at different periods. In the case of Quiriquina this is a rather
interesting conclusion, for these eight parallel dikes cut through the
metamorphic schists in a N.W. and S.E. line, and since their injection
the overlying cretaceous or tertiary strata have been tilted (whilst
still under the sea) from a N.W. by N. and S.E. by S. line; and again,
during the great earthquake of February 1835, the ground in this
neighbourhood was fissured in N.W. and S.E. lines; and from the manner
in which buildings were
thrown down, it was evident that the surface undulated in this same
direction.[24]

 [24] “Geolog. Trans.,” vol. vi, pp. 602 and 617. “Journal of
 Researches” (2nd edit.), p. 307.

_Central and Northern Chile._—Northward of Concepcion, as far as
Copiapo, the shores of the Pacific consist, with the exception of some
small tertiary basins, of gneiss, mica-schist, altered clay-slate,
granite, greenstone and syenite: hence the coast from Tres Montes to
Copiapo, a distance of 1,200 miles, and I have reason to believe for a
much greater space, is almost similarly constituted.

Near Valparaiso the prevailing rock is gneiss, generally including much
hornblende: concretionary balls formed of feldspar, hornblende and
mica, from two or three feet in diameter, are in very many places
conformably enfolded by the foliated gneiss: veins of quartz and
feldspar, including black schorl and well-crystallised epidote, are
numerous. Epidote likewise occurs in the gneiss in thin layers,
parallel to the foliation of the mass. One large vein of a coarse
granitic character was remarkable from in one part quite changing its
character, and insensibly passing into a blackish porphyry, including
acicular crystals of glassy feldspar and of hornblende: I have never
seen any other such case.[25]

 [25] Humboldt (“Personal Narrative,” vol. iv, p. 60) has described
 with much surprise, concretionary balls, with concentric divisions,
 composed of partially vitreous feldspar, hornblende, and garnets,
 included within great veins of gneiss, which cut across the mica-slate
 near Venezuela.

I shall in the few following remarks on the rocks of Chile allude
exclusively to their foliation and cleavage. In the gneiss round
Valparaiso the strike of the foliation is very variable, but I think
about N. by W. and S. by E. is the commonest direction; this likewise
holds good with the cleavage of the altered feldspathic clay-slates,
occasionally met with on the coast for ninety miles north of
Valparaiso. Some feldspathic slate, alternating with strata of
claystone porphyry in the Bell of Quillota and at Jajuel, and
therefore, perhaps, belonging to a later period than the metamorphic
schists on the coast, cleaved in this same direction. In the Eastern
Cordillera, in the Portillo Pass, there is a grand mass of mica-slate,
foliated in a north and south line, and with a high westerly dip: in
the Uspallata range, clay-slate and grauwacke have a highly inclined,
nearly north and south cleavage, though in some parts the strike is
irregular: in the main or Cumbre range, the direction of the cleavage
in the feldspathic clay-slate is N.W. and S.E.

Between Coquimbo and Guasco there are two considerable formations of
mica-slate, in one of which the rock passed sometimes into common
clay-slate and sometimes into a glossy black variety, very like that in
the Chonos Archipelago. The folia and cleavage of these rocks ranged
between [N. and N.W. by N.] and [S. and S.W. by S.]. Near the Port of
Guasco several varieties of altered clay-slate have a quite irregular
cleavage. Between Guasco and Copiapo, there are some siliceous and
talcaceous slates cleaving in a north and south line, with an easterly
dip of between 60° and 70°: high up, also, the main valley of Copiapo,
there is mica-slate with a high easterly dip. In the whole space
between Valparaiso and Copiapo an easterly dip is much more common than
an opposite or westerly one.

_Concluding Remarks on Cleavage and Foliation._

In this southern part of the southern hemisphere, we have seen that the
cleavage-laminæ range over wide areas with remarkable uniformity,
cutting straight through the planes of stratification,[26] but yet
being parallel in strike to the main axes of elevation, and generally
to the outlines of the coast. The dip, however, is as variable, both in
angle and in direction (that is, sometimes being inclined to the one
side and sometimes to the directly opposite side), as the strike is
uniform. In all these respects there is a close agreement with the
facts given by Professor Sedgwick in his celebrated memoir in the
“Geological Transactions,” and by Sir R. I. Murchison in his various
excellent discussions on this subject. The Falkland Islands, and more
especially Tierra del Fuego, offer striking instances of the lines of
cleavage, the principle axes of elevation, and the outlines of the
coast, gradually changing together their courses. The direction which
prevails throughout Tierra del Fuego and the Falkland Islands, namely,
from west with some northing to east with some southing, is also common
to the several ridges in Northern Patagonia and in the western parts of
Banda Oriental: in this latter province, in the Sierra Tapalguen, and
in the Western Falkland Island, the W. by N., or W.N.W. and E.S.E.,
ridges, are crossed at right angles by others ranging N.N.E. and S.S.W.

 [26] In my paper on the Falkland Islands (_Geological Journal,_ vol.
 iii, p. 267), I have given a curious case on the authority of Captain
 Sulivan, R.N., of much folded beds of clay-slate, in some of which the
 cleavage is perpendicular to the horizon, and in others it is
 perpendicular to each curvature or fold of the bed: this appears a new
 case.


The fact of the cleavage-laminæ in the clay-slate of Tierra del Fuego,
where seen cutting straight through the planes of stratification, and
where consequently there could be no doubt about their nature,
differing slightly in colour, texture, and hardness, appears to me very
interesting. In a thick mass of laminated, feldspathic and altered
clay-slate, interposed between two great strata of porphyritic
conglomerate in Central Chile, and where there could be but little
doubt about the bedding, I observed similar slight differences in
composition, and likewise some distinct thin layers of epidote,
parallel to the highly inclined cleavage of the mass. Again, I
incidentally noticed in North Wales,[27] where glaciers had passed over
the truncated edges of the highly inclined laminæ of clay-slate, that
the surface, though smooth, was worn into small parallel undulations,
caused by the competent laminæ being of slightly different degrees of
hardness. With reference to the slates of North Wales, Professor
Sedgwick describes the planes of cleavage, as “coated over with
chlorite and semi-crystalline matter, which not only merely define the
planes in question, but strike in parallel flakes through the whole
mass of the rock.”[28] In some of those
glossy and hard varieties of clay-slate, which may often be seen
passing into mica-schist, it has appeared to me that the
cleavage-planes were formed of excessively thin, generally slighted
convoluted, folia, composed of microscopically minute scales of mica.
From these several facts, and more especially from the case of the
clay-slate in Tierra del Fuego, it must, I think, be concluded, that
the same power which has impressed on the slate its fissile structure
or cleavage has tended to modify its mineralogical character in
parallel planes.

 [27] _London Phil. Mag._, vol. xxi, p. 182.


 [28] “Geological Trans.,” vol. iii, p. 471.


Let us now turn to the foliation of the metamorphic schists, a subject
which has been much less attended to. As in the case of
cleavage-laminæ, the folia preserve over very large areas a uniform
strike: thus Humboldt[29] found for a distance of 300 miles in
Venezuela, and indeed over a much larger space, gneiss, granite, mica,
and clay-slate, striking very uniformly N.E. and S.W., and dipping at
an angle of between 60° and 70° to N.W.; it would even appear from the
facts given in this chapter, that the metamorphic rocks throughout the
north-eastern part of South America are generally foliated within two
points of N.E. and S.W. Over the eastern parts of Banda Oriental, the
foliation strikes with a high inclination, very uniformly N.N.E. to
S.S.W., and over the western parts, in a W. by N. and E. by S. line.
For a space of 300 miles on the shores of the Chonos and Chiloe
Islands, we have seen that the foliation seldom deviates more than a
point of the compass from a N. 19° W. and S. 19° E. strike. As in the
case of cleavage, the angle of the dip in foliated rocks is generally
high but variable, and alternates from one side of the line of strike
to the other side, sometimes being vertical: in the Northern Chonos
Islands, however, the folia are inclined almost always to the west; in
nearly the same manner, the cleavage-laminæ in Southern Tierra del
Fuego certainly dip much more frequently to S.S.W. than to the opposite
point. In Eastern Banda Oriental, in parts of Brazil, and in some other
districts, the foliation runs in the same direction with the
mountain-ranges and adjoining coast-lines: amongst the Chonos Islands,
however, this coincidence fails, and I have given my reasons for
suspecting that one granitic axis has burst through and tilted the
already inclined folia of mica-schist: in the case of cleavage,[30] the
coincidence between its strike and that of the main stratification
seems sometimes to fail. Foliation and cleavage resemble each other in
the planes winding round concretions, and in becoming tortuous where
veins of quartz abound.[31] On the flanks of
the mountains both in Tierra del Fuego and in other countries, I have
observed that the cleavage-planes frequently dip at a high angle
inwards; and this was long ago observed by Von Buch to be the case in
Norway: this fact is perhaps analogous to the folded, fan-like or
radiating structure in the metamorphic schists of the Alps,[32] in
which the folia in the central crests are vertical and on the two
flanks inclined inwards. Where masses of fissile and foliated rocks
alternate together, the cleavage and foliation, in all cases which I
have seen, are parallel. Where in one district the rocks are fissile,
and in another adjoining district they are foliated, the planes of
cleavage and foliation are likewise generally parallel: this is the
case with the feldspathic homogeneous slates in the southern part of
the Chonos group, compared with the fine foliated mica-schists of the
northern part; so again the clay-slate of the whole eastern side of
Tierra del Fuego cleaves in exactly the same line with the foliated
gneiss and mica-slate of the western coast; other analogous instances
might have been adduced.[33]

 [29] “Personal Narrative,” vol. vi, p. 59 _et seq._


 [30] Cases are given by Mr. Jukes in his “Geology of Newfoundland,” p.
 130.


 [31] I have seen in Brazil and Chile concretions thus enfolded by
 foliated gneiss; and Macculloch (“Highlands,” vol. i, p. 64) has
 described a similar case. For analogous cases in clay-slate, see
 Professor Henslow’s Memoir in “Cambridge Phil. Trans.,” vol. i, p.
 379, and Macculloch’s “Class. of Rocks,” p. 351. With respect to both
 foliation and cleavage becoming tortuous where quartz-veins abound, I
 have seen instances near Monte Video, at Concepcion, and in the Chonos
 Islands. See also Mr. Greenough’s “Critical Examination,” p. 78.


 [32] Studer in _Edin. New Phil. Journal,_ vol. xxiii, p. 144.


 [33] I have given a case in Australia. See my “Volcanic Islands.”


With respect to the origin of the folia of quartz, mica, feldspar, and
other minerals composing the metamorphic schists, Professor Sedgwick,
Mr. Lyell, and most authors believe, that the constituent parts of each
layer were separately deposited as sediment, and then metamorphosed.
This view, in the majority of cases, I believe to be quite untenable.
In those not uncommon instances, where a mass of clay-slate, in
approaching granite, gradually passes into gneiss,[34] we clearly see
that folia of distinct minerals can originate through the metamorphosis
of a homogeneous fissile rock. The deposition, it may be remarked, of
numberless alternations of pure quartz, and of the elements of mica or
feldspar does not appear a probable event.[35] In those districts in
which the metamorphic schists are foliated in planes parallel to the
cleavage of the rocks in an adjoining district, are we to believe that
the folia are due to sedimentary layers, whilst the cleavage-laminæ,
though parallel, have no relation whatever to such planes of
deposition? On this view, how can we reconcile the vastness of the
areas over which the strike of the foliation is uniform, with what we
see in disturbed districts composed of true strata: and especially, how
can we understand the high and even vertical dip throughout many wide
districts, which are not mountainous, and throughout some, as in
Western Banda Oriental, which are not even hilly? Are we to admit that
in the northern part of the Chonos Archipelago, mica-slate was first
accumulated in parallel horizontal folia to a thickness of about four
geographical miles, and then upturned at an angle of forty degrees;
whilst, in the southern part of this same Archipelago, the
cleavage-laminæ of closely allied rocks, which none would imagine had
ever been horizontal, dip at nearly the same angle, to nearly the same
point?

 [34] I have described in “Volcanic Islands” a good instance of such a
 passage at the Cape of Good Hope.


 [35] See some excellent remarks on this subject, in D’Aubuisson’s
 “Traité de Géog.,” tome i, p. 297. Also some remarks by Mr. Dana in
 _Silliman’s American Journ.,_ vol. xlv, p. 108.

Seeing, then, that foliated schists indisputably are sometimes produced
by the metamorphosis of homogeneous fissile rocks; seeing that
foliation and cleavage are so closely analogous in the several
above-enumerated respects; seeing that some fissile and almost
homogeneous rocks show incipient mineralogical changes along the planes
of their cleavage, and that other rocks with a fissile structure
alternate with, and pass into varieties with a foliated structure, I
cannot doubt that in most cases foliation and cleavage are parts of the
same process: in cleavage there being only an incipient separation of
the constituent minerals; in foliation a much more complete separation
and crystallisation.

The fact often referred to in this chapter, of the foliation and the
so-called strata in the metamorphic series,—that is, the alternating
masses of different varieties of gneiss, mica-schist, and
hornblende-slate, etc.,—being parallel to each other, at first appears
quite opposed to the view, that the folia have no relation to the
planes of original deposition. Where the so-called beds are not very
thick and of widely different mineralogical composition from each
other, I do not think that there is any difficulty in supposing that
they have originated in an analogous manner with the separate folia. We
should bear in mind what thick strata, in ordinary sedimentary masses,
have obviously been formed by a concretionary process. In a pile of
volcanic rocks on the Island of Ascension, there are strata, differing
quite as much in appearance as the ordinary varieties of the
metamorphic schists, which undoubtedly have been produced, not by
successive flowings of lava, but by internal molecular changes. Near
Monte Video, where the stratification, as it would be called, of the
metamorphic series is, in most parts, particularly well developed,
being as usual, parallel to the foliation, we have seen that a mass of
chloritic schist, netted with quartz-veins, is entangled in gneiss, in
such a manner as to show that it had certainly originated in some
process of segregation: again, in another spot, the gneiss tended to
pass into hornblendic schist by alternating with layers of quartz; but
these layers of quartz almost certainly had never been separately
deposited, for they were absolutely continuous with the numerous
intersecting veins of quartz. I have never had an opportunity of
tracing for any distance, along the line both of strike and of dip, the
so-called beds in the metamorphic schists, but I strongly suspect that
they would not be found to extend with the same character, very far in
the line either of their dip or strike. Hence I am led to believe, that
most of the so-called beds are of the nature of complex folia, and have
not been separately deposited. Of course, this view cannot be extended
to _thick_ masses included in the metamorphic series, which are of
totally different composition from the adjoining schists, and which are
far extended, as is sometimes the case with quartz and marble; these
must generally be of the nature of true
strata.[36] Such strata, however, will almost always strike in the same
direction with the folia, owing to the axes of elevation being in most
countries parallel to the strike of the foliation; but they will
generally dip at a different angle from that of the foliation; and the
angle of the foliation in itself almost always varies much: hence, in
crossing a metamorphosed schistose district, it would require especial
attention to discriminate between true strata of deposition and complex
foliated masses. The mere presence of true strata in the midst of a set
of metamorphic schists, is no argument that the foliation is of
sedimentary origin, without it be further shown in each case, that the
folia not only strike, but dip throughout in parallel planes with those
of the true stratification.

 [36] Macculloch states (“Classification of Rocks,” p. 364) states that
 primary limestones are often found in irregular masses or great
 nodules, “which can scarcely be said to possess a stratified shape!”


As in some cases it appears that where a fissile rock has been exposed
to partial metamorphic action, for instance from the irruption of
granite, the foliation has supervened on the already existing
cleavage-planes; so perhaps in some instances, the foliation of a rock
may have been determined by the original planes of deposition or of
oblique current-laminæ: I have, however, myself, never seen such a
case, and I must maintain that in most extensive metamorphic areas, the
foliation is the extreme result of that process, of which cleavage is
the first effect. That foliation may arise without any previous
structural arrangement in the mass, we may infer from injected, and
therefore once liquified, rocks, both of volcanic and plutonic origin,
sometimes having a “grain” (as expressed by Professor Sedgwick), and
sometimes being composed of distinct folia or laminæ of different
compositions. In my work on “Volcanic Islands,” I have given several
instances of this structure in volcanic rocks, and it is not uncommonly
seen in plutonic masses—thus, in the Cordillera of Chile, there are
gigantic mountain-like masses of red granite, which have been injected
whilst liquified, and which, nevertheless, display in parts a decidedly
laminar structure.[37]

 [37] As remarked in a former part of this chapter, I suspect that the
 boldly conical mountains of gneiss-granite, near Rio de Janeiro, in
 which the constituent minerals are arranged in parallel planes, are of
 intrusive origin. We must not, however, forget the lesson of caution
 taught by the curious claystone porphyries of Port Desire, in which we
 have seen that the breaking up and aggregation of a thinly stratified
 tufaceous mass, has yielded a rock semi-porphyritic with crystals of
 feldspar, arranged in the planes of original deposition.


Finally, we have seen that the planes of cleavage and of foliation,
that is, of the incipient process and of the final result, generally
strike parallel to the principal axes of elevation, and to the outline
of the land: the strike of the axes of elevation (that is, of the lines
of fissures with the strata on their edges upturned), according to the
reasoning of Mr. Hopkins, is determined by the form of the area
undergoing changes of level, and the consequent direction of the lines
of tension and fissure. Now, in that remarkable pile of volcanic rocks
at Ascension, which has
several times been alluded to (and in some other cases), I have
endeavoured to show,[38] that the lamination of the several varieties,
and their alternations, have been caused by the moving mass, just
before its final consolidation, having been subjected (as in a glacier)
to planes of different tension; this difference in the tension
affecting the crystalline and concretionary processes. One of the
varieties of rock thus produced at Ascension, at first sight,
singularly resembles a fine-grained gneiss; it consists of quite
straight and parallel zones of excessive tenuity, of more or less
coloured crystallised feldspar, of distinct crystals of quartz,
diopside, and oxide of iron. These considerations, notwithstanding the
experiments made by Mr. Fox, showing the influence of electrical
currents in producing a structure like that of cleavage, and
notwithstanding the apparently inexplicable variation, both in the
inclination of the cleavage-laminæ and in their dipping first to one
side and then to the other side of the line of strike, lead me to
suspect that the planes of cleavage and foliation are intimately
connected with the planes of different tension, to which the area was
long subjected, _after_ the main fissures or axes of upheavement had
been formed, but _before_ the final consolidation of the mass and the
total cessation of all molecular movement.

 [38] In “Volcanic Islands.”


[Illustration: Geological sections through the Cordilleras.]

For enlargements of the above plate use the following links:
left section
center section
right section




Chapter VII CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.


Central Chile.—Basal formations of the Cordillera.—Origin of the
porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section
of the Cordillera by the Peuquenes are Portillo Pass.—Great gypseous
formation.—Peuquenes line; thickness of strata, fossils of.—Portillo
line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks
of.—Concluding remarks on the denudation and elevation of the Portillo
line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous
strata.—Section near the Puente del Inca; fossils of.—Great
subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the
Uspallata chain.—Structure and nature of the strata.—Silicified
vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding
remarks on the Uspallata range; origin subsequent to that of the main
Cordillera; two periods of subsidence; comparison with the Portillo
chain.


The district between the Cordillera and the Pacific, on a rude average,
is from about eighty to one hundred miles in width. It is crossed by
many chains of mountains, of which the principal ones, in the latitude
of Valparaiso and southward of it, range nearly north and south; but in
the more northern parts of the province, they run in almost every
possible direction. Near the Pacific, the mountain-ranges are generally
formed of syenite or granite, and or of an allied euritic porphyry; in
the low country, besides these granitic rocks and greenstone, and much
gneiss, there are, especially northward of Valparaiso, some
considerable districts of true clay-slate with quartz veins, passing
into a feldspathic and porphyritic slate; there is also some grauwacke
and quartzose and jaspery rocks, the latter occasionally assuming the
character of the basis of claystone porphyry: trap-dikes are numerous.
Nearer the Cordillera the ranges (such as those of S. Fernando, the
Prado,[1] and Aconcagua) are formed partly of granitic rocks, and
partly of purple porphyritic conglomerates, claystone porphyry,
greenstone porphyry, and other rocks, such as we shall immediately see,
form the basal strata of the main Cordillera. In the more northern
parts of Chile, this porphyritic series extends over large tracts of
country far from the Cordillera; and even in Central Chile such
occasionally occur in outlying positions.

 [1] Meyen “Reise um Erde” Th. I, s. 235.

I will describe the Campana of Quillota, which stands only fifteen
miles from the Pacific, as an instance of one of these outlying masses.
This hill is conspicuous from rising to the height of 6,400 feet: its
summit shows a nucleus, uncovered for a height of 800 feet, of fine
greenstone, including epidote and octahedral magnetic iron ore; its
flanks are formed of great strata of porphyritic claystone conglomerate
associated with various true porphyries and amygdaloids, alternating
with thick masses of a highly feldspathic, sometimes porphyritic,
pale-coloured slaty rock, with its cleavage-laminæ dipping inwards at a
high angle. At the base of the hill there are syenites, a granular
mixture of quartz and feldspar, and harsh quartzose rocks, all
belonging to the basal metamorphic series. I may observe that at the
foot of several hills of this class, where the porphyries are first
seen (as near S. Fernando, the Prado, Las Vacas, etc.), similar harsh
quartzose rocks and granular mixtures of quartz and feldspar occur, as
if the more fusible constituent parts of the granitic series had been
drawn off to form the overlying porphyries.

In Central Chile, the flanks of the main Cordillera, into which I
penetrated by four different valleys, generally consist of distinctly
stratified rocks. The strata are inclined at angles varying from
sometimes even under ten, to twenty degrees, very rarely exceeding
forty degrees: in some, however, of the quite small, exterior,
spur-like ridges, the inclination was not unfrequently greater. The dip
of the strata in the main outer lines was usually outwards or from the
Cordillera, but in Northern Chile frequently inwards,—that is, their
basset-edges fronted the Pacific. Dikes occur in extraordinary numbers.
In the great, central, loftiest ridges, the strata, as we shall
presently see, are almost always highly inclined and often vertical.
Before giving a detailed account of my two sections across the
Cordillera, it will, I think, be convenient to describe the basal
strata as seen, often to a thickness of four or five thousand feet, on
the flanks of the outer lines.

_Basal strata of the Cordillera._—The prevailing rock is a purplish or
greenish, porphyritic claystone conglomerate. The embedded fragments
vary in size from mere particles to blocks as much as six or eight
inches (rarely more) in diameter; in many places, where the fragments
were minute, the signs of aqueous deposition were unequivocally
distinct; where they were large, such evidence could rarely be
detected. The basis is generally porphyritic with perfect crystals of
feldspar, and resembles that of a true injected claystone porphyry:
often, however, it has a mechanical or sedimentary aspect, and
sometimes (as at Jajuel) is jaspery. The included fragments are either
angular, or partially or quite rounded;[2] in some parts the rounded,
in others the angular fragments prevail, and usually both kinds are
mixed together: hence the word _breccia_ ought strictly to be appended
to the term _porphyritic conglomerate._ The fragments consist of many
varieties of claystone porphyry, usually of nearly the same colour with
the surrounding basis, namely, purplish-reddish, brownish, mottled or
bright green; occasionally fragments of a laminated, pale-coloured,
feldspathic rock, like altered clay-slate are included; as are
sometimes grains of quartz, but only in one instance in Central Chile
(namely, at the mines of Jajuel) a few pebbles of quartz. I nowhere
observed mica in this formation, and rarely hornblende; where the
latter mineral did occur, I was generally in doubt whether the mass
really belonged to this formation, or was of intrusive origin.
Calcareous spar occasionally occurs in small cavities; and nests and
layers of epidote are common. In some few places in the finer-grained
varieties (for instance, at Quillota), there were short, interrupted
layers of earthy feldspar, which could be traced, exactly as at Port
Desire, passing into large crystals of feldspar: I doubt, however,
whether in this instance the layers had ever been separately deposited
as tufaceous sediment.

 [2] Some of the rounded fragments in the porphyritic conglomerate near
 the Baths of Cauquenes, were marked with radii and concentric zones of
 different shades of colour: any one who did not know that pebbles, for
 instance flint pebbles from the chalk, are sometimes zoned
 concentrically with their worn and rounded surfaces, might have been
 led to infer, that these balls of porphyry were not true pebbles, but
 had originated in concretionary action.)


All the varieties of porphyritic conglomerates and breccias pass into
each other, and by innumerable gradations into porphyries no longer
retaining the least trace of mechanical origin: the transition appears
to have been effected much more easily in the finer-grained, than in
the coarser-grained varieties. In one instance, near Cauquenes, I
noticed that a porphyritic conglomerate assumed a spheroidal structure,
and tended to become columnar. Besides the porphyritic conglomerates
and the perfectly characterised porphyries, of metamorphic origin,
there are other porphyries, which, though differing not at all or only
slightly in composition, certainly have had a different origin: these
consist of pink or purple claystone porphyries, sometimes including
grains of quartz,—of greenstone porphyry, and of other dusky rocks, all
generally porphyritic with fine, large, tabular, opaque crystals, often
placed crosswise, of feldspar cleaving like albite (judging from
several measurements), and often amygdaloidal with silex, agate,
carbonate of
lime, green and brown bole.[3] These several porphyritic and
amygdaloidal varieties never show any signs of passing into masses of
sedimentary origin: they occur both in great and small intrusive
masses, and likewise in strata alternating with those of the
porphyritic conglomerate, and with the planes of junction often quite
distinct, yet not seldom blended together. In some of these intrusive
masses, the porphyries exhibit, more or less plainly, a brecciated
structure, like that often seen in volcanic masses. These brecciated
porphyries could generally be distinguished at once from the
metamorphosed, porphyritic breccia-conglomerates, by all the fragments
being angular and being formed of the same variety, and by the absence
of every trace of aqueous deposition. One of the porphyries above
specified, namely, the greenstone porphyry with large tabular crystals
of albite, is particularly abundant, and in some parts of the
Cordillera (as near St. Jago) seemed more common even than the purplish
porphyritic conglomerate. Numerous dikes likewise consist of this
greenstone porphyry; others are formed of various fine-grained trappean
rocks; but very few of claystone porphyry: I saw no true basaltic
dikes.

 [3] This bole is a very common mineral in the amygdaloidal rocks; it
 is generally of a greenish-brown colour, with a radiating structure;
 externally it is black with an almost metallic lustre, but often
 coated by a bright green film. It is soft and can be scratched by a
 quill; under the blowpipe swells greatly and becomes scaly, then fuses
 easily into a black magnetic bead. This substance is evidently similar
 to that which often occurs in submarine volcanic rocks. An examination
 of some very curious specimens of a fine porphyry (from Jajuel) leads
 me to suspect that some of these amygdaloidal balls, instead of having
 been deposited in pre-existing air-vesicles, are of concretionary
 origin; for in these specimens, some of the pea-shaped little masses
 (often externally marked with minute pits) are formed of a mixture of
 green earth with stony matter, like the basis of the porphyry,
 including minute imperfect crystals of feldspar; and these pea-shaped
 little masses are themselves amygdaloidal with minute spheres of the
 green earth, each enveloped by a film of white, apparently
 feldspathic, earthy matter: so that the porphyry is doubly
 amygdaloidal. It should not, however, be overlooked, that all the
 strata here have undergone metamorphic action, which may have caused
 crystals of feldspar to appear, and other changes to be effected, in
 the originally simple amygdaloidal balls. Mr. J. D. Dana, in an
 excellent paper on Trap-rocks (_Edin. New Phil. Journ.,_ vol. xli, p.
 198), has argued with great force, that all amygdaloidal minerals have
 been deposited by aqueous infiltration. I may take this opportunity of
 alluding to a curious case, described in my work on “Volcanic
 Islands,” of an amygdaloid with many of its cells only half filled up
 with a mesotypic mineral.
    M. Rose has described an amygdaloid, brought by Dr. Meyen (“Reise
    um Erde,” Th. I, s. 316) from Chile, as consisting of crystallised
    quartz, with crystals of stilbite within, and lined externally by
    green earth.


In several places in the lower part of the series, but not everywhere,
thick masses of a highly feldspathic, often porphyritic, slaty rock
occur interstratified with the porphyritic conglomerate; I believe in
one or two cases blackish limestone has been found in a similar
position. The feldspathic rock is of a pale grey or greenish colour; it
is easily fusible;
where porphyritic, the crystals of feldspar are generally small and
vitreous: it is distinctly laminated, and sometimes includes parallel
layers of epidote;[4] the lamination appears to be distinct from
stratification. Occasionally this rock is somewhat curious; and at one
spot, namely, at the C. of Quillota, it had a brecciated structure.
Near the mines of Jajuel, in a thick stratum of this feldspathic,
porphyritic slate, there was a layer of hard, blackish, siliceous,
infusible, compact clay-slate, such as I saw nowhere else; at the same
place I was able to follow for a considerable distance the junction
between the slate and the conformably underlying porphyritic
conglomerate, and they certainly passed gradually into each other.
Wherever these slaty feldspathic rocks abound, greenstone seems common;
at the C. of Quillota a bed of well-crystallised greenstone lay
conformably in the midst of the feldspathic slate, with the upper and
lower junctions passing insensibly into it. From this point, and from
the frequently porphyritic condition of the slate, I should perhaps
have considered this rock as an erupted one (like certain laminated
feldspathic lavas in the trachytic series), had I not seen in Tierra
del Fuego how readily true clay-slate becomes feldspathic and
porphyritic, and had I not seen at Jajuel the included layer of black,
siliceous clay-slate, which no one could have thought of igneous
origin. The gentle passage of the feldspathic slate, at Jajuel, into
the porphyritic conglomerate, which is certainly of aqueous origin,
should also be taken in account.

 [4] This mineral is extremely common in all the formations of Chile;
 in the gneiss near Valparaiso and in the granitic veins crossing it,
 in the injected greenstone crowning the C. of Quillota, in some
 granitic porphyries, in the porphyritic conglomerate, and in the
 feldspathic clay-slates.


The alternating strata of porphyries and porphyritic conglomerate, and
with the occasionally included beds of feldspathic slate, together make
a grand formation; in several places within the Cordillera, I estimated
its thickness at from six to seven thousand feet. It extends for many
hundred miles, forming the western flank of the Chilean Cordillera; and
even at Iquique in Peru, 850 miles north of the southernmost point
examined by me in Chile, the coast-escarpment which rises to a height
of between two and three thousand feet is thus composed. In several
parts of Northern Chile this formation extends much further towards the
Pacific, over the granitic and metamorphic lower rocks, than it does in
Central Chile; but the main Cordillera may be considered as its central
line, and its breadth in an east and west direction is never great. At
first the origin of this thick, massive, long but narrow formation,
appeared to me very anomalous: whence were derived, and how were
dispersed the innumerable fragments, often of large size, sometimes
angular and sometimes rounded, and almost invariably composed of
porphyritic rocks? Seeing that the interstratified porphyries are never
vesicular and often not even amygdaloidal, we must conclude that the
pile was formed in deep water; how then came so many fragments to be
well rounded and so many to remain angular, sometimes the two kinds
being equally mingled, sometimes one and sometimes the other
preponderating? That the claystone,
greenstone, and other porphyries and amygdaloids, which lie
_conformably_ between the beds of conglomerate, are ancient submarine
lavas, I think there can be no doubt; and I believe we must look to the
craters whence these streams were erupted, as the source of the
breccia-conglomerate; after the great explosion, we may fairly imagine
that the water in the heated and scarcely quiescent crater would remain
for a considerable time[5] sufficiently agitated to triturate and round
the loose fragments, few or many in number, would be shot forth at the
next eruption, associated with few or many angular fragments, according
to the strength of the explosion. The porphyritic conglomerate being
purple or reddish, even when alternating with dusty-coloured or bright
green porphyries and amygdaloids, is probably an analogous circumstance
to the scoriæ of the blackish basalts being often bright red. The
ancient submarine orifices whence the porphyries and their fragments
were ejected having been arranged in a band, like most still active
volcanoes, accounts for the thickness, the narrowness, and linear
extension of this formation.

 [5] This certainly seems to have taken place in some recent volcanic
 archipelagos, as at the Galapagos, where numerous craters are
 exclusively formed of tuff and fragments of lava.

This whole great pile of rock has suffered much metamorphic action, as
is very obvious in the gradual formation and appearance of the crystals
of albitic feldspar and of epidote—in the bending together of the
fragments—in the appearance of a laminated structure in the feldspathic
slate—and, lastly, in the disappearance of the planes of
stratification, which could sometimes be seen on the same mountain
quite distinct in the upper part, less and less plain on the flanks,
and quite obliterated at the base. Partly owing to this metamorphic
action, and partly to the close relationship in origin, I have seen
fragments of porphyries—taken from a metamorphosed conglomerate—from a
neighbouring stream of lava—from the nucleus or centre (as it appeared
to me) of the whole submarine volcano—and lastly from an intrusive mass
of quite subsequent origin, all of which were absolutely
undistinguishable in external characters.

One other rock, of plutonic origin, and highly important in the history
of the Cordillera, from having been injected in most of the great axes
of elevation, and from having apparently been instrumental in
metamorphosing the superincumbent strata, may be conveniently described
in this preliminary discussion. It has been called by some authors
_Andesite_: it mainly consists of well-crystallised white albite[6] (as
determined with the goniometer in numerous specimens both by
Professor Miller and myself), of less perfectly crystallised green
hornblende, often associated with much mica, with chlorite and epidote,
and occasionally with a few grains of quartz: in one instance in
Northern Chile, I found crystals of orthitic or potash feldspar,
mingled with those of albite. Where the mica and quartz are abundant,
the rock cannot be distinguished from granite; and it may be called
andesitic granite. Where these two minerals are quite absent, and when,
as often then happens, the crystals of albite are imperfect and blend
together, the rock may be called andesitic porphyry, which bears nearly
the same relation to andesitic granite that euritic porphyry does to
common granite. These andesitic rocks form mountain masses of a white
colour, which, in their general outline and appearance—in their
joints—in their occasionally including dark-coloured, angular
fragments, apparently of some pre-existing rock—and in the great dikes
branching from them into the superincumbent strata, manifest a close
and striking resemblance to masses of common granite and syenite: I
never, however, saw in these andesitic rocks, those granitic veins of
segregation which are so common in true granites. We have seen that
andesite occurs in three places in Tierra del Fuego; in Chile, from S.
Fernando to Copiapo, a distance of 450 miles, I found it under most of
the axes of elevation; in a collection of specimens from the Cordillera
of Lima in Peru, I immediately recognised it; and Erman[7] states that
it occurs in Eastern Kamtschatka. From its wide range, and from the
important part it has played in the history of the Cordillera, I think
this rock has well deserved its distinct name of Andesite.

 [6] I here, and elsewhere, call by this name, those feldspathic
 minerals which cleave like albite: but it now appears (_Edin. New
 Phil. Journal.,_ vol. xxiv, p. 181) that Abich has analysed a mineral
 from the Cordillera, associated with hornblende and quartz (probably
 the same rock with that here under discussion), which cleaves like
 albite, but which is a new and distinct kind, called by him
 _Andesine._ It is allied to leucite, with the greater proportion of
 its potash replaced by lime and soda. This mineral seems scarcely
 distinguishable from albite, except by analysis.


 [7] _Geograph. Journal,_ vol. ix, p. 510.


The few still active volcanoes in Chile are confined to the central and
loftiest ranges of the Cordillera; and volcanic matter, such as appears
to have been of subaerial eruption, is everywhere rare. According to
Meyen,[8] there is a hill of pumice high up the valley of the Maypu,
and likewise a trachytic formation at Colina, a village situated north
of St. Jago. Close to this latter city, there are two hills formed of a
pale feldspathic porphyry, remarkable from being doubly columnar, great
cylindrical columns being subdivided into smaller four- or five-sided
ones; and a third hillock (Cerro Blanco) is formed of a fragmentary
mass of rock, which I believed to be of volcanic origin, intermediate
in character between the above feldspathic porphyry and common
trachyte, and containing needles of hornblende and granular oxide of
iron. Near the Baths of Cauquenes, between two short parallel lines of
elevation, where they are intersected by the valley, there is a small,
though distinct volcanic district; the rock is a dark grey (andesitic)
trachyte, which fuses into a greenish-grey bead, and is formed of long
crystals of fractured glassy albite (judging from one measurement)
mingled with well-formed crystals, often twin, of augite. The whole
mass is vesicular, but the surface is darker coloured and much more
vesicular than any other part. This trachyte forms a cliff-bounded,
horizontal, narrow strip on the steep southern side of the valley, at
the height of four or five hundred feet above the river-bed; judging
from an apparently
corresponding line of cliff on the northern side, the valley must once
have been filled up to this height by a field of lava. On the summit of
a lofty mountain some leagues higher up this same valley of the
Cachapual, I found columnar pitchstone porphyritic with feldspar; I do
not suppose this rock to be of volcanic origin, and only mention it
here, from its being intersected by masses and dikes of a _vesicular_
rock, approaching in character to trachyte; in no other part of Chile
did I observe vesicular or amygdaloidal dikes, though these are so
common in ordinary volcanic districts.

 [8] “Reise um Erde,” Th. I, ss. 338 and 362.

