The Student’s Elements of Geology

By SIR CHARLES LYELL, BART., F.R.S.

AUTHOR OF
“THE PRINCIPLES OF GEOLOGY,” “THE ANTIQUITY OF MAN,” ETC.

Thecosmilia annularis

WITH MORE THAN 600 ILLUSTRATIONS ON WOOD.

NEW YORK
HARPER & BROTHERS, PUBLISHERS
1878

Tertiary or Cainozoic, Secondary or Mesozoic, Primary or Paleozoic

CONTENTS.




PREFACE

Chapter I—ON THE DIFFERENT CLASSES OF ROCKS.
Geology defined. — Successive Formation of the Earth’s Crust. —
Classification of Rocks according to their Origin and Age. — Aqueous
Rocks. — Their Stratification and imbedded Fossils. — Volcanic Rocks,
with and without Cones and Craters. — Plutonic Rocks, and their
Relation to the Volcanic. — Metamorphic Rocks, and their probable
Origin. — The term Primitive, why erroneously applied to the
Crystalline Formations. — Leading Division of the Work.

Chapter II—AQUEOUS ROCKS—THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral Composition of Strata. — Siliceous Rocks. — Argillaceous. —
Calcareous. — Gypsum. — Forms of Stratification. — Original
Horizontality. — Thinning out. — Diagonal Arrangement. — Ripple-mark.

Chapter III—ARRANGEMENT OF FOSSILS IN STRATA—FRESH-WATER AND MARINE.
Successive Deposition indicated by Fossils. — Limestones formed of
Corals and Shells. — Proofs of gradual Increase of Strata derived from
Fossils. — Serpula attached to Spatangus. — Wood bored by Teredina. —
Tripoli formed of Infusoria. — Chalk derived principally from Organic
Bodies. — Distinction of Fresh-water from Marine Formations. — Genera
of Fresh-water and Land Shells. — Rules for recognising Marine
Testacea. — Gyrogonite and Chara. — Fresh-water Fishes. — Alternation
of Marine and Fresh-water Deposits. — Lym-Fiord.

Chapter IV—CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and Mechanical Deposits. — Cementing together of Particles. —
Hardening by Exposure to Air. — Concretionary Nodules. — Consolidating
Effects of Pressure. — Mineralization of Organic Remains. — Impressions
and Casts: how formed. — Fossil Wood. — Goppert’s Experiments. —
Precipitation of Stony Matter most rapid where Putrefaction is going
on. — Sources of Lime and Silex in Solution.

Chapter V—ELEVATION OF STRATA ABOVE THE SEA.—HORIZONTAL AND INCLINED
STRATIFICATION.
Why the Position of Marine Strata, above the Level of the Sea, should
be referred to the rising up of the Land, not to the going down of the
Sea. — Strata of Deep-sea and Shallow-water Origin alternate. — Also
Marine and Fresh-water Beds and old Land Surfaces. — Vertical,
inclined, and folded Strata. — Anticlinal and Synclinal Curves. —
Theories to explain Lateral Movements. — Creeps in Coal-mines. — Dip
and Strike. — Structure of the Jura. — Various Forms of Outcrop. —
Synclinal Strata forming Ridges. — Connection of Fracture and Flexure
of Rocks. — Inverted Strata. — Faults described. — Superficial Signs of
the same obliterated by Denudation. — Great Faults the Result of
repeated Movements. — Arrangement and Direction of parallel Folds of
Strata. — Unconformability. — Overlapping Strata.

Chapter VI—DENUDATION.
Denudation defined. — Its Amount more than equal to the entire Mass of
Stratified Deposits in the Earth’s Crust. — subaërial Denudation. —
Action of the Wind. — Action of Running Water. — Alluvium defined. —
Different Ages of Alluvium. — Denuding Power of Rivers affected by Rise
or Fall of Land. — Littoral Denudation. — Inland Sea-Cliffs. —
Escarpments. — Submarine Denudation. — Dogger-bank. — Newfoundland
Bank. — Denuding Power of the Ocean during Emergence of Land.

Chapter VII—JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN
REMODELLING THE EARTH’S CRUST.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at
great Depths. — Why the Height of the successive Strata in a given
Region is so disproportionate to their Thickness. — Computation of the
average annual Amount of subaërial Denudation. — Antagonism of Volcanic
Force to the Levelling Power of running Water. — How far the Transfer
of Sediment from the Land to a neighbouring Sea-bottom may affect
Subterranean Movements. — Permanence of Continental and Oceanic Areas.

Chapter VIII—CHRONOLOGICAL CLASSIFICATION OF ROCKS.
Aqueous, Plutonic, volcanic, and metamorphic Rocks considered
chronologically. — Terms Primary, Secondary, and Tertiary; Palæozoic,
Mesozoic, and Cainozoic explained. — On the different Ages of the
aqueous Rocks. — Three principal Tests of relative Age: Superposition,
Mineral Character, and Fossils. — Change of Mineral Character and
Fossils in the same continuous Formation. — Proofs that distinct
Species of Animals and Plants have lived at successive Periods. —
Distinct Provinces of indigenous Species. — Great Extent of single
Provinces. — Similar Laws prevailed at successive Geological Periods. —
Relative Importance of mineral and palæontological Characters. — Test
of Age by included Fragments. — Frequent Absence of Strata of
intervening Periods. — Tabular Views of fossiliferous Strata.

Chapter IX—CLASSIFICATION OF TERTIARY FORMATIONS.
Order of Succession of Sedimentary Formations. — Frequent
Unconformability of Strata. — Imperfection of the Record. —
Defectiveness of the Monuments greater in Proportion to their
Antiquity. — Reasons for studying the newer Groups first. —
Nomenclature of Formations. — Detached Tertiary Formations scattered
over Europe. — Value of the Shell-bearing Mollusca in Classification. —
Classification of Tertiary Strata. — Eocene, Miocene, and Pliocene
Terms explained.

Chapter X—RECENT AND POST-PLIOCENE PERIODS.
Recent and Post-pliocene Periods. — Terms defined. — Formations of the
Recent Period. — Modern littoral Deposits containing Works of Art near
Naples. — Danish Peat and Shell-mounds. — Swiss Lake-dwellings. —
Periods of Stone, Bronze, and Iron. — Post-pliocene Formations. —
Coexistence of Man with extinct Mammalia. — Reindeer Period of South of
France. — Alluvial Deposits of Paleolithic Age. — Higher and
Lower-level Valley-gravels. — Loess or Inundation-mud of the Nile,
Rhine, etc. — Origin of Caverns. — Remains of Man and extinct
Quadrupeds in Cavern Deposits. — Cave of Kirkdale. — Australian
Cave-breccias. — Geographical Relationship of the Provinces of living
Vertebrata and those of extinct Post-pliocene Species. — Extinct
struthious Birds of New Zealand. — Climate of the Post-pliocene Period.
— Comparative Longevity of Species in the Mammalia and Testacea. —
Teeth of Recent and Post-pliocene Mammalia.

Chapter XI—POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS.
Geographical Distribution, Form, and Characters of Glacial Drift. —
Fundamental Rocks, polished, grooved, and scratched. — Abrading and
striating Action of Glaciers. — Moraines, Erratic Blocks, and “Roches
Moutonnees”. — Alpine Blocks on the Jura. — Continental Ice of
Greenland. — Ancient Centres of the Dispersion of Erratics. —
Transportation of Drift by floating Icebergs. — Bed of the Sea furrowed
and polished by the running aground of floating Ice-islands.

Chapter XII—POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS,
concluded.
Glaciation of Scandinavia and Russia. — Glaciation of Scotland. —
Mammoth in Scotch Till. — Marine Shells in Scotch Glacial Drift. —
Their Arctic Character. — Rarity of Organic Remains in Glacial
Deposits. — Contorted Strata in Drift. — Glaciation of Wales, England,
and Ireland. — Marine Shells of Moel Tryfaen. — Erratics near
Chichester. — Glacial Formations of North America. — Many Species of
Testacea and Quadrupeds survived the Glacial Cold. — Connection of the
Predominance of Lakes with Glacial Action. — Action of Ice in
preventing the silting up of Lake-basins. — Absence of Lakes in the
Caucasus. — Equatorial Lakes of Africa.

Chapter XIII—PLIOCENE PERIOD.
Glacial Formations of Pliocene Age. — Bridlington Beds. — Glacial
Drifts of Ireland. — Drift of Norfolk Cliffs. — Cromer Forest-bed. —
Aldeby and Chillesford Beds. — Norwich Crag. — Older Pliocene Strata. —
Red Crag of Suffolk. — Coprolitic Bed of Red Crag. — White or Coralline
Crag. — Relative Age, Origin, and Climate of the Crag Deposits. —
Antwerp Crag. — Newer Pliocene Strata of Sicily. — Newer Pliocene
Strata of the Upper Val d’Arno. — Older Pliocene of Italy. —
Subapennine Strata. — Older Pliocene Flora of Italy.

Chapter XIV—MIOCENE PERIOD.—UPPER MIOCENE.
Upper Miocene Strata of France. — Faluns of Touraine. — Tropical
Climate implied by Testacea. — Proportion of recent Species of Shells.
— faluns more ancient than the Suffolk Crag. — Upper Miocene of
Bordeaux and the South of France. — Upper Miocene of Oeningen, in
Switzerland. — Plants of the Upper Fresh-water Molasse. — Fossil Fruit
and Flowers as well as Leaves. — Insects of the Upper Molasse. — Middle
or Marine Molasse of Switzerland. — Upper Miocene Beds of the
Bolderberg, in Belgium. — Vienna Basin. — Upper Miocene of Italy and
Greece. — Upper Miocene of India; Siwalik Hills. — Older Pliocene and
Miocene of the United States.

Chapter XV—LOWER MIOCENE.
Lower Miocene Strata of France. — Line between Miocene and Eocene. —
Lacustrine Strata of Auvergne. — Fossil Mammalia of the Limagne
d’Auvergne. — Lower Molasse of Switzerland. — Dense Conglomerates and
Proofs of Subsidence. — Flora of the Lower Molasse. — American
Character of the Flora. — Theory of a Miocene Atlantis. — Lower Miocene
of Belgium. — Rupelian Clay of Hermsdorf near Berlin. — Mayence Basin.
— Lower Miocene of Croatia. — Oligocene Strata of Beyrich. — Lower
Miocene of Italy. — Lower Miocene of England. — Hempstead Beds. — Bovey
Tracey Lignites in Devonshire. — Isle of Mull Leaf-Beds. — Arctic
Miocene Flora. — Disco Island. — Lower Miocene of United States. —
Fossils of Nebraska.

Chapter XVI—EOCENE FORMATIONS.
Eocene Areas of North of Europe. — Table of English and French Eocene
Strata. — Upper Eocene of England. — Bembridge Beds. — Osborne or St.
Helen’s Beds. — Headon Series. — Fossils of the Barton Sands and Clays.
— Middle Eocene of England. — Shells, Nummulites, Fish and Reptiles of
the Bracklesham Beds and Bagshot Sands. — Plants of Alum Bay and
Bournemouth. — Lower Eocene of England. — London Clay Fossils. —
Woolwich and Reading Beds formerly called “Plastic Clay”. — Fluviatile
Beds underlying Deep-sea Strata. — Thanet Sands. — Upper Eocene Strata
of France. — Gypseous Series of Montmartre and Extinct Quadrupeds. —
Fossil Footprints in Paris Gypsum. — Imperfection of the Record. —
Calcaire Silicieux. — Gres de Beauchamp. — Calcaire Grossier. —
Miliolite Limestone. — Soissonnais Sands. — Lower Eocene of France. —
Nummulitic Formations of Europe, Africa, and Asia. — Eocene Strata in
the United States. — Gigantic Cetacean.

Chapter XVII—UPPER CRETACEOUS GROUP.
Lapse of Time between Cretaceous and Eocene Periods. — Table of
successive Cretaceous Formations. — Maestricht Beds. — Pisolitic
Limestone of France. — Chalk of Faxoe. — Geographical Extent and Origin
of the White Chalk. — Chalky Matter now forming in the Bed of the
Atlantic. — Marked Difference between the Cretaceous and existing
Fauna. — Chalk-flints. — Pot-stones of Horstead. — Vitreous Sponges in
the Chalk. — Isolated Blocks of Foreign Rocks in the White Chalk
supposed to be ice-borne. — Distinctness of Mineral Character in
contemporaneous Rocks of the Cretaceous Epoch. — Fossils of the White
Chalk. — Lower White Chalk without Flints. — Chalk Marl and its
Fossils. — Chloritic Series or Upper Greensand. — Coprolite Bed near
Cambridge. — Fossils of the Chloritic Series. — Gault. — Connection
between Upper and Lower Cretaceous Strata. — Blackdown Beds. — Flora of
the Upper Cretaceous Period. — Hippurite Limestone. — Cretaceous Rocks
in the United States.

Chapter XVIII—LOWER CRETACEOUS OR NEOCOMIAN FORMATION.
Classification of marine and fresh-water Strata. — Upper Neocomian. —
Folkestone and Hythe Beds. — Atherfield Clay. — Similarity of
Conditions causing Reappearance of Species after short Intervals. —
Upper Speeton Clay. — Middle Neocomian. — Tealby Series. — Middle
Speeton Clay. — Lower Neocomian. — Lower Speeton Clay. — Wealden
Formation. — Fresh-water Character of the Wealden. — Weald Clay. —
Hastings Sands. — Punfield Beds of Purbeck, Dorsetshire. — Fossil
Shells and Fish of the Wealden. — Area of the Wealden. — Flora of the
Wealden.

Chapter XIX—JURASSIC GROUP.—PURBECK BEDS AND OOLITE.
The Purbeck Beds a Member of the Jurassic Group. — Subdivisions of that
Group. — Physical Geography of the Oolite in England and France. —
Upper Oolite. — Purbeck Beds. — New Genera of fossil Mammalia in the
Middle Purbeck of Dorsetshire. — Dirt-bed or ancient Soil. — Fossils of
the Purbeck Beds. — Portland Stone and Fossils. — Kimmeridge Clay. —
Lithographic Stone of Solenhofen. — Archæopteryx. — Middle Oolite. —
Coral Rag. — Nerinæa Limestone. — Oxford Clay, Ammonites and
Belemnites. — Kelloway Rock. — Lower, or Bath, Oolite. — Great Plants
of the Oolite. — Oolite and Bradford Clay. — Stonesfield Slate. —
Fossil Mammalia. — Fuller’s Earth. — Inferior Oolite and Fossils. —
Northamptonshire Slates. — Yorkshire Oolitic Coal-field. — Brora Coal.
— Palæontological Relations of the several Subdivisions of the Oolitic
group.

Chapter XX—JURASSIC GROUP, CONTINUED.—LIAS.
Mineral Character of Lias. — Numerous successive Zones in the Lias,
marked by distinct Fossils, without Unconformity in the Stratification,
or Change in the Mineral Character of the Deposits. — Gryphite
Limestone. — Shells of the Lias. — Fish of the Lias. — Reptiles of the
Lias. — Ichthyosaur and Plesiosaur. — Marine Reptile of the Galapagos
Islands. — Sudden Destruction and Burial of Fossil Animals in Lias. —
Fluvio-marine Beds in Gloucestershire, and Insect Limestone. — Fossil
Plants. — The origin of the Oolite and Lias, and of alternating
Calcareous and Argillaceous Formations.

Chapter XXI—TRIAS, OR NEW RED SANDSTONE GROUP.
Beds of Passage between the Lias and Trias, Rhætic Beds. — Triassic
Mammifer. — Triple Division of the Trias. — Keuper, or Upper Trias of
England. — Reptiles of the Upper Trias. — Foot-prints in the Bunter
formation in England. — Dolomitic Conglomerate of Bristol. — Origin of
Red Sandstone and Rock-salt. — Precipitation of Salt from inland Lakes
and Lagoons. — Trias of Germany. — Keuper. — St. Cassian and Hallstadt
Beds. — Peculiarity of their Fauna. — Muschelkalk and its Fossils. —
Trias of the United States. — Fossil Foot-prints of Birds and Reptiles
in the Valley of the Connecticut. — Triassic Mammifer of North
Carolina. — Triassic Coal-field of Richmond, Virginia. — Low Grade of
early Mammals favourable to the Theory of Progressive Development.

Chapter XXII—PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Line of Separation between Mesozoic and Palæozoic Rocks. — Distinctness
of Triassic and Permian Fossils. — Term Permian. — Thickness of
calcareous and sedimentary Rocks in North of England. — Upper, Middle,
and Lower Permian. — Marine Shells and Corals of the English Magnesian
Limestone. — Reptiles and Fish of Permian Marl-slate. — Foot-prints of
Reptiles. — Angular Breccias in Lower Permian. — Permian Rocks of the
Continent. — Zechstein and Rothliegendes of Thuringia. — Permian Flora.
— Its generic Affinity to the Carboniferous.

Chapter XXIII—THE COAL OR CARBONIFEROUS GROUP.
Principal Subdivisions of the Carboniferous Group. — Different
Thickness of the sedimentary and calcareous Members in Scotland and the
South of England. — Coal-measures. — Terrestrial Nature of the Growth
of Coal. — Erect fossil Trees. — Uniting of many Coal-seams into one
thick Bed. — Purity of the Coal explained. — Conversion of Coal into
Anthracite. — Origin of Clay-ironstone. — Marine and brackish-water
Strata in Coal. — Fossil Insects. — Batrachian Reptiles. —
Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia
Coal-measures with successive Growths of erect fossil Trees. —
Similarity of American and European Coal. — Air-breathers of the
American Coal. — Changes of Condition of Land and Sea indicated by the
Carboniferous Strata of Nova Scotia.

Chapter XXIV—FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD.
Vegetation of the Coal Period. — Ferns, Lycopodiaceæ, Equisetaceæ,
Sigillariæ, Stigmariæ, Coniferæ. — Angiosperms. — Climate of the Coal
Period. — Mountain Limestone. — Marine Fauna of the Carboniferous
Period. — Corals. — Bryozoa, Crinoidea. — Mollusca. — Great Number of
fossil Fish. — Foraminifera.

Chapter XXV—DEVONIAN OR OLD RED SANDSTONE GROUP.
Classification of the Old Red Sandstone in Scotland and in Devonshire.
— Upper Old Red Sandstone in Scotland, with Fish and Plants. — Middle
Old Red Sandstone. — Classification of the Ichthyolites of the Old Red,
and their Relation to Living Types. — Lower Old Red Sandstone, with
Cephalaspis and Pterygotus. — Marine or Devonian Type of Old Red
Sandstone. — Table of Devonian Series. — Upper Devonian Rocks and
Fossils. — Middle. — Lower. — Eifel Limestone of Germany. — Devonian of
Russia. — Devonian Strata of the United States and Canada. — Devonian
Plants and Insects of Canada.

Chapter XXVI—SILURIAN GROUP.
Classification of the Silurian Rocks. — Ludlow Formation and Fossils. —
Bone-bed of the Upper Ludlow. — Lower Ludlow Shales with Pentamerus. —
Oldest known Remains of fossil Fish. — Table of the progressive
Discovery of Vertebrata in older Rocks. — Wenlock Formation, Corals,
Cystideans and Trilobites. — Llandovery Group or Beds of Passage. —
Lower Silurian Rocks. — Caradoc and Bala Beds. — Brachiopoda. —
Trilobites. — Cystideæ. — Graptolites. — Llandeilo Flags. — Arenig or
Stiper-stones Group. — Foreign Silurian Equivalents in Europe. —
Silurian Strata of the United States. — Canadian Equivalents. — Amount
of specific Agreement of Fossils with those of Europe.

Chapter XXVII—CAMBRIAN AND LAURENTIAN GROUPS.
Classification of the Cambrian Group, and its Equivalent in Bohemia. —
Upper Cambrian Rocks. — Tremadoc Slates and their Fossils. — Lingula
Flags. — Lower Cambrian Rocks. — Menevian Beds. — Longmynd Group. —
Harlech Grits with large Trilobites. — Llanberis Slates. — Cambrian
Rocks of Bohemia. — Primordial Zone of Barrande. — Metamorphosis of
Trilobites. — Cambrian Rocks of Sweden and Norway. — Cambrian Rocks of
the United States and Canada. — Potsdam Sandstone. — Huronian Series. —
Laurentian Group, upper and lower. — Eozoon Canadense, oldest known
Fossil. — Fundamental Gneiss of Scotland.

Chapter XXVIII—VOLCANIC ROCKS.
External Form, Structure, and Origin of Volcanic Mountains. — Cones and
Craters. — Hypothesis of “Elevation Craters” considered. — Trap Rocks.
— Name whence derived. — Minerals most abundant in Volcanic Rocks. —
Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks. —
Similar Minerals in Meteorites. — Theory of Isomorphism. — Basaltic
Rocks. — Trachytic Rocks. — Special Forms of Structure. — The columnar
and globular Forms. — Trap Dikes and Veins. — Alteration of Rocks by
volcanic Dikes. — Conversion of Chalk into Marble. — Intrusion of Trap
between Strata. — Relation of trappean Rocks to the Products of active
Volcanoes.

Chapter XXIX—ON THE AGES OF VOLCANIC ROCKS.
Tests of relative Age of Volcanic Rocks. — Why ancient and modern Rocks
cannot be identical. — Tests by Superposition and intrusion. — Test by
Alteration of Rocks in Contact. — Test by Organic Remains. — Test of
Age by Mineral Character. — Test by Included Fragments. — Recent and
Post-pliocene volcanic Rocks. — Vesuvius, Auvergne, Puy de Come, and
Puy de Pariou. — Newer Pliocene volcanic Rocks. — Cyclopean Isles,
Etna, Dikes of Palagonia, Madeira. — Older Pliocene volcanic Rocks. —
Italy. — Pliocene Volcanoes of the Eifel. — Trass.

Chapter XXX—AGE OF VOLCANIC ROCKS—CONTINUED.
Volcanic Rocks of the Upper Miocene Period. — Madeira. — Grand Canary.
— Azores. — Lower Miocene Volcanic Rocks. — Isle of Mull. — Staffa and
Antrim. — The Eifel. — Upper and Lower Miocene Volcanic Rocks of
Auvergne. — Hill of Gergovia. — Eocene Volcanic Rocks of Monte Bolca. —
Trap of Cretaceous Period. — Oolitic Period. — Triassic Period. —
Permian Period. — Carboniferous Period. — Erect Trees buried in
Volcanic Ash in the Island of Arran. — Old Red Sandstone Period. —
Silurian Period. — Cambrian Period. — Laurentian Volcanic Rocks.

Chapter XXXI—PLUTONIC ROCKS.
General Aspect of Plutonic Rocks. — Granite and its Varieties. —
Decomposing into Spherical Masses. — Rude columnar Structure. — Graphic
Granite. — Mutual Penetration of Crystals of Quartz and Feldspar. —
Glass Cavities in Quartz of Granite. — Porphyritic, talcose, and
syenitic Granite. — Schorlrock and Eurite. — Syenite. — Connection of
the Granites and Syenites with the Volcanic Rocks. — Analogy in
Composition of Trachyte and Granite. — Granite Veins in Glen Tilt, Cape
of Good Hope, and Cornwall. — Metalliferous Veins in Strata near their
Junction with Granite. — Quartz Veins. — Exposure of Plutonic Rocks at
the surface due to Denudation.

Chapter XXXII—ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.
Difficulty in ascertaining the precise Age of a Plutonic Rock. — Test
of Age by Relative Position. — Test by Intrusion and Alteration. — Test
by Mineral Composition. — Test by included Fragments. — Recent and
Pliocene Plutonic Rocks, why invisible. — Miocene Syenite of the Isle
of Skye. — Eocene Plutonic Rocks in the Andes. — Granite altering
Cretaceous Rocks. — Granite altering Lias in the Alps and in Skye. —
Granite of Dartmoor altering Carboniferous Strata. — Granite of the Old
Red Sandstone Period. — Syenite altering Silurian Strata in Norway. —
Blending of the same with Gneiss. — Most ancient Plutonic Rocks. —
Granite protruded in a solid Form.

Chapter XXXIII—METAMORPHIC ROCKS.
General Character of Metamorphic Rocks. — Gneiss. — Hornblende-schist.
— Serpentine. — Mica-schist. — Clay-slate. — Quartzite. —
Chlorite-schist. — Metamorphic Limestone. — Origin of the metamorphic
Strata. — Their Stratification. — Fossiliferous Strata near intrusive
Masses of Granite converted into Rocks identical with different Members
of the metamorphic Series. — Arguments hence derived as to the Nature
of Plutonic Action. — Hydrothermal Action, or the Influence of Steam
and Gases in producing Metamorphism. — Objections to the metamorphic
Theory considered.

Chapter XXXIV—METAMORPHIC ROCKS—continued.
Definition of slaty Cleavage and Joints. — Supposed Causes of these
Structures. — Crystalline Theory of Cleavage. — Mechanical Theory of
Cleavage. — Condensation and Elongation of slate Rocks by lateral
Pressure. — Lamination of some volcanic Rocks due to Motion. — Whether
the Foliation of the crystalline Schists be usually parallel with the
original Planes of Stratification. — Examples in Norway and Scotland. —
Causes of Irregularity in the Planes of Foliation.

Chapter XXXV—ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.
Difficulty of ascertaining the Age of metamorphic Strata. — Metamorphic
Strata of Eocene date in the Alps of Switzerland and Savoy. — Limestone
and Shale of Carrara. — Metamorphic Strata of older date than the
Silurian and Cambrian Rocks. — Order of Succession in metamorphic
Rocks. — Uniformity of mineral Character. — Supposed Azoic Period. —
Connection between the Absence of Organic Remains and the Scarcity of
calcareous Matter in metamorphic Rocks.

Chapter XXXVI—MINERAL VEINS.
Different Kinds of mineral Veins. — Ordinary metalliferous Veins or
Lodes. — Their frequent Coincidence with Faults. — Proofs that they
originated in Fissures in solid Rock. — Veins shifting other Veins. —
Polishing of their Walls or “Slicken sides”. — Shells and Pebbles in
Lodes. — Evidence of the successive Enlargement and Reopening of veins.
— Examples in Cornwall and in Auvergne. — Dimensions of Veins. — Why
some alternately swell out and contract. — Filling of Lodes by
Sublimation from below. — Supposed relative Age of the precious Metals.
— Copper and lead Veins in Ireland older than Cornish Tin. — Lead Vein
in Lias, Glamorganshire. — Gold in Russia, California, and Australia. —
Connection of hot Springs and mineral Veins.

INDEX



PREFACE.

THE LAST or sixth EDITION of my “Elements of Geology” was already out
of print before the end of 1868, in which year I brought out the tenth
edition of my “Principles of Geology.”

In writing the last-mentioned work I had been called upon to pass in
review almost all the leading points of speculation and controversy to
which the rapid advance of the science had given rise, and when I
proposed to bring out a new edition of the “Elements” I was strongly
urged by my friends not to repeat these theoretical discussions, but to
confine myself in the new treatise to those parts of the “Elements”
which were most indispensable to a beginner. This was to revert, to a
certain extent, to the original plan of the first edition; but I found,
after omitting a great number of subjects, that the necessity of
bringing up to the day those which remained, and adverting, however
briefly, to new discoveries, made it most difficult to confine the
proposed abridgment within moderate limits. Some chapters had to be
entirely recast, some additional illustrations to be introduced, and
figures of some organic remains to be replaced by new ones from
specimens more perfect than those which had been at my command on
former occasions. By these changes the work assumed a form so different
from the sixth edition of the “Elements,” that I resolved to give it a
new title and call it the “Student’s Elements of Geology.”

In executing this task I have found it very difficult to meet the
requirements of those who are entirely ignorant of the science. It is
only the adept who has already overcome the first steps as an observer,
and is familiar with many of the technical terms, who can profit by a
brief and concise manual. Beginners wish for a short and cheap book in
which they may find a full explanation of the leading facts and
principles of Geology. Their wants, I fear, somewhat resemble those of
the old woman in New England, who asked a bookseller to supply her with
“the cheapest Bible in the largest possible print.”

But notwithstanding the difficulty of reconciling brevity with the
copiousness of illustration demanded by those who have not yet mastered
the rudiments of the science, I have endeavoured to abridge the work in
the manner above hinted at, so as to place it within the reach of many
to whom it was before inaccessible.

CHARLES LYELL.

73 HARLEY STREET, LONDON,
_December_, 1870.




CHAPTER I.
ON THE DIFFERENT CLASSES OF ROCKS.


Geology defined. — Successive Formation of the Earth’s Crust. —
Classification of Rocks according to their Origin and Age. — Aqueous
Rocks. — Their Stratification and imbedded Fossils. — Volcanic Rocks,
with and without Cones and Craters. — Plutonic Rocks, and their
Relation to the Volcanic. — Metamorphic Rocks, and their probable
Origin. — The term Primitive, why erroneously applied to the
Crystalline Formations. — Leading Division of the Work.

Of what materials is the earth composed, and in what manner are these
materials arranged? These are the first inquiries with which Geology is
occupied, a science which derives its name from the Greek _ge_, the
earth, and _logos_, a discourse. Previously to experience we might have
imagined that investigations of this kind would relate exclusively to
the mineral kingdom, and to the various rocks, soils, and metals, which
occur upon the surface of the earth, or at various depths beneath it.
But, in pursuing such researches, we soon find ourselves led on to
consider the successive changes which have taken place in the former
state of the earth’s surface and interior, and the causes which have
given rise to these changes; and, what is still more singular and
unexpected, we soon become engaged in researches into the history of
the animate creation, or of the various tribes of animals and plants
which have, at different periods of the past, inhabited the globe.

All are aware that the solid parts of the earth consist of distinct
substances, such as clay, chalk, sand, limestone, coal, slate, granite,
and the like; but previously to observation it is commonly imagined
that all these had remained from the first in the state in which we now
see them—that they were created in their present form, and in their
present position. The geologist soon comes to a different conclusion,
discovering proofs that the external parts of the earth were not all
produced in the beginning of things in the state in which we now behold
them, nor in an instant of time. On the contrary, he can show that they
have acquired their actual configuration and condition gradually, under
a great variety of circumstances, and at successive periods, during
each of which distinct races of living beings have flourished on the
land and in the waters, the remains of these creatures still lying
buried in the crust of the earth.

By the “earth’s crust,” is meant that small portion of the exterior of
our planet which is accessible to human observation. It comprises not
merely all of which the structure is laid open in mountain precipices,
or in cliffs overhanging a river or the sea, or whatever the miner may
reveal in artificial excavations; but the whole of that outer covering
of the planet on which we are enabled to reason by observations made at
or near the surface. These reasonings may extend to a depth of several
miles, perhaps ten miles; and even then it may be said, that such a
thickness is no more than 1/400 part of the distance from the surface
to the centre. The remark is just: but although the dimensions of such
a crust are, in truth, insignificant when compared to the entire globe,
yet they are vast, and of magnificent extent in relation to man, and to
the organic beings which people our globe. Referring to this standard
of magnitude, the geologist may admire the ample limits of his domain,
and admit, at the same time, that not only the exterior of the planet,
but the entire earth, is but an atom in the midst of the countless
worlds surveyed by the astronomer.

The materials of this crust are not thrown together confusedly; but
distinct mineral masses, called rocks, are found to occupy definite
spaces, and to exhibit a certain order of arrangement. The term _rock_
is applied indifferently by geologists to all these substances, whether
they be soft or stony, for clay and sand are included in the term, and
some have even brought peat under this denomination. Our old writers
endeavoured to avoid offering such violence to our language, by
speaking of the component materials of the earth as consisting of rocks
and _soils._ But there is often so insensible a passage from a soft and
incoherent state to that of stone, that geologists of all countries
have found it indispensable to have one technical term to include both,
and in this sense we find _roche_ applied in French, _rocca_ in
Italian, and _felsart_ in German. The beginner, however, must
constantly bear in mind that the term rock by no means implies that a
mineral mass is in an indurated or stony condition.

The most natural and convenient mode of classifying the various rocks
which compose the earth’s crust, is to refer, in the first place, to
their origin, and in the second to their relative age. I shall
therefore begin by endeavouring briefly to explain to the student how
all rocks may be divided into four great classes by reference to their
different origin, or, in other words, by reference to the different
circumstances and causes by which they have been produced.

The first two divisions, which will at once be understood as natural,
are the aqueous and volcanic, or the products of watery and those of
igneous action at or near the surface.

Aqueous Rocks.—The aqueous rocks, sometimes called the sedimentary, or
fossiliferous, cover a larger part of the earth’s surface than any
others. They consist chiefly of mechanical deposits (pebbles, sand, and
mud), but are partly of chemical and some of them of organic origin,
especially the limestones. These rocks are _stratified,_ or divided
into distinct layers, or strata. The term _stratum_ means simply a bed,
or any thing spread out or _strewed_ over a given surface; and we infer
that these strata have been generally spread out by the action of
water, from what we daily see taking place near the mouths of rivers,
or on the land during temporary inundations. For, whenever a running
stream charged with mud or sand, has its velocity checked, as when it
enters a lake or sea, or overflows a plain, the sediment, previously
held in suspension by the motion of the water, sinks, by its own
gravity to the bottom. In this manner layers of mud and sand are thrown
down one upon another.

If we drain a lake which has been fed by a small stream, we frequently
find at the bottom a series of deposits, disposed with considerable
regularity, one above the other; the uppermost, perhaps, may be a
stratum of peat, next below a more dense and solid variety of the same
material; still lower a bed of shell-marl, alternating with peat or
sand, and then other beds of marl, divided by layers of clay. Now, if a
second pit be sunk through the same continuous lacustrine _formation_
at some distance from the first, nearly the same series of beds is
commonly met with, yet with slight variations; some, for example, of
the layers of sand, clay, or marl, may be wanting, one or more of them
having thinned out and given place to others, or sometimes one of the
masses first examined is observed to increase in thickness to the
exclusion of other beds.

The term _formation,_ which I have used in the above explanation,
expresses in geology any assemblage of rocks which have some character
in common, whether of origin, age, or composition. Thus we speak of
stratified and unstratified, fresh-water and marine, aqueous and
volcanic, ancient and modern, metalliferous and non-metalliferous
formations.

In the estuaries of large rivers, such as the Ganges and the
Mississippi, we may observe, at low water, phenomena analogous to those
of the drained lakes above mentioned, but on a grander scale, and
extending over areas several hundred miles in length and breadth. When
the periodical inundations subside, the river hollows out a channel to
the depth of many yards through horizontal beds of clay and sand, the
ends of which are seen exposed in perpendicular cliffs. These beds vary
in their mineral composition, or colour, or in the fineness or
coarseness of their particles, and some of them are occasionally
characterised by containing drift-wood. At the junction of the river
and the sea, especially in lagoons nearly separated by sand-bars from
the ocean, deposits are often formed in which brackish and salt-water
shells are included.

In Egypt, where the Nile is always adding to its delta by filling up
part of the Mediterranean with mud, the newly deposited sediment is
_stratified,_ the thin layer thrown down in one season differing
slightly in colour from that of a previous year, and being separable
from it, as has been observed in excavations at Cairo and other
places.[1]

When beds of sand, clay, and marl, containing shells and vegetable
matter, are found arranged in a similar manner in the interior of the
earth, we ascribe to them a similar origin; and the more we examine
their characters in minute detail, the more exact do we find the
resemblance. Thus, for example, at various heights and depths in the
earth, and often far from seas, lakes, and rivers, we meet with layers
of rounded pebbles composed of flint, limestone, granite, or other
rocks, resembling the shingles of a sea-beach or the gravel in a
torrent’s bed. Such layers of pebbles frequently alternate with others
formed of sand or fine sediment, just as we may see in the channel of a
river descending from hills bordering a coast, where the current sweeps
down at one season coarse sand and gravel, while at another, when the
waters are low and less rapid, fine mud and sand alone are carried
seaward.[2]

If a stratified arrangement, and the rounded form of pebbles, are alone
sufficient to lead us to the conclusion that certain rocks originated
under water, this opinion is farther confirmed by the distinct and
independent evidence of _fossils,_ so abundantly included in the
earth’s crust. By a _fossil_ is meant any body, or the traces of the
existence of any body, whether animal or vegetable, which has been
buried in the earth by natural causes. Now the remains of animals,
especially of aquatic species, are found almost everywhere imbedded in
stratified rocks, and sometimes, in the case of limestone, they are in
such abundance as to constitute the entire mass of the rock itself.
Shells and corals are the most frequent, and with them are often
associated the bones and teeth of fishes, fragments of wood,
impressions of leaves, and other organic substances. Fossil shells, of
forms such as now abound in the sea, are met with far inland, both near
the surface, and at great depths below it. They occur at all heights
above the level of the ocean, having been observed at elevations of
more than 8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the
Andes, and above 18,000 feet in the Himalaya.[3]

These shells belong mostly to marine testacea, but in some places
exclusively to forms characteristic of lakes and rivers. Hence it is
concluded that some ancient strata were deposited at the bottom of the
sea, and others in lakes and estuaries.

We have now pointed out one great class of rocks, which, however they
may vary in mineral composition, colour, grain, or other characters,
external and internal, may nevertheless be grouped together as having a
common origin. They have all been formed under water, in the same
manner as modern accumulations of sand, mud, shingle, banks of shells,
reefs of coral, and the like, and are all characterised by
stratification or fossils, or by both.

Volcanic Rocks.—The division of rocks which we may next consider are
the volcanic, or those which have been produced at or near the surface
whether in ancient or modern times, not by water, but by the action of
fire or subterranean heat. These rocks are for the most part
unstratified, and are devoid of fossils. They are more partially
distributed than aqueous formations, at least in respect to horizontal
extension. Among those parts of Europe where they exhibit characters
not to be mistaken, I may mention not only Sicily and the country round
Naples, but Auvergne, Velay, and Vivarais, now the departments of Puy
de Dome, Haute Loire, and Ardêche, towards the centre and south of
France, in which are several hundred conical hills having the forms of
modern volcanoes, with craters more or less perfect on many of their
summits. These cones are composed moreover of lava, sand, and ashes,
similar to those of active volcanoes. Streams of lava may sometimes be
traced from the cones into the adjoining valleys, where they have
choked up the ancient channels of rivers with solid rock, in the same
manner as some modern flows of lava in Iceland have been known to do,
the rivers either flowing beneath or cutting out a narrow passage on
one side of the lava. Although none of these French volcanoes have been
in activity within the period of history or tradition, their forms are
often very perfect. Some, however, have been compared to the mere
skeletons of volcanoes, the rains and torrents having washed their
sides, and removed all the loose sand and scoriæ, leaving only the
harder and more solid materials. By this erosion, and by earthquakes,
their internal structure has occasionally been laid open to view, in
fissures and ravines; and we then behold not only many successive beds
and masses of porous lava, sand, and scoriæ, but also perpendicular
walls, or _dikes,_ as they are called, of volcanic rock, which have
burst through the other materials. Such dikes are also observed in the
structure of Vesuvius, Etna, and other active volcanoes. They have been
formed by the pouring of melted matter, whether from above or below,
into open fissures, and they commonly traverse deposits of _volcanic
tuff,_ a substance produced by the showering down from the air, or
incumbent waters, of sand and cinders, first shot up from the interior
of the earth by the explosions of volcanic gases.

Besides the parts of France above alluded to, there are other
countries, as the north of Spain, the south of Sicily, the Tuscan
territory of Italy, the lower Rhenish provinces, and Hungary, where
spent volcanoes may be seen, still preserving in many cases a conical
form, and having craters and often lava-streams connected with them.

There are also other rocks in England, Scotland, Ireland, and almost
every country in Europe, which we infer to be of igneous origin,
although they do not form hills with cones and craters. Thus, for
example, we feel assured that the rock of Staffa, and that of the
Giant’s Causeway, called basalt, is volcanic, because it agrees in its
columnar structure and mineral composition with streams of lava which
we know to have flowed from the craters of volcanoes. We find also
similar basaltic and other igneous rocks associated with beds of _tuff_
in various parts of the British Isles, and forming _dikes,_ such as
have been spoken of; and some of the strata through which these dikes
cut are occasionally altered at the point of contact, as if they had
been exposed to the intense heat of melted matter.

The absence of cones and craters, and long narrow streams of
superficial lava, in England and many other countries, is principally
to be attributed to the eruptions having been submarine, just as a
considerable proportion of volcanoes in our own times burst out beneath
the sea. But this question must be enlarged upon more fully in the
chapters on Igneous Rocks, in which it will also be shown, that as
different sedimentary formations, containing each their characteristic
fossils, have been deposited at successive periods, so also volcanic
sand and scoriæ have been thrown out, and lavas have flowed over the
land or bed of the sea, at many different epochs, or have been injected
into fissures; so that the igneous as well as the aqueous rocks may be
classed as a chronological series of monuments, throwing light on a
succession of events in the history of the earth.

Plutonic Rocks (_Granite,_ etc).—We have now pointed out the existence
of two distinct orders of mineral masses, the aqueous and the volcanic:
but if we examine a large portion of a continent, especially if it
contain within it a lofty mountain range, we rarely fail to discover
two other classes of rocks, very distinct from either of those above
alluded to, and which we can neither assimilate to deposits such as are
now accumulated in lakes or seas, nor to those generated by ordinary
volcanic action. The members of both these divisions of rocks agree in
being highly crystalline and destitute of organic remains. The rocks of
one division have been called Plutonic, comprehending all the granites
and certain porphyries, which are nearly allied in some of their
characters to volcanic formations. The members of the other class are
stratified and often slaty, and have been called by some the
_crystalline schists,_ in which group are included gneiss,
micaceous-schist (or mica-slate), hornblende-schist, statuary marble,
the finer kinds of roofing slate, and other rocks afterwards to be
described.

As it is admitted that nothing strictly analogous to these crystalline
productions can now be seen in the progress of formation on the earth’s
surface, it will naturally be asked, on what data we can find a place
for them in a system of classification founded on the origin of rocks.
I cannot, in reply to this question, pretend to give the student, in a
few words, an intelligible account of the long chain of facts and
reasonings from which geologists have been led to infer the nature of
the rocks in question. The result, however, may be briefly stated. All
the various kinds of granites which constitute the Plutonic family are
supposed to be of igneous or aqueo-igneous origin, and to have been
formed under great pressure, at a considerable depth in the earth, or
sometimes, perhaps, under a certain weight of incumbent ocean. Like the
lava of volcanoes, they have been melted, and afterwards cooled and
crystallised, but with extreme slowness, and under conditions very
different from those of bodies cooling in the open air. Hence they
differ from the volcanic rocks, not only by their more crystalline
texture, but also by the absence of tuffs and breccias, which are the
products of eruptions at the earth’s surface, or beneath seas of
inconsiderable depth. They differ also by the absence of pores or
cellular cavities, to which the expansion of the entangled gases gives
rise in ordinary lava.

Metamorphic, or Stratified Crystalline Rocks.—The fourth and last great
division of rocks are the crystalline strata and slates, or schists,
called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and
the like, the origin of which is more doubtful than that of the other
three classes. They contain no pebbles, or sand, or scoriæ, or angular
pieces of imbedded stone, and no traces of organic bodies, and they are
often as crystalline as granite, yet are divided into beds,
corresponding in form and arrangement to those of sedimentary
formations, and are therefore said to be stratified. The beds sometimes
consist of an alternation of substances varying in colour, composition,
and thickness, precisely as we see in stratified fossiliferous
deposits. According to the Huttonian theory, which I adopt as the most
probable, and which will be afterwards more fully explained, the
materials of these strata were originally deposited from water in the
usual form of sediment, but they were subsequently so altered by
subterranean heat, as to assume a new texture. It is demonstrable, in
some cases at least, that such a complete conversion has actually taken
place, fossiliferous strata having exchanged an earthy for a highly
crystalline texture for a distance of a quarter of a mile from their
contact with granite. In some cases, dark limestones, replete with
shells and corals, have been turned into white statuary marble; and
hard clays, containing vegetable or other remains, into slates called
mica-schist or hornblende-schist, every vestige of the organic bodies
having been obliterated.

Although we are in a great degree ignorant of the precise nature of the
influence exerted in these cases, yet it evidently bears some analogy
to that which volcanic heat and gases are known to produce; and the
action may be conveniently called Plutonic, because it appears to have
been developed in those regions where Plutonic rocks are generated, and
under similar circumstances of pressure and depth in the earth.
Intensely heated water or steam permeating stratified masses under
great pressure have no doubt played their part in producing the
crystalline texture and other changes, and it is clear that the
transforming influence has often pervaded entire mountain masses of
strata.

In accordance with the hypothesis above alluded to, I proposed in the
first edition of the Principles of Geology (1833) the term
“Metamorphic” for the altered strata, a term derived from meta, _
trans,_ and morphe, _forma._

Hence there are four great classes of rocks considered in reference to
their origin—the aqueous, the volcanic, the Plutonic, and the
metamorphic. In the course of this work it will be shown that portions
of each of these four distinct classes have originated at many
successive periods. They have all been produced contemporaneously, and
may even now be in the progress of formation on a large scale. It is
not true, as was formerly supposed, that all granites, together with
the crystalline or metamorphic strata, were first formed, and therefore
entitled to be called “primitive,” and that the aqueous and volcanic
rocks were afterwards superimposed, and should, therefore, rank as
secondary in the order of time. This idea was adopted in the infancy of
the science, when all formations, whether stratified or unstratified,
earthy or crystalline, with or without fossils, were alike regarded as
of aqueous origin. At that period it was naturally argued that the
foundation must be older than the superstructure; but it was afterwards
discovered that this opinion was by no means in every instance a
legitimate deduction from facts; for the inferior parts of the earth’s
crust have often been modified, and even entirely changed, by the
influence of volcanic and other subterranean causes, while superimposed
formations have not been in the slightest degree altered. In other
words, the destroying and renovating processes have given birth to new
rocks below, while those above, whether crystalline or fossiliferous,
have remained in their ancient condition. Even in cities, such as
Venice and Amsterdam, it cannot be laid down as universally true that
the upper parts of each edifice, whether of brick or marble, are more
modern than the foundations on which they rest, for these often consist
of wooden piles, which may have rotted and been replaced one after the
other, without the least injury to the buildings above; meanwhile,
these may have required scarcely any repair, and may have been
constantly inhabited. So it is with the habitable surface of our globe,
in its relation to large masses of rock immediately below; it may
continue the same for ages, while subjacent materials, at a great
depth, are passing from a solid to a fluid state, and then
reconsolidating, so as to acquire a new texture.

As all the crystalline rocks may, in some respects, be viewed as
belonging to one great family, whether they be stratified or
unstratified, metamorphic or Plutonic, it will often be convenient to
speak of them by one common name. It being now ascertained, as above
stated, that they are of very different ages, sometimes newer than the
strata called secondary, the terms primitive and primary which were
formerly used for the whole must be abandoned, as they would imply a
manifest contradiction. It is indispensable, therefore, to find a new
name, one which must not be of chronological import, and must express,
on the one hand, some peculiarity equally attributable to granite and
gneiss (to the Plutonic as well as the _altered_ rocks), and, on the
other, must have reference to characters in which those rocks differ,
both from the volcanic and from the _unaltered_ sedimentary strata. I
proposed in the Principles of Geology (first edition, vol. iii) the
term “hypogene” for this purpose, derived from upo, _ under,_ and
ginomai, _to be,_ or _to be born_; a word implying the theory that
granite, gneiss, and the other crystalline formations are alike
_netherformed_ rocks, or rocks which have not assumed their present
form and structure at the surface. They occupy the lowest place in the
order of superposition. Even in regions such as the Alps, where some
masses of granite and gneiss can be shown to be of comparatively modern
date, belonging, for example, to the period hereafter to be described
as tertiary, they are still _underlying_ rocks. They never repose on
the volcanic or trappean formations, nor on strata containing organic
remains. They are _hypogene,_ as “being under” all the rest.

From what has now been said, the reader will understand that each of
the four great classes of rocks may be studied under two distinct
points of view; first, they may be studied simply as mineral masses
deriving their origin from particular causes, and having a certain
composition, form, and position in the earth’s crust, or other
characters both positive and negative, such as the presence or absence
of organic remains. In the second place, the rocks of each class may be
viewed as a grand chronological series of monuments, attesting a
succession of events in the former history of the globe and its living
inhabitants.

I shall accordingly proceed to treat of each family of rocks; first, in
reference to those characters which are not chronological, and then in
particular relation to the several periods when they were formed.

 [1] See Principles of Geology, by the Author, Index, “Nile,” “Rivers,”
 etc.

 [2] See p. 44, Fig. 7.

 [3] Col. R. J. Strachey found oolitic fossils 18,400 feet high in the
 Himalaya.




CHAPTER II.
AQUEOUS ROCKS.—THEIR COMPOSITION AND FORMS OF STRATIFICATION.


Mineral Composition of Strata. — Siliceous Rocks. — Argillaceous. —
Calcareous. — Gypsum. — Forms of Stratification. — Original
Horizontality. — Thinning out. — Diagonal Arrangement. — Ripple-mark.

In pursuance of the arrangement explained in the last chapter, we shall
begin by examining the aqueous or sedimentary rocks, which are for the
most part distinctly stratified, and contain fossils. We may first
study them with reference to their mineral composition, external
appearance, position, mode of origin, organic contents, and other
characters which belong to them as aqueous formations, independently of
their age, and we may afterwards consider them chronologically or with
reference to the successive geological periods when they originated.

I have already given an outline of the data which led to the belief
that the stratified and fossiliferous rocks were originally deposited
under water; but, before entering into a more detailed investigation,
it will be desirable to say something of the ordinary materials of
which such strata are composed. These may be said to belong principally
to three divisions, the siliceous, the argillaceous, and the
calcareous, which are formed respectively of flint, clay, and carbonate
of lime. Of these, the siliceous are chiefly made up of sand or flinty
grains; the argillaceous, or clayey, of a mixture of siliceous matter
with a certain proportion, about a fourth in weight, of aluminous
earth; and, lastly, the calcareous rocks, or limestones, of carbonic
acid and lime.

Siliceous and Arenaceous Rocks.—To speak first of the sandy division:
beds of loose sand are frequently met with, of which the grains consist
entirely of silex, which term comprehends all purely siliceous
minerals, as quartz and common flint. Quartz is silex in its purest
form. Flint usually contains some admixture of alumina and oxide of
iron. The siliceous grains in sand are usually rounded, as if by the
action of running water. Sandstone is an aggregate of such grains,
which often cohere together without any visible cement, but more
commonly are bound together by a slight quantity of siliceous or
calcareous matter, or by oxide of iron or clay.

Pure siliceous rocks may be known by not effervescing when a drop of
nitric, sulphuric or other acid is applied to them, or by the grains
not being readily scratched or broken by ordinary pressure. In nature
there is every intermediate gradation, from perfectly loose sand to the
hardest sandstone. In _micaceous sandstones_ mica is very abundant; and
the thin silvery plates into which that mineral divides are often
arranged in layers parallel to the planes of stratification, giving a
slaty or laminated texture to the rock.

When sandstone is coarse-grained, it is usually called _ grit._ If the
grains are rounded, and large enough to be called pebbles, it becomes a
_conglomerate_ or _pudding-stone,_ which may consist of pieces of one
or of many different kinds of rock. A conglomerate, therefore, is
simply gravel bound together by cement.

Argillaceous Rocks.—Clay, strictly speaking, is a mixture of silex or
flint with a large proportion, usually about one fourth, of alumina, or
argil; but in common language, any earth which possesses sufficient
ductility, when kneaded up with water, to be fashioned like paste by
the hand, or by the potter’s lathe, is called a _clay_; and such clays
vary greatly in their composition, and are, in general, nothing more
than mud derived from the decomposition or wearing down of rocks. The
purest clay found in nature is porcelain clay, or kaolin, which results
from the decomposition of a rock composed of feldspar and quartz, and
it is almost always mixed with quartz. The kaolin of China consists of
71·15 parts of silex, 15·86 of alumine, 1·92 of lime, and 6·73 of
water;[1] but other porcelain clays differ materially, that of Cornwall
being composed, according to Boase, of nearly equal parts of silica and
alumine, with 1 per cent of magnesia.[2] _ Shale_ has also the
property, like clay, of becoming plastic in water: it is a more solid
form of clay, or argillaceous matter, condensed by pressure. It always
divides into laminæ more or less regular.

One general character of all argillaceous rocks is to give out a
peculiar, earthy odour when breathed upon, which is a test of the
presence of alumine, although it does not belong to pure alumine, but,
apparently, to the combination of that substance with oxide of iron.[3]

Calcareous Rocks.—This division comprehends those rocks which, like
chalk, are composed chiefly of lime and carbonic acid. Shells and
corals are also formed of the same elements, with the addition of
animal matter. To obtain pure lime it is necessary to calcine these
calcareous substances, that is to say, to expose them to heat of
sufficient intensity to drive off the carbonic acid, and other volatile
matter. White chalk is sometimes pure carbonate of lime; and this rock,
although usually in a soft and earthy state, is occasionally
sufficiently solid to be used for building, and even passes into a
_compact_ stone, or a stone of which the separate parts are so minute
as not to be distinguishable from each other by the naked eye.

Many limestones are made up entirely of minute fragments of shells and
coral, or of calcareous sand cemented together. These last might be
called “calcareous sandstones;” but that term is more properly applied
to a rock in which the grains are partly calcareous and partly
siliceous, or to quartzose sandstones, having a cement of carbonate of
lime.

The variety of limestone called _oolite_ is composed of numerous small
egg-like grains, resembling the roe of a fish, each of which has
usually a small fragment of sand as a nucleus, around which concentric
layers of calcareous matter have accumulated.

Any limestone which is sufficiently hard to take a fine polish is
called _marble._ Many of these are fossiliferous; but statuary marble,
which is also called saccharoid limestone, as having a texture
resembling that of loaf-sugar, is devoid of fossils, and is in many
cases a member of the metamorphic series.

_Siliceous limestone_ is an intimate mixture of carbonate of lime and
flint, and is harder in proportion as the flinty matter predominates.

The presence of carbonate of lime in a rock may be ascertained by
applying to the surface a small drop of diluted sulphuric, nitric, or
muriatic acid, or strong vinegar; for the lime, having a greater
chemical affinity for any one of these acids than for the carbonic,
unites immediately with them to form new compounds, thereby becoming a
sulphate, nitrate or muriate of lime. The carbonic acid, when thus
liberated from its union with the lime, escapes in a gaseous form, and
froths up or effervesces as it makes its way in small bubbles through
the drop of liquid. This effervescence is brisk or feeble in proportion
as the limestone is pure or impure, or, in other words, according to
the quantity of foreign matter mixed with the carbonate of lime.
Without the aid of this test, the most experienced eye cannot always
detect the presence of carbonate of lime in rocks.

The above-mentioned three classes of rocks, the siliceous,
argillaceous, and calcareous, pass continually into each other, and
rarely occur in a perfectly separate and pure form. Thus it is an
exception to the general rule to meet with a limestone as pure as
ordinary white chalk, or with clay as aluminous as that used in
Cornwall for porcelain, or with sand so entirely composed of siliceous
grains as the white sand of Alum Bay, in the Isle of Wight, employed in
the manufacture of glass, or sandstone so pure as the grit of
Fontainebleau, used for pavement in France. More commonly we find sand
and clay, or clay and marl, intermixed in the same mass. When the sand
and clay are each in considerable quantity, the mixture is called
_loam._ If there is much calcareous matter in clay it is called _marl_;
but this term has unfortunately been used so vaguely, as often to be
very ambiguous. It has been applied to substances in which there is no
lime; as, to that red loam usually called red marl in certain parts of
England. Agriculturists were in the habit of calling any soil a marl
which, like true marl, fell to pieces readily on exposure to the air.
Hence arose the confusion of using this name for soils which,
consisting of loam, were easily worked by the plough, though devoid of
lime.

_Marl slate_ bears the same relation to marl which shale bears to clay,
being a calcareous shale. It is very abundant in some countries, as in
the Swiss Alps. Argillaceous or marly limestone is also of common
occurrence.

There are few other kinds of rock which enter so largely into the
composition of sedimentary strata as to make it necessary to dwell here
on their characters. I may, however, mention two others—magnesian
limestone or dolomite, and gypsum. _Magnesian limestone_ is composed of
carbonate of lime and carbonate of magnesia; the proportion of the
latter amounting in some cases to nearly one half. It effervesces much
more slowly and feebly with acids than common limestone. In England
this rock is generally of a yellowish colour; but it varies greatly in
mineralogical character, passing from an earthy state to a white
compact stone of great hardness. _Dolomite,_ so common in many parts of
Germany and France, is also a variety of magnesian limestone, usually
of a granular texture.

_Gypsum_ is a rock composed of sulphuric acid, lime, and water. It is
usually a soft whitish-yellow rock, with a texture resembling that of
loaf-sugar, but sometimes it is entirely composed of lenticular
crystals. It is insoluble in acids, and does not effervesce like chalk
and dolomite, because it does not contain carbonic acid gas, or fixed
air, the lime being already combined with sulphuric acid, for which it
has a stronger affinity than for any other. Anhydrous gypsum is a rare
variety, into which water does not enter as a component part. _
Gypseous marl_ is a mixture of gypsum and marl. _Alabaster_ is a
granular and compact variety of gypsum found in masses large enough to
be used in sculpture and architecture. It is sometimes a pure
snow-white substance, as that of Volterra in Tuscany, well known as
being carved for works of art in Florence and Leghorn. It is a softer
stone than marble, and more easily wrought.

Forms of Stratification.—A series of strata sometimes consists of one
of the above rocks, sometimes of two or more in alternating beds.

Thus, in the coal districts of England, for example, we often pass
through several beds of sandstone, some of finer, others of coarser
grain, some white, others of a dark colour, and below these, layers of
shale and sandstone or beds of shale, divisible into leaf-like laminæ,
and containing beautiful impressions of plants. Then again we meet with
beds of pure and impure coal, alternating with shales and sandstones,
and underneath the whole, perhaps, are calcareous strata, or beds of
limestone, filled with corals and marine shells, each bed
distinguishable from another by certain fossils, or by the abundance of
particular species of shells or zoophytes.

This alternation of different kinds of rock produces the most distinct
stratification; and we often find beds of limestone and marl,
conglomerate and sandstone, sand and clay, recurring again and again,
in nearly regular order, throughout a series of many hundred strata.
The causes which may produce these phenomena are various, and have been
fully discussed in my treatise on the modern changes of the earth’s
surface.[4] It is there seen that rivers flowing into lakes and seas
are charged with sediment, varying in quantity, composition, colour,
and grain according to the seasons; the waters are sometimes flooded
and rapid, at other periods low and feeble; different tributaries,
also, draining peculiar countries and soils, and therefore charged with
peculiar sediment, are swollen at distinct periods. It was also shown
that the waves of the sea and currents undermine the cliffs during
wintry storms, and sweep away the materials into the deep, after which
a season of tranquillity succeeds, when nothing but the finest mud is
spread by the movements of the ocean over the same submarine area.

It is not the object of the present work to give a description of these
operations, repeated as they are, year after year, and century after
century; but I may suggest an explanation of the manner in which some
micaceous sandstones have originated, namely, those in which we see
innumerable thin layers of mica dividing layers of fine quartzose sand.
I observed the same arrangement of materials in recent mud deposited in
the estuary of Laroche St. Bernard in Brittany, at the mouth of the
Loire. The surrounding rocks are of gneiss, which, by its waste,
supplies the mud: when this dries at low water, it is found to consist
of brown laminated clay, divided by thin seams of mica. The separation
of the mica in this case, or in that of micaceous sandstones, may be
thus understood. If we take a handful of quartzose sand, mixed with
mica, and throw it into a clear running stream, we see the materials
immediately sorted by the water, the grains of quartz falling almost
directly to the bottom, while the plates of mica take a much longer
time to reach the bottom, and are carried farther down the stream. At
the first instant the water is turbid, but immediately after the flat
surfaces of the plates of mica are seen all alone, reflecting a silvery
light, as they descend slowly, to form a distinct micaceous lamina. The
mica is the heavier mineral of the two; but it remains a longer time
suspended in the fluid, owing to its greater extent of surface. It is
easy, therefore, to perceive that where such mud is acted upon by a
river or tidal current, the thin plates of mica will be carried
farther, and not deposited in the same places as the grains of quartz;
and since the force and velocity of the stream varies from time to
time, layers of mica or of sand will be thrown down successively on the
same area.

Original Horizontality.—It is said generally that the upper and under
surfaces of strata, or the “planes of stratification,” are parallel.
Although this is not strictly true, they make an approach to
parallelism, for the same reason that sediment is usually deposited at
first in nearly horizontal layers. Such an arrangement can by no means
be attributed to an original evenness or horizontality in the bed of
the sea: for it is ascertained that in those places where no matter has
been recently deposited, the bottom of the ocean is often as uneven as
that of the dry land, having in like manner its hills, valleys, and
ravines. Yet if the sea should go down, or be removed from near the
mouth of a large river where a delta has been forming, we should see
extensive plains of mud and sand laid dry, which, to the eye, would
appear perfectly level, although, in reality, they would slope gently
from the land towards the sea.

This tendency in newly-formed strata to assume a horizontal position
arises principally from the motion of the water, which forces along
particles of sand or mud at the bottom, and causes them to settle in
hollows or depressions where they are less exposed to the force of a
current than when they are resting on elevated points. The velocity of
the current and the motion of the superficial waves diminish from the
surface downward, and are least in those depressions where the water is
deepest.


A good illustration of the principle here alluded to may be sometimes
seen in the neighbourhood of a volcano, when a section, whether natural
or artificial, has laid open to view a succession of various-coloured
layers of sand and ashes, which have fallen in showers upon uneven
ground. Thus let A B (Fig. 1) be two ridges, with an intervening
valley. These original inequalities of the surface have been gradually
effaced by beds of sand and ashes _c, d, e,_ the surface at _e_ being
quite level. It will be seen that, although the materials of the first
layers have accommodated themselves in a great degree to the shape of
the ground A B, yet each bed is thickest at the bottom. At first a
great many particles would be carried by their own gravity down the
steep sides of A and B, and others would afterwards be blown by the
wind as they fell off the ridges, and would settle in the hollow, which
would thus become more and more effaced as the strata accumulated from
_c_ to _e._ Now, water in motion can exert this levelling power on
similar materials more easily than air, for almost all stones lose in
water more than a third of the weight which they have in air, the
specific gravity of rocks being in general as 2½ when compared to that
of water, which is estimated at 1. But the buoyancy of sand or mud
would be still greater in the sea, as the density of salt-water exceeds
that of fresh.

Fig. 2. Section of strata of sandtone, grit, and congolmerate.

Yet, however uniform and horizontal may be the surface of new deposits
in general, there are still many disturbing causes, such as eddies in
the water, and currents moving first in one and then in another
direction, which frequently cause irregularities. We may sometimes
follow a bed of limestone, shale, or sandstone, for a distance of many
hundred yards continuously; but we generally find at length that each
individual stratum thins out, and allows the beds which were previously
above and below it to meet. If the materials are coarse, as in grits
and conglomerates, the same beds can rarely be traced many yards
without varying in size, and often coming to an end abruptly. (See Fig.
2.)

Fig. 3: Section of sand at Sandy Hill, near Biggleswade, Bedfordshire.

Diagonal or Cross Stratification.—There is also another phenomenon of
frequent occurrence. We find a series of larger strata, each of which
is composed of a number of minor layers placed obliquely to the general
planes of stratification. To this diagonal arrangement the name of
“false or cross bedding” has been given. Thus in the section (Fig. 3)
we see seven or eight large beds of loose sand, yellow and brown, and
the lines _a, b, c_ mark some of the principal planes of
stratification, which are nearly horizontal. But the greater part of
the subordinate laminæ do not conform to these planes, but have often a
steep slope, the inclination being sometimes towards opposite points of
the compass. When the sand is loose and incoherent, as in the case here
represented, the deviation from parallelism of the slanting laminæ
cannot possibly be accounted for by any rearrangement of the particles
acquired during the consolidation of the rock. In what manner, then,
can such irregularities be due to original deposition? We must suppose
that at the bottom of the sea, as well as in the beds of rivers, the
motions of waves, currents, and eddies often cause mud, sand, and
gravel to be thrown down in heaps on particular spots, instead of being
spread out uniformly over a wide area. Sometimes, when banks are thus
formed, currents may cut passages through them, just as a river forms
its bed.

Fig. 4Fig. 5

Suppose the bank A (Fig. 4) to be thus formed with a steep sloping
side, and, the water being in a tranquil state, the layer of sediment
No. 1 is thrown down upon it, conforming nearly to its surface.
Afterwards the other layers, 2, 3, 4, may be deposited in succession,
so that the bank B C D is formed. If the current then increases in
velocity, it may cut away the upper portion of this mass down to the
dotted line e, and deposit the materials thus removed farther on, so as
to form the layers 5, 6, 7, 8. We have now the bank B, C, D, E (Fig.
5), of which the surface is almost level, and on which the nearly
horizontal layers, 9, 10, 11, may then accumulate. It was shown in Fig.
3 that the diagonal layers of successive strata may sometimes have an
opposite slope. This is well seen in some cliffs of loose sand on the
Suffolk coast. A portion of one of these is represented in Fig. 6,
where the layers, of which there are about six in the thickness of an
inch, are composed of quartzose grains. This arrangement may have been
due to the altered direction of the tides and currents in the same
place.

Fig. 6: Cliff between Mismer and Dunwich.

Fig. 7: Section from Monte Calvo to the sea by the valley of the
Magnan, near Nice.

The description above given of the slanting position of the minor
layers constituting a single stratum is in certain cases applicable on
a much grander scale to masses several hundred feet thick, and many
miles in extent. A fine example may be seen at the base of the Maritime
Alps near Nice. The mountains here terminate abruptly in the sea, so
that a depth of one hundred fathoms is often found within a stone’s
throw of the beach, and sometimes a depth of 3000 feet within half a
mile. But at certain points, strata of sand, marl, or conglomerate
intervene between the shore and the mountains, as in the section (Fig.
7), where a vast succession of slanting beds of gravel and sand may be
traced from the sea to Monte Calvo, a distance of no less than nine
miles in a straight line. The dip of these beds is remarkably uniform,
being always southward or towards the Mediterranean, at an angle of
about 25°. They are exposed to view in nearly vertical precipices,
varying from 200 to 600 feet in height, which bound the valley through
which the river Magnan flows. Although, in a general view, the strata
appear to be parallel and uniform, they are nevertheless found, when
examined closely, to be wedge-shaped, and to thin out when followed for
a few hundred feet or yards, so that we may suppose them to have been
thrown down originally upon the side of a steep bank where a river or
Alpine torrent discharged itself into a deep and tranquil sea, and
formed a delta, which advanced gradually from the base of Monte Calvo
to a distance of nine miles from the original shore. If subsequently
this part of the Alps and bed of the sea were raised 700 feet, the
delta may have emerged, a deep channel may then have been cut through
it by the river, and the coast may at the same time have acquired its
present configuration.

It is well known that the torrents and streams which now descend from
the Alpine declivities to the shore, bring down annually, when the snow
melts, vast quantities of shingle and sand, and then, as they subside,
fine mud, while in summer they are nearly or entirely dry; so that it
may be safely assumed that deposits like those of the valley of the
Magnan, consisting of coarse gravel alternating with fine sediment, are
still in progress at many points, as, for instance, at the mouth of the
Var. They must advance upon the Mediterranean in the form of great
shoals terminating in a steep talus; such being the original mode of
accumulation of all coarse materials conveyed into deep water,
especially where they are composed in great part of pebbles, which
cannot be transported to indefinite distances by currents of moderate
velocity. By inattention to facts and inferences of this kind, a very
exaggerated estimate has sometimes been made of the supposed depth of
the ancient ocean. There can be no doubt, for example, that the strata
_a_, Fig. 7, or those nearest to Monte Calvo, are older than those
indicated by _b_, and these again were formed before _c_; but the
vertical depth of gravel and sand in any one place cannot be proved to
amount even to 1000 feet, although it may perhaps be much greater, yet
probably never exceeding at any point 3000 or 4000 feet. But were we to
assume that all the strata were once horizontal, and that their present
dip or inclination was due to subsequent movements, we should then be
forced to conclude that a sea several miles deep had been filled up
with alternate layers of mud and pebbles thrown down one upon another.

In the locality now under consideration, situated a few miles to the
west of Nice, there are many geological data, the details of which
cannot be given in this place, all leading to the opinion that, when
the deposit of the Magnan was formed, the shape and outline of the
Alpine declivities and the shore greatly resembled what we now behold
at many points in the neighbourhood. That the beds _a, b, c, d_ are of
comparatively modern date is proved by this fact, that in seams of
loamy marl intervening between the pebbly beds are fossil shells, half
of which belong to species now living in the Mediterranean.

Fig. 8: Slab of ripple-marked (New Red) sandstone from Cheshire.

Ripple-mark.—The ripple-mark, so common on the surface of sandstones of
all ages (see Fig. 8), and which is so often seen on the sea-shore at
low tide, seems to originate in the drifting of materials along the
bottom of the water, in a manner very similar to that which may explain
the inclined layers above described. This ripple is not entirely
confined to the beach between high and low water mark, but is also
produced on sands which are constantly covered by water. Similar
undulating ridges and furrows may also be sometimes seen on the surface
of drift snow and blown sand.

The ripple-mark is usually an indication of a sea-beach, or of water
from six to ten feet deep, for the agitation caused by waves even
during storms extends to a very slight depth. To this rule, however,
there are some exceptions, and recent ripple-marks have been observed
at the depth of 60 or 70 feet. It has also been ascertained that
currents or large bodies of water in motion may disturb mud and sand at
the depth of 300 or even 450 feet.[5] Beach ripple, however, may
usually be distinguished from current ripple by frequent changes in its
direction. In a slab of sandstone, not more than an inch thick, the
furrows or ridges of an ancient ripple may often be seen in several
successive laminæ to run towards different points of the compass.

 [1] W. Phillips, Mineralogy, p.33.

 [2] Phil. Mag., vol. x, 1837.

 [3] See W. Phillips’s Mineralogy, “Alumine.”

 [4] Consult Index to Principles of Geology, “Stratification,”
 “Currents,” “Deltas,” “Water,” etc.

 [5] Darwin, Volcanic Islands, p. 134.




CHAPTER III.
ARRANGEMENT OF FOSSILS IN STRATA.—FRESH-WATER AND MARINE FOSSILS.


Successive Deposition indicated by Fossils. — Limestones formed of
Corals and Shells. — Proofs of gradual Increase of Strata derived from
Fossils. — Serpula attached to Spatangus. — Wood bored by Teredina. —
Tripoli formed of Infusoria. — Chalk derived principally from Organic
Bodies. — Distinction of Fresh-water from Marine Formations. — Genera
of Fresh-water and Land Shells. — Rules for recognising Marine
Testacea. — Gyrogonite and Chara. — Fresh-water Fishes. — Alternation
of Marine and Fresh-water Deposits. — Lym-Fiord.

Having in the last chapter considered the forms of stratification so
far as they are determined by the arrangement of inorganic matter, we
may now turn our attention to the manner in which organic remains are
distributed through stratified deposits. We should often be unable to
detect any signs of stratification or of successive deposition, if
particular kinds of fossils did not occur here and there at certain
depths in the mass. At one level, for example, univalve shells of some
one or more species predominate; at another, bivalve shells; and at a
third, corals; while in some formations we find layers of vegetable
matter, commonly derived from land plants, separating strata.

It may appear inconceivable to a beginner how mountains, several
thousand feet thick, can have become full of fossils from top to
bottom; but the difficulty is removed, when he reflects on the origin
of stratification, as explained in the last chapter, and allows
sufficient time for the accumulation of sediment. He must never lose
sight of the fact that, during the process of deposition, each separate
layer was once the uppermost, and immediately in contact with the water
in which aquatic animals lived. Each stratum, in fact, however far it
may now lie beneath the surface, was once in the state of shingle, or
loose sand or soft mud at the bottom of the sea, in which shells and
other bodies easily became enveloped.

Rate of Deposition indicated by Fossils.—By attending to the nature of
these remains, we are often enabled to determine whether the deposition
was slow or rapid, whether it took place in a deep or shallow sea, near
the shore or far from land, and whether the water was salt, brackish,
or fresh. Some limestones consist almost exclusively of corals, and in
many cases it is evident that the present position of each fossil
zoophyte has been determined by the manner in which it grew originally.
The axis of the coral, for example, if its natural growth is erect,
still remains at right angles to the plane of stratification. If the
stratum be now horizontal, the round spherical heads of certain species
continue uppermost, and their points of attachment are directed
downward. This arrangement is sometimes repeated throughout a great
succession of strata. From what we know of the growth of similar
zoophytes in modern reefs, we infer that the rate of increase was
extremely slow, and some of the fossils must have flourished for ages
like forest-trees, before they attained so large a size. During these
ages, the water must have been clear and transparent, for such corals
cannot live in turbid water.

Fossil Gryphæ, covered both on the outside and inside with fossil
serpulæ.
In like manner, when we see thousands of full-grown shells dispersed
everywhere throughout a long series of strata, we cannot doubt that
time was required for the multiplication of successive generations; and
the evidence of slow accumulation is rendered more striking from the
proofs, so often discovered, of fossil bodies having lain for a time on
the floor of the ocean after death before they were imbedded in
sediment. Nothing, for example, is more common than to see fossil
oysters in clay, with Serpulæ, or barnacles (acorn-shells), or corals,
and other creatures, attached to the inside of the valves, so that the
mollusk was certainly not buried in argillaceous mud the moment it
died. There must have been an interval during which it was still
surrounded with clear water, when the creatures whose remains now
adhere to it grew from an embryonic to a mature state. Attached shells
which are merely external, like some of the Serpulæ (_a_) in Fig. 9,
may often have grown upon an oyster or other shell while the animal
within was still living; but if they are found on the inside, it could
only happen after the death of the inhabitant of the shell which
affords the support. Thus, in Fig. 9, it will be seen that two Serpulæ
have grown on the interior, one of them exactly on the place where the
adductor muscle of the _Gryphæa_ (a kind of oyster) was fixed.

Fig. 10: Serpula attached to a fossil. Fig. 11: Recent Spatangus with
spines removed from one side.
Some fossil shells, even if simply attached to the _ outside_ of
others, bear full testimony to the conclusion above alluded to, namely,
that an interval elapsed between the death of the creature to whose
shell they adhere, and the burial of the same in mud or sand. The
sea-urchins, or _Echini_, so abundant in white chalk, afford a good
illustration. It is well known that these animals, when living, are
invariably covered with spines supported by rows of tubercles. These
last are only seen after the death of the sea-urchin, when the spines
have dropped off. In Fig. 11 a living species of _Spatangus_, common on
our coast, is represented with one half of its shell stripped of the
spines. In Fig. 10 a fossil of a similar and allied genus from the
white chalk of England shows the naked surface which the individuals of
this family exhibit when denuded of their bristles. The full-grown _
Serpula_, therefore, which now adheres externally, could not have begun
to grow till the _Micraster_ had died, and the spines became detached.

Fig. 12: Ananchytes from the chalk.
Now the series of events here attested by a single fossil may be
carried a step farther. Thus, for example, we often meet with a
sea-urchin (_Ananchytes_) in the chalk (see Fig. 12) which has fixed to
it the lower valve of a _Crania_, a genus of bivalve mollusca. The
upper valve (_b_, Fig. 12) is almost invariably wanting, though
occasionally found in a perfect state of preservation in white chalk at
some distance. In this case, we see clearly that the sea-urchin first
lived from youth to age, then died and lost its spines, which were
carried away. Then the young _Crania_ adhered to the bared shell, grew
and perished in its turn; after which the upper valve was separated
from the lower before the _Ananchytes_ became enveloped in chalky mud.

Fig. 13: Fossil wood bored by Teredina.

Fig. 14: Recent wood bored by Teredo.

It may be well to mention one more illustration of the manner in which
single fossils may sometimes throw light on a former state of things,
both in the bed of the ocean and on some adjoining land. We meet with
many fragments of wood bored by ship-worms at various depths in the
clay on which London is built. Entire branches and stems of trees,
several feet in length, are sometimes found drilled all over by the
holes of these borers, the tubes and shells of the mollusk still
remaining in the cylindrical hollows. In Fig. 14, _ e_, a
representation is given of a piece of recent wood pierced by the
_Teredo navalis_, or common ship-worm, which destroys wooden piles and
ships. When the cylindrical tube _d_ has been extracted from the wood,
the valves are seen at the larger or anterior extremity, as shown at
_c._ In like manner, a piece of fossil wood (_a_, Fig. 13) has been
perforated by a kindred but extinct genus, the _Teredina_ of Lamarck.
The calcareous tube of this mollusk was united and, as it were,
soldered on to the valves of the shell (_b_), which therefore cannot be
detached from the tube, like the valves of the recent _Teredo._ The
wood in this fossil specimen is now converted into a stony mass, a
mixture of clay and lime; but it must once have been buoyant and
floating in the sea, when the _ Teredinæ_ lived upon, and perforated
it. Again, before the infant colony settled upon the drift wood, part
of a tree must have been floated down to the sea by a river, uprooted,
perhaps, by a flood, or torn off and cast into the waves by the wind:
and thus our thoughts are carried back to a prior period, when the tree
grew for years on dry land, enjoying a fit soil and climate.

Strata of Organic Origin.—It has been already remarked that there are
rocks in the interior of continents, at various depths in the earth,
and at great heights above the sea, almost entirely made up of the
remains of zoophytes and testacea. Such masses may be compared to
modern oyster-beds and coral-reefs; and, like them, the rate of
increase must have been extremely gradual. But there are a variety of
stone deposits in the earth’s crust, now proved to have been derived
from plants and animals of which the organic origin was not suspected
until of late years, even by naturalists. Great surprise was therefore
created some years since by the discovery of Professor Ehrenberg, of
Berlin, that a certain kind of siliceous stone, called tripoli, was
entirely composed of millions of the remains of organic beings, which
were formerly referred to microscopic Infusoria, but which are now
admitted to be plants. They abound in rivulets, lakes, and ponds in
England and other countries, and are termed Diatomaceæ by those
naturalists who believe in their vegetable origin. The subject alluded
to has long been well-known in the arts, under the name of infusorial
earth or mountain meal, and is used in the form of powder for polishing
stones and metals. It has been procured, among other places, from the
mud of a lake at Dolgelly, in North Wales, and from Bilin, in Bohemia,
in which latter place a single stratum, extending over a wide area, is
no less than fourteen feet thick. This stone, when examined with a
powerful microscope, is found to consist of the siliceous plates or
frustules of the above-figured Diatomaceæ, united together without any
visible cement. It is difficult to convey an idea of their extreme
minuteness; but Ehrenberg estimates that in the Bilin tripoli there are
41,000 millions of individuals of the _Gaillonella distans_ (see Fig.
16) in every cubic inch (which weighs about 220 grains), or about 187
millions in a single grain. At every stroke, therefore, that we make
with this polishing powder, several millions, perhaps tens of millions,
of perfect fossils are crushed to atoms.

Figs 15 and 16: Gaillonella; Fig. 17: Bacillaria parodoxa
A well-known substance, called bog-iron ore, often met with in
peat-mosses, has often been shown by Ehrenberg to consist of
innumerable articulated threads, of a yellow ochre colour, composed of
silica, argillaceous matter, and peroxide of iron. These threads are
the cases of a minute microscopic body, called _Gaillonella ferruginea_
(Fig. 15), associated with the siliceous frustules of other fresh-water
algæ. Layers of this iron ore occurring in Scotch peat bogs are often
called “the pan,” and are sometimes of economical value.

It is clear much time must have been required for the accumulation of
strata to which countless generations of Diatomaceæ have contributed
their remains; and these discoveries lead us naturally to suspect that
other deposits, of which the materials have been supposed to be
inorganic, may in reality be composed chiefly of microscopic organic
bodies. That this is the case with the white chalk, has often been
imagined, and is now proved to be the fact. It has, moreover, been
lately discovered that the chambers into which these Foraminifera are
divided are actually often filled with thousands of well-preserved
organic bodies, which abound in every minute grain of chalk, and are
especially apparent in the white coating of flints, often accompanied
by innumerable needle-shaped spiculæ of sponges (see Chapter XVII).

“The dust we tread upon was once alive!”—BYRON.

How faint an idea does this exclamation of the poet convey of the real
wonders of nature! for here we discover proofs that the calcareous and
siliceous dust of which hills are composed has not only been once
alive, but almost every particle, albeit invisible to the naked eye,
still retains the organic structure which, at periods of time
incalculably remote, was impressed upon it by the powers of life.

Fresh-water and Marine Fossils.—Strata, whether deposited in salt or
fresh water, have the same forms; but the imbedded fossils are very
different in the two cases, because the aquatic animals which frequent
lakes and rivers are distinct from those inhabiting the sea. In the
northern part of the Isle of Wight formations of marl and limestone,
more than 50 feet thick occur, in which the shells are of extinct
species. Yet we recognise their fresh-water origin, because they are of
the same genera as those now abounding in ponds, lakes, and rivers,
either in our own country or in warmer latitudes.

In many parts of France—in Auvergne, for example—strata occur of
limestone, marl, and sandstone hundreds of feet thick, which contain
exclusively fresh-water and land shells, together with the remains of
terrestrial quadrupeds. The number of land-shells scattered through
some of these fresh-water deposits is exceedingly great; and there are
districts in Germany where the rocks scarcely contain any other fossils
except snail-shells (_helices_); as, for instance, the limestone on the
left bank of the Rhine, between Mayence and Worms, at Oppenheim,
Findheim, Budenheim, and other places. In order to account for this
phenomenon, the geologist has only to examine the small deltas of
torrents which enter the Swiss lakes when the waters are low, such as
the newly-formed plain where the Kander enters the Lake of Thun. He
there sees sand and mud strewn over with innumerable dead land-shells,
which have been brought down from the valleys in the Alps in the
preceding spring, during the melting of the snows. Again, if we search
the sands on the borders of the Rhine, in the lower part of its course,
we find countless land-shells mixed with others of species belonging to
lakes, stagnant pools, and marshes. These individuals have been washed
away from the alluvial plains of the great river and its tributaries,
some from mountainous regions, others from the low country.

Although fresh-water formations are often of great thickness, yet they
are usually very limited in area when compared to marine deposits, just
as lakes and estuaries are of small dimensions in comparison with seas.

The absence of many fossil forms usually met with in marine strata,
affords a useful negative indication of the fresh-water origin of a
formation. For example, there are no sea-urchins, no corals, no
chambered shells, such as the nautilus, nor microscopic Foraminifera in
lacustrine or fluviatile deposits. In distinguishing the latter from
formations accumulated in the sea, we are chiefly guided by the forms
of the mollusca. In a fresh-water deposit, the number of individual
shells is often as great as in a marine stratum, if not greater; but
there is a smaller variety of species and genera. This might be
anticipated from the fact that the genera and species of recent
fresh-water and land shells are few when contrasted with the marine.
Thus, the genera of true mollusca according to Woodward’s system,
excluding those altogether extinct and those without shells, amount to
446 in number, of which the terrestrial and fresh-water genera scarcely
form more than a fifth.[1]

Fig. 18: Cyrena obovata. Fig. 19: Cyrena fluminatis.

Fig. 20: Anodonta Cordierii. Fig. 21: Anodonta latimarginata. Fig. 22:
Unio littoralis.

Almost all bivalve shells, or those of acephalous mollusca, are marine,
about sixteen only out of 140 genera being fresh-water. Among these
last, the four most common forms, both recent and fossil, are _Cyclas,
Cyrena, Unio,_ and _Anodonta_ (see Figures); the two first and two last
of which are so nearly allied as to pass into each other.

Fig. 23: Gryphæa incurva.

Lamarck divided the bivalve mollusca into the Dimyary, or those having
two large muscular impressions in each valve, as _a b_ in the Cyclas,
Fig. 18, and Unio, Fig. 22, and the _ Monomyary,_ such as the oyster
and scallop, in which there is only one of these impressions, as is
seen in Fig. 23. Now, as none of these last, or the unimuscular
bivalves, are fresh-water,[2] we may at once presume a deposit
containing any of them to be marine.

Fig. 24: Planorbis enomphalus. Fig. 25: Limnæa longiscala. Fig. 26:
Pauldina lenta. Fig. 27: Succinea amphibia. Fig. 28: Ancylus velletia.
Fig. 29: Valvata piscinalis. Fig. 30: Physa hypnorum. Fig. 31:
Auricula. Fig. 32: Melania inquinata. Fig. 33: Physa columnaris. Fig.
34: Melanopsis buccinoidea.

Fig. 35: Neritina globulud. Fig. 36: Nerita granulosa.
The univalve shells most characteristic of fresh-water deposits are,
_Planorbis, Limnæa,_ and _Paludina._ (See Figures.) But to these are
occasionally added _Physa, Succinea, Ancylus, Valvata, Melanopsis,
Melania, Potamides,_ and _ Neritina_ (see Figures), the four last being
usually found in estuaries.

Fig. 37: Potamides cinctus.

Some naturalists include _Neritina_ (Fig. 35) and the marine _Nerita_
(Fig. 36) in the same genus, it being scarcely possible to distinguish
the two by good generic characters. But, as a general rule, the
fluviatile species are smaller, smoother, and more globular than the
marine; and they have never, like the _Neritæ,_ the inner margin of the
outer lip toothed or crenulated. (See Fig. 36.)

The Potamides inhabit the mouths of rivers in warm latitudes, and are
distinguishable from the marine Cerithia by their orbicular and
multispiral opercula. The genus Auricula (Fig. 31) is amphibious,
frequenting swamps and marshes within the influence of the tide.

The terrestrial shells are all univalves. The most important genera
among these, both in a recent and fossil state, are _ Helix_ (Fig. 38),
_Cyclostoma_ (Fig. 39), _Pupa_ (Fig. 40), _Clausilia_ (Fig. 41),
_Bulimus_ (Fig. 42), _ Glandina_ and _Achatina._

Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa
tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus.

_Ampullaria_ (Fig. 43) is another genus of shells inhabiting rivers and
ponds in hot countries. Many fossil species formerly referred to this
genus, and which have been met with chiefly in marine formations, are
now considered by conchologists to belong to _Natica_ and other marine
genera.

Fig. 43: Ampullaria glauca.
All univalve shells of land and fresh-water species, with the exception
of _Melanopsis_ (Fig. 34), and _Achatina,_ which has a slight
indentation, have entire mouths; and this circumstance may often serve
as a convenient rule for distinguishing fresh-water from marine strata;
since, if any univalves occur of which the mouths are not entire, we
may presume that the formation is marine. The aperture is said to be
entire in such shells as the fresh-water _Ampullaria_ and the
land-shells (Figs 38-42), when its outline is not interrupted by an
indentation or notch, such as that seen at _b_ in _ Ancillaria_ (Fig.
45); or is not prolonged into a canal, as that seen at _a_ in
_Pleurotoma_ (Fig. 44).

Fig. 44: Pleurotoma exorta. Fig. 45: Ancillaria subulata.

The mouths of a large proportion of the marine univalves have these
notches or canals, and almost all species are carnivorous; whereas
nearly all testacea having entire mouths are plant-eaters, whether the
species be marine, fresh-water, or terrestrial.

There is, however, one genus which affords an occasional exception to
one of the above rules. The _Potamides_ (Fig. 37), a subgenus of
Cerithium, although provided with a short canal, comprises some species
which inhabit salt, others brackish, and others fresh-water, and they
are said to be all plant-eaters.

Among the fossils very common in fresh-water deposits are the shells of
_Cypris,_ a minute bivalve crustaceous animal.[3] Many minute living
species of this genus swarm in lakes and stagnant pools in Great
Britain; but their shells are not, if considered separately, conclusive
as to the fresh-water origin of a deposit, because the majority of
species in another kindred genus of the same order, the _Cytherina_ of
Lamarck, inhabit salt-water; and, although the animal differs slightly,
the shell is scarcely distinguishable from that of the Cypris.

Fresh-water Fossil Plants.—The seed-vessels and stems of _ Chara,_ a
genus of aquatic plants, are very frequent in fresh-water strata. These
seed-vessels were called, before their true nature was known,
gyrogonites, and were supposed to be foraminiferous shells. (See Fig.
46, _a_.)

The _Charæ_ inhabit the bottom of lakes and ponds, and flourish mostly
where the water is charged with carbonate of lime. Their seed-vessels
are covered with a very tough integument, capable of resisting
decomposition; to which circumstance we may attribute their abundance
in a fossil state. The annexed figure (Fig. 47) represents a branch of
one of many new species found by Professor Amici in the lakes of
Northern Italy. The seed-vessel in this plant is more globular than in
the British _Charæ,_) and therefore more nearly resembles in form the
extinct fossil species found in England, France, and other countries.
The stems, as well as the seed-vessels, of these plants occur both in
modern shell-marl and in ancient fresh-water formations. They are
generally composed of a large central tube surrounded by smaller ones;
the whole stem being divided at certain intervals by transverse
partitions or joints. (See _b,_ Fig. 46.)

Fig. 46: Chara medicaginula. Fig. 47: Chara elastica.

It is not uncommon to meet with layers of vegetable matter, impressions
of leaves, and branches of trees, in strata containing fresh-water
shells; and we also find occasionally the teeth and bones of land
quadrupeds, of species now unknown. The manner in which such remains
are occasionally carried by rivers into lakes, especially during
floods, has been fully treated of in the “Principles of Geology.”

Fresh-water and Marine Fish.—The remains of fish are occasionally
useful in determining the fresh-water origin of strata. Certain genera,
such as carp, perch, pike, and loach (_Cyprinus, Perca, Esox,_ and
_Cobitis_), as also _Lebias,_ being peculiar to fresh-water. Other
genera contain some fresh-water and some marine species, as _Cottus,
Mugil,_ and _Anguilla,_ or eel. The rest are either common to rivers
and the sea, as the salmon; or are exclusively characteristic of
salt-water. The above observations respecting fossil fishes are
applicable only to the more modern or tertiary deposits; for in the
more ancient rocks the forms depart so widely from those of existing
fishes, that it is very difficult, at least in the present state of
science, to derive any positive information from ichthyolites
respecting the element in which strata were deposited.

The alternation of marine and fresh-water formations, both on a small
and large scale, are facts well ascertained in geology. When it occurs
on a small scale, it may have arisen from the alternate occupation of
certain spaces by river-water and the sea; for in the flood season the
river forces back the ocean and freshens it over a large area,
depositing at the same time its sediment; after which the salt-water
again returns, and, on resuming its former place, brings with it sand,
mud, and marine shells.

There are also lagoons at the mouth of many rivers, as the Nile and
Mississippi, which are divided off by bars of sand from the sea, and
which are filled with salt and fresh water by turns. They often
communicate exclusively with the river for months, years, or even
centuries; and then a breach being made in the bar of sand, they are
for long periods filled with salt-water.

Lym-Fiord.—The Lym-Fiord in Jutland offers an excellent illustration of
analogous changes; for, in the course of the last thousand years, the
western extremity of this long frith, which is 120 miles in length,
including its windings, has been four times fresh and four times salt,
a bar of sand between it and the ocean having been often formed and
removed. The last irruption of salt water happened in 1824, when the
North Sea entered, killing all the fresh-water shells, fish, and
plants; and from that time to the present, the sea-weed _Fucus
vesiculosus,_ together with oysters and other marine mollusca, have
succeeded the _Cyclas, Lymnæa, Paludina,_ and _Charæ._[4]

But changes like these in the Lym-Fiord, and those before mentioned as
occurring at the mouths of great rivers, will only account for some
cases of marine deposits of partial extent resting on fresh-water
strata. When we find, as in the south-east of England (Chapter XVIII),
a great series of fresh-water beds, 1000 feet in thickness, resting
upon marine formations and again covered by other rocks, such as the
Cretaceous, more than 1000 feet thick, and of deep-sea origin, we shall
find it necessary to seek for a different explanation of the phenomena.

 [1] See Woodward’s Manual of Mollusca, 1856.

 [2] The fresh-water Mulleria, when young, forms a single exception to
 the rule, as it then has two muscular impressions, but it has only one
 in the adult state.

 [3] For figures of fossil species of Purbeck, see Chapter XIX

 [4] See Principles, Index, “Lym-Fiord.”




CHAPTER IV.
CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.


Chemical and Mechanical Deposits. — Cementing together of Particles. —
Hardening by Exposure to Air. — Concretionary Nodules. — Consolidating
Effects of Pressure. — Mineralization of Organic Remains. — Impressions
and Casts: how formed. — Fossil Wood. — Goppert’s Experiments. —
Precipitation of Stony Matter most rapid where Putrefaction is going
on. — Sources of Lime and Silex in Solution.

Having spoken in the preceding chapters of the characters of
sedimentary formations, both as dependent on the deposition of
inorganic matter and the distribution of fossils, I may next treat of
the consolidation of stratified rocks, and the petrifaction of imbedded
organic remains.

Chemical and Mechanical Deposits.— A distinction has been made by
geologists between deposits of a mechanical, and those of a chemical,
origin. By the name mechanical are designated beds of mud, sand, or
pebbles produced by the action of running water, also accumulations of
stones and scoriæ thrown out by a volcano, which have fallen into their
present place by the force of gravitation. But the matter which forms a
chemical deposit has not been mechanically suspended in water, but in a
state of solution until separated by chemical action. In this manner
carbonate of lime is occasionally precipitated upon the bottom of lakes
in a solid form, as may be well seen in many parts of Italy, where
mineral springs abound, and where the calcareous stone, called
travertin, is deposited. In these springs the lime is usually held in
solution by an excess of carbonic acid, or by heat if it be a hot
spring, until the water, on issuing from the earth, cools or loses part
of its acid. The calcareous matter then falls down in a solid state,
incrusting shells, fragments of wood and leaves, and binding them
together.

That similar travertin is formed at some points in the bed of the sea
where calcareous springs issue cannot be doubted, but as a general rule
the quantity of lime, according to Bischoff, spread through the waters
of the ocean is very small, the free carbonic acid gas in the same
waters being five times as much as is necessary to keep the lime in a
fluid state. Carbonate of lime, therefore, can rarely be precipitated
at the bottom of the sea by chemical action alone, but must be produced
by vital agency as in the case of coral reefs.

In such reefs, large masses of limestone are formed by the stony
skeletons of zoophytes; and these, together with shells, become
cemented together by carbonate of lime, part of which is probably
furnished to the sea-water by the decomposition of dead corals. Even
shells, of which the animals are still living on these reefs, are very
commonly found to be incrusted over with a hard coating of limestone.

If sand and pebbles are carried by a river into the sea, and these are
bound together immediately by carbonate of lime, the deposit may be
described as of a mixed origin, partly chemical, and partly mechanical.

Now, the remarks already made in Chapter II, on the original
horizontality of strata are strictly applicable to mechanical deposits,
and only partially to those of a mixed nature. Such as are purely
chemical may be formed on a very steep slope, or may even incrust the
vertical walls of a fissure, and be of equal thickness throughout; but
such deposits are of small extent, and for the most part confined to
vein-stones.

Consolidation of Strata.—It is chiefly in the case of calcareous rocks
that solidification takes place at the time of deposition. But there
are many deposits in which a cementing process comes into operation
long afterwards. We may sometimes observe, where the water of
ferruginous or calcareous springs has flowed through a bed of sand or
gravel, that iron or carbonate of lime has been deposited in the
interstices between the grains or pebbles, so that in certain places
the whole has been bound together into a stone, the same set of strata
remaining in other parts loose and incoherent.

Proofs of a similar cementing action are seen in a rock at Kelloway, in
Wiltshire. A peculiar band of sandy strata belonging to the group
called Oolite by geologists may be traced through several counties, the
sand being for the most part loose and unconsolidated, but becoming
stony near Kelloway. In this district there are numerous fossil shells
which have decomposed, having for the most part left only their casts.
The calcareous matter hence derived has evidently served, at some
former period, as a cement to the siliceous grains of sand, and thus a
solid sandstone has been produced. If we take fragments of many other
argillaceous grits, retaining the casts of shells, and plunge them into
dilute muriatic or other acid, we see them immediately changed into
common sand and mud; the cement of lime, derived from the shells,
having been dissolved by the acid.

Traces of impressions and casts are often extremely faint. In some
loose sands of recent date we meet with shells in so advanced a stage
of decomposition as to crumble into powder when touched. It is clear
that water percolating such strata may soon remove the calcareous
matter of the shell; and unless circumstances cause the carbonate of
lime to be again deposited, the grains of sand will not be cemented
together; in which case no memorial of the fossil will remain.

In what manner silex and carbonate of lime may become widely diffused
in small quantities through the waters which permeate the earth’s crust
will be spoken of presently, when the petrifaction of fossil bodies is
considered; but I may remark here that such waters are always passing
in the case of thermal springs from hotter to colder parts of the
interior of the earth; and, as often as the temperature of the solvent
is lowered, mineral matter has a tendency to separate from it and
solidify. Thus a stony cement is often supplied to sand, pebbles, or
any fragmentary mixture. In some conglomerates, like the pudding-stone
of Hertfordshire (a Lower Eocene deposit), pebbles of flint and grains
of sand are united by a siliceous cement so firmly, that if a block be
fractured, the rent passes as readily through the pebbles as through
the cement.

It is probable that many strata became solid at the time when they
emerged from the waters in which they were deposited, and when they
first formed a part of the dry land. A well-known fact seems to confirm
this idea: by far the greater number of the stones used for building
and road-making are much softer when first taken from the quarry than
after they have been long exposed to the air; and these, when once
dried, may afterwards be immersed for any length of time in water
without becoming soft again. Hence it is found desirable to shape the
stones which are to be used in architecture while they are yet soft and
wet, and while they contain their “quarry-water,” as it is called; also
to break up stone intended for roads when soft, and then leave it to
dry in the air for months that it may harden. Such induration may
perhaps be accounted for by supposing the water, which penetrates the
minutest pores of rocks, to deposit, on evaporation, carbonate of lime,
iron, silex, and other minerals previously held in solution, and
thereby to fill up the pores partially. These particles, on
crystallising, would not only be themselves deprived of freedom of
motion, but would also bind together other portions of the rock which
before were loosely aggregated. On the same principle wet sand and mud
become as hard as stone when frozen; because one ingredient of the
mass, namely, the water, has crystallised, so as to hold firmly
together all the separate particles of which the loose mud and sand
were composed.

Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like
dough when first found; and some simple minerals, which are rigid and
as hard as glass in our cabinets, are often flexible and soft in their
native beds: this is the case with asbestos, sahlite, tremolite, and
chalcedony, and it is reported also to happen in the case of the
beryl.[1]

The marl recently deposited at the bottom of Lake Superior, in North
America, is soft, and often filled with fresh-water shells; but if a
piece be taken up and dried, it becomes so hard that it can only be
broken by a smart blow of the hammer. If the lake, therefore, was
drained, such a deposit would be found to consist of strata of
marlstone, like that observed in many ancient European formations, and,
like them, containing fresh-water shells.

Fig. 48: Calcareous nodules in Lias.
Concretionary Structure.—It is probable that some of the heterogeneous
materials which rivers transport to the sea may at once set under
water, like the artificial mixture called pozzolana, which consists of
fine volcanic sand charged with about twenty per cent of oxide of iron,
and the addition of a small quantity of lime. This substance hardens,
and becomes a solid stone in water, and was used by the Romans in
constructing the foundations of buildings in the sea. Consolidation in
such cases is brought about by the action of chemical affinity on
finely comminuted matter previously suspended in water. After
deposition similar particles seem often to exert a mutual attraction on
each other, and congregate together in particular spots, forming lumps,
nodules, and concretions. Thus in many argillaceous deposits there are
calcareous balls, or spherical concretions, ranged in layers parallel
to the general stratification; an arrangement which took place after
the shale or marl had been thrown down in successive laminæ; for these
laminæ are often traceable through the concretions, remaining parallel
to those of the surrounding unconsolidated rock. (See Fig. 48.) Such
nodules of limestone have often a shell or other foreign body in the
centre.

Among the most remarkable examples of concretionary structure are those
described by Professor Sedgwick as abounding in the magnesian limestone
of the north of England. The spherical balls are of various sizes, from
that of a pea to a diameter of several feet, and they have both a
concentric and radiated structure, while at the same time the laminæ of
original deposition pass uninterruptedly through them. In some cliffs
this limestone resembles a great irregular pile of cannon-balls. Some
of the globular masses have their centre in one stratum, while a
portion of their exterior passes through to the stratum above or below.
Thus the larger spheroid in the section (Fig. 49) passes from the
stratum _b_ upward into _a._ In this instance we must suppose the
deposition of a series of minor layers, first forming the stratum _b,_
and afterwards the incumbent stratum _a_; then a movement of the
particles took place, and the carbonates of lime and magnesia separated
from the more impure and mixed matter forming the still unconsolidated
parts of the stratum. Crystallisation, beginning at the centre, must
have gone on forming concentric coats around the original nucleus
without interfering with the laminated structure of the rock.

Fig. 49: Spheroidal concretions in magnesian limestone. Fig. 50:
Section through strata of grit.
When the particles of rocks have been thus rearranged by chemical
forces, it is sometimes difficult or impossible to ascertain whether
certain lines of division are due to original deposition or to the
subsequent aggregation of several particles. Thus suppose three strata
of grit, A, B, C, are charged unequally with calcareous matter, and
that B is the most calcareous. If consolidation takes place in B, the
concretionary action may spread upward into a part of A, where the
carbonate of lime is more abundant than in the rest; so that a mass, _d
e f,_ forming a portion of the superior stratum, becomes united with B
into one solid mass of stone. The original line of division, _d e,_
being thus effaced, the line _d f_ would generally be considered as the
surface of the bed B, though not strictly a true plane of
stratification.

Pressure and Heat.—When sand and mud sink to the bottom of a deep sea,
the particles are not pressed down by the enormous weight of the
incumbent ocean; for the water, which becomes mingled with the sand and
mud, resists pressure with a force equal to that of the column of fluid
above. The same happens in regard to organic remains which are filled
with water under great pressure as they sink, otherwise they would be
immediately crushed to pieces and flattened. Nevertheless, if the
materials of a stratum remain in a yielding state, and do not set or
solidify, they will be gradually squeezed down by the weight of other
materials successively heaped upon them, just as soft clay or loose
sand on which a house is built may give way. By such downward pressure
particles of clay, sand, and marl may become packed into a smaller
space, and be made to cohere together permanently.

Analogous effects of condensation may arise when the solid parts of the
earth’s crust are forced in various directions by those mechanical
movements hereafter to be described, by which strata have been bent,
broken, and raised above the level of the sea. Rocks of more yielding
materials must often have been forced against others previously
consolidated, and may thus by compression have acquired a new
structure. A recent discovery may help us to comprehend how fine
sediment derived from the detritus of rocks may be solidified by mere
pressure. The graphite or “black lead” of commerce having become very
scarce, Mr. Brockedon contrived a method by which the dust of the purer
portions of the mineral found in Borrowdale might be recomposed into a
mass as dense and compact as native graphite. The powder of graphite is
first carefully prepared and freed from air, and placed under a
powerful press on a strong steel die, with air-tight fittings. It is
then struck several blows, each of a power of 1000 tons; after which
operation the powder is so perfectly solidified that it can be cut for
pencils, and exhibits when broken the same texture as native graphite.

But the action of heat at various depths in the earth is probably the
most powerful of all causes in hardening sedimentary strata. To this
subject I shall refer again when treating of the metamorphic rocks, and
of the slaty and jointed structure.

Mineralisation of Organic Remains.—The changes which fossil organic
bodies have undergone since they were first imbedded in rocks, throw
much light on the consolidation of strata. Fossil shells in some modern
deposits have been scarcely altered in the course of centuries, having
simply lost a part of their animal matter. But in other cases the shell
has disappeared, and left an impression only of its exterior, or,
secondly, a cast of its interior form, or, thirdly, a cast of the shell
itself, the original matter of which has been removed. These different
forms of fossilisation may easily be understood if we examine the mud
recently thrown out from a pond or canal in which there are shells. If
the mud be argillaceous, it acquires consistency on drying, and on
breaking open a portion of it we find that each shell has left
impressions of its external form. If we then remove the shell itself,
we find within a solid nucleus of clay, having the form of the interior
of the shell. This form is often very different from that of the outer
shell. Thus a cast such as _a,_ Fig. 51, commonly called a fossil
screw, would never be suspected by an inexperienced conchologist to be
the internal shape of the fossil univalve, _ b,_ Fig. 51. Nor should we
have imagined at first sight that the shell a and the cast _b,_ Fig.
52, belong to one and the same fossil. The reader will observe, in the
last-mentioned figure (_b,_ Fig. 52), that an empty space shaded dark,
which the _ shell itself_ once occupied, now intervenes between the
enveloping stone and the cast of the smooth interior of the whorls. In
such cases the shell has been dissolved and the component particles
removed by water percolating the rock. If the nucleus were taken out, a
hollow mould would remain, on which the external form of the shell with
its tubercles and striæ, as seen in _a,_ Fig. 52, would be seen
embossed. Now if the space alluded to between the nucleus and the
impression, instead of being left empty, has been filled up with
calcareous spar, flint, pyrites, or other mineral, we then obtain from
the mould an exact cast both of the external and internal form of the
original shell. In this manner silicified casts of shells have been
formed; and if the mud or sand of the nucleus happen to be incoherent,
or soluble in acid, we can then procure in flint an empty shell, which
in shape is the exact counterpart of the original. This cast may be
compared to a bronze statue, representing merely the superficial form,
and not the internal organisation; but there is another description of
petrifaction by no means uncommon, and of a much more wonderful kind,
which may be compared to certain anatomical models in wax, where not
only the outward forms and features, but the nerves, blood-vessels, and
other internal organs are also shown. Thus we find corals, originally
calcareous, in which not only the general shape, but also the minute
and complicated internal organisation is retained in flint.

Fig. 51: Phasianella Heddingtonensis. Fig. 52: Pleurotomaria Anglica.

Fig. 53: Section of a tree from the coal-measures.

Such a process of petrifaction is still more remarkably exhibited in
fossil wood, in which we often perceive not only the rings of annual
growth, but all the minute vessels and medullary rays. Many of the
minute cells and fibres of plants, and even those spiral vessels which
in the living vegetable can only be discovered by the microscope, are
preserved. Among many instances, I may mention a fossil tree,
seventy-two feet in length, found at Gosforth, near Newcastle, in
sandstone strata associated with coal. By cutting a transverse slice so
thin as to transmit light, and magnifying it about fifty-five times,
the texture, as seen in Fig. 53, is exhibited. A texture equally minute
and complicated has been observed in the wood of large trunks of fossil
trees found in the Craigleith quarry near Edinburgh, where the stone
was not in the slightest degree siliceous, but consisted chiefly of
carbonate of lime, with oxide of iron, alumina, and carbon. The
parallel rows of vessels here seen are the rings of annual growth, but
in one part they are imperfectly preserved, the wood having probably
decayed before the mineralising matter had penetrated to that portion
of the tree.

In attempting to explain the process of petrifaction in such cases, we
may first assume that strata are very generally permeated by water
charged with minute portions of calcareous, siliceous, and other earths
in solution. In what manner they become so impregnated will be
afterwards considered. If an organic substance is exposed in the open
air to the action of the sun and rain, it will in time putrefy, or be
dissolved into its component elements, consisting usually of oxygen,
hydrogen, nitrogen, and carbon. These will readily be absorbed by the
atmosphere or be washed away by rain, so that all vestiges of the dead
animal or plant disappear. But if the same substances be submerged in
water, they decompose more gradually; and if buried in earth, still
more slowly; as in the familiar example of wooden piles or other buried
timber. Now, if as fast as each particle is set free by putrefaction in
a fluid or gaseous state, a particle equally minute of carbonate of
lime, flint, or other mineral, is at hand ready to be precipitated, we
may imagine this inorganic matter to take the place just before left
unoccupied by the organic molecule. In this manner a cast of the
interior of certain vessels may first be taken, and afterwards the more
solid walls of the same may decay and suffer a like transmutation. Yet
when the whole is lapidified, it may not form one homogeneous mass of
stone or metal. Some of the original ligneous, osseous, or other
organic elements may remain mingled in certain parts, or the
lapidifying substance itself may be differently coloured at different
times, or so crystallised as to reflect light differently, and thus the
texture of the original body may be faithfully exhibited.

The student may perhaps ask whether, on chemical principles, we have
any ground to expect that mineral matter will be thrown down precisely
in those spots where organic decomposition is in progress? The
following curious experiments may serve to illustrate this point:
Professor Goppert of Breslau, with a view of imitating the natural
process of petrifaction, steeped a variety of animal and vegetable
substances in waters, some holding siliceous, others calcareous, others
metallic matter in solution. He found that in the period of a few
weeks, or sometimes even days, the organic bodies thus immersed were
mineralised to a certain extent. Thus, for example, thin vertical
slices of deal, taken from the Scotch fir (_Pinus sylvestris_), were
immersed in a moderately strong solution of sulphate of iron. When they
had been thoroughly soaked in the liquid for several days they were
dried and exposed to a red-heat until the vegetable matter was burnt up
and nothing remained but an oxide of iron, which was found to have
taken the form of the deal so exactly that casts even of the dotted
vessels peculiar to this family of plants were distinctly visible under
the microscope.

The late Dr. Turner observes, that when mineral matter is in a “nascent
state,” that is to say, just liberated from a previous state of
chemical combination, it is most ready to unite with other matter, and
form a new chemical compound. Probably the particles or atoms just set
free are of extreme minuteness, and therefore move more freely, and are
more ready to obey any impulse of chemical affinity. Whatever be the
cause, it clearly follows, as before stated, that where organic matter
newly imbedded in sediment is decomposing, there will chemical changes
take place most actively.

An analysis was lately made of the water which was flowing off from the
rich mud deposited by the Hooghly River in the Delta of the Ganges
after the annual inundation. This water was found to be highly charged
with carbonic acid holding lime in solution.[2] Now if newly-deposited
mud is thus proved to be permeated by mineral matter in a state of
solution, it is not difficult to perceive that decomposing organic
bodies, naturally imbedded in sediment, may as readily become petrified
as the substances artificially immersed by Professor Goppert in various
fluid mixtures.

It is well known that the waters of all springs are more or less
charged with earthy, alkaline, or metallic ingredients derived from the
rocks and mineral veins through which they percolate. Silex is
especially abundant in hot springs, and carbonate of lime is almost
always present in greater or less quantity. The materials for the
petrifaction of organic remains are, therefore, usually at hand in a
state of chemical solution wherever organic remains are imbedded in new
strata.

 [1] Dr. MacCulloch, Syst. of Geol., vol. i, p. 123.

 [2] Piddington, Asiat. Research., vol. xviii, p. 226.




CHAPTER V.
ELEVATION OF STRATA ABOVE THE SEA.—HORIZONTAL AND INCLINED
STRATIFICATION.


Why the Position of Marine Strata, above the Level of the Sea, should
be referred to the rising up of the Land, not to the going down of the
Sea. — Strata of Deep-sea and Shallow-water Origin alternate. — Also
Marine and Fresh-water Beds and old Land Surfaces. — Vertical,
inclined, and folded Strata. — Anticlinal and Synclinal Curves. —
Theories to explain Lateral Movements. — Creeps in Coal-mines. — Dip
and Strike. — Structure of the Jura. — Various Forms of Outcrop. —
Synclinal Strata forming Ridges. — Connection of Fracture and Flexure
of Rocks. — Inverted Strata. — Faults described. — Superficial Signs of
the same obliterated by Denudation. — Great Faults the Result of
repeated Movements. — Arrangement and Direction of parallel Folds of
Strata. — Unconformability. — Overlapping Strata.

Land has been raised, not the Sea lowered.—It has been already stated
that the aqueous rocks containing marine fossils extend over wide
continental tracts, and are seen in mountain chains rising to great
heights above the level of the sea (p. 29). Hence it follows, that what
is now dry land was once under water. But if we admit this conclusion,
we must imagine, either that there has been a general lowering of the
waters of the ocean, or that the solid rocks, once covered by water,
have been raised up bodily out of the sea, and have thus become dry
land. The earlier geologists, finding themselves reduced to this
alternative, embraced the former opinion, assuming that the ocean was
originally universal, and had gradually sunk down to its actual level,
so that the present islands and continents were left dry. It seemed to
them far easier to conceive that the water had gone down, than that
solid land had risen upward into its present position. It was, however,
impossible to invent any satisfactory hypothesis to explain the
disappearance of so enormous a body of water throughout the globe, it
being necessary to infer that the ocean had once stood at whatever
height marine shells might be detected. It moreover appeared clear, as
the science of geology advanced, that certain spaces on the globe had
been alternately sea, then land, then estuary, then sea again, and,
lastly, once more habitable land, having remained in each of these
states for considerable periods. In order to account for such phenomena
without admitting any movement of the land itself, we are required to
imagine several retreats and returns of the ocean; and even then our
theory applies merely to cases where the marine strata composing the
dry land are horizontal, leaving unexplained those more common
instances where strata are inclined, curved, or placed on their edges,
and evidently not in the position in which they were first deposited.

Geologists, therefore, were at last compelled to have recourse to the
doctrine that the solid land has been repeatedly moved upward or
downward, so as permanently to change its position relatively to the
sea. There are several distinct grounds for preferring this conclusion.
First, it will account equally for the position of those elevated
masses of marine origin in which the stratification remains horizontal,
and for those in which the strata are disturbed, broken, inclined, or
vertical. Secondly, it is consistent with human experience that land
should rise gradually in some places and be depressed in others. Such
changes have actually occurred in our own days, and are now in
progress, having been accompanied in some cases by violent convulsions,
while in others they have proceeded so insensibly as to have been
ascertainable only by the most careful scientific observations, made at
considerable intervals of time. On the other hand, there is no evidence
from human experience of a rising or lowering of the sea’s level in any
region, and the ocean cannot be raised or depressed in one place
without its level being changed all over the globe.

These preliminary remarks will prepare the reader to understand the
great theoretical interest attached to all facts connected with the
position of strata, whether horizontal or inclined, curved or vertical.

Now the first and most simple appearance is where strata of marine
origin occur above the level of the sea in horizontal position. Such
are the strata which we meet with in the south of Sicily, filled with
shells for the most part of the same species as those now living in the
Mediterranean. Some of these rocks rise to the height of more than 2000
feet above the sea. Other mountain masses might be mentioned, composed
of horizontal strata of high antiquity, which contain fossil remains of
animals wholly dissimilar from any now known to exist. In the south of
Sweden, for example, near Lake Wener, the beds of some of the oldest
fossiliferous deposits, called Silurian and Cambrian by geologists,
occur in as level a position as if they had recently formed part of the
delta of a great river, and been left dry on the retiring of the annual
floods. Aqueous rocks of equal antiquity extend for hundreds of miles
over the lake-district of North America, and exhibit in like manner a
stratification nearly undisturbed. The Table Mountain at the Cape of
Good Hope is another example of highly elevated yet perfectly
horizontal strata, no less than 3500 feet in thickness, and consisting
of sandstone of very ancient date.

Instead of imagining that such fossiliferous rocks were always at their
present level, and that the sea was once high enough to cover them, we
suppose them to have constituted the ancient bed of the ocean, and to
have been afterwards uplifted to their present height. This idea,
however startling it may at first appear, is quite in accordance, as
before stated, with the analogy of changes now going on in certain
regions of the globe. Thus, in parts of Sweden, and the shores and
islands of the Gulf of Bothnia, proofs have been obtained that the land
is experiencing, and has experienced for centuries, a slow upheaving
movement.[1]

It appears from the observations of Mr. Darwin and others, that very
extensive regions of the continent of South America have been
undergoing slow and gradual upheaval, by which the level plains of
Patagonia, covered with recent marine shells, and the Pampas of Buenos
Ayres, have been raised above the level of the sea. On the other hand,
the gradual sinking of the west coast of Greenland, for the space of
more than 600 miles from north to south, during the last four
centuries, has been established by the observations of a Danish
naturalist, Dr. Pingel. And while these proofs of continental elevation
and subsidence, by slow and insensible movements, have been recently
brought to light, the evidence has been daily strengthened of continued
changes of level effected by violent convulsions in countries where
earthquakes are frequent. There the rocks are rent from time to time,
and heaved up or thrown down several feet at once, and disturbed in
such a manner as to show how entirely the original position of strata
may be modified in the course of centuries.

Mr. Darwin has also inferred that, in those seas where circular coral
islands and barrier reefs abound, there is a slow and continued sinking
of the submarine mountains on which the masses of coral are based;
while there are other areas of the South Sea where the land is on the
rise, and where coral has been upheaved far above the sea-level.

Alternations of Marine and Fresh-water Strata.—It has been shown in the
third chapter that there is such a differencebetween land, fresh-water,
and marine fossils as to enable the geologist to determine whether
particular groups of strata were formed at the bottom of the ocean or
in estuaries, rivers, or lakes. If surprise was at first created by the
discovery of marine corals and shells at the height of several miles
above the sea-level, the imagination was afterwards not less startled
by observing that in the successive strata composing the earth’s crust,
especially if their total thickness amounted to thousands of feet, they
comprised in some parts formations of shallow-sea as well as of
deep-sea origin; also beds of brackish or even of purely fresh-water
formation, as well as vegetable matter or coal accumulated on ancient
land. In these cases we as frequently find fresh-water beds below a
marine set or shallow-water under those of deep-sea origin as the
reverse. Thus, if we bore an artesian well below London, we pass
through a marine clay, and there reach, at the depth of several hundred
feet, a shallow-water and fluviatile sand, beneath which comes the
white chalk originally formed in a deep sea. Or if we bore vertically
through the chalk of the North Downs, we come, after traversing marine
chalky strata, upon a fresh-water formation many hundreds of feet
thick, called the Wealden, such as is seen in Kent and Surrey, which is
known in its turn to rest on purely marine beds. In like manner, in
various parts of Great Britain we sink vertical shafts through marine
deposits of great thickness, and come upon coal which was formed by the
growth of plants on an ancient land-surface sometimes hundreds of
square miles in extent.

Vertical, Inclined, and Curved Strata.—It has been stated that marine
strata of different ages are sometimes found at a considerable height
above the sea, yet retaining their original horizontality; but this
state of things is quite exceptional. As a general rule, strata are
inclined or bent in such a manner as to imply that their original
position has been altered.

Fig. 54: Vertical conglomerate and sandstone.
The most unequivocal evidence of such a change is afforded by their
standing up vertically, showing their edges, which is by no means a
rare phenomenon, especially in mountainous countries. Thus we find in
Scotland, on the southern skirts of the Grampians, beds of
pudding-stone alternating with thin layers of fine sand, all placed
vertically to the horizon. When Saussure first observed certain
conglomerates in a similar position in the Swiss Alps, he remarked that
the pebbles, being for the most part of an oval shape, had their longer
axes parallel to the planes of stratification (see Fig. 54 on preceding
page). From this he inferred that such strata must, at first, have been
horizontal, each oval pebble having settled at the bottom of the water,
with its flatter side parallel to the horizon, for the same reason that
an egg will not stand on either end if unsupported. Some few, indeed,
of the rounded stones in a conglomerate occasionally afford an
exception to the above rule, for the same reason that in a river’s bed,
or on a shingle beach, some pebbles rest on their ends or edges; these
having been shoved against or between other stones by a wave or
current, so as to assume this position.

Anticlinal and Synclinal Curves.—Vertical strata, when they can be
traced continuously upward or downward for some depth, are almost
invariably seen to be parts of great curves, which may have a diameter
of a few yards, or of several miles. I shall first describe two curves
of considerable regularity, which occur in Forfarshire, extending over
a country twenty miles in breadth, from the foot of the Grampians to
the sea near Arbroath.

Fig. 55: Section of Forfarshire, from N.W. to S.E.

The mass of strata here shown may be 2000 feet in thickness, consisting
of red and white sandstone, and various coloured shales, the beds being
distinguishable into four principal groups, namely, No. 1, red marl or
shale; No. 2, red sandstone, used for building; No. 3, conglomerate;
and No. 4, grey paving-stone, and tile-stone, with green and reddish
shale, containing peculiar organic remains. A glance at the section
will show that each of the formations 2, 3, 4 are repeated thrice at
the surface, twice with a southerly, and once with a northerly
inclination or _dip_, and the beds in No. 1, which are nearly
horizontal, are still brought up twice by a slight curvature to the
surface, once on each side of A. Beginning at the north-west extremity,
the tile-stones and conglomerates, No. 4 and No. 3, are vertical, and
they generally form a ridge parallel to the southern skirts of the
Grampians. The superior strata, Nos. 2 and 1, become less and less
inclined on descending to the valley of Strathmore, where the strata,
having a concave bend, are said by geologists to lie in a “trough”
or“basin.” Through the centre of this valley runs an imaginary line A,
called technically a “synclinal line,” where the beds, which are tilted
in opposite directions, may be supposed to meet. It is most important
for the observer to mark such lines, for he will perceive by the
diagram that, in travelling from the north to the centre of the basin,
he is always passing from older to newer beds; whereas, after crossing
the line A, and pursuing his course in the same southerly direction, he
is continually leaving the newer, and advancing upon older strata. All
the deposits which he had before examined begin then to recur in
reversed order, until he arrives at the central axis of the Sidlaw
hills, where the strata are seen to form an arch, or _saddle_, having
an _anticlinal_ line, B, in the centre. On passing this line, and
continuing towards the S.E., the formations 4, 3, and 2, are again
repeated, in the same relative order of superposition, but with a
southerly dip. At Whiteness (see Fig. 55) it will be seen that the
inclined strata are covered by a newer deposit, _a_, in horizontal
beds. These are composed of red conglomerate and sand, and are newer
than any of the groups, 1, 2, 3, 4, before described, and rest
_unconformably_ upon strata of the sandstone group, No. 2.