_Passage of the Andes by the Portillo or Pequenes Pass._

Although I crossed the Cordillera only once by this pass, and only once
by that of the Cumbre or Uspallata (presently to be described), riding
slowly and halting occasionally to ascend the mountains, there are many
circumstances favourable to obtaining a more faithful sketch of their
structure than would at first be thought possible from so short an
examination. The mountains are steep and absolutely bare of vegetation;
the atmosphere is resplendently clear; the stratification distinct; and
the rocks brightly and variously coloured: some of the natural sections
might be truly compared for distinctness to those coloured ones in
geological works. Considering how little is known of the structure of
this gigantic range, to which I particularly attended, most travellers
having collected only specimens of the rocks, I think my
sketch-sections, though necessarily imperfect, possess some interest.
Plate V sections (between  and 441) which I will now describe in
detail, is on a horizontal scale of a third of an inch to a nautical
mile, and on a vertical scale of one inch to a mile (or 6,000 feet).
The width of the range (excluding a few outlying hillocks), from the
plain on which St. Jago the capital of Chile stands, to the Pampas, is
sixty miles, as far as I can judge from the maps, which differ from
each other and are all _ exceedingly_ imperfect. The St. Jago plain at
the mouth of the Maypu, I estimate from adjoining known points at 2,300
feet, and the Pampas at 3,500 feet, both above the level of the sea.
The height of the Pequenes line, according to Dr. Gillies,[9] is 13,210
feet; and that of the Portillo line (both in the gaps where the road
crosses them) is 14,345 feet; the lowest part of the intermediate
valley of Tenuyan is 7,530 feet—all above the level of the sea.

 [9] _Journal of Nat. and Geograph. Science,_ August 1830.

The Cordillera here, and indeed I believe throughout Chile, consist of
several parallel, anticlinal and uniclinal mountain-lines, ranging
north, or north with a little westing, and south. Some exterior and
much lower ridges often vary considerably from this course, projecting
like oblique spurs from the main ranges: in the district towards the
Pacific, the mountains, as before remarked, extend in various
directions, even east and west. In the main exterior lines, the strata,
as also before remarked, are seldom inclined at a high angle; but in
the central lofty ridges they are almost always highly inclined, broken
by many
great faults, and often vertical. As far as I could judge, few of the
ranges are of great length: and in the central parts of the Cordillera,
I was frequently able to follow with my eye a ridge gradually becoming
higher and higher, as the stratification increased in inclination, from
one end where its height was trifling and its strata gently inclined to
the other end where vertical strata formed snow-clad pinnacles. Even
outside the main Cordillera, near the baths of Cauquenes, I observed
one such case, where a north and south ridge had its strata in the
valley inclined at 37°, and less than a mile south of it at 67°:
another parallel and similarly inclined ridge rose at the distance of
about five miles, into a lofty mountain with absolutely vertical
strata. Within the Cordillera, the height of the ridges and the
inclination of the strata often became doubled and trebled in much
shorter distances than five miles; this peculiar form of upheaval
probably indicates that the stratified crust was thin, and hence
yielded to the underlying intrusive masses unequally, at certain points
on the lines of fissure.

The valleys, by which the Cordillera are drained, follow the anticlinal
or rarely synclinal troughs, which deviate most from the usual north
and south course; or still more commonly those lines of faults or of
unequal curvature (that is, lines with the strata on both hands dipping
in the same direction, but at a somewhat different angle) which deviate
most from a northerly course. Occasionally the torrents run for some
distance in the north and south valleys, and then recover their eastern
or western course by bursting through the ranges at those points where
the strata have been least inclined and the height consequently is
less. Hence the valleys, along which the roads run, are generally
zigzag; and, in drawing an east and west section, it is necessary to
contract greatly that which is actually seen on the road.

Commencing at the western end of our section [Plate V] where the R.
Maypu debouches on the plain of St. Jago, we immediately enter on the
porphyritic conglomerate formation, and in the midst of it find some
hummocks [A] of granite and syenite, which probably (for I neglected to
collect specimens) belong to the andesitic class. These are succeeded
by some rugged hills [B] of dark-green, crystalline, feldspathic and in
some parts slaty rocks, which I believe belong to the altered
clay-slate formation. From this point, great mountains of purplish and
greenish, generally thinly stratified, highly porphyritic
conglomerates, including many strata of amygdaloidal and greenstone
porphyries, extend up the valley to the junction of the rivers Yeso and
Volcan. As the valley here runs in a very southerly course, the width
of the porphyritic conglomerate formation is quite conjectural; and
from the same cause, I was unable to make out much about the
stratification. In most of the exterior mountains the dip was gentle
and directed inwards; and at only one spot I observed an inclination as
high as 50°. Near the junction of the R. Colorado with the main stream,
there is a hill of whitish, brecciated, partially decomposed
feldspathic porphyry, having a volcanic aspect but not being really of
that nature: at Tolla, however, in this valley, Dr. Meyen[10] met with
a hill
of pumice containing mica. At the junction of the Yeso and Volcan [D]
there is an extensive mass, in white conical hillocks, of andesite,
containing some mica, and passing either into andesitic granite, or
into a spotted, semi-granular mixture of albitic (?) feldspar and
hornblende: in the midst of this formation Dr. Meyen found true
trachyte. The andesite is covered by strata of dark-coloured,
crystalline, obscurely porphyritic rocks, and above them by the
ordinary porphyritic conglomerates,—the strata all dipping away at a
small angle from the underlying mass. The surrounding lofty mountains
appear to be entirely composed of the porphyritic conglomerate, and I
estimated its thickness here at between six and seven thousand feet.

 [10] “Reise um Erde,” Th. I, ss. 338, 341.)

Beyond the junction of the Yeso and Volcan, the porphyritic strata
appear to dip towards the hillocks of andesite at an angle of 40°; but
at some distant points on the same ridge they are bent up and vertical.
Following the valley of the Yeso, trending N.E. (and therefore still
unfavourable for our transverse section), the same porphyritic
conglomerate formation is prolonged to near the Cuestadel Indio,
situated at the western end of the basin (like a drained lake) of Yeso.
Some way before arriving at this point, distant lofty pinnacles capped
by coloured strata belonging to the great gypseous formation could
first be seen. From the summit of the Cuesta, looking southward, there
is a magnificent sectional view of a mountain-mass, at least 2,000 feet
in thickness [E], of fine andesite granite (containing much black mica,
a little chlorite and quartz), which sends great white dikes far into
the superincumbent, dark-coloured, porphyritic conglomerates. At the
line of junction the two formations are wonderfully interlaced
together: in the lower part of the porphyritic conglomerate, the
stratification has been quite obliterated, whilst in the upper part it
is very distinct, the beds composing the crests of the surrounding
mountains being inclined at angles of between 70 and 80 degrees, and
some being even vertical. On the northern side of the valley, there is
a great corresponding mass of andesitic granite, which is encased by
porphyritic conglomerate, dipping both on the western and eastern
sides, at about 80° to west, but on the eastern side with the tips of
the strata bent in such a manner, as to render it probable that the
whole mass has been on that side thrown over and inverted.

In the valley basin of the Yeso, which I estimated at 7,000 feet above
the level of the sea, we first reach at [F] the gypseous formation. Its
thickness is very great. It consists in most parts of snow-white, hard,
compact gypsum, which breaks with a saccharine fracture, having
translucent edges; under the blowpipe gives out much vapour; it
frequently includes nests and exceedingly thin layers of crystallised,
blackish carbonate of lime. Large, irregularly shaped concretions
(externally still exhibiting lines of aqueous deposition) of
blackish-grey, but sometimes white, coarsely and brilliantly
crystallised, hard anhydrite, abound within the common gypsum.
Hillocks, formed of the hardest and purest varieties of the white
gypsum, stand up above the surrounding parts, and have their surfaces
cracked and marked, just like newly baked bread. There is much pale
brown, soft argillaceous
gypsum; and there were some intercalated green beds which I had not
time to reach. I saw only one fragment of selenite or transparent
gypsum, and that perhaps may have come from some subsequently formed
vein. From the mineralogical characters here given, it is probable that
these gypseous beds have undergone some metamorphic action. The strata
are much hidden by detritus, but they appeared in most parts to be
highly inclined; and in an adjoining lofty pinnacle they could be
distinctly seen bending up, and becoming vertical, conformably with the
underlying porphyritic conglomerate. In very many parts of the great
mountain-face [F], composed of thin gypseous beds, there were
innumerable masses, irregularly shaped and not like dikes, yet with
well-defined edges, of an imperfectly granular, pale greenish, or
yellowish-white rock, essentially composed of feldspar, with a little
chlorite or hornblende, epidote, iron-pyrites, and ferruginous powder:
I believe that these curious trappean masses have been injected from
the not far distant mountain-mass [E] of andesite whilst still fluid,
and that owing to the softness of the gypseous strata they have not
acquired the ordinary forms of dikes. Subsequently to the injection of
these feldspathic rocks, a great dislocation has taken place; and the
much shattered gypseous strata here overlie a hillock [G], composed of
vertical strata of impure limestone and of black highly calcareous
shale including threads of gypsum: these rocks, as we shall presently
see, belong to the upper parts of the gypseous series, and hence must
here have been thrown down by a vast fault.

Proceeding up the valley-basin of the Yeso, and taking our section
sometimes on one hand and sometimes on the other, we come to a great
hill of stratified porphyritic conglomerate [H] dipping at 45° to the
west; and a few hundred yards farther on, we have a bed between three
or four hundred feet thick of gypsum [I] dipping eastward at a very
high angle: here then we have a fault and anticlinal axis. On the
opposite side of the valley, a vertical mass of red conglomerate,
conformably underlying the gypsum, appears gradually to lose its
stratification and passes into a mountain of porphyry. The gypsum [I]
is covered by a bed [K], at least 1,000 feet in thickness, of a
purplish-red, compact, heavy, fine-grained sandstone or mudstone, which
fuses easily into a white enamel, and is seen under a lens to contain
triturated crystals. This is succeeded by a bed [L], 1,000 feet thick
(I believe I understate the thickness) of gypsum, exactly like the beds
before described; and this again is capped by another great bed [M] of
purplish-red sandstone. All these strata dip eastward; but the
inclination becomes less and less, as we leave the first and almost
vertical bed [I] of gypsum.

Leaving the basin-plain of Yeso, the road rapidly ascends, passing by
mountains composed of the gypseous and associated beds, with their
stratification greatly disturbed and therefore not easily intelligible:
hence this part of the section has been left uncoloured. Shortly before
reaching the great Pequenes ridge, the lowest stratum visible [N] is a
red sandstone or mudstone, capped by a vast thickness of black,
compact, calcareous, shaly rock [O], which has been thrown into four
lofty,
though small ridges: looking northward, the strata in these ridges are
seen gradually to rise in inclination, becoming in some distant
pinnacles absolutely vertical.

The ridge of Pequenes, which divides the waters flowing into the
Pacific and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E.
line; its strata dip eastward at an angle of between 30° and 45°, but
in the higher peaks bending up and becoming almost vertical. Where the
road crosses this range, the height is 13,210 feet above the sea-level,
and I estimated the neighbouring pinnacles at from fourteen to fifteen
thousand feet. The lowest stratum visible in this ridge is a red
stratified sandstone [P]; on it are superimposed two great masses [Q
and S] of black, hard, compact, even having a conchoidal fracture,
calcareous, more or less laminated shale, passing into limestone: this
rock contains organic remains, presently to be enumerated. The
compacter varieties fuse easily in a white glass; and this I may add is
a very general character with all the sedimentary beds in the
Cordillera: although this rock when broken is generally quite black, it
everywhere weathers into an ash-grey tint. Between these two great
masses [Q and S], a bed [R] of gypsum is interposed, about three
hundred feet in thickness, and having the same characters as heretofore
described. I estimated the total thickness of these three beds [Q, R,
S] at nearly three thousand feet; and to this must be added, as will be
immediately seen, a great overlying mass of red sandstone.

In descending the eastern slope of this great central range, the
strata, which in the upper part dip eastward at about an angle of 40°,
become more and more curved, till they are nearly vertical; and a
little further onwards there is seen on the further side of a ravine, a
thick mass of strata of bright red sandstone [T], with their upper
extremities slightly curved, showing that they were once conformably
prolonged over the beds [S]: on the southern and opposite side of the
road, this red sandstone and the underlying black shaly rocks stand
vertical, and in actual juxtaposition. Continuing to descend, we come
to a synclinal valley filled with rubbish, beyond which we have the red
sandstone [T2] corresponding with [T], and now dipping, as is seen both
north and south of the road, at 45° to the west; and under it, the beds
[S2, R2, Q2, and I believe P2] in corresponding order and of similar
composition, with those on the western flank of the Pequenes range, but
dipping westward. Close to the synclinal valley the dip of these strata
is 45°, but at the eastern or farther end of the series it increases to
60°. Here the great gypseous formation abruptly terminates, and is
succeeded eastward by a pile of more modern strata. Considering how
violently these central ranges have been dislocated, and how very
numerous dikes are in the exterior and lower parts of the Cordillera,
it is remarkable that I did not here notice a single dike. The
prevailing rock in this neighbourhood is the black, calcareous, compact
shale, whilst in the valley-basin of the Yeso the purplish red
sandstone or mudstone predominates,—both being associated with gypseous
strata of exactly the same nature. It would be very difficult to
ascertain the relative superposition of these several masses, for we
shall afterwards see in the Cumbre Pass that
the gypseous and intercalated beds are lens-shaped, and that they thin
out, even where very thick, and disappear in short horizontal
distances: it is quite possible that the black shales and red
sandstones may be contemporaneous, but it is more probable that the
former compose the uppermost parts of the series.

The fossils above alluded to in the black calcareous shales are few in
number, and are in an imperfect condition; they consist, as named for
me by M. d’Orbigny, of:—

Ammonite, indeterminable, near to _A. recticostatus,_ d’Orbigny, “Pal.
Franc.” (Neocomian formation).

Gryphæa, near to _G. Couloni_ (Neocomian formations of France and
Neufchâtel).

Natica, indeterminable.

Cyprina rostrata, d’Orbigny, “Pal. Franc.” (Neocomian formation).

Rostellaria angulosa (?), d’Orbigny, “Pal. de l’Amér. Mer.”

Terebratula (?).


Some of the fragments of Ammonites were as thick as a man’s arm: the
Gryphæa is much the most abundant shell. These fossils M. d’Orbigny
considers as belonging to the Neocomian stage of the Cretaceous system.
Dr. Meyen,[11] who ascended the valley of the Rio Volcan, a branch of
the Yeso, found a nearly similar, but apparently more calcareous
formation, with much gypsum, and no doubt the equivalent of that here
described: the beds were vertical, and were prolonged up to the limits
of perpetual snow; at the height of 9,000 feet above the sea, they
abounded with fossils, consisting, according to Von Buch,[12] of:—

Exogyra (Gryphæa) Couloni, absolutely identical with specimens from the
Jura and South of France.

Trigonia costata, identical with those found in the upper Jurassic beds
at Hildesheim.

Pecten striatus, identical with those found in the upper Jurassic beds
at Hildesheim.

Cucullæa, corresponding in form to _C. longirostris,_ so frequent in
the upper Jurassic beds of Westphalia.

Ammonites resembling _A. biplex._


 [11] “Reise um Erde,” etc., Th. I, s. 355.


 [12] “Descript. Phys. des Iles Canaries,” p. 471.

Von Buch concludes that this formation is intermediate between the
limestone of the Jura and the chalk, and that it is analogous with the
uppermost Jurassic beds forming the plains of Switzerland. Hence M.
D’Orbigny and Von Buch, under different terms, compare these fossils to
those from the same late stage in the secondary formations of Europe.

Some of the fossils which I collected were found a good way down the
western slope of the main ridge, and hence must originally have been
covered up by a great thickness of the black shaly rock, independently
of the now denuded, thick, overlying masses of red sandstone. I
neglected at the time to estimate how many hundred or rather thousand
feet thick the superincumbent strata must have been: and I will not now
attempt to do so. This, however, would have been a highly
interesting point, as indicative of a great amount of subsidence, of
hich we shall hereafter find in other parts of the Cordillera analogous
evidence during this same period. The altitude of the Peuquenes Range,
considering its not great antiquity, is very remarkable; many of the
fossils were embedded at the height of 13,210 feet, and the same beds
are prolonged up to at least from fourteen to fifteen thousand feet
above the level of the sea.

_The Portillo or Eastern Chain._—The valley of Tenuyan, separating the
Peuquenes and Portillo lines, is, as estimated by Dr. Gillies and
myself, about twenty miles in width; the lowest part, where the road
crosses the river, being 7,500 feet above the sea-level. The pass on
the Portillo line is 14,365 feet high (1,100 feet higher than that on
the Peuquenes), and the neighbouring pinnacles must, I conceive, rise
to nearly 16,000 feet above the sea. The river draining the
intermediate valley of Tenuyan, passes through the Portillo line. To
return to our section:—shortly after leaving the lower beds [P2] of the
gypseous formation, we come to grand masses of a coarse, red
conglomerate [V], totally unlike any strata hitherto seen in the
Cordillera. This conglomerate is distinctly stratified, some of the
beds being well defined by the greater size of the pebbles: the cement
is calcareous and sometimes crystalline, though the mass shows no signs
of having been metamorphosed. The included pebbles are either perfectly
or only partially rounded: they consist of purplish sandstones, of
various porphyries, of brownish limestone, of black calcareous, compact
shale precisely like that in situ in the Peuquenes range, and
_containing some of the same fossil shells_; also very many pebbles of
quartz, some of micaceous schist, and numerous, broken, rounded
crystals of a reddish orthitic or potash feldspar (as determined by
Professor Miller), and these from their size must have been derived
from a coarse-grained rock, probably granite. From this feldspar being
orthitic, and even from its external appearance, I venture positively
to affirm that it has not been derived from the rocks of the western
ranges; but, on the other hand, it may well have come, together with
the quartz and metamorphic schists, from the eastern or Portillo line,
for this line mainly consists of coarse orthitic granite. The pebbles
of the fossiliferous slate and of the purple sandstone, certainly have
been derived from the Peuquenes or western ranges.

The road crosses the valley of Tenuyan in a nearly east and west line,
and for several miles we have on both hands the conglomerate,
everywhere dipping west and forming separate great mountains. The
strata, where first met with, after leaving the gypseous formation, are
inclined westward at an angle of only 20°, which further on increases
to about 45°. The gypseous strata, as we have seen, are also inclined
westward: hence, when looking from the eastern side of the valley
towards the Peuquenes range, a most deceptive appearance is presented,
as if the newer beds of conglomerate dipped directly under the much
older beds of the gypseous formation. In the middle of the valley, a
bold mountain of unstratified lilac-coloured porphyry (with crystals of
hornblende) projects; and further on, a little south of the road, there
is another mountain, with its strata inclined at a small angle
eastwards,
which in its general aspect and colour, resembles the porphyritic
conglomerate formation, so rare on this side of the Peuquenes line and
so grandly developed throughout the western ranges.

The conglomerate is of great thickness: I do not suppose that the
strata forming the separate mountain-masses [V, V, V] have ever been
prolonged over each other, but that one mass has been broken up by
several, distinct, parallel, uniclinal lines of elevation. Judging
therefore of the thickness of the conglomerate, as seen in the separate
mountain-masses, I estimated it at least from one thousand five hundred
to two thousand feet. The lower beds rest conformably on some
singularly coloured, soft strata [W], which I could not reach to
examine; and these again rest conformably on a thick mass of micaceous,
thinly laminated, siliceous sandstone [X], associated with a little
black clay-slate. These lower beds are traversed by several dikes of
decomposing porphyry. The laminated sandstone is directly superimposed
on the vast masses of granite [Y, Y] which mainly compose the Portillo
range. The line of junction between this latter rock, which is of a
bright red colour, and the whitish sandstone was beautifully distinct;
the sandstone being penetrated by numerous, great, tortuous dikes
branching from the granite, and having been converted into a granular
quartz rock (singularly like that of the Falkland Islands), containing
specks of an ochrey powder, and black crystalline atoms, apparently of
imperfect mica. The quartzose strata in one spot were folded into a
regular dome.

The granite which composes the magnificent bare pinnacles and the steep
western flank of the Portillo chain, is of a brick-red colour, coarsely
crystallised, and composed of orthitic or potash feldspar, quartz, and
imperfect mica in small quantity, sometimes passing into chlorite.
These minerals occasionally assume a laminar or foliated arrangement.
The fact of the feldspar being orthitic in this range, is very
remarkable, considering how rare, or rather, as I believe, entirely
absent, this mineral is throughout the western ranges, in which
soda-feldspar, or at least a variety cleaving like albite, is so
extremely abundant. In one spot on the western flank, and on the
eastern flank near Los Manantiales and near the crest, I noticed some
great masses of a whitish granite, parts of it fine- grained, and parts
containing large crystals of feldspar; I neglected to collect
specimens, so I do not know whether this feldspar is also orthitic,
though I am inclined to think so from its general appearance. I saw
also some syenite and one mass which resembled andesite, but of which I
likewise neglected to collect specimens. From the manner in which the
whitish granites formed separate mountain-masses in the midst of the
brick-red variety, and from one such mass near the crest being
traversed by numerous veins of flesh-coloured and greenish eurite (into
which I occasionally observed the brick-red granite insensibly
passing), I conclude that the white granites probably belong to an
older formation, almost overwhelmed and penetrated by the red granite.

On the crest I saw also, at a short distance, some coloured stratified
beds, apparently like those [W] at the western base, but was prevented
examining them by a snowstorm: Mr. Caldcleugh,[13] however, collected
here specimens of ribboned jasper, magnesian limestone, and other
minerals. A little way down the eastern slope a few fragments of quartz
and mica-slate are met with; but the great formation of this latter
rock [Z], which covers up much of the eastern flank and base of the
Portillo range, cannot be conveniently examined until much lower down
at a place called Mal Paso. The mica-schist here consists of thick
layers of quartz, with intervening folia of finely-scaly mica, often
passing into a substance like black glossy clay-slate: in one spot, the
layers of the quartz having disappeared, the whole mass became
converted into glossy clay-slate. Where the folia were best defined,
they were inclined at a high angle westward, that is, towards the
range. The line of junction between the dark mica-slate and the coarse
red granite was most clearly distinguishable from a vast distance: the
granite sent many small veins into the mica-slate, and included some
angular fragments of it. As the sandstone on the western base has been
converted by the red granite into a granular quartz-rock, so this great
formation of mica-schist may possibly have been metamorphosed at the
same time and by the same means; but I think it more probable,
considering its more perfect metamorphic character and its
well-pronounced foliation, that it belongs to an anterior epoch,
connected with the white granites: I am the more inclined to this view,
from having found at the foot of the range the mica-schist surrounding
a hummock [Y2], exclusively composed of white granite. Near Los
Arenales, the mountains on all sides are composed of the mica-slate;
and looking backwards from this point up to the bare gigantic peaks
above, the view was eminently interesting. The colours of the red
granite and the black mica-slate are so distinct, that with a bright
light these rocks could be readily distinguished even from the Pampas,
at a level of at least 9,000 feet below. The red granite, from being
divided by parallel joints, has weathered into sharp pinnacles, on some
of which, even on some of the loftiest, little caps of mica-schist
could be clearly seen: here and there isolated patches of this rock
adhered to the mountain-flanks, and these often corresponded in height
and position on the opposite sides of the immense valleys. Lower down
the schist prevailed more and more, with only a few quite small points
of granite projecting through. Looking at the entire eastern face of
the Portillo range, the red colour far exceeds in area the black; yet
it was scarcely possible to doubt that the granite had once been almost
wholly encased by the mica-schist.

 [13] “Travels,” etc., vol. i, p. 308.


At Los Arenales, low down on the eastern flank, the mica-slate is
traversed by several closely adjoining, broad dikes, parallel to each
other and to the foliation of the schist. The dikes are formed of three
different varieties of rock, of which a pale brown feldspathic porphyry
with grains of quartz was much the most abundant. These dikes with
their granules of quartz, as well as the mica-schist itself, strikingly
resemble the rocks of the Chonos Archipelago. At a height of about
twelve hundred feet above the dikes, and perhaps connected with them,
there is a range of cliffs formed of successive lava-streams [AA],
between three and four hundred feet in thickness, and in places finely
columnar. The lava consists of dark-greyish, harsh rocks, intermediate
in character between trachyte and basalt, containing glassy feldspar,
olivine, and a little mica, and sometimes amygdaloidal with zeolite:
the basis is either quite compact, or crenulated with air-vesicles
arranged in laminæ. The streams are separated from each other by beds
of fragmentary brown scoriæ, firmly cemented together, and including a
few well-rounded pebbles of lava. From their general appearance, I
suspect that these lava-streams flowed at an ancient period under the
pressure of the sea, when the Atlantic covered the Pampas and washed
the eastern foot of the Cordillera.[14] On the opposite and northern
side of the valley there is another line of lava-cliffs at a
corresponding height; the valley between being of considerable breadth,
and as nearly as I could estimate 1,500 feet in depth. This field of
lava is confined on both sides by the mountains of mica-schist, and
slopes down rapidly but irregularly to the edge of the Pampas, where,
having a thickness of about two hundred feet, it terminates against a
little range of claystone porphyry. The valley in this lower part
expands into a bay-like, gentle slope, bordered by the cliffs of lava,
which must certainly once have extended across this wide expanse. The
inclination of the streams from Los Arenales to the mouth of the valley
is so great, that at the time (though ignorant of M. Elie de Beaumont’s
researches on the extremely small slope over which lava can flow, and
yet retain a compact structure and considerable thickness) I concluded
that they must subsequently to their flowing have been upheaved and
tilted from the mountains; of this conclusion I can now entertain not
the smallest doubt.

 [14] This conclusion might, perhaps, even have been anticipated, from
 the general rarity of volcanic action, except near the sea or large
 bodies of water. Conformably with this rule, at the present day, there
 are no active volcanoes on this eastern side of the Cordillera; nor
 are severe earthquakes experienced here.

At the mouth of the valley, within the cliffs of the above lava-field,
there are remnants, in the form of separate small hillocks and of lines
of low cliffs, of a considerable deposit of compact white tuff
(quarried for filtering-stones), composed of broken pumice, volcanic
crystals, scales of mica, and fragments of lava. This mass has suffered
much denudation; and the hard mica-schist has been deeply worn, since
the period of its deposition; and this period must have been subsequent
to the denudation of the basaltic lava-streams, as attested by their
encircling cliffs standing at a higher level. At the present day, under
the existing arid climate, ages might roll past without a square yard
of rock of any kind being denuded, except perhaps in the rarely
moistened drainage-channel of the valley. Must we then look back to
that ancient period, when the waves of the sea beat against the eastern
foot of the Cordillera, for a power sufficient to denude extensively,
though superficially, this tufaceous deposit, soft although it be?

There remains only to mention some little water-worn hillocks [BB],
a few hundred feet in height, and mere mole-hills compared with the
gigantic mountains behind them, which rise out of the sloping,
shingle-covered margin of the Pampas. The first little range is
composed of a brecciated purple porphyritic claystone, with obscurely
marked strata dipping at 70° to the S.W.; the other ranges consist of—a
pale-coloured feldspathic porphyry,—a purple claystone porphyry with
grains of quartz,—and a rock almost exclusively composed of brick-red
crystals of feldspar. These outermost small lines of elevation extend
in a N.W. by W. and S.E. by S. direction.

_Concluding remarks on the Portillo range._—When on the Pampas and
looking southward, and whilst travelling northward, I could see for
very many leagues the red granite and dark mica-schist forming the
crest and eastern flank of the Portillo line. This great range,
according to Dr. Gillies, can be traced with little interruption for
140 miles southward to the R. Diamante, where it unites with the
western ranges: northward, according to this same author, it terminates
where the R. Mendoza debouches from the mountains; but a little further
north in the eastern part of the Cumbre section, there are, as we shall
hereafter see, some mountain-masses of a brick-red porphyry, the last
injected amidst many other porphyries, and having so close an analogy
with the coarse red granite of the Portillo line, that I am tempted to
believe that they belong to the same axis of injection; if so, the
Portillo line is at least 200 miles in length. Its height, even in the
lowest gap in the road, is 14,365 feet, and some of the pinnacles
apparently attain an elevation of about 16,000 feet above the sea. The
geological history of this grand chain appears to me eminently
interesting. We may safely conclude, that at a former period the valley
of Tenuyan existed as an arm of the sea, about twenty-miles in width,
bordered on one hand by a ridge or chain of islets of the black
calcareous shales and purple sandstones of the gypseous formation; and
on the other hand, by a ridge or chain of islets composed of
mica-slate, white granite, and perhaps to a partial extent of red
granite. These two chains, whilst thus bordering the old sea-channel,
must have been exposed for a vast lapse of time to alluvial and
littoral action, during which the rocks were shattered, the fragments
rounded, and the strata of conglomerate accumulated to a thickness of
at least fifteen hundred or two thousand feet. The red orthitic granite
now forms, as we have seen, the main part of the Portillo chain: it is
injected in dikes not only into the mica-schist and white granites, but
into the laminated sandstone, which it has metamorphosed, and which it
has thrown off, together with the conformably overlying coloured beds
and stratified conglomerate, at an angle of forty-five degrees. To have
thrown off so vast a pile of strata at this angle, is a proof that the
main part of the red granite (whether or not portions, as perhaps is
probable, previously existed) was injected in a liquified state after
the accumulation both of the laminated sandstone and of the
conglomerate; this conglomerate, we know, was accumulated, not only
after the deposition of the fossiliferous strata of the Peuquenes line,
but after their elevation and long-continued denudation: and these
fossiliferous strata belong to the early part of the Cretaceous system.
Late, therefore, in a geological sense, as must be the age of the main
part of the red granite, I can conceive nothing more impressive than
the eastern view of this great range, as forcing the mind to grapple
with the idea of the thousands of thousands of years requisite for the
denudation of the strata which originally encased it,—for that the
fluidified granite was once encased, its mineralogical composition and
structure, and the bold conical shape of the mountain-masses, yield
sufficient evidence. Of the encasing strata we see the last vestiges in
the coloured beds on the crest, in the little caps of mica-schist on
some of the loftiest pinnacles, and in the isolated patches of this
same rock at corresponding heights on the now bare and steep flanks.

The lava-streams at the eastern foot of the Portillo are interesting,
not so much from the great denudation which they have suffered at a
comparatively late period as from the evidence they afford by their
inclination taken conjointly with their thickness and compactness, that
after the great range had assumed its present general outline, it
continued to rise as an axis of elevation. The plains extending from
the base of the Cordillera to the Atlantic show that the continent has
been upraised in mass to a height of 3,500 feet, and probably to a much
greater height, for the smooth shingle-covered margin of the Pampas is
prolonged in a gentle unbroken slope far up many of the great valleys.
Nor let it be assumed that the Peuquenes and Portillo ranges have
undergone only movements of elevation; for we shall hereafter see, that
the bottom of the sea subsided several thousand feet during the
deposition of strata, occupying the same relative place in the
Cordillera, with those of the Peuquenes ridge; moreover, we shall see
from the unequivocal evidence of buried upright trees, that at a
somewhat later period, during the formation of the Uspallata chain,
which corresponds geographically with that of the Portillo, there was
another subsidence of many thousand feet: here, indeed, in the valley
of Tenuyan, the accumulation of the coarse stratified conglomerate to a
thickness of fifteen hundred or two thousand feet, offers strong
presumptive evidence of subsidence; for all existing analogies lead to
the belief that large pebbles can be transported only in shallow water,
liable to be affected by currents and movements of undulation—and if
so, the shallow bed of the sea on which the pebbles were first
deposited must necessarily have sunk to allow of the accumulation of
the superincumbent strata. What a history of changes of level, and of
wear and tear, all since the age of the latter secondary formations of
Europe, does the structure of this one great mountain-chain reveal!

_Passage of the Andes by the Cumbre or Uspallata Pass._

This Pass crosses the Andes about sixty miles north of that just
described: the section given in Plate V, Section 1/2, [see map page
440] is on the same scale as before, namely, at one-third of an inch to
a mile in distance, and one inch to a mile (or 6,000 feet) in height.
Like the last section, it is a mere sketch, and cannot pretend to
accuracy, though made under favourable circumstances. We will commence
as before, with the
western half, of which the main range bears the name of the Cumbre
(that is the Ridge), and corresponds to the Peuquenes line in the
former section; as does the Uspallata range, though on a much smaller
scale, to that of the Portillo. Near the point where the river
Aconcagua debouches on the basin plain of the same name, at a height of
about two thousand three hundred feet above the sea, we meet with the
usual purple and greenish porphyritic claystone conglomerate. Beds of
this nature, alternating with numerous compact and amygdaloidal
porphyries, which have flowed as submarine lavas, and associated with
great mountain-masses of various, injected, non-stratified porphyries,
are prolonged the whole distance up to the Cumbre or central ridge. One
of the commonest stratified porphyries is of a green colour, highly
amygdaloidal with the various minerals described in the preliminary
discussion, and including fine tabular crystals of albite. The
mountain-range north (often with a little westing) and south. The
stratification, wherever I could clearly distinguish it, was inclined
westward or towards the Pacific, and, except near the Cumbre, seldom at
angles above 25°. Only at one spot on this western side, on a lofty
pinnacle not far from the Cumbre, I saw strata apparently belonging to
the gypseous formation, and conformably capping a pile of stratified
porphyries. Hence, both in composition and in stratification, the
structure of the mountains on this western side of the divortium
aquarum, is far more simple than in the corresponding part of the
Peuquenes section. In the porphyritic claystone conglomerate, the
mechanical structure and the planes of stratification have generally
been much obscured and even quite obliterated towards the base of the
series, whilst in the upper parts, near the summits of the mountains,
both are distinctly displayed. In these upper portions the porphyries
are generally lighter coloured. In three places [X, Y, Z] masses of
andesite are exposed: at [Y], this rock contained some quartz, but the
greater part consisted of andesitic porphyry, with only a few
well-developed crystals of albite, and forming a great white mass,
having the external aspect of granite, capped by much dark unstratified
porphyry. In many parts of the mountains, there are dikes of a green
colour, and other white ones, which latter probably spring from
underlying masses of andesite.

The Cumbre, where the road crosses it, is, according to Mr. Pentland,
12,454 feet above the sea; and the neighbouring peaks, composed of dark
purple and whitish porphyries, some obscurely stratified with a
westerly dip, and others without a trace of stratification, must exceed
13,000 feet in height. Descending the eastern slope of the Cumbre, the
structure becomes very complicated, and generally differs on the two
sides of the east and west line of road and section. First we come to a
great mass [A] of nearly vertical, singularly contorted strata,
composed of highly compact red sandstones, and of often calcareous
conglomerates, and penetrated by green, yellow, and reddish dikes; but
I shall presently have an opportunity of describing in some detail an
analogous pile of strata. These vertical beds are abruptly succeeded by
others [B], of apparently nearly the same nature but more
metamorphosed,
alternating with porphyries and limestones; these dip for a short space
westward, but there has been here an extraordinary dislocation, which,
on the north side of the road, appears to have determined the
excavation of the north and south valley of the R. de las Cuevas. On
this northern side of the road, the strata [B] are prolonged till they
come in close contact with a jagged lofty mountain [D] of
dark-coloured, unstratified, intrusive porphyry, where the beds have
been more highly inclined and still more metamorphosed. This mountain
of porphyry seems to form a short axis of elevation, for south of the
road in its line there is a hill [C] of porphyritic conglomerate with
absolutely vertical strata.

We now come to the gypseous formation: I will first describe the
structure of the several mountains, and then give in one section a
detailed account of the nature of the rocks. On the north side of the
road, which here runs in an east and west valley, the mountain of
porphyry [D] is succeeded by a hill [E] formed of the upper gypseous
strata tilted, at an angle of between 70° and 80° to the west, by a
uniclinal axis of elevation which does not run parallel to the other
neighbouring ranges, and which is of short length; for on the south
side of the valley its prolongation is marked only by a small flexure
in a pile of strata inclined by a quite separate axis. A little further
on the north and south valley of Horcones enters at right angles our
line of section; its western side is bounded by a hill of gypseous
strata [F] dipping westward at about 45°, and its eastern side by a
mountain of similar strata [G] inclined westward at 70°, and
superimposed by an oblique fault on another mass of the same strata
[H], also inclined westward, but at an angle of about 30°: the
complicated relation of these three masses [F, G, H] is explained by
the structure of a great mountain-range lying some way to the north, in
which a regular anticlinal axis (represented in the section by dotted
lines) is seen, with the strata on its eastern side again bending up
and forming a distinct uniclinal axis, of which the beds marked [H]
form the lower part. This great uniclinal line is intersected, near the
Puente del Inca, by the valley along which the road runs, and the
strata composing it will be immediately described. On the south side of
the road, in the space corresponding with the mountains [E, F, and G],
the strata everywhere dip westward generally at an angle of 30°,
occasionally mounting up to 45°, but not in an unbroken line, for there
are several vertical faults, forming separate uniclinal masses, all
dipping in the same direction,—a form of elevation common in the
Cordillera. We thus see that within a narrow space, the gypseous strata
have been upheaved and crushed together by a great uniclinal,
anticlinal, and one lesser uniclinal line [E] of elevation; and that
between these three lines and the Cumbre, in the sandstones,
conglomerates and porphyritic formation, there have been at least two
or three other great elevatory axes.

The uniclinal axis [I] intersected near the Puente del Inca[15] (of
which
the strata at [H] form a part) ranges N. by W. and S. by E., forming a
chain of mountains, apparently little inferior in height to the Cumbre:
the strata, as we have seen, dip at an average angle of 30° to the
west. The flanks of the mountains are here quite bare and steep,
affording an excellent section; so that I was able to inspect the
strata to a thickness of about 4,000 feet, and could clearly
distinguish their general nature for 1,000 feet higher, making a total
thickness of 5,000 feet, to which must be added about 1,000 feet of the
inferior strata seen a little lower down the valley, I will describe
this one section in detail, beginning at the bottom.

 [15] At this place, there are some hot and cold springs, the warmest
 having a temperature, according to Lieutenant Brand (“Travels,” p.
 240), of 91°; they emit much gas. According to Mr. Brande, of the
 Royal Institution, ten cubical inches contain forty-five grains of
 solid matter, consisting chiefly of salt, gypsum, carbonate of lime,
 and oxide of iron. The water is charged with carbonic acid and
 sulphuretted hydrogen. These springs deposit much tufa in the form of
 spherical balls. They burst forth, as do those of Cauquenes, and
 probably those of Villa Vicencio, on a line of elevation.

1st. The lowest mass is the altered clay-slate described in the
preliminary discussion, and which in this line of section was here
first met with. Lower down the valley, at the R. de las Vacas, I had a
better opportunity of examining it; it is there in some parts well
characterised, having a distinct, nearly vertical, tortuous cleavage,
ranging N.W. and S.E., and intersected by quartz veins: in most parts,
however, it is crystalline and feldspathic, and passes into a true
greenstone often including grains of quartz. The clay-slate, in its
upper half, is frequently brecciated, the embedded angular fragments
being of nearly the same nature with the paste.

2nd. Several strata of purplish porphyritic conglomerate, of no very
great thickness, rest conformably upon the feldspathic slate. A thick
bed of fine, purple, claystone porphyry, obscurely brecciated (but not
of metamorphosed sedimentary origin), and capped by porphyritic
conglomerate, was the lowest bed actually examined in this section at
the Puente del Inca.

3rd. A stratum, eighty feet thick, of hard and very compact impure
whitish limestone, weathering bright red, with included layers
brecciated and recemented. Obscure marks of shell are distinguishable
in it.

4th. A red, quartzose, fine-grained conglomerate, with grains of
quartz, and with patches of white earthy feldspar, apparently due to
some process of concretionary crystalline action; this bed is more
compact and metamorphosed than any of the overlying conglomerates.

5th. A whitish cherty limestone, with nodules of bluish argillaceous
limestone.

6th. A white conglomerate, with many particles of quartz, almost
blending into the paste.

7th. Highly siliceous, fine-grained white sandstone.

8th and 9th. Red and white beds not examined.

10th. Yellow, fine-grained, thinly stratified, magnesian (judging from
its slow dissolution in acids) limestone: it includes some white quartz
pebbles, and little cavities, lined with calcareous spar, some
retaining the form of shells.


11th. A bed between twenty and thirty feet thick, quite conformable
with the underlying ones, composed of a hard basis, tinged lilac-grey
porphyritic with _numerous_ crystals of whitish feldspar, with black
mica and little spots of soft ferruginous matter: evidently a submarine
lava.

12th. Yellow magnesian limestone, as before, part-stained purple.

13th. A most singular rock; basis purplish grey, obscurely crystalline,
easily fusible into a dark green glass, not hard, thickly speckled with
crystals more or less perfect of white carbonate of lime, of red
hydrous oxide of iron, of a white and transparent mineral like
analcime, and of a green opaque mineral like soap-stone; the basis is
moreover amygdaloidal with many spherical balls of white crystallised
carbonate of lime, of which some are coated with the red oxide of iron.
I have no doubt, from the examination of a superincumbent stratum (19),
that this is a submarine lava; though in Northern Chile, some of the
metamorphosed sedimentary beds are almost as crystalline, and of as
varied composition.

14th. Red sandstone, passing in the upper part into a coarse, hard, red
conglomerate, 300 feet thick, having a calcareous cement, and including
grains of quartz and broken crystals of feldspar; basis infusible; the
pebbles consist of dull purplish porphyries, with some of quartz, from
the size of a nut to a man’s head. This is the coarsest conglomerate in
this part of the Cordillera: in the middle there was a white layer not
examined.

15th. Grand thick bed, of a very hard, yellowish-white rock, with a
crystalline feldspathic base, including large crystals of white
feldspar, many little cavities mostly full of soft ferruginous matter,
and numerous hexagonal plates of black mica. The upper part of this
great bed is slightly cellular; the lower part compact: the thickness
varied a little in different parts. Manifestly a submarine lava; and is
allied to bed 11.

16th and 17th. Dull purplish, calcareous, fine-grained, compact
sandstones, which pass into coarse white conglomerates with numerous
particles of quartz.

18th. Several alternations of red conglomerate, purplish sandstone, and
submarine lava, like that singular rock forming bed 13.

19th. A very heavy, compact, greenish-black stone, with a fine-grained
obviously crystalline basis, containing a few specks of white
calcareous spar, many specks of the crystallised hydrous red oxide of
iron, and some specks of a green mineral; there are veins and nests
filled with epidote: certainly a submarine lava.

20th. Many thin strata of compact, fine-grained, pale purple sandstone.

21st. Gypsum in a nearly pure state, about three hundred feet in
thickness: this bed, in its concretions of anhydrite and layers of
small blackish crystals of carbonate of lime, exactly resembles the
great gypseous beds in the Peuquenes range.

22nd. Pale purple and reddish sandstone, as in bed 20: about three
hundred feet in thickness.

23rd. A thick mass composed of layers, often as thin as paper and
convoluted, of pure gypsum with others very impure, of a purplish
colour.

24th. Pure gypsum, thick mass.

25th. Red sandstones, of great thickness.

26th. Pure gypsum, of great thickness.

27th. Alternating layers of pure and impure gypsum, of great thickness.

I was not able to ascend to these few last great strata, which compose
the neighbouring loftiest pinnacles. The thickness, from the lowest to
the uppermost bed of gypsum, cannot be less than 2,000 feet: the beds
beneath I estimated at 3,000 feet, and this does not include either the
lower parts of the porphyritic conglomerate, or the altered clay-slate;
I conceive the total thickness must be about six thousand feet. I
distinctly observed that not only the gypsum, but the alternating
sandstones and conglomerates were lens-shaped, and repeatedly thinned
out and replaced each other: thus in the distance of about a mile, a
bed 300 feet thick of sandstone between two beds of gypsum, thinned out
to nothing and disappeared. The lower part of this section differs
remarkably,—in the much greater diversity of its mineralogical
composition,—in the abundance of calcareous matter,—in the greater
coarseness of some of the conglomerates,—and in the numerous particles
and well-rounded pebbles, sometimes of large size, of quartz,— from any
other section hitherto described in Chile. From these peculiarities and
from the lens-form of the strata, it is probable that this great pile
of strata was accumulated on a shallow and very uneven bottom, near
some pre-existing land formed of various porphyries and quartz-rock.
The formation of porphyritic claystone conglomerate does not in this
section attain nearly its ordinary thickness; this may be PARTLY
attributed to the metamorphic action having been here much less
energetic than usual, though the lower beds have been affected to a
certain degree. If it had been as energetic as in most other parts of
Chile, many of the beds of sandstone and conglomerate, containing
rounded masses of porphyry, would doubtless have been converted into
porphyritic conglomerate; and these would have alternated with, and
even blended into, crystalline and porphyritic strata without a trace
of mechanical structure,—namely, into those which, in the present state
of the section, we see are unquestionably submarine lavas.

The beds of gypsum, together with the red alternating sandstones and
conglomerates, present so perfect and curious a resemblance with those
seen in our former section in the basin-valley of Yeso, that I cannot
doubt the identity of the two formations: I may add, that a little
westward of the P. del Inca, a mass of gypsum passed into a
fine-grained, hard, brown sandstone, which contained some layers of
black, calcareous, compact, shaly rock, precisely like that seen in
such vast masses on the Peuquenes range.

Near the Puente del Inca, numerous fragments of limestone, containing
some fossil remains, were scattered on the ground: these fragments so
perfectly resemble the limestone of bed No. 3, in which I saw
impressions of shells, that I have no doubt they have fallen from it.


The yellow magnesian limestone of bed No. 10, which also includes
traces of shells, has a different appearance. These fossils (as named
by M. d’Orbigny) consist of:—
    Gryphæa, near to _G. Couloni_ (Neocomian formation).
    Arca, perhaps _A. Gabrielis,_ d’Orbigny, “Pal. Franc.” (Neocomian
    formation).

Mr. Pentland made a collection of shells from this same spot, and Von
Buch[16] considers them as consisting of:—
    Trigonia, resembling in form _T. costata._
    Pholadomya, like one found by M. Dufresnoy near Alencon.
    Isocardi excentrica, Voltz., identical with that from the Jura.

 [16] “Description Phys. des Iles Can.,” p. 472.

Two of these shells, namely, the Gryphæa and Trigonia, appear to be
identical with species collected by Meyen and myself on the Peuquenes
range; and in the opinion of Von Buch and M. d’Orbigny, the two
formations belong to the same age. I must here add, that Professor E.
Forbes, who has examined my specimens from this place and from the
Peuquenes range, has likewise a strong impression that they indicate
the Cretaceous period, and probably an early epoch in it: so that all
the palæontologists who have seen these fossils nearly coincide in
opinion regarding their age. The limestone, however, with these fossils
here lies at the very base of the formation, just above the porphyritic
conglomerate, and certainly several thousand feet lower in the series,
than the equivalent, fossiliferous, black, shaly rocks high up on the
Peuquenes range.

It is well worthy of remark that these shells, or at least those of
which I saw impressions in the limestone (bed No. 3), must have been
covered up, on the _least_ computation, by 4,000 feet of strata: now we
know from Professor E. Forbes’s researches, that the sea at greater
depths than 600 feet becomes exceedingly barren of organic beings,—a
result quite in accordance with what little I have seen of deep-sea
soundings. Hence, after this limestone with its shells was deposited,
the bottom of the sea where the main line of the Cordillera now stands,
must have subsided some thousand feet to allow of the deposition of the
superincumbent submarine strata. Without supposing a movement of this
kind, it would, moreover, be impossible to understand the accumulation
of the several lower strata of _coarse,_ well-rounded conglomerates,
which it is scarcely possible to believe were spread out in profoundly
deep water, and which, especially those containing pebbles of quartz,
could hardly have been rounded in submarine craters and afterwards
ejected from them, as I believe to have been the case with much of the
porphyritic conglomerate formation. I may add that, in Professor
Forbes’s opinion, the above-enumerated species of mollusca probably did
not live at a much greater depth than twenty fathoms, that is only 120
feet.

To return to our section down the valley; standing on the great N. by
W. and S. by E. uniclinal axis of the Puente del Inca, of which a
section has just been given, and looking north-east, greater tabular
masses of gypseous formation (KK) could be seen in the distance, very
slightly inclined towards the east. Lower down the valley, the
mountains are almost exclusively composed of porphyries, many of them
of intrusive origin and non-stratified, others stratified, but with the
stratification seldom distinguishable except in the upper parts.
Disregarding local disturbances, the beds are either horizontal or
inclined at a small angle eastwards: hence, when standing on the plain
of Uspallata and looking to the west or backwards, the Cordillera
appear composed of huge, square, nearly horizontal, tabular masses: so
wide a space, with such lofty mountains so equably elevated, is rarely
met with within the Cordillera. In this line of section, the interval
between the Puente del Inca and the neighbourhood of the Cumbre,
includes all the chief axes of dislocation.

The altered clay-slate formation, already described, is seen in several
parts of the valley as far down as Las Vacas, underlying the
porphyritic conglomerate. At the Casa de Pujios [L], there is a hummock
of (andesitic?) granite; and the stratification of the surrounding
mountains here changes from W. by S. to S.W. Again, near the R. Vacas
there is a larger formation of (andesitic?) granite [M], which sends a
meshwork of veins into the superincumbent clay-slate, and which locally
throws off the strata, on one side to N.W. and on the other to S.E. but
not at a high angle: at the junction, the clay-slate is altered into
fine-grained greenstone. This granitic axis is intersected by a green
dike, which I mention, because I do not remember having elsewhere seen
dikes in this lowest and latest intrusive rock. From the R. Vacas to
the plain of Uspallata, the valley runs N.E., so that I have had to
contract my section; it runs exclusively through porphyritic rocks. As
far as the Pass of Jaula, the claystone conglomerate formation, in most
parts highly porphyritic, and crossed by numerous dikes of greenstone
porphyry, attains a great thickness: there is also much intrusive
porphyry. From the Jaula to the plain, the stratification has been in
most places obliterated, except near the tops of some of the mountains;
and the metamorphic action has been extremely great. In this space, the
number and bulk of the intrusive masses of differently coloured
porphyries, injected one into another and intersected by dikes, is
truly extraordinary. I saw one mountain of whitish porphyry, from which
two huge dikes, thinning out, branched _downwards_ into an adjoining
blackish porphyry. Another hill of white porphyry, which had burst
through dark-coloured strata, was itself injected by a purple,
brecciated, and recemented porphyry, both being crossed by a green
dike, and both having been upheaved and injected by a granitic dome.
One brick-red porphyry, which above the Jaula forms an isolated mass in
the midst of the porphyritic conglomerate formation, and lower down the
valley a magnificent group of peaked mountains, differs remarkably from
all the other porphyries. It consists of a red feldspathic base,
including some rather large crystals of red feldspar, numerous large
angular grains of quartz, and little bits of a soft green mineral
answering in most of its characters to soapstone. The crystals of red
feldspar resemble in external appearance those of orthite, though, from
being
partially decomposed, I was unable to measure them; and they certainly
are quite unlike the variety, so abundantly met with in almost all the
other rocks of this line of section, and which, wherever I tried it,
cleaved like albite. This brick-red porphyry appears to have burst
through all the other porphyries, and numerous red dikes traversing the
neighbouring mountains have proceeded from it: in some few places,
however, it was intersected by white dikes. From this posteriority of
intrusive origin,—from the close general resemblance between this red
porphyry and the red granite of the Portillo line, the only difference
being that the feldspar here is less perfectly granular, and that
soapstone replaces the mica, which is there imperfect and passes into
chlorite,—and from the Portillo line a little southward of this point
appearing to blend (according to Dr. Gillies) into the western
ranges,—I am strongly urged to believe (as formerly remarked) that the
grand mountain-masses composed of this brick-red porphyry belong to the
same axis of injection with the granite of the Portillo line; if so,
the injection of this porphyry probably took place, as long
subsequently to the several axes of elevation in the gypseous formation
near the Cumbre, as the injection of the Portillo granite has been
shown to have been subsequent to the elevation of the gypseous strata
composing the Peuquenes range; and this interval, we have seen, must
have been a very long one.

The Plain of Uspallata has been briefly described in Chapter 3; it
resembles the basin-plains of Chile; it is ten or fifteen miles wide,
and is said to extend for 180 miles northward; its surface is nearly
six thousand feet above the sea; it is composed, to a thickness of some
hundred feet of loosely aggregated, stratified shingle, which is
prolonged with a gently sloping surface up the valleys in the mountains
on both sides. One section in this plain [Z] is interesting, from the
unusual[17] circumstance of alternating layers of almost loose red and
white sand with lines of pebbles (from the size of a nut to that of an
apple), and beds of gravel, being inclined at an angle of 45°, and in
some spots even at a higher angle. These beds are dislocated by small
faults: and are capped by a thick mass of horizontally stratified
gravel, evidently of subaqueous origin. Having been accustomed to
observe the irregularities of beds accumulated under currents, I feel
sure that the inclination here has not been thus produced. The pebbles
consist chiefly of the brick-red porphyry just described and of white
granite, both probably derived from the ranges to the west, and of
altered clay-slate and of certain porphyries, apparently belonging to
the rocks of the Uspallata chain. This plain corresponds geographically
with the valley of Tenuyan between the Portillo and Peuquenes ranges;
but in that valley the shingle, which likewise has been derived both
from the eastern and western ranges, has been cemented into a hard
conglomerate, and has been throughout tilted at a considerable
inclination; the gravel there apparently attains a much greater
thickness, and is probably of higher antiquity.

 [17] I find that Mr. Smith of Jordan Hill has described (_Edinburgh
 New Phil. Journ.,_ vol. xxv, p. 392) beds of sand and gravel, near
 Edinburgh, tilted at an angle of 60°, and dislocated by miniature
 faults.


_The Uspallata range._—The road by the Villa Vicencio Pass does not
strike directly across the range, but runs for some leagues northward
along its western base: and I must briefly describe the rocks here
seen, before continuing with the coloured east and west section. At the
mouth of the valley of Canota, and at several points northwards, there
is an extensive formation of a glossy and harsh, and of a feldspathic
clay-slate, including strata of grauwacke, and having a tortuous,
nearly vertical cleavage, traversed by numerous metalliferous veins and
others of quartz. The clay-slate is in many parts capped by a thick
mass of fragments of the same rock, firmly recemented; and both
together have been injected and broken up by very numerous hillocks,
ranging north and south, of lilac, white, dark and salmon-coloured
porphyries: one steep, now denuded, hillock of porphyry had its face as
distinctly impressed with the angles of a fragmentary mass of the
slate, with some of the points still remaining embedded, as sealing-wax
could be by a seal. At the mouth of this same valley of Canota, in a
fine escarpment having the strata dipping from 50° to 60° to the
N.E.,[18] the clay-slate formation is seen to be covered by—(1st) a
purple, claystone porphyry resting unconformably in some parts on the
solid slate, and in others on a thick fragmentary mass; (2nd), a
conformable stratum of compact blackish rock, having a spheroidal
structure, full of minute acicular crystals of glassy feldspar, with
red spots of oxide of iron; (3rd), a great stratum of purplish-red
claystone porphyry, abounding with crystals of opaque feldspar, and
laminated with thin, parallel, often short, layers, and likewise with
great irregular patches of white, earthy, semi-crystalline feldspar;
this rock (which I noticed in other neighbouring places) perfectly
resembles a curious variety described at Port Desire, and occasionally
occurs in the great porphyritic conglomerate formation of Chile; (4th),
a thin stratum of greenish white, indurated tuff, fusible and
containing broken crystals and particles of porphyries; (5th), a grand
mass, imperfectly columnar and divided into three parallel and closely
joined strata, of cream-coloured claystone porphyry; (6th), a thick
stratum of lilac-coloured porphyry, which I could see was capped by
another bed of the cream-coloured variety; I was unable to examine the
still higher parts of the escarpment. These conformably stratified
porphyries, though none are either vesicular are amygdaloidal, have
evidently flowed as submarine lavas: some of them are separated from
each other by seams of indurated tuff, which, however, are quite
insignificant in thickness compared with the porphyries. This whole
pile resembles, but not very closely, some of the less brecciated parts
of the great porphyritic conglomerate formation of Chile; but it does
not probably belong to the same age, as the porphyries here rest
unconformably on the altered feldspathic clay-slate, whereas the
porphyritic conglomerate formation alternates with
and rests conformably on it. These porphyries, moreover, with the
exception of the one blackish stratum, and of the one indurated, white
tufaceous bed, differ from the beds composing the Uspallata range in
the line of the Villa Vicencio Pass.

 [18] Nearly opposite to this escarpment, there is another
 corresponding one, with the strata dipping not to the exactly opposite
 point, or S.W., but to S.S.W.: consequently the two escarpments trend
 towards each other, and some miles southward they become actually
 united: this is a form of elevation which I have not elsewhere seen.

I will now give, first, a sketch of the structure of the range, as
represented in the section, and will then describe its composition and
interesting history. At its western foot, a hillock [N] is seen to rise
out of the plain, with its strata dipping at 70° to the west, fronted
by strata [O] inclined at 45° to the east, thus forming a little north
and south anticlinal axis. Some other little hillocks of similar
composition, with their strata highly inclined, range N.E. and S.W.,
obliquely to the main Uspallata line. The cause of these dislocations,
which, though on a small scale, have been violent and complicated, is
seen to lie in hummocks of lilac, purple and red porphyries, which have
been injected in a liquified state through and into the underlying
clay-slate formation. Several dykes were exposed here, but in no other
part, that I saw of this range. As the strata consist of black, white,
greenish and brown-coloured rocks, and as the intrusive porphyries are
so brightly tinted, a most extraordinary view was presented, like a
coloured geological drawing. On the gently inclined main western slope
[PP], above the little anticlinal ridges just mentioned, the strata dip
at an average angle of 25° to the west; the inclination in some places
being only 19°, in some few others as much as 45°. The masses having
these different inclinations, are separated from each other by parallel
vertical faults [as represented at Pa], often giving rise to separate,
parallel, uniclinal ridges. The summit of the main range is broad and
undulatory, with the stratification undulatory and irregular: in a few
places granitic and porphyritic masses [Q] protrude, which, from the
small effect they have locally produced in deranging the strata,
probably form the upper points of a regular, great underlying dome.
These denuded granitic points, I estimated at about nine thousand feet
in height above the sea. On the eastern slope, the strata in the upper
part are regularly inclined at about 25° to the east, so that the
summit of this chain, neglecting small irregularities, forms a broad
anticlinal axis. Lower down, however, near Los Hornillos [R], there is
a well-marked synclinal axis, beyond which the strata are inclined at
nearly the same angle, namely from 20° to 30°, inwards or westward.
Owing to the amount of denudation which this chain has suffered, the
outline of the gently inclined eastern flank scarcely offers the
slightest indication of this synclinal axis. The stratified beds, which
we have hitherto followed across the range, a little further down are
seen to lie, I believe unconformably, on a broad mountainous band of
clay-slate and grauwacke. The strata and laminæ of this latter
formation, on the extreme eastern flank, are generally nearly vertical;
further inwards they become inclined from 45° to 80° to the west: near
Villa Vicencio [S] there is apparently an anticlinal axis, but the
structure of this outer part of the clay-slate formation is so obscure,
that I have not marked the planes of stratification in the section. On
the margin of the Pampas, some low, much dislocated spurs of this same
formation,
project in a north-easterly line, in the same oblique manner as do the
ridges on the western foot, and as is so frequently the case with those
at the base of the main Cordillera.

I will now describe the nature of the beds, beginning at the base on
the eastern side. First, for the clay-slate formation: the slate is
generally hard and bluish, with the laminæ coated by minute micaceous
scales; it alternates many times with a coarse-grained, greenish
grauwacke, containing rounded fragments of quartz and bits of slate in
a slightly calcareous basis. The slate in the upper part generally
becomes purplish, and the cleavage so irregular that the whole consists
of mere splinters. Transverse veins of quartz are numerous. At the
Calera, some leagues distant, there is a dark crystalline limestone,
apparently included in this formation. With the exception of the
grauwacke being here more abundant, and the clay-slate less altered,
this formation closely resembles that unconformably underlying the
porphyries at the western foot of this same range; and likewise that
alternating with the porphyritic conglomerate in the main Cordillera.
This formation is a considerable one, and extends several leagues
southward to near Mendoza: the mountains composed of it rise to a
height of about two thousand feet above the edge of the Pampas, or
about seven thousand feet above the sea.[19]

 [19] I infer this from the height of V. Vicencio, which was
 ascertained by Mr. Miers to be 5,328 feet above the sea.


Secondly: the most usual bed on the clay-slate is a coarse, white,
slightly calcareous conglomerate, of no great thickness, including
broken crystals of feldspar, grains of quartz, and numerous pebbles of
brecciated claystone porphyry, but without any pebbles of the
underlying clay-slate. I nowhere saw the actual junction between this
bed and the clay-slate, though I spent a whole day in endeavouring to
discover their relations. In some places I distinctly saw the white
conglomerate and overlying beds inclined at from 25° to 30° to the
west, and at the bottom of the same mountain, the clay-slate and
grauwacke inclined to the same point, but at an angle from 70° to 80°:
in one instance, the clay-slate dipped not only at a different angle,
but to a different point from the overlying formation. In these cases
the two formations certainly appeared quite unconformable: moreover, I
found in the clay-slate one great, vertical, dike-like fissure, filled
up with an indurated whitish tuff, quite similar to some of the upper
beds presently to be described; and this shows that the clay-slate must
have been consolidated and dislocated before their deposition. On the
other hand, the stratification of the slate and grauwacke,[20] in some
cases gradually and entirely disappeared in approaching the overlying
white conglomerate;
in other cases the stratification of the two formations became strictly
conformable; and again in other cases, there was some tolerably well
characterised clay-slate lying above the conglomerate. The most
probable conclusion appears to be, that after the clay-slate formation
had been dislocated and tilted, but whilst under the sea, a fresh and
more recent deposition of clay-slate took place, on which the white
conglomerate was conformably deposited, with here and there a thin
intercalated bed of clay-slate. On this view the white conglomerates
and the presently to be described tuffs and lavas are really
unconformable to the main part of the clay-slate; and this, as we have
seen, certainly is the case with the clay-stone lavas in the valley of
Canota, at the western and opposite base of the range.

 [20] The coarse, mechanical structure of many grauwackes has always
 appeared to me a difficulty; for the texture of the associated
 clay-slate and the nature of the embedded organic remains where
 present, indicate that the whole has been a deep-water deposit. Whence
 have the sometimes included angular fragments of clay-slate, and the
 rounded masses of quartz and other rocks, been derived? Many
 deep-water limestones, it is well known, have been brecciated, and
 then firmly recemented.

Thirdly: on the white conglomerate, strata several hundred feet in
thickness are superimposed, varying much in nature in short distances:
the commonest variety is a white, much indurated tuff, sometimes
slightly calcareous, with ferruginous spots and water-lines, often
passing into whitish or purplish compact, fine-grained grit or
sandstones; other varieties become semi-porcellanic, and tinted faint
green or blue; others pass into an indurated shale: most of these
varieties are easily fusible.

Fourthly: a bed, about one hundred feet thick of a compact, partially
columnar, pale-grey, feldspathic lava, stained with iron, including
very numerous crystals of opaque feldspar, and with some crystallised
and disseminated calcareous matter. The tufaceous stratum on which this
feldspathic lava rests is much hardened, stained purple, and has a
spherico-concretionary structure; it here contains a good many pebbles
of claystone porphyry.

Fifthly: thin beds, 400 feet in thickness, varying much in nature,
consisting of white and ferruginous tuffs, in some parts having a
concretionary structure, in others containing rounded grains and a few
pebbles of quartz; also passing into hard gritstones and into greenish
mudstones: there is, also, much of a bluish-grey and green
semi-porcellanic stone.

Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a
nature that I do not believe a score of specimens would show all the
varieties; much is highly amygdaloidal, much compact; there are
greenish, blackish, purplish, and grey varieties, rarely including
crystals of green augite and minute acicular ones of feldspar, but
often crystals and amygdaloidal masses of white, red, and black
carbonate of lime. Some of the blackish varieties of this rock have a
conchoidal fracture and resemble basalt; others have an irregular
fracture. Some of the grey and purplish varieties are thickly speckled
with green earth and with white crystalline carbonate of lime; others
are largely amygdaloidal with green earth and calcareous spar. Again,
other earthy varieties, of greenish, purplish and grey tints, contain
much iron, and are almost half composed of amygdaloidal balls of dark
brown bole, of a whitish indurated feldspathic matter, of bright green
earth, of agate, and of black and white crystallised carbonate of lime.
All these varieties are easily fusible. Viewed from a distance, the
line of junction with the underlying semi-porcellanic strata was
distinct; but when examined
closely, it was impossible to point out within a foot where the lava
ended and where the sedimentary mass began: the rock at the time of
junction was in most places hard, of a bright green colour, and
abounded with irregular amygdaloidal masses of ferruginous and pure
calcareous spar, and of agate.

Seventhly: strata, eighty feet in thickness, of various indurated
tuffs, as before; many of the varieties have a fine basis including
rather coarse extraneous particles; some of them are compact and
semi-porcellanic, and include vegetable impressions.

Eighthly: a bed, about fifty feet thick, of greenish-grey, compact,
feldspathic lava, with numerous small crystals of opaque feldspar,
black augite, and oxide of iron. The junction with the bed on which it
rested, was ill defined; balls and masses of the feldspathic rock being
enclosed in much altered tuff.

Ninthly: indurated tuffs, as before.

Tenthly: a conformable layer, less than two feet in thickness, of
pitchstone, generally brecciated, and traversed by veins of agate and
of carbonate of lime: parts are composed of apparently concretionary
fragments of a more perfect variety, arranged in horizontal lines in a
less perfectly characterised variety. I have much difficulty in
believing that this thin layer of pitchstone flowed as lava.

Eleventhly: sedimentary and tufaceous beds as before, passing into
sandstone, including some conglomerate: the pebbles in the latter are
of claystone porphyry, well rounded, and some as large as
cricket-balls.

Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of
bed No. 8, divided by numerous joints into large angular blocks.

Thirteenthly: sedimentary beds as before.

Fourteenthly: a thick bed of greenish or greyish black, compact basalt
(fusing into a black enamel), with small crystals, occasionally
distinguishable, of feldspar and augite: the junction with the
underlying sedimentary bed, differently from that in most of the
foregoing streams, here was quite distinct:—the lava and tufaceous
matter preserving their perfect characters within two inches of each
other. This rock closely resembles certain parts of that varied and
singular lava-stream No. 6; it likewise resembles, as we shall
immediately see, many of the great upper beds on the western flank and
on the summit of this range.

The pile of strata here described attains a great thickness; and above
the last-mentioned volcanic stratum, there were several other great
tufaceous beds alternating with submarine lavas, which I had not time
to examine; but a corresponding series, several thousand feet in
thickness, is well exhibited on the crest and western flank of the
range. Most of the lava-streams on the western side are of a jet-black
colour and basaltic nature; they are either compact and fine-grained,
including minute crystals of augite and feldspar, or they are
coarse-grained and abound with rather large coppery-brown crystals of
an augitic mineral.[21] Another variety was of a dull-red colour,
having a claystone brecciated basis, including specks of oxide of iron
and of calcareous spar, and
amygdaloidal with green earth: there were apparently several other
varieties. These submarine lavas often exhibit a spheroidal, and
sometimes an imperfect columnar structure: their upper junctions are
much more clearly defined than their lower junctions; but the latter
are not so much blended into the underlying sedimentary beds as is the
case in the eastern flank. On the crest and western flank of the range,
the streams, viewed as a whole, are mostly basaltic; whilst those on
the eastern side, which stand lower in the series, are, as we have
seen, mostly feldspathic.

 [21] Very easily fusible into a jet-black bead, attracted by the
 magnet: the crystals are too much tarnished to be measured by the
 goniometer.


The sedimentary strata alternating with the lavas on the crest and
western side, are of an almost infinitely varying nature; but a large
proportion of them closely resemble those already described on the
eastern flank: there are white and brown, indurated, easily fusible
tuffs,—some passing into pale blue and green semi-porcellanic
rocks,—others into brownish and purplish sandstones and gritstones,
often including grains of quartz,—others into mudstone containing
broken crystals and particles of rock, and occasionally single large
pebbles. There was one stratum of a bright red, coarse, volcanic
gritstone; another of conglomerate; another of a black, indurated,
carbonaceous shale marked with imperfect vegetable impressions; this
latter bed, which was thin, rested on a submarine lava, and followed
all the considerable inequalities of its upper surface. Mr. Miers
states that coal has been found in this range. Lastly, there was a bed
(like No. 10 on the eastern flank) evidently of sedimentary origin, and
remarkable from closely approaching in character to an imperfect
pitchstone, and from including extremely thin layers of perfect
pitchstone, as well as nodules and irregular fragments (but not
resembling extraneous fragments) of this same rock arranged in
horizontal lines: I conceive that this bed, which is only a few feet in
thickness, must have assumed its present state through metamorphic and
concretionary action. Most of these sedimentary strata are much
indurated, and no doubt have been partially metamorphosed: many of them
are extraordinarily heavy and compact; others have agate and
crystalline carbonate of lime disseminated throughout them. Some of the
beds exhibit a singular concretionary arrangement, with the curves
determined by the lines of fissure. There are many veins of agate and
calcareous spar, and innumerable ones of iron and other metals, which
have blackened and curiously affected the strata to considerable
distances on both sides.

Many of these tufaceous beds resemble, with the exception of being more
indurated, the upper beds of the Great Patagonian tertiary formation,
especially those variously coloured layers high up the River Santa
Cruz, and in a remarkable degree the tufaceous formation at the
northern end of Chiloe. I was so much struck with this resemblance,
that I particularly looked out for silicified wood, and found it under
the following extraordinary circumstances. High up on this western
flank,[22]
at a height estimated at 7,000 feet above the sea, in a broken
escarpment of thin strata, composed of compact green gritstone passing
into a fine mudstone, and alternating with layers of coarser, brownish,
very heavy mudstone, including broken crystals and particles of rock
almost blended together, I counted the stumps of fifty-two trees. They
projected between two and five feet above the ground, and stood at
exactly right angles to the strata, which were here inclined at an
angle of about 25° to the west. Eleven of these trees were silicified
and well preserved; Mr. R. Brown has been so kind as to examine the
wood when sliced and polished; he says it is coniferous, partaking of
the characters of the Araucarian tribe, with some curious points of
affinity with the Yew. The bark round the trunks must have been
circularly furrowed with irregular lines, for the mudstone round them
is thus plainly marked. One cast consisted of dark argillaceous
limestone; and forty of them of coarsely crystallised carbonate of
lime, with cavities lined by quartz crystals: these latter white
calcareous columns do not retain any internal structure, but their
external form plainly shows their origin. All the stumps have nearly
the same diameter, varying from one foot to eighteen inches; some of
them stand within a yard of each other; they are grouped in a clump
within a space of about sixty yards across, with a few scattered round
at the distance of 150 yards. They all stand at about the same level.
The longest stump stood seven feet out of the ground: the roots, if
they are still preserved, are buried and concealed. No one layer of the
mudstone appeared much darker than the others, as if it had formerly
existed as soil, nor could this be expected, for the same agents which
replaced with silex and lime the wood of the trees, would naturally
have removed all vegetable matter from the soil. Besides the fifty-two
upright trees, there were a few fragments, like broken branches,
horizontally embedded. The surrounding strata are crossed by veins of
carbonate of lime, agate, and oxide of iron; and a poor gold vein has
been worked not far from the trees.

 [22] For the information of any future traveller, I will describe the
 spot in detail. Proceeding eastward from the Agua del Zorro, and
 afterwards leaving on the north side of the road a rancho attached to
 some old goldmines, you pass through a gully with low but steep rocks
 on each hand: the road then bends, and the ascent becomes steeper. A
 few hundred yards farther on, a stone’s throw on the south side of the
 road, the white calcareous stumps may be seen. The spot is about half
 a mile east of the Agua del Zorro.


The green and brown mudstone beds including the trees, are conformably
covered by much indurated, compact, white or ferruginous tuffs, which
pass upwards into a fine-grained, purplish sedimentary rock: these
strata, which, together, are from four to five hundred feet in
thickness, rest on a thick bed of submarine lava, and are conformably
covered by another great mass of fine-grained basalt,[23] which I
estimated at 1,000 feet in thickness, and which probably has been
formed by more than one stream. Above this mass I could clearly
distinguish five conformable alternations, each several hundred feet in
thickness,
of stratified sedimentary rocks and lavas, such as have been previously
described. Certainly the upright trees have been buried under several
thousand feet in thickness of matter, accumulated under the sea. As the
trees obviously must once have grown on dry land, what an enormous
amount of subsidence is thus indicated! Nevertheless, had it not been
for the trees there was no appearance which would have led any one even
to have conjectured that these strata had subsided. As the land,
moreover, on which the trees grew, is formed of subaqueous deposits, of
nearly if not quite equal thickness with the superincumbent strata, and
as these deposits are regularly stratified and fine-grained, not like
the matter thrown up on a sea-beach, a previous upward movement, aided
no doubt by the great accumulation of lavas and sediment, is also
indicated.[24]

 [23] This rock is quite black, and fuses into a black bead, attracted
 strongly by the magnet; it breaks with a conchoidal fracture; the
 included crystals of augite are distinguishable by the naked eye, but
 are not perfect enough to be measured: there are many minute acicular
 crystals of glassy feldspar.


 [24] At first I imagined, that the strata with the trees might have
 been accumulated in a lake: but this seems highly improbable; for,
 first, a very deep lake was necessary to receive the matter below the
 trees, then it must have been drained for their growth, and afterwards
 re-formed and made profoundly deep, so as to receive a subsequent
 accumulation of matter _several thousand_ feet in thickness. And all
 this must have taken place necessarily before the formation of the
 Uspallata range, and therefore on the margin of the wide level expanse
 of the Pampas! Hence I conclude, that it is infinitely more probable
 that the strata were accumulated under the sea: the vast amount of
 denudation, moreover, which this range has suffered, as shown by the
 wide valleys, by the exposure of the very trees and by other
 appearances, could have been effected, I conceive, only by the
 long-continued action of the sea; and this shows that the range was
 either upheaved from under the sea, or subsequently let down into it.
 From the natural manner in which the stumps (fifty-two in number) are
 _grouped in a clump_, and from their all standing vertically to the
 strata, it is superfluous to speculate on the chance of the trees
 having been drifted from adjoining land, and deposited upright: I may,
 however, mention that the late Dr. Malcolmson assured me, that he once
 met in the Indian Ocean, fifty miles from land, several cocoa-nut
 trees floating upright, owing to their roots being loaded with earth.


In nearly the middle of the range, there are some hills [Q], before
alluded to, formed of a kind of granite externally resembling andesite,
and consisting of a white, imperfectly granular, feldspathic basis,
including some perfect crystals apparently of albite (but I was unable
to measure them), much black mica, epidote in veins, and very little or
no quartz. Numerous small veins branch from this rock into the
surrounding strata; and it is a singular fact that these veins, though
composed of the same kind of feldspar and small scales of mica as in
the solid rock, abound with innumerable minute _rounded_ grains of
quartz: in the veins or dikes also, branching from the great granitic
axis in the peninsula of Tres Montes, I observed that quartz was more
abundant in them than in the main rock: I have heard of other analogous
cases: can we account for this fact, by the long-continued vicinity of
quartz[25] when cooling, and by its having been thus more easily
sucked into fissures than the other constituent minerals of granite?
The strata encasing the flanks of these granitic or andesite masses,
and forming a thick cap on one of their summits, appear originally to
have been of the same tufaceous nature with the beds already described,
but they are now changed into porcellanic, jaspery, and crystalline
rocks, and into others of a white colour with a harsh texture, and
having a siliceous aspect, though really of a feldspathic nature and
fusible. Both the granitic intrusive masses and the encasing strata are
penetrated by innumerable metallic veins, mostly ferruginous and
auriferous, but some containing copper-pyrites and a few silver: near
the veins, the rocks are blackened as if blasted by gunpowder. The
strata are only slightly dislocated close round these hills, and hence,
perhaps, it may be inferred that the granitic masses form only the
projecting points of a broad continuous axis-dome, which has given to
the upper parts of this range its anticlinal structure.