An example of curved strata, in which the bends or convolutions of the
rock are sharper and far more numerous within an equal space, has been
well described by Sir James Hall.[2] It occurs near St. Abb’s Head, on
the east coast of Scotland, where the rocks consist principally of a
bluish slate, having frequently a ripple-marked surface. The
undulations of the beds reach from the top to the bottom of cliffs from
200 to 300 feet in height, and there are sixteen distinct bendings in
the course of about six miles, the curvatures being alternately concave
and convex upward.

Folding by Lateral Movement.—An experiment was made by Sir James Hall,
with a view of illustrating the manner in which such strata, assuming
them to have been originally horizontal, may have been forced into
their present position. A set of layers of clay were placed under a
weight, and their opposite ends pressed towards each other with such
force as to cause them to approach more nearly together. On the removal
of the weight, the layers of clay were found to be curved and folded,
so as to bear a miniature resemblance to the strata in the cliffs. We
must, however, bear in mind that in the natural section or sea-cliff we
only see the foldings imperfectly, one part being invisible beneath the
sea, and the other, or upper portion, being supposed to have been
carried away by _denudation_, or that action of water which will be
explained in the next chapter. The dark lines in the plan (Fig. 57)
represent what is actually seen of the strata in the line of cliff
alluded to; the fainter lines, that portion which is concealed beneath
the sea-level, as also that which is supposed to have once existed
above the present surface.

Fig. 56: Curved strata of slate near St. Abb’s Head, Berwickshire.

Fig. 57

We may still more easily illustrate the effects which a lateral thrust
might produce on flexible strata, by placing several pieces of
differently coloured cloths upon a table, and when they are spread out
horizontally, cover them with a book. Then apply other books to each
end, and force them towards each other. The folding of the cloths (see
Fig. 58) will imitate those of the bent strata; the incumbent book
being slightly lifted up, and no longer touching the two volumes on
which it rested before, because it is supported by the tops of the
anticlinal ridges formed by the curved cloths. In like manner there can
be no doubt that the squeezed strata, although laterally condensed and
more closely packed, are yet elongated and made to rise upward, in a
direction perpendicular to the pressure.

Fig. 58

Whether the analogous flexures in stratified rocks have really been due
to similar sideway movements is a question which we can not decide by
reference to our own observation. Our inability to explain the nature
of the process is, perhaps, not simply owing to the inaccessibility of
the subterranean regions where the mechanical force is exerted, but to
the extreme slowness of the movement. The changes may sometimes be due
to variation in the temperature of mountain masses of rock causing
them, while still solid, to expand or contract; or melting them, and
then again cooling them and allowing them to crystallise. If such be
the case, we have scarcely more reason to expect to witness the
operation of the process within the limited periods of our scientific
observation than to see the swelling of the roots of a tree, by which,
in the course of years, a wall of solid masonry may be lifted up, rent
or thrown down. In both instances the force may be irresistible, but
though adequate, it need not be visible by us, provided the time
required for its development be very great. The lateral pressure
arising from the unequal expansion of rocks by heat may cause one mass
lying in the same horizontal plane gradually to occupy a larger space,
so as to press upon another rock, which, if flexible, may be squeezed
into a bent and folded form. It will also appear, when the volcanic and
granitic rocks are described, that some of them have, when melted in
the interior of the earth’s crust, been injected forcibly into
fissures, and after the solidification of such intruded matter, other
sets of rents, crossing the first, have been formed and in their turn
filled by melted rock. Such repeated injections imply a stretching, and
often upheaval, of the whole mass.

We also know, especially by the study of regions liable to earthquakes,
that there are causes at work in the interior of the earth capable of
producing a sinking in of the ground, sometimes very local, but often
extending over a wide area. The continuance of such a downward
movement, especially if partial and confined to linear areas, may
produce regular folds in the strata.

Creeps in Coal-mines.—The “creeps,” as they are called in coal-mines,
afford an excellent illustration of this fact.—First, it may be stated
generally, that the excavation of coal at a considerable depth causes
the mass of overlying strata to sink down bodily, even when props are
left to support the roof of the mine. “In Yorkshire,” says Mr. Buddle,
“three distinct subsidences were perceptible at the surface, after the
clearing out of three seams of coal below, and innumerable vertical
cracks were caused in the incumbent mass of sandstone and shale which
thus settled down.”[3] The exact amount of depression in these cases
can only be accurately measured where water accumulates on the surface,
or a railway traverses a coal-field.

When a bed of coal is worked out, pillars or rectangular masses of coal
are left at intervals as props to support the roof, and protect the
colliers. Thus in Fig. 59, representing a section at Wallsend,
Newcastle, the galleries which have been excavated are represented by
the white spaces _a, b,_ while the adjoining dark portions are parts of
the original coal seam left as props, beds of sandy clay or shale
constituting the floor of the mine. When the props have been reduced in
size, they are pressed down by the weight of overlying rocks (no less
than 630 feet thick) upon the shale below, which is thereby squeezed
and forced up into the open spaces.

Now it might have been expected that, instead of the floor rising up,
the ceiling would sink down, and this effect, called a “thrust,” does,
in fact, take place where the pavement is more solid than the roof. But
it usually happens, in coal-mines, that the roof is composed of hard
shale, or occasionally of sandstone, more unyielding than the
foundation, which often consists of clay. Even where the argillaceous
substrata are hard at first, they soon become softened and reduced to a
plastic state when exposed to the contact of air and water in the floor
of a mine.

Fig. 59: Section of carboniferous strata at Wallsend showing ‘creeps’.

The first symptom of a “creep,” says Mr. Buddle, is a slight curvature
at the bottom of each gallery, as at _a_, Fig. 59: then the pavement,
continuing to rise, begins to open with a longitudinal crack, as at
_b_; then the points of the fractured ridge reach the roof, as at _c_;
and, lastly, the upraised beds close up the whole gallery, and the
broken portions of the ridge are reunited and flattened at the top,
exhibiting the flexure seen at _d._ Meanwhile the coal in the props has
become crushed and cracked by pressure. It is also found that below the
creeps _a, b, c, d,_ an inferior stratum, called the “metal coal,”
which is 3 feet thick, has been fractured at the points _e, f, g, h,_
and has risen, so as to prove that the upward movement, caused by the
working out of the “main coal,” has been propagated through a thickness
of 54 feet of argillaceous beds, which intervene between the two
coal-seams. This same displacement has also been traced downward more
than 150 feet below the metal coal, but it grows continually less and
less until it becomes imperceptible.

No part of the process above described is more deserving of our notice
than the slowness with which the change in the arrangement of the beds
is brought about. Days, months, or even years, will sometimes elapse
between the first bending of the pavement and the time of its reaching
the roof. Where the movement has been most rapid, the curvature of the
beds is most regular, and the reunion of the fractured ends most
complete; whereas the signs of displacement or violence are greatest in
those creeps which have required months or years for their entire
accomplishment. Hence we may conclude that similar changes may have
been wrought on a larger scale in the earth’s crust by partial and
gradual subsidences, especially where the ground has been undermined
throughout long periods of time; and we must be on our guard against
inferring sudden violence, simply because the distortion of the beds is
excessive.

Engineers are familiar with the fact that when they raise the level of
a railway by heaping stone or gravel on a foundation of marsh,
quicksand, or other yielding formation, the new mound often sinks for a
time as fast as they attempt to elevate it; when they have persevered
so as to overcome this difficulty, they frequently find that some of
the adjoining flexible ground has risen up in one or more parallel
arches or folds, showing that the vertical pressure of the sinking
materials has given rise to a lateral folding movement.

In like manner, in the interior of the earth, the solid parts of the
earth’s crust may sometimes, as before mentioned, be made to expand by
heat, or may be pressed by the force of steam against flexible strata
loaded with a great weight of incumbent rocks. In this case the
yielding mass, squeezed, but unable to overcome the resistance which it
meets with in a vertical direction, may be gradually relieved by
lateral folding.

Fig. 60
Dip and Strike.—In describing the manner in which strata depart from
their original horizontality, some technical terms, such as “dip” and
“strike,” “anticlinal” and “synclinal” line or axis, are used by
geologists. I shall now proceed to explain some of these to the
student. If a stratum or bed of rock, instead of being quite level, be
inclined to one side, it is said to _dip_; the point of the compass to
which it is inclined is called the _point of dip_, and the degree of
deviation from a level or horizontal line is called _ the amount of
dip_, or _the angle of dip._ Thus, in the annexed diagram (Fig. 60), a
series of strata are inclined, and they dip to the north at an angle of
forty-five degrees. The _strike_, or _line of bearing_, is the
prolongation or extension of the strata in a direction _at right
angles_ to the dip; and hence it is sometimes called the _ direction_
of the strata. Thus, in the above instance of strata dipping to the
north, their strike must necessarily be east and west. We have borrowed
the word from the German geologists, _ streichen_ signifying to extend,
to have a certain direction. Dip and strike may be aptly illustrated by
a row of houses running east and west, the long ridge of the roof
representing the strike of the stratum of slates, which dip on one side
to the north, and on the other to the south.

A stratum which is horizontal, or quite level in all directions, has
neither dip nor strike.

It is always important for the geologist, who is endeavouring to
comprehend the structure of a country, to learn how the beds dip in
every part of the district; but it requires some practice to avoid
being occasionally deceived, both as to the point of dip and the amount
of it.

Fig. 61: Apparent horizontality of inclined strata.

If the upper surface of a hard stony stratum be uncovered, whether
artificially in a quarry, or by waves at the foot of a cliff, it is
easy to determine towards what point of the compass the slope is
steepest, or in what direction water would flow if poured upon it. This
is the true dip. But the edges of highly inclined strata may give rise
to perfectly horizontal lines in the face of a vertical cliff, if the
observer see the strata in the line of the strike, the dip being inward
from the face of the cliff. If, however, we come to a break in the
cliff, which exhibits a section exactly at right angles to the line of
the strike, we are then able to ascertain the true dip. In the drawing
(Fig. 61), we may suppose a headland, one side of which faces to the
north, where the beds would appear perfectly horizontal to a person in
the boat; while in the other side facing the west, the true dip would
be seen by the person on shore to be at an angle of 40°. If, therefore,
our observations are confined to a vertical precipice facing in one
direction, we must endeavour to find a ledge or portion of the plane of
one of the beds projecting beyond the others, in order to ascertain the
true dip.

Fig. 62: Two hands used to determine the inclination of strata.
If not provided with a clinometer, a most useful instrument, when it is
of consequence to determine with precision the inclination of the
strata, the observer may measure the angle within a few degrees by
standing exactly opposite to a cliff where the true dip is exhibited,
holding the hands immediately before the eyes, and placing the fingers
of one in a perpendicular, and of the other in a horizontal position,
as in Fig. 62. It is thus easy to discover whether the lines of the
inclined beds bisect the angle of 90°, formed by the meeting of the
hands, so as to give an angle of 45°, or whether it would divide the
space into two equal or unequal portions. You have only to change hands
to get the line of dip on the upper side of the horizontal hand.

Fig. 63: Section illustrating the structure of the Swiss Jura.

It has been already seen, in describing the curved strata on the east
coast of Scotland, in Forfarshire and Berwickshire, that a series of
concave and convex bendings are occasionally repeated several times.
These usually form part of a series of parallel waves of strata, which
are prolonged in the same direction, throughout a considerable extent
of country. Thus, for example, in the Swiss Jura, that lofty chain of
mountains has been proved to consist of many parallel ridges, with
intervening longitudinal valleys, as in Fig. 63, the ridges being
formed by curved fossiliferous strata, of which the nature and dip are
occasionally displayed in deep transverse gorges, called “cluses,”
caused by fractures at right angles to the direction of the chain.[4]
Now let us suppose these ridges and parallel valleys to run north and
south, we should then say that the _strike_ of the beds is north and
south, and the _dip_ east and west. Lines drawn along the summits of
the ridges, A, B, would be anticlinal lines, and one following the
bottom of the adjoining valleys a synclinal line.

Fig. 64: Ground-plan of the denuded ridge C, Fig. 63. Fig. 65:
Transverse section.

Outcrop of Strata.—It will be observed that some of these ridges, A, B,
are unbroken on the summit, whereas one of them, C, has been fractured
along the line of strike, and a portion of it carried away by
denudation, so that the ridges of the beds in the formations _a, b, c_
come out to the day, or, as the miners say, _crop out_, on the sides of
a valley. The ground-plan of such a denuded ridge as C, as given in a
geological map, may be expressed by the diagram, Fig. 64, and the
cross-section of the same by Fig. 65. The line D E, Fig. 64, is the
anticlinal line, on each side of which the dip is in opposite
directions, as expressed by the arrows. The emergence of strata at the
surface is called by miners their _outcrop_, or _basset._

If, instead of being folded into parallel ridges, the beds form a boss
or dome-shaped protuberance, and if we suppose the summit of the dome
carried off, the ground-plan would exhibit the edges of the strata
forming a succession of circles, or ellipses, round a common centre.
These circles are the lines of strike, and the dip being always at
right angles is inclined in the course of the circuit to every point of
the compass, constituting what is termed a quâ-quâversal dip—that is,
turning every way.

There are endless variations in the figures described by the
basset-edges of the strata, according to the different inclination of
the beds, and the mode in which they happen to have been denuded. One
of the simplest rules, with which every geologist should be acquainted,
relates to the V-like form of the beds as they crop out in an ordinary
valley. First, if the strata be horizontal, the V-like form will be
also on a level, and the newest strata will appear at the greatest
heights.

Fig. 66: Slope of valley 40°, dip of strata 20°. Fig. 67: Slope of
valley 20°, dip of strata 50°. Fig. 68: Slope of valley 20°, dip of
strata 20°, in opposite directions.
Secondly, if the beds be inclined and intersected by a valley sloping
in the same direction, and the dip of the beds be less steep than the
slope of the valley, then the V’s, as they are often termed by miners,
will point upward (see Fig. 66), those formed by the newer beds
appearing in a superior position, and extending highest up the valley,
as A is seen above B.

Thirdly, if the dip of the beds be steeper than the slope of the
valley, then the V’s will point downward (see Fig. 67), and those
formed of the older beds will now appear uppermost, as B appears above
A.

Fourthly, in every case where the strata dip in a contrary direction to
the slope of the valley, whatever be the angle of inclination, the
newer beds will appear the highest, as in the first and second cases.
This is shown by the drawing (Fig. 68), which exhibits strata rising at
an angle of 20°, and crossed by a valley, which declines in an opposite
direction at 20°.

These rules may often be of great practical utility; for the different
degrees of dip occurring in the two cases represented in Figs. 66 and
67 may occasionally be encountered in following the same line of
flexure at points a few miles distant from each other. A miner
unacquainted with the rule, who had first explored the valley Fig. 66,
may have sunk a vertical shaft below the coal-seam A, until he reached
the inferior bed, B. He might then pass to the valley, Fig. 67, and
discovering there also the outcrop of two coal-seams, might begin his
workings in the uppermost in the expectation of coming down to the
other bed A, which would be observed cropping out lower down the
valley. But a glance at the section will demonstrate the futility of
such hopes.[5]

Section of carboniferous rocks of Lancashire. Section of carboniferous
rocks of Lancashire. (E. Hull.[6])

Synclinal Strata forming Ridges.—Although in many cases an anticlinal
axis forms a ridge, and a synclinal axis a valley, as in A B, Fig. 63,
yet this can by no means be laid down as a general rule, as the beds
very often slope inward from either side of a mountain, as at _a, b,_
Fig. 69, while in the intervening valley, _c_, they slope upward,
forming an arch.

It would be natural to expect the fracture of solid rocks to take place
chiefly where the bending of the strata has been sharpest, and such
rending may produce ravines giving access to running water and exposing
the surface to atmospheric waste. The entire absence, however, of such
cracks at points where the strain must have been greatest, as at _a_,
Fig. 63, is often very remarkable, and not always easy of explanation.
We must imagine that many strata of limestone, chert, and other rocks
which are now brittle, were pliant when bent into their present
position. They may have owed their flexibility in part to the fluid
matter which they contained in their minute pores, as before described
p. 62 and in part to the permeation of sea-water while they were yet
submerged.

Fig. 70: Strata of chert, grit, and marl, near St. Jean de Luz.

At the western extremity of the Pyrenees, great curvatures of the
strata are seen in the sea-cliffs, where the rocks consist of marl,
grit, and chert. At certain points, as at _a_, Fig. 70, some of the
bendings of the flinty chert are so sharp that specimens might be
broken off well fitted to serve as ridge-tiles on the roof of a house.
Although this chert could not have been brittle as now, when first
folded into this shape, it presents, nevertheless, here and there, at
the points of greatest flexure, small cracks, which show that it was
solid, and not wholly incapable of breaking at the period of its
displacement. The numerous rents alluded to are not empty, but filled
with chalcedony and quartz.

Fig. 71: Bent and undulating gypseous marl.
Between San Caterina and Castrogiovanni, in Sicily, bent and undulating
gypseous marls occur, with here and there thin beds of solid gypsum
interstratified. Sometimes these solid layers have been broken into
detached fragments, still preserving their sharp edges (_g, g,_ Fig.
71), while the continuity of the more pliable and ductile marls, _m,
m,_ has not been interrupted.

Fig. 72: Folded strata.
We have already explained, Fig. 69, that stratified rocks have usually
their strata bent into parallel folds forming anticlinal and synclinal
axes, a group of several of these folds having often been subjected to
a common movement, and having acquired a uniform strike or direction.
In some disturbed regions these folds have been doubled back upon
themselves in such a manner that it is often difficult for an
experienced geologist to determine correctly the relative age of the
beds by superposition. Thus, if we meet with the strata seen in the
section, Fig. 72, we should naturally suppose that there were twelve
distinct beds, or sets of beds, No. 1 being the newest, and No. 12 the
oldest of the series. But this section may perhaps exhibit merely six
beds, which have been folded in the manner seen in Fig. 73, so that
each of them is twice repeated, the position of one half being
reversed, and part of No. 1, originally the uppermost, having now
become the lowest of the series.

Fig. 73
These phenomena are observable on a magnificent scale in certain
regions in Switzerland, in precipices often more than 2000 feet in
perpendicular height, and there are flexures not inferior in dimensions
in the Pyrenees. The upper part of the curves seen in this diagram,
Fig. 73, and expressed in fainter lines, has been removed by what is
called denudation, to be afterwards explained.

Fractures of the Strata and Faults.—Numerous rents may often be seen in
rocks which appear to have been simply broken, the fractured parts
still remaining in contact; but we often find a fissure, several inches
or yards wide, intervening between the disunited portions. These
fissures are usually filled with fine earth and sand, or with angular
fragments of stone, evidently derived from the fracture of the
contiguous rocks.

The face of each wall of the fissure is often beautifully polished, as
if glazed, striated, or scored with parallel furrows and ridges, such
as would be produced by the continued rubbing together of surfaces of
unequal hardness. These polished surfaces are called by miners
“slickensides.” It is supposed that the lines of the striæ indicate the
direction in which the rocks were moved. During one of the minor
earthquakes in Chili, in 1840, the brick walls of a building were rent
vertically in several places, and made to vibrate for several minutes
during each shock, after which they remained uninjured, and without any
opening, although the line of each crack was still visible. When all
movement had ceased, there were seen on the floor of the house, at the
bottom of each rent, small heaps of fine brick-dust, evidently produced
by trituration.

It is not uncommon to find the mass of rock on one side of a fissure
thrown up above or down below the mass with which it was once in
contact on the other side. “This mode of displacement is called a
fault, shift, slip, or throw.” “The miner,” says Playfair, describing a
fault, “is often perplexed, in his subterranean journey, by a
derangement in the strata, which changes at once all those lines and
bearings which had hitherto directed his course. When his mine reaches
a certain plane, which is sometimes perpendicular, as in A B, Fig. 74,
sometimes oblique to the horizon (as in C D, ibid.), he finds the beds
of rock broken asunder, those on the one side of the plane having
changed their place, by sliding in a particular direction along the
face of the others. In this motion they have sometimes preserved their
parallelism, as in Fig. 74, so that the strata on each side of faults A
B, C D, continue parallel to one another; in other cases, the strata on
each side are inclined, as in _a, b, c, d_ (Fig. 75), though their
identity is still to be recognised by their possessing the same
thickness and the same internal characters.”[7]

Fig. 74: Faults.

Fig. 75: E F, fault or fissure filled with rubbish, on each side of
which the shifted strata are not parallel.

In Coalbrook Dale, says Mr. Prestwich[8], deposits of sandstone, shale,
and coal, several thousand feet thick, and occupying an area of many
miles, have been shivered into fragments, and the broken remnants have
been placed in very discordant positions, often at levels differing
several hundred feet from each other. The sides of the faults, when
perpendicular, are commonly several yards apart, and are sometimes as
much as 50 yards asunder, the interval being filled with broken
_débris_ of the strata. In following the course of the same fault it is
sometimes found to produce in different places very unequal changes of
level, the amount of shift being in one place 300, and in another 700
feet, which arises from the union of two or more faults. In other
words, the disjointed strata have in certain districts been subjected
to renewed movements, which they have not suffered elsewhere.

We may occasionally see exact counterparts of these slips, on a small
scale, in pits of loose sand and gravel, many of which have doubtless
been caused by the drying and shrinking of argillaceous and other beds,
slight subsidences having taken place from failure of support.
Sometimes, however, even these small slips may have been produced
during earthquakes; for land has been moved, and its level, relatively
to the sea, considerably altered, within the period when much of the
alluvial sand and gravel now covering the surface of continents was
deposited.

I have already stated that a geologist must be on his guard, in a
region of disturbed strata, against inferring repeated alternations of
rocks, when, in fact, the same strata, once continuous, have been bent
round so as to recur in the same section, and with the same dip. A
similar mistake has often been occasioned by a series of faults.

Fig. 76: Apparent alternations of strata caused by vertical faults.

If, for example, the dark line A H (Fig. 76) represent the surface of a
country on which the strata _a, b, c_ frequently crop out, an observer
who is proceeding from H to A might at first imagine that at every step
he was approaching new strata, whereas the repetition of the same beds
has been caused by vertical faults, or downthrows. Thus, suppose the
original mass, A, B, C, D, to have been a set of uniformly inclined
strata, and that the different masses under E F, F G, and G D sank down
successively, so as to leave vacant the spaces marked in the diagram by
dotted lines, and to occupy those marked by the continuous lines, then
let denudation take place along the line A H, so that the protruding
masses indicated by the fainter lines are swept away—a miner, who has
not discovered the faults, finding the mass _a_, which we will suppose
to be a bed of coal four times repeated, might hope to find four beds,
workable to an indefinite depth, but first, on arriving at the fault G,
he is stopped suddenly in his workings, for he comes partly upon the
shale _b_, and partly on the sandstone _ c_; the same result awaits him
at the fault F, and on reaching E he is again stopped by a wall
composed of the rock _d._

The very different levels at which the separated parts of the same
strata are found on the different sides of the fissure, in some faults,
is truly astonishing. One of the most celebrated in England is that
called the “ninety-fathom dike,” in the coal-field of Newcastle. This
name has been given to it, because the same beds are ninety fathoms
(540 feet) lower on the northern than they are on the southern side.
The fissure has been filled by a body of sand, which is now in the
state of sandstone, and is called the dike, which is sometimes very
narrow, but in other places more than twenty yards wide.[9] The walls
of the fissure are scored by grooves, such as would have been produced
if the broken ends of the rock had been rubbed along the plane of the
fault.[10] In the Tynedale and Craven faults, in the north of England,
the vertical displacement is still greater, and the fracture has
extended in a horizontal direction for a distance of thirty miles or
more.

Great Faults the Result of Repeated Movements.—It must not, however, be
supposed that faults generally consist of single linear rents; there
are usually a number of faults springing off from the main one, and
sometimes a long strip of country seems broken up into fragments by
sets of parallel and connecting transverse faults. Oftentimes a great
line of fault has been repeated, or the movements have been continued
through successive periods, so that, newer deposits having covered the
old line of displacement, the strata both newer and older have given
way along the old line of fracture. Some geologists have considered it
necessary to imagine that the upward or downward movement in these
cases was accomplished at a single stroke, and not by a series of
sudden but interrupted movements. They appear to have derived this idea
from a notion that the grooved walls have merely been rubbed in one
direction, which is far from being a constant phenomenon. Not only are
some sets of striæ not parallel to others, but the clay and rubbish
between the walls, when squeezed or rubbed, have been streaked in
different directions, the grooves which the harder minerals have
impressed on the softer being frequently curved and irregular.

Fig. 77: Faults and denuded coal-strata, Ashby de la Zouch.

The usual absence of protruding masses of rock forming precipices or
ridges along the lines of great faults has already been alluded to in
explaining Fig. 76, p. 89, and the same remarkable fact is well
exemplified in every coal-field which has been extensively worked. It
is in such districts that the former relation of the beds which have
been shifted is determinable with great accuracy. Thus in the
coal-field of Ashby de la Zouch, in Leicestershire (see Fig. 77), a
fault occurs, on one side of which the coal-beds _a, b, c, d_ must once
have risen to the height of 500 feet above the corresponding beds on
the other side. But the uplifted strata do not stand up 500 feet above
the general surface; on the contrary, the outline of the country, as
expressed by the line _z z_, is uniformly undulating, without any
break, and the mass indicated by the dotted outline must have been
washed away.[11]

The student may refer to Mr. Hull’s measurement of faults, observed in
the Lancashire coal-field, where the vertical displacement has amounted
to thousands of feet, and yet where all the superficial inequalities
which must have resulted from such movements have been obliterated by
subsequent denudation. In the same memoir proofs are afforded of there
having been two periods of vertical movement in the same fault—one, for
example, before, and another after, the Triassic epoch.[12]

The shifting of the beds by faults is often intimately connected with
those same foldings which constitute the anticlinal and synclinal axes
before alluded to, and there is no doubt that the subterranean causes
of both forms of disturbance are to a great extent the same. A fault in
Virginia, believed to imply a displacement of several thousand feet,
has been traced for more than eighty miles in the same direction as the
foldings of the Appalachian chain.[13] An hypothesis which attributes
such a change of position to a succession of movements, is far
preferable to any theory which assumes each fault to have been
accomplished by a single upcast or downthrow of several thousand feet.
For we know that there are operations now in progress, at great depths
in the interior of the earth, by which both large and small tracts of
ground are made to rise above and sink below their former level, some
slowly and insensibly, others suddenly and by starts, a few feet or
yards at a time; whereas there are no grounds for believing that,
during the last 3000 years at least, any regions have been either
upheaved or depressed, at a single stroke, to the amount of several
hundred, much less several thousand feet.

It is certainly not easy to understand how in the subterranean regions
one mass of solid rock should have been folded up by a continued series
of movements, while another mass in contact, or only separated by a
line of fissure, has remained stationary or has perhaps subsided. But
every volcano, by the intermittent action of the steam, gases, and lava
evolved during an eruption, helps us to form some idea of the manner in
which such operations take place. For eruptions are repeated at
uncertain intervals throughout the whole or a large part of a
geological period, some of the surrounding and contiguous districts
remaining quite undisturbed. And in most of the instances with which we
are best acquainted the emission of lava, scoria, and steam is
accompanied by the uplifting of the solid crust. Thus in Vesuvius,
Etna, the Madeiras, the Canary Islands, and the Azores there is
evidence of marine deposits of recent and tertiary date having been
elevated to the height of a thousand feet, and sometimes more, since
the commencement of the volcanic explosions. There is, moreover, a
general tendency in contemporaneous volcanic vents to affect a linear
arrangement, extending in some instances, as in the Andes or the Indian
Archipelago, to distances equalling half the circumference of the
globe. Where volcanic heat, therefore, operates at such a depth as not
to obtain vent at the surface, in the form of an eruption, it may
nevertheless be conceived to give rise to upheavals, foldings, and
faults in certain linear tracts. And marine denudation, to be treated
of in the next chapter, will help us to understand why that which
should be the protruding portion of the faulted rocks is missing at the
surface.

Arrangement and Direction of Parallel Folds of Strata.—The possible
causes of the folding of strata by lateral movements have been
considered in a former part of this chapter. No European chain of
mountains affords so remarkable an illustration of the persistency of
such flexures for a great distance as the Appalachians before alluded
to, and none has been studied and described by many good observers with
more accuracy. The chain extends from north to south, or rather N.N.E.
to S.S.W., for nearly 1500 miles, with a breadth of 50 miles,
throughout which the Palæozoic strata have been so bent as to form a
series of parallel anticlinal and synclinal ridges and troughs,
comprising usually three or four principal and many smaller plications,
some of them forming broad and gentle arches, others narrower and
steeper ones, while some, where the bending has been greatest, have the
position of their beds inverted, as before shown in Fig. 73, p. 87.

The strike of the parallel ridges, after continuing in a straight line
for many hundred miles, is then found to vary for a more limited
distance as much as 30°, the folds wheeling round together in the new
direction and continuing to be parallel, as if they had all obeyed the
same movement. The date of the movements by which the great flexures
were brought about must, of course, be subsequent to the formation of
the uppermost part of the coal or the newest of the bent rocks, but the
disturbance must have ceased before the Triassic strata were deposited
on the denuded edges of the folded beds.

The manner in which the numerous parallel folds, all simultaneously
formed, assume a new direction common to the whole of them, and
sometimes varying at an angle of 30° from the normal strike of the
chain, shows what deviation from an otherwise uniform strike of the
beds may be experienced when the geographical area through which they
are traced is on so vast a scale.

The disturbances in the case here adverted to occurred between the
Carboniferous period and that of the Trias, and this interval is so
vast that they may have occupied a great lapse of time, during which
their parallelism was always preserved. But, as a rule, wherever after
a long geological interval the recurrence of lateral movements gives
rise to a new set of folds, the strike of these last is different.
Thus, for example, Mr. Hull has pointed out that three principal lines
of disturbance, all later than the Carboniferous period, have affected
the stratified rocks of Lancashire. The first of these, having an
E.N.E. direction, took place at the close of the Carboniferous period.
The next, running north and south, at the close of the Permian, and the
third, having a N.N.W. direction, at the close of the Jurassic
period.[14]

Fig. 78: Unconformable junction of old red sandstone and Silurian
schist at the Siccar Point, near St. Abb’s Head, Berwickshire.

Unconformability of Strata.— Strata are said to be unconformable when
one series is so placed over another that the planes of the superior
repose on the edges of the inferior (see Fig. 78). In this case it is
evident that a period had elapsed between the production of the two
sets of strata, and that, during this interval, the older series had
been tilted and disturbed. Afterwards the upper series was thrown down
in horizontal strata upon it. If these superior beds, _d, d,_ Fig. 78,
are also inclined, it is plain that the lower strata _a, a,_ have been
twice displaced; first, before the deposition of the newer beds, _d,
d,_ and a second time when these same strata were upraised out of the
sea, and thrown slightly out of the horizontal position.

Fig. 79: Junction of unconformable strata near Mons, in Belgium.

It often happens that in the interval between the deposition of two
sets of unconformable strata, the inferior rock has not only been
denuded, but drilled by perforating shells. Thus, for example, at
Autreppe and Gusigny, near Mons, beds of an ancient (primary or
palæozoic) limestone, highly inclined, and often bent, are covered with
horizontal strata of greenish and whitish marls of the Cretaceous
formation. The lowest, and therefore the oldest, bed of the horizontal
series is usually the sand and conglomerate, _a_, in which are rounded
fragments of stone, from an inch to two feet in diameter. These
fragments have often adhering shells attached to them, and have been
bored by perforating mollusca. The solid surface of the inferior
limestone has also been bored, so as to exhibit cylindrical and
pear-shaped cavities, as at _c_, the work of saxicavous mollusca; and
many rents, as at _b_, which descend several feet or yards into the
limestone, have been filled with sand and shells, similar to those in
the stratum _a._

Overlapping Strata.—Strata are said to overlap when an upper bed
extends beyond the limits of a lower one. This may be produced in
various ways; as, for example, when alterations of physical geography
cause the arms of a river or channels of discharge to vary, so that
sediment brought down is deposited over a wider area than before, or
when the sea-bottom has been raised up and again depressed without
disturbing the horizontal position of the strata. In this case the
newer strata may rest for the most part conformably on the older, but,
extending farther, pass over their edges. Every intermediate state
between unconformable and over-lapping beds may occur, because there
may be every gradation between a slight derangement of position, and a
considerable disturbance and denudation of the older formation before
the newer beds come on.

 [1] See “Principles of Geology,” 1867, p. 314.

 [2] Edin. Trans., vol. vii, pl. 3.

 [3] Proceedings of Geol. Soc., vol. iii, p. 148.

 [4] Thurmann, “Essai sur les Soulèvemens Jurassiques de Porrentruy,”
 Paris, 1832.

 [5] I am indebted to the kindness of T. Sopwith, Esq., for three
 models which I have copied in the above diagrams; but the beginner may
 find it by no means easy to understand such copies, although, if he
 were to examine and handle the originals, turning them about in
 different ways, he would at once comprehend their meaning, as well as
 the import of others far more complicated, which the same engineer has
 constructed to illustrate _faults._

 [6] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 324, 1868.

 [7] Playfair, Illust. of Hutt. Theory, § 42.

 [8] Geol. Trans., second series. vol. v, p. 452.

 [9] Conybeare and Phillips Outlines, etc., p. 376.

 [10] Phillips, Geology, Lardner’s Cyclop., p. 41.

 [11] See Mammatt’s Geological Facts, etc., p. 90 and plate.

 [12] Hull, Quart. Geol. Journ., vol. xxiv, p. 318, 1868.

 [13] H. D. Rogers, Geol. of Pennsylvania, p. 897.

 [14] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 323.




CHAPTER VI.
DENUDATION


Denudation defined. — Its Amount more than equal to the entire Mass of
Stratified Deposits in the Earth’s Crust. — Subaërial Denudation. —
Action of the Wind. — Action of Running Water. — Alluvium defined. —
Different Ages of Alluvium. — Denuding Power of Rivers affected by Rise
or Fall of Land. — Littoral Denudation. — Inland Sea-Cliffs. —
Escarpments. — Submarine Denudation. — Dogger-bank. — Newfoundland
Bank. — Denuding Power of the Ocean during Emergence of Land.

Denudation, which has been occasionally spoken of in the preceding
chapters, is the removal of solid matter by water in motion, whether of
rivers or of the waves and currents of the sea, and the consequent
laying bare of some inferior rock. This operation has exerted an
influence on the structure of the earth’s crust as universal and
important as sedimentary deposition itself; for denudation is the
necessary antecedent of the production of all new strata of mechanical
origin. The formation of every new deposit by the transport of sediment
and pebbles necessarily implies that there has been, somewhere else, a
grinding down of rock into rounded fragments, sand, or mud, equal in
quantity to the new strata. All deposition, therefore, except in the
case of a shower of volcanic ashes, and the outflow of lava, and the
growth of certain organic formations, is the sign of superficial waste
going on contemporaneously, and to an equal amount, elsewhere. The gain
at one point is no more than sufficient to balance the loss at some
other. Here a lake has grown shallower, there a ravine has been
deepened. Here the depth of the sea has been augmented by the removal
of a sandbank during a storm, there its bottom has been raised and
shallowed by the accumulation in its bed of the same sand transported
from the bank.

When we see a stone building, we know that somewhere, far or near, a
quarry has been opened. The courses of stone in the building may be
compared to successive strata, the quarry to a ravine or valley which
has suffered denudation. As the strata, like the courses of hewn stone,
have been laid one upon another gradually, so the excavation both of
the valley and quarry have been gradual. To pursue the comparison still
farther, the superficial heaps of mud, sand, and gravel, usually called
alluvium, may be likened to the rubbish of a quarry which has been
rejected as useless by the workmen, or has fallen upon the road between
the quarry and the building, so as to lie scattered at random over the
ground.

But we occasionally find in a conglomerate large rounded pebbles of an
older conglomerate, which had previously been derived from a variety of
different rocks. In such cases we are reminded that, the same materials
having been used over and over again, it is not enough to affirm that
the entire mass of stratified deposits in the earth’s crust affords a
monument and measure of the denudation which has taken place, for in
truth the quantity of matter now extant in the form of stratified rock
represents but a fraction of the material removed by water and
redeposited in past ages.

Subaërial Denudation.—Denudation may be divided into subaërial, or the
action of wind, rain, and rivers; and submarine, or that effected by
the waves of the sea, and its tides and currents. With the operation of
the first of these we are best acquainted, and it may be well to give
it our first attention.

_Action of the Wind._—In desert regions where no rain falls, or where,
as in parts of the Sahara, the soil is so salt as to be without any
covering of vegetation, clouds of dust and sand attest the power of the
wind to cause the shifting of the unconsolidated or disintegrated rock.

In examining volcanic countries I have been much struck with the great
superficial changes brought about by this power in the course of
centuries. The highest peak of Madeira is about 6050 feet above the
sea, and consists of the skeleton of a volcanic cone now 250 feet high,
the beds of which once dipped from a centre in all directions at an
angle of more than 30°. The summit is formed of a dike of basalt with
much olivine, fifteen feet wide, apparently the remains of a column of
lava which once rose to the crater. Nearly all the scoriæ of the upper
part of the cone have been swept away, those portions only remaining
which were hardened by the contact or proximity of the dike. While I
was myself on this peak on January 25, 1854, I saw the wind, though it
was not stormy weather, removing sand and dust derived from the
decomposing scoriæ. There had been frost in the night, and some ice was
still seen in the crevices of the rock.

On the highest platform of the Grand Canary, at an elevation of 6000
feet, there is a cylindrical column of hard lava, from which the softer
matter has been carried away; and other similar remnants of the dikes
of cones of eruption attest the denuding power of the wind at points
where running water could never have exerted any influence. The waste
effected by wind aided by frost and snow, may not be trifling, even in
a single winter, and when multiplied by centuries may become
indefinitely great.

Fig. 80: Section through several eroded formations.

_Action of Running Water._—There are different classes of phenomena
which attest in a most striking manner the vast spaces left vacant by
the erosive power of water. I may allude, first, to those valleys on
both sides of which the same strata are seen following each other in
the same order, and having the same mineral composition and fossil
contents. We may observe, for example, several formations, as Nos. 1,
2, 3, 4, in the diagram (Fig. 80): No. 1, conglomerate, No. 2, clay,
No. 3, grit, and No. 4, limestone, each repeated in a series of hills
separated by valleys varying in depth. When we examine the subordinate
parts of these four formations, we find, in like manner, distinct beds
in each, corresponding, on the opposite sides of the valleys, both in
composition and order of position. No one can doubt that the strata
were originally continuous, and that some cause has swept away the
portions which once connected the whole series. A torrent on the side
of a mountain produces similar interruptions; and when we make
artificial cuts in lowering roads, we expose, in like manner,
corresponding beds on either side. But in nature, these appearances
occur in mountains several thousand feet high, and separated by
intervals of many miles or leagues in extent.

In the “Memoirs of the Geological Survey of Great Britain” (vol. i),
Professor Ramsay has shown that the missing beds, removed from the
summit of the Mendips, must have been nearly a mile in thickness; and
he has pointed out considerable areas in South Wales and some of the
adjacent counties of England, where a series of primary (or palæozoic)
strata, no less than 11,000 feet in thickness, have been stripped off.
All these materials have of course been transported to new regions, and
have entered into the composition of more modern formations. On the
other hand, it is shown by observations in the same “Survey,” that the
Palæozoic strata are from 20,000 to 30,000 feet thick. It is clear that
such rocks, formed of mud and sand, now for the most part consolidated,
are the monuments of denuding operations, which took place on a grand
scale at a very remote period in the earth’s history. For, whatever has
been given to one area must always have been borrowed from another; a
truth which, obvious as it may seem when thus stated, must be
repeatedly impressed on the student’s mind, because in many geological
speculations it is taken for granted that the external crust of the
earth has been always growing thicker in consequence of the
accumulation, period after period, of sedimentary matter, as if the new
strata were not always produced at the expense of pre-existing rocks,
stratified or unstratified. By duly reflecting on the fact that all
deposits of mechanical origin imply the transportation from some other
region, whether contiguous or remote, of an equal amount of solid
matter, we perceive that the stony exterior of the planet must always
have grown thinner in one place, whenever, by accessions of new strata,
it was acquiring thickness in another.

It is well known that generally at the mouths of large rivers, deltas
are forming and the land is encroaching upon the sea; these deltas are
monuments of recent denudation and deposition; and it is obvious that
if the mud, sand, and gravel were taken from them and restored to the
continents they would fill up a large part of the gullies and valleys
which are due to the excavating and transporting power of torrents and
rivers.

Alluvium.—Between the superficial covering of vegetable mould and the
subjacent rock there usually intervenes in every district a deposit of
loose gravel, sand, and mud, to which when it occurs in valleys the
name of alluvium has been popularly applied. The term is derived from
_alluvio_, an inundation, or _alluo_, to wash, because the pebbles and
sand commonly resemble those of a river’s bed or the mud and gravel
washed over low lands by a flood.

In the course of those changes in physical geography which may take
place during the gradual emergence of the bottom of the sea and its
conversion into dry land, any spot may either have been a sunken reef,
or a bay, or estuary, or sea-shore, or the bed of a river. The
drainage, moreover, may have been deranged again and again by
earthquakes, during which temporary lakes are caused by landslips, and
partial deluges occasioned by the bursting of the barriers of such
lakes. For this reason it would be unreasonable to hope that we should
ever be able to account for all the alluvial phenomena of each
particular country, seeing that the causes of their origin are so
various. Besides, the last operations of water have a tendency to
disturb and confound together all pre-existing alluviums. Hence we are
always in danger of regarding as the work of a single era, and the
effect of one cause, what has in reality been the result of a variety
of distinct agents, during a long succession of geological epochs. Much
useful instruction may therefore be gained from the exploration of a
country like Auvergne, where the superficial gravel of very different
eras happens to have been preserved and kept separate by sheets of
lava, which were poured out one after the other at periods when the
denudation, and probably the upheaval, of rocks were in progress. That
region had already acquired in some degree its present configuration
before any volcanoes were in activity, and before any igneous matter
was superimposed upon the granitic and fossiliferous formations. The
pebbles therefore in the older gravels are exclusively constituted of
granite and other aboriginal rocks; and afterwards, when volcanic vents
burst forth into eruption, those earlier alluviums were covered by
streams of lava, which protected them from intermixture with gravel of
subsequent date. In the course of ages, a new system of valleys was
excavated, so that the rivers ran at lower levels than those at which
the first alluviums and sheets of lava were formed. When, therefore,
fresh eruptions gave rise to new lava, the melted matter was poured out
over lower grounds; and the gravel of these plains differed from the
first or upland alluvium, by containing in it rounded fragments of
various volcanic rocks, and often fossil bones belonging to species of
land animals different from those which had previously flourished in
the same country and been buried in older gravels.

Fig. 81: Lavas of Auvergne resting on alluviums of different ages.

The annexed drawing (Fig. 81) will explain the different heights at
which beds of lava and gravel, each distinct from the other in
composition and age, are observed, some on the flat tops of hills, 700
or 800 feet high, others on the slope of the same hills, and the newest
of all in the channel of the existing river where there is usually
gravel alone, although in some cases a narrow strip of solid lava
shares the bottom of the valley with the river.

The proportion of extinct species of quadrupeds is more numerous in the
fossil remains of the gravel No. 1 than in that indicated as No. 2; and
in No. 3 they agree more closely, sometimes entirely, with those of the
existing fauna. The usual absence or rarity of organic remains in beds
of loose gravel and sand is partly owing to the friction which
originally ground down the rocks into small fragments, and partly to
the porous nature of alluvium, which allows the free percolation
through it of rain-water, and promotes the decomposition and removal of
fossil remains.

The loose transported matter on the surface of a large part of the land
now existing in the temperate and arctic regions of the northern
hemisphere, must be regarded as being in a somewhat exceptional state,
in consequence of the important part which ice has played in
comparatively modern geological times. This subject will be more
specially alluded to when we describe, in the eleventh chapter, the
deposits called “glacial.”

Denuding Power of Rivers affected by Rise or Fall of Land.—It has long
been a matter of common observation that most rivers are now cutting
their channels through alluvial deposits of greater depth and extent
than could ever have been formed by the present streams. From this fact
it has been inferred that rivers in general have grown smaller, or
become less liable to be flooded than formerly. It may be true that in
the history of almost every country the rivers have been both larger
and smaller than they are at the present moment. For the rainfall in
particular regions varies according to climate and physical geography,
and is especially governed by the elevation of the land above the sea,
or its distance from it and other conditions equally fluctuating in the
course of time. But the phenomenon alluded to may sometimes be
accounted for by oscillations in the level of the land, experienced
since the existing valleys originated, even where no marked diminution
in the quantity of rain and in the size of the rivers has occurred.

We know that many large areas of land are rising and others sinking,
and unless it could be assumed that both the upward and downward
movements are everywhere uniform, many of the existing hydrographical
basins ought to have the appearance of having been temporary lakes
first filled with fluviatile strata and then partially re-excavated.

Suppose, for example, part of a continent, comprising within it a large
hydrographical basin like that of the Mississippi, to subside several
inches or feet in a century, as the west coast of Greenland, extending
600 miles north and south, has been sinking for three or four
centuries, between the latitudes 60° and 69° N.[1] It will rarely
happen that the rate of subsidence will be everywhere equal, and in
many cases the amount of depression in the interior will regularly
exceed that of the region nearer the sea. Whenever this happens, the
fall of the waters flowing from the upland country will be diminished,
and each tributary stream will have less power to carry its sand and
sediment into the main river, and the main river less power to convey
its annual burden of transported matter to the sea. All the rivers,
therefore, will proceed to fill up partially their ancient channels,
and, during frequent inundations, will raise their alluvial plains by
new deposits. If then the same area of land be again upheaved to its
former height, the fall, and consequently the velocity, of every river
will begin to augment. Each of them will be less given to overflow its
alluvial plain; and their power of carrying earthy matter seaward, and
of scouring out and deepening their channels, will be sustained till,
after a lapse of many thousand years, each of them has eroded a new
channel or valley through a fluviatile formation of comparatively
modern date. The surface of what was once the river-plain at the period
of greatest depression, will then remain fringing the valley-sides in
the form of a terrace apparently flat, but in reality sloping down with
the general inclination of the river. Everywhere this terrace will
present cliffs of gravel and sand, facing the river. That such a series
of movements has actually taken place in the main valley of the
Mississippi and in its tributary valleys during oscillations of level,
I have endeavoured to show in my description of that country;[2] and
the fresh-water shells of existing species and bones of land
quadrupeds, partly of extinct races, preserved in the terraces of
fluviatile origin, attest the exclusion of the sea during the whole
process of filling up and partial re-excavation.

Littoral Denudation.—Part of the action of the waves between high and
low watermark must be included in subaërial denudation, more especially
as the undermining of cliffs by the waves is facilitated by
land-springs, and these often lead to the sliding down of great masses
of land into the sea. Along our coasts we find numerous submerged
forests, only visible at low water, having the trunks of the trees
erect and their roots attached to them and still spreading through the
ancient soil as when they were living. They occur in too many places,
and sometimes at too great a depth, to be explained by a mere change in
the level of the tides, although as the coasts waste away and alter in
shape, the height to which the tides rise and fall is always varying,
and the level of high tide at any given point may, in the course of
many ages, differ by several feet or even fathoms. It is this
fluctuation in the height of the tides, and the erosion and destruction
of the sea-coast by the waves, that makes it exceedingly difficult for
us in a few centuries, or even perhaps in a few thousand years, to
determine whether there is a change by subterranean movement in the
relative level of sea and land.

We often behold, as on the coasts of Devonshire and Pembrokeshire,
facts which appear to lead to opposite conclusions. In one place a
raised beach with marine littoral shells, and in another immediately
adjoining a submerged forest. These phenomena indicate oscillations of
level, and as the movements are very gradual, they must give repeated
opportunities to the breakers to denude the land which is thus again
and again exposed to their fury, although it is evident that the
submergence is sometimes effected in such a manner as to allow the
trees which border the coast not to be carried away.

Inland Sea-cliffs.—In countries where hard limestone rocks abound,
inland cliffs have often retained faithfully for ages the characters
which they acquired when they constituted the boundary of land and sea.
Thus, in the Morea, no less than three or even four ranges of cliffs
are well-preserved, rising one above the other at different distances
from the actual shore, the summit of the highest and oldest
occasionally attaining 1000 feet in elevation. A consolidated beach
with marine shells is usually found at the base of each cliff, and a
line of littoral caverns. These ranges of cliff probably imply pauses
in the process of upheaval when the waves and currents had time to
undermine and clear away considerable masses of rock.

But the beginner should be warned not to expect to find evidence of the
former sojourn of the sea on all those lands which we are nevertheless
sure have been submerged at periods comparatively modern; for
notwithstanding the enduring nature of the marks left by littoral
action on some rocks, especially limestones, we can by no means detect
sea-beaches and inland cliffs everywhere. On the contrary, they are,
upon the whole, extremely partial, and are often entirely wanting in
districts composed of argillaceous and sandy formations, which must,
nevertheless, have been upheaved at the same time, and by the same
intermittent movements, as the adjoining harder rocks.

Escarpments.—Besides the inland cliffs above alluded to which mark the
ancient limits of the sea, there are other abrupt terminations of rocks
of various kinds which resemble sea-cliffs, but which have in reality
been due to subaërial denudation. These have been called “escarpments,”
a term which it is useful to confine to the outcrop of particular
formations having a scarped outline, as distinct from cliffs due to
marine action.

I formerly supposed that the steep line of cliff-like slopes seen along
the outcrop of the chalk, when we follow the edge of the North or South
Downs, was due to marine action; but Professor Ramsay has shown[3] that
the present outline of the physical geography is more in favour of the
idea of the escarpments having been due to gradual waste since the
rocks were exposed in the atmosphere to the action of rain and rivers.

Mr. Whittaker has given a good summary of the grounds for ascribing
these apparent sea-cliffs to waste in the open air. 1. There is an
absence of all signs of ancient sea-beaches or littoral deposits at the
base of the escarpment. 2. Great inequality is observed in the level of
the base line. 3. The escarpments do not intersect, like sea-cliffs, a
series of distinct rocks, but are always confined to the boundary-line
of the same formation. 4. There are sometimes different contiguous and
parallel escarpments—those, for example, of the greensand and
chalk—which are so near each other, and occasionally so similar in
altitude, that we cannot imagine any existing archipelago if converted
into dry land to present a like outline.

The above theory is by no means inconsistent with the opinion that the
limits of the outcrop of the chalk and greensand which the escarpments
now follow, were originally determined by marine denudation. When the
south-east of England last emerged from beneath the level of the sea,
it was acted upon, no doubt, by the tide, waves, and currents, and the
chalk would form from the first a mass projecting above the more
destructible clay called Gault. Still the present escarpments so much
resembling sea-cliffs have no doubt, for reasons above stated, derived
their most characteristic features subsequently to emergence from
subaërial waste by rain and rivers.

Submarine Denudation.—When we attempt to estimate the amount of
submarine denudation, we become sensible of the disadvantage under
which we labour from our habitual incapacity of observing the action of
marine currents on the bed of the sea. We know that the agitation of
the waves, even during storms, diminishes at a rapid rate, so as to
become very insignificant at the depth of a few fathoms, and is quite
imperceptible at the depth of about sixteen fathoms; but when large
bodies of water are transferred by a current from one part of the ocean
to another, they are known to maintain at great depths such a velocity
as must enable them to remove the finer, and sometimes even the
coarser, materials of the rocks over which they flow. As the
Mississippi when more than 150 feet deep can keep open its channel and
even carry down gravel and sand to its delta, the surface velocity
being not more than two or three miles an hour, so a gigantic current,
like the Gulf Stream, equal in volume to many hundred Mississippis, and
having in parts a surface velocity of more than three miles, may act as
a propelling and abrading power at still greater depths. But the
efficacy of the sea as a denuding agent, geologically considered, is
not dependent on the power of currents to preserve at great depths a
velocity sufficient to remove sand and mud, because, even where the
deposition or removal of sediment is not in progress, the depth of
water does not remain constant throughout geological time. Every page
of the geological record proves to us that the relative levels of land
and sea, and the position of the ocean and of continents and islands,
has been always varying, and we may feel sure that some portions of the
submarine area are now rising and others sinking. The force of tidal
and other currents and of the waves during storms is sufficient to
prevent the emergence of many lands, even though they may be undergoing
continual upheaval. It is not an uncommon error to imagine that the
waste of sea-cliffs affords the measure of the amount of marine
denudation of which it probably constitutes an insignificant portion.

Dogger-bank.—That great shoal called the Dogger-bank, about sixty miles
east of the coast of Northumberland, and occupying an area about as
large as Wales, has nowhere a depth of more than ninety feet, and in
its shallower parts is less than forty feet under water. It might
contribute towards the safety of the navigation of our seas to form an
artificial island, and to erect a light-house on this bank; but no
engineer would be rash enough to attempt it, as he would feel sure that
the ocean in the first heavy gale would sweep it away as readily as it
does every temporary shoal that accumulates from time to time around a
sunk vessel on the same bank.[4]

No observed geographical changes in historical times entitle us to
assume that where upheaval may be in progress it proceeds at a rapid
rate. Three or four feet rather than as many yards in a century may
probably be as much as we can reckon upon in our speculations; and if
such be the case, the continuance of the upward movement might easily
be counteracted by the denuding force of such currents aided by such
waves as, during a gale, are known to prevail in the German Ocean. What
parts of the bed of the ocean are stationary at present, and what areas
may be rising or sinking, is a matter of which we are very ignorant, as
the taking of accurate soundings is but of recent date.

_Newfoundland Bank._—The great bank of Newfoundland may be compared in
size to the whole of England. This part of the bottom of the Atlantic
is surrounded on three sides by a rapidly deepening ocean, the bank
itself being from twenty to fifty fathoms (or from 120 to 300 feet)
under water. We are unable to determine by the comparison of different
charts made at distant periods, whether it is undergoing any change of
level, but if it be gradually rising we cannot anticipate on that
account that it will become land, because the breakers in an open sea
would exercise a prodigious force even on solid rock brought up to
within a few yards of the surface. We know, for example, that when a
new volcanic island rose in the Mediterranean in 1831, the waves were
capable in a few years of reducing it to a sunken rock.

In the same way currents which flow over the Newfoundland bank a great
part of the year at the rate of two miles an hour, and are known to
retain a considerable velocity to near the bottom, may carry away all
loose sand and mud, and make the emergence of the shoal impossible, in
spite of the accessions of mud, sand, and boulders derived occasionally
from melting icebergs which, coming from the northern glaciers, are
frequently stranded on various parts of the bank. They must often leave
at the bottom large erratic blocks which the marine currents may be
incapable of moving, but the same rocky fragments may be made to sink
by the undermining of beds consisting of finer matter on which the
blocks and gravel repose. In this way gravel and boulders may continue
to overspread a submarine bottom after the latter has been lowered for
hundreds of feet, the surface never having been able to emerge and
become land. It is by no means improbable that the annual removal of an
average thickness of half an inch of rock might counteract the ordinary
upheaval which large submarine areas are undergoing; and the real
enigma which the geologist has to solve is not the extensive denudation
of the white chalk or of our tertiary sands and clays, but the fact
that such incoherent materials have ever succeeded in lifting up their
heads above water in an open sea. Why were they not swept away during
storms into some adjoining abysses, the highest parts of each shoal
being always planed off down to the depth of a few fathoms? The
hardness and toughness of some rocks already exposed to windward and
acting as breakwaters may perhaps have assisted; nor must we forget the
protection afforded by a dense and unbroken covering of barnacles,
limpets, and other creatures which flourish most between high and low
water and shelter some newly risen coasts from the waves.

 [1] Principles of Geology 7th ed., p. 506; 10th ed., vol. ii, p. 196.

 [2] Second Visit to the United States, vol. i, chap. xxxiv.

 [3] Physical Geography and Geology of Great Britain, p. 78, 1864.

 [4] Principles, 10th ed., vol. i, p. 569.




CHAPTER VII.
JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE
EARTH’S CRUST.


How we obtain an Insight at the Surface, of the Arrangement of Rocks at
great Depths. — Why the Height of the successive Strata in a given
Region is so disproportionate to their Thickness. — Computation of the
average annual Amount of subaërial Denudation. — Antagonism of Volcanic
Force to the Levelling Power of running Water. — How far the Transfer
of Sediment from the Land to a neighbouring Sea-bottom may affect
Subterranean Movements. — Permanence of Continental and Oceanic Areas.

How we obtain an Insight at the Surface, of the Arrangement of Rocks at
Great Depths.— The reader has been already informed that, in the
structure of the earth’s crust, we often find proofs of the direct
superposition of marine to fresh-water strata, and also evidence of the
alternation of deep-sea and shallow-water formations. In order to
explain how such a series of rocks could be made to form our present
continents and islands, we have not only to assume that there have been
alternate upward and downward movements of great vertical extent, but
that the upheaval in the areas which we at present inhabit has, in
later geological times, sufficiently predominated over subsidence to
cause these portions of the earth’s crust to be land instead of sea.
The sinking down of a delta beneath the sea-level may cause strata of
fluviatile or even terrestrial origin, such as peat with trees proper
to marshes, to be covered by deposits of deep-sea origin. There is also
no end to the thickness of mud and sand which may accumulate in shallow
water, provided that fresh sediment is brought down from the wasting
land at a rate corresponding to that of the sinking of the bed of the
sea. The latter, again, may sometimes sink so fast that the earthy
matter, being intercepted in some new landward depression, may never
reach its former resting-place, where, the water becoming clear may
favour the growth of shells and corals, and calcareous rocks of organic
origin may thus be superimposed on mechanical deposits.

The succession of strata here alluded to would be consistent with the
occurrence of gradual downward and upward movements of the land and bed
of the sea without any disturbance of the horizontality of the several
formations. But the arrangement of rocks composing the earth’s crust
differs materially from that which would result from a mere series of
vertical movements. Had the volcanic forces been confined to such
movements, and had the stratified rocks been first formed beneath the
sea and then raised above it, without any lateral compression, the
geologist would never have obtained an insight into the monuments of
various ages, some of extremely remote antiquity.

What we have said in Chapter V of dip and strike, of the folding and
inversion of strata, of anticlinal and synclinal flexures, and in
Chapter VI of denudation at different periods, whether subaërial or
submarine, must be understood before the student can comprehend what
may at first seem to him an anomaly, but which it is his business
particularly to understand. I allude to the small height above the
level of the sea attained by strata often many miles in thickness, and
about the chronological succession of which, in one and the same
region, there is no doubt whatever. Had stratified rocks in general
remained horizontal, the waves of the sea would have been enabled
during oscillations of level to plane off entirely the uppermost beds
as they rose or sank during the emergence or submergence of the land.
But the occurrence of a series of formations of widely different ages,
all remaining horizontal and in conformable stratification, is
exceptional, and for this reason the total annihilation of the
uppermost strata has rarely taken place. We owe, indeed, to the side
way movements of _lateral compression_ those anticlinal and synclinal
curves of the beds already described (Fig. 55), which, together with
denudation, subaërial and submarine, enable us to investigate the
structure of the earth’s crust many miles below those points which the
miner can reach. I have already shown in Fig. 56, how, at St. Abb’s
Head, a series of strata of indefinite thickness may become vertical,
and then denuded, so that the edges of the beds alone shall be exposed
to view, the altitude of the upheaved ridges being reduced to a
moderate height above the sea-level; and it may be observed that
although the incumbent strata of Old Red Sandstone are in that place
nearly horizontal, yet these same newer beds will in other places be
found so folded as to present vertical strata, the edges of which are
abruptly cut off, as in 2, 3, 4 on the right-hand side of the diagram,
Fig. 55.

Why the Height of the Successive Strata in a given Region is so
Disproportionate to their Thickness.—We cannot too distinctly bear in
mind how dependent we are on the joint action of the volcanic and
aqueous forces, the one in
disturbing the original position of rocks, and the other in destroying
large portions of them, for our power of consulting the different pages
and volumes of those stony records of which the crust of the globe is
composed. Why, it may be asked, if the ancient bed of the sea has been
in many regions uplifted to the height of two or three miles, and
sometimes twice that altitude, and if it can be proved that some single
formations are of themselves two or three miles thick, do we so often
find several important groups resting one upon the other, yet attaining
only the height of a few hundred feet above the level of the sea?

The American geologists, after carefully studying the Allegheny or
Appalachian mountains, have ascertained that the older fossiliferous
rocks of that chain (from the Silurian to the Carboniferous inclusive)
are not less than 42,000 feet thick, and if they were now superimposed
on each other in the order in which they were thrown down, they ought
to equal in height the Himalayas with the Alps piled upon them. Yet
they rarely reach an altitude of 5000 feet, and their loftiest peaks
are no more than 7000 feet high. The Carboniferous strata forming the
highest member of the series, and containing beds of coal, can be shown
to be of shallow-water origin, or even sometimes to have originated in
swamps in the open air. But what is more surprising, the lowest part of
this great Palæozoic series, instead of having been thrown down at the
bottom of an abyss more than 40,000 feet deep, consists of sediment
(the Potsdam sandstone), evidently spread out on the bottom of a
shallow sea, on which ripple-marked sands were occasionally formed.
This vast thickness of 40,000 feet is not obtained by adding together
the maximum density attained by each formation in distant parts of the
chain, but by measuring the successive groups as they are exposed in a
very limited area, and where the denuded edges of the vertical strata
forming the parallel folds alluded to at page 87 “crop out” at the
surface. Our attention has been called by Mr. James Hall,
Palæontologist of New York, to the fact that these Palæozoic rocks of
the Appalachian chain, which are of such enormous density, where they
are almost entirely of mechanical origin, thin out gradually as they
are traced to the westward, where evidently the contemporaneous seas
allowed organic rocks to be formed by corals, echinoderms, and
encrinites in clearer water, and where, although the same successive
periods are represented, the total mass of strata from the Silurian to
the Carboniferous, instead of being 40,000 is only 4000 feet thick.

A like phenomenon is exhibited in every mountainous country, as, for
example, in the European Alps; but we need not go farther than the
north of England for its illustration. Thus in Lancashire and central
England the thickness of the Carboniferous formation, including the
Millstone Grit and Yoredale beds, is computed to be more than 18,000
feet; to this we may add the Mountain Limestone, at least 2000 feet in
thickness, and the overlying Permian and Triassic formations, 3000 or
4000 feet thick. How then does it happen that the loftiest hills of
Yorkshire and Lancashire, instead of being 24,000 feet high, never rise
above 3000 feet? For here, as before pointed out in the Alleghenies,
all the great thicknesses are sometimes found in close approximation
and in a region only a few miles in diameter. It is true that these
same sets of strata do not preserve their full force when followed for
indefinite distances. Thus the 18,000 feet of Carboniferous grits and
shales in Lancashire, before alluded to, gradually thin out, as Mr.
Hull has shown, as they extend southward, by attenuation or original
deficiency of sediment, and not in consequence of subsequent
denudation, so that when we have followed them for about 100 miles into
Leicestershire, they have dwindled away to a thickness of only 3000
feet. In the same region the Carboniferous limestone attains so unusual
a thickness—namely, more than 4000 feet—as to appear to compensate in
some measure for the deficiency of contemporaneous sedimentary rock.[1]

It is admitted that when two formations are unconformable their fossil
remains almost always differ considerably. The break in the continuity
of the organic forms seems connected with a great lapse of time, and
the same interval has allowed extensive disturbance of the strata, and
removal of parts of them by denudation, to take place. The more we
extend our investigations the more numerous do the proofs of these
breaks become, and they extend to the most ancient rocks yet
discovered. The oldest examples yet brought to light in the British
Isles are on the borders of Rosshire and Sutherlandshire, and have been
well described by Sir Roderick Murchison, by whom their chronological
relations were admirably worked out, and proved to be very different
from those which previous observers had imagined them to be. I had an
opportunity in the autumn of 1869 of verifying the splendid section
given in Fig. 82 by climbing in a few hours from the banks of Loch
Assynt to the summit of the mountain called Queenaig, 2673 feet high.

The formations 1, 2, 3, the Laurentian, Cambrian, and
Silurian, to be explained in Chapters XXV and XXVI, not only occur in
succession in this one mountain, but their unconformable junctions are
distinctly exposed to view.

Fig. 82: Unconformable Palæozoic stata, Sutherlandshire (Murchison).

To begin with the oldest set of rocks, No. 1; they consist chiefly of
hornblendic gneiss, and in the neighbouring Hebrides form whole
islands, attaining a thickness of thousands of feet, although they have
suffered such contortions and denudation that they seldom rise more
than a few hundred feet above the sea-level. In discordant
stratification upon the edges of this gneiss reposes No. 2, a group of
conglomerate and purple sandstone referable to the Cambrian (or
Longmynd) formation, which can elsewhere be shown to be characterised
by its peculiar organic remains. On this again rests No. 3, a lower
member of the important group called Silurian, an outlier of which, 3′,
caps the summit of Queenaig, attesting the removal by denudation of
rocks of the same age, which once extended from the great mass 3 to 3′.
Although this rock now consists of solid quartz, it is clear that in
its original state it was formed of fine sand, perforated by numerous
lob-worms or annelids, which left their burrows in the shape of tubular
hollows Fig. 563 of _Arenicolites_), hundreds, nay thousands, of which
I saw as I ascended the mountain.

Fig. 83: Diagrammatic section of the same groups near Queenaig
(Murchison).

In Queenaig we only behold this single quartzose member of the Silurian
series, but in the neighbouring country (see Fig. 83) it is seen to the
eastward to be followed by limestones, 3_a_, and schists, 3_b_,
presenting numerous folds, and becoming more and more metamorphic and
crystalline, until at length, although very different in age and
strike, they much resemble in appearance the group No. 1. It is very
seldom that in the same country one continuous formation, such as the
Silurian, is, as in this case, more fossiliferous and less altered by
volcanic heat in its older than in its newer strata, and still more
rare to find an underlying and unconformable group like the Cambrian
retaining its original condition of a conglomerate and sandstone more
perfectly than the overlying formation. Here also we may remark in
regard to the origin of these Cambrian rocks that they were evidently
produced at the expense of the underlying Laurentian, for the rounded
pebbles occurring in them are identical in composition and texture with
that crystalline gneiss which constitutes the contorted beds of the
inferior formation No. 1. When the reader has studied the chapter on
metamorphism, and has become aware how much modification by heat,
pressure, and chemical action is required before the conversion of
sedimentary into crystalline strata can be brought about, he will
appreciate the insight which we thus gain into the date of the changes
which had already been effected in the Laurentian rocks long before the
Cambrian pebbles of quartz and gneiss were derived from them. The
Laurentian is estimated by Sir William Logan to amount in Canada to
30,000 feet in thickness. As to the Cambrian, it is supposed by Sir
Roderick Murchison that the fragment left in Sutherlandshire is about
3500 feet thick, and in Wales and the borders of Shropshire this
formation may equal 10,000 feet, while the Silurian strata No. 3,
difficult as it may be to measure them in their various foldings to the
eastward, where they have been invaded by intrusive masses of granite,
are supposed many times to surpass the Cambrian in volume and density.

But although we are dealing here with stratified rocks, each of which
would be several miles in thickness, if they were fully represented,
the whole of them do not attain the elevation of a single mile above
the level of the sea.

Computation of the Average Annual Amount of Subaërial Denudation.—The
geology of the district above alluded to may assist our imagination in
conceiving the extent to which groups of ancient rocks, each of which
may in their turn have formed continents and oceanic basins, have been
disturbed, folded, and denuded even in the course of a few out of many
of those geological periods to which our imperfect records relate. It
is not easy for us to overestimate the effects which causes in every
day action must produce when the multiplying power of time is taken
into account.

Attempts were made by Manfredi in 1736, and afterwards by Playfair in
1802, to calculate the time which it would require to enable the rivers
to deliver over the whole of the land into the basin of the ocean. The
data were at first too imperfect and vague to allow them even to
approximate to safe conclusions. But in our own time similar
investigations have been renewed with more prospect of success, the
amount brought down by many large rivers to the sea having been more
accurately ascertained. Mr. Alfred Tylor, in 1850, inferred that the
quantity of detritus now being distributed over the sea-bottom would,
at the end of 10,000 years, cause an elevation of the sea-level to the
extent of at least three inches.[2] Subsequently Mr. Croll, in 1867,
and again, with more exactness, in 1868, deduced from the latest
measurement of the sediment transported by European and American rivers
the rate of subaërial denudation to which the surface of large
continents is exposed, taking especially the hydrographical basin of
the Mississippi as affording the best available measure of the average
waste of the land. The conclusion arrived at in his able memoir,[3] was
that the whole terrestrial surface is denuded at the rate of one foot
in 6000 years and this opinion was simultaneously enforced by his
fellow-labourer, Mr. Geikie, who, being jointly engaged in the same
line of inquiry, published a luminous essay on the subject in 1868.

The student, by referring to my “Principles of Geology,”[4] may see
that Messrs. Humphrey and Abbot, during their survey of the
Mississippi, attempted to make accurate measurements of the proportion
of sediment carried down annually to the sea by that river, including
not only the mud held in suspension, but also the sand and gravel
forced along the bottom.

It is evident that when we know the dimensions of the area which is
drained, and the annual quantity of earthy matter taken from it and
borne into the sea, we can affirm how much on an average has been
removed from the general surface in one year, and there seems no danger
of our overrating the mean rate of waste by selecting the Mississippi
as our example, for that river drains a country equal to more than half
the continent of Europe, extends through twenty degrees of latitude,
and therefore through regions enjoying a great variety of climate, and
some of its tributaries descend from mountains of great height. The
Mississippi is also more likely to afford us a fair test of ordinary
denudation, because, unlike the St. Lawrence and its tributaries, there
are no great lakes in which the fluviatile sediment is thrown down and
arrested in its way to the sea. In striking a general average we have
to remember that there are large deserts in which there is scarcely any
rainfall, and tracts which are as rainless as parts of Peru, and these
must not be neglected as counterbalancing others, in the tropics, where
the quantity of rain is in excess. If then, argues Mr. Geikie, we
assume that the Mississippi is lowering the surface of the great basin
which it drains at the rate of one foot in 6000 years, 10 feet in
60,000 years, 100 feet in 600,000 years, and 1000 feet in 6,000,000
years, it would not require more than about 4,500,000 years to wear
away the whole of the North American continent if its mean height is
correctly estimated by Humboldt at 748 feet. And if the mean height of
all the land now above the sea throughout the globe is 1000 feet, as
some geographers believe, it would only require six million years to
subject a mass of rock equal in volume to the whole of the land to the
action of subaërial denudation. It may be objected that the annual
waste is partial, and not equally derived from the general surface of
the country, inasmuch as plains, water-sheds, and level ground at all
heights remain comparatively unaltered; but this, as Mr. Geikie has
well pointed out, does not affect our estimate of the sum total of
denudation. The amount remains the same, and if we allow too little for
the loss from the surface of table-lands we only increase the
proportion of the loss sustained by the sides and bottoms of the
valleys, and _vice versa._[5]

Antagonism of Volcanic Force to the Levelling Power of Running
Water.—In all these estimates it is assumed that the entire quantity of
land above the sea-level remains on an average undiminished in spite of
annual waste. Were it otherwise the subaërial denudation would be
continually lessened by the diminution of the height and dimensions of
the land exposed to waste. Unfortunately we have as yet no accurate
data enabling us to measure the action of that force by which the
inequalities of the surface of the earth’s crust may be restored, and
the height of the continents and depth of the seas made to continue
unimpaired. I stated in 1830 in the “Principles of Geology,”[6] that
running water and volcanic action are two antagonistic forces; the one
labouring continually to reduce the whole of the land to the level of
the sea, the other to restore and maintain the inequalities of the
crust on which the very existence of islands and continents depends. I
stated, however, that when we endeavour to form some idea of the
relation of these destroying and renovating forces, we must always bear
in mind that it is not simply by upheaval that subterranean movements
can counteract the levelling force of running water. For whereas the
transportation of sediment from the land to the ocean would raise the
general sea-level, the subsidence of the sea-bottom, by increasing its
capacity, would check this rise and prevent the submergence of the
land. I have, indeed, endeavoured to show that unless we assume that
there is, on the whole, more subsidence than upheaval, we must suppose
the diameter of the planet to be always increasing, by that quantity of
volcanic matter which is annually poured out in the shape of lava or
ashes, whether on the land or in the bed of the sea, and which is
derived from the interior of the earth. The abstraction of this matter
causes, no doubt, subterranean vacuities and a corresponding giving way
of the surface; if it were not so, the average density of parts of the
interior would be always lessening and the size of the planet
increasing.[7]

Our inability to estimate the amount or direction of the movements due
to volcanic power by no means renders its efficacy as a land-preserving
force in past times a mere matter of conjecture. The student will see
in Chapter XXIV that we have proofs of Carboniferous forests hundreds
of miles in extent which grew on the lowlands or deltas near the sea,
and which subsided and gave place to other forests, until in some
regions fluviatile and shallow-water strata with occasional seams of
coal were piled one over the other, till they attained a thickness of
many thousand feet. Such accumulations, observed in Great Britain and
America on opposite sides of the Atlantic, imply the long-continued
existence of land vegetation, and of rivers draining a former continent
placed where there is now deep sea.

It will be also seen in Chapter XXV that we have evidence of a rich
terrestrial flora, the Devonian, even more ancient than the
Carboniferous; while on the other hand, the later Triassic, Oolitic,
Cretaceous, and successive Tertiary periods have all supplied us with
fossil plants, insects, or terrestrial mammalia; showing that, in spite
of great oscillations of level and continued changes in the position of
land and sea, the volcanic forces have maintained a due proportion of
dry land. We may appeal also to fresh-water formations, such as the
Purbeck and Wealden, to prove that in the Oolitic and Neocomian eras
there were rivers draining ancient lands in Europe in times when we
know that other spaces, now above water, were submerged.

How far the Transfer of Sediment from the Land to a Neighbouring
Sea-bottom may affect Subterranean Movements.—Little as we understand
at present the laws which govern the distribution of volcanic heat in
the interior and crust of the globe, by which mountain chains, high
table-lands, and the abysses of the ocean are formed, it seems clear
that this heat is the prime mover on which all the grander features in
the external configuration of the planet depend.

It has been suggested that the stripping off by denudation of dense
masses from one part of a continent and the delivery of the same into
the bed of the ocean must have a decided effect in causing changes of
temperature in the earth’s crust below, or, in other words, in causing
the subterranean isothermals to shift their position. If this be so,
one part of the crust may be made to rise, and another to sink, by the
expansion and contraction of the rocks, of which the temperature is
altered.

I cannot, at present, discuss this subject, of which I have treated
more fully elsewhere,[8] but may state here that I believe this
transfer of sediment to play a very subordinate part in modifying those
movements on which the configuration of the earth’s crust depends. In
order that strata of shallow-water origin should be able to attain a
thickness of several thousand feet, and so come to exert a considerable
downward pressure, there must have been first some independent and
antecedent causes at work which have given rise to the incipient
shallow receptacle in which the sediment began to accumulate. The same
causes there continuing to depress the sea-bottom, room would be made
for fresh accessions of sediment, and it would only be by a long
repetition of the depositing process that the new matter could acquire
weight enough to affect the temperature of the rocks far below, so as
to increase or diminish their volume.

Permanence of Continental and Oceanic Areas.—If the thickness of more
than 40,000 feet of sedimentary strata before alluded to in the
Appalachians proves a preponderance of downward movements in Palæozoic
times in a district now forming the eastern border of North America, it
also proves, as before hinted, the continued existence and waste of
some neighbouring continent, probably formed of Laurentian rocks, and
situated where the Atlantic now prevails. Such an hypothesis would be
in perfect harmony with the conclusions forced upon us by the study of
the present configuration of our continents, and the relation of their
height to the depth of the oceanic basins; also to the considerable
elevation and extent sometimes reached by drift containing shells of
recent species, and still more by the fact of sedimentary strata,
several thousand feet thick, as those of central Sicily, or such as
flank the Alps and Apennines, containing fossil Mollusca sometimes
almost wholly identical with species still living.

I have remarked elsewhere[9] that upward and downward movements of 1000
feet or more would turn much land into sea and sea into land in the
continental areas and their borders, whereas oscillations of equal
magnitude would have no corresponding effect in the bed of the ocean
generally, believed as it is to have a mean depth of 15,000 feet, and
which, whether this estimate be correct or not, is certainly of great
profundity. Subaërial denudation would not of itself lessen the area of
the land, but would tend to fill up with sediment seas of moderate
depth adjoining the coast. The coarser matter falls to the bottom near
the shore in the first still water which it reaches, and whenever the
sea-bottom on which this matter has been thrown is slightly elevated,
it becomes land, and an upheaval of a thousand feet causes it to attain
the mean elevation of continents in general.

Suppose, therefore, we had ascertained that the triturating power of
subaërial denudation might in a given time—in three, or six, or a
greater number of millions of years—pulverise a volume of rock equal in
dimensions to all the present land, we might yet find, could we revisit
the earth at the end of such a period, that the continents occupied
very much the same position which they held before; we should find the
rivers employed in carrying down to the sea the very same mud, sand,
and pebbles with which they had been charged in our own time, the
superficial alluvial matter as well as a great thickness of sedimentary
strata would inclose shells, all or a great part of which we should
recognise as specifically identical with those already known to us as
living. Every geologist is aware that great as have been the
geographical changes in the northern hemisphere since the commencement
of the Glacial Period, there having been submergence and re-emergence
of land to the extent of 1000 feet vertically, and in the temperate
latitudes great vicissitudes of climate, the marine mollusca have not
changed, and the same drift which had been carried down to the sea at
the beginning of the period is now undergoing a second transportation
in the same direction.

As when we have measured a fraction of time in an hour-glass we have
only to reverse the position of our chronometer and we make the same
sand measure over again the duration of a second equal period, so when
the volcanic force has remoulded the form of a continent and the
adjoining sea-bottom, the same materials are made to do duty a second
time. It is true that at each oscillation of level the solid rocks
composing the original continent suffer some fresh denudation, and do
not remain unimpaired like the wooden and glass framework of the
hour-glass, still the wear and tear suffered by the larger area exposed
to subaërial denudation consists either of loose drift or of
sedimentary strata, which were thrown down in seas near the land, and
subsequently upraised, the same continents and oceanic basins remaining
in existence all the while.

From all that we know of the extreme slowness of the upward and
downward movements which bring about even slight geographical changes,
we may infer that it would require a long succession of geological
periods to cause the submarine and supramarine areas to change places,
even if the ascending movements in the one region and the descending in
the other were continuously in one direction. But we have only to
appeal to the structure of the Alps, where there are so many shallow
and deep water formations of various ages crowded into a limited area,
to convince ourselves that mountain chains are the result of great
oscillations of level. High land is not produced simply by uniform
upheaval, but by a predominance of elevatory over subsiding movements.
Where the ocean is extremely deep it is because the sinking of the
bottom has been in excess, in spite of interruptions by upheaval.

Yet persistent as may be the leading features of land and sea on the
globe, they are not immutable. Some of the finest mud is doubtless
carried to indefinite distances from the coast by marine currents, and
we are taught by deep-sea dredgings that in clear water at depths
equalling the height of the Alps organic beings may flourish, and their
spoils slowly accumulate on the bottom. We also occasionally obtain
evidence that submarine volcanoes are pouring out ashes and streams of
lava in mid-ocean as well as on land (see Principles, vol. ii, p. 64),
and that wherever mountains like Etna, Vesuvius, and the Canary Islands
are now the site of eruptions, there are signs of accompanying
upheaval, by which beds of ashes full of recent marine shells have been
uplifted many hundred feet. We need not be surprised, therefore, if we
learn from geology that the continents and oceans were not always
placed where they now are, although the imagination may well be
overpowered when it endeavours to contemplate the quantity of time
required for such revolutions.

We shall have gained a great step if we can approximate to the number
of millions of years in which the average aqueous denudation going on
upon the land would convey seaward a quantity of matter equal to the
average volume of our continents, and this might give us a gauge of the
minimum of volcanic force necessary to counteract such levelling power
of running water; but to discover a relation between these great
agencies and the rate at which species of organic beings vary, is at
present wholly beyond the reach of our computation, though perhaps it
may not prove eventually to transcend the powers of man.

 [1] Hull, Quart. Geol. Journ., vol. xxiv, p. 322, 1868.

 [2] Tylor, Phil. Mag., 4th series, p. 268, 1850.

 [3] Croll, Phil. Mag., 1868, p. 381.

 [4] Vol. i, p. 442, 1867.

 [5] Trans. Geol. Soc. Glasgow, vol. iii, p. 169.

 [6] 1st ed., chap. x, p. 167, 1830; see also 10th ed., vol. i, chap.
 xv, p. 327, 1867.

 [7] Principles, vol. ii, p. 237; also 1st ed., p. 447, 1830.

 [8] Principles, vol. ii, p. 229, 1868.

 [9] Principles, vol. i, p. 265, 1867.




CHAPTER VIII.
CHRONOLOGICAL CLASSIFICATION OF ROCKS.


Aqueous, Plutonic, volcanic, and metamorphic Rocks considered
chronologically. — Terms Primary, Secondary, and Tertiary; Palæozoic,
Mesozoic, and Cainozoic explained. — On the different Ages of the
aqueous Rocks. — Three principal Tests of relative Age: Superposition,
Mineral Character, and Fossils. — Change of Mineral Character and
Fossils in the same continuous Formation. — Proofs that distinct
Species of Animals and Plants have lived at successive Periods. —
Distinct Provinces of indigenous Species. — Great Extent of single
Provinces. — Similar Laws prevailed at successive Geological Periods. —
Relative Importance of mineral and palæontological Characters. — Test
of Age by included Fragments. — Frequent Absence of Strata of
intervening Periods. — Tabular Views of fossiliferous Strata.

Chronology of Rocks.— In the first chapter it was stated that the four
great classes of rocks, the aqueous, the volcanic, the Plutonic, and
the metamorphic, would each be considered not only in reference to
their mineral characters, and mode of origin, but also to their
relative age. In regard to the aqueous rocks, we have already seen that
they are stratified, that some are calcareous, others argillaceous or
siliceous, some made up of sand, others of pebbles; that some contain
fresh-water, others marine fossils, and so forth; but the student has
still to learn which rocks, exhibiting some or all of these characters,
have originated at one period of the earth’s history, and which at
another.

To determine this point in reference to the fossiliferous formations is
more easy than in any other class, and it is therefore the most
convenient and natural method to begin by establishing a chronology for
these strata, and then to refer as far as possible to the same
divisions, the several groups of Plutonic, volcanic, and metamorphic
rocks. Such a system of classification is not only recommended by its
greater clearness and facility of application, but is also best fitted
to strike the imagination by bringing into one view the contemporaneous
revolutions of the inorganic and organic creations of former times. For
the sedimentary formations are most readily distinguished by the
different species of fossil animals and plants which they inclose, and
of which one assemblage after another has flourished and then
disappeared from the earth in succession.

In the present work, therefore, the four great classes of rocks, the
aqueous, Plutonic, volcanic, and metamorphic, will form four parallel,
or nearly parallel, columns in one chronological table. They will be
considered as four sets of monuments relating to four contemporaneous,
or nearly contemporaneous, series of events. I shall endeavour, in a
subsequent chapter on the Plutonic rocks, to explain the manner in
which certain masses belonging to each of the four classes of rocks may
have originated simultaneously at every geological period, and how the
earth’s crust may have been continually remodelled, above and below, by
aqueous and igneous causes, from times indefinitely remote. In the same
manner as aqueous and fossiliferous strata are now formed in certain
seas or lakes, while in other places volcanic rocks break out at the
surface, and are connected with reservoirs of melted matter at vast
depths in the bowels of the earth, so, at every era of the past,
fossiliferous deposits and superficial igneous rocks were in progress
contemporaneously with others of subterranean and Plutonic origin, and
some sedimentary strata were exposed to heat, and made to assume a
crystalline or metamorphic structure.

It can by no means be taken for granted, that during all these changes
the solid crust of the earth has been increasing in thickness. It has
been shown, that so far as aqueous action is concerned, the gain by
fresh deposits, and the loss by denudation, must at each period have
been equal (see above, Chap. VI, p. 96); and in like manner, in the
inferior portion of the earth’s crust, the acquisition of new
crystalline rocks, at each successive era, may merely have
counterbalanced the loss sustained by the melting of materials
previously consolidated. As to the relative antiquity of the
crystalline foundations of the earth’s crust, when compared to the
fossiliferous and volcanic rocks which they support, I have already
stated, in the first chapter, that to pronounce an opinion on this
matter is as difficult as at once to decide which of the two, whether
the foundations or superstructure of an ancient city built on wooden
piles may be the oldest. We have seen that, to answer this question, we
must first be prepared to say whether the work of decay and restoration
had gone on most rapidly above or below; whether the average duration
of the piles has exceeded that of the buildings, or the contrary. So
also in regard to the relative age of the superior and inferior
portions of the earth’s crust; we cannot hazard even a conjecture on
this point, until we know whether, upon an average, the power of water
above, or that of heat below, is most efficacious in giving new forms
to solid matter.

The early geologists gave to all the crystalline and non-fossiliferous
rocks the name of Primitive or Primary, under the idea that they were
formed anterior to the appearance of life upon the earth, while the
aqueous or fossiliferous strata were termed Secondary, and alluviums or
other superficial deposits, Tertiary. The meaning of these terms, has,
however, been gradually modified with advancing knowledge, and they are
now used to designate three great chronological divisions under which
all geological formations can be classed, each of them being
characterised by the presence of distinctive groups of organic remains
rather than by any mechanical peculiarities of the strata themselves.
If, therefore, we retain the term “primary,” it must not be held to
designate a set of crystalline rocks some of which have been proved to
be even of Tertiary age, but must be applied to all rocks older than
the secondary formations. Some geologists, to avoid misapprehension,
have introduced the term Palæozoic for primary, from _palaion,_
“ancient,” and _zoon,_ “an organic being,” still retaining the terms
secondary and tertiary; Mr. Phillips, for the sake of uniformity, has
proposed Mesozoic, for secondary, from _mesos,_ “middle,” etc.; and
Cainozoic, for tertiary, from _kainos,_ “recent,” etc.; but the terms
primary, secondary, and tertiary have the claim of priority in their
favour, and are of corresponding value.

It may perhaps be suggested that some metamorphic strata, and some
granites, may be anterior in date to the oldest of the primary
fossiliferous rocks. This opinion is doubtless true, and will be
discussed in future chapters; but I may here observe, that when we
arrange the four classes of rocks in four parallel columns in one table
of chronology, it is by no means assumed that these columns are all of
equal length; one may begin at an earlier period than the rest, and
another may come down to a later point of time, and we may not be yet
acquainted with the most ancient of the primary fossiliferous beds, or
with the newest of the hypogene.

For reasons already stated, I proceed first to treat of the aqueous or
fossiliferous formations considered in chronological order or in
relation to the different periods at which they have been deposited.

There are three principal tests by which we determine the age of a
given set of strata; first, superposition; secondly, mineral character;
and, thirdly, organic remains. Some aid can occasionally be derived
from a fourth kind of proof, namely, the fact of one deposit including
in it fragments of a pre-existing rock, by which the relative ages of
the two may, even in the absence of all other evidence, be determined.

Superposition.—The first and principal test of the age of one aqueous
deposit, as compared to another, is relative position. It has been
already stated, that, where strata are horizontal, the bed which lies
uppermost is the newest of the whole, and that which lies at the bottom
the most ancient. So, of a series of sedimentary formations, they are
like volumes of history, in which each writer has recorded the annals
of his own times, and then laid down the book, with the last written
page uppermost, upon the volume in which the events of the era
immediately preceding were commemorated. In this manner a lofty pile of
chronicles is at length accumulated; and they are so arranged as to
indicate, by their position alone, the order in which the events
recorded in them have occurred.

In regard to the crust of the earth, however, there are some regions
where, as the student has already been informed, the beds have been
disturbed, and sometimes extensively thrown over and turned upside
down. (See p. 73, p. 87.) But an experienced geologist can rarely be
deceived by these exceptional cases. When he finds that the strata are
fractured, curved, inclined, or vertical, he knows that the original
order of superposition must be doubtful, and he then endeavours to find
sections in some neighbouring district where the strata are horizontal,
or only slightly inclined. Here, the true order of sequence of the
entire series of deposits being ascertained, a key is furnished for
settling the chronology of those strata where the displacement is
extreme.

Mineral Character.—The same rocks may often be observed to retain for
miles, or even hundreds of miles, the same mineral peculiarities, if we
follow the planes of stratification, or trace the beds, if they be
undisturbed, in a horizontal direction. But if we pursue them
vertically, or in any direction transverse to the planes of
stratification, this uniformity ceases almost immediately. In that case
we can scarcely ever penetrate a stratified mass for a few hundred
yards without beholding a succession of extremely dissimilar rocks,
some of fine, others of coarse grain, some of mechanical, others of
chemical origin; some calcareous, others argillaceous, and others
siliceous. These phenomena lead to the conclusion that rivers and
currents have dispersed the same sediment over wide areas at one
period, but at successive periods have been charged, in the same
region, with very different kinds of matter. The first observers were
so astonished at the vast spaces over which they were able to follow
the same homogeneous rocks in a horizontal direction, that they came
hastily to the opinion, that the whole globe had been environed by a
succession of distinct aqueous formations, disposed round the nucleus
of the planet, like the concentric coats of an onion. But, although, in
fact, some formations may be continuous over districts as large as half
of Europe, or even more, yet most of them either terminate wholly
within narrower limits, or soon change their lithological character.
Sometimes they thin out gradually, as if the supply of sediment had
failed in that direction, or they come abruptly to an end, as if we had
arrived at the borders of the ancient sea or lake which served as their
receptacle. It no less frequently happens that they vary in mineral
aspect and composition, as we pursue them horizontally. For example, we
trace a limestone for a hundred miles, until it becomes more
arenaceous, and finally passes into sand, or sandstone. We may then
follow this sandstone, already proved by its continuity to be of the
same age, throughout another district a hundred miles or more in
length.

Organic Remains.—This character must be used as a criterion of the age
of a formation, or of the contemporaneous origin of two deposits in
distant places, under very much the same restrictions as the test of
mineral composition.

First, the same fossils may be traced over wide regions, if we examine
strata in the direction of their planes, although by no means for
indefinite distances. Secondly, while the same fossils prevail in a
particular set of strata for hundreds of miles in a horizontal
direction, we seldom meet with the same remains for many fathoms, and
very rarely for several hundred yards, in a vertical line, or a line
transverse to the strata. This fact has now been verified in almost all
parts of the globe, and has led to a conviction that at successive
periods of the past, the same area of land and water has been inhabited
by species of animals and plants even more distinct than those which
now people the antipodes, or which now co-exist in the arctic,
temperate, and tropical zones. It appears that from the remotest
periods there has been ever a coming in of new organic forms, and an
extinction of those which pre-existed on the earth; some species having
endured for a longer, others for a shorter, time; while none have ever
reappeared after once dying out. The law which has governed the
succession of species, whether we adopt or reject the theory of
transmutation, seems to be expressed in the verse of the poet:—

                Natura il fece, e poi ruppe la stampa. ARIOSTO.
                Nature made him, and then broke the die.

And this circumstance it is, which confers on fossils their highest
value as chronological tests, giving to each of them, in the eyes of
the geologist, that authority which belongs to contemporary medals in
history.

The same cannot be said of each peculiar variety of rock; for some of
these, as red marl and red sandstone, for example, may occur at once at
the top, bottom, and middle of the entire sedimentary series;
exhibiting in each position so perfect an identity of mineral aspect as
to be undistinguishable. Such exact repetitions, however, of the same
mixtures of sediment have not often been produced, at distant periods,
in precisely the same parts of the globe; and even where this has
happened, we are seldom in any danger of confounding together the
monuments of remote eras, when we have studied their imbedded fossils
and their relative position.

Zoological Provinces.—It was remarked that the same species of organic
remains cannot be traced horizontally, or in the direction of the
planes of stratifications for indefinite distances. This might have
been expected from analogy; for when we inquire into the present
distribution of living beings, we find that the habitable surface of
the sea and land may be divided into a considerable number of distinct
provinces, each peopled by a peculiar assemblage of animals and plants.
In the “Principles of Geology,” I have endeavoured to point out the
extent and probable origin of these separate divisions; and it was
shown that climate is only one of many causes on which they depend, and
that difference of longitude as well as latitude is generally
accompanied by a dissimilarity of indigenous species.

As different seas, therefore, and lakes are inhabited, at the same
period, by different aquatic animals and plants, and as the lands
adjoining these may be peopled by distinct terrestrial species, it
follows that distinct fossils will be imbedded in contemporaneous
deposits. If it were otherwise—if the same species abounded in every
climate, or in every part of the globe where, so far as we can
discover, a corresponding temperature and other conditions favourable
to their existence are found—the identification of mineral masses of
the same age, by means of their included organic contents, would be a
matter of still greater certainty.

Nevertheless, the extent of some single zoological provinces,
especially those of marine animals, is very great; and our geological
researches have proved that the same laws prevailed at remote periods;
for the fossils are often identical throughout wide spaces, and in
detached deposits, consisting of rocks varying entirely in their
mineral nature.

The doctrine here laid down will be more readily understood, if we
reflect on what is now going on in the Mediterranean. That entire sea
may be considered as one zoological province; for although certain
species of testacea and zoophytes may be very local, and each region
has probably some species peculiar to it, still a considerable number
are common to the whole Mediterranean. If, therefore, at some future
period, the bed of this inland sea should be converted into land, the
geologist might be enabled, by reference to organic remains, to prove
the contemporaneous origin of various mineral masses scattered over a
space equal in area to half of Europe.

Deposits, for example, are well known to be now in progress in this sea
in the deltas of the Po, Rhone, Nile, and other rivers, which differ as
greatly from each other in the nature of their sediment as does the
composition of the mountains which their drain. There are also other
quarters of the Mediterranean, as off the coast of Campania, or near
the base of Etna, in Sicily, or in the Grecian Archipelago, where
another class of rocks is now forming; where showers of volcanic ashes
occasionally fall into the sea, and streams of lava overflow its
bottom; and where, in the intervals between volcanic eruptions, beds of
sand and clay are frequently derived from the waste of cliffs, or the
turbid waters of rivers. Limestones, moreover, such as the Italian
travertins, are here and there precipitated from the waters of mineral
springs, some of which rise up from the bottom of the sea. In all these
detached formations, so diversified in their lithological characters,
the remains of the same shells, corals, crustacea, and fish are
becoming inclosed; or, at least, a sufficient number must be common to
the different localities to enable the zoologist to refer them all to
one contemporaneous assemblage of species.

There are, however, certain combinations of geographical circumstances
which cause distinct provinces of animals and plants to be separated
from each other by very narrow limits; and hence it must happen that
strata will be sometimes formed in contiguous regions, differing widely
both in mineral contents and organic remains. Thus, for example, the
testacea, zoophytes, and fish of the Red Sea are, as a group, extremely
distinct from those inhabiting the adjoining parts of the
Mediterranean, although the two seas are separated only by the narrow
isthmus of Suez. Calcareous formations have accumulated on a great
scale in the Red Sea in modern times, and fossil shells of existing
species are well preserved therein; and we know that at the mouth of
the Nile large deposits of mud are amassed, including the remains of
Mediterranean species. It follows, therefore, that if at some future
period the bed of the Red Sea should be laid dry, the geologist might
experience great difficulties in endeavouring to ascertain the relative
age of these formations, which, although dissimilar both in organic and
mineral characters, were of synchronous origin.

But, on the other hand, we must not forget that the north-western
shores of the Arabian Gulf, the plains of Egypt, and the Isthmus of
Suez, are all parts of one province of _terrestrial_ species. Small
streams, therefore, occasional land- floods, and those winds which
drift clouds of sand along the deserts, might carry down into the Red
Sea the same shells of fluviatile and land testacea which the Nile is
sweeping into its delta, together with some remains of terrestrial
plants and the bones of quadrupeds, whereby the groups of strata before
alluded to might, notwithstanding the discrepancy of their mineral
composition and _marine_ organic fossils, be shown to have belonged to
the same epoch.

Yet, while rivers may thus carry down the same fluviatile and
terrestrial spoils into two or more seas inhabited by different marine
species, it will much more frequently happen that the coexistence of
terrestrial species of distinct zoological and botanical provinces will
be proved by the identity of the marine beings which inhabited the
intervening space. Thus, for example, the land quadrupeds and shells of
the valley of the Mississippi, of central America, and of the West
India islands differ very considerably, yet their remains are all
washed down by rivers flowing from these three zoological provinces
into the Gulf of Mexico.

In some parts of the globe, at the present period, the line of
demarkation between distinct provinces of animals and plants is not
very strongly marked, especially where the change is determined by
temperature, as it is in seas extending from the temperate to the
tropical zone, or from the temperate to the arctic regions. Here a
gradual passage takes place from one set of species to another. In like
manner the geologist, in studying particular formations of remote
periods, has sometimes been able to trace the gradation from one
ancient province to another, by observing carefully the fossils of all
the intermediate places. His success in thus acquiring a knowledge of
the zoological or botanical geography of very distant eras has been
mainly owing to this circumstance, that the mineral character has no
tendency to be affected by climate. A large river may convey yellow or
red mud into some part of the ocean, where it may be dispersed by a
current over an area several hundred leagues in length, so as to pass
from the tropics into the temperate zone. If the bottom of the sea be
afterwards upraised, the organic remains imbedded in such yellow or red
strata may indicate the different animals or plants which once
inhabited at the same time the temperate and equatorial regions.

It may be true, as a general rule, that groups of the same species of
animals and plants may extend over wider areas than deposits of
homogeneous composition; and if so, palæontological characters will be
of more importance in geological classification than the test of
mineral composition; but it is idle to discuss the relative value of
these tests, as the aid of both is indispensable, and it fortunately
happens, that where the one criterion fails, we can often avail
ourselves of the other.

Test by included Fragments of older Rocks.—It was stated, that proof
may sometimes be obtained of the relative date of two formations by
fragments of an older rock being included in a newer one. This evidence
may sometimes be of great use, where a geologist is at a loss to
determine the relative age of two formations from want of clear
sections exhibiting their true order of position, or because the strata
of each group are vertical. In such cases we sometimes discover that
the more modern rock has been in part derived from the degradation of
the older. Thus, for example, we may find chalk in one part of a
country, and in another strata of clay, sand, and pebbles. If some of
these pebbles consist of that peculiar flint, of which layers more or
less continuous are characteristic of the chalk, and which include
fossil shells, sponges, and foraminifera of cretaceous species, we may
confidently infer that the chalk was the oldest of the two formations.

Chronological Groups.—The number of groups into which the fossiliferous
strata may be separated are more or less numerous, according to the
views of classification which different geologists entertain; but when
we have adopted a certain system of arrangement, we immediately find
that a few only of the entire series of groups occur one upon the other
in any single section or district.

The thinning out of individual strata was before described (p. 42).But
let the diagram (Fig. 84) represent seven fossiliferous groups, instead
of as many strata. It will then be seen that in the middle all the
superimposed formations are present; but in consequence of some of them
thinning out, No. 2 and No. 5 are absent at one extremity of the
section, and No. 4 at the other.

Fig. 84: Seven fossiliferous groups.

In another diagram (Fig. 85), a real section of the geological
formations in the neighbourhood of Bristol and the Mendip Hills is
presented to the reader, as laid down on a true scale by Professor
Ramsay, where the newer groups 1, 2, 3, 4 rest unconformably on the
formations 5, 6, 7 and 8. At the southern end of the line of section we
meet with the beds No. 3 (the New Red Sandstone) resting immediately on
Nos. 7 and 8, while farther north as at Dundry Hill in Somersetshire,
we behold eight groups superimposed one upon the other, comprising all
the strata from the inferior Oolite, No. 1, to the coal and
carboniferous limestone. The limited horizontal extension of the groups
1 and 2 is owing to denudation, as these formations end abruptly, and
have left outlying patches to attest the fact of their having
originally covered a much wider area.

Section South of Bristol.

In order, therefore, to establish a chronological succession of
fossiliferous groups, a geologist must begin with a single section in
which several sets of strata lie one upon the other. He must then trace
these formations, by attention to their mineral character and fossils,
continuously, as far as possible, from the starting-point. As often as
he meets with new groups, he must ascertain by superposition their age
relatively to those first examined, and thus learn how to intercalate
them in a tabular arrangement of the whole.

By this means the German, French, and English geologists have
determined the succession of strata throughout a great part of Europe,
and have adopted pretty generally the following groups, almost all of
which have their representatives in the British Islands.

Abridged General Table of Fossiliferous Strata.

TABULAR VIEW OF THE FOSSILIFEROUS STRATA,
SHOWING THE ORDER OF SUPERPOSITION OR CHRONOLOGICAL SUCCESSION OF THE
PRINCIPAL GROUPS DESCRIBED IN THIS WORK.


POST-TERTIARY
EXAMPLES

POST-TERTIARY

EXAMPLES



POST-

TERTIARY 1.

RECENT

Shells and mammals, all of living species. British

Clyde marine strata, with canoes (p.
146).

Foreign

Danish kitchen middens (p.
146).

Lacustrine mud, with remains of Swiss lake-dwellings (p. 148).

Marine strata inclosing Temple of Serapis, at Puzzuoli (p. 146).
2.

POST-

PLIOCENE.

Shells, recent mammalia in part extinct. British

Loam of Brixham cave, with flint implements and bones of extinct
and living quadrupeds (p.
157)

Drift near Salisbury, with bones of mammoth, Spermophilus, and
stone implements (p. 161).

Glacial drift of Scotland, with marine shells and remains of
mammoth (p. 176.

Erratics of Pagham and Selsey Bill (p. 182).

Glacial drift of Wales, with marine fossil shells, about 1400
feet high, on Moel Tryfaen (p.
181).

Foreign

Dordogne caves of the reindeer period (p. 150).

Older valley-gravels of Amiens, with flint implements and bones
of extinct mammalia (p.
152).

Loess of Rhine (p. 154).

Ancient Nile-mud forming river-terraces (p. 154).

Loam and breccia of Liege caverns, with human remains (pp. 156, 157).

Australian cave breccias, with bones of extinct marsupials (p. 158).

Glacial drift of Northern Europe (p.
166, p. 174).


TERTIARY OR CAINOZOIC

PLIOCENE 3.
NEWER
PLIOCENE.
The shells almost all of living species. British
Bridlington beds, marine Arctic fauna (p. 189).
Glacial boulder formation of Norfolk cliffs (p. 190).
Forest-bed of Norfolk cliffs, with bones of _Elephas meridionalis,_
etc. (p. 191).
Chillesford and Aldeby beds, with marine shells, chiefly Arctic (p.
192).
Norwich crag (p. 193).
Foreign
Eastern base of Mount Etna, with marine shells (p. 204).
Sicilian calcareous and tufaceous strata (p. 205, 206).
Lacustrine strata of Upper Val d’Arno (p. 207).
Madeira leaf-bed and land-shells (p. 532). 4.
OLDER
PLIOCENE.
Extinct species of
shells forming a
large minority. British
Red crag of Suffolk, marine shells, some of northern forms (p. 194,
195).
White or coralline crag of Suffolk (p. 197).
Foreign
Antwerp crag (p. 204).
Subapennine marls and sands (p. 208).

EXAMPLES

MIOCENE 5.
UPPER
MIOCENE.
Majority of the
shells extinct. British
Wanting.
Foreign
Faluns of Touraine (p. 211).
Faluns, proper, of Bordeaux (p. 214).
Fresh-water strata of Gers (p. 215).
Swiss Oeningen beds, rich in plants and insects (pp. 215-23).
Marine Molasse, Switzerland (p. 223).
Bolderberg beds of Belgium (p. 224).
Vienna basin (p. 224).
Beds of the Superga, near Turin (p. 226).
Deposit at Pikermé, near Athens (p. 226).
Strata of the Siwâlik hills, India (p. 226).
Marine strata of the Atlantic border in the United States (p. 227).
Volcanic tuff and limestone of Madeira, the Canaries, and the Azores
(). 6.
LOWER
MIOCENE.
Nearly all the
shells extinct. British
Hempstead beds, marine and fresh-water strata (p. 244).
Lignites and clays of Bovey Tracey (p. 245).
Isle of Mull leaf-bed, volcanic tuff (p. 247).
Foreign
Calcaire de la Beauce, etc. (p. 230).
Grès de Fontainebleau (p. 230).
Lacustrine strata of the Limagne d’Auvergne, and the Cantal (p. 233).
Mayence basin (p. 242).
Radaboj beds of Croatia (p. 242).
Brown coal of Germany (p. 244).
Lower Molasse of Switzerland, fresh-water and brackish (p. 235-9).
Rupelmonde, Kleynspawen, and Tongrian beds of Belgium (p. 241, 242).
Nebraska beds, United States (p. 248).
Lower Miocene beds of Italy (p. 244).
Miocene flora of North Greenland (p. 239). EOCENE 7.
UPPER
EOCENE. British
Bembridge fluvio-marine strata (p. 252).
Osborne or St. Helen’s series (p. 255).
Headon series, with marine and fresh-water shells (p. 255).
Barton sands and clays (p. 258).
Foreign
Gypsum of Montmartre, fresh-water with _Palæotherium_ (p. 270).
Calcaire silicieux, or Travertin inférieur (p. 273),
Grès de Beauchamp, or Sables moyens (p. 273). 8.
MIDDLE
EOCENE. British
Bracklesham beds and Bagshot sands (p. 259).
White clays of Alum Bay and Bournemouth (p. 262).
Foreign
Calcaire grossier, miliolitic limestone (p. 274).
Soissonnais sands, or Lits coquilliers, with _Nummulites planulata_ (p.
275).
Claiborne beds of the United States, with _Orbitoides_ and _Zeuglodon_
(p. 279).
Nummulitic formation of Europe, Asia, etc. (p. 277). 9.
LOWER
EOCENE. British
London clay proper (p. 263).
Woolwich and Reading series, fluvio-marine (p. 267).
Thanet sands (p. 269).
Foreign
Argile de Londres, near Dunkirk (p. 252).
Argile plastique (p. 276).
Sables de Bracheux (p. 276).

SECONDARY OR MESOZOIC.

CRETACEOUS 10.
UPPER
CRETACEOUS. British
Upper white chalk, with flints (p. 290).
Lower white chalk, without flints (p. 298).
Chalk marl (p. 298).
Chloritic series (or Upper Greensand), fire-stone of Surrey (p. 298).
Gault (p. 300).
Blackdown beds (p. 301).

EXAMPLES

CRETACEOUS 10.
UPPER
CRETACEOUS. Foreign
Maetricht beds and Faxoe chalk (p. 233).
Pisolitic limestone of France (p. 285).
White chalk of France, Sweden, and Russia (p. 286, 287).
Planer-kalk of Saxony (p. 293).
Sands and clays of Aix-la-Chapelle (p. 302).
Hippurite limestone of South of France (p. 305).
New Jersey, U.S., sands and marls (p. 307). 11.
LOWER
CRETACEOUS or
NEOCOMIAN. British
Sands of Folkestone, Sandgate, and Hythe (p. 308).
Atherfield clay, with _Perna mulleti_ (p. 309).
Punfield marine beds, with _Vicarya lujana_ (p. 318).
Speeton clay of Flamborough Head and Tealby (p. 311).
Weald clay of Surrey, Kent, and Sussex, fresh-water, with _Cypris_ (p.
313-5).
Hastings sands (p. 316-8).
Foreign
Neocomian of Neufchatel, and Hils conglomerate of North Germany (p.
312).
Wealden beds of Hanover (p. 319). OOLITE 12.
UPPER OOLITE. British
Upper Purbeck beds, fresh-water (p. 323).
Middle Purbeck, with numerous marsupial quadrupeds, etc. (p. 324).
Lower Purbeck, fresh-water, with intercalated dirt-bed (p. 330).
Portland stone and sand. (p. 334).
Kimmeridge clay (p. 335).
Foreign
Marnes à gryphées virgules of Argonne (p. 336).
Lithographic-stone of Solenhofen, with _Archæopteryx_ (p. 337). 13.
MIDDLE OOLITE. British
Coral rag of Berkshire, Wilts, and Yorkshire (p. 339).
Oxford clay, with belemnites and Ammonite (p. 340).
Kelloway rock of Wilts and Yorkshire (p. 341).
Foreign
Nerinæan limestone of the Jura (p. 339). 14.
LOWER OOLITE. British
Cornbrash and forest marble (p. 341).
Great or Bath oolite of Bradford (p. 342).
Stonesfield slate, with marsupials and _Araucaria_ (p. 345).
Fuller’s earth of Bath (p. 348).
Inferior oolite (p. 349). LIAS 15.
LIAS. Upper Lias, argillaceous, with _Ammonites striatulus_ (p.
353).
Shale and limestone, with _Ammonites bifrons_ (p. 353).
Middle Lias or Marlstone series, with zones containing characteristic
Ammonites (p. 353).
Lower Lias, also with zones characterised by peculiar Ammonites (p.
356). TRIAS 16.
UPPER TRIAS. British
Rhætic, Penarth or _Avicula contorta_ beds (beds of passage) (p. 366).
Keuper or Upper New Red sandstone, etc. (p. 369).
Red shales of Cheshire and Lancashire, with rock-salt (p. 371).
Dolomite conglomerate of Bristol (p. 373).
Foreign
Keuper beds of Germany (p. 375).
St. Cassian or Hallstadt beds, with rich marine fauna (p. 376).
Coal-field of Richmond, Virginia (p. 382).
Chatham coal-field, North Carolina (p. 383). 17.
MIDDLE TRIAS. British
Wanting.
Foreign
Muschelkalk of Germany (p. 378). 18.
LOWER TRIAS. British
Bunter or Lower New Red sandstone of Lancashire and Cheshire (p. 372).
Foreign
Bunter-sandstein of Germany (p. 380).
Red sandstone of Connecticut Valley, with footprints of birds and
reptiles (p. 381).

PRIMARY OR PALÆOZOIC
EXAMPLES

PERMIAN 19.
PERMIAN. British
Upper Permian of St. Bees’ Head, Cumberland (p. 386).
Middle Permian, magnesian limestone, and marl-slate of Durham and
Yorkshire, with _Protosaurus_ (p. 387).
Lower Permian sandstones and breccias of Penrith and Dumfriesshire,
intercalated (p. 390).
Foreign
Dark-coloured shales of Thuringia (p. 392).
Zechstein or Dolomitic limestone (p. 392).
Mergel-schiefer or Kupfer-schiefer (p. 392).
Rothliegendes of Thuringia, with _Psaronius_ (p. 392).
Magnesian limestones, etc., of Russia (p. 393). CARBONIFEROUS 20.
UPPER CARBONIFEROUS. British
Coal-measures of South Wales, with underclays inclosing _Stigmaria_ (p.
397).
Coal-measures of north and central England (p. 395).
Millstone grit (p. 395).
Yoredale series of Yorkshire (p. 395).
Coal-field of Kilkenny with _Labyrinthodont_ (p. 407).
Foreign
Coal-field of Saarbruck, with _Archegosaurus_ (p. 406).
Carboniferous strata of South Joggins, Nova Scotia (p. 409).
Pennsylvania coal-field (p. 403). 21.
LOWER CARBONIFEROUS. British
Mountain limestone of Wales and South of England (p. 430).
Same in Ireland (p. 437437).
Carboniferous limestone of Scotland alternating with coal-bearing
sandstones (p. 396).
Erect trees in volcanic ash in the Island of Arran (p. 546).
Foreign
Mountain limestone of Belgium (p. 436). DEVONIAN or
OLD RED SANDSTONE 22.
UPPER
DEVONIAN. British
Yellow sandstone of Dura Den, with _Holoptychius_, etc. (p. 440); and
of Ireland with _Anodon Jukesii_ (p. 441).
Sandstones of Forfarshire and Perthshire, with _Holoptychius_, etc. (p.
442).
Pilton group of North Devon (p. 449).
Petherwyn group of Cornwall, with _Clymenia_ and _Cypridina_ (p. 451).
Foreign
Clymenien-kalk and Cypridinen-schiefer of Germany (p. 450) 23.
MIDDLE
DEVONIAN. British
Bituminous schists of Gamrie, Caithness, etc., with numerous fish (p.
443).
Ilfracombe beds with peculiar trilobites and corals (p. 450).
Limestones of Torquay, with broad-winged Spirifers (p. 451).
Foreign
Eifel limestone, with underlying schists containing _Calceola_ (p.
453).
Devonian strata of Russia (p. 454). 24.
LOWER
DEVONIAN. British
Arbroath paving-stones, with _Cephalaspis_ and _Pterygotus_ (p. 446).
Lower sandstones of Forfarshire, with _Pterygotus_ (p. 446).
Sandstones and slates of the Foreland and Linton (p. 454).
Foreign
Oriskany sandstone of Western Canada and New York (p. 456).
Sandstones of Gaspe, with _Cephalaspis_ (p. 455 ).

EXAMPLES

SILURIAN 25.
UPPER SILURIAN British
Upper Ludlow formation, Downton sandstone, with bone-bed (p. 459).
Lower Ludlow formation, with oldest known fish remains (p. 461).
Wenlock limestone and shale (p. 465).
Woolhope limestone and grit (p. 467).
Tarannon shales (p. 468).
_Beds of passage between Upper and Lower Silurian:_
Upper Llandovery, or May-hill sandstone, with _Pentamerus oblongus_,
etc. (p. 468).
Lower Llandovery slates (p. 469).
Foreign
Niagara limestone, with _Calymene, Homalonotus_, etc. (p. 479).
Clinton group of America, with _Pentamerus oblongus_, etc. (p. 479).
Silurian strata of Russia, with _Pentamerus_ (p. 477). 26.
LOWER SILURIAN. British
Bala and Caradoc beds (p. 470).
Llandeilo flags (p. 473).
Arenig or Stiper-stones group (Lower Llandeilo of Murchison) (p. 475).
Foreign
Ungulite or Obolus grit of Russia (p. 477).
Trenton limestone, and other Lower Silurian groups of North America (p.
479).
Lower Silurian of Sweden (p. 477). CAMBRIAN 27.
UPPER CAMBRIAN. British
Tremadoc slates (p. 483).
Lingula flags, with _Lingula Davisii_ (p. 484).
Foreign
“Primordial” zone of Bohemia in part, with trilobites of the genera
_Paradoxides_, etc. (p. 487).
Alum schists of Sweden and Norway (p. 489).
Potsdam sandstone, with _Dikelocephalus_ and _Obolella_ (p. 489). 28.
LOWER CAMBRIAN. British
Menevian beds of Wales, with _Paradoxides Davidis_, etc. (p. 484).
Longmynd group, comprising the Harlech grits and Llanberis slates (p.
485).
Foreign
Lower portion of Barrande’s “Primordial” zone in Bohemia (p. 486).
Fucoid sandstones of Sweden (p. 489).
Huronian series of Canada? (p. 490). LAURENTIAN 29.
UPPER LAURENTIAN. British
Fundamental gneiss of the Hebrides? (p. 493).
Hypersthene rocks of Skye? (p. 491).
Foreign
Labradorite series north of the river St. Lawrence in Canada (p. 491).
Adirondack mountains of New York (p. 491). 30.
LOWER LAURENTIAN. British
Wanting?
Foreign
Beds of gneiss and quartzite, with interstratified limestones, in one
of which, 1000 feet thick, occurs a foraminifer, _Eozoon Canadense_,
the oldest known fossil (p. 491).




CHAPTER IX.
CLASSIFICATION OF TERTIARY FORMATIONS.


Order of Succession of Sedimentary Formations. — Frequent
Unconformability of Strata. — Imperfection of the Record. —
Defectiveness of the Monuments greater in Proportion to their
Antiquity. — Reasons for studying the newer Groups first. —
Nomenclature of Formations. — Detached Tertiary Formations scattered
over Europe. — Value of the Shell-bearing Mollusca in Classification. —
Classification of Tertiary Strata. — Eocene, Miocene, and Pliocene
Terms explained.

By reference to the tables given at the end of the last chapter the
reader will see that when the fossiliferous rocks are arranged
chronologically, we have first to consider the Post-tertiary and then
the Tertiary or Cainozoic formations, and afterwards to pass on to
those of older date.

Fig. 86: Order of Superposition of Deposits

Order of Superposition.—The diagram (Fig. 86) will show the order of
superposition of these deposits, assuming them all to be visible in one
continuous section. In nature, as before hinted (p. 107), we have never
an opportunity of seeing the whole of them so displayed in a single
region; first, because sedimentary deposition is confined, during any
one geological period, to limited areas; and secondly, because strata,
after they have been formed, are liable to be utterly annihilated over
wide areas by denudation. But wherever certain members of the series
are present, they overlie one another in the order indicated in the
diagram, though not always in the exact manner there represented,
because some of them repose occasionally in unconformable
stratification on others. This mode of superposition has been already
explained (p. 94, p. 111), where I pointed out that the discordance
which implies a considerable lapse of time between two formations in
juxtaposition is almost invariably accompanied by a great dissimilarity
in the species of organic remains.

Frequent Unconformability of Strata.—Where the widest gaps appear in
the sequence of the fossil forms, as between the Permian and Triassic
rocks, or between the Cretaceous and Eocene, examples of such
unconformability are very frequent. But they are also met with in some
part or other of the world at the junction of almost all the other
principal formations, and sometimes the subordinate divisions of any
one of the leading groups may be found lying unconformably on another
subordinate member of the same—the Upper, for example, on the Lower
Silurian, or the superior division of the Old Red Sandstone on a lower
member of the same, and so forth. Instances of such irregularities in
the mode of succession of the strata are the more intelligible the more
we extend our survey of the fossiliferous formations, for we are
continually bringing to light deposits of intermediate date, which have
to be intercalated between those previously known, and which reveal to
us a long series of events, of which antecedently to such discoveries
we had no knowledge.

But while unconformability invariably bears testimony to a lapse of
unrepresented time, the conformability of two sets of strata in contact
by no means implies that the newer formation immediately succeeded the
older one. It simply implies that the ancient rocks were subjected to
no movements of such a nature as to tilt, bend, or break them before
the more modern formation was superimposed. It does not show that the
earth’s crust was motionless in the region in question, for there may
have been a gradual sinking or rising, extending uniformly over a large
surface, and yet, during such movement, the stratified rocks may have
retained their original horizontality of position. There may have been
a conversion of a wide area from sea into land and from land into sea,
and during these changes of level some strata may have been slowly
removed by aqueous action, and after this new strata may be
superimposed, differing perhaps in date by thousands of years or
centuries, and yet resting conformably on the older set. There may even
be a blending of the materials constituting the older deposit with
those of the newer, so as to give rise to a passage in the mineral
character of the one rock into the other as if there had been no break
or interruption in the depositing process.

Imperfection of the Record.—Although by the frequent discovery of new
sets of intermediate strata the transition from one type of organic
remains to another is becoming less and less abrupt, yet the entire
series of records appears to the geologists now living far more
fragmentary and defective than it seemed to their predecessors half a
century ago. The earlier inquirers, as often as they encountered a
break in the regular sequence of formations, connected it theoretically
with a sudden and violent catastrophe, which had put an end to the
regular course of events that had been going on uninterruptedly for
ages, annihilating at the same time all or nearly all the organic
beings which had previously flourished, after which, order being
re-established, a new series of events was initiated. In proportion as
our faith in these views grows weaker, and the phenomena of the organic
or inorganic world presented to us by geology seem explicable on the
hypothesis of gradual and insensible changes, varied only by occasional
convulsions, on a scale comparable to that witnessed in historical
times; and in proportion as it is thought possible that former
fluctuations in the organic world may be due to the indefinite
modifiability of species without the necessity of assuming new and
independent acts of creation, the number and magnitude of the gaps
which still remain, or the extreme imperfection of the record, become
more and more striking, and what we possess of the ancient annals of
the earth’s history appears as nothing when contrasted with that which
has been lost.