 [25] See a paper by M. Elie de Beaumont, “Soc. Philomath.,” May 1839
 (“L’Institut.,” 1839, p. 161.)


_Concluding remarks on the Uspallata range._—I will not attempt to
estimate the total thickness of the pile of strata forming this range,
but it must amount to many thousand feet. The sedimentary and tufaceous
beds have throughout a general similarity, though with infinite
variations. The submarine lavas in the lower part of the series are
mostly feldspathic, whilst in the upper part, on the summit and western
flank, they are mostly basaltic. We are thus reminded of the relative
position in most recent volcanic districts of the trachytic and
basaltic lavas,—the latter from their greater weight having sunk to a
lower level in the earth’s crust, and having consequently been erupted
at a later period over the lighter and upper lavas of the trachytic
series.[26] Both the basaltic and feldspathic submarine streams are
very compact; none being vesicular, and only a few amygdaloidal: the
effects which some of them, especially those low in the series, have
produced on the tufaceous beds over which they have flowed is highly
curious. Independently of this local metamorphic action, all the strata
undoubtedly display an indurated and altered character; and all the
rocks of this range—the lavas, the alternating sediments, the intrusive
granite and porphyries, and the underlying clay-slate—are intersected
by metalliferous veins. The lava-strata can often be seen extending for
great distances, conformably with the under and overlying beds; and it
was obvious that they thickened towards the west. Hence the points of
eruption must have been situated westward of the present range, in the
direction of the main Cordillera: as, however, the flanks of the
Cordillera are entirely composed of various porphyries, chiefly
claystone and greenstone, some intrusive, and others belonging to the
porphyritic conglomerate formation, but all quite unlike these
submarine lava-streams, we must in all probability look to the plain of
Uspallata for the now deeply buried points of eruption.

 [26] See on this subject, “Volcanic Islands,” etc., by the Author.

Comparing our section of the Uspallata range with that of the Cumbre,
we see, with the exception of the underlying clay-slate, and perhaps of
the intrusive rocks of the axes, a striking dissimilarity in the strata
composing them. The great porphyritic conglomerate formation
has not extended as far as this range; nor have we here any of the
gypseous strata, the magnesian and other limestones, the red
sandstones, the siliceous beds with pebbles of quartz, and
comparatively little of the conglomerates, all of which form such vast
masses over the basal series in the main Cordillera. On the other hand,
in the Cordillera, we do not find those endless varieties of indurated
tuffs, with their numerous veins and concretionary arrangement, and
those grit and mud stones, and singular semi-porcellanic rocks, so
abundant in the Uspallata range. The submarine lavas, also, differ
considerably; the feldspathic streams of the Cordillera contain much
mica, which is absent in those of the Uspallata range: in this latter
range we have seen on how grand a scale, basaltic lava has been poured
forth, of which there is not a trace in the Cordillera. This
dissimilarity is the more striking, considering that these two parallel
chains are separated by a plain only between ten and fifteen miles in
width; and that the Uspallata lavas, as well as no doubt the
alternating tufaceous beds, have proceeded from the west, from points
apparently between the two ranges. To imagine that these two piles of
strata were contemporaneously deposited in two closely adjoining, very
deep, submarine areas, separated from each other by a lofty ridge,
where a plain now extends, would be a gratuitous hypothesis. And had
they been contemporaneously deposited, without any such dividing ridge,
surely some of the gypseous and other sedimentary matter forming such
immensely thick masses in the Cordillera, would have extended this
short distance eastwards; and surely some of the Uspallata tuffs and
basalts also accumulated to so great a thickness, would have extended a
little westward. Hence I conclude, that it is far from probable that
these two series are not contemporaneous; but that the strata of one of
the chains were deposited, and even the chain itself uplifted, before
the formation of the other:—which chain, then, is the oldest?
Considering that in the Uspallata range the lowest strata on the
western flank lie unconformably on the clay-slate, as probably is the
case with those on the eastern flank, whereas in the Cordillera all the
overlying strata lie conformably on this formation:—considering that in
the Uspallata range some of the beds, both low down and high up in the
series, are marked with vegetable impressions, showing the continued
existence of neighbouring land;—considering the close general
resemblance between the deposits of this range and those of tertiary
origin in several parts of the continent;—and lastly, even considering
the lesser height and outlying position of the Uspallata range,—I
conclude that the strata composing it are in all probability of
subsequent origin, and that they were accumulated at a period when a
deep sea studded with submarine volcanoes washed the eastern base of
the already partially elevated Cordillera.

This conclusion is of much importance, for we have seen that in the
Cordillera, during the deposition of the Neocomian strata, the bed of
the sea must have subsided many thousand feet: we now learn that at a
later period an adjoining area first received a great accumulation of
strata, and was upheaved into land on which coniferous trees grew, and
that this area then subsided several thousand feet to receive the
superincumbent
submarine strata, afterwards being broken up, denuded, and elevated in
mass to its present height. I am strengthened in this conclusion of
there having been two distinct, great periods of subsidence, by
reflecting on the thick mass of coarse stratified conglomerate in the
valley of Tenuyan, between the Peuquenes and Portillo lines; for the
accumulation of this mass seems to me, as previously remarked, almost
necessarily to have required a prolonged subsidence; and this
subsidence, from the pebbles in the conglomerate having been to a great
extent derived from the gypseous or Neocomian strata of the Peuquenes
line, we know must have been quite distinct from, and subsequent to,
that sinking movement which probably accompanied the deposition of the
Peuquenes strata, and which certainly accompanied the deposition of the
equivalent beds near the Puente del Inca, in this line of section.

The Uspallata chain corresponds in geographical position, though on a
small scale, with the Portillo line; and its clay-slate formation is
probably the equivalent of the mica-schist of the Portillo, there
metamorphosed by the old white granites and syenites. The coloured beds
under the conglomerate in the valley of Tenuyan, of which traces are
seen on the crest of the Portillo, and even the conglomerate itself,
may perhaps be synchronous with the tufaceous beds and submarine lavas
of the Uspallata range; an open sea and volcanic action in the latter
case, and a confined channel between two bordering chains of islets in
the former case, having been sufficient to account for the
mineralogical dissimilarity of the two series. From this correspondence
between the Uspallata and Portillo ranges, perhaps in age and certainly
in geographical position, one is tempted to consider the one range as
the prolongation of the other; but their axes are formed of totally
different intrusive rocks; and we have traced the apparent continuation
of the red granite of the Portillo in the red porphyries diverging into
the main Cordillera. Whether the axis of the Uspallata range was
injected before, or as perhaps is more probable, after that of the
Portillo line, I will not pretend to decide; but it is well to remember
that the highly inclined lava-streams on the eastern flank of the
Portillo line, prove that its angular upheavement was not a single and
sudden event; and therefore that the anticlinal elevation of the
Uspallata range may have been contemporaneous with some of the later
angular movements by which the gigantic Portillo range gained its
present height above the adjoining plain.




Chapter VIII NORTHERN CHILE.—CONCLUSION.


Section from Illapel to Combarbala; gypseous formation with silicified
wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley;
fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas,
silicified wood.—Conglomerates, nature of former land, fossils,
thickness of strata, great subsidence.—Valley of Despoblado, fossils,
tufaceous deposit, complicated dislocations of.—Relations between
ancient orifices of eruption and subsequent axes of injection.—Iquique,
Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the
porphyritic conglomerate and gypseous formations.—Great subsidence with
partial elevations during the cretaceo-oolitic period.—On the elevation
and structure of the Cordillera.—Recapitulation on the tertiary
series.—Relation between movements of subsidence and volcanic
action.—Pampean formation.—Recent elevatory movements. Long-continued
volcanic action in the Cordillera.—Conclusion.

_Valparaiso to Coquimbo._ I have already described the general nature
of the rocks in the low country north of Valparaiso, consisting of
granites, syenites, greenstones, and altered feldspathic clay-slate.
Near Coquimbo there is much hornblendic rock and various dusky-coloured
porphyries. I will describe only one section in this district, namely,
from near Illapel in a N.E. line to the mines of Los Hornos, and thence
in a north by east direction to Combarbala, at the foot of the main
Cordillera.

Near Illapel, after passing for some distance over granite, andesite,
and andesitic porphyry, we come to a greenish stratified feldspathic
rock, which I believe is altered clay-slate, conformably capped by
porphyries and porphyritic conglomerate of great thickness, dipping at
an average angle of 20° to N.E. by N. The uppermost beds consist of
conglomerates and sandstone only a little metamorphosed, and
conformably covered by a gypseous formation of very great thickness,
but much denuded. This gypseous formation, where first met with, lies
in a broad valley or basin, a little southward of the mines of Los
Hornos: the lower half alone contains gypsum, not in great masses as in
the Cordillera, but in innumerable thin layers, seldom more than an
inch or two in thickness. The gypsum is either opaque or transparent,
and is associated with carbonate of lime. The layers alternate with
numerous varying ones of a calcareous clay-shale (with strong aluminous
odour, adhering to the tongue, easily fusible into a pale green glass),
more or less indurated, either earthy and cream-coloured, or greenish
and hard. The more indurated varieties have a compact, homogeneous,
almost crystalline fracture, and contain granules of crystallised oxide
of iron. Some of the varieties almost resemble honestones. There is
also a little black, hardly fusible, siliceo-calcareous clay-slate,
like some of the varieties alternating with gypsum on the Peuquenes
range.

The upper half of this gypseous formation is mainly formed of the
same calcareous clay-shale rock, but without any gypsum, and varying
extremely in nature: it passes from a soft, coarse, earthy, ferruginous
state, including particles of quartz, into compact claystones with
crystallised oxide of iron,—into porcellanic layers, alternating with
seams of calcareous matter,—and into green porcelain-jasper,
excessively hard, but easily fusible. Strata of this nature alternate
with much black and brown siliceo-calcareous slate, remarkable from the
wonderful number of huge embedded logs of silicified wood. This wood,
according to Mr. R. Brown, is (judging from several specimens) all
coniferous. Some of the layers of the black siliceous slate contained
irregular angular fragments of imperfect pitchstone, which I believe,
as in the Uspallata range, has originated in a metamorphic process.
There was one bed of a marly tufaceous nature, and of little specific
gravity. Veins of agate and calcareous spar are numerous. The whole of
this gypseous formation, especially the upper half, has been injected,
metamorphosed, and locally contorted by numerous hillocks of intrusive
porphyries crowded together in an extraordinary manner. These hillocks
consist of purple claystone and of various other porphyries, and of
much white feldspathic greenstone passing into andesite; this latter
variety included in one case crystals of orthitic and albitic feldspar
touching each other, and others of hornblende, chlorite, and epidote.
The strata surrounding these intrusive hillocks at the mines of Los
Hornos, are intersected by many veins of copper-pyrites, associated
with much micaceous iron-ore, and by some of gold: in the neighbourhood
of these veins the rocks are blackened and much altered. The gypsum
near the intrusive masses is always opaque. One of these hillocks of
porphyry was capped by some stratified porphyritic conglomerate, which
must have been brought up from below, through the whole immense
thickness of the overlying gypseous formation. The lower beds of the
gypseous formation resemble the corresponding and probably
contemporaneous strata of the main Cordillera; whilst the upper beds in
several respects resemble those of the Uspallata chain, and possibly
may be contemporaneous with them; for I have endeavoured to show that
the Uspallata beds were accumulated subsequently to the gypseous or
Neocomian formations of the Cordillera.

This pile of strata dips at an angle of about 20 degrees to N.E. by N.,
close up to the foot of the Cuesta de Los Hornos, a crooked range of
mountains formed of intrusive rocks of the same nature with the above
described hillocks. Only in one or two places, on this south-eastern
side of the range, I noticed a narrow fringe of the upper gypseous
strata brushed up and inclined south-eastward from it. On its
north-eastern flank, and likewise on a few of the summits, the
stratified porphyritic conglomerate is inclined N.E.: so that, if we
disregard the very narrow anticlinal fringe of gypseous strata at its
S.E. foot, this range forms a second uniclinal axis of elevation.
Proceeding in a north-by-east direction to the village of Combarbala,
we come to a third escarpment of the porphyritic conglomerate, dipping
eastwards, and forming the outer range of the main Cordillera. The
lower beds were here more jaspery than usual, and they included some
white cherty strata and red
sandstones, alternating with purple claystone porphyry. Higher up in
the Cordillera there appeared to be a line of andesitic rocks; and
beyond them, a fourth escarpment of the porphyritic conglomerate, again
dipping eastwards or inwards. The overlying gypseous strata, if they
ever existed here, have been entirely removed.

_Copper mines of Panuncillo._—From Combarbala to Coquimbo, I traversed
the country in a zigzag direction, crossing and recrossing the
porphyritic conglomerate and finding in the granitic districts an
unusual number of mountain-masses composed of various intrusive,
porphyritic rocks, many of them andesitic. One common variety was
greenish-black, with large crystals of blackish albite. At Panuncillo a
short N.N.W. and S.S.E. ridge, with a nucleus formed of greenstone and
of a slate-coloured porphyry including crystals of glassy feldspar,
deserves notice, from the very singular nature of the almost vertical
strata composing it. These consist chiefly of a finer and coarser
granular mixture, not very compact, of white carbonate of lime, of
protoxide of iron and of yellowish garnets (ascertained by Professor
Miller), each grain being an almost perfect crystal. Some of the
varieties consist exclusively of granules of the calcareous spar; and
some contain grains of copper ore, and, I believe, of quartz. These
strata alternate with a bluish, compact, fusible, feldspathic rock.
Much of the above granular mixture has, also, a pseudo-brecciated
structure, in which fragments are obscurely arranged in planes parallel
to those of the stratification, and are conspicuous on the weathered
surfaces. The fragments are angular or rounded, small or large, and
consist of bluish or reddish compact feldspathic matter, in which a few
acicular crystals of feldspar can sometimes be seen. The fragments
often blend at their edges into the surrounding granular mass, and seem
due to a kind of concretionary action.

These singular rocks are traversed by many copper veins, and appear to
rest conformably on the granular mixture (in parts as fine-grained as a
sandstone) of quartz, mica, hornblende, and feldspar; and this on
fine-grained, common gneiss; and this on a laminated mass, composed of
pinkish _orthitic_ feldspar, including a few specks of hornblende; and
lastly, this on granite, which together with andesitic rocks, form the
surrounding district.

_Coquimbo: Mining district of Arqueros._—At Coquimbo the porphyritic
conglomerate formation approaches nearer to the Pacific than in any
other part of Chile visited by me, being separated from the coast by a
tract only a few miles broad of the usual plutonic rocks, with the
addition of a porphyry having a red euritic base. In proceeding to the
mines of Arqueros, the strata of porphyritic conglomerate are at first
nearly horizontal, an unusual circumstance, and afterwards they dip
gently to S.S.E. After having ascended to a considerable height, we
come to an undulatory district in which the famous silver mines are
situated; my examination was chiefly confined to those of S. Rosa. Most
of the rocks in this district are stratified, dipping in various
directions, and many of them are of so singular a nature, that at the
risk of being tedious I must briefly describe them. The commonest
variety is a dull-red, compact, finely brecciated stone, containing
much iron and innumerable white crystallised particles of carbonate of
lime, and minute extraneous fragments. Another variety is almost
equally common near S. Rosa; it has a bright green, scanty basis,
including distinct crystals and patches of white carbonate of lime, and
grains of red, semi-micaceous oxide of iron; in parts the basis becomes
dark green, and assumes an obscure crystalline arrangement, and
occasionally in parts it becomes soft and slightly translucent like
soapstone. These red and green rocks are often quite distinct, and
often pass into each other; the passage being sometimes affected by a
fine brecciated structure, particles of the red and green matter being
mingled together. Some of the varieties appear gradually to become
porphyritic with feldspar; and all of them are easily fusible into pale
or dark-coloured beads, strongly attracted by the magnet. I should
perhaps have mistaken several of these stratified rocks for submarine
lavas, like some of those described at the Puente del Inca, had I not
examined, a few leagues eastward of this point, a fine series of
analogous but less metamorphosed, sedimentary beds belonging to the
gypseous formation, and probably derived from a volcanic source.

This formation is intersected by numerous metalliferous veins, running,
though irregularly, N.W. and S.E., and generally at right angles to the
many dikes. The veins consist of native silver, of muriate of silver,
an amalgam of silver, cobalt, antimony, and arsenic,[1] generally
embedded in sulphate of barytes. I was assured by Mr. Lambert, that
native copper without a trace of silver has been found in the same vein
with native silver without a trace of copper. At the mines of Aristeas,
the silver veins are said to be unproductive as soon as they pass into
the green strata, whereas at S. Rosa, only two or three miles distant,
the reverse happens; and at the time of my visit, the miners were
working through a red stratum, in the hope of the vein becoming
productive in the underlying green sedimentary mass. I have a specimen
of one of these green rocks, with the usual granules of white
calcareous spar and red oxide of iron, abounding with disseminated
particles of glittering native and muriate of silver, yet taken at the
distance of one yard from any vein,—a circumstance, as I was assured,
of very rare occurrence.

 [1] See the Report on M. Domeyko’s account of those mines, in the
 “Comptes Rendus,” tome xiv, p. 560.

_Section eastward, up the Valley of Coquimbo._—After passing for a few
miles over the coast granitic series, we come to the porphyritic
conglomerate, with its usual characters, and with some of the beds
distinctly displaying their mechanical origin. The strata, where first
met with, are, as before stated, only slightly inclined; but near the
Hacienda of Pluclaro, we come to an anticlinal axis, with the beds much
dislocated and shifted by a great fault, of which not a trace is
externally seen in the outline of the hill. I believe that this
anticlinal axis can be traced northwards, into the district of
Arqueros, where a conspicuous hill called Cerro Blanco, formed of a
harsh, cream-coloured euritic rock, including a few crystals of reddish
feldspar, and associated with some
purplish claystone porphyry, seems to fall on a line of elevation. In
descending from the Arqueros district, I crossed on the northern border
of the valley, strata inclined eastward from the Pluclaro axis: on the
porphyritic conglomerate there rested a mass, some hundred feet thick,
of brown argillaceous limestone, in parts crystalline, and in parts
almost composed of _Hippurites Chilensis,_ d’Orbigny; above this came a
black calcareous shale, and on it a red conglomerate. In the brown
limestone, with the Hippurites, there was an impression of a Pecten and
a coral, and great numbers of a large Gryphæa, very like, and,
according to Professor E. Forbes, probably identical with _G.
Orientalis,_ Forbes MS.,—a cretaceous species (probably upper
greensand) from Verdachellum, in Southern India. These fossils seem to
occupy nearly the same position with those at the Puente del
Inca,—namely, at the top of the porphyritic conglomerate, and at the
base of the gypseous formation.

A little above the Hacienda of Pluclaro, I made a detour on the
northern side of the valley, to examine the superincumbent gypseous
strata, which I estimated at 6,000 feet in thickness. The uppermost
beds of the porphyritic conglomerate, on which the gypseous strata
conformably rest, are variously coloured, with one very singular and
beautiful stratum composed of purple pebbles of various kinds of
porphyry, embedded in white calcareous spar, including cavities lined
with bright-green crystallised epidote. The whole pile of strata
belonging to both formations is inclined, apparently from the
above-mentioned axis of Pluclaro, at an angle of between 20 and 30
degrees to the east. I will here give a section of the principal beds
met with in crossing the entire thickness of the gypseous strata.

Firstly: above the porphyritic conglomerate formation, there is a
fine-grained, red, crystalline sandstone.

Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly
rock, often marked with dendritic manganese, and having, where most
compact, the external appearance of honestone. It is easily fusible. I
shall for the future, for convenience’ sake, call this variety
pseudo-honestone. Some of the varieties are quite black when freshly
broken, but all weather into a yellowish-ash coloured, soft, earthy
substance, precisely as is the case with the compact shaly rocks of the
Peuquenes range. This stratum is of the same general nature with many
of the beds near Los Hornos in the Illapel section. In this second bed,
or in the underlying red sandstone (for the surface was partially
concealed by detritus), there was a thick mass of gypsum, having the
same mineralogical characters with the great beds described in our
sections across the Cordillera.

Thirdly: a thick stratum of fine-grained, red, sedimentary matter,
easily fusible into a white glass, like the basis of claystone
porphyry; but in parts jaspery, in parts brecciated, and including
crystalline specks of carbonate of lime. In some of the jaspery layers,
and in some of the black siliceous slaty bands, there were irregular
seams of imperfect pitchstone, undoubtedly of metamorphic origin, and
other seams of brown, crystalline limestone. Here, also, were masses,
externally resembling ill-preserved silicified wood.


Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum
concealed by detritus.

Sixthly: a thinly stratified mass of bright green, compact,
smooth-grained, calcareo-argillaceous stone, easily fusible, and
emitting a strong aluminous odour: the whole has a highly
angulo-concretionary structure; and it resembles, to a certain extent,
some of the upper tufaceo-infusorial deposits of the Patagonian
tertiary formation. It is in its nature allied to our pseudo-honestone,
and it includes well characterised layers of that variety; and other
layers of a pale green, harder, and brecciated variety; and others of
red sedimentary matter, like that of bed Three. Some pebbles of
porphyries are embedded in the upper part.

Seventhly: red sedimentary matter or sandstone like that of bed One,
several hundred feet in thickness, and including jaspery layers, often
having a finely brecciated structure.

Eighthly: white, much indurated, almost crystalline tuff, several
hundred feet in thickness, including rounded grains of quartz and
particles of green matter like that of bed Six. Parts pass into a very
pale green, semi-porcellanic stone.

Ninthly: red or brown coarse conglomerate, three or four hundred feet
thick, formed chiefly of pebbles of porphyries, with volcanic
particles, in an arenaceous, non-calcareous, fusible basis: the upper
two feet are arenaceous without any pebbles.

Tenthly: the last and uppermost stratum here exhibited, is a compact,
slate-coloured porphyry, with numerous elongated crystals of glassy
feldspar, from one hundred and fifty to two hundred feet in thickness;
it lies strictly conformably on the underlying conglomerate, and is
undoubtedly a submarine lava.

This great pile of strata has been broken up in several places by
intrusive hillocks of purple claystone porphyry, and by dikes of
porphyritic greenstone: it is said that a few poor metalliferous veins
have been discovered here. From the fusible nature and general
appearance of the finer-grained strata, they probably owe their origin
(like the allied beds of the Uspallata range, and of the Upper
Patagonian tertiary formations), to gentle volcanic eruptions, and to
the abrasion of volcanic rocks. Comparing these beds with those in the
mining district of Arqueros, we see at both places rocks easily
fusible, of the same peculiar bright green and red colours, containing
calcareous matter, often having a finely brecciated structure, often
passing into each other, and often alternating together: hence I cannot
doubt that the only difference between them, lies in the Arqueros beds
having been more metamorphosed (in conformity with their more
dislocated and injected condition), and consequently in the calcareous
matter, oxide of iron and green colouring matter, having been
segregated under a more crystalline form.

The strata are inclined, as before stated, from 20° to 30° eastward,
towards an irregular north and south chain of andesitic porphyry and of
porphyritic greenstone, where they are abruptly cut off. In the valley
of Coquimbo, near to the H. of Gualliguaca, similar plutonic rocks are
met with, apparently a southern prolongation of the above chain; and
eastward of it we have an escarpment of the porphyritic conglomerate,
with the strata inclined at a small angle eastward, which makes the
third escarpment, including that nearest the coast. Proceeding up the
valley we come to another north and south line of granite, andesite,
and blackish porphyry, which seem to lie in an irregular trough of the
porphyritic conglomerate. Again, on the south side of the R. Claro,
there are some irregular granitic hills, which have thrown off the
strata of porphyritic conglomerate to the N.W. by W.; but the
stratification here has been much disturbed. I did not proceed any
farther up the valley, and this point is about two-thirds of the
distance between the Pacific and the main Cordillera.

I will describe only one other section, namely, on the north side of
the R. Claro, which is interesting from containing fossils: the strata
are much dislocated by faults and dikes, and are inclined to the north,
towards a mountain of andesite and porphyry, into which they appear to
become almost blended. As the beds approach this mountain, their
inclination increases up to an angle of 70°, and in the upper part, the
rocks become highly metamorphosed. The lowest bed visible in this
section, is a purplish hard sandstone. Secondly, a bed two or three
hundred feet thick, of a white siliceous sandstone, with a calcareous
cement, containing seams of slaty sandstone, and of hard
yellowish-brown (dolomitic?) limestone; numerous, well-rounded, little
pebbles of quartz are included in the sandstone. Thirdly, a dark
coloured limestone with some quartz pebbles, from fifty to sixty feet
in thickness, containing numerous silicified shells, presently to be
enumerated. Fourthly, very compact, calcareous, jaspery sandstone,
passing into (fifthly) a great bed, several hundred feet thick, of
conglomerate, composed of pebbles of white, red, and purple porphyries,
of sandstone and quartz, cemented by calcareous matter. I observed that
some of the finer parts of this conglomerate were much indurated within
a foot of a dike eight feet in width, and were rendered of a paler
colour with the calcareous matter segregated into white crystallised
particles; some parts were stained green from the colouring matter of
the dike. Sixthly, a thick mass, obscurely stratified, of a red
sedimentary stone or sandstone, full of crystalline calcareous matter,
imperfect crystals of oxide of iron, and I believe of feldspar, and
therefore closely resembling some of the highly metamorphosed beds at
Arqueros: this bed was capped by, and appeared to pass in its upper
part into, rocks similarly coloured, containing calcareous matter, and
abounding with minute crystals, mostly elongated and glassy, of reddish
albite. Seventhly, a conformable stratum of fine reddish porphyry with
large crystals of (albitic?) feldspar; probably a submarine lava.
Eighthly, another conformable bed of green porphyry, with specks of
green earth and cream-coloured crystals of feldspar. I believe that
there are other superincumbent crystalline strata and submarine lavas,
but I had not time to examine them.

The upper beds in this section probably correspond with parts of the
great gypseous formation; and the lower beds of red sandstone
conglomerate and fossiliferous limestone no doubt are the equivalents
of
the Hippurite stratum, seen in descending from Arqueros to Pluclaro,
which there lies conformably upon the porphyritic conglomerate
formation. The fossils found in the third bed, consist of:—

Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.”
This species, which occurs here in vast numbers, according to M.
D’Orbigny, resembles certain cretaceous forms.

Ostrea hemispherica, d’Orbigny, “Voyage” etc.
Also resembles, according to the same author, cretaceous forms.

Terebratula ænigma, d’Orbigny, “Voyage” etc. (Pl. XXII, Figs. 10-12.)
Is allied, according to M. d’Orbigny, to T. concinna from the Forest
Marble. A series of this species, collected in several localities
hereafter to be referred to, has been laid before Professor Forbes; and
he informs me that many of the specimens are almost undistinguishable
from our oolitic T. tetrædra, and that the varieties amongst them are
such as are found in that variable species. Generally speaking, the
American specimens of T. ænigma may be distinguished from the British
T. tetrædra, by the surface having the ribs sharp and well-defined to
the beak, whilst in the British species they become obsolete and
smoothed down; but this difference is not constant. Professor Forbes
adds, that, possibly, internal characters may exist, which would
distinguish the American species from its European allies.

Spirifer linguiferoides, E. Forbes.
Professor Forbes states that this species is very near to S. linguifera
of Phillips (a carboniferous limestone fossil), but probably distinct.
M. d’Orbigny considers it as perhaps indicating the Jurassic period.

Ammonites, imperfect impression of.

M. Domeyko has sent to France a collection of fossils, which, I
presume, from the description given, must have come from the
neighbourhood of Arqueros; they consist of:—

Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal.
Ostrea hemispherica, d’Orbigny, “Voyage” Part Pal.
Turritella Andii, d’Orbigny, “Voyage” Part Pal. (Pleurotomaria
Humboldtii of Von Buch).
Hippurites Chilensis, d’Orbigny, “Voyage” Part Pal.
The specimens of this Hippurite, as well as those I collected in my
descent from Arqueros, are very imperfect; but in M. d’Orbigny’s
opinion they resemble, as does the Turritella Andii, cretaceous (upper
greensand) forms.

Nautilus Domeykus, d’Orbigny, “Voyage” Part Pal.
Terebratula ænigma, d’Orbigny, “Voyage” Part Pal.
Terebratula ignaciana, d’Orbigny, “Voyage” Part Pal.
This latter species was found by M. Domeyko in the same block of
limestone with the T. ænigma. According to M. d’Orbigny, it comes near
to T. ornithocephala from the Lias. A series of this species collected
at Guasco, has been examined by Professor E. Forbes, and he states that
it is difficult
to distinguish between some of the specimens and the T. hastata from
the mountain limestone; and that it is equally difficult to draw a line
between them and some Marlstone Terebratulæ. Without a knowledge of the
internal structure, it is impossible at present to decide on their
identity with analogous European forms.

The remarks given on the several foregoing shells, show that, in M.
d’Orbigny’s opinion, the Pecten, Ostrea, Turritella, and Hippurite
indicate the cretaceous period; and the Gryphæa appears to Professor
Forbes to be identical with a species, associated in Southern India
with unquestionably cretaceous forms. On the other hand, the two
Terebratulæ and the Spirifer point, in the opinion both of M. d’Orbigny
and Professor Forbes, to the oolitic series. Hence M. d’Orbigny, not
having himself examined this country, has concluded that there are here
two distinct formations; but the Spirifer and T. ænigma were certainly
included in the same bed with the Pecten and Ostrea, whence I extracted
them; and the geologist M. Domeyko sent home the two Terebratulæ with
the other-named shells, from the same locality, without specifying that
they came from different beds. Again, as we shall presently see, in a
collection of shells given me from Guasco, the same species, and others
presenting analogous differences, are mingled together, and are in the
same condition; and lastly, in three places in the valley of Copiapo, I
found some of these same species similarly grouped. Hence there cannot
be any doubt, highly curious though the fact be, that these several
fossils, namely, the Hippurites, Gryphæa, Ostrea, Pecten, Turritella,
Nautilus, two Terebratulæ, and Spirifer all belong to the same
formation, which would appear to form a passage between the oolitic and
cretaceous systems of Europe. Although aware how unusual the term must
sound, I shall, for convenience’ sake, call this formation
cretaceo-oolitic. Comparing the sections in this valley of Coquimbo
with those in the Cordillera described in the last chapter, and bearing
in mind the character of the beds in the intermediate district of Los
Hornos, there is certainly a close general mineralogical resemblance
between them, both in the underlying porphyritic conglomerate, and in
the overlying gypseous formation. Considering this resemblance, and
that the fossils from the Puente del Inca at the base of the gypseous
formation, and throughout the greater part of its entire thickness on
the Peuquenes range, indicate the Neocomian period,—that is, the dawn
of the cretaceous system, or, as some have believed, a passage between
this latter and the oolitic series—I conclude that probably the
gypseous and associated beds in all the sections hitherto described,
belong to the same great formation, which I have
denominated—cretaceo-oolitic. I may add, before leaving Coquimbo, that
M. Gay found in the neighbouring Cordillera, at the height of 14,000
feet above the sea, a fossiliferous formation, including a Trigonia and
Pholadomya;[2]—both of which genera occur at the Puente del Inca.

 [2] D’Orbigny, “Voyage,” Part Géolog., p. 242.

_Coquimbo to Guasco._—The rocks near the coast, and some way inland, do
not differ from those described northwards of Valparaiso: we have
much greenstone, syenite, feldspathic and jaspery slate, and grauwackes
having a basis like that of claystone; there are some large tracts of
granite, in which the constituent minerals are sometimes arranged in
folia, thus composing an imperfect gneiss. There are two large
districts of mica-schists, passing into glossy clay-slate, and
resembling the great formation in the Chonos Archipelago. In the valley
of Guasco, an escarpment of porphyritic conglomerate is first seen high
up the valley, about two leagues eastward of the town of Ballenar. I
heard of a great gypseous formation in the Cordillera; and a collection
of shells made there was given me. These shells are all in the same
condition, and appear to have come from the same bed: they consist of:—

Turritella Andii, d’Orbigny, “Voyage,” Part Pal.
Pecten Dufreynoyi, d’Orbigny, “Voyage,” Part Pal.
Terebatula ignaciana, d’Orbigny, “Voyage,” Part Pal.
The relations of these species have been given under the head of
Coquimbo.

Terebratula ænigma, d’Orbigny, “Voyage,” Part Pal.
This shell M. d’Orbigny does not consider identical with his T. ænigma,
but near to T. obsoleta. Professor Forbes thinks that it is certainly a
variety of T. ænigma: we shall meet with this variety again at Copiapo.

Spirifer Chilensis, E. Forbes.
Professor Forbes remarks that this fossil resembles several
carboniferous limestone Spirifers; and that it is also related to some
liassic species, as S. Wolcotii.


If these shells had been examined independently of the other
collections, they would probably have been considered, from the
characters of the two Terebratulæ, and from the Spirifer, as oolitic;
but considering that the first species, and according to Professor
Forbes, the four first, are identical with those from Coquimbo, the two
formations no doubt are the same, and may, as I have said, be
provisionally called cretaceo-oolitic.

_Valley of Copiapo._—The journey from Guasco to Copiapo, owing to the
utterly desert nature of the country, was necessarily so hurried, that
I do not consider my notes worth giving. In the valley of Copiapo some
of the sections are very interesting. From the sea to the town of
Copiapo, a distance estimated at thirty miles, the mountains are
composed of greenstone, granite, andesite, and blackish porphyry,
together with some dusky-green feldspathic rocks, which I believe to be
altered clay-slate: these mountains are crossed by many brown-coloured
dikes, running north and south. Above the town, the main valley runs in
a south-east and even more southerly course towards the Cordillera,
where it is divided into three great ravines, by the northern one of
which, called Jolquera, I penetrated for a short distance. The section,
Fig. 3 in Plate V, gives an eye-sketch of the structure and composition
of the mountains on both sides of this valley: a straight east and west
line from the town to the Cordillera is perhaps
not more than thirty miles, but along the valley the distance is much
greater. Wherever the valley trended very southerly, I have endeavoured
to contract the section into its true proportion. This valley, I may
add, rises much more gently than any other valley which I saw in Chile.

To commence with our section, for a short distance above the town we
have hills of the granitic series, together with some of that rock [A],
which I suspect to be altered clay-slate, but which Professor G. Rose,
judging from specimens collected by Meyen at P. Negro, states is
serpentine passing into greenstone. We then come suddenly to the great
gypseous formation [B], without having passed over, differently from,
in all the sections hitherto described, any of the porphyritic
conglomerate. The strata are at first either horizontal or gently
inclined westward; then highly inclined in various directions, and
contorted by underlying masses of intrusive rocks; and lastly, they
have a regular eastward dip, and form a tolerably well pronounced north
and south line of hills. This formation consists of thin strata, with
innumerable alternations, of black, calcareous slate-rock, of
calcareo-aluminous stones like those at Coquimbo, which I have called
pseudo-honestones of green jaspery layers, and of pale-purplish,
calcareous, soft rotten-stone, including seams and veins of gypsum.
These strata are conformably overlaid by a great thickness of thinly
stratified, compact limestone with included crystals of carbonate of
lime. At a place called Tierra Amarilla, at the foot of a mountain thus
composed there is a broad vein, or perhaps stratum, of a beautiful and
curious crystallised mixture, composed, according to Professor G.
Rose,[3] of sulphate of iron under two forms, and of the sulphates of
copper and alumina: the section is so obscure that I could not make out
whether this vein or stratum occurred in the gypseous formation, or
more probably in some underlying masses [A], which I believe are
altered clay-slate.

 [3] Meyen’s “Reise,” etc., Th. I, s. 394.

_Second axis of elevation._—After the gypseous masses [B], we come to a
line of hills of unstratified porphyry [C], which on their eastern side
blend into strata of great thickness of porphyritic conglomerate,
dipping eastward. This latter formation, however, here has not been
nearly so much metamorphosed as in most parts of Central Chile; it is
composed of beds of true purple claystone porphyry, repeatedly
alternating with thick beds of purplish-red conglomerate with the
well-rounded, large pebbles of various porphyries, not blended
together. _Third axis of elevation._—Near the ravine of Los Hornitos,
there is a well-marked line of elevation, extending for many miles in a
N.N.E. and S.S.W. direction, with the strata dipping in most parts (as
in the second axis) only in one direction, namely, eastward at an
average angle of between 30° and 40°. Close to the mouth of the valley,
however, there is, as represented in the section, a steep and high
mountain [D], composed of various green and brown intrusive porphyries
enveloped with strata, apparently belonging to the upper parts of the
porphyritic
conglomerate, and dipping both eastward and westward. I will describe
the section seen on the eastern side of this mountain [D], beginning at
the base with the lowest bed visible in the porphyritic conglomerate,
and proceeding upwards through the gypseous formation. Bed 1 consists
of reddish and brownish porphyry varying in character, and in many
parts highly amygdaloidal with carbonate of lime, and with bright green
and brown bole. Its upper surface is throughout clearly defined, but
the lower surface is in most parts indistinct, and towards the summit
of the mountain [D] quite blended into the intrusive porphyries. Bed 2,
a pale lilac, hard but not heavy stone, slightly laminated, including
small extraneous fragments, and imperfect as well as some perfect and
glassy crystals of feldspar; from one hundred and fifty to two hundred
feet in thickness. When examining it in situ, I thought it was
certainly a true porphyry, but my specimens now lead me to suspect that
it possibly may be a metamorphosed tuff. From its colour it could be
traced for a long distance, overlying in one part, quite conformably to
the porphyry of bed 1, and in another not distant part, a very thick
mass of conglomerate, composed of pebbles of a porphyry chiefly like
that of bed 1: this fact shows how the nature of the bottom formerly
varied in short horizontal distances. Bed 3, white, much indurated
tuff, containing minute pebbles, broken crystals, and scales of mica,
varies much in thickness. This bed is remarkable from containing many
globular and pear-shaped, externally rusty balls, from the size of an
apple to a man’s head, of very tough, slate-coloured porphyry, with
imperfect crystals of feldspar: in shape these balls do not resemble
pebbles, _and i believe that they are subaqueous volcanic bombs_; they
differ from _subaerial_ bombs only in not being vesicular. Bed 4; a
dull purplish-red, hard conglomerate, with crystallised particles and
veins of carbonate of lime, from three hundred to four hundred feet in
thickness. The pebbles are of claystone porphyries of many varieties;
they are tolerably well rounded, and vary in size from a large apple to
a man’s head. This bed includes three layers of coarse, black,
calcareous, somewhat slaty rock: the upper part passes into a compact
red sandstone.

In a formation so highly variable in mineralogical nature, any division
not founded on fossil remains, must be extremely arbitrary:
nevertheless, the beds below the last conglomerate may, in accordance
with all the sections hitherto described, be considered as belonging to
the porphyritic conglomerate, and those above it to the gypseous
formation, marked [E] in the section. The part of the valley in which
the following beds are seen is near Potrero Seco. Bed 5, compact,
fine-grained, pale greenish-grey, non-calcareous, indurated mudstone,
easily fusible into a pale green and white glass. Bed 6, purplish,
coarse-grained, hard sandstone, with broken crystals of feldspar and
crystallised particles of carbonate of lime; it possesses a slightly
nodular structure. Bed 7, blackish-grey, much indurated, calcareous
mudstone, with extraneous particles of unequal size; the whole being in
parts finely brecciated. In this mass there is a stratum, twenty feet
in thickness, of impure gypsum. Bed 8, a greenish mudstone, with
several layers of gypsum. Bed 9,
a highly indurated, easily fusible, white tuff, thickly mottled with
ferruginous matter, and including some white semi-porcellanic layers,
which are interlaced with ferruginous veins. This stone closely
resembles some of the commonest varieties in the Uspallata chain. Bed
10, a thick bed of rather bright green, indurated mudstone or tuff,
with a concretionary nodular structure so strongly developed that the
whole mass consists of balls. I will not attempt to estimate the
thickness of the strata in the gypseous formation hitherto described,
but it must certainly be very many hundred feet. Bed 11 is at least 800
feet in thickness: it consists of thin layers of whitish, greenish, or
more commonly brown, fine-grained, indurated tuffs, which crumble into
angular fragments: some of the layers are semi-porcellanic, many of
them highly ferruginous, and some are almost composed of carbonate of
lime and iron with drusy cavities lined with quartzf-crystals. Bed 12,
dull purplish or greenish or dark-grey, very compact and much indurated
mudstone: estimated at 1,500 feet in thickness: in some parts this rock
assumes the character of an imperfect coarse clay-slate; but viewed
under a lens, the basis always has a mottled appearance, with the edges
of the minute component particles blending together. Parts are
calcareous, and there are numerous veins of highly crystalline
carbonate of lime charged with iron. The mass has a nodular structure,
and is divided by only a few planes of stratification: there are,
however, two layers, each about eighteen inches thick, of a dark brown,
finer-grained stone, having a conchoidal, semi-porcellanic fracture,
which can be followed with the eye for some miles across the country.

I believe this last great bed is covered by other nearly similar
alternations; but the section is here obscured by a tilt from the next
porphyritic chain, presently to be described. I have given this section
in detail, as being illustrative of the general character of the
mountains in this neighbourhood; but it must not be supposed that any
one stratum long preserves the same character. At a distance of between
only two and three miles the green mudstones and white indurated tuffs
are to a great extent replaced by red sandstone and black calcareous
shaly rocks, alternating together. The white indurated tuff, bed 11,
here contains little or no gypsum, whereas on the northern and opposite
side of the valley, it is of much greater thickness and abounds with
layers of gypsum, some of them alternating with thin seams of
crystalline carbonate of lime. The uppermost, dark-coloured, hard
mudstone, bed 12, is in this neighbourhood the most constant stratum.
The whole series differs to a considerable extent, especially in its
upper part, from that met with at [BB], in the lower part of the
valley; nevertheless, I do not doubt that they are equivalents. _Fourth
axis of elevation (Valley of Copiapo)._—This axis is formed of a chain
of mountains [F], of which the central masses (near La Punta) consist
of andesite containing green hornblende and coppery mica, and the outer
masses of greenish and black porphyries, together with some fine
lilac-coloured claystone porphyry; all these porphyries being injected
and broken up by small hummocks of andesite. The
central great mass of this latter rock, is covered on the eastern side
by a black, fine-grained, highly micaceous slate, which, together with
the succeeding mountains of porphyry, are traversed by numerous white
dikes, branching from the andesite, and some of them extending in
straight lines, to a distance of at least two miles. The mountains of
porphyry eastward of the micaceous schist soon, but gradually, assume
(as observed in so many other cases) a stratified structure, and can
then be recognised as a part of the porphyritic conglomerate formation.
These strata [G] are inclined at a high angle to the S.E., and form a
mass from fifteen hundred to two thousand feet in thickness. The
gypseous masses to the west already described, dip directly towards
this axis, with the strata only in a few places (one of which is
represented in the section) thrown from it: hence this fourth axis is
mainly uniclinal towards the S.E., and just like our third axis, only
locally anticlinal.

The above strata of porphyritic conglomerate [G] with their
south-eastward dip, come abruptly up against beds of the gypseous
formation [H], which are gently, but irregularly, inclined westward: so
that there is here a synclinal axis and great fault. Further up the
valley, here running nearly north and south, the gypseous formation is
prolonged for some distance; but the stratification is unintelligible,
the whole being broken up by faults, dikes, and metalliferous veins.
The strata consist chiefly of red calcareous sandstones, with numerous
veins in the place of layers, of gypsum; the sandstone is associated
with some black calcareous slate-rock, and with green
pseudo-honestones, passing into porcelain-jasper. Still further up the
valley, near Las Amolanas [I], the gypseous strata become more regular,
dipping at an angle of between 30 and 40 degrees to W.S.W., and
conformably overlying, near the mouth of the ravine of Jolquera, strata
[K] of porphyritic conglomerate. The whole series has been tilted by a
partially concealed axis [L], of granite, andesite, and a granitic
mixture of white feldspar, quartz, and oxide of iron.

_Fifth axis of elevation (Valley of Copiapo, near Las Amolanas)._—I
will describe in some detail the beds [I] seen here, which, as just
stated, dip to W.S.W., at an angle of from 30° to 40°. I had not time
to examine the underlying porphyritic conglomerate, of which the lowest
beds, as seen at the mouth of the Jolquera, are highly compact, with
crystals of red oxide of iron; and I am not prepared to say whether
they are chiefly of volcanic or metamorphic origin. On these beds there
rests a coarse purplish conglomerate, very little metamorphosed,
composed of pebbles of porphyry, but remarkable from containing one
pebble of granite;—of which fact no instance has occurred in the
sections hitherto described. Above this conglomerate, there is a black
siliceous claystone, and above it numerous alternations of
dark-purplish and green porphyries, which may be considered as the
uppermost limit of the porphyritic conglomerate formation.

Above these porphyries comes a coarse, arenaceous conglomerate, the
lower half white and the upper half of a pink colour, composed chiefly
of pebbles of various porphyries, but with some of red sandstone
and jaspery rocks. In some of the more arenaceous parts of the
conglomerate, there was an oblique or current lamination; a
circumstance which I did not elsewhere observe. Above this
conglomerate, there is a vast thickness of thinly stratified,
pale-yellowish, siliceous sandstone, passing into a granular
quartz-rock, used for grindstones (hence the name of the place _ Las
Amolanas_), and certainly belonging to the gypseous formation, as does
probably the immediately underlying conglomerate. In this yellowish
sandstone there are layers of white and pale-red siliceous
conglomerate; other layers with small, well-rounded pebbles of white
quartz, like the bed at the R. Claro at Coquimbo; others of a greenish,
fine-grained, less siliceous stone, somewhat resembling the
pseudo-honestones lower down the valley; and lastly, others of a black
calcareous shale-rock. In one of the layers of conglomerate, there was
embedded a fragment of mica-slate, of which this is the first instance;
hence perhaps, it is from a formation of mica-slate, that the numerous
small pebbles of quartz, both here and at Coquimbo, have been derived.
Not only does the siliceous sandstone include layers of the black,
thinly stratified, not fissile, calcareous shale-rock, but in one place
the whole mass, especially the upper part, was, in a marvellously short
horizontal distance, after frequent alternations, replaced by it. When
this occurred, a mountain-mass, several thousand feet in thickness was
thus composed; the black calcareous shale-rock, however, always
included some layers of the pale-yellowish siliceous sandstone, of the
red conglomerate, and of the greenish jaspery and pseudo-honestone
varieties. It likewise included three or four widely separated layers
of a brown limestone, abounding with shells immediately to be
described. This pile of strata was in parts traversed by many veins of
gypsum. The calcareous shale-rock, though when freshly broken quite
black, weathers into an ash- colour: in which respect and in general
appearance, it perfectly resembles those great fossiliferous beds of
the Peuquenes range, alternating with gypsum and red sandstone,
described in the last chapter.

The shells out of the layers of brown limestone, included in the black
calcareous shale-rock, which latter, as just stated, replaces the white
siliceous sandstone, consist of:—

Pecten Dufreynoyi, d’Orbigny, “Voyage,” Part Pal.
Turritella Andii, d’Orbigny, “Voyage,” Part Pal.

 Astarte Darwinii, E. Forbes.
Gryphæa Darwinii, E. Forbes.
An intermediate form between G. gigantea and G. incurva.

Gryphæa nov. spec.?, E. Forbes.
Perna Americana, E. Forbes.
Avicula, nov. spec.
Considered by Mr. G. B. Sowerby as the A. echinata, by M. d’Orbigny as
certainly a new and distinct species, having a Jurassic aspect. The
specimen has been unfortunately lost.


Terebratula ænigma, d’Orbigny, (var. of do. E. Forbes.)
This is the same variety, with that from Guasco, considered by M.
D’Orbigny to be a distinct species from his T. ænigma, and related to
T. obsoleta.

Plagiostoma and Ammonites, fragments of.

The lower layers of the limestone contained thousands of the Gryphæa;
and the upper ones as many of the Turritella, with the Gryphæa (nov.
species) and Serpulæ adhering to them; in all the layers, the
Terebratula and fragments of the Pecten were included. It was evident,
from the manner in which species were grouped together, that they had
lived where now embedded. Before making any further remarks, I may
state, that higher up this same valley we shall again meet with a
similar association of shells; and in the great Despoblado Valley,
which branches off near the town from that of Copiapo, the Pecten
Dufreynoyi, some Gryphites (I believe G. Darwinii), and the _ true_
Terebratula ænigma of d’Orbigny were found together in an equivalent
formation, as will be hereafter seen. A specimen also, I may add, of
the true T. ænigma, was given me from the neighbourhood of the famous
silver mines of Chanuncillo, a little south of the valley of the
Copiapo, and these mines, from their position, I have no doubt, lie
within the great gypseous formation: the rocks close to one of the
silver veins, judging from fragments shown me, resemble those singular
metamorphosed deposits from the mining district of Arqueros near
Coquimbo.

I will reiterate the evidence on the association of these several
shells in the several localities.

_Coquimbo._

In the same bed, Rio Claro:
    Pecten Dufreynoyi.
    Ostrea hemispherica.
    Terebratula ænigma.
    Spirifer linguiferoides.

Same bed, near Arqueros:
    Hippurites Chilensis.
    Gryphæa orientalis.

Collected by M. Domeyko from the same locality, apparently near
Arqueros:
    Terebratula ænigma and Terebratula ignaciana, in same block of
    limestone.
    Pecten Dufreynoyi.
    Ostrea hemispherica.
    Hippurites Chilensis.
    Turritella Andii.
    Nautilus Domeykus.

_Guasco._

In a collection from the Cordillera, given me: the specimens all in the
same condition:
    Pecten Dufreynoyi.
    Turritella Andii.
    Terebratula ignaciana.
    Terebratula ænigma, _var._
    Spirifer Chilensis.

_Copiapo._

Mingled together in alternating beds in the main valley of Copiapo near
Las Amolanas, and likewise higher up the valley:
    Pecten Dufreynoyi.
    Turritella Andii.
    Terebratula ænigma, _var._, as at Guasco.
    Astarte Darwinii.
    Gryphæa Darwinii.
    Gryphæa nov. species?
    Perna Americana.
    Avicula, nov. species.

Main valley of Copiapo, apparently same formation with that of
Amolanas:
    Terebratula ænigma (true).

In the same bed, high up the great lateral valley of the Despoblado, in
the ravine of Maricongo:
    Terebratula ænigma (true).
    Pecten Dufreynoyi.
    Gryphæa Darwinii?

Considering this table, I think it is impossible to doubt that all
these fossils belong to the same formation. If, however, the species
from Las Amolanas, in the Valley of Copiapo, had, as in the case of
those from Guasco, been separately examined, they would probably have
been ranked as oolitic; for, although no Spirifers were found here, all
the other species, with the exception of the Pecten, Turritella, and
Astarte, have a more ancient aspect than cretaceous forms. On the other
hand, taking into account the evidence derived from the cretaceous
character of these three shells, and of the Hippurites, Gryphæa
orientalis, and Ostrea, from Coquimbo, we are driven back to the
provisional name already used of cretaceo-oolitic. From geological
evidence, I believe this formation to be the equivalent of the
Neocomian beds of the Cordillera of Central Chile.

To return to our section near Las Amolanas:—Above the yellow siliceous
sandstone, or the equivalent calcareous slate-rock, with its bands of
fossil-shells, according as the one or other prevails, there is a pile
of strata, which cannot be less than from two to three thousand feet in
thickness, in main part composed of a coarse, bright red conglomerate,
with many intercalated beds of red sandstone, and some of green and
other coloured porcelain-jaspery layers. The included pebbles are
well-rounded, varying from the size of an egg to that of a
cricket-ball, with a few larger; and they consist chiefly of
porphyries. The basis of the conglomerate, as well as some of the
alternating thin beds, are formed of a red, rather harsh, easily
fusible sandstone, with crystalline calcareous particles. This whole
great pile is remarkable from the thousands of huge, embedded,
silicified trunks of trees, one of which was eight feet long, and
another eighteen feet in circumference: how marvellous it is, that
every vessel in so thick a mass of wood should have been converted into
silex! I brought home many specimens, and all of them, according to Mr.
R. Brown, present a coniferous structure.

Above this great conglomerate, we have from two to three hundred feet
in thickness of red sandstone; and above this, a stratum of black
calcareous slate-rock, like that which alternates with and
replaces the underlying yellowish-white, siliceous sandstone. Close to
the junction between this upper black slate-rock and the upper red
sandstone, I found the Gryphæa Darwinii, the Turritella Andii, and vast
numbers of a bivalve, too imperfect to be recognised. Hence we see
that, as far as the evidence of these two shells serves—and the
Turritella is an eminently characteristic species—the whole thickness
of this vast pile of strata belongs to the same age. Again, above the
last-mentioned upper red sandstone, there were several alternations of
the black, calcareous slate-rock; but I was unable to ascend to them.
All these uppermost strata, like the lower ones, vary extremely in
character in short horizontal distances. The gypseous formation, as
here seen, has a coarser, more mechanical texture, and contains much
more siliceous matter than the corresponding beds lower down the
valley. Its total thickness, together with the upper beds of the
porphyritic conglomerate, I estimated at least at 8,000 feet; and only
a small portion of the porphyritic conglomerate, which on the eastern
flank of the fourth axis of elevation appeared to be from fifteen
hundred to two thousand feet thick, is here included. As corroborative
of the great thickness of the gypseous formation, I may mention that in
the Despoblado Valley (which branches from the main valley a little
above the town of Copiapo) I found a corresponding pile of red and
white sandstones, and of dark, calcareous, semi-jaspery mudstones,
rising from a nearly level surface and thrown into an absolutely
vertical position; so that, by pacing, I ascertained their thickness to
be nearly two thousand seven hundred feet; taking this as a standard of
comparison, I estimated the thickness of the strata _above_ the
porphyritic conglomerate at 7,000 feet.

The fossils before enumerated, from the limestone-layers in the whitish
siliceous sandstone, are now covered, on the least computation, by
strata from 5,000 to 6,000 feet in thickness. Professor E. Forbes
thinks that these shells probably lived at a depth of from about 30 to
40 fathoms, that is from 180 to 240 feet; anyhow, it is impossible that
they could have lived at the depth of from 5,000 to 6,000 feet. Hence
in this case, as in that of the Puente del Inca, we may safely conclude
that the bottom of the sea on which the shells lived, subsided, so as
to receive the superincumbent submarine strata: and this subsidence
must have taken place during the existence of these shells; for, as I
have shown, some of them occur high up as well as low down in the
series. That the bottom of the sea subsided, is in harmony with the
presence of the layers of coarse, well-rounded pebbles included
throughout this whole pile of strata, as well as of the great upper
mass of conglomerate from 2,000 to 3,000 feet thick; for coarse gravel
could hardly have been formed or spread out at the profound depths
indicated by the thickness of the strata. The subsidence, also, must
have been slow to have allowed of this often-recurrent spreading out of
the pebbles. Moreover, we shall presently see that the surfaces of some
of the streams of porphyritic lava beneath the gypseous formation, are
so highly amygdaloidal that it is scarcely possible to believe that
they flowed under the vast pressure of a deep ocean. The conclusion of
a
great subsidence during the existence of these cretaceo-oolitic
fossils, may, I believe, be extended to the district of Coquimbo,
although owing to the fossiliferous beds there not being directly
covered by the upper gypseous strata, which in the section north of the
valley are about 6,000 feet in thickness, I did not there insist on
this conclusion.

The pebbles in the above conglomerates, both in the upper and lower
beds, are all well rounded, and, though chiefly composed of various
porphyries, there are some of red sandstone and of a jaspery stone,
both like the rocks intercalated in layers in this same gypseous
formation; there was one pebble of mica-slate and some of quartz,
together with many particles of quartz. In these respects there is a
wide difference between the gypseous conglomerates and those of the
porphyritic-conglomerate formation, in which latter, angular and
rounded fragments, almost exclusively composed of porphyries, are
mingled together, and which, as already often remarked, probably were
ejected from craters deep under the sea. From these facts I conclude,
that during the formation of the conglomerates, land existed in the
neighbourhood, on the shores of which the innumerable pebbles were
rounded and thence dispersed, and on which the coniferous forests
flourished—for it is improbable that so many thousand logs of wood
should have drifted from any great distance. This land, probably
islands, must have been mainly formed of porphyries, with some
mica-slate, whence the quartz was derived, and with some red sandstone
and jaspery rocks. This latter fact is important, as it shows that in
this district, even previously to the deposition of the lower gypseous
or cretaceo-oolitic beds, strata of an analogous nature had elsewhere,
no doubt in the more central ranges of the Cordillera, been elevated;
thus recalling to our minds the relations of the Cumbre and Uspallata
chains. Having already referred to the great lateral valley of the
Despoblado, I may mention that above the 2,700 feet of red and white
sandstone and dark mudstone, there is a vast mass of coarse, hard, red
conglomerate, some thousand feet in thickness, which contains much
silicified wood, and evidently corresponds with the great upper
conglomerate at Las Amolanas: here, however, the conglomerate consists
almost exclusively of pebbles of granite, and of disintegrated crystals
of reddish feldspar and quartz firmly recemented together. In this
case, we may conclude that the land whence the pebbles were derived,
and on which the now silicified trees once flourished, was formed of
granite.

The mountains near Las Amolanas, composed of the cretaceo-oolitic
strata, are interlaced with dikes like a spider’s web, to an extent
which I have never seen equalled, except in the denuded interior of a
volcanic crater: north and south lines, however, predominate. These
dikes are composed of green, white, and blackish rocks, all porphyritic
with feldspar, and often with large crystals of hornblende. The white
varieties approach closely in character to andesite, which composes as
we have seen, the injected axes of so many of the lines of elevation.
Some of the green varieties are finely laminated, parallel to the walls
of the dikes.

_Sixth axis of elevation (Valley of Copiapo)._—This axis consists of
a broad mountainous mass [O] of andesite, composed of albite, brown
mica, and chlorite, passing into andesitic granite, with quartz: on its
western side it has thrown off, at a considerable angle, a thick mass
of stratified porphyries, including much epidote [NN], and remarkable
only from being divided into very thin beds, as highly amygdaloidal on
their surfaces as subaerial lava-streams are often vesicular. This
porphyritic formation is conformably covered, as seen some way up the
ravine of Jolquera, by a mere remnant of the lower part of the
cretaceo-oolitic formation [MM], which in one part encases, as
represented in the coloured section, the foot of the andesitic axis
[L], of the already described fifth line, and in another part entirely
conceals it: in this latter case, the gypseous or cretaceo-oolitic
strata falsely appeared to dip under the porphyritic conglomerate of
the fifth axis. The lowest bed of the gypseous formation, as seen here
[M], is of yellowish siliceous sandstone, precisely like that of
Amolanas, interlaced in parts with veins of gypsum, and including
layers of the black, calcareous, non-fissile slate-rock: the _
Turritella Andii, Pecten Dufreynoyi, Terebratula ænigma, var.,_ and
some Gryphites were embedded in these layers. The sandstone varies in
thickness from only twenty to eighty feet; and this variation is caused
by the inequalities in the upper surface of an underlying stream of
purple claystone porphyry. Hence the above fossils here lie at the very
base of the gypseous or cretaceo-oolitic formation, and hence they were
probably once covered up by strata about seven thousand feet in
thickness: it is, however, possible, though from the nature of all the
other sections in this district not probable, that the porphyritic
claystone lava may in this case have invaded a higher level in the
series. Above the sandstone there is a considerable mass of much
indurated, purplish-black, calcareous claystone, allied in nature to
the often-mentioned black calcareous slate-rock.

Eastward of the broad andesitic axis of this sixth line, and penetrated
by many dikes from it, there is a great formation [P] of mica-schist,
with its usual variations, and passing in one part into a ferruginous
quartz-rock. The folia are curved and highly inclined, generally
dipping eastward. It is probable that this mica-schist is an old
formation, connected with the granitic rocks and metamorphic schists
near the coast; and that the one fragment of mica-slate, and the
pebbles of quartz low down in the gypseous formation at Las Amolanas,
have been derived from it. The mica-schist is succeeded by stratified
porphyritic conglomerate [Q] of great thickness, dipping eastward with
a high inclination: I have included this latter mountain-mass in the
same anticlinal axis with the porphyritic streams [NN]; but I am far
from sure that the two masses may not have been independently upheaved.

_Seventh axis of elevation._—Proceeding up the ravine, we come to
another mass [R] of andesite; and beyond this, we again have a very
thick, stratified porphyritic formation [S], dipping at a small angle
eastward, and forming the basal part of the main Cordillera. I did not
ascend the ravine any higher; but here, near Castano, I examined
several sections, of which I will not give the details, only observing,
that the porphyritic beds, or submarine lavas, preponderate greatly in
bulk over the alternating sedimentary layers, which have been but
little metamorphosed: these latter consist of fine-grained red tuffs
and of whitish volcanic grit-stones, together with much of a singular,
compact rock, having an almost crystalline basis, finely brecciated
with red and green fragments, and occasionally including a few large
pebbles. The porphyritic lavas are highly amygdaloidal, both on their
upper and lower surfaces; they consist chiefly of claystone porphyry,
but with one common variety, like some of the streams at the Puente del
Inca, having a grey mottled basis, abounding with crystals of red
hydrous oxide of iron, green ones apparently of epidote, and a few
glassy ones of feldspar. This pile of strata differs considerably from
the basal strata of the Cordillera in Central Chile, and may possibly
belong to the upper and gypseous series: I saw, however, in the bed of
the valley, one fragment of porphyritic breccia-conglomerate, exactly
like those great masses met with in the more southern parts of Chile.

Finally, I must observe, that though I have described between the town
of Copiapo and the western flank of the main Cordillera seven or eight
axes of elevation, extending nearly north and south, it must not be
supposed that they all run continuously for great distances. As was
stated to be the case in our sections across the Cordillera of Central
Chile, so here most of the lines of elevation, with the exception of
the first, third, and fifth, are very short. The stratification is
everywhere disturbed and intricate; nowhere have I seen more numerous
faults and dikes. The whole district, from the sea to the Cordillera,
is more or less metalliferous; and I heard of gold, silver, copper,
lead, mercury, and iron veins. The metamorphic action, even in the
lower strata, has certainly been far less here than in Central Chile.

_Valley of the Despoblado._—This great barren valley, which has already
been alluded to, enters the main valley of Copiapo a little above the
town: it runs at first northerly, then N.E., and more easterly into the
Cordillera; I followed its dreary course to the foot of the first main
ridge. I will not give a detailed section, because it would be
essentially similar to that already given, and because the
stratification is exceedingly complicated. After leaving the plutonic
hills near the town, I met first, as in the main valley, with the
gypseous formation, having the same diversified character as before,
and soon afterwards with masses of porphyritic conglomerate, about one
thousand feet in thickness. In the lower part of this formation there
were very thick beds composed of fragments of claystone porphyries,
both angular and rounded, with the smaller ones partially blended
together and the basis rendered porphyritic; these beds separated
distinct streams, from sixty to eighty feet in thickness, of claystone
lavas. Near Paipote, also, there was much true porphyritic
breccia-conglomerate: nevertheless, few of these masses were
metamorphosed to the same degree with the corresponding formation in
Central Chile. I did not meet in this valley with any true andesite,
but only with imperfect andesitic porphyry, including large crystals of
hornblende: numerous as have been the varieties of intrusive porphyries
already mentioned, there were here
mountains composed of a new kind, having a compact, smooth,
cream-coloured basis, including only a few crystals of feldspar, and
mottled with dendritic spots of oxide of iron. There were also some
mountains of a porphyry with a brick-red basis, containing irregular,
often lens-shaped, patches of compact feldspar, and crystals of
feldspar, which latter to my surprise I find to be orthite.

At the foot of the first ridge of the main Cordillera, in the ravine of
Maricongo, and at an elevation which, from the extreme coldness and
appearance of the vegetation, I estimated at about ten thousand feet, I
found beds of white sandstone and of limestone including the Pecten
Dufreynoyi, Terebratula ænigma, and some Gryphites. This ridge throws
the water on the one hand into the Pacific, and on the other, as I was
informed, into a great gravel-covered, basin-like plain, including a
salt-lake, and without any drainage-exit. In crossing the Cordillera by
this Pass, it is said that three principal ridges must be traversed,
instead of two, or only one as in Central Chile.

The crest of this first main ridge and the surrounding mountains, with
the exception of a few lofty pinnacles, are capped by a great thickness
of a horizontally stratified, tufaceous deposit. The lowest bed is of a
pale purple colour, hard, fine-grained, and full of broken crystals of
feldspar and scales of mica. The middle bed is coarser, and less hard,
and hence weathers into very sharp pinnacles; it includes very small
fragments of granite, and innumerable ones of all sizes of grey
vesicular trachyte, some of which were distinctly rounded. The
uppermost bed is about two hundred feet in thickness, of a darker
colour and apparently hard: but I had not time to ascend to it. These
three horizontal beds may be seen for the distance of many leagues,
especially westward or in the direction of the Pacific, capping the
summits of the mountains, and standing on the opposite sides of the
immense valleys at exactly corresponding heights. If united they would
form a plain, inclined very slightly towards the Pacific; the beds
become thinner in this direction, and the tuff (judging from one point
to which I ascended, some way down the valley) finer-grained and of
less specific gravity, though still compact and sonorous under the
hammer. The gently inclined, almost horizontal stratification, the
presence of some rounded pebbles, and the compactness of the lowest
bed, though rendering it probable, would not have convinced me that
this mass had been of subaqueous origin, for it is known that volcanic
ashes falling on land and moistened by rain often become hard and
stratified; but beds thus originating, and owing their consolidation to
atmospheric moisture, would have covered almost equally every
neighbouring summit, high and low, and would not have left those above
a certain exact level absolutely bare; this circumstance seems to me to
prove that the volcanic ejections were arrested at their present,
widely extended, equable level, and there consolidated by some other
means than simple atmospheric moisture; and this no doubt must have
been a sheet of water. A lake at this great height, and without a
barrier on any one side, is out of the question; consequently we must
conclude that the tufaceous matter was anciently deposited beneath the
sea. It was certainly
deposited before the excavation of the valleys, or at least before
their final enlargement;[4] and I may add, that Mr. Lambert, a
gentleman well acquainted with this country, informs me, that in
ascending the ravine of Santandres (which branches off from the
Despoblado) he met with streams of lava and much erupted matter capping
all the hills of granite and porphyry, with the exception of some
projecting points; he also remarked that the valleys had been excavated
subsequently to these eruptions.

 [4] I have endeavoured to show in my “Journal,” etc. (2nd edit.), p.
 355, that this arid valley was left by the retreating sea, as the land
 slowly rose, in the state in which we now see it.


This volcanic formation, which I am informed by Mr. Lambert extends far
northward, is of interest, as typifying what has taken place on a
grander scale on the corresponding western side of the Cordillera of
Peru. Under another point of view, however, it possesses a far higher
interest, as confirming that conclusion drawn from the structure of the
fringes of stratified shingle which are prolonged from the plains at
the foot of the Cordillera far up the valleys,—namely, that this great
range has been elevated in mass to a height of between eight and nine
thousand feet;[5] and now, judging from this tufaceous deposit, we may
conclude that the horizontal elevation has been in the district of
Copiapo about ten thousand feet.

 [5] I may here mention that on the south side of the main valley of
 Copiapo, near Potrero Seco, the mountains are capped by a thick mass
 of horizontally stratified shingle, at a height which I estimated at
 between fifteen hundred and two thousand feet above the bed of the
 valley. This shingle, I believe, forms the edge of a wide plain, which
 stretches southwards between two mountain ranges.


No. 40


[Illustration]

In the valley of the Despoblado, the stratification, as before remarked
has been much disturbed, and in some points to a greater degree than I
have anywhere else seen. I will give two cases: a very thick mass of
thinly stratified red sandstone, including beds of conglomerate, has
been crushed together (as represented in figure no. 24) into a yoke or
urn-formed trough, so that the strata on both sides have been folded
inwards: on the right hand the properly underlying porphyritic
claystone conglomerate is seen overlying the sandstone, but it soon
becomes vertical, and then is inclined towards the trough, so that the
beds
radiate like the spokes of a wheel: on the left hand, the inverted
porphyritic conglomerate also assumes a dip towards the trough, not
gradually, as on the right hand, but by means of a vertical fault and
synclinal break; and a little still further on towards the left, there
is a second great oblique fault (both shown by the arrow-lines), with
the strata dipping to a directly opposite point; these mountains are
intersected by infinitely numerous dikes, some of which can be seen to
rise from hummocks of greenstone, and can be traced for thousands of
feet. In the second case, two low ridges trend together and unite at
the head of a little wedge-shaped valley: throughout the right-hand
ridge, the strata dip at 45° to the east; in the left-hand ridge, we
have the very same strata and at first with exactly the same dip; but
in following this ridge up the valley, the strata are seen very
regularly to become more and more inclined until they stand vertical,
they then gradually fall over (the basset edges forming symmetrical
serpentine lines along the crest), till at the very head of the valley
they are reversed at an angle of 45°: so that at this point the beds
have been turned through an angle of 135°; and here there is a kind of
anticlinal axis, with the strata on both sides dipping to opposite
points at an angle of 45°, but those on the left hand upside down.

_On the eruptive sources of the porphyritic claystone and greenstone
lavas._—In Central Chile, from the extreme metamorphic action, it is in
most parts difficult to distinguish between the streams of porphyritic
lava and the porphyritic breccia-conglomerate, but here, at Copiapo,
they are generally perfectly distinct, and in the Despoblado, I saw for
the first time, two great strata of purple claystone porphyry, after
having been for a considerable space closely united together, one above
the other, become separated by a mass of fragmentary matter, and then
both thin out;—the lower one more rapidly than the upper and greater
stream. Considering the number and thickness of the streams of
porphyritic lava, and the great thickness of the beds of
breccia-conglomerate, there can be little doubt that the sources of
eruption must originally have been numerous: nevertheless, it is now
most difficult even to conjecture the precise point of any one of the
ancient submarine craters. I have repeatedly observed mountains of
porphyries, more or less distinctly stratified towards their summits or
on their flanks, without a trace of stratification in their central and
basal parts: in most cases, I believe this is simply due either to the
obliterating effects of metamorphic action, or to such parts having
been mainly formed of intrusive porphyries, or to both causes
conjoined; in some instances, however, it appeared to me very probable
that the great central unstratified masses of porphyry were the now
partially denuded nuclei of the old submarine volcanoes, and that the
stratified parts marked the points whence the streams flowed. In one
case alone, and it was in this Valley of the Despoblado, I was able
actually to trace a thick stratum of purplish porphyry, which for a
space of some miles conformably overlay the usual alternating beds of
breccia-conglomerates and claystone lavas, until it became united with,
and blended into, a mountainous mass of various unstratified
porphyries.


The difficulty of tracing the streams of porphyries to their ancient
and doubtless numerous eruptive sources, may be partly explained by the
very general disturbance which the Cordillera in most parts has
suffered; but I strongly suspect that there is a more specific cause,
namely, _that the original points of eruption tend to become the points
of injection._ This in itself does not seem improbable; for where the
earth’s crust has once yielded, it would be liable to yield again,
though the liquified intrusive matter might not be any longer enabled
to reach the submarine surface and flow as lava. I have been led to
this conclusion, from having so frequently observed that, where part of
an unstratified mountain-mass resembled in mineralogical character the
adjoining streams or strata, there were several other kinds of
intrusive porphyries and andesitic rocks injected into the same point.
As these intrusive mountain-masses form most of the axes-lines in the
Cordillera, whether anticlinal, uniclinal, or synclinal, and as the
main valleys have generally been hollowed out along these lines, the
intrusive masses have generally suffered much denudation. Hence they
are apt to stand in some degree isolated, and to be situated at the
points where the valleys abruptly bend, or where the main tributaries
enter. On this view of there being a tendency in the old points of
eruption to become the points of subsequent injection and disturbance,
and consequently of denudation, it ceases to be surprising that the
streams of lava in the porphyritic claystone conglomerate formation,
and in other analogous cases, should most rarely be traceable to their
actual sources.

_Iquique, Southern Peru._—Differently from what we have seen throughout
Chile, the coast here is formed not by the granitic series, but by an
escarpment of the porphyritic conglomerate formation, between two and
three thousand feet in height.[6] I had time only for a very short
examination; the chief part of the escarpment appears to be composed of
various reddish and purple, sometimes laminated, porphyries, resembling
those of Chile; and I saw some of the porphyritic breccia-conglomerate;
the stratification appeared but little inclined. The uppermost part,
judging from the rocks near the famous silver mine of
Huantajaya,[7]consists of laminated, impure, argillaceous,
purplish-grey limestone, associated, I believe, with some purple
sandstone. In the limestone shells are found: the three following
species were given me:—

    Lucina Americana, E. Forbes.
    Terebratula inca, E. Forbes.
    Terebratula ænigma, D’Orbigny.

 [6] The lowest point, where the road crosses the coast-escarpment, is
 1,900 feet by the barometer above the level of the sea.


 [7] Mr. Bollaert has described (“Geolog. Proceedings,” vol. ii, p.
 598, a singular mass of stratified detritus, gravel, and sand,
 eighty-one yards in thickness, overlying the limestone, and abounding
 with loose masses of silver ore. The miners believe that they can
 attribute these masses to their proper veins.

This latter species we have seen associated with the fossils of which
lists have been given in this chapter, in two places in the valley of
Coquimbo, and in the ravine of Maricongo at Copiapo. Considering this
fact, and the superposition of these beds on the porphyritic
conglomerate formation; and, as we shall immediately see, from their
containing much gypsum, and from their otherwise close general
resemblance in mineralogical nature with the strata described in the
valley of Copiapo, I have little doubt that these fossiliferous beds of
Iquique belong to the great cretaceo-oolitic formation of Northern
Chile. Iquique is situated seven degrees latitude north of Copiapo; and
I may here mention, that an Ammonites, nov. species, and an Astarte,
nov. species, were given me from the Cerro Pasco, about ten degrees of
latitude north of Iquique, and M. D’Orbigny thinks that they probably
indicate a Neocomian formation. Again, fifteen degrees of latitude
northward, in Colombia, there is a grand fossiliferous deposit, now
well known from the labours of Von Buch, Lea, d’Orbigny, and Forbes,
which belongs to the earlier stages of the cretaceous system. Hence,
bearing in mind the character of the few fossils from Tierra del Fuego,
there is some evidence that a great portion of the stratified deposits
of the whole vast range of the South American Cordillera belongs to
about the same geological epoch.