When we examine a large area such as Europe, the average as well as the
extreme height above the sea attained by the older formations is
usually found to exceed that reached by the more modern ones, the
primary or palaeozoic rising higher than the secondary, and these in
their turn than the tertiary; while in reference to the three divisions
of the tertiary, the lowest or Eocene group attains a higher
summit-level than the Miocene, and these again a greater height than
the Pliocene formations. Lastly, the post-tertiary deposits, such, at
least, as are of marine origin, are most commonly restricted to much
more moderate elevations above the sea-level than the tertiary strata.

It is also observed that strata, in proportion as they are of newer
date, bear the nearest resemblance in mineral character to those which
are now in the progress of formation in seas or lakes, the newest of
all consisting principally of soft mud or loose sand, in some places
full of shells, corals, or other organic bodies, animal or vegetable,
in others wholly devoid of such remains. The farther we recede from the
present time, and the higher the antiquity of the formations which we
examine, the greater are the changes which the sedimentary deposits
have undergone. Time, as I have explained in Chapters V, VI, and VII,
has multiplied the effects of condensation by pressure and cementation,
and the modification produced by heat, fracture, contortion, upheaval,
and denudation. The organic remains also have sometimes been
obliterated entirely, or the mineral matter of which they were composed
has been removed and replaced by other substances.

Why newer Groups should be studied first.—We likewise observe that the
older the rocks the more widely do their organic remains depart from
the types of the living creation. First, we find in the newer tertiary
rocks a few species which no longer exist, mixed with many living ones,
and then, as we go farther back, many genera and families at present
unknown make their appearance, until we come to strata in which the
fossil relics of existing species are nowhere to be detected, except a
few of the lowest forms of invertebrate, while some orders of animals
and plants wholly unrepresented in the living world begin to be
conspicuous.

When we study, therefore, the geological records of the earth and its
inhabitants, we find, as in human history, the defectiveness and
obscurity of the monuments always increasing the remoter the era to
which we refer, and the difficulty of determining the true
chronological relations of rocks is more and more enhanced, especially
when we are comparing those which were formed simultaneously in very
distant regions of the globe. Hence we advance with securer steps when
we begin with the study of the geological records of later times,
proceeding from the newer to the older, or from the more to the less
known.

In thus inverting what might at first seem to be the more natural order
of historical research, we must bear in mind that each of the periods
above enumerated, even the shortest, such as the Post-tertiary, or the
Pliocene, Miocene, or Eocene, embrace a succession of events of vast
extent, so that to give a satisfactory account of what we already know
of any one of them would require many volumes. When, therefore, we
approach one of the newer groups before endeavouring to decipher the
monuments of an older one, it is like endeavouring to master the
history of our own country and that of some contemporary nations,
before we enter upon Roman History, or like investigating the annals of
Ancient Italy and Greece before we approach those of Egypt and Assyria.

Nomenclature.—The origin of the terms Primary and Secondary, and the
synonymous terms Palaeozoic, and Mesozoic, were explained in Chapter
VIII, p. 123.

The Tertiary or Cainozoic strata (see p. 123) were so called because
they were all posterior in date to the Secondary series, of which last
the Chalk of Cretaceous, No. 9, Fig. 86, constitutes the newest group.
The whole of them were at first confounded with the superficial
alluviums of Europe; and it was long before their real extent and
thickness, and the various ages to which they belong, were fully
recognised. They were observed to occur in patches, some of
fresh-water, others of marine origin, their geographical area being
usually small as compared to the secondary formations, and their
position often suggesting the idea of their having been deposited in
different bays, lakes, estuaries, or inland seas, after a large portion
of the space now occupied by Europe had already been converted into dry
land.

The first deposits of this class, of which the characters were
accurately determined, were those occurring in the neighbourhood of
Paris, described in 1810 by MM. Cuvier and Brongniart. They were
ascertained to consist of successive sets of strata, some of marine,
others of fresh-water origin, lying one upon the other. The fossil
shells and corals were perceived to be almost all of unknown species,
and to have in general a near affinity to those now inhabiting warmer
seas. The bones and skeletons of land animals, some of them of large
size, and belonging to more than forty distinct species, were examined
by Cuvier, and declared by him not to agree specifically, nor most of
them even generically, with any hitherto observed in the living
creation.

Strata were soon afterwards brought to light in the vicinity of London,
and in Hampshire, which, although dissimilar in mineral composition,
were justly inferred by Mr. T. Webster to be of the same age as those
of Paris, because the greater number of the fossil shells were
specifically identical. For the same reason, rocks found on the
Gironde, in the South of France, and at certain points in the North of
Italy, were suspected to be of contemporaneous origin.

Another important discovery was soon afterwards made by Brocchi in
Italy, who investigated the argillaceous and sandy deposits, replete
with shells, which form a low range of hills, flanking the Apennines on
both sides, from the plains of the Po to Calabria. These lower hills
were called by him the Subapennines, and were formed of strata chiefly
marine, and newer than those of Paris and London.

Another tertiary group occurring in the neighbourhood of Bordeaux and
Dax, in the South of France, was examined by M. de Basterot in 1825,
who described and figured several hundred species of shells, which
differed for the most part both from the Parisian series and those of
the Subapennine hills. It was soon, therefore, suspected that this
fauna might belong to a period intermediate between that of the
Parisian and Subapennine strata, and it was not long before the
evidence of superposition was brought to bear in support of this
opinion; for other strata, contemporaneous with those of Bordeaux, were
observed in one district (the Valley of the Loire), to overlie the
Parisian formation, and in another (in Piedmont) to underlie the
Subapennine beds. The first example of these was pointed out in 1829 by
M. Desnoyers, who ascertained that the sand and marl of marine origin
called faluns, near Tours, in the basin of the Loire, full of
sea-shells and corals, rested upon a lacustrine formation, which
constitutes the uppermost subdivision of the Parisian group, extending
continuously throughout a great table-land intervening between the
basin of the Seine and that of the Loire. The other example occurs in
Italy, where strata containing many fossils similar to those of
Bordeaux were observed by Bonelli and others in the environs of Turin,
subjacent to strata belonging to the Subapennine group of Brocchi.

Value of Testacean Fossils in Classification.—It will be observed that
in the foregoing allusions to organic remains, the testacea or the
shell-bearing mollusca are selected as the most useful and convenient
class for the purposes of general classification. In the first place,
they are more universally distributed through strata of every age than
any other organic bodies. Those families of fossils which are of rare
and casual occurrence are absolutely of no avail in establishing a
chronological arrangement. If we have plants alone in one group of
strata and the bones of mammalia in another, we can draw no conclusion
respecting the affinity or discordance of the organic beings of the two
epochs compared; and the same may be said if we have plants and
vertebrated animals in one series and only shells in another. Although
corals are more abundant, in a fossil state, than plants, reptiles, or
fish, they are still rare when contrasted with shells, because they are
more dependent for their well-being on the constant clearness of the
water, and are, therefore, less likely to be included in rocks which
endure in consequence of their thickness and the copiousness of
sediment which prevailed when they originated. The utility of the
testacea is, moreover, enhanced by the circumstance that some forms are
proper to the sea, others to the land, and others to fresh water.
Rivers scarcely ever fail to carry down into their deltas some
land-shells, together with species which are at once fluviatile and
lacustrine. By this means we learn what terrestrial, fresh-water, and
marine species coexisted at particular eras of the past: and having
thus identified strata formed in seas with others which originated
contemporaneously in inland lakes, we are then enabled to advance a
step farther, and show that certain quadrupeds or aquatic plants, found
fossil in lacustrine formations, inhabited the globe at the same period
when certain fish, reptiles, and zoophytes lived in the ocean.

Among other characters of the molluscous animals, which render them
extremely valuable in settling chronological questions in geology, may
be mentioned, first, the wide geographical range of many species; and,
secondly, what is probably a consequence of the former, the great
duration of species in this class, for they appear to have surpassed in
longevity the greater number of the mammalia and fish. Had each species
inhabited a very limited space, it could never, when imbedded in
strata, have enabled the geologist to identify deposits at distant
points; or had they each lasted but for a brief period, they could have
thrown no light on the connection of rocks placed far from each other
in the chronological, or, as it is often termed, vertical series.

Classification of Tertiary Strata.—Many authors have divided the
European Tertiary strata into three groups—lower, middle, and upper;
the lower comprising the oldest formations of Paris and London before
mentioned; the middle those of Bordeaux and Touraine; and the upper all
those newer than the middle group.

In the first edition of the Principles of Geology, I divided the whole
of the Tertiary formations into four groups, characterised by the
percentage of recent shells which they contained. The lower tertiary
strata of London and Paris were thought by M. Deshayes to contain only
3½ per cent of recent species, and were termed Eocene. The middle
tertiary of the Loire and Gironde had, according to the specific
determinations of the same conchologist, 17 per cent, and formed the
Miocene division. The Subapennine beds contained 35 to 50 per cent, and
were termed Older Pliocene, while still more recent beds in Sicily,
which had from 90 to 95 per cent of species identical with those now
living, were called Newer Pliocene. The first of the above terms,
Eocene, is derived from eos, _dawn_, and cainos, _recent_, because the
fossil shells of this period contain an extremely small proportion of
living species, which may be looked upon as indicating the dawn of the
existing state of the testaceous fauna, no recent species having been
detected in the older or secondary rocks.

The term Miocene (from meion, _less_, and cainos, _recent_) is intended
to express a minor proportion of recent species (of testacea), the term
Pliocene (from pleion, _ more_, and cainos, _recent_) a comparative
plurality of the same. It may assist the memory of students to remind
them, that the _Mi_ocene contain a _mi_nor proportion, and _ Pl_iocene
a comparative _pl_urality of recent species; and that the greater
number of recent species always implies the more modern origin of the
strata.

It has sometimes been objected to this nomenclature that certain
species of infusoria found in the chalk are still existing, and, on the
other hand, the Miocene and Older Pliocene deposits often contain the
remains of mammalia, reptiles, and fish, exclusively of extinct
species. But the reader must bear in mind that the terms Eocene,
Miocene, and Pliocene were originally invented with reference purely to
conchological data, and in that sense have always been and are still
used by me.

Since the year 1830 the number of known shells, both recent and fossil,
has largely increased, and their identification has been more
accurately determined. Hence some modifications have been required in
the classifications founded on less perfect materials. The Eocene,
Miocene, and Pliocene periods have been made to comprehend certain sets
of strata of which the fossils do not always conform strictly in the
proportion of recent to extinct species with the definitions first
given by me, or which are implied in the etymology of those terms.




CHAPTER X.
RECENT AND POST-PLIOCENE PERIODS.


Recent and Post-pliocene Periods. — Terms defined. — Formations of the
Recent Period. — Modern littoral Deposits containing Works of Art near
Naples. — Danish Peat and Shell-mounds. — Swiss Lake-dwellings. —
Periods of Stone, Bronze, and Iron. — Post-pliocene Formations. —
Coexistence of Man with extinct Mammalia. — Reindeer Period of South of
France. — Alluvial Deposits of Paleolithic Age. — Higher and
Lower-level Valley-gravels. — Loess or Inundation-mud of the Nile,
Rhine, etc. — Origin of Caverns. — Remains of Man and extinct
Quadrupeds in Cavern Deposits. — Cave of Kirkdale. — Australian
Cave-breccias. — Geographical Relationship of the Provinces of living
Vertebrata and those of extinct Post-pliocene Species. — Extinct
struthious Birds of New Zealand. — Climate of the Post-pliocene Period.
— Comparative Longevity of Species in the Mammalia and Testacea. —
Teeth of Recent and Post-pliocene Mammalia.

We have seen in the last chapter that the uppermost or newest strata
are called Post-tertiary, as being more modern than the Tertiary. It
will also be observed that the Post-tertiary formations are divided
into two subordinate groups: the Recent, and Post-pliocene. In the
former, or the Recent, the mammalia as well as the shells are identical
with species now living: whereas in the Post-pliocene, the shells being
all of living forms, a part, and often a considerable part, of the
mammalia belonged to extinct species. To this nomenclature it may be
objected that the term Post-pliocene should in strictness include all
geological monuments posterior in date to the Pliocene; but when I have
occasion to speak of the whole collectively, I shall call them
Post-tertiary, and reserve the term Post-pliocene for the older
Post-tertiary formations, while the Upper or newer ones will be called
“Recent.”

Cases will occur where it may be scarcely possible to draw the boundary
line between the Recent and Post-pliocene deposits; and we must expect
these difficulties to increase rather than diminish with every advance
in our knowledge, and in proportion as gaps are filled up in the series
of records.

RECENT PERIOD

It was stated in the sixth chapter, when I treated of denudation, that
the dry land, or that part of the earth’s surface which is not covered
by the waters of lakes or seas, is generally wasting away by the
incessant action of rain and rivers, and in some cases by the
undermining and removing power of waves and tides on the sea-coast. But
the rate of waste is very unequal, since the level and gently sloping
lands, where they are protected by a continuous covering of vegetation,
escape nearly all wear and tear, so that they may remain for ages in a
stationary condition, while the removal of matter is constantly
widening and deepening the intervening ravines and valleys.

The materials, both fine and coarse, carried down annually by rivers
from the higher regions to the lower, and deposited in successive
strata in the basins of seas and lakes, must be of enormous volume. We
are always liable to underrate their magnitude, because the
accumulation of strata is going on out of sight.

There are, however, causes at work which, in the course of centuries,
tend to render visible these modern formations, whether of marine or
lacustrine origin. For a large portion of the earth’s crust is always
undergoing a change of level, some areas rising and others sinking at
the rate of a few inches, or a few feet, perhaps sometimes yards, in a
century; so that spaces which were once subaqueous are gradually
converted into land, and others which were high and dry become
submerged. In consequence of such movements we find in certain regions,
as in Cashmere, for example, where the mountains are often shaken by
earthquakes, deposits which were formed in lakes in the historical
period, but through which rivers have now cut deep and wide channels.
In lacustrine strata thus intersected, works of art and fresh-water
shells are seen. In other districts on the borders of the sea, usually
at very moderate elevations above its level, raised beaches occur, or
marine littoral deposits, such as those in which, on the borders of the
Bay of Baiæ, near Naples, the well-known temple of Serapis was
imbedded. In that case the date of the monument buried in the marine
strata is ascertainable, but in many other instances the exact age of
the remains of human workmanship is uncertain, as in the estuary of the
Clyde at Glasgow, where many canoes have been exhumed, with other works
of art, all assignable to some part of the Recent Period.

Danish Peat and Shell-mounds or Kitchen-middens.—Sometimes we obtain
evidence, without the aid of a change of level, of events which took
place in pre-historic times. The combined labours, for example, of the
antiquary, zoologist, and botanist have brought to light many monuments
of the early inhabitants buried in peat-mosses in Denmark. Their
geological age is determined by the fact that, not only the
contemporaneous fresh-water and land shells, but all the quadrupeds,
found in the peat, agree specifically with those now inhabiting the
same districts, or which are known to have been indigenous in Denmark
within the memory of man. In the lower beds of peat (a deposit varying
from 20 to 30 feet in thickness), weapons of stone accompany trunks of
the Scotch fir, _Pinus sylvestris._ This peat may be referred to that
part of the stone period for which Sir John Lubbock proposed the name
of “Neolithic”[1] in contradistinction to a still older era, termed by
him “Paleolithic,” and which will be described in the sequel. In the
higher portions of the same Danish bogs, bronze implements are
associated with trunks and acorns of the common oak. It appears that
the pine has never been a native of Denmark in historical times, and it
seems to have given place to the oak about the time when articles and
instruments of bronze superseded those of stone. It also appears that,
at a still later period, the oak itself became scarce, and was nearly
supplanted by the beech, a tree which now flourishes luxuriantly in
Denmark. Again, at the still later epoch when the beech-tree abounded,
tools of iron were introduced, and were gradually substituted for those
of bronze.

On the coasts of the Danish islands in the Baltic, certain mounds,
called in those countries “Kjökken-mödding,” or “kitchen-middens,”
occur, consisting chiefly of the castaway shells of the oyster, cockle,
periwinkle, and other eatable kinds of molluscs. The mounds are from
three to ten feet high, and from 100 to 1000 feet in their longest
diameter. They greatly resemble heaps of shells formed by the Red
Indians of North America along the eastern shores of the United States.
In the old refuse-heaps, recently studied by the Danish antiquaries and
naturalists with great skill and diligence, no implements of metal have
ever been detected. All the knives, hatchets, and other tools, are of
stone, horn, bone, or wood. With them are often intermixed fragments of
rude pottery, charcoal and cinders, and the bones of quadrupeds on
which the rude people fed. These bones belong to wild species still
living in Europe, though some of them, like the beaver, have long been
extirpated in Denmark. The only animal which they seem to have
domesticated was the dog.

As there is an entire absence of metallic tools, these refuse-heaps are
referred to the Neolithic division of the age of stone, which
immediately preceded in Denmark the age of bronze. It appears that a
race more advanced in civilisation, armed with weapons of that mixed
metal, invaded Scandinavia, and ousted the aborigines.

Lacustrine Habitations of Switzerland.—In Switzerland a different class
of monuments, illustrating the successive ages of stone, bronze, and
iron, has been of late years investigated with great success, and
especially since 1854, in which year Dr. F. Keller explored near the
shore at Meilen, in the bottom of the lake of Zurich, the ruins of an
old village, originally built on numerous wooden piles, driven, at some
unknown period, into the muddy bed of the lake. Since then a great many
other localities, more than a hundred and fifty in all, have been
detected of similar pile-dwellings, situated near the borders of the
Swiss lakes, at points where the depth of water does not exceed 15
feet.[2] The superficial mud in such cases is filled with various
articles, many hundreds of them being often dredged up from a very
limited area. Thousands of piles, decayed at their upper extremities,
are often met with still firmly fixed in the mud.

As the ages of stone, bronze, and iron merely indicate successive
stages of civilisation, they may all have coexisted at once in
different parts of the globe, and even in contiguous regions, among
nations having little intercourse with each other. To make out,
therefore, a distinct chronological series of monuments is only
possible when our observations are confined to a limited district, such
as Switzerland.

The relative antiquity of the pile-dwellings, which belong respectively
to the ages of stone and bronze, is clearly illustrated by the
associations of the tools with certain groups of animal remains. Where
the tools are of stone, the castaway bones which served for the food of
the ancient people are those of deer, the wild boar, and wild ox, which
abounded when society was in the hunter state. But the bones of the
later or bronze epoch were chiefly those of the domestic ox, goat, and
pig, indicating progress in civilisation. Some villages of the stone
age are of later date than others, and exhibit signs of an improved
state of the arts. Among their relics are discovered carbonised grains
of wheat and barley, and pieces of bread, proving that the cultivation
of cereals had begun. In the same settlements, also, cloth, made of
woven flax and straw, has been detected.

The pottery of the bronze age in Switzerland is of a finer texture, and
more elegant in form, than that of the age of stone. At Nidau, on the
lake of Bienne, articles of iron have also been discovered, so that
this settlement was evidently not abandoned till that metal had come
into use.

At La Thène, in the northern angle of the lake of Neufchâtel, a great
many articles of iron have been obtained, which in form and
ornamentation are entirely different both from those of the bronze
period and from those used by the Romans. Gaulish and Celtic coins have
also been found there by MM. Schwab and Desor. They agree in character
with remains, including many iron swords, which have been found at
Tiefenau, near Berne, in ground supposed to have been a battle-field;
and their date appears to have been anterior to the great Roman
invasion of Northern Europe, though perhaps not long before that
event.[3] Coins, which sometimes occur in deposits of the age of iron,
have never yet been found in formations of the ages of bronze or stone.

The period of bronze must have been one of foreign commerce, as tin,
which enters into this metallic mixture in the proportion of about ten
per cent to the copper, was obtained by the ancients chiefly from
Cornwall.[4] Very few human bones of the bronze period have been met
with in the Danish peat, or in the Swiss lake-dwellings, and this
scarcity is generally attributed by archæologists to the custom of
burning the dead, which prevailed in the age of bronze.

POST-PLIOCENE PERIOD

From the foregoing observations we may infer that the ages of iron and
bronze in Northern and Central Europe were preceded by a stone age, the
Neolithic, referable to that division of the post-tertiary epoch which
I have called Recent, when the mammalia as well as the other organic
remains accompanying the stone implements were of living species. But
memorials have of late been brought to light of a still older age of
stone, for which, as above stated, the name Paleolithic has been
proposed, when man was contemporary in Europe with the elephant and
rhinoceros, and various other animals, of which many of the most
conspicuous have long since died out.

Reindeer Period in South of France.—In the larger number of the caves
of Europe, as for example in those of England, Belgium, Germany, and
many parts of France, the animal remains agree specifically with the
fauna of this oldest division of the age of stone, or that to which
belongs the drift of Amiens and Abbeville presently to be mentioned,
containing flint implements of a very antique type. But there are some
caves in the departments of Dordogne, Aude, and other parts of the
south of France, which are believed by M. Lartet to be of intermediate
date between the Paleolithic and Neolithic periods. To this
intermediate era M. Lartet gave, in 1863, the name of the “reindeer
period,” because vast quantities of the bones and horns of that deer
have been met with in the French caverns. In some cases separate plates
of molars of the mammoth, and several teeth of the great Irish deer,
_Cervus megaceros,_ and of the cave-lion, _Felis spelæa,_ have been
found mixed up with cut and carved bones of reindeer. On one of these
sculptured bones in the cave of Perigord, a rude representation of the
mammoth, with its long curved tusks and covering of wool, occurs, which
is regarded by M. Lartet as placing beyond all doubt the fact that the
early inhabitants of these caves must have seen this species of
elephant still living in France. The presence of the marmot, as well as
the reindeer and some other northern animals, in these caverns seems to
imply a colder climate than that of the Swiss lake-dwellings, in which
no remains of reindeer have as yet been discovered. The absence of this
last in the old lacustrine habitations of Switzerland is the more
significant, because in a cave in the neighbourhood of the lake of
Geneva, namely, that of Mont Saleve, bones of the reindeer occur with
flint implements similar to those of the caverns of Dordogne and
Perigord.

The state of the arts, as exemplified by the instruments found in these
caverns of the reindeer period, is somewhat more advanced than that
which characterises the tools of the Amiens drift, but is nevertheless
more rude than that of the Swiss lake-dwellings. No metallic articles
occur, and the stone hatchets are not ground after the fashion of
celts; the needles of bone are shaped in a workmanlike style, having
their eyes drilled with consummate skill.

The formations above alluded to, which are as yet but imperfectly
known, may be classed as belonging to the close of the Paleolithic era,
of the monuments of which I am now about to treat.

Alluvial Deposits of the Paleolithic Age.—The alluvial and marine
deposits of the Paleolithic age, the earliest to which any vestiges of
man have yet been traced back, belong to a time when the physical
geography of Europe differed in a marked degree from that now
prevailing. In the Neolithic period, the valleys and rivers coincided
almost entirely with those by which the present drainage of the land is
effected, and the peat-mosses were the same as those now growing. The
situation of the shell-mounds and lake-dwellings above alluded to is
such as to imply that the topography of the districts where they are
observed has not subsequently undergone any material alteration.
Whereas we no sooner examine the Post-pliocene formations, in which the
remains of so many extinct mammalia are found, than we at once perceive
a more decided discrepancy between the former and present outline of
the surface. Since those deposits originated, changes of considerable
magnitude have been effected in the depth and width of many valleys, as
also in the direction of the superficial and subterranean drainage,
and, as is manifest near the sea-coast, in the relative position of
land and water. In Fig. 87 an ideal section is given, illustrating the
different position which the Recent and Post-pliocene alluvial deposits
occupy in many European valleys.

Fig. 87: Recent and Post-pliocene alluvial deposits.

The peat, No. 1, has been formed in a low part of the modern alluvial
plain, in parts of which gravel No. 2 of the recent period is seen.
Over this gravel the loam or fine sediment 2′ has in many places been
deposited by the river during floods which covered nearly the whole
alluvial plain.

No. 3 represents an older alluvium, composed of sand and gravel, formed
before the valley had been excavated to its present depth. It contains
the remains of fluviatile shells of living species associated with the
bones of mammalia, in part of recent, and in part of extinct species.
Among the latter, the mammoth (_E. primigenius_) and the Siberian
rhinoceros (_R. tichorhinus_) are the most common in Europe. No. 3′ is
a remnant of the loam or brick-earth by which No. 3 was overspread. No.
4 is a still older and more elevated terrace, similar in its
composition and organic remains to No. 3, and covered in like manner
with its inundation-mud, 4′. Sometimes the valley-gravels of older date
are entirely missing, or there is only one, and occasionally there are
more than two, marking as many successive stages in the excavation of
the valley. They usually occur at heights varying from 10 to 100 feet,
sometimes on the right and sometimes on the left side of the existing
river-plain, but rarely in great strength on exactly opposite sides of
the valley.

Among the genera of extinct quadrupeds most frequently met with in
England, France, Germany, and other parts of Europe, are the elephant,
rhinoceros, hippopotamus, horse, great Irish deer, bear, tiger, and
hyæna. In the peat, No. 1 (Fig. 87), and in the more modern gravel and
silt (No. 2), works of art of the ages of iron and bronze, and of the
later or Neolithic stone period, already described, are met with. In
the more ancient or Paleolithic gravels, 3 and 4, there have been found
of late years in several valleys in France and England—as, for example,
in those of the Seine and Somme, and of the Thames and Ouse, near
Bedford—stone implements of a rude type, showing that man coexisted in
those districts with the mammoth and other extinct quadrupeds of the
genera above enumerated. In 1847, M. Boucher de Perthes observed in an
ancient alluvium at Abbeville, in Picardy, the bones of extinct
mammalia associated in such a manner with flint implements of a rude
type as to lead him to infer that both the organic remains and the
works of art were referable to one and the same period. This inference
was soon after confirmed by Mr. Prestwich, who found in 1859 a flint
tool in situ in the same stratum at Amiens that contained the remains
of extinct mammalia.

The flint implements found at Abbeville and Amiens are most of them
considered to be hatchets and spear-heads, and are different from those
commonly called “celts.” These celts, so often found in the recent
formations, have a more regular oblong shape, the result of grinding,
by which also a sharp edge has been given to them. The Abbeville tools
found in gravel at different levels, as in Nos. 3 and 4, Fig. 87, in
which bones of the elephant, rhinoceros, and other extinct mammalia
occur, are always unground, having evidently been brought into their
present form simply by the chipping off of fragments of flint by
repeated blows, such as could be given by a stone hammer.

Some of them are oval, others of a spear-headed form, no two exactly
alike, and yet the greater number of each kind are obviously fashioned
after the same general pattern. Their outer surface is often white, the
original black flint having been discoloured and bleached by exposure
to the air, or by the action of acids, as they lay in the gravel. They
are most commonly stained of the same ochreous colour as the flints of
the gravel in which they are imbedded. Occasionally their antiquity is
indicated not only by their colour but by superficial incrustations of
carbonate of lime, or by dendrites formed of oxide of iron and
manganese. The edges also of most of them are worn, sometimes by having
been used as tools, or sometimes by having been rolled in the old
river’s bed. They are met with not only in the lower-level gravels, as
in No. 3, Fig. 87, but also in No. 4, or the higher gravels, as at St.
Acheul, in the suburbs of Amiens, where the old alluvium lies at an
elevation of about 100 feet above the level of the river Somme. At both
levels fluviatile and land-shells are met with in the loam as well as
in the gravel, but there are no marine shells associated, except at
Abbeville, in the lowest part of the gravel, near the sea, and a few
feet only above the present high-water mark. Here with fossil shells of
living species are mingled the bones of _Elephas primigenius_ and _E.
antiquus, Rhinoceros tichorhinus, Hippopotamus, Felis spelæa, Hyæna
spelæa,_ reindeer, and many others, the bones accompanying the flint
implements in such a manner as to show that both were buried in the old
alluvium at the same period.

Nearly the entire skeleton of a rhinoceros was found at one point,
namely, in the Menchecourt drift at Abbeville, the bones being in such
juxtaposition as to show that the cartilage must have held them
together at the time of their inhumation.

The general absence here and elsewhere of human bones from gravel and
sand in which flint tools are discovered, may in some degree be due to
the present limited extent of our researches. But it may also be
presumed that when a hunter population, always scanty in numbers,
ranged over this region, they were too wary to allow themselves to be
overtaken by the floods which swept away many herbivorous animals from
the low river-plains where they may have been pasturing or sleeping.
Beasts of prey prowling about the same alluvial flats in search of food
may also have been surprised more readily than the human tenant of the
same region, to whom the signs of a coming tempest were better known.

Inundation-mud of Rivers.—Brick-earth.—Fluviatile Loam, or Loess.—As a
general rule, the fluviatile alluvia of different ages (Nos. 2, 3, 4,
Fig. 87) are severally made up of coarse materials in their lower
portions, and of fine silt or loam in their upper parts. For rivers are
constantly shifting their position in the valley-plain, encroaching
gradually on one bank, near which there is deep water, and deserting
the other or opposite side, where the channel is growing shallower,
being destined eventually to be converted into land. Where the current
runs strongest, coarse gravel is swept along, and where its velocity is
slackened, first sand, and then only the finest mud, is thrown down. A
thin film of this fine sediment is spread, during floods, over a wide
area, on one, or sometimes on both sides, of the main stream, often
reaching as far as the base of the bluffs or higher grounds which bound
the valley. Of such a description are the well-known annual deposits of
the Nile, to which Egypt owes its fertility. So thin are they, that the
aggregate amount accumulated in a century is said rarely to exceed five
inches, although in the course of thousands of years it has attained a
vast thickness, the bottom not having been reached by borings extending
to a depth of 60 feet towards the central parts of the valley.
Everywhere it consists of the same homogeneous mud, destitute of
stratification—the only signs of successive accumulation being where
the Nile has silted up its channel, or where the blown sands of the
Libyan desert have invaded the plain, and give rise to alternate layers
of sand and mud.

In European river-loams we occasionally observe isolated pebbles and
angular pieces of stone which have been floated by ice to the places
where they now occur; but no such coarse materials are met with in the
plains of Egypt.

In some parts of the valley of the Rhine the accumulation of similar
loam, called in Germany “loess,” has taken place on an enormous scale.
Its colour is yellowish-grey, and very homogeneous; and Professor
Bischoff has ascertained, by analysis, that it agrees in composition
with the mud of the Nile. Although for the most part unstratified, it
betrays in some places marks of stratification, especially where it
contains calcareous concretions, or in its lower part where it rests on
subjacent gravel and sand which alternate with each other near the
junction. About a sixth part of the whole mass is composed of carbonate
of lime, and there is usually an intermixture of fine quartzose and
micaceous sand.

Although this loam of the Rhine is unsolidified, it usually terminates
where it has been undermined by running water in a vertical cliff, from
the face of which shells of terrestrial, fresh-water and amphibious
mollusks project in relief. These shells do not imply the permanent
sojourn of a body of fresh water on the spot, for the most aquatic of
them, the _Succinea_, inhabits marshes and wet grassy meadows. The
_Succinea elongata_ (or _S. oblongata_), Fig. 88, is very
characteristic both of the loess of the Rhine and of some other
European river-loams.

Fig. 88: Succinea elongata; Fig. 89: Pupa muscorum (Linn.); Fig. 90:
Helix hispida (Linn.) (plebia).

Among the land-shells of the Rhenish loess, _Helix hispida_, Fig. 90,
and _Pupa muscorum_, Fig. 89, are very common. Both the terrestrial and
aquatic shells are of most fragile and delicate structure, and yet they
are almost invariably perfect and uninjured. They must have been broken
to pieces had they been swept along by a violent inundation. Even the
colour of some of the land-shells, as that of _Helix nemoralis_, is
occasionally preserved.

In parts of the valley of the Rhine, between Bingen and Basle, the
fluviatile loam or loess now under consideration is several hundred
feet thick, and contains here and there throughout that thickness land
and amphibious shells. As it is seen in masses fringing both sides of
the great plain, and as occasionally remnants of it occur in the centre
of the valley, forming hills several hundred feet in height, it seems
necessary to suppose, first, a time when it slowly accumulated; and
secondly, a later period, when large portions of it were removed, or
when the original valley, which had been partially filled up with it,
was re-excavated.

Such changes may have been brought about by a great movement of
oscillation, consisting first of a general depression of the land, and
then of a gradual re-elevation of the same. The amount of continental
depression which first took place in the interior, must be imagined to
have exceeded that of the region near the sea, in which case the higher
part of the great valley would have its alluvial plain gradually raised
by an accumulation of sediment, which would only cease when the
subsidence of the land was at an end. If the direction of the movement
was then reversed, and, during the re-elevation of the continent, the
inland region nearest the mountains should rise more rapidly than that
near the coast, the river would acquire a denuding power sufficient to
enable it to sweep away gradually nearly all the loam and gravel with
which parts of its basin had been filled up. Terraces and hillocks of
mud and sand would then alone remain to attest the various levels at
which the river had thrown down and afterwards removed alluvial matter.

Cavern Deposits containing Human Remains and Bones of Extinct
Animals.—In England, and in almost all countries where limestone rocks
abound, caverns are found, usually consisting of cavities of large
dimensions, connected together by low, narrow, and sometimes torturous
galleries or tunnels. These subterranean vaults are usually filled in
part with mud, pebbles, and breccia, in which bones occur belonging to
the same assemblage of animals as those characterising the
Post-pliocene alluvia above described. Some of these bones are
referable to extinct and others to living species, and they are
occasionally intermingled, as in the valley-gravels, with implements of
one or other of the great divisions of the stone age, and these are not
unfrequently accompanied by human bones, which are much more common in
cavern deposits than in valley-alluvium.

Each suite of caverns, and the passages by which they communicate the
one with the other, afford memorials to the geologist of successive
phases through which they must have passed. First, there was a period
when the carbonate of lime was carried out gradually by springs;
secondly, an era when engulfed rivers or occasional floods swept
organic and inorganic debris into the subterranean hollows previously
formed; and thirdly, there were such changes in the configuration of
the region as caused the engulfed rivers to be turned into new
channels, and springs to be dried up, after which the cave-mud,
breccia, gravel, and fossil bones would bear the same kind of relation
to the existing drainage of the country as the older valley-drifts with
their extinct mammalian remains and works of art bear to the present
rivers and alluvial plains.

The quarrying away of large masses of Carboniferous and Devonian
limestone, near Liege, in Belgium, has afforded the geologist
magnificent sections of some of these caverns, and the former
communication of cavities in the interior of the rocks with the old
surface of the country by means of vertical or oblique fissures, has
been demonstrated in places where it would not otherwise have been
suspected, so completely have the upper extremities of these fissures
been concealed by superficial drift, while their lower ends, which
extended into the roofs of the caves, are masked by stalactitic
incrustations.

The origin of the stalactite is thus explained by the eminent chemist
Liebig. Mould or humus, being acted on by moisture and air, evolves
carbonic acid, which is dissolved by rain. The rain-water, thus
impregnated, permeates the porous limestone, dissolves a portion of it,
and afterwards, when the excess of carbonic acid evaporates in the
caverns, parts with the calcareous matter, and forms stalactite. Even
while caverns are still liable to be occasionally flooded such
calcareous incrustations accumulate, but it is generally when they are
no longer in the line of drainage that a solid floor of hard stalagmite
is formed on the bottom.

The late Dr. Schmerling examined forty caves near Liege, and found in
all of them the remains of the same fauna, comprising the mammoth,
tichorhine rhinoceros, cave-bear, cave-hyæna, cave-lion, and many
others, some of extinct and some of living species, and in all of them
flint implements. In four or five caves only parts of human skeletons
were met with, comprising sometimes skulls with a few other bones,
sometimes nearly every part of the skeleton except the skull. In one of
the caves, that of Engihoul, where Schmerling had found the remains of
at least three human individuals, they were mingled in such a manner
with bones of extinct mammalia, as to leave no doubt on his mind (in
1833) of man having co-existed with them.

In 1860, Professor Malaise, of Liege, explored with me this same cave
of Engihoul, and beneath a hard floor of stalagmite we found mud full
of bones of extinct and recent animals, such as Schmerling had
described, and my companion, persevering in his researches after I had
returned to England, extracted from the same deposit two human lower
jaw-bones retaining their teeth. The skulls from these Belgian caverns
display no marked deviation from the normal European type of the
present day.

The careful investigations carried on by Dr. Falconer, Mr. Pengelly,
and others, in the Brixham cave near Torquay, in 1858, demonstrated
that flint knives were there imbedded in such a manner in loam
underlying a floor of stalagmite as to prove that man had been an
inhabitant of that region when the cave-bear and other members of the
ancient post-pliocene fauna were also in existence.

The absence of gnawed bones had led Dr. Schmerling to infer that none
of the Belgian caves which he explored had served as the dens of wild
beasts; but there are many caves in Germany and England which have
certainly been so inhabited, especially by the extinct hyæna and bear.

A fine example of a hyæna’s den was afforded by the cave of Kirkdale,
so well described by the late Dr. Buckland in his _Reliquiæ Diluvianæ._
In that cave, above twenty-five miles north-north-east of York, the
remains of about 300 hyænas, belonging to individuals of every age,
were detected. The species (_Hyæna spelæa_) has been considered by
palæontologists as extinct; it was larger than the fierce _Hyæna
crocuta_ of South Africa, which it closely resembled, and of which it
is regarded by Mr. Boyd Dawkins as a variety. Dr. Buckland, after
carefully examining the spot, proved that the hyænas must have lived
there; a fact attested by the quantity of their dung, which, as in the
case of the living hyæna, is of nearly the same composition as bone,
and almost as durable. In the cave were found the remains of the ox,
young elephant, hippopotamus, rhinoceros, horse, bear, wolf, hare,
water-rat, and several birds. All the bones have the appearance of
having been broken and gnawed by the teeth of the hyænas; and they
occur confusedly mixed in loam or mud, or dispersed through a crust of
stalagmite which covers it. In these and many other cases it is
supposed that portions of herbivorous quadrupeds have been dragged into
caverns by beasts of prey, and have served as their food—an opinion
quite consistent with the known habits of the living hyæna.

_Australian Cave-breccias._—Ossiferous breccias are not confined to
Europe, but occur in all parts of the globe; and those discovered in
fissures and caverns in Australia correspond closely in character with
what has been called the bony breccia of the Mediterranean, in which
the fragments of bone and rock are firmly bound together by a red
ochreous cement.

Fig. 91: Part of a lower jaw of Macropus atlas.

Some of these caves were examined by the late Sir T. Mitchell in the
Wellington Valley, about 210 miles west of Sidney, on the river Bell,
one of the principal sources of the Macquarie, and on the Macquarie
itself. The caverns often branch off in different directions through
the rock, widening and contracting their dimensions, and the roofs and
floors are covered with stalactite. The bones are often broken, but do
not seem to be water-worn. In some places they lie imbedded in loose
earth, but they are usually included in a breccia.

The remains belong to marsupial animals. Among the most abundant are
those of the kangaroo, of which there are four species, while others
belong to the genera _Phascolomys_, the wombat; _Dasyurus_), the ursine
opossum; _Phalangista_, the vulpine opossum; and _Hypsiprymnus_, the
kangaroo-rat.

Fig. 92: Lower jaw of largest living species of kangaroo.

In the fossils above enumerated, several species are larger than the
largest living ones of the same genera now known in Australia. Fig. 91
of the right side of a lower jaw of a kangaroo (_Macropus atlas_, Owen)
will at once be seen to exceed in magnitude the corresponding part of
the largest living kangaroo, which is represented in Fig. 92. In both
these specimens part of the substance of the jaw has been broken open,
so as to show the permanent false molar (_a_, Fig. 91), concealed in
the socket. From the fact of this molar not having been cut, we learn
that the individual was young, and had not shed its first teeth.

The reader will observe that all these extinct quadrupeds of Australia
belong to the marsupial family, or, in other words, that they are
referable to the same peculiar type of organisation which now
distinguishes the Australian mammalia from those of other parts of the
globe. This fact is one of many pointing to a general law deducible
from the fossil vertebrate and invertebrate animals of times
immediately antecedent to our own, namely, that the present
geographical distribution of organic _forms_ dates back to a period
anterior to the origin of existing _species_; in other words, the
limitation of particular genera or families of quadrupeds, mollusca,
etc., to certain existing provinces of land and sea, began before the
larger part of the species now contemporary with man had been
introduced into the earth.

Professor Owen, in his excellent “History of British Fossil Mammals,”
has called attention to this law, remarking that the fossil quadrupeds
of Europe and Asia differ from those of Australia or South America. We
do not find, for example, in the Europæo-Asiatic province fossil
kangaroos, or armadillos, but the elephant, rhinoceros, horse, bear,
hyæna, beaver, hare, mole, and others, which still characterise the
same continent.

In like manner, in the Pampas of South America the skeletons of
Megatherium, Megalonyx, Glyptodon, Mylodon, Toxodon, Macrauchenia, and
other extinct forms, are analogous to the living sloth, armadillo,
cavy, capybara, and llama. The fossil quadrumana, also associated with
some of these forms in the Brazilian caves, belong to the Platyrrhine
family of monkeys, now peculiar to South America. That the extinct
fauna of Buenos Ayres and Brazil was very modern has been shown by its
relation to deposits of marine shells, agreeing with those now
inhabiting the Atlantic.

The law of geographical relationship above alluded to, between the
living vertebrata of every great zoological province and the fossils of
the period immediately antecedent, even where the fossil species are
extinct, is by no means confined to the mammalia. New Zealand, when
first examined by Europeans, was found to contain no indigenous land
quadrupeds, no kangaroos, or opossums, like Australia; but a wingless
bird abounded there, the smallest living representative of the ostrich
family, called the Kiwi by the natives (_Apteryx_). In the fossils of
the Post-pliocene period in this same island, there is the like absence
of kangaroos, opossums, wombats, and the rest; but in their place a
prodigious number of well-preserved specimens of gigantic birds of the
struthious order, called by Owen _Dinornis_ and _Palapteryx_, which are
entombed in superficial deposits. These genera comprehended many
species, some of which were four, some seven, others nine, and others
eleven feet in height! It seems doubtful whether any contemporary
mammalia shared the land with this population of gigantic feathered
bipeds.

Mr. Darwin, when describing the recent and fossil mammalia of South
America, has dwelt much on the wonderful relationship of the extinct to
the living types in that part of the world, inferring from such
geographical phenomena that the existing species are all related to the
extinct ones which preceded them by a bond of common descent.

Climate of the Post-pliocene Period.—The evidence as to the climate of
Europe during this epoch is somewhat conflicting. The fluviatile and
land-shells are all of existing species, but their geographical range
has not always been the same as at present. Some, for example, which
then lived in Britain are now only found in Norway and Finland,
probably implying that the Post-pliocene climate of Britain was colder,
especially in the winter. So also the reindeer and the musk-ox (_Ovibos
moschatus_), now inhabitants of the Arctic regions, occur fossil in the
valleys of the Thames and Avon, and also in France and Germany,
accompanied in most places by the mammoth and the woolly rhinoceros. At
Grays in Essex, on the other hand, another species both of elephant and
rhinoceros occurs, together with a hippopotamus and the _Cyrena
fluminalis_, a shell now extinct in Europe but still an inhabitant of
the Nile and some Asiatic rivers. With it occurs the _Unio littoralis_,
now living in the Seine and Loire. In the valley of the Somme flint
tools have been found associated with _Hippopotamus major_ and _Cyrena
fluminalis_ in the lower-level Post-pliocene gravels; while in the
higher-level (and more ancient) gravels similar tools are more
abundant, and are associated with the bones of the mammoth and other
Post-pliocene quadrupeds indicative of a colder climate.

It is possible that we may here have evidence of summer and winter
migrations rather than of a general change of temperature. Instead of
imagining that the hippopotamus lived all the year round with the
musk-ox and lemming, we may rather suppose that the apparently
conflicting evidence may be due to the place of our observations being
near the boundary line of a northern and southern fauna, either of
which may have advanced or receded during comparatively slight and
temporary fluctuations of climate. There may then have been a
continuous land communication between England and the North of Siberia,
as well as in an opposite direction with Africa, then united to
Southern Europe.

In drift at Fisherton, near Salisbury, thirty feet above the river
Wiley, the Greenland lemming and a new species of the Arctic genus
_Spermophilus_ have been found, along with the mammoth, reindeer,
cave-hyæna, and other mammalia suited to a cold climate. A flint
implement was taken out from beneath the bones of the mammoth. In a
higher and older deposit in the vicinity, flint tools like those of
Amiens have been discovered. Nearly all the known Post-pliocene
quadrupeds have now been found accompanying flint knives or hatchets in
such a way as to imply their coexistence with man; and we have thus the
concurrent testimony of several classes of geological facts to the vast
antiquity of the human race. In the first place, the disappearance of a
great variety of species of wild animals from every part of a wide
continent must have required a vast period for its accomplishment; yet
this took place while man existed upon the earth, and was completed
before that early period when the Danish shell-mounds were formed or
the oldest of the Swiss lake-dwellings constructed. Secondly, the
deepening and widening of valleys, indicated by the position of the
river gravels at various heights, implies an amount of change of which
that which has occurred during the historical period forms a scarcely
perceptible part. Thirdly, the change in the course of rivers which
once flowed through caves now removed from any line of drainage, and
the formation of solid floors of stalagmite, must have required a great
lapse of time. Lastly, ages must have been required to change the
climate of wide regions to such an extent as completely to alter the
geographical distribution of many mammalia as well as land and
fresh-water shells. The 3000 or 4000 years of the historical period
does not furnish us with any appreciable measure for calculating the
number of centuries which would suffice for such a series of changes,
which are by no means of a local character, but have operated over a
considerable part of Europe.

Relative Longevity of Species in the Mammalia and Testacea.—I called
attention in 1830[5] to the fact, which had not at that time attracted
notice, that the association in the Post-pliocene deposits of shells,
exclusively of living species, with many extinct quadrupeds betokened a
longevity of species in the testacea far exceeding that in the
mammalia. Subsequent researches seem to show that this greater duration
of the same specific forms in the class mollusca is dependent on a
still more general law, namely, that the lower the grade of animals, or
the greater the simplicity of their structure, the more persistent are
they in general in their specific characters throughout vast periods of
time. Not only have the invertebrata, as shown by geological data,
altered at a less rapid rate than the vertebrata, but if we take one of
the classes of the former, as for example the mollusca, we find those
of more simple structure to have varied at a slower rate than those of
a higher and more complex organisation; the Brachiopoda, for example,
more slowly than the lamellibranchiate bivalves, while the latter have
been more persistent than the univalves, whether gasteropoda or
cephalopoda. In like manner the specific identity of the characters of
the foraminifera, which are among the lowest types of the invertebrata,
has outlasted that of the mollusca in an equally decided manner.

Teeth of Post-pliocene Mammalia.—To those who have never studied
comparative anatomy, it may seem scarcely credible that a single bone
taken from any part of the skeleton may enable a skilful osteologist to
distinguish, in many cases, the genus, and sometimes the species, of
quadrupeds to which it belonged. Although few geologists can aspire to
such knowledge, which must be the result of long practice and study,
they will nevertheless derive great advantage from learning, what is
comparatively an easy task, to distinguish the principal divisions of
the mammalia by the forms and characters of their teeth.

Fig. 93: Elephas primigenius (or Mammoth) molar of upper jaw, right
side. Post-pliocene; Fig. 94: Elephas antiquus, Falconer. Penultimate
molar. Post-pliocene and Pliocene.

Figures 93 through 105 represent the teeth of some of the more common
species and genera found in alluvial and cavern deposits.

Figs. 95 to 100: Teeth of extinct mammalia.

Figs. 101 to 105: Teeth of extinct mammalia.

On comparing the grinding surfaces of the corresponding molars of the
three species of elephants, Figs. 93, 94, 95 it will be seen that the
folds of enamel are most numerous in the mammoth, fewer and wider, or
more open, in _E. antiquus_; and most open and fewest in _E.
meridionalis._ It will be also seen that the enamel in the molar of the
_Rhinoceros tichorhinus_ (Fig. 97), is much thicker than in that of the
_Rhinoceros leptorhinus_ (Fig. 96).

 [1] Sir John Lubbock, Pre-historic Times, p. 3, 1865.

 [2] Bulletin de la Sociétié Vaudoise des Sci. Nat., tome vi, Lausanne
 1860; and Antiquity of Man, by the author, chap. ii.

 [3] Sir J. Lubbock’s Lecture, Royal Institution, Feb. 27th, 1863.

 [4] Diodorus, v, 21, 22 and Sir H. James Note on Block of Tin dredged
 up in Falmouth Harbour. Royal Institution of Cornwall, 1863.

 [5] Principles of Geology, 1st ed., vol. iii, p. 140.




CHAPTER XI.
POST-PLIOCENE PERIOD, continued—GLACIAL CONDITIONS.[1]


Geographical Distribution, Form, and Characters of Glacial Drift. —
Fundamental Rocks, polished, grooved, and scratched. — Abrading and
striating Action of Glaciers. — Moraines, Erratic Blocks, and “Roches
Moutonnees.” Alpine Blocks on the Jura. — Continental Ice of Greenland.
— Ancient Centres of the Dispersion of Erratics. — Transportation of
Drift by floating Icebergs. — Bed of the Sea furrowed and polished by
the running aground of floating Ice-islands.

Character and Distribution of Glacial Drift.—In speaking of the loose
transported matter commonly found on the surface of the land in all
parts of the globe, I alluded to the exceptional character of what has
been called the boulder formation in the temperate and Arctic latitudes
of the northern hemisphere. The peculiarity of its form in Europe north
of the 50th, and in North America north of the 40th parallel of
latitude, is now universally attributed to the action of ice, and the
difference of opinion respecting it is now chiefly restricted to the
question whether land-ice or floating icebergs have played the chief
part in its distribution. It is wanting in the warmer and equatorial
regions, and reappears when we examine the lands which lie south of the
40th and 50th parallels in the southern hemisphere, as, for example, in
Patagonia, Tierra del Fuego, and New Zealand. It consists of sand and
clay, sometimes stratified, but often wholly devoid of stratification
for a depth of 50, 100, or even a greater number of feet. To this
unstratified form of the deposit the name of _till_ has long been
applied in Scotland. It generally contains a mixture of angular and
rounded fragments of rock, some of large size, having occasionally one
or more of their sides flattened and smoothed, or even highly polished.
The smoothed surfaces usually exhibit many scratches parallel to each
other, one set of which often crosses an older set. The till is almost
everywhere wholly devoid of organic remains, except those washed into
it from older formations, though in some places it contains marine
shells, usually of northern or Arctic species, and frequently in a
fragmentary state. The bulk of the till has usually been derived from
the grinding down into mud of rocks in the immediate neighbourhood, so
that it is red in a region of Red Sandstone, as in Strathmore in
Forfarshire; grey or black in a district of coal and bituminous shale,
as around Edinburgh; and white in a chalk country, as in parts of
Norfolk and Denmark. The stony fragments dispersed irregularly through
the till usually belong, especially in mountainous countries, to rocks
found in some part of the same hydrographical basin; but there are
regions where the whole of the boulder clay has come from a distance,
and huge blocks, or “erratics,” as they have been called, many feet in
diameter, have not unfrequently travelled hundreds of miles from their
point of departure, or from the parent rocks from which they have
evidently been detached. These are commonly angular, and have often one
or more of their sides polished and furrowed.

The rock on which the boulder formation reposes, if it consists of
granite, gneiss, marble, or other hard stone, capable of permanently
retaining any superficial markings which may have been imprinted upon
it, is usually smoothed or polished, like the erratics above described,
and exhibits parallel striæ and furrows having a determinate direction.
This direction, both in Europe and North America, agrees generally in a
marked manner with the course taken by the erratic blocks in the same
district. The boulder clay, when it was first studied, seemed in many
of its characters so singular and anomalous, that geologists despaired
of ever being able to interpret the phenomena by reference to causes
now in action. In those exceptional cases where marine shells of the
same date as the boulder clay were found, nearly all of them were
recognised as living species—a fact conspiring with the superficial
position of the drift to indicate a comparatively modern origin.

The term “diluvium” was for a time the most popular name of the boulder
formation, because it was referred by many to the deluge of Noah, while
others retained the name as expressive of their opinion that a series
of diluvial waves raised by hurricanes and storms, or by earthquakes,
or by the sudden upheaval of land from the bed of the sea, had swept
over the continents, carrying with them vast masses of mud and heavy
stones, and forcing these stones over rocky surfaces so as to polish
and imprint upon them long furrows and striæ. But geologists were not
long in seeing that the boulder formation was characteristic of high
latitudes, and that on the whole the size and number of erratic blocks
increases as we travel towards the Arctic regions. They could not fail
to be struck with the contrast which the countries bordering the Baltic
presented when compared with those surrounding the Mediterranean. The
multitude of travelled blocks and striated rocks in the one region, and
the absence of such appearances in the other, were too obvious to be
overlooked. Even the great development of the boulder formation, with
large erratics so far south as the Alps, offered an exception to the
general rule favourable to the hypothesis that there was some intimate
connection between it and accumulations of snow and ice.

Fig. 106: Limestone, polished, furrowed, and scratched by the glacier
of Rosenlau in Switzerland.

Transporting and abrading Power of Glaciers.—I have described elsewhere
(“Principles” vol. i, chap. xvi, 1867) the manner in which the snow of
the Alpine heights is prevented from accumulating indefinitely in
thickness by the constant descent of a large portion of it by
gravitation. Becoming converted into ice it forms what are termed
glaciers, which glide down the principal valleys. On their surface are
seen mounds of rubbish or large heaps of sand and mud, with angular
fragments of rock which fall from the steep slopes or precipices
bounding the glaciers. When a glacier, thus laden, descends so far as
to reach a region about 3500 feet above the level of the sea, the
warmth of the air is such that it melts rapidly in summer, and all the
mud, sand, and pieces of rock are slowly deposited at its lower end,
forming a confused heap of unstratified rubbish called a _moraine_, and
resembling the _till_ before described (p. 166).

Besides the blocks thus carried down on the top of the glacier, many
fall through fissures in the ice to the bottom, where some of them
become firmly frozen into the mass, and are pushed along the base of
the glacier, abrading, polishing, and grooving the rocky floor below,
as a diamond cuts glass, or as emery-powder polishes steel. The striæ
which are made, and the deep grooves which are scooped out by this
action, are rectilinear and parallel to an extent never seen in those
produced on loose stones or rocks, where shingle is hurried along by a
torrent, or by the waves on a sea-beach. In addition to these polished,
striated, and grooved surfaces of rock, another mark of the former
action of a glacier is the “roche moutonnee.” Projecting eminences of
rock so called have been smoothed and worn into the shape of flattened
domes by the glacier as it passed over them. They have been traced in
the Alps to great heights above the present glaciers, and to great
horizontal distances beyond them.

Alpine Blocks on the Jura.—The moraines, erratics, polished surfaces,
domes, and striæ, above described, are observed in the great valley of
Switzerland, fifty miles broad; and almost everywhere on the Jura, a
chain which lies to the north of this valley. The average height of the
Jura is about one-third that of the Alps, and it is now entirely
destitute of glaciers; yet it presents almost everywhere similar
moraines, and the same polished and grooved surfaces. The erratics,
moreover, which cover it, present a phenomenon which has astonished and
perplexed the geologist for more than half a century. No conclusion can
be more incontestable than that these angular blocks of granite,
gneiss, and other crystalline formations came from the Alps, and that
they have been brought for a distance of fifty miles and upward across
one of the widest and deepest valleys in the world; so that they are
now lodged on a chain composed of limestone and other formations,
altogether distinct from those of the Alps. Their great size and
angularity, after a journey of so many leagues, has justly excited
wonder; for hundreds of them are as large as cottages; and one in
particular, composed of gneiss, celebrated under the name of Pierre à
Bot, rests on the side of a hill about 900 feet above the lake of
Neufchâtel, and is no less than 40 feet in diameter.

In the year 1821, M. Venetz first announced his opinion that the Alpine
glaciers must formerly have extended far beyond their present limits,
and the proofs appealed to by him in confirmation of this doctrine were
acknowledged by all subsequent observers, and greatly strengthened by
new observations and arguments. M. Charpentier supposed that when the
glaciers extended continuously from the Alps to the Jura, the former
mountains were 2000 or 3000 feet higher than at present. Other writers,
on the contrary, conjectured that the whole country had been submerged,
and the moraines and erratic blocks transported on floating icebergs;
but a careful study of the distribution of the travelled masses, and
the total absence of marine shells from the old glacial drift of
Switzerland, have entirely disproved this last hypothesis. In addition
to the many evidences of the action of ice in the northern parts of
Europe which we have already mentioned, there occur here and there in
some of these countries, what are wanting in Switzerland, deposits of
marine fossil shells, which exhibit so arctic a character that they
must have led the geologist to infer the former prevalence of a much
colder climate, even had he not encountered so many accompanying signs
of ice-action. The same marine shells demonstrate the submergence of
large areas in Scandinavia and the British Isles, during the glacial
cold.

A characteristic feature of the deposits under consideration in all
these countries is the occurrence of large erratic blocks, and
sometimes of moraine matter, in situations remote from lofty mountains,
and separated from the nearest points where the parent rocks appear at
the surface by great intervening valleys, or arms of the sea. We also
often observe striæ and furrows, as in Norway, Sweden, and Scotland,
which deviate from the direction which they ought to follow if they had
been connected with the present line of drainage, and they, therefore,
imply the prevalence of a very distinct condition of things at the time
when the cold was most intense. The actual state of North Greenland
seems to afford the best explanation of such abnormal glacial markings.

Greenland Continental Ice.—Greenland is a vast unexplored continent,
buried under one continuous and colossal mass of ice that is always
moving seaward, a very small part of it in an easterly direction, and
all the rest westward, or towards Baffin’s Bay. All the minor ridges
and valleys are levelled and concealed under a general covering of
snow, but here and there some steep mountains protrude abruptly from
the icy slope, and a few superficial lines of stones or moraines are
visible at certain seasons, when no snow has fallen for many months,
and when evaporation, promoted by the wind and sun, has caused much of
the upper snow to disappear. The height of this continent is unknown,
but it must be very great, as the most elevated lands of the outskirts,
which are described as comparatively low, attain altitudes of 4000 to
6000 feet. The icy slope gradually lowers itself towards the outskirts,
and then terminates abruptly in a mass about 2000 feet in thickness,
the great discharge of ice taking place through certain large friths,
which, at their upper ends, are usually about four miles across. Down
these friths the ice is protruded in huge masses, several miles wide,
which continue their course—grating along the rocky bottom like
ordinary glaciers long after they have reached the salt water. When at
last they arrive at parts of Baffin’s Bay deep enough to buoy up
icebergs from 1000 to 1500 feet in vertical thickness, broken masses of
them float off, carrying with them on their surface not only fine mud
and sand but large stones. These fragments of rock are often polished
and scored on one or more sides, and as the ice melts, they drop down
to the bottom of the sea, where large quantities of mud are deposited,
and this muddy bottom is inhabited by many mollusca.

Although the direction of the ice-streams in Greenland may coincide in
the main with that which separate glaciers would take if there were no
more ice than there is now in the Swiss Alps, yet the striation of the
surface of the rocks on an ice-clad continent would, on the whole, vary
considerably in its minor details from that which would be imprinted on
rocks constituting a region of separate glaciers. For where there is a
universal covering of ice there will be a general outward movement from
the higher and more central regions towards the circumference and lower
country, and this movement will be, to a certain extent, independent of
the minor inequalities of hill and valley, when these are all reduced
to one level by the snow. The moving ice may sometimes cross even at
right angles deep narrow ravines, or the crests of buried ridges, on
which last it may afterwards seem strange to detect glacial striæ and
polishing after the liquefaction of the snow and ice has taken place.

Rink mentions that in North Greenland powerful springs of clayey water
escape in winter from under the ice, where it descends to “the
outskirts,” and where, as already stated, it is often 2000 feet thick—a
fact showing how much grinding action is going on upon the surface of
the subjacent rocks. I also learn from Dr. Torell that there are large
areas in the outskirts, now no longer covered with permanent snow or
glaciers, which exhibit on their surface unmistakable signs of ancient
ice-action, so that, vast as is the power now exerted by ice in
Greenland, it must once have operated on a still grander scale. The
land, though now very elevated, may perhaps have been formerly much
higher. It is well-known that the south coast of Greenland, from
latitude 60° to about 70° N., has for the last four centuries been
sinking at the rate of several feet in a century. By this means a
surface of rock, well scored and polished by ice, is now slowly
subsiding beneath the sea, and is becoming strewed over, as the
icebergs melt, with impalpable mud and smoothed and scratched stones.
It is not precisely known how far north this downward movement extends.

Drift carried by Icebergs.—An account was given so long ago as the year
1822, by Scoresby, of icebergs seen by him in the Arctic seas drifting
along in latitudes 69° and 70° N., which rose above the surface from
100 to 200 feet, and some of which measured a mile in circumference.
Many of them were loaded with beds of earth and rock, of such thickness
that the weight was conjectured to be from 50,000 to 100,000 tons. A
similar transportation of rocks is known to be in progress in the
southern hemisphere, where boulders included in ice are far more
frequent than in the north. One of these icebergs was encountered in
1839, in mid-ocean, in the antarctic regions, many hundred miles from
any known land, sailing northward, with a large erratic block firmly
frozen into it. Many of them, carefully measured by the officers of the
French exploring expedition of the Astrolabe, were between 100 and 225
feet high above water, and from two to five miles in length. Captain
d’Urville ascertained one of them which he saw floating in the Southern
Ocean to be 13 miles long and 100 feet high, with walls perfectly
vertical. The submerged portions of such islands must, according to the
weight of ice relatively to sea-water, be from six to eight times more
considerable than the part which is visible, so that when they are once
fairly set in motion, the mechanical force which they might exert
against any obstacle standing in their way would be prodigious.

We learn, therefore, from a study both of the arctic and antarctic
regions, that a great extent of land may be entirely covered throughout
the whole year by snow and ice, from the summits of the loftiest
mountains to the sea-coast, and may yet send down angular erratics to
the ocean. We may also conclude that such land will become in the
course of ages almost everywhere scored and polished like the rocks
which underlie a glacier. The discharge of ice into the surrounding sea
will take place principally through the main valleys, although these
are hidden from our sight. Erratic blocks and moraine matter will be
dispersed somewhat irregularly after reaching the sea, for not only
will prevailing winds and marine currents govern the distribution of
the drift, but the shape of the submerged area will have its influence;
inasmuch as floating ice, laden with stones, will pass freely through
deep water, while it will run a ground where there are reefs and
shallows. Some icebergs in Baffin’s Bay have been seen stranded on a
bottom 1000 or even 1500 feet deep. In the course of ages such a
sea-bed may become densely covered with transported matter, from which
some of the adjoining greater depths may be free. If, as in West
Greenland, the land is slowly sinking, a large extent of the bottom of
the ocean will consist of rock polished and striated by land-ice, and
then overspread by mud and boulders detached from melting bergs.

The mud, sand, and boulders thus let fall in still water must be
exactly like the moraines of terrestrial glaciers, devoid of
stratification and organic remains. But occasionally, on the outer side
of such packs of stranded bergs, the waves and currents may cause the
detached earthy and stony materials to be sorted according to size and
weight before they reach the bottom, and to acquire a stratified
arrangement.

I have already alluded (p. 172) to the large quantity of ice,
containing great blocks of stone, which is sometimes seen floating far
from land, in the southern or Antarctic seas. After the emergence,
therefore, of such a submarine area, the superficial detritus will have
no necessary relation to the hills, valleys, and river-plains over
which it will be scattered. Many a water-shed may intervene between the
starting-point of each erratic or pebble and its final resting-place,
and the only means of discovering the country from which it took its
departure will consist in a careful comparison of its mineral or fossil
contents with those of the parent rocks.

 [1] As to the former excess of cold, whether brought about by
 modifications in the height and distribution of the land or by altered
 astronomical conditions, see Principles, vol. i, (10th ed., 1867),
 chaps. xii and xiii, “Vicissitudes of Climate.”




CHAPTER XII.
POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS, concluded.


Glaciation of Scandinavia and Russia. — Glaciation of Scotland. —
Mammoth in Scotch Till. — Marine Shells in Scotch Glacial Drift. —
Their Arctic Character. — Rarity of Organic Remains in Glacial
Deposits. — Contorted Strata in Drift. — Glaciation of Wales, England,
and Ireland. — Marine Shells of Moel Tryfaen. — Erratics near
Chichester. — Glacial Formations of North America. — Many Species of
Testacea and Quadrupeds survived the Glacial Cold. — Connection of the
Predominance of Lakes with Glacial Action. — Action of Ice in
preventing the silting up of Lake-basins. — Absence of Lakes in the
Caucasus. — Equatorial Lakes of Africa.

Glaciation of Scandinavia and Russia.—In large tracts of Norway and
Sweden, where there have been no glaciers in historical times, the
signs of ice-action have been traced as high as 6000 feet above the
level of the sea. These signs consist chiefly of polished and furrowed
rock-surfaces, of moraines and erratic blocks. The direction of the
erratics, like that of the furrows, has usually been conformable to the
course of the principal valleys; but the lines of both sometimes
radiate outward in all directions from the highest land, in a manner
which is only explicable by the hypothesis above alluded to of a
general envelope of continental ice, like that of Greenland (page 170).
Some of the far-transported blocks have been carried from the central
parts of Scandinavia towards the Polar regions; others southward to
Denmark; some south-westward, to the coast of Norfolk in England;
others south-eastward, to Germany, Poland, and Russia.

In the immediate neighbourhood of Upsala, in Sweden, I had observed, in
1834, a ridge of stratified sand and gravel, in the midst of which
occurs a layer of marl, evidently formed originally at the bottom of
the Baltic, by the slow growth of the mussel, cockle, and other marine
shells of living species, intermixed with some proper to fresh water.
The marine shells are all of dwarfish size, like those now inhabiting
the brackish waters of the Baltic; and the marl, in which many of them
are imbedded, is now raised more than 100 feet above the level of the
Gulf of Bothnia. Upon the top of this ridge repose several huge
erratics, consisting of gneiss for the most part unrounded, from nine
to sixteen feet in diameter, and which must have been brought into
their present position since the time when the neighbouring gulf was
already characterised by its peculiar fauna. Here, therefore, we have
proof that the transport of erratics continued to take place, not
merely when the sea was inhabited by the existing testacea, but when
the north of Europe had already assumed that remarkable feature of its
physical geography which separates the Baltic from the North Sea, and
causes the Gulf of Bothnia to have only one-fourth of the saltness
belonging to the ocean. In Denmark, also, recent shells have been found
in stratified beds, closely associated with the boulder clay.

Glaciation of Scotland.—Mr. T. F. Jamieson, in 1858, adduced a great
body of facts to prove that the Grampians once sent down glaciers from
the central regions in all directions towards the sea. “The glacial
grooves,” he observed, “radiate outward from the central heights
towards all points of the compass, though they do not always strictly
conform to the actual shape and contour of the minor valleys and
ridges.”

These facts and other characteristics of the Scotch drift lead us to
the inference that when the glacial cold first set in, Scotland stood
higher above the sea than at present, and was covered for the most part
with snow and ice, as Greenland is now. This sheet of land-ice sliding
down to lower levels, ground down and polished the subjacent rocks,
sweeping off nearly all superficial deposits of older date, and leaving
only till and boulders in their place. To this continental state
succeeded a period of depression and partial submergence. The sea
advanced over the lower lands, and Scotland was converted into an
archipelago, some marine sand with shells being spread over the bottom
of the sea. On this sand a great mass of boulder clay usually quite
devoid of fossils was accumulated. Lastly, the land re-emerged from the
water, and, reaching a level somewhat above its present height, became
connected with the continent of Europe, glaciers being formed once more
in the higher regions, though the ice probably never regained its
former extension.[1] After all these changes, there were some minor
oscillations in the level of the land, on which, although they have had
important geographical consequences, separating Ireland from England,
for example, and England from the Continent, we need not here enlarge.

_Mammoth in Scotch Till._—Almost all remains of the terrestrial fauna
of the Continent which preceded the period of submergence have been
lost; but a few patches of estuarine and fresh-water formations escaped
denudation by submergence. To these belong the peaty clay from which
several mammoths’ tusks and horns of reindeer were obtained at
Kilmaurs, in Ayrshire as long ago as 1816. Mr. Bryce in 1865
ascertained that the fresh-water formation containing these fossils
rests on carboniferous sandstone, and is covered, first by a bed of
marine sand with arctic shells, and then with a great mass of till with
glaciated boulders.[2] Still more recent explorations in the
neighbourhood of Kilmaurs have shown that the fresh-water formation
contains the seed of the pond-weed _Potamogeton_ and the aquatic
Ranunculus; and Mr. Young of the Glasgow Museum washed the mud adhering
to the reindeer horns of Kilmaurs and that which filled the cracks of
the associated elephants’ tusks, and detected in these fossils (which
had been in the Glasgow Museum for half a century) abundance of the
same seeds.

All doubts, therefore, as to the true position of the remains of the
mammoth, a fossil so rare in Scotland, have been set at rest, and it
serves to prove that part of the ancient continent sank beneath the sea
at a period of great cold, as the shells of the overlying sand attest.
The incumbent till or boulder clay is about 40 feet thick, but it often
attains much greater thickness in the same part of Scotland.

Figs. 107-112: Northern shells common in the drift of the Clyde, in
Scotland.

_Marine Shells of Scotch Drift._—The greatest height to which marine
shells have yet been traced in this boulder clay is at Airdie, in
Lanarkshire, ten miles east of Glasgow, 524 feet above the level of the
sea. At that spot they were found imbedded in stratified clays with
till above and below them. There appears no doubt that the overlying
deposit was true glacial till, as some boulders of granite were
observed in it, which must have come from distances of sixty miles at
the least.

Fig. 113: Leda truncata; Fig. 114: Tellina calcarea, Chem.

The shells figured in Figs. 107 to 112 are only a few out of a large
assemblage of living species, which, taken as a whole, bear testimony
to conditions far more arctic than those now prevailing in the Scottish
seas. But a group of marine shells, indicating a still greater excess
of cold, has been brought to light since 1860 by the Reverend Thomas
Brown, from glacial drift or clay on the borders of the estuaries of
the Forth and Tay. This clay occurs at Elie, in Fife, and at Errol, in
Perthshire; and has already afforded about 35 shells, all of living
species, and now inhabitants of arctic regions, such as _Leda truncata,
Tellina proxima_ (see Figs. 113 and 114), _Pecten Grœnlandicus,
Crenella lævigata, Crenella nigra,_ and others, some of them first
brought by Captain Sir E. Parry from the coast of Melville Island,
latitude 76° N. These were all identified in 1863 by Dr. Torell, who
had just returned from a survey of the seas around Spitzbergen, where
he had collected no less than 150 species of mollusca, living chiefly
on a bottom of fine mud derived from the moraines of melting glaciers
which there protrude into the sea. He informed me that the fossil fauna
of this Scotch glacial deposit exhibits not only the species but also
the peculiar varieties of mollusca now characteristic of very high
latitudes. Their large size implies that they formerly enjoyed a
colder, or, what was to them a more genial climate, than that now
prevailing in the latitude where the fossils occur. Marine shells have
also been found in the glacial drift of Caithness and Aberdeenshire at
heights of 250 feet, and in Banff of 350 feet, and stratified drift
continuous with the above ascends to heights of 500 feet. Already 75
species are enumerated from Caithness, and the same number from
Aberdeenshire and Banff, and in both cases all but six are arctic
species.

I formerly suggested that the absence of all signs of organic life in
the Scotch drift might be connected with the severity of the cold, and
also in some places with the depth of the sea during the period of
extreme submergence; but my faith in such an hypothesis has been shaken
by modern investigations, an exuberance of life having been observed
both in arctic and antarctic seas of great depth, and where floating
ice abounds. The difficulty, moreover, of accounting for the entire
dearth of marine shells in till is removed when once we have adopted
the theory of this boulder clay being the product of land-ice. For
glaciers coming down from a continental ice-sheet like that which
covers Greenland may fill friths many hundred feet below the sea-level,
and even invade parts of a bay a thousand feet deep, before they find
water enough to float off their terminal portions in the form of
icebergs. In such a case till without marine shells may first
accumulate, and then, if the climate becomes warmer and the ice melts,
a marine deposit may be superimposed on the till without any change of
level being required.

Another curious phenomenon bearing on this subject was styled by the
late Hugh Miller the “striated pavements” of the boulder clay. Where
portions of the till have been removed by the sea on the shores of the
Forth, or in the interior by railway cuttings, the boulders imbedded in
what remains of the drift are seen to have been all subjected to a
process of abrasion and striation, the striæ and furrows being parallel
and persistent across them all, exactly as if a glacier or iceberg had
passed over them and scored them in a manner similar to that so often
undergone by the solid rocks below the glacial drift. It is possible,
as Mr. Geikie conjectures, that this second striation of the boulders
may be referable to floating ice.[3]

_Contorted Strata in Drift._—In Scotland the till is often covered with
stratified gravel, sand, and clay, the beds of which are sometimes
horizontal and sometimes contorted for a thickness of several feet.
Such contortions are not uncommon in Forfarshire, where I observed
them, among other places, in a vertical cutting made in 1840 near the
left bank of the South Esk, east of the bridge of Cortachie. The
convolutions of the beds of fine and coarse sand, gravel, and loam,
extend through a thickness of no less than 25 feet vertical, or from
_b_ to _c_, Fig. 115, the horizontal stratification being resumed very
abruptly at a short distance, as to the right of _f_, _g._ The
overlying coarse gravel and sand, _ a_, is in some places horizontal,
in others it exhibits cross bedding, and does not partake of the
disturbances which the strata _b_, _c_, have undergone. The underlying
till is exposed for a depth of about 20 feet; and we may infer from
sections in the neighbourhood that it is considerably thicker.

Fig. 15: Section of contorted drift overlying till, seen on left bank
of South Esk, near Cortachie, in 1840.

In some cases I have seen fragments of stratified clays and sands, bent
in like manner, in the middle of a great mass of till. Mr. Trimmer has
suggested, in explanation of such phenomena, the intercalation in the
glacial period of large irregular masses of snow or ice between layers
of sand and gravel. Some of the cliffs near Behring’s Straits, in which
the remains of elephants occur, consist of ice mixed with mud and
stones; and Middendorf describes the occurrence in Siberia of masses of
ice, found at various depths from the surface after digging through
drift. Whenever the intercalation of snow and ice with drift, whether
stratified or unstratified, has taken place, the melting of the ice
will cause such a failure of support as may give rise to flexures, and
sometimes to the most complicated foldings. But in many cases the
strata may have been bent and deranged by the mechanical pressure of an
advancing glacier, or by the sideway thrust of huge islands of ice
running aground against sandbanks; in which case, the position of the
beds forming the foundation of the banks may not be at all disturbed by
the shock.

There are indeed many signs in Scotland of the action of floating ice,
as might have been expected where proofs of submergence in the Glacial
Period are not wanting. Among these are the occurrence of large erratic
blocks, frequently in clusters at or near the tops of hills or ridges,
places which may have formed islets or shallows in the sea where
floating ice would mostly ground and discharge its cargo on melting.
Glaciers or land-ice would, on the contrary, chiefly discharge their
cargoes at the bottom of valleys. Traces of an earlier and independent
glaciation have also been observed in some regions where the striation,
apparently produced by ice proceeding from the north-west, is not
explicable by the radiation of land-ice from a central mountainous
region.[4]

Glaciation of Wales and England.—The mountains of North Wales were
recognised, in 1842, by Dr. Buckland, as having been an independent
centre of the dispersion of erratics—great glaciers, long since
extinct, having radiated from the Snowdonian heights in Carnarvonshire,
through seven principal valleys towards as many points of the compass,
carrying with them large stony fragments, and grooving the subjacent
rocks in as many directions.

Besides this evidence of land-glaciers, Mr. Trimmer had previously, in
1831, detected the signs of a great submergence in Wales in the
Post-pliocene period. He had observed stratified drift, from which he
obtained about a dozen species of marine shells, near the summit of
Moel Tryfaen, a hill 1400 feet high, on the south side of the Menai
Straits. I had an opportunity of examining in the summer of 1863,
together with the Reverend W. S. Symonds, a long and deep cutting made
through this drift by the Alexandra Mining Company in search of slates.
At the top of the hill above-mentioned we saw a stratified mass of
incoherent sand and gravel 35 feet thick, from which no less than 54
species of mollusca, besides three characteristic arctic varieties—in
all 57 forms—have been obtained by Mr. Darbishire. They belong without
exception to species still living in British or more northern seas;
eleven of them being exclusively arctic, four common to the arctic and
British seas, and a large proportion of the remainder having a
northward range, or, if found at all in the southern seas of Britain,
being comparatively less abundant. In the lowest beds of the drift were
large heavy boulders of far-transported rocks, glacially polished and
scratched on more than one side. Underneath the whole we saw the edges
of vertical slates exposed to view, which here, like the rocks in other
parts of Wales, both at greater and less elevations, exhibit beneath
the drift unequivocal marks of prolonged glaciation. The whole deposit
has much the appearance of an accumulation in shallow water or on a
beach, and it probably acquired its thickness during the gradual
subsidence of the coast—an hypothesis which would require us to ascribe
to it a high antiquity, since we must allow time, first for its
sinking, and then for its re-elevation.

The height reached by these fossil shells on Moel Tryfaen is no less
than 1300 feet—a most important fact when we consider how very few
instances we have on record beyond the limits of Wales, whether in
Europe or North America, of marine shells having been found in glacial
drift at half the height above indicated. A marine molluscous fauna,
however, agreeing in character with that of Moel Tryfaen, and
comprising as many species, has been found in drift at Macclesfield and
other places in central England, sometimes reaching an elevation of
1200 feet.

Professor Ramsay[5] estimated the probable amount of submergence during
some part of the glacial period at about 2300 feet; for he was unable
to distinguish the superficial sands and gravel which reached that high
elevation from the drift which, at Moel Tryfaen and at lower points,
contains shells of living species. The evidence of the marine origin of
the highest drift is no doubt inconclusive in the absence of shells, so
great is the resemblance of the gravel and sand of a sea beach and of a
river’s bed, when organic remains are wanting; but, on the other hand,
when we consider the general rarity of shells in drift which we know to
be of marine origin, we cannot suppose that, in the shelly sands of
Moel Tryfaen, we have hit upon the exact uppermost limit of marine
deposition, or, in other words, a precise measure of the submergence of
the land beneath the sea since the glacial period.

We are gradually obtaining proofs of the larger part of England, north
of a line drawn from the mouth of the Thames to the Bristol Channel,
having been under the sea and traversed by floating ice since the
commencement of the glacial epoch. Among recent observations
illustrative of this point, I may allude to the discovery, by Mr. J. F.
Bateman, near Blackpool, in Lancashire, fifty miles from the sea, and
at the height of 568 feet above its level, of till containing rounded
and angular stones and marine shells, such as _Turritella communis,
Purpura lapillus, Cardium edule,_ and others, among which _Trophon
clathratum_ (=_Fusus Bamffius_), though still surviving in North
British seas, indicates a cold climate.

_Erratics near Chichester._—The most southern memorials of ice-action
and of a Post-pliocene fauna in Great Britain is on the coast of the
county of Sussex, about 25 miles west of Brighton, and 15 south of
Chichester. A marine deposit exposed between high and low tide occurs
on both sides of the promontory called Selsea Bill, in which Mr.
Godwin-Austen found thirty-eight species of shells, and the number has
since been raised to seventy.

This assemblage is interesting because on the whole, while all the
species are recent, they have a somewhat more southern aspect than
those of the present British Channel. It is true that about forty of
them range from British to high northern latitudes; but several of
them, as, for example, _Lutraria rugosa_ and _ Pecten polymorphous_,
which are abundant, are not known at present to range farther north
than the coast of Portugal, and seem to indicate a warmer temperature
than now prevails on the coast where we find them fossil. What renders
this curious is the fact that the sandy loam in which they occur is
overlaid by yellow clayey gravel with large erratic blocks which must
have been drifted into their present position by ice when the climate
had become much colder. These transported fragments of granite,
syenite, and greenstone, as well as of Devonian and Silurian rocks, may
have come from the coast of Normandy and Brittany, and are many of them
of such large size that we must suppose them to have been drifted into
their present site by coast-ice. I measured one of granite, at Pagham,
21 feet in circumference. In the gravel of this drift with erratics are
a few littoral shells of living species, indicating an ancient
coast-line.

Glacial Formations of North America.—In the western hemisphere, both in
Canada and as far south as the 40th and even 38th parallel of latitude
in the United States, we meet with a repetition of all the
peculiarities which distinguish the European boulder formation.
Fragments of rock have travelled for great distances, especially from
north to south: the surface of the subjacent rock is smoothed,
striated, and fluted; unstratified mud or _till_ containing boulders is
associated with strata of loam, sand, and clay, usually devoid of
fossils. Where shells are present, they are of species still living in
northern seas, and not a few of them identical with those belonging to
European drift, including most of those already given in Figs. 107 to
112, p. 176. The fauna also of the glacial epoch in North America is
less rich in species than that now inhabiting the adjacent sea, whether
in the Gulf of St. Lawrence, or off the shores of Maine, or in the Bay
of Massachusetts.

The extension on the American continent of the range of erratics during
the Post-pliocene period to lower latitudes than they reached in
Europe, agrees well with the present southward deflection of the
isothermal lines, or rather the lines of equal winter temperature. It
seems that formerly, as now, a more extreme climate and a more abundant
supply of ice prevailed on the western side of the Atlantic. Another
resemblance between the distribution of the drift fossils in Europe and
North America has yet to be pointed out. In Canada and the United
States, as in Europe, the marine shells are generally confined to very
moderate elevations above the sea (between 100 and 700 feet), while the
erratic blocks and the grooved and polished surfaces of rock extend to
elevations of several thousand feet.

I have already mentioned that in Europe several quadrupeds of living,
as well as extinct, species were common to pre-glacial and post-glacial
times. In like manner there is reason to suppose that in North America
much of the ancient mammalian fauna, together with nearly all the
invertebrata, lived through the ages of intense cold. That in the
United States the _Mastodon giganteus_ was very abundant after the
drift period, is evident from the fact that entire skeletons of this
animal are met with in bogs and lacustrine deposits occupying hollows
in the glacial drift. They sometimes occur in the bottom even of small
ponds recently drained by the agriculturist for the sake of the
shell-marl. In 1845 no less than six skeletons of the same species of
Mastodon were found in Warren county, New Jersey, six feet below the
surface, by a farmer who was digging out the rich mud from a small pond
which he had drained. Five of these skeletons were lying together, and
a large part of the bones crumbled to pieces as soon as they were
exposed to the air.

It would be rash, however, to infer from such data that these
quadrupeds were mired in _modern_ times, unless we use that term
strictly in a geological sense. I have shown that there is a fluviatile
deposit in the valley of the Niagara, containing shells of the genera
_Melania, Lymnea, Planorbis, Velvata, Cyclaz, Unio, Helix,_ etc., all
of recent species, from which the bones of the great Mastodon have been
taken in a very perfect state. Yet the whole excavation of the ravine,
for many miles below the Falls, has been slowly effected since that
fluviatile deposit was thrown down. Other extinct animals accompany the
_Mastodon giganteus_ in the post-glacial deposits of the United States,
and this, taken with the fact that so few of the mollusca, even of the
commencement of the cold period, differ from species now living is
important, as refuting the hypothesis, for which some have contended,
that the intensity of the glacial cold annihilated all the species in
temperate and arctic latitudes.

Connection of the Predominance of Lakes with Glacial Action.—It was
first pointed out by Professor Ramsay in 1862, that lakes are
exceedingly numerous in those countries where erratics, striated
blocks, and other signs of ice-action abound; and that they are
comparatively rare in tropical and sub-tropical regions. Generally in
countries where the winter cold is intense, such as Canada,
Scandinavia, and Finland, even the plains and lowlands are thickly
strewn with innumerable ponds and small lakes, together with some
others of a larger size; while in more temperate regions, such as Great
Britain, Central and Southern Europe, the United States, and New
Zealand, lake districts occur in all such mountainous tracts as can be
proved to have been glaciated in times comparatively modern or since
the geographical configuration of the surface bore a considerable
resemblance to that now prevailing. In the same countries, beyond the
glaciated regions, lakes abruptly cease, and in warmer and tropical
countries are either entirely absent, or consist, as in equatorial
Africa, of large sheets of water unaccompanied so far as we yet know by
numerous smaller ponds and tarns.

The southern limits of the lake districts of the Northern Hemisphere
are found at about 40° N. latitude on the American continent, and about
50° in Europe, or where the Alps intervene four degrees farther south.
A large proportion of the smaller lakes are dammed up by barriers of
unstratified drift, having the exact character of the moraines of
glaciers, and are termed by geologists “morainic,” but some of them are
true rock-basins, and would hold water even if all the loose drift now
resting on their margins were removed.

In a paper read before the Geological Society of London in 1862,
Professor Ramsay maintained that the first formation of most existing
lakes took place during the glacial epoch, and was due, not to
elevation or subsidence, but to actual erosion of their basins by
glaciers. M. Mortillet in the same year advanced the theory that after
the Alpine lake-basins had been filled up with loose fluviatile
deposits, they were re-excavated by the great glaciers which passed
down the valleys at the time of the greatest cold, a doctrine which
would attribute to moving ice almost as great a capacity of erosion as
that which assumed that the original basins were scooped out of solid
rock by glaciers. It is impossible to deny that the mere geographical
distribution of lakes points to the intimate connection of their origin
with the abundance of ice during a former excess of cold, but how far
the erosive action of moving ice has been the sole or even the
principal cause of lake-basins, is a question still open to discussion.

The lakes of Switzerland and the north of Italy are some of them twenty
and thirty miles in length, and so deep that their bottoms are in some
cases from 1000 to 2000 feet beneath the level of the sea. It is
admitted on all hands that they were once filled with ice, and as the
existing glaciers polish and grind down, as before stated, the surface
of the rocks, we are prepared to find that every lake-basin in
countries once covered by ice should bear the marks of superficial
glaciation, and also that the ice during its advance and retreat should
have left behind it much transported matter as well as some evidence of
its having enlarged the pre-existing cavity. But much more than this is
demanded by the advocates of glacial erosion. They suggest that as the
old extinct glaciers were several thousand feet thick, they were able
in some places gradually to scoop out of the solid rock cavities twenty
or thirty miles in length, and as in the case of Lago Maggiore from a
thousand to two thousand six hundred feet below the previous level of
the river-channel, and also that the ice had the power to remove from
the cavity formed by its grinding action all the materials of the
missing rocks. A constant supply, it is argued, of fine mud issues from
the termination of every glacier in the stream which is produced by the
melting of the ice, and this result of friction is exhibited both
during winter and summer, affording evidence of the continual deepening
and widening of the valleys through which glaciers pass. As the fine
mud is carried away by a river from the deep pool which is formed from
the base of every cataract, so it seems to be imagined that lake-basins
may be gradually emptied of the mud formed by abrasion during the
glacial period.

I am by no means disposed to object to this theory on the ground of the
insufficiency of the time during which the extreme cold endured, but we
must carefully consider whether that same time is not so vast as to
make it probable that other forces, besides the motion of glaciers,
must have cooperated in converting some parts of the ancient valley
courses into lake-basins. They who have formed the most exalted
conceptions of the erosive energy of moving ice do not deny that during
the period termed “Glacial” there have been movements of the earth’s
crust sufficient to produce oscillations of level in Europe amounting
to 1000 feet or more in both directions. M. Charpentier, indeed,
attributed some of the principal changes of climate in Switzerland,
during the glacial period, to a depression of the central Alps to the
extent of 3000 feet, and Swiss geologists have long been accustomed to
attribute their lake basins, in part, to those convulsions by which the
shape and course of the valleys may have been modified. Our experience,
in the lifetime of the present generation, of the changes of level
witnessed in New Zealand during great earthquakes is entirely opposed
to the notion that the movements, whether upward or downward, are
uniform in amount or direction throughout areas of indefinite extent.
On the contrary, the land has been permanently raised in one region
several feet or yards, and the rise has been found gradually to die
out, so as to be imperceptible at a distance of twenty miles, and in
some areas is even exchanged for a simultaneous downward movement of
several feet.

But, it is asked, if such inequality of movement can have contributed
towards the production of lake basins, does it not leave unexplained
the comparative rarity of lakes in tropical and subtropical countries.
In reply to this question it may be observed that in our endeavour to
estimate the effects of subterranean movements in modifying the
superficial geography of a country we must remember that each
convulsion effects a very slight change. If it interferes with the
drainage, whether by raising the lower or sinking the higher portion of
a hydrographical basin, the upheaval or depression will only amount to
a few feet at a time, and there may be an interval of years or
centuries before any further movement takes place in the same region.
In the mean time an incipient lake if produced may be filled up with
sediment, and the recently-formed barrier will then be cut through by
the river, whereas in a country where glacial conditions prevail no
such obliteration of the temporary lake-basin would take place; for
however deep it became by repeated sinking of the upper or rising of
the lower extremity, being always filled with ice it might remain,
throughout the greater part of its extent, free from sediment or drift
until the ice melted at the close of the glacial period.

One of the most serious objections to the exclusive origin by
ice-erosion of wide and deep lake-basins arises from their capricious
distribution, as for example in Piedmont, both to the eastward and
westward of Turin, where great lakes are wanting,[6] although some of
the largest extinct glaciers descending from Mont Blanc and Monte Rosa
came down from the Alps, leaving their gigantic moraines in the low
country. Here, therefore, we might have expected to find lakes of the
first magnitude rivalling the contiguous Lago Maggiore in importance.

A still more striking illustration of the same absence of lakes where
large glaciers abound is afforded by the Caucasus, a chain more than
300 miles long, and the loftiest peaks of which attain heights from
16,000 to 18,000 feet. This greatest altitude is reached by Elbruz, a
mountain in lat. 43° N. three degrees south of Mont Blanc, but on the
other hand 3000 feet higher. The present Caucasian glaciers are equal
or superior in dimensions to those of Switzerland, and like them give
rise occasionally to temporary lakes by obstructing the course of
rivers, and causing great floods when the icy barriers give way. Mr.
Freshfield, a careful observer, writing in 1869, says:[7] “A total
absence of lakes on both sides of the chains is the most marked
feature. Not only are there no great subalpine sheets of water, like
Como or Geneva, but mountain tarns, such as the Dauben See on the
Gemmi, or the Klonthal See near Glarus, are equally wanting.” The same
author states on the authority of the eminent Swiss geologist, Mons. E.
Favre, who also explored the Caucasus in 1868, that moraines of great
height and huge erratics of granite and other rocks “justify the
assertion that the present glaciers of the Caucasus, like those of the
Alps, are only the shadows of their former selves.”

It seems safe to assume that the chain of lakes, of which the Albert
Nyanza forms one in equatorial Africa, was due to causes other than
glacial. Yet if we could imagine a glacial period to visit that region
filling the lakes with ice and scoring the rocks which form their sides
and bottoms, we should be unable to decide how much the capacity of the
basins had been enlarged and the surface modified by glacial erosion.
The same may be true of the Lago Maggiore and Lake Superior, although
the present basins of both of them afford abundant superficial markings
due to ice-action.

But to whatever combination of causes we attribute the great Alpine
lakes one thing is clear, namely, that they are, geologically speaking,
of modern origin. Every one must admit that the upper valley of the
Rhone has been chiefly caused by fluviatile denudation, and it is
obvious that the quantity of matter removed from that valley previous
to the glacial period would have been amply sufficient to fill up with
sediment the basin of the Lake of Geneva, supposing it to have been in
existence, even if its capacity had been many times greater than it is
now.[8]

On the whole, it appears to me, in accordance with the views of
Professor Ramsay, M. Mortillet, Mr. Geikie, and others, that the
abrading action of ice has formed some mountain tarns and many morainic
lakes; but when it is a question of the origin of larger and deeper
lakes, like those of Switzerland or the north of Italy, or inland
fresh-water seas, like those of Canada, it will probably be found that
ice has played a subordinate part in comparison with those movements by
which changes of level in the earth’s crust are gradually brought
about.

 [1] Jamieson, Quart. Geol. Journ., 1860, vol. xvi, p. 370.

 [2] Bryce, Quart. Geol. Journ., vol. xxi, p. 217, 1865.

 [3] Geikie, Trans. Geol. Soc. Glasgow, vol. i, part ii, p. 68, 1863.

 [4] Milne Home, Trans. Royal Soc. Edinburgh, vol. xxv, 1868-9.

 [5] Quart. Geol. Journ., 1852, vol. viii, p. 372.

 [6] Antiquity of Man, p. 313.

 [7] Travels in Central Caucasus, 1869, p. 452.

 [8] See Principles, vol. i, p. 420, 10th ed., 1867.


TERTIARY OR CAINOZOIC PERIOD




CHAPTER XIII.
PLIOCENE PERIOD


Glacial Formations of Pliocene Age. — Bridlington Beds. — Glacial
Drifts of Ireland. — Drift of Norfolk Cliffs. — Cromer Forest-bed. —
Aldeby and Chillesford Beds. — Norwich Crag. — Older Pliocene Strata. —
Red Crag of Suffolk. — Coprolitic Bed of Red Crag. — White or Coralline
Crag. — Relative Age, Origin, and Climate of the Crag Deposits. —
Antwerp Crag. — Newer Pliocene Strata of Sicily. — Newer Pliocene
Strata of the Upper Val d’Arno. — Older Pliocene of Italy. —
Subapennine Strata. — Older Pliocene Flora of Italy.

It will be seen in the description given in the last chapter of the
Post-pliocene formations of the British Isles that they comprise a
large proportion of those commonly termed glacial, characterised by
shells which, although referable to living species, usually indicate a
colder climate than that now belonging to the latitudes where they
occur fossil. But in parts of England, more especially in Yorkshire,
Norfolk, and Suffolk, there are superficial formations of clay with
glaciated boulders, and of sand and pebbles, containing occasional,
though rare, patches of shells, in which the marine fauna begins to
depart from that now inhabiting the neighbouring sea, and comprises
some species of mollusca not yet known as living, as well as extinct
varieties of others, entitling us to class them as Newer Pliocene,
although belonging to the close of that period and chronologically on
the verge of the later or Post-pliocene epoch.

Bridlington Drift.—To this era belongs the well-known locality of
Bridlington, near the mouth of the Humber, in Yorkshire, where about
seventy species or well-marked varieties of shells have been found on
the coast, near the sea-level, in a bed of sand several feet thick
resting on glacial clay with much chalk débris, and covered by a
deposit of purple clay with glaciated boulders. More than a third of
the species in this drift are now inhabitants of arctic regions, none
of them extending southward to the British seas; which is the more
remarkable as Bridlington is situated in lat. 54° north. Fifteen
species are British and Arctic, a very few belong to those species
which range south of our British seas. Five species or well-marked
varieties are not known living, namely, the variety of _Astarte
borealis_ (called _A. Withami_); _ A. mutabilis_; the sinistral form of
_Tritonium carinatum, Cardita analis,_ and _Tellina obliqua,_ Fig. 120,
p. 194. Mr. Searles Wood also inclines to consider _Nucula Cobboldiæ,_
Fig. 119, p. 194, now absent from the European seas and the Atlantic,
as specifically distinct from a closely-allied shell now living in the
seas surrounding Vancouver’s Island, which some conchologists regard as
a variety. _Tellina obliqua_ also approaches very near to a shell now
living in Japan.

Glacial Drift of Ireland.—Marine drift containing the last-mentioned
Nucula and other glacial shells reaches a height of from 1000 to 1200
feet in the county of Wexford, south of Dublin. More than eighty
species have already been obtained from this formation, of which two,
_Conovulus pyramidalis_ and _ Nassa monensis,_ are not known as living;
while _Turritella incrassata_ and _Cypræa lucida_ no longer inhabit the
British seas, but occur in the Mediterranean. The great elevation of
these shells, and the still greater height to which the surface of the
rocks in the mountainous regions of Ireland have been smoothed and
striated by ice-action, has led geologists to the opinion that that
island, like the greater part of England and Scotland, after having
been united with the continent of Europe, from whence it received the
plants and animals now inhabiting it, was in great part submerged. The
conversion of this and other parts of Great Britain into an archipelago
was followed by a re-elevation of land and a second continental period.
After all these changes the final separation of Ireland from Great
Britain took place, and this event has been supposed to have preceded
the opening of the straits of Dover.[1]

Fig. 116: Tellina balthica
Drift of Norfolk Cliffs.—There are deposits of boulder clay and till in
the Norfolk cliffs principally made up of the waste of white chalk and
flints which, in the opinion of Mr. Searles Wood, jun., and others, are
older than the Bridlington drift, and contain a larger proportion of
shells common to the Norwich and Red Crag, including a certain number
of extinct forms, but also abounding in _Tellina balthica_ (_T.
solidula,_ Fig. 116), which is found fossil at Bridlington, and living
in our British seas, but wanting in all the formations, even the
newest, afterwards to be described as Crag. As the greater part of
these drifts are barren of organic remains, their classification is at
present a matter of great uncertainty.

They can nowhere be so advantageously studied as on the coast between
Happisburgh and Cromer. Here we may see vertical cliffs, sometimes 300
feet and more in height, exposed for a distance of fifty miles, at the
base of which the chalk with flints crops out in nearly horizontal
strata. Beds of gravel and sand repose on this undisturbed chalk. They
are often strangely contorted, and envelop huge masses or erratics of
chalk with layers of vertical flint. I measured one of these fragments
in 1839 at Sherringham, and found it to be eighty feet in its longest
diameter. It has been since entirely removed by the waves of the sea.
In the floor of the chalk beneath it the layers of flint were
horizontal. Such erratics have evidently been moved bodily from their
original site, probably by the same glacial action which has polished
and striated some of the accompanying granitic and other boulders,
occasionally six feet in diameter, which are imbedded in the drift.

Cromer Forest-bed.—Intervening between these glacial formations and the
subjacent chalk lies what has been called the Cromer Forest-bed. This
buried forest has been traced from Cromer to near Kessingland, a
distance of more than forty miles, being exposed at certain seasons
between high and low water mark. It is the remains of an old land and
estuarine deposit, containing the submerged stumps of trees standing
erect with their roots in the ancient soil. Associated with the stumps
and overlying them, are lignite beds with fresh-water shells of recent
species, and laminated clay without fossils. Through the lignite and
forest-bed are scattered cones of the Scotch and spruce firs with the
seeds of recent plants, and the bones of at least twenty species of
terrestrial mammalia. Among these are two species of elephant, _E.
meridionalis,_ Nesti, and _E. antiquus,_ the former found in the Newer
Pliocene beds of the Val d’Arno, near Florence. In the same bed occur
_Hippopotamus major, Rhinoceros etruscus,_ both of them also Val d’Arno
species, many species of deer considered by Mr. Boyd Dawkins to be
characteristic of warmer countries, and also a horse, beaver, and
field-mouse. Half of these mammalia are extinct, and the rest still
survive in Europe. The vegetation taken alone does not imply a
temperature higher than that now prevailing in the British Isles. There
must have been a subsidence of the forest to the amount of 400 or 500
feet, and a re-elevation of the same to an equal extent in order to
allow the ancient surface of the chalk or covering of soil, on which
the forest grew, to be first covered with several hundred feet of
drift, and then upheaved so that the trees should reach their present
level. Although the relative antiquity of the forest-bed to the
overlying glacial till is clear, there is some difference of opinion as
to its relation to the crag presently to be described.

Fig. 117: Natica helicoides
Chillesford and Aldeby Beds.—It is in the counties of Norfolk, Suffolk,
and Essex, that we obtain our most valuable information respecting the
British Pliocene strata, whether newer or older. They have obtained in
those counties the provincial name of “Crag,” applied particularly to
masses of shelly sand which have long been used in agriculture to
fertilise soils deficient in calcareous matter. At Chillesford, between
Woodbridge and Aldborough in Suffolk, and Aldeby, near Beccles, in the
same county, there occur stratified deposits, apparently older than any
of the preceding drifts of Yorkshire, Norfolk, and Suffolk. They are
composed at Chillesford of yellow sands and clays, with much mica,
forming horizontal beds about twenty feet thick. Messrs. Prestwich and
Searles Wood, senior, who first described these beds, point out that
the shells indicate on the whole a colder climate than the Red Crag;
two-thirds of them being characteristic of high latitudes. Among these
are _Cardium Grœnlandicum, Leda limatula, Tritonium carinatum,_ and
_Scalaria Grœnlandica._ In the upper part of the laminated clays a
skeleton of a whale was found associated with casts of the
characteristic shells, _Nucula Cobboldiæ_ and _Tellina obliqua,_
already referred to as no longer inhabiting our seas, and as being
extinct varieties if not species. The same shells occur in a perfect
state in the lower part of the formation. _ Natica helicoides_ (Fig.
117) is an example of a species formerly known only as fossil, but
which has now been found living in our seas.

At Aldeby, where beds occur decidedly similar in mineral character as
well as fossil remains, Messrs. Crowfoot and Dowson have now obtained
sixty-six species of mollusca, comprising the Chillesford species and
some others. Of these about nine-tenths are recent. They are in a
perfect state, clearly indicating a cold climate; as two-thirds of them
are now met with in arctic regions. As a rule, the lamellibranchiate
molluscs have both valves united, and many of them, such as _Mya
arenaria,_ stand with the siphonal end upward, as when in a living
state. _Tellina balthica,_ before mentioned (Fig. 116) as so
characteristic of the glacial beds, including the drift of Bridlington,
has not yet been found in deposits of Chillesford and Aldeby age,
whether at Sudbourn, East Bavent, Horstead, Coltishall, Burgh, or in
the highest beds overlying the Norwich Crag proper at Bramerton and
Thorpe.

Fig. 118: <i>Mastodon arvernensis,</i> third milk molar, left side,
upper jaw: grinding surface. Norwich Crag, Postwick, also found in Red
Crag, see p. 197.

Norwich or Fluvio-marine Crag.—The beds above alluded to ought,
perhaps, to be regarded as beds of passage between the glacial
formations and those called from a provincial name “Crag,” the newest
member of which has been commonly called the “Norwich Crag.” It is
chiefly seen in the neighbourhood of Norwich, and consists of beds of
incoherent sand, loam, and gravel, which are exposed to view on both
banks of the Yare, as at Bramerton and Thorpe. As they contain a
mixture of marine, land, and fresh-water shells, with bones of fish and
mammalia, it is clear that these beds have been accumulated at the
bottom of a sea near the mouth of a river. They form patches rarely
exceeding twenty feet in thickness, resting on white chalk. At their
junction with the chalk there invariably intervenes a bed called the
“Stone-bed,” composed of unrolled chalk-flints, commonly of large size,
mingled with the remains of a land fauna comprising _ Mastodon
arvernensis, Elephas meridionalis,_ and an extinct species of deer. The
mastodon, which is a species characteristic of the Pliocene strata of
Italy and France, is the most abundant fossil, and one not found in the
Cromer forest before mentioned. When these flints, probably long
exposed in the atmosphere, became submerged, they were covered with
barnacles, and the surface of the chalk became perforated by the
_Pholas crispata,_ each fossil shell still remaining at the bottom of
its cylindrical cavity, now filled up with loose sand from the
incumbent crag. This species of Pholas still exists, and drills the
rocks between high and low water on the British coast. The name of
“Fluvio-marine” has often been given to this formation, as no less than
twenty species of land and fresh-water shells have been found in it.
They are all of living species; at least only one univalve, _Paludina
lenta,_ has any, and that a very doubtful, claim to be regarded as
extinct.

Fig. 119: Nucula Cobboldiæ; Fig. 120: Tellina obliqua.

Of the marine shells, 124 in number, about 18 per cent are extinct,
according to the latest estimate given me by Mr. Searles Wood; but, for
reasons presently to be mentioned, this percentage must be only
regarded as provisional. It must also be borne in mind that the
proportion of recent shells would be augmented if the uppermost beds at
Bramerton, near Norwich, which belong to the most modern or Chillesford
division of the Crag, had been included, as they were formerly, by Mr.
Woodward and myself, in the Norwich series. Arctic shells, which formed
so large a proportion in the Chillesford and Aldeby beds, are more rare
in the Norwich Crag, though many northern species—such as _Rhynchonella
psittacea, Scalaria Grœnlandica, Astarte borealis, Panopæa Norvegia,_
and others—still occur. The _Nucula Cobboldiæ_ and _Tellina obliqua,_
Figs. 119 and 120, before mentioned, p. 194, are frequent in these
beds, as are also _Littorina littorea, Cardium edule,_ and _Turritella
communis,_ of our seas, proving the littoral origin of the beds.

OLDER PLIOCENE STRATA.

Red Crag.—Among the English Pliocene beds the next in antiquity is the
Red Crag, which often rests immediately on the London Clay, as in the
county of Essex, illustrated in Fig. 121.

 Fig. 121: Red Crag, London clay and chalk.

It is chiefly in the county of Suffolk that it is found, rarely
exceeding twenty feet in thickness, and sometimes overlying another
Pliocene deposit, the Coralline Crag, to be mentioned in the sequel. It
has yielded—exclusive of 25 species regarded by Mr. Wood as
derivative—256 species of mollusca, of which 65, or 25 per cent, are
extinct. Thus, apart from its order of superposition, its greater
antiquity than the Norwich and glacial beds, already described, is
proved by the greater departure from the fauna of our seas. It may also
be observed that in most of the deposits of this Red Crag, the northern
forms of the Norwich Crag, and of such glacial formations as
Bridlington, are less numerous, while those having a more southern
aspect begin to make their appearance. Both the quartzose sand, of
which it chiefly consists, and the included shells, are most commonly
distinguished by a deep ferruginous or ochreous colour, whence its
name. The shells are often rolled, sometimes comminuted, and the beds
have much the appearance of having been shifting sand-banks, like those
now forming on the Dogger-bank, in the sea, sixty miles east of the
coast of Northumberland. Cross stratification is almost always present,
the planes of the strata being sometimes directed towards one point of
the compass, sometimes to the opposite, in beds immediately overlying.
That such a structure is not deceptive or due to any subsequent
concretionary rearrangement of particles, or to mere bands of colour
produced by the iron, is proved by each bed being made up of flat
pieces of shell which lie parallel to the planes of the smaller strata.

It has long been suspected that the different patches of Red Crag are
not all of the same age, although their chronological relation cannot
be decided by superposition. Separate masses are characterised by
shells specifically distinct or greatly varying in relative abundance,
in a manner implying that the deposits containing them were separated
by intervals of time. At Butley, Tunstall, Sudbourn, and in the Red
Crag of Chillesford, the mollusca appear to assume their most modern
aspect when the climate was colder than when the earliest deposits of
the same period were formed. At Butley, _Nucula Cobboldiæ_, so common
in the Norwich and certain glacial beds, is found, and _Purpura
tetragona_ (Fig. 122) is very abundant. On the other hand, at
Walton-on-the-Naze, in Essex, we seem to have an exhibition of the
oldest phase of the Red Crag; and a warmer climate seems indicated, not
only by the absence of many northern forms, but also by the abundance
of some now living in the British seas and the Mediterranean. _Voluta
Lamberti_ (see Figs. 123 and 124), an extinct form, which seems to have
flourished chiefly in the antecedent Coralline Crag period, is still
represented here by individuals of every age.

Fig. 122: Purpura tetragona.
The reversed whelk (Fig. 125) is common at Walton, where the dextral
form of that shell is unknown. Here also we find most frequently
specimens of lamellibranchiate molluscs, with both the valves united,
showing that they belonged to this sea of the Upper Crag, and were not
washed in from an older bed, such as the Coralline, in which case the
ligament would not have held together the valves in strata so often
showing signs of the boisterous action of the waves. No less than forty
species of lamellibranchiate molluscs, with double valves, have been
collected by Mr. Bell from the various localities of the Red Crag.

Fig. 123: Voluta Lamberti; Fig. 124: Voluta Lamberti; Fig. 125: Trophon
antiquum.

At and near the base of the Red Crag is a loose bed of brown nodules,
first noticed by Professor Henslow as containing a large percentage of
earthy phosphates. This bed of coprolites (as it is called, because
they were originally supposed to be the fæces of animals) does not
always occur at one level, but is generally in largest quantity at the
junction of the Crag and the underlying formation. In thickness it
usually varies from six to eighteen inches, and in some rare cases
amounts to many feet. It has been much used in agriculture for manure,
as not only the nodules, but many of the separate bones associated with
them, are largely impregnated with phosphate of lime, of which there is
sometimes as much as sixty per cent. They are not unfrequently covered
with barnacles, showing that they were not formed as concretions in the
stratum where they now lie buried, but had been previously
consolidated. The phosphatic nodules often collect fossil crabs and
fishes from the London Clay, together with the teeth of gigantic
sharks. In the same bed have been found many ear-bones of whales, and
the teeth of _Mastodon arvernensis, Rhinoceros Schleiermacheri, Tapirus
priscus,_ and Hipparion (a quadruped of the horse family), and antlers
of a stag, _Cervus anoceros._ Organic remains also of the older chalk
and Lias are met with, showing how great was the denudation of previous
formations during the Pliocene period. As the older White Crag,
presently to be mentioned, contains similar phosphatic nodules near its
base, those of the Red Crag may be partly derived from this source.

White or Coralline Crag.—The lower or Coralline Crag is of very limited
extent, ranging over an area about twenty miles in length, and three or
four in breadth, between the rivers Stour and Alde, in Suffolk. It is
generally calcareous and marly—often a mass of comminuted shells, and
the remains of bryozoa[2] (or polyzoa), passing occasionally into a
soft building-stone. At Sudbourn and Gedgrave, near Orford, this
building-stone has been largely quarried. At some places in the
neighbourhood the softer mass is divided by thin flags of hard
limestone, and bryozoa placed in the upright position in which they
grew. From the abundance of these coralloid mollusca the lowest or
White Crag obtained its popular name, but true corals, as now defined,
or zoantharia, are very rare in this formation.

The Coralline Crag rarely, if ever, attains a thickness of thirty feet
in any one section. Mr. Prestwich imagines that if the beds found at
different localities were united in the probable order of their
succession, they might exceed eighty feet in thickness, but Mr. Searles
Wood does not believe in the possibility of establishing such a
chronological succession by aid of the organic remains, and questions
whether proof could be obtained of more than forty feet. I was unable
to come to any satisfactory opinion on the subject, although at Orford,
especially at Gedgrave, in the neighbourhood of that place, I saw many
sections in pits, where this crag is cut through. These pits are so
unconnected, and of such limited extent, that no continuous section of
any length can be obtained, so that speculations as to the thickness of
the whole deposit must be very vague. At the base of the formation at
Sutton a bed of phosphatic nodules, very similar to that before alluded
to in the Red Crag, with remains of mammalia, has been met with.

Fig. 126: Section near Woodbridge, in Suffolk.

Whenever the Red and Coralline Crag occur in the same district, the Red
Crag lies uppermost; and in some cases, as in the section represented
in Fig. 126, which I had an opportunity of seeing exposed to view in
1839, it is clear that the older deposit, or Coralline Crag, _b_, had
suffered denudation, before the newer formation, _a_, was thrown down
upon it. At D there was not only seen a distinct cliff, eight or ten
feet high, of Coralline Crag, running in a direction N.E. and S.W.,
against which the Red Crag abuts with its horizontal layers, but this
cliff occasionally overhangs. The rock composing it is drilled
everywhere by _ Pholades_, the holes which they perforated having been
afterwards filled with sand, and covered over when the newer beds were
thrown down. The older formation is shown by its fossils to have
accumulated in a deeper sea, and contains none of those littoral forms
such as the limpet, _Patella_, found in the Red Crag. So great an
amount of denudation could scarcely take place, in such incoherent
materials, without some of the fossils of the inferior beds becoming
mixed up with the overlying crag, so that considerable difficulty must
be occasionally experienced by the palæontologist in deciding which
species belong severally to each group.

Fig. 127: Fascicularia aurantium, from the inferior or Coralline Crag,
Suffolk. Fig. 128: Astarte Omalii, species common to Upper and Lower
Crag.

Mr. Searles Wood estimates the total number of marine testaceous
mollusca of the Coralline Crag at 350, of which 110 are not known as
living, being in the proportion of thirty-one per cent extinct. No less
than 130 species of bryozoa have been found in the Coralline Crag, and
some belong to genera unknown in the living creation, and of a very
peculiar structure; as, for example, that represented in Fig. 127,
which is one of several species having a globular form. Among the
testacea the genus _Astarte_ (see Fig. 128) is largely represented, no
less than fourteen species being known, and many of these being rich in
individuals. There is an absence of genera peculiar to hot climates,
such as _Conus, Oliva, Fasciolaria, Crassatella_, and others. The
absence also of large cowries (_Cyprea_), those found belonging
exclusively to the section _Trivia_, is remarkable. The large volute,
called _Voluta Lamberti_ (Fig. 123, p. 196), may seem an exception; but
it differs in form from the volutes of the torrid zone, and, like the
living _Voluta Magellanica_, must have been fitted for an
extra-tropical climate.

Fig. 129: Lingula Dumortieri. Fig. 130: Pyrula reticulata. Fig. 131:
Temnechinus excavatus.

The occurrence of a species of _Lingula_ at Sutton (see Fig. 129) is
worthy of remark, as these _Brachiopoda_ seem now confined to more
equatorial latitudes; and the same may be said still more decidedly of
a species of _Pyrula_, supposed by Mr. Wood to be identical with _P.
reticulata_ (Fig. 130), now living in the Indian Ocean. A genus also of
echinoderms, called by Professor Forbes _Temnechinus_ (Fig. 131),
occurs in the Red and Coralline Crag of Suffolk, and until lately was
unknown in a living state, but it has been brought to light as an
existing form by the deep-sea dredgings, both of the United States
survey, off Florida, at a depth of from 180 to 480 feet, and more
recently (1869), in the British seas, during the explorations of the
“Porcupine.”

Climate of the Crag Deposits.—One of the most interesting conclusions
deduced from a careful comparison of the shells of the British Pliocene
strata and the fauna of our present seas has been pointed out by
Professor E. Forbes. It appears that, during the Glacial period, a
period intermediate, as we have seen, between that of the Crag and our
own time, many shells, previously established in the temperate zone,
retreated southward to avoid an uncongenial climate, and they have been
found fossil in the Newer Pliocene strata of Sicily, Southern Italy,
and the Grecian Archipelago, where they may have enjoyed, during the
era of floating icebergs, a climate resembling that now prevailing in
higher European latitudes.[3] The Professor gave a list of fifty shells
which inhabited the British seas while the Coralline and Red Crag were
forming, and which, though now living in our seas, were wanting, as far
as was then known, in the glacial deposits. Some few of these species
have subsequently been found in the glacial drift, but the general
conclusion of Forbes remains unshaken.

The transport of blocks by ice, when the Red Crag was being deposited,
appears to me evident from the large size of some huge, irregular,
quite unrounded chalk flints, retaining their white coating, and 2 feet
long by 18 inches broad, in beds worked for phosphatic nodules at
Foxhall, four miles south-east of Ipswich. These must have been
tranquilly drifted to the spot by floating ice. Mr. Prestwich also
mentions the occurrence of a large block of porphyry in the base of the
Coralline Crag at Sutton, which would imply that the ice-action had
begun in our seas even in this older period. The cold seems to have
gone on increasing from the time of the Coralline to that of the
Norwich Crag, and became more and more severe, not perhaps without some
oscillations of temperature, until it reached its maximum in what has
been called the Glacial period, or at the close of the Newer Pliocene,
and in the Post-pliocene periods.

Relation of the Fauna of the Crag to that of the recent Seas.—By far
the greater number of the recent marine species occurring in the
several Crag formations are still inhabitants of the British seas; but
even these differ considerably in their relative abundance, some of the
commonest of the Crag shells being now extremely scarce—as, for
example, _Buccinum Dalei_—while others, rarely met with in a fossil
state, are now very common, as _Murex erinaceus_ and _Cardium
echinatum._ Some of the species also, the identity of which with the
living would not be disputed by any conchologist, are nevertheless
distinguishable as varieties, whether by slight deviations in form or a
difference in average dimensions. Since Mr. Searles Wood first
described the marine testacea of the Crags, the additions made to that
fossil fauna have not been considerable, whereas we have made in the
same period immense progress in our knowledge of the living testacea of
the British and arctic seas, and of the Mediterranean. By this means
the naturalist has been enabled to identify with existing species many
forms previously supposed to be extinct.

In the forthcoming supplement to the invaluable monograph communicated
by Mr. Wood to the Palæontographical Society, in which he has completed
his figures and descriptions of the British crag shells of every age,
list will be found of all the fossil shells, of which a summary is
given in the table, p. 202.

To begin with the uppermost or Chillesford beds, it will be seen that
about 9 per cent only are extinct, or not known as living, whereas in
the Norwich, which succeeds in the descending order, seventeen in a
hundred are extinct. Formerly, when the Norwich or Fluvio-marine Crag
was spoken of, both these formations were included under the same head,
for both at Bramerton and Thorpe, the chief localities where the
Norwich Crag was studied, an overlying deposit occurs referable to the
Chillesford age. If now the two were fused together as of old, their
shells would, according to Mr. Wood, yield a percentage of fifteen in a
hundred of species extinct or not known as living.

NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN THE CRAG.

CHILLESFORD AND ALDEBY BEDS Total
number Not known
as living Percentage of
Shells not known
as living Bivalves   61   4 9·5 Univalves   33   5
Brachiopods     0   0 NORWICH OR FLUVIO-MARINE CRAG Bivalves  
61 10 17·5 Univalves   64 12 Brachiopods     1   0
RED CRAG
_(Exclusive to many derivative shells)_ Bivalves 128 31 25·0
Univalves 127 33 Brachiopods     1   1 CORALLINE CRAG
Bivalves 161 47 31·5 Univalves 184 60 Brachiopods    
5   3

To come next to the Red Crag, the reader will observe that a percentage
of 25 is given of shells unknown as living, and this increases to 31 in
the antecedent Coralline Crag. But the gap between these two stages of
our Pliocene deposits is really wider than these numbers would
indicate, for several reasons. In the first place, the Coralline Crag
is more strictly the product of a single period, the Red Crag, as we
have seen, consisting of separate and independent patches, slightly
varying in age, of which the newest is probably not much anterior to
the Norwich Crag. Secondly, there was a great change of conditions,
both as to the depth of the sea and climate, between the periods of the
Coralline and Red Crag, causing the fauna in each to differ far more
widely than would appear from the above numerical results.

The value of the analysis given in the above table of the shells of the
Red and Coralline Crags is in no small degree enhanced by the fact that
they were all either collected by Mr. Wood himself, or obtained by him
direct from their discoverers, so that he was enabled in each case to
test their authenticity, and as far as possible to avoid those errors
which arise from confounding together shells belonging to the sea of a
newer deposit, and those washed into it from a formation of older date.
The danger of this confusion may be conceived when we remember that the
number of species rejected from the Red Crag as derivative by Mr. Wood
is no less than 25. Some geologists have held that on the same grounds
it is necessary to exclude as spurious some of the species found in the
Norwich Crag proper; but Mr. Wood does not entertain this view,
believing that the spurious shells which have sometimes found their way
into the lists of this crag have been introduced by want of care from
strata of Red Crag.

There can be no doubt, on the other hand, that conchologists have
occasionally rejected from the Red and Norwich Crags, as derivative,
shells which really belonged to the seas of those periods, because they
were extinct or unknown as living, which in their eyes afforded
sufficient ground for suspecting them to be intruders. The derivative
origin of a species may sometimes be indicated by the extreme scarcity
of the individuals, their colour, and worn condition; whereas an
opposite conclusion may be arrived at by the integrity of the shells,
especially when they are of delicate and tender structure, or their
abundance, and, in the case of the lamellibranchiata, by their being
held together by the ligament, which often happens when the shells have
been so broken that little more than the hinges of the two valves are
preserved. As to the univalves, I have seen from a pit of Red Crag,
near Woodbridge, a large individual of the extinct _Voluta Lamberti_,
seven inches in length, of which the lip, then perfect, had in former
stages of its growth been frequently broken, and as often repaired. It
had evidently lived in the sea of the Red Crag, where it had been
exposed to rough usage, and sustained injuries like those which the
reversed whelk, _Trophon antiquum_, so characteristic of the same
formation, often exhibits. Additional proofs, however, have lately been
obtained by Mr. Searles Wood that this shell had not died out in the
era of the Red Crag by the discovery of the same fossil near Southwold,
in beds of the later Norwich Crag.