Proceeding from the coast escarpment inwards, I crossed, in a space of
about thirty miles, an elevated undulatory district, with the beds
dipping in various directions. The rocks are of many kinds,—white
laminated, sometimes siliceous sandstone,—purple and red sandstone,
sometimes so highly calcareous as to have a crystalline
fracture,—argillaceous limestone,—black calcareous slate-rock, like
that so often described at Copiapo and other places,—thinly laminated,
fine-grained, greenish, indurated, sedimentary, fusible rocks,
approaching in character to the so-called pseudo-honestone of Chile,
including thin contemporaneous veins of gypsum,—and lastly, much
calcareous, laminated porcelain jasper, of a green colour, with red
spots, and of extremely easy fusibility: I noticed one conformable
stratum of a freckled-brown, feldspathic lava. I may here mention that
I heard of great beds of gypsum in the Cordillera. The only novel point
in this formation, is the presence of innumerable thin layers of
rock-salt, alternating with the laminated and hard, but sometimes
earthy, yellowish, or bright red and ferruginous sandstones. The
thickest layer of salt was only two inches, and it thinned out at both
ends. On one of these saliferous masses I noticed a stratum about
twelve feet thick, of dark-brown, hard brecciated, easily fusible rock,
containing grains of quartz and of black oxide of iron, together with
numerous imperfect fragments of shells. The problem of the origin of
salt is so obscure, that every fact, even geographical position, is
worth recording.[8] With the exception of
these saliferous beds, most of the rocks as already remarked, present a
striking general resemblance with the upper parts of the gypseous or
cretaceo-oolitic formation of Chile.

 [8] It is well known that stratified salt is found in several places
 on the shores of Peru. The island of San Lorenzo, off Lima, is
 composed of a pile of thin strata, about eight hundred feet in
 thickness, composed of yellowish and purplish, hard siliceous, or
 earthy sandstones, alternating with thin layers of shale, which in
 places passes into a greenish, semi-porcellanic, fusible rock. There
 are some thin beds of reddish mudstone, and soft ferruginous
 rotten-stones, with layers of gypsum. In nearly all these varieties,
 especially in the softer sandstones, there are numerous thin seams of
 rock-salt: I was informed that one layer has been found two inches in
 thickness. The manner in which the minutest fissures of the dislocated
 beds have been penetrated by the salt, apparently by subsequent
 infiltration, is very curious. On the south side of the island, layers
 of coal and of impure limestone have been discovered. Hence we here
 have salt, gypsum, and coal associated together. The strata include
 veins of quartz, carbonate of lime, and iron pyrites; they have been
 dislocated by an injected mass of greenish-brown feldspathic trap.
    Not only is salt abundant on the extreme western limits of the
    district between the Cordillera and the Pacific, but, according to
    Helms, it is found in the outlying low hills on the eastern flank
    of the Cordillera. These facts appear to me opposed to the theory,
    that rock-salt is due to the sinking of water, charged with salt,
    in mediterranean spaces of the ocean. The general character of the
    geology of these countries would rather lead to the opinion, that
    its origin is in some way connected with volcanic heat at the
    bottom of the sea: see on this subject Sir R. Murchison’s
    “Anniversary Address to Geolog. Soc., 1843,” p. 65.)

_Metalliferous Veins._

I have only a few remarks to make on this subject: in nine mining
districts, some of them of considerable extent, which I visited in
_Central_ Chile, I found the _principal_ veins running from between [N.
and N.W.] to [S. and S.E.];[9] at the C. de los Hornos (further
northward), it is N.N.W. and S.S.E.; at Panuncillo, it is N.N.W. and
S.S.E.; and, lastly, at Arqueros, the direction is N.W. and S.E.): in
some other places, however, their courses appeared quite irregular, as
is said to be generally the case in the whole valley of Copiapo: at
Tambillos, south of Coquimbo, I saw one large copper vein extending
east and west. It is worthy of notice, that the foliation of the gneiss
and mica-slate, where such rocks occur, certainly tend to run like the
metalliferous veins, though often irregularly, in a direction a little
westward of north. At Yaquil, I observed that the principal auriferous
veins ran nearly parallel to the grain or imperfect cleavage of the
surrounding _granitic_ rocks. With respect to the distribution of the
different metals, copper, gold, and iron are generally associated
together, and are most frequently found (but with many exceptions, as
we shall presently see) in the rocks of the lower series, between the
Cordillera and the Pacific, namely, in granite, syenite, altered
feldspathic clay-slate, gneiss, and as near Guasco mica-schist.
The copper-ores consist of sulphurets, oxides, and carbonates,
sometimes with laminæ of native metal: I was assured that in some cases
(as at Panuncillo S.E. of Coquimbo), the upper part of the same vein
contains oxides, and the lower part sulphurets of copper.[10] Gold
occurs in its native form; it is believed that, in many cases, the
upper part of the vein is the most productive part: this fact probably
is connected with the abundance of this metal in the stratified
detritus of Chile, which must have been chiefly derived from the
degradation of the upper portions of the rocks. These superficial beds
of well-rounded gravel and sand, containing gold, appeared to me to
have been formed under the sea close to the beach, during the slow
elevation of the land: Schmidtmeyer[11] remarks that in Chile gold is
sought for in shelving banks at the height of some feet on the sides of
the streams, and not in their beds, as would have been the case had
this metal been deposited by common alluvial action. Very frequently
the copper-ores, including some gold, are associated with abundant
micaceous specular iron. Gold is often found in iron-pyrites: at two
gold mines at Yaquil (near Nancagua), I was informed by the proprietor
that in one the gold was always associated with copper-pyrites, and in
the other with iron-pyrites: in this latter case, it is said that if
the vein ceases to contain iron-pyrites, it is yet worth while to
continue the search, but if the iron-pyrites, when it reappears, is not
auriferous, it is better at once to give up working the vein. Although
I believe copper and gold are most frequently found in the lower
granitic and metamorphic schistose series, yet these metals occur both
in the porphyritic conglomerate formation (as on the flanks of the Bell
of Quillota and at Jajuel), and in the superincumbent strata. At Jajuel
I was informed that the copper-ore, with some gold, is found only in
the greenstones and altered feldspathic clay-slate, which alternate
with the purple porphyritic conglomerate. Several gold veins and some
of copper-ore are worked in several parts of the Uspallata range, both
in the metamorphosed strata, which have been shown to have been of
probably subsequent origin to the Neocomian or gypseous formation of
the main Cordillera, and in the intrusive andesitic rocks of that
range. At Los Hornos (N.E. of Illapel), likewise, there are numerous
veins of copper-pyrites and of gold, both in the strata of the gypseous
formation and in the injected hills of andesite and various porphyries.

 [9] These mining districts are Yaquil near Nancagua, where the
 direction of the chief veins, to which only in all cases I refer, is
 north and south; in the Uspallata range, the prevailing line is N.N.W.
 and S.S.E.; in the C. de Prado, it is N.N.W. and S.S.E.; near Illapel,
 it is N. by W. and S. by E.; at Los Hornos the direction varies from
 between [N. and N.W.] to [S. and S.E.].


 [10] The same fact has been observed by Mr. Taylor in Cuba: _London
 Phil. Journ.,_ vol. xi, p. 21.


 [11] “Travels in Chile,” p. 29.

Silver, in the form of a chloride, sulphuret, or an amalgam, or in its
native state, and associated with lead and other metals, and at
Arqueros with pure native copper, occurs chiefly in the upper great
gypseous or cretaceo-oolitic formation which forms probably the richest
mass in Chile. We may instance the mining districts of Arqueros near
Coquimbo, and of nearly the whole valley of Copiapo, and of Iquique
(where the principal veins run N.E. by E. and S.W. by W.), in Peru.
Hence comes Molina’s remark, that silver is born in the cold and
solitary deserts of the Upper Cordillera. There are, however,
exceptions to
this rule: at Paral (S.E. of Coquimbo) silver is found in the
porphyritic conglomerate formation; as I suspect is likewise the case
at S. Pedro de Nolasko in the Peuquenes Pass. Rich argentiferous lead
is found in the clay-slate of the Uspallata range; and I saw an old
silver-mine in a hill of syenite at the foot of the Bell of Quillota: I
was also assured that silver has been found in the andesitic and
porphyritic region between the town of Copiapo and the Pacific. I have
stated in a previous part of this chapter, that in two neighbouring
mines at Arqueros the veins in one were productive when they traversed
the singular green sedimentary beds, and unproductive when crossing the
reddish beds; whereas at the other mine exactly the reverse takes
place; I have also described the singular and rare case of numerous
particles of native silver and of the chloride being disseminated in
the green rock at the distance of a yard from the vein. Mercury occurs
with silver both at Arqueros and at Copiapo: at the base of C. de los
Hornos (S.E. of Coquimbo, a different place from Los Hornos, before
mentioned) I saw in a syenitic rock numerous quartzose veins,
containing a little cinnabar in nests: there were here other parallel
veins of copper and of a ferrugino-auriferous ore. I believe tin has
never been found in Chile.

From information given me by Mr. Nixon of Yaquil,[12] and by others, it
appears that in Chile those veins are generally most permanently
productive, which, consisting of various minerals (sometimes differing
but slightly from the surrounding rocks), include parallel strings
_rich_ in metals; such a vein is called a _veta real._ More commonly
the mines are worked only where one, two, or more thin veins or strings
running in a different direction, intersect a _poor_ “veta real:” it is
unanimously believed that at such points of intersection (_cruceros_),
the quantity of metal is much greater than that contained in other
parts of the intersecting veins. In some _ cruceros_ or points of
intersection, the metals extend even beyond the walls of the main,
broad, stony vein. It is said that the greater the angle of
intersection, the greater the produce; and that nearly parallel strings
attract each other; in the Uspallata range, I observed that numerous
thin auri-ferruginous veins repeatedly ran into knots, and then
branched out again. I have already described the remarkable manner in
which rocks of the Uspallata range are indurated and blackened (as if
by a blast of gunpowder) to a considerable distance from the metallic
veins.

 [12] At the Durazno mine, the gold is associated with copper-pyrites,
 and the veins contain large prisms of plumbago. Crystallised carbonate
 of lime is one of the commonest minerals in the matrix of the Chilean
 veins.


Finally, I may observe, that the presence of metallic veins seems
obviously connected with the presence of intrusive rocks, and with the
degree of metamorphic action which the different districts of Chile
have undergone.[13] Such metamorphosed areas are generally accompanied
by numerous dikes and injected masses of andesite and various
porphyries: I have in several places traced the metalliferous veins
from
the intrusive masses into the encasing strata. Knowing that the
porphyritic conglomerate formation consists of alternate streams of
submarine lavas and of the debris of anciently erupted rocks, and that
the strata of the upper gypseous formation sometimes include submarine
lavas, and are composed of tuffs, mudstones, and mineral substances,
probably due to volcanic exhalations,—the richness of these strata is
highly remarkable when compared with the erupted beds, often of
submarine origin, but _not metamorphosed,_ which compose the numerous
islands in the Pacific, Indian, and Atlantic Oceans; for in these
islands metals are entirely absent, and their nature even unknown to
the aborigines.

 [13] Sir R. Murchison and his fellow travellers have given some
 striking facts on this subject in their account of the Ural Mountains
 (“Geolog. Proc.,” vol. iii, p. 748.

_Summary of the Geological History of the Chilean Cordillera, and of
the Southern Parts of South America._

We have seen that the shores of the Pacific, for a space of 1,200 miles
from Tres Montes to Copiapo, and I believe for a very much greater
distance, are composed, with the exception of the tertiary basins, of
metamorphic schists, plutonic rocks, and more or less altered
clay-slate. On the floor of the ocean thus constituted, vast streams of
various purplish claystone and greenstone porphyries were poured forth,
together with great alternating piles of angular and rounded fragments
of similar rocks ejected from the submarine craters. From the
compactness of the streams and fragments, it is probable that, with the
exception of some districts in Northern Chile, the eruptions took place
in profoundly deep water. The orifices of eruption appear to have been
studded over a breadth, with some outliers, of from fifty to one
hundred miles: and closely enough together, both north and south, and
east and west, for the ejected matter to form a continuous mass, which
in Central Chile is more than a mile in thickness. I traced this
mould-like mass, for only 450 miles; but judging from what I saw at
Iquique, from specimens, and from published accounts, it appears to
have a manifold greater length. In the basal parts of the series, and
especially towards the flanks of the range, mud, since converted into a
feldspathic slaty rock, and sometimes into greenstone, was occasionally
deposited between the beds of erupted matter: with this exception the
uniformity of the porphyritic rocks is very remarkable. At the period
when the claystone and greenstone porphyries nearly or quite ceased
being erupted, that great pile of strata which, from often abounding
with gypsum, I have generally called the gypseous formation was
deposited, and feldspathic lavas, together with other singular volcanic
rocks, were occasionally poured forth: I am far from pretending that
any distinct line of demarcation can be drawn between this formation
and the underlying porphyries and porphyritic conglomerate, but in a
mass of such great thickness, and between beds of such widely different
mineralogical nature, some division was necessary. At about the
commencement of the gypseous period, the bottom of the sea here seems
first to have been peopled by shells, not many in kind,
but abounding in individuals. At the P. del Inca the fossils are
embedded near the base of the formation; in the Peuquenes range, at
different levels, halfway up, and even higher in the series; hence, in
these sections, the whole pile of strata belongs to the same period:
the same remark is applicable to the beds at Copiapo, which attain a
thickness of between seven and eight thousand feet. The fossil shells
in the Cordillera of Central Chile, in the opinion of all the
palæontologists who have examined them, belong to the earlier stages of
the cretaceous system; whilst in Northern Chile there is a most
singular mixture of cretaceous and oolitic forms: from the geological
relations, however, of these two districts, I cannot but think that
they all belong to nearly the same epoch, which I have provisionally
called cretaceo-oolitic.

The strata in this formation, composed of black calcareous shaly-rocks
of red and white, and sometimes siliceous sandstone, of coarse
conglomerates, limestones, tuffs, dark mudstones, and those singular
fine-grained rocks which I have called pseudo-honestones, vast beds of
gypsum, and many other jaspery and scarcely describable varieties, vary
and replace each other in short horizontal distances, to an extent, I
believe, unequalled even in any tertiary basin. Most of these
substances are easily fusible, and have apparently been derived either
from volcanoes still in quiet action, or from the attrition of volcanic
products. If we picture to ourselves the bottom of the sea, rendered
uneven in an extreme degree, with numerous craters, some few
occasionally in eruption, but the greater number in the state of
solfataras, discharging calcareous, siliceous, ferruginous matters, and
gypsum or sulphuric acid to an amount surpassing, perhaps, even the
existing sulphureous volcanoes of Java,[14] we shall probably
understand the circumstances under which this singular pile of varying
strata was accumulated. The shells appear to have lived at the
quiescent periods when only limestone or calcareo-argillaceous matter
was depositing. From Dr. Gillies’ account, this gypseous or
cretaceo-oolitic formation extends as far south as the Pass of
Planchon, and I followed it northward at intervals for 500 miles:
judging from the character of the beds with the _Terebratula ænigma,_
at Iquique, it extends from four to five hundred miles further: and
perhaps even for ten degrees of latitude north of Iquique to the Cerro
Pasco, not far from Lima: again, we know that a cretaceous formation,
abounding with fossils, is largely developed north of the equator, in
Colombia: in Tierra del Fuego, at about this same period, a wide
district of clay-slate was deposited, which in its mineralogical
characters and external features, might be compared to the Silurian
regions of North Wales. The gypseous formation, like that of the
porphyritic breccia-conglomerate on which it rests, is of
inconsiderable breadth; though of greater breadth in Northern than in
Central Chile.

 [14] Von Buch’s “Descript. Physique des Iles Canaries,” p. 428.

As the fossil shells in this formation are covered, in the Peuquenes
ridge, by a great thickness of strata; at the Puente del Inca, by at
least five thousand feet; at Coquimbo, though the superposition there
is less plainly seen, by about six thousand feet; and at Copiapo,
certainly by five or
six thousand, and probably by seven thousand feet (the same species
there recurring in the upper and lower parts of the series), we may
feel confident that the bottom of the sea subsided during this
cretaceo-oolitic period, so as to allow of the accumulation of the
superincumbent submarine strata. This conclusion is confirmed by, or
perhaps rather explains, the presence of the many beds at many levels
of coarse conglomerate, the well-rounded pebbles in which we cannot
believe were transported in very deep water. Even the underlying
porphyries at Copiapo. with their highly amygdaloidal surfaces, do not
appear to have flowed under great pressure. The great sinking movement
thus plainly indicated, must have extended in a north and south line
for at least four hundred miles, and probably was co-extensive with the
gypseous formation.

The beds of conglomerate just referred to, and the extraordinarily
numerous silicified trunks of fir-trees at Los Hornos, perhaps at
Coquimbo and at two distant points in the valley of Copiapo, indicate
that land existed at this period in the neighbourhood. This land, or
islands, in the northern part of the district of Copiapo, must have
been almost exclusively composed, judging from the nature of the
pebbles of granite: in the southern parts of Copiapo, it must have been
mainly formed of claystone porphyries, with some mica-schist, and with
much sandstone and jaspery rocks exactly like the rocks in the gypseous
formation, and no doubt belonging to its basal series. In several other
places also, during the accumulation of the gypseous formation, its
basal parts and the underlying porphyritic conglomerate must likewise
have been already partially upheaved and exposed to wear and tear; near
the Puente del Inca and at Coquimbo, there must have existed masses of
mica-schist or some such rock, whence were derived the many small
pebbles of opaque quartz. It follows from these facts, that in some
parts of the Cordillera the upper beds of the gypseous formation must
lie unconformably on the lower beds; and the whole gypseous formation,
in parts, unconformably on the porphyritic conglomerate; although I saw
no such cases, yet in many places the gypseous formation is entirely
absent; and this, although no doubt generally caused by quite
subsequent denudation, may in others be due to the underlying
porphyritic conglomerate having been locally upheaved before the
deposition of the gypseous strata, and thus having become the source of
the pebbles of porphyry embedded in them. In the porphyritic
conglomerate formation, in its lower and middle parts, there is very
rarely any evidence, with the exception of the small quartz pebbles at
Jajuel near Aconcagua, and of the single pebble of granite at Copiapo,
of the existence of neighbouring land: in the upper parts, however, and
especially in the district of Copiapo, the number of thoroughly
well-rounded pebbles of compact porphyries make me believe, that, as
during the prolonged accumulation of the gypseous formation the lower
beds had already been locally upheaved and exposed to wear and tear, so
it was with the porphyritic conglomerate. Hence in following thus far
the geological history of the Cordillera, it may be inferred that the
bed of a deep and open, or nearly open, ocean was filled up by
porphyritic
eruptions, aided probably by some general and some local elevations, to
that comparatively shallow level at which the cretaceo-oolitic shells
first lived. At this period, the submarine craters yielded at intervals
a prodigious supply of gypsum and other mineral exhalations, and
occasionally, in certain places poured forth lavas, chiefly of a
feldspathic nature: at this period, islands clothed with fir-trees and
composed of porphyries, primary rocks, and the lower gypseous strata
had already been locally upheaved, and exposed to the action of the
waves;—the general movement, however, at this time having been over a
very wide area, one of slow subsidence, prolonged till the bed of the
sea sank several thousand feet.

In Central Chile, after the deposition of a great thickness of the
gypseous strata, and after their upheaval, by which the Cumbre and
adjoining ranges were formed, a vast pile of tufaceous matter and
submarine lava was accumulated, where the Uspallata chain now stands;
also after the deposition and upheaval of the equivalent gypseous
strata of the Peuquenes range, the great thick mass of conglomerate in
the valley of Tenuyan was accumulated: during the deposition of the
Uspallata strata, we know absolutely, from the buried vertical trees,
that there was a subsidence of some thousand feet; and we may infer
from the nature of the conglomerate in the valley of Tenuyan, that a
similar and perhaps contemporaneous movement there took place. We have,
then, evidence of a second great period of subsidence; and, as in the
case of the subsidence which accompanied the accumulation of the
cretaceo-oolitic strata, so this latter subsidence appears to have been
complicated by alternate or local elevatory movement— for the vertical
trees, buried in the midst of the Uspallata strata, must have grown on
dry land, formed by the upheaval of the lower submarine beds. Presently
I shall have to recapitulate the facts, showing that at a still later
period, namely, at nearly the commencement of the old tertiary deposits
of Patagonia and of Chile, the continent stood at nearly its present
level, and then, for the third time, slowly subsided to the amount of
several hundred feet, and was afterwards slowly re-uplifted to its
present level.

The highest peaks of the Cordillera appear to consist of active or more
commonly dormant volcanoes,—such as Tupungato, Maypu, and Aconcagua,
which latter stands 23,000 feet above the level of the sea, and many
others. The next highest peaks are formed of the gypseous and
porphyritic strata, thrown into vertical or highly inclined positions.
Besides the elevation thus gained by angular displacements, I infer,
without any hesitation—from the stratified gravel-fringes which gently
slope up the valleys of the Cordillera from the gravel-capped plains at
their base, which latter are connected with the plains, still covered
with recent shells on the coast—that this great range has been upheaved
in mass by a slow movement, to an amount of at least 8,000 feet. In the
Despoblado Valley, north of Copiapo, the horizontal elevation, judging
from the compact, stratified tufaceous deposit, capping the distant
mountains at corresponding heights, was about ten thousand feet. It is
very possible, or rather probable, that this elevation in mass may not
have
been strictly horizontal, but more energetic under the Cordillera, than
towards the coast on either side; nevertheless, movements of this kind
may be conveniently distinguished from those by which strata have been
abruptly broken and upturned. When viewing the Cordillera, before
having read Mr. Hopkins’s profound “Researches on Physical Geology,”
the conviction was impressed on me, that the angular dislocations,
however violent, were quite subordinate in importance to the great
upward movement in mass, and that they had been caused by the edges of
the wide fissures, which necessarily resulted from the tension of the
elevated area, having yielded to the inward rush of fluidified rock,
and having thus been upturned.

The ridges formed by the angularly upheaved strata are seldom of great
length: in the central parts of the Cordillera they are generally
parallel to each other, and run in north and south lines; but towards
the flanks they often extend more or less obliquely. The angular
displacement has been much more violent in the central than in the
exterior _main_ lines; but it has likewise been violent in some of the
_minor_ lines on the extreme flanks. The violence has been very unequal
on the same short lines; the crust having apparently tended to yield on
certain points along the lines of fissures. These points, I have
endeavoured to show, were probably first foci of eruption, and
afterwards of injected masses of porphyry and andesite.[15] The close
similarity of the andesitic granites and porphyries, throughout Chile,
Tierra del Fuego, and even in Peru, is very remarkable. The prevalence
of feldspar cleaving like albite, is common not only to the andesites,
but (as I infer from the high authority of Professor G. Rose, as well
as from my own measurements) to the various claystone and greenstone
porphyries, and to the trachytic lavas of the Cordillera. The andesitic
rocks have in most cases been the last injected ones, and they probably
form a continuous dome under this great range: they stand in intimate
relationship with the modern lavas; and they seem to have been the
immediate agent in metamorphosing the porphyritic conglomerate
formation, and often likewise the gypseous strata, to the extraordinary
extent to which they have suffered.

 [15] Sir R. Murchison and his companions state (“Geolog. Proc.,” vol.
 iii, p. 747), that no true granite appears in the higher Ural
 Mountains; but that syenitic greenstone—a rock closely analogous to
 our andesite—is far the most abundant of the intrusive masses.


With respect to the age at which the several parallel ridges composing
the Cordillera were upthrown, I have little evidence. Many of them may
have been contemporaneously elevated and injected in the same
manner[16] as in volcanic archipelagoes lavas are contemporaneously
ejected on the parallel lines of fissure. But the pebbles apparently
derived from the wear and tear of the porphyritic conglomerate
formation, which are occasionally present in the upper parts of this
same formation, and are often present in the gypseous formation,
together with the pebbles from the basal parts of the latter formation
in its upper strata, render it almost certain that portions, we may
infer ridges,
of these two formations were successively upheaved. In the case of the
gigantic Portillo range, we may feel almost certain that a preexisting
granitic line was upraised (not by a single blow, as shown by the
highly inclined basaltic streams in the valley on its eastern flank) at
a period long subsequent to the upheavement of the parallel Peuquenes
range.[17] Again, subsequently to the upheavement of the Cumbre chain,
that of Uspallata was formed and elevated; and afterwards, I may add,
in the plain of Uspallata, beds of sand and gravel were violently
upthrown. The manner in which the various kinds of porphyries and
andesites have been injected one into the other, and in which the
infinitely numerous dikes of various composition intersect each other,
plainly show that the stratified crust has been stretched and yielded
many times over the same points. With respect to the age of the axes of
elevation between the Pacific and the Cordillera, I know little: but
there are some lines which must—namely, those running north and south
in Chiloe, those eight or nine east and west, parallel, far-extended,
most symmetrical uniclinal lines at P. Rumena, and the short N.W.-S.E.
and N.E.-S.W. lines at Concepcion—have been upheaved long after the
formation of the Cordillera. Even during the earthquake of 1835, when
the linear north and south islet of St. Mary was uplifted several feet
above the surrounding area, we perhaps see one feeble step in the
formation of a subordinate mountain-axis. In some cases, moreover, for
instance, near the baths of Cauquenes, I was forcibly struck with the
small size of the breaches cut through the exterior mountain-ranges,
compared with the size of the same valleys higher up where entering the
Cordillera; and this circumstance appeared to me scarcely explicable,
except on the idea of the exterior lines having been subsequently
upthrown, and therefore having been exposed to a less amount of
denudation. From the manner in which the fringes of gravel are
prolonged in unbroken slopes up the valleys of the Cordillera, I infer
that most of the greater dislocations took place during the earlier
parts of the great elevation in mass: I have, however, elsewhere given
a case, and M. de Tschudi[18] has given another, of a ridge thrown up
in Peru across the bed of a river, and consequently after the final
elevation of the country above the level of the sea.

 [16] “Volcanic Islands,” etc.)


 [17] I have endeavoured to show in my “Journal” (2nd edit., p. 321),
 that the singular fact of the river, which drains the valley between
 these two ranges, passing through the Portillo and higher line, is
 explained by its slow and subsequent elevation. There are many
 analogous cases in the drainage of rivers: see _ Edinburgh New Phil.
 Journal,_ vol. xxviii, pp. 33 and 44.


 [18] “Reise in Peru,” Band 2, s. 8: Author’s “Journal,” 2nd edit., p.
 359.

Ascending to the older tertiary formations, I will not again
recapitulate the remarks already given at the end of the Fifth
Chapter,—on their great extent, especially along the shores of the
Atlantic—on their antiquity, perhaps corresponding with that of the
eocene deposits of Europe,—on the almost entire dissimilarity, though
the formations are apparently contemporaneous, of the fossils from the
eastern and western coasts, as is likewise the case, even in a still
more marked degree, with the shells now living in these opposite though
approximate
seas,—on the climate of this period not having been more tropical than
what might have been expected from the latitudes of the places under
which the deposits occur; a circumstance rendered well worthy of
notice, from the contrast with what is known to have been the case
during the older tertiary periods of Europe, and likewise from the fact
of the southern hemisphere having suffered at a much later period,
apparently at the same time with the northern hemisphere, a colder or
more equable temperature, as shown by the zones formerly affected by
ice-action. Nor will I recapitulate the proofs of the bottom of the
sea, both on the eastern and western coast, having subsided seven or
eight hundred feet during this tertiary period; the movement having
apparently been co-extensive, or nearly co-extensive, with the deposits
of this age. Nor will I again give the facts and reasoning on which the
proposition was founded, that when the bed of the sea is either
stationary or rising, circumstances are far less favourable than when
its level is sinking, to the accumulation of conchiferous deposits of
sufficient thickness, extension, and hardness to resist, when upheaved,
the ordinary vast amount of denudation. We have seen that the highly
remarkable fact of the absence of any _ extensive_ formations
containing recent shells, either on the eastern or western coasts of
the continent,—though these coasts now abound with living
mollusca,—though they are, and apparently have always been, as
favourable for the deposition of sediment as they were when the
tertiary formations were copiously deposited,—and though they have been
upheaved to an amount quite sufficient to bring up strata from the
depths the most fertile for animal life—can be explained in accordance
with the above proposition. As a deduction, it was also attempted to be
shown, first, that the want of close sequence in the fossils of
successive formations, and of successive stages in the same formation,
would follow from the improbability of the same area continuing slowly
to subside from one whole period to another, or even during a single
entire period; and secondly, that certain epochs having been favourable
at distant points, in the same quarter of the world for the synchronous
accumulation of fossiliferous strata, would follow from movements of
subsidence having apparently, like those of elevation,
contemporaneously affected very large areas.

There is another point which deserves some notice, namely, the analogy
between the upper parts of the Patagonian tertiary formation, as well
as of the upper possibly contemporaneous beds at Chiloe and Concepcion,
with the great gypseous formation of Cordillera; for in both
formations, the rocks, in their fusible nature, in their containing
gypsum, and in many other characters, show a connection, either
intimate or remote, with volcanic action; and as the strata in both
were accumulated during subsidence, it appears at first natural to
connect this sinking movement with a state of high activity in the
neighbouring volcanoes. During the cretaceo-oolitic period this
certainly appears to have been the case at the Puente del Inca, judging
from the number of intercalated lava-streams in the lower 3,000 feet of
strata; but generally, the volcanic orifices seem at this time to have
existed as submarine solfataras, and were certainly quiescent compared
with their state
during the accumulation of the porphyritic conglomerate formation.
During the deposition of the tertiary strata we know that at S. Cruz,
deluges of basaltic lava were poured forth; but as these lie in the
upper part of the series, it is possible that the subsidence may at
that time have ceased: at Chiloe, I was unable to ascertain to what
part of the series the pile of lavas belonged. The Uspallata tuffs and
great streams of submarine lavas, were probably intermediate in age
between the cretaceo-oolitic and older tertiary formations, and we know
from the buried trees that there was a great subsidence during their
accumulation; but even in this case, the subsidence may not have been
strictly contemporaneous with the great volcanic eruptions, for we must
believe in at least one intercalated period of elevation, during which
the ground was upraised on which the now buried trees grew. I have been
led to make these remarks, and to throw some doubt on the strict
contemporaneousness of high volcanic activity and movements of
subsidence, from the conviction impressed on my mind by the study of
coral formations,[19] that these two actions do not generally go on
synchronously;—on the contrary, that in volcanic districts, subsidence
ceases as soon as the orifices burst forth into renewed action, and
only recommences when they again have become dormant.

 [19] “The Structure, etc., of Coral Reefs.”

At a later period, the Pampean mud, of estuary origin, was deposited
over a wide area,—in one district conformably on the underlying old
tertiary strata, and in another district unconformably on them, after
their upheaval and denudation. During and before the accumulation,
however, of these old tertiary strata, and, therefore, at a very remote
period, sediment, strikingly resembling that of the Pampas, was
deposited; showing during how long a time in this case the same
agencies were at work in the same area. The deposition of the Pampean
estuary mud was accompanied, at least in the southern parts of the
Pampas, by an elevatory movement, so that the M. Hermoso beds probably
were accumulated after the upheaval of those round the S. Ventana; and
those at P. Alta after the upheaval of the M. Hermoso strata; but there
is some reason to suspect that one period of subsidence intervened,
during which mud was deposited over the coarse sand of the Barrancas de
S. Gregorio, and on the higher parts of Banda Oriental. The mammiferous
animals characteristic of this formation, many of which differ as much
from the present inhabitants of South America, as do the eocene mammals
of Europe from the present ones of that quarter of the globe, certainly
co-existed at B. Blanca with twenty species of mollusca, one balanus,
and two corals, all now living in the adjoining sea: this is likewise
the case in Patagonia with the Macrauchenia, which co-existed with
eight shells, still the commonest kinds on that coast. I will not
repeat what I have elsewhere said, on the place of habitation, food,
wide range, and extinction of the numerous gigantic mammifers, which at
this late period inhabited the two Americas.

The nature and grouping of the shells embedded in the old tertiary
formations of Patagonia and Chile show us, that the continent at that
period must have stood only a few fathoms below its present level, and
that afterwards it subsided over a wide area, seven or eight hundred
feet. The manner in which it has since been rebrought up to its actual
level, was described in detail in the First and Second Chapters. It was
there shown that recent shells are found on the shores of the Atlantic,
from Tierra del Fuego northward for a space of at least 1,180 nautical
miles, and at the height of about 100 feet in La Plata, and of 400 feet
in Patagonia. The elevatory movements on this side of the continent
have been slow; and the coast of Patagonia, up to the height in one
part of 950 feet and in another of 1,200 feet, is modelled into eight
great, step-like, gravel-capped plains, extending for hundreds of miles
with the same heights; this fact shows that the periods of denudation
(which, judging from the amount of matter removed, must have been long
continued) and of elevation were synchronous over surprisingly great
lengths of coasts. On the shores of the Pacific, upraised shells of
recent species, generally, though not always, in the same proportional
numbers as in the adjoining sea, have actually been found over a north
and south space of 2,075 miles, and there is reason to believe that
they occur over a space of 2,480 miles. The elevation on this western
side of the continent has not been equable; at Valparaiso, within the
period during which upraised shells have remained undecayed on the
surface, it has been 1,300 feet, whilst at Coquimbo, 200 miles
northward, it has been within this same period only 252 feet. At Lima,
the land has been uplifted at least 80 feet since Indian man inhabited
that district; but the level within historical times apparently has
subsided. At Coquimbo, in a height of 364 feet, the elevation has been
interrupted by five periods of comparative rest. At several places the
land has been lately, or still is, rising both insensibly and by sudden
starts of a few feet during earthquake-shocks; this shows that these
two kinds of upward movement are intimately connected together. For a
space of 775 miles, upraised recent shells are found on the two
opposite sides of the continent; and in the southern half of this
space, it may be safely inferred from the slope of the land up to the
Cordillera, and from the shells found in the central part of Tierra del
Fuego, and high up the River Santa Cruz, that the entire breadth of the
continent has been uplifted. From the general occurrence on both coasts
of successive lines of escarpments, of sand-dunes and marks of erosion,
we must conclude that the elevatory movement has been normally
interrupted by periods, when the land either was stationary, or when it
rose at so slow a rate as not to resist the average denuding power of
the waves, or when it subsided. In the case of the present high
sea-cliffs of Patagonia and in other analogous instances, we have seen
that the difficulty in understanding how strata can be removed at those
depths under the sea, at which the currents and oscillations of the
water are depositing a smooth surface of mud, sand, and sifted pebbles,
leads to the suspicion that the formation or denudation of such cliffs
has been accompanied by a sinking movement.

In South America, everything has taken place on a grand scale, and all
geological phenomena are still in active operation. We know how violent
at the present day the earthquakes are, we have seen how great
an area is now rising, and the plains of tertiary origin are of vast
dimensions; an almost straight line can be drawn from Tierra del Fuego
for 1,600 miles northward, and probably for a much greater distance,
which shall intersect no formation older than the Patagonian deposits;
so equable has been the upheaval of the beds, that throughout this long
line, not a fault in the stratification or abrupt dislocation was
anywhere observable. Looking to the basal, metamorphic, and plutonic
rocks of the continent, the areas formed of them are likewise vast; and
their planes of cleavage and foliation strike over surprisingly great
spaces in uniform directions. The Cordillera, with its pinnacles here
and there rising upwards of twenty thousand feet above the level of the
sea, ranges in an unbroken line from Tierra del Fuego, apparently to
the Arctic circle. This grand range has suffered both the most violent
dislocations, and slow, though grand, upward and downward movements in
mass; I know not whether the spectacle of its immense valleys, with
mountain-masses of once liquified and intrusive rocks now bared and
intersected, or whether the view of those plains, composed of shingle
and sediment hence derived, which stretch to the borders of the
Atlantic Ocean, is best adapted to excite our astonishment at the
amount of wear and tear which these mountains have undergone.

The Cordillera from Tierra del Fuego to Mexico, is penetrated by
volcanic orifices, and those now in action are connected in great
trains. The intimate relation between their recent eruptions and the
slow elevation of the continent in mass,[20] appears to me highly
important, for no explanation of the one phenomenon can be considered
as satisfactory which is not applicable to the other. The permanence of
the volcanic action on this chain of mountains is, also, a striking
fact; first, we have the deluges of submarine lavas alternating with
the porphyritic conglomerate strata, then occasionally feldspathic
streams and abundant mineral exhalations during the gypseous or
cretaceo-oolitic period: then the eruptions of the Uspallata range, and
at an ancient but unknown period, when the sea came up to the eastern
foot of the Cordillera, streams of basaltic lava at the foot of the
Portillo range; then the old tertiary eruptions; and lastly, there are
here and there amongst the mountains, much worn and apparently very
ancient volcanic formations without any craters; there are, also,
craters quite extinct, and others in the condition of solfataras, and
others occasionally or habitually in fierce action. Hence it would
appear that the Cordillera has been, probably with some quiescent
periods, a source of volcanic matter from an epoch anterior to our
cretaceo-oolitic formation to the present day; and now the earthquakes,
daily recurrent on some part of the western coast, give little hope
that the subterranean energy is expended.

 [20] On the Connection of certain Volcanic Phenomena in South America:
 “Geolog. Transact.,” vol. v, p. 609.


Recurring to the evidence by which it was shown that some at least of
the parallel ridges, which together compose the Cordillera, were
successively and slowly upthrown at widely different periods; and that
the whole range certainly once, and almost certainly twice, subsided
some thousand feet, and being then brought up by a slow movement
in mass, again, during the old tertiary formations, subsided several
hundred feet, and again was brought up to its present level by a slow
and often interrupted movement; we see how opposed is this complicated
history of changes slowly effected, to the views of those geologists
who believe that this great mountain-chain was formed in late times by
a single blow. I have endeavoured elsewhere to show,[21] that the
excessively disturbed condition of the strata in the Cordillera, so far
from indicating single periods of extreme violence, presents
insuperable difficulties, except on the admission that the masses of
once liquified rocks of the axes were repeatedly injected with
intervals sufficiently long for their successive cooling and
consolidation. Finally, if we look to the analogies drawn from the
changes now in progress in the earth’s crust, whether to the manner in
which volcanic matter is erupted, or to the manner in which the land is
historically known to have risen and sunk: or again, if we look to the
vast amount of denudation which every part of the Cordillera has
obviously suffered, the changes through which it has been brought into
its present condition, will appear neither to have been too slowly
effected, nor to have been too complicated.

 [21] “Geolog. Transact.,” vol. v, p. 626.

NOTE.—As, both in France and England, translations of a passage in
Professor Ehrenberg’s Memoir, often referred to in the Fourth Chapter
of this volume, have appeared, implying that Professor Ehrenberg
believes, from the character of the infusoria, that the Pampean
formation was deposited by a sea-debacle rushing over the land, I may
state, on the authority of a letter to me, that these translations are
incorrect. The following is the passage in question:—“Durch Beachtung
der mikroscopischen Formen hat sich nun feststellen lassen, das die
Mastodonten-Lager am La Plata und die Knochen-Lager am Monte Hermoso,
who wie die der Riesen-Gürtelthiere in den Dünenhügeln bei Bahia
Blanca, beides in Patagonien, unveränderte brakische
Süsswasserbildungen sind, die einst wohl sämmtlich zum obersten
Fluthgebiethe des Meeres im tieferen Festlande
gehörten.”—_Monatsberichten der königl. Akad., etc.,_ zu Berlin vom
April 1845.




INDEX TO CORAL-REEFS.


The names in italics are all names of places, and refer exclusively to
the Appendix: in well-defined archipelagoes, or groups of islands, the
name of each separate island is not given.

Abrolhos, Brazil, coated by corals 50
_Abrolhos (Australia)_ 130
Absence of coral-reefs from certain coasts 51
_Acaba, gulf of_ 147
_Admiralty group_ 124
Africa, east coast, fringing-reef of 48
—— Madreporitic rock of 101
_Africa, east coast_ 141
Age of individual corals 57, 64
_Aiou_ 128
_Aitutaki_ 114
_Aldabra_ 139
_Alert reef_ 123
_Alexander, Grand Duke, island_ 115
Allan, Dr., on Holuthuriæ feeding on corals 21
—— on quick growth of corals at Madagascar 62
—— on reefs affected by currents 9
_Alloufatou_ 119
_Alphonse_ 139
_Amargoura (Amargura)_ 119
_Amboina_ 128
_America, west coast_ 111
_Amirantes_ 138
_Anachorites_ 125
_Anambas_ 133
Anamouka, description of 99
_Anamouka_ 119
_Anadaman islands_ 132
_Antilles_ 153
_Appoo reef_ 134
_Arabia Felix_ 143
Areas, great extent of, interspersed with low islands
—— of subsidence and of elevation 106
—— of subsidence appear to be elongated 106
—— of subsidence alternating with areas of elevation 108
_Arru group_ 128
_Arzobispo_ 127
Ascidia, depth at which found 67
_Assomption_ 139
_Astova_ 139
_Atlantic islands_ 121
Atolls, breaches in their reefs 31, 81
—— dimensions of 25
—— dimensions of groups of 71
—— not based on craters or on banks of sediment, or of ck 69, 71, 72,
73, 108
—— of irregular forms 25, 84
—— steepness of their flanks 26
—— width of their reef and islets 25
—— their lowness 70
—— lagoons 29
—— general range 94
—— with part of their reef submerged, and theory of 29, 81
_Augustine, St._ 120
Aurora island, an upraised atoll 64, 71, 104
_Aurora_ 112
Austral islands, recently elevated 99
_Austral islands_ 114
_Australia, N.W. coast_ 130
Australian barrier-reef 42, 93
_Australian barrier_ 123

_Babuyan group_ 134
_Bahama banks_ 149, 150
_Balahac_ 133
_Bally_ 131
_Baring_ 121
Barrier-reef of Australia 42, 93
—— of New Caledonia 44
Barrier-reefs, breaches through 77
—— not based on worn down margin of rock 43
—— on banks of sediment 43
—— on submarine craters 44
—— steepness of their flanks 39
—— their probable vertical thickness 43, 76
—— theory of their formation 76, 78
_Bampton shoal_ 123
_Banks islands_ 122
_Banks in the West Indies_ 147
_Bashee islands_ 134
_Bass island_ 115
_Batoa_ 119
_Beaupré reef_ 123
Beechey, Captain, obligations of the author to 26
—— on submerged reefs 27
—— account of Matilda island 60
Belcher, Captain, on boring through coral-reef 59
_Belize reef, off_ 151
_Bellinghausen_ 113
_Bermuda islands_ 153
_Beveridge reef_ 118
_Bligh_ 122
Bolabola, view of 12
_Bombay shoal_ 136
_Bonin Bay_ 131
_Bonin group_ 127
Borings through coral-reefs 59
Borneo, W. coast, recently elevated 101
_Borneo, E. coast_ 131
—— _S.W. and W. coast_ 133
—— _N. coast_ 133
—— _western bank_ 136
_Boscawen_ 119
_Boston_ 121
_Bouka_ 124
_Bourbon_ 138
_Bourou_ 128
_Bouton_ 132
Brazil, fringing-reefs on coast of 48
Breaches through barrier-reefs 71
_Brook_ 115
_Bunker_ 115
_Bunoa_ 133
Byron 121

_Cagayanes_ 133
_Candelaria_ 124
_Cargados Carajos_ 138
_Caroline archipelago_ 125
_Caroline island_ 115
_Carteret shoal_ 128
Caryophyllia, depth at which it lives 66
_Cavilli_ 133
_Cayman island_ 152
_Celebes_ 129
_Ceram_ 128
Ceylon, recently elevated 101
_Ceylon_ 137
Chagos Great Bank, description and theory of 37, 85
Chagos group 86
_Chagos group_ 137
Chama-shells embedded in coral-rock 68
Chamisso, on corals preferring the surf 52
Changes in the state of Keeling atoll 21
—— of atolls 74
Channels leading into the lagoons of atolls 30, 82
—— —— into the Maldiva atolls 33, 35
—— through barrier-reefs 77
_Chase_ 120
_China sea_ 135
Christmas atoll 60, 97
_Christmas atoll_ 116
_Christmas island_ (Indian Ocean) 137
_Clarence_ 116
_Clipperton rock_ 111
Cocos, or Keeling atoll 15
_Cocos (or Keeling)_ 137
_Cocos island_ (Pacific) 111
Cochin China, encroachments of the sea on the coast 95
_Cochin China_ 183
_Coetivi_ 139
_Comoro group_ 139
Composition of coral-formations 88
Conglomerate coral-rock on Keeling atoll 20
—— on other atolls 28
—— coral-rock 88
Cook islands, recently elevated 98, 103
_Cook islands_ 114
Coral-blocks bored by vermiform animals 21, 88
Coral-reefs, their distribution and absence from certain areas 50
—— destroyed by loose sediment 53
Coral-rock at Keeling atoll 20
—— Mauritius 47
—— organic remains of 88
Corals dead but upright in Keeling lagoon 22
—— depths at which they live 64
—— off Keeling atoll 17
—— killed by a short exposure 16
—— living in the lagoon of Keeling atoll 20
—— quick growth of, in Keeling lagoon 21
—— merely coating the bottom of the sea 50
—— standing exposed in the Low archipelago 96
Corallian sea 94
_Corallian sea_ 123
_Cornwallis_ 121
_Cosmoledo_ 139
Couthouy, Mr., alleged proofs of recent elevation of the Low
archipelago 96
—— on coral-rock at Mangaia and Aurora islands 64
—— on external ledges round coral-islands 80
—— remarks confirmatory of the author’s theory 96
Crescent-formed reefs 84
_Cuba_ 150
Cuming, Mr., on the recent elevation of the Philippines 101

_Dangerous, or Low archipelago_ 111
_Danger islands_ 116
Depths at which reef-building corals live 63
—— at Mauritius, the Red Sea, and in the Maldiva archipelago 66
—— at which other corals and corallines can live 67
_Dhalac group_ 144
Diego Garcia, slow growth of reef 56
Dimensions of the larger groups of atolls 71
Disseverment of the Maldiva atolls, and theory of 37, 82
Distribution of coral-reefs 50
_Domingo, St._ 152
Dory, Port, recently elevated 100
_Dory, Port_ 127
_Duff islands_ 122
_Durour_ 125

_Eap_ 126
arthquakes at Keeling atoll 23
—— in groups of atolls 75
—— in Navigator archipelago 100
ast Indian archipelago, recently elevated 100
_Easter_ 111
_Echequier_ 125
hrenberg, on the banks of the Red Sea 49, 143
—— on depths at which corals live in the Red Sea 66
—— on corals preferring the surf 53
—— on the antiquity of certain corals 57
_Eimeo_ 112
levated reef of Mauritius 47
levations, recent proofs of 98
—— immense areas of 106
_Elivi_ 126
lizabeth island 59
—— recently elevated 98, 104
_Elizabeth island_ 112
_Ellice group_ 120
ncircled islands, their height 41
—— geological composition 42, 44
ua, description of 99
_Eoua_ 118
upted matter probably not associated with thick masses of coral-rock 89

Fais, recently elevated 100, 104
_Fais_ 126
_Fanning_ 116
_Farallon de Medinilla_ 127
_Farson group_ 144
_Fataka_ 122
Fiji archipelago 119
Fish, feeding on corals 21
—— killed in Keeling lagoon by heavy rain 24
Fissures across coral-islands 75
Fitzroy, Captain, on a submerged shed at Keeling atoll 23
—— on an inundation in the Low archipelago 74
_Flint_ 115
_Flores_ 130
_Florida_ 149
_Folger_ 127
_Formosa_ 135
Forster, theory of coral-formations 73
_Frederick reef_ 123
_Freewill_ 128
Friendly group recently elevated 99, 105
_Friendly archipelago_ 118
Fringing-reefs, absent where coast precipitous 5
—— breached in front of streams 54
—— described by MM. Quoy and Gaimard 98
—— not closely attached to shelving coasts 46
—— of east coast of Africa
—— of Cuba 48
—— of Mauritius 45
—— on worn down banks of rock 9
—— on banks of sediment 49
—— their appearance when elevated 7
—— their growth influenced by currents 49
—— by shallowness of sea 49

_Galapagos archipelago_ 111
_Galega_ 139
Gambier islands, section of 43
_Gambier islands_ 112
_Gardner_ 116
_Gaspar rico_ 121
Geological composition of coral-formations

_Gilbert archipelago_ 120
_Gilolo_ 129
_Glorioso_ 139
Gloucester island 74
_Glover reef_ 152
_Gomez_ 111
_Gouap_ 126
_Goulou_ 126
_Grampus_ 127
_Gran Cocal_ 120
Great Chagos Bank, description and theory of 37, 85
Grey, Captain, on sandbars 46
Grouping of the different classes of reefs 93
_Guedes_ 128

Hall, Captain B., on Loo Choo 101
Harvey islands, recently elevated 104
Height of encircled islands 41
_Hermites_ 125
_Hervey or Cook islands_ 114
_Hogoleu_ 125
Holothuriæ (Holuthuriæ) feeding on coral 21
Houden island, height of 71
_Honduras, reef off_ 151
_Horn_ 119
_Houtman Abrolhos_ 130
Huaheine; alleged proofs of its recent elevation 103
_Huaheine_ 113
_Humphrey_ 115
_Hunter_ 119
Hurricanes, effects of, on coral-islands 74

_Immaum_ 143
_Independence_ 120
India, west coast, recently elevated 101
_India_ 143
Irregular reefs in shallow seas 49
Islets of coral-rock, their formation 19
—— their destruction in the Maldiva atolls 36

_Jamaica_ 152
_Jarvis_ 115
Java, recently elevated 100
_Java_ 131
_Johnston island_ 116
_Juan de Nova_ 139
_Juan de Nova (Madagascar)_ 140

_Kalatoa_ 131
Kamtschatka, proofs of its recent elevation 105
_Karkalang_ 129
Keeling atoll, section of reef 15
_Keeling, south atoll_ 137
—— _north atoll_ 137
_Keffing_ 128
_Kemin_ 115, 116
_Kennedy_ 123
_Keppel_ 119
_Kumi_ 135

_Laccadive group_ 137
Ladrones, or Marianas, recently elevated 100
_Ladrones archipelago_ 127
Lagoon of Keeling atoll 20
Lagoons bordered by inclined ledges and walls, and theory of their
formation 32, 79
—— of small atolls filled up with sediment 32
Lagoon-channels within barrier-reefs 40
Lagoon-reefs, all submerged in some atolls, and rising to the surface
in others 55
_Lancaster reef_ 115
_Latte_ 119
_Lauglan islands_ 123
Ledges round certain lagoons 32, 79
_Lette_ 129
_Lighthouse reef_ 152
Lloyd, Mr., on corals refixing themselves 62
Loo Choo, recently elevated 101
_Loo Choo_ 135
_Louisiade_ 123
Low archipelago, alleged proofs of its recent elevation 96
_Low archipelago_ 111
Lowness of coral-islands 70
_Loyalty group_ 123
_Lucepara_ 133
Lutké, Admiral, on fissures across coral-islands 75
Luzon, recently elevated 101
_Luzon_ 134
Lyell, Mr., on channels into the lagoons of atolls 31
—— on the lowness of their leeward sides 82
—— on the antiquity of certain corals 58
—— on the apparent continuity of distinct coral-islands 89
—— on the recently elevated beds of the Red Sea 102
—— on the outline of the areas of subsidence 106

_Macassar strait_ 131
_Macclesfield bank_ 136
Madagascar, quick growth of corals at 62
—— madreporitic rock of 101
_Madagascar_ 140
_Madjiko-sima_ 135
_Madura (Java)_ 131
_Madura (India)_ 137
Mahlos Mahdoo, theory of formation 88
Malacca, recently elevated 100
_Malacca_ 133
Malcolmson, Dr., on recent elevation of W. coast of India 100
—— on recent elevation of Camaran island 102
_Malden_ 115
Maldiva atolls, and theory of their formation 33, 80, 82
—— steepness of their flanks 26
—— growth of coral at 62
_Maldiva archipelago_ 137
Mangaia island 64
—— recently elevated 99, 104
_Mangaia_ 114
_Mangs_ 127
Marianas, recently elevated 100
_Mariana archipelago_ 127
_Mariere_ 126
_Marquesas archipelago_ 113
_Marshall archipelago_ 121
_Marshall island_ 127
_Martinique_ 153
_Martires_ 126
Mary’s St. in Madagascar, harbour made in reefs 54
_Mary island_ 116
_Matia, or Aurora_ 112
Matilda atoll 60
Mauritius, fringing-reefs of 45
—— depths at which corals live there 64
—— recently elevated 101
_Mauritius_ 138
Maurua, section of 43
_Maurua_ 113
Menchikoff atoll 25,

_Mendana archipelago_ 113
_Mendana isles_ 122
_Mexico, gulf of_ 149
Millepora complanata at Keeling atoll 16
_Mindoro_ 134
_Mohilla (Mohila)_ 139
Molucca islands, recently elevated 100
_Mopeha_ 113
Moresby, Captain, on boring through coral-reefs 59
_Morty_ 129
_Mosquito coast_ 152
Musquillo atoll 84
_Mysol_ 129

Namourrek group 84
_Natunas_ 133
Navigator archipelago, elevation of 99
_Navigator archipelago_ 117
_Nederlandisch_ 120
Nelson, Lieutenant, on the consolidation of coral-rocks under water 59
—— theory of coral-formations 73
—— on the Bermuda islands 154
_New Britain_ 124
New Caledonia, steepness of its reefs 39
—— —— barrier-reef of , 79, 83, 93
_New Caledonia_ 123
_New Guinea (E. end)_ 124
_New Guinea (W. end)_ 127
_New Hanover_ 124
New Hebrides, recently elevated 100
_New Hebrides_ 121
New Ireland, recently elevated 100
_New Ireland_ 124
_New Nantucket_ 116
_Nicobar islands_ 132
_Niouha_ 119
Nulliporæ at Keeling atoll 18
—— on the reefs of atolls 28
—— on barrier-reefs 39
—— their wide distribution and abundance 68

Objections to the theory of subsidence 7
_Ocean islands_ 117, 121
_Ono_ 120
_Onouafu (Onouafou)_ 119
_Ormuz_ 143
_Oscar group_ 120
Oscillations of level 103, 108
_Ouallan, or Ualan (Oualan)_ 125
Ouluthy atoll 60
_Outong Java_ 124

_Palawan, S.W. coast 133
—— N.W. coast 134
—— western bank 136
_ Palmerston 114
_Palmyra_ 116
_Paracells_ 136
_Paraquas_ 136
_Patchow_ 135
_Pelew islands_ 126
Pemba island, singular form of 102
_Pemba_ 142
_Penrhyn_ 115
_Peregrino_ 115
Pernambuco, bar of sandstone at 47
Persian gulf, recently elevated 102
_Persian gulf_ 143
Pescado 115
_Pescadores_ 135
_Peyster group_ 120
_Philip_ 126
Philippine archipelago, recently elevated 101
_Philippine archipelago_ 134
_Phœnix_ 116
_Piguiram_ 126
_Pitcairn_ 112
Pitt’s bank 86
_Pitt island_ 120
_Platte_ 139
_Pleasant_ 121
Porites, chief coral on margin of Keeling atoll 16
_Postillions_ 131
Pouynipète 95
—— its probable subsidence 95
_Pouynipète_ 125
_Pratas shoal_ 135
_Proby_ 119
_Providence_ 139
_Puerto Rico_ 152
_Pulo Anna_ 126
Pumice floated to coral-islands 88
_Pylstaart_ 118
Pyrard de Laval, astonishment at the atolls in the Indian Ocean 11

Quoy and Gaimard, depths at which corals live 66
—— description of reefs applicable only to fringing-reefs 98

Range of atolls 94
_Rapa_ 115
_Rearson_ 115
Red Sea, banks of rock coated by reefs 49
—— proofs of its recent elevation 102
—— supposed subsidence of 103
_Red Sea_ 143
Reefs, irregular in shallow seas 49
—— rising to the surface in some lagoons and all submerged in others 55
—— their distribution 50
—— their absence from some coasts 51
_Revilla-gigedo_ 111
Ring-formed reefs of the Maldiva atolls, and theory of , 80
_Rodriguez_ 138
_Rosario_ 127
_Rose island_ 118
_Rotches_ 120
_Rotoumah_ 120
_Roug_ 125
_Rowley shoals_ 130
Rüppell, Dr., on the recent deposits of Red Sea 102

_Sable, ile de_ 138
_Sahia de Malha_ 137
_St. Pierre_ 139
_Sala_ 111
_Salomon (Solomon) archipelago_ 123
Samoa, or Navigator archipelago, elevation of 99
_Samoa archipelago_ 117
Sand-bars parallel to coasts 46
_Sandal-wood_ 129
Sandwich archipelago, recently elevated 98
_Sandwich archipelago_ 117
_Sanserot_ 126
_Santa-Cruz group_ 122
Savage island, recently elevated 59, 99, 104
_Savage_ 118
_Savu_ 129
_Saya, or Sahia de Malha_ 137
_Scarborough shoal_ 136
Scarus feeding on corals 21
_Schouten_ 124
_Scilly_ 113
Scoriæ floated to coral-islands 89
_Scott’s reef_ 130
Sections of islands encircled by barrier-reefs 43, 176
—— of Bolabola 76
Sediment in Keeling lagoon 21
—— in other atolls 29, 35
—— injurious to corals 53
—— transported from coral-islands far seaward 89
 _Seniavine_ 125
_Serangani_ 129
_Seychelles_ 138
Ship-bottom quickly coated with coral 62
_Smyth island_ 116
Society archipelago, stationary condition of 96
—— alleged proofs of recent elevation 103
_Society archipelago_ 112
_Socotra_ 143
_Solor_ 130
Sooloo islands, recently elevated 101
_Sooloo islands_ 133
_Souvaroff_ 115
_Spanish_ 126
Sponge, depths at which found 67
_Starbuck (Slarbuck)_ 115
Stones transported in roots of trees 89
Storms, effects of, on coral-islands 74
Stutchbury, Mr., on the growth of an Agaricia 63
—— on upraised corals in Society archipelago 103
Subsidence of Keeling atoll 28
—— extreme slowness of 87, 108
—— areas of, apparently elongated 106
—— areas of immense 106
—— great amount of 108
_Suez, gulf of_ 147
_Sulphur islands_ 127
Sumatra, recently elevated 100
_Sumatra_ 132
_Sumbawa_ 130
Surf favourable to the growth of massive corals 52
_Swallow shoal_ 136
_Sydney island_ 116

Tahiti, alleged proofs of its recent elevation 103
_Tahiti_ 112
Temperature of the sea at the Galapagos archipelago 51
_Tenasserim_ 133
_Tenimber island_ 128
_Teturoa_ 113
Theories on coral-formations 69, 73
Theory of subsidence, and objections to 72, 86
Thickness, vertical, of barrier-reefs 43, 76
_Thomas, St._ 153
_Tikopia_ 122
Timor, recently elevated 100
_Timor_ 129
_Timor-laut_ 128
_Tokan-Bessees_ 131
_Tongatabou_ 118
_Tonquin_ 137
_Toubai_ 113
_Toufoa (Toofoa)_ 119
_Toupoua_ 122
Traditions of change in coral-islands 73
Tridacnæ embedded in coral-rock 63
—— left exposed in the Low archipelago 96
Tubularia, quick growth of 63
_Tumbelan_ 133
_Turneffe reef_ 152
_Turtle_ 119

_Ualan_ 125

Vanikoro, section of 43
—— its state and changes in its reefs 95
_Vanikoro_ 122
_Vine reef_ 125
_Virgin Gorda_ 153
_Viti archipelago_ 119
Volcanic islands, with living corals on their shores 51
—— matter, probably not associated with thick masses of coral-rock 88
Volcanoes, authorities for their position on the map 90
—— their presence determined by the movements in progress 104
—— absent or extinct in the areas of subsidence 105

_Waigiou_ 128
_Wallis island_ 119
_Washington_ 116
_Wells’ reef_ 123
Wellstead, Lieutenant, account of a ship coated with corals 62
West Indies, banks of sediment fringed by reefs 49
—— recently elevated 102
_West Indies_ 147
Whitsunday island, view of 12
—— changes in its state 74
Williams, Rev. J., on traditions of the natives regarding coral-islands
74
—— on antiquity of certain corals 64
_Wolchonsky_ 111
_Wostock_ 115

_Xulla islands_ 128

_York island_ 116
_Yucutan, coast of_ 151

Zones of different kinds of corals outside the same reefs 55, 60




INDEX TO VOLCANIC ISLANDS.


Abel, M., on calcareous casts at the Cape of Good Hope 261
 Abingdon island 234
 Abrolhos islands, incrustation on 188
 Aeriform explosions at Ascension 191
 _Albatross_, driven from St. Helena 225
 Albemarle island 234
 Albite, at the Galapagos archipelago 234
 Amygdaloidal cells, half filled 184
 Amygdaloids, calcareous origin of 176
 Ascension, arborescent incrustation on rocks of 188
—— absence of dikes, freedom from volcanic action, and state of
lava-streams 226
 Ascidia, extinction of 258
 Atlantic Ocean, new volcanic focus in 226
 Augite, fused 239
 Australia 251
 Azores 182, 248

 Bahia in Brazil, dikes at 247
 Bailly, M., on the mountains of Mauritius 185
 Bald Head 260
 Banks’ Cove 234, 236
 Barn, The, St. Helena 216
 Basalt, specific gravity of 245
 Basaltic coast-mountains at Mauritiu 185
—— at St. Helena 218
—— at St. Jago 178
 Beaumont, M. Elie de, on circular subsidences in lava 233
—— on dikes indicating elevation 228
—— on inclination of lava-streams 227
—— on laminated dikes 212
 Bermuda, calcareous rocks of 260, 262
 Beudant, M., on bombs 191
—— on jasper 197
—— on laminated trachyte 211
—— on obsidian of Hungary 207
—— on silex in trachyte 270, 197
 Bole 257
 Bombs, volcanic 189
 Bory St. Vincent, on bombs 190
 Boulders, absence in Australia and Cape of Good Hope 265
 Brattle island 238
 Brewster, Sir D., on a calcareo-animal substance 201
—— on decomposed glass 252
 Brown, Mr. R., on extinct plants from Van Diemen’s land 257
—— on sphærulitic bodies in silicified wood 207
 Buch, Von, on cavernous lava 233
—— on central volcanoes 249
—— on crystals sinking in obsidian 243
—— on laminated lava 209
—— on obsidian streams 208
—— on olivine in basalt 234
—— on superficial calcareous beds in the Canary islands 224

 Calcareous deposit at St. Jago affected by heat 169, 171
—— fibrous matter, entangled in streaks in scoriæ 174
—— freestone at Ascension 198
—— incrustations at Ascension 199
—— sandstone at St. Helena 222
—— superficial beds at King George’s sound 260
 Cape of Good Hope 263
 Carbonic acid, expulsion of, by heat 171, 176
 Carmichael, Capt., on glassy coatings to dikes 216
 Casts, calcareous, of branches 261
 Chalcedonic nodules 257
 Chalcedony in basalt and in silicified wood 196
 Chatham island 231, 235, 241, 248, 259
 Chlorophæite 257
 Clarke, Rev. W., on the Cape of Good Hope 258, 263
 Clay-slate, its decomposition and junction with granite at the Cape of
 Good Hope 264
 Cleavage of clay-slate in Australia 252
 Cleavage, cross, in sandstone 253
 Coast denudation at St. Helena 226
 Columnar basalt 173
 “Comptes Rendus,” account of volcanic phenomena in the Atlantic 226
 Concepcion, earthquake of 228, 249
 Concretions in aqueous and igneous rocks compared 206
—— in tuff 197
—— of obsidian 206, 208
 Conglomerate, recent, at St. Jago 181
 Coquimbo, curious rock of 261
 Corals, fossil, from Van Diemen’s Land 256
 Crater, segment of, at the Galapagos 238
—— great central one at St. Helena 219
—— internal ledges round, and parapet on 220
 Craters, basaltic, at Ascension 189
—— form of, affected by the trade wind 189
—— of elevation 227
—— of tuff at Terceira 182
—— of tuff at the Galapagos archipelago 230, 231, 235, 237
—— their breached state 240
—— small basaltic at St. Jago 177
—— —— at the Galapagos archipelago 232
 Crystallisation favoured by space 211

 Dartigues, M., on sphærulites 207
 Daubeny, Dr., on a basin-formed island 237
—— on fragments in trachyte 193
 D’Aubuisson on hills of phonolite 222
—— on the composition of obsidian 206
—— on the lamination of clay-slate 210
 De la Beche, Sir H., on magnesia in erupted lime 174
—— on specific gravity of limestones 198
 Denudation of coast at St. Helena 226
 Diana’s Peak, St. Helena 220
 Dieffenbach, Dr., on the Chatham Islands 259
 Dikes, truncated, on central crateriform ridge of St. Helena 219
—— at St. Helena; number of; coated by a glossy layer; uniform
thickness of 216
—— great parallel ones at St. Helena 222
—— not observed at Ascension 226
—— of tuff 231
—— of trap in the plutonic series 247
—— remnants of, extending far into the sea round St. Helena 226
 Dislocations at Ascension 192
—— at St. Helena 217, 221
 Distribution of volcanic islands 248
 Dolomieu, on decomposed trachyte 182
—— on laminated lava 210, 211
—— on obsidian 208
 Drée, M., on crystals sinking in lava 243
 Dufrenoy, M., on the composition of the surface of certain
 lava-streams 209, 243
—— on the inclination of tuff-strata 236

 Eggs of birds embedded at St. Helena 224
—— of turtle at Ascension 198
 Ejected fragments at Ascension 192
—— at the Galapagos archipelago 239
 Elevation of St. Helena 225
—— the Galapagos archipelago 241
—— Van Diemen’s Land, Cape of Good Hope, New Zealand, Australia, and
Chatham island 258
—— of volcanic islands 250
 Ellis, Rev. W., on ledges within the great crater at Hawaii 220
—— on marine remains at Otaheite 184
 Eruption, fissures of 224, 249, 250
 Extinction of land-shells at St. Helena 224

 Faraday, Mr., on the expulsion of carbonic acid gas 171
 Feldspar, fusibility of 246
—— in radiating crystals 263
—— Labrador, ejected 193
 Feldspathic lavas 179
—— at St. Helena 219
—— rock, alternating with obsidian 202
—— lamination, and origin of 209
 Fernando Noronha 181, 210
 Ferruginous superficial beds 259
 Fibrous calcareous matter at St. Jago 174
 Fissures of eruption 242, 249, 250
 Fitton, Dr., on calcareous breccia 262
 Flagstaff Hill, St. Helena 216
 Fleurian de Bellevue on sphærulites 207
 Fluidity of lavas 234, 235
 Forbes, Professor, on the structure of glaciers 212
 Fragments ejected at Ascension 192
—— at the Galapagos archipelago 239
 Freshwater Bay 238, 243
 Fuerteventura (Feurteventura), calcareous beds of 224

 Galapagos archipelago 229
—— parapets round craters 220
 Gay Lussac, on the expulsion of carbonic acid gas 171
 Glaciers, their structure 212
 Glossiness of texture, origin of 206
 Gneiss, derived from clay-slate 264
—— with a great embedded fragment 252
 Gneiss-granite, form of hills of 259
 Good Hope, Cape of 263
 Gorges, narrow, at St. Helena 225
 Granite, junction with clay-slate, at the Cape of Good Hope 263
 Granitic ejected fragments 192, 239
 Gravity, specific, of lavas 243-8
 Gypsum, at Ascension 201
—— in volcanic strata at St. Helena 215
—— on surface of the ground at ditto 223

 Hall, Sir J., on the expulsion of carbonic acid gas 171
 Heat, action of, on calcareous matter 170
 Hennah, Mr., on ashes at Ascension 189
 Henslow, Prof., on chalcedony 197
 Hoffmann, on decomposed trachyte 182
 Holland, Dr., on Iceland 228
 Horner, Mr., on a calcareo-animal substance 201
—— on fusibility of feldspar 246
 Hubbard, Dr., on dikes 247
 Humboldt on ejected fragments 193
—— on obsidian formations 207, 209
—— on parapets round craters 220
—— on sphærulites 210
 Hutton on amygdaloids 176
 Hyalite in decomposed trachyte 182

 Iceland, stratification of the circumferential hills 228
 Islands, volcanic, distribution of 248
—— their elevation 250
 Incrustation, on St. Paul’s rocks 187
 Incrustations, calcareous, at Ascension 199

 Jago, St. 167
 James island 234, 237, 242
 Jasper, origin of 196
 Jonnès, M. Moreau de, on craters affected by wind 189
 Juan Fernandez 250

 Keilhau, M., on granite 264
 Kicker Rock 232
 King George’s sound 259

 Labrador feldspar, ejected 193
 Lakes at bases of volcanoes 229
 Lamination of volcanic rocks 209
 Land-shells, extinct, at St. Helena 224
 Lanzarote, calcareous beds of 223
 Lava, adhesion to sides of a gorge 177
—— feldspathic 179
—— with cells semi-amygdaloidal 184
 Lavas, specific gravity of 243, 247
 Lava-streams blending together at St. Jago 177
—— composition of surface of 208
—— differences in the state of their surfaces 244
—— extreme thinness of 238
—— heaved up into hillocks at the Galapagos archipelago 233
—— their fluidity 234, 235
—— with irregular hummocks at Ascension 189
 Lead, separation from silver 244
 Lesson, M., on craters at Ascension 189
 Leucite 234
 Lime, sulphate of, at Ascension 200
 Lonsdale, Mr., on fossil-corals from Van Diemen’s land 256
 Lot, St. Helena 221
 Lyell, Mr., on craters of elevation 227
—— on embedded turtles’ eggs 198
—— on glossy coating to dikes 216

 Macaulay, Dr., on calcareous casts at Madeira 262
 MacCulloch, Dr., on an amygdaloid 184
—— on chlorophæite 287
—— on laminated pitchstone 209
 Mackenzie, Sir G., on cavernous lava-streams 233
—— on glossy coatings to dikes 216
—— on obsidian streams 208
—— on stratification in Iceland 228
 Madeira, calcareous casts at 262
 _Magazine, Nautical,_—account of volcanic phenomena in the Atlantic
 226
 Marekanite 206
 Mauritius, crater of elevation of 184, 227
 Mica, in rounded nodules 168
—— origin in metamorphic slate 264
—— radiating form of 263
 Miller, Prof., on ejected Labrador feldspar 193
—— on quartz crystals in obsidian beds 202
 Mitchell, Sir T., on bombs 191
—— on the Australian valleys 254
 Mud streams at the Galapagos archipelago 236

 Narborough island 234
 Nelson, Lieut., on the Bermuda islands 260, 262
 New Caledonia 248
 New Red sandstone, cross cleavage of 253
 New South Wales 251
 New Zealand 259
 Nulliporæ (fossil), resembling concretions 169

 Obsidian, absent at the Galapagos archipelago 241
—— bombs of 191
—— composition and origin of 207, 208
—— crystals of feldspar sink in 243
—— its irruption from lofty craters 246
—— passage of beds into 202
—— specific gravity of 243, 246
—— streams of 208
 Olivine decomposed at St. Jago 178
—— at Van Diemen’s land 257
—— in the lavas at the Galapagos archipelago 234
 Oolitic structure of recent calcareous beds at St. Helena 223
 Otaheite 183
 Oysters, extinction of 258

 Panza islands, laminated trachyte of 209
 Pattinson, Mr., on the separation of lead and silver 244
 Paul’s, St., rocks of 187
 Pearlstone 206
 Peperino 232
 Péron, M., on calcareous rocks of Australia 262, 263
 Phonolite, hills of 179, 181, 221
—— laminated 210
—— with more fusible hornblende 246
 Pitchstone 204
—— dikes of 209
 Plants, extinct 257
 Plutonic rocks, separation of constituent parts of, by gravity 246
 Porto Praya 167
 Prevost, M. C., on rarity of great dislocations in volcanic islands
 217
 Prosperous hill, St. Helena 218
 Pumice, absent at the Galapagos archipelago 241
—— laminated 209, 210, 211
 Puy de Dome, trachyte of 193

 Quail island, St. Jago 168, 170, 173
 Quartz, crystals of, in beds alternating with obsidian 202
—— crystallised in sandstone 252
—— fusibility of 246
—— rock, mottled from metamorphic action with earthy matter 170

 Red hill 173
 Resin-like altered scoriæ 171
 Rio de Janeiro, gneiss of 252
 Robert, M., on strata of Iceland 228
 Rogers, Professor, on curved lines of elevation 249

 Salses, compared with tuff craters 240
 Salt deposited by the sea 200
—— in volcanic strata 201, 215
—— lakes of, in craters 240
 Sandstone of Brazil 265
—— of the Cape of Good Hope 265
—— platforms of, in New South Wales 252, 265
 Schorl, radiating 263
 Scrope, Mr. P., on laminated trachyte 209, 210, 212
—— on obsidian 208
—— on separation of trachyte and basalt 244
—— on silex in trachyte 176
—— on sphærulites 210
 Seale, Mr., geognosy of St. Helena 215
—— on dikes 226
—— on embedded birds’ bones 225
 Seale, on extinct shells of St. Helena 224
 Sedgwick, Professor, on concretions 206
 Septaria, in concretions in tuff 198
 Serpulæ on upraised rocks 185
 Seychelles 248
 Shells, colour of, affected by light 201
—— from Van Diemen’s land 256
—— land, extinct, at St. Helena 224
—— particles of, drifted by the wind at St. Helena 223
 Shelly matter deposited by the waves 200
 Siau, M., on ripples 254
 Signal Post Hill 168, 175, 176
 Silica, deposited by steam 182
—— large proportion of, in obsidian 206, 208
—— specific gravity of 246
 Siliceous sinter 196
 Smith, Dr. A., on junction of granite and clay-slate 264
 Spallanzani on decomposed trachyte 182
 Specific gravity of recent calcareous rocks and of limestone 198
—— of lavas 245
 Sphærulites in glass and in silicified wood 207
—— in obsidian 204, 210
 Sowerby, Mr. G. B., on fossil-shells from Van Diemen’s land 256
—— from St. Jago 169
—— land-shells from St. Helena 224
 St. Helena 214
—— crater of elevation of 227
 St. Jago, crater of elevation of 227
—— effects of calcareous matter on lava 231
 St. Paul’s rocks 187, 248
 Stokes, Mr., collections of sphærulites and of obsidians 207, 212
 Stony-top, Little 218, 222
—— Great 218
 Stratification of sandstone in New South Wales 253, 255
 Streams of obsidian 208
 Stutchbury, Mr., on marine remains at Otaheite 184
 Subsided space at Ascension 192

 Tahiti 183
 Talus, stratified, within tuff craters 236
 Terceira 182
 Tertiary deposit of St. Jago 169
 Trachyte, absent at the Galapagos archipelago 241
—— at Ascension 193
—— at Terceira 182
—— decomposition of, by steam 182
—— its lamination 200, 210
—— its separation from basalt 244
—— softened at Ascension 194
—— specific gravity of 245
—— with singular veins 195
 Trap-dikes in the plutonic series 247
—— at King George’s sound 259
 Travertin at Van Diemen’s land 257
 Tropic-bird, now rare, at St. Helena 225
 Tuff, craters of 231, 235, 236
—— their breached state 240
—— peculiar kind of 231
 Turner, Mr., on the separation of molten metals 244
 Tyerman and Bennett on marine remains at Huaheine 184

 Valleys, gorge-like, at St. Helena 225
—— in New South Wales 254
—— in St. Jago 180
 Van Diemen’s land 256
 Veins in trachyte 195
—— of jasper 195
 Vincent, Bory St., on bombs 190
 Volcanic bombs 189
—— island in process of formation in the Atlantic 226
—— islands, their distribution 248

 Wacke, its passage into lava 183, 257
 Wackes, argillaceous 168, 178
 Webster, Dr., on a basin-formed island 237
—— on gypsum at Ascension 201
 White, Martin, on soundings 254
 Wind, effects of, on the form of craters




INDEX TO SOUTH AMERICAN GEOLOGY.