Antwerp Crag.—Strata of the same age as the Red and Coralline Crag of
Suffolk have been long known in the country round Antwerp, and on the
banks of the Scheldt, below that city; and the lowest division, or
Black Crag, there found, is shown by the shells to be somewhat more
ancient than any of our British series, and probably forms the first
links of a downward passage from the strata of the Pliocene to those of
the Upper Miocene period.

Fig. 132: Murex vaginatus

Newer Pliocene Strata of Sicily.—At several points north of Catania, on
the eastern sea-coast of Sicily—as at Aci-Castello, for example,
Trezza, and Nizzeti—marine strata, associated with volcanic tuffs and
basaltic lavas, are seen, which belong to a period when the first
igneous eruptions of Mount Etna were taking place in a shallow bay of
the Mediterranean. They contain numerous fossil shells, and out of 142
species that have been collected all but eleven are identical with
species now living. Some few of these eleven shells may possibly still
linger in the depths of the Mediterranean, like _Murex vaginatus_, see
Fig. 132. The last-mentioned shell had already become rare when the
associated marine and volcanic strata above alluded to were formed. On
the whole, the modern character of the testaceous fauna under
consideration is expressed not only by the small proportion of extinct
species, but by the relative number of individuals by which most of the
other species are represented, for the proportion agrees with that
observed in the present fauna of the Mediterranean. The rarity of
individuals in the extinct species is such as to imply that they were
already on the point of dying out, having flourished chiefly in the
earlier Pliocene times, when the Subapennine strata were in progress.

Yet since the accumulation of these Newer Pliocene sands and clays, the
whole cone of Etna, 11,000 feet in height and about 90 miles in
circumference at its base, has been slowly built up; an operation
requiring many tens of thousands of years for its accomplishment, and
to estimate the magnitude of which it is necessary to study in detail
the internal structure of the mountain, and to see the proofs of its
double axis, or the evidence of the lavas of the present great centre
of eruption having gradually overwhelmed and enveloped a more ancient
cone, situated 3½ miles to the east of the present one.[4]

It appears that while Etna was increasing in bulk by a series of
eruptions, its whole mass, comprising the foundations of subaqueous
origin above alluded to, was undergoing a slow upheaval, by which those
marine strata were raised to the height of 1200 feet above the sea, as
seen at Catera, and perhaps to greater heights, for we cannot trace
their extension westward, owing to the dense and continuous covering of
modern lava under which they are buried. During the gradual rise of
these Newer Pliocene formations (consisting of clays, sands, and
basalts) other strata of Post-pliocene date, marine as well as
fluviatile, accumulated round the base of the mountain, and these, in
their turn, partook of the upward movement, so that several inland
cliffs and terraces at low levels, due partly to the action of the sea
and partly to the river Simeto, originated in succession. Fossil
remains of the elephant, and other extinct quadrupeds, have been found
in these Post-Pliocene strata, associated with recent shells.

There is probably no part of Europe where the Newer Pliocene formations
enter so largely into the structure of the earth’s crust, or rise to
such heights above the level of the sea, as Sicily. They cover nearly
half the island, and near its centre, at Castrogiovanni, reach an
elevation of 3000 feet. They consist principally of two divisions, the
upper calcareous and the lower argillaceous, both of which may be seen
at Syracuse, Girgenti, and Castrogiovanni. According to Philippi, to
whom we are indebted for the best account of the tertiary shells of
this island, thirty-five species out of one hundred and twenty-four
obtained from the beds in central Sicily are extinct.

A geologist, accustomed to see nearly all the Newer Pliocene formations
in the north of Europe occupying low grounds and very incoherent in
texture, is naturally surprised to behold formations of the same age so
solid and stony, of such thickness, and attaining so great an elevation
above the level of the sea. The upper or calcareous member of this
group in Sicily consists in some places of a yellowish-white stone,
like the Calcaire Grossier of Paris; in others, of a rock nearly as
compact as marble. Its aggregate thickness amounts sometimes to 700 or
800 feet. It usually occurs in regular horizontal beds, and is
occasionally intersected by deep valleys, such as those of Sortino and
Pentalica, in which are numerous caverns. The fossils are in every
stage of preservation, from shells retaining portions of their animal
matter and colour to others which are mere casts. The limestone passes
downward into a sandstone and conglomerate, below which is clay and
blue marl, from which perfect shells and corals may be disengaged. The
clay sometimes alternates with yellow sand.

South of the plain of Catania is a region in which the tertiary beds
are intermixed with volcanic matter, which has been for the most part
the product of submarine eruptions. It appears that, while the clay,
sand, and yellow limestone before mentioned were in course of
deposition at the bottom of the sea, volcanoes burst out beneath the
waters, like that of Graham Island, in 1831, and these explosions
recurred again and again at distant intervals of time. Volcanic ashes
and sand were showered down and spread by the waves and currents so as
to form strata of tuff, which are found intercalated between beds of
limestone and clay containing marine shells, the thickness of the whole
mass exceeding 2000 feet. The fissures through which the lava rose may
be seen in many places, forming what are called _dikes._

Fig. 133: Pecten jacobæus

No shell is more conspicuous in these Sicilian strata than the great
scallop, _Pecten jacobæus_ (Fig. 133), now so common in the
neighbouring seas. The more we reflect on the preponderating number of
this and other recent shells, the more we are surprised at the great
thickness, solidity, and height above the sea of the rocky masses in
which they are entombed, and the vast amount of geographical change
which has taken place since their origin. It must be remembered that,
before they began to emerge, the uppermost strata of the whole must
have been deposited under water. In order, therefore, to form a just
conception of their antiquity, we must first examine singly the
innumerable minute parts of which the whole is made up, the successive
beds of shells, corals, volcanic ashes, conglomerates, and sheets of
lava; and we must afterwards contemplate the time required for the
gradual upheaval of the rocks, and the excavation of the valleys. The
historical period seems scarcely to form an appreciable unit in this
computation, for we find ancient Greek temples, like those of Girgenti
(Agrigentum), built of the modern limestone of which we are speaking,
and resting on a hill composed of the same; the site having remained to
all appearances unaltered since the Greeks first colonised the island.

It follows, from the modern geological date of these rocks, that the
fauna and flora of a large part of Sicily are of higher antiquity than
the country itself. The greater part of the island has been raised
above the sea since the epoch of existing species, and the animals and
plants now inhabiting it must have migrated from adjacent countries,
with whose productions the species are now identical. The average
duration of species would seem to be so great that they are destined to
outlive many important changes in the configuration of the earth’s
surface, and hence the necessity for those innumerable contrivances by
which they are enabled to extend their range to new lands as they are
formed, and to escape from those which sink beneath the sea.

Newer Pliocene Strata of the Upper Val D’arno.—When we ascend the Arno
for about ten miles above Florence, we arrive at a deep narrow valley
called the Upper Val d’Arno, which appears once to have been a lake, at
a time when the valley below Florence was an arm of the sea. The
horizontal lacustrine strata of this upper basin are twelve miles long
and two broad. The depression which they fill has been excavated out of
Eocene and Cretaceous rocks, which form everywhere the sides of the
valley in highly inclined stratification. The thickness of the more
modern and unconformable beds is about 750 feet, of which the upper 200
feet consist of Newer Pliocene strata, while the lower are Older
Pliocene. The newer series are made up of sands and a conglomerate
called “sansino.” Among the imbedded fossil mammalia are _Mastodon
arvernensis, Elephas meridionalis, Rhinoceros etruscus, Hippopotamus
major,_ and remains of the genera bear, hyæna, and felis, nearly all of
which occur in the Cromer forest-bed (see Chap. 13, p. 191).

In the same upper strata are found, according to M. Gaudin, the leaves
and cones of _Glyptostrobus europæus_, a plant closely allied to _G.
heterophyllus_, now inhabiting the north of China and Japan. This
conifer had a wide range in time, having been traced back to the Lower
Miocene strata of Switzerland, and being common at Œningen in the Upper
Miocene, as we shall see in the sequel (p. 218).

Older Pliocene of Italy.—Subapennine Strata.—The Apennines, it is
well-known, are composed chiefly of Secondary or Mesozoic rocks,
forming a chain which branches off from the Ligurian Alps and passes
down the middle of the Italian peninsula. At the foot of these
mountains, on the side both of the Adriatic and the Mediterranean, are
found a series of tertiary strata, which form, for the most part, a
line of low hills occupying the space between the older chain and the
sea. Brocchi was the first Italian geologist who described this newer
group in detail, giving it the name of the Subapennine. Though chiefly
composed of Older Pliocene strata, it belongs, nevertheless, in part,
both to older and newer members of the tertiary series. The strata, for
example, of the Superga, near Turin, are Miocene; those of Asti and
Parma Older Pliocene, as is the blue marl of Sienna; while the shells
of the incumbent yellow sand of the same territory approach more nearly
to the recent fauna of the Mediterranean, and may be Newer Pliocene.

We have seen that most of the fossil shells of the Older Pliocene
strata of Suffolk which are of recent species are identical with
testacea now living in British seas, yet some of them belong to
Mediterranean species, and a few even of the genera are those of warmer
climates. We might therefore expect, in studying the fossils of
corresponding age in countries bordering the Mediterranean, to find
among them some species and genera of warmer latitudes. Accordingly, in
the marls belonging to this period at Asti, Parma, Sienna, and parts of
the Tuscan and Roman territories, we observe the genera _Conus, Cypræa,
Strombus, Pyrula, Mitra, Fasciolaria, Sigaretus, Delphinula,
Ancillaria, Oliva, Terebellum, Terebra, Perna, Plicatula,_ and
_Corbis_, some characteristic of tropical seas, others represented by
species more numerous or of larger size than those now proper to the
Mediterranean.

Older Pliocene Flora of Italy.—I have already alluded to the Newer
Pliocene deposits of the Upper Val d’Arno above Florence, and stated
that below those sands and conglomerates, containing the remains of the
_Elephas meridionalis_ and other associated quadrupeds, lie an older
horizontal and conformable series of beds, which may be classed as
Older Pliocene. They consist of blue clays with some subordinate layers
of lignite, and exhibit a richer flora than the overlying Newer
Pliocene beds, and one receding farther from the existing vegetation of
Europe. They also comprise more species common to the antecedent
Miocene period. Among the genera of flowering plants, M. Gaudin
enumerates pine, oak, evergreen oak, plum, plane, alder, elm, fig,
laurel, maple, walnut, birch, buckthorn, hickory, sumach, sarsaparilla,
sassafras, cinnamon, Glyptostrobus, Taxodium, Sequoia, Persea,
Oreodaphne (Fig. 134), Cassia, and Psoralea, and some others. This
assemblage of plants indicates a warm climate, but not so subtropical
an one as that of the Upper Miocene period, which will presently be
considered.

Fig. 134: Creodaphne Heerii. Fig. 135: Liquidambar europæum, var.
trilobatum Fig. 134: _Creodaphne Heerii_.
Leaf[5]
Fig. 135: _Liquidambar europæum_, var. _trilobatum_, A. Br. (sometimes
four-lobed, and more commonly five-lobed).
_a._ Leaf. _b._ Part of same. _c._ Fruit. _d._ Seed Œningen.

M. Gaudin, jointly with the Marquis Strozzi, has thrown much light on
the botany of beds of the same age in another part of Tuscany, at a
place called Montajone, between the rivers Elsa and Evola, where, among
other plants, is found the _Oreodaphne Heerii_, Gaud. (see Fig. 134),
which is probably only a variety of _Oreodaphne foetens_, or the laurel
called the Til in Madeira, where, as in the Canaries, it constitutes a
large portion of the native woods, but cannot now endure the climate of
Europe. In the fossil specimens the same glands or protuberances are
preserved[6] (see Fig. 134) as those which are seen in the axils of the
primary veins of the leaves in the recent Til. Another plant also
indicating a warmer climate is the _ Liquidambar europæum_, Brong. (see
Fig. 135), a species nearly allied to _L. styracifluum_, L., which
flourishes in most places in the Southern States of North America, on
the borders of the Gulf of Mexico.

 [1] See Antiquity of Man, chap. xiv.

 [2] Ehrenberg proposed in 1831 the term _ Bryozoum_, or “Moss-animal,”
 for the molluscous or ascidian form of polyp, characterised by having
 two openings to the digestive sack, as in _Eschara, Flustra,
 Retepora,_ and other zoophytes popularly included in the corals, but
 now classed by naturalists as mollusca. The term _Polyzoum,_
 synonymous with _ Bryozoum,_ was, it seems, proposed in 1830, or the
 year before, by Mr. J. O. Thompson.

 [3] E. Forbes Mem. Geol. Survey of Gt. Brit., vol. i, p. 386.

 [4] See a Memoir on the Lavas and Mode of Origin of Mount Etna by the
 Author in Phil. Trans., 1858.

 [5] Feuilles fossiles de la Toscane.

 [6] Contributions à la Flore fossile Italienne. Gaudin and Strozzi.
 Plate 11, Fig. 3. Gaudin, p. 22.




CHAPTER XIV.
MIOCENE PERIOD—UPPER MIOCENE.


Upper Miocene Strata of France—Faluns of Touraine. — Tropical Climate
implied by Testacea. — Proportion of recent Species of Shells. — faluns
more ancient than the Suffolk Crag. — Upper Miocene of Bordeaux and the
South of France. — Upper Miocene of Œningen, in Switzerland. — Plants
of the Upper Fresh-water Molasse. — Fossil Fruit and Flowers as well as
Leaves. — Insects of the Upper Molasse. — Middle or Marine Molasse of
Switzerland. — Upper Miocene Beds of the Bolderberg, in Belgium. —
Vienna Basin. — Upper Miocene of Italy and Greece. — Upper Miocene of
India; Siwalik Hills. — Older Pliocene and Miocene of the United
States.

Upper Miocene Strata of France—Faluns of Touraine.—The strata which we
meet with next in the descending order are those called by many
geologists “Middle Tertiary,” for which in 1833 I proposed the name of
Miocene, selecting the “faluns” of the valley of the Loire, in France,
as my example or type. I shall now call these falunian deposits Upper
Miocene, to distinguish them from others to which the name of Lower
Miocene will be given.

No British strata have a distinct claim to be regarded as Upper
Miocene, and as the Lower Miocene are also but feebly represented in
the British Isles, we must refer to foreign examples in illustration of
this important period in the earth’s history. The term “faluns” is
given provincially by French agriculturists to shelly sand and marl
spread over the land in Touraine, just as similar shelly deposits were
formerly much used in Suffolk to fertilise the soil, before the
coprolitic or phosphatic nodules came into use. Isolated masses of such
faluns occur from near the mouth of the Loire, in the neighbourhood of
Nantes, to as far inland as a district south of Tours. They are also
found at Pontlevoy, on the Cher, about seventy miles above the junction
of that river with the Loire, and thirty miles south-east of Tours.
Deposits of the same age also appear under new mineral conditions near
the towns of Dinan and Rennes, in Brittany. I have visited all the
localities above enumerated, and found the beds on the Loire to consist
principally of sand and marl, in which are shells and corals, some
entire, some rolled, and others in minute fragments. In certain
districts, as at Doué, in the Department of Maine and Loire, ten miles
south-west of Saumur, they form a soft building-stone, chiefly composed
of an aggregate of broken shells, bryozoa, corals, and echinoderms,
united by a calcareous cement; the whole mass being very like the
Coralline Crag near Aldborough, and Sudbourn in Suffolk. The scattered
patches of faluns are of slight thickness, rarely exceeding fifty feet;
and between the district called Sologne and the sea they repose on a
great variety of older rocks; being seen to rest successively upon
gneiss, clay-slate, various secondary formations, including the chalk;
and, lastly, upon the upper fresh-water limestone of the Parisian
tertiary series, which, as before mentioned (p. 142), stretches
continuously from the basin of the Seine to that of the Loire.

Fig. 136: Dinotherium giganteum.
At some points, as at Louans, south of Tours, the shells are stained of
a ferruginous colour, not unlike that of the Red Crag of Suffolk. The
species are, for the most part, marine, but a few of them belong to
land and fluviatile genera. Among the former, _ Helix turonensis)_
(Fig. 38) is the most abundant. Remains of terrestrial quadrupeds are
here and there intermixed, belonging to the genera Dinotherium (Fig.
136), Mastodon, Rhinoceros, Hippopotamus, Chæropotamus, Dichobune,
Deer, and others, and these are accompanied by cetacea, such as the
Lamantin, Morse, Sea-calf, and Dolphin, all of extinct species.

The fossil testacea of the faluns of the Loire imply, according to the
late Edward Forbes, that the beds were formed partly on the shore
itself at the level of low water, and partly at very moderate depths,
not exceeding ten fathoms below that level. The molluscan fauna is, on
the whole, much more littoral than that of the Pliocene Red and
Coralline Crag of Suffolk, and implies a shallower sea. It is,
moreover, contrasted with the Suffolk Crag by the indications it
affords of an extra-European climate. Thus it contains seven species of
Cypræa, some larger than any existing cowry of the Mediterranean,
several species of _Oliva, Ancillaria, Mitra, Terebra, Pyrula,
Fasciolaria,_ and _ Conus._ Of the cones there are no less than eight
species, some very large, whereas the only European cone now living is
of diminutive size. The genus _Nerita,_ and many others, are also
represented by individuals of a type now characteristic of equatorial
seas, and wholly unlike any Mediterranean forms. These proofs of a more
elevated temperature seem to imply the higher antiquity of the faluns
as compared with the Suffolk Crag, and are in perfect accordance with
the fact of the smaller proportion of testacea of recent species found
in the faluns.

Out of 290 species of shells, collected by myself in 1840 at Pontlevoy,
Louans, Bossée, and other villages twenty miles south of Tours, and at
Savigné, about fifteen miles north-west of that place, seventy-two only
could be identified with recent species, which is in the proportion of
twenty-five per cent. A large number of the 290 species are common to
all the localities, those peculiar to each not being more numerous than
we might expect to find in different bays of the same sea.

The total number of species of testaceous mollusca from the faluns in
my possession is 302, of which forty-five only, or fourteen per cent,
were found by Mr. Wood to be common to the Suffolk Crag. The number of
corals, including bryozoa and zoantharia, obtained by me at Doué and
other localities before adverted to, amounts to forty-three, as
determined by Mr. Lonsdale, of which seven (one of them a zoantharian)
agree specifically with those of the Suffolk Crag. Some of the genera
occurring fossil in Touraine, as the corals Astrea and _
Dendrophyllia_, and the bryozoan _Lunulites_, have not been found in
European seas north of the Mediterranean; nevertheless, the zoantharia
of the faluns do not seem to indicate, on the whole, so warm a climate
as would be inferred from the shells.

It was stated that, on comparing about 300 species of Touraine shells
with about 450 from the Suffolk Crag, forty-five only were found to be
common to both, which is in the proportion of only fifteen per cent.
The same small amount of agreement is found in the corals also. I
formerly endeavoured to reconcile this marked difference in species
with the supposed co-existence of the two faunas, by imagining them to
have severally belonged to distinct zoological provinces or two seas,
the one opening to the north and the other to the south, with a barrier
of land between them, like the Isthmus of Suez, now separating the Red
Sea and the Mediterranean. But I now abandon that idea for several
reasons; among others, because I succeeded in 1841 in tracing the Crag
fauna southward in Normandy to within seventy miles of the Falunian
type, near Dinan, yet found that both assemblages of fossils retained
their distinctive characters, showing no signs of any blending of
species or transition of climate.

The principal grounds, however, for referring the English Crag to the
older Pliocene and the French faluns to the Upper Miocene epochs,
consist in the predominance of fossil shells in the British strata
identifiable with species not only still living, but which are now
inhabitants of neighbouring seas, while the accompanying extinct
species are of genera such as characterise Europe. In the faluns, on
the contrary, the recent species are in a decided minority; and most of
them are now inhabitants of the Mediterranean, the coast of Africa, and
the Indian Ocean; in a word, less northern in character, and pointing
to the prevalence of a warmer climate. They indicate a state of things
receding farther from the present condition of Central Europe in
physical geography and climate, and doubtless, therefore, receding
farther from our era in time.

Fig. 137: Voluta Lamberti.
Among the conspicuous fossils common to the faluns of the Loire and the
Suffolk Crag is a variety of the _Voluta Lamberti_, a shell already
alluded to (Fig. 123). The specimens of this shell which I have myself
collected in Touraine, or have seen in museums, are thicker and heavier
than British individuals of the same species, and shorter in proportion
to their width, and have the folds on the columella less oblique, as
represented in Fig. 137.

Upper Miocene of Bordeaux and the South of France.—A great extent of
country between the Pyrenees and the Gironde is overspread by tertiary
deposits of various ages, and chiefly of Miocene date. Some of these,
near Bordeaux, coincide in age with the faluns of Touraine, already
mentioned, but many of the species of shells are peculiar to the south.
The succession of beds in the basin of the Gironde implies several
oscillations of level by which the same wide area was alternately
converted into sea and land and into brackish-water lagoons, and
finally into fresh-water ponds and lakes.

Among the fresh-water strata of this age near the base of the Pyrenees
are marls, limestones and sands, in which the eminent comparative
anatomist, M. Lartet, has obtained a great number of fossil mammalia
common to the faluns of the Loire and the Upper Miocene beds of
Switzerland, such as _Dinotherium giganteum_ and _Mastodon
angustidens_; also the bones of quadrumana, or of the ape and monkey
tribe, which were discovered in 1837, the first of that order of
quadrupeds detected in Europe. They were found near Auch, in the
Department of Gers, in latitude 43° 39′ N. About forty miles west of
Toulouse. They were referred by MM. Lartet and Blainville to a genus
closely allied to the Gibbon, to which they gave the name of
_Pliopithecus._ Subsequently, in 1856, M. Lartet described another
species of the same family of long-armed apes (_Hylobates_), which he
obtained from strata of the same age at Saint-Gaudens, in the Haute
Garonne. The fossil remains of this animal consisted of a portion of a
lower jaw with teeth and the shaft of a humerus. It is supposed to have
been a tree-climbing frugivorous ape, equalling man in stature. As the
trunks of oaks are common in the lignite beds in which it lay, it has
received the generic name of _Dryopithecus._ The angle formed by the
ascending ramus of the jaw and the alveolar border is less open, and
therefore more like the human subject, than in the Chimpanzee, and what
is still more remarkable, the fossil, a young but adult individual, had
all its milk teeth replaced by the second set, while its last true
molar (or wisdom-tooth) was still undeveloped, or only existed as a
germ in the jaw-bone. In the mode, therefore, of the succession of its
teeth (which, as in all the old-World apes, exactly agree in number
with those in man) it differed from the Gorilla and Chimpanzee, and
corresponded with the human species.

Upper Miocene Beds of Œningen, in Switzerland.—The faluns of the Loire
first served, as already stated (p. 211), as the type of the Miocene
formations in Europe. They yielded a plentiful harvest of marine fossil
shells and corals, but were entirely barren of plants and insects. In
Switzerland, on the other hand, deposits of the same age have been
discovered, remarkable for their botanical and entomological treasures.
We are indebted to Professor Heer, of Zurich, for the description,
restoration, and classification of several hundred species and
varieties of these fossil plants, the whole of which he has illustrated
by excellent figures in his “Flora Tertiaria Helvetiæ.” This great
work, and those of Adolphe Brongniart, Unger, Goppert and others, show
that this class of fossils is beginning to play the same important part
in the classification of the tertiary strata containing lignite or
brown coal as an older flora has long played in enabling us to
understand the ancient coal or carboniferous formation. No small
skepticism has always prevailed among botanists as to whether the
leaves alone and the wood of plants could ever afford sufficient data
for determining even genera and families in the vegetable kingdom. In
truth, before such remains could be rendered available a new science
had to be created. It was necessary to study the outlines, nervation,
and microscopic structure of the leaves, with a degree of care which
had never been called for in the classification of living plants, where
the flower and fruit afforded characters so much more definite and
satisfactory. As geologists, we cannot be too grateful to those who,
instead of despairing when so difficult a task was presented to them,
or being discouraged when men of the highest scientific attainments
treated the fossil leaves as worthless, entered with full faith and
enthusiasm into this new and unexplored field. That they should
frequently have fallen into errors was unavoidable, but it is
remarkable, especially if we inquire into the history of Professor
Heer’s researches, how often early conjectures as to the genus and
family founded on the leaves alone were afterwards confirmed when
fuller information was obtained. As examples to be found on comparing
Heer’s earlier and later works, I may instance the chestnut, elm,
maple, cinnamon, magnolia, buckbean or Menyanthes, vine, buckthorn
(_Rhamnus_), _Andromeda_ and _Myrica,_ and among the conifers _Sequoia_
and _ Taxodium._ In all these cases the plants were first recognised by
their leaves, and the accuracy of the determination was afterwards
confirmed when the fruit, and in some instances both fruit and flower,
were found attached to the same stem as the leaves.

But let us suppose that no fruit, seed, or flower had ever been met
with in a fossil state, we should still have been indebted to the
persevering labours of botanical palæontologists for one of the
grandest scientific discoveries for which the present century is
remarkable—namely, the proofs now established of the prevalence of a
mild climate and a rich arborescent flora in the arctic regions in that
Miocene epoch on the history of which we are now entering. It may be
useful if I endeavour to give the reader in a few words some idea of
the nature of the evidence of these important conclusions, to show how
far they may be safely based on fossil leaves alone. When we begin by
studying the fossils of the Newer Pliocene deposits, such as those of
the Upper Val d’Arno, before alluded to, we perceive that the fossil
foliage agrees almost entirely with the trees and shrubs of a modern
European forest. In the plants of the Older Pliocene strata of the same
region we observe a larger proportion of species and genera which,
although they may agree with well-known Asiatic or other foreign types,
are at present wanting in Italy. If we then examine the Miocene
formations of the same country, exotic forms become more abundant,
especially the palms, whether they belong to the European or American
fan-palms, _Chamærops_ and _Sabal_, or to the more tropical family of
the date-palms or _Phœnicites_, which last are conspicuous in the Lower
Miocene beds of Central Europe. Although we have not found the fruit or
flower of these palms in a fossil state, the leaves are so
characteristic that no one doubts the family to which they belong, or
hesitates to accept them as indications of a warm and sub-tropical
climate.

When the Miocene formations are traced to the northward of the 50th
degree of latitude, the fossil palms fail us, but the greater
proportion of the leaves, whether identical with those of existing
European trees or of forms now unknown in Europe, which had accompanied
the Miocene palms, still continue to characterise rocks of the same
age, until we meet with them not only in Iceland, but in Greenland, in
latitude 70° N., and in Spitzbergen, latitude 78° 56′, or within about
11 degrees of the pole, and under circumstances which clearly show them
to have been indigenous in those regions, and not to have been drifted
from the south (see p. 240). Not only, therefore, has the botanist
afforded the geologist much palæontological assistance in identifying
distinct tertiary formations in distant places by his power of
accurately discriminating the forms, veining, and microscopic structure
of leaves or wood, but, independently of that exact knowledge derivable
from the organs of fructification, we are indebted to him for one of
the most novel, unexpected results of modern scientific inquiry.

The Miocene formations of Switzerland have been called _ Molasse_, a
term derived from the French _mol_, and applied to a _soft_,
incoherent, greenish sandstone, occupying the country between the Alps
and the Jura. This molasse comprises three divisions, of which the
middle one is marine, and being closely related by its shells to the
faluns of Touraine, may be classed as Upper Miocene. The two others are
fresh-water, the upper of which may be also grouped with the faluns,
while the lower must be referred to the Lower Miocene, as defined in
the next chapter.

Upper Fresh-water Molasse.—This formation is best seen at Œningen, in
the valley of the Rhine, between Constance and Schaffhausen, a locality
celebrated for having produced in the year 1700 the supposed human
skeleton called by Scheuchzer “homo diluvii testis,” a fossil
afterwards demonstrated by Cuvier to be a reptile, or aquatic
salamander, of larger dimensions than even its great living
representative, the salamander of Japan.

The Œningen strata consist of a series of marls and limestones, many of
them thinly laminated, and which appear to have slowly accumulated in a
lake probably fed by springs holding carbonate of lime in solution. The
elliptical area over which this fresh-water formation has been traced
extends, according to Sir Roderick Murchison, for a distance of ten
miles east and west from Berlingen, on the right bank of the river to
Wangen, and to Œningen, near Stein, on the left bank. The organic
remains have been chiefly derived from two quarries, the lower of which
is about 550 feet above the level of the Lake of Constance, while the
upper quarry is 150 feet higher. In this last, a section thirty feet
deep displays a great succession of beds, most of them splitting into
slabs and some into very thin laminæ. Twenty-one beds are enumerated by
Professor Heer, the uppermost a bluish-grey marl seven feet thick, with
organic remains, resting on a limestone with fossil plants, including
leaves of poplar, cinnamon, and pond-weed (_Potamogeton_), together
with some insects; while in the bed No. 4, below, is a bituminous rock,
in which the _Mastodon tapiroides_, a characteristic Upper Miocene
quadruped, has been met with. The 5th bed, two or three inches thick,
contains fossil fish, e.g., _Leuciscus_ (roach), and the larvæ of
dragon-flies, with plants such as the elm (_Ulmus_), and the aquatic
Chara. Below this are other plant-beds; and then, in No. 9, the stone
in which the great salamander (Andrias Scheuchzeri) and some fish were
found. Below this other strata occur with fish, tortoises, the great
salamander before alluded to, fresh-water mussels, and plants. In No.
16 the fossil fox of Œningen, _Galecynus Œningensis,_ Owen, was
obtained by Sir R. Murchison. To this succeed other beds with mammalia
(_Lagomys_), reptiles, (_Emys_), fish, and plants, such as walnut,
maple, and poplar. In the 19th bed are numerous fish, insects, and
plants, below which are marls of a blue indigo colour.

In the lower quarry eleven beds are mentioned, in which, as in the
upper, both land and fresh-water plants and many insects occur. In the
6th, reckoning from the top, many plants have been obtained, such as
_Liquidambar, Daphnogene, Podogonium,_ and _ Ulmus_, together with
tortoises, besides the bones and teeth of a ruminant quadruped, named
by H. von Meyer _Palæomeryx eminens._ No. 9 is called the insect-bed, a
layer only a few inches thick, which, when exposed to the frost, splits
into leaves as thin as paper. In these thin laminæ plants such as _
Liquidambar, Daphnogene,_ and _Glyptostrobus_, occur, with innumerable
insects in a wonderful state of preservation, usually found singly.
Below this is an indigo-blue marl, like that at the bottom of the
higher quarry, resting on yellow marl ascertained to be at least thirty
feet thick.

Fig. 138: Cinnamomum polymorphum.
All the above fossil-bearing strata were evidently formed with extreme
slowness. Although the fossiliferous beds are, in the aggregate, no
more than a few yards in thickness, and have only been examined in the
small area comprised in the two quarries just alluded to, they give us
an insight into the state of animal and vegetable life in part of the
Upper Miocene period, such as no other region in the world has
elsewhere supplied. In the year 1859, Professor Heer had already
determined no less than 475 species of plants and more than 800 insects
from these Œningen beds. He supposes that a river entering a lake
floated into it some of the leaves and land insects, together with the
carcasses of quadrupeds, among others a great Mastodon. Occasionally,
during tempests, twigs and even boughs of trees with their leaves were
torn off and carried for some distance so as to reach the lake.
Springs, containing carbonate of lime, seem at some points to have
supplied calcareous matter in solution, giving origin locally to a kind
of travertin, in which organic bodies sinking to the bottom became
hermetically sealed up. The laminæ, says Heer, which immediately
succeed each other were not all formed at the same season, for it can
be shown that, when some of them originated, certain plants were in
flower, whereas, when the next of these layers was produced, the same
plants had ripened their fruit. This inference is confirmed by
independent proofs derived from insects. The principal insect-bed is
rarely two inches thick, and is composed, says Heer, of about 250
leaf-like laminæ, some of which were deposited in the spring, when the
_Cinnamomum polymorphum_ (Fig. 138) was in flower, others in summer,
when winged ants were numerous, and when the poplar and willow had
matured their seed; others, again, in autumn, when the same _
Cinnamomum polymorphum_ (Fig. 138) was in fruit, as well as the
liquidambar, oak, clematis, and many other plants. The ancient lake
seems to have had a belt of poplars and willows round its borders,
countless leaves of which were imbedded in mud, and together with them,
at some points, a species of reed, _Arundo_, which was very common.

One of the most characteristic shrubs is a papilionaceous and
leguminous plant of an extinct genus, called by Heer _ Podogonium_, of
which two species are known. Entire twigs have been found with flowers,
and always without leaves, as the flowers evidently came out, as in the
poplar and willow tribe, before any leaves made their appearance. Other
specimens have been obtained with ripe fruits accompanied by leaves,
which resemble those of the tamarind, to which it was evidently allied,
being of the family Cæsalpineæ, now proper to warmer regions.

Fig. 138: Acer trilobatum.
The Upper Miocene flora of Œningen is peculiarly important, in
consequence of the number of genera of which not merely the leaves,
but, as in the case of the _Podogonium_ just mentioned, the fruit also
and even the flower are known. Thus there are nineteen species of
maple, ten of which have already been found with fruit. Although in no
one region of the globe do so many maples now flourish, we need not
suspect Professor Heer of having made too many species in this genus
when we consider the manner in which he has dealt with one of them,
_Acer trilobatum_, Figs. 139 and 140. Of this plant the number of
marked varieties figured and named is very great, and no less than
three of them had been considered as distinct species by other
botanists, while six of the others might have laid claim, with nearly
equal propriety, to a like distinction. The common form, called _Acer
trilobatum_, Fig. 139, may be taken as a normal representative of the
Œningen fossil, and Fig. 140, as one of the most divergent varieties,
having almost four lobes in the leaf instead of three.

Fig. 140: Acer trilobatum.

Among the conspicuous genera which abounded in the Miocene period in
Europe is the plane-tree, _Platanus,_ the fossil species being
considered by Heer to come nearer to the American _ P. occidentalis_
than to _P. orientalis_ of Greece and Asia Minor. In some of the fossil
specimens the male flowers are preserved. Among other points of
resemblance with the living plane-trees, as we see them in the parks
and squares of London, fossil fragments of the trunk are met with,
having pieces of their bark peeling off.

Platanus aceroides.
The vine of Œningen, _Vitis teutonica_, Ad. Brong, is of a North
American type. Both the leaves and seeds have been found at Œningen,
and bunches of compressed grapes of the same species have been met with
in the brown coal of Wetteravia in Germany. No less than eight species
of smilax, a monocotyledonous genus, occur at Œningen and in other
Upper Miocene localities, the flowers of some of them, as well as the
leaves, being preserved; as in the case of the very common fossil, _S.
sagittifera_, Fig. 142, _a._

Leaves of plants supposed to belong to the order Proteaceæ have been
obtained partly from Œningen and partly from the lacustrine formation
of the same age at Locle in the Jura. They have been referred to the
genera _Banksia, Grevillea, Hakea,_ and _Persoonia._ Of Hakea there is
the impression of a supposed seed-vessel, with its characteristic thick
stalk and seeds, but as the fruit is without structure, and has not yet
been found attached to the same stem as the leaf, the proof is
incomplete.

Fig. 142: Smilax sagittifera.
To whatever family the foliage hitherto regarded as proteaceous by many
able palæontologists may eventually be shown to belong, we must be
careful not to question their affinity to that order of plants on those
geographical considerations which have influenced some botanists. The
nearest living Proteaceæ now feel the in Abyssinia in lat. 20° N., but
the greatest number are confined to the Cape and Australia. The
ancestors, however, of the Œningen fossils ought not to be looked for
in such distant regions, but from that European land which in Lower
Miocene times bore trees with similar foliage, and these had doubtless
an Eocene source, for cones admitted by all botanists to be proteaceous
have been met with in one division of that older Tertiary group (see
Fig. 206). The source of these last, again, must not be sought in the
antipodes, for in the white chalk of Aix-la-Chapelle leaves like those
of Grevillea and other proteaceous genera have been found in abundance,
and, as we shall see (p. 304) in a most perfect state of preservation.
All geologists agree that the distribution of the Cretaceous land and
sea had scarcely any connection with the present geography of the
globe.

Fig. 143: Fruit of the fossil and recent species of Hakea, a genus of
Proteaceæ.

In the same beds with the supposed Proteaceæ there occurs at Locle a
fan-palm of the American type Sabal (for genus see Fig. 151), a genus
which ranges throughout the low country near the sea from the Carolinas
to Florida and Louisiana. Among the Coniferæ of Upper Miocene age is
found a deciduous cypress nearly allied to the _Taxodium distichum_ of
North America, and a _Glyptostrobus_ (Fig. 144), very like the Japanese
_G. heterophyllus,_ now common in our shrubberies.

Fig. 144: Glyptostrobus Europæus.
Before the appearance of Heer’s work on the Miocene Flora of
Switzerland, Unger and Goppert had already pointed out the large
proportion of living North American genera which distinguished the
vegetation of the Miocene period in Central Europe. Next in number,
says Heer, to these American forms at Œningen the European genera
preponderate, the Asiatic ranking in the third, the African in the
fourth, and the Australian in the fifth degree. The American forms are
more numerous than in the Italian Pliocene flora, and the whole
vegetation indicates a warmer climate than the Pliocene, though not so
high a temperature as that of the older or Lower Miocene period.

The conclusions drawn from the insects are for the most part in perfect
harmony with those derived from the plants, but they have a somewhat
less tropical and less American aspect, the South European types being
more numerous. On the whole, the insect fauna is richer than that now
inhabiting any part of Europe. No less than 844 species are reckoned by
Heer from the Œningen beds alone, the number of specimens which he has
examined being 5080. The entire list of Swiss species from the Upper
and Lower Miocene together amount to 1322. Almost all the living
families of Coleoptera are represented, but, as we might have
anticipated from the preponderance of arborescent and ligneous plants,
the wood-eating beetles play the most conspicuous part, the Buprestidæ
and other long-horned beetles being particularly abundant.

The patterns and some remains of the colours both of _ Coleoptera_ and
_Hemiptera_ are preserved at Œningen, as, for example in _Harpactor_
(Fig. 145), in which the antennæ, one of the eyes, and the legs and
wings are retained. The characters, indeed, of many of the insects are
so well defined as to incline us to believe that if this class of the
invertebrata were not so rare and local, they might be more useful than
even the plants and shells in settling chronological points in geology.

Middle or Marine Molasse (Upper Miocene) of Switzerland.—It was before
stated that the Miocene formation of Switzerland consisted of, first,
the upper fresh-water molasse, comprising the lacustrine marls of
Œningen; secondly, the marine molasse, corresponding in age to the
faluns of Touraine; and thirdly, the lower fresh-water molasse. Some of
the beds of the marine or middle series reach a height of 2470 feet
above the sea. A large number of the shells are common to the faluns of
Touraine, the Vienna basin, and other Upper Miocene localities. The
terrestrial plants play a subordinate part in the fossiliferous beds,
yet more than ninety of them are enumerated by Heer as belonging to
this falunian division, and of these more than half are common to
subjacent Lower Miocene beds, while a proportion of about forty-five in
one hundred are common to the overlying Œningen flora. Twenty-six of
the ninety-two species are peculiar.

Fig. 145: Harpactor maculipes.

Fig. 146: Olica Dufresnii.

Upper Miocene of the Bolderberg, in Belgium.—In a small hill or ridge
called the Bolderberg, which I visited in 1851, situated near Hasselt,
about forty miles E.N.E. of Brussels, strata of sand and gravel occur,
to which M. Dumont first called attention as appearing to constitute a
northern representative of the faluns of Touraine. On the whole, they
are very distinct in their fossils from the two upper divisions of the
Antwerp Crag before mentioned (p. 204), and contain shells of the
genera _Oliva, Conus, Ancillaria, Pleurotoma,_ and _ Cancellaria_ in
abundance. The most common shell is an Olive (Fig. 146), called by Nyst
_Oliva Dufresnii_; and constituting, as M. Bosquet observes, a smaller
and shorter variety of the Bordeaux species.

So far as the shells of the Bolderberg are known, the proportion of
recent species agrees with that in the faluns of Touraine, and the
climate must have been warmer than that of the Coralline Crag of
England.

Upper Miocene Beds of the Vienna Basin.—In South Germany the general
resemblance of the shells of the Vienna tertiary basin with those of
the faluns of Touraine has long been acknowledged. In the late Dr.
Hörnes’s excellent work on the fossil mollusca of that formation, we
see accurate figures of many shells, clearly of the same species as
those found in the falunian sands of Touraine.

According to Professor Suess, the most ancient and purely marine of the
Miocene strata in this basin consist of sands, conglomerates,
limestones, and clays, and they are inclined inward, or from the
borders of the trough towards the centre, their outcropping edges
rising much higher than the newer beds, whether Miocene or Pliocene,
which overlie them, and which occupy a smaller area at an inferior
elevation above the sea. M. Hornes has described no less than 500
species of gasteropods, of which he identifies one-fifth with living
species of the Mediterranean, Indian, or African seas, but the
proportion of existing species among the lamellibranchiate bivalves
exceeds this average. Among many univalves agreeing with those of
Africa on the eastern side of the Atlantic are _Cypræa sanguinolenta,
Buccinum lyratum,_ and _Oliva flammulata._ In the lowest marine beds of
the Vienna basin the remains of several mammalia have been found, and
among them a species of _Dinotherium_, a Mastodon of the _Trilophodon_
family, a Rhinoceros (allied to _R. megarhinus_, Christol), also an
animal of the hog tribe, _ Listriodon_, von Meyer, and a carnivorous
animal of the canine family. The _Helix turonensis_  (Fig. 38), the
most common land shell of the French faluns, accompanies the above land
animals. In a higher member of the Vienna Miocene series are found
_Dinotherium giganteum_ (Fig. 136), _Mastodon longirostris, Rhinoceros
Schleiermacheri, Acerotherium incisivum,_ and _ Hippotherium gracile,_
all of them equally characteristic of an Upper Miocene deposit
occurring at Eppelsheim, in Hesse Darmstadt; a locality also remarkable
as having furnished in latitude 49° 50′ N. the bone of a large ape of
the Gibbon kind, the most northerly example yet discovered of a
quadrumanous animal.

Fig. 147: Amphistegina Hauerina.
M. Alcide d’Orbigny has shown that the foraminifera of the Vienna basin
differ alike from the Eocene and Pliocene species, and agree with those
of the faluns, so far as the latter are known. Among the Vienna
foraminifera, the genus _Amphistegina_ (Fig. 147) is very
characteristic, and is supposed by d’Archiac to take the same place
among the Rhizopods of the Upper Miocene era which the Nummulites
occupy in the Eocene period.

The flora of the Vienna basin exhibits some species which have a
general range through the whole Miocene period, such as _Cinnamomum
polymorphum_ (Fig. 138), and _C. Scheuchzeri,_ also Planera Richardi,
Mich., _Liquidambar europæum_ (Fig. 135) _Juglans bilinica, Cassia
ambigua,_ and _C. lignitum._ Among the plants common to the Upper
Miocene beds of Œningen, in Switzerland, are _Platanus aceroides_ (Fig.
141), _Myrica vindobonensis,_ and others.

Upper Miocene Strata of Italy.—We are indebted to Signor Michelotti for
a valuable work on the Miocene shells of Northern Italy. Those found in
the hill called the Superga, near Turin, have long been known to
correspond in age with the faluns of Touraine, and they contain so many
species common to the Upper Miocene strata of Bordeaux as to lead to
the conclusion that there was a free communication between the northern
part of the Mediterranean and the Bay of Biscay in the Upper Miocene
period.

Upper Miocene Formations of Greece.—At Pikermé, near Athens, MM. Wagner
and Roth have described a deposit in which they found the remains of
the genera _Mastodon, Dinotherium, Hipparion,_ two species of _Giraffe,
Antelope,_ and others, some living and some extinct. With them were
also associated fossil bones of the _Semnopithecus,_ showing that here,
as in the south of France, the quadrumana were characteristic of this
period. The whole fauna attests the former extension of a vast expanse
of grassy plains where we have now the broken and mountainous country
of Greece; plains, which were probably united with Asia Minor,
spreading over the area where the deep Ægean Sea and its numerous
islands are now situated. We are indebted to M. Gaudry, who visited
Pikermé, for a treatise on these fossil bones, showing how many data
they contribute to the theory of a transition from the mammalia of the
Upper Miocene through the Pliocene and Post-pliocene forms to those of
living genera and species.

Upper Miocene of India. Siwâlik Hills.—The Siwâlik Hills lie at the
southern foot of the Himalayan chain, rising to the height of 2000 and
3000 feet. Between the Jumna and the Ganges they consist of inclined
strata of sandstone, shingle, clay, and marl. We are indebted to the
indefatigable researches of Dr. Falconer and Sir Proby Cautley,
continued for fifteen years, for the discovery in these marls and
sandstones of a great variety of fossil mammalia and reptiles, together
with many fresh-water shells. Out of fifteen species of shells of the
genera _Paludina, Melania, Ampullaria,_ and _Unio,_ all are extinct or
unknown species with the exception of four, which are still inhabitants
of Indian rivers. Such a proportion of living to extinct mollusca
agrees well with the usual character of an Upper Miocene or Falunian
fauna, as observed in Touraine, or in the basin of Vienna and
elsewhere.

The genera of mammalia point in the same direction. One of them, of the
genus _Chalicotherium_ (or _Anisodon_ of Lartet), is a pachyderm
intermediate between the _Rhinoceros_ and _ Anoplothere,_ and
characteristic of the Upper Miocene strata of Eppelsheim, and of the
south of France. With it occurs also an extinct form of Hippopotamus,
called Hexaprotodon, and a species of Hippotherium and pig, also two
species of _Mastodon_, two of elephant, and three other elephantine
proboscidians; none of them agreeing with any fossil forms of Europe,
and being intermediate between the genera Elephas and Mastodon,
constituting the sub-genus _Stegodon_ of Falconer. With these are
associated a monkey, allied to the _Semnopithecus entellus_, now living
in the Himalaya, and many ruminants. Among these last, besides the
giraffe, camel, antelope, stag, and others, we find a remarkable new
type, the _Sivatherium,_ like a gigantic four-horned deer. There are
also new forms of carnivora, both feline and canine, the _Machairodus_
among the former, also hyænas, and a subursine form called the
Hyænarctos, and a genus allied to the otter (_Enhydriodon_), of
formidable size.

The giraffe, camel, and a large ostrich may be cited as proofs that
there were formerly extensive plains where now a steep chain of hills,
with deep ravines, runs for many hundred miles east and west. Among the
accompanying reptiles are several crocodiles, some of huge dimensions,
and one not distinguishable, says Dr. Falconer, from a species now
living in the Ganges (_C. Gangeticus_); and there is still another
saurian which the same anatomist has identified with a species now
inhabiting India. There was also an extinct species of tortoise of
gigantic proportions (_Colossochelys Atlas_), the curved shell of which
was twelve feet three inches long and eight feet in diameter, the
entire length of the animal being estimated at eighteen feet, and its
probable height seven feet.

Numerous fossils of the Siwâlik type have also been found in Perim
Island, in the Gulf of Cambay, and among these a species of
_Dinotherium,_ a genus so characteristic of the Upper Miocene period in
Europe.

Older Pliocene and Miocene Formations in the United States.—Between the
Alleghany Mountains, formed of older rocks, and the Atlantic, there
intervenes, in the United States, a low region occupied principally by
beds of marl, clay, and sand, consisting of the cretaceous and tertiary
formations, and chiefly of the latter. The general elevation of this
plain bordering the Atlantic does not exceed 100 feet, although it is
sometimes several hundred feet high. Its width in the middle and
southern states is very commonly from 100 to 150 miles. It consists, in
the South, as in Georgia, Alabama, and South Carolina, almost
exclusively of Eocene deposits; but in North Carolina, Maryland,
Virginia, Delaware, more modern strata predominate, of the age of the
English Crag and faluns of Touraine.[1]

Fig. 148: Fulgur canaliculatus. Fig. 149: Fusus quadricostatus.

In the Virginian sands, we find in great abundance a species of Astarte
(_A. undulata,_ Conrad), which resembles closely, and may possibly be a
variety of, one of the commonest fossils of the Suffolk Crag (_A.
Omalii_); the other shells also, of the genera _Natica, Fissurella,
Artemis, Lucina, Chama, Pectunculus,_ and _Pecten,_ are analagous to
shells both of the English Crag and French faluns, although the species
are almost all distinct. Out of 147 of these American fossils I could
only find thirteen species common to Europe, and these occur partly in
the Suffolk Crag, and partly in the faluns of Touraine; but it is an
important characteristic of the American group, that it not only
contains many peculiar extinct forms, such as _Fusus quadricostatus,_
Say (see Fig. 149), and _Venus tridacnoides,_ abundant in these same
formations, but also some shells which, like _Fulgur carica_ of Say and
_F. canaliculatus_ (see Fig. 148), _Calyptræa costata, Venus
mercenaria,_ Lam., _Modiola glandula,_ Totten, and _ Pecten
magellanicus,_ Lam., are recent species, yet of forms now confined to
the western side of the Atlantic—a fact implying that some traces of
the beginning of the present geographical distribution of mollusca date
back to a period as remote as that of the Miocene strata.

Fig. 150: Astrangia lineata.
Of ten species of corals which I procured on the banks of the James
River, one agrees generically with a coral now living on the coast of
the United States. Mr. Lonsdale regarded these corals as indicating a
temperature exceeding that of the Mediterranean, and the shells would
lead to similar conclusions. Those occurring on the James River are in
the 37th degree of N. latitude, while the French faluns are in the
47th; yet the forms of the American fossils would scarcely imply so
warm a climate as must have prevailed in France when the Miocene strata
of Touraine originated.

Among the remains of fish in these post-eocene strata of the United
States are several large teeth of the shark family, not distinguishable
specifically from fossils of the faluns of Touraine.

 [1] Proceedings of the Geol. Soc., vol. iv, pt. iii, 1845, p. 547.




CHAPTER XV.
LOWER MIOCENE (OLIGOCENE OF BEYRICH).


Lower Miocene Strata of France. — Line between Miocene and Eocene. —
Lacustrine Strata of Auvergne. — Fossil mammalia of the Limagne
d’Auvergne. — Lower Molasse of Switzerland. — Dense Conglomerates and
Proofs of Subsidence. — Flora of the Lower Molasse. — American
Character of the Flora. — Theory of a Miocene Atlantis. — Lower Miocene
of Belgium. — Rupelian Clay of Hermsdorf near Berlin. — Mayence Basin.
— Lower Miocene of Croatia. — Oligocene Strata of Beyrich. — Lower
Miocene of Italy. — Lower Miocene of England. — Hempstead Beds. — Bovey
Tracey Lignites in Devonshire. — Isle of Mull Leaf-Beds. — Arctic
Miocene Flora. — Disco Island. — Lower Miocene of United States. —
Fossils of Nebraska.

Line between Miocene and Eocene Formations.—The marine faluns of the
valley of the Loire have been already described as resting in some
places on a fresh-water tertiary limestone, fragments of which have
been broken off and rolled on the shores and in the bed of the Miocene
sea. Such pebbles are frequent at Pontlevoy on the Cher, with hollows
drilled in them in which the perforating marine shells of the Falunian
period still remain. Such a mode of superposition implies an interval
of time between the origin of the fresh-water limestone and its
submergence beneath the waters of the Upper Miocene sea. The limestone
in question forms a part of the formation called the Calcaire de la
Beauce, which constitutes a large table-land between the basins of the
Loire and the Seine. It is associated with marls and other deposits,
such as may have been formed in marshes and shallow lakes in the newest
part of a great delta. Beds of flint, continuous or in nodules,
accumulated in these lakes, and aquatic plants called Charae, left
their stems and seed-vessels imbedded both in the marl and flint,
together with fresh-water and land shells. Some of the siliceous rocks
of this formation are used extensively for mill-stones. The flat
summits or platforms of the hills round Paris, and large areas in the
forest of Fontainebleau, as well as the Plateau de la Beauce, already
alluded to, are chiefly composed of these fresh-water strata. Next to
these in the descending order are marine sands and sandstone, commonly
called the Gres de Fontainebleau, from which a considerable number of
shells, very distinct from those of the faluns, have been obtained at
Etampes, south of Paris, and at Montmartre and other hills in Paris
itself, or in its suburbs. At the bottom of these sands a green clay
occurs, containing a small oyster, _Ostrea cyathula,_ Lam., which,
although of slight thickness, is spread over a wide area. This clay
rests immediately on the Paris gypsum, or that series of beds of gypsum
and gypseous marl from which Cuvier first obtained several species of
Palæotherium and other extinct mammalia.[1]

At this junction of the clay and the gypsum the majority of French
geologists have always drawn the line between the Middle and Lower
Tertiary, or between the Miocene and Eocene formations, regarding the
Fontainebleau sands and the _Ostrea cyathula_ clay as the base of the
Miocene, and the gypsum, with its mammalia, as the top of the Eocene
group. I formerly dissented from this division, but I now find that I
must admit it to be the only one which will agree with the distribution
of the Miocene mammalia, while even the mollusca of the Fontainebleau
sands, which were formerly supposed to present at preponderance of
affinities to an Eocene fauna, have since been shown to agree more
closely with the fossils of certain deposits always regarded as Middle
Tertiary at Mayence and in Belgium. In fact, we are now arriving at
that stage of progress when the line, wherever it be drawn between
Miocene and Eocene, will be an arbitrary one, or one of mere
convenience, as I shall have an opportunity of showing when the Upper
Eocene formations in the Isle of Wight are described in the sixteenth
chapter.

Lower Miocene of Central France.—Lacustrine strata, belonging, for the
most part, to the same Miocene system as the Calcaire de la Beauce, are
again met with farther south in Auvergne, Cantal, and Vélay. They
appear to be the monuments of ancient lakes, which, like some of those
now existing in Switzerland, once occupied the depressions in a
mountainous region, and have been each fed by one or more rivers and
torrents. The country where they occur is almost entirely composed of
granite and different varieties of granitic schist, with here and there
a few patches of Secondary strata, much dislocated, and which have
suffered great denudation. There are also some vast piles of volcanic
matter, the greater part of which is newer than the fresh-water strata,
and is sometimes seen to rest upon them, while a small part has
evidently been of contemporaneous origin. Of these igneous rocks I
shall treat more particularly in the sequel.

The study of these regions possesses a peculiar interest very distinct
in kind from that derivable from the investigation either of the
Parisian or English Tertiary areas. For we are presented in Auvergne
with the evidence of a series of events of astonishing magnitude and
grandeur, by which the original form and features of the country have
been greatly changed, yet never so far obliterated but that they may
still, in part at least, be restored in imagination. Great lakes have
disappeared—lofty mountains have been formed, by the reiterated
emission of lava, preceded and followed by showers of sand and
scoriæ—deep valleys have been subsequently furrowed out through masses
of lacustrine and volcanic origin—at a still later date, new cones have
been thrown up in these valleys—new lakes have been formed by the
damming up of rivers—and more than one assemblage of quadrupeds, birds,
and plants, Eocene, Miocene, and Pliocene, have followed in succession;
yet the region has preserved from first to last its geographical
identity; and we can still recall to our thoughts its external
condition and physical structure before these wonderful vicissitudes
began, or while a part only of the whole had been completed. There was
first a period when the spacious lakes, of which we still may trace the
boundaries, lay at the foot of mountains of moderate elevation,
unbroken by the bold peaks and precipices of Mont Dor, and unadorned by
the picturesque outline of the Puy de Dome, or of the volcanic cones
and craters now covering the granitic platform. During this earlier
scene of repose deltas were slowly formed; beds of marl and sand,
several hundred feet thick, deposited; siliceous and calcareous rocks
precipitated from the waters of mineral springs; shells and insects
imbedded, together with the remains of the crocodile and tortoise, the
eggs and bones of water-birds, and the skeletons of quadrupeds, most of
them of genera and species characteristic of the Miocene period. To
this tranquil condition of the surface succeeded the era of volcanic
eruptions, when the lakes were drained, and when the fertility of the
mountainous district was probably enhanced by the igneous matter
ejected from below, and poured down upon the more sterile granite.
During these eruptions, which appear to have taken place towards the
close of the Miocene epoch, and which continued during the Pliocene,
various assemblages of quadrupeds successively inhabited the district,
among which are found the genera mastodon, rhinoceros, elephant, tapir,
hippopotamus, together with the ox, various kinds of deer, the bear,
hyæna, and many beasts of prey which ranged the forest or pastured on
the plain, and were occasionally overtaken by a fall of burning
cinders, or buried in flows of mud, such as accompany volcanic
eruptions. Lastly, these quadrupeds became extinct, and gave place in
their turn to the species now existing. There are no signs, during the
whole time required for this series of events, of the sea having
intervened, nor of any denudation which may not have been accomplished
by currents in the different lakes, or by rivers and floods
accompanying repeated earthquakes, or subterranean movements, during
which the levels of the district have in some places been materially
modified, and perhaps the whole upraised relatively to the surrounding
parts of France.

_Auvergne._—The most northern of the fresh-water groups is situated in
the valley-plain of the Allier, which lies within the department of the
Puy de Dome, being the tract which went formerly by the name of the
Limagne d’Auvergne. The average breadth of this tract is about twenty
miles; and it is for the most part composed of nearly horizontal strata
of sand, sandstone, calcareous marl, clay, and limestone, none of which
observe a fixed and invariable order of superposition. The ancient
borders of the lake wherein the fresh-water strata were accumulated may
generally be traced with precision, the granite and other ancient rocks
rising up boldly from the level country. The actual junction, however,
of the lacustrine beds and the granite is rarely seen, as a small
valley usually intervenes between them. The fresh-water strata may
sometimes be seen to retain their horizontality within a very slight
distance of the border-rocks, while in some places they are inclined,
and in few instances vertical. The principal divisions into which the
lacustrine series may be separated are the following:—first, Sandstone,
grit, and conglomerate, including red marl and red sandstone; secondly,
Green and white foliated marls; thirdly, Limestone, or travertin, often
oolitic in structure; fourthly, Gypseous marls.

The relations of these different groups cannot be learnt by the study
of any one section; and the geologist who sets out with the expectation
of finding a fixed order of succession may perhaps complain that the
different parts of the basin give contradictory results. The arenaceous
division, the marls, and the limestone may all be seen in some places
to alternate with each other; yet it can by no means be affirmed that
there is no order of arrangement. The sands, sandstone, and
conglomerate constitute in general a littoral group; the foliated white
and green marl, a contemporaneous central deposit more than 700 feet
thick, and thinly foliated, a character which often arises from the
innumerable thin shells or carapace valves shed by the small crustacean
called _Cypris_ in the ancient lakes of Auvergne; and lastly the
limestone is for the most part subordinate to the newer portions of
both the above formations.

It seems that, when the ancient lake of the Limagne first began to be
filled with sediment, no volcanic action had yet produced lava and
scoriæ on any part of the surface of Auvergne. No pebbles, therefore,
of lava were transported into the lake—no fragments of volcanic rocks
imbedded in the conglomerate. But at a later period, when a
considerable thickness of sandstone and marl had accumulated, eruptions
broke out, and lava and tuff were deposited, at some spots, alternately
with the lacustrine strata. It is not improbable that cold and thermal
springs, holding different mineral ingredients in solution, became more
numerous during the successive convulsions attending this development
of volcanic agency, and thus deposits of carbonate and sulphate of
lime, silex, and other minerals were produced. Hence these minerals
predominate in the uppermost strata. The subterranean movements may
then have continued until they altered the relative levels of the
country, and caused the waters of the lakes to be drained off, and the
further accumulation of regular fresh-water strata to cease.

Lower Miocene Mammalia of the Limagne.—It is scarcely possible to
determine the age of the oldest part of the fresh-water series of the
Limagne, large masses both of the sandy and marly strata being devoid
of fossils. Some of the lowest beds may be of Upper Eocene date,
although, according to M. Pomel, only one bone of a _Palæotherium_ has
been discovered in Auvergne. But in Vélay, in strata containing some
species of fossil mammalia common to the Limagne, no less than four
species of Palæothere have been found by M. Aymard, and one of these is
generally supposed to be identical with _Palæotherium magnum,_ an
undoubted Upper Eocene fossil, of the Paris gypsum, the other three
being peculiar.

Not a few of the other mammalia of the Limagne belong undoubtedly to
genera and species elsewhere proper to the Lower Miocene. Thus, for
example, the Cainotherium of Bravard, a genus not far removed from the
Anoplotherium, is represented by several species, one of which, as I
learn from Mr. Waterhouse, agrees with _Microtherium Renggeri_ of the
Mayence basin. In like manner, the _Amphitragulus elegans_ of Pomel, an
Auvergne fossil, is identified by Waterhouse with _Dorcatherium nanum_
of Kaup, a Rhenish species from Weissenau, near Mayence. A small
species, also, of rodent, of the genus Titanomys of H. von Meyer, is
common to the Lower Miocene of Mayence and the Limagne d’Auvergne, and
there are many other points of agreement which the discordance of
nomenclature tends to conceal. A remarkable carnivorous genus, the
Hyænodon of Laizer, is represented by more than one species. The same
genus has also been found in the Upper Eocene marls of Hordwell Cliff,
Hampshire, just below the level of the Bembridge Limestone, and
therefore a formation older than the Gypsum of Paris. Several species
of opossum (_Didelphis_) are met with in the same strata of the
Limagne. The total number of mammalia enumerated by M. Pomel as
appertaining to the Lower Miocene fauna of the Limagne and Velay falls
little short of a hundred, and with them are associated some large
crocodiles and tortoises, and some Ophidian and Batrachian reptiles.

Lower Molasse of Switzerland.—The two upper divisions of the Swiss
Molasse—the one fresh-water, the other marine—have already been
described in the preceding chapter. I shall now proceed to treat of the
third division, which is of Lower Miocene age. Nearly the whole of this
Lower Molasse is fresh-water, yet some of the inferior beds contain a
mixture of marine and fluviatile shells, the _Cerithium margaritaceum,_
a well-known Lower Miocene fossil, being one of the marine species.
Notwithstanding, therefore, that some of these Lower Miocene strata
consist of old shingle-beds several thousand feet in thickness, as in
the Rigi, near Lucerne, and in the Speer, near Wesen, mountains 5000
and 7000 feet above the sea, the deposition of the whole series must
have begun at or below the sea-level.

The conglomerates, as might be expected, are often very unequal in
thickness, in closely adjoining districts, since in a littoral
formation accumulations of pebbles would swell out in certain places
where rivers entered the sea, and would thin out to comparatively small
dimensions where no streams or only small ones came down to the coast.
For ages, in spite of a gradual depression of the land and adjacent
sea-bottom, the rivers continued to cover the sinking area with their
deltas; until finally, the subsidence being in excess, the sea of the
Middle Molasse gained upon the land, and marine beds were thrown down
over the dense mass of fresh-water and brackish-water deposit, called
the Lower Molasse, which had previously accumulated.

Flora of the Lower Molasse.—In part of the Swiss Molasse, which belongs
exclusively to the Lower Miocene period, the number of plants has been
estimated at more than 500 species, somewhat exceeding those which were
before enumerated as occurring in the two upper divisions. The Swiss
Lower Miocene may best be studied on the northern borders of the Lake
of Geneva, between Lausanne and Vevay, where the contiguous villages of
Monod and Rivaz are situated. The strata there, which I have myself
examined, consist of alternations of conglomerate, sandstone, and
finely laminated marls with fossil plants. A small stream falls in a
succession of cascades over the harder beds of pudding-stone, which
resist, while the sandstone and plant-bearing shales and marls give
way. From the latter no less than 193 species of plants have been
obtained by the exertions of MM. Heer and Gaudin, and they are
considered to afford a true type of the vegetation of the Lower Miocene
formations of Switzerland—a vegetation departing farther in its
character from that now flourishing in Europe than any of the higher
members of the series before alluded to, and yet displaying so much
affinity to the flora of Œningen as to make it natural for the botanist
to refer the whole to one and the same Miocene period. There are,
indeed, no less than 81 species of these Older Miocene plants which
pass up into the flora of Œningen.

This fact is important as bearing on the propriety of classing the
Lower Molasse of Switzerland as belonging to the Miocene rather than to
the latter part of the Eocene period. There are, indeed, so many types
among the fossils, both specific and generic, which have a wide range
through the whole of the Molasse, that a unity of character is thereby
stamped on the whole flora, in spite of the contrast between the plants
of the uppermost and lowest formations, or between Oeningen and Monod.
The proofs of a warmer climate, and the excess of arborescent over
herbaceous plants, and of evergreen trees over deciduous species, are
characters common to the whole flora, but which are intensified as we
descend to the inferior deposits.

Nearly all the plants at Monod are contained in three layers of marl
separated by two of soft sandstone. The thickness of the marls is ten
feet, and vegetable matter predominates so much in some layers as to
form an imperfect lignite. One bed is filled with large leaves of a
species of fig (_Ficus populina_), and of a hornbeam (_Carpinus
grandis_), the strength of the wind having probably been great when
they were blown into the lake; whereas another contiguous layer
contains almost exclusively smaller leaves, indicating, apparently, a
diminished strength in the wind. Some of the upper beds at Monod abound
in leaves of Proteaceæ, Cyperaceæ, and ferns, while in some of the
lower ones _Sequoia, Cinnamomum,_ and _Sparganium_ are common. In one
bed of sandstone the trunk of a large palm-tree was found unaccompanied
by other fossils, and near Vevay, in the same series of Lower Miocene
strata, the leaves of a palm of the genus _Sabal_ (Fig. 151), a genus
now proper to America, were obtained.

Fig. 151: Sabal major
Among other genera of the same class is a _Flabellaria_ occurring near
Lausanne, and a magnificent _Phœnicites_ allied to the date palm. When
these plants flourished the climate must have been much hotter than
now. The Alps were no doubt much lower, and the palms now found fossil
in strata elevated 2000 feet above the sea grew nearly at the
sea-level, as is demonstrated by the brackish-water character of some
of the beds into which they were carried by winds or rivers from the
adjoining coast.

In the same plant-bearing deposits of the Lower Molasse in Switzerland
leaves have been found which have been ascribed to the order Proteaceæ
already spoken of as well represented in the Œningen beds (see p. 221).
The Proteas and other plants of this family now flourish at the Cape of
Good Hope; while the Banksias, and a set of genera distinct from those
of Africa, grow most luxuriantly in the southern and temperate parts of
Australia. They were probably inhabitants, says Heer, of dry hilly
ground, and the stiff leathery character of their leaves must have been
favourable to their preservation, allowing them to float on a river for
great distances without being injured, and then to sink, when
water-logged, to the bottom. It has been objected that the fruit of the
Proteaceæ is of so tough and enduring a texture that it ought to have
been more commonly met with; but in the first place we must not forget
the numerous cones found in the Eocene strata of Sheppey, which all
admit to be proteaceous and to belong to at least two species (see p.
222). Secondly, besides the fruit of Hakea before mentioned (p. 221),
Heer found associated with fossil leaves, having the exact form and
nervation of Banksia, fruit precisely such as may have come from a cone
of that plant, and lately he has received another similar fruit from
the Lower Miocene strata of Lucerne. They may have fallen out of a
decayed cone in the same way as often happens to the seeds of the
spruce fir, _Pinus abies,_ found scattered over the ground in our
woods. It is a known fact that among the living Proteaceæ the cones are
very firmly attached to the branches, so that the seeds drop out
without the cone itself falling to the ground, and this may perhaps be
the reason why, in some instances in which fossil seeds have been
found, no traces of the cone have been observed.

Fig. 152: Fruit of fossil Banksia and leaf of Banksia. Fig. 153:
Sequoia Langsdorfii.

Among the Coniferæ the Sequoia here figured is common at Rivaz, and is
one of the most universal plants in the Lowest Miocene of Switzerland,
while it also characterises the Miocene Brown Coals of Germany and
certain beds of the Val d’Arno, which I have called Older Pliocene, p.
208.

Fig. 154: Lastræa stiriaca.
Among the ferns met with in profusion at Monod is the _ Lastræa
stiriaca,_ Unger, which has a wide range in the Miocene period from
strata of the age of Œningen to the lowest part of the Swiss Molasse.
In some specimens, as shown in Fig. 154, the fructification is
distinctly seen.

Among the laurels several species of _Cinnamomum_ are very conspicuous.
Besides the _C. polymorphum,_ before figured, p. 219, another species
also ranges from the Lower to the Upper Molasse of Switzerland, and is
very characteristic of different deposits of Brown Coal in Germany. It
has been called _Cinnamomum Rossmässleri_ by Heer (see Fig. 155). The
leaves are easily recognised as having two side veins, which run up
uninterruptedly to their point.

Fig. 155: Cinnamomum Rossmässleri.
American Character of the Flora.—If we consider not merely the number
of species but those plants which constitute the mass of the Lower
Miocene vegetation, we find the European part of the fossil flora very
much less prominent than in the Œningen beds, while the foreground is
occupied by American forms, by evergreen oaks, maples, poplars, planes,
Liquidambar, Robinia, Sequoia, Taxodium, and ternate-leaved pines.
There is also a much greater fusion of the characters now belonging to
distinct botanical provinces than in the Upper Miocene flora, and we
shall find this fusion still more strikingly exemplified as we go back
to the antecedent Eocene and Cretaceous periods.

Professor Heer has advocated the doctrine, first advanced by Unger to
explain the large number of American genera in the Miocene flora of
Europe, that the present basin of the Atlantic was occupied by land
over which the Miocene flora could pass freely. But other able
botanists have shown that it is far more probable that the American
plants came from the east and not from the west, and instead of
reaching Europe by the shortest route over an imaginary Atlantis,
migrated in an opposite direction, crossing the whole of Asia.

Arctic Miocene Flora.—But when we indulge in speculations as to the
geographical origin of the Miocene plants of Central Europe, we must
take into account the discoveries recently made of a rich terrestrial
flora having flourished in the Arctic Regions in the Miocene period
from which many species may have migrated from a common centre so as to
reach the present continents of Europe, Asia, and America. Professor
Heer has examined the various collections of fossil plants that have
been obtained in North Greenland (lat. 70°), Iceland, Spitzbergen, and
other parts of the Arctic regions, and has determined that they are of
Miocene age and indicate a temperate climate.[2] Including the
collections recently brought from Greenland by Mr. Whymper, the Arctic
Miocene flora now comprises 194 species, and that of Greenland 137
species, of which 46, or exactly one-third, are identical with plants
found in the Miocene beds of Central Europe. Considerably more than
half the number are trees, which is the more remarkable since, at the
present day, trees do not exist in any part of Greenland even 10
degrees farther south.

More than thirty species of Coniferæ have been found, including several
Sequoias (allied to the gigantic Wellingtonia of California), with
species of Thujopsis and Salisburia now peculiar to Japan. There are
also beeches, oaks, planes, poplars, maples, walnuts, limes, and even a
magnolia, two cones of which have recently been obtained, proving that
this splendid evergreen not only lived but ripened its fruit within the
Arctic circle. Many of the limes, planes, and oaks were large-leaved
species, and both flowers and fruit, besides immense quantities of
leaves, are in many cases preserved. Among the shrubs were many
evergreens, as _ Andromeda,_ and two extinct genera, _Daphnogene_ and _
M’Clintockia,_ with fine leathery leaves, together with hazel,
blackthorn, holly, logwood, and hawthorn. A species of Zamia
(_Zamites_) grew in the swamps, with _Potamogeton, Sparganium,_ and
_Menyanthes,_ while ivy and vines twined around the forest trees and
broad-leaved ferns grew beneath their shade. Even in Spitzbergen, as
far north as latitude 78° 56′, no less than ninety-five species of
fossil plants have been obtained, including _Taxodium_ of two species,
hazel, poplar, alder, beech, plane-tree, and lime. Such a vigorous
growth of trees within 12 degrees of the pole, where now a dwarf willow
and a few herbaceous plants form the only vegetation, and where the
ground is covered with almost perpetual snow and ice, is truly
remarkable.

The identity of so many of the fossils with Miocene species of Central
Europe and Italy not only proves that the climate of Greenland was much
warmer than it is now, but also renders it probable that a much more
uniform climate prevailed over the entire northern hemisphere. This is
also indicated by the whole character of the Upper Miocene flora of
Central Europe, which does not necessitate a mean temperature very much
greater than exists at present, if we suppose such absence of winter
cold as is proper to insular climates. Professor Heer believes that the
mean temperature of North Greenland must have been at least 30 degrees
higher than at present, while an addition of 10 degrees to the mean
temperature of Central Europe would probably be as much as was
required. The chief locality where this wonderful flora is preserved is
at Atanekerdluk in North Greenland (lat. 70°), on a hill at an
elevation of about 1200 feet above the sea. There is here a
considerable succession of sedimentary strata pierced by volcanic
rocks. Fossil plants occur in all the beds, and the erect trunks as
thick as a man’s body which are sometimes found, together with the
abundance of specimens of flowers and fruit in good preservation,
sufficiently prove that the plants grew where they are now found. At
Disco island and other localities on the same part of the coast, good
coal is abundant, interstratified with beds of sandstone, in some of
which fossil plants have also been found, similar to those at
Atanekerdluk.

Fig. 156: Leda (Nucula) Deshayesiana. Lower Miocene, Belgium.—The Upper
Miocene Bolderberg beds, mentioned in p. 224, rest on a Lower Miocene
formation called the Rupelian of Dumont. This formation is best seen at
the villages of Rupelmonde and Boom, ten miles south of Antwerp, on the
banks of the Scheldt and near the junction with it of a small stream
called the Rupel. A stiff clay abounding in fossils is extensively
worked at the above localities for making tiles. It attains a thickness
of about 100 feet, and though very different in age, much resembles in
mineral character the “London clay,” containing, like it, septaria or
concretions of argillaceous limestone traversed by cracks in the
interior, which are filled with calc-spar. The shells, referable to
about forty species, have been described by MM. Nyst and De Koninck.
Among them _Leda_ (or Nucula) _Deshayesiana_ (see Fig. 156) is by far
the most abundant; a fossil unknown as yet in the English tertiary
strata, but when young much resembling Leda amygdaloides of the London
Clay proper (see Fig. 213). Among other characteristic shells are
_Pecten Hœninghausii,_ and a species of _ Cassidaria,_ and several of
the genus _Pleurotoma._ Not a few of these testacea agree with English
Eocene species, such as _Actæon simulatus,_ Sowb, _Cancellaria evulsa,_
Brander, _Corbula pisum_ (Fig. 157), and _Nautilus (Aturia) ziczac._
They are accompanied by many teeth of sharks, as _Lamna contortidens,_
Ag., _ Oxyrhinaxiphodon,_ Ag., _Carcharodon angustidens_ (see Fig.
196), Ag., and other fish, some of them common to the Middle Eocene
strata.

_Kleyn Spawen beds._—The succession of the Lower Miocene strata of
Belgium can be best studied in the environs of Kleyn Spawen, a village
situated about seven miles west of Maestricht, in the old province of
Limburg in Belgium. In that region, about 200 species of testacea,
marine and fresh-water, have been obtained, with many foraminifera and
remains of fish. In none of the Belgian Lower Miocene strata could I
find any nummulites; and M. d’Archiac had previously observed that
these foraminifera characterise his “Lower Tertiary Series,” as
contrasted with the Middle, and they therefore serve as a good test of
age between Eocene and Miocene, at least in Belgium and the North of
France.[3] Between the Bolderberg beds and the Rupelian clay there is a
great gap in Belgium, which seems, according to M. Beyrich, to be
filled up in the North of Germany by what he calls the Sternberg beds,
and which, had Dumont found them in Belgium, he might probably have
termed Upper Rupelian.

Lower Miocene of Germany.—_Rupelian Clay of Hermsdorf, near
Berlin._—Professor Beyrich has described a mass of clay, used for
making tiles, within seven miles of the gates of Berlin, near the
village of Hermsdorf, rising up from beneath the sands with which that
country is chiefly overspread. This clay is more than forty feet thick,
of a dark bluish-grey colour, and, like that of Rupelmonde, contains
septaria. Among other shells, the _Leda Deshayesiana,_ before mentioned
(Fig. 156), abounds, together with many species of _ Pleurotoma,
Voluta,_ etc., a certain proportion of the fossils being identical in
species with those of Rupelmonde.

_Mayence Basin._—An elaborate description has been published by Dr. F.
Sandberger of the Mayence tertiary area, which occupies a tract from
five to twelve miles in breadth, extending for a great distance along
the left bank of the Rhine from Mayence to the neighbourhood of
Manheim, and which is also found to the east, north, and south-west of
Frankfort. M. De Koninck, of Liege, first pointed out to me that the
purely marine portion of the deposit contained many species of shells
common to the Kleyn Spawen beds, and to the clay of Rupelmonde, near
Antwerp. Among these he mentioned _Cassidaria depressa, Tritonium
argutum,_ Brander (_T. flandricum,_ De Koninck), _ Tornatella simulata,
Aporrhais Sowbyi, Leda Deshayesiana_ (Fig. 156), _Corbula pisum,_ (Fig.
158) and others.

Lower Miocene Beds of Croatia.—The Brown Coal of Radaboj, near Angram
in Croatia, not far from the borders of Styria, is covered, says Von
Buch, by beds containing the marine shells of the Vienna basin, or, in
other words, by Upper Miocene or Falunian strata. They appear to
correspond in age to the Mayence basin, or to the Rupelian strata of
Belgium. They have yielded more than 200 species of fossil plants,
described by the late Professor Unger. These plants are well preserved
in a hard marlstone, and contain several palms; among them the Sabal,
Fig. 151, p. 237, and another genus allied to the date-palm _
Phœnicites spectabilis._ The only abundant plant among the Radaboj
fossils which is characteristic of the Upper Miocene period is the
_Populus mutabilis,_ whereas no less than fifty of the Radaboj species
are common to the more ancient flora of the Lower Molasse of
Switzerland.

Fig. 157: Vanessa Pluto.

The insect fauna is very rich, and, like the plants, indicates a more
tropical climate than do the fossils of Œningen presently to be
mentioned. There are ten species of Termites, or white ants, some of
gigantic size, and large dragon-flies with speckled wings, like those
of the Southern States in North America; there are also grasshoppers of
considerable size, and even the Lepidoptera are not unrepresented. In
one instance, the pattern of a butterfly’s wing has escaped
obliteration in the marl-stone of Radaboj; and when we reflect on the
remoteness of the time from which it has been faithfully transmitted to
us, this fact may inspire the reader with some confidence as to the
reliable nature of the characters which other insects of a more durable
texture, such as the beetles, may afford for specific determination.
The Vanessa above figured retains, says Heer, some of its colours, and
corresponds with _V. Hadena_ of India.

Professor Beyrich has made known to us the existence of a long
succession of marine strata in North Germany, which lead by an almost
gradual transition from beds of Upper Miocene age to others of the age
of the base of the Lower Miocene. Although some of the German lignites
called Brown Coal belong to the upper parts of this series, the most
important of them are of Lower Miocene date, as, for example, those of
the Siebengebirge, near Bonn, which are associated with volcanic rocks.

Professor Beyrich confines the term “Miocene” to those strata which
agree in age with the faluns of Touraine, and he has proposed the term
“Oligocene” for those older formations called Lower Miocene in this
work.

Lower Miocene of Italy.—In the hills of which the Superga forms a part
there is a great series of Tertiary strata which pass downward into the
Lower Miocene. Even in the Superga itself there are some fossil plants
which, according to Heer, have never been found in Switzerland so high
as the marine Molasse, such as _Banksia longifolia,_ and _Carpinus
grandis._ In several parts of the Ligurian Apennines, as at Dégo and
Carcare, the Lower Miocene appears, containing some nummulites, and at
Cadibona, north of Savona, fresh-water strata of the same age occur,
with dense beds of lignite inclosing remains of the _ Anthracotherium
magnum_ and _A. minimum,_ besides other mammalia enumerated by
Gastaldi. In these beds a great number of the Lower Miocene plants of
Switzerland have been discovered.

Lower Miocene of England—Hempstead Beds.—We have already stated that
the Upper Miocene formation is nowhere represented in the British
Isles; but strata referable to the Lower Miocene period are found both
in England, Scotland, and Ireland. In the Hampshire basin these occupy
a very small superficial area, having been discovered by the late
Edward Forbes at Hempstead near Yarmouth, in the northern part of the
Isle of Wight, where they are 170 feet thick, and rich in
characteristic marine shells. They overlie the uppermost of an
extensive series of Eocene deposits of marine, brackish, and
fresh-water formations, which rest on the Chalk and terminate upward in
strata corresponding in age to the Paris gypsum, and containing the
same extinct genera of quadrupeds, _Palæotherium, Anoplotherium,_ and
others which Cuvier first described. The following is the succession of
these Lower Miocene strata, most of them exposed in a cliff east of
Yarmouth:

1. The uppermost or Corbula beds, consisting of marine sands and clays,
contain _Voluta Rathieri,_ a characteristic Lower Miocene shell;
_Corbula pisum_ (Fig. 158), a species common to the Upper Eocene clay
of Barton; Cyrena semistriata (Fig. 159), several Cerithia, and other
shells peculiar to this series.

Fig. 158: Corbula pisum. Fig. 159: Cyrena semistriata. Fig. 160:
Cerithium plicatum. Fig. 161: Cerithium elegans. Fig. 162: Rissoa
Chastelii. Fig. 163: Paludina lenta.

2. Next are fresh-water and estuary marls and carbonaceous clays in the
brackish-water portion of which are found abundantly _ Cerithium
plicatum,_ Lam. (Fig. 160), _Cerithium elegans_ (Fig. 161), and
_Cerithium tricinctum_; also _Rissoa Chastelii_ (Fig. 162), a very
common Kleyn Spawen shell, and which occurs in each of the four
subdivisions of the Hempstead series down to its base, where it passes
into the Bembridge beds. In the fresh-water portion of the same beds
_Paludina lenta_ (Fig. 163) occurs; a shell identified by some
conchologists with a species now living, _P. unicolor_; also several
species of _ Lymneus, Planorbis,_ and _Unio._

3. The next series, or middle fresh-water and estuary marls, are
distinguished by the presence of _Melania fasciata, Paludina lenta,_
and clays with _Cypris_; the lowest bed contains _Cyrena semistriata_
(Fig. 159), mingled with Cerithia and a _panopæa._

4. The lower fresh-water and estuary marls contain _Melania costata,_
Sowerby, _Melanopsis,_ etc. The bottom bed is carbonaceous, and called
the “Black band,” in which _ Rissoa Chastelii_ (Fig. 162), before
alluded to, is common. This bed contains a mixture of Hempstead shells
with those of the underlying Upper Eocene or Bembridge series. The
mammalia, among which is _Hyopotamus bovinus,_ differ, so far as they
are known, from those of the Bembridge beds. Among the plants,
Professor Heer has recognised four species common to the lignite of
Bovey Tracey, a Lower Miocene formation presently to be described:
namely, _Sequoia Couttsiæ,_ Heer; _Andromeda reticulata,_ Ettings.;
_Nelumbium (Nymphœa) doris,_ Heer; and _Carpolithes Websteri,_
Brong.[4] The seed-vessels of _Chara medicaginula,_ Brong, and _C.
helicteres_ are characteristic of the Hempstead beds generally.

The _Hyopotamus_ belongs to the hog tribe, or the same family as the
Anthracotherium, of which seven species, varying in size from the
hippopotamus to the wild boar, have been found in Italy and other part
of Europe associated with the lignites of the Lower Miocene period.

Lignites and Clays of Bovey Tracey, Devonshire.—Surrounded by the
granite and other rocks of the Dartmoor hills in Devonshire, is a
formation of clay, sand, and lignite, long known to geologists as the
Bovey Coal formation, respecting the age of which, until the year 1861,
opinions were very unsettled. This deposit is situated at Bovey Tracey,
a village distant eleven miles from Exeter in a south-west, and about
as far from Torquay in a north-west direction. The strata extend over a
plain nine miles long, and they consist of the materials of decomposed
and worn-down granite and vegetable matter, and have evidently filled
up an ancient hollow or lake-like expansion of the valleys of the Bovey
and Teign.

The lignite is of bad quality for economical purposes, as there is a
great admixture in it of iron pyrites, and it emits a sulphurous odour,
but it has been successfully applied to the baking of pottery, for
which some of the fine clays are well adapted. Mr. Pengelly has
confirmed Sir H. De la Beche’s opinion that much of the upper portion
of this old lacustrine formation has been removed by denudation.[5]

At the surface is a dense covering of clay and gravel with angular
stones probably of the Post-pliocene period, for in the clay are three
species of willow and the dwarf birch, _Betula nana,_ indicating a
climate colder than that of Devonshire at the present day.

Below this are Lower Miocene strata about 300 feet in thickness, in the
upper part of which are twenty-six beds of lignite, clay, and sand, and
at their base a ferruginous quartzose sand, varying in thickness from
two to twenty-seven feet. Below this sand are forty-five beds of
alternating lignite and clay. No shells or bones of mammalia, and no
insect, with the exception of one fragment of a beetle (_Buprestis_);
in a word, no organic remains, except plants, have as yet been found.
These plants occur in fourteen of the beds—namely, in two of the clays,
and the rest in the lignites. One of the beds is a perfect mat of the
debris of a coniferous tree, called by Heer _Sequoia Couttsiæ,_
intermixed with leaves of ferns. The same Sequoia (before mentioned as
a Hempstead fossil, p. 246) is spread through all parts of the
formation, its cones, and seeds, and branches of every age being
preserved. It is a species supplying a link between _Sequoia
Langsdorfii_ (see Fig. 153, p. 238) and _S. Sternbergi,_ the widely
spread fossil representatives of the two living trees _S. sempervirens_
and _S. gigantea_ (or Wellingtonia), both now confined to California.
Another bed is full of the large rhizomes of ferns, while two others
are rich in dicotyledonous leaves. In all, Professor Heer enumerates
forty-nine species of plants, twenty of which are common to the Miocene
beds of the Continent, a majority of them being characteristic of the
Lower Miocene. The new species, also of Bovey, are allied to plants of
the older Miocene deposits of Switzerland, Germany, and other
Continental countries. The grape-stones of two species of vine occur in
the clays, and leaves of the fig and seeds of a water-lily. The oak and
laurel have supplied many leaves. Of the triple-nerved laurels several
are referred to Cinnamomum. There are leaves also of a palm of which
the genus is not determined. Leaves also of proteaceous forms, like
some of the Continental fossils before mentioned, occur, and ferns like
the well-known _ Lastræa stiriaca_ (Fig. 154, p. 238), displaying at
Bovey, as in Switzerland, its fructification.

The croziers of some of the young ferns are very perfect, and were at
first mistaken by collectors for shells of the genus _ Planorbis._ On
the whole, the vegetation of Bovey implies the existence of a
sub-tropical climate in Devonshire, in the Lower Miocene period.

Scotland: Isle of Mull.—In the sea-cliffs forming the headland of
Ardtun, on the west coast of Mull, in the Hebrides, several bands of
tertiary strata containing leaves of dicotyledonous plants were
discovered in 1851 by the Duke of Argyll.[6] From his description it
appears that there are three leaf-beds, varying in thickness from 1½ to
5½ feet, which are interstratified with volcanic tuff and trap, the
whole mass being about 130 feet in thickness. A sheet of basalt 40 feet
thick covers the whole; and another columnar bed of the same rock, ten
feet thick, is exposed at the bottom of the cliff. One of the leaf-beds
consists of a compressed mass of leaves unaccompanied by any stems, as
if they had been blown into a marsh where a species of _Equisetum_
grew, of which the remains are plentifully imbedded in clay.

It is supposed by the Duke of Argyll that this formation was
accumulated in a shallow lake or marsh in the neighbourhood of a
volcano, which emitted showers of ashes and streams of lava. The
tufaceous envelope of the fossils may have fallen into the lake from
the air as volcanic dust, or have been washed down into it as mud from
the adjoining land. Even without the aid of organic remains we might
have decided that the deposit was newer than the chalk, for
chalk-flints containing cretaceous fossils were detected by the duke in
the principal mass of volcanic ashes or tuff.[7]

The late Edward Forbes observed that some of the plants of this
formation resembled those of Croatia, described by Unger, and his
opinion has been confirmed by Professor Heer, who found that the
conifer most prevalent was the _Sequoia Langsdorfii_ (Fig. 153, p.
238), also _Corylus grossedentata,_ a Lower Miocene species of
Switzerland and of Menat in Auvergne. There is likewise a plane-tree,
the leaves of which seem to agree with those of _Platanus aceroides_
(Fig. 141), and a fern which is as yet peculiar to Mull, _Filicites
hebridica,_ Forbes.

These interesting discoveries in Mull led geologists to suspect that
the basalt of Antrim, in Ireland, and of the celebrated Giant’s
Causeway, might be of the same age. The volcanic rocks that overlie the
chalk, and some of the strata associated with and interstratified
between masses of basalt, contain leaves of dicotyledonous plants,
somewhat imperfect, but resembling the beech, oak, and plane, and also
some coniferæ of the genera pine and Sequoia. The general dearth of
strata in the British Isles, intermediate in age between the formation
of the Eocene and Pliocene periods, may arise, says Professor Forbes,
from the extent of dry land which prevailed in that vast interval of
time. If land predominated, the only monuments we are likely ever to
find of Miocene date are those of lacustrine and volcanic origin, such
as the Bovey Coal in Devonshire, the Ardtun beds in Mull, or the
lignites and associated basalts in Antrim.

Lower Miocene, United states: Nebraska.—In the territory of Nebraska,
on the Upper Missouri, near the Platte River, lat. 42° N., a tertiary
formation occurs, consisting of white limestone, marls, and siliceous
clay, described by Dr. D. Dale Owen,[8] in which many bones of extinct
quadrupeds, and of chelonians of land or fresh-water forms, are met
with. Among these, Dr. Leidy describes a gigantic quadruped, called by
him _Titanotherium,_ nearly allied to the _Palæotherium,_ but larger
than any of the species found in the Paris gypsum. With these are
several species of the genus _Oreodon,_ Leidy, uniting the characters
of pachyderms and ruminants also; _Eucrotaphus,_ another new genus of
the same mixed character; two species of rhinoceros of the sub-genus
_Acerotherium,_ a Lower Miocene form of Europe before mentioned; two
species of _Archæotherium,_ a pachyderm allied to _Chæropotamus_ and _
Hyracotherium_; also _Pæbrotherium,_ an extinct ruminant allied to
_Dorcatherium,_ Kaup; also _ Agriochoerus,_ of Leidy, a ruminant allied
to _ Merycopotamus_ of Falconer and Cautley; and, lastly, a large
carnivorous animal of the genus _Machairodus,_ the most ancient example
of which in Europe occurs in the Lower Miocene strata of Auvergne, but
of which some species are found in Pliocene deposits. The turtles are
referred to the genus _Testudo,_ but have some affinity to _Emys._ On
the whole, the Nebraska formation is probably newer than the Paris
gypsum, and referable to the Lower Miocene period, as above defined.

 [1] Bulletin, 1856, Journ., vol. xii, p. 768.

 [2] Heer “Miocene baltische Flora” and “Fossil-flora von Alaska” 1869.

 [3] D’Archiac Monogr., pp. 79, 100.

 [4] Pengelly, preface to The Lignite Formation of Bovey Tracey, p.
 xvii, London, 1863.

 [5] Philos. Trans., 1863. Paper by W. Pengelly, F.R.S., and Dr. Oswald
 Heer.

 [6] Quart. Geol. Journal, 1851, p. 19.

 [7] Quart. Geol. Journal, 1851, p. 90.

 [8] David Dale Owen, Geol. Survey of Wisconsin, etc., Philad., 1852.




CHAPTER XVI.
EOCENE FORMATIONS.


Eocene Areas of North of Europe. — Table of English and French Eocene
Strata. — Upper Eocene of England. — Bembridge Beds. — Osborne or St.
Helen’s Beds. — Headon Series. — Fossils of the Barton Sands and Clays.
— Middle Eocene of England. — Shells, Nummulites, Fish and Reptiles of
the Bracklesham Beds and Bagshot Sands. — Plants of Alum Bay and
Bournemouth. — Lower Eocene of England. — London Clay Fossils. —
Woolwich and Reading Beds formerly called “Plastic Clay.” Fluviatile
Beds underlying Deep-sea Strata. — Thanet Sands. — Upper Eocene Strata
of France. — Gypseous Series of Montmartre and Extinct Quadrupeds. —
Fossil Footprints in Paris Gypsum. — Imperfection of the Record. —
Calcaire Silicieux. — Gres de Beauchamp. — Calcaire Grossier. —
Miliolite Limestone. — Soissonnais Sands. — Lower Eocene of France. —
Nummulitic Formations of Europe, Africa, and Asia. — Eocene Strata in
the United States. — Gigantic Cetacean.

Eocene Areas of the North of Europe.—The strata next in order in the
descending series are those which I term Eocene.

Fig. 164: Map of the principal Eocene areas of North-western Europe.

In the map (Fig. 164) the position of several Eocene areas in the north
of Europe is pointed out. When this map was constructed I classed as
the newer part of the Eocene those Tertiary strata which have been
described in the last chapter as Lower Miocene, and to which M. Beyrich
has given the name of Oligocene. None of these occur in the London
Basin, and they occupy in that of Hampshire, as we have seen at p. 244,
too insignificant a superficial area to be noticed in a map on this
scale. They fill a larger space in the Paris Basin between the Seine
and the Loire, and constitute also part of the northern limits of the
area of the Netherlands which are shaded in the map.

It is in the northern part of the Isle of Wight that we have the
uppermost beds of the true Eocene best exhibited—namely, those which
correspond in their fossils with the celebrated gypsum of the Paris
basin before alluded to, p. 231 (see Table, p. 252). That gypsum has
been selected by almost all Continental geologists as affording the
best line of demarkation between the Middle and Lower Tertiary, or, in
other words, between the Lower Miocene and Eocene formations.

In reference to the Table I may observe, that the correlation of the
French and English subdivisions here laid down is often a matter of
great doubt and difficulty, notwithstanding their geographical
proximity. This arises from various circumstances, partly from the
former prevalence of marine conditions in one basin simultaneously with
fluviatile or lacustrine in the other, and sometimes from the existence
of land in one area causing a break or absence of all records during a
period when deposits may have been in progress in the other basin. As
bearing on this subject, it may be stated that we have unquestionable
evidence of oscillations of level shown by the superposition of salt or
brackish-water strata to fluviatile beds; and those of deep-sea origin
to strata formed in shallow water. Even if the upward and downward
movements were uniform in amount and direction, which is very
improbable, their effect in producing the conversion of sea into land
or land into sea would be different, according to the previous shape
and varying elevation of the land and bottom of the sea. Lastly,
denudation, marine and subaërial, has frequently caused the absence of
deposits in one basin of corresponding age to those in the other, and
this destructive agency has been more than ordinarily effective on
account of the loose and unconsolidated nature of the sands and clays.

TABLE OF ENGLISH AND FRENCH EOCENE STRATA.
UPPER EOCENE

English subdivisions French equivalents A.1. Bembridge series, Isle
of Wight, p. 252. A.1. Gypseous series of Montmartre, p. 270. A.2.
Osborne or St. Helen’s series, Isle of Wight, p. 255. A.2 and 3.
Calcaire siliceux, or Travertin Inférieur, p. 273. A.3. Headon series,
Isle of Wight, p. 255. A.4. Barton series. Sands and clays of Barton
Cliff, Hants, p. 258. A.4. Grès de Beauchamp, or Sables Moyens, p.
273. MIDDLE EOCENE B.1. Bracklesham series, p. 259. B.1. Calcaire
Grossier p. 274 B.2. Alum Bay and Bournemouth beds, p. 259. B.2.
Wanting in France? B.2. Wanting in England? B.2. Soissonnais Sands,
or Lits Coquilliers, p. 275 LOWER EOCENE C.1. London Clay, p.
263. C.1. Argile de Londres, Cassel, near Dunkirk. C.2. Woolwich and
Reading series, p. 267. C.2. Argile plastique and lignite, p. 276
C.3. Thanet sands, p. 269. C.3. Sables de Bracheux, p. 276

UPPER EOCENE, ENGLAND.

Bembridge Series, A.1.—These beds are about 120 feet thick, and, as
stated in p. 245, lie immediately under the Hempstead beds, near
Yarmouth, in the Isle of Wight, being conformable with those Lower
Miocene strata. They consist of marls, clays, and limestones of
fresh-water, brackish, and marine origin. Some of the most abundant
shells, as _Cyrena semistriata_ var., and _Paludina lenta,_ Fig. 163,
are common to this and to the overlying Hempstead series; but the
majority of the species are distinct. The following are the
subdivisions described by the late Professor Forbes:
     _a._ Upper marls, distinguished by the abundance of _Melania
     turritissima,_ Forbes (Fig. 165).
     _b._ Lower marls, characterised by _Cerithium mutabile, Cyrena
     pulchra,_ etc., and by the remains of _Trionyx_ (see Fig. 166).
     _c._ Green marls, often abounding in a peculiar species of oyster,
     and accompanied by _ Cerithium, Mytilus, Arca, nucula,_ etc.
     _d._ Bembridge limestones, compact cream-coloured limestones
     alternating with shales and marls, in all of which land-shells are
     common, especially at Sconce, near Yarmouth, as described by Mr.
     F. Edwards. The _Bulimus ellipticus,_ Fig. 167, and _Helix
     occlusa,_ Fig. 168, are among its best known land-shells.
     _Paludina orbicularis,_ Fig. 169, is also of frequent occurrence.
     One of the bands is filled with a little globular _Paludina._
     Among the fresh-water pulmonifera, _ Lymnea longiscata_ (Fig. 171)
     and _Planorbis discus_ (Fig. 170) are the most generally
     distributed: the latter represents or takes the place of the
     _Planorbis euomphalus_ (see Fig. 175) of the more ancient Headon
     series. _Chara tuberculata_ (Fig. 172) is the characteristic
     Bembridge gyrogonite or seed-vessel.

Fig. 165: Melania turritissima, Fig. 166: Fragment of Carapace of
Trionyx, Fig. 167: Bulimus ellipticus, Fig. 168: Helix occlusa, Fig.
169: Paludina orbicularis, Fig. 170: Planorbis discus, Fig. 171: Lymnea
longiscata, Fig. 172: Chara tuberculata.

Fig. 173: Lower molar tooth.
From this formation on the shores of Whitecliff Bay, Dr. Mantell
obtained a fine specimen of a fan palm, _Flabellaria Lamanonis,_
Brong., a plant first obtained from beds of corresponding age in the
suburbs of Paris. The well-known building-stone of Binstead, near Ryde,
a limestone with numerous hollows caused by Cyrenæ which have
disappeared and left the moulds of their shells, belongs to this
subdivision of the Bembridge series. In the same Binstead stone Mr.
Pratt and the Reverend Darwin Fox first discovered the remains of
mammalia characteristic of the gypseous series of Paris, as _
Palæotherium magnum_ (Fig. 174), _P. medium, P. minus, P. minimum, P.
curtum, P. crassum_; also _Anoplotherium commune_ (Fig. 173), _A.
secundarium, Dichobune cervinum,_ and _Chæropotamus Cuvieri._ The
Palæothere above alluded to resembled the living tapir in the form of
the head, and in having a short proboscis, but its molar teeth were
more like those of the rhinoceros. _Palæotherium magnum_ was of the
size of a horse, three or four feet high. The woodcut, Fig. 174, is one
of the restorations which Cuvier attempted of the outline of the living
animal, derived from the study of the entire skeleton. As the vertical
range of particular species of quadrupeds, so far as our knowledge
extends, is far more limited than that of the testacea, the occurrence
of so many species at Binstead, agreeing with fossils of the Paris
gypsum, strengthens the evidence derived from shells and plants of the
synchronism of the two formations.

Fig. 174: Palæotherium magnum.

Osborne or St. Helen’s Series, A.2.—This group is of fresh and
brackish-water origin, and very variable in mineral character and
thickness. Near Ryde, it supplies a freestone much used for building,
and called by Professor Forbes the Nettlestone grit. In one part
ripple-marked flagstones occur, and rocks with fucoidal markings. The
Osborne beds are distinguished by peculiar species of _Paludina,
Melania,_ and _ Melanopsis,_ as also of _Cypris_ and the seeds of _
Chara._

Fig. 175: Planorbis euomphalus, Fig. 176: Helix labyrinthica.

Headon Series A.3.—These beds are seen both in Whitecliff Bay, Headon
Hill, and Alum Bay, or at the east and west extremities of the Isle of
Wight. The upper and lower portions are fresh-water, and the middle of
mixed origin, sometimes brackish and marine. Everywhere _Planorbis
euomphalus,_ Fig. 175, characterises the fresh-water deposits, just as
the allied form, P. discus, Fig. 170, does the Bembridge limestone. The
brackish-water beds contain _Potamomya plana, Cerithium mutabile,_ and
_Potamides cinctus_ (Fig. 37), and the marine beds _ Venus_ (or
_Cytherea_) _incrassata,_ a species common to the Limburg beds and Grès
de Fontainebleau, or the Lower Miocene series. The prevalence of
salt-water remains is most conspicuous in some of the central parts of
the formation.

Fig. 177: Neritina concava.

Among the shells which are widely distributed through the Headon series
are _Neritina concava_ (Fig. 177), _Lymnea caudata_ (Fig. 178), and _
Cerithium concavum_ (Fig. 179). _Helix labyrinthica,_ Say (Fig. 176), a
land-shell now inhabiting the United States, was discovered in this
series by Mr. Searles Wood in Hordwell Cliff. It is also met with in
Headon Hill, in the same beds. At Sconce, in the Isle of Wight, it
occurs in the Bembridge series, and affords a rare example of an Eocene
fossil of a species still living, though, as usual in such cases,
having no local connection with the actual geographical range of the
species. The lower and middle portion of the Headon series is also met
with in Hordwell Cliff (or Hordle, as it is often spelt), near
Lymington, Hants. Among the shells which abound in this cliff are
_Paludina lenta_ and various species of _Lymnea, Planorbis, Melania,
Cyclas, Unio, Potamomya, Dreissena,_ etc.

Fig. 178: Lymnea caudata, Fig. 179: Cerithium concavum.
Among the chelonians we find a species of _Emys,_ and no less than six
species of _Trionyx_; among the saurians an alligator and a crocodile;
among the ophidians two species of land-snakes (_Paleryx,_ Owen); and
among the fish Sir P. Egerton and Mr. Wood have found the jaws, teeth,
and hard shining scales of the genus _Lepidosteus,_ or bony pike of the
American rivers. This same genus of fresh-water ganoids has also been
met with in the Hempstead beds in the Isle of Wight. The bones of
several birds have been obtained from Hordwell, and the remains of
quadrupeds of the genera _Palæotherium (P. minus), Anoplotherium,
Anthracotherium, Dichodon, Dichobune, Spalacodon,_ and _Hyænodon._ The
latter offers, I believe, the oldest known example of a true
carnivorous animal in the series of British fossils, although I attach
very little theoretical importance to the fact, because herbivorous
species are those most easily met with in a fossil state in all save
cavern deposits. In another point of view, however, this fauna deserves
notice. Its geological position is considerably lower than that of the
Bembridge or Montmartre beds, from which it differs almost as much in
species as it does from the still more ancient fauna of the Lower
Eocene beds to be mentioned in the sequel. It therefore teaches us what
a grand succession of distinct assemblages of mammalia flourished on
the earth during the Eocene period.

Many of the marine shells of the brackish-water beds of the above
series, both in the Isle of Wight and Hordwell Cliff, are common to the
underlying Barton Clay: and, on the other hand, there are some
fresh-water shells, such as _Cyrena obovata,_ which are common to the
Bembridge beds, notwithstanding the intervention of the St. Helen’s
series. The white and green marls of the Headon series, and some of the
accompanying limestones, often resemble the Eocene strata of France in
mineral character and colour in so striking a manner as to suggest the
idea that the sediment was derived from the same region or produced
contemporaneously under very similar geographical circumstances.

At Brockenhurst, near Lyndhurst, in the New Forest, marine strata have
recently been found containing fifty-nine shells, of which many have
been described by Mr. Edwards. These beds rest on the Lower Headon, and
are considered as the equivalent of the middle part of the Headon
series, many of the shells being common to the brackish-water or Middle
Headon beds of Colwell and Whitecliff Bays, such as _Cancellaria
muricata,_ Sowerby, _ Fusus labiatus,_ Sowerby, etc. In these beds at
Brockenhurst, corals, ably described by Dr. Duncan, have recently been
found in abundance and perfection; see Fig. 180, _Solenastræa
cellulosa._

Fig. 180: Solenastræa cellulosa.

Baron von Könen[1] has pointed out that no less than forty-six out of
the fifty-nine Brockenhurst shells, or a proportion of 78 per cent,
agree with species occurring in Dumont’s Lower Tongrian formation in
Belgium. This being the case, we might fairly expect that if we had a
marine equivalent of the Bembridge series or of the contemporaneous
Paris gypsum, we should find it to contain a still greater number of
shells common to the Tongrian beds of Belgium, but the exact
correlation of these fresh-water groups of France, Belgium, and Britain
has not yet been fully made out. It is possible that the Tongrian of
Dumont may be newer than the Bembridge series, and therefore referable
to the Lower Miocene. If ever the whole series should be complete, we
must be prepared to find the marine equivalent of the Bembridge beds,
or the uppermost Eocene, passing by imperceptible shades into the
inferior beds of the overlying Miocene strata.

Among the fossils found in the Middle Headon are _Cytherea incrassata_
and _Cerithium plicatum_ (Fig. 160). These shells, especially the
latter, are very characteristic of the Lower Miocene, and their
occurrence in the Headon series has been cited as an objection to the
line proposed to be drawn between Miocene and Eocene. But if we were to
attach importance to such occasional passages, we should soon find that
no lines of division could be drawn anywhere, for in the present state
of our knowledge of the Tertiary series there will always be species
common to beds above and below our boundary-lines.

Fig. 181: Chama squamosa.
Barton Series (_Sands and Clays_), A.4 Table—Both in the Isle of Wight,
and in Hordwell Cliff, Hants, the Headon beds, above-mentioned, rest on
white sands usually devoid of fossils, and used in the Isle of Wight
for making glass. In one of these sands Dr. Wright found _ Chama
squamosa,_ a Barton Clay shell, in great plenty, and certain
impressions of marine shells have been found in sands supposed to be of
the same age in Whitecliff Bay. These sands have been called Upper
Bagshot in the maps of our Government Survey, but this identification
of a fossiliferous series in the Isle of Wight with an unfossiliferous
formation in the London Basin can scarcely be depended upon. The Barton
Clay, which immediately underlies these sands, is seen vertical in Alum
Bay, Isle of Wight, and nearly horizontal in the cliffs of the mainland
near Lymington. This clay, together with the Bracklesham beds,
presently to be described, has been termed Middle Bagshot by the
Survey. In Barton Cliff, where it attains a thickness of about 300
feet, it is rich in marine fossils.

It was formerly confounded with the London Clay, an older Eocene
deposit of very similar mineral character, to be mentioned (p. 263),
which contains many shells in common, but not more than one-fourth of
the whole. In other words, there are known at present 247 species in
the London Clay and 321 in that of Barton, and only 70 common to the
two formations. Fifty-six of these have been found in the intermediate
Bracklesham beds, and the reappearance of the other 14 may imply a
return of similar conditions, whether of temperature or depth or of a
muddy argillaceous bottom, common to the two periods of the London and
Barton Clays. According to M. Hebert, the most characteristic Barton
Clay fossils correspond to those of the Gres de Beauchamp, or Sables
Moyens, of the Paris Basin, but it also contains many common to the
older Calcaire Grossier.

SHELLS OF THE BARTON CLAY.

Certain foraminifera called Nummulites begin, when we study the
Tertiary formations in a descending order, to make their first
appearance in these beds. A small species called _ Nummulites
variolaria,_ Fig. 190, is found both on the Hampshire coast and in beds
of the same age in Whitecliff Bay, in the Isle of Wight. Several marine
shells, such as _Corbula pisum_ (Fig. 158), are common to the Barton
beds and the Hempstead or Lower Miocene series, and a still greater
number, as before stated, are common to the Headon series.

Fig. 182: Mitra scabra, Fig. 183: Voluta ambigua, Fig. 184: Typhis
pungens, Fig. 185: Voluta athleta, Fig. 186: Terebellum fusiforme, Fig.
187: Terebellum sopita, Fig. 188: Cardita sulcata, Fig. 189:
Crassatella sulcata, Fig. 190: Nummulites variolaria.

MIDDLE EOCENE, ENGLAND.

Bracklesham Beds and Bagshot Sands, B.1, Table—Beneath the Barton Clay
we find in the north of the Isle of Wight, both in Alum and Whitecliff
Bays, a great series of various coloured sands and clays for the most
part unfossiliferous, and probably of estuarine origin. As some of
these beds contain _Cardita planicosta_ (Fig. 191) they have been
identified with the marine beds much richer in fossils seen in the
coast section in Bracklesham Bay near Chichester in Sussex, where the
strata consist chiefly of green clayey sands with some lignite. Among
the Bracklesham fossils besides the Cardita, the huge _Cerithium
giganteum_ is seen, so conspicuous in the Calcaire Grossier of Paris,
where it is sometimes two feet in length. The _Nummulites lævigata_
(see Fig. 192), so characteristic of the lower beds of the Calcaire
Grossier in France, where it sometimes forms stony layers, as near
Compiègne, is very common in these beds, together with _N. scabra_ and
_N. variolaria._ Out of 193 species of testacea procured from the
Bagshot and Bracklesham beds in England, 126 occur in the Calcaire
Grossier in France. It was clearly, therefore, coeval with that part of
the Parisian series more nearly than with any other.

Fig. 191: Cardita (Venericardia) planicosta, Fig. 192: Nummulites
(Nummularia) lavigata.

According to tables compiled from the best authorities by Mr.
Etheridge, the number of mollusca now known from the Bracklesham beds
in Great Britain is 393, of which no less than 240 are peculiar to this
subdivision of the British Eocene series, while 70 are common to the
Older London Clay, and 140 to the Newer Barton Clay. The volutes and
cowries of this formation, as well as the lunulites and corals, favour
the idea of a warm climate having prevailed, which is borne out by the
discovery of a serpent, _Palæophis typhœus_ (see Fig. 193), exceeding,
according to Professor Owen, twenty feet in length, and allied in its
osteology to the Boa, Python, Coluber, and Hydrus. The compressed form
and diminutive size of certain caudal vertebræ indicate so much analogy
with Hydrus as to induce Professor Owen to pronounce this extinct
ophidian to have been marine.[2] Among the companions of the sea-snake
of Bracklesham was an extinct crocodile (_Gavialis Dixoni,_ Owen), and
numerous fish, such as now frequent the seas of warm latitudes, as the
Ostracion of the family Balistidæ, of which a dorsal spine is figured
(see Fig. 194), and gigantic rays of the genus _ Myliobates_ (see Fig.
195).

Fig. 193: Palæophis typhoeus, Owen; an Eocene sea-serpent, Fig. 194:
Defensive spine of Ostracion.

Fig. 195: Dental plates of Myliobates Edwardsi.
The teeth of sharks also, of the genera _Carcharodon, Otodus, Lamna,
Galeocerdo,_ and others, are abundant. (See Figs. 196, 197, 198, 199.)

Fig. 196: Carcharodon angustidens, Fig. 197: Otodus obliquus, Fig. 198:
Lamna elegans, Fig. 199: Galcocerdo latidens.

MARINE SHELLS OF BRACKLESHAM BEDS.

Alum Bay and Bournemouth Beds. (_Lower Bagshot of English Survey_),
B.2, Table—To that great series of sands and clays which intervene
between the equivalents of the Bracklesham Beds and the London Clay or
Lower Eocene, our Government Survey has given the name of the Lower
Bagshot sands, for they are supposed to agree in age with the inferior
unfossiliferous sands of the country round Bagshot in the London Basin.
This part of the series is finely exposed in the vertical beds of Alum
bay, in the Isle of Wight, and east and west of Bournemouth, on the
south coast of Hampshire. In some of the close and white compact clays
of this locality, there are not only dicotyledonous leaves, but
numerous fronds of ferns allied to Gleichenia which are well preserved
with their fruit.

Fig. 200: Pleurotoma attenuata, Fig. 201: Voluta Selseïensis, Fig. 202:
Turritella multisulcata, Fig. 203: Lucina serrata, Fig. 204: Conus
deperditus.

None of the beds are of great horizontal extent, and there is much
cross-stratification in the sands, and in some places black
carbonaceous seams and lignite. In the midst of these leaf-beds in
Studland Bay, Purbeck shells of the genus Unio attest the fresh-water
origin of the white clay.

No less than forty species of plants are mentioned by MM. de la Harpe
and Gaudin from this formation in Hampshire, among which the Proteaceæ
(_Dryandra,_ etc.) and the fig tribe are abundant, as well as the
cinnamon and several other laurineæ, with some papilionaceous plants.
On the whole, they remind the botanist of the types of subtropical
India and Australia.[3]

Heer has mentioned several species which are common to this Alum Bay
flora and that of Monte Bolca, near Verona, so celebrated for its
fossil fish, and where the strata contain nummulites and other Middle
Eocene fossils. He has particularly alluded to _Aralia primigenia_ (of
which genus a fruit has since been found by Mr. Mitchell at
Bournemouth), _Daphnogene Veronensis,_ and _ Ficus granadilla,_ as
among the species common to and characteristic of the Isle of Wight and
Italian Eocene beds; and he observes that in the flora of this period
these forms of a temperate climate which constitute a marked feature in
the European Miocene formations, such as the willow, poplar, birch,
alder, elm, hornbeam, oak, fir, and pine, are wanting. The American
types are also absent, or much more feebly represented than in the
Miocene period, although fine specimens of the fan-palm (_Sabal_) have
been found in these Eocene clays at Studland. The number of exotic
forms which are common to the Eocene and Miocene strata of Europe, like
those to be alluded to in the sequel which are common to the Eocene and
Cretaceous fauna, demonstrate the remoteness of the times in which the
geographical distribution of living plants originated. A great majority
of the Eocene genera have disappeared from our temperate climates, but
not the whole of them; and they must all have exerted some influence on
the assemblages of species which succeeded them. Many of these last
occurring in the Upper Miocene are indeed so closely allied to the
flora now surviving as to make it questionable, even in the opinion of
naturalists opposed to the doctrine of transmutation, whether they are
not genealogically related the one to the other.

LOWER EOCENE FORMATIONS, ENGLAND.

London Clay, C.1, Table—This formation underlies the preceding, and
sometimes attains a thickness of 500 feet. It consists of tenacious
brown and bluish-grey clay, with layers of concretions called septaria,
which abound chiefly in the brown clay, and are obtained in sufficient
numbers from sea-cliffs near Harwich, and from shoals off the coast of
Essex and the Isle of Sheppey, to be used for making Roman cement. The
total number of British fossil mollusca known at present (January,
1870) in this formation are 254, of which 166 are peculiar, or not
found in other Eocene beds in this country. The principal localities of
fossils in the London clay are Highgate Hill, near London, the Island
of Sheppey at the mouth of the Thames, and Bognor on the Sussex coast.
Out of 133 fossil shells, Mr. Prestwich found only 20 to be common to
the Calcaire Grossier (from which 600 species have been obtained),
while 33 are common to the “Lits Coquilliers” (p. 275), in which 200
species are known in France.

In the Island of Sheppey near the mouth of the Thames, the thickness of
the London Clay is estimated by Mr. Prestwich to be more than 500 feet,
and it is in the uppermost 50 feet that a great number of fossil fruits
were obtained, being chiefly found on the beach when the sea has washed
away the clay of the rapidly wasting cliffs.

Fig. 205: Nipadites ellipticus.
Mr. Bowerbank, in a valuable publication on these fossil fruits and
seeds, has described no less than thirteen fruits of palms of the
recent type _Nipa,_ now only found in the Molucca and Philippine
Islands, and in Bengal (see Fig. 205). In the delta of the Ganges, Dr.
Hooker observed the large nuts of _Nipa fruticans_ floating in such
numbers in the various arms of that great river, as to obstruct the
paddle-wheels of steamboats. These plants are allied to the cocoanut
tribe on the one side, and on the other to the _Pandanus,_ or
screw-pine. There are also met with three species of _Anona,_ or
custard-apple; and cucurbitaceous fruits (of the gourd and melon
family), and fruits of various species of _Acacia._

Besides fir-cones or fruit of true Coniferæ there are cones of
Proteaceæ in abundance, and the celebrated botanist the late Robert
Brown pointed out the affinity of these to the New Holland types
_Petrophila_ and _Isopogon._ Of the first there are about fifty, and of
the second thirty described species now living in Australia.

Ettingshausen remarked in 1851 that five of the fossil species from
Sheppey, named by Bowerbank[4] were specimens of the same fruit (see
Fig. 206), in different states of preservation; and Mr. Carruthers,
having examined the original specimens now in the British Museum, tells
me that all these cones from Sheppey may be reduced to two species,
which have an undoubted affinity to the two existing Australian genera
above mentioned, although their perfect identity in structure cannot be
made out.

Fig. 206: Eocene Proteaceous Fruit (Petrophiloides Richardsoni.

The contiguity of land may be inferred not only from these vegetable
productions, but also from the teeth and bones of crocodiles and
turtles, since these creatures, as Dean Conybeare remarked, must have
resorted to some shore to lay their eggs. Of turtles there were
numerous species referred to extinct genera. These are, for the most
part, not equal in size to the largest living tropical turtles. A
sea-snake, which must have been thirteen feet long, of the genus
_Palæophis_ before mentioned (p. 261) has also been described by
Professor Owen from Sheppey, of a different species from that of
Bracklesham, and called _P. toliapicus._ A true crocodile, also,
_Crocodilus toliapicus,_ and another saurian more nearly allied to the
gavial, accompany the above fossils; also the relics of several birds
and quadrupeds. One of these last belongs to the new genus
_Hyracotherium_ of Owen, of the hog tribe, allied to Chæropotamus,
another is a _Lophiodon_; a third a pachyderm called _Coryphodon
eocænus_ by Owen, larger than any existing tapir. All these animals
seem to have inhabited the banks of the great river which floated down
the Sheppey fruits. They imply the existence of a mammiferous fauna
antecedent to the period when nummulites flourished in Europe and Asia,
and therefore before the Alps, Pyrenees, and other mountain-chains now
forming the backbones of great continents, were raised from the deep;
nay, even before a part of the constituent rocky masses now entering
into the central ridges of these chains had been deposited in the sea.

The marine shells of the London Clay confirm the inference derivable
from the plants and reptiles in favour of a high temperature. Thus many
species of _Conus_ and _ Voluta_ occur, a large _Cypræa, C. oviformis,_
a very large _Rostellaria_ (Fig. 209), a species of _Cancellaria,_ six
species of _Nautilus_ (Fig. 211), besides other Cephalopoda of extinct
genera, one of the most remarkable of which is the _ Belosepia_ (Fig.
212). Among many characteristic bivalve shells are _Leda amygdaloides_
(Fig. 213) and _Cryptodon angulatum_ (Fig. 214), and among the Radiata
a star-fish, _Astropecten_ (Fig. 215.)

Fig. 207: Voluta nodosa, Fig. 208: Phorus extensus, Fig. 209:
Rostellaria (Hippocrenes) ampla, Fig. 210: Nautilus centralis, Fig.
211: Aturia ziczac, Fig. 212: Belosepia sepioidea, Fig. 213: Leda
amygdaloides, Fig. 214: Cyptodon (Axinus) angulatum, Fig. 215:
Astropecten crispatus.

These fossils are accompanied by a sword-fish (_Tetrapterus priscus,_
Agassiz), about eight feet long, and a saw-fish (_Pristis bisulcatus,_
Agassiz), about ten feet in length; genera now foreign to the British
seas. On the whole, about eighty species of fish have been described by
M. Agassiz from these beds of Sheppey, and they indicate, in his
opinion, a warm climate.

In the lower part of the London clay at Kyson, a few miles east of
Woodbridge, the remains of mammalia have been detected. Some of these
have been referred by Professor Owen to an opossum, and others to the
genus _Hyracotherium._ The teeth of this last-mentioned pachyderm were
at first, in 1840, supposed to belong to a monkey, an opinion
afterwards abandoned by Owen when more ample materials for comparison
were obtained.

Woolwich and Reading Series, C.2, Table—This formation was formerly
called the Plastic Clay, as it agrees with a similar clay used in
pottery which occupies the same position in the French series, and it
has been used for the like purposes in England.[5]

No formations can be more dissimilar, on the whole, in mineral
character than the Eocene deposits of England and Paris; those of our
own island being almost exclusively of mechanical origin—accumulations
of mud, sand, and pebbles; while in the neighbourhood of Paris we find
a great succession of strata composed of limestones, some of them
siliceous, and of crystalline gypsum and siliceous sandstone, and
sometimes of pure flint used for millstones. Hence it is often
impossible, as before stated, to institute an exact comparison between
the various members of the English and French series, and to settle
their respective ages. But in regard to the division which we have now
under consideration, whether we study it in the basins of London,
Hampshire, or Paris, we recognise as a general rule the same mineral
character, the beds consisting over a large area of mottled clays and
sand, with lignite, and with some strata of well-rolled flint pebbles,
derived from the chalk, varying in size, but occasionally several
inches in diameter. These strata may be seen in the Isle of Wight in
contact with the chalk, or in the London basin, at Reading, Blackheath,
and Woolwich. In some of the lowest of them, banks of oysters are
observed, consisting of _Ostrea bellovacina,_ so common in France in
the same relative position. In these beds at Bromley, Dr. Buckland
found a large pebble to which five full-grown oysters were affixed, in
such a manner as to show that they had commenced their first growth
upon it, and remained attached to it through life.

Fig. 216: Cyrena cuneiformis, Fig. 217: Melania (Melanatria) inquinata.

In several places, as at Woolwich on the Thames, at Newhaven in Sussex,
and elsewhere, a mixture of marine and fresh-water testacea
distinguishes this member of the series. Among the latter, _ Cyrena
cuneiformis_ (see Fig. 216) and _Melania inquinata_ (see Fig. 217) are
very common, as in beds of corresponding age in France. They clearly
indicate points where rivers entered the Eocene sea. Usually there is a
mixture of brackish, fresh-water, and marine shells, and sometimes, as
at Woolwich, proofs of the river and the sea having successively
prevailed on the same spot. At New Charlton, in the suburbs of
Woolwich, Mr. de la Condamine discovered in 1849, and pointed out to
me, a layer of sand associated with well-rounded flint pebbles in which
numerous individuals of the _Cyrena tellinella_ were seen standing
endwise with both their valves united, the siphonal extremity of each
shell being uppermost, as would happen if the mollusks had died in
their natural position. I have described[6] a bank of sandy mud, in the
delta of the Alabama River at Mobile, on the borders of the Gulf of
Mexico, where in 1846 I dug out at low tide specimens of living species
of _Cyrena_ and of a _ Gnathodon,_ which were similarly placed with
their shells erect, or in a posture which enables the animal to
protrude its siphon upward, and draw in or reject water at pleasure.
The water at Mobile is usually fresh, but sometimes brackish. At
Woolwich a body of river-water must have flowed permanently into the
sea where the _Cyrenæ_ lived, and they may have been killed suddenly by
an influx of pure salt-water, which invaded the spot when the river was
low, or when a subsidence of land took place. Traced in one direction,
or eastward towards Herne Bay, the Woolwich beds assume more and more
of a marine character; while in an opposite, or south-western
direction, they become, as near Chelsea and other places, more
fresh-water, and contain _Unio, Paludina,_ and layers of lignite, so
that the land drained by the ancient river seems clearly to have been
to the south-west of the present site of the metropolis.

_Fluviatile Beds underlying Deep-sea Strata._—Before the minds of
geologists had become familiar with the theory of the gradual sinking
of land, and its conversion into sea at different periods, and the
consequent change from shallow to deep water, the fluviatile and
littoral character of this inferior group appeared strange and
anomalous. After passing through hundreds of feet of London clay,
proved by its fossils to have been deposited in deep salt-water, we
arrive at beds of fluviatile origin, and associated with them masses of
shingle, attaining at Blackheath, near London, a thickness of 50 feet.
These shingle banks are probably of marine origin, but they indicate
the proximity of land, and the existence of a shore where the flints of
the chalk were rolled into sand and pebbles, and spread over a wide
space. We have, therefore, first, as before stated (p. 268), evidence
of oscillations of level during the accumulation of the Woolwich
series, then of a great submergence, which allowed a marine deposit 500
thick to be laid over the antecedent beds of fresh and brackish water
origin.

Thanet Sands, C.3, Table—The Woolwich or plastic clay above described
may often be seen in the Hampshire basin in actual contact with the
chalk, constituting in such places the lowest member of the British
Eocene series. But at other points another formation of marine origin,
characterised by a somewhat different assemblage of organic remains,
has been shown by Mr. Prestwich to intervene between the chalk and the
Woolwich series. For these beds he has proposed the name of “Thanet
Sands,” because they are well seen in the Isle of Thanet, in the
northern part of Kent, and on the sea-coast between Herne Bay and the
Reculvers, where they consist of sands with a few concretionary masses
of sandstone, and contain, among other fossils, _Pholadomya cuneata,
Cyprina morrisii, Corbula longirostris, Scalaria Bowerbankii,_ etc. The
greatest thickness of these beds is 90 feet.

UPPER EOCENE FORMATIONS OF FRANCE.

The tertiary formations in the neighbourhood of Paris consist of a
series of marine and fresh-water strata, alternating with each other,
and filling up a depression in the chalk. The area which they occupy
has been called the Paris Basin, and is about 180 miles in its greatest
length from north to south, and about 90 miles in breadth from east to
west. MM. Cuvier and Brongniart attempted, in 1810, to distinguish five
different groups, comprising three fresh-water and two marine, which
were supposed to imply that the waters of the ocean, and of rivers and
lakes, had been by turns admitted into and excluded from the same area.
Investigations since made in the Hampshire and London basins have
rather tended to confirm these views, at least so far as to show that
since the commencement of the Eocene period there have been great
movements of the bed of the sea, and of the adjoining lands, and that
the superposition of deep-sea to shallow-water deposits (the London
Clay, for example, to the Woolwich beds) can only be explained by
referring to such movements. It appears, notwithstanding, from the
researches of M. Constant Prevost, that some of the minor alternations
and intermixtures of fresh-water and marine deposits, in the Paris
basin, may be accounted for without such changes of level, by imagining
both to have been simultaneously in progress, in the same bay of the
same sea, or a gulf into which many rivers entered.

Gypseous Series of Montmartre, A.1, Table—To enlarge on the numerous
subdivisions of the Parisian strata would lead me beyond my present
limits; I shall therefore give some examples only of the most important
formations. Beneath the Grès de Fontainebleau, belonging to the Lower
Miocene period, as before stated, we find, in the neighbourhood of
Paris, a series of white and green marls, with subordinate beds of
gypsum. These are most largely developed in the central parts of the
Paris basin, and, among other places, in the hill of Montmartre, where
its fossils were first studied by Cuvier.

The gypsum quarried there for the manufacture of plaster of Paris
occurs as a granular crystalline rock, and, together with the
associated marls, contains land and fluviatile shells, together with
the bones and skeletons of birds and quadrupeds. Several land-plants
are also met with, among which are fine specimens of the fan-palm or
palmetto tribe (_Flabellaria_). The remains also of fresh-water fish,
and of crocodiles and other reptiles, occur in the gypsum. The
skeletons of mammalia are usually isolated, often entire, the most
delicate extremities being preserved; as if the carcasses, clothed with
their flesh and skin, had been floated down soon after death, and while
they were still swollen by the gases generated by their first
decomposition. The few accompanying shells are of those light kinds
which frequently float on the surface of rivers, together with wood.

In this formation the relics of about fifty species of quadrupeds,
including the genera _Palæotherium_ (see Fig. 174), _Anoplotherium_
(see Fig. 218), and others, have been found, all extinct, and nearly
four-fifths of them belonging to the Perissodactyle or odd-toed
division of the order _Pachydermata,_ which now contains only four
living genera, namely, rhinoceros, tapir, horse, and hyrax. With them a
few carnivorous animals are associated, among which are the _Hyænodon
dasyuroides,_ a species of dog, _Canis Parisiensis,_ and a weasel,
_Cynodon Parisiensis._ Of the _Rodentia_ are found a squirrel; of the
_Cheiroptera,_ a bat; while the _Marsupalia_ (an order now confined to
America, Australia, and some contiguous islands) are represented by an
opossum.

Of birds, about ten species have been ascertained, the skeletons of
some of which are entire. None of them are referable to existing
species.[7] The same remark, according to MM. Cuvier and Agassiz,
applies both to the reptiles and fish. Among the last are crocodiles
and tortoises of the genera _Emys_ and _ Trionyx._

Fig. 218: Xiphodon gracile, or Anoplotherium gracile.

The tribe of land quadrupeds most abundant in this formation is such as
now inhabits alluvial plains and marshes, and the banks of rivers and
lakes, a class most exposed to suffer by river inundations. Among these
were several species of _ Palæotherium,_ a genus before alluded to.
These were associated with the Anoplotherium, a tribe intermediate
between pachyderms and ruminants. One of the three divisions of this
family was called by Cuvier _Xiphodon._ Their forms were slender and
elegant, and one, named _Xiphodon gracile_ (Fig. 218), was about the
size of the chamois; and Cuvier inferred from the skeleton that it was
as light, graceful, and agile as the gazelle.

_Fossil Footprints._—There are three superimposed masses of gypsum in
the neighbourhood of Paris, separated by intervening deposits of
laminated marl. In the uppermost of the three, in the valley of
Montmorency, M. Desnoyers discovered in 1859 many footprints of animals
occurring at no less than six different levels.[8] The gypsum to which
they belong varies from thirty to fifty feet in thickness, and is that
which has yielded to the naturalist the largest number of bones and
skeletons of mammalia, birds, and reptiles. I visited the quarries,
soon after the discovery was made known, with M. Desnoyers, who also
showed me large slabs in the Museum at Paris, where, on the upper
planes of stratification, the indented foot-marks were seen, while
corresponding casts in relief appeared on the lower surfaces of the
strata of gypsum which were immediately superimposed. A thin film of
marl, which before it was dried and condensed by pressure must have
represented a much thicker layer of soft mud, intervened between the
beds of solid gypsum. On this mud the animals had trodden, and made
impressions which had penetrated to the gypseous mass below, then
evidently unconsolidated. Tracks of the _ Anoplotherium_ with its
bisulcate hoof, and the trilobed footprints of _Palæotherium,_ were
seen of different sizes, corresponding to those of several species of
these genera which Cuvier had reconstructed, while in the same beds
were foot-marks of carnivorous mammalia. The tracks also of fluviatile,
lacustrine, and terrestrial tortoises (_Emys, Trionyx,_ etc.) were
discovered, also those of crocodiles, iguanas, geckos, and great
batrachians, and the footprints of a huge bird, apparently a wader, of
the size of the gastornis, to be mentioned in the sequel. There were
likewise the impressions of the feet of other creatures, some of them
clearly distinguishable from any of the fifty extinct types of mammalia
of which the bones have been found in the Paris gypsum. The whole
assemblage, says Desnoyers, indicate the shores of a lake, or several
small lakes communicating with each other, on the borders of which many
species of pachyderms wandered, and beasts of prey which occasionally
devoured them. The tooth-marks of these last had been detected by
palæontologists long before on the bones and skulls of Paleotheres
entombed in the gypsum.

_Imperfection of the Record._—These foot-marks have revealed to us new
and unexpected proofs that the air-breathing fauna of the Upper Eocene
period in Europe far surpassed in the number and variety of its species
the largest estimate which had previously been formed of it. We may now
feel sure that the mammalia, reptiles, and birds which have left
portions of their skeletons as memorials of their existence in the
solid gypsum constituted but a part of the then living creation.
Similar inferences may be drawn from the study of the whole succession
of geological records. In each district the monuments of periods
embracing thousands, and probably in some instances hundreds of
thousands of years, are totally wanting. Even in the volumes which are
extant the greater number of the pages are missing in any given region,
and where they are found they contain but few and casual entries of the
physical events or living beings of the times to which they relate. It
may also be remarked that the subordinate formations met with in two
neighbouring countries, such as France and England (the minor Tertiary
groups above enumerated), commonly classed as equivalents and referred
to corresponding periods, may nevertheless have been by no means
strictly coincident in date. Though called contemporaneous, it is
probable that they were often separated by intervals of many thousands
of years. We may compare them to double stars, which appear single to
the naked eye because seen from a vast distance in space, and which
really belong to one and the same stellar system, though occupying
places in space extremely remote if estimated by our ordinary standard
of terrestrial measurements.

Calcaire silicieux, or Travertin inférieur, A.2 and 3, Table—This
compact siliceous limestone extends over a wide area. It resembles a
precipitate from the waters of mineral springs, and is often traversed
by small empty sinuous cavities. It is, for the most part, devoid of
organic remains, but in some places contains fresh-water and land
species, and never any marine fossils. The calcaire siliceux and the
calcaire grossier usually occupy distinct parts of the Paris basin, the
one attaining its fullest development in those places where the other
is of slight thickness. They are described by some writers as
alternating with each other towards the centre of the basin, as at
Sergy and Osny.

The gypsum, with its associated marls before described, is in greatest
force towards the centre of the basin, where the calcaire grossier and
calcaire silicieux are less fully developed.

Grès de Beauchamp, or Sables Moyens, A.4, Table—In some parts of the
Paris basin, sands and marls, called the Grès de Beauchamp, or Sables
moyens, divide the gypseous beds from the calcaire grossier proper.
These sands, in which a small nummulite (N. variolaria) is very
abundant, contain more than 300 species of marine shells, many of them
peculiar, but others common to the next division.

MIDDLE EOCENE FORMATIONS OF FRANCE.

Calcaire Grossier, upper and middle, B.1, Table—The upper division of
this group consists in great part of beds of compact, fragile
limestone, with some intercalated green marls. The shells in some parts
are a mixture of _Cerithium, Cyclostoma,_ and _Corbula_; in others
_Limnea, Cerithium, Paludina,_ etc. In the latter, the bones of
reptiles and mammalia, _Palæotherium_ and _ Lophiodon,_ have been
found. The middle division, or calcaire grossier proper, consists of a
coarse limestone, often passing into sand. It contains the greater
number of the fossil shells which characterise the Paris basin. No less
than 400 distinct species have been procured from a single spot near
Grignon, where they are imbedded in a calcareous sand, chiefly formed
of comminuted shells, in which, nevertheless, individuals in a perfect
state of preservation, both of marine, terrestrial, and fresh-water
species, are mingled together. Some of the marine shells may have lived
on the spot; but the _Cyclostoma_ and _Limnea,_ being land and
fresh-water shells, must have been brought thither by rivers and
currents, and the quantity of triturated shells implies considerable
movement in the waters.

Nothing is more striking in this assemblage of fossil testacea than the
great proportion of species referable to the genus _ Cerithium_ (see p.
245). There occur no less than 137 species of this genus in the Paris
basin, and almost all of them in the calcaire grossier. Most of the
living _Cerithia_ inhabit the sea near the mouths of rivers, where the
waters are brackish; so that their abundance in the marine strata now
under consideration is in harmony with the hypothesis that the Paris
basin formed a gulf into which several rivers flowed.

In some parts of the calcaire grossier round Paris, certain beds occur
of a stone used in building, and called by the French geologists
“Miliolite limestone.” It is almost entirely made up of millions of
microscopic shells, of the size of minute grains of sand, which all
belong to the class Foraminifera. Examples of some of these are given
in Figs. 219 to 221. As this miliolitic stone never occurs in the
Faluns, or Upper Miocene strata of Brittany and Touraine, it often
furnishes the geologist with a useful criterion for distinguishing the
detached Eocene and Upper Miocene formations scattered over those and
other adjoining provinces. The discovery of the remains of Palæotherium
and other mammalia in some of the upper beds of the calcaire grossier
shows that these land animals began to exist before the deposition of
the overlying gypseous series had commenced.

Fig. 219: Calcarina rarispina, Fig. 220: Spirolina stenostoma, Fig.
221: Triloculina inflata.

Lower Calcaire grossier, or Glauconie grossiere, B.1, Table—The lower
part of the calcaire grossier, which often contains much green earth,
is characterised at Auvers, near Pontoise, to the north of Paris, and
still more in the environs of Compiègne, by the abundance of
nummulites, consisting chiefly of _N. lævigata, N. scabra,_ and _ N.
Lamarcki,_ which constitute a large proportion of some of the stony
strata, though these same foraminifera are wanting in beds of similar
age in the immediate environs of Paris.

Fig. 222: Nerita conoidea.

Soissonnais sands, or Lits coquilliers, B.2, Table—Below the preceding
formation, shelly sands are seen, of considerable thickness, especially
at Cuisse-Lamotte, near Compiègne, and other localities in the
Soissonnais, about fifty miles N.E. of Paris, from which about 300
species of shells have been obtained, many of them common to the
calcaire grossier and the Bracklesham beds of England, and many
peculiar. The _Nummulites planulata_ is very abundant, and the most
characteristic shell is the _Nerita conoidea,_ Lam., a fossil which has
a very wide geographical range; for, as M. d’Archiac remarks, it
accompanies the nummulitic formation from Europe to India, having been
found in Cutch, near the mouths of the Indus, associated with
_Nummulites scabra._ No less than 33 shells of this group are said to
be identical with shells of the London clay proper, yet, after visiting
Cuisse-Lamotte and other localities of the “Sables inférieurs” of
Archiac, I agree with Mr. Prestwich, that the latter are probably newer
than the London clay, and perhaps older than the Bracklesham beds of
England. The London clay seems to be unrepresented in the Paris basin,
unless partially so, by these sands.[9]

LOWER EOCENE FORMATIONS OF FRANCE.

Argile Plastique, C.2, Table—At the base of the tertiary system in
France are extensive deposits of sands, with occasional beds of clay
used for pottery, and called “argile plastique.” Fossil oysters
(_Ostrea bellovacina_) abound in some places, and in others there is a
mixture of fluviatile shells, such as _Cyrena cuneiformis_ (Fig. 216),
_ Melania inquinata_ (Fig. 216), and others, frequently met with in
beds occupying the same position in the London Basin. Layers of lignite
also accompany the inferior clays and sands.

Immediately upon the chalk at the bottom of all the tertiary strata in
France there generally is a conglomerate or breccia of rolled and
angular chalk-flints, cemented by siliceous sand. These beds appear to
be of littoral origin, and imply the previous emergence of the chalk,
and its waste by denudation. In the year 1855, the tibia and femur of a
large bird equalling at least the ostrich in size were found at Meudon,
near Paris, at the base of the Plastic clay. This bird, to which the
name of _Gastornis Parisiensis_ has been assigned, appears, from the
Memoirs of MM. Hébert, Lartet, and Owen, to belong to an extinct genus.
Professor Owen refers it to the class of wading land birds rather than
to an aquatic species.[10]

That a formation so much explored for economical purposes as the Argile
plastique around Paris, and the clays and sands of corresponding age
near London, should never have afforded any vestige of a feathered
biped previously to the year 1855, shows what diligent search and what
skill in osteological interpretation are required before the existence
of birds of remote ages can be established.

Sables de Bracheux, C.3, Table—The marine sands called the Sables de
Bracheux (a place near Beauvais), are considered by M. Hébert to be
older than the Lignites and Plastic clay, and to coincide in age with
the Thanet Sands of England. At La Fère, in the Department of Aisne, in
a deposit of this age, a fossil skull has been found of a quadruped
called by Blainville _Arctocyon primævus,_ and supposed by him to be
related both to the bear and to the Kinkajou (_Cercoleptes_). This
creature appears to be the oldest known tertiary mammifer.

Nummulitic Formations of Europe, Asia, etc.—Of all the rocks of the
Eocene period, no formations are of such great geographical importance
as the Upper and Middle Eocene, as above defined, assuming that the
older tertiary formation, commonly called nummulitic, is correctly
ascribed to this group. It appears that of more than fifty species of
these foraminifera described by D’Archiac, one or two species only are
found in other tertiary formations whether of older or newer date.
_Nummulites intermedia,_ a Middle Eocene form, ascends into the Lower
Miocene, but it seems doubtful whether any species descends to the
level of the London clay, still less to the Argile plastique or
Woolwich beds. Separate groups of strata are often characterised by
distinct species of nummulite; thus the beds between the lower Miocene
and the lower Eocene may be divided into three sections, distinguished
by three different species of nummulites, _N. variolaria_ in the upper,
_N. lævigata_ in the middle, and _N. planulata_ in the lower beds. The
nummulitic limestone of the Swiss Alps rises to more than 10,000 feet
above the level of the sea, and attains here and in other mountain
chains a thickness of several thousand feet. It may be said to play a
far more conspicuous part than any other tertiary group in the solid
framework of the earth’s crust, whether in Europe, Asia, or Africa. It
occurs in Algeria and Morocco, and has been traced from Egypt, where it
was largely quarried of old for the building of the Pyramids, into Asia
Minor, and across Persia by Bagdad to the mouths of the Indus. It has
been observed not only in Cutch, but in the mountain ranges which
separate Scinde from Persia, and which form the passes leading to
Caboul; and it has been followed still farther eastward into India, as
far as eastern Bengal and the frontiers of China.

Dr. T. Thompson found nummulites at an elevation of no less than 16,500
feet above the level of the sea, in Western Thibet. One of the species,
which I myself found very abundant on the flanks of the Pyrenees, in a
compact crystalline marble (Fig. 223) is called by M. d’Archiac
_Nummulites Puschi._ The same is also very common in rocks of the same
age in the Carpathians. In many distant countries, in Cutch, for
example, some of the same shells, such as _Nerita conoidea_ (Fig. 222),
accompany the nummulites, as in France. The opinion of many observers,
that the Nummulitic formation belongs partly to the cretaceous era,
seems chiefly to have arisen from confounding an allied genus,
Orbitoides, with the true Nummulite.

Fig. 223: Nummulites Puschi.

When we have once arrived at the conviction that the nummulitic
formation occupies a middle and upper place in the Eocene series, we
are struck with the comparatively modern date to which some of the
greatest revolutions in the physical geography of Europe, Asia, and
Northern Africa must be referred. All the mountain-chains, such as the
Alps, Pyrenees, Carpathians, and Himalayas, into the composition of
whose central and loftiest parts the nummulitic strata enter bodily,
could have had no existence till after the Middle Eocene period. During
that period the sea prevailed where these chains now rise, for
nummulites and their accompanying testacea were unquestionably
inhabitants of salt water. Before these events, comprising the
conversion of a wide area from a sea to a continent, England had been
peopled, as I before pointed out (p. 267), by various quadrupeds, by
herbivorous pachyderms, by insectivorous bats, and by opossums.

Almost all the volcanoes which preserve any remains of their original
form, or from the craters of which lava streams can be traced, are more
modern than the Eocene fauna now under consideration; and besides these
superficial monuments of the action of heat, Plutonic influences have
worked vast changes in the texture of rocks within the same period.
Some members of the nummulitic and overlying tertiary strata called
_flysch_ have actually been converted in the central Alps into
crystalline rocks, and transformed into marble, quartz-rock,
micha-schist, and gneiss.[11]

Eocene Strata in the United States.—In North America the Eocene
formations occupy a large area bordering the Atlantic, which increases
in breadth and importance as it is traced southward from Delaware and
Maryland to Georgia and Alabama. They also occur in Louisiana and other
States both east and west of the valley of the Mississippi. At
Claiborne, in Alabama, no less than 400 species of marine shells, with
many echinoderms and teeth of fish, characterise one member of this
system. Among the shells, the _Cardita planicosta,_ before mentioned
(Fig. 191), is in abundance; and this fossil and some others identical
with European species, or very nearly allied to them, make it highly
probable that the Claiborne beds agree in age with the central or
Bracklesham group of England, and with the calcaire grossiere of
Paris.[12]

Higher in the series is a remarkable calcareous rock, formerly called
“the nummulite limestone,” from the great number of discoid bodies
resembling nummulites which it contains, fossils now referred by A.
d’Orbigny to the genus _Orbitoides,_ which has been demonstrated by Dr.
Carpenter to belong to the foraminifera.[13] That naturalist, moreover,
is of opinion that the Orbitoides alluded to (_O. Mantelli_) is of the
same species as one found in Cutch, in the Middle Eocene or nummulitic
formation of India.

Above the orbitoidal limestone is a white limestone, sometimes soft and
argillaceous, but in parts very compact and calcareous. It contains
several peculiar corals, and a large Nautilus allied to _N. ziczac_;
also in its upper bed a gigantic cetacean, called _Zeuglodon_ by
Owen.[14]

The colossal bones of this cetacean are so plentiful in the interior of
Clarke County, Alabama, as to be characteristic of the formation. The
vertebral column of one skeleton found by Dr. Buckley at a spot visited
by me, extended to the length of nearly seventy feet, and not far off
part of another backbone nearly fifty feet long was dug up. I obtained
evidence, during a short excursion, of so many localities of this
fossil animal within a distance of ten miles, as to lead me to conclude
that they must have belonged to at least forty distinct individuals.

Professor Owen first pointed out that this huge animal was not
reptilian, since each tooth was furnished with double roots (Fig. 224),
implanted in corresponding double sockets; and his opinion of the
cetacean nature of the fossil was afterwards confirmed by Dr. Wyman and
Dr. R. W. Gibbes. That it was an extinct mammal of the whale tribe has
since been placed beyond all doubt by discovery of the entire skull of
another fossil species of the same family, having the double occipital
condyles only met with in mammals, and the convoluted tympanic bones
which are characteristic of cetaceans.

Fig. 224: Zeuglodon cetoides, Fig 225: Basilosaurus.

 [1] Quart. Geol. Journal, vol. xx, p. 97, 1864.

 [2] Palæont. Soc. Monograph, Rept., pt. ii, p. 61.

 [3] Heer, Climat et Végétation du Pays Tertiaire, p. 172.

 [4] Bowerbank, Fossil Fruits and Seeds of London Clay, Plates ix and
 x.

 [5] Prestwich, Quart. Geol. Journ., vol. x.

 [6] Second Visit to the United States, vol. ii, p. 104.

 [7] Cuvier, Oss. Foss., tome iii, p. 255.

 [8] Sur des Empreintes de Pas d’Animaux par M. J. Desnoyers. Compte
 rendu de l’Institut, 1859.

 [9] D’Archiac, Bulletin, tome x; and Prestwich, Quart. Geol. Journ.,
 1847, p. 377.

 [10] Quart. Geol. Journ., vol. xii, p. 204, 1856.

 [11] Murchison, Quart. Journ. of Geol. Soc., vol. v, and Lyell, vol.
 vi, 1850. Anniversary Address.

 [12] See paper by the Author, Quart. Journ. of Geol. Soc., vol. iv, p.
 12; and Second Visit to the United States, vol. ii, p. 59.

 [13] Quart. Journ. of Geol. Soc., vol. vi, p. 32.

 [14] See Memoir by R. W. Gibbes, Journ. of Acad. Nat. Sci. Philad.,
 vol. i, 1847.




CHAPTER XVII.
UPPER CRETACEOUS GROUP.


Lapse of Time between Cretaceous and Eocene Periods. — Table of
successive Cretaceous Formations. — Maestricht Beds. — Pisolitic
Limestone of France. — Chalk of Faxoe. — Geographical Extent and Origin
of the White Chalk. — Chalky Matter now forming in the Bed of the
Atlantic. — Marked Difference between the Cretaceous and existing
Fauna. — Chalk-flints. — Pot-stones of Horstead. — Vitreous Sponges in
the Chalk. — Isolated Blocks of Foreign Rocks in the White Chalk
supposed to be ice-borne. — Distinctness of Mineral Character in
contemporaneous Rocks of the Cretaceous Epoch. — Fossils of the White
Chalk. — Lower White Chalk without Flints. — Chalk Marl and its
Fossils. — Chloritic Series or Upper Greensand. — Coprolite Bed near
Cambridge. — Fossils of the Chloritic Series. — Gault. — Connection
between Upper and Lower Cretaceous Strata. — Blackdown Beds. — Flora of
the Upper Cretaceous Period. — Hippurite Limestone. — Cretaceous Rocks
in the United States.

We have treated in the preceding chapters of the Tertiary or Cainozoic
strata, and have next to speak of the Secondary or Mesozoic formations.
The uppermost of these last is commonly called the chalk or the
cretaceous formation, from creta, the latin name for that remarkable
white earthy limestone, which constitutes an upper member of the group
in those parts of Europe where it was first studied. The marked
discordance in the fossils of the tertiary, as compared with the
cretaceous formations, has long induced many geologists to suspect that
an indefinite series of ages elapsed between the respective periods of
their origin. Measured, indeed, by such a standard, that is to say, by
the amount of change in the Fauna and Flora of the earth effected in
the interval, the time between the Cretaceous and Eocene may have been
as great as that between the Eocene and Recent periods, to the history
of which the last seven chapters have been devoted. Several deposits
have been met with here and there, in the course of the last half
century, of an age intermediate between the white chalk and the plastic
clays and sands of the Paris and London districts, monuments which have
the same kind of interest to a geologist which certain medieval records
excite when we study the history of nations. For both of them throw
light on ages of darkness, preceded and followed by others of which the
annals are comparatively well-known to us. But these newly-discovered
records do not fill up the wide gap, some of them being closely allied
to the Eocene, and others to the Cretaceous type, while none appear as
yet to possess so distinct and characteristic a fauna as may entitle
them to hold an independent place in the great chronological series.

Among the formations alluded to, the Thanet Sands of Prestwich have
been sufficiently described in the last chapter, and classed as Lower
Eocene. To the same tertiary series belong the Belgian formations,
called by Professor Dumont, Landenian. On the other hand, the
Maestricht and Faxoe limestones are very closely connected with the
chalk, to which also the Pisolitic limestone of France is referable.

Classification of the Cretaceous Rocks.—The cretaceous group has
generally been divided into an Upper and a Lower series, the Upper
called familiarly _the chalk,_ and the Lower _the greensand_; the one
deriving its name from the predominance of white earthy limestone and
marl, of which it consists in a great part of France and England, the
other or lower series from the plentiful mixture of green or chloritic
grains contained in some of the sands and cherts of which it largely
consists in the same countries. But these mineral characters often
fail, even when we attempt to follow out the same continuous
subdivisions throughout a small portion of the north of Europe, and are
worse than valueless when we desire to apply them to more distant
regions. It is only by aid of the organic remains which characterise
the successive marine subdivisions of the formation that we are able to
recognise in remote countries, such as the south of Europe or North
America, the formations which were there contemporaneously in progress.
To the English student of geology it will be sufficient to begin by
enumerating those groups which characterise the series in this country
and others immediately contiguous, alluding but slightly to those of
more distant regions. In the table (p. 283) it will be seen that I have
used the term Neocomian for that commonly called “Lower Greensand;” as
this latter term is peculiarly objectionable, since the green grains
are an exception to the rule in many of the members of this group even
in districts where it was first studied and named.

UPPER CRETACEOUS OR CHALK PERIOD.

Maestricht Beds and Faxoe Limestone.

Upper White Chalk, with flints.

Lower White Chalk, without flints.

Chalk Marl.

Chloritic series (or Upper Greensand).

Gault.

LOWER CRETACEOUS OR NEOCOMIAN. Marine Fresh-water

Marine: Upper Neocomian, see p.308

Marine: Middle Neocomian, see p.312

Marine: Lower Neocomian, see p.312

Wealden Beds (upper part).

Belemnitella mucronata. Belemnitella mucronata,
Maestricht, Faxoe, and White Chalk.
_a/_ Entire specimen, showing vascular impression on outer surface, and
characteristic slit. _b._ Section of same, showing place of
phragmocone.[1]

_Maestricht Beds._—On the banks of the Meuse, at Maestricht, reposing
on ordinary white chalk with flints, we find an upper calcareous
formation about 100 feet thick, the fossils of which are, on the whole,
very peculiar, and all distinct from tertiary species. Some few are of
species common to the inferior white chalk, among which may be
mentioned _Belemnitella mucronata_ (Fig. 226) and _Pecten
quadricostatus,_ a shell regarded by many as a mere variety of _P.
quinquecostatus_ (see Fig. 270). Besides the Belemnite there are other
_genera,_ such as _Baculites_ and _Hamites,_ never found in strata
newer than the cretaceous, but frequently met with in these Maestricht
beds. On the other hand, _Voluta, Fasciolaria,_ and other genera of
univalve shells, usually met with only in tertiary strata, occur.

The upper part of the rock, about 20 feet thick, as seen in St. Peter’s
Mount, in the suburbs of Maestricht, abounds in corals and Bryozoa,
often detachable from the matrix; and these beds are succeeded by a
soft yellowish limestone 50 feet thick, extensively quarried from time
immemorial for building. The stone below is whiter, and contains
occasional nodules of grey chert or chalcedony.

M. Bosquet, with whom I examined this formation (August, 1850), pointed
out to me a layer of chalk from two to four inches thick, containing
green earth and numerous encrinital stems, which forms the line of
demarkation between the strata containing the fossils peculiar to
Maestricht and the white chalk below. The latter is distinguished by
regular layers of black flint in nodules, and by several shells, such
as _Terebratula carnea_ (see Fig. 246), wholly wanting in beds higher
than the green band. Some of the organic remains, however, for which
St. Peter’s Mount is celebrated, occur both above and below that
parting layer, and, among others, the great marine reptile called
_Mosasaurus_ (see Fig. 227), a saurian supposed to have been 24 feet in
length, of which the entire skull and a great part of the skeleton have
been found. Such remains are chiefly met with in the soft freestone,
the principal member of the Maestricht beds. Among the fossils common
to the Maestricht and white chalk may be instanced the echinoderm, Fig.
228.

Mosasaurus Camperi.

Hemipneustes radiatus.
I saw proofs of the previous denudation of the white chalk exhibited in
the lower bed of the Maestricht formation in Belgium, about 30 miles
S.W. of Maestricht, at the village of Jendrain, where the base of the
newer deposit consisted chiefly of a layer of well-rolled, black
chalk-flint pebbles, in the midst of which perfect specimens of
_Thecidea papillata_ and _Belemnitella mucronata_ are imbedded. To a
geologist accustomed in England to regard rolled pebbles of chalk-flint
as a common and distinctive feature of tertiary beds of different ages,
it is a new and surprising phenomenon to behold strata made up of such
materials, and yet to feel no doubt that they were accumulated in a sea
in which the belemnite and other cretaceous mollusca flourished.

Pisolitic Limestone of France.—Geologists were for many years at
variance respecting the chronological relations of this rock, which is
met with in the neighbourhood of Paris, and at places north, south,
east, and west of that metropolis, as between Vertus and Laversines,
Meudon and Montereau. By many able palæontologists the species of
fossils, more than fifty in number, were declared to be more Eocene in
their appearance than Cretaceous. But M. Hébert found in this formation
at Montereau, near Paris, the _Pecten quadricostatus,_ a well-known
Cretaceous species, together with some other fossils common to the
Maestricht chalk and to the Baculite limestone of the Cotentin, in
Normandy. He therefore, as well as M. Alcide d’Orbigny, who had
carefully studied the fossils, came to the opinion that it was an upper
member of the Cretaceous group. It is usually in the form of a coarse
yellowish or whitish limestone, and the total thickness of the series
of beds already known is about 100 feet. Its geographical range,
according to M. Hébert, is not less than 45 leagues from east to west,
and 35 from north to south. Within these limits it occurs in small
patches only, resting unconformably on the white chalk.

The _Nautilus Danicus,_ Fig. 230, and two or three other species found
in this rock, are frequent in that of Faxoe, in Denmark, but as yet no
Ammonites, Hamites, Scaphites, Turrilites, Baculites, or Hippurites
have been met with. The proportion of peculiar species, many of them of
tertiary aspect, is confessedly large; and great aqueous erosion
suffered by the white chalk, before the pisolitic limestone was formed,
affords an additional indication of the two deposits being widely
separated in time. The pisolitic formation, therefore, may eventually
prove to be somewhat more intermediate in date between the secondary
and tertiary epochs than the Maestricht rock.

Chalk of Faxoe.— In the island of Seeland, in Denmark, the newest
member of the chalk series, seen in the sea-cliffs at Stevensklint
resting on white chalk with flints, is a yellow limestone, a portion of
which, at Faxoe, where it is used as a building stone, is composed of
corals, even more conspicuously than is usually observed in recent
coral reefs. It has been quarried to the depth of more than 40 feet,
but its thickness is unknown. The imbedded shells are chiefly casts,
many of them of univalve mollusca, which are usually very rare in the
white chalk of Europe. Thus, there are two species of _Cypræa,_ one of
_Oliva,_ two of _Mitra,_ four of the genus _Cerithium,_ six of _Fusus,_
two of _Trochus,_ one of _Patella,_ one of _Emarginula,_ etc.; on the
whole, more than thirty univalves, spiral or patelliform. At the same
time, some of the accompanying bivalve shells, echinoderms, and
zoophytes, are specifically identical with fossils of the true
Cretaceous series. Among the cephalopoda of Faxoe may be mentioned
_Baculites Faujasii_ (Fig. 229), and _Belemnitella mucronata_ (Fig.
226), shells of the white chalk. The _Nautilus Danicus_ (see Fig. 230)
is characteristic of this formation; and it also occurs in France in
the calcaire pisolitique of Laversin (Department of Oise). The claws
and entire skull of a small crab, _Brachyurus rugosus_ (Schlott.), are
scattered through the Faxoe stone, reminding us of similar crustaceans
inclosed in the rocks of modern coral reefs. Some small portions of
this coralline formation consist of white earthy chalk.

Fig. 229: Portion of Baculites Faujasii, Fig. 230: Nautilus Danicus.

Composition, Extent and Origin of the White Chalk.—The highest beds of
chalk in England and France consist of a pure, white, calcareous mass,
usually too soft for a building-stone, but sometimes passing into a
more solid state. It consists, almost purely, of carbonate of lime; the
stratification is often obscure, except where rendered distinct by
interstratified layers of flint, a few inches thick, occasionally in
continuous beds, but oftener in nodules, and recurring at intervals
generally from two to four feet distant from each other. This upper
chalk is usually succeeded, in the descending order, by a great mass of
white chalk without flints, below which comes the chalk marl, in which
there is a slight admixture of argillaceous matter. The united
thickness of the three divisions in the south of England equals, in
some places, 1000 feet. The section in Fig. 231 will show the manner in
which the white chalk extends from England into France, covered by the
tertiary strata described in former chapters, and reposing on lower
cretaceous beds.

The area over which the white chalk preserves a nearly homogeneous
aspect is so vast, that the earlier geologists despaired of discovering
any analogous deposits of recent date. Pure chalk, of nearly uniform
aspect and composition, is met with in a north-west and south-east
direction, from the north of Ireland to the Crimea, a distance of about
1140 geographical miles, and in an opposite direction it extends from
the south of Sweden to the south of Bordeaux, a distance of about 840
geographical miles. In Southern Russia, according to Sir R. Murchison,
it is sometimes 600 feet thick, and retains the same mineral character
as in France and England, with the same fossils, including _Inoceramus
Cuvieri, Belemnitella mucronata,_ and _Ostrea vesicularis_ (Fig. 251).

Diagrammatic section from Hertfordshire, in England, to Sens, in
France.

Great light has recently been thrown upon the origin of the
unconsolidated white chalk by the deep soundings made in the North
Atlantic, previous to laying down, in 1858, the electric telegraph
between Ireland and Newfoundland. At depths sometimes exceeding two
miles, the mud forming the floor of the ocean was found, by Professor
Huxley, to be almost entirely composed (more than nineteen-twentieths
of the whole) of minute Rhizopods, or foraminiferous shells of the
genus Globigerina, especially the species _Globigerina bulloides_ (see
Fig. 232.) the organic bodies next in quantity were the siliceous
shells called _ Polycystineæ,_ and next to them the siliceous skeletons
of plants called _Diatomaceæ_ (Figs. 233, 234, 235), and occasionally
some siliceous spiculæ of sponges (Fig. 236) were intermixed. These
were connected by a mass of living gelatinous matter to which he has
given the name of _Bathybius,_ and which contains abundance of very
minute bodies termed Coccoliths and Coccospheres, which have also been
detected fossil in chalk.

Sir Leopold MacClintock and Dr. Wallich have ascertained that 95 per
cent of the mud of a large part of the North Atlantic consists of
Globigerina shells. But Captain Bullock, R.N., lately brought up from
the enormous depth of 16,860 feet a white, viscid, chalky mud, wholly
devoid of Globigerinæ. This mud was perfectly homogeneous in
composition, and contained no organic remains visible to the naked eye.
Mr. Etheridge, however, has ascertained by microscopical examination
that it is made up of _ Coccoliths, Discoliths,_ and other minute
fossils like those of the Chalk classed by Huxley as _Bathybius,_ when
this term is used in its widest sense. This mud, more than three miles
deep, was dredged up in latitude 20° 19′ N., longitude 4° 36′ E., or
about midway between Madeira and the Cape of Good Hope.

Fig. 232: Globigerina bulloides, Calcareous Rhizopod. Fig. 233:
Actinocyclus, Fig. 234: Pinnularia, Fig. 235: Eunotia bidens, Siliceous
Diatomaceæ. Fig. 236: Spicula of sponge, Siliceous sponge.

The recent deep-sea dredgings in the Atlantic conducted by Dr. Wyville
Thomson, Dr. Carpenter, Mr. Gwyn Jeffreys, and others, have shown that
on the same white mud there sometimes flourish Mollusca, Crustacea, and
Echinoderms, besides abundance of siliceous sponges, forming, on the
whole, a marine fauna bearing a striking resemblance in its general
character to that of the ancient chalk.

Popular Error as to the Geological Continuity of the Cretaceous
Period.—We must be careful, however, not to overrate the points of
resemblance which the deep-sea investigations have placed in a strong
light. They have been supposed by some naturalists to warrant a
conclusion expressed in these words: “We are still living in the
Cretaceous epoch;” a doctrine which has led to much popular delusion as
to the bearing of the new facts on geological reasoning and
classification. The reader should be reminded that in geology we have
been in the habit of founding our great chronological divisions, not on
foraminifera and sponges, nor even on echinoderms and corals, but on
the remains of the most highly organised beings available to us, such
as the mollusca; these being met with, as explained (p. 142), in
stratified rocks of almost every age. In dealing with the mollusca, it
is those of the highest or most specialised organisation, which afford
us the best characters in proportion as their vertical range is the
most limited. Thus the Cephalopoda are the most valuable, as having a
more restricted range in time than the Gasteropoda; and these, again,
are more characteristic of the particular stratigraphical subdivisions
than are the Lamellibranchiate Bivalves, while these last, again, are
more serviceable in classification than the Brachiopoda, a still lower
class of shell-fish, which are the most enduring of all.

When told that the new dredgings prove that “we are still living in the
Chalk Period,” we naturally ask whether some cuttle-fish has been found
with a Belemnite forming part of its internal framework; or have
Ammonites, Baculites, Hamites, Turrilites, with four or five other
Cephalopodous genera characteristic of the chalk and unknown as
tertiary, been met with in the abysses of the ocean? Or, in the absence
of these long-extinct forms, has a single spiral univalve, or species
of Cretaceous Gasteropod, been found living? Or, to descend still lower
in the scale, has some characteristic Cretaceous genus of
Lamellibranchiate Bivalve, such as the Inoceramus, or Hippurite,
foreign to the Tertiary seas, been proved to have survived down to our
time? Or, of the numerous genera of lamellibranchiates common to the
Cretaceous and Recent seas, has one species been found living? The
answer to all these questions is—not one has been found. Even of the
humblest shell-fish, the Brachiopods, no new species common to the
Cretaceous and recent seas has yet been met with. It has been very
generally admitted by conchologists that out of a hundred species of
this tribe occurring fossil in the Upper Chalk—one, and one only,
_Terebratulina striata,_ is still living, being thought to be identical
with _Terebratula caput-serpentis._ Although this identity is still
questioned by some naturalists of authority, it would certainly not
surprise us if another lamp-shell of equal antiquity should be met with
in the deep sea.

Had it been declared that we are living in the Eocene epoch, the idea
would not be so extravagant, for the great reptiles of the Upper Chalk,
the Mosasaurus, Pliosaurus, and Pterodactyle, and many others, as well
as so many genera of chambered univalves, had already disappeared from
the earth, and the marine fauna had made a greater approach to our own
by nearly the entire difference which separates it from the fauna of
the Cretaceous seas. The Eocene nummulitic limestone of Egypt is a rock
mainly composed, like the more ancient white chalk, of globigerine mud;
and if the reader will refer to what we have said of the extent to
which the nummulitic marine strata, formed originally at the bottom of
the sea, now enter into the frame-work of mountain chains of the
principal continents, he will at once perceive that the present
Atlantic, Pacific, and Indian Oceans are geographical terms, which must
be wholly without meaning when applied to the Eocene, and still more to
the Cretaceous Period; so that to talk of the chalk having been
uninterruptedly forming in the Atlantic from the Cretaceous Period to
our own, is as inadmissible in a geographical as in a geological sense.

Chalk-flints.—The origin of the layers of flint, whether in the form of
nodules, or continuous sheets, or in veins or cracks not parallel to
the stratification, has always been more difficult to explain than that
of the white chalk. But here, again, the late deep-sea soundings have
suggested a possible source of such mineral matter. During the cruise
of the “Bulldog,” already alluded to, it was ascertained that while the
calcareous _Globigerinæ_ had almost exclusive possession of certain
tracts of the sea-bottom, they were wholly wanting in others, as
between Greenland and Labrador. According to Dr. Wallich, they may
flourish in those spaces where they derive nutriment from organic and
other matter, brought from the south by the warm waters of the Gulf
Stream, and they may be absent where the effects of that great current
are not felt. Now, in several of the spaces where the calcareous
Rhizopods are wanting, certain microscopic plants, called _Diatomaceæ,_
above mentioned (Figs. 233-235), the solid parts of which are
siliceous, monopolise the ground at a depth of nearly 400 fathoms, or
2400 feet.

The large quantities of silex in solution required for the formation of
these plants may probably arise from the disintegration of feldspathic
rocks, which are universally distributed. As more than half of their
bulk is formed of siliceous earth, they may afford an endless supply of
silica to all the great rivers which flow into the ocean. We may
imagine that, after a lapse of many years or centuries, changes took
place in the direction of the marine currents, favouring at one time a
supply in the same area of siliceous, and at another of calcareous
matter in excess, giving rise in the one case to a preponderance of
Globigerinæ, and in the other of Diatomaceæ. These last, and certain
sponges, may by their decomposition have furnished the silex, which,
separating from the chalky mud, collected round organic bodies, or
formed nodules, or filled shrinkage cracks.

Pot-stones.—A more difficult enigma is presented by the occurrence of
certain huge flints, or pot-stones, as they are called in Norfolk,
occurring singly, or arranged in nearly continuous columns at right
angles to the ordinary and horizontal layers of small flints. I visited
in the year 1825 an extensive range of quarries then open on the river
Bure, near Horstead, about six miles from Norwich, which afforded a
continuous section, a quarter of a mile in length, of white chalk,
exposed to the depth of about twenty-six feet, and covered by a bed of
gravel. The pot-stones, many of them pear-shaped, were usually about
three feet in height and one foot in their transverse diameter, placed
in vertical rows, like pillars, at irregular distances from each other,
but usually from twenty to thirty feet apart, though sometimes nearer
together, as in Figure 237. These rows did not terminate downward in
any instance which I could examine, nor upward, except at the point
where they were cut off abruptly by the bed of gravel. On breaking open
the pot-stones, I found an internal cylindrical nucleus of pure chalk,
much harder than the ordinary surrounding chalk, and not crumbling to
pieces like it, when exposed to the winter’s frost. At the distance of
half a mile, the vertical piles of pot-stones were much farther apart
from each other. Dr. Buckland has described very similar phenomena as
characterising the white chalk on the north coast of Antrim, in
Ireland.[2]

View of a chalk-pit at Horstead, near Norwich, showing the position of
the pot-stones.

Vitreous Sponges of the Chalk.—These pear-shaped masses of flint often
resemble in shape and size the large sponges called Neptune’s Cups
(_Spongia patera,_ Hardw.), which grow in the seas of Sumatra; and if
we could suppose a series of such gigantic sponges to be separated from
each other, like trees in a forest, and the individuals of each
successive generation to grow on the exact spot where the parent sponge
died and was enveloped in calcareous mud, so that they should become
piled one above the other in a vertical column, their growth keeping
pace with the accumulation of the enveloping calcareous mud, a
counterpart of the phenomena of the Horstead pot-stones might be
obtained.

Fig. 238: Ventriculites radiatus. White chalk.
Professor Wyville Thomson, describing the modern soundings in 1869 off
the north coast of Scotland, speaks of the ooze or chalk mud brought
from a depth of about 3000 feet, and states that at one haul they
obtained forty specimens of vitreous sponges buried in the mud. He
suggests that the Ventriculites of the chalk were nearly allied to
these sponges, and that when the silica of their spicules was removed,
and was dissolved out of the calcareous matrix, it set into flint.

Boulders and Groups of Pebbles in Chalk.—The occurrence here and there,
in the white chalk of the south of England, of isolated pebbles of
quartz and green schist has justly excited much wonder. It was at first
supposed that they had been dropped from the roots of some floating
tree, by which means stones are carried to some of the small coral
islands of the Pacific. But the discovery in 1857 of a group of stones
in the white chalk near Croydon, the largest of which was syenite and
weighed about forty pounds, accompanied by pebbles and fine sand like
that of a beach, has been shown by Mr. Godwin Austen to be inexplicable
except by the agency of floating ice. If we consider that icebergs now
reach 40 degrees north latitude in the Atlantic, and several degrees
nearer the equator in the southern hemisphere, we can the more easily
believe that even during the Cretaceous epoch, assuming that the
climate was milder, fragments of coast ice may have floated
occasionally as far as the south of England.

Distinctness of Mineral Character in Contemporaneous Rocks of the
Cretaceous Period.—But we must not imagine that because pebbles are so
rare in the white chalk of England and France there are no proofs of
sand, shingle, and clay having been accumulated contemporaneously even
in European seas. The siliceous sandstone called “upper quader” by the
Germans overlies white argillaceous chalk or “pläner-kalk,” a deposit
resembling in composition and organic remains the chalk marl of the
English series. This sandstone contains as many fossil shells common to
our white chalk as could be expected in a sea-bottom formed of such
different materials. It sometimes attains a thickness of 600 feet, and,
by its jointed structure and vertical precipices, plays a conspicuous
part in the picturesque scenery of Saxon Switzerland, near Dresden. It
demonstrates that in the Cretaceous sea, as in our own, distinct
mineral deposits were simultaneously in progress. The quartzose
sandstone alluded to, derived from the detritus of the neighbouring
granite, is absolutely devoid of carbonate of lime, yet it was formed
at the distance only of four hundred miles from a sea-bottom now
constituting part of France, where the purely calcareous white chalk
was forming. In the North American continent, on the other hand, where
the Upper Cretaceous formations are so widely developed, true white
chalk, in the ordinary sense of that term, does not exist.

Fig. 239: Ananchytes ovatus. White chalk, upper and lower.

Fossils of the White Chalk.—Among the fossils of the white chalk,
echinoderms are very numerous; and some of the genera, like
_Ananchytes_ (see Fig. 239), are exclusively cretaceous. Among the
Crinoidea, the _Marsupites_ (Fig. 242) is a characteristic genus. Among
the mollusca, the cephalopoda are represented by Ammonites, Baculites
(Fig. 229), and Belemnites (Fig. 226). Although there are eight or more
species of Ammonites and six of them peculiar to it, this genus is much
less fully represented than in each of the other subdivisions of the
Upper Cretaceous group.

Among the brachiopoda in the white chalk, the _ Terebratulæ_ are very
abundant (see Figs. 243-247). With these are associated some forms of
oyster (see Fig. 251), and other bivalves (Figs. 249, 250).

Fig. 240: Micraster cor-angumum. White chalk. Fig. 241: Galerites
albogalerus. White chalk. Fig. 242: Marsupites Milleri. White chalk.
Fig. 243: Terebratulina striata. Upper white chalk. Fig. 244:
Rhynchonella octoplicata. Upper white chalk. Fig. 245: Magas pumila.
Upper white chalk. Fig. 246: Terebratula carnea. Upper white chalk.
Fig. 247: Terebratula biplicata. Upper cretaceous. Fig. 248: Crania
Parisiensis. Inferior or attached valve. Upper white chalk. Fig. 249:
Peten Beaveri. Lower white chalk and chalk marl. Fig. 250: Lima
spinosa. Upper white chalk.

Among the bivalve mollusca, no form marks the Cretaceous era in Europe,
America, and India in a more striking manner than the extinct genus
_Inoceramus_ (_Catillus_ of Lam.; see Fig. 252), the shells of which
are distinguished by a fibrous texture, and are often met with in
fragments, having probably been extremely friable.

Of the singular family called _Rudistes_ by Lamarck, hereafter to be
mentioned as extremely characteristic of the chalk of southern Europe,
a single representative only (Fig. 253) has been discovered in the
white chalk of England.

Fig. 251: Ostrea vesicularis. Upper chalk and upper greensand. Fig.
252: Inoceramus Lamarckii. White chalk.

_Radiolites Mortoni_, Mantell. Houghton, Sussex. White chalk.
Diameter one-seventh nat. size.
Fig. 253. Two individuals deprived of their upper valves, adhering
together.
Fig. 254. Same seen from above.
Fig. 255. Transverse section of part of the wall of the shell,
magnified to show the structure.
Fig. 256. Vertical section of the same.
On the side where the shell is thinnest, there is one external furrow
and corresponding internal ridge, _a_, _b_, figs. 255, 256; but they
are usually less prominent than in these figures. The upper or
opercular valve is wanting.

The general absence of univalve mollusca in the white chalk is very
marked. Of bryozoa there is an abundance, such as _Eschara_ and
_Escharina_ (Figs. 257, 258). These and other organic bodies,
especially sponges, such as _Ventriculites_ (Fig. 238), are dispersed
indifferently through the soft chalk and hard flint, and some of the
flinty nodules owe their irregular forms to inclosed sponges, such as
Fig. 259, _a,_ where the hollows in the exterior are caused by the
branches of a sponge (Fig. 259, _b_), seen on breaking open the flint.

Fig. 257: Eschara disticha. White chalk. Fig. 258: Escharina oceani.
White chalk. Fig. 259: A branching sponge in a flint, from the white
chalk.

The remains of fishes of the Upper Cretaceous formations consist
chiefly of teeth belonging to the shark family. Some of the genera are
common to the Tertiary formations, and some are distinct. To the latter
belongs the genus _Ptychodus_ (Fig. 260), which is allied to the living
Port Jackson shark, _Cestracion Phillippi,_ the anterior teeth of which
(see Fig. 261, _a_) are sharp and cutting, while the posterior or
palatal teeth (_b_) are flat (Fig. 260). But we meet with no bones of
land-animals, nor any terrestrial or fluviatile shells, nor any plants,
except sea-weeds, and here and there a piece of drift-wood. All the
appearances concur in leading us to conclude that the white chalk was
the product of an open sea of considerable depth.

Fig. 260: Palatal tooth of Ptychodus decurrens. Lower white chalk.

The existence of turtles and oviparous saurians, and of a Pterodactyl
or winged lizard, found in the white chalk of Maidstone, implies, no
doubt, some neighbouring land; but a few small islets in mid-ocean,
like Ascension, formerly so much frequented by migratory droves of
turtle, might perhaps have afforded the required retreat where these
creatures laid their eggs in the sand, or from which the flying species
may have been blown out to sea. Of the vegetation of such islands we
have scarcely any indication, but it consisted partly of cycadaceous
plants; for a fragment of one of these was found by Captain Ibbetson in
the Chalk Marl of the Isle of Wight, and is referred by A. Brongniart
to _ Clathraria Lyellii,_ Mantell, a species common to the antecedent
Wealden period. The fossil plants, however, of beds corresponding in
age to the white chalk at Aix-la-Chapelle, presently to be described,
like the sandy beds of Saxony, before alluded to (p. 293), afford such
evidence of land as to prove how vague must be any efforts of ours to
restore the geography of that period.

Fig. 261: Cestracion Phillipi; recent.
The Pterodactyl of the Kentish chalk, above alluded to, was of gigantic
dimensions, measuring 16 feet 6 inches from tip to tip of its
outstretched wings. Some of its elongated bones were at first mistaken
by able anatomists for those of birds; of which class no osseous
remains have as yet been derived from the white chalk, although they
have been found (as will be seen on page 299) in the Chloritic sand.

The collector of fossils from the white chalk was formerly puzzled by
meeting with certain bodies which they call larch-cones, which were
afterwards recognised by Dr. Buckland to be the excrement of fish (see
Fig. 262). They are composed in great part of phosphate of lime.

Fig. 262: Coprolites of fish, from the chalk. Fig. 263: Baculites
anceps. Lower chalk. Fig. 264: Ammonites Rhotomagensis. Chalk marl.

Lower White Chalk.—The Lower White Chalk, which is several hundred feet
thick, without flints, has yielded 25 species of Ammonites, of which
half are peculiar to it. The genera Baculite, Hamite, Scaphite,
Turrilite, Nautilus, Belemnite, and Belemnitella, are also represented.

Chalk Marl.—The lower chalk without flints passes gradually downward,
in the south of England, into an argillaceous limestone, “the chalk
marl,” already alluded to. It contains 32 species of Ammonites, seven
of which are peculiar to it, while eleven pass up into the overlying
lower white chalk. _ A. Rhotomagensis_ is characteristic of this
formation. Among the British cephalopods of other genera may be
mentioned _Scaphites æqualis_ (Fig. 266) and _Turrilites costatus_
(Fig. 265).

Chloritic Series (or Upper Greensand).—According to the old
nomenclature, this subdivision of the chalk was called Upper Greensand,
in order to distinguish it from those members of the Neocomian or Lower
Cretaceous series below the Gault to which the name of Greensand had
been applied. Besides the reasons before given (p. 282) for abandoning
this nomenclature, it is objectionable in this instance as leading the
uninitiated to suppose that the divisions thus named Upper and Lower
Greensand are of co-ordinate value, instead of which the chloritic sand
is quite a subordinate member of the Upper Cretaceous group, and the
term Greensand has very commonly been used for the whole of the Lower
Cretaceous rocks, which are almost comparable in importance to the
entire Upper Cretaceous series. The higher portion of the Chloritic
series in some districts has been called chloritic marl, from its
consisting of a chalky marl with chloritic grains. In parts of Surrey,
where calcareous matter is largely intermixed with sand, it forms a
stone called malm-rock or firestone. In the cliffs of the southern
coast of the Isle of Wight it contains bands of calcareous limestone
with nodules of chert.

Fig. 265: Turrilites costatus. Lower chalk and chalk marl. Fig. 266:
Scaphites æqualis. Chloritic marl and sand, Dorsetshire.
_Coprolite Bed._—The so-called coprolite bed, found near Farnham, in
Surrey, and near Cambridge, contains nodules of phosphate of lime in
such abundance as to be largely worked for the manufacture of
artificial manure. It belongs to the upper part of the Chloritic
series, and is doubtless chiefly of animal origin, and may perhaps be
partly coprolitic, derived from the excrement of fish and reptiles. The
late Mr. Barrett discovered in it, near Cambridge, in 1858, the remains
of a bird, which was rather larger than the common pigeon, and probably
of the order Natatores, and which, like most of the Gull tribe, had
well-developed wings. Portions of the metacarpus, metatarsus, tibia,
and femur have been detected, and the determinations of Mr. Barrett
have been confirmed by Professor Owen.

This phosphatic bed in the suburbs of Cambridge must have been formed
partly by the denudation of pre-existing rocks, mostly of Cretaceous
age. The fossil shells and bones of animals washed out of these denuded
strata, now forming a layer only a few feet thick, have yielded a rich
harvest to the collector. A large Rudist of the genus Radiolite, no
less than two feet in height, may be seen in the Cambridge Museum,
obtained from this bed. The number of reptilian remains, all apparently
of Cretaceous age, is truly surprising; more than ten species of
Pterodactyl, five or six of Ichthyosaurus, one of Pliosaurus, one of
Dinosaurus, eight of Chelonians, besides other forms, having been
recognised.

The chloritic sand is regarded by many geologists as a littoral deposit
of the Chalk Ocean, and therefore contemporaneous with part of the
chalk marl, and even, perhaps, with some part of the white chalk. For,
as the land went on sinking, and the cretaceous sea widened its area,
white mud and chloritic sand were always forming somewhere, but the
line of sea-shore was perpetually shifting its position. Hence, though
both sand and mud originated simultaneously, the one near the land, the
other far from it, the sands in every locality where a shore became
submerged might constitute the underlying deposit.

Fig. 267: Ostrea columba. Chloritic sand. Fig. 268: Ostrea carinata.
Chalk marl and chloritic sand.

Among the characteristic mollusca of the chloritic sand may be
mentioned _Terebrirostra lyra_ (Fig. 269), _Plagiostoma Hoperi_ (Fig.
271), _Pecten quinque-costatus_ (Fig. 270), and _Ostrea columba_ (Fig.
267).

Fig. 269: Terebrirostra lyra. Chloritic sand. Fig. 270: Pecten
5-costatus. White chalk and chloritic sand. Fig. 271: Plagiostoma
Hoperi. White chalk and chloritic sand.

The Cephalopoda are abundant, among which 40 species of Ammonites are
now known, 10 being peculiar to this subdivision, and the rest common
to the beds immediately above or below.

Gault.—The lowest member of the Upper Cretaceous group, usually about
100 feet thick in the S.E. of England, is provincially termed Gault. It
consists of a dark blue marl, sometimes intermixed with green sand.
Many peculiar forms of cephalopoda, such as the _Hamite_ (Fig. 272),
and _ Scaphite,_ with other fossils, characterise this formation,
which, small as is its thickness, can be traced by its organic remains
to distant parts of Europe, as, for example, to the Alps.

Fig. 272: Ancyloceras spinigerum. Near Folkestone.
Twenty-one species of British Ammonites are recorded as found in the
Gault, of which only eight are peculiar to it, ten being common to the
overlying Chloritic series.

Connection between Upper and Lower Cretaceous Strata.—Blackdown
Beds.—The break between the Upper and Lower Cretaceous formations will
be appreciated when it is stated that, although the Neocomian contains
31 species of Ammonite, and the Gault, as we have seen, 21, there are
only three of those common to both divisions. Nevertheless, we may
expect the discovery in England, and still more when we extend our
survey to the Continent, of beds of passage intermediate between the
Upper and Lower Cretaceous. Even now the Blackdown beds in Devonshire,
which rest immediately on Triassic strata, and which evidently belong
to some part of the Cretaceous series, have been referred by some
geologists to the Upper group, by others to the Lower or Neocomian.
They resemble the Folkestone beds of the latter series in mineral
character, and 59 out of 156 of their fossil mollusca are common to
them; but they have also 16 species common to the Gault, and 20 to the
overlying Chloritic series; and what is very important, out of seven
Ammonites six are found also in the Gault and Chloritic series, only
one being peculiar to the Blackdown beds.

Professor Ramsay has remarked that there is a stratigraphical break;
for in Kent, Surrey, and Sussex, at those few points where there are
exposures of junctions of the Gault and Neocomian, the surface of the
latter has been much eroded or denuded, while to the westward of the
great chalk escarpment the unconformability of the two groups is
equally striking. At Blackdown this unconformability is still more
marked, for though distant only 100 miles from Kent and Surrey, no
formation intervenes between these beds and the Trias; all intermediate
groups, such as the Lower Neocomian and Oolite, having either not been
deposited or destroyed by denudation.

Flora of the Upper Cretaceous Period.—As the Upper Cretaceous rocks of
Europe are, for the most part, of purely marine origin, and formed in
deep water usually far from the nearest shore, land-plants of this
period, as we might naturally have anticipated, are very rarely met
with. In the neighbourhood of Aix-la-Chapelle, however, an important
exception occurs, for there certain white sands and laminated clays,
400 feet in thickness, contain the remains of terrestrial plants in a
beautiful state of preservation. These beds are the equivalents of the
white chalk and chalk marl of England, or Senonien of d’Orbigny,
although the white siliceous sands of the lower beds, and the green
grains in the upper part of the formation, cause it to differ in
mineral character from our white chalk.

Beds of fine clay, with fossil plants, and with seams of lignite, and
even perfect coal, are intercalated. Floating wood, containing
perforating shells, such as Pholas and Gastrochoena, occur. There are
likewise a few beds of a yellowish-brown limestone, with marine shells,
which enable us to prove that the lowest and highest plant-beds belong
to one group. Among these shells are _Pecten quadricostatus,_ and
several others which are common to the upper and lower part of the
series, and _ Trigonia limbata,_ D’Orbigny, a shell of the white chalk.
On the whole, the organic remains and the geological position of the
strata prove distinctly that in the neighbourhood of Aix-la-Chapelle a
gulf of the ancient Cretaceous sea was bounded by land composed of
Devonian rocks. These rocks consisted of quartzose and schistose beds,
the first of which supplied white sand and the other argillaceous mud
to a river which entered the sea at this point, carrying down in its
turbid waters much drift-wood and the leaves of plants. Occasionally,
when the force of the river abated, marine shells of the genera
_Trigonia, Turritella, Pecten,_ etc., established themselves in the
same area, and plants allied to _Zostera_ and _Fucus_ grew on the
bottom.

The fossil plants of this member of the upper chalk at Aix have been
diligently collected and studied by Dr. Debey, and as they afford the
only example yet known of a terrestrial flora older than the Eocene, in
which the great divisions of the vegetable kingdom are represented in
nearly the same proportions as in our own times, they deserve
particular attention. Dr. Debey estimates the number of species as
amounting to more than two hundred, of which sixty-seven are
cryptogamous, chiefly ferns, twenty species of which can be well
determined, most of them being in fructification. The scars on the bark
of one or two are supposed to indicate tree-ferns. Of thirteen genera
three are still existing, namely, _Gleichenia,_ now inhabiting the Cape
of Good Hope, and New Holland; Lygodium, now spread extensively through
tropical regions, but having some species which live in Japan and North
America; and _ Asplenium,_ a cosmopolite form. Among the phænogamous
plants, the Conifers are abundant, the most common belonging to a genus
called Cycadopteris by Debey, and hardly separable from Sequoia (or
Wellingtonia), of which both the cones and branches are preserved. When
I visited Aix, I found the silicified wood of this plant very
plentifully dispersed through the white sands in the pits near that
city. In one silicified trunk 200 rings of annual growth could be
counted. Species of Araucaria like those of Australia are also found.
Cycads are extremely rare, and of Monocotyledons there are but few. No
palms have been recognised with certainty, but the genus Pandanus, or
screw pine, has been distinctly made out. The number of the
Dicotyledonous Angiosperms is the most striking feature in so ancient a
flora.[3]

Among them we find the familiar forms of the Oak, Fig, and Walnut
(Quercus, Ficus, and Juglans), of the last both the nuts and leaves;
also several genera of the Myrtaceæ. But the predominant order is the
Proteaceæ, of which there are between sixty and seventy supposed
species, many of extinct genera, but some referred to the following
living forms—Dryandra, Grevillea, Hakea, Banksia, Persoonia—all now
belonging to Australia, and Leucospermum, species of which form small
bushes at the Cape.

  Brongniart. Lindley. Cryptogamic. 1. Cryptogamous amphigens,
  or cellular cryptogamic. Thallogens. Lichens, sea-weeds,
  fungi. 2. Cryptogamous acrogens. Acrogens. Mosses,
  equisetums, ferns, lycopodiums,—Lepidodendra.
Phænerogamic. 3. Dicotyledonous
gymnosperms. Gymnogens. Conifers and Cycads. 4. Dicot.
angiosperms. Exogens. Compositæ, leguminosæ, cruciferæ, healths,
etc. All native European trees except conifers. 5.
Monocotyledons. Endogens. Palms, lilies, aloes, rushes, grasses,
etc.

The epidermis of the leaves of many of these Aix plants, especially of
the Proteaceæ, is so perfectly preserved in an envelope of fine clay,
that under the microscope the stomata, or polygonal cellules, can be
detected, and their peculiar arrangement is identical with that known
to characterise some living Proteaceæ (Grevillea, for example).
Although this peculiarity of the structure of stomata is also found in
plants of widely distant orders, it is, on the whole, but rarely met
with, and being thus observed to characterise a foliage previously
suspected to be proteaceous, it adds to the probability that the
botanical evidence had been correctly interpreted.

An occasional admixture at Aix-la-Chapelle of Fucoids and Zosterites
attests, like the shells, the presence of salt-water. Of insects, Dr.
Debey has obtained about ten species of the families Curculionidæ and
Carabidæ.

The resemblance of the flora of Aix-la-Chapelle to the tertiary and
living floras in the proportional number of dicotyledonous angiosperms
as compared to the gymnogens, is a subject of no small theoretical
interest, because we can now affirm that these Aix plants flourished
before the rich reptilian fauna of the secondary rocks had ceased to
exist. The Ichthyosaurus, Pterodactyl, and Mosasaurus were of coeval
date with the oak, the walnut, and the fig. Speculations have often
been hazarded respecting a connection between the rarity of Exogens in
the older rocks and a peculiar state of the atmosphere. A denser air,
it was suggested, had in earlier times been alike adverse to the
well-being of the higher order of flowering plants, and of the
quick-breathing animals, such as mammalia and birds, while it was
favourable to a cryptogamic and gymnospermous flora, and to a
predominance of reptile life. But we now learn that there is no
incompatibility in the co-existence of a vegetation like that of the
present globe, and some of the most remarkable forms of the extinct
reptiles of the age of gymnosperms.

If the passage seem at present to be somewhat sudden from the flora of
the Lower or Neocomian to that of the Upper Cretaceous period, the
abruptness of the change will probably disappear when we are better
acquainted with the fossil vegetation of the uppermost beds of the
Neocomian and that of the lowest strata of the Gault or true Cretaceous
series.

Hippurite limestone.—_Difference between the Chalk of the North and
South of Europe._—By the aid of the three tests, superposition, mineral
character, and fossils, the geologist has been enabled to refer to the
same Cretaceous period certain rocks in the north and south of Europe,
which differ greatly both in their fossil contents and in their mineral
composition and structure.

Fig. 273: Map.
If we attempt to trace the cretaceous deposits from England and France
to the countries bordering the Mediterranean, we perceive, in the first
place, that in the neighbourhood of London and Paris they form one
great continuous mass, the Straits of Dover being a trifling
interruption, a mere valley with chalk cliffs on both sides. We then
observe that the main body of the chalk which surrounds Paris stretches
from Tours to near Poitiers (see Fig. 273, in which the shaded part
represents chalk).

Between Poitiers and La Rochelle, the space marked A on the map
separates two regions of chalk. This space is occupied by the Oolite
and certain other formations older than the Chalk and Neocomian, and
has been supposed by M. E. de Beaumont to have formed an island in the
Cretaceous sea. South of this space we again meet with rocks which we
at once recognise to be cretaceous, partly from the chalky matrix and
partly from the fossils being very similar to those of the white chalk
of the north: especially certain species of the genera _Spatangus,
Ananchytes, Cidarites, Nucula, Ostrea,_ _Gryphæa (Exogyra), Pecten,
Plagiostoma (Lima), Trigonia, Catillus (Inoceramus),_ and
_Terebratula._[4] But Ammonites, as M. d’Archiac observes, of which so
many species are met with in the chalk of the north of France, are
scarcely ever found in the southern region; while the genera _Hamite,
Turrilite,_ and _Scaphite,_ and perhaps _Belemnite,_ are entirely
wanting.

Fig. 274: Radiolites. White chalk of France. Fig. 275: Radiolites
foliaceus. White chalk of France.

Fig. 276: Hippurites organisans. Upper chalk:—chalk marl of Pyrenees?
Fig. 276: Hippurites organisans. Upper chalk:—chalk marl of
Pyrenees?[5]

On the other hand, certain forms are common in the south which are rare
or wholly unknown in the north of France. Among these may be mentioned
many _Hippurites, Sphærulites,_ and other members of that great family
of mollusca called _Rudistes_ by Lamarck, to which nothing analogous
has been discovered in the living creation, but which is quite
characteristic of rocks of the Cretaceous era in the south of France,
Spain, Sicily, Greece, and other countries bordering the Mediterranean.
The species called _ Hippurites organisans_ (Fig. 276) is more abundant
than any other in the south of Europe; and the geologist should make
himself well acquainted with the cast of the interior, _d,_ which is
often the only part preserved in many compact marbles of the Upper
Cretaceous period. The flutings on the interior of the Hippurite, which
are represented on the cast by smooth, rounded longitudinal ribs, and
in some individuals attain a great size and length, are wholly unlike
the markings on the exterior of the shell.

Cretaceous Rocks in the United States.—If we pass to the American
continent, we find in the State of New Jersey a series of sandy and
argillaceous beds wholly unlike in mineral character to our Upper
Cretaceous system; which we can, nevertheless, recognise as referable,
palæontologically, to the same division.

That they were about the same age generally as the European chalk and
Neocomian, was the conclusion to which Dr. Morton and Mr. Conrad came
after their investigation of the fossils in 1834. The strata consist
chiefly of green sand and green marl, with an overlying coralline
limestone of a pale yellow colour, and the fossils, on the whole, agree
most nearly with those of the Upper European series, from the
Maestricht beds to the Gault inclusive. I collected sixty shells from
the New Jersey deposits in 1841, five of which were identical with
European species—_Ostrea larva, O. vesicularis, Gryphæa costata, Pecten
quinque-costatus, Belemnitella mucronata._ As some of these have the
greatest vertical range in Europe, they might be expected more than any
others to recur in distant parts of the globe. Even where the species
were different, the generic forms, such as the Baculite and certain
sections of Ammonites, as also the _Inoceramus_ (see Fig. 252) and
other bivalves, have a decidedly cretaceous aspect. Fifteen out of the
sixty shells above alluded to were regarded by Professor Forbes as good
geographical representatives of well-known cretaceous fossils of
Europe. The correspondence, therefore, is not small, when we reflect
that the part of the United States where these strata occur is between
3000 and 4000 miles distant from the chalk of Central and Northern
Europe, and that there is a difference of ten degrees in the latitude
of the places compared on opposite sides of the Atlantic. Fish of the
genera _Lamna, Galeus,_ and _ Carcharodon_ are common to New Jersey and
the European cretaceous rocks. So also is the genus _Mosasaurus_ among
reptiles.

It appears from the labours of Dr. Newberry and others, that the
Cretaceous strata of the United States east and west of the
Appalachians are characterised by a flora decidedly analogous to that
of Aix-la-Chapelle above-mentioned, and therefore having considerable
resemblance to the vegetation of the Tertiary and Recent Periods.

 [1] For particulars of structure see p. 318.

 [2] Geol. Trans., 1st Series, vol. iv, p. 413.

 [3] In this and subsequent remarks on fossil plants I shall often use
 Dr. Lindley’s terms, as most familiar in this country; but as those of
 M. A. Brongniart are much cited, it may be useful to geologists to
 give a table explaining the corresponding names of groups so much
 spoken of in palæontology.

 [4] D’Archiac, Sur la form. Crétacée du S.-O. de la France Mém. de la
 Soc. Géol. de France, tome ii.

 [5] D’Orbigny’s Paléontologie français, pl. 533.




CHAPTER XVIII.
LOWER CRETACEOUS OR NEOCOMIAN FORMATION.


Classification of marine and fresh-water Strata. — Upper Neocomian. —
Folkestone and Hythe Beds. — Atherfield Clay. — Similarity of
Conditions causing Reappearance of Species after short Intervals. —
Upper Speeton Clay. — Middle Neocomian. — Tealby Series. — Middle
Speeton Clay. — Lower Neocomian. — Lower Speeton Clay. — Wealden
Formation. — Fresh-water Character of the Wealden. — Weald Clay. —
Hastings Sands. — Punfield Beds of Purbeck, Dorsetshire. — Fossil
Shells and Fish of the Wealden. — Area of the Wealden. — Flora of the
Wealden.

We now come to the Lower Cretaceous Formation which was formerly called
Lower Greensand, and for which it will be useful for reasons before
explained (p. 282) to use the term “Neocomian.”

LOWER CRETACEOUS OR NEOCOMIAN GROUP.

Marine Fresh-water

Upper Neocomian—Greensand of Folkestone, Sandgate, and Hythe,
Atherfield clay, upper part of Speeton clay.

Middle Neocomian—Punfield Marine bed, Tealby beds, middle part of
Speeton clay.

Lower Neocomian—Lower part of Speeton clay.

Part of Wealden beds of Kent, Surrey, Sussex, Hants, and Dorset.

In Western France, the Alps, the Carpathians, Northern Italy, and the
Apennines, an extensive series of rocks has been described by
Continental geologists under the name of Tithonian. These beds, which
are without any marine equivalent in this country, appear completely to
bridge over the interval between the Neocomian and the Oolites. They
may, perhaps, as suggested by Mr. Judd, be of the same age as part of
the Wealden series.

UPPER NEOCOMIAN.

Folkstone and Hythe Beds.—The sands which crop out beneath the Gault in
Wiltshire, Surrey, and Sussex are sometimes in the uppermost part pure
white, at others of a yellow and ferruginous colour, and some of the
beds contain much green matter. At Folkestone they contain layers of
calcareous matter and chert, and at Hythe, in the neighbourhood, as
also at Maidstone and other parts of Kent, the limestone called Kentish
Rag is intercalated. This somewhat clayey and calcareous stone forms
strata two feet thick, alternating with quartzose sand. The total
thickness of these Folkestone and Hythe beds is less than 300 feet, and
they are seen to rest immediately on a grey clay, to which we shall
presently allude as the Atherfield clay. Among the fossils of the
Folkestone and Hythe beds we may mention _Nautilus plicatus_ (Fig.
277), _ Ancyloceras (Scaphites) gigas_ (Fig. 278), which has been aptly
described as an Ammonite more or less uncoiled; _Trigonia caudata_
(Fig. 280), _Gervillia anceps_ (Fig. 279), a bivalve genus allied to
Avicula, and _Terebratula sella_ (Fig. 281). In ferruginous beds of the
same age in Wiltshire is found a remarkable shell called _Diceras
Lonsdalii_ (Fig. 282), which abounds in the Upper and Middle Neocomian
of Southern Europe. This genus is closely allied to Chama, and the cast
of the interior has been compared to the horns of a goat.

Fig. 277: Nautilus licatus. Fig. 278: Ancyloceras gigas. Fig. 279:
Gervillia anceps. Fig. 280: Trigonia caudata.

Atherfield Clay.—We mentioned before that the Folkstone and Hythe
series rest on a grey clay. This clay is only of slight thickness in
Kent and Surrey, but acquires great dimensions at Atherfield, in the
Isle of Wight. The difference, indeed, in mineral character and
thickness of the Upper Neocomian formation near Folkestone, and the
corresponding beds in the south of the Isle of Wight, about 100 miles
distant, is truly remarkable. In the latter place we find no limestone
answering to the Kentish Rag, and the entire thickness from the bottom
of the Atherfield clay to the top of the Neocomian, instead of being
less than 300 feet as in Kent, is given by the late Professor E. Forbes
as 843 feet, which he divides into sixty-three strata, forming three
groups. The uppermost of these consists of ferruginous sands, the
second of sands and clay, and the third or lowest of a brown clay,
abounding in fossils.

Fig. 281: Terebratula sella. Fig. 282: Diceras Lonsdalii. a. The
bivavle shell, b. Cast of one of the valves enlarged.

Pebbles of quartzose sandstone, jasper, and flinty slate, together with
grains of chlorite and mica, and, as Mr. Godwin-Austen has shown,
fragments and water-worn fossils of the oolitic rocks, speak plainly of
the nature of the pre-existing formations, by the wearing down of which
the Neocomian beds were formed. The land, consisting of such rocks, was
doubtless submerged before the origin of the white chalk, a deposit
which was formed in a more open sea, and in clearer waters.

Fig. 283: Perna mulleti.
Among the shells of the Atherfield clay the biggest and most abundant
shell is the large _Perna Mulleti,_ of which a reduced figure is given
in Fig. 283.

_Similarity of Conditions causing Reappearance of Species._—Some
species of mollusca and other fossils range through the whole series,
while others are confined to particular subdivisions, and Forbes laid
down a law which has since been found of very general application in
regard to estimating the chronological relations of consecutive strata.
Whenever similar conditions, he says, are repeated, the same species
reappear, provided too great a lapse of time has not intervened;
whereas if the length of the interval has been geologically great, the
same genera will reappear represented by distinct species. Changes of
depth, or of the mineral nature of the sea-bottom, the presence or
absence of lime or of peroxide of iron, the occurrence of a muddy, or a
sandy, or a gravelly bottom, are marked by the banishment of certain
species and the predominance of others. But these differences of
conditions being mineral, chemical, and local in their nature, have no
necessary connection with the extinction, throughout a large area, of
certain animals or plants. When the forms proper to loose sand or soft
clay, or to perfectly clear water, or to a sea of moderate or great
depth, recur with all the same species, we may infer that the interval
of time has been, geologically speaking, small, however dense the mass
of matter accumulated. But if, the genera remaining the same, the
species are changed, we have entered upon a new period; and no
similarity of climate, or of geographical and local conditions, can
then recall the old species which a long series of destructive causes
in the animate and inanimate world has gradually annihilated.

Fig. 284: Ammonites Deshayesii.

Speeton Clay, Upper Division.—On the coast, beneath the white chalk of
Flamborough Head, in Yorkshire, an argillaceous formation crops out,
called the Speeton clay, several hundred feet in thickness, the
palæontological relations of which have been ably worked out by Mr.
John W. Judd,[1] and he has shown that it is separable into three
divisions, the uppermost of which, 150 feet thick, and containing 87
species of mollusca, decidedly belongs to the Atherfield clay and
associated strata of Hythe and Folkestone, already described. It is
characterised by the _Perna Mulleti_ (Fig. 283) and _Terebratula sella_
(Fig. 281), and by _ Ammonites Deshayesii_ (Fig. 284), a well-known
Hythe fossil. Fine skeletons of reptiles of the genera Pliosaurus and
Teleosaurus have been obtained from this clay. At the base of this
upper division of the Speeton clay there occurs a layer of large
Septaria, formerly worked for the manufacture of cement. This bed is
crowded with fossils, especially Ammonites, one species of which, three
feet in diameter, was observed by Mr. Judd.

MIDDLE NEOCOMIAN.

Tealby Series.—At Tealby, a village in the Lincolnshire Wolds, there
crop out beneath the white chalk some non-fossiliferous ferruginous
sands about twenty-feet thick, beneath which are beds of clay and
limestone, about fifty feet thick, with an interesting suite of
fossils, among which are _ Pecten cinctus_ (Fig. 285), from 9 to 12
inches in diameter, _ Ancyloceras Duvallei_ (Fig. 286), and some forty
other shells, many of them common to the Middle Speeton clay, about to
be mentioned. Mr. Judd remarks that as _Ammonites clypeiformis_ and
_Terebratula hippopus_ characterise the Middle Neocomian of the
Continent, it is to this stage that the Tealby series containing the
same fossils may be assigned.[2]

Fig. 285: Pecten cinctus. Fig. 286: Ancyloceras (Crioceras) Duvallei.

The middle division of the Speeton clay, occurring at Speeton below the
cement-bed, before alluded to, is 150 feet thick, and contains about 39
species of mollusca, half of which are common to the overlying clay.
Among the peculiar shells, _Pecten cinctus_ (Fig. 285) and _Ancyloceras
(Crioceras) Duvallei_ (Fig. 286) occur.

LOWER NEOCOMIAN.

In the lower division of the Speeton clay, 200 feet thick, 46 species
of mollusca have been found, and three divisions, each characterised by
its peculiar ammonite, have been noticed by Mr. Judd. The central zone
is marked by _Ammonites Noricus_ (see Fig. 287). On the Continent these
beds are well-known by their corresponding fossils, the Hils clay and
conglomerate of the north of Germany agreeing with the Middle and Lower
Speeton, the latter of which, with the same mineral characters and
fossils as in Yorkshire, is also found in the little island of
Heligoland. Yellow limestone, which I have myself seen near Neuchatel,
in Switzerland, represents the Lower Neocomian at Speeton.

Fig. 287: Ammonites Noricus.

WEALDEN FORMATION.

Beneath the Atherfield clay or Upper Neocomian of the S.E. of England,
a fresh-water formation is found, called the Wealden, which, although
it occupies a small horizontal area in Europe, as compared to the White
Chalk and the marine Neocomian beds, is nevertheless of great
geological interest, since the imbedded remains give us some insight
into the nature of the terrestrial fauna and flora of the Lower
Cretaceous epoch. The name of Wealden was given to this group because
it was first studied in parts of Kent, Surrey, and Sussex, called the
Weald; and we are indebted to Dr. Mantell for having shown, in 1822, in
his “Geology of Sussex,” that the whole group was of fluviatile origin.
In proof of this he called attention to the entire absence of
Ammonites, Belemnites, Brachiopoda, Echinodermata, Corals, and other
marine fossils, so characteristic of the Cretaceous rocks above, and of
the Oolitic strata below, and to the presence in the Weald of Paludinæ,
Melaniæ, Cyrenæ, and various fluviatile shells, as well as the bones of
terrestrial reptiles and the trunks and leaves of land-plants.

The evidence of so unexpected a fact as that of a dense mass of purely
fresh-water origin underlying a deep-sea deposit (a phenomenon with
which we have since become familiar) was received, at first, with no
small doubt and incredulity. But the relative position of the beds is
unequivocal; the Weald Clay being distinctly seen to pass beneath the
Atherfield Clay in various parts of Surrey, Kent, and Sussex, and to
reappear in the Isle of Wight at the base of the Cretaceous series,
being, no doubt, continuous far beneath the surface, as indicated by
the dotted lines in Fig. 288. They are also found occupying the same
relative position below the chalk in the peninsula of Purbeck,
Dorsetshire, where, as we shall see in the sequel, they repose on
strata referable to the Upper Oolite.

_Weald Clay._—The Upper division, or Weald Clay, is, in great part, of
fresh-water origin, but in its highest portion contains beds of oysters
and other marine shells which indicate fluvio-marine conditions. The
uppermost beds are not only conformable, as Dr. Fitton observes, to the
inferior strata of the overlying Neocomian, but of similar mineral
composition. To explain this, we may suppose that, as the delta of a
great river was tranquilly subsiding, so as to allow the sea to
encroach upon the space previously occupied by fresh-water, the river
still continued to carry down the same sediment into the sea. In
confirmation of this view it may be stated that the remains of the
_Iguanodon Mantelli,_ a gigantic terrestrial reptile, very
characteristic of the Wealden, has been discovered near Maidstone, in
the overlying Kentish Rag, or marine limestone of the Upper Neocomian.
Hence we may infer that some of the saurians which inhabited the
country of the great river continued to live when part of the district
had become submerged beneath the sea. Thus, in our own times, we may
suppose the bones of large alligators to be frequently entombed in
recent fresh-water strata in the delta of the Ganges. But if part of
that delta should sink down so as to be covered by the sea, marine
formations might begin to accumulate in the same space where
fresh-water beds had previously been formed; and yet the Ganges might
still pour down its turbid waters in the same direction, and carry
seaward the carcasses of the same species of alligator, in which case
their bones might be included in marine as well as in subjacent
fresh-water strata.

Fig. 288

The Iguanodon, first discovered by Dr. Mantell, was an herbivorous
reptile, of which the teeth, though bearing a great analogy, in their
general form and crenated edges (see Figs. 289 _a_ and _b_), to the
modern Iguanas which now frequent the tropical woods of America and the
West Indies, exhibit many important differences. It appears that they
have often been worn by the process of mastication; whereas the
existing herbivorous reptiles clip and gnaw off the vegetable
productions on which they feed, but do not chew them. Their teeth
frequently present an appearance of having been chipped off, but never,
like the fossil teeth of the Iguanodon, have a flat ground surface (see
Fig. 290, _b_) resembling the grinders of herbivorous mammalia. Dr.
Mantell computes that the teeth and bones of this species which passed
under his examination during twenty years must have belonged to no less
than seventy-one distinct individuals, varying in age and magnitude
from the reptile just burst from the egg, to one of which the femur
measured twenty-four inches in circumference. Yet, notwithstanding that
the teeth were more numerous than any other bones, it is remarkable
that it was not until the relics of all these individuals had been
found, that a solitary example of part of a jaw-bone was obtained. Soon
afterwards remains both of the upper and lower jaw were met with in the
Hastings beds in Tilgate Forest, near Cuckfield. In the same sands at
Hastings, Mr. Beckles found large tridactyle impressions which it is
conjectured were made by the hind feet of this animal, on which it is
ascertained that there were only three well-developed toes.

Fig. 289 a, b: Tooth of Iguanodon Mantelli. Fig. 290: a. Partially worn
tooth of young individual of the same; <i>b.</i> Crown of tooth in
adult worn down.

Fig. 291: Cypris spinigera.
Occasionally bands of limestone, called Sussex Marble, occur in the
Weald Clay, almost entirely composed of a species of _ Paludina,_
closely resembling the common _P. vivipara_ of English rivers. Shells
of the _Cypris,_ a genus of Crustaceans mentioned (p. 57) as abounding
in lakes and ponds, are also plentifully scattered through the clays of
the Wealden, sometimes producing, like plates of mica, a thin
lamination (see Fig. 292).

Fig. 292: Weald clay with Cyprides.
Hastings Sands.—This lower division of the Wealden consists of sand,
sandstone, calciferous grit, clay, and shale; the argillaceous strata,
notwithstanding the name, predominating somewhat over the arenaceous,
as will be seen by reference to the following table, drawn up by
Messrs. Drew and Foster, of the Geological Survey of Great Britain:

  Names of Subordinate
Formations. Mineral Composition
of the Strata. Thickness
in feet. Hastings Sand Tunbridge Wells Sand Sandstone and
loam 150 Wadhurst Clay Blue and brown shale and clay, with
a little calc-grit 100 Ashdown Sand Hard sand, with some beds of
calc-grit 160 Ashburnham Beds Mottled white and red clay, with
some sandstone 330

The picturesque scenery of the “High Rocks” and other places in the
neighbourhood of Tunbridge Wells is caused by the steep natural cliffs,
to which a hard bed of white sand, occurring in the upper part of the
Tunbridge Wells Sand, mentioned in the above table, gives rise. This
bed of “rock-sand” varies in thickness from 25 to 48 feet. Large masses
of it, which were by no means hard or capable of making a good
building-stone, form, nevertheless, projecting rocks with perpendicular
faces, and resist the degrading action of the river because, says Mr.
Drew, they present a solid mass without planes of division. The
calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in
which the remains of the Iguanodon and Hylæosaurus were first found by
Dr. Mantell, constitute an upper member of the Tunbridge Wells Sand,
while the “sand-rock” of the Hastings cliffs, about 100 feet thick, is
one of the lower members of the same. The reptiles, which are very
abundant in this division, consist partly of saurians, referred by Owen
and Mantell to eight genera, among which, besides those already
enumerated, we find the Megalosaurus and Plesiosaurus. The Pterodactyl
also, a flying reptile, is met with in the same strata, and many
remains of Chelonians of the genera _Trionyx_ and _Emys,_ now confined
to tropical regions.

The fishes of the Wealden are chiefly referable to the Ganoid and
Placoid orders. Among them the teeth and scales of _ Lepidotus_ are
most widely diffused (see Fig. 293, next page). These ganoids were
allied to the _Lepidosteus,_ or Gar-pike, of the American rivers. The
whole body was covered with large rhomboidal scales, very thick, and
having the exposed part coated with enamel. Most of the species of this
genus are supposed to have been either river-fish, or inhabitants of
the sea at the mouth of estuaries.

Fig. 293: Lepidotus Mantelli, a. Palate and teeth, b. Side view of
teeth, c. Scale.

Fig. 294: Unio Valdensis. Fig. 295: Under side of slab of sandstone
about one yard in diameter.
At different heights in the Hastings Sands, we find again and again
slabs of sandstone with a strong ripple-mark, and between these slabs
beds of clay many yards thick. In some places, as at Stammerham,
Horsham, near there, are indications of this clay having been exposed
so as to dry and crack before the next layer was thrown down upon it.
The open cracks in the clay have served as moulds, of which casts have
been taken in relief, and which are, therefore, seen on the lower
surface of the sandstone (see Fig. 295).

Near the same place a reddish sandstone occurs in which are innumerable
traces of a fossil vegetable, apparently _ Sphenopteris,_ the stems and
branches of which are disposed as if the plants were standing erect on
the spot where they originally grew, the sand having been gently
deposited upon and around them; and similar appearances have been
remarked in other places in this formation.[3] In the same division
also of the Wealden, at Cuckfield, is a bed of gravel or conglomerate,
consisting of water-worn pebbles of quartz and jasper, with rolled
bones of reptiles. These must have been drifted by a current, probably
in water of no great depth.

Fig. 296: Sphenopteris gracilis.
From such facts we may infer that, notwithstanding the great thickness
of this division of the Wealden, the whole of it was a deposit in water
of a moderate depth, and often extremely shallow. This idea may seem
startling at first, yet such would be the natural consequence of a
gradual and continuous sinking of the ground in an estuary or bay, into
which a great river discharged its turbid waters. By each foot of
subsidence, the fundamental rock would be depressed one foot farther
from the surface; but the bay would not be deepened, if newly-deposited
mud and sand should raise the bottom one foot. On the contrary, such
new strata of sand and mud might be frequently laid dry at low water,
or overgrown for a season by a vegetation proper to marshes.

Punfield Beds, Brackish and Marine.—The shells of the Wealden beds
belong to the genera _Melanopsis, Melania, Paludina, Cyrena, Cyclas,
Unio_ (see Fig. 294), and others, which inhabit rivers or lakes; but
one band has been found at Punfield, in Dorsetshire, indicating a
brackish state of the water, where the genera _Corbula, Mytilus,_ and
_Ostrea_ occur; and in some places this bed becomes purely marine,
containing some well-known Neocomian fossils, among which _Ammonites
Deshayesii_ (Fig. 284) may be mentioned. Others are peculiar as
British, but very characteristic of the Upper and Middle Neocomian of
Spain, and among these the _ Vicarya Lujani_ (Fig. 297), a shell allied
to Nerinea, is conspicuous.

By reference to table (p. 308) it will be seen that the Wealden beds
are given as the fresh-water equivalents of the Marine Neocomian. The
highest part of them in England may, for reasons just given, be
regarded as Upper Neocomian, while some of the inferior portions may
correspond in age to the Middle and Lower divisions of that group. In
favour of this latter view, M. Marcou mentions that a fish called
_Asteracanthus granulosus,_ occurring in the Tilgate beds, is
characteristic of the lowest beds of the Neocomian of the Jura, and it
is well known that _Corbula alata,_ common in the Ashburnham beds, is
found also at the base of the Neocomian of the Continent.

Fig. 297: Vicarya Lujani. Fig. 297: _Vicarya Lujani_, De Verneuil.[4]
Wealden, Punfield.

_Area of the Wealden._—In regard to the geographical extent of the
Wealden, it cannot be accurately laid down, because so much of it is
concealed beneath the newer marine formations. It has been traced about
320 English miles from west to east, from the coast of Dorsetshire to
near Boulogne, in France; and nearly 200 miles from north-west to
south-east, from Surrey and Hampshire to Vassy, in France. If the
formation be continuous throughout this space, which is very doubtful,
it does not follow that the whole was contemporaneous; because, in all
likelihood, the physical geography of the region underwent frequent
changes throughout the whole period, and the estuary may have altered
its form, and even shifted its place. Dr. Dunker, of Cassel, and H. von
Meyer, in an excellent monograph on the Wealdens of Hanover and
Westphalia, have shown that they correspond so closely, not only in
their fossils, but also in their mineral characters, with the English
series, that we can scarcely hesitate to refer the whole to one great
delta. Even then, the magnitude of the deposit may not exceed that of
many modern rivers. Thus, the delta of the Quorra or Niger, in Africa,
stretches into the interior for more than 170 miles, and occupies, it
is supposed, a space of more than 300 miles along the coast, thus
forming a surface of more than 25,000 square miles, or equal to about
one-half of England.[5] Besides, we know not, in such cases, how far
the fluviatile sediment and organic remains of the river and the land
may be carried out from the coast, and spread over the bed of the sea.
I have shown, when treating of the Mississippi, that a more ancient
delta, including species of shells such as now inhabit Louisiana, has
been upraised, and made to occupy a wide geographical area, while a
newer delta is forming;[6] and the possibility of such movements and
their effects must not be lost sight of when we speculate on the origin
of the Wealden.

It may be asked where the continent was placed, from the ruins of which
the Wealden strata were derived, and by the drainage of which a great
river was fed. If the Wealden was gradually going downward 1000 feet or
more perpendicularly, a large body of fresh-water would not continue to
be poured into the sea at the same point. The adjoining land, if it
participated in the movement, could not escape being submerged. But we
may suppose such land to have been stationary, or even undergoing
contemporaneous slow upheaval. There may have been an ascending
movement in one region, and a descending one in a contiguous parallel
zone of country. But even if that were the case, it is clear that
finally an extensive depression took place in that part of Europe where
the deep sea of the Cretaceous period was afterwards brought in.

_Thickness of the Wealden._—In the Weald area itself, between the North
and South Downs, fresh-water beds to the thickness of 1600 feet are
known, the base not being reached. Probably the thickness of the whole
Wealden series, as seen in Swanage Bay, cannot be estimated as less
than 2000 feet.

_Wealden Flora._—The flora of the Wealden is characterised by a great
abundance of Coniferæ, Cycadeæ, and Ferns, and by the absence of leaves
and fruits of Dicotyledonous Angiosperms. The discovery in 1855, in the
Hastings beds of the Isle of Wight, of Gyrogonites, or spore-vessels of
the Chara, was the first example of that genus of plants, so common in
the tertiary strata, being found in a Secondary or Mesozoic rock.

 [1] Judd, Speeton clay, Quart. Geol. Journ., vol. xxiv, 1868, p. 218.

 [2] Judd, Quart. Geol. Journ., 1867, vol. xxiii, p. 249.

 [3] Mantell, Geol. of S.E. of England, p. 244.

 [4] Foss. de Utrillas.

 [5] Fitton, Geol. of Hastings, p. 58, who cites Lander’s Travels.

 [6] See p. 102 and Second Visit to the United States, vol. ii, chap.
 xxxiv.




CHAPTER XIX.
JURASSIC GROUP.—PURBECK BEDS AND OOLITE.


The Purbeck Beds a Member of the Jurassic Group. — Subdivisions of that
Group. — Physical Geography of the Oolite in England and France. —
Upper Oolite. — Purbeck Beds. — New Genera of fossil Mammalia in the
Middle Purbeck of Dorsetshire. — Dirt-bed or ancient Soil. — Fossils of
the Purbeck Beds. — Portland Stone and Fossils. — Kimmeridge Clay. —
Lithographic Stone of Solenhofen. — Archæopteryx. — Middle Oolite. —
Coral Rag. — Nerinæa Limestone. — Oxford Clay, Ammonites and
Belemnites. — Kelloway Rock. — Lower, or Bath, Oolite. — Great Plants
of the Oolite. — Oolite and Bradford Clay. — Stonesfield Slate. —
Fossil Mammalia. — Fuller’s Earth. — Inferior Oolite and Fossils. —
Northamptonshire Slates. — Yorkshire Oolitic Coal-field. — Brora Coal.
— Palæontological Relations of the several Subdivisions of the Oolitic
group.

Classification of the Oolite.—Immediately below the Hastings Sands we
find in Dorsetshire another remarkable fresh-water formation, called
_the Purbeck,_ because it was first studied in the sea-cliffs of the
peninsula of Purbeck in that county. These beds are for the most part
of fresh-water origin, but the organic remains of some few intercalated
beds are marine, and show that the Purbeck series has a closer affinity
to the Oolitic group, of which it may be considered as the newest or
uppermost member.

In England generally, and in the greater part of Europe, both the
Wealden and Purbeck beds are wanting, and the marine cretaceous group
is followed immediately, in the descending order, by another series
called the Jurassic. In this term, the formations commonly designated
as “the Oolite and Lias” are included, both being found in the Jura
Mountains. The Oolite was so named because in the countries where it
was first examined the limestones belonging to it had an Oolitic
structure (see p. 37). These rocks occupy in England a zone nearly
thirty miles in average breadth, which extends across the island, from
Yorkshire in the north-east, to Dorsetshire in the south-west. Their
mineral characters are not uniform throughout this region; but the
following are the names of the principal subdivisions observed in the
central and south-eastern parts of England.

OOLITE

Upper _a._  Purbeck beds.
_b._  Portland stone and sand.
_c._  Kimmeridge clay. Middle _d._  Coral rag.
_e._  Oxford clay, and Kelloway rock. Lower _f._   Cornbrash and
Forest marble.
_g._  Great Oolite and Stonesfield slate.
_h._  Fuller’s earth.
_i._   Inferior Oolite.

The Upper Oolitic system of the above table has usually the Kimmeridge
clay for its base; the Middle Oolitic system, the Oxford clay. The
Lower system reposes on the Lias, an argillo-calcareous formation,
which some include in the Lower Oolite, but which will be treated of
separately in the next chapter. Many of these subdivisions are
distinguished by peculiar organic remains; and, though varying in
thickness, may be traced in certain directions for great distances,
especially if we compare the part of England to which the
above-mentioned type refers with the north-east of France and the Jura
Mountains adjoining. In that country, distant above 400 geographical
miles, the analogy to the accepted English type, notwithstanding the
thinness or occasional absence of the clays, is more perfect than in
Yorkshire or Normandy.

Physical Geography.—The alternation, on a grand scale, of distinct
formations of clay and limestone has caused the oolitic and liassic
series to give rise to some marked features in the physical outline of
parts of England and France. Wide valleys can usually be traced
throughout the long bands of country where the argillaceous strata crop
out; and between these valleys the limestones are observed, forming
ranges of hills or more elevated grounds. These ranges terminate
abruptly on the side on which the several clays rise up from beneath
the calcareous strata.

Fig. 298: Configuration of surface.

Fig. 298 will give the reader an idea of the configuration of the
surface now alluded to, such as may be seen in passing from London to
Cheltenham, or in other parallel lines, from east to west, in the
southern part of England. It has been necessary, however, in this
drawing, greatly to exaggerate the inclination of the beds, and the
height of the several formations, as compared to their horizontal
extent. It will be remarked, that the lines of steep slope, or
escarpment, face towards the west in the great calcareous eminences
formed by the chalk and the Upper, Middle, and Lower Oolites; and at
the base of which we have respectively the Gault, Kimmeridge clay,
Oxford clay, and Lias. This last forms, generally, a broad vale at the
foot of the escarpment of inferior Oolite, but where it acquires
considerable thickness, and contains solid beds of marlstone, it
occupies the lower part of the escarpment.

The external outline of the country which the geologist observes in
travelling eastward from Paris to Metz, is precisely analogous, and is
caused by a similar succession of rocks intervening between the
tertiary strata and the Lias; with this difference, however, that the
escarpments of Chalk, Upper, Middle, and Lower Oolites face towards the
east instead of the west. It is evident, therefore, that the denuding
causes (see p. 105) have acted similarly over an area several hundred
miles in diameter, removing the softer clays more extensively than the
limestones, and causing these last to form steep slopes or escarpments
wherever the harder calcareous rock was based upon a more yielding and
destructible formation.

UPPER OOLITE.

Purbeck Beds.—These strata, which we class as the uppermost member of
the Oolite, are of limited geographical extent in Europe, as already
stated, but they acquire importance when we consider the succession of
three distinct sets of fossil remains which they contain. Such repeated
changes in organic life must have reference to the history of a vast
lapse of ages. The Purbeck beds are finely exposed to view in
Durdlestone Bay, near Swanage, Dorsetshire, and at Lulworth Cove and
the neighbouring bays between Weymouth and Swanage. At Meup’s Bay, in
particular, Professor E. Forbes examined minutely, in 1850, the organic
remains of this group, displayed in a continuous sea-cliff section, and
it appears from his researches that the Upper, Middle, and Lower
Purbecks are each marked by peculiar species of organic remains, these
again being different, so far as a comparison has yet been instituted,
from the fossils of the overlying Hastings Sands and Weald Clay.

_Upper Purbeck._—The highest of the three divisions is purely
fresh-water, the strata, about fifty feet in thickness, containing
shells of the genera _Paludina, Physa, Limnæa, Planorbis, Valvata,
Cyclas,_ and _Unio,_ with _ Cyprides_ and fish. All the species seem
peculiar, and among these the _Cyprides_ are very abundant and
characteristic (see Fig. 299, _a, b, c._)

The stone called “Purbeck Marble,” formerly much used in ornamental
architecture in the old English cathedrals of the southern counties, is
exclusively procured from this division.

Fig. 299: Cyprides from the Upper Purbecks.

_Middle Purbeck._—Next in succession is the Middle Purbeck, about
thirty feet thick, the uppermost part of which consists of fresh-water
limestone, with cyprides, turtles, and fish, of different species from
those in the preceding strata. Below the limestone are brackish-water
beds full of _Cyrena,_ and traversed by bands abounding in _Corbula_
and _ Melania._ These are based on a purely marine deposit, with _
Pecten, Modiola, Avicula,_ and _Thracia._ Below this, again, come
limestones and shales, partly of brackish and partly of fresh-water
origin, in which many fish, especially species of _ Lepidotus_ and
_Microdon radiatus,_ are found, and a crocodilian reptile named
_Macrorhynchus._ Among the mollusks, a remarkable ribbed _Melania,_ of
the section _Chilina,_ occurs.

Fig. 300: Ostrea distorta. Fig. 301: Hemicidaris Purbeckensis.

Immediately below is a great and conspicuous stratum, twelve feet
thick, formed of a vast accumulation of shells of _Ostrea distorta_
(Fig. 300), long familiar to geologists under the local name of
“Cinder-bed.” In the uppermost part of this bed Professor Forbes
discovered the first echinoderm (Fig. 301) as yet known in the Purbeck
series, a species of _ Hemicidaris,_ a genus characteristic of the
Oolitic period, and scarcely, if at all, distinguishable from a
previously known Oolitic fossil. It was accompanied by a species of
_Perna._ Below the Cinder-bed fresh-water strata are again seen, filled
in many places with species of _Cypris_ (Fig. 302, _a, b, c_), and with
_Valvata, Paludina, Planorbis, Limnæa, Physa_ (Fig. 303), and _Cyclas,_
all different from any occurring higher in the series. It will be seen
that _Cypris fasciculata_ (Fig. 302, _b_) has tubercles at the end only
of each valve, a character by which it can be immediately recognised.
In fact, these minute crustaceans, almost as frequent in some of the
shales as plates of mica in a micaceous sandstone, enable geologists at
once to identify the Middle Purbeck in places far from the Dorsetshire
cliffs, as, for example, in the Vale of Wardour in Wiltshire. Thick
beds of chert occur in the Middle Purbeck filled with mollusca and
cyprides of the genera already enumerated, in a beautiful state of
preservation, often converted into chalcedony. Among these Professor
Forbes met with gyrogonites (the spore-vessels of _Chara_), plants
never until 1851 discovered in rocks older than the Eocene. About
twenty feet below the “Cinder-bed” is a stratum two or three inches
thick, in which fossil mammalia presently to be mentioned occur, and
beneath this a thin band of greenish shales, with marine shells and
impressions of leaves like those of a large _Zostera,_ forming the base
of the Middle Purbeck.

Fig. 302: Cyprides from the Middle Purbecks.

Fig. 303: Physa Bristovii

_Fossil Mammalia of the Middle Purbeck._—In 1852,[1] after alluding to
the discovery of numerous insects and air-breathing mollusca in the
Purbeck strata, I remarked that, although no mammalia had then been
found, “it was too soon to infer their non-existence on mere negative
evidence.” Only two years after this remark was in print, Mr. W. R.
Brodie found in the Middle Purbeck, about twenty feet below the
“Cinder-bed” above alluded to, in Durdlestone Bay, portions of several
small jaws with teeth, which Professor Owen recognised as belonging to
a small mammifer of the insectivorous class, more closely allied in its
dentition to the _ Amphitherium_ (or _Thylacotherium_) than to any
existing type.

Four years later (in 1856) the remains of several other species of
warm-blooded quadrupeds were exhumed by Mr. S. H. Beckles, F.R.S., from
the same thin bed of marl near the base of the Middle Purbeck. In this
marly stratum many reptiles, several insects, and some fresh-water
shells of the genera _Paludina, Planorbis,_ and _Cyclas,_ were found.

Mr. Beckles had determined thoroughly to explore the thin layer of
calcareous mud from which in the suburbs of Swanage the bones of the
Spalacotherium had already been obtained, and in three weeks he brought
to light from an area forty feet long and ten wide, and from a layer
the average thickness of which was only five inches, portions of the
skeletons of six new species of mammalia, as interpreted by Dr.
Falconer, who first examined them. Before these interesting inquiries
were brought to a close, the joint labours of Professor Owen and Dr.
Falconer had made it clear that twelve or more species of mammalia
characterised this portion of the Middle Purbeck, most of them
insectivorous or predaceous, varying in size from that of a mole to
that of the common polecat, _Mustela putorius._ While the majority had
the character of insectivorous marsupials, Dr. Falconer selected one as
differing widely from the rest, and pointed out that in certain
characters it was allied to the living Kangaroo-rat, or _Hypsiprymnus,_
ten species of which now inhabit the prairies and scrub-jungle of
Australia, feeding on plants, and gnawing scratched-up roots. A
striking peculiarity of their dentition, one in which they differ from
all other quadrupeds, consists in their having a single large
pre-molar, the enamel of which is furrowed with vertical grooves,
usually seven in number.

The largest pre-molar (see Fig. 305) in the fossil genus exhibits in
like manner seven parallel grooves, producing by their termination a
similar serrated edge in the crown; but their direction is diagonal—a
distinction, says Dr. Falconer, which is “trivial, not typical.” As
these oblique furrows form so marked a character of the majority of the
teeth, Dr. Falconer gave to the fossil the generic name of _
Plagiaulax._ The shape and relative size of the incisor, _ a,_ Fig.
306, exhibit a no less striking similarity to Hypsiprymnus.
Nevertheless, the more sudden upward curve of this incisor, as well as
other characters of the jaw, indicate a great deviation in the form of
Plagiaulax from that of the living kangaroo-rats.

Fig. 304: Pre-molar of the recent Australian Hypsiprymnus Gaimardi,
showing 7 grooves at right angles to the length of the jaw. Fig. 305:
Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii,
showing 7 diagonal grooves.

Fig. 306: Plagiaulax Becklessi. Right ramus of lower jaw.
There are two fossil specimens of lower jaws of this genus evidently
referable to two distinct species extremely unequal in size and
otherwise distinguishable. The _Plagiaulax Becklesii_ (Fig. 306) was
about as big as the English squirrel or the flying phalanger of
Australia (_Petaurus Australis,_ Waterhouse). The smaller fossil,
having only half the linear dimensions of the other, was probably only
one-twelfth of its bulk. It is of peculiar geological interest,
because, as shown by Dr. Falconer, its two back molars bear a decided
resemblance to those of the Triassic _ Microlestes_ (Fig. 389), the
most ancient of known mammalia, of which an account will be given in
Chapter XXI.

Up to 1857 all the mammalian remains discovered in secondary rocks had
consisted solely of single branches of the lower jaw, but in that year
Mr. Beckles obtained the upper portion of a skull, and on the same slab
the lower jaw of another quadruped with eight molars, a large canine,
and a broad and thick incisor. It has been named Triconodon from its
bicuspid teeth, and is supposed to have been a small insectivorous
marsupial, about the size of a hedgehog. Other jaws have since been
found indicating a larger species of the same genus.

Professor Owen has proposed the name of _Galestes_ for the largest of
the mammalia discovered in 1858 in Purbeck, equalling the polecat
(_Mustela putorius_) in size. It is supposed to have been predaceous
and marsupial.

Between forty and fifty pieces or sides of lower jaws with teeth have
been found in oolitic strata in Purbeck; only five upper maxillaries,
together with one portion of a separate cranium, occur at Stonesfield,
and it is remarkable that with these there were no examples in Purbeck
of an entire skeleton, nor of any considerable number of bones in
juxtaposition. In several portions of the matrix there were detached
bones, often much decomposed, and fragments of others apparently
mammalian; but if all of them were restored, they would scarcely
suffice to complete the five skeletons to which the five upper
maxillaries above alluded to belonged. As the average number of pieces
in each mammalian skeleton is about 250, there must be many thousands
of missing bones; and when we endeavour to account for their absence,
we are almost tempted to indulge in speculations like those once
suggested to me by Dr. Buckland, when he tried to solve the enigma in
reference to Stonesfield; “The corpses,” he said, “of drowned animals,
when they float in a river, distended by gases during putrefaction,
have often their lower jaw hanging loose, and sometimes it has dropped
off. The rest of the body may then be drifted elsewhere, and sometimes
may be swallowed entire by a predaceous reptile or fish, such as an
ichthyosaur or a shark.”

As all the above-mentioned Purbeck marsupials, belonging to eight or
nine genera and to about fourteen species, insectivorous, predaceous,
and herbivorous, have been obtained from an area less than 500 square
yards in extent, and from a single stratum no more than a few inches
thick, we may safely conclude that the whole lived together in the same
region, and in all likelihood they constituted a mere fraction of the
mammalia which inhabited the lands drained by one river and its
tributaries. They afford the first positive proof as yet obtained of
the co-existence of a varied fauna of the highest class of vertebrata
with that ample development of reptile life which marks all the periods
from the Trias to the Lower Cretaceous inclusive, and with a
gymnospermous flora, or that state of the vegetable kingdom when cycads
and conifers predominated over all kinds of plants, except the ferns,
so far, at least, as our present imperfect knowledge of fossil botany
entitles us to speak.

The following table will enable the reader to see at a glance how
conspicuous a part, numerically considered, the mammalian species of
the Middle Purbeck now play when compared with those of other
formations more ancient than the Paris gypsum, and, at the same time,
it will help him to appreciate the enormous hiatus in the history of
fossil mammalia which at present occurs between the Eocene and Purbeck
periods, and between the latter and the Stonesfield Oolite, and between
this again and the Trias.

_Number and Distribution of all the known Species of Fossil Mammalia
from Strata older than the Paris Gypsum, or than the Bembridge Series
of the Isle of Wight._

TERTIARY Headon Series and beds between the Paris Gypsum and the
Grès de Beauchamp 14 10 English
   4 French Barton Clay and Sables de Beauchamp 0 Bagshot Beds,
   Calcaire Grossier, and Upper Soissonnais of
   Cuisse-Lamotte 20 16 French
   1 English
   3 U. States[2] London Clay, including the Kyson Sand 7 English
   Plastic Clay and Lignite 9 7 French
2 English Sables de Bracheux 1 French Thanet Sands and Lower
Landenian of Belgium 0
SECONDARY Maestricht Chalk 0 White Chalk 0 Chalk Marl 0
Chloritic Series (Upper Greensand) 0 Gault 0 Neocomian (Lower
Greensand) 0 Wealden 0 Upper Purbeck Oolite 0 Middle Purbeck
Oolite 14 Swanage Lower Purbeck Oolite 0 Portland Oolite 0
Kimmeridge Clay 0 Coral Rag 0 Oxford Clay 0 Great
Oolite 4 Stonesfield Inferior Oolite 0 Lias 0 Upper
Trias 4 Wurtemberg
Somersetshire
N. Carolina Middle Trias 0 Lower Trias 0
PRIMARY Permian 0 Carboniferous 0 Devonian 0 Silurian 0
Cambrian 0 Laurentian 0

The Sables de Bracheux, enumerated in the Tertiary division of the
table, supposed by Mr. Prestwich to be somewhat newer than the Thanet
Sands, and by M. Hébert to be of about that age, have yielded at La
Fere the _Arctocyon (Palæocyon) primævus,_ the oldest known tertiary
mammal.

It is worthy of notice, that in the Hastings Sands there are certain
layers of clay and sandstone in which numerous footprints of quadrupeds
have been found by Mr. Beckles, and traced by him in the same set of
rocks through Sussex and the Isle of Wight. They appear to belong to
three or four species of reptiles, and no one of them to any
warm-blooded quadruped. They ought, therefore, to serve as a warning to
us, when we fail in like manner to detect mammalian footprints in older
rocks (such as the New Red Sandstone), to refrain from inferring that
quadrupeds, other than reptilian, did not exist or pre-exist.

But the most instructive lesson read to us by the Purbeck strata
consists in this: They are all, with the exception of a few
intercalated brackish and marine layers, of fresh-water origin; they
are 160 feet in thickness, have been well searched by skillful
collectors, and by the late Edward Forbes in particular, who studied
them for months consecutively. They have been numbered, and the
contents of each stratum recorded separately, by the officers of the
Geological Survey of Great Britain. They have been divided into three
distinct groups by Forbes, each characterised by the same genera of
pulmoniferous mollusca and cyprides, these genera being represented in
each group by different species; they have yielded insects of many
orders, and the fruits of several plants; and lastly, they contain
“dirt-beds,” or old terrestrial surfaces and vegetable soils at
different levels, in some of which erect trunks and stumps of cycads
and conifers, with their roots still attached to them, are preserved.
Yet when the geologist inquires if any land-animals of a higher grade
than reptiles lived during any one of these three periods, the rocks
are all silent, save one thin layer a few inches in thickness; and this
single page of the earth’s history has suddenly revealed to us in a few
weeks the memorials of so many species of fossil mammalia, that they
already outnumber those of many a subdivision of the tertiary series,
and far surpass those of all the other secondary rocks put together!

_Lower Purbeck._—Beneath the thin marine band mentioned at p. 324 as
the base of the Middle Purbeck, some purely fresh-water marls occur,
containing species of _Cypris_ (Fig. 307 _a, c_), _Valvata,_ and
_Limnæa,_ different from those of the Middle Purbeck. This is the
beginning of the inferior division, which is about 80 feet thick. Below
the marls are seen, at Meup’s Bay, more than thirty feet of
brackish-water strata, abounding in a species of _Serpula,_ allied to,
if not identical with, _Serpula coacervites,_ found in beds of the same
age in Hanover. There are also shells of the genus _ Rissoa_ (of the
subgenus _Hydrobia_), and a little _ Cardium_ of the subgenus
_Protocardium,_ in these marine beds, together with _Cypris._ Some of
the cypris-bearing shales are strangely contorted and broken up, at the
west end of the Isle of Purbeck. The great dirt-bed or vegetable soil
containing the roots and stools of _Cycadeæ,_ which I shall presently
describe, underlies these marls, and rests upon the lowest fresh-water
limestone, a rock about eight feet thick, containing _Cyclas, Valvata,_
and _Limnæa,_ of the same species as those of the uppermost part of the
Lower Purbeck, or above the dirt-bed. The fresh-water limestone in its
turn rests upon the top beds of the Portland stone, which, although it
contains purely marine remains, often consists of a rock
undistinguishable in mineral character from the Lowest Purbeck
limestone.

Fig. 307: Cyprides from the Lower Purbeck.
Fig. 308: Mantellia nidiformis.
_Dirt-bed or ancient Surface-soil._—The most remarkable of all the
varied succession of beds enumerated in the above list is that called
by the quarrymen “the dirt,” or “black dirt,” which was evidently an
ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark
brown or black colour, and contains a large proportion of earthy
lignite. Through it are dispersed rounded and sub-angular fragments of
stone, from 3 to 9 inches in diameter, in such numbers that it almost
deserves the name of gravel. I also saw in 1866, in Portland, a smaller
dirt-bed six feet below the principal one, six inches thick, consisting
of brown earth with upright _Cycads_ of the same species, _Mantellia
nidiformis,_ as those found in the upper bed, but no _Coniferæ._ The
weight of the incumbent strata squeezing down the compressible dirt-bed
has caused the _Cycads_ to assume that form which has led the quarrymen
to call them “petrified birds’ nests,” which suggested to Brongniart
the specific name of _nidiformis._ I am indebted to Mr. Carruthers for
Figure 308 of one of these Purbeck specimens, in which the original
cylindrical figure has been less distorted than usual by pressure.

Many silicified trunks of coniferous trees, and the remains of plants
allied to _Zamia_ and _Cycas,_ are buried in this dirt-bed, and must
have become fossil on the spots where they grew. The stumps of the
trees stand erect for a height of from one to three feet, and even in
one instance to six feet, with their roots attached to the soil at
about the same distances from one another as the trees in a modern
forest. The carbonaceous matter is most abundant immediately around the
stumps, and round the remains of fossil _Cycadeæ._

Besides the upright stumps above mentioned, the dirt-bed contains the
stems of silicified trees laid prostrate. These are partly sunk into
the black earth, and partly enveloped by a calcareous slate which
covers the dirt-bed. The fragments of the prostrate trees are rarely
more than three or four feet in length; but by joining many of them
together, trunks have been restored, having a length from the root to
the branches of from 20 to 23 feet, the stems being undivided for 17 or
20 feet, and then forked. The diameter of these near the root is about
one foot; but I measured one myself, in 1866, which was 3½ feet in
diameter, said by the quarrymen to be unusually large. Root-shaped
cavities were observed by Professor Henslow to descend from the bottom
of the dirt-bed into the subjacent fresh-water stone, which, though now
solid, must have been in a soft and penetrable state when the trees
grew. The thin layers of calcareous slate (Fig. 309) were evidently
deposited tranquilly, and would have been horizontal but for the
protrusion of the stumps of the trees, around the top of each of which
they form hemispherical concretions.

Fig. 309: Section in Isle of Portland, Dorset.

The dirt-bed is by no means confined to the island of Portland, where
it has been most carefully studied, but is seen in the same relative
position in the cliffs east of Lulworth Cove, in Dorsetshire, where, as
the strata have been disturbed, and are now inclined at an angle of
45°, the stumps of the trees are also inclined at the same angle in an
opposite direction—a beautiful illustration of a change in the position
of beds originally horizontal (see Fig. 310).

Fig. 310: Section of cliff east of Lulworth Cove.

From the facts above described we may infer, first, that those beds of
the Upper Oolite, called “the Portland,” which are full of marine
shells, were overspread with fluviatile mud, which became dry land, and
covered by a forest, throughout a portion of the space now occupied by
the south of England, the climate being such as to permit the growth of
the _Zamia_ and _Cycas._ Secondly. This land at length sank down and
was submerged with its forests beneath a body of fresh-water, from
which sediment was thrown down enveloping fluviatile shells. Thirdly.
The regular and uniform preservation of this thin bed of black earth
over a distance of many miles, shows that the change from dry land to
the state of a fresh-water lake or estuary, was not accompanied by any
violent denudation, or rush of water, since the loose black earth,
together with the trees which lay prostrate on its surface, must
inevitably have been swept away had any such violent catastrophe taken
place.

The forest of the dirt-bed, as before hinted, was not everywhere the
first vegetation which grew in this region. Besides the lower bed
containing upright _Cycadeæ,_ before mentioned, another has sometimes
been found above it, which implies oscillations in the level of the
same ground, and its alternate occupation by land and water more than
once.

_Subdivisions of the Purbeck._—It will be observed that the division of
the Purbecks into upper, middle, and lower, was made by Professor
Forbes strictly on the principle of the entire distinctness of the
species of organic remains which they include. The lines of demarkation
are not lines of disturbance, nor indicated by any striking physical
characters or mineral changes. The features which attract the eye in
the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth,
and the Cinder-bed, do not indicate any breaks in the distribution of
organised beings. “The causes which led to a complete change of life
three times during the deposition of the fresh-water and brackish
strata must,” says this naturalist, “be sought for, not simply in
either a rapid or a sudden change of their area into land or sea, but
in the great lapse of time which intervened between the epochs of
deposition at certain periods during their formation.”

Each dirt-bed may, no doubt, be the memorial of many thousand years or
centuries, because we find that two or three feet of vegetable soil is
the only monument which many a tropical forest has left of its
existence ever since the ground on which it now stands was first
covered with its shade. Yet, even if we imagine the fossil soils of the
Lower Purbeck to represent as many ages, we need not be surprised to
find that they do not constitute lines of separation between strata
characterised by different zoological types. The preservation of a
layer of vegetable soil, when in the act of being submerged, must be
regarded as a rare exception to a general rule. It is of so perishable
a nature, that it must usually be carried away by the denuding waves or
currents of the sea, or by a river; and many Purbeck dirt-beds were
probably formed in succession and annihilated, besides those few which
now remain.

The plants of the Purbeck beds, so far as our knowledge extends at
present, consist chiefly of Ferns, Coniferæ, and Cycadeæ (Fig. 308),
without any angiosperms; the whole more allied to the Oolitic than to
the Cretaceous vegetation. The same affinity is indicated by the
vertebrate and invertebrate animals. Mr. Brodie has found the remains
of beetles and several insects of the homopterous and trichopterous
orders, some of which now live on plants, while others are of such
forms as hover over the surface of our present rivers.

Portland Oolite and Sand (_b,_ Table p. 321).—The Portland Oolite has
already been mentioned as forming in Dorsetshire the foundation on
which the fresh-water limestone of the Lower Purbeck reposes (see p.
331). It supplies the well-known building-stone of which St. Paul’s and
so many of the principal edifices of London are constructed. About
fifty species of mollusca occur in this formation, among which are some
ammonites of large size. The cast of a spiral univalve called by the
quarrymen the “Portland screw” (_a,_ Figure 311), is common; the shell
of the same (_b_) being rarely met with. Also _Trigonia gibbosa_ (Fig.
313) and _Cardium dissimile_ (Fig. 314). This upper member rests on a
dense bed of sand, called the Portland Sand, containing similar marine
fossils, below which is the Kimmeridge Clay. In England these Upper
Oolite formations are almost wholly confined to the southern counties.
But some fragments of them occur beneath the Neocomian or Speeton Clay
on the coast of Yorkshire, containing many more fossils common to the
Portlandian of the Continent than does the same formation in
Dorsetshire. Corals are rare in this formation, although one species is
found plentifully at Tisbury, Wiltshire, in the Portland Sand,
converted into flint and chert, the original calcareous matter being
replaced by silex (Fig. 312).

Fig. 311: Cerithium Portlandicum.
Fig. 312: Isastræa oblonga. Fig. 313: Trigonia gibbosa.

Kimmeridge Clay.—The _Kimmeridge Clay_ consists, in great part, of a
bituminous shale, sometimes forming an impure coal, several hundred
feet in thickness. In some places in Wiltshire it much resembles peat;
and the bituminous matter may have been, in part at least, derived from
the decomposition of vegetables. But as impressions of plants are rare
in these shales, which contain ammonites, oysters, and other marine
shells, with skeletons of fish and saurians, the bitumen may perhaps be
of animal origin. Some of the saurians (Pliosaurus) in Dorsetshire are
among the most gigantic of their kind.

Fig. 314: Cardium dissimile. Fig. 315: Ostrea expansa. Fig. 316:
Cardium striatulum. Fig. 317: Ostrea deltoidea. Fig. 318: Gryphæa
(Exogyra) virgula.

Among the fossils, amounting to nearly 100 species, may be mentioned
_Cardium striatulum_ (Fig. 316) and _Ostrea deltoidea_ (Fig. 317), the
latter found in the Kimmeridge Clay throughout England and the north of
France, and also in Scotland, near Brora. The _Gryphæa virgula_ (Fig.
318), also met with in the Kimmeridge Clay near Oxford, is so abundant
in the Upper Oolite of parts of France as to have caused the deposit to
be termed “marnes à gryphées virgules.” Near Clermont, in Argonne, a
few leagues from St. Menehould, where these indurated marls crop out
from beneath the Gault, I have seen them, on decomposing, leave the
surface of every ploughed field literally strewed over with this fossil
oyster.

Fig. 319: Trigonellites latus.
The _Trigonellites latus_ (_Aptychus_ of some authors) (Fig. 319) is
also widely dispersed through this clay. The real nature of the shell,
of which there are many species in oolitic rocks, is still a matter of
conjecture. Some are of opinion that the two plates have been the
gizzard of a cephalopod; others, that it may have formed a bivalve
operculum of the same.

Solenhofen Stone.—The celebrated lithographic stone of Solenhofen in
Bavaria, appears to be of intermediate age between the Kimmeridge clay
and the Coral Rag, presently to be described. It affords a remarkable
example of the variety of fossils which may be preserved under
favourable circumstances, and what delicate impressions of the tender
parts of certain animals and plants may be retained where the sediment
is of extreme fineness. Although the number of testacea in this slate
is small, and the plants few, and those all marine, count Munster had
determined no less than 237 species of fossils when I saw his
collection in 1833; and among them no less than seven _species_ of
flying reptiles or pterodactyls (see Fig. 320), six saurians, three
tortoises, sixty species of fish, forty-six of crustacea, and
twenty-six of insects. These insects, among which is a libellula, or
dragon-fly, must have been blown out to sea, probably from the same
land to which the pterodactyls, and other contemporaneous
air-breathers, resorted.

Fig. 320: Skeleton of Pterodactylus crassirostris.

In the same slate of Solenhofen a fine example was met with in 1862 of
the skeleton of a bird almost entire, and retaining even its feathers
so perfect that the vanes as well as the shaft are preserved. The head
was at first supposed to be wanting, but Mr. Evans detected on the slab
what seems to be the impression of the cranium and beak, much
resembling in size and shape that of the jay or woodcock. This valuable
specimen is now in the British Museum, and has been called by Professor
Owen _Archæopteryx macrura._ Although anatomists agree that it is a
true bird, yet they also find that in the length of the bones of the
tail, and some other minor points of its anatomy, it approaches more
nearly to reptiles than any known living bird. In the living
representatives of the class Aves, the tail-feathers are attached to a
coccygian bone, consisting of several vertebræ united together, whereas
in the Archæopteryx the tail is composed of twenty vertebræ, each of
which supports a pair of quill-feathers. The first five only of the
vertebræ, as seen in A, have transverse processes, the fifteen
remaining ones become gradually longer and more tapering. The feathers
diverge outward from them at an angle of 45°.

Fig. 321: Tail and feather of Archæopteryx, from Solenhofen, and tail
of living bird for comparison.

Professor Huxley in his late memoirs on the order of reptiles called
Dinosaurians, which are largely represented in all the formations, from
the Neocomian to the Trias inclusive, has shown that they present in
their structure many remarkable affinities to birds. But a reptile
about two feet long, called Compsognathus, lately found in the
Stonesfield slate, makes a much greater approximation to the class Aves
than any Dinosaur, and therefore forms a closer link between the
classes Aves and Reptilia than does the Archæopteryx.

It appears doubtful whether any species of British fossil, whether of
the vertebrate or invertebrate class, is common to the Oolite and
Chalk. But there is no similar break or discordance as we proceed
downward, and pass from one to another of the several leading members
of the Jurassic group, the Upper, Middle, and Lower Oolite, and the
Lias, there being often a considerable proportion of the mollusca,
sometimes as much as a fourth, common to such divisions as the Upper
and Middle Oolite.

MIDDLE OOLITE.

Coral Rag.—One of the limestones of the Middle Oolite has been called
the “Coral Rag,” because it consists, in part, of continuous beds of
petrified corals, most of them retaining the position in which they
grew at the bottom of the sea. In their forms they more frequently
resemble the reef-building polyparia of the Pacific than do the corals
of any other member of the Oolite. They belong chiefly to the genera
_Thecosmilia_ (Fig. 322), _Protoseris,_ and _Thamnastræa,_ and
sometimes form masses of coral fifteen feet thick.

Fig. 322: Thecosmilia annularis. Fig. 323: Thamnastræa.

Fig. 324: Ostrea gregaria. Fig. 325: Nerinæa Goodhallii.
In Fig. 323 of a _Thamnastræa_ from this formation, it will be seen
that the cup-shaped cavities are deepest on the right-hand side, and
that they grow more and more shallow, until those on the left side are
nearly filled up. The last-mentioned stars are supposed to represent a
perfected condition, and the others an immature state. These coralline
strata extend through the calcareous hills of the north-west of
Berkshire, and north of Wilts, and again recur in Yorkshire, near
Scarborough. The _ Ostrea gregarea_ (Fig. 324) is very characteristic
of the formation in England and on the Continent.

One of the limestones of the Jura, referred to the age of the English
Coral Rag, has been called “Nerinæan limestone” (Calcaire à Nérinées)
by M. Thirria; _Nerinæa_ being an extinct genus of univalve shells
(Fig. 325) much resembling the _Cerithium_ in external form. The
section shows the curious and continuous ridges on the columnella and
whorls.

Oxford Clay.—The coralline limestone, or “Coral Rag,” above described,
and the accompanying sandy beds, called “calcareous grits,” of the
Middle Oolite, rest on a thick bed of clay, called the “Oxford Clay,”
sometimes not less than 600 feet thick. In this there are no corals,
but great abundance of cephalopoda, of the genera Ammonite and
Belemnite (Figs. 326 and 327). In some of the finely laminated clays
ammonites are very perfect, although somewhat compressed, and are
frequently found with the lateral lobe extended on each side of the
opening of the mouth into a horn-like projection (Figure 327). These
were discovered in the cuttings of the Great Western Railway, near
Chippenham, in 1841, and have been described by Mr. Pratt (_An. Nat.
Hist.,_ Nov., 1841).

Fig. 326: Belemnites hastatus. Fig. 327: Ammonites Jason.

Similar elongated processes have been also observed to extend from the
shells of some Belemnites discovered by Dr. Mantell in the same clay
(see Figure 328), who, by the aid of this and other specimens, has been
able to throw much light on the structure of singular extinct forms of
cuttle-fish.[3]

Kelloway Rock.—The arenaceous limestone which passes under this name is
generally grouped as a member of the Oxford clay, in which it forms, in
the south-west of England, lenticular masses, 8 or 10 feet thick,
containing at Kelloway, in Wiltshire, numerous casts of ammonites and
other shells. But in Yorkshire this calcareo-arenaceous formation
thickens to about 30 feet, and constitutes the lower part of the Middle
Oolite, extending inland from Scarborough in a southerly direction. The
number of mollusca which it contains is, according to Mr. Etheridge,
143, of which only 34, or 23½ per cent, are common to the Oxford clay
proper. Of the 52 Cephalopoda, 15 (namely 13 species of ammonite, the
_Ancyloceras Calloviense_ and one Belemnite) are common to the Oxford
Clay, giving a proportion of nearly 30 per cent.

Fig. 328: Belemnites Puzosianus.

LOWER OOLITE.

Cornbrash and Forest Marble.—The upper division of this series, which
is more extensive than the preceding or Middle Oolite, is called in
England the Cornbrash, as being a brashy, easily broken rock, good for
corn land. It consists of clays and calcareous sandstones, which pass
downward into the Forest Marble, an argillaceous limestone, abounding
in marine fossils. In some places, as at Bradford, this limestone is
replaced by a mass of clay. The sandstones of the Forest Marble of
Wiltshire are often ripple-marked and filled with fragments of broken
shells and pieces of drift-wood, having evidently been formed on a
coast. Rippled slabs of fissile oolite are used for roofing, and have
been traced over a broad band of country from Bradford in Wilts, to
Tetbury in Gloucestershire. These calcareous tile-stones are separated
from each other by thin seams of clay, which have been deposited upon
them, and have taken their form, preserving the undulating ridges and
furrows of the sand in such complete integrity, that the impressions of
small footsteps, apparently of crustaceans, which walked over the soft
wet sands, are still visible. In the same stone the claws of crabs,
fragments of echini, and other signs of a neighbouring beach, are
observed.[4]

Great (or Bath) Oolite.—Although the name of Coral Rag has been
appropriated, as we have seen, to a member of the Middle Oolite before
described, some portions of the Lower Oolite are equally entitled in
many places to be called coralline limestones. Thus the Great Oolite
near Bath contains various corals, among which the _Eunomia radiata_
(Fig. 329) is very conspicuous, single individuals forming masses
several feet in diameter; and having probably required, like the large
existing brain-coral (_Meandrina_) of the tropics, many centuries
before their growth was completed.

Fig. 329: Eunomia radiata.

Different species of crinoids, or stone-lilies, are also common in the
same rocks with corals; and, like them, must have enjoyed a firm
bottom, where their base of attachment remained undisturbed for years
(_c,_ Fig. 330). Such fossils, therefore, are almost confined to the
limestones; but an exception occurs at Bradford, near Bath, where they
are enveloped in clay sometimes 60 feet thick. In this case, however,
it appears that the solid upper surface of the “Great Oolite” had
supported, for a time, a thick submarine forest of these beautiful
zoophytes, until the clear and still water was invaded by a current
charged with mud, which threw down the stone-lilies, and broke most of
their stems short off near the point of attachment. The stumps still
remain in their original position; but the numerous articulations, once
composing the stem, arms, and body of the encrinite, were scattered at
random through the argillaceous deposit in which some now lie
prostrate. These appearances are represented in the section _b,_ Fig.
330, where the darker strata represent the Bradford clay, which is
however a formation of such local development that in many places it
cannot easily be separated from the clays of the overlying
“forest-marble” and underlying “fuller’s earth.” The upper surface of
the calcareous stone below is completely incrusted over with a
continuous pavement, formed by the stony roots or attachments of the
Crinoidea; and besides this evidence of the length of time they had
lived on the spot, we find great numbers of single joints, or circular
plates of the stem and body of the encrinite, covered over with
_serpulæ._ Now these _serpulæ_ could only have begun to grow after the
death of some of the stone-lilies, parts of whose skeletons had been
strewed over the floor of the ocean before the irruption of
argillaceous mud. In some instances we find that, after the parasitic
_serpulæ_ were full grown, they had become incrusted over with a
bryozoan, called _Diastopora diluviana_ (see _b,_ Fig. 331); and many
generations of these molluscoids had succeeded each other in the pure
water before they became fossil.

Fig. 330: Apiocrinites rotundus, or Pear Eucrinite.

Fig. 331: a. Aingle plate of body of Apiocrinus, overgrown with serpulæ
and bryozoa; b. Portion of same magnified, showing the bryozoan
Diastopora diluviana covering one of the serpulæ.

We may, therefore, perceive distinctly that, as the pines and cycadeous
plants of the ancient “dirt-bed,” or fossil forest, of the Lower
Purbeck were killed by submergence under fresh water, and soon buried
beneath muddy sediment, so an invasion of argillaceous matter put a
sudden stop to the growth of the Bradford Encrinites, and led to their
preservation in marine strata.

Such differences in the fossils as distinguish the calcareous and
argillaceous deposits from each other, would be described by
naturalists as arising out of a difference in the _stations_ of
species; but besides these, there are variations in the fossils of the
higher, middle, and lower part of the oolitic series, which must be
ascribed to that great law of change in organic life by which distinct
assemblages of species have been adapted, at successive geological
periods, to the varying conditions of the habitable surface. In a
single district it is difficult to decide how far the limitation of
species to certain minor formations has been due to the local influence
of _stations,_ or how far it has been caused by time or the law of
variation above alluded to. But we recognise the reality of the
last-mentioned influence, when we contrast the whole oolitic series of
England with that of parts of the Jura, Alps, and other distant
regions, where, although there is scarcely any lithological
resemblance, yet some of the same fossils remain peculiar in each
country to the Upper, Middle, and Lower Oolite formations respectively.
Mr. Thurmann has shown how remarkably this fact holds true in the
Bernese Jura, although the argillaceous divisions, so conspicuous in
England, are feebly represented there, and some entirely wanting.

The calcareous portion of the Great Oolite consists of several shelly
limestones, one of which, called the Bath Oolite, is much celebrated as
a building-stone. In parts of Gloucestershire, especially near
Minchinhampton, the Great Oolite, says Mr. Lycett, “must have been
deposited in a shallow sea, where strong currents prevailed, for there
are frequent changes in the mineral character of the deposit, and some
beds exhibit false stratification. In others, heaps of broken shells
are mingled with pebbles of rocks foreign to the neighbourhood, and
with fragments of abraded madrepores, dicotyledonous wood, and crabs’
claws. The shelly strata, also, have occasionally suffered denudation,
and the removed portions have been replaced by clay.” In such
shallow-water beds shells of the genera _Patella, Nerita, Rimula,
Cylindrites_ are common (see Figs. 334 to 337); while cephalopods are
rare, and instead of ammonites and belemnites, numerous genera of
carnivorous trachelipods appear. Out of 224 species of univalves
obtained from the Minchinhampton beds, Mr. Lycett found no less than 50
to be carnivorous. They belong principally to the genera _Buccinum,
Pleurotoma, Rostellaria, Murex, Purpuroidea_ (Fig. 333), and Fusus, and
exhibit a proportion of zoophagous species not very different from that
which obtains in seas of the Recent period. These zoological results
are curious and unexpected, since it was imagined that we might look in
vain for the carnivorous trachelipods in rocks of such high antiquity
as the Great Oolite, and it was a received doctrine that they did not
begin to appear in considerable numbers till the Eocene period, when
those two great families of cephalopoda, the ammonites and belemnites,
and a great number of other representatives of the same class of
chambered shells, had become extinct.

Fig. 332: Terebratula digona. Fig. 333: Purpuroidea nodulata. Fig. 334:
Cylindrites acutus. Fig. 335: Patella rugosa. Fig. 336: Nerita
costulata. Fig. 337: Rimula (Emarginula) clathrata.

Stonesfield Slate: Mammalia.—The slate of Stonesfield has been shown by
Mr. Lonsdale to lie at the base of the Great Oolite.[5] It is a
slightly oolitic shelly limestone, forming large lenticular masses
imbedded in sand only six feet thick, but very rich in organic remains.
It contains some pebbles of a rock very similar to itself, and which
may be portions of the deposit, broken up on a shore at low water or
during storms, and redeposited. The remains of belemnites, trigoniæ,
and other marine shells, with fragments of wood, are common, and
impressions of ferns, cycadeæ, and other plants. Several insects, also,
and, among the rest, the elytra or wing-covers of beetles, are
perfectly preserved (see Fig. 338), some of them approaching nearly to
the genus Buprestis. The remains, also, of many genera of reptiles,
such as _Plesiosaur, Crocodile,_ and _ Pterodactyl,_ have been
discovered in the same limestone.

Fig. 338: Elytron of Buprestis?
But the remarkable fossils for which the Stonesfield slate is most
celebrated are those referred to the mammiferous class. The student
should be reminded that in all the rocks described in the preceding
chapters as older than the Eocene, no bones of any land-quadruped, or
of any cetacean, had been discovered until the _Spalacotherium_ of the
Purbeck beds came to light in 1854. Yet we have seen that terrestrial
plants were not wanting in the Upper Cretaceous formation (see p. 302),
and that in the Wealden there was evidence of fresh-water sediment on a
large scale, containing various plants, and even ancient vegetable
soils. We had also in the same Wealden many land-reptiles and winged
insects, which render the absence of terrestrial quadrupeds the more
striking. The want, however, of any bones of whales, seals, dolphins,
and other aquatic mammalia, whether in the chalk or in the upper or
middle oolite, is certainly still more remarkable.

These observations are made to prepare the reader to appreciate more
justly the interest felt by every geologist in the discovery in the
Stonesfield slate of no less than ten specimens of lower jaws of
mammiferous quadrupeds, belonging to four different species and to
three distinct genera, for which the names of _ Amphitherium,
Phascolotherium,_ and _Stereognathus_ have been adopted.

Fig. 339: Tupaia Tana. Right ramus of lower jaw.

It is now generally admitted that these or really the remains of
mammalia (although it was at first suggested that they might be
reptiles), and the only question open to controversy is limited to this
point, whether the fossil mammalia found in the Lower Oolite of
Oxfordshire ought to be referred to the marsupial quadrupeds, or to the
ordinary placental series. Cuvier had long ago pointed out a
peculiarity in the form of the angular process (_c,_ Figs. 342 and 343)
of the lower jaw, as a character of the genus _Didelphys_; and
Professor Owen has since confirmed the doctrine of its generality in
the entire marsupial series. In all these pouched quadrupeds this
process is turned inward, as at _c, d,_ Fig. 342, in the Brazilian
opossum, whereas in the placental series, as at _c,_ Figs. 340 and 341,
there is an almost entire absence of such inflection. The _Tupaia Tana_
of Sumatra has been selected by Mr. Waterhouse for this illustration,
because the jaws of that small insectivorous quadruped bear a great
resemblance to those of the Stonesfield _Amphitherium._ By clearing
away the matrix from the specimen of _Amphitherium Prevostii_ here
represented (Fig. 344), Professor Owen ascertained that the angular
process (_c_) bent inward in a slighter degree than in any of the known
marsupialia; in short, the inflection does not exceed that of the mole
or hedgehog. This fact made him doubt whether the _Amphitherium_ might
not be an insectivorous placental, although it offered some points of
approximation in its osteology to the marsupials, especially to the
_Myrmecobius,_ a small insectivorous quadruped of Australia, which has
nine molars on each side of the lower jaw, besides a canine and three
incisors.[6] Another species of _Amphitherium_ has been found at
Stonesfield (Fig. 345), which differs from the former (Fig. 344)
principally in being larger.

Fig. 340: Part of lower jaw of Tupaia Tana. Fig. 341: Side view of
same. Fig. 342: Part of lower jaw of Didelphys Azaræ. Fig. 343: Side
view of same. Fig. 344: Amphitherium Prevostii.

Fig. 344: Amphitherium Prevostii.

Fig. 345: Amphitherium Broderipii. Fig. 346: Phascolotherium
Bucklandii.

The second mammiferous genus discovered in the same slates was named
originally by Mr. Broderip _Didelphys Bucklandi_ (see Fig. 346), and
has since been called _Phascolotherium_ by Owen. It manifests a much
stronger likeness to the marsupials in the general form of the jaw, and
in the extent and position of its inflected angle, while the agreement
with the living genus Didelphys in the number of the pre-molar and
molar teeth is complete.[7]

In 1854 the remains of another mammifer, small in size, but larger than
any of those previously known, was brought to light. The generic name
of _Stereognathus_ was given to it, and, as is usually the case in
these old rocks (see p. 328), it consisted of part of a lower jaw, in
which were implanted three double-fanged teeth, differing in structure
from those of all other known recent or extinct mammals.

Plants of the Oolite.—The Araucarian pines, which are now abundant in
Australia and its islands, together with marsupial quadrupeds, are
found in like manner to have accompanied the marsupials in Europe
during the Oolitic period (see Fig. 348). In the same rock endogens of
the most perfect structure are met with, as, for example, fruits allied
to the Pandanus, such as the _Kaidacarpum ooliticum_ of Carruthers in
the Great Oolite, and the _Podocarya_ of Buckland (see Fig. 347) in the
Inferior Oolite.

Fuller’s Earth.—Between the Great and Inferior Oolite near Bath, an
argillaceous deposit, called “the fuller’s earth,” occurs; but it is
wanting in the north of England. It abounds in the small oyster
represented in Fig. 349. The number of mollusca known in this deposit
is about seventy; namely, fifty Lamellibranchiate Bivalves, ten
Brachiopods, three Gasteropods, and seven or eight Cephalopods.

Fig. 347: Portion of a fossil fruit of Podocarya Bucklandii. Fig. 348:
Cone of fossil Araucaria sphærocarpa.

Fig. 349: Ostrea acuminata.
Inferior Oolite.—This formation consists of a calcareous freestone,
usually of small thickness, but attaining in some places, as in the
typical area of Cheltenham and the Western Cotswolds, a thickness of
250 feet. It sometimes rests upon yellow sands, formerly classed as the
sands of the Inferior Oolite, but now regarded as a member of the Upper
Lias. These sands repose upon the Upper Lias clays in the south and
west of England. The Collyweston slate, formerly classed with the Great
Oolite, and supposed to represent in Northamptonshire the Stonesfield
slate, is now found to belong to the Inferior Oolite, both by community
of species and position in the series. The Collyweston beds, on the
whole, assume a much more marine character than the Stonesfield slate.
Nevertheless, one of the fossil plants _Aroides Stutterdi,_ Carruthers,
remarkable, like the Pandanaceous species before mentioned (Fig. 347)
as a representative of the monocotyledonous class, is common to the
Stonesfield beds in Oxfordshire.

The Inferior Oolite of Yorkshire consists largely of shales and
sandstones, which assume much the aspect of a true coal-field, thin
seams of coal having actually been worked in them for more than a
century. A rich harvest of fossil ferns has been obtained from them, as
at Gristhorpe, near Scarborough (Fig. 350). They contain also Cycadeæ,
of which family a magnificent specimen has been described by Mr.
Williamson under the name Zamia gigas, and a fossil called _Equisetum
Columnare_ (see Fig. 397), which maintains an upright position in
sandstone strata over a wide area. Shells of _Estheria_ and _Unio,_
collected by Mr. Bean from these Yorkshire coal-bearing beds, point to
the estuary or fluviatile origin of the deposit.

Fig. 350: Hemitelites Brownii.
At Brora, in Sutherlandshire, a coal formation, probably coeval with
the above, or at least belonging to some of the lower divisions of the
Oolitic period, has been mined extensively for a century or more. It
affords the thickest stratum of pure vegetable matter hitherto detected
in any secondary rock in England. One seam of coal of good quality has
been worked three and a half feet thick, and there are several feet
more of pyritous coal resting upon it.

Fig. 351: Terebratula fimbria. Fig. 352: Rhynchonella spinosa. Fig.
353: Pholadomya fidicula.

Among the characteristic shells of the Inferior Oolite, I may instance
_Terebratula fimbria_ (Fig. 351), _Rhynchonella spinosa_ (Fig. 352),
and _Pholadomya fidicula_ (Fig. 353). The extinct genus _Pleurotomaria_
is also a form very common in this division as well as in the Oolitic
system generally. It resembles the _Trochus_ in form, but is marked by
a deep cleft (_a,_ Figs. 354, 355) on one side of the mouth. The
_Collyrites (Dysaster) ringens_ (Fig. 356) is an Echinoderm common to
the Inferior Oolite of England and France, as are the two Ammonites
(Figs. 357, 358).

Fig. 354: Pleurotomaria granulata. Fig. 355: Pleurotomaria ornata. Fig.
356: Collyrites (Dysaster) ringens. Fig. 357: Ammonites Humphresianus.
Fig. 358: Ammonites Braikenridgii. Fig. 359: Ostrea Marshii.

Palæontological Relations of the Oolitic Strata.—Observations have
already been made on the distinctness of the organic remains of the
Oolitic and Cretaceous strata, and the proportion of species common to
the different members of the Oolite. Between the Lower Oolite and the
Lias there is a somewhat greater break, for out of 256 mollusca of the
Upper Lias, thirty-seven species only pass up into the Inferior Oolite.

Fig. 360: Ammonites macrocephalus.
In illustration of shells having a great vertical range, it may be
stated that in England some few species pass up from the Lower to the
Upper Oolite, as, for example, _Rhynchonella obsoleta, Lithodomus
inclusus, Pholadomya ovalis,_ and _Trigonia costata._

Of all the Jurassic Ammonites of Great Britain, _A. macrocephalus_
(Fig. 360), which is common to the Great Oolite and Oxford Clay, has
the widest range.

We have every reason to conclude that the gaps which occur, both
between the larger and smaller sections of the English Oolites, imply
intervals of time, elsewhere represented by fossiliferous strata,
although no deposit may have taken place in the British area. This
conclusion is warranted by the partial extent of many of the minor and
some of the larger divisions even in England.

 [1] Elements of Geology, 4th edition.

 [2] I allude to several Zeuglodons found in Alabama, and referred by
 some zoologists to three species.

 [3] See Phil. Trans. 1850, p. 363; also Huxley, Memoirs of Geol.
 Survey, 1864; Phillips, Palæont. Soc.

 [4] P. Scrope, Proc. Geol. Soc., March, 1831.

 [5] Proceedings Geol. Soc., vol. i, p. 414.

 [6] A figure of this recent _Myrmecobius_ will be found in my
 Principles of Geology, chap. ix.

 [7] Owen’s British Fossil Mammals, p. 62.




CHAPTER XX.
JURASSIC GROUP—_continued_—LIAS.


Mineral Character of Lias. — Numerous successive Zones in the Lias,
marked by distinct Fossils, without Unconformity in the Stratification,
or Change in the Mineral Character of the Deposits. — Gryphite
Limestone. — Shells of the Lias. — Fish of the Lias. — Reptiles of the
Lias. — Ichthyosaur and Plesiosaur. — Marine Reptile of the Galapagos
Islands. — Sudden Destruction and Burial of Fossil Animals in Lias. —
Fluvio-marine Beds in Gloucestershire, and Insect Limestone. — Fossil
Plants. — The origin of the Oolite and Lias, and of alternating
Calcareous and Argillaceous Formations.

Lias.—The English provincial name of Lias has been very generally
adopted for a formation of argillaceous limestone, marl, and clay,
which forms the base of the Oolite, and is classed by many geologists
as part of that group. The peculiar aspect which is most characteristic
of the Lias in England, France, and Germany, is an alternation of thin
beds of blue or grey limestone, having a surface which becomes
light-brown when weathered, these beds being separated by
dark-coloured, narrow argillaceous partings, so that the quarries of
this rock, at a distance, assume a striped and ribbon-like appearance.

The Lias has been divided in England into three groups, the Upper,
Middle, and Lower. The Upper Lias consists first of sands, which were
formerly regarded as the base of the Oolite, but which, according to
Dr. Wright, are by their fossils more properly referable to the Lias;
secondly, of clay shale and thin beds of limestone. The Middle Lias, or
marl-stone series, has been divided into three zones; and the Lower
Lias, according to the labours of Quenstedt, Oppel, Strickland, Wright,
and others, into seven zones, each marked by its own group of fossils.
This Lower Lias averages from 600 to 900 feet in thickness.

From Devon and Dorsetshire to Yorkshire all these divisions, observes
Professor Ramsay, are constant; and from top to bottom we cannot assert
that anywhere there is actual unconformity between any two
subdivisions, whether of the larger or smaller kind.

In the whole of the English Lias there are at present known about 937
species of mollusca, and of these 267 are Cephalopods, of which class
more than two-thirds are Ammonites, the Nautilus and Belemnite also
abounding. The whole series has been divided by zones characterised by
particular Ammonites; for while other families of shells pass from one
division to another in numbers varying from about 20 to 50 per cent,
these cephalopods are almost always limited to single zones, as
Quenstedt and Oppel have shown for Germany, and Dr. Wright and others
for England.

As no actual unconformity is known from the top of the Upper to the
bottom of the Lower Lias, and as there is a marked uniformity in the
mineral character of almost all the strata, it is somewhat difficult to
account even for such partial breaks as have been alluded to in the
succession of species, if we reject the hypothesis that the old species
were in each case destroyed at the close of the deposition of the rocks
containing them, and replaced by the creation of new forms when the
succeeding formation began. I agree with Professor Ramsay in not
accepting this hypothesis. No doubt some of the old species
occasionally died out, and left no representatives in Europe or
elsewhere; others were locally exterminated in the struggle for life by
species which invaded their ancient domain, or by varieties better
fitted for a new state of things. Pauses also of vast duration may have
occurred in the deposition of strata, allowing time for the
modification of organic life throughout the globe, slowly brought about
by variation accompanied by extinction of the original forms.

Fig. 361: Plagiostoma (Lima) giganteum. Fig. 362: Gryphæa incurva.

Fossils of the Lias.—The name of Gryphite limestone has sometimes been
applied to the Lias, in consequence of the great number of shells which
it contains of a species of oyster, or _ Gryphæa_ (Fig. 362). A large
heavy shell called _Hippopodium_ (Fig. 365), allied to _Cypricardia,_
is also characteristic of the upper part of the Lower Lias. In this
formation occur also the Aviculas, Figs. 363 and 364. The Lias
formation is also remarkable for being the newest of the secondary
rocks in which brachiopoda of the genera _Spirifer_ and _ Leptæna_
(Figs. 366, 367) occur, although the former is slightly modified in
structure so as to constitute the subgenus Spiriferina, Davidson, and
the Leptæna has dwindled to a shell smaller in size than a pea. No less
than eight or nine species of Spiriferina are enumerated by Mr.
Davidson as belonging to the Lias. Palliobranchiate mollusca
predominate greatly in strata older than the Trias; but, so far as we
yet know, they did not survive the Liassic epoch.

Fig. 363: Avicula inæquivalvis. Fig. 364: Avicula cygnipes. Fig. 365:
Hippopodium ponderosum. Fig. 366: Spiriferina (Spirifera). Fig. 367:
Leptæna Moorei.

Fig. 368: Ammonites Bucklandi. Fig. 369: Ammonites planorbis. Fig. 370:
Nautilus truncatus. Fig. 371: Ammonites bifrons.

Allusion has already been made, p. 354, to numerous zones in the Lias
having each their peculiar Ammonites. Two of these occur near the base
of the Lower Lias, having a united thickness, varying from 40 to 80
feet. The upper of these is characterised by _Ammonites Bucklandi,_ and
the lower by _Ammonites planorbis_ (see Figs. 368, 369).[1] Sometimes,
however, there is a third intermediate zone, that of _Ammonites
angulatus,_ which is the equivalent of the zone called the infra-lias
on the Continent, the species of which are for the most part common to
the superior group marked by _Ammonites Bucklandi._

Fig. 372: Ammonites margaritatus.
Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are
conspicuous. (See Fig. 373.) Of _ Palæocoma (Ophioderma) Egertoni_
(Fig. 374), referable to the _Ophiuridæ_ of Muller, perfect specimens
have been met with in the Middle Lias beds of Dorset and Yorkshire.

Fig. 373: Extracrinus (Pentacrinus) Briareus. Fig. 374: Palæocoma
(Ophioderma) tenuibrachiata.

The _Extracrinus Briareus_ (removed by Major Austin from Pentacrinus on
account of generic differences) occurs in tangled masses, forming thin
beds of considerable extent, in the Lower Lias of Dorset,
Gloucestershire, and Yorkshire. The remains are often highly charged
with pyrites. This Crinoid, with its innumerable tentacular arms,
appears to have been frequently attached to the driftwood of the
liassic sea, in the same manner as Barnacles float about on wood at the
present day. There is another species of _ Extracrinus_ and several of
_Pentacrinus_ in the Lias; and the latter genus is found in nearly all
the formations from the Lias to the London Clay inclusive. It is
represented in the present seas by the delicate and rare _Pentacrinus
caput-medusæ_ of the Antilles, which, with Comatula, is one of the few
surviving members of the ancient family of the Crinoids, represented by
so many extinct genera in the older formations.

Fig. 375: Scales of Lepidotus gigas. Fig. 376: a. Scales of Æchmodus
Leachii, b. Æchmodus (restored outline), c. Scales of Dapedius
monilifer.

Fishes of the Lias.—The fossil fish, of which there are no less than
117 species known as British, resemble generically those of the Oolite,
but differ, according to M. Agassiz, from those of the Cretaceous
period. Among them is a species of _ Lepidotus_ (_L. gigas,_ Agassiz),
Fig. 375, which is found in the Lias of England, France, and
Germany.[2] This genus was before mentioned (p. 316) as occurring in
the Wealden, and is supposed to have frequented both rivers and
sea-coasts. Another genus of Ganoids (or fish with hard, shining, and
enamelled scales), called _Æchmodus_ (Fig. 376), is almost exclusively
Liassic. The teeth of a species of _ Acrodus,_ also, are very abundant
in the Lias (Fig. 377).

Fig. 377: Acrodus nobilis. Fig. 378: Hybodus reticulatus, a. Part of
fin, commonly called Ichthyodorylite, b. Tooth.

But the remains of fish which have excited more attention than any
others are those large bony spines called ichthyodorulites (a, Figure
378), which were once supposed by some naturalists to be jaws, and by
others weapons, resembling those of the living Balistes and Silurus;
but which M. Agassiz has shown to be neither the one nor the other. The
spines, in the genera last mentioned, articulate with the backbone,
whereas there are no signs of any such articulation in the
ichthyodorulites.

Fig. 379: Chimæra monstrosa. Fig. 379: Chimæra monstrosa.[3]

These last appear to have been bony spines which formed the anterior
part of the dorsal fin, like that of the living genera _ Cestracion_
and _Chimæra_ (see _a,_ Figure 379). In both of these genera, the
posterior concave face is armed with small spines, as in that of the
fossil _Hybodus_ (Fig. 378), a placoid fish of the shark family found
fossil at Lyme Regis. Such spines are simply imbedded in the flesh, and
attached to strong muscles. “They serve,” says Dr. Buckland, “as in the
_Chimæra_ (Fig. 379), to raise and depress the fin, their action
resembling that of a movable mast, raising and lowering backward the
sail of a barge.”[4]

Reptiles of the Lias.—It is not, however, the fossil fish which form
the most striking feature in the organic remains of the Lias; but the
_Enaliosaurian_ reptiles, which are extraordinary for their number,
size, and structure. Among the most singular of these are several
species of _Ichthyosaurus_ and _Plesiosaurus_ (Figs. 380, 381). The
genus _ Ichthyosaurus,_ or fish-lizard, is not confined to this
formation, but has been found in strata as high as the White Chalk of
England, and as low as the Trias of Germany, a formation which
immediately succeeds the Lias in the descending order. It is evident
from their fish-like vertebræ, their paddles, resembling those of a
porpoise or whale, the length of their tail, and other parts of their
structure, that the Ichthyosaurs were aquatic. Their jaws and teeth
show that they were carnivorous; and the half-digested remains of
fishes and reptiles, found within their skeletons, indicate the precise
nature of their food.

Mr. Conybeare was enabled, in 1824, after examining many skeletons
nearly perfect, to give an ideal restoration of the osteology of this
genus, and of that of the _ Plesiosaurus._[5] (See Figs. 380, 381.) The
latter animal had an extremely long neck and small head, with teeth
like those of the crocodile, and paddles analogous to those of the _
Ichthyosaurus,_ but larger. It is supposed to have lived in shallow
seas and estuaries, and to have breathed air like the Ichthyosaur and
our modern cetacea.[6] Some of the reptiles above mentioned were of
formidable dimensions. One specimen of _ Ichthyosaurus platydon,_ from
the Lias at Lyme, now in the British Museum, must have belonged to an
animal more than 24 feet in length; and there are species of
_Plesiosaurus_ which measure from 18 to 20 feet in length. The form of
the _ Ichthyosaurus_ may have fitted it to cut through the waves like
the porpoise; as it was furnished besides its paddles with a tail-fin
so constructed as to be a powerful organ of motion; but it is supposed
that the _Plesiosaurus,_ at least the long-necked species (Fig. 381),
was better suited to fish in shallow creeks and bays defended from
heavy breakers.

It is now very generally agreed that these extinct saurians must have
inhabited the sea; and it was urged that as there are now chelonians,
like the tortoise, living in fresh water, and others, as the turtle,
frequenting the ocean, so there may have been formerly some saurians
proper to salt, others to fresh water. The common crocodile of the
Ganges is well-known to frequent equally that river and the brackish
and salt water near its mouth; and crocodiles are said in like manner
to be abundant both in the rivers of the Isla de Pinos (Isle of Pines),
south of Cuba, and in the open sea round the coast. In 1835 a curious
lizard (_Amblyrhynchus cristatus_) was discovered by Mr. Darwin in the
Galapagos Islands.[7] It was found to be exclusively marine, swimming
easily by means of its flattened tail, and subsisting chiefly on
seaweed. One of them was sunk from the ship by a heavy weight, and on
being drawn up after an hour was quite unharmed.

Fig. 380: Skeleton of Ichthyosaurus communis, restored by Conybeare and
Cuvier. Fig. 381: Skeleton of Plesiosaurus dolichodeirus, restored by
Rev. W. D. Conybeare.

The families of Dinosauria, crocodiles, and Pterosauria or winged
reptiles, are also represented in the Lias.

Sudden Destruction of Saurians.—It has been remarked, and truly, that
many of the fish and saurians, found fossil in the Lias, must have met
with sudden death and immediate burial; and that the destructive
operation, whatever may have been its nature, was often repeated.

“Sometimes,” says Dr. Buckland, “scarcely a single bone or scale has
been removed from the place it occupied during life; which could not
have happened had the uncovered bodies of these saurians been left,
even for a few hours, exposed to putrefaction, and to the attacks of
fishes and other smaller animals at the bottom of the sea.”[8] Not only
are the skeletons of the Ichthyosaurs entire, but sometimes the
contents of their stomachs still remain between their ribs, as before
remarked, so that we can discover the particular species of fish on
which they lived, and the form of their excrements. Not unfrequently
there are layers of these coprolites, at different depths in the Lias,
at a distance from any entire skeletons of the marine lizards from
which they were derived; “as if,” says Sir H. De la Beche, “the muddy
bottom of the sea received small sudden accessions of matter from time
to time, covering up the coprolites and other exuviæ which had
accumulated during the intervals.”[9] It is further stated that, at
Lyme Regis, those surfaces only of the coprolites which lay uppermost
at the bottom of the sea have suffered partial decay, from the action
of water before they were covered and protected by the muddy sediment
that has afterwards permanently enveloped them.

Numerous specimens of the Calamary or pen-and-ink fish, (_Geoteuthis
bollensis_) have also been met with in the Lias at Lyme, with the
ink-bags still distended, containing the ink in a dried state, chiefly
composed of carbon, and but slightly impregnated with carbonate of
lime. These Cephalopoda, therefore, must, like the saurians, have been
soon buried in sediment; for, if long exposed after death, the membrane
containing the ink would have decayed.[10]

As we know that river-fish are sometimes stifled, even in their own
element, by muddy water during floods, it cannot be doubted that the
periodical discharge of large bodies of turbid fresh water in the sea
may be still more fatal to marine tribes. In the “Principles of
Geology” I have shown that large quantities of mud and drowned animals
have been swept down into the sea by rivers during earthquakes, as in
Java in 1699; and that indescribable multitudes of dead fishes have
been seen floating on the sea after a discharge of noxious vapours
during similar convulsions. But in the intervals between such
catastrophes, strata may have accumulated slowly in the sea of the
Lias, some being formed chiefly of one description of shell, such as
ammonites, others of gryphites.

Fig. 382: Wing of a neuropterous insect.

Fresh-water Deposits.—Insect-beds.—From the above remarks the reader
will infer that the Lias is for the most part a marine deposit. Some
members, however, of the series have an estuarine character, and must
have been formed within the influence of rivers. At the base of the
Upper and Lower Lias respectively, insect-beds appear to be almost
everywhere present throughout the Midland and South-western districts
of England. These beds are crowded with the remains of insects, small
fish, and crustaceans, with occasional marine shells. One band in
Gloucestershire, rarely exceeding a foot in thickness, has been named
the “insect limestone.” It passes upward, says the Reverend P. B.
Brodie,[11] into a shale containing _Cypris_ and _ Estheria,_ and is
full of the wing-cases of several genera of Coleoptera, with some
nearly entire beetles, of which the eyes are preserved. The nervures of
the wings of neuropterous insects (Figure 382) are beautifully perfect
in this bed. Ferns, with Cycads and leaves of monocotyledonous plants,
and some apparently brackish and fresh-water shells, accompany the
insects in several places, while in others marine shells predominate,
the fossils varying apparently as we examine the bed nearer or farther
from the ancient land, or the source whence the fresh water was
derived. After studying 300 specimens of these insects from the Lias,
Mr. Westwood declares that they comprise both wood-eating and
herb-devouring beetles, of the Linnean genera _Elater, Carabus,_ etc.,
besides grasshoppers (_Gryllus_), and detached wings of dragon-flies
and may-flies, or insects referable to the Linnean genera _Libellula,
Ephemera, Hemerobius,_ and _Panorpa,_ in all belonging to no less than
twenty-four families. The size of the species is usually small, and
such as taken alone would imply a temperate climate; but many of the
associated organic remains of other classes must lead to a different
conclusion.

Fossil Plants.—Among the vegetable remains of the Lias, several species
of _Zamia_ have been found at Lyme Regis, and the remains of coniferous
plants at Whitby. M. Ad. Brongniart enumerates forty-seven liassic
acrogens, most of them ferns; and fifty gymnosperms, of which
thirty-nine are cycads, and eleven conifers. Among the cycads the
predominance of _ Zamites,_ and among the ferns the numerous genera
with leaves having reticulated veins (as in Fig. 349), are mentioned as
botanical characteristics of this era.[12] The absence as yet from the
Lias and Oolite of all signs of dicotyledonous angiosperms is worthy of
notice. The leaves of such plants are frequent in tertiary strata, and
occur in the Cretaceous, though less plentifully (see p. 303). The
angiosperms seem, therefore, to have been at the least comparatively
rare in these older secondary periods, when more space was occupied by
the Cycads and Conifers.

Origin of the Oolite and Lias.—The entire group of Oolite and Lias
consists of repeated alternations of clay, sandstone, and limestone,
following each other in the same order. Thus the clays of the Lias are
followed by the sands now considered (see p. 353) as belonging to the
same formation, though formerly referred to the Inferior Oolite, and
these sands again by the shelly and coralline limestone called the
Great or Bath Oolite. So, in the Middle Oolite, the Oxford Clay is
followed by calcareous grit and coral rag; lastly, in the Upper Oolite,
the Kimmeridge Clay is followed by the Portland Sand and limestone (see
Fig. 298).[13] The clay beds, however, as Sir H. de la Beche remarks,
can be followed over larger areas than the sand or sandstones.[14] It
should also be remembered that while the Oolite system becomes
arenaceous and resembles a coal-field in Yorkshire, it assumes in the
Alps an almost purely calcareous form, the sands and clays being
omitted; and even in the intervening tracts it is more complicated and
variable than appears in ordinary descriptions. Nevertheless, some of
the clays and intervening limestones do retain, in reality, a pretty
uniform character for distances of from 400 to 600 miles from east to
west and north to south.

In order to account for such a succession of events, we may imagine,
first, the bed of the ocean to be the receptacle for ages of fine
argillaceous sediment, brought by oceanic currents, which may have
communicated with rivers, or with part of the sea near a wasting coast.
This mud ceases, at length, to be conveyed to the same region, either
because the land which had previously suffered denudation is depressed
and submerged, or because the current is deflected in another direction
by the altered shape of the bed of the ocean and neighbouring dry land.
By such changes the water becomes once more clear and fit for the
growth of stony zoophytes. Calcareous sand is then formed from
comminuted shell and coral, or, in some cases, arenaceous matter
replaces the clay; because it commonly happens that the finer sediment,
being first drifted farthest from coasts, is subsequently overspread by
coarse sand, after the sea has grown shallower, or when the land,
increasing in extent, whether by upheaval or by sediment filling up
parts of the sea, has approached nearer to the spots first occupied by
fine mud.

The increased thickness of the limestones in those regions, as in the
Alps and Jura, where the clays are comparatively thin, arises from the
calcareous matter having been derived from species of corals and other
organic beings which live in clear water, far from land, to the growth
of which the influx of mud would be unfavourable. Portions therefore of
these clays and limestones have probably been formed contemporaneously
to a greater extent than we can generally prove, for the distinctness
of the species of organic beings would be caused by the difference of
conditions between the more littoral and the more pelagic areas and the
different depths and nature of the sea-bottom. Independently of those
ascending and descending movements which have given rise to the
superposition of the limestones and clays, and by which the position of
land and sea are made in the course of ages to vary, the geologist has
the difficult task of allowing for the contemporaneous thinning out in
one direction and thickening in another, of the successive organic and
inorganic deposits of the same era.

 [1] Quart. Journ., vol. xvi, p. 376.

 [2] Agassiz, Poissons Fossiles, vol. ii, tab. 28, 29.

 [3] Agassiz, Poissons Fossiles, vol. iii, tab. C, Fig. 1.

 [4] Bridgewater Treatise, p. 290.

 [5] Geol. Soc. Transactions, Second Series, vol. i, p. 49.

 [6] Conybeare and De la Beche, Geol. Trans., First Series, vol. v, p.
 559; and Buckland, Bridgewater Treatise, p. 203.

 [7] See Darwin, Naturalist’s Voyage, p. 385. Murray.

 [8] Bridgewater Treatise, p. 115.

 [9] Geological Researches, p. 334.

 [10] Buckland, Bridgewater Treatise, p. 307.

 [11] A History of Fossil Insects, etc., 1846. London.

 [12] Tableau des Vég. Foss., 1849, p. 105.

 [13] Conybeare and Philips’s Outlines, etc., p. 166.

 [14] Geological Researches, p. 337.




CHAPTER XXI.
TRIAS, OR NEW RED SANDSTONE GROUP.


Beds of Passage between the Lias and Trias, Rhætic Beds. — Triassic
Mammifer. — Triple Division of the Trias. — Keuper, or Upper Trias of
England. — Reptiles of the Upper Trias. — Foot-prints in the Bunter
formation in England. — Dolomitic Conglomerate of Bristol. — Origin of
Red Sandstone and Rock-salt. — Precipitation of Salt from inland Lakes
and Lagoons. — Trias of Germany. — Keuper. — St. Cassian and Hallstadt
Beds. — Peculiarity of their Fauna. — Muschelkalk and its Fossils. —
Trias of the United States. — Fossil Foot-prints of Birds and Reptiles
in the Valley of the Connecticut. — Triassic Mammifer of North
Carolina. — Triassic Coal-field of Richmond, Virginia. — Low Grade of
early Mammals favourable to the Theory of Progressive Development.

Beds of Passage between the Lias and Trias—Rhætic Beds.—We have
mentioned in the last chapter (p. 356) that the base of the Lower Lias
is characterised, both in England and Germany, by beds containing
distinct species of Ammonites, the lowest subdivision having been
called the zone of _Ammonites planorbis._ Below this zone, on the
boundary line between the Lias and the strata of which we are about to
treat, called “Trias,” certain cream-coloured limestones devoid of
fossils are usually found. These white beds were called by William
Smith the White Lias, and they have been shown by Mr. Charles Moore to
belong to a formation similar to one in the Rhætian Alps of Bavaria, to
which Mr. Gumbel has applied the name of Rhætic. They have also long
been known as the Koessen beds in Germany, and may be regarded as beds
of passage between the Lias and Trias. They are named the Penarth beds
by the Government surveyors of Great Britain, from Penarth, near
Cardiff, in Glamorganshire, where they sometimes attain a thickness of
fifty feet.

The principal member of this group has been called by Dr. Wright the
_Avicula contorta_ bed,[1] as this shell is very abundant, and has a
wide range in Europe. General Portlock first described the formation as
it occurs at Portrush, in Antrim, where the _ Avicula contorta_ is
accompanied by _Pecten Valoniensis,_ as in Germany.

The best known member of the group, a thin band or bone-breccia, is
conspicuous among the black shales in the neighbourhood of Axmouth in
Devonshire, and in the cliffs of Westbury-on-Severn, as well as at Aust
and other places on the borders of the Bristol Channel. It abounds in
the remains of saurians and fish, and was formerly classed as the
lowest bed of the Lias; but Sir P. Egerton first pointed out, in 1841,
that it should be referred to the Upper New Red Sandstone, because it
contained an assemblage of fossil fish which are either peculiar to
this stratum, or belong to species well-known in the Muschelkalk of
Germany. These fish belong to the genera _Acrodus, Hybodus, Gyrolepis,_
and _Saurichthys._

Fig. 383: Cardium rhæticum. Fig. 384: Pecten Valoniensis. Fig. 385:
Avicula contorta. Fig. 386: Hybodus plica ilis. Fig. 387: Saurichthys
apicalis. Fig. 388: Gyrolepsis tenuistriatus.

Among those common to the English bone-bed and the Muschelkalk of
Germany are _Hybodus plicatilis_ (Fig. 386), _Saurychthys apicalis_
(Fig. 387), _Gyrolepis tenuistriatus_ (Fig. 388), and _G. Albertii._
Remains of saurians, _Plesiosaurus_ among others, have also been found
in the bone-bed, and plates of an _Encrinus._ It may be questioned
whether some of those fossils which have the most Triassic character
may not have been derived from the destruction of older strata, since
in bone-beds, in general, many of the organic remains are undoubtedly
derivative.

Fig. 389: Microlestes antiquus, molar tooth.
_Triassic Mammifer._—In North-western Germany, as in England, there
occurs beneath the Lias a remarkable bone breccia. It is filled with
shells and with the remains of fishes and reptiles, almost all the
genera of which, and some even of the species, agree with those of the
subjacent Trias. This breccia has accordingly been considered by
Professor Quenstedt, and other German geologists of high authority, as
the newest or uppermost part of the Trias. Professor Plieninger found
in it, in 1847, the molar tooth of a small Triassic mammifer, called by
him _ Microlestes antiquus._ He inferred its true nature from its
double fangs, and from the form and number of the protuberances or
cusps on the flat crown; and considering it as predaceous, probably
insectivorous, he called it _Microlestes_ from micros, little, and
lestes, a beast of prey. Soon afterwards he found a second tooth, also
at the same locality, Diegerloch, about two miles to the south-east of
Stuttgart.

No anatomist had been able to give any feasible conjecture as to the
affinities of this minute quadruped until Dr. Falconer, in 1857,
recognised an unmistakable resemblance between its teeth and the two
back molars of his new genus _Plagiaulax_ (Fig. 306), from the Purbeck
strata. This would lead us to the conclusion that Microlestes was
marsupial and plant-eating.

In Würtemberg there are two bone-beds, namely, that containing the
Microlestes, which has just been described, which constitutes, as we
have seen, the uppermost member of the Trias, and another of still
greater extent, and still more rich in the remains of fish and
reptiles, which is of older date, intervening between the Keuper and
Muschelkalk.

The genera _Saurichthys, Hybodus,_ and _Gyrolepis_ are found in both
these breccias, and one of the species, _ Saurichthys Mongeoti,_ is
common to both bone-beds, as is also a remarkable reptile called
_Nothosaurus mirabilis._ The saurian called _Belodon_ by H. von Meyer,
of the Thecodont family, is another Triassic form, associated at
Diegerloch with Microlestes.

TRIAS OF ENGLAND.

Between the Lias and the Coal (or Carboniferous group) there is
interposed, in the midland and western counties of England, a great
series of red loams, shales, and sandstones, to which the name of the
“New Red Sandstone formation” was first given, to distinguish it from
other shales and sandstones called the “Old Red,” often identical in
mineral character, which lie immediately beneath the coal. The name of
“Red Marl” has been incorrectly applied to the red clays of this
formation, as before explained (p. 38), for they are remarkably free
from calcareous matter. The absence, indeed, of carbonate of lime, as
well as the scarcity of organic remains, together with the bright red
colour of most of the rocks of this group, causes a strong contrast
between it and the Jurassic formations before described.

The group in question is more fully developed in Germany than in
England or France. It has been called the Trias by German writers, or
the Triple Group, because it is separable into three distinct
formations, called the “Keuper,” the “Muschelkalk,” and the
“Bunter-sandstein.” Of these the middle division, or the Muschelkalk,
is wholly wanting in England, and the uppermost (Keuper) and lowest
(Bunter) members of the series are not rich in fossils.

Upper Trias or Keuper.—In certain grey indurated marls below the
bone-bed Mr. Boyd Dawkins has found at Watchet, on the coast of
Somersetshire, a molar tooth of Microlestes, enabling him to refer to
the Trias strata formerly supposed to be Liassic. Mr. Charles Moore had
previously discovered many teeth of mammalia of the same family near
Frome, in Somersetshire, in the contents of a vertical fissure
traversing a mass of carboniferous limestone. The top of this fissure
must have communicated with the bed of the Triassic sea, and probably
at a point not far from the ancient shore on which the small marsupials
of that era abounded.

This upper division of the Trias called the Keuper is of great
thickness in the central counties of England, attaining, according to
Mr. Hull’s estimate, no less than 3450 feet in Cheshire, and it covers
a large extent of country between Lancashire and Devonshire.

In Worcestershire and Warwickshire in sandstone belonging to the
uppermost part of the Keuper the bivalve crustacean _Estheria minuta_
occurs. The member of the English “New Red” containing this shell, in
those parts of England, is, according to Sir Roderick Murchison and Mr.
Strickland, 600 feet thick, and consists chiefly of red marl or slate,
with a band of sandstone. Ichthyodorulites, or spines of _ Hybodus,_
teeth of fishes, and footprints of reptiles were observed by the same
geologists in these strata.

Fig. 390: Estheria minuta.
Fig. 391: Hyperodapedon Gordoni. Left Plate, Maxillary.
In the Upper Trias or Keuper the remains of two saurians of the order
Lacertilia have been found. The one called _ Rhynchosaurus_ occurred at
Grinsell near Shrewsbury, and is characterised by having a small
bird-like skull and jaws without teeth. The other _Hyperodapedon_ (Fig.
391) was first noticed in 1858, near Elgin, in strata now recognised as
Upper Triassic, and afterwards in beds of about the same age in the
neighbourhood of Warwick. Remains of the same genus have been found
both in Central India and Southern Africa in rocks believed to be of
Triassic age. The Hyperodapedon has been shown by Professor Huxley to
be a terrestrial reptile having numerous palatal teeth, and closely
allied to the living Sphenodon of New Zealand.

The recent discoveries of a living saurian in New Zealand so closely
allied to this supposed extinct division of the Lacertilia seems to
afford an illustration of a principle pointed out by Mr. Darwin of the
survival in insulated tracts, after many changes in physical geography,
of orders of which the congeners have become extinct on continents
where they have been exposed to the severer competition of a larger
progressive fauna.

Fig. 392: Tooth of Labyrinthodon.
Teeth of Labyrinthodon (Fig. 392) found in the Keuper in Warwickshire
were examined microscopically by Professor Owen, and compared with
other teeth from the German Keuper. He found after careful
investigation that neither of them could be referred to true saurians,
although they had been named _Mastodonsaurus_ and _Phytosaurus_ by
Jäger. It appeared that they were of the _Batrachian_ order, and of
gigantic dimensions in comparison with any representatives of that
order now living. Both the Continental and English fossil teeth
exhibited a most complicated texture, differing from that previously
observed in any reptile, whether recent or extinct, but most nearly
analogous to the _Ichthyosaurus._ A section of one of these teeth
exhibits a series of irregular folds, resembling the labyrinthic
windings of the surface of the brain; and from this character Professor
Owen has proposed the name Labyrinthodon for the new genus. Fig. 393 of
part of one is given from his “Odontography,” plate 64, A. The entire
length of this tooth is supposed to have been about three inches and a
half, and the breadth at the base one inch and a half.

Fig. 393: Transverse section of upper part of tooth of Labyrinthodon
Jaegeri.

_Rock-salt._—In Cheshire and Lancashire there are red clays containing
gypsum and salt of the age of the Trias which are between 1000 and 1500
feet thick. In some places lenticular masses of pure rock-salt nearly
100 feet thick are interpolated between the argillaceous beds. At the
base of the formation beneath the rock-salt occur the Lower Sandstones
and Marl, called provincially in Cheshire “water-stones,” which are
largely quarried for building. They are often ripple-marked, and are
impressed with numerous footprints of reptiles.

The basement beds of the Keuper rest with a slight unconformability
upon an eroded surface of the “Bunter” next to be described.

Fig. 394: Single footstep of Cheirotherium.

Lower Trias or Bunter.—The lower division or English representative of
the “Bunter” attains a thickness of 1500 feet in the counties last
mentioned, according to Professor Ramsay. Besides red and green shales
and red sandstones, it comprises much soft white quartzose sandstone,
in which the trunks of silicified trees have been met with at Allesley
Hill, near Coventry. Several of them were a foot and a half in
diameter, and some yards in length, decidedly of coniferous wood, and
showing rings of annual growth.[2] Impressions, also, of the footsteps
of animals have been detected in Lancashire and Cheshire in this
formation. Some of the most remarkable occur a few miles from
Liverpool, in the whitish quartzose sandstone of Storton Hill, on the
west side of the Mersey. They bear a close resemblance to tracks first
observed in this member of the Upper New Red Sandstone, at the village
of Hesseberg, near Hildburghausen, in Saxony. For many years these
footprints have been referred to a large unknown quadruped,
provisionally named _Cheirotherium_ by Professor Kaup, because the
marks both of the fore and hind feet resembled impressions made by a
human hand. (See Fig. 394.) The foot-marks at Hesseberg are partly
concave, and partly in relief, the former, or the depressions, are seen
upon the upper surface of the sandstone slabs, but those in relief are
only upon the lower surfaces, being, in fact, natural casts, formed in
the subjacent footprints as in moulds. The larger impressions, which
seem to be those of the hind foot, are generally eight inches in
length, and five in width, and one was twelve inches long. Near each
large footstep, and at a regular distance (about an inch and a half)
before it, a smaller print of a fore foot, four inches long and three
inches wide, occurs. The footsteps follow each other in pairs, each
pair in the same line, at intervals of fourteen inches from pair to
pair. The large as well as the small steps show the great toes
alternately on the right and left side; each step makes the print of
five toes, the first, or great toe, being bent inward like a thumb.
Though the fore and hind foot differ so much in size, they are nearly
similar in form.

Fig. 395: Line of footsteps on slab of sandstone.

As neither in Germany nor in England had any bones or teeth been met
with in the same identical strata as the footsteps, anatomists
indulged, for several years, in various conjectures respecting the
mysterious animals from which they might have been derived. Professor
Kaup suggested that the unknown quadruped might have been allied to the
_Marsupialia_; for in the kangaroo the first toe of the fore foot is in
a similar manner set obliquely to the others, like a thumb, and the
disproportion between the fore and hind feet is also very great. But M.
Link conceived that some of the four species of animals of which the
tracks had been found in Saxony might have been gigantic _Batrachians,_
and when it was afterwards inferred that the Labyrinthodon was an
air-breathing reptile, it was conjectured by Professor Owen that it
might be one and the same as the Cheirotherium.

Dolomitic Conglomerate of Bristol.—Near Bristol, in Somersetshire, and
in other counties bordering the Severn, the lowest strata belonging to
the Triassic series consist of a conglomerate or breccia resting
unconformably upon the Old Red Sandstone, and on different members of
the Carboniferous rocks, such as the Coal Measures, Millstone Grit, and
Mountain Limestone. This mode of superposition will be understood by
reference to the section below Dundry Hill (Fig. 85), where No. 4 is
the dolomitic conglomerate. Such breccias may have been partly the
result of the subÆrial waste of an old land-surface which gradually
sank down and suffered littoral denudation in proportion as it became
submerged. The pebbles and fragments of older rocks which constitute
the conglomerate are cemented together by a red or yellow base of
dolomite, and in some places the encrinites and other fossils derived
from the Mountain Limestone are so detached from the parent rocks that
they have the deceptive appearance of belonging to a fauna
contemporaneous with the dolomitic beds in which they occur. The
imbedded fragments are both rounded and angular, some consisting of
sandstone from the coal-measures, being of vast size, and weighing
nearly a ton. Fractured bones and teeth of saurians which are truly of
contemporaneous origin are dispersed through some parts of the breccia,
and two of these reptiles called Thecodont saurians, named from the
manner in which the teeth were implanted in the jawbone, obtained great
celebrity because the patches of red conglomerate in which they were
found, near Bristol, were originally supposed to be of Permian or
Palæozoic age, and therefore the only representatives in England of
vertebrate animals of so high a grade in rocks of such antiquity. The
teeth of these saurians are conical, compressed, and with finely
serrated edges (see Fig. 396); they are referred by Professor Huxley to
the Dinosaurian order.

Fig. 396: Tooth of Thecodontosaurus.
Origin of Red Sandstone and Rock-salt.—In various parts of the world,
red and mottled clays and sandstones, of several distinct geological
epochs, are found associated with salt, gypsum, and magnesian
limestone, or with one or all of these substances. There is, therefore,
in all likelihood, a general cause for such a coincidence.
Nevertheless, we must not forget that there are dense masses of red and
variegated sandstones and clays, thousands of feet in thickness, and of
vast horizontal extent, wholly devoid of saliferous or gypseous matter.
There are also deposits of gypsum and of common salt, as in the
blue-clay formation of Sicily, without any accompanying red sandstone
or red clay.

These red deposits may be accounted for by the decomposition of gneiss
and mica schist, which in the eastern Grampians of Scotland has
produced a mass of detritus of precisely the same colour as the Old Red
Sandstone.

It is a general fact, and one not yet accounted for, that scarcely any
fossil remains are ever preserved in stratified rocks in which this
oxide of iron abounds; and when we find fossils in the New or Old Red
Sandstone in England, it is in the grey, and usually calcareous beds,
that they occur. The saline or gypseous interstratified beds may have
been produced by submarine gaseous emanations, or hot mineral springs,
which often continue to flow in the same spots for ages. Beds of
rock-salt are, however, more generally attributed to the evaporation of
lakes or lagoons communicating at intervals with the ocean. In Cheshire
two beds of salt occur of the extraordinary thickness of 90 or even 100
feet, and extending over an area supposed to be 150 miles in diameter.
The adjacent beds present ripple-marked sandstones and footprints of
animals at so many levels as to imply that the whole area underwent a
slow and gradual depression during the formation of the red sandstone.

Major Harris, in his “Highlands of Ethiopia,” describes a salt lake,
called the Bahr Assal, near the Abyssinian frontier, which once formed
the prolongation of the Gulf of Tadjara, but was afterwards cut off
from the gulf by a broad bar of lava or of land upraised by an
earthquake. “Fed by no rivers, and exposed in a burning climate to the
unmitigated rays of the sun, it has shrunk into an elliptical basin,
seven miles in its transverse axis, half filled with smooth water of
the deepest cærulean hue, and half with a solid sheet of glittering
snow-white salt, the offspring of evaporation.” “If,” says Mr. Hugh
Miller, “we suppose, instead of a barrier of lava, that sand-bars were
raised by the surf on a flat arenaceous coast during a slow and equable
sinking of the surface, the waters of the outer gulf might occasionally
topple over the bar, and supply fresh brine when the first stock had
been exhausted by evaporation.”

The Runn of Cutch, as I have shown elsewhere,[3] is a low region near
the delta of the Indus, equal in extent to about a quarter of Ireland,
which is neither land nor sea, being dry during part of every year, and
covered by salt water during the monsoons. Here and there its surface
is incrusted over with a layer of salt caused by the evaporation of
sea-water. A subsiding movement has been witnessed in this country
during earthquakes, so that a great thickness of pure salt might result
from a continuation of such sinking.

TRIAS OF GERMANY.

In Germany, as before hinted, chapter 21, the Trias first received its
name as a Triple Group, consisting of two sandstones with an
intermediate marine calcareous formation, which last is wanting in
England.

NOMENCLATURE OF TRIAS.

German French English Keuper Marnes irisées Saliferous and
gypseous
shales and sandstone. Muschelkalk Muschelkalk, on calcaire
coquillier Wanting in England. Bunter-sandstein Grès
bigarré Sandtone and quartzose conglomerate.

Keuper.—The first of these, or the Keuper, underlying the beds before
described as Rhætic, attains in Würtemberg a thickness of about 1000
feet. It is divided by Alberti into sandstone, gypsum, and carbonaceous
clay-slate.[4] Remains of reptiles called _Nothosaurus_ and
_Phytosaurus,_ have been found in it with Labyrinthodon; the detached
teeth, also, of placoid fish and of Rays, and of the genera
_Saurichthys_ and _Gyrolepis_ (Figs. 387, 388). The plants of the
Keuper are generically very analogous to those of the oolite and lias,
consisting of ferns, equisetaceous plants, cycads, and conifers, with a
few doubtful monocotyledons. A few species such as _Equisetites
columnaris,_ are common to this group and the oolite.

Fig. 397: Equisetites columnaris.
_St. Cassian and Hallstadt Beds_ (see Map, Fig. 398).— The sandstones
and clay of the Keuper resemble the deposits of estuaries and a shallow
sea near the land, and afford, in the N.W. of Germany, as in France and
England, but a scanty representation of the marine life of that period.
We might, however, have anticipated, from its rich reptilian fauna,
that the contemporaneous inhabitants of the sea of the Keuper period
would be very numerous, should we ever have an opportunity of bringing
their remains to light. This, it is believed, has at length been
accomplished, by the position now assigned to certain Alpine rocks
called the “St. Cassian beds,” the true place of which in the series
was until lately a subject of much doubt and discussion. It has been
proved that the Hallstadt beds on the northern flanks of the Austrian
Alps correspond in age with the St. Cassian beds on their southern
declivity, and the Austrian geologists, M. Suess of Vienna and others,
have satisfied themselves that the Hallstadt formation is referable to
the period of the Upper Trias. Assuming this conclusion to be correct,
we become acquainted suddenly and unexpectedly with a rich marine fauna
belonging to a period previously believed to be very barren of organic
remains, because in England, France, and Northern Germany the upper
Trias is chiefly represented by beds of fresh or brackish water origin.

Fig. 398: Map of Tyrol and Styria showing St. Cassian and Hallstadt
Beds.

Fig. 399: Scoliotoma. Fig. 400: Koninckia Leonhardi.
About 600 species of invertebrate fossils occur in the Hallstadt and
St. Cassian beds, many of which are still undescribed; some of the
Mollusca are of new and peculiar genera, as _Scoliostoma,_ Fig. 399,
and _Platystoma,_ Fig. 400, among the Gasteropoda; and _Koninckia,_
Fig. 401, among the Brachiopoda.

Fig. 401: Koninckia Leonhardi.
The following table of genera of marine shells from the Hallstadt and
St. Cassian beds, drawn up first on the joint authority of M. Suess and
the late Dr. Woodward, and since corrected by Messrs. Etheridge and
Tate, shows how many connecting links between the fauna of primary and
secondary Palæozoic and Mesozoic rocks are supplied by the St. Cassian
and Hallstadt beds.

GENERA OF FOSSIL MOLLUSCA IN THE ST. CASSIAN AND HALLSTADT BEDS.

Common to Older Rocks Characteristic Triassic Genera Common to
Newer Rocks Orthoceras
Bactrites
Macrocheilus
Loxonema
Holopella
Murchisonia
Porcellia
Athyris
Retzia
Cyrtina
Euomphalus  Ceratites
Cochloceras
Choristoceras
Rhabdoceras
Aulacoceras
Scoliostoma [5]
Naticella
Platystoma
Ptychostoma
Euchrysalis
Halobia
Hornesia
Amphiclina
Koninckia
Cassianella [6]
Myophoria [6]  Ammonites
Chemnitzia
Cerithium
Monodonta
Opis
Sphoera
Cardita
Myoconcha
Hinnites
Monotis
Plicatula
Pachyrisma
Thecidium

The first column marks the last appearance of several genera which are
characteristic of Palæozoic strata. The second shows those genera which
are characteristic of the Upper Trias, either as peculiar to it, or, as
in the three cases marked by asterisks, reaching their maximum of
development at this era. The third column marks the first appearance in
Triassic rocks of genera destined to become more abundant in later
ages.

It is only, however, when we contemplate the number of species by which
each of the above-mentioned genera are represented that we comprehend
the peculiarities of what is commonly called the St. Cassian fauna.
Thus, for example, the Ammonite, which is not common to older rocks, is
represented by no less than seventy-three species; whereas Loxonema,
which is only known as common to older rocks, furnishes fifteen
Triassic species. Cerithium, so abundant in tertiary strata, and which
still lives, is represented by no less than fourteen species. As the
Orthoceras had never been met with in the marine Muschelkalk, much
surprise was naturally felt that seven or eight species of the genus
should appear in the Hallstadt beds, assuming these last to belong to
the Upper Trias. Among these species are some of large dimensions,
associated with large Ammonites with foliated lobes, a form never seen
before so low in the series, while the Orthoceras had never been seen
so high.

On the whole, the rich marine fauna of Hallstadt and St. Cassian, now
generally assigned to the lowest members of the Upper Trias or Keuper,
leads us to suspect that when the strata of the Triassic age are better
known, especially those belonging to the period of the Bunter
sandstone, the break between the Palæozoic and Mesozoic Periods may be
almost effaced. Indeed some geologists are not yet satisfied that the
true position of the St. Cassian beds (containing so great an admixture
of types, having at once both Mesozoic and Palæozoic affinities) is
made out, and doubt whether they have yet been clearly proved to be
newer than the Muschelkalk.

Muschelkalk.—The next member of the Trias in Germany, the
_Muschelkalk,_ which underlies the _Keuper_ before described, consists
chiefly of a compact greyish limestone, but includes beds of dolomite
in many places, together with gypsum and rock-salt. This limestone, a
formation wholly unrepresented in England, abounds in fossil shells, as
the name implies. Among the Cephalopoda there are no belemnites, and no
ammonites with foliated sutures, as in the Lias, and Oolite, and the
Hallstadt beds; but we find instead a genus allied to the Ammonite,
called _Ceratites_ by de Haan, in which the descending lobes (Fig. 402)
terminate in a few small denticulations pointing inward. Among the
bivalve crustacea, the _Estheria minuta,_ Bronn (see Fig. 390), is
abundant, ranging through the Keuper, Muschelkalk, and
Bunter-sandstein; and _Gervillia socialis_ (Fig. 403), having a similar
range, is found in great numbers in the Muschelkalk of Germany, France,
and Poland.

Fig. 402: Ceratites nodosus. Fig. 403: Gervillia (Avicula) socialis.
Fig. 404: Enerinus liliiformis. Fig. 405: Aspidura loricata.

The abundance of the heads and stems of lily encrinites, _ Encrinus
liliiformis_ (Fig. 404), (or _Encrinites moniliformis_), shows the slow
manner in which some beds of this limestone have been formed in clear
sea-water. The star-fish called _Aspidura loricata_ (Fig. 405) is as
yet peculiar to the Muschelkalk. In the same formation are found the
skull and teeth of a reptile of the genus _Placodus_ (see Fig. 406),
which was referred originally by Munster, and afterwards by Agassiz, to
the class of fishes. But more perfect specimens enabled Professor Owen,
in 1858, to show that this fossil animal was a Saurian reptile, which
probably fed on shell-bearing mollusks, and used its short and flat
teeth, so thickly coated with enamel, for pounding and crushing the
shells.

Fig. 406: Palatal teeth of Placodus gigas.
Fig. 407: Voltzia heterophylla.
Bunter-sandstein.—The _Bunter-sandstein_ consists of various-coloured
sandstones, dolomites, and red clays, with some beds, especially in the
Hartz, of calcareous pisolite or roe-stone, the whole sometimes
attaining a thickness of more than 1000 feet. The sandstone of the
Vosges is proved, by its fossils, to belong to this lowest member of
the Triassic group. At Sulzbad (or Soultz-les-bains), near Strasburg,
on the flanks of the Vosges, many plants have been obtained from the
“bunter,” especially conifers of the extinct genus _Voltzia,_ of which
the fructification has been preserved. (See Fig. 407.) Out of thirty
species of ferns, cycads, conifers, and other plants, enumerated by M.
Ad. Brongniart, in 1849, as coming from the “Grès bigarré,” or Bunter,
not one is common to the Keuper.

The footprints of Labyrinthodon observed in the clays of this formation
at Hildburghausen, in Saxony, have already been mentioned. Some idea of
the variety and importance of the terrestrial vertebrate fauna of the
three members of the Trias in Northern Germany may be derived from the
fact that in the great monograph by the late Hermann von Meyer on the
reptiles of the Trias, the remains of no less than eighty distinct
species are described and figured.

TRIAS OF THE UNITED STATES.

New Red Sandstone of the Valley of the Connecticut River.—In a
depression of the granitic or hypogene rocks in the States of
Massachusetts and Connecticut strata of red sandstone, shale, and
conglomerate are found, occupying an area more than 150 miles in length
from north to south, and about five to ten miles in breadth, the beds
dipping to the eastward at angles varying from 5 to 50 degrees. The
extreme inclination of 50 degrees is rare, and only observed in the
neighbourhood of masses of trap which have been intruded into the red
sandstone while it was forming, or before the newer parts of the
deposit had been completed. Having examined this series of rocks in
many places, I feel satisfied that they were formed in shallow water,
and for the most part near the shore, and that some of the beds were
from time to time raised above the level of the water, and laid dry,
while a newer series, composed of similar sediment, was forming.

Fig. 408: Foot-prints of a bird, Turner’s Falls, Valley of the
Connecticut.

According to Professor Hitchcock, the footprints of no less than
thirty-two species of bipeds, and twelve of quadrupeds, have been
already detected in these rocks. Thirty of these are believed to be
those of birds, four of lizards, two of chelonians, and six of
batrachians. The tracks have been found in more than twenty places,
scattered through an extent of nearly 80 miles from north to south, and
they are repeated through a succession of beds attaining at some points
a thickness of more than 1000 feet.[7]

The bipedal impressions are, for the most part, trifid, and show the
same number of joints as exist in the feet of living tridactylous
birds. Now, such birds have three phalangeal bones for the inner toe,
four for the middle, and five for the outer one (see Fig. 408); but the
impression of the terminal joint is that of the nail only. The fossil
footprints exhibit regularly, where the joints are seen, the same
number; and we see in each continuous line of tracks the three-jointed
and five-jointed toes placed alternately outward, first on the one
side, and then on the other. In some specimens, besides impressions of
the three toes in front, the rudiment is seen of the fourth toe behind.
It is not often that the matrix has been fine enough to retain
impressions of the integument or skin of the foot; but in one fine
specimen found at Turner’s Falls, on the Connecticut, by Dr. Deane,
these markings are well preserved, and have been recognised by
Professor Owen as resembling the skin of the ostrich, and not that of
reptiles.

The casts of the footprints show that some of the fossil bipeds of the
red sandstone of Connecticut had feet four times as large as the living
ostrich, but scarcely, perhaps, larger than the Dinornis of New
Zealand, a lost genus of feathered giants related to the Apteryx, of
which there were many species which have left their bones and almost
entire skeletons in the superficial alluvium of that island. By
referring to what was said of the Iguanodon of the Wealden, the reader
will perceive that the Dinosaur was somewhat intermediate between
reptiles and birds, and left a series of tridactylous impressions on
the sand.

To determine the exact age of the red sandstone and shale containing
these ancient footprints, in the United States, is not possible at
present. No fossil shells have yet been found in the deposit, nor
plants in a determinable state. The fossil fish are numerous and very
perfect; but they are of a peculiar type, called _Ischypterus,_ by Sir
Philip Egerton, from the great size and strength of the fulcral rays of
the dorsal fin, from ischus, strength, and pteron, a fin.

The age of the Connecticut beds cannot be proved by direct
superposition, but may be presumed from the general structure of the
country. That structure proves them to be newer than the movements to
which the Appalachian or Allegheny chain owes its flexures, and this
chain includes the ancient or palæozoic coal-formation among its
contorted rocks.

Coal-field of Richmond, Virginia.—In the State of Virginia, at the
distance of about 13 miles eastward of Richmond, the capital of that
State, there is a coal-field occurring in a depression of the granite
rocks, and occupying a geological position analogous to that of the New
Red Sandstone, above-mentioned, of the Connecticut valley. It extends
26 miles from north to south, and from four to twelve from east to
west.

The plants consist chiefly of zamites, calamites, equiseta, and ferns,
and, upon the whole, are considered by Professor Heer to have the
nearest affinity to those of the European Keuper.

The equiseta are very commonly met with in a vertical position more or
less compressed perpendicularly. It is clear that they grew in the
places where they are now buried in strata of hardened sand and mud. I
found them maintaining their erect attitude, at points many miles
apart, in beds both above and between the seams of coal. In order to
explain this fact, we must suppose such shales and sandstones to have
been gradually accumulated during the slow and repeated subsidence of
the whole region.

Fig. 409: Triassic coal-shale, Richmond, Virginia.
The fossil fish are Ganoids, some of them of the genus _ Catopterus,_
others belonging to the liassic genus _ Tetragonolepis (Æchmodus),_ see
Fig. 376. Two species of _ Entomostraca_ called _Estheria_ are in such
profusion in some shaly beds as to divide them like the plates of mica
in micaceous shales (see Fig. 409).

These Virginian coal-measures are composed of grits, sandstones, and
shales, exactly resembling those of older or primary date in America
and Europe, and they rival, or even surpass, the latter in the richness
and thickness of the coal-seams. One of these, the main seam, is in
some places from 30 to 40 feet thick, composed of pure bituminous coal.
The coal is like the finest kinds shipped at Newcastle, and when
analysed yields the same proportions of carbon and hydrogen—a fact
worthy of notice, when we consider that this fuel has been derived from
an assemblage of plants very distinct specifically, and in part
generically, from those which have contributed to the formation of the
ancient or palæozoic coal.

Triassic Mammifer.—In North Carolina, the late Professor Emmons has
described the strata of the Chatham coal-field, which correspond in age
to those near Richmond, in Virginia. In beds underlying them he has met
with three jaws of a small insectivorous mammal which he has called
_Dromatherium sylvestre,_ closely allied to _Spalacotherium._ Its
nearest living analogue, says Professor Owen, “is found in Myrmecobius;
for each ramus of the lower jaw contained ten small molars in a
continuous series, one canine, and three conical incisors—the latter
being divided by short intervals.”

Low Grade of Early Mammals favourable to the Theory of Progressive
Development.—There is every reason to believe that this fossil
quadruped is at least as ancient as the Microlestes of the European
Trias described in p. 368; and the fact is highly important, as proving
that a certain low grade of marsupials had not only a wide range in
time, from the Trias to the Purbeck, or uppermost oolitic strata of
Europe, but had also a wide range in space, namely, from Europe to
North America, in an east and west direction, and, in regard to
latitude, from Stonesfield, in 52° N., to that of North Carolina, 35°
N.

If the three localities in Europe where the most ancient mammalia have
been found—Purbeck, Stonesfield, and Stuttgart—had belonged all of them
to formations of the same age, we might well have imagined so limited
an area to have been peopled exclusively with pouched quadrupeds, just
as Australia now is, while other parts of the globe were inhabited by
placentals; for Australia now supports one hundred and sixty species of
marsupials, while the rest of the continents and islands are tenanted
by about seventeen hundred species of mammalia, of which only forty-six
are marsupial, namely, the opossums of North and South America. But the
great difference of age of the strata in each of these three localities
seems to indicate the predominance throughout a vast lapse of time
(from the era of the Upper Trias to that of the Purbeck beds) of a low
grade of quadrupeds; and this persistency of similar generic and
ordinal types in Europe while the species were changing, and while the
fish, reptiles, and mollusca were undergoing great modifications, would
naturally lead us to suspect that there must also have been a vast
extension in space of the same marsupial forms during that portion of
the Secondary or Mesozoic epoch which has been termed “the age of
reptiles.” Such an inference as to the wide geographical range of the
ancient marsupials has been confirmed by the discovery in the Trias of
North America of the above-mentioned Dromatherium. The predominance in
earlier ages of these mammalia of a low grade, and the absence, so far
as our investigations have yet gone, of species of higher organisation,
whether aquatic or terrestrial, is certainly in favour of the theory of
progressive development.

 [1] Dr. Wright, on Lias and Bone Bed, Quart. Geol. Journ., 1860, vol.
 xvi.

 [2] Buckland, Proc. Geol. Soc., vol. ii, p. 439; and Murchison and
 Strickland, Geol. Trans., Second Series., vol. v, p. 347.

 [3] Principles of Geology, chap. xxvii.

 [4] Monog. des Bunter-Sandsteins.

 [5] Reaches its maximum in the Trias, but passes down to older rocks.

 [6]
Reach their maximum in the Trias, but pass up to newer rocks.

 [7] Hitchcock, Mem. of Amer. Acad., New Series, vol. iii, p. 129,
 1848.


PRIMARY OR PALÆOZOIC SERIES




CHAPTER XXII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.


Line of Separation between Mesozoic and Palæozoic Rocks. — Distinctness
of Triassic and Permian Fossils. — Term Permian. — Thickness of
calcareous and sedimentary Rocks in North of England. — Upper, Middle,
and Lower Permian. — Marine Shells and Corals of the English Magnesian
Limestone. — Reptiles and Fish of Permian Marl-slate. — Foot-prints of
Reptiles. — Angular Breccias in Lower Permian. — Permian Rocks of the
Continent. — Zechstein and Rothliegendes of Thuringia. — Permian Flora.
— Its generic Affinity to the Carboniferous.

In pursuing our examination of the strata in descending order, we have
next to pass from the base of the Secondary or Mesozoic to the
uppermost or newest of the Primary or Palæozoic formations. As this
point has been selected as a line of demarkation for one of the three
great divisions of the fossiliferous series, the student might
naturally expect that by aid of lithological and palæontological
characters he would be able to recognise without difficulty a distinct
break between the newer and older group. But so far is this from being
the case in Great Britain, that nowhere have geologists found more
difficulty in drawing the line of separation than between the Secondary
and Primary series. The obscurity has arisen from the great resemblance
in colour and mineral character of the Triassic and Permian red marls
and sandstones, and the scarcity and often total absence in them of
organic remains. The thickness of the strata belonging to each group
amounts in some places to several thousand feet; and by dint of a
careful examination of their geological position, and of those fossil,
animal, and vegetable forms which are occasionally met with in some
members of each series, it has at length been made clear that the older
or Permian rocks are more connected with the Primary or Palæozoic than
with the Secondary or Mesozoic strata already described.

The term Permian has been proposed for this group by Sir R. Murchison,
from Perm, a Russian province, where it occupies an area twice the size
of France, and contains a great abundance and variety of fossils, both
vertebrate and invertebrate. Professor Sedgwick in 1832[1] described
what is now recognised as the central member of this group, the
Magnesian limestone, showing that it attained a thickness of 600 feet
along the north-east of England, in the counties of Durham, Yorkshire,
and Nottinghamshire, its lower part often passing into a fossiliferous
marl-slate and resting on an inferior Red Sandstone, the equivalent of
the Rothliegendes of Germany. It has since been shown that some of the
Red Sandstones of newer date also belong to the Permian group; and it
appears from the observations of Mr. Binney, Sir R. Murchison, Mr.
Harkness, and others, that it is in the region where the limestone is
most largely developed, as, for example, in the county of Durham, that
the associated red sandstones or sedimentary rocks are thinnest,
whereas in the country where the latter are thickest the calcareous
member is reduced to thirty, or even sometimes to ten feet. It is
clear, therefore, says Mr. Hull, that the sedimentary region in the
north of England area has been to the westward, and the calcareous area
to the eastward; and that in this group there has been a development
from opposite directions of the two types of strata.

In illustration of this he has given us the following table:

THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND.

  N.W. of England N.E. of England Feet Feet Upper Permian
  (Sedimentary) 600 50–100 Middle Permian
  (Calcareous) 10–30 600 Lower Permian
  (Sedimentary) 3000 100–250[2]

Upper Permian.—What is called in this table the Upper Permian will be
seen to attain its chief thickness in the north-west, or on the coast
of Cumberland, as at St. Bee’s Head, where it is described by Sir
Roderick Murchison as consisting of massive red sandstones with gypsum
resting on a thin course of Magnesian Limestone with fossils, which
again is connected with the Lower Red Sandstone, resembling the upper
one in such a manner that the whole forms a continuous series. No
fossil footprints have been found in this Upper as in the Lower Red
Sandstone.

Middle Permian—Magnesian Limestone and Marl-slate.—This formation is
seen upon the coast of Durham and Yorkshire, between the Wear and the
Tees. Among its characteristic fossils are _Schizodus Schlotheimi_
(Fig. 410) and _Mytilus septifer_ (Fig. 412). These shells occur at
Hartlepool and Sunderland, where the rock assumes an oolitic and
botryoidal character. Some of the beds in this division are
ripple-marked. In some parts of the coast of Durham, where the rock is
not crystalline, it contains as much as 44 per cent of carbonate of
magnesia, mixed with carbonate of lime. In other places—for it is
extremely variable in structure—it consists chiefly of carbonate of
lime, and has concreted into globular and hemispherical masses, varying
from the size of a marble to that of a cannon-ball, and radiating from
the centre. Occasionally earthy and pulverulent beds pass into compact
limestone or hard granular dolomite. Sometimes the limestone appears in
a brecciated form, the fragments which are united together not
consisting of foreign rocks but seemingly composed of the breaking-up
of the Permian limestone itself, about the time of its consolidation.
Some of the angular masses in Tynemouth cliff are two feet in diameter.

Fig. 410: Schidozus Schlotheimi, Permian crystalline limestone. Fig.
411: The hinge of Schizodus truncatus, Permian. Fig. 412: Mytilus
septifer, Permian crystalline limestone.

The magnesian limestone sometimes becomes very fossiliferous and
includes in it delicate bryozoa, one of which, _Fenestella retiformis_
(Fig. 413), is a very variable species, and has received many different
names. It sometimes attains a large size, single specimens measuring
eight inches in width. The same bryozoan, with several other British
species, is also found abundantly in the Permian of Germany.

The total known fauna of the Permian series of Great Britain at present
numbers 147 species, of which 77, or more than half, are mollusca. Not
one of these is common to rocks newer than the Palæozoic, and the
brachiopods are the only group which have furnished species common to
the more ancient or Carboniferous rocks. Of these _Lingula Crednerii_
(Fig. 415) is an example. There are 25 Gasteropods and only one
cephalopod, _Nautilus Freieslebeni,_ which is also found in the German
Zechstein.

Fig. 413: Magnesian Limestone. Fig. 413: Magnesian Limestone, Humbleton
Hill, near Sunderland.[3]

Shells of the genera _Productus_ (Fig. 414) and _ Strophalosia_ (the
latter of allied form with hinge teeth), which do not occur in strata
newer than the Permian, are abundant in the ordinary yellow magnesian
limestone, as will be seen in the valuable memoirs of Messrs. King and
Howse. They are accompanied by certain species of _Spirifera_ (Fig.
416), _Lingula Crednerii_ (Fig. 415), and other brachiopoda of the true
primary or palæozoic type. Some of this same tribe of shells, such as
Camarophoria, allied to Rhynchonella, Spiriferina, and two species of
_Lingula,_ are specifically the same as fossils of the carboniferous
rocks. _Avicula, Arca,_ and _Schizodus_ (Fig. 410), and other
lamellibranchiate bivalves, are abundant, but spiral univalves are very
rare.

Fig. 414: Productus horridus. Fig. 415: Lingula Crednerii. Fig. 416:
Spirifera alata.

Beneath the limestone lies a formation termed the marl-stone, which
consists of hard calcareous shales, marl-slate, and thin-bedded
limestones. At East Thickley, in Durham, where it is thirty feet thick,
this slate has yielded many fine specimens of fossil fish—of the genera
_Palæoniscus_ ten species, _Pygopterus_ two species, _Coelacanthus_ two
species, and _Platysomus_ two species, which as genera are common to
the older Carboniferous formation, but the Permian species are
peculiar, and, for the most part, identical with those found in the
marl-slate or copper-slate of Thuringia.

Fig. 417: Restored outline of a fish of the genus Palæoniscus. Fig.
418: Shark, Heterocercal. Fig. 419: Shad. (Clupea. Herring tribe.)
Homocereal.

The _Palæoniscus_ above mentioned belongs to that division of fishes
which M. Agassiz has called “Heterocercal,” which have their tails
unequally bilobate, like the recent shark and sturgeon, and the
vertebral column running along the upper caudal lobe. (See Fig. 418.)
The “Homocercal” fish, which comprise almost all the 9000 species at
present known in the living creation, have the tail-fin either single
or equally divided; and the vertebral column stops short, and is not
prolonged into either lobe. (See Fig. 419.) Now it is a singular fact,
first pointed out by Agassiz, that the heterocercal form, which is
confined to a small number of genera in the existing creation, is
universal in the magnesian limestone, and all the more ancient
formations. It characterises the earlier periods of the earth’s
history, whereas in the secondary strata, or those newer than the
Permian, the homocercal tail predominates.

A full description has been given by Sir Philip Egerton of the species
of fish characteristic of the marl-slate, in Professor King’s monograph
before referred to, where figures of the ichthyolites, which are very
entire and well preserved, will be found. Even a single scale is
usually so characteristically marked as to indicate the genus, and
sometimes even the particular species. They are often scattered through
the beds singly, and may be useful to a geologist in determining the
age of the rock.

Fig. 420: Palæoniscus comptus. Fig. 421: Palæoniscus elegans. Fig. 422:
Palæoniscus glaphyrus. Fig. 423: Cœlacanthus granulatus. Fig. 424:
Pygopterus mandibularis. Fig. 425: Acrolepis Sedgwickii.

We are indebted to Messrs. Hancock and Howse for the discovery in this
marl-slate at Midderidge, Durham, of two species of _ Protosaurus,_ a
genus of reptiles, one representative of which, _P. Speneri,_ has been
celebrated ever since the year 1810 as characteristic of the
Kupfer-schiefer or Permian of Thuringia. Professor Huxley informs us
that the agreement of the Durham fossil with Hermann von Meyer’s figure
of the German specimen is most striking. Although the head is wanting
in all the examples yet found, they clearly belong to the Lacertian
order, and are therefore of a higher grade than any other vertebrate
animal hitherto found fossil in a Palæozoic rock. Remains of
Labyrinthodont reptiles have also been met with in the same slate near
Durham.

Lower Permian.—The inferior sandstones which lie beneath the marl-slate
consist of sandstone and sand, separating the Magnesian Limestone from
the coal, in Yorkshire and Durham. In some instances, red marl and
gypsum have been found associated with these beds. They have been
classed with the Magnesian Limestone by Professor Sedgwick, as being
nearly co-extensive with it in geographical range, though their
relations are very obscure. But the principal development of Lower
Permian is, as we have seen by Mr. Hull’s table p. 386, in the
northwest, where the Penrith sandstone, as it has been called, and the
associated breccias and purple shales are estimated by Professor
Harkness to attain a thickness of 3000 feet. Organic remains are
generally wanting, but the leaves and wood of coniferous plants, and in
one case a cone, have been found. Also in the purple marls of
Corncockle Muir near Dumfries, very distinct footprints of reptiles
occur, originally referred to the Trias, but shown by Mr. Binney in
1856 to be Permian. No bones of the animals which they represent have
yet been discovered.

_Angular Breccias in Lower Permian._—A striking feature in these beds
is the occasional occurrence, especially at the base of the formation,
of angular and sometimes rounded fragments of Carboniferous and older
rocks of the adjoining districts being included in a paste of red marl.
Some of the angular masses are of huge size.

In the central and southern counties, where the Middle Permian or
Magnesian Limestone is wanting, it is difficult to separate the upper
and lower sandstones, and Mr. Hull is of opinion that the patches of
this formation found here and there in Worcestershire, Shropshire, and
other counties may have been deposited in a sea separated from the
northern basin by a barrier of Carboniferous rocks running east and
west, and now concealed under the Triassic strata of Cheshire. Similar
breccias to those before described are found in the more southern
counties last mentioned, where their appearance is rendered more
striking by the marked contrast they present to the beds of well-rolled
and rounded pebbles of the Trias occupying a large area in the same
region.

Professor Ramsay refers the angular form and large size of the
fragments composing these breccias to the action of floating ice in the
sea. These masses of angular rock, some of them weighing more than half
a ton, and lying confusedly in a red, unstratified marl, like stones in
boulder-drift, are in some cases polished, striated, and furrowed like
erratic blocks in the moraine of a glacier. They can be shown in some
cases to have travelled from the parent rocks, thirty or more miles
distant, and yet not to have lost their angular shape.[4]

Permian Rocks of the Continent.—Germany is the classic ground of the
Magnesian Limestone now called Permian. The formation was well studied
by the miners of that country a century ago as containing a thin band
of dark-coloured cupriferous shale, characterised at Mansfield in
Thuringia by numerous fossil fish. Beneath some variegated sandstones
(not belonging to the Trias, though often confounded with it) they came
down first upon a dolomitic limestone corresponding to the upper part
of our Middle Permian, and then upon a marl-slate richly impregnated
with copper pyrites, and containing fish and reptiles (Protosaurus)
identical in species with those of the corresponding marl-slate of
Durham. To the limestone they gave the name of Zechstein, and to the
marl-slate that of Mergel-schiefer or Kupfer-schiefer. Beneath the
fossiliferous group lies the Rothliegendes or Rothtodt-liegendes,
meaning the red-lyer or red-dead-lyer, so-called by the German miners
from its colour, and because the copper had _died out_ when they
reached this underlying non-metalliferous member of the series. This
red under-lyer is, in fact, a great deposit of red sandstone, breccia,
and conglomerate with associated porphyry, basalt, and amygdaloid.

According to Sir R. Murchison, the Permian rocks are composed, in
Russia, of white limestone, with gypsum and white salt; and of red and
green grits, occasionally with copper ore; also magnesian limestones,
marl-stones, and conglomerates.

Fig. 426: Walchia piniformis.

Permian Flora.—About 18 or 20 species of plants are known in the
Permian rocks of England. None of them pass down into the Carboniferous
series, but several genera, such as _ Alethopteris, Neuropteris,
Walchia,_ and _Ullmania,_ are common to the two groups. The Permian
flora on the Continent appears, from the researches of MM. Murchison
and de Verneuil in Russia, and of MM. Geinitz and von Gutbier in
Saxony, to be, with a few exceptions, distinct from that of the coal.

Fig. 27: Cardiocarpon Ottonis.

In the Permian rocks of Saxony no less than 60 species of fossil plants
have been met with. Two or three of these, as _Calamites gigas,
Sphenopteris erosa,_ and _S. lobata,_ are also met with in the
government of Perm in Russia. Seven others, and among them _Neuropteris
Loshii, Pecopteris arborescens,_ and _P. similis,_ and several species
of _Walchia_ (see Fig. 426), a genus of Conifers, called _Lycopodites_
by some authors, are said by Geinitz to be common to the coal-measures.

Fig. 428: Noeggerathia cuneifolia. Fig. 428: Noeggerathia cuneifolia.
Brongniart.[5]

Among the genera also enumerated by Colonel Gutbier are the fruit
called _Cardiocarpon_ (see Fig. 427), _ Asterophyllites,_ and
Annularia, so characteristic of the Carboniferous period; also
_Lepidodendron,_ which is common to the Permian of Saxony, Thuringia,
and Russia, although not abundant. _Neoggerathia_ (see Fig. 428), the
leaves of which have parallel veins without a midrib, and to which
various generic synonyms, such as _Cordaites, Flabellaria,_ and _
Poacites,_ have been given, is another link between the Permian and
Carboniferous vegetation. Coniferæ, of the Araucarian division, also
occur; but these are likewise met with both in older and newer rocks.
The plants called _Sigillaria_ and _ Stigmaria,_ so marked a feature in
the Carboniferous period, are as yet wanting in the true Permian.

Among the remarkable fossils of the Rothliegendes, or lowest part of
the Permian in Saxony and Bohemia, are the silicified trunks of
tree-ferns called generically _Psaronius._ Their bark was surrounded by
a dense mass of air-roots, which often constituted a great addition to
the original stem, so as to double or quadruple its diameter. The same
remark holds good in regard to certain living extra-tropical
arborescent ferns, particularly those of New Zealand.

Upon the whole, it is evident that the Permian plants approach much
nearer to the Carboniferous flora than to the Triassic; and the same
may be said of the Permian fauna.

 [1] Trans. Geol. Soc. Lond., Second Series, vol. iii, p. 37.

 [2] Edward Hull, Ternary Classification, Quart. Journ. Science, No.
 xxiii, 1869.

 [3] King’s Monograph, pl. 2.

 [4] Ramsay, Quart. Geol. Journ., 1855; and Lyell, Principles of
 Geology, vol. i, p. 223, 10th edit.

 [5] Murchison’s Russia, vol. ii, pl. A, fig. 3.




CHAPTER XXIII.
THE COAL OR CARBONIFEROUS GROUP.


Principal Subdivisions of the Carboniferous Group. — Different
Thickness of the sedimentary and calcareous Members in Scotland and the
South of England. — Coal-measures. — Terrestrial Nature of the Growth
of Coal. — Erect fossil Trees. — Uniting of many Coal-seams into one
thick Bed. — Purity of the Coal explained. — Conversion of Coal into
Anthracite. — Origin of Clay-ironstone. — Marine and brackish-water
Strata in Coal. — Fossil Insects. — Batrachian Reptiles. —
Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia
Coal-measures with successive Growths of erect fossil Trees. —
Similarity of American and European Coal. — Air-breathers of the
American Coal. — Changes of Condition of Land and Sea indicated by the
Carboniferous Strata of Nova Scotia.

Principal Subdivisions of the Carboniferous Group.—The next group which
we meet with in the descending order is the Carboniferous, commonly
called “The Coal,” because it contains many beds of that mineral, in a
more or less pure state, interstratified with sandstones, shales, and
limestones. The coal itself, even in Great Britain and Belgium, where
it is most abundant, constitutes but an insignificant portion of the
whole mass. In South Wales, for example, the thickness of the
coal-bearing strata has been estimated at between 11,000 and 12,000
feet, while the various coal seams, about 80 in number, do not,
according to Professor Phillips, exceed in the aggregate 120 feet.

The Carboniferous formation assumes various characters in different
parts even of the British Islands. It usually comprises two very
distinct members: first, the sedimentary beds, usually called the
Coal-measures, of mixed fresh-water, terrestrial, and marine origin,
often including seams of coal; second, that named in England the
Mountain or Carboniferous Limestone, of purely marine origin, and made
up chiefly of corals, shells, and encrinites, and resting on shales
called the shales of the Mountain Limestone.

In the south-western part of our island, in Somersetshire and South
Wales, the three divisions usually spoken of are:

Coal-measures: Strata of shale, sandstone, and grit, from 600 to 12,000
feet thick, with occasional seams of coal.

Millstone grit: A coarse quartzose sandstone passing into a
conglomerate, sometimes used for millstones, with beds of shale;
usually devoid of coal; occasionally above 600 feet thick.

Mountain or Carboniferous Limestone: A calcareous rock containing
marine shells, corals, and encrinites; devoid of coal; thickness
variable, sometimes more than 1500 feet.

If the reader will refer to the section in Fig. 85, he will see that
the Upper and Lower Coal-measures of the coal-field near Bristol are
divided by a micaceous flaggy sandstone called the Pennant Rock. The
Lower Coal-measures of the same section rest sometimes, especially in
the north part of the basin, on a base of coarse grit called the
Millstone Grit (No. 2 on the previous page).

In the South Welsh coal-field Millstone Grit occurs in like manner at
the base of the productive coal. It is called by the miners the
“Farewell Rock,” as when they reach it they have no longer any hopes of
obtaining coal at a greater depth in the same district. In the central
and northern coal-fields of England this same grit, including quartz
pebbles, with some accompanying sandstones and shales containing coal
plants, acquires a thickness of several thousand feet, lying beneath
the productive coal-measures, which are nearly 10,000 feet thick.

Below the Millstone Grit is a continuation of similar sandstones and
shales called by Professor Phillips the Yoredale series, from Yoredale,
in Yorkshire, where they attain a thickness of from 800 to 1000 feet.
At several intervals bands of limestone divide this part of the series,
one of which, called the Main Limestone or Upper Scar Limestone,
composed in great part of encrinites, is 70 feet thick. Thin seams of
coal also occur in these lower Yoredale beds in Yorkshire, showing that
in the same region there were great alternations in the state of the
surface. For at successive periods in the same area there prevailed
first terrestrial conditions favourable to the growth of pure coal,
secondly, a sea of some depth suited to the formation of Carboniferous
Limestone, and, thirdly, a supply of muddy sediment and sand,
furnishing the materials for sandstone and shale. There is no clear
line of demarkation between the Coal-measures and the Millstone Grit,
nor between the Millstone Grit and underlying Yoredale rocks.

On comparing a series of vertical sections in a north-westerly
direction from Leicestershire and Warwickshire into North Lancashire,
we find, says Mr. Hull, within a distance of 120 miles an augmentation
of the sedimentary materials to the extent of 16,000 feet.

Leicestershire and Warwickshire 2,600 feet North
Staffordshire 9,000 feet South Lancashire 12,130 feet North
Lancashire 18,700 feet

In central England, where the sedimentary beds are reduced to about
3000 feet in all, the Carboniferous Limestone attains an enormous
thickness, as much as 4000 feet at Ashbourne, near Derby, according to
Mr. Hull’s estimate. To a certain extent, therefore, we may consider
the calcareous member of the formation as having originated
simultaneously with the accumulation of the materials of grit,
sandstone, and shale, with seams of coal; just as strata of mud, sand,
and pebbles, several thousand feet thick, with layers of vegetable
matter, are now in the process of formation in the cypress swamps and
delta of the Mississippi, while coral reefs are forming on the coast of
Florida and in the sea of the Bermuda islands. For we may safely
conclude that in the ancient Carboniferous ocean those marine animals
which were limestone builders were never freely developed in areas
where the rivers poured in fresh water charged with sand or clay; and
the limestone could only become several thousand feet thick in parts of
the ocean which remained perfectly clear for ages.

The calcareous strata of the Scotch coal-fields, those of Lanarkshire,
the Lothians, and Fife, for example, are very insignificant in
thickness when compared to those of England. They consist of a few beds
intercalated between the sandstones and shales containing coal and
ironstone, the combined thickness of all the limestones amounting to no
more than 150 feet. The vegetation of some of these northern
sedimentary beds containing coal may be older than any of the
coal-measures of central and southern England, as being coeval with the
Mountain Limestone of the south. In Ireland the limestone predominates
over the coal-bearing sands and shales. We may infer the former
continuity of several of the coal-fields in northern and central
England, not only from the abrupt manner in which they are cut off at
their outcrop, but from their remarkable correspondence in the
succession and character of particular beds. But the limited extent to
which these strata are exposed at the surface is not merely owing to
their former denudation, but even in a still greater degree to their
having been largely covered by the New Red Sandstone, as in Cheshire,
and here and there by the Permian strata, as in Durham.

It has long been the opinion of the most eminent geologists that the
coal-fields of Yorkshire and Lancashire were once united, the upper
Coal-measures and the overlying Millstone Grit and Yoredale rocks
having been subsequently removed; but what is remarkable, is the
ancient date now assigned to this denudation, for it seems that a
thickness of no less than 10,000 feet of the coal-measures had been
carried away before the deposition even of the lower Permian rocks
which were thrown down upon the already disturbed truncated edges of
the coal-strata.[1] The carboniferous strata most productive of
workable coal have so often a basin-shaped arrangement that these
troughs have sometimes been supposed to be connected with the original
conformation of the surface upon which the beds were deposited. But it
is now admitted that this structure has been owing to movements of the
earth’s crust of upheaval and subsidence, and that the flexure and
inclination of the beds has no connection with the original
geographical configuration of the district.

COAL-MEASURES.

I shall now treat more particularly of the productive coal-measures,
and their mode of origin and organic remains.

Coal formed on Land.—In South Wales, already alluded to, where the
coal-measures attain a thickness of 12,000 feet, the beds throughout
appear to have been formed in water of moderate depth, during a slow,
but perhaps intermittent, depression of the ground, in a region to
which rivers were bringing a never-failing supply of muddy sediment and
sand. The same area was sometimes covered with vast forests, such as we
see in the deltas of great rivers in warm climates, which are liable to
be submerged beneath fresh or salt water should the ground sink
vertically a few feet.

In one section near Swansea, in South Wales, where the total thickness
of strata is 3246 feet, we learn from Sir H. De la Beche that there are
ten principal masses of sandstone. One of these is 500 feet thick, and
the whole of them make together a thickness of 2125 feet. They are
separated by masses of shale, varying in thickness from 10 to 50 feet.
The intercalated coal-beds, sixteen in number, are generally from one
to five feet thick, one of them, which has two or three layers of clay
interposed, attaining nine feet. At other points in the same coal-field
the shales predominate over the sandstones. Great as is the diversity
in the horizontal extent of individual coal-seams, they all present one
characteristic feature, in having, each of them, what is called its
_underclay._ These underclays, co-extensive with every layer of coal,
consist of arenaceous shale, sometimes called fire-stone, because it
can be made into bricks which stand the fire of a furnace. They vary in
thickness from six inches to more than ten feet; and Sir William Logan
first announced to the scientific world in 1841 that they were regarded
by the colliers in South Wales as an essential accompaniment of each of
the eighty or more seams of coal met with in their coal-field. They are
said to form the _floor_ on which the coal rests; and some of them have
a slight admixture of carbonaceous matter, while others are quite
blackened by it.

All of them, as Sir William Logan pointed out, are characterised by
inclosing a peculiar species of fossil vegetable called _ Stigmaria,_
to the exclusion of other plants. It was also observed that, while in
the overlying shales, or “roof” of the coal, ferns and trunks of trees
abound without any _ Stigmariæ,_ and are flattened and compressed,
those singular plants of the underclay most commonly retain their
natural forms, unflattened and branching freely, and sending out their
slender rootlets, formerly thought to be leaves, through the mud in all
directions. Several species of _Stigmaria_ had long been known to
botanists, and described by them, before their position under each seam
of coal was pointed out, and before their true nature as the roots of
trees (some having been actually found attached to the base of
_Sigillaria_ stumps) was recognised. It was conjectured that they might
be aquatic, perhaps floating plants, which sometimes extended their
branches and leaves freely in fluid mud, in which they were finally
enveloped.

Now that all agree that these underclays are ancient soils, it follows
that in every instance where we find them they attest the terrestrial
nature of the plants which formed the overlying coal, which consists of
the trunks, branches, and leaves of the same plants. The trunks have
generally fallen prostrate in the coal, but some of them still remain
at right angles to the ancient soils (see Fig. 440). Professor Goppert,
after examining the fossil vegetables of the coal-fields of Germany,
has detected, in beds of pure coal, remains of plants of every family
hitherto known to occur fossil in the carboniferous rocks. Many seams,
he remarks, are rich in _Sigillariæ, Lepidodendra,_ and _Stigmariæ,_
the latter in such abundance as to appear to form the bulk of the coal.
In some places, almost all the plants were calamites, in others
ferns.[2]

Between the years 1837 and 1840, six fossil trees were discovered in
the coal-fields of Lancashire, where it is intersected by the Bolton
railway. They were all at right angles to the plane of the bed, which
dips about 15 degrees to the south. The distance between the first and
the last was more than 100 feet, and the roots of all were imbedded in
a soft argillaceous shale. In the same plane with the roots is a bed of
coal, eight or ten inches thick, which has been found to extend across
the railway, or to the distance of at least ten yards. Just above the
covering of the roots, yet beneath the coal-seam, so large a quantity
of the _Lepidostrobus variabilis_ was discovered inclosed in nodules of
hard clay, that more than a bushel was collected from the small
openings around the base of some of the trees (see Fig. 457 of this
genus). The exterior trunk of each was marked by a coating of friable
coal, varying from one-quarter to three-quarters of an inch in
thickness; but it crumbled away on removing the matrix. The dimensions
of one of the trees is 15½ feet in circumference at the base, 7½ feet
at the top, its height being eleven feet. All the trees have large
spreading roots, solid and strong, sometimes branching, and traced to a
distance of several feet, and presumed to extend much farther.

In a colliery near Newcastle a great number of _ Sigillariæ_ occur in
the rock as if they had retained the position in which they grew. No
less than thirty, some of them four or five feet in diameter, were
visible within an area of 50 yards square, the interior being
sandstone, and the bark having been converted into coal. Such vertical
stems are familiar to our coal-miners, under the name of coal-pipes.
They are much dreaded, for almost every year in the Bristol, Newcastle,
and other coal-fields, they are the cause of fatal accidents. Each
cylindrical cast of a tree, formed of solid sandstone, and increasing
gradually in size towards the base, and being without branches, has its
whole weight thrown downward, and receives no support from the coating
of friable coal which has replaced the bark. As soon, therefore, as the
cohesion of this external layer is overcome, the heavy column falls
suddenly in a perpendicular or oblique direction from the roof of the
gallery whence coal has been extracted, wounding or killing the workman
who stands below. It is strange to reflect how many thousands of these
trees fell originally in their native forests in obedience to the law
of gravity; and how the few which continued to stand erect, obeying,
after myriads of ages, the same force, are cast down to immolate their
human victims.

It has been remarked that if, instead of working in the dark, the miner
was accustomed to remove the upper covering of rock from each seam of
coal, and to expose to the day the soils on which ancient forests grew,
the evidence of their former growth would be obvious. Thus in South
Staffordshire a seam of coal was laid bare in the year 1844, in what is
called an open work at Parkfield colliery, near Wolverhampton. In the
space of about a quarter of an acre the stumps of no less than 73 trees
with their roots attached appeared, as shown in Fig. 429, some of them
more than eight feet in circumference. The trunks, broken off close to
the root, were lying prostrate in every direction, often crossing each
other. One of them measured 15, another 30 feet in length, and others
less. They were invariably flattened to the thickness of one or two
inches, and converted into coal. Their roots formed part of a stratum
of coal ten inches thick, which rested on a layer of clay two inches
thick, below which was a second forest resting on a two-foot seam of
coal. Five feet below this, again, was a third forest with large stumps
of _Lepidodendra, Calamites,_ and other trees.

Fig. 429: Ground plan of fossil forest, Parkfield Colliery, near
Wolverhampton, showing the position of 73 trees in a quarter of an ace.
Blending of Coal-seams.—Both in England and North America seams of coal
are occasionally observed to be parted from each other by layers of
clay and sand, and, after they have been persistent for miles, to come
together and blend in one single bed, which is then found to be equal
in the aggregate to the thickness of the several seams. I was shown by
Mr. H. D. Rogers a remarkable example of this in Pennsylvania. In the
Shark Mountain, near Pottsville, in that State, there are thirteen
seams of anthracite coal, some of them more than six feet thick,
separated by beds of white quartzose grit and a conglomerate of quartz
pebbles, often of the size of a hen’s egg. Between Pottsville and the
Lehigh Summit Mine, seven of these seams of coal, at first widely
separated, are, in the course of several miles, brought nearer and
nearer together by the gradual thinning out of the intervening
coarse-grained strata and their accompanying shales, until at length
they successively unite and form one mass of coal between forty and
fifty feet thick, very pure on the whole, though with a few thin
partings of clay. This mass of coal I saw quarried in the open air at
Mauch Chunk, on the Bear Mountain. The origin of such a vast thickness
of vegetable remains, so unmixed, on the whole, with earthy
ingredients, can be accounted for in no other way than by the growth,
during thousands of years, of trees and ferns in the manner of peat—a
theory which the presence of the Stigmaria _in situ_ under each of the
seven layers of anthracite fully bears out. The rival hypothesis, of
the drifting of plants into a sea or estuary, leaves the
non-intermixture of sediment, or of clay, sand, and pebbles, with the
pure coal wholly unexplained.

The late Mr. Bowman was the first who gave a satisfactory explanation
of the manner in which distinct coal-seams, after maintaining their
independence for miles, may at length unite, and then persist
throughout another wide area with a thickness equal to that which the
separate seams had previously maintained.

Fig. 430: Uniting of distinct coal-seams.

Let A C (Fig. 430) be a three-foot seam of coal originally laid down as
a mass of vegetable matter on the level area of an extensive swamp,
having an under-clay, _f g,_ through which the Stigmariæ or roots of
the trees penetrate as usual. One portion, B C, of this seam of coal is
now inclined; the area of the swamp having subsided as much as 25 feet
at E C, and become for a time submerged under salt, fresh, or brackish
water. Some of the trees of the original forest A B C fell down, others
continued to stand erect in the new lagoon, their stumps and part of
their trunks becoming gradually enveloped in layers of sand and mud,
which at length filled up the new piece of water C E.

When this lagoon has been entirely silted up and converted into land,
the forest-covered surface A B will extend once more over the whole
area A B E, and a second mass of vegetable matter, D E, forming three
feet more of coal, will accumulate. We then find in the region E C two
seams of coals, each three feet thick, with their respective
under-clays, with erect buried trees based upon the surface of the
lower coal, the two seams being separated by 25 feet of intervening
shale and sandstone. Whereas in the region A B, where the growth of the
forest has never been interrupted by submergence, there will simply be
one seam, two yards thick, corresponding to the united thickness of the
beds B E and B C. It may be objected that the uninterrupted growth of
plants during the interval of time required for the filling up of the
lagoon will have caused the vegetable matter in the region D A B to be
thicker than the two distinct seams E and C, and no doubt there would
actually be a slight excess representing one or more generation of
trees and plants forming the undergrowth; but this excess of vegetable
matter, when compressed into coal, would be so insignificant in
thickness that the miner might still affirm that the seam D A
throughout the area D A B was equal to the two seams C and E.

Cause of the Purity of Coal.—The purity of the coal itself, or the
absence in it of earthy particles and sand, throughout areas of vast
extent, is a fact which appears very difficult to explain when we
attribute each coal-seam to a vegetation growing in swamps. It has been
asked how, during river inundations capable of sweeping away the leaves
of ferns and the stems and roots of _Sigillariæ_ and other trees, could
the waters fail to transport some fine mud into the swamps? One
generation after another of tall trees grew with their roots in mud,
and their leaves and prostrate trunks formed layers of vegetable
matter, which was afterwards covered with mud since turned to shale.
Yet the coal itself, or altered vegetable matter, remained all the
while unsoiled by earthy particles. This enigma, however perplexing at
first sight, may, I think, be solved by attending to what is now taking
place in deltas. The dense growth of reeds and herbage which
encompasses the margins of forest-covered swamps in the valley and
delta of the Mississippi is such that the fluviatile waters, in passing
through them, are filtered and made to clear themselves entirely before
they reach the areas in which vegetable matter may accumulate for
centuries, forming coal if the climate be favourable. There is no
possibility of the least intermixture of earthy matter in such cases.
Thus in the large submerged tract called the “Sunk Country,” near New
Madrid, forming part of the western side of the valley of the
Mississippi, erect trees have been standing ever since the year
1811-12, killed by the great earthquake of that date; lacustrine and
swamp plants have been growing there in the shallows, and several
rivers have annually inundated the whole space, and yet have been
unable to carry in any sediment within the outer boundaries of the
morass, so dense is the marginal belt of reeds and brush-wood. It may
be affirmed that generally, in the “cypress swamps” of the Mississippi,
no sediment mingles with the vegetable matter accumulated there from
the decay of trees and semi-aquatic plants. As a singular proof of this
fact, I may mention that whenever any part of a swamp in Louisiana is
dried up, during an unusually hot season, and the wood set on fire,
pits are burnt into the ground many feet deep, or as far down as the
fire can descend without meeting with water, and it is then found that
scarcely any residuum or earthy matter is left. At the bottom of all
these “cypress swamps” a bed of clay is found, with roots of the tall
cypress (_Taxodium distichum_), just as the under-clays of the coal are
filled with _Stigmaria._

Conversion of Coal into Anthracite.—It appears from the researches of
Liebig and other eminent chemists, that when wood and vegetable matter
are buried in the earth exposed to moisture, and partially or entirely
excluded from the air, they decompose slowly and evolve carbonic acid
gas, thus parting with a portion of their original oxygen. By this
means they become gradually converted into lignite or wood-coal, which
contains a larger proportion of hydrogen than wood does. A continuance
of decomposition changes this lignite into common or bituminous coal,
chiefly by the discharge of carbureted hydrogen, or the gas by which we
illuminate our streets and houses. According to Bischoff, the
inflammable gases which are always escaping from mineral coal, and are
so often the cause of fatal accidents in mines, always contain carbonic
acid, carbureted hydrogen, nitrogen, and olefiant gas. The
disengagement of all these gradually transforms ordinary or bituminous
coal into anthracite, to which the various names of glance-coal, coke,
hard-coal, culm, and many others, have been given.

There is an intimate connection between the extent to which the coal
has in different regions parted with its gaseous contents, and the
amount of disturbance which the strata have undergone. The coincidence
of these phenomena may be attributed partly to the greater facility
afforded for the escape of volatile matter, when the fracturing of the
rocks has produced an infinite number of cracks and crevices. The gases
and water which are made to penetrate these cracks are probably
rendered the more effective as metamorphic agents by increased
temperature derived from the interior. It is well known that, at the
present period, thermal waters and hot vapours burst out from the earth
during earthquakes, and these would not fail to promote the
disengagement of volatile matter from the Carboniferous rocks.

In Pennsylvania the strata of coal are horizontal to the westward of
the Alleghany Mountains, where the late Professor H. D. Rogers pointed
out that they were most bituminous; but as we travel south-eastward,
where they no longer remain level and unbroken, the same seams become
progressively debitumenized in proportion as the rocks become more bent
and distorted. At first, on the Ohio River, the proportion of hydrogen,
oxygen, and other volatile matters ranges from forty to fifty per cent.
Eastward of this line, on the Monongahela, it still approaches forty
per cent, where the strata begin to experience some gentle flexures. On
entering the Alleghany Mountains, where the distinct anticlinal axes
begin to show themselves, but before the dislocations are considerable,
the volatile matter is generally in the proportion of eighteen or
twenty per cent. At length, when we arrive at some insulated
coal-fields associated with the boldest flexures of the Appalachian
chain, where the strata have been actually turned over, as near
Pottsville, we find the coal to contain only from six per cent of
volatile matter, thus becoming a genuine anthracite.

Clay-ironstone.—Bands and nodules of clay-ironstone are common in
coal-measures, and are formed, says Sir H. De la Beche, of carbonate of
iron mingled mechanically with earthy matter, like that constituting
the shales. Mr. Hunt, of the Museum of Practical Geology, instituted a
series of experiments to illustrate the production of this substance,
and found that decomposing vegetable matter, such as would be
distributed through all coal strata, prevented the further oxidation of
the proto-salts of iron, and converted the peroxide into protoxide by
taking a portion of its oxygen to form carbonic acid. Such carbonic
acid, meeting with the protoxide of iron in solution, would unite with
it and form a carbonate of iron; and this mingling with fine mud, when
the excess of carbonic acid was removed, might form beds or nodules of
argillaceous ironstone.[3]

Intercalated Marine Beds in Coal.—Both in the coal-fields of Europe and
America the association of fresh, brackish-water, and marine strata
with coal-seams of terrestrial origin is frequently recognised. Thus,
for example, a deposit near Shrewsbury, probably formed in brackish
water, has been described by Sir R. Murchison as the youngest member of
the coal-measures of that district, at the point where they are in
contact with the overlying Permian group. It consists of shales and
sandstones about 150 feet thick, with coal and traces of plants;
including a bed of limestone varying from two to nine feet in
thickness, which is cellular, and resembles some lacustrine limestones
of France and Germany. It has been traced for 30 miles in a straight
line, and can be recognised at still more distant points. The
characteristic fossils are a small bivalve, having the form of a
_Cyclas_ or _Cyrena,_ also a small entomostracan, _Cythere inflata_
(Fig. 432), and the microscopic shell of an annelid of an extinct genus
called _ Microconchus_ (Fig. 431), allied to _Spirorbis._ In the
coal-field of Yorkshire there are fresh-water strata, some of which
contain shells referred to the family Unionidæ; but in the midst of the
series there is one thin but very widely-spread stratum, abounding in
fishes and marine shells, such as _ Goniatites Listeri_ (Fig. 433),
_Orthoceras,_ and _ Aviculopecten papyraceus,_ Goldf. (Fig. 434).

Fig. 431: Microconchus (Spirorbis) carbonarius. Fig. 432: Cythere
(Leperditia) inflata. Fig. 433: Goniatites Listeri. Fig. 434:
Aviculopecten papyraceus.
Insects in European Coal.—Articulate animals of the genus Scorpion were
found by Count Sternberg in 1835 in the coal-measures of Bohemia, and
about the same time in those of Coalbrook Dale by Mr. Prestwich, were
also true insects, such as beetles of the family _Curculionidæ,_ a
neuropterous insect of the genus _Corydalis,_ and another related to
the _Phasmidæ,_ have been found.

From the coal of Wetting, in Westphalia, several specimens of the
cockroach or _Blatta_ family, and the wing of a cricket (_Acridites_)
have been described by Germar. Professor Goldenberg published, in 1854,
descriptions of no less than twelve species of insects from the nodular
clay-ironstone of Saarbrück, near Trèves.[4] Among them are several _
Blattinæ,_ three species of _Neuroptera,_ one beetle of the _Scarabæus_
family, a grasshopper or locust, _ Gryllacris_ (see Fig. 435), and
several white ants or Termites. Professor Goldenberg showed me, in
1864, the wing of a white ant, found low down in the productive
coal-measures of Saarbrück, in the interior of a flattened
Lepidodendron. It is much larger than that of any known living species
of the same genus.

Fig. 435: Wing of a Grasshopper. Gryllacris lithanthraca.

Fig. 436: Archegosaurus minor. Fossil reptile from the coal-measures,
Saarbrück.
Batrachian Reptiles in Coal.—No vertebrated animals more highly
organised than fish were known in rocks of higher antiquity than the
Permian until the year 1844, when the _Apateon pedestris,_ Meyer, was
discovered in the coal-measures of Munster-Appel in Rhenish Bavaria,
and three years later, in 1847, Professor von Dechen found three other
distinct species of the same family of Amphibia in the Saarbruck
coal-field above alluded to. These were described by the late Professor
Goldfuss under the generic name of _Archegosaurus._ The skulls, teeth,
and the greater portions of the skeleton, nay, even a large part of the
skin, of two of these reptiles have been faithfully preserved in the
centre of spheroidal concretions of clay-ironstone. The largest of
these, _Archegosaurus Decheni,_ must have been three feet six inches
long. Figure 436 represents the skull and neck bones of the smallest of
the three, of the natural size. They were considered by Goldfuss as
saurians, but by Herman von Meyer as most nearly allied to the
_Labyrinthodon_ before mentioned (p. 371), and the remains of the
extremities leave no doubt they were quadrupeds, “provided,” says Von
Meyer, “with hands and feet terminating in distinct toes; but these
limbs were weak, serving only for swimming or creeping.” The same
anatomist has pointed out certain points of analogy between their bones
and those of the _Proteus anguinus_; and Professor Owen has observed
that they make an approach to the _Proteus_ in the shortness of their
ribs. Two specimens of these ancient reptiles retain a large part of
the outer skin, which consisted of long, narrow, wedge-shaped,
tile-like, and horny scales, arranged in rows (see Fig. 437).

Fig. 437: Imbricated covering of skin of Archegosaurus medius.
In 1865, several species belonging to three different genera of the
same family of perennibranchiate Batrachians were found in the
coal-field of Kilkenny in bituminous shale at the junction of the coal
with the underlying Stigmaria-bearing clay. They were, probably,
inhabitants of a marsh, and the large processes projecting from the
vertebræ of their tail imply, according to Professor Huxley, great
powers of swimming. They were of the Labyrinthodont family, and their
association with the fish of the coal, of which so large a proportion
are ganoids, reminds us that the living perennibranchiate amphibia of
America frequent the same rivers as the ganoid Lepidostei or bony
pikes.

_Labyrinthodont footprints in coal-measures._—In 1844, the very year
when the Apateon, before mentioned, of the coal was first met with in
the country between the Moselle and the Rhine, Dr. King published an
account of the footprints of a large reptile discovered by him in North
America. These occur in the coal-strata of Greensburg, in Westmoreland
County, Pennsylvania; and I had an opportunity of examining them when
in that country in 1846. The footmarks were first observed standing out
in relief from the lower surface of slabs of sandstone, resting on thin
layers of fine unctuous clay. I brought away one of these masses, which
is represented in Fig. 438. It displays, together with footprints, the
casts of cracks (_a, a′_) of various sizes. The origin of such cracks
in clay, and casts of the same, has before been explained, and referred
to the drying and shrinking of mud, and the subsequent pouring of sand
into open crevices. It will be seen that some of the cracks, as at _b,
c,_ traverse the footprints, and produce distortion in them, as might
have been expected, for the mud must have been soft when the animal
walked over it and left the impressions; whereas, when it afterwards
dried up and shrank, it would be too hard to receive such indentations.

Fig. 438: Slab of sandstone from the coal-measures of Pennsylvania,
with foot-prints of air-breathing reptile and casts of cracks.

We may assume that the reptile which left these prints on the ancient
sands of the coal-measures was an air-breather, because its weight
would not have been sufficient under water to have made impressions so
deep and distinct. The same conclusion is also borne out by the casts
of the cracks above described, for they show that the clay had been
exposed to the air and sun, so as to have dried and shrunk.

Nova Scotia Coal-measures.—The sedimentary strata in which thin seams
of coal occur attain a thickness, as we have seen, of 18,000 feet in
the north of England exclusive of the Mountain Limestone, and are
estimated by Von Dechen at over 20,000 feet in Rhenish Prussia. But the
finest example in the world of a natural exposure in a continuous
section ten miles long, occurs in the sea-cliffs bordering a branch of
the Bay of Fundy, in Nova Scotia. These cliffs, called the “South
Joggins,” which I first examined in 1842, and afterwards with Dr.
Dawson in 1845, have lately been admirably described by the
last-mentioned geologist[5] in detail, and his evidence is most
valuable as showing how large a portion of this dense mass was formed
on land, or in swamps where terrestrial vegetation flourished, or in
fresh-water lagoons. His computation of the thickness of the whole
series of carboniferous strata as exceeding three miles, agrees with
the measurement made independently by Sir William Logan in his survey
of this coast.

There is no reason to believe that in this vast succession of strata,
comprising some marine as well as many fresh-water and terrestrial
formations, there is any repetition of the same beds. There are no
faults to mislead the geologist, and cause him to count the same beds
over more than once, while some of the same plants have been traced
from the top to the bottom of the whole series, and are distinct from
the flora of the antecedent Devonian formation of Canada. Eighty-one
seams of coal, varying in thickness from an inch to about five feet,
have been discovered, and no less than seventy-one of these have been
actually exposed in the sea-cliffs.

In the section (Fig. 439), which I examined in 1842, the beds from _c_
to _i_ are seen all dipping the same way, their average inclination
being at an angle of 24° S.S.W. The vertical height of the cliffs is
from 150 to 200 feet; and between _d_ and _g_—in which space I observed
seventeen trees in an upright position, or, to speak more correctly, at
right angles to the planes of stratification—I counted nineteen seams
of coal, varying in thickness from two inches to four feet. At low tide
a fine horizontal section of the same beds is exposed to view on the
beach, which at low tide extends sometimes 200 yards from the base of
the cliff. The thickness of the beds alluded to, between _d_ and _g,_
is about 2500 feet, the erect trees consisting chiefly of large
_Sigillariæ,_ occurring at ten distinct levels, one above the other.
The usual height of the buried trees seen by me was from six to eight
feet; but one trunk was about 25 feet high and four feet in diameter,
with a considerable bulge at the base. In no instance could I detect
any trunk intersecting a layer of coal, however thin; and most of the
trees terminated downward in seams of coal. Some few only were based on
clay and shale; none of them, except _ Calamites,_ on sandstone. The
erect trees, therefore, appeared in general to have grown on beds of
vegetable matter. In the underclays _Stigmaria_ abounds.

Fig. 439: Section of the cliffs of the South Joggins, near Minudie,
Nova Scotia.

These root-bearing beds have been found under all the coal-seams, and
such old soils are at present the most destructible masses in the whole
cliff, the sandstones and laminated shales being harder and more
capable of resisting the action of the waves and the weather.
Originally the reverse was doubtless true, for in the existing delta of
the Mississippi those clays in which the innumerable roots of the
deciduous cypress and other swamp trees ramify in all directions are
seen to withstand far more effectually the undermining power of the
river, or of the sea at the base of the delta, than do beds of loose
sand or layers of mud not supporting trees. It is obvious that if this
sand or mud be afterwards consolidated and turned to sandstone and hard
shale, it would be the least destructible.

In regard to the plants, they belonged to the same genera, and most of
them to the same species, as those met with in the distant coal-fields
of Europe. Dr. Dawson has enumerated more than 150 species, two-thirds
of which are European, a greater agreement than can be said to exist
between the same Nova Scotia flora and that of the coal-fields of the
United States. By referring to the section, Fig. 439, the position of
the four-foot coal will be perceived, and in Fig. 440 (a section made
by me in 1842 of a small portion) that from _e_ to _f_ of the same
cliff is exhibited, in order to show the manner of occurrence of erect
fossil trees at right angles to the planes of the inclined strata.

Fig. 440: Erect fossil trees, Coal-measures, Nova Scotia.
In the sandstone which filled their interiors, I frequently observed
fern-leaves, and sometimes fragments of _Stigmaria,_ which had
evidently entered together with sediment after the trunk had decayed
and become hollow, and while it was still standing under water. Thus
the tree, _a,_ Fig. 440, represented in the bed _e_ in the section,
Fig. 439, is a hollow trunk five feet eight inches in length,
traversing various strata, and cut off at the top by a layer of clay
two feet thick, on which rests a seam of coal (_b,_ Fig. 440) one foot
thick. On this coal again stood two large trees (_c_ and _d_), while at
a greater height the trees _f_ and _g_ rest upon a thin seam of coal
(_e_), and above them is an underclay, supporting the four-foot coal.

Occasionally the layers of matter in the inside of the tree are more
numerous than those without; but it is more common in the coal-measures
of all countries to find a cylinder of pure sandstone—the cast of the
interior of a tree—intersecting a great many alternating beds of shale
and sandstone, which originally enveloped the trunk as it stood erect
in the water. Such a want of correspondence in the materials outside
and inside, is just what we might expect if we reflect on the
difference of time at which the deposition of sediment will take place
in the two cases; the imbedding of the tree having gone on for many
years before its decay had made much progress. In many places distinct
proof is seen that the enveloping strata took years to accumulate, for
some of the sandstones surrounding erect sigillarian trunks support at
different levels roots and stems of _Calamites_; the _Calamites_ having
begun to grow after the older _Sigillariæ_ had been partially buried.

The general absence of structure in the interior of the large fossil
trees of the Coal implies the very durable nature of their bark, as
compared with their woody portion. The same difference of durability of
bark and wood exists in modern trees, and was first pointed out to me
by Dr. Dawson, in the forests of Nova Scotia, where the Canoe Birch
(_Betula papyracea_) has such tough bark that it may sometimes be seen
in the swamps looking externally sound and fresh, although consisting
simply of a hollow cylinder with all the wood decayed and gone. When
portions of such trunks have become submerged in the swamps they are
sometimes found filled with mud. One of the erect fossil trees of the
South Joggins fifteen feet in height, occurring at a higher level than
the main coal, has been shown by Dr. Dawson to have a coniferous
structure, so that some _Coniferæ_ of the Coal period grew in the same
swamps as _Sigillariæ,_ just as now the deciduous Cypress (_Taxodium
distichum_) abounds in the marshes of Louisiana even to the edge of the
sea.

When the carboniferous forests sank below high-water mark, a species of
_Spirorbis_ or _Serpula_ (Fig. 431), attached itself to the outside of
the stumps and stems of the erect trees, adhering occasionally even to
the interior of the bark—another proof that the process of envelopment
was very gradual. These hollow upright trees, covered with innumerable
marine annelids, reminded me of a “cane-brake,” as it is commonly
called, consisting of tall reeds, _Arundinaria macrosperma,_ which I
saw in 1846, at the Balize, or extremity of the delta of the
Mississippi. Although these reeds are fresh-water plants, they were
covered with barnacles, having been killed by an incursion of
salt-water over an extent of many acres, where the sea had for a season
usurped a space previously gained from it by the river. Yet the dead
reeds, in spite of this change, remained standing in the soft mud,
enabling us to conceive how easily the larger _ Sigillariæ,_ hollow as
they were but supported by strong roots, may have resisted an incursion
of the sea.

The high tides of the Bay of Fundy, rising more than 60 feet, are so
destructive as to undermine and sweep away continually the whole face
of the cliffs, and thus a new crop of erect fossil trees is brought
into view every three or four years. They are known to extend over a
space between two and three miles from north to south, and more than
twice that distance from east to west, being seen in the banks of
streams intersecting the coal-field.

_Structure of Coal._—The bituminous coal of Nova Scotia is similar in
composition and structure to that of Great Britain, being chiefly
derived from sigillarioid trees mixed with leaves of ferns and of a
Lycopodiaceous tree called _ Cordaites_ (_Noeggerathia,_ etc., for
genus, see Fig. 428), supposed by Dawson to have been deciduous, and
which had broad parallel veined leaves without a mid-rib. On the
surface of the seams of coal are large quantities of mineral charcoal,
which doubtless consist, as Dr. Dawson suggests, of fragments of wood
which decayed in the open air, as would naturally be expected in swamps
where so many erect trees were preserved. Beds of cannel-coal display,
says Dr. Dawson, such a microscopical structure and chemical
composition as shows them to have been of the nature of fine vegetable
mud such as accumulates in the shallow ponds of modern swamps. The
underclays are loamy soils, which must have been sufficiently above
water to admit of drainage, and the absence of sulphurets, and the
occurrence of carbonate of iron in them, prove that when they existed
as soils, rain-water, and not sea-water, percolated them. With the
exception, perhaps, of _Asterophyllites_ (see Fig. 461), there is a
remarkable absence from the coal-measures of any form of vegetation
properly aquatic, the true coal being a sub-aërial accumulation in soil
that was wet and swampy but not permanently submerged.

Air-breathers of the Coal.—If we have rightly interpreted the evidence
of the former existence at more than eighty different levels of forests
of trees, some of them of vast extent, and which lasted for ages,
giving rise to a great accumulation of vegetable matter, it is natural
to ask whether there were not many air-breathing inhabitants of these
same regions. As yet no remains of mammalia or birds have been found, a
negative character common at present to all the Palæozoic formations;
but in 1852 the osseous remains of a reptile, the first ever met with
in the carboniferous strata of the American continent, were found by
Dr. Dawson and myself. We detected them in the interior of one of the
erect Sigillariæ before alluded to as of such frequent occurrence in
Nova Scotia. The tree was about two feet in diameter, and consisted of
an external cylinder of bark, converted into coal, and an internal
stony axis of black sandstone, or rather mud and sand stained black by
carbonaceous matter, and cemented together with fragments of wood into
a rock. These fragments were in the state of charcoal, and seem to have
fallen to the bottom of the hollow tree while it was rotting away. The
skull, jaws, and vertebræ of a reptile, probably about 2½ feet in
length (_Dendrerpeton Acadianum,_ Owen), were scattered through this
stony matrix. The shell, also, of a _ Pupa_ (see Fig. 442), the first
land-shell ever met with in the coal or in beds older than the
tertiary, was observed in the same stony mass. Dr. Wyman of Boston
pronounced the reptile to be allied in structure to _Menobranchus_ and
_Menopoma,_ species of batrachians, now inhabiting the North American
rivers. The same view was afterwards confirmed by Professor Owen, who
also pointed out the resemblance of the cranial plates to those seen in
the skull of _Archegosaurus_ and _Labyrinthodon._[6] Whether the
creature had crept into the hollow tree while its top was still open to
the air, or whether it was washed in with mud during a flood, or in
whatever other manner it entered, must be matter of conjecture.

Footprints of two reptiles of different sizes had previously been
observed by Dr. Harding and Dr. Gesner on ripple-marked flags of the
lower coal-measures in Nova Scotia (No. 2, Fig. 447), evidently made by
quadrupeds walking on the ancient beach, or out of the water, just as
the recent Menopoma is sometimes observed to do. The remains of a
second and smaller species of Dendrerpeton, _D. Oweni,_ were also found
accompanying the larger one, and still retaining some of its dermal
appendages; and in the same tree were the bones of a third small
lizard-like reptile, _Hylonomus Lyelli,_ seven inches long, with stout
hind limbs, and fore limbs comparatively slender, supposed by Dr.
Dawson to be capable of walking and running on land.[7]

Fig. 441: Xylobius Sigillariæ. Coal, Nova Scotia.

In a second specimen of an erect stump of a hollow tree 15 inches in
diameter, the ribbed bark of which showed that it was a Sigillaria, and
which belonged to the same forest as the specimen examined by us in
1852, Dr. Dawson obtained not only fifty specimens of Pupa vetusta
(Fig. 442), and nine skeletons of reptiles belonging to four species,
but also several examples of an articulated animal resembling the
recent centipede or gally-worm, a creature which feeds on decayed
vegetable matter (see Fig. 441). Under the microscope, the head, with
the eyes, mandible, and labrum, are well seen. It is interesting, as
being the earliest known representative of the myriapods, none of which
had previously been met with in rocks older than the oolite or
lithographic slate of Germany.

Fig. 442: Pupa vetusta.
Some years after the discovery of the first Pupa, Dr. Dawson, carefully
examining the same great section containing so many buried forests in
the cliffs of Nova Scotia, discovered another bed, separated from the
tree containing Dendrerpeton by a mass of strata more than 1200 feet
thick. As there were 21 seams of coal in this intervening mass, the
length of time comprised in the interval is not to be measured by the
mere thickness of the sandstones and shales. This lower bed is an
underclay seven feet thick, with stigmarian rootlets, and the small
land-shells occurring in it are in all stages of growth. They are
chiefly confined to a layer about two inches thick, and are unmixed
with any aquatic shells. They were all originally entire when imbedded,
but are most of them now crushed, flattened, and distorted by pressure;
they must have been accumulated, says Dr. Dawson, in mud deposited in a
pond or creek.

Fig. 443: Zonites (Conulus) priseus.

The surface striæ of _Pupa vetusta,_ when magnified 50 diameters,
present exactly the same appearance as a portion corresponding in size
of the common English _Pupa juniperi,_ and the internal hexagonal
cells, magnified 500 diameters, show the internal structure of the
fossil and recent Pupa to be identical. In 1866[8] Dr. Dawson
discovered in this lower bed, so full of the Pupa, another land-shell
of the genus Helix (sub-genus Zonites), see Fig. 443.

None of the reptiles obtained from the coal-measures of the South
Joggins are of a higher grade than the Labyrinthodonts, but some of
these were of very great size, two caudal vertebræ found by Mr. Marsh
in 1862 measuring two and a half inches in diameter, and implying a
gigantic aquatic reptile with a powerful swimming tail.

Except some obscure traces of an insect found by Dr. Dawson in a
coprolite of a terrestrial reptile occurring in a fossil tree, no
specimen of this class has been brought to light in the Joggins. But
Mr. James Barnes found in a bed of shale at Little Grace Bay, Cape
Breton, the wing of an Ephemera, which must have measured seven inches
from tip to tip of the expanded wings—larger than any known living
insect of the Neuropterous family.

That we should have made so little progress in obtaining a knowledge of
the terrestrial fauna of the Coal is certainly a mystery, but we have
no reason to wonder at the extreme rarity of insects, seeing how few
are known in the carboniferous rocks of Europe, worked for centuries
before America was discovered, and now quarried on so enormous a scale.
These European rocks have not yet produced a single land-shell, in
spite of the millions of tons of coal annually extracted, and the many
hundreds of soils replete with the fossil roots of trees, and the erect
trunks and stumps preserved in the position in which they grew. In many
large coal-fields we continue as much in the dark respecting the
invertebrate air-breathers then living, as if the coal had been thrown
down in mid-ocean. The early date of the carboniferous strata cannot
explain the enigma, because we know that while the land supported a
luxuriant vegetation, the contemporaneous seas swarmed with life—with
Articulata, Mollusca, Radiata, and Fishes. The perplexity in which we
are involved when we attempt to solve this problem may be owing partly
to our want of diligence as collectors, but still more perhaps to
ignorance of the laws which govern the fossilisation of land-animals,
whether of high or low degree.

Carboniferous Rain-prints.—At various levels in the coal measures of
Nova Scotia, ripple-marked sandstones, and shales with rain-prints,
were seen by Dr. Dawson and myself, but still more perfect impressions
of rain were discovered by Mr. Brown, near Sydney, in the adjoining
island of cape Breton. They consist of very delicate markings on
greenish slates, accompanied by worm-tracks (_a, b,_ Fig. 444), such as
are often seen between high and low water mark on the recent mud of the
Bay of Fundy.

The great humidity of the climate of the Coal period had been
previously inferred from the number of its ferns and the continuity of
its forests for hundreds of miles; but it is satisfactory to have at
length obtained such positive proofs of showers of rain, the drops of
which resembled in their average size those which now fall from the
clouds. From such data we may presume that the atmosphere of the
Carboniferous period corresponded in density with that now investing
the globe, and that different currents of air varied then as now in
temperature, so as to give rise, by their mixture, to the condensation
of aqueous vapour.

Fig. 444: Carboniferous rain-prints with worm tracks on green shale,
from Cape Breton, Nova Scotia. Fig. 445: Casts of rain-prints on a
portion of the same slab (Fig. 444), seen to project on the underside
of an incumbent layer of arenaceous shale.

Folding and Denudation of the Beds indicated by the Nova Scotia
Coal-strata.—The series of events which are indicated by the great
section of the coal-strata in Nova Scotia consist of a gradual and
long-continued subsidence of a tract which throughout most of the
period was in the state of a delta, though occasionally submerged
beneath a sea of moderate depth. Deposits of mud and sand were first
carried down into a shallow sea on the low shores of which the
footprints of reptiles were sometimes impressed (see p. 407).

Fig. 446: Cone and branch of Lepidodendron corrugatum.
Though no regular seams of coal were formed, the characteristic
imbedded coal-plants are of the genera _Cyclopteris_ and _
Alethopteris,_ agreeing with species occurring at much higher levels,
and distinct from those of the antecedent Devonian group. The
_Lepidodendron corrugatum_ (see Fig. 446), a plant predominating in the
Lower Carboniferous group of Europe, is also conspicuous in these
shallow-water beds, together with many fishes and entomostracans. A
more rapid rate of subsidence sometimes converted part of the sea into
deep clear water, in which there was a growth of coral which was
afterwards turned into crystalline limestone, and parts of it,
apparently by the action of sulphuric acid, into gypsum. In spite of
continued sinking, amounting to several thousand feet, the sea might in
time have been rendered shallow by the growth of coral, had not its
conversion into land or swampy ground been accelerated by the pouring
in of sand and the advance of the delta accompanied with such
fluviatile and brackish-water formations as are common in lagoons.

The amount to which the bed of the sea sank down in order to allow of
the formation of so vast a thickness of rock of sedimentary and organic
origin is expressed by the total thickness of the Carboniferous strata,
including the coal-measures, No. 1, and the rocks which underlie them,
No. 2, Fig. 447.

Fig. 447: Diagram showing the curvature and supposed denudation of the
Carboniferous strata in Nova Scotia.

After the strata No. 2 had been elaborated, the conditions proper to a
great delta exclusively prevailed, the subsidence still continuing so
that one forest after another grew and was submerged until their
under-clays with roots, and usually seams of coal, were left at more
than eighty distinct levels. Here and there, also, deposits bearing
testimony to the existence of fresh or brackish-water lagoons, filled
with calcareo-bituminous mud, were formed. In these beds (_h_ and _i,_
Fig. 439) are found fresh-water bivalves or mussels allied to Anodon,
though not identical with that or any living genus, and called
_Naiadites carbonarius_ by Dawson. They are associated with small
entomostracous crustaceans of the genus Cythere, and scales of small
fishes. Occasionally some of the calamite brakes and forests of
Sigillariæ and Coniferæ were exposed in the flood season, or sometimes,
perhaps, by slight elevatory movements to the denuding action of the
river or the sea.

In order to interpret the great coast section exposed to view on the
shores of the Bay of Fundy, the student must, in the first place,
understand that the newest or last-mentioned coal formations would have
been the only ones known to us (for they would have covered all the
others), had there not been two great movements in opposite directions,
the first consisting of a general sinking of three miles, which took
place during the Carboniferous Period, and the second an upheaval of
more limited horizontal extent, by which the anticlinal axis A was
formed. That the first great change of level was one of subsidence is
proved by the fact that there are shallow-water deposits at the base of
the Carboniferous series, or in the lowest beds of No. 2.

Subsequent movements produced in the Nova Scotia and the adjoining New
Brunswick coal-fields the usual anticlinal and synclinal flexures. In
order to follow these, we must survey the country for about thirty
miles round the South Joggins, or the region where the erect trees
described in the foregoing pages are seen. As we pass along the cliffs
for miles in a southerly direction, the beds containing these fossil
trees, which were mentioned as dipping about 18° south, are less and
less inclined, until they become nearly horizontal in the valley of a
small river called the Shoulie, as ascertained by Dr. Dawson. After
passing this synclinal line the beds begin to dip in an opposite or
north-easterly direction, acquiring a steep dip where they rest
unconformably on the edges of the Upper Silurian strata of the Cobequid
Hills, as shown in Fig. 447. But if we travel northward towards Minudie
from the region of the coal-seams and buried forests, we find the dip
of the coal-strata increasing from an angle of 18° to one of more than
40°, lower beds being continually exposed to view until we reach the
anticlinal axis A and see the lower Carboniferous formation, No. 2, at
the surface. The missing rocks removed by denudation are expressed by
the faint lines at A, and thus the student will see that, according to
the principles laid down in the seventh chapter, we are enabled, by the
joint operations of upheaval and denudation, to look, as it were, about
three miles into the interior of the earth without passing beyond the
limits of a single formation.

 [1] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 327.

 [2] Quart. Geol. Journ., vol. v, Mem., p. 17.

 [3] Memoirs of the Geol. Survey, pp. 51, 255, etc.

 [4] Dunker and V. Meyer, Palæont., vol. iv, p. 17.

 [5] Acadian Geology, 2nd edit., 1868.

 [6] Quart. Geol. Journ., vol. ix, p. 58.

 [7] Dawson, Air-Breathers of the Coal in Nova Scotia, Montreal, 1863.

 [8] Dawson, Acadian Geology, 1868, p. 385.




CHAPTER XXIV.
FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD.


Vegetation of the Coal Period. — Ferns, Lycopodiaceæ, Equisetaceæ,
Sigillariæ, Stigmariæ, Coniferæ. — Angiosperms. — Climate of the Coal
Period. — Mountain Limestone. — Marine Fauna of the Carboniferous
Period. — Corals. — Bryozoa, Crinoidea. — Mollusca. — Great Number of
fossil Fish. — Foraminifera.

Vegetation of the Coal Period.—In the last chapter we have seen that
the seams of coal, whether bituminous or anthracitic, are derived from
the same species of plants, and Goppert has ascertained that the
remains of every family of plants scattered through the shales and
sandstones of the coal-measures are sometimes met with in the pure coal
itself—a fact which adds greatly to the geological interest of this
flora.

The coal-period was called by Adolphe Brongniart the age of
Acrogens,[1] so great appears to have been the numerical preponderance
of flowerless or cryptogamic plants of the families of ferns,
club-mosses, and horse-tails. He reckoned the known species in 1849 at
500, and the number has been largely increased by recent research in
spite of reductions owing to the discovery that different parts of even
the same plants had been taken for distinct species. Notwithstanding
these changes, Brongniart’s generalisation concerning this flora still
holds true, namely, that the state of the vegetable world was then
extremely different from that now prevailing, not only because the
cryptogamous plants constituted nearly the whole flora, but also
because they were, on the whole, more highly developed than any
belonging to the same class now existing, and united some forms of
structure now only found separately and in distinct orders. The only
phænogamous plants were constitute any feature in the coal are the
coniferæ; monocotyledonous angiosperms appear to have been very rare,
and the dicotyledonous, with one or two doubtful exceptions, were
wanting. For this we are in some measure prepared by what we have seen
of the Secondary or Mesozoic floras if, consistently with the belief in
the theory of evolution, we expect to find the prevalence of simpler
and less specialised organisms in older rocks.

Ferns.—We are struck at the first glance with the similarity of the
ferns to those now living. In the fossil genus _Pecopteris,_ for
example (Fig. 448), it is not easy to decide whether the fossils might
not be referred to the same genera as those established for living
ferns; whereas, in regard to some of the other contemporary families of
plants, with the exception of the fir tribe, it is not easy to guess
even the class to which they belong. The ferns of the Carboniferous
period are generally without organs of fructification, but in the few
instances in which these do occur in a fit state for microscopical
investigations they agree with those of the living ferns.

Fig. 448: Pecopteris elliptica. Fig. 449: Caulopteris primæva. Fig.
448: _Pecopteris elliptica_, Bunbury.[2] Frostburg.
Fig. 449: _Caulopteris primæva_, Lindley.

When collecting fossil specimens from the coal-measures of Frostburg,
in Maryland, I found in the iron-shales several species with
well-preserved rounded spots or marks of the sori (see Fig. 448). In
the general absence of such characters they have been divided into
genera distinguished chiefly by the branching of the fronds and the way
in which the veins of the leaves are disposed. The larger portion are
supposed to have been of the size of ordinary European ferns, but some
were decidedly arborescent, especially the group called _Caulopteris_
(see Fig. 449) by Lindley, and the _Psaronius_ of the upper or newest
coal-measures, before alluded to (p. 393). All the recent tree-ferns
belong to one tribe (_Polypodiaceæ_), and to a small number only of
genera in that tribe, in which the surface of the trunk is marked with
scars, or cicatrices, left after the fall of the fronds. These scars
resemble those of _Caulopteris._

No less than 130 species of ferns are enumerated as having been
obtained from the British coal-strata, and this number is more than
doubled if we include the Continental and American species. Even if we
make some reduction on the ground of varieties which have been
mistaken, in the absence of their fructification, for species, still
the result is singular, because the whole of Europe affords at present
no more than sixty-seven indigenous species.

Living tree-ferns of different genera. Fig. 450: Tree-fern from Isle of
Bourbon. Fig. 451: Cyathea glauca, Mauritius. Fig. 452: Tree-fern from
Brazil.

Lycopodiaceæ—_Lepidodendron._—About forty species of fossil plants of
the Coal have been referred to this genus, more than half of which are
found in the British coal-measures. They consist of cylindrical stems
or trunks, covered with leaf-scars. In their mode of branching, they
are always dichotomous (see Fig. 454). They belong to the
_Lycopodiaceæ,_ bearing sporangia and spores similar to those of the
living representatives of this family (Fig. 457); and although most of
the Carboniferous species grew to the size of large trees, Mr.
Carruthers has found by careful measurement that the volume of the
fossil spores did not exceed that of the recent club-moss, a fact of
some geological importance, as it may help to explain the facility with
which these seeds may have been transported by the wind, causing the
same wide distribution of the species of the fossil forests in Europe
and America which we now observe in the geographical distribution of so
many living families of cryptogamous plants.

Lepidodendrum Sternbergii. Coal-measures, near Newcastle. Fig. 453:
Branching trunk, 49 feet long, supposed to have belonged to L.
Sternbergii. Fig. 454: Branching stem with bark and leaves of L.
Sternbergii. Fig. 455: Portion of same, nearer the root.

Fig. 456: a. Lycopodium densum. Living species, New Zealand; b. Branch;
c. Part of same, magnified.
The Figs. 453–455 represent a fossil _Lepidodendron,_ 49 feet long,
found in Jarrow Colliery, near Newcastle, lying in shale parallel to
the planes of stratification. Fragments of others, found in the same
shale, indicate, by the size of the rhomboidal scars which cover them,
a still greater magnitude.

The living club-mosses, of which there are about 200 species, are most
abundant in tropical climates. They usually creep on the ground, but
some stand erect, as the _Lycopodium densum_ from New Zealand (see Fig.
456), which attains a height of three feet.

In the Carboniferous strata of Coalbrook Dale, and in many other
coal-fields, elongated cylindrical bodies, called fossil cones, named
_Lepidostrobus_ by M. Adolphe Brongniart, are met with. (See Fig. 457.)
They often form the nucleus of concretionary balls of clay-ironstone,
and are well preserved, exhibiting a conical axis, around which a great
quantity of scales were compactly imbricated. The opinion of M.
Brongniart that the _ Lepidostrobus_ is the fruit of _Lepidodendron_
has been confirmed, for these _strobili_ or fruits have been found
terminating the tip of a branch of a well-characterised _
Lepidodendron_ in Coalbrook Dale and elsewhere.

Fig. 457: a. Lepidostrobus ornatus; b. Portion of a section, showing
the large sporangia in their natural position, and each supported by
its bract or scale; c. Spores in these sporangia, highly magnified.

Fig. 458: Calamites Sucowii, common throughout Europe. Fig. 459: Stem
of Fig. 458, as retored by Dr. Dawson.

Equisetaceæ.—To this family belong two fossil genera of the coal,
_Equisetites_ and _Calamites._ The Calamites were evidently closely
related to the modern horse-tails (Equiseta) differing principally in
their great size, the want of sheaths at the joints, and some details
of fructification. They grew in dense brakes on sandy and muddy flats
in the manner of modern Equisetaceæ, and their remains are frequent in
the coal. Seven species of this plant occur in the great Nova Scotia
section before described, where the stems of some of them five inches
in diameter, and sometimes eight feet high, may be seen terminating
downward in a tapering root (see Fig. 460).

Fig. 460: Radical termination of a Calamite. Fig. 461: Asterophyllites
foliosus, Coal-measures, Newcastle.

Botanists are not yet agreed whether the _Asterophyllites,_ a species
of which is represented in Fig. 461, can form a separate genus from the
Calamite, from which, however, according to Dr. Dawson, its foliage is
distinguished by a true mid-rib, which is wanting in the leaves known
to belong to some Calamites.

Fig. 462: Annularia sphenophylloides.Fig. 463: Sphenophyllum erosum.
Figs. 462 and 463 represent leaves of _Annularia_ and _ Sphenophyllum,_
common in the coal, and believed by Mr. Carruthers to be leaves of
Calamites. Dr. Williamson, who has carefully studied the Calamites,
thinks that they had a fistular pith, exogenous woody stem, and thick
smooth bark, which last having always disappeared, leaves a fluted
stem, as represented in Fig. 459.

Sigillaria.—A large portion of the trees of the Carboniferous period
belonged to this genus, of which as many as 28 species are enumerated
as British. The structure, both internal and external, was very
peculiar, and, with reference to existing types, very anomalous. They
were formerly referred, by M. Ad. Brongniart, to ferns, which they
resemble in the scalariform texture of their vessels and, in some
degree, in the form of the cicatrices left by the base of the
leaf-stalks which have fallen off (see Fig. 464). But some of them are
ascertained to have had long linear leaves, quite unlike those of
ferns. They grew to a great height, from 30 to 60, or even 70 feet,
with regular cylindrical stems, and without branches, although some
species were dichotomous towards the top. Their fluted trunks, from one
to five feet in diameter, appear to have decayed more rapidly in the
interior than externally, so that they became hollow when standing; and
when thrown prostrate, they were squeezed down and flattened. Hence, we
find the bark of the two opposite sides (now converted into bright
shining coal) constitute two horizontal layers, one upon the other,
half an inch, or an inch, in their united thickness. These same trunks,
when they are placed obliquely or vertically to the planes of
stratification, retain their original rounded form, and are
uncompressed, the cylinder of bark having been filled with sand, which
now affords a cast of the interior.

Fig. 464: Sigillaria lævigata.
Dr. Hooker inclined to the belief that the _ Sigillariæ_ may have been
cryptogamous, though more highly developed than any flowerless plants
now living. Dr. Dawson having found in some species what he regards as
medullary rays, thinks with Brongniart that they have some relation to
gymnogens, while Mr. Carruthers leans to the opinion that they belong
to the Lycopodiaceæ.

_Stigmaria._—This fossil, the importance of which has already been
pointed out in p. 398, was originally conjectured to be an aquatic
plant. It is now ascertained to be the root of _Sigillaria._ The
connection of the roots with the stem, previously suspected, on
botanical grounds, by Brongniart, was first proved, by actual contact,
in the Lancashire coal-field, by Mr. Binney. The fact has lately been
shown, even more distinctly, by Mr. Richard Brown, in his description
of the _Stigmariæ_ occurring in the under-clays of the coal-seams of
the Island of Cape Breton, in Nova Scotia. In a specimen of one of
these, represented in Fig. 465, the spread of the roots was sixteen
feet, and some of them sent out rootlets, in all directions, into the
surrounding clay.

Fig. 465: Stigmaria attached to a trunk of Sigillaria.

In the sea-cliffs of the South Joggins in Nova Scotia, I examined
several erect _Sigillariæ,_ in company with Dr. Dawson, and we found
that from the lower extremities of the trunk they sent out _Stigmariæ_
as roots. All the stools of the fossil trees dug out by us divided into
four parts, and these again bifurcated, forming eight roots, which were
also dichotomous when traceable far enough. The cylindrical rootlets
formerly regarded as leaves are now shown by more perfect specimens to
have been attached to the root by fitting into deep cylindrical pits.
In the fossil there is rarely any trace of the form of these cavities,
in consequence of the shrinkage of the surrounding tissues. Where the
rootlets are removed, nothing remains on the surface of the Stigmaria
but rows of mammillated tubercles (see Figs. 466, 467), which have
formed the base of each rootlet.

Fig. 466: Stigmaria ficoides. Fig. 467: Surface of another individual
of same species, showing form of tubercles.

These protuberances may possibly indicate the place of a joint at the
lower extremity of the rootlet. Rows of these tubercles are arranged
spirally round each root, which have always a medullary axis and woody
system much resembling that of _Sigillaria,_ the structure of the
vessels being, like it, scalariform.

Coniferæ.—The coniferous trees of this period are referred to five
genera; the woody structure of some of them showing that they were
allied to the Araucarian division of pines, more than to any of our
common European firs. Some of their trunks exceeded forty-four feet in
height. Many, if not all of them, seem to have differed from living
_Coniferæ_ in having large piths; for Professor Williamson has
demonstrated the fossil of the coal-measures called _Sternbergia_ to be
the pith of these trees, or rather the cast of cavities formed by the
shrinking or partial absorption of the original medullary axis (see
Figs. 468, 469). This peculiar type of pith is observed in living
plants of very different families, such as the common Walnut and the
White Jasmine, in which the pith becomes so reduced as simply to form a
thin lining of the medullary cavity, across which transverse plates of
pith extend horizontally, so as to divide the cylindrical hollow into
discoid interspaces. When these interspaces have been filled up with
inorganic matter, they constitute an axis to which, before their true
nature was known, the provisional name of _Sternbergia_ (_d, d,_ Fig.
468) was given. In the above specimen the structure of the wood (_b,_
Figs. 468 and 469) is coniferous, and the fossil is referable to
Endlicher’s fossil genus _ Dadoxylon._

Fig. 468: Fragment of coniferous wood. Fig. 468: Fragment of coniferous
wood, _Dadoxylon_, of Endlicher, fractured longitudinally; from
Coalbrook Dale.
W.C. Williamson[3]

Fig. 469: Magnified portion of Fig. 468; transverse section.

The fossil named _Trigonocarpon_ (Figs. 470 and 471), formerly supposed
to be the fruit of a palm, may now, according to Dr. Hooker, be
referred, like the _Sternbergia,_ to the _ Coniferæ._ Its geological
importance is great, for so abundant is it in the coal-measures, that
in certain localities the fruit of some species may be procured by the
bushel; nor is there any part of the formation where they do not occur,
except the under-clays and limestone. The sandstone, ironstone, shales,
and coal itself, all contain them. Mr. Binney has at length found in
the clay-ironstone of Lancashire several specimens displaying
structure, and from these, says Dr. Hooker, we learn that the _
Trigonocarpon_ belonged to that large section of existing coniferous
plants which bear fleshy solitary fruits, and not cones. It resembled
very closely the fruit of the Chinese genus _ Salisburia,_ one of the
Yew tribe, or Taxoid conifers.

Fig. 470: Trigonocarpum ovatum.Fig. 471: Trigonocarpum olivæforme.
Fig. 472: Antholithes.
Angiosperms.—The curious fossils called _ Antholithes_ by Lindley have
usually been considered to be flower spikes, having what seems a calyx
and linear petals (see Fig. 472). Dr. Hooker, after seeing very perfect
specimens, also thought that they resembled the spike of a
highly-organised plant in full flower, such as one of the
_Bromeliaceæ,_ to which Professor Lindley had at first compared them.
Mr. Carruthers, who has lately examined a large series in different
museums, considers it to be a dicotyledonous angiosperm allied to _
Orobanche_ (broom-rape), which grew, not on the soil, but parasitically
on the trees of the coal forests.

In the coal-measures of Granton, near Edinburgh, a remarkable fossil
(Fig. 473) was found and described in 1840,[4] by Dr. Robert Paterson.
It was compressed between layers of bituminous shale, and consists of a
stem bearing a cylindrical spike, _a,_ which in the portion preserved
in the slate exhibits two subdivisions and part of a third. The spike
is covered on the exposed surface with the four-cleft calyces of the
flowers arranged in parallel rows. The stem shows, at _b,_ a little
below the spike, remains of a lateral appendage, which is supposed to
indicate the beginning of the spathe. The fossil has been referred to
the _ Aroidiæ,_ and there is every probability that it is a true member
of this order. There can at least be no doubt as to the high grade of
its organisation, and that it belongs to the monocotyledonous
angiosperms. Mr. Carruthers has carefully examined the original
specimen in the Botanical Museum, Edinburgh, and thinks it may have
been an epiphyte.

Fig. 473: Pothocites Grantonii.
Climate of the Coal Period.—As to the climate of the Coal, the Ferns
and the Coniferæ are perhaps the two classes of plants which may be
most relied upon as leading us to safe conclusions, as the genera are
nearly allied to living types. All botanists admit that the abundance
of ferns implies a moist atmosphere. But the coniferæ, says Hooker, are
of more doubtful import, as they are found in hot and dry, and in cold
and dry climates; in hot and moist, and in cold and moist regions. In
New Zealand the coniferæ attain their maximum in numbers, constituting
1/62 part of all the flowering plants; whereas in a wide district
around the Cape of Good Hope they do not form 1/1600 of the phenogamic
flora. Besides the conifers, many species of ferns flourish in New
Zealand, some of them arborescent, together with many lycopodiums; so
that a forest in that country may make a nearer approach to the
carboniferous vegetation than any other now existing on the globe.

MARINE FAUNA OF THE CARBONIFEROUS PERIOD.

It has already been stated that the Carboniferous or Mountain Limestone
underlies the coal-measures in the South of England and Wales, whereas
in the North, and in Scotland, marine calcareous rocks partly of the
age of the Mountain Limestone alternate with shales and sandstones,
containing seams of coal. In its most calcareous form the Mountain
Limestone is destitute of land-plants, and is loaded with marine
remains—the greater part, indeed, of the rock being made up bodily of
crinoids, corals, and bryozoa with interspersed mollusca.

Corals.—The corals deserve especial notice, as the cup-and-star corals,
which have the most massive and stony skeletons, display peculiarities
of structure by which they may be distinguished generally, as MM. Milne
Edwards and Haime first pointed out, from all species found in strata
newer than the Permian. There is, in short, an ancient or _Palæozoic,_
and a modern or _Neozoic_ type, if, by the latter term, we designate
(as proposed by Professor E. Forbes) all strata from the triassic to
the most modern, inclusive. The accompanying diagrams (Figs. 474, 475)
may illustrate these types.

Fig. 474: Palæozoic type of lamelliferous cup-shaped Coral.

Vertical section of _Campophyllum flexuosum,_ (_Cyathophyllum,_
Goldfuss); from the Devonian of the Eifel. The lamellæ are seen around
the inside of the cup; the walls consist of cellular tissue; and large
transverse plates, called _ tubulæ,_ divide the interior into chambers.

Arrangement of the _lamellæ_ in _Polycoelia profunda,_ Germar, sp.;
from the Magnesian Limestone, Durham. This diagram shows the
quadripartite arrangement of the primary septa, characteristic of
palæozoic corals, there being four principal and eight intermediate
lamellæ, the whole number in this type being always a multiple of four.

_Stauria astræiformis,_ Milne Edwards. Young group, natural size. Upper
Silurian, Gothland. The lamellæ or septal system in each cup are
divided by four prominent ridges into four groups.

Fig. 475: Neozoic type of lamelliferous cup-shaped Coral.
_Parasmilia centralis,_ Mantell, sp. Vertical section. Upper Chalk,
Gravesend. In this type the lamellæ are massive, and extend to the axis
or columella composed of loose cellular tissue, without any transverse
plates like those in Fig. 474, _ a._

_Cyathina Bowerbankii,_ Ed. and H. Transverse section, enlarged. Gault,
Folkestone. In this coral the primary septa are a multiple of six. The
twelve principal plates reach the columella, and between each pair
there are three secondaries, in all forty-eight. The short intermediate
plates which proceed from the columella are not counted. They are
called _pali._

_Fungia patellaris,_ Lamarck. Recent; very young state. Diagram of its
six primary and six secondary septa, magnified. The sextuple
arrangement is always more manifest in the young than in the adult
state.

It will be seen that the more ancient corals have what is called a
quadripartite arrangement of the chief plates or _ lamellæ_—parts of
the skeleton which support the organs of reproduction. The number of
these lamellæ in the Palæozoic type is 4, 8, 16, etc.; while in the
Neozoic type the number is 6, 12, 24, or some other multiple of six;
and this holds good, whether they be simple forms, as in Figs. 474, _
a,_ and 475, _a,_ or aggregate clusters of corallites, as in 474, _c._
But further investigations have shown in this, as in all similar grand
generalisations in natural history, that there are excepions to the
rule. Thus in the Lower Greensand _ Holocystis elegans_ (Ed. and H.)
and other forms have the Palæozoic type, and Dr. Duncan has shown to
what extent the Neozoic forms penetrate downward into the Carboniferous
and Devonian rocks.

Fig. 476: Lithostrotion basaltiforme. Fig. 477: Lonsdaleia floriformis.

From a great number of lamelliferous corals met with in the Mountain
Limestone, two species (Figs. 476, 477) have been selected, as having a
very wide range, extending from the eastern borders of Russia to the
British Isles, and being found almost everywhere in each country. These
fossils, together with numerous species of _Zaphrentis, Amplexus,
Cyathophyllum, Clisiophyllum, Syringopora,_ and _Michelinia,_[5] form a
group of rugose corals widely different from any that followed them.

Bryozoa and Crinoidea.—Of the _Bryozoa,_ the prevailing forms are
_Fenestella, Hemitrypa,_ and _ Polypora,_ and these often form
considerable beds. Their net-like fronds are easily recognised.
_Crinoidea_ are also numerous in the Mountain Limestone (see Figs. 478,
479), two genera, _Pentremites_ and _Codonaster,_ being peculiar to
this formation in Europe and North America.

Fig. 478: Cyathocrinus planus. Fig. 479: Cyathocrinus caryocrinoides.

Fig. 480: Palæchinus gigas.
In the greater part of them, the cup or pelvis, Figure 479, _ b,_ is
greatly developed in size in proportion to the arms, although this is
not the case in Fig. 478. The genera _ Poteriocrinus, Cyathocrinus,
Pentremites, Actinocrinus,_ and _ Platycrinus,_ are all of them
characteristic of this formation. Other Echinoderms are rare, a few
Sea-Urchins only being known: these have a complex structure, with many
more plates on their surface than are seen in the modern genera of the
same group. One genus, the _Palæchinus_ (Fig. 480), is the analogue of
the modern _Echinus,_ but has four, five, or six rows of plates in the
interambulacral region or area, whereas the modern genera have only
two. The other, _Archæocidaris,_ represents, in like manner, the
_Cidaris_ of the present seas.

Mollusca.—The British Carboniferous mollusca enumerated by Mr.
Etheridge[6] comprise 653 species referable to 86 genera, occurring
chiefly in the Mountain Limestone. Of this large number only 40 species
are common to the underlying Devonian rocks, 9 of them being
Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda
(or Palliobranchiates). This latter group constitutes the larger part
of the Carboniferous Mollusca, 157 species being known in Great Britain
alone, and it will be found to increase in importance in the fauna of
the primary rocks the lower we descend in the series. Perhaps the most
characteristic shells of the formation are large species of _
Productus,_ such as _P. giganteus, p. hemisphericus, P.
semireticulatus_ (Fig. 481), and _P. scabriculus._ Large plaited
spirifers, as _Spirifera striata, S. rotundata,_ and _S. trigonalis_
(Fig. 482), also abound; and smooth species, such as _Spirifera glabra_
(Fig. 483), with its numerous varieties.

Fig. 481: Productus semireticulatus. Fig. 482: Spirifera trigonalis.

Fig. 483: Spirifera glabra.
Fig. 484: Terebratula hastata. Fig. 485: Aviculopecten sublobatus. Fig.
486: Pleurotomaria carinata.

Among the brachiopoda, _Terebratula hastata_ (Fig. 484) deserves
mention, not only for its wide range, but because it often retains the
pattern of the original coloured stripes which ornamented the living
shell. These coloured bands are also preserved in several
lamellibranchiate bivalves, as in _ Aviculopecten_ (Fig. 485), in which
dark stripes alternate with a light ground. In some also of the spiral
univalves the pattern of the original painting is distinctly retained,
as in _ Pleurotomaria_ (Fig. 486), which displays wavy blotches,
resembling the colouring in many recent trochidæ.

Fig. 487: Euomphalus pentagulatus.

Some few of the carboniferous mollusca, such as Avicula, _ Nucula_
(sub-genus _Ctenodonta_), _Solemya,_ and _ Lithodomus,_ belong no doubt
to existing genera; but the majority, though often referred to as
living types, such as _ Isocardia, Turritella,_ and _Buccinum,_ belong
really to forms which appear to have become extinct at the close of the
Palæozoic epoch. _Euomphalus_ is a characteristic univalve shell of
this period. In the interior it is divided into chambers (Fig. 487,
_d_), the septa or partitions not being perforated as in foraminiferous
shells, or in those having siphuncles, like the Nautilus. The animal
appears to have retreated at different periods of its growth from the
internal cavity previously formed, and to have closed all communication
with it by a septum. The number of chambers is irregular, and they are
generally wanting in the innermost whorl. The animal of the recent
_Turritella communis_ partitions off in like manner as it advances in
age a part of its spire, forming a shelly septum.

More than twenty species of the genus _Bellerophon_ (see Fig. 488), a
shell like the living Argonaut without chambers, occur in the Mountain
Limestone. The genus is not met with in strata of later date. It is
most generally regarded as belonging to the pelagic Nucleobranchiata
and the family Atlantidæ, partly allied to the Glass-Shell,
_Carinaria_; but by some few it is thought to be a simple form of
Cephalopod.

Fig. 488: Bellerophon costatus.
Fig. 489: Portion of Orthoxeras laterale. Fig. 490: Goniatites
crenistra.
The carboniferous Cephalopoda do not depart so widely from the living
type (the Nautilus) as do the more ancient Silurian representatives of
the same order; yet they offer some remarkable forms. Among these is
_Orthoceras,_ a siphuncled and chambered shell, like a Nautilus
uncoiled and straightened (Fig. 489). Some species of this genus are
several feet long. The _Goniatite_ is another genus, nearly allied to
the _Ammonite,_ from which it differs in having the lobes of the septa
free from lateral denticulations, or crenatures; so that the outline of
these is angular, continuous, and uninterrupted. The species
represented in Fig. 490 is found in most localities, and presents the
zigzag character of the septal lobes in perfection. The dorsal position
of the siphuncle, however, clearly distinguishes the Goniatite from the
Nautilus, and proves it to have belonged to the family of the
Ammonites, from which, indeed, some authors do not believe it to be
generically distinct.

Fossil Fish.—The distribution of these is singularly partial; so much
so, that M. De Koninck of Liége, the eminent palæontologist, once
stated to me that, in making his extensive collection of the fossils of
the Mountain Limestone of Belgium, he had found no more than four or
five examples of the bones or teeth of fishes. Judging from Belgian
data, he might have concluded that this class of vertebrata was of
extreme rarity in the Carboniferous seas; whereas the investigation of
other countries has led to quite a different result. Thus, near
Clifton, on the Avon, as well as at numerous places around the Bristol
basin from the Mendip Hills to Tortworth, there is a celebrated
“bone-bed,” almost entirely made up of ichthyolites. It occurs at the
base of the Lower Limestone shales immediately resting upon the passage
beds of the Old Red Sandstone. Similar bone-beds occur in the
Carboniferous Limestone of Armagh, in Ireland, where they are made up
chiefly of the teeth of fishes of the Placoid order, nearly all of them
rolled as if drifted from a distance. Some teeth are sharp and pointed,
as in ordinary sharks, of which the genus _Cladodus_ afford an
illustration; but the majority, as in _Psammodus_ and _ Cochliodus,_
are, like the teeth of the Cestracion of Port Jackson (see Fig. 261),
massive palatal teeth fitted for grinding. (See Figs. 491, 492.)

Fig. 491: Psammodus porosus.
Fig. 492: Cochliodus controtus.
There are upward of seventy other species of fossil fish known in the
Mountain Limestone of the British Islands. The defensive fin-bones of
these creatures are not infrequent at Armagh and Bristol; those known
as _Oracanthus, Ctenocanthus,_ and _ Onchus_ are often of a very large
size. Ganoid fish, such as _ Holoptychius,_ also occur; but these are
far less numerous. The great _Megalichthys Hibberti_ appears to range
from the Upper Coal-measures to the lowest Carboniferous strata.

Foraminifera.—In the upper part of the Mountain Limestone group in the
S.W. of England, near Bristol, limestones having a distinct oolitic
structure alternate with shales. In these rocks the nucleus of every
minute spherule is seen, under the microscope, to consist of a small
rhizopod or foraminifer. This division of the lower animals, which is
represented so fully at later epochs by the Nummulites and their
numerous minute allies, appears in the Mountain Limestone to be
restricted to a very few species, among which _Textularia, Nodosaria,
Endothyra,_ and _Fusulina_ (Fig. 493), have been recognised. The first
two genera are common to this and all the after periods; the third has
been found in the Upper Silurian, but is not known above the
Carboniferous strata; the fourth (Fig. 493) is characteristic of the
Mountain Limestone in the United States, Arctic America, Russia, and
Asia Minor, but is also known in the Permian.

Fig. 493: Fusulina cylindrica.

 [1] For botanical nomenclature see p. 304.

 [2] Sir C. Bunbury, Quart. Geol. Journ., vol. ii, 1845.

 [3] Manchester Phil. Mem., vol. ix, 1851.

 [4] Trans. of Bot. Soc. of Edinburgh, vol. i, 1844.

 [5] For figures of these corals, see Palæontographical Society’s
 Monographs, 1852.

 [6] Quart. Geol. Journ., vol. xxiii, p. 674, 1867.




CHAPTER XXV.
DEVONIAN OR OLD RED SANDSTONE GROUP.


Classification of the Old Red Sandstone in Scotland and in Devonshire.
— Upper Old Red Sandstone in Scotland, with Fish and Plants. — Middle
Old Red Sandstone. — Classification of the Ichthyolites of the Old Red,
and their Relation to Living Types. — Lower Old Red Sandstone, with
Cephalaspis and Pterygotus. — Marine or Devonian Type of Old Red
Sandstone. — Table of Devonian Series. — Upper Devonian Rocks and
Fossils. — Middle. — Lower. — Eifel Limestone of Germany. — Devonian of
Russia. — Devonian Strata of the United States and Canada. — Devonian
Plants and Insects of Canada.

Classification of the two Types of Old Red Sandstone.—We have seen that
the Carboniferous strata are surmounted by the Permian and Trias, both
originally included in England under the name “New Red Sandstone,” from
the prevailing red colour of the strata. Under the coal came other red
sandstones and shales which were distinguished by the title of “Old Red
Sandstone.” Afterwards the name of “Devonian” was given by Sir R.
Murchison and Professor Sedgwick to marine fossiliferous strata which,
in the south of England, occupy a similar position between the
overlying coal and the underlying Silurian formations.

It may be truly said that in the British Isles the rocks of this age
present themselves in their mineral aspect, and even to some extent in
their fossil contents, under two very different forms; the one as
distinct from the other as are often lacustrine or fluviatile from
marine strata. It has indeed been suggested that by far the greater
part of the deposits belonging to what may be termed the Old Red
Sandstone type are of fresh-water origin. The number of land-plants,
the character of the fishes, and the fact that the only shell yet
discovered belongs to the genus _ Anodonta,_ must be allowed to lend no
small countenance to this opinion. In this case the difficulty of
classification when the strata of this type are compared in different
regions, even where they are contiguous, may arise partly from their
having been formed in distinct hydrographical basins, or in the
neighbourhood of the land in shallow parts of the sea into which large
bodies of fresh-water entered, and where no marine mollusca or corals
could flourish. Under such geographical conditions the limited extent
of some kinds of sediment, as well as the absence of those marine forms
by which we are able to identify or contrast marine formations, may be
explained, while the great thickness of the rocks, which might seem at
first sight to require a corresponding depth of water, can often be
shown to have been due to the gradual sinking down of the bottom of the
estuary or sea where the sediment was accumulated.

Another active cause of local variation in Scotland was the frequency
of contemporaneous volcanic eruptions; some of the rocks derived from
this source, as between the Grampians and the Tay, having formed
islands in the sea, and having been converted into shingle and
conglomerate, before the upper portions of the red shales and
sandstones were superimposed.

The dearth of calcareous matter over wide areas is characteristic of
the Old Red Sandstone. This is, no doubt, in great part due to the
absence of shells and corals; but why should these be so generally
wanting in all sedimentary rocks the colour of which is determined by
the red oxide of iron? Some geologists are of opinion that the waters
impregnated with this oxide were prejudicial to living beings, others
that strata permeated with this oxide would not preserve such fossil
remains.

In regard to the two types, the Old Red Sandstone and the Devonian, I
shall first treat of them separately, and then allude to the proofs of
their having been to a great extent contemporaneous. That they
constitute a series of rocks intermediate in date between the lowest
Carboniferous and the uppermost Silurian is not disputed by the ablest
geologists; and it can no longer be contended that the Upper, Middle,
and Lower Old Red Sandstone preceded in date the three divisions to
which, by aid of the marine shells, the Devonian rocks have been
referred, while, on the other hand, we have not yet data for enabling
us to affirm to what extent the subdivisions of the one series may be
the equivalents in time of those of the other.

Upper Old Red Sandstone.—The highest beds of the series in Scotland,
lying immediately below the coal in Fife, are composed of yellow
sandstone well seen at Dura Den, near Coupar, in Fife, where, although
the strata contain no mollusca, fish have been found abundantly, and
have been referred to the genera _ Holoptychius, Pamphractus,
Glyptopomus,_ and many others. In the county of Cork, in Ireland, a
similar yellow sandstone occurs containing fish of genera
characteristic of the Scotch Old Red Sandstone, as for example
Coccosteus (a form represented by many species in the Old Red Sandstone
and by one only in the Carboniferous group), and _Glytolepis_ and
_Asterolepis,_ both exclusively confined to the “Old Red.” In the same
Irish sandstone at Kiltorkan has been found an _Anodonta_ or
fresh-water mussel, the only shell hitherto discovered in the Old Red
Sandstone of the British Isles (see Fig. 494).

Fig. 494: Anodonta Jukesii.
Fig. 495: Bifurcating branch of Lepidendron Griffithsii.
Fig. 496: Palæopteris Hibernia.
In the same formation are found the fern (Fig. 496) and the _
Lepidodendron_ (Fig. 495), and other species of plants, some of which,
Professor Heer remarks, agree specifically with species from the lower
carboniferous beds. This induces him to lean to the opinion long ago
advocated by Sir Richard Griffiths, that the yellow sandstone, in spite
of its fish remains, should be classed as Lower Carboniferous, an
opinion which I am not yet prepared to adopt. Between the Mountain
Limestone and the yellow sandstone in the south-west of Ireland there
intervenes a formation no less than 5000 feet thick, called the
“Carboniferous slate,” and at the base of this, in some places, are
local deposits, such as the Glengariff Grits, which appear to be beds
of passage between the Carboniferous and Old Red Sandstone groups.

It is a remarkable result of the recent examination of the fossil flora
of Bear Island, latitude 74° 30′ N., that Professor Heer has described
as occurring in that part of the Arctic region (nearly twenty-six
degrees to the north of the Irish locality) a flora agreeing in several
of its species with that of the yellow sandstones of Ireland. This Bear
Island flora is believed by Professor Heer to comprise species of
plants some of which ascend even to the higher stages of the European
Carboniferous formation, or as high as the Mountain Limestone and
Millstone Grit. Palæontologists have long maintained that the same
species which have a wide range in space are also the most persistent
in time, which may prepare us to find that some plants having a vast
geographical range may also have endured from the period of the Upper
Devonian to that of the Millstone Grit.

Fig. 497: Scale of Holoptychius nobilissimus.

Outliers of the Upper “Old Red” occur unconformably on older members of
the group, and the formation represented at Whiteness, near Arbroath,
_a,_ Fig. 55, may probably be one of these outliers, though the want of
organic remains renders this uncertain. It is not improbable that the
beds given in this section as Nos. 1, 2, and 3, may all belong to the
early part of the period of the Upper Old Red, as some scales of
_Holoptychius nobilissimus_ have been found scattered through these
beds, No. 2, in Strathmore. Another nearly allied _Holoptychius_ occurs
in Dura Den, see Fig. 498 of this fish and also Fig. 497 of one of its
scales, as these last are often the only parts met with; being
scattered in Forfarshire through red-coloured shales and sandstones, as
are scales of a large species of the same genus in a corresponding
matrix in Herefordshire.[1] The number of fish obtained from the
British Upper Old Red Sandstone amounts to fifteen species referred to
eleven genera.

Fig. 498: Holoptychius, as restored by Professor Huxley.

Sir R. Murchison groups with this upper division of the Old Red of
Scotland certain light-red and yellow sandstones and grits which occur
in the northernmost part of the mainland, and extend also into the
Orkney and Shetland Islands. They contain Calamites and other plants
which agree generically with Carboniferous forms.

Middle Old Red Sandstone.—In the northern part of Scotland there occur
a great series of bituminous schists and flagstones, to the fossil fish
of which attention was first called by the late Hugh Miller. They were
afterwards described by Agassiz, and the rocks containing them were
examined by Sir R. Murchison and Professor Sedgwick, in Caithness,
Cromarty, Moray, Nairn, Gamrie in Banff, and the Orkneys and Shetlands,
in which great numbers of fossil fish have been found. These were at
first supposed to be the oldest known vertebrate animals, as in
Cromarty the beds in which they occur seem to form the base of the Old
Red system resting almost immediately on the crystalline or metamorphic
rocks. But in fact these fish-bearing beds, when they are traced from
north to south, or to the central parts of Scotland, thin out, so that
their relative age to the Lower Old Red Sandstone, presently to be
mentioned, was not at first detected, the two formations not appearing
in superposition in the same district. In Caithness, however, many
hundred feet below the fish-zone of the middle division, remains of
_Pteraspis_ were found by Mr. Peach in 1861. This genus has never yet
been found in either of the two higher divisions of the Old Red
Sandstone, and confirms Sir R. Murchison’s previous suspicion that the
rocks in which it occurs belong to the Lower “Old Red,” or agree in age
with the Arbroath paving-stone.[2]

_Fossil Fish of the Middle Old Red Sandstone._—The Devonian fish were
referred by Agassiz to two of his great orders, namely, the Placoids
and Ganoids. Of the first of these, which in the Recent period comprise
the shark, the dog-fish, and the ray, no entire skeletons are
preserved, but fin-spines, called ichthyodorulites, and teeth occur. On
such remains the genera _ Onchus, Odontacanthus,_ and _Ctenodus,_ a
supposed cestraciont, and some others, have been established.

By far the greater number of the Old Red Sandstone fishes belong to a
sub-order of Ganoids instituted by Huxley in 1861, and for which he has
proposed the name of _ Crossopterygidæ_,[3] or the fringe-finned, in
consideration of the peculiar manner in which the fin-rays of the
paired fins are arranged so as to form a fringe round a central lobe,
as in the Polypterus (see _a,_ Fig. 499), a genus of which there are
several species now inhabiting the Nile and other African rivers. The
reader will at once recognise in _ Osteolepis_ (Fig. 500), one of the
common fishes of the Old Red Sandstone, many points of analogy with
_Polypterus._ They not only agree in the structure of the fin, at first
pointed out by Huxley, but also in the position of the pectoral,
ventral, and anal fins, and in having an elongated body and rhomboidal
scales. On the other hand, the tail is more symmetrical in the recent
fish, which has also an apparatus of dorsal finlets of a very abnormal
character, both as to number and structure. As to the dorsals of
_Osteolepis,_ they are regular in structure and position, having
nothing remarkable about them, except that there are two of them, which
is comparatively unusual in living fish.

Fig. 499: Polypterus. Living in the Nile and other rivers.

Fig. 500: Restoration of Osteolepis.

Among the “fringe-finned” Ganoids we find some with rhomboidal scales,
such as _Osteolepis,_ Fig. 500; others with cycloidal scales, as
_Holoptychius,_ before mentioned (see Fig. 498). In the genera
_Dipterus_ and _Diplopterus,_ as Hugh Miller pointed out, and in
several other of the fringe-finned genera, as in _Gyroptychius_ and
_Glyptolepis,_ the two dorsals are placed far backward, or directly
over the ventral and anal fins. The _Asterolepis_ was a ganoid fish of
gigantic dimensions. _A. Asmusii,_ Eichwald, a species characteristic
of the Old Red Sandstone of Russia, as well as that of Scotland,
attained the length of between twenty and thirty feet. It was clothed
with strong bony armour, embossed with star-like tubercles, but it had
only a cartilaginous skeleton. The mouth was furnished with two rows of
teeth, the outer ones small and fish-like, the inner larger and with a
reptilian character. The _Asterolepis_ occurs also in the Devonian
rocks of North America.

If we except the Placoids already alluded to, and a few other families
of doubtful affinities, all the Old Red Sandstone fishes are Ganoids,
an order so named by Agassiz from the shining outer surface of their
scales; but Professor Huxley has also called our attention to the fact
that, while a few of the primary and the great majority of the
secondary Ganoids resemble the living bony pike, _Lepidosteus,_ or the
_Amia,_ genera now found in North American rivers, and one of them,
_Lepidosteus,_ extending as far south as Guatemala, the Crossopterygii,
or fringe-finned Ichthyolites, of the Old Red are closely related to
the African _Polypterus,_ which is represented by five or six species
now inhabiting the Nile and the rivers of Senegal. These North American
and African Ganoids are quite exceptional in the living creation; they
are entirely confined to the northern hemisphere, unless some species
of _Polypterus_ range to the south of the line in Africa; and, out of
about 9000 living species of fish known to M. Günther, and of which
more than 6000 are now preserved in the British Museum, they probably
constitute no more than nine.

Fig. 501: Pterichthys. Upper side, showing mouth.
If many circumstances favour the theory of the fresh-water origin of
the Old Red Sandstone, this view of its nature is not a little
confirmed by our finding that it is in Llake Superior and the other
inland Canadian seas of fresh water, and in the Mississippi and African
rivers, that we at present find those fish which have the nearest
affinity to the fossil forms of this ancient formation.

Among the anomalous forms of Old Red fishes not referable to Huxley’s
Crossopterygii is the _Pterichthys,_ of which five species have been
found in the middle division of the Old Red of Scotland. Some writers
have compared their shelly covering to that of Crustaceans, with which,
however, they have no real affinity. The wing-like appendages, whence
the genus is named, were first supposed by Hugh Miller to be paddles,
like those of the turtle; and there can now be no doubt that they do
really correspond with the pectoral fins.

The number of species of fish already obtained from the middle division
of the Old Red Sandstone in Great Britain is about 70, and the
principal genera, besides _Osteolepis_ and _ Pterichthys,_ already
mentioned, are _Glyptolepis, Diplacanthus, Dendrodus, Coccosteus,
Cheirancanthus,_ and _ Acanthoides._

Fig. 502: Cephalapsis Lyellii.

Lower Old Red Sandstone.—The third or lowest division south of the
Grampians consists of grey paving-stone and roofing-slate, with
associated red and grey shales; these strata underlie a dense mass of
conglomerate. In these grey beds several remarkable fish have been
found of the genus named by Agassiz _ Cephalaspis,_ or
“buckler-headed,” from the extraordinary shield which covers the head
(see Fig. 502), and which has often been mistaken for that of a
trilobite, such as _ Asaphus._ A species of _Pteraspis,_ of the same
family, has also been found by the Reverend Hugh Mitchell in beds of
corresponding age in Perthshire; and Mr. Powrie enumerates no less than
five genera of the family Acanthodidæ, the spines, scales, and other
remains of which have been detected in the grey flaggy sandstones.[4]

Fig. 503: Pteygotus anglicus.
In the same formation at Carmylie, in Forfarshire, commonly known as
the Arbroath paving-stone, fragments of a huge crustacean have been met
with from time to time. They are called by the Scotch quarrymen the
“Seraphim,” from the wing-like form and feather-like ornament of the
thoracic appendage, the part most usually met with. Agassiz, having
previously referred some of these fragments to the class of fishes, was
the first to recognise their crustacean character, and, although at the
time unable correctly to determine the true relation of the several
parts, he figured the portions on which he founded his opinion, in the
first plate of his “Poissons Fossiles du Vieux Grès Rouge.”

Fig. 504: Pterygotus anglicus. Ventral aspect.
Carapace, showing the large sessile eyes at the anterior angles.

The _metastoma_ or post-oral plate (serving the office of a lower lip).

Chelate appendages (antennules).

First pair of simple palpi (antennæ).

Second pair of simple palpi (mandibles).

Third pair of simple palpi (first maxillæ).

Pair of swimming feet with their broad basal joints, whose serrated
edges serve the office of maxillæ.

Thoracic plate covering the first two thoracic segments, which are
indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic
segments. 7-12. Abdominal segments. 13. Telson, or tail-plate.)

A restoration in correct proportion to the size of the fragments of _P.
anglicus_ (Fig. 504), from the Lower Old Red Sandstone of Perthshire
and Forfarshire, would give us a creature measuring from five to six
feet in length, and more than one foot across.

The largest crustaceans living at the present day are the _ Inachus
Kaempferi,_ of De Haan, from Japan (a brachyurous or short-tailed
crab), chiefly remarkable for the extraordinary length of its limbs;
the fore-arm measuring four feet in length, and the others in
proportion, so that it covers about 25 square feet of ground; and the
_Limulus Moluccanus,_ the great King Crab of China and the Eastern
seas, which, when adult, measures 1½ foot across its carapace, and is
three feet in length.

Besides some species of _Pterygotus,_ several of the allied genus
_Eurypterus_ occur in the Lower Old Red Sandstone, and with them the
remains of grass-like plants so abundant in Forfarshire and
Kincardineshire as to be useful to the geologist by enabling him to
identify the inferior strata at distant points. Some botanists have
suggested that these plants may be of the family _Fluviales,_ and of
fresh-water genera. They are accompanied by fossils, called “berries”
by the quarrymen, which they compared to a compressed blackberry (see
Figs. 505, 506), and which were called “Parka” by Dr. Fleming. They are
now considered by Mr. Powrie to be the eggs of crustaceans, which is
highly probable, for they have not only been found with _Pterygotus
anglicus_ in Forfarshire and Perthshire, but also in the Upper Silurian
strata of England, in which species of the same genus, Pterygotus,
occur.

Fig. 505: Parka decipiens. In sandstone of lower beds of Old Red, Ley’s
Mill, Forfarshire. Fig. 506: Parka decipiens. In shale of Lower Old
Red, Park Hill, Fife.

Fig. 507: Shale of Old Red Sandstone. Forfarshire. With impression of
plants and eggs of Crustaceans.
The grandest exhibitions, says Sir R. Murchison, of the Old Red
Sandstone in England and Wales appear in the escarpments of the Black
Mountains and in the Fans of Brecon and Carmarthen, the one 2862, and
the other 2590 feet above the sea. The mass of red and brown sandstone
in these mountains is estimated at not less than 10,000 feet, clearly
intercalated between the Carboniferous and Silurian strata. No shells
or corals have ever been found in the whole series, not even where the
beds are calcareous, forming irregular courses of concretionary lumps
called “corn-stones,” which may be described as mottled red and green
earthy limestones. The fishes of this lowest English Old Red are
_Cephalaspis_ and _Pteraspis,_ specifically different from species of
the same genera which occur in the uppermost Ludlow or Silurian
tilestones. Crustaceans also of the genus _Eurypterus_ are met with.

Marine or Devonian Type.—We may now speak of the marine type of the
British strata intermediate between the Carboniferous and Silurian, in
treating of which we shall find it much more easy to identify the
Upper, Middle, and Lower divisions with strata of the same age in other
countries. It was not until the year 1836 that Sir R. Murchison and
Professor Sedgwick discovered that the culmiferous or anthracitic
shales and sandstones of North Devon, several thousand feet thick,
belonged to the coal, and that the beds below them, which are of still
greater thickness, and which, like the carboniferous strata, had been
confounded under the general name “graywacke,” occupied a geological
position corresponding to that of the Old Red Sandstone already
described. In this reform they were aided by a suggestion of Mr.
Lonsdale, who, after studying the Devonshire fossils, perceived that
they belonged to a peculiar palæontological type of intermediate
character between the Carboniferous and Silurian.

It is in the north of Devon that these formations may best be studied,
where they have been divided into an Upper, Middle, and Lower Group,
and where, although much contorted and folded, they have for the most
part escaped being altered by intrusive trap-rocks and by granite,
which in Dartmoor and the more southern parts of the same county have
often reduced them to a crystalline or metamorphic state.

DEVONIAN SERIES IN NORTH DEVON.

UPPER DEVONIAN OR PILTON GROUP (a) Sandy slates and schists with
fossils, 36 species out of 110 common to the Carboniferous group
(Pilton, Barnstaple, etc.), resting on soft schists in which fossils
are very abundant (Croyde, etc.), and which pass down into
(b) Yellow, brown, and red sandstone, with land plants (_Cyclopteris,_
etc.) and marine shells. One zone, characterised by the abundance of
cucullæa (Baggy Point, Marwood, Sloly, etc.) resting on hard grey and
reddish sandstone and micaceous flags, no fossils yet found (Dulverton,
Pickwell, Down, etc.) MIDDLE DEVONIAN OR ILFRACOMBE GROUP. (a) Green
glossy slates of considerable thickness, no fossils yet recorded from
these beds (Mortenoe, Lee Bay, etc.).
(b) Slates and schists, with several irregular courses of limestone
containing shells and corals like those of the Plymouth Limestone
(Combe Martin, Ilfracombe, etc.). LOWER DEVONIAN OR LYNTON
GROUP. (a) Hard, greenish, red, and purple sandstone—no fossils yet
found (Hangman Hill, etc.).
(b) Soft slates with subordinate sandstones—fossils numerous at various
horizons—Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth,
etc.).

The above table exhibits the sequence of the strata or subdivisions as
seen both on the sea-coast of the British Channel and in the interior
of Devon. It will be seen that in all main points it agrees with the
table drawn up in 1864 for the sixth edition of my “Elements.” Mr.
Etheridge[5] has since published an excellent account of the different
subdivisions of the rocks and their fossils, and has also pointed out
their relation to the corresponding marine strata of the Continent. The
slight modifications introduced in my table since 1864 are the result
of a tour made in 1870 in company with Mr. T. Mck. Hughes, when we had
the advantage of Mr. Etheridge’s memoir as our guide.

The place of the sandstones of the Foreland is not yet clearly made
out, as they are cut off by a great fault and disturbance.

Fig. 508: Spirifera disjuncta. Fig. 509: Phacops latifrons.
Upper Devonian Rocks.—The slates and sandstones of Barnstaple (_a_ and
_b_ of the preceding section) contain the shell _Spirifera disjuncta,_
Sowerby (S. Verneuilii, Murch.), (see Fig. 508), which has a very wide
range in Europe, Asia Minor, and even China; also _Strophalosia
caperata,_ together with the large trilobite _Phacops latifrons,_
Bronn. (See Fig. 509), which is all but world-wide in its distribution.
The fossils are numerous, and comprise about 150 species of mollusca, a
fifth of which pass up into the overlying Carboniferous rocks. To this
Upper Devonian belong a series of limestones and slates well developed
at Petherwyn, in Cornwall, where they have yielded 75 species of
fossils. The genus of Cephalopoda called _ Clymenia_ (Fig. 510) is
represented by no less than eleven species, and strata occupying the
same position in Germany are called Clymenien-Kalk, or sometimes
Cypridinen-Schiefer, on account of the number of minute bivalve shells
of the crustacean called _ Cypridina serrato-striata_ (Fig. 511), which
is found in these beds, in the Rhenish provinces, the Harz, Saxony, and
Silesia, as well as in Cornwall and Belgium.

Middle Devonian Rocks.—We come next to the most typical portion of the
Devonian system, including the great limestones of Plymouth and Torbay,
replete with shells, trilobites, and corals. Of the corals 51 species
are enumerated by Mr. Etheridge, none of which pass into the
Carboniferous formation. Among the genera we find _Favosites,
Heliolites,_ and _Cyathophyllum._ The two former genera are very
frequent in Silurian rocks: some few even of the species are said to be
common to the Devonian and Silurian groups, as, for example, _Favosites
cervicornis_ (Fig. 513), one of the commonest of all the Devonshire
fossils. The _Cyathophyllum cæspitosum_ (Fig. 514) and _Heliolites
pyriformis_ (Fig. 512) are species peculiar to this formation.

Fig. 510: Clymenia linearis. Fig. 511: Cypridina serrato-striata.

Fig. 512: Heliolites porosa.
Fig. 513: Favosites cervicornis. Fig. 514: Cyathophyllum cæspitosum.

Fig. 515: Stringocephalus Burtini. Fig. 516: Uncites Gryphus.

With the above are found no less than eleven genera of stone-lilies or
crinoids, some of them, such as _ Cupressocrinites,_ distinct from any
Carboniferous forms. The mollusks, also, are no less characteristic; of
68 species of Brachiopoda, ten only are common to the Carboniferous
Limestone. The _Stringocephalus Burtini_ (Fig. 515) and _Uncites
Gryphus_ (Fig. 516) may be mentioned as exclusively Middle Devonian
genera, and extremely characteristic of the same division in Belgium.
The _Stringocephalus_ is also so abundant in the Middle Devonian of the
banks of the Rhine as to have suggested the name of Stringocephalus
Limestone.

Fig. 517: Megalodon cucullatus.
The only two species of Brachiopoda common to the Silurian and Devonian
formations are _Atrypa reticularis_ (Fig. 532), which seems to have
been a cosmopolite species, and _Strophomena rhomboidalis._

Among the peculiar lamellibranchiate bivalves common to the Plymouth
limestone of Devonshire and the Continent, we find the _ Megalodon_
(Fig. 517). There are also twelve genera of Gasteropods which have
yielded 36 species, four of which pass to the Carboniferous group,
namely _Macrocheilus,_ _Acroculia, Euomphalus,_ and _Murchisonia._
Pteropods occur, such as _Conularia_ (Fig. 518), and Cephalopods, such
as _Cyrtoceras, Gyroceras, Orthoceras,_ and others, nearly all of
genera distinct from those prevailing in the Upper Devonian Limestone,
or Clymenien-kalk of the Germans already mentioned. Although but few
species of Trilobites occur, the characteristic _Bronteus flabellifer_
(Fig. 519) is far from rare, and all collectors are familiar with its
fan-like tail. In this same group, called, as before stated, the
Stringocephalus, or Eifel Limestone, in Germany, several fish remains
have been detected, and among others the remarkable genus Coccosteus,
covered with its tuberculated bony armour; and these ichthyolites
serve, as Sir R. Murchison observes (Siluria, p. 362), to identify this
middle marine Devonian with the Old Red Sandstone of Britain and
Russia.

Fig. 518: Conularia ornata.Fig. 519: Bronteus flabellifer.
Fig. 520: Calceola sandalina.
Beneath the Eifel Limestone (the great central and typical member of
“the Devonian” on the Continent) lie certain schists called by German
writers “Calceola-schiefer,” because they contain in abundance a fossil
body of very curious structure, _Calceola sandalina_ (Fig. 520), which
has been usually considered a brachiopod, but which some naturalists
have lately referred to a Goniophyllum, supposing it to be an abnormal
form of the order _Zoantharia rugosa_ (see Fig. 474), differing from
all other corals in being furnished with a strong operculum. This is by
no means a rare fossil in the slaty limestone of South Devon, and, like
the Eifel form, is confined to the middle group of this country.

Lower Devonian Rocks.—A great series of sandstones and glossy slates,
with Crinoids, Brachiopods, and some corals, occurring on the coast at
Lynmouth and the neighbourhood, and called the Lynton Group (see Table
p. 449, form the lowest member of the Devonian in North Devon. Among
the 18 species of all classes enumerated by Mr. Etheridge, two-thirds
are common to the Middle Devonian, but only one, the ubiquitous _
Atrypa reticularis,_ can with certainty be identified with Silurian
species. Among the characteristic forms are _Alveolites
suborbicularis,_ also common to this formation in the Rhine, and
_Orthis arcuata,_ very widely spread in the North Devon localities. But
we may expect a large addition to the number of fossils whenever these
strata shall have been carefully searched. The Spirifer Sandstone of
Sandberger, as exhibited in the rocks bordering the Rhine between
Coblentz and Caub, belong to this Lower division, and the same
broad-winged Spirifers distinguish the Devonian strata of North
America.

Fig. 521: Spirifora mucronata.
Fig. 522: Homalonotus armatus.
Among the Trilobites of this era several large species of _
Homalonotus_ (Fig. 522) are conspicuous. The genus is still better
known as a Silurian form, but the spinose species appear to belong
exclusively to the “Lower Devonian,” and are found in Britain, Europe,
and the Cape of Good Hope.

Devonian of Russia.—The Devonian strata of Russia extend, according to
Sir R. Murchison, over a region more spacious than the British Isles;
and it is remarkable that, where they consist of sandstone like the
“Old Red” of Scotland and Central England, they are tenanted by fossil
fishes often of the same species and still oftener of the same genera
as the British, whereas when they consist of limestone they contain
shells similar to those of Devonshire, thus confirming, as Sir Roderick
has pointed out, the contemporaneous origin which had been previously
assigned to formations exhibiting two very distinct mineral types in
different parts of Britain.[6] The calcareous and the arenaceous rocks
of Russia above alluded to alternate in such a manner as to leave no
doubt of their having been deposited in different parts of the same
great period.

Fig. 523: Psilophyton princeps.
Devonian Strata in the United States and Canada.—Between the
Carboniferous and Silurian strata there intervenes, in the United
States and Canada, a great series of formations referable to the
Devonian group, comprising some strata of marine origin abounding in
shells and corals, and others of shallow-water and littoral origin in
which terrestrial plants abound. The fossils, both of the deep and
shallow water strata, are very analogous to those of Europe, the
species being in some cases the same. In Eastern Canada Sir W. Logan
has pointed out that in the peninsula of Gaspe, south of the estuary of
St. Lawrence, a mass of sandstone, conglomerate, and shale referable to
this period occurs, rich in vegetable remains, together with some
fish-spines. Far down in the sandstones of Gaspe, Dr. Dawson found, in
1869, an entire specimen of the genus _Cephalaspis,_ a form so
characteristic, as we have already seen, of the Scotch Lower Old Red
Sandstone. Some of the sandstones are ripple-marked, and towards the
upper part of the whole series a thin seam of coal has been observed,
measuring, together with some associated carbonaceous shale, about
three inches in thickness. It rests on an under-clay in which are the
roots of Psilophyton (see Fig. 523). At many other levels rootlets of
this same plant have been shown by Principal Dawson to penetrate the
clays, and to play the same part as do the rootlets of Stigmaria in the
coal formation.

We had already learnt from the works of Göppert, Unger, and Bronn that
the European plants of the Devonian epoch resemble generically, with
few exceptions, those already known as Carboniferous; and Dr. Dawson,
in 1859, enumerated 32 genera and 69 species which he had then obtained
from the State of New York and Canada. A perusal of his catalogue,[7]
comprising _Coniferæ, Sigillariæ, Calamites, Asterophyllites,
Lepidodendra,_ and ferns of the genera _Cyclopteris, Neuropteris,
Sphenopteris,_ and others, together with fruits, such as _Cardiocarpum_
and _Trigonocarpum,_ might dispose geologists to believe that they were
presented with a list of Carboniferous fossils, the difference of the
species from those of the coal-measures, and even a slight admixture of
genera unknown in Europe, being naturally ascribed to geographical
distribution and the distance of the New from the Old World. But
fortunately the coal formation is fully developed on the other side of
the Atlantic, and is singularly like that of Europe, both
lithologically and in the species of its fossil plants. There is also
the most unequivocal evidence of relative age afforded by
superposition, for the Devonian strata in the United States are seen to
crop out from beneath the Carboniferous on the borders of Pennsylvania
and New York, where both formations are of great thickness.

The number of American Devonian plants has now been raised by Dr.
Dawson to 120, to which we may add about 80 from the European flora of
the same age, so that already the vegetation of this period is
beginning to be nearly half as rich as that of the coal-measures which
have been studied for so much longer a time and over so much wider an
area. The Psilophyton above alluded to is believed by Dr. Dawson to be
a lycopodiaceous plant, branching dichotomously (see _P. princeps,_
Fig. 523), with stems springing from a rhizome, which last has circular
areoles, much resembling those of Stigmaria, and like it sending forth
cylindrical rootlets. The extreme points of some of the branchlets are
rolled up so as to resemble the croziers or circinate vernation of
ferns; the leaves or bracts, _a,_ supposed to belong to the same plant,
are described by Dawson as having inclosed the fructification. The
remains of _Psilophyton princeps_ have been traced through all the
members of the Devonian series in America, and Dr. Dawson has lately
recognised it in specimens of Old Red Sandstone from the north of
Scotland.

The monotonous character of the Carboniferous flora might be explained
by imagining that we have only the vegetation handed down to us of one
set of stations, consisting of wide swampy flats. But Dr. Dawson
supposes that the geographical conditions under which the Devonian
plants grew were more varied, and had more of an upland character. If
so, the limitation of this more ancient flora, represented by so many
genera and species, to the gymnospermous and cryptogamous orders, and
the absence or extreme rarity of plants of higher grade, lead us
naturally to speculate on the theory of progressive development,
however difficult it may be to avail ourselves of this explanation, so
long as we meet with even a few exceptional cases of what may seem to
be monocotyledonous or dicotyledonous exogens.

Devonian Insects of Canada.—The earliest known insects were brought to
light in 1865 in the Devonian strata of St. John’s, New Brunswick, and
are referred by Mr. Scudder to four species of _Neuroptera._ One of
them is a gigantic Ephemera, and measured five inches in expanse of
wing.

Like many other ancient animals, says Dr. Dawson, they show a
remarkable union of characters now found in distinct orders of insects,
or constitute what have been named “synthetic types.” Of this kind is a
stridulating or musical apparatus like that of the cricket in an insect
otherwise allied to the _ Neuroptera._ This structure, as Dr. Dawson
observes, if rightly interpreted by Mr. Scudder, introduces us to the
sounds of the Devonian woods, bringing before our imagination the trill
and hum of insect life that enlivened the solitudes of these strange
old forests.

 [1] Siluria, 4th ed., p. 265.

 [2] Siluria, 4th ed., p. 258.

 [3] Abridged from _crossotos,_ a fringe, and _ pteryx,_ a fin.

 [4] Powrie, Geol. Quart. Journ., vol. xx, p. 417.

 [5] Quart. Geol. Journ., vol. xxiii., 1867.

 [6] Murchison’s Siluria, p. 329.

 [7] Quart. Geol. Journ., vol. xv, p. 477, 1859; also vol. xviii, p.
 296, 1862.




CHAPTER XXVI.
SILURIAN GROUP.


Classification of the Silurian Rocks. — Ludlow Formation and Fossils. —
Bone-bed of the Upper Ludlow. — Lower Ludlow Shales with Pentamerus. —
Oldest known Remains of fossil Fish. — Table of the progressive
Discovery of Vertebrata in older Rocks. — Wenlock Formation, Corals,
Cystideans and Trilobites. — Llandovery Group or Beds of Passage. —
Lower Silurian Rocks. — Caradoc and Bala Beds. — Brachiopoda. —
Trilobites. — Cystideæ. — Graptolites. — Llandeilo Flags. — Arenig or
Stiper-stones Group. — Foreign Silurian Equivalents in Europe. —
Silurian Strata of the United States. — Canadian Equivalents. — Amount
of specific Agreement of Fossils with those of Europe.

Classification of the Silurian Rocks.—We come next in descending order
to that division of Primary or Palæozoic rocks which immediately
underlie the Devonian group or Old Red Sandstone. For these strata Sir
Roderick Murchison first proposed the name of Silurian when he had
studied and classified them in that part of Wales and some of the
contiguous counties of England which once constituted the kingdom of
the _Silures,_ a tribe of ancient Britons. The following table will
explain the two principal divisions, Upper and Lower, of the Silurian
rocks, and the minor subdivisions usually adopted, comprehending all
the strata originally embraced in the Silurian system by Sir Roderick
Murchison. The formations below the Arenig or Stiper-stones group are
treated of in the next chapter, when the “Primordial” or Cambrian group
is described.

UPPER SILURIAN ROCKS. Thickness
in feet 1. LUDLOW FORMATION:
     _a._  Upper Ludlow beds 780 _b._ Lower Ludlow beds: 1,050 2.
     WENLOCK FORMATION:
     _a._  Wenlock limestone and shale above 4,000 _b._ Woolhope
     limestone and shale, and Denbighshire grits: 3. LLANDOVERY
     FORMATION
     (Beds of passage between Upper and Lower Silurian):
     _a._  Upper Llandovery (May-Hill beds): 800 _b._ Lower
     Llandovery: 600–1,000 LOWER SILURIAN ROCKS. 1. BALA AND CARADOC
     BEDS, including volcanic rocks: 12,000 2. LLANDEILO FLAGS,
     including volcanic rocks: 4,500 3. ARENIG OR STIPER-STONES
     GROUP, including volcanic rocks: above 10,000

UPPER SILURIAN ROCKS.

1. Ludlow Formation.—This member of the Upper Silurian group, as will
be seen by above table, is of great thickness, and subdivided into two
parts—the Upper Ludlow and the Lower Ludlow. Each of these may be
distinguished near the town of Ludlow, and at other places in
Shropshire and Herefordshire, by peculiar organic remains; but out of
more than 500 species found in the Ludlow formation as a whole, not
more than five species per hundred are common to the overlying
Devonian. The student may refer to the excellent tables given in the
last edition of Sir R. Murchison’s Siluria for a list of the organic
remains of all classes distributed through the different subdivisions
of the Upper and Lower Silurian.

_a._ Upper Ludlow: _Downton Sandstone._—At the top of this subdivision
there occur beds of fine-grained yellowish sandstone and hard reddish
grits which were formerly referred by Sir R. Murchison to the Old Red
Sandstone, under the name of “Tilestones.” In mineral character this
group forms a transition from the Silurian to the Old Red Sandstone,
the strata of both being conformable; but it is now ascertained that
the fossils agree in great part specifically, and in general character
entirely, with those of the underlying Upper Ludlow rocks. Among these
are _Orthoceras bullatum, Platyschisma helicites, Bellerophon
trilobatus, Chonetes lata,_ etc., with numerous defenses of fishes.

These beds, therefore, now generally called the “Downton Sandstone,”
are classed as the newest member of the Upper Silurian. They are well
seen at Downton Castle, near Ludlow, where they are quarried for
building, and at Kington, in Herefordshire. In the latter place, as
well as at Ludlow, crustaceans of the genera Pterygotus (for genus see
Fig. 504) and Eurypterus are met with.

_Bone-bed of the Upper Ludlow._—At the base of the Downton sandstones
there occurs a bone-bed which deserves especial notice as affording the
most ancient example of fossil fish occurring in any considerable
quantity. It usually consists of one or two thin layers of brown bony
fragments near the junction of the Old Red Sandstone and the Ludlow
rocks, and was first observed by Sir R. Murchison near the town of
Ludlow, where it is three or four inches thick. It has since been
traced to a distance of 45 miles from that point into Gloucestershire
and other counties, and is commonly not more than an inch thick, but
varies to nearly a foot. Near Ludlow two bone-beds are observable, with
14 feet of intervening strata full of Upper Ludlow fossils.[1] At that
point immediately above the upper fish-bed numerous small globular
bodies have been found, which were determined by Dr. Hooker to be the
sporangia of a cryptogamic land-plant, probably lycopodiaceous.

Fig. 524: Onchus tenuistriatus. Fig. 525: Shagreen-scales of a placoid
fish, Thelodus parvidens.

Most of the fish have been referred by Agassiz to his placoid order,
some of them to the genus Onchus, to which the spine (Fig. 524) and the
minute scales (Fig. 525) are supposed to belong. It has been suggested,
however, that Onchus may be one of those Acanthodian fish referred by
Agassiz to his Ganoid order, which are so characteristic of the base of
the Old Red Sandstone in Forfarshire, although the species of the Old
Red are all different from these of the Silurian beds now under
consideration.

Fig. 526: Plectrodus mirabilis.
The jaw and teeth of another predaceous genus (Fig. 526) have also been
detected, together with some specimens of _Pteraspis Ludensis._ As
usual in bone-beds, the teeth and bones are, for the most part,
fragmentary and rolled.

Fig. 527: Orthis elegantula.Fig. 528: Rhynchonella navicula.
_Grey Sandstone and Mudstone, etc._—The next subdivision of the Upper
Ludlow consists of grey calcareous sandstone, or very commonly a
micaceous stone, decomposing into soft mud, and contains, besides the
shells mentioned aon page 459, _Lingula cornea, Orthis orbicularis,_ a
round variety of _O. elegantula, Modiolopsis platyphylla, Grammysia
cingulata,_ all characteristic of the Upper Ludlow. The lowest or
mud-stone beds contain _Rhynchonella navicula_ (Fig. 528), which is
common to this bed and the Lower Ludlow. As usual in Palæozoic strata
older than the coal, the brachiopodous mollusca greatly outnumber the
lamellibranchiate (see p. 470); but the latter are by no means
unrepresented. Among other genera, for example, we observe _Avicula_
and _Pterinea, Cardiola, Ctenodonta_ (sub-genus of _ Nucula_),
_Orthonota, Modiolopsis,_ and _ Palæarca._

Some of the Upper Ludlow sandstones are ripple-marked, thus affording
evidence of gradual deposition; and the same may be said of the
accompanying fine argillaceous shales, which are of great thickness,
and have been provincially named “mud-stones.” In some of these shales
stems of crinoidea are found in an erect position, having evidently
become fossil on the spots where they grew at the bottom of the sea.
The facility with which these rocks, when exposed to the weather, are
resolved into mud, proves that, notwithstanding their antiquity, they
are nearly in the state in which they were first thrown down.

Fig. 529: Pentamerus Knightii.

_b._ Lower Ludlow Beds.—The chief mass of this formation consists of a
dark grey argillaceous shale with calcareous concretions, having a
maximum thickness of 1000 feet. In some places, and especially at
Aymestry, in Herefordshire, a subcrystalline and argillaceous
limestone, sometimes 50 feet thick, overlies the shale. Sir R.
Murchison therefore classes this Aymestry limestone as holding an
intermediate position between the Upper and Lower Ludlow, but Mr.
Lightbody remarks that at Mocktrie, near Leintwardine, the Lower Ludlow
shales, with their characteristic fossils, occur both above and below a
similar limestone. This limestone around Aymestry and Sedgeley is
distinguished by the abundance of _Pentamerus Knightii,_ Sowerby (Fig.
529), also found in the Lower Ludlow and Wenlock shale. This genus of
brachiopoda was first found in Silurian strata, and is exclusively a
palæozoic form. The name was derived from _pente,_ five, and _meros,_ a
part, because both valves are divided by a central septum, making four
chambers, and in one valve the septum itself contains a small chamber,
making five. The size of these septa is enormous compared with those of
any other brachiopod shell; and they must nearly have divided the
animal into two equal halves; but they are, nevertheless, of the same
nature as the septa or plates which are found in the interior of _
Spirifera, Terebratula,_ and many other shells of this order. Messrs.
Murchison and De Verneuil discovered this species dispersed in myriads
through a white limestone of Upper Silurian age, on the banks of the
Is, on the eastern flank of the Urals in Russia, and a similar species
is frequent in Sweden.

Fig. 530: Lingula Lewisii.
Three other abundant shells in the Aymestry limestone are, first,
_Lingula Lewisii_ (Fig. 530); second, _Rhynchonella Wilsoni,_ Sowerby
(Fig. 531), which is also common to the Lower Ludlow and Wenlock
limestone; third, _Atrypa reticularis,_ Linn. (Fig. 532), which has a
very wide range, being found in every part of the Upper Silurian
system, and even ranging up into the Middle Devonian series.

Fig. 531: Rhynchonella (Terebratula) Wilsoni.

The Aymestry Limestone contains many shells, especially brachiopoda,
corals, trilobites, and other fossils, amounting on the whole to 74
species, all except three or four being common to the beds either above
or below.

Fig. 532: Atrypa reticularis. The Lower Ludlow Shale contains, among
other fossils, many large cephalopoda not known in newer rocks, as the
_Phragmoceras_ of Broderip, and the _Lituites_ of Breynius (see Figs.
533, 534). The latter is partly straight and partly convoluted in a
very flat spire. The _Orthoceras Ludense_ (Fig. 535), as well as the
cephalopod last mentioned, occurs in this member of the species.

Fig. 533: Phragmoceras ventricosum.

A species of Graptolite, _G. priodon,_ Bronn (Fig. 545), occurs
plentifully in the Lower Ludlow. This fossil, referred, though somewhat
doubtfully, to a form of hydrozoid or sertularian polyp, has not yet
been met with in strata above the Silurian.

Star-fish, as Sir R. Murchison points out, are by no means rare in the
Lower Ludlow rock. These fossils, of which six extinct genera are now
known in the Ludlow series, represented by 18 species, remind us of
various living forms now found in our British seas, both of the
families _Asteriadæ_ and _ Ophiuridæ._

Fig. 534: Lituites (Trochoceras) giganteus. Fig. 535: Fragment of
Orthoceras Ludense.

Oldest known Fossil Fish.—Until 1859 there was no example of a fossil
fish older than the bone-bed of the Upper Ludlow, but in that year a
specimen of Pteraspis was found at Church Hill, near Leintwardine, in
Shropshire, by Mr. J. E. Lee of Caerleon, F.G.S., in shale below the
Aymestry limestone, associated with fossil shells of the Lower Ludlow
formation—shells which differ considerably from those characterising
the Upper Ludlow already described. This discovery is of no small
interest as bearing on the theory of progressive development, because,
according to Professor Huxley, the genus Pteraspis is allied to the
sturgeon, and therefore by no means of low grade in the piscine class.

It is a fact well worthy of notice that no remains of vertebrata have
yet been met with in any strata older than the Lower Ludlow.

When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites,
Corals, and other fossils already obtained from more ancient Silurian
formations, Upper, Middle, and Lower, we may well ask whether any set
of fossiliferous rocks newer in the series were ever studied with equal
diligence, and over so vast an area, without yielding a single
ichthyolite. Yet we must hesitate before we accept, even on such
evidence, so sweeping a conclusion, as that the globe, for ages after
it was inhabited by all the great classes of invertebrata, remained
wholly untenanted by vertebrate animals.

_Dates of the Discovery of different Classes of Fossil Vertebrata;
showing the gradual progress made in tracing them to rocks of higher
antiquity._

  Year Formations Geographical localities
  Mammalia 1798 Upper Eocene Paris (Gypsum of Montmartre).1
  1818 Lower Oolite Stonesfield.2 1847 Upper
  Trias Stuttgart.3 Aves 1782 Upper Eocene Paris (Gypsum
  of Montmartre).4 1839 Lower Eocene Isle of Sheppey (London
  Clay).5 1854 Lower Eocene Woolwich Beds.6 1855 Lower
  Eocene Mendon (Plastic Clay).7 1858 Chloritic Series, or
  Upper Greensand Cambridge.8 1863 Upper
  Oolite Solenhofen.9 Reptilia 1710 Permian (or
  Zechstein) Thuringia.10 1844 Carboniferous Saarbrück,
  near Trèves.11 Pisces 1709 Permian (or
  Kupferschiefer) Thuringia.12 1793 Carboniferous (Mountain
  Limestone) Glasgow.13 1828 Devonian Caithness.14
  1840 Upper Ludlow Ludlow.15 1859 Lower
  Ludlow Leintwardine.16

1. George Cuvier, Bulletin Soc. Philom. xx.
2. In 1818, Cuvier, visiting the Museum of Oxford, decided on the
mammalian character of a jaw from Stonesfield. See also p. 347.
3. Prof. Plieninger. See p. 368.
4. Cuvier, Ossemens Foss. Art. “Oiseaux.”
5. Prof. Owen, Geol. Trans., 2nd series, vol. vi, p. 203, 1839.
6. Upper part of the Woolwich beds. Prestwich, Quart. Geol. Journ.,
vol. x, p. 157.
7. _Gastornis Parisiensis._ Owen, Quart. Geol. Journ., vol. xii, p.
204, 1856.
8. Coprolitic bed, in the Upper Greensand. See p. 299.
9. The _Archæopteryx macrura,_ Owen. See p. 338.
10. The fossil monitor of Thuringia (_Protosaurus Speneri,_ V. Meyer)
was figured by Spener of Berlin in 1810. (Miscel. Berlin.)
11. See p. 406.
12. Memorabilia Saxoniæ Subterr., Leipsic, 1709.
13. History of Rutherglen by Rev. David Ure, 1793.
14. Sedgwick and Murchison, Geol. Trans., 2nd series, vol. ii, p. 141,
1828.
15. Sir R. Murchison. See p. 459.
16. See p. 461.

Obs.—The evidence derived from foot-prints, though often to be relied
on, is omitted in the above table, as being less exact than that
founded on bones and teeth.

In the preceding Table a few dates are set before the reader of the
discovery of different classes of animals in ancient rocks, to enable
him to perceive at a glance how gradual has been our progress in
tracing back the signs of vertebrata to formations of high antiquity.
Such facts may be useful in warning us not to assume too hastily that
the point which our retrospect may have reached at the present moment
can be regarded as fixing the date of the first introduction of any one
class of beings upon the earth.

2. Wenlock Formation.—We next come to the Wenlock formation, which has
been divided (see Table, p. 458) into Wenlock limestone, Wenlock shale,
and Woolhope limestone and Denbighshire grits.

Fig. 536: Halysites catenularius.

_a. Wenlock Limestone._—This limestone, otherwise well known to
collectors by the name of the Dudley Limestone, forms a continuous
ridge in Shropshire, ranging for about 20 miles from S.W. to N.E.,
about a mile distant from the nearly parallel escarpment of the
Aymestry limestone. This ridgy prominence is due to the solidity of the
rock, and to the softness of the shales above and below it. Near
Wenlock it consists of thick masses of grey subcrystalline limestone,
replete with corals, encrinites, and trilobites. It is essentially of a
concretionary nature; and the concretions, termed “ball-stones” in
Shropshire, are often enormous, even 80 feet in diameter. They are of
pure carbonate of lime, the surrounding rock being more or less
argillaceous[2] Sometimes in the Malvern Hills this limestone,
according to Professor Phillips, is oolitic.

Fig. 537: Favosites Gothlandica.
Among the corals, in which this formation is so rich, 53 species being
known, the “chain-coral,” _Halysites catenularius_ (Fig. 536), may be
pointed out as one very easily recognised, and widely spread in Europe,
ranging through all parts of the Silurian group, from the Aymestry
limestone to near the bottom of the Llandeilo rocks. Another coral, the
_Favosites Gothlandica_ (Fig. 537), is also met with in profusion in
large hemispherical masses, which break up into columnar and prismatic
fragments, like that here figured (Fig. 537, _b_). Another common form
in the Wenlock limestone is the _Omphyma turbinatum_ (Fig. 538), which,
like many of its modern companions, reminds us of some cup-corals; but
all the Silurian genera belong to the palæozoic type before mentioned
(p. 432), exhibiting the quadripartite arrangement of the septalamellæ
within the cup.

Fig. 538: Omphyma turbinatum.Fig. 539: Pseudocrinites bifasciatus.

Among the numerous Crinoids, several peculiar species of _
Cyathocrinus_ (for genus see Figs. 478, 479) contribute their
calcareous stems, arms, and cups towards the composition of the Wenlock
limestone. Of Cystideans there are a few very remarkable forms, most of
them peculiar to the Upper Silurian formation, as, for example, the _
Pseudocrinites,_ which was furnished with pinnated fixed arms,[3] as
represented in Fig. 539.

Fig. 540: Strophomena (Leptæna) depressa.
The Brachiopoda are, many of them, of the same species as those of the
Aymestry limestone; as, for example, _Atrypa reticularis_ (Fig. 532),
and _Strophomena depressa_ (Fig. 540); but the latter species ranges
also from the Ludlow rocks, through the Wenlock shale, to the Caradoc
Sandstone.

Fig. 541: Calymene Blumenbachii.
The crustaceans are represented almost exclusively by Trilobites, which
are very conspicuous, 22 being peculiar. The _ Calymene Blumenbachii_
(Fig. 541), called the ”Dudley Trilobite,” was known to collectors long
before its true place in the animal kingdom was ascertained. It is
often found coiled up like the common _Oniscus_ or wood-louse, and this
is so usual a circumstance among certain genera of trilobites as to
lead us to conclude that they must have habitually resorted to this
mode of protecting themselves when alarmed. The other common species is
the _Phacops caudatus (Asaphus caudatus),_ Brong. (see Fig. 542), and
this is conspicuous for its large size and flattened form.
_Sphærexochus mirus_ (Fig. 543) is almost a globe when rolled up, the
forehead or glabellum of this species being extremely inflated. The
_Homalonotus,_ a form of Trilobite in which the tripartite division of
the dorsal crust is almost lost (see Fig. 544), is very characteristic
of this division of the Silurian series.

Fig. 542: Phacops (Asaphus) caudatus.Fig. 543: Sphærexochus mirus.
_Wenlock Shale._—This, observes Sir R. Murchison, is infinitely the
largest and most persistent member of the Wenlock formation, for the
limestone often thins out and disappears. The shale, like the Lower
Ludlow, often contains elliptical concretions of impure earthy
limestone.

Fig. 544: Homalonotus delphinocephalus.
In the Malvern district it is a mass of finely levigated argillaceous
matter, attaining, according to Professor Phillips, a thickness of 640
feet, but it is sometimes more than 1000 feet thick in Wales, and is
worked for flag-stones and slates. The prevailing fossils, besides
corals and trilobites, and some crinoids, are several small species of
_Orthis, Cardiola,_ and numerous thin-shelled species of
_Orthoceratites._

About six species of _Graptolite,_ a peculiar group of sertularian
fossils before alluded to (p. 463) as being confined to Silurian rocks,
occur in this shale. Of fossils of this genus, which is very
characteristic of the Lower Silurian, I shall again speak in the sequel
(p. 474).

Fig. 545: Graptolithus priodon.
_b. Woolhope Beds._—Though not always recognised as a separate
subdivision of the Wenlock, the Woolhope beds, which underlie the
Wenlock shale, are of great importance. Usually they occur as massive
or nodular limestones, underlaid by a fine shale or flag-stone; and in
other cases, as in the noted Denbighshire sandstones, as a coarse grit
of very great thickness. This grit forms mountain ranges through North
and South Wales, and is generally marked by the great sterility of the
soil where it occurs. It contains the usual Wenlock fossils, but with
the addition of some common in the uppermost Ludlow rock, such as _
Chonetes lata_ and _Bellerophon trilobatus._ The chief fossils of the
Woolhope limestone are _Illænus Barriensis, Homalonotus
delphinocephalus_ (Fig. 544), _Strophomena imbrex,_ and _Rhynchonella
Wilsoni_ (Fig. 531). The latter attains in the Woolhope beds an unusual
size for the species, the specimens being sometimes twice as large as
those found in the Wenlock limestone.

In some places below the Wenlock formation there are shales of a pale
or purple colour, which near Tarannon attain a thickness of about 1000
feet; they can be traced through Radnor and Montgomery to North Wales,
according to Messrs. Jukes and Aveline. By the latter geologist they
have been identified with certain shales above the May-Hill Sandstone,
near Llandovery, but, owing to the extreme scarcity of fossils, their
exact position remains doubtful.

3. Llandovery Group—Beds of Passage.—We now come to beds respecting the
classification of which there has been much difference of opinion, and
which in fact must be considered as beds of passage between Upper and
Lower Silurian. I formerly adopted the plan of those who class them as
Middle Silurian, but they are scarcely entitled to this distinction,
since after about 1400 Silurian species have been compared the number
peculiar to the group in question only gives them an importance equal
to such minor subdivisions as the Ludlow or Bala groups. I therefore
prefer to regard them as the base of the Upper Silurian, to which group
they are linked by more than twice as many species as to the Lower
Silurian. By this arrangement the line of demarkation between the two
great divisions, though confessedly arbitrary, is less so than by any
other. They are called Llandovery Rocks, from a town in South Wales, in
the neighbourhood of which they are well developed, and where,
especially at a hill called Noeth Grug, in spite of several faults,
their relations to one another can be clearly seen.

_a.  Upper Llandovery or May-Hill Sandstone._—The May-Hill group, which
has also been named ”Upper Llandovery,” by Sir R. Murchison, ranges
from the west of the Longmynd to Builth, Llandovery, and Llandeilo, and
to the sea in Marlow’s Bay, where it is seen in the cliffs. It consists
of brownish and yellow sandstones with calcareous nodules, having
sometimes a conglomerate at the base derived from the waste of the
Lower Silurian rocks. These May-Hill beds were formerly supposed to be
part of the Caradoc formation, but their true position was determined
by Professor Sedgwick[4] to be at the base of the Upper Silurian
proper. The more calcareous portions of the rock have been called the
Pentamerus limestone, because _Pentamerus oblongus_ (Fig. 546) is very
abundant in them. It is usually accompanied by _P. (Stricklandinia)
lirata_ (Fig. 547); both forms have a wide geographical range, being
also met with in the same part of the Silurian series in Russia and the
United States.

Fig. 546: Pentamerus oblongus.
About 228 species of fossils are known in the May-Hill division, more
than half of which are Wenlock species. They consist of trilobites of
the genera _Illænus_ and _Calymene_; Brachiopods of the genera _Orthis,
Atrypa, Leptæna, Pentamerus, Strophomena,_ and others; Gasteropods of
the genera _Turbo, Murchisonia_ (for genus, see Fig. 567), and
_Bellerophon_; and Pteropods of the genus _Conularia._ The Brachiopods,
of which there are 66 species, are almost all Upper Silurian.

Fig. 547: Stricklandinia (Pentamerus) lirata. Fig. 548: Tentaculites
annulatus.
Among the fossils of the May-Hill shelly sandstone at Malvern,
_Tentaculites annulatus_ (Fig. 548), an annelid, probably allied to
Serpula, is found.

_Lower Llandovery Rocks._—Below the May-Hill Group are the Lower
Llandovery Rocks, which consist chiefly of hard slaty rocks, and beds
of conglomerate from 600 to 1000 feet in thickness. The fossils, which
are somewhat rare in the lower beds, consist of 128 known species, only
eleven of which are peculiar, 83 being common to the May-Hill group
above, and 93 common to the rocks below. _Stricklandinia (Pentamerus)
levis,_ which is common in the Lower Llandovery, becomes rare in the
Upper, while _ Pentamerus oblongus_ (Fig. 546), which is the
characteristic shell of the Upper Llandovery, occurs but seldom in the
Lower.

LOWER SILURIAN ROCKS.

The Lower Silurian has been divided into, first, the Bala Group;
second, the Llandeilo flags; and, third, the Arenig or Lower Llandeilo
formation.

Bala and Caradoc Beds.—The Caradoc sandstone was originally so named by
Sir R. I. Murchison from the mountain called Caer Caradoc, in
Shropshire; it consists of shelly sandstones of great thickness, and
sometimes containing much calcareous matter. The rock is frequently
laden with the beautiful trilobite called by Murchison _Trinucleus
Caractaci_ (see Fig. 553), which ranges from the base to the summit of
the formation, usually accompanied by _Strophomena grandis_ (see Fig.
551), and _Orthis vespertilio_ (Fig. 550), with many other fossils.

Fig. 549: Orthis tricenaria. Fig. 550: Orthis vespertilio. Fig. 551:
Orthis (Strophomena) grandis.

_Brachiopoda._—Nothing is more remarkable in these beds, and in the
Silurian strata generally of all countries, than the preponderance of
brachiopoda over other forms of mollusca. Their proportional numbers
can by no means be explained by supposing them to have inhabited seas
of great depth, for the contrast between the palæozoic and the present
state of things has not been essentially altered by the late
discoveries made in our deep-sea dredgings. We find the living
brachiopoda so rare as to form about one forty-fourth of the whole
bivalve fauna, whereas in the Lower Silurian rocks of which we are now
about to treat, and where the brachiopoda reach their maximum, they are
represented by more than twice as many species as the Lamellibranchiate
bivalves.

There may, indeed, be said to be a continued decrease of the
proportional number of this lower tribe of mollusca as we proceed from
older to newer rocks. In the British Devonian, for example, the
Brachiopoda number 99, the Lamellibranchiata 58; while in the
Carboniferous their proportions are more than reversed, the
Lamellibranchiata numbering 334 species, and the Brachiopoda only 157.
In the Secondary or Cainozoic formations the preponderance of the
higher grade of bivalves becomes more and more marked, till in the
tertiary strata it approaches that observed in the living creation.

While on this subject, it may be useful to the student to know that a
Brachiopod differs from ordinary bivalves, mussels, cockles, etc., in
being always equal-sided and never quite equi-valved; the form of each
valve being symmetrical, it may be divided into two equal parts by a
line drawn from the apex to the centre of the margin.

_Trilobites._—In the Bala and Caradoc beds the trilobites reach their
maximum, being represented by 111 species referred to 23 genera.

Burmeister, in his work on the organisation of trilobites, supposes
that they swam at the surface of the water in the open sea and near
coasts, feeding on smaller marine animals, and to have had the power of
rolling themselves into a ball as a defence against injury. He was also
of opinion that they underwent various transformations analogous to
those of living crustaceans. M. Barrande, author of an admirable work
on the Silurian rocks of Bohemia, confirms the doctrine of their
metamorphosis, having traced more than twenty species through different
stages of growth from the young state just after its escape from the
egg to the adult form. He has followed some of them from a point in
which they show no eyes, no joints, or body rings, and no distinct
tail, up to the complete form with the full number of segments. This
change is brought about before the animal has attained a tenth part of
its full dimensions, and hence such minute and delicate specimens are
rarely met with. Some of his figures of the metamorphoses of the common
_Trinucleus_ are copied in Figs. 552 and 553. It was not till 1870 that
Mr. Billings was enabled, by means of a specimen found in Canada, to
prove that the trilobite was provided with eight legs.

It has been ascertained that a great thickness of slaty and crystalline
rocks of South Wales, as well as those of Snowdon and Bala, in North
Wales, which were first supposed to be of older date than the Silurian
sandstones and mudstones of Shropshire, are in fact identical in age,
and contain the same organic remains. At Bala, in Merionethshire, a
limestone rich in fossils occurs, in which two genera of star-fish,
_Protaster_ and _Palæaster,_ are found; the fossil specimen of the
latter (Fig. 554) being almost as uncompressed as if found just washed
up on the sea-beach. Besides the star-fish there occur abundance of
those peculiar bodies called _Cystideæ._ They are the _Sphæronites_ of
old authors, and were considered by Professor E. Forbes as intermediate
between the crinoids and echinoderms. The _Echinosphæronite_ here
represented (Fig. 555) is characteristic of the Caradoc beds in Wales,
and of their equivalents in Sweden and Russia.

Fig. 552: Young individuals of Trinucleus concentricus. Fig. 553:
Trinucleus concentricus.

Fig. 554: Palæaster asperimus.
Fig. 555: Echinosphæronites ballicus.
With it have been found several other genera of the same family, such
as _Sphæronites, Hemicosmites,_ etc. Among the mollusca are Pteropods
of the genus _Conularia_ of large size (for genus, see Fig. 518). About
eleven species of Graptolite are reckoned as belonging to this
formation; they are chiefly found in peculiar localities where black
mud abounded. The formation, when traced into South Wales and Ireland,
assumes a greatly altered mineral aspect, but still retains its
characteristic fossils. The known fauna of the Bala group comprises 565
species, 352 of which are peculiar, and 93, as before stated, are
common to the overlying Llandovery rocks. It is worthy of remark that,
when it occurs under the form of trappean tuff (volcanic ashes of De la
Beche), as in the crest of Snowdon, the peculiar species which
distinguish it from the Llandeilo beds are still observable. The
formation generally appears to be of shallow-water origin, and in that
respect is contrasted with the group next to be described. Professor
Ramsay estimates the thickness of the Bala Beds, including the
contemporaneous volcanic rocks, stratified and unstratified, as being
from 10,000 to 12,000 feet.

Fig. 556: Didymograpsus (Graptolites) Murchisonii.
Llandeilo Flags.—The Lower Silurian strata were originally divided by
Sir R. Murchison into the upper group already described, under the name
of Caradoc Sandstone, and a lower one, called, from a town in
Carmarthenshire, the _Llandeilo_ flags. The last mentioned strata
consist of dark-coloured micaceous flags, frequently calcareous, with a
great thickness of shales, generally black, below them. The same beds
are also seen at Builth, in Radnorshire, where they are interstratified
with volcanic matter.

A still lower part of the Llandeilo rocks consists of a black
carbonaceous slate of great thickness, frequently containing sulphate
of alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It
has been conjectured that this carbonaceous matter may be due in great
measure to large quantities of imbedded animal remains, for the number
of Graptolites included in these slates was certainly very great. In
Great Britain eleven genera and about 40 species of Graptolites occur
in the Llandeilo flags and underlying Arenig beds. The double
Graptolites, or those with two rows of cells, such as Diplograpsus
(Fig. 557), are conspicuous.

Fig. 557: Diplograpsus pristis. Fig. 558: Rastrites peregrinus.

Fig. 559: Diplograpsus folium.
The brachiopoda of the Llandeilo flags, which number 47 species, are in
the main the same as those of the Caradoc Sandstone, but the other
mollusca are in great part of different species.

In Europe generally, as, for example, in Sweden and Russia, no shells
are so characteristic of this formation as Orthoceratites, usually of
great size, and with a wide siphuncle placed on one side instead of
being central (see Fig. 560).

Fig. 560: Orthoceras duplex.

Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the
same beds also are found Bellerophon (see Fig. 488) and some Pteropod
shells (_Conularia, Theca,_ etc.), also in spots where sand abounded,
lamellibranchiate bivalves of large size. The Crustaceans were
plentifully represented by the Trilobites, which appear to have swarmed
in the Silurian seas just as crabs and shrimps do in our own; no less
than 263 species have been found in the British Silurian fauna. The
genera _Asaphus_ (Fig. 561), _Ogygia_ (Fig. 562), and _Trinucleus_
(Figs. 552 and 553) form a marked feature of the rich and varied
Trilobitic fauna of this age.

Fig. 561: Asaphus tyrannus. Fig. 562: Ogygia Buchii.

Beneath the black slates above described of the Llandeilo formation,
Graptolites are still found in great variety and abundance, and the
characteristic genera of shells and trilobites of the Lower Silurian
rocks are still traceable downward, in Shropshire, Cumberland, and
North and South Wales, through a vast depth of shaly beds, in some
districts interstratified with trappean formations of contemporaneous
origin; these consist of tuffs and lavas, the tuffs being formed of
such materials as are ejected from craters and deposited immediately on
the bed of the ocean, or washed into it from the land. According to
Professor Ramsay, their thickness is about 3300 feet in North Wales,
including those of the Lower Llandeilo. The lavas are feldspathic, and
of porphyritic structure, and, according to the same authority, of an
aggregate thickness of 2500 feet.

Fig. 563: Arenicolites linearis.
Arenig or Stiper-Stones Group _(Lower Llandeilo of Murchison)._—Next in
the descending order are the shales and sandstones in which the
quartzose rocks called Stiper-Stones in Shropshire occur. Originally
these Stiper-Stones were only known as arenaceous quartzose strata in
which no organic remains were conspicuous, except the tubular burrows
of annelids (see Fig. 563, _Arenicolites linearis_), which are
remarkably common in the Lowest Silurian in Shropshire, and in the
State of New York, in America. They have already been alluded to as
occurring by thousands in the Silurian strata unconformably overlying
the Cambrian, in the mountain of Queenaig, in Sutherlandshire (Fig.
82). I have seen similar burrows now made on the retiring of the tides
in the sands of the Bristol Channel, near Minehead, by lob-worms which
are dug out by fishermen and used as bait. When the term Silurian was
given by Sir R. Murchison, in 1835, to the whole series, he considered
the Stiper-Stones as the base of the Silurian system, but no fossil
fauna had then been obtained, such as could alone enable the geologist
to draw a line between this member of the series and the Llandeilo
flags above, or a vast thickness of rock below, which was seen to form
the Longmynd hills, and was called ”unfossiliferous graywacke.”
Professor Sedgwick had described, in 1843, strata now ascertained to be
of the same age as largely developed in the Arenig mountain, in
Merionethshire; and the Skiddaw slates in the Lake-District of
Cumberland, studied by the same author, were of corresponding date,
though the number of fossils was, in both cases, too few for the
determination of their true chronological relations. The subsequent
researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir
R. I. Murchison and the Government surveyors in Shropshire, have
increased the species to more than sixty. These were examined by Mr.
Salter, and shown in the third edition of ”Siluria” (p. 52, 1859) to be
quite distinct from the fossils of the overlying Llandeilo flags. Among
these the _Obolella plumbea, Æglina binodosa, Ogygia Selwynii,_ and
_Didymograpsus geminus_ (Fig. 564), and _D. Hirundo,_ are
characteristic.

Fig. 564: Didymograpsus geminus.

But, although the species are distinct, the genera are the same as
those which characterise the Silurian rocks above, and none of the
characteristic primordial or Cambrian forms, presently to be mentioned,
are intermixed. The same may be said of a set of beds underlying the
Arenig rocks at Ramsay Island and other places in the neighbourhood of
St. David’s. These beds, which have only lately become known to us
through the labours of Dr. Hicks,[5] present already twenty new
species, the greater part of them allied generically to the Arenig
rocks. This Arenig group may therefore be conveniently regarded as the
base of the great Silurian system, a system which, by the thickness of
its strata and the changes in animal life of which it contains the
record, is more than equal in value to the Devonian, or Carboniferous,
or other principal divisions, whether of primary or secondary date.

It would be unsafe to rely on the mere thickness of the strata,
considered apart from the great fluctuations in organic life which took
place between the era of the Llandeilo and that of the Ludlow
formation, especially as the enormous pile of Silurian rocks observed
in Great Britain (in Wales more particularly) is derived in great part
from igneous action, and is not confined to the ordinary deposition of
sediment from rivers or the waste of cliffs.

In volcanic archipelagoes, such as the Canaries, we see the most active
of all known causes, aqueous and igneous, simultaneously at work to
produce great results in a comparatively moderate lapse of time. The
outpouring of repeated streams of lava—the showering down upon land and
sea of volcanic ashes—the sweeping seaward of loose sand and cinders,
or of rocks ground down to pebbles and sand, by rivers and torrents
descending steeply inclined channels—the undermining and eating away of
long lines of sea-cliff exposed to the swell of a deep and open
ocean—these operations combine to produce a considerable volume of
superimposed matter, without there being time for any extensive change
of species. Nevertheless, there would seem to be a limit to the
thickness of stony masses formed even under such favourable
circumstances, for the analogy of tertiary volcanic regions lends no
countenance to the notion that sedimentary and igneous rocks 25,000,
much less 45,000 feet thick, like those of Wales, could originate while
one and the same fauna should continue to people the earth. If, then,
we allow that about 25,000 feet of matter may be ascribed to one
system, such as the Silurian, as above described, we may be prepared to
discover in the next series of subjacent rocks a distinct assemblage of
species, or even in great part of genera, of organic remains. Such
appears to be the fact, and I shall therefore conclude with the Arenig
beds my enumeration of the Silurian formations in Great Britain, and
proceed to say something of their foreign equivalents, before treating
of rocks older than the Silurian.

Silurian Strata of the Continent of Europe.—When we turn to the
continent of Europe, we discover the same ancient series occupying a
wide area, but in no region as yet has it been observed to attain great
thickness. Thus, in Norway and Sweden, the total thickness of strata of
Silurian age is considerably less than 1000 feet, although the
representatives both of the Upper and Lower Silurian of England are not
wanting there. In Russia the Silurian strata, so far as they are yet
known, seem to be even of smaller vertical dimensions than in
Scandinavia, and they appear to consist chiefly of the Llandovery
group, or of a limestone containing _ Pentamerus oblongus,_ below which
are strata with fossils corresponding to those of the Llandeilo beds of
England. The lowest rock with organic remains yet discovered is ”the
Ungulite or Obolus grit” of St. Petersburg, probably coeval with the
Llandeilo flags of Wales.

The shales and grits near St. Petersburg, above alluded to, contain
green grains in their sandy layers, and are in a singularly unaltered
state, taking into account their high antiquity. The prevailing
Brachiopods consist of the _Obolus_ _Shells of the lowest known
Fossiliferous Beds in Russia._

Fig. 565: Siphonotreta unguiculata. Fig. 566: Obolus Apollinis.

or Ungulite of Pander, and a _Siphonotreta_ (Figs. 565, 566).
Notwithstanding the antiquity of this Russian formation, it should be
stated that both of these genera of brachiopods have been also found in
the Upper Silurian of England, i.e., in the Wenlock limestone.

Among the green grains of the sandy strata above-mentioned, Professor
Ehrenberg announced in 1854 his discovery of remains of foraminifera.
These are casts of the cells; and among five or six forms three are
considered by him as referable to existing genera (e.g., _Textularia,
Rotalia,_ and _Guttulina_).

Silurian Strata of the United States.—The Silurian formations can be
advantageously studied in the States of New York, Ohio, and other
regions north and south of the great Canadian lakes. Here they are
often found, as in Russia, nearly in horizontal position, and are more
rich in well-preserved fossils than in almost any spot in Europe. In
the State of New York, where the succession of the beds and their
fossils have been most carefully worked out by the Government
surveyors, the subdivisions given in the first column of the table
below have been adopted.

_Subdivisions of the Silurian Strata of New York.
(Strata below the Oriskany sandstone or base of the Devonian.)_

New York Names British equivalents 1.  Upper Pentamerus Limestone
   2.  Encrinal Limestone
   3.  Delthyris Shaly Limestone
   4.  Pentamerus and Tentaculite Limestones
   5.  Water Lime Group
   6.  Onondaga Salt Group
   7.  Niagara Group Upper Silurian (or Ludlow
and Wenlock formations 8.  Clinton Group
   9.  Medina Sandstone
10.  Oneida Conglomerate
11.  Gray Sandstone Beds of Passage, Llandovery Group. 12.  Hudson
River Group
13.  Trenton Limestone
14.  Black-River Limestone
15.  Bird’s-eye Limestone
16.  Chazy Limestone
17.  Calciferous Sandstone Lower Silurian (or Caradoc and Bala,
Llandeilo and Arenig Formations).

In the second column of the same table I have added the supposed
British equivalents. All Palæontologists, European and American, such
as MM. De Verneuil, D. Sharpe, Professor Hall, E. Billings, and others,
who have entered upon this comparison, admit that there is a marked
general correspondence in the succession of fossil forms, and even
species, as we trace the organic remains downward from the highest to
the lowest beds; but it is impossible to parallel each minor
subdivision.

That the Niagara Limestone, over which the river of that name is
precipitated at the great cataract, together with its underlying
shales, corresponds to the Wenlock limestone and shale of England there
can be no doubt. Among the species common to this formation in America
and Europe are _Calymene Blumenbachii, Homalonotus delphinocephalus_
(Fig. 544), with several other trilobites; _Rhynchonella Wilsoni,_ Fig.
531, and _Retzia cuneata; Orthis elegantula, Pentamerus galeatus,_ with
many more brachiopods; _Orthoceras annulatum,_ among the cephalopodous
shells; and _Favosites gothlandica,_ with other large corals.

Fig. 567: Murchisonia gracilis.
The Clinton Group, containing _Pentamerus oblongus_ and _
Stricklandinia,_ and related more nearly by its fossil species with the
beds above than with those below, is the equivalent of the Llandovery
Group or beds of passage.

The Hudson River Group, and the Trenton Limestone, agree
palæontologically with the Caradoc or Bala group, containing in common
with them several species of trilobites, such as _ Asaphus (Isotelus)
gigas, Trinucleus concentricus_ (Fig. 553); and various shells, such as
_ Orthis striatula, Orthis biforata_ (or _O. lynx_), _O. porcata_ (_O.
occidentalis_ of Hall), and _Bellerophon bilobatus._ In the Trenton
limestone occurs _Murchisonia gracilis,_ Fig. 567, a fossil also common
to the Llandeilo beds in England.

Mr. D. Sharpe, in his report on the mollusca collected by me from these
strata in North America,[6] has concluded that the number of species
common to the Silurian rocks on both sides of the Atlantic is between
30 and 40 per cent; a result which, although no doubt liable to future
modification, when a larger comparison shall have been made, proves,
nevertheless, that many of the species had a wide geographical range.
It seems that comparatively few of the gasteropods and
lamellibranchiate bivalves of North America can be identified
specifically with European fossils, while no less than two-fifths of
the brachiopoda, of which my collection chiefly consisted, are the
same. In explanation of these facts, it is suggested that most of the
recent brachiopoda (especially the orthidiform ones) are inhabitants of
deep water, and that they may have had a wider geographical range than
shells living near shore. The predominance of bivalve mollusca of this
peculiar class has caused the Silurian period to be sometimes styled
”the age of brachiopods.”

In Canada, as in the State of New York, the Potsdam Sandstone underlies
the above-mentioned calcareous rocks, but contains a different suite of
fossils, as will be hereafter explained. In parts of the globe still
more remote from Europe the Silurian strata have also been recognised,
as in South America, Australia, and India. In all these regions the
facies of the fauna, or the types of organic life, enable us to
recognise the contemporaneous origin of the rocks; but the fossil
species are distinct, showing that the old notion of a universal
diffusion throughout the ”primæval seas” of one uniform specific fauna
was quite unfounded, geographical provinces having evidently existed in
the oldest as in the most modern times.

 [1] Murchison’s Siluria, p. 140.

 [2] Murchison’s Siluria, chap. vi.

 [3] E. Forbes, Mem. Geol. Surv., vol. ii, p. 496.

 [4] Quart. Geol. Journ., vol. iv, p. 215, 1853.

 [5] Trans. Brit. Assoc., 1866. Proc. Liverpool Geol. Soc., 1869.

 [6] Quart. Geol. Journ., vol. iv.




CHAPTER XXVII.
CAMBRIAN AND LAURENTIAN GROUPS.


Classification of the Cambrian Group, and its Equivalent in Bohemia. —
Upper Cambrian Rocks. — Tremadoc Slates and their Fossils. — Lingula
Flags. — Lower Cambrian Rocks. — Menevian Beds. — Longmynd Group. —
Harlech Grits with large Trilobites. — Llanberis Slates. — Cambrian
Rocks of Bohemia. — Primordial Zone of Barrande. — Metamorphosis of
Trilobites. — Cambrian Rocks of Sweden and Norway. — Cambrian Rocks of
the United States and Canada. — Potsdam Sandstone. — Huronian Series. —
Laurentian Group, upper and lower. — Eozoon Canadense, oldest known
Fossil. — Fundamental Gneiss of Scotland.

CAMBRIAN  GROUP.

The characters of the Upper and Lower Silurian rocks were established
so fully, both on stratigraphical and palæontological data, by Sir
Roderick Murchison after five years’ labour, in 1839, when his
“Silurian System” was published, that these formations could from that
period be recognised and identified in all other parts of Europe and in
North America, even in countries where most of the fossils differed
specifically from those of the classical region in Britain, where they
were first studied.

While Sir R. I. Murchison was exploring in 1833, in Shropshire and the
borders of Wales, the strata which in 1835 he first called Silurian,
Professor Sedgwick was surveying the rocks of North Wales, which both
these geologists considered at that period as of older date, and for
which in 1836 Sedgwick proposed the name of Cambrian. It was afterwards
found that a large portion of the slaty rocks of North Wales, which had
been considered as more ancient than the Llandeilo beds and
Stiper-Stones before alluded to, were, in reality, not inferior in
position to those Lower Silurian beds of Murchison, but merely
extensive undulations of the same, bearing fossils identical in
species, though these were generally rarer and less perfectly
preserved, owing to the changes which the rocks had undergone from
metamorphic action. To such rocks the term “Cambrian” was no longer
applicable, although it continued to be appropriate to strata inferior
to the Stiper-Stones, and which were older than those of the Lower
Silurian group as originally defined. It was not till 1846 that fossils
were found in Wales in the Lingula flags, the place of which will be
seen in the table below. By this time Barrande had already published an
account of a rich collection of fossils which he had discovered in
Bohemia, portions of which he recognised as of corresponding age with
Murchison’s Upper and Lower Silurian, while others were more ancient,
to which he gave the name of “Primordial,” for the fossils were
sufficiently distinct to entitle the rocks to be referred to a new
period. They consisted chiefly of trilobites of genera distinct from
those occurring in the overlying Silurian formations. These peculiar
genera were afterwards found in rocks holding a corresponding position
in Wales, and I shall retain for them the term Cambrian, as recent
discoveries in our own country seem to carry the first fauna of
Barrande, or his primordial type, even into older strata than any which
he found to be fossiliferous in Bohemia.

The term primordial was intended to express M. Barrande’s own belief
that the fossils of the rocks so-called afforded evidence of the first
appearance of vital phenomena on this planet, and that consequently no
fossiliferous strata of older date would or could ever be discovered.
The acceptance of such a nomenclature would seem to imply that we
despaired of extending our discoveries of new and more ancient fossil
groups at some future day when vast portions of the globe, hitherto
unexplored, should have been thoroughly surveyed. Already the discovery
of the Laurentian Eozoon in Canada, presently to be mentioned,
discountenances such views.

The following table will show the succession of the strata in England
and Wales which belong to the Cambrian group or the fossiliferous rocks
older than the Arenig or Lower Llandeilo rocks:

UPPER CAMBRIAN TREMADOC SLATES _(Primordial of Barrande in part)_
LINGULA FLAGS _(Primordial of Barrande)_ LOWER CAMBRIAN MENEVIAN
BEDS _(Primordial of Barrande)_ LONGMYND GROUP _a._ Harlech Grits
_b._ Llanberis Slates

Tremadoc Slates.—The Tremadoc slates of Sedgwick are more than 1000
feet in thickness, and consist of dark earthy slates occurring near the
little town of Tremadoc, situated on the north side of Cardigan Bay, in
Carnarvonshire. These slates were first examined by Sedgwick in 1831,
and were re-examined by him and described in 1846,[1] after some
fossils had been found in the underlying Lingula flags by Mr. Davis.
The inferiority in position of these Lingula flags to the Tremadoc beds
was at the same time established. The overlying Tremadoc beds were
traced by their pisolitic ore from Tremadoc to Dolgelly. No fossils
proper to the Tremadoc slates were then observed, but subsequently,
thirty-six species of all classes have been found in them, thanks to
the researches of Messrs. Salter, Homfray, and Ash. We have already
seen that in the Arenig or Stiper-Stones group, where the species are
distinct, the genera agree with Silurian types; but in these Tremadoc
slates, where the species are also peculiar, there is about an equal
admixture of Silurian types with those which Barrande has termed
“primordial.” Here, therefore, it may truly be said that we are
entering upon a new domain of life in our retrospective survey of the
past. The trilobites of new species, but of Lower Silurian genera,
belong to _Ogygia, Asaphus,_ and _Cheirurus_; whereas those belonging
to primordial types, or Barrande’s first fauna as well as to the
Lingula flags of Wales, comprise _Dikelocephalus, Conocoryphe_ (for
genera see Fig. 577 and 581),[2] _Olenus,_ and _Angelina._

Fig. 568: Theca (Cleidotheca operculata.
In the Tremadoc slates are found _Bellerophon, Orthoceras,_ and
_Cyrtoceras,_ all specifically distinct from Lower Silurian fossils of
the same genera: the Pteropods _Theca_ (Fig. 568) and _Conularia_ range
throughout these slates; there are no Graptolites. The _Lingula
(Lingulella) Davisii_ ranges from the top to the bottom of the
formation, and links it with the zone next to be described. The
Tremadoc slates are very local, and seem to be confined to a small part
of North Wales; and Professor Ramsay supposes them to lie unconformably
on the Lingula flags, and that a long interval of time elapsed between
these formations. Cephalopoda have not yet been found lower than this
group, but it will be observed that they occur here associated with
genera of Trilobites considered by Barrande as characteristically
Primordial, some of which belong to all the divisions of the British
Cambrian about to be mentioned. This renders the absence of cephalopoda
of less importance as bearing on the theory of development.

Lingula Flags.—Next below the Tremadoc slates in North Wales lie
micaceous flagstones and slates, in which, in 1846, Mr. E. Davis
discovered the _Lingula (Lingulella),_ Fig. 570, named after him, and
from which was derived the name of Lingula flags. These beds, which are
palæontologically the equivalents of Barrande’s primordial zone, are
represented by more than 5000 feet of strata, and have been studied
chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in
North Wales, and at St. David’s in South Wales. They have yielded about
forty species of fossils, of which six only are common to the overlying
Tremadoc rocks, but the two formations are closely allied by having
several characteristic “primordial” genera in common. _Dikelocephalus,
Olenus_ (Fig. 571), and _Conocoryphe_ are prominent forms, as is also
_Hymenocaris_ (Fig. 569), a genus of phyllopod crustacean entirely
confined to the Lingula Flags. According to Mr. Belt, who has devoted
much attention to these beds, there are already palæontological data
for subdividing the Lingula Flags into three sections.[3]

“Lingula Flags” of Dolgelly, and Ffestiniog; N. Wales.

In Merionethshire, according to Professor Ramsay, the Lingula Flags
attain their greatest development; in Carnarvonshire they thin out so
as to have lost two-thirds of their thickness in eleven miles, while in
Anglesea and on the Menai Straits both they and the Tremadoc beds are
entirely absent, and the Lower Silurian rests directly on Lower
Cambrian strata.

LOWER CAMBRIAN.

Menevian Beds.—Immediately beneath the Lingula Flags there occurs a
series of dark grey and black flags and slates alternating at the upper
part with some beds of sandstone, the whole reaching a thickness of
from 500 to 600 feet. These beds were formerly classed, on purely
lithological grounds, as the base of the Lingula Flags, but Messrs.
Hicks and Salter, to whose exertions we owe almost all our knowledge of
the fossils, have pointed out[4] that the most characteristic genera
found in them are quite unknown in the Lingula Flags, while they
possess many of the strictly Lower Cambrian genera, such as _
Microdiscus_ and _Paradoxides._ They therefore proposed to place them,
and it seems to me with good reason, at the top of the Lower Cambrian
under the term “Menevian,” Menevia being the classical name of St.
David’s. The beds are well exhibited in the neighbourhood of St.
David’s in South Wales, and near Dolgelly and Maentwrog in North Wales.
They are the equivalents of the lowest part of Barrande’s Primordial
Zone (Étage C). More than forty species have been found in them, and
the group is altogether very rich in fossils for so early a period.

Fig. 572: Paradoxides Davidis.
The trilobites are of large size; _Paradoxides Davidis_ (see Fig. 572),
the largest trilobite known in England, 22 inches or nearly two feet
long, is peculiar to the Menevian Beds. By referring to the Bohemian
trilobite of the same genus (Fig. 576), the reader will at once see how
these fossils (though of such different dimensions) resemble each other
in Bohemia and Wales, and other closely allied species from the two
regions might be added, besides some which are common to both
countries. The Swedish fauna, presently to be mentioned, will be found
to be still more nearly connected with the Welsh Menevian. In all these
countries there is an equally marked difference between the Cambrian
fossils and those of the Upper and Lower Silurian rocks. The trilobite
with the largest number of rings, _ Erinnys venulosa,_ occurs here in
conjunction with _ Agnostus_ and _Microdiscus,_ the genera with the
smallest number. Blind trilobites are also found as well as those which
have the largest eyes, such as _Microdiscus_ on the one hand, and
_Anoplenus_ on the other.

LONGMYND GROUP.

Older than the Menevian Beds are a thick series of olive green, purple,
red and grey grits and conglomerates found in North and South Wales,
Shropshire, and parts of Ireland and Scotland. They have been called by
Professor Sedgwick the Longmynd or Bangor Group, comprising, first, the
Harlech and Barmouth sandstones; and secondly, the Llanberis slates.

Fig. 573: Histioderma Hibernica.
Harlech Grits.—The sandstones of this period attain in the Longmynd
hills a thickness of no less than 6000 feet without any interposition
of volcanic matter; in some places in Merionethshire they are still
thicker. Until recently these rocks possessed but a very scanty fauna.

With the exception of five species of annelids (see Fig. 460) brought
to light by Mr. Salter in Shropshire, and Dr. Kinahan in Wicklow, and
an obscure crustacean form, _Palæopyge Ramsayi,_ they were supposed to
be barren of organic remains. Now, however, through the labours of Mr.
Hicks,[5] they have yielded at St. David’s a rich fauna of trilobites,
brachiopods, phyllopods, and pteropods, showing, together with other
fossils, a by no means low state of organisation at this early period.
Already the fauna amounts to 20 species referred to 17 genera.

A new genus of trilobite called _Plutonia Sedgwickii,_ not yet figured
and described, has been met with in the Harlech grits. It is comparable
in size to the large _Paradoxides Davidis_ before mentioned, has
well-developed eyes, and is covered all over with tubercles. In the
same strata occur other genera of trilobites, namely, _Conocoryphe,
Paradoxides, Microdiscus,_ and the Pteropod _Theca_ (Fig. 568), all
represented by species peculiar to the Harlech grits. The sands of this
formation are often rippled, and were evidently left dry at low tides,
so that the surface was dried by the sun and made to shrink and present
sun-cracks. There are also distinct impressions of rain-drops on many
surfaces, like those in Fig. 444 and 445.

Lanberis Slates.—The slates of Llanberis and Penrhyn in Carnarvonshire,
with their associated sandy strata, attain a great thickness, sometimes
about 3000 feet. They are perhaps not more ancient than the Harlech and
Barmouth beds last mentioned, for they may represent the deposits of
fine mud thrown down in the same sea, on the borders of which the sands
above-mentioned were accumulating. In some of these slaty rocks in
Ireland, immediately opposite Anglesea and Carnarvon, two species of
fossils have been found, to which the late Professor E. Forbes gave the
name of _Oldhamia._ The nature of these organisms is still a matter of
discussion among naturalists.

Fig. 574: Oldhamia radiata.
Fig. 575: Oldhamia antiqua.
Cambrian Rocks of Bohemia _(Primordial zone of Barrande)._—In the year
1846, as before stated, M. Joachim Barrande, after ten years’
exploration of Bohemia, and after collecting more than a thousand
species of fossils, had ascertained the existence in that country of
three distinct faunas below the Devonian. To his first fauna, which was
older than any then known in this country, he gave the name of Étage C;
his two first stages A and B consisting of crystalline and metamorphic
rocks and unfossiliferous schists. This Étage C or primordial zone
proved afterwards to be the equivalent of those subdivisions of the
Cambrian groups which have been above described under the names of
Menevian and Lingula Flags. The second fauna tallies with Murchison’s
Lower Silurian, as originally defined by him when no fossils had been
discovered below the Stiper-Stones. The third fauna agrees with the
Upper Silurian of the same author. Barrande, without government
assistance, had undertaken single-handed the geological survey of
Bohemia, the fossils previously obtained from that country having
scarcely exceeded 20 in number, whereas he had already acquired, in
1850, no less than 1100 species, namely, 250 crustaceans (chiefly
Trilobites), 250 Cephalopods, 160 gasteropods and pteropods, 130
acephalous mollusks, 210 brachiopods, and 110 corals and other fossils.
These numbers have since been almost doubled by subsequent
investigations in the same country.

In the primordial zone C, he discovered trilobites of the genera
_Paradoxides, Conocoryphe, Ellipsocephalus, Sao, Arionellus,
Hydrocephalus,_ and _Agnostus._ M. Barrande pointed out that these
primordial trilobites have a peculiar facies of their own dependent on
the multiplication of their thoracic segments and the diminution of
their caudal shield or pygidium.

_Fossils of the lowest Fossiliferous Beds in Bohemia, or
“Primordial Zone” of Barrande._

Fig. 576: Paradoxides Bohemicus. Fig. 577: Conocoryphe striata. Fig.
578: Agnostus integer. Fig. 579: Agnostus Rex. Fig. 580: Sao hirsuta in
its various stages of growth.

One of the “primordial” or Upper Cambrian Trilobites of the genus
_Sao,_ a form not found as yet elsewhere in the world, afforded M.
Barrande a fine illustration of the metamorphosis of these creatures,
for he traced them through no less than twenty stages of their
development. A few of these changes have been selected for
representation in Figure 580, that the reader may learn the gradual
manner in which different segments of the body and the eyes make their
appearance.

In Bohemia the primordial fauna of Barrande derived its importance
exclusively from its numerous and peculiar trilobites. Besides these,
however, the same ancient schists have yielded two genera of
brachiopods, _Orthis_ and _Orbicula,_ a Pteropod of the genus _Theca,_
and four echinoderms of the cystidean family.

Cambrian of Sweden and Norway.—The Cambrian beds of Wales are
represented in Sweden by strata the fossils of which have been
described by a most able naturalist, M. Angelin, in his “Palæontologica
Suecica” (1852-4). The “alum-schists,” as they are called in Sweden,
are horizontal argillaceous rocks which underlie conformably certain
Lower Silurian strata in the mountain called Kinnekulle, south of the
great Wener Lake in Sweden. These schists contain trilobites belonging
to the genera _Paradoxides, Olenus, Agnostus,_ and others, some of
which present rudimentary forms, like the genus last mentioned, without
eyes, and with the body segments scarcely developed, and others, again,
have the number of segments excessively multiplied, as in
_Paradoxides._ Such peculiarities agree with the characters of the
crustaceans met with in the Cambrian strata of Wales; and Dr. Torell
has recently found in Sweden the _Paradoxides Hicksii,_ a well-known
Lower Cambrian fossil.

At the base of the Cambrian strata in Sweden, which in the
neighbourhood of Lake Wener are perfectly horizontal, lie ripple-marked
quartzose sandstones with worm-tracks and annelid borings, like some of
those found in the Harlech grits of the Longmynd. Among these are some
which have been referred doubtfully to plants. These sandstones have
been called in Sweden “fucoid sandstones.” The whole thickness of the
Cambrian rocks of Sweden does not exceed 300 feet from the equivalents
of the Tremadoc beds to these sandstones, which last seem to correspond
with the Longmynd, and are regarded by Torell as older than any
fossiliferous primordial rocks in Bohemia.

Cambrian of the United States and Canada _(Potsdam Sandstone)._—This
formation, as we learn from Sir W. Logan, is 700 feet thick in Canada;
the upper part consists of sandstone containing fucoids, and perforated
by small vertical holes, which are very characteristic of the rock, and
appear to have been made by annelids _(Scolithus linearis)._ The lower
portion is a conglomerate with quartz pebbles. I have seen the Potsdam
sandstone on the banks of the St. Lawrence, and on the borders of Lake
Champlain, where, as at Keesville, it is a white quartzose fine-grained
grit, almost passing into quartzite. It is divided into horizontal
ripple-marked beds, very like those of the Lingula Flags of Britain,
and replete with a small round-shaped _ Obolella,_ in such numbers as
to divide the rock into parallel planes, in the same manner as do the
scales of mica in some micaceous sandstones. Among the shells of this
formation in Wisconsin are species of _Lingula_ and _Orthis,_ and
several trilobites of the primordial genus _Dikelocephalus_ (Fig. 581).
On the banks of the St. Lawrence, near Beauharnois and elsewhere, many
fossil footprints have been observed on the surface of the rippled
layers. They are supposed by Professor Owen to be the trails of more
than one species of articulate animal, probably allied to the King
Crab, or _Limulus._

Fig. 581: Dikelocephalus Minnesotensis.
Recent investigations by the naturalists of the Canadian survey have
rendered it certain that below the level of the Potsdam Sandstone there
are slates and schists extending from New York to Newfoundland,
occupied by a series of trilobitic forms similar in genera, though not
in species, to those found in the European Upper Cambrian strata.

Huronian Series.—Next below the Upper Cambrian occur strata called the
Huronian by Sir W. Logan, which are of vast thickness, consisting
chiefly of quartzite, with great masses of greenish chloritic slate,
which sometimes include pebbles of crystalline rocks derived from the
Laurentian formation, next to be described. Limestones are rare in this
series, but one band of 300 feet in thickness has been traced for
considerable distances to the north of Lake Huron. Beds of greenstone
are intercalated conformably with the quartzose and argillaceous
members of this series. No organic remains have yet been found in any
of the beds, which are about 18,000 feet thick, and rest unconformably
on the Laurentian rocks.

LAURENTIAN GROUP.

In the course of the geological survey carried on under the direction
of Sir W.E. Logan, it has been shown that, northward of the river St.
Lawrence, there is a vast series of crystalline rocks of gneiss,
mica-schist, quartzite, and limestone, more than 30,000 feet in
thickness, which have been called Laurentian, and which are already
known to occupy an area of about 200,000 square miles. They are not
only more ancient than the fossiliferous Cambrian formations above
described, but are older than the Huronian last mentioned, and had
undergone great disturbing movements before the Potsdam sandstone and
the other “primordial” or Cambrian rocks were formed. The older half of
this Laurentian series is unconformable to the newer portion of the
same.

Upper Laurentian or Labrador Series.—The Upper Group, more than 10,000
feet thick, consists of stratified crystalline rocks in which no
organic remains have yet been found. They consist in great part of
feldspars, which vary in composition from anorthite to andesine, or
from those kinds in which there is less than one per cent of potash and
soda to those in which there is more than seven per cent of these
alkalies, the soda preponderating greatly. These feldsparites sometimes
form mountain masses almost without any admixture of other minerals;
but at other times they include augite, which passes into hypersthene.
They are often granitoid in structure. One of the varieties is the same
as the apolescent labradorite rock of Labrador. The Adirondack
Mountains in the State of New York are referred to the same series, and
it is conjectured that the hypersthene rocks of Skye, which resemble
this formation in mineral character, may be of the same geological age.

Lower Laurentian.—This series, about 20,000 feet in thickness, is, as
before stated, unconformable to that last mentioned; it consists in
great part of gneiss of a reddish tint with orthoclase feldspar. Beds
of nearly pure quartz, from 400 to 600 feet thick, occur in some
places. Hornblendic and micaceous schists are often interstratified,
and beds of limestone, usually crystalline. Beds of plumbago also
occur. That this pure carbon may have been of organic origin before
metamorphism has naturally been conjectured.

There are several of these limestones which have been traced to great
distances, and one of them is from 700 to 1500 feet thick. In the most
massive of them Sir W. Logan observed, in 1859, what he considered to
be an organic body much resembling the Silurian fossil called
_Stromatopora rugosa._ It had been obtained the year before by Mr. J.
MacMullen at the Grand Calumet, on the river Ottawa. This fossil was
examined in 1864 by Dr. Dawson of Montreal, who detected in it, by aid
of the microscope, the distinct structure of a Rhizopod or Foraminifer.
Dr. Carpenter and Professor T. Rupert Jones have since confirmed this
opinion, comparing the structure to that of the well-known nummulite.
It appears to have grown one layer over another, and to have formed
reefs of limestone as do the living coral-building polyp animals. Parts
of the original skeleton, consisting of carbonate of lime, are still
preserved; while certain inter-spaces in the calcareous fossil have
been filled up with serpentine and white augite. On this oldest of
known organic remains Dr. Dawson has conferred the name of _ Eozoon_
_Canadense_ (see Figs. 582, 583); its antiquity is such that the
distance of time which separated it from the Upper Cambrian period, or
that of the Potsdam sandstone, may, says Sir W. Logan, be equal to the
time which elapsed between the Potsdam sandstone and the nummulitic
limestones of the Tertiary period. The Laurentian and Huronian rocks
united are about 50,000 feet in thickness, and the Lower Laurentian was
disturbed before the newer series was deposited. We may naturally
expect the other proofs of unconformability will hereafter be detected
at more than one point in so vast a succession of strata.

Fig. 582 and 583: Eozoon Canadense.

Fig. 582. _a._ Chambers of lower tier communicating at +, and separated
from adjoining chambers at O by an intervening septum, traversed by
passages. _b._ Chambers of an upper tier. _c._ Walls of the chambers
traversed by fine tubules. (These tubules pass with uniform parallelism
from the inner to the outer surface, opening at regular distances from
each other.) _d._ Intermediate skeleton, composed of homogeneous shell
substance, traversed by _f._ Stoloniferous passages connecting the
chambers of the two tiers. _e._ Canal system in intermediate skeleton,
showing the arborescent saceodic prolongations. (Fig. 583 shows these
bodies in a decalcified state.) _f._ Stoloniferous passages.
Fig. 583. Decalcified portion of natural rock, showing _canal system_
and the several layers; the acuteness of the planes prevents more than
one or two parallel tiers being observed.

The mineral character of the Upper Laurentian differs, as we have seen,
from that of the Lower, and the pebbles of gneiss in the Huronian
conglomerates are thought to prove that the Laurentian strata were
already in a metamorphic state before they were broken up to supply
materials for the Huronian. Even if we had not discovered the Eozoon,
we might fairly have inferred from analogy that as the quartzites were
once beds of sand, and the gneiss and mica-schist derived from shales
and argillaceous sandstones, so the calcareous masses, from 400 to 1000
feet and more in thickness, were originally of organic origin. This is
now generally believed to have been the case with the Silurian,
Devonian, Carboniferous, Oolitic, and Cretaceous limestones and those
nummulitic rocks of tertiary date which bear the closest affinity to
the Eozoon reefs of the Lower Laurentian. The oldest stratified rock in
Scotland is that called by Sir R. Murchison “the fundamental gneiss,”
which is found in the north-west of Ross-shire, and in Sutherlandshire
(see Fig. 82), and forms the whole of the adjoining island of Lewis, in
the Hebrides. It has a strike from north-west to south-east, nearly at
right angles to the metamorphic strata of the Grampians. On this
Laurentian gneiss, in parts of the western Highlands, the Lower
Cambrian and various metamorphic rocks rest unconformably. It seems
highly probable that this ancient gneiss of Scotland may correspond in
date with part of the great Laurentian group of North America.

 [1] Quart. Geol. Journ., vol. iii, p. 156.

 [2] This genus has been substituted for Barrande’s _ Conocephalus,_ as
 the latter term had been preoccupied by the entomologists.

 [3] Geol. Mag., vol iv.

 [4] British Association Report 1865, 1866, 1868 and Quart. Geol.
 Journ., vols. xxi, xxv.

 [5] Brit. Assoc. Report, 1868.




CHAPTER XXVIII.
VOLCANIC ROCKS.


External Form, Structure, and Origin of Volcanic Mountains. — Cones and
Craters. — Hypothesis of “Elevation Craters” considered. — Trap Rocks.
— Name whence derived. — Minerals most abundant in Volcanic Rocks. —
Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks. —
Similar Minerals in Meteorites. — Theory of Isomorphism. — Basaltic
Rocks. — Trachytic Rocks. — Special Forms of Structure. — The columnar
and globular Forms. — Trap Dikes and Veins. — Alteration of Rocks by
volcanic Dikes. — Conversion of Chalk into Marble. — Intrusion of Trap
between Strata. — Relation of trappean Rocks to the Products of active
Volcanoes.

The aqueous or fossiliferous rocks having now been described, we have
next to examine those which may be called volcanic, in the most
extended sense of that term. In the diagram (Fig. 584) suppose _a, a_
to represent the crystalline formations, such as the granitic and
metamorphic; _b, b_ the fossiliferous strata; and _c, c_ the volcanic
rocks. These last are sometimes found, as was explained in the first
chapter, breaking through _a_ and _b,_ sometimes overlying both, and
occasionally alternating with the strata _b, b._

Fig. 584: a. Hypogene formations, stratified and unstratified. b.
Aqueous formations. c. Volcanic rocks.

External Form, Structure, and Origin of Volcanic Mountains.—The origin
of volcanic cones with crater-shaped summits has been explained in the
“Principles of Geology” (Chapters 23 to 27), where Vesuvius, Etna,
Santorin, and Barren Island are described. The more ancient portions of
those mountains or islands, formed long before the times of history,
exhibit the same external features and internal structure which belong
to most of the extinct volcanoes of still higher antiquity; and these
last have evidently been due to a complicated series of operations,
varied in kind according to circumstances; as, for example, whether the
accumulation took place above or below the level of the sea, whether
the lava issued from one or several contiguous vents, and, lastly,
whether the rocks reduced to fusion in the subterranean regions
happened to have contained more or less silica, potash, soda, lime,
iron, and other ingredients. We are best acquainted with the effects of
eruptions above water, or those called subÆrial or supramarine; yet the
products even of these are arranged in so many ways that their
interpretation has given rise to a variety of contradictory opinions,
some of which will have to be considered in this chapter.

Fig. 585: Part of the chain of extinct volcanoes called the Monts Dome,
Aurvergne.

_Cones and Craters._—In regions where the eruption of volcanic matter
has taken place in the open air, and where the surface has never since
been subjected to great aqueous denudation, cones and craters
constitute the most striking peculiarity of this class of formations.
Many hundreds of these cones are seen in central France, in the ancient
provinces of Auvergne, Velay, and Vivarais, where they observe, for the
most part, a linear arrangement, and form chains of hills. Although
none of the eruptions have happened within the historical era, the
streams of lava may still be traced distinctly descending from many of
the craters, and following the lowest levels of the existing valleys.
The origin of the cone and crater-shaped hill is well understood, the
growth of many having been watched during volcanic eruptions. A chasm
or fissure first opens in the earth, from which great volumes of steam
are evolved. The explosions are so violent as to hurl up into the air
fragments of broken stone, parts of which are shivered into minute
atoms. At the same time melted stone or _lava_ usually ascends through
the chimney or vent by which the gases make their escape. Although
extremely heavy, this lava is forced up by the expansive power of
entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in
the same manner as water is made to boil over the edge of a vessel when
steam has been generated at the bottom by heat. Large quantities of the
lava are also shot up into the air, where it separates into fragments,
and acquires a spongy texture by the sudden enlargement of the included
gases, and thus forms _scoriæ,_ other portions being reduced to an
impalpable powder or dust. The showering down of the various ejected
materials round the orifice of eruption gives rise to a conical mound,
in which the successive envelopes of sand and scoriæ form layers,
dipping on all sides from a central axis. In the mean time a hollow,
called a _ crater,_ has been kept open in the middle of the mound by
the continued passage upward of steam and other gaseous fluids. The
lava sometimes flows over the edge of the crater, and thus thickens and
strengthens the sides of the cone; but sometimes it breaks down the
cone on one side (see Fig. 585), and often it flows out from a fissure
at the base of the hill, or at some distance from its base.

Some geologists had erroneously supposed, from observations made on
recent cones of eruption, that lava which consolidates on steep slopes
is always of a scoriaceous or vesicular structure, and never of that
compact texture which we find in those rocks which are usually termed
“trappean.” Misled by this theory, they have gone so far as to believe
that if melted matter has originally descended a slope at an angle
exceeding four or five degrees, it never, on cooling, acquires a stony
compact texture. Consequently, whenever they found in a volcanic
mountain sheets of stony materials inclined at angles of from 5° to 20°
or even more than 30°, they thought themselves warranted in assuming
that such rocks had been originally horizontal, or very slightly
inclined, and had acquired their high inclination by subsequent
upheaval. To such dome-shaped mountains with a cavity in the middle,
and with the inclined beds having what was called a quâquâversal dip or
a slope outward on all sides, they gave the name of “Elevation
craters.”

As the late Leopold Von Buch, the author of this theory, had selected
the Isle of Palma, one of the Canaries, as a typical illustration of
this form of volcanic mountain, I visited that island in 1854, in
company with my friend Mr. Hartung, and I satisfied myself that it owes
its origin to a series of eruptions of the same nature as those which
formed the minor cones, already alluded to. In some of the more ancient
or Miocene volcanic mountains, such as Mont Dor and Cantal in central
France, the mode of origin by upheaval as above described is attributed
to those dome-shaped masses, whether they possess or not a great
central cavity, as in Palma. Where this cavity is present, it has
probably been due to one or more great explosions similar to that which
destroyed a great part of ancient Vesuvius in the time of Pliny.
Similar paroxysmal catastrophes have caused in historical times the
truncation on a grand scale of some large cones in Java and
elsewhere.[1]

Among the objections which may be considered as fatal to Von Buch’s
doctrine of upheaval in these cases, I may state that a series of
volcanic formations extending over an area six or seven miles in its
shortest diameter, as in Palma, could not be accumulated in the form of
lavas, tuffs, and volcanic breccias or agglomerates without producing a
mountain as lofty as that which they now constitute. But assuming that
they were first horizontal, and then lifted up by a force acting most
powerfully in the centre and tilting the beds on all sides, a central
crater having been formed by explosion or by a chasm opening in the
middle, where the continuity of the rocks was interrupted, we should
have a right to expect that the chief ravines or valleys would open
towards the central cavity, instead of which the rim of the great
crater in Palma and other similar ancient volcanoes is entire for more
than three parts of the whole circumference.

If dikes are seen in the precipices surrounding such craters or central
cavities, they certainly imply rents which were filled up with liquid
matter. But none of the dislocations producing such rents can have
belonged to the supposed period of terminal and paroxysmal upheaval,
for had a great central crater been already formed before they
originated, or at the time when they took place, the melted matter,
instead of filling the narrow vents, would have flowed down into the
bottom of the cavity, and would have obliterated it to a certain
extent. Making due allowance for the quantity of matter removed by
subaërial denudation in volcanic mountains of high antiquity, and for
the grand explosions which are known to have caused truncation in
active volcanoes, there is no reason for calling in the violent
hypothesis of elevation craters to explain the structure of such
mountains as Teneriffe, the Grand Canary, Palma, or those of central
France, Etna, or Vesuvius, all of which I have examined. With regard to
Etna, I have shown, from observations made by me in 1857, that modern
lavas, several of them of known date, have formed continuous beds of
compact stone even on slopes of 15, 36, and 38 degrees, and, in the
case of the lava of 1852, more than 40 degrees. The thickness of these
tabular layers varies from 1½ foot to 26 feet. And their planes of
stratification are parallel to those of the overlying and underlying
scoriæ which form part of the same currents.[2]

Nomenclature of Trappean Rocks.—When geologists first began to examine
attentively the structure of the northern and western parts of Europe,
they were almost entirely ignorant of the phenomena of existing
volcanoes. They found certain rocks, for the most part without
stratification, and of a peculiar mineral composition, to which they
gave different names, such as basalt, greenstone, porphyry, trap tuff,
and amygdaloid. All these, which were recognised as belonging to one
family, were called “trap” by Bergmann, from _trappa,_ Swedish for a
flight of steps—a name since adopted very generally into the
nomenclature of the science; for it was observed that many rocks of
this class occurred in great tabular masses of unequal extent, so as to
form a succession of terraces or steps. It was also felt that some
general term was indispensable, because these rocks, although very
diversified in form and composition, evidently belonged to one group,
distinguishable from the Plutonic as well as from the non-volcanic
fossiliferous rocks.

By degrees familiarity with the products of active volcanoes convinced
geologists more and more that they were identical with the trappean
rocks. In every stream of modern lava there is some variation in
character and composition, and even where no important difference can
be recognised in the proportions of silica, alumina, lime, potash,
iron, and other elementary materials, the resulting materials are often
not the same, for reasons which we are as yet unable to explain. The
difference also of the lavas poured out from the same mountain at two
distinct periods, especially in the quantity of silica which they
contain, is often so great as to give rise to rocks which are regarded
as forming distinct families, although there may be every intermediate
gradation between the two extremes, and although some rocks, forming a
transition from the one class to the other, may often be so abundant as
to demand special names. These species might be multiplied
indefinitely, and I can only afford space to name a few of the
principal ones, about the composition and aspect of which there is the
least discordance of opinion.

Minerals most abundant in Volcanic Rocks.—The minerals which form the
chief constituents of these igneous rocks are few in number. Next to
quartz, which is nearly pure silica or silicic acid, the most important
are those silicates commonly classed under the several heads of
feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in
drawing up which I have received the able assistance of Mr. David
Forbes, the chemical analysis of these minerals and their varieties is
shown, and he has added the specific gravity of the different mineral
species, the geological application of which in determining the rocks
formed by these minerals will be explained in the sequel (p.504).

_Analysis of Minerals most abundant in the Volcanic and Hypogene
Rocks._

THE QUARTZ GROUP QUARTZ 100·0
2·6 Silica
Specific gravity TRIDYMITE 100·0
2·3 Silica
Specific gravity THE FELDSPAR GROUP ORTHOCLASE.
—— Carisbad, in granite (bulk) 65·23
16·26
0·27
nil
trace
nil
14·66
1·45
nil
2·55 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Sanadine, Drachenfels in trachyte
(Rammelsberg) 65·87
18·53
nil
nil
0·95
0·30
10·32
3·49
W. 0·44
2·55 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity ALBITE.
—— Arendal, in granite (G. Rose) 68·46
19·30
nil
0·28
0·68
nil
nil
11·27
nil
2·61 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity OLIGOCLASE.
—— Ytterby, in granite (Berzelius) 61·55
23·80
nil
nil
3·18
0·80
0·38
9·67
nil
2·65 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Teneriffe, in trachyte (Deville) 61·55
22·03
nil
nil
2·81
0·47
3·44
7·74
nil
2·59 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity LABRADORITE.
—— Hitteroe, in Labrador-rock (Waage) 51·39
29·42
2·90
nil
9·44
0·37
1·10
5·03
W. 0·71
2·72 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Iceland, in volcanic (Damour) 52·17
29·22
1·90
nil
13·11
nil
nil
3·40
nil
2·71 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity ANORTHITE.
—— Harzburg, in diorite (Streng) 45·37
34·81
0·59
nil
16·52
0·83
0·40
1·45
W. 0·87
2·74 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Hecla, in volcanic (Waltershausen) 45·14
32·10
2·03
0·78
18·32
nil
0·22
1·06
nil
2·74 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity LEUCITE.
—— Vesuvius, 1811, in lava (Rammelsberg) 56·10
23·22
nil
nil
nil
nil
20·59
0·57
nil
2·48 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity NEPHELINE.
—— Miask, in Miascite (Scheerer) 44·30
33·25
0·82
nil
0·32
0·07
5·82
16·02
nil
2·59 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Vesuvius, in volcanic (Arfvedson) 44·11
33·73
nil
nil
nil
nil
nil
20·46
W. 0·62
2·60 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity THE MICA GROUP MUSCOVITE.
—— Finland, in grante (Rose) 46·36
36·80
4·53
nil
nil
nil
9·22
nil
F. 0·67
W. 1·84
2·90 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents

Specific gravity LEPIDOLITE.
—— Cornwall, in granite (Regnault) 52·40
26·80
nil
1·50
nil
nil
9·14
nil
F. 4·18
Li. 4·85
2·90 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents

Specific gravity BIOTITE.
—— Bodennais (V. Kobel> 40·86
15·13
13·00
nil
nil
22·00
8·83
nil
W. 0·44
2·70 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Vesuvius, in volcanic (Chodnef) 40·91
17·71
11·02
nil
0·30
19·04
9·96
nil
nil
2·75 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity PHLOGOPITE.
—— New York, in metamorphic limestone (Rammelsberg) 41·96
13·47
nil
2·67
0·34
27·12
9·37
nil
F. 2·93
W. 0·60
2·81 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents

Specific gravity MARGARITE.
—— Nexos (Smith) 30·02
49·52
1·65
nil
10·82
0·48
1·25

W. 5·55
2·99 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
=Potash
=Soda
Other constituents
Specific gravity RAPIDOLITE.
—— Pyrenees (Delesse) 32·10
18·50
nil
0·06
nil
36·70
nil
nil
W. 12·10
2·61 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity TALC.
—— Zillerthal (Delesse) 63·00
nil
nil
trace
nil
33·60
nil
nil
W. 3·10
2·78 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity THE AMPHIBOLE AND PYROXENE GROUP TREMOLITE.
—— St. Gothard (Rammelsbeg) 58·55
nil
nil
nil
13·90
26·63
nil
nil
F.W. 0·34
2·93 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity ACTINOLITE.
—— Arendal, in granite (Rammelsberg) 56·77
0·97
nil
5·88
13·56
21·48
nil
nil
W. 2·20
3·02 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity HORNBLENDE.
—— Faymont, in diorite (Deville) 41·99
11·66
nil
22·22
9·55
12·59
nil
1·02
W. 1·47
3·20 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Etna, in volcanic (Waltershausen) 40·91
13·68
nil
17·49
13·44
13·19
nil
nil
W. 0·85
3·01 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity URALITE.
—— Ural, (Rammelsberg) 50·75
5·65
nil
17·27
11·59
12·28
nil
nil
W. 1·80
3·14 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity AUGITE.
—— Bohemia, in dolerite (Rammelsberg) 51·12
3·38
0·95
8·08
23·54
12·82
nil
nil
nil
3·35 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Vesuvius, in lava of 1858 (Rammelsberg) 49·61
4·42
nil
9·08
22·83
14·22
nil
nil
nil
3·25 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity DIALLAGE.
—— Harz, in Gabbro (Rammelsberg) 52·00
3·10
nil
9·36
16·29
18·51
nil
nil
W. 1·10
3·23 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity HYPERSTHENE.
—— Labrador, in Labrador-Rock (Damour) 51·36
0·37
nil
22·59
3·09
21·31
nil
nil
nil
3·39 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity THE OLIVINE GROUP BRONZITE.
—— Greenland (V. Kobell) 58·00
1·33
11·14
nil
nil
29·66
nil
nil
nil
3·20 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity OLIVINE.
—— Carlsbad, in basalt (Rammelsberg) 39·34
nil
nil
14·85
nil
45·81
nil
nil
nil
3·40 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Mount Somma, in volcanic (Walmstedt) 10·08
0·18
nil
15·74
nil
44·22
nil
nil
nil
3·33 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity

In the “Other constituents” the following signs are used: F=Fluorine,
Li=Lithia, W=Loss on igniting the mineral, in most instances only
Water.

From the table above it will be observed that many minerals are omitted
which, even if they are of common occurrence, are more to be regarded
as accessory than as essential components of the rocks in which they
are found.[3] Such are, for example, Garnet, Epidote, Tourmaline,
Idocrase, Andalusite, Scapolite, the various Zeolites, and several
other silicates of somewhat rarer occurrence. Magnetite, Titanoferrite,
and Iron-pyrites also occur as normal constituents of various igneous
rocks, although in very small amount, as also Apatite, or phosphate of
lime. The other salts of lime, including its carbonate or calcite,
although often met with, are invariably products of secondary chemical
action.

The Zeolites, above mentioned, so named from the manner in which they
froth up under the blow-pipe and melt into a glass, differ in their
chemical composition from all the other mineral constituents of
volcanic rocks, since they are hydrated silicates containing from 10 to
25 per cent of water. They abound in some trappean rocks and ancient
lavas, where they fill up vesicular cavities and interstices in the
substance of the rocks, but are rarely found in any quantity in recent
lavas; in most cases they are to be regarded as secondary products
formed by the action of water on the other constituents of the rocks.
Among them the species Analcime, Stilbite, Natrolite, and Chabazite may
be mentioned as of most common occurrence.

Quartz Group.—The microscope has shown that pure quartz is oftener
present in lavas than was formerly supposed. It had been argued that
the quartz in granite having a specific gravity of 2·6, was not of
purely igneous origin, because the silica resulting from fusion in the
laboratory has only a specific gravity of 2·3. But Mr. David Forbes has
ascertained that the free quartz in trachytes, which are known to have
flowed as lava, has the same specific gravity as the ordinary quartz of
granite; and the recent researches of Von Rath and others prove that
the mineral Tridymite, which is crystallised silica of specific gravity
2·3 (see Table, p. 499), is of common occurrence in the volcanic rocks
of Mexico, Auvergne, the Rhine, and elsewhere, although hitherto
entirely overlooked.

Feldspar Group.—In the Feldspar group (Table, p. 499) the five mineral
species most commonly met with as rock constituents are: 1. Orthoclase,
often called common or potash-feldspar. 2. Albite, or soda-feldspar, a
mineral which plays a more subordinate part than was formerly supposed,
this name having been given to much which has since been proved to be
Oligoclase. 3. Oligoclase, or soda-lime feldspar, in which soda is
present in much larger proportion than lime, and of which mineral
andesite are andesine, is considered to be a variety. 4. Labradorite,
or lime-soda-feldspar, in which the proportions of lime and soda are
the reverse to what they are in Oligoclase. 5. Anorthite or
lime-feldspar. The two latter feldspars are rarely if ever found to
enter into the composition of rocks containing quartz.

In employing such terms as potash-feldspar, etc., it must, however,
always be borne in mind that it is only intended to direct attention to
the predominant alkali or alkaline earth in the mineral, not to assert
the absence of the others, which in most cases will be found to be
present in minor quantity. Thus potash-feldspar (orthoclase) almost
always contains a little soda, and often traces of lime or magnesia;
and in like manner with the others. The terms “glassy” and “compact”
feldspars only refer to structure, and not to species or composition;
the student should be prepared to meet with any of the above feldspars
in either of these conditions: the glassy state being apparently due to
quick cooling, and the compact to conditions unfavourable to
crystallisation; the so-called “compact feldspar” is also very commonly
found to be an admixture of more than one feldspar species, and
frequently also contains quartz and other extraneous mineral matter
only to be detected by the microscope.

Feldspars when arranged according to their system of crystallisation
are _monoclinic,_ having one axis obliquely inclined; or _triclinic,_
having the three axes all obliquely inclined to each other. If arranged
with reference to their cleavage they are _orthoclastic,_ the fracture
taking place always at a right angle; or _plagioclastic,_ in which the
cleavages are oblique to one another. Orthoclase is orthoclastic and
monoclinic; all the other feldspars are plagioclastic and triclinic.

_Minerals in Meteorites._—That variety of the Feldspar Group which is
called Anorthite has been shown by Rammelsberg to occur in a meteoric
stone, and his analysis proves it to be almost identical in its
chemical proportions to the same mineral in the lavas of modern
volcanoes. So also Bronzite (Enstatite) and Olivine have been met with
in meteorites shown by analysis to come remarkably near to these
minerals in ordinary rocks.

Mica Group.—With regard to the micas, the four principal species
(Table, p. 499) all contain potash in nearly the same proportion, but
differ greatly in the proportion and nature of their other ingredients.
Muscovite is often called common or potash mica; Lepidolite is
characterised by containing lithia in addition; Biotite contains a
large amount of magnesia and oxide of iron; whilst Phlogopite contains
still more of the former substance. In rocks containing quartz,
muscovite or lepidolite are most common. The mica in recent volcanic
rocks, gabbros, and diorites is usually Biotite, while that so common
in metamorphic limestones is usually, if not always, Phlogopite.

Amphibole and Pyroxene Group.—The minerals included in the table under
the Amphibole and Pyroxene Group differ somewhat in their
crystallisation form, though they all belong to the monoclinic system.
Amphibole is a general name for all the different varieties of
Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes
Augite, Diallage, Malacolite, Sahlite, etc. The two divisions are so
much allied in chemical composition and crystallographic characters,
and blend so completely one into the other in Uralite (see page 499),
that it is perhaps best to unite them in one group.

Theory of Isomorphism.—The history of the changes of opinion on this
point is curious and instructive. Werner first distinguished augite
from hornblende; and his proposal to separate them obtained afterwards
the sanction of Haüy, Mohs, and other celebrated mineralogists. It was
agreed that the form of the crystals of the two species was different,
and also their structure, as shown by _cleavage_—that is to say, by
breaking or cleaving the mineral with a chisel, or a blow of the
hammer, in the direction in which it yields most readily. It was also
found by analysis that augite usually contained more lime, less
alumina, and no fluoric acid; which last, though not always found in
hornblende, often enters into its composition in minute quantity. In
addition to these characters, it was remarked as a geological fact,
that augite and hornblende are very rarely associated together in the
same rock. It was also remarked that in the crystalline slags of
furnaces augitic forms were frequent, the hornblendic entirely absent;
hence it was conjectured that hornblende might be the result of slow,
and augite of rapid cooling. This view was confirmed by the fact that
Mitscherlich and Berthier were able to make augite artificially, but
could never succeed in forming hornblende. Lastly, Gustavus Rose fused
a mass of hornblende in a porcelain furnace, and found that it did not,
on cooling, assume its previous shape, but invariably took that of
augite. The same mineralogist observed certain crystals called Uralite
(see Table, p. 499) in rocks from Siberia, which possessed the cleavage
and chemical composition of hornblende, while they had the external
form of augite.

If, from these data, it is inferred that the same substance may assume
the crystalline forms of hornblende or augite indifferently, according
to the more or less rapid cooling of the melted mass, it is
nevertheless certain that the variety commonly called augite, and
recognised by a peculiar crystalline form, has usually more lime in it,
and less alumina, than that called hornblende, although the quantities
of these elements do not seem to be always the same. Unquestionably the
facts and experiments above mentioned show the very near affinity of
hornblende and augite; but even the convertibility of one into the
other, by melting and recrystallising, does not perhaps demonstrate
their absolute identity. For there is often some portion of the
materials in a crystal which are not in perfect chemical combination
with the rest. Carbonate of lime, for example, sometimes carries with
it a considerable quantity of silex into its own form of crystal, the
silex being mechanically mixed as sand, and yet not preventing the
carbonate of lime from assuming the form proper to it. This is an
extreme case, but in many others some one or more of the ingredients in
a crystal may be excluded from perfect chemical union; and after
fusion, when the mass recrystallises, the same elements may combine
perfectly or in new proportions, and thus a new mineral may be
produced. Or some one of the gaseous elements of the atmosphere, the
oxygen for example, may, when the melted matter reconsolidates, combine
with some one of the component elements.

The different quantity of the impurities or the refuse above alluded
to, which may occur in all but the most transparent and perfect
crystals, may partly explain the discordant results at which
experienced chemists have arrived in their analysis of the same
mineral. For the reader will often find that crystals of a mineral
determined to be the same by physical characters, crystalline form, and
optical properties, have been declared by skilful analysers to be
composed of distinct elements. This disagreement seemed at first
subversive of the atomic theory, or the doctrine that there is a fixed
and constant relation between the crystalline form and structure of a
mineral and its chemical composition. The apparent anomaly, however,
which threatened to throw the whole science of mineralogy into
confusion, was reconciled to fixed principles by the discoveries of
Professor Mitscherlich at Berlin, who ascertained that the composition
of the minerals which had appeared so variable was governed by a
general law, to which he gave the name of _isomorphism_ (from _ isos,_
equal, and _morphe,_ form). According to this law, the ingredients of a
given species of mineral are not absolutely fixed as to their kind and
quality; but one ingredient may be replaced by an equivalent portion of
some analogous ingredient. Thus, in augite, the lime may be in part
replaced by portions of protoxide of iron, or of manganese, while the
form of the crystal, and the angle of its cleavage planes, remain the
same. These vicarious substitutions, however, of particular elements
cannot exceed certain defined limits.

Basaltic Rocks.—The two principal families of trappean or volcanic
rocks are the basalts and the trachytes, which differ chiefly from each
other in the quantity of silica which they contain. The basaltic rocks
are comparatively poor in silica, containing less than 50 per cent of
that mineral, and none in a pure state or as free quartz, apart from
the rest of the matrix. They contain a larger proportion of lime and
magnesia than the trachytes, so that they are heavier, independently of
the frequent presence of the oxides of iron which in some cases forms
more than a fourth part of the whole mass. Abich has, therefore,
proposed that we should weigh these rocks, in order to appreciate their
composition in cases where it is impossible to separate their component
minerals. Thus, basalt from Staffa, containing 47·80 per cent of
silica, has a specific gravity of 2·95; whereas trachyte, which has 66
per cent of silica, has a specific gravity of only 2·68; trachytic
porphyry, containing 69 per cent of silica, a specific gravity of only
2·58. If we then take a rock of intermediate composition, such as that
prevailing in the Peak of Teneriffe, which Abich calls
Trachyte-dolerite, its proportion of silica being intermediate, or 58
per cent, it weighs 2·78, or more than trachyte, and less than
basalt.[4]

_Basalt._—The different varieties of this rock are distinguished by the
names of basalts, anamezites, and dolerites, names which, however, only
denote differences in texture without implying any difference in
mineral or chemical composition: the term _Basalt_ being used only when
the rock is compact, amorphous, and often semi-vitreous in texture, and
when it breaks with a perfect conchoidal fracture; when, however, it is
uniformly crystalline in appearance, yet very close-grained, the name _
Anamesite_ (from _anamesos,_ intermediate) is employed, but if the rock
be so coarsely crystallised that its different mineral constituents can
be easily recognised by the eye, it is called _ Dolerite_ (from
_doleros,_ deceitful), in allusion to the difficulty of distinguishing
it from some of the rocks known as Plutonic.

_Melaphyre_ is often quite undistinguishable in external appearance
from basalt, for although rarely so heavy, dark-coloured, or compact,
it may present at times all these varieties of texture. Both these
rocks are composed of triclinic feldspar and augite with more or less
olivine, magnetic or titaniferous oxide of iron, and usually a little
nepheline, leucite, and apatite; basalt usually contains considerably
more olivine than melaphyre, but chemically they are closely allied,
although the melaphyres usually contain more silica and alumina, with
less oxides of iron, lime, and magnesia, than the basalts. The Rowley
Hills in Staffordshire, commonly known as Rowley Ragstone, are
melaphyre.

_Greenstone._—This name has usually been extended to all granular
mixtures, whether of hornblende and feldspar, or of augite and
feldspar. The term _diorite_ has been applied exclusively to compounds
of hornblende and triclinic feldspar. _ Labrador-rock_ is a term used
for a compound of labradorite or labrador-feldspar and hypersthene;
when the hypersthene predominates it is sometimes known under the name
of _ Hypersthene-rock._ _Gabbro_ and _Diabase_ are rocks mainly
composed of triclinic feldspars and diallage. All these rocks become
sometimes very crystalline, and help to connect the volcanic with the
Plutonic formations, which will be treated of in Chapter XXXI.

Trachytic Rocks.—The name trachyte (from [**Greek]_ trachus,_ rough)
was originally given to a coarse granular feldspathic rock which was
rough and gritty to the touch. The term was subsequently made to
include other rocks, such as clinkstone and obsidian, which have the
same mineral composition, but to which, owing to their different
texture, the word in its original meaning would not apply. The
feldspars which occur in Trachytic rocks are invariably those which
contain the largest proportion of silica, or from 60 to 70 per cent of
that mineral. Through the base are usually disseminated crystals of
glassy feldspar, mica, and sometimes hornblende. Although quartz is not
a necessary ingredient in the composition of this rock, it is very
frequently present, and the quartz trachytes are very largely developed
in many volcanic districts. In this respect the trachytes differ
entirely from the members of the Basaltic family, and are more nearly
allied to the granites.

_Obsidian._—Obsidian, Pitchstone, and Pearlstone are only different
forms of a volcanic glass produced by the fusion of trachytic rocks.
The distinction between them is caused by different rates of cooling
from the melted state, as has been proved by experiment. Obsidian is of
a black or ash-grey colour, and though opaque in mass is transparent in
thin edges.

_Clinkstone or Phonolite._—Among the rocks of the trachytic family, or
those in which the feldspars are rich in silica, that termed Clinkstone
or Phonolite is conspicuous by its fissile structure, and its tendency
to lamination, which is such as sometimes to render it useful as
roofing-slate. It rings when struck with the hammer, whence its name;
is compact, and usually of a greyish blue or brownish colour; is
variable in composition, but almost entirely composed of feldspar. When
it contains disseminated crystals of feldspar, it is called _Clinkstone
porphyry._

Volcanic Rocks distinguished by special Forms of Structure.—Many
volcanic rocks are commonly spoken of under names denoting structure
alone, which must not be taken to imply that they are distinct rocks,
i.e., that they differ from one another either in mineral or chemical
composition. Thus the terms Trachytic porphyry, Trachytic tuff, etc.,
merely refer to the same rock under different conditions of mechanical
aggregation or crystalline development which would be more correctly
expressed by the use of the adjective, as porphyritic trachyte, etc.,
but as these terms are so commonly employed it is considered advisable
to direct the student’s attention to them.

Fig. 586: Porphyry. White crystals of feldspar in a dark base of
hornblende and feldspar.
_Porphyry_ is one of this class, and very characteristic of the
volcanic formations. When distinct crystals of one or more minerals are
scattered through an earthy or compact base, the rock is termed a
porphyry (see Fig. 586). Thus trachyte is usually porphyritic; for in
it, as in many modern lavas, there are crystals of feldspar; but in
some porphyries the crystals are of augite, olivine, or other minerals.
If the base be greenstone, basalt, or pitchstone, the rock may be
denominated greenstone-porphyry, pitchstone-porphyry, and so forth. The
old classical type of this form of rock is the red porphyry of Egypt,
or the well-known “Rosso antico.” It consists, according to Delesse, of
a red feldspathic base in which are disseminated rose-coloured crystals
of the feldspar called oligoclase, with some plates of blackish
hornblende and grains of oxide of iron (iron-glance). _ Red
quartziferous porphyry_ is a much more siliceous rock, containing about
70 or 80 per cent of silex, while that of Egypt has only 62 per cent.

_Amygdaloid._—This is also another form of igneous rock, admitting of
every variety of composition. It comprehends any rock in which round or
almond-shaped nodules of some mineral, such as agate, chalcedony,
calcareous spar, or zeolite, are scattered through a base of wacke,
basalt, greenstone, or other kind of trap. It derives its name from the
Greek word _amygdalon,_ an almond. The origin of this structure cannot
be doubted, for we may trace the process of its formation in modern
lavas. Small pores or cells are caused by bubbles of steam and gas
confined in the melted matter. After or during consolidation, these
empty spaces are gradually filled up by matter separating from the
mass, or infiltered by water permeating the rock. As these bubbles have
been sometimes lengthened by the flow of the lava before it finally
cooled, the contents of such cavities have the form of almonds. In some
of the amygdaloidal traps of Scotland, where the nodules have
decomposed, the empty cells are seen to have a glazed or vitreous
coating, and in this respect exactly resemble scoriaceous lavas, or the
slags of furnaces.

Fig. 587: Scoriaceous lava in part converted into an amygdaloid.
Fig. 587 represents a fragment of stone taken from the upper part of a
sheet of basaltic lava in Auvergne. One-half is scoriaceous, the pores
being perfectly empty; the other part is amygdaloidal, the pores or
cells being mostly filled up with carbonate of lime, forming white
kernels.

_Lava._—This term has a somewhat vague signification, having been
applied to all melted matter observed to flow in streams from volcanic
vents. When this matter consolidates in the open air, the upper part is
usually scoriaceous, and the mass becomes more and more stony as we
descend, or in proportion as it has consolidated more slowly and under
greater pressure. At the bottom, however, of a stream of lava, a small
portion of scoriaceous rock very frequently occurs, formed by the first
thin sheet of liquid matter, which often precedes the main current, and
solidifies under slight pressure.

The more compact lavas are often porphyritic, but even the scoriaceous
part sometimes contains imperfect crystals, which have been derived
from some older rocks, in which the crystals pre-existed, but were not
melted, as being more infusible in their nature. Although melted matter
rising in a crater, and even that which enters a rent on the side of a
crater, is called lava, yet this term belongs more properly to that
which has flowed either in the open air or on the bed of a lake or sea.
If the same fluid has not reached the surface, but has been merely
injected into fissures below ground, it is called trap. There is every
variety of composition in lavas; some are trachytic, as in the Peak of
Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne;
others are andesitic, as those of Chili; some of the most modern in
Vesuvius consist of green augite, and many of those of Etna of augite
and labrador-feldspar.[5]

_Scoriæ_ and _Pumice_ may next be mentioned, as porous rocks produced
by the action of gases on materials melted by volcanic heat. _Scoriæ_
are usually of a reddish-brown and black colour, and are the cinders
and slags of basaltic or augitic lavas. _Pumice_ is a light, spongy,
fibrous substance, produced by the action of gases on trachytic and
other lavas; the relation, however, of its origin to the composition of
lava is not yet well understood. Von Buch says that it never occurs
where only labrador-feldspar is present.

_Volcanic Ash or Tuff, Trap Tuff._—Small angular fragments of the
scoriæ and pumice, above-mentioned, and the dust of the same, produced
by volcanic explosions, form the tuffs which abound in all regions of
active volcanoes, where showers of these materials, together with small
pieces of other rocks ejected from the crater, and more or less burnt,
fall down upon the land or into the sea. Here they often become mingled
with shells, and are stratified. Such tuffs are sometimes bound
together by a calcareous cement, and form a stone susceptible of a
beautiful polish. But even when little or no lime is present, there is
a great tendency in the materials of ordinary tuffs to cohere together.
The term _ volcanic ash_ has been much used for rocks of all ages
supposed to have been derived from matter ejected in a melted state
from volcanic orifices. We meet occasionally with extremely compact
beds of volcanic materials, interstratified with fossiliferous rocks.
These may sometimes be tuffs, although their density or compactness is
such as the cause them to resemble many of those kinds of trap which
are found in ordinary dikes.

_Wacke_ is a name given to a decomposed state of various trap rocks of
the basaltic family, or those which are poor in silica. It resembles
clay of a yellowish or brown colour, and passes gradually from the soft
state to the hard dolerite, greenstone, or other trap rock from which
it has been derived.

_Agglomerate._—In the neighbourhood of volcanic vents, we frequently
observe accumulations of angular fragments of rocks formed during
eruptions by the explosive action of steam, which shatters the
subjacent stony formations, and hurls them up into the air. They then
fall in showers around the cone or crater, or may be spread for some
distance over the surrounding country. The fragments consist usually of
different varieties of scoriaceous and compact lavas; but other kinds
of rock, such as granite or even fossiliferous limestones, may be
intermixed; in short, any substance through which the expansive gases
have forced their way. The dispersion of such materials may be aided by
the wind, as it varies in direction or intensity, and by the slope of
the cone down which they roll, or by floods of rain, which often
accompany eruptions. But if the power of running water, or of the waves
and currents of the sea, be sufficient to carry the fragments to a
distance, it can scarcely fail to wear off their angles, and the
formation then becomes a _conglomerate._ If occasionally globular
pieces of scoriæ abound in an agglomerate, they may not owe their round
form to attrition. When all the angular fragments are of volcanic rocks
the mass is usually termed a volcanic breccia.

_Laterite_ is a red or brick-like rock composed of silicate of alumina
and oxide of iron. The red layers called “ochre beds,” dividing the
lavas of the Giant’s Causeway, are laterites. These were found by
Delesse to be trap impregnated with the red oxide of iron, and in part
reduced to kaolin. When still more decomposed, they were found to be
clay coloured by red ochre. As two of the lavas of the Giant’s Causeway
are parted by a bed of lignite, it is not improbable that the layers of
laterite seen in the Antrim cliffs resulted from atmospheric
decomposition. In Madeira and the Canary Islands streams of lava of
subaërial origin are often divided by red bands of laterite, probably
ancient soils formed by the decomposition of the surfaces of
lava-currents, many of these soils having been coloured red in the
atmosphere by oxide of iron, others burnt into a red brick by the
overflowing of heated lavas. These red bands are sometimes prismatic,
the small prisms being at right angles to the sheets of lava. Red clay
or red marl, formed as above stated by the disintegration of lava,
scoriæ, or tuff, has often accumulated to a great thickness in the
valleys of Madeira, being washed into them by alluvial action; and some
of the thick beds of laterite in India may have had a similar origin.
In India, however, especially in the Deccan, the term “laterite” seems
to have been used too vaguely to answer the above definition. The
vegetable soil in the gardens of the suburbs of Catania which was
overflowed by the lava of 1669 was turned or burnt into a layer of red
brick-coloured stone, or in other words, into laterite, which may now
be seen supporting the old lava-current.

Columnar and Globular Structure.—One of the characteristic forms of
volcanic rocks, especially of basalt, is the columnar, where large
masses are divided into regular prisms, sometimes easily separable, but
in other cases adhering firmly together. The columns vary, in the
number of angles, from three to twelve; but they have most commonly
from five to seven sides. They are often divided transversely, at
nearly equal distances, like the joints in a vertebral column, as in
the Giant’s Causeway, in Ireland. They vary exceedingly in respect to
length and diameter. Dr. MacCulloch mentions some in Skye which are
about 400 feet long; others, in Morven, not exceeding an inch. In
regard to diameter, those of Ailsa measure nine feet, and those of
Morven an inch or less.[6] They are usually straight, but sometimes
curved; and examples of both these occur in the island of Staffa. In a
horizontal bed or sheet of trap the columns are vertical; in a vertical
dike they are horizontal.

Fig. 588: Lava of La Coupe d’Ayzac, near Antraigue, in the Department
of Ardêche.

It being assumed that columnar trap has consolidated from a fluid
state, the prisms are said to be always at right angles to the _cooling
surfaces._ If these surfaces, therefore, instead of being either
perpendicular or horizontal, are curved, the columns ought to be
inclined at every angle to the horizon; and there is a beautiful
exemplification of this phenomenon in one of the valleys of the
Vivarais, a mountainous district in the South of France, where, in the
midst of a region of gneiss, a geologist encounters unexpectedly
several volcanic cones of loose sand and scoriæ. From the crater of one
of these cones, called La Coupe d’Ayzac, a stream of lava has descended
and occupied the bottom of a narrow valley, except at those points
where the river Volant, or the torrents which join it, have cut away
portions of the solid lava. Fig. 588 represents the remnant of the lava
at one of these points. It is clear that the lava once filled the whole
valley up to the dotted line _d a_; but the river has gradually swept
away all below that line, while the tributary torrent has laid open a
transverse section; by which we perceive, in the first place, that the
lava is composed, as usual in this country, of three parts: the
uppermost, at _a,_ being scoriaceous, the second _b,_ presenting
irregular prisms; and the third, _c,_ with regular columns, which are
vertical on the banks of the Volant, where they rest on a horizontal
base of gneiss, but which are inclined at an angle of 45°, at _g,_ and
are nearly horizontal at _f,_ their position having been everywhere
determined, according to the law before mentioned, by the form of the
original valley.

Fig. 589: Columnar basalt in the Vicentin.

In Fig. 589, a view is given of some of the inclined and curved columns
which present themselves on the sides of the valleys in the hilly
region north of Vicenza, in Italy, and at the foot of the higher
Alps.[7] Unlike those of the Vivarais, last mentioned, the basalt of
this country was evidently submarine, and the present valleys have
since been hollowed out by denudation.

The columnar structure is by no means peculiar to the trap rocks in
which augite abounds; it is also observed in trachyte, and other
feldspathic rocks of the igneous class, although in these it is rarely
exhibited in such regular polygonal forms. It has been already stated
that basaltic columns are often divided by cross-joints. Sometimes each
segment, instead of an angular, assumes a spheroidal form, so that a
pillar is made up of a pile of balls, usually flattened, as in the
Cheese-grotto at Bertrich-Baden, in the Eifel, near the Moselle (Fig.
590). The basalt there is part of a small stream of lava, from 30 to 40
feet thick, which has proceeded from one of several volcanic craters,
still extant, on the neighbouring heights.

Fig. 590: Basaltic pillars of Käsegrotte, Bertrich-Baden, half-way
between Trèves and Coblenz.

In some masses of decomposing greenstone, basalt, and other trap rocks,
the globular structure is so conspicuous that the rock has the
appearance of a heap of large cannon balls. According to M. Delesse,
the centre of each spheroid has been a centre of crystallisation,
around which the different minerals of the rock arranged themselves
symmetrically during the process of cooling. But it was also, he says,
a centre of contraction, produced by the same cooling, the globular
form, therefore, of such spheroids being the combined result of
crystallisation and contraction.[8]

Fig. 591: Globiform pitchstone. Chiaja di Luna, Isle of Ponza.

Mr. Scrope gives as an illustration of this structure a resinous
trachyte or pitchstone-porphyry in one of the Ponza islands, which rise
from the Mediterranean, off the coast of Terracina and Gaeta. The
globes vary from a few inches to three feet in diameter, and are of an
ellipsoidal form (see Fig. 591). The whole rock is in a state of
decomposition, “and when the balls,” says Mr. Scrope, “have been
exposed a short time to the weather, they scale off at a touch into
numerous concentric coats, like those of a bulbous root, inclosing a
compact nucleus. The laminæ of this nucleus have not been so much
loosened by decomposition; but the application of a ruder blow will
produce a still further exfoliation.”[9]

Fig. 592: Dike in valley, near Brazen Head, Madeira. (From a drawing of
Captain Basil Hall, R.N.)
Volcanic or Trap Dikes.—The leading varieties of the trappean
rocks—basalt, greenstone, trachyte, and the rest—are found sometimes in
dikes penetrating stratified and unstratified formations, sometimes in
shapeless masses protruding through or overlying them, or in horizontal
sheets intercalated between strata. Fissures have already been spoken
of as occurring in all kinds of rocks, some a few feet, others many
yards in width, and often filled up with earth or angular pieces of
stone, or with sand and pebbles. Instead of such materials, suppose a
quantity of melted stone to be driven or injected into an open rent,
and there consolidated, we have then a tabular mass resembling a wall,
and called a trap dike. It is not uncommon to find such dikes passing
through strata of soft materials, such as tuff, scoriæ, or shale,
which, being more perishable than the trap, are often washed away by
the sea, rivers, or rain, in which case the dike stands prominently out
in the face of precipices, or on the level surface of a country (see
Fig. 592).

In the islands of Arran and Skye, and in other parts of Scotland, where
sandstone, conglomerate, and other hard rocks are traversed by dikes of
trap, the converse of the above phenomenon is seen. The dike, having
decomposed more rapidly than the containing rock, has once more left
open the original fissure, often for a distance of many yards inland
from the sea-coast. There is yet another case, by no means uncommon in
Arran and other parts of Scotland, where the strata in contact with the
dike, and for a certain distance from it, have been hardened, so as to
resist the action of the weather more than the dike itself, or the
surrounding rocks. When this happens, two parallel walls of indurated
strata are seen protruding above the general level of the country and
following the course of the dike. In Fig. 593, a ground plan is given
of a ramifying dike of greenstone, which I observed cutting through
sandstone on the beach near Kildonan Castle, in Arran. The larger
branch varies from five to seven feet in width, which will afford a
scale of measurement for the whole.

Fig. 593: Ground-plan of greenstone dikes traversing sandstone.

In the Hebrides and other countries, the same masses of trap which
occupy the surface of the country far and wide, concealing the
subjacent stratified rocks, are seen also in the sea-cliffs, prolonged
downward in veins or dikes, which probably unite with other masses of
igneous rock at a greater depth. The largest of the dikes represented
in Fig. 594, and which are seen in part of the coast of Skye, is no
less than 100 feet in width.

Fig. 594: Trap dividing and covering sandstone near Suishnish, in Skye.

Every variety of trap-rock is sometimes found in dikes, as basalt,
greenstone, feldspar-porphyry, and trachyte. The amygdaloidal traps
also occur, though more rarely, and even tuff and breccia, for the
materials of these last may be washed down into open fissures at the
bottom of the sea, or during eruption on the land may be showered into
them from the air. Some dikes of trap may be followed for leagues
uninterruptedly in nearly a straight direction, as in the north of
England, showing that the fissures which they fill must have been of
extraordinary length.

Rocks altered by Volcanic Dikes.—After these remarks on the form and
composition of dikes themselves, I shall describe the alterations which
they sometimes produce in the rocks in contact with them. The changes
are usually such as the heat of melted matter and of the entangled
steam and gases might be expected to cause.

_Plas-Newydd: Dike cutting through Shale._—A striking example, near
Plas-Newydd, in Anglesea, has been described by Professor Henslow.[10]
The dike is 134 feet wide, and consists of a rock which is a compound
of feldspar and augite (dolerite of some authors). Strata of shale and
argillaceous limestone, through which it cuts perpendicularly, are
altered to a distance of 30, or even, in some places, of 35 feet from
the edge of the dike. The shale, as it approaches the trap, becomes
gradually more compact, and is most indurated where nearest the
junction. Here it loses part of its schistose structure, but the
separation into parallel layers is still discernible. In several places
the shale is converted into hard porcelanous jasper. In the most
hardened part of the mass the fossil shells, principally _Producti,_
are nearly obliterated; yet even here their impressions may frequently
be traced. The argillaceous limestone undergoes analogous mutations,
losing its earthy texture as it approaches the dike, and becoming
granular and crystalline. But the most extraordinary phenomenon is the
appearance in the shale of numerous crystals of analcime and garnet,
which are distinctly confined to those portions of the rock affected by
the dike.[11] Some garnets contain as much as 20 per cent of lime,
which they may have derived from the decomposition of the fossil shells
or _Producti._ The same mineral has been observed, under very analogous
circumstances, in High Teesdale, by Professor Sedgwick, where it also
occurs in shale and limestone, altered by basalt.[12]

_Antrim: Dike cutting through Chalk._—In several parts of the county of
Antrim, in the north of Ireland, chalk with flints is traversed by
basaltic dikes. The chalk is there converted into granular marble near
the basalt, the change sometimes extending eight or ten feet from the
wall of the dike, being greatest near the point of contact, and thence
gradually decreasing till it becomes evanescent. “The extreme effect,”
says Dr. Berger, “presents a dark brown crystalline limestone, the
crystals running in flakes as large as those of coarse primitive
(_metamorphic_) limestone; the next state is saccharine, then fine
grained and arenaceous; a compact variety, having a porcelanous aspect
and a bluish-grey colour, succeeds: this, towards the outer edge,
becomes yellowish-white, and insensibly graduates into the unaltered
chalk. The flints in the altered chalk usually assume a grey yellowish
colour.”[13] All traces of organic remains are effaced in that part of
the limestone which is most crystalline.

Fig. 595: Basaltic dikes in chalk in Island of Rathlin, Antrim.
Ground-plan as seen on the beach. Fig. 595: Basaltic dikes in chalk in
Island of Rathlin, Antrim. Ground-plan as seen on the beach. (Conybeare
and Buckland[14])

Fig. 595 represents three basaltic dikes traversing the chalk, all
within the distance of 90 feet. The chalk contiguous to the two outer
dikes is converted into a finely granular marble, _m, m,_ as are the
whole of the masses between the outer dikes and the central one. The
entire contrast in the composition and colour of the intrusive and
invaded rocks, in these cases, renders the phenomena peculiarly clear
and interesting. Another of the dikes of the north-east of Ireland has
converted a mass of red sandstone into hornstone. By another, the shale
of the coal-measures has been indurated, assuming the character of
flinty slate; and in another place the slate-clay of the lias has been
changed into flinty slate, which still retains numerous impressions of
ammonites.[15]

It might have been anticipated that beds of coal would, from their
combustible nature, be affected in an extraordinary degree by the
contact of melted rock. Accordingly, one of the greenstone dikes of
Antrim, on passing through a bed of coal, reduces it to a cinder for
the space of nine feet on each side. At Cockfield Fell, in the north of
England, a similar change is observed. Specimens taken at the distance
of about thirty yards from the trap are not distinguishable from
ordinary pit-coal; those nearer the dike are like cinders, and have all
the character of coke; while those close to it are converted into a
substance resembling soot.[16]

It is by no means uncommon to meet with the same rocks, even in the
same districts, absolutely unchanged in the proximity of volcanic
dikes. This great inequality in the effects of the igneous rocks may
often arise from an original difference in their temperature, and in
that of the entangled gases, such as is ascertained to prevail in
different lavas, or in the same lava near its source and at a distance
from it. The power also of the invaded rocks to conduct heat may vary,
according to their composition, structure, and the fractures which they
may have experienced, and perhaps, also, according to the quantity of
water (so capable of being heated) which they contain. It must happen
in some cases that the component materials are mixed in such
proportions as to prepare them readily to enter into chemical union,
and form new minerals; while in other cases the mass may be more
homogeneous, or the proportions less adapted for such union.

We must also take into consideration, that one fissure may be simply
filled with lava, which may begin to cool from the first; whereas in
other cases the fissure may give passage to a current of melted matter,
which may ascend for days or months, feeding streams which are
overflowing the country above, or being ejected in the shape of scoriæ
from some crater. If the walls of a rent, moreover, are heated by hot
vapour before the lava rises, as we know may happen on the flanks of a
volcano, the additional heat supplied by the dike and its gases will
act more powerfully.

Intrusion of Trap between Strata.—Masses of trap are not unfrequently
met with intercalated between strata, and maintaining their parallelism
to the planes of stratification throughout large areas. They must in
some places have forced their way laterally between the divisions of
the strata, a direction in which there would be the least resistance to
an advancing fluid, if no vertical rents communicated with the surface,
and a powerful hydrostatic pressure were caused by gases propelling the
lava upward.

Relation of Trappean Rocks to the Products of active Volcanoes.—When we
reflect on the changes above described in the strata near their contact
with trap dikes, and consider how complete is the analogy or often
identity in composition and structure of the rocks called trappean and
the lavas of active volcanoes, it seems difficult at first to
understand how so much doubt could have prevailed for half a century as
to whether trap was of igneous or aqueous origin. To a certain extent,
however, there was a real distinction between the trappean formations
and those to which the term volcanic was almost exclusively confined. A
large portion of the trappean rocks first studied in the north of
Germany, and in Norway, France, Scotland, and other countries, were
such as had been formed entirely under water, or had been injected into
fissures and intruded between strata, and which had never flowed out in
the air, or over the bottom of a shallow sea. When these products,
therefore, of submarine or subterranean igneous action were contrasted
with loose cones of scoriæ, tuff, and lava, or with narrow streams of
lava in great part scoriaceous and porous, such as were observed to
have proceeded from Vesuvius and Etna, the resemblance seemed remote
and equivocal. It was, in truth, like comparing the roots of a tree
with its leaves and branches, which, although the belong to the same
plant, differ in form, texture, colour, mode of growth, and position.
The external cone, with its loose ashes and porous lava, may be likened
to the light foliage and branches, and the rocks concealed far below,
to the roots. But it is not enough to say of the volcano,

                  “Quantum vertice in auras
     Ætherias, tantum radice in Tartara tendit,”

for its roots do literally reach downward to Tartarus, or to the
regions of subterranean fire; and what is concealed far below is
probably always more important in volume and extent than what is
visible above ground.

Fig. 596: Strata intercepted by a trap dike, and covered with alluvium.

We have already stated how frequently dense masses of strata have been
removed by denudation from wide areas (see Chapter VI); and this fact
prepares us to expect a similar destruction of whatever may once have
formed the uppermost part of ancient submarine or subaërial volcanoes,
more especially as those superficial parts are always of the lightest
and most perishable materials. The abrupt manner in which dikes of trap
usually terminate at the surface (see Fig. 596), and the water-worn
pebbles of trap in the alluvium which covers the dike, prove
incontestably that whatever was uppermost in these formations has been
swept away. It is easy, therefore, to conceive that what is gone in
regions of trap may have corresponded to what is now visible in active
volcanoes.

As to the absence of porosity in the trappean formations, the
appearances are in a great degree deceptive, for all amygdaloids are,
as already explained, porous rocks, into the cells of which mineral
matter such as silex, carbonate of lime, and other ingredients, have
been subsequently introduced (see p. 507); sometimes, perhaps, by
secretion during the cooling and consolidation of lavas. In the Little
Cumbray, one of the Western Islands, near Arran, the amygdaloid
sometimes contains elongated cavities filled with brown spar; and when
the nodules have been washed out, the interior of the cavities is
glazed with the vitreous varnish so characteristic of the pores of
slaggy lavas. Even in some parts of this rock which are excluded from
air and water, the cells are empty, and seem to have always remained in
this state, and are therefore undistinguishable from some modern
lavas.[17]

Dr. MacCulloch, after examining with great attention these and the
other igneous rocks of Scotland, observes, “that it is a mere dispute
about terms, to refuse to the ancient eruptions of trap the name of
submarine volcanoes; for they are such in every essential point,
although they no longer eject fire and smoke.” The same author also
considers it not improbable that some of the volcanic rocks of the same
country may have been poured out in the open air.[18]

It will be seen in the following chapters that in the earth’s crust
there are volcanic tuffs of all ages, containing marine shells, which
bear witness to eruptions at many successive geological periods. These
tuffs, and the associated trappean rocks, must not be compared to lava
and scoriæ which had cooled in the open air. Their counterparts must be
sought in the products of modern submarine volcanic eruptions. If it be
objected that we have no opportunity of studying these last, it may be
answered, that subterranean movements have caused, almost everywhere in
regions of active volcanoes, great changes in the relative level of
land and sea, in times comparatively modern, so as to expose to view
the effects of volcanic operations at the bottom of the sea.

 [1] Principles, vol. ii, pp. 56 and 145.

 [2] Memoir on Mount Etna, Phil. Trans., 1858.

 [3] For analyses of these minerals see the Mineralogies of Dana and
 Bristow.

 [4] Dr. Daubeny on Volcanoes, 2nd ed., pp. 14, 15.

 [5] G. Hose, Ann. des Mines, tome viii, p. 32.

 [6] MacCulloch Sys. of Geol., vol. ii, p. 137.

 [7] Fortis, Mém. sur l’Hist. Nat. de l’Italie, tome 1., p. 233, plate
 7.

 [8] Delesse, sur les Roches Globuleuses, Mém. de la Soc. Géol. de
 France, 2 sér., tome iv.

 [9] Scrope, Geol. Trans., 2nd series, vol. ii, p. 205.

 [10] Cambridge Transactions, vol. i, p. 402.

 [11] Ibid., vol. i, p. 410.

 [12] Ibid., vol. ii, p. 175.

 [13] Dr. Berger, Geol. Trans., 1st series, vol. iii, p. 172.

 [14] Geol. Trans., 1st series, vol. iii, p. 210 and plate 10.

 [15] Ibid., vol. iii, p. 213; and Playfair, Illus. of Hutt. Theory, s.
 253.

 [16] Sedgwick, Camb. Trans., vol. ii, p. 37.)

 [17] MacCulloch, West. Islands, vol. ii, p. 487.

 [18] Syst. of Geol., vol. ii, p. 114.




CHAPTER XXIX.
ON THE AGES OF VOLCANIC ROCKS.


Tests of relative Age of Volcanic Rocks. — Why ancient and modern Rocks
cannot be identical. — Tests by Superposition and intrusion. — Test by
Alteration of Rocks in Contact. — Test by Organic Remains. — Test of
Age by Mineral Character. — Test by Included Fragments. — Recent and
Post-pliocene volcanic Rocks. — Vesuvius, Auvergne, Puy de Côme, and
Puy de Pariou. — Newer Pliocene volcanic Rocks. — Cyclopean Isles,
Etna, Dikes of Palagonia, Madeira. — Older Pliocene volcanic Rocks. —
Italy. — Pliocene Volcanoes of the Eifel. — Trass.

Having in the former part of this work referred the sedimentary strata
to a long succession of geological periods, we have now to consider how
far the volcanic formations can be classed in a similar chronological
order. The tests of relative age in this class of rocks are four:
first, superposition and intrusion, with or without alteration of the
rocks in contact; second, organic remains; third, mineral characters;
fourth, included fragments of older rocks.

Besides these four tests it may be said, in a general way, that
volcanic rocks of Primary or Palæozoic antiquity differ from those of
the Secondary or Mesozoic age, and these again from the Tertiary and
Recent. Not, perhaps, that they differed originally in a greater degree
than the modern volcanic rocks of one region, such as that of the
Andes, differ from those of another, such as Iceland, but because all
rocks permeated by water, especially if its temperature be high, are
liable to undergo a slow transmutation, even when they do not assume a
new crystalline form like that of the hypogene rocks.

Although subaërial and submarine denudation, as before stated, remove,
in the course of ages, large portions of the upper or more superficial
products of volcanoes, yet these are sometimes preserved by subsidence,
becoming covered by the sea or by superimposed marine deposits. In this
way they may be protected for ages from the waves of the sea, or the
destroying action of rivers, while, at the same time, they may not sink
so deep as to be exposed to that Plutonic action (to be spoken of in
Chapter XXXI) which would convert them into crystalline rocks. But even
in this case they will not remain unaltered, because they will be
percolated by water often of high temperature, and charged with
carbonate of lime, silex, iron, and other mineral ingredients, whereby
gradual changes in the constitution of the rocks may be superinduced.
Every geologist is aware how often silicified trees occur in volcanic
tuffs, the perfect preservation of their internal structure showing
that they have not decayed before the petrifying material was supplied.

The porous and vesicular nature of a large part, both of the basaltic
and trachytic lavas, affords cavities in which silex and carbonate of
lime are readily deposited. Minerals of the zeolite family, the
composition of which has already been alluded to, p. 500, occur in
amygdaloids and other trap-rocks in great abundance, and Daubrée’s
observations have proved that they are not always simple deposits of
substances held in solution by the percolating waters, being
occasionally products of the chemical action of that water on the rock
through which they are filtered, and portions of which are decomposed.
From these considerations it follows that the perfect identity of very
ancient and very modern volcanic formations is scarcely possible.

Fig. 597: Showing melted matter forced between two strata.

Tests by Superposition.—If a volcanic rock rest upon an aqueous
deposit, the volcanic must be the newest of the two; but the like rule
does not hold good where the aqueous formation rests upon the volcanic,
for melted matter, rising from below, may penetrate a sedimentary mass
without reaching the surface, or may be forced in conformably between
two strata, as _ b_ below D in Fig. 597, after which it may cool down
and consolidate. Superposition, therefore, is not of the same value as
a test of age in the unstratified volcanic rocks as in fossiliferous
formations. We can only rely implicitly on this test where the volcanic
rocks are contemporaneous, not where they are intrusive. Now, they are
said to be contemporaneous if produced by volcanic action which was
going on simultaneously with the deposition of the strata with which
they are associated. Thus in the section at D (Fig. 597), we may
perhaps ascertain that the trap _b_ flowed over the fossiliferous bed
_c,_ and that, after its consolidation, _a_ was deposited upon it, _a_
and _c_ both belonging to the same geological period. But, on the other
hand, we must conclude the trap to be intrusive, if the stratum _a_ be
altered by _b_ at the point of contact, or if, in pursuing _b_ for some
distance, we find at length that it cuts through the stratum _a,_ and
then overlies it as at E.

Fig. 598: Section through sedimentary mass with melted matter.
We may, however, be easily deceived in supposing the volcanic rock to
be intrusive, when in reality it is contemporaneous; for a sheet of
lava, as it spreads over the bottom of the sea, cannot rest everywhere
upon the same stratum, either because these have been denuded, or
because, if newly thrown down, they thin out in certain places, thus
allowing the lava to cross their edges. Besides, the heavy igneous
fluid will often, as it moves along, cut a channel into beds of soft
mud and sand. Suppose the submarine lava F (Fig. 598) to have come in
contact in this manner with the strata _a, b, c,_ and that after its
consolidation the strata _d, e_ are thrown down in a nearly horizontal
position, yet so as to lie unconformably to F, the appearance of
subsequent intrusion will here be complete, although the trap is in
fact contemporaneous. We must not, therefore, hastily infer that the
rock F is intrusive, unless we find the overlying strata, _d, e,_ to
have been altered at their junction, as if by heat.

The test of age by superposition is strictly applicable to all
stratified volcanic tuffs, according to the rules already explained in
the case of sedimentary deposits (see p. 124).

Test of Age by Organic Remains.—We have seen how, in the vicinity of
active volcanoes, scoriæ, pumice, fine sand, and fragments of rock are
thrown up into the air, and then showered down upon the land, or into
neighbouring lakes or seas. In the tuffs so formed shells, corals, or
any other durable organic bodies which may happen to be strewed over
the bottom of a lake or sea will be imbedded, and thus continue as
permanent memorials of the geological period when the volcanic eruption
occurred. Tufaceous strata thus formed in the neighbourhood of
Vesuvius, Etna, Stromboli, and other volcanoes now in islands or near
the sea, may give information of the relative age of these tuffs at
some remote future period when the fires of these mountains are
extinguished. By evidence of this kind we can establish a coincidence
in age between volcanic rocks and the different primary, secondary, and
tertiary fossiliferous strata.

The tuffs alluded to may not always be marine, but may include, in some
places, fresh-water shells; in others, the bones of terrestrial
quadrupeds. The diversity of organic remains in formations of this
nature is perfectly intelligible, if we reflect on the wide dispersion
of ejected matter during late eruptions, such as that of the volcano of
Coseguina, in the province of Nicaragua, January 19, 1835. Hot cinders
and fine scoriæ were then cast up to a vast height, and covered the
ground as they fell to the depth of more than ten feet, for a distance
of eight leagues from the crater, in a southerly direction. Birds,
cattle, and wild animals were scorched to death in great numbers, and
buried in ashes. Some volcanic dust fell at Chiapa, upward of 1200
miles, not to leeward of the volcano, as might have been anticipated,
but to windward, a striking proof of a counter-current in the upper
region of the atmosphere; and some on Jamaica, about 700 miles distant
to the north-east. In the sea, also, at the distance of 1100 miles from
the point of eruption, Captain Eden of the “Conway” sailed 40 miles
through floating pumice, among which were some pieces of considerable
size.[1]

Test of Age by Mineral Composition.—As sediment of homogeneous
composition, when discharged from the mouth of a large river, is often
deposited simultaneously over a wide space, so a particular kind of
lava flowing from a crater during one eruption may spread over an
extensive area; thus in Iceland, in 1783, the melted matter, pouring
from Skaptar Jokul, flowed in streams in opposite directions, and
caused a continuous mass the extreme points of which were 90 miles
distant from each other. This enormous current of lava varied in
thickness from 100 feet to 600 feet, and in breadth from that of a
narrow river gorge to 15 miles.[2] Now, if such a mass should
afterwards be divided into separate fragments by denudation, we might
still, perhaps, identify the detached portions by their similarity in
mineral composition. Nevertheless, this test will not always avail the
geologist; for, although there is usually a prevailing character in
lava emitted during the same eruption, and even in the successive
currents flowing from the same volcano, still, in many cases, the
different parts even of one lava-stream, or, as before stated, of one
continuous mass of trap, vary much in mineral composition and texture.

In Auvergne, the Eifel, and other countries where trachyte and basalt
are both present, the trachytic rocks are for the most part older than
the basaltic. These rocks do, indeed, sometimes alternate partially, as
in the volcano of Mont Dor, in Auvergne; and in Madeira trachytic rocks
overlie an older basaltic series; but the trachyte occupies more
generally an inferior position, and is cut through and overflowed by
basalt. It can by no means be inferred that trachyte predominated at
one period of the earth’s history and basalt at another, for we know
that trachytic lavas have been formed at many successive periods, and
are still emitted from many active craters; but it seems that in each
region, where a long series of eruptions have occurred, the lavas
containing feldspar more rich in silica have been first emitted, and
the escape of the more augitic kinds has followed. The hypothesis
suggested by Mr. Scrope may, perhaps, afford a solution of this
problem. The minerals, he observes, which abound in basalt are of
greater specific gravity than those composing the feldspathic lavas;
thus, for example, hornblende, augite, and olivine are each more than
three times the weight of water; whereas common feldspar and albite
have each scarcely more than 2½ times the specific gravity of water;
and the difference is increased in consequence of there being much more
iron in a metallic state in basalt and greenstone than in trachyte and
other allied feldspathic lavas. If, therefore, a large quantity of rock
be melted up in the bowels of the earth by volcanic heat, the denser
ingredients of the boiling fluid may sink to the bottom, and the
lighter remaining above would in that case be first propelled upward to
the surface by the expansive power of gases. Those materials,
therefore, which occupy the lowest place in the subterranean reservoir
will always be emitted last, and take the uppermost place on the
exterior of the earth’s crust.

Test by Included Fragments.—We may sometimes discover the relative age
of two trap-rocks, or of an aqueous deposit and the trap on which it
rests, by finding fragments of one included in the other in cases such
as those before alluded to, where the evidence of superposition alone
would be insufficient. It is also not uncommon to find a conglomerate
almost exclusively composed of rolled pebbles of trap, associated with
some fossiliferous stratified formation in the neighbourhood of massive
trap. If the pebbles agree generally in mineral character with the
latter, we are then enabled to determine its relative age by knowing
that of the fossiliferous strata associated with the conglomerate. The
origin of such conglomerates is explained by observing the shingle
beaches composed of trap-pebbles in modern volcanoes, as at the base of
Etna.

Recent and Post-pliocene Volcanic Rocks.—I shall now select examples of
contemporaneous volcanic rocks of successive geological periods, to
show that igneous causes have been in activity in all past ages of the
world. They have been perpetually shifting the places where they have
broken out at the earth’s surface, and we can sometimes prove that
those areas which are now the great theatres of volcanic action were in
a state of perfect tranquillity at remote geological epochs, and that,
on the other hand, in places where at former periods the most violent
eruptions took place at the surface and continued for a great length of
time, there has been an entire suspension of igneous action in
historical times, and even, as in the British Isles, throughout a large
part of the antecedent Tertiary Period.

In the absence of British examples of volcanic rocks newer than the
Upper Miocene, I may state that in other parts of the world, especially
in those where volcanic eruptions are now taking place from time to
time, there are tuffs and lavas belonging to that part of the Tertiary
era the antiquity of which is proved by the presence of the bones of
extinct quadrupeds which co-existed with terrestrial, fresh-water, and
marine mollusca of species still living. One portion of the lavas,
tuffs, and trap-dikes of Etna, Vesuvius, and the island of Ischia has
been produced within the historical era; another and a far more
considerable part originated at times immediately antecedent, when the
waters of the Mediterranean were already inhabited by the existing
testacea, but when certain species of elephant, rhinoceros, and other
quadrupeds now extinct, inhabited Europe.

_Vesuvius._—I have traced in the “Principles of Geology” the history of
the changes which the volcanic region of Campania is known to have
undergone during the last 2000 years. The aggregate effect of igneous
operations during that period is far from insignificant, comprising as
it does the formation of the modern cone of Vesuvius since the year 79,
and the production of several minor cones in Ischia, together with that
of Monte Nuovo in the year 1538. Lava-currents have also flowed upon
the land and along the bottom of the sea—volcanic sand, pumice, and
scoriæ have been showered down so abundantly that whole cities were
buried—tracts of the sea have been filled up or converted into
shoals—and tufaceous sediment has been transported by rivers and
land-floods to the sea. There are also proofs, during the same recent
period, of a permanent alteration of the relative levels of the land
and sea in several places, and of the same tract having, near Puzzuoli,
been alternately upheaved and depressed to the amount of more than
twenty feet. In connection with these convulsions, there are found, on
the shores of the Bay of Baiæ, recent tufaceous strata, filled with
articles fabricated by the hands of man, and mingled with marine
shells.

It has also been stated (p. 206), that when we examine this same
region, it is found to consist largely of tufaceous strata, of a date
anterior to human history or tradition, which are of such thickness as
to constitute hills from 500 to more than 2000 feet in height. Some of
these strata contain marine shells which are exclusively of living
species, others contain a slight mixture, one or two per cent of
species not known as living.

The ancient part of Vesuvius is called Somma, and consists of the
remains of an older cone which appears to have been partly destroyed by
explosion. In the great escarpment which this remnant of the ancient
mountain presents towards the modern cone of Vesuvius, there are many
dikes which are for the most part vertical, and traverse the inclined
beds of lava and scoriæ which were successively superimposed during
those eruptions by which the old cone was formed. They project in
relief several inches, or sometimes feet, from the face of the cliff,
being extremely compact, and less destructible than the intersected
tuffs and porous lavas. In vertical extent they vary from a few yards
to 500 feet, and in breadth from one to twelve feet. Many of them cut
all the inclined beds in the escarpment of Somma from top to bottom,
others stop short before they ascend above halfway. In mineral
composition they scarcely differ from the lavas of Somma, the rock
consisting of a base of leucite and augite, through which large
crystals of augite and some of leucite are scattered.

Nothing is more remarkable than the usual parallelism of the opposite
sides of the dikes, which correspond almost as regularly as the two
opposite faces of a wall of masonry. This character appears at first
the more inexplicable, when we consider how jagged and uneven are the
rents caused by earthquakes in masses of heterogeneous composition,
like those composing the cone of Somma. In explanation of this
phenomenon, M. Necker refers us to Sir W. Hamilton’s account of an
eruption of Vesuvius in the year 1779, who records the following fact:
“The lavas, when they either boiled over the crater, or broke out from
the conical parts of the volcano, constantly formed channels as regular
as if they had been cut by art down the steep part of the mountain; and
whilst in a state of perfect fusion, continued their course in those
channels, which were sometimes full to the brim, and at other times
more or less so, according to the quantity of matter in motion.

”These channels (says the same observer), I have found, upon
examination after an eruption, to be in general from two to five or six
feet wide, and seven or eight feet deep. They were often hid from the
sight by a quantity of scoriæ that had formed a crust over them; and
the lava, having been conveyed in a covered way for some yards, came
out fresh again into an open channel. After an eruption, I have walked
in some of those subterraneous or covered galleries, which were
exceedingly curious, the sides, top, and bottom _being worn perfectly
smooth and even_ in most parts by the violence of the currents of the
red-hot lavas which they had conveyed for many weeks successively.” I
was able to verify this phenomenon in 1858, when a stream of lava
issued from a lateral cone.[3] Now, the walls of a vertical fissure,
through which lava has ascended in its way to a volcanic vent, must
have been exposed to the same erosion as the sides of the channels
before adverted to. The prolonged and uniform friction of the heavy
fluid, as it is forced and made to flow upward, cannot fail to wear and
smooth down the surfaces on which it rubs, and the intense heat must
melt all such masses as project and obstruct the passage of the
incandescent fluid.

The rock composing the dikes both in the modern and ancient part of
Vesuvius is far more compact than that of ordinary lava, for the
pressure of a column of melted matter in a fissure greatly exceeds that
in an ordinary stream of lava; and pressure checks the expansion of
those gases which give rise to vesicles in lava. There is a tendency in
almost all the Vesuvian dikes to divide into horizontal prisms, a
phenomenon in accordance with the formation of vertical columns in
horizontal beds of lava; for in both cases the divisions which give
rise to the prismatic structure are at right angles to the cooling
surfaces. (See  p. 510.)

_Auvergne._—Although the latest eruptions in central France seem to
have long preceded the historical era, they are so modern as to have a
very intimate connection with the present superficial outline of the
country and with the existing valleys and river-courses. Among a great
number of cones with perfect craters, one called the Puy de Tartaret
sent forth a lava-current which can be traced up to its crater, and
which flowed for a distance of thirteen miles along the bottom of the
present valley to the village of Nechers, covering the alluvium of the
old valley in which were preserved the bones of an extinct species of
horse, and of a lagomys and other quadrupeds all closely allied to
recent animals, while the associated land-shells were of species now
living, such as _Cyclostoma elegans, Helix hortensis, H. nemoralis,_
_H. lapicida,_ and _Clausilia rugosa._ That the current which has
issued from the Puy de Tartaret may, nevertheless, be very ancient in
reference to the events of human history, we may conclude, not only
from the divergence of the mammiferous fauna from that of our day, but
from the fact that a Roman bridge of such form and construction as
continued in use only down to the fifth century, but which may be
older, is now seen at a place about a mile and a half from St.
Nectaire. This ancient bridge spans the river Couze with two arches,
each about fourteen feet wide. These arches spring from the lava of
Tartaret, on both banks, showing that a ravine precisely like that now
existing had already been excavated by the river through that lava
thirteen or fourteen centuries ago.

While the river Couze has in most cases, as at the site of this ancient
bridge, been simply able to cut a deep channel through the lava, the
lower portion of which is shown to be columnar, the same torrent has in
other places, where the valley was contracted to a narrow gorge, had
power to remove the entire mass of basaltic rock, causing for a short
space a complete breach of continuity in the volcanic current. The work
of erosion has been very slow, as the basalt is tough and hard, and one
column after another must have been undermined and reduced to pebbles,
and then to sand. During the time required for this operation, the
perishable cone of Tartaret, occupying the lowest part of the great
valley descending from Mont Dor (see p. 542), and damming up the river
so as to cause the Lake of Chambon, has stood uninjured, proving that
no great flood or deluge can have passed over this region in the
interval between the eruption of Tartaret and our own times.

_Puy de Côme._—The Puy de Côme and its lava-current, near Clermont, may
be mentioned as another minor volcano of about the same age. This
conical hill rises from the granitic platform, at an angle of between
30° and 40°, to the height of more than 900 feet. Its summit presents
two distinct craters, one of them with a vertical depth of 250 feet. A
stream of lava takes its rise at the western base of the hill instead
of issuing from either crater, and descends the granitic slope towards
the present site of the town of Pont Gibaud. Thence it pours in a broad
sheet down a steep declivity into the valley of the Sioule, filling the
ancient river-channel for the distance of more than a mile. The Sioule,
thus dispossessed of its bed, has worked out a fresh one between the
lava and the granite of its western bank; and the excavation has
disclosed, in one spot, a wall of columnar basalt about fifty feet
high.[4]

The excavation of the ravine is still in progress, every winter some
columns of basalt being undermined and carried down the channel of the
river, and in the course of a few miles rolled to sand and pebbles.
Meanwhile the cone of Côme remains unimpaired, its loose materials
being protected by a dense vegetation, and the hill standing on a ridge
not commanded by any higher ground, so that no floods of rain-water can
descend upon it. There is no end to the waste which the hard basalt may
undergo in future, if the physical geography of the country continue
unchanged—no limit to the number of years during which the heap of
incoherent and transportable materials called the Puy de Côme may
remain in an almost stationary condition.

_Puy de Pariou._—The brim of the crater of the Puy de Pariou, near
Clermont, is so sharp, and has been so little blunted by time, that it
scarcely affords room to stand upon. This and other cones in an equally
remarkable state of integrity have stood, I conceive, uninjured, not
_in spite_ of their loose porous nature, as might at first be naturally
supposed, but in consequence of it. No rills can collect where all the
rain is instantly absorbed by the sand and scoriæ, as is remarkably the
case on Etna; and nothing but a water-spout breaking directly upon the
Puy de Pariou could carry away a portion of the hill, so long as it is
not rent or ingulfed by earthquakes.

Newer Pliocene Volcanic Rocks.—The more ancient portion of Vesuvius and
Etna originated at the close of the Newer Pliocene period, when less
than ten, sometimes only one, in a hundred of the shells differed from
those now living. In the case of Etna, it was before stated (p. 205)
that Post-pliocene formations occur in the neighbourhood of Catania,
while the oldest lavas of the great volcano are Pliocene. These last
are seen associated with sedimentary deposits at Trezza and other
places on the southern and eastern flanks of the great cone (see p.
205).

_Cyclopean Islands._—The Cyclopean Islands, called by the Sicilians Dei
Faraglioni, in the sea-cliffs of which these beds of clay, tuff, and
associated lava are laid open to view, are situated in the Bay of
Trezza, and may be regarded as the extremity of a promontory severed
from the main land. Here numerous proofs are seen of submarine
eruptions, by which the argillaceous and sandy strata were invaded and
cut through, and tufaceous breccias formed. Inclosed in these breccias
are many angular and hardened fragments of laminated clay in different
states of alteration by heat, and intermixed with volcanic sands.

Fig. 599: View of the Isle of Cyclops, in the Bay of Trezza.

The loftiest of the Cyclopean islets, or rather rocks, is about 200
feet in height, the summit being formed of a mass of stratified clay,
the laminæ of which are occasionally subdivided by thin arenaceous
layers. These strata dip to the N.W., and rest on a mass of columnar
lava (see Fig. 599) in which the tops of the pillars are weathered, and
so rounded as to be often hemispherical.

Fig. 600: Contortions of strata in the largest of the Cyclopean
Islands.
In some places in the adjoining and largest islet of the group, which
lies to the north-eastward of that represented in Figure 599), the
overlying clay has been greatly altered and hardened by the igneous
rock, and occasionally contorted in the most extraordinary manner; yet
the lamination has not been obliterated, but, on the contrary, rendered
much more conspicuous, by the indurating process.

In Fig. 600 I have represented a portion of the altered rock, a few
feet square, where the alternating thin laminæ of sand and clay are
contorted in a manner often observed in ancient metamorphic schists. A
great fissure, running from east to west, nearly divides this larger
island into two parts, and lays open its internal structure. In the
section thus exhibited, a dike of lava is seen, first cutting through
an older mass of lava, and then penetrating the superincumbent tertiary
strata. In one place the lava ramifies and terminates in thin veins,
from a few feet to a few inches in thickness (see Fig. 601). The
arenaceous laminæ are much hardened at the point of contact, and the
clays are converted into siliceous schist. In this island the altered
rocks assume a honey-comb structure on their weathered surface,
singularly contrasted with the smooth and even outline which the same
beds present in their usual soft and yielding state. The pores of the
lava are sometimes coated, or entirely filled with carbonate of lime,
and with a zeolite resembling analcime, which has been called
cyclopite. The latter mineral has also been found in small fissures
traversing the altered marl, showing that the same cause which
introduced the minerals into the cavities of the lava, whether we
suppose sublimation or aqueous infiltration, conveyed it also into the
open rents of the contiguous sedimentary strata.

Fig. 601: Newer pliocene strata invaded by lava. Isle of Cyclops
(horizontal section).
_Dikes of Palagonia._—Dikes of vesicular and amygdaloidal lava are also
seen traversing marine tuff or peperino, west of Palagonia, some of the
pores of the lava being empty, while others are filled with carbonate
of lime. In such cases we may suppose the tuff to have resulted from
showers of volcanic sand and scoriæ, together with fragments of
limestone, thrown out by a submarine explosion, similar to that which
gave rise to Graham Island in 1831. When the mass was, to a certain
degree, consolidated, it may have been rent open, so that the lava
ascended through fissures, the walls of which were perfectly even and
parallel. In one case, after the melted matter that filled the rent
(Fig. 602) had cooled down, it must have been fractured and shifted
horizontally by a lateral movement.

In Fig. 603, the lava has more the appearance of a vein, which forced
its way through the peperino. It is highly probable that similar
appearances would be seen, if we could examine the floor of the sea in
that part of the Mediterranean where the waves have recently washed
away the new volcanic island; for when a superincumbent mass of ejected
fragments has been removed by denudation, we may expect to see sections
of dikes traversing tuff, or, in other words, sections of the channels
of communication by which the subterranean lavas reached the surface.

Figs. 602 and 603: Ground-plan of dikes near Palagonia.
_Madeira._—Although the more ancient portion of the volcanic eruptions
by which the island of Madeira and the neighbouring one of Porto Santo
were built up occurred, as we shall presently see, in the Upper Miocene
Period, a still larger part of the island is of Pliocene date. That the
latest outbreaks belonged to the Newer Pliocene Period, I infer from
the close affinity to the present flora of Madeira of the fossil plants
preserved in a leaf-bed in the north-eastern part of the island. These
fossils, associated with some lignite in the ravine of the river San
Jorge, can none of them be proved to be of extinct species, but their
antiquity may be inferred from the following considerations:
Firstly—The leaf-bed, discovered by Mr. Hartung and myself in 1853, at
the height of 1000 feet above the level of the sea, crops out at the
base of a cliff formed by the erosion of a gorge cut through
alternating layers of basalt and scoriæ, the product of a vast
succession of eruptions of unknown date, piled up to a thickness of
1000 feet, and which were all poured out after the plants, of which
about twenty species have been recognised