Abich, on a new variety of feldspar 446
 Abrolhos islands 415
 Absence of recent formations on the S. American coasts 409
 Aguerros on elevation of Imperial 305
 Albite, constituent mineral in andesite 446
—— in rocks of Tierra del Fuego 427
—— in porphyries 444
—— crystals of, with orthite 447
 Alison, Mr., on elevation of Valparaiso 307, 310
 Alumina, sulphate of 439
Ammonites from Concepcion 400, 405
 Amolanas, Las 493
 Amygdaloid, curious varieties of 444
Amygdaloids of the Uspallata range 471
—— of Copiapo 498
 Andesite of Chile 446
—— in the valley of Maypu 449, 450
—— of the Cumbre pass 460, 466
—— of the Uspallata range 475
—— of Los Hornos 480
—— of Copiapo 488, 491
 Anhydrite, concretions of 450, 463
 Araucaria, silicified wood of 394, 474
 Arica, elevation of 323
 Arqueros, mines of 481
 Ascension, gypsum deposited on 328
—— laminated volcanic rocks of 439, 440
 Augite in fragments, in gneiss 414
—— with albite, in lava 347
 Austin, Mr. R. A. C., on bent cleavage lamina 434
 Austin, Captain, on sea-bottom 302
 Australia, foliated rocks of 438
 _Azara labiata_, beds of, at San Pedro 277, 352

 _Baculites vagina_ 400
 Bahia Blanca, elevation of 280
—— formations near 355
—— character of living shells of 408
 Bahia (Brazil), elevation near 280
—— crystalline rocks of 414
 Ballard, M., on the precipitation of sulphate of soda 349
 Banda Oriental, tertiary formations of 365
—— crystalline rocks of 418
 Barnacles above sea-level 311
—— adhering to upraised shells 306
 Basalt of S. Cruz 389
—— streams of, in the Portillo range 456
—— in the Uspallata range 472
 Basin chains of Chile 333
 Beagle Channel 427, 430
 Beaumont, Elie de, on inclination of lava-streams 390, 457
—— on viscid quartz-rocks 475
 Beech-tree, leaves of fossil 391
 Beechey, Captain, on sea-bottom 299
 Belcher, Lieutenant, on elevated shells from Concepcion 306
 Bella Vista, plain of 325
 Benza, Dr., on decomposed granite 417
 Bettington, Mr., on quadrupeds transported by rivers 374
 Blake, Mr., on the decay of elevated shells near Iquique 322
—— on nitrate of soda 346
 Bole 444
 Bollaert, Mr., on mines of Iquique 503
 Bones, silicified 402
—— fossil, fresh condition of 366
 Bottom of sea off Patagonia 292, 298
 Bougainville, on elevation of the Falkland islands 290
 Boulder formation of S. Cruz 285, 295
—— of Falkland islands 290
—— anterior to certain extinct quadrupeds 371
—— of Tierra del Fuego 391
 Boulders in the Cordillera 339, 341
—— transported by earthquake-waves 344
—— in fine-grained tertiary deposits 401
 Brande, Mr., on a mineral spring 461
 Bravais, M., on elevation of Scandinavia 320
 Brazil, elevation of 279
—— crystalline rocks of 414, 418
 Broderip, Mr., on elevated shells from Concepcion 306
 Brown, Mr. R., on silicified wood of Uspallata range 474
 Brown, on silicified wood 495
 Bucalema, elevated shells near 307
 Buch, Von, on cleavage 438
—— on cretaceous fossils of the Cordillera 453, 465
—— on the sulphureous volcanoes of Java 509
 Buenos Ayres 352
 Burchell, Mr., on elevated shells of Brazil 279
 Byron, on elevated shells 303

 Cachapual, boulders in valley of 339, 341
 Caldcleugh, Mr., on elevation of Coquimbo 314
—— on rocks of the Portillo range 456
 Callao, elevation near 323
—— old town of 327
 Cape of Good Hope, metamorphic rocks of 439
 _Carcharias megalodon_ 402
 Carpenter, Dr., on microscopic organisms 352
 Castro (Chiloe), beds near 394
 Cauquenes Baths, boulders near 339, 341
—— pebbles in porphyry near 443
—— volcanic formation near 447
—— stratification near 449
 Caves above sea-level 303, 307, 322
 _Cervus pumilus,_ fossil-horns of 304
 Chevalier, M., on elevation near Lima 323
 Chile, structure of country between the Cordillera and the Pacific 333
—— tertiary formations of 337
—— crystalline rocks in 435
—— central, geology of 441
—— northern, geology of 479
 Chiloe, gravel on coast 294
—— elevation of 303
—— tertiary formation of 337, 405
—— crystalline rocks of 433
 Chlorite-schist, near M. Video 419
 Chonos archipelago, tertiary formations of 393
—— crystalline rocks of 430
 Chupat, Rio, scoriæ transported by 280
 Claro, Rio, fossiliferous beds of 485
 Clay-shale of Los Hornos 480
 Clay-slate, formation of, Tierra del Fuego 424
—— of Concepcion 433
—— feldspathic, of Chile 442, 444, 448
—— —— of the Uspallata range 468, 470
—— black siliceous, band of, in porphyritic formations of Chile 445
 Claystone porphyry, formation of, in Chile 442
—— origin of 445
—— eruptive sources of 444
 Cleavage, definition of 414
—— at Bahia 415
—— Rio de Janeiro 415
—— Maldonado 418
—— Monte Video 420
—— S. Guitru-gueyu 421
—— Falkland I. 424
—— Tierra del Fuego 428
—— Chonos I. 434
—— Chiloe 435
—— Concepcion 434
—— Chile 435
—— discussion on 436
 Cleavage-laminæ superficially bent 434
 Cliffs, formation of 301
 Climate, late changes in 345
—— of Chile during tertiary period 408
 Coal of Concepcion 399
—— S. Lorenzo 504
 Coast-denudation of St. Helena 301
 Cobija, elevation of 322
 Colombia, cretaceous formation of 504
 Colonia del Sacramiento, elevation of 278
—— Pampean formation near 355
 Colorado, Rio, gravel of 295
—— sand-dunes of 281, 294
—— Pampean formation near 355
 Combarbala 479, 481
 Concepcion, elevation of 305
—— deposits of 399, 405
—— crystalline rocks of 433
 Conchalee, gravel-terraces of 311
 Concretions of gypsum, at Iquique 345
—— in sandstone at S. Cruz 387
—— in tufaceous tuff of Chiloe 387
—— in gneiss 414
—— in claystone-porphyry at Port Desire 421
—— in gneiss at Valparaiso 435
—— in metamorphic rocks 436
—— of anhydrite 450
—— relations of, to veins 473
 Conglomerate claystone of Chile 443, 445
—— of Tenuyan 454, 458, 478
—— of the Cumbre Pass 462, 466
—— of Rio Claro 485
—— of Copiapo 496, 499
 Cook, Captain, on form of sea-bottom 300
 Copiapo, elevation of 321
—— tertiary formations of 403
—— secondary formations of 489
 Copper, sulphate of 489
—— native, at Arqueros 482
—— mines of, at Panuncillo 481
—— veins, distribution of 505
 Coquimbo, elevation and terraces of 312
—— tertiary formations of 404
—— secondary formations of 482
 Corallines living on pebbles 299
 Cordillera, valleys bordered by gravel fringes 337
—— basal strata of 442
—— fossils of 453, 465, 486, 487, 493, 503
—— elevation of 442, 459, 474, 476, 500, 502, 510, 512, 517
—— gypseous formations of 450, 452, 461, 463, 479, 483, 489, 491, 503
—— claystone-porphyries of 442
—— andesitic rocks of 446
—— volcanoes of 447, 511, 517
 Coste, M., on elevation of Lemus 303
 Coy inlet, tertiary formation of 390
 _Crassatella Lyellii_ 392
 Cruickshanks, Mr., on elevation near Lima 327
 Crystals of feldspar, gradual formation of, at Port Desire 422
 Cumbre, Pass of, in Cordillera 502
 Cuming, Mr., on habits of the Mesodesma 310
—— on range of living shells on west coast 407

 Dana, Mr., on foliated rocks 438
—— on amygdaloids 444
 Darwin, Mount 427
 D’Aubuisson, on concretions 397
—— on foliated rocks 438
 Decay, gradual, of upraised shells 323, 327
 Decomposition of granite rocks 417
 De la Beche, Sir H., his theoretical researches in geology 299
—— on the action of salt on calcareous rocks 327
—— on bent cleavage-laminæ 434
 Denudation on coast of Patagonia 292, 300, 409
—— great powers of 410
—— of the Portillo range 456, 458
 Deposits, saline 344
 Despoblado, valley of 496, 497, 499
 Detritus, nature of, in Cordillera 338
 Devonshire, bent cleavage in 434
 Dikes, in gneiss of Brazil 414, 418
—— near Rio de Janeiro 417
—— pseudo, at Port Desire 423
—— in Tierra del Fuego 426
—— in Chonos archipelago, containing quartz 432
—— near Concepcion, with quartz 434
—— granitic-porphyritic, at Valparaiso 435
—— rarely vesicular in Cordillera 347
—— absent in the central ridges of the Portillo pass 452
—— of the Portillo range, with grains of quartz 456
—— intersecting each other often 466
—— numerous at Copiapo 498
 Domeyko, M., on the silver mines of Coquimbo 482
—— on the fossils of Coquimbo 486
 D’Orbigny, M. A., on upraised shells of Monte Video 278
—— on elevated shells at St. Pedro 278
—— on elevated shells near B. Ayres 279
—— on elevation of S. Blas 281
—— on the sudden elevation of La Plata 293
—— on elevated shells near Cobija 322
—— on elevated shells near Arica 322
—— on the climate of Peru 324
—— on salt deposits of Cobija 345
—— on crystals of gypsum in salt-lakes 349
—— on absence of gypsum in the Pampean formation 353
—— on fossil remains from Bahia Blanca 359, 360
—— on fossil remains from the banks of the Parana 362
—— on the geology of St. Fé 363
—— on the age of Pampean formation 367, 376
—— on the _Mastodon Andium_ 379
—— on the geology of the Rio Negro 381
—— on the character of the Patagonian fossils 391
—— on fossils from Concepcion 399
—— —— from Coquimbo 404
—— —— from Payta 405
—— on fossil tertiary shells of Chile 406
—— on cretaceous fossils of Tierra del Fuego 426
—— —— from the Cordillera of Chile 453, 465, 486, 488, 493, 504

 Earth, marine origin of 304, 308
 Earthenware, fossil 326
 Earthquake, effect of, at S. Maria 293
—— elevation during, at Lemus 303
—— of 1822, at Valparaiso 310
—— effects of, in shattering surface 325
—— fissures made by 325
—— probable effects on cleavage 325
 Earthquakes in Pampas 290
 Earthquake-waves, power of, in throwing up shells 310
—— effects of, near Lima 327
—— power of, in transporting boulders 344
 Edmonston, Mr., on depths at which shells live at Valparaiso 309
 Ehrenberg, Professor, on infusoria in the Pampean formation 355, 359,
 362
—— on infusoria in the Patagonian formation 383, 384, 386, 391, 392
 Elevation of La Plata 278
—— Brazil 279
—— Bahia Blanca 280, 357
—— San Blas 281
—— Patagonia 281, 291, 293
—— Tierra del Fuego 288
—— Falkland islands 290
—— Pampas 289, 377
—— Chonos archipelago 303
—— Chiloe 304
—— Chile 304
—— Valparaiso 307, 310
—— Coquimbo 312, 320
—— Guasco 320
—— Iquique 322
—— Cobija 322
—— Lima 323
—— sudden, at S. Maria 293
—— —— at Lemus 303
—— insensible, at Chiloe 304
—— —— at Valparaiso 311
—— —— at Coquimbo 314
—— axes of, at Chiloe 398, 405
—— —— at P. Rumena 398, 405
—— —— at Concepcion 398, 405
—— unfavourable for the accumulation of permanent deposits 410
—— lines of, parallel to cleavage and foliation 416, 417, 424, 428,
432, 434, 438
—— lines of, oblique to foliation 431
—— areas of, causing lines of elevation and cleavage 441
—— lines of, in the Cordillera 442
—— slow, in the Portillo range 475
—— two periods of, in Cordillera of Central Chile 476
—— of the Uspallata range 474
—— two periods of, in Cumbre Pass 476
—— horizontal, in the Cordillera of Copiapo 500
—— axes of, coincident with volcanic orifices 503
—— of the Cordillera, summary on 510, 513, 517
 Elliott, Captain, on human remains 279
 Ensenada, elevated shells of 278
 Entre Rios, geology of 363
 _Equus curvidens_ 364, 379
 Epidote in Tierra del Fuego 426
—— in gneiss 435
—— frequent in Chile 445
—— in the Uspallata range 475
—— in porphyry of Coquimbo 482
 Erman, M., on andesite 347
 Escarpments, recent, of Patagonia 301
 Extinction of fossil mammifers 370

 Falkland islands, elevation of 290
—— pebbles on coast 297, 299
—— geology of 424
 Falkner, on saline incrustations 347
 Faults, great, in Cordillera 461, 469
 Feldspar, earthy, metamorphosis of, at Port Desire 422
—— albitic 347
—— crystals of, with albite 347
—— orthitic, in conglomerate of Tenuyan 454
—— in granite of Portillo range 455
—— in porphyries in the Cumbre Pass 466
 Feuillée on sea-level at Coquimbo 314
 Fissures, relations of, to concretions 397
—— upfilled, at Port Desire 424
—— in clay-slate 470
 Fitton, Dr., on the geology of Tierra del Fuego 427
 Fitzroy, Captain, on the elevation of the Falkland islands 427
—— on the elevation of Concepcion 305
 Foliation, definition of 414
—— of rocks at Bahia 414
—— Rio de Janeiro 415
—— Maldonado 418
—— Monte Video 420
—— S. Guitru-gueyu 421
—— Falkland I. 424
—— Tierra del Fuego 427
—— Chonos archipelago 430
—— Chiloe 433
—— Concepcion 434
—— Chile 435
—— discussion on 435
 Forbes, Professor E., on cretaceous fossils of Concepcion 400
—— on cretaceous fossils and subsidence in Cumbre Pass 465
—— on fossils from Guasco 488
—— —— from Coquimbo 483, 487
—— —— from Copiapo 493
—— on depths at which shells live 409, 496
 Formation, Pampean 352
—— —— area of 371
—— —— estuary origin 373
—— tertiary of Entre Rios 363
—— of Banda Oriental 365
—— volcanic, in Banda Oriental 367
—— of Patagonia 381
—— summary on 391
—— tertiary of Tierra del Fuego 391
—— —— of the Chonos archipelago 393
—— —— of Chiloe 394
—— —— of Chile 394
—— —— of Concepcion 398, 404
—— —— of Navidad 400
—— —— of Coquimbo 402
—— —— of Peru 404
—— —— subsidence during 402
—— volcanic, of Tres Montes 393
—— —— of Chiloe 394
—— —— old, near Maldonado 418
—— —— with laminar structure 440
—— —— ancient, in Tierra del Fuego 426
—— recent, absent on S. American coast 409
—— metamorphic, of claystone-porphyry of Patagonia 421, 440
—— foliation of 436
—— plutonic, with laminar structure 440
—— palaeozoic, of the Falkland I. 424
—— claystone, at Concepcion 433
—— Jurassic, of Cordillera 512
—— Neocomian, of the Portillo Pass 453
—— volcanic, of Cumbre Pass 465
—— gypseous, of Los Hornos 479, 487
—— —— of Coquimbo 482
—— —— of Guasco 487
—— —— of Copiapo 488
—— —— of Iquique 503
—— cretaceo-oolitic, of Coquimbo 486, 495
—— —— of Guasco 487, 494
—— —— of Copiapo 495
—— —— of Iquique 504
 Fossils, Neocomian, of Portillo Pass 453
—— —— of Cumbre Pass 465
—— secondary, of Coquimbo 485
—— —— of Guasco 487
—— —— of Copiapo 494
—— —— of Iquique 503
—— palæozoic, from the Falklands 424
 Fragments of hornblende-rock in gneiss 414
—— of gneiss in gneiss 416
 Freyer, Lieutenant, on elevated shells of Arica 323
 Frezier on sea-level at Coquimbo 314

 Galapagos archipelago, pseudo-dikes of 424
 Gallegos, Port, tertiary formation of 390
 Garnets in gneiss 415
—— in mica-slate 427
—— at Panuncillo 481
 Gardichaud, M., on granites of Brazil 417
 Gay, M., on elevated shells 306
—— on boulders in the Cordillera 339, 341
—— on fossils from Cordillera of Coquimbo 487
 Gill, Mr., on brickwork transported by an earthquake-wave 327
 Gillies, Dr., on heights in the Cordillera 448
—— on extension of the Portillo range 458
 Glen Roy, parallel roads of 319
—— sloping terraces of 340
 Gneiss, near Bahia 414
—— of Rio de Janeiro 415
—— decomposition of 417
 Gold, distribution of 506
 Gorodona, formations near 362
 Granite, axis of oblique, to foliation 431
—— andesitic 446
—— of Portillo range 455
—— veins of, quartzose 432, 475
—— pebble of, in porphyritic conglomerate 493
—— conglomerate 497
 Grauwacke of Uspallata range 468
 Gravel at bottom of sea 293, 298
—— formation of, in Patagonia 295
—— means of transportation of 298
—— strata of, inclined 467
 Gravel-terraces in Cordillera 337
 Greenough, Mr., on quartz veins 437
 Greenstone, resulting from metamorphose hornblende-rock 419
—— of Tierra del Fuego 426
—— on the summit of the Campana of Quillota 442
—— porphyry 443
—— relation of, to clay-slate 443
 _Gryphæa orientalis_ 483
 Guasco, elevation of 321
—— secondary formation of 487
 Guitru-gueyu, Sierra 421
 Guyana, gneissic rocks of 415
 Gypsum, nodules of, in gravel at Rio Negro 296
—— deposited from sea-water 327
—— deposits of, at Iquique 345
—— crystals of, in salt lakes 346
—— in Pampean formation 353
—— in tertiary formation of Patagonia 382, , ,
—— great formation of, in the Portillo Pass 461, 463
—— —— in the Cumbre Pass 461, 463
—— —— near Los Hornos 479
—— —— at Coquimbo 482
—— —— at Copiapo 490, 492
—— —— near Iquique 504
—— of San Lorenzo 504

 Hall, Captain, on terraces at Coquimbo 316
 Hamilton, Mr., on elevation near Tacna 323
 Harlan, Dr., on human remains 279
 Hayes, Mr. A., on nitrate of soda 346
 Henslow, Professor, on concretions 437
 Herbert, Captain, on valleys in the Himalaya 335
 Herradura Bay, elevated shells of 315
—— tertiary formations of 402
 Himalaya, valleys in 335
 _Hippurites Chilensis_ 483, 486
 Hitchcock, Professor, on dikes 414
 Honestones, pseudo, of Coquimbo 483
—— of Copiapo 489
 Hooker, Dr. J. D., on fossil beech-leaves 391
 Hopkins, Mr., on axes of elevation oblique to foliation 432
—— on origin of lines of elevation 440, 512
 Hornblende-rock, fragments of, in gneiss 414
 Hornblende-schist, near M. Video 420
 Hornos, Los, section near 479
 Hornstone, dike of 433, 434
 Horse, fossil tooth of 358, 364
 Huafo island 393, 404
—— subsidence at 411
 Huantajaya, mines of 503
 Humboldt, on saline incrustations 347
—— on foliations of gneiss 415
—— on concretions in gneiss 435

 Icebergs, action on cleavage 434, 436
 Illapele, section near 479
 Imperial, beds of shells near 305
 Incrustations, saline 347
 Infusoria in Pampean formation 352, 355, 360, 363
—— in Patagonian formation 382, 383, 384, 391
 Iodine, salts of 347, 348
 Iquique, elevation of 322
—— saliferous deposits of 344
—— cretaceo-oolitic formation of 503
 Iron, oxide of, in lavas 463, 499
—— in sedimentary beds 480, 482
—— tendency in, to produce hollow concretions 398
—— sulphate of 489
 Isabelle, M., on volcanic rocks of Banda Oriental 368

 Joints in clay-slate 428
 Jukes, Mr., on cleavage in Newfoundland 437

 Kamtschatka, andesite of 347
 Kane, Dr., on the production of carbonate of soda 328
 King George’s sound, calcareous beds of 312

 Lakes, origin of 300
—— fresh-water, near salt lakes 350
 Lava, basaltic, of S. Cruz 389
—— claystone-porphyry, at Chiloe 395
—— —— ancient submarine 446
—— basaltic, of the Portillo range 457
—— feldspathic, of the Cumbre Pass 463
—— submarine, of the Uspallata range 471, 473, 476
—— basaltic, of the Uspallata range 475
—— submarine, of Coquimbo 484, 486
—— of Copiapo 490, 496, 499
 Lemus island 393, 404
 Lemuy islet 394
 Lignite of Chiloe 395
—— of Concepcion 398
 Lima, elevation of 323
 Lime, muriate of 328, 344, 347
 Limestone of Cumbre Pass 462
—— of Coquimbo 483, 485
—— of Copiapo 493
 Lund and Clausen on remains of caves in Brazil 378, 380
 Lund, M., on granites of Brazil 417
 Lyell, M., on upraised shells retaining their colours 289
—— on terraces at Coquimbo 315
—— on elevation near Lima 327
—— on fossil horse’s tooth 364
—— on the boulder-formation being anterior to the extinction of North
American mammifers 371
—— on quadrupeds washed down by floods 374
—— on age of American fossil mammifers 379
—— on changes of climate 409
—— on denudation 410
—— on foliation 438

 MacCulloch, Dr., on concretions 437
—— on beds of marble 440
 Maclaren, Mr., letter to, on coral-formations 413
 _Macrauchenia Patachonica_ 358, 370
 Madeira, subsidence of 302
 Magellan, Strait, elevation near, of 288
 Magnesia, sulphate of, in veins 387
 Malcolmson, Dr., on trees carried out to sea 475
 Maldonado, elevation of 277
—— Pampean formation of 365
—— crystalline rocks of 418
 Mammalia, fossil, of Bahia Blanca 356, 364
—— —— near St. Fé 363
—— —— of Banda Oriental 366
—— —— of St. Julian 369
—— —— at Port Gallegos 391
—— washed down by floods 373
—— number of remains of, and range of, in Pampas 376
 Man, skeletons of (Brazil) 279
—— remains of, near Lima 325
—— Indian, antiquity of 325
 Marble, beds of 418
 Maricongo, ravine of 500
 Marsden, on elevation of Sumatra 305
 _Mastodon Andium_, remains of 362
—— range of 378
 Maypu, Rio, mouth of, with upraised shells 307
—— gravel fringes of 339
—— debouchement from the Cordillera 449
 Megalonyx, range of 379
 Megatherium, range of 379
 Miers, Mr., on elevated shells 311
—— on the height of the Uspallata plain 335
 Minas, Las 418
 Mocha Island, elevation of 305
—— tertiary form of 398
—— subsidence at 411
 Molina, on a great flood 341
 Monte Hermoso, elevation of 280
—— fossils of 355
 Monte Video, elevation of 278
—— Pampean formation of 365
—— crystalline rocks of 419
 Morris and Sharpe, Messrs., on the palæozoic fossils of the Falklands
 424
 Mud, Pampean 352
—— long deposited on the same area 376
 Murchison, Sir R., on cleavage 436
—— on waves transporting gravel 299
—— on origin of salt formations 505
—— on the relations of metalliferous veins and intrusive rocks 507
—— on the absence of granite in the Ural 512

 _Nautilus d’Orbignyanus_ 400, 405
 Navidad, tertiary formations of, subsidence of 400, 411
 Negro, Rio, pumice of pebbles of 281
—— gravel of 295
—— salt lakes of 295
—— tertiary strata of 384
 North America, fossil remains of 379
 North Wales, sloping terraces absent in 340
—— bent cleavage of 434
 Neuvo Gulf, plains of 282
—— tertiary formation of 384

 Owen, Professor, on fossil mammiferous remains 356, 358, 364, 366, 370

 Palmer, Mr., on transportation of gravel 300
 Pampas, elevation of 290
—— earthquakes of 290
—— formation of 295, 350
—— localities in which fossil mammifers have been found 380
 Panuncillo, mines of 481
 Parana, Rio, on saline incrustations 347
—— Pampean formations near 361
—— on the S. Tandil 420
 Parish, Sir W., on elevated shells near Buenos Ayres 278, 279
—— on earthquakes in the Pampas 290
—— on fresh-water near salt lakes 350
—— on origin of Pampean formation 373
 Patagonia, elevation and plains of 281
—— denudation of 291
—— gravel-formation of 295
—— sea-cliffs of 301
—— subsidence during tertiary period 411
—— crystalline rocks of 421
 Payta, tertiary formations of 404
 Pebbles of pumice 280
—— decrease in size on the coast of Patagonia 293
—— means of transportation 298
—— encrusted with living corallines 299
—— distribution of, at the eastern foot of Cordillera 337
—— dispersal of, in the Pampas 354
—— zoned with colour 443
 Pentland, Mr., on heights in the Cordillera 460
—— on fossils of the Cordillera 465
 Pernambuco 279
 Peru, tertiary formations of 403
 Peuquenes, Pass of, in the Cordillera 448
—— ridge of 452
 Pholas, elevated shells of 303
 Pitchstone of Chiloe 395
—— of Port Desire 421
—— near Cauquenes 448
—— layers of, in the Uspallata range 472
—— of Los Hornos 480
—— of Coquimbo 483
 Plains of Patagonia 282, 291
—— of Chiloe 304
—— of Chile 333
—— of Uspallata 335
—— on eastern foot of Cordillera 336
—— of Iquique 346
 Plata, La, elevation of 277
—— tertiary formation of 295, 353
—— crystalline rocks of 418
 Playfair, Professor, on the transportation of gravel 300
 Pluclaro, axis of 483
 Pondicherry, fossils of 400
 Porcelain rocks of Port Desire 422
—— of the Uspallata range 471, 473, 476
 Porphyry, pebbles of, strewed over Patagonia 296
 Porphyry, claystone, of Chiloe 395
—— —— of Patagonia 421
—— —— of Chile 442, 445
—— greenstone, of Chile 444
—— doubly columnar 448
—— claystone, rare, on the eastern side of the Portillo Pass 454
—— brick-red and orthitic, of Cumbre Pass 458, 467
—— intrusive, repeatedly injected 467
—— claystone of the Uspallata range 468
—— —— of Copiapo 489, 499
—— —— eruptive sources of 502
 Port Desire, elevation and plains of 283
—— tertiary formation of 383
—— porphyries of 421
 Portillo Pass in the Cordillera 448
 Portillo chain 454, 458
—— compared with that of the Uspallata 478
 Prefil or sea-wall of Valparaiso 310
 Puente del Inca, section of 461
 Pumice, pebbles of 230
—— conglomerate of R. Negro 382
—— hills of, in the Cordillera 347
 Punta Alta, elevation of 280
—— beds of 356

 Quartz-rock of the S. Ventana 421
—— C. Blanco 421
—— Falkland islands 424
—— Portillo range 455
—— viscidity of 475
—— veins of, near Monte Video 420
—— —— in dike of greenstone 426
—— grains of, in mica slate 430
—— —— in dikes 432, 434
—— veins of, relations to cleavage 437
 Quillota, Campana of 442
 Quintero, elevation of 311
 Quiriquina, elevation of 306
—— deposits of 399

 Rancagua, plain of 334
 Rapel, R., elevation near 307
 Reeks, Mr. T., his analysis of decomposed shells 328
—— his analysis of salts 344
 Remains, human 324
 Rio de Janeiro, elevation near 279
—— crystalline rocks of 415
 Rivers, small power of transporting pebbles 298
—— small power of, in forming valleys 343
—— drainage of, in the Cordillera 449, 513
 Roads, parallel, of Glen Roy 319
 Rocks, volcanic, of Banda Oriental 367
—— Tres Montes 393
—— Chiloe 394
—— Tierra del Fuego 426
—— with laminar structure 440
 Rodents, fossil, remains of 356
 Rogers, Professor, address to Association of American Geologists 412
 Rose, Professor G., on sulphate of iron at Copiapo 489

 S. Blas, elevation of 281
 S. Cruz, elevation and plains of 284
—— valley of 285
—— nature of gravel in valley of 296
—— boulder formation of 371
—— tertiary formation of 386
—— subsidence at 412
 S. Fé Bajada, formations of 363
 S. George’s bay, plains of 282
 S. Helena island, sea-cliffs, and subsidence of 301
 S. Josef, elevation of 281
—— tertiary formation of 383
 S. Juan, elevation near 278
 S. Julian, elevation and plains of 284
—— salt lake of 348
—— earthy deposit with mammiferous remains 369
—— tertiary formations of 384
—— subsidence at 411
 S. Lorenzo, elevation of 323
—— old salt formation of 504
 S. Mary, island of, elevation of 305
 S. Pedro, elevation of 278
 Salado, R., elevated shells of 279
—— Pampean formation of 353
 Salines 348
 Salt, with upraised shell 324, 327
—— lakes of 348
—— purity of, in salt lakes 349
—— deliquescent, necessary for the preservation of meat 349
—— ancient formation of, at Iquique 504
—— —— at S. Lorenzo 504
—— strata of, origin of 505
 Salts, superficial deposits of 344
 Sand-dunes of the Uruguay 279
—— of the Pampas 281
—— near Bahia Blanca 281, 293
—— of the Colorado 281, 294
—— of S. Cruz 286
—— of Arica 323
 Sarmiento, Mount 427
 Schmidtmeyer on auriferous detritus 506
 Schomburghk, Sir R., on sea-bottom 299
—— on the rocks of Guyana 415
 Scotland, sloping terraces of 340
 Sea, nature of bottom of, off Patagonia 292
—— power of, in forming valleys 343
 Sea cliffs, formation of 301
 Seale, Mr., model of St. Helena 301
 Sebastian Bay, tertiary formation of 391
 Sedgwick, Professor, on cleavage 336
 Serpentine of Copiapo 489
 Serpulæ, on upraised rocks 325
 Shale-rock, of the Portillo Pass 452
—— of Copiapo 493
 Shells, upraised state of, in Patagonia 288
—— elevated, too small for human food 308
—— transported far inland, for food 309
—— upraised, proportional numbers varying 312, 324
—— —— gradual decay of 323, 324, 327
—— —— absent on high plains of Chile 335
—— —— near Bahia Blanca 358
—— preserved in concretions 394, 397
—— living and fossil range of, on west coast 406, 408
—— living, different on the east and west coast 411
 Shingle of Patagonia 295
 Siau, M., on sea-bottom 299
 Silver mines of Arqueros 431
—— of Chanuncillo 494
—— of Iquique 503
—— distribution of 506
 Slip, great, at S. Cruz 387
 Smith, Mr., of Jordan Hill, on upraised shells retaining their colours
 289
—— on Madeira 302
—— on elevated seaweed 325
—— on inclined gravel beds 467
 Soda, nitrate of 346
—— sulphate of, near Bahia Blanca 348, 349
—— carbonate of 347
 Soundings off Patagonia 293, 299
—— in Tierra del Fuego 300
 Spirifers 486, 488
 Spix and Martius on Brazil 417
 Sprengel on the production of carbonate of soda 328
 Springs, mineral, in the Cumbre Pass 461
 Stratification of sandstone in metamorphic rocks 414
—— of clay-slate in Tierra del Fuego 428
—— of the Cordillera of Central Chile 442, 448, 461
—— little disturbed in Cumbre Pass 460, 466
—— disturbance of, near Copiapo 501
 Streams of lava at S. Cruz, inclination of 390
—— in the Portillo range 457
 String of cotton with fossil-shells 325
 _Struthiolaria ornata_ 392
 Studer, M., on metamorphic rocks 438
 Subsidence during formation of sea-cliffs 301
—— near Lima 327
—— probable, during Pampean formation 376
—— necessary for the accumulation of permanent deposits 411
—— during the tertiary formations of Chile and Patagonia 413
—— probable during the Neocomian formation of the Portillo Pass 453
—— probable during the formation of conglomerate of Tenuyan 459
—— during the Neocomian formation of the Cumbre Pass 465
—— of the Uspallata range 474, 477
—— great, at Copiapo 496
—— —— during the formation of the Cordillera 510
 Sulphur, volcanic exhalations of 509
 Sumatra, promontories of 305
 Summary on the recent elevatory movements 259, 329, 514
—— on the Pampean formation 371, 515
—— on the tertiary formations of Patagonia and Chile 391, 404, 513
—— on the Chilean Cordillera 508
—— on the cretaceo-oolitic formation 508
—— on the subsidences of the Cordillera 509
—— on the elevation of the Cordillera 511, 517

 Tacna, elevation of 323
 Tampico, elevated shells near 329
 Tandil, crystalline rocks of 420
 Tapalguen, Pampean formation of 353
—— crystalline rocks of 420
 Taylor, Mr., on copper veins of Cuba 506
 Temperature of Chile during the tertiary period 408
 Tension, lines of, origin of, axes of elevation and of cleavage 440
 Tenuy Point, singular section of 395
 Tenuyan, valley of 454, 478
 Terraces of the valley of S. Cruz 286
—— of equable heights throughout Patagonia 290
—— of Patagonia, formation of 294
—— of Chiloe 304
—— at Conchalee 311
—— of Coquimbo 316
—— not horizontal at Coquimbo 317
—— of Guasco 320
—— of S. Lorenzo 323
—— of gravel within the Cordillera 337
 Theories on the origin of the Pampean formation 372
 Tierra Amarilla 489
 Tierra del Fuego, form of sea-bottom 300
—— tertiary formations of 391
—— clay-slate formation of 424
—— cretaceous formation of 426
—— crystalline rocks of 426
—— cleavage of clay-slate 427, 436
 Tosca rock 352
 Trachyte of Chiloe 394
—— of Port Desire 421
—— in the Cordillera 347
 Traditions of promontories having been islands 305
—— on changes of level near Lima 327
 Trees buried in plain of Iquique 346
—— silicified, vertical, of the Uspallata range 473
 Tres Montes, elevation of 303
—— volcanic rocks of 393
 _Trigonocelia insolita_ 392
 Tristan Arroyo, elevated shells of 278
 Tschudi, Mr., on subsidence near Lima 327
 Tuff, calcareous, at Coquimbo 313
—— on basin-plain near St. Jago 334
—— structure of, in Pampas 352
—— origin of, in Pampas 374
—— pumiceous, of R. Negro 382
—— Nuevo Gulf 383
—— Port Desire 383
—— S. Cruz 386
—— Patagonia, summary on Chiloe 391
—— formation of, in Portillo chain 395
—— great deposit of, at Copiapo 457
 Tuffs, volcanic, metamorphic, of Uspallata 471
—— of Coquimbo 484

 Ulloa, on rain in Peru 324
—— on elevation near Lima 327
 Uruguay, Rio, elevation of country near 278
 Uspallata, plain of 335, 515
—— pass of 459
—— range of 368
—— concluding remarks on 476

 Valdivia, tertiary beds of 398
—— mica-slate of 433
 Valley of S. Cruz, structure of 285
—— Coquimbo 314
—— Guasco, structure of 320
—— Copiapo, structure of 321
—— S. Cruz, tertiary formations of 386
—— Coquimbo, geology of 482
—— Guasco, secondary formations of 487
—— Copiapo, secondary formations of 488
—— Despoblado 496, 497, 499
 Valleys in the Cordillera bordered by gravel fringes 337
—— formation of 338
—— in the Cordillera 449
 Valparaiso, elevation of 307
—— gneiss of 435
 Vein of quartz near Monte Video 419
—— in mica-slate 430
—— relations of, to cleavage 437
—— in a trap dike 426
—— of granite, quartzose 432, 475
—— remarkable, in gneiss, near Valparaiso 435
 Veins, relations of, to concretions 396
—— metalliferous, of the Uspallata range 475
—— metalliferous, discussion on 505
 Venezuela, gneissic rocks of 415
 Ventana, Sierra, Pampean formation near 353
—— quartz-rock of 421
 Villa Vincencio Pass 468
 Volcan, Rio, mouth of 449
—— fossils of 453
 Volcanoes of the Cordillera 392, 447, 511
—— absent, except near bodies of water 457
—— ancient submarine, in Cordillera 502
—— action of, in relation to changes of level 514
—— long action of, in the Cordillera 517

 Wafer on elevated shells 322
 Waves caused by earthquakes, power of, in transporting boulders 326,
 344
—— power of, in throwing up shells 309
 Weaver, Mr., on elevated shells 329
 White, Martin, on sea-bottom 299
 Wood, silicified, of Entre Rios 364
—— S. Cruz 388
—— Chiloe 394, 396
—— Uspallata range 473
—— Los Hornos 479
—— Copiapo 495, 497

 Yeso, Rio, and plain of 450
 Ypun Island, tertiary formation of 393

 Zeagonite 426




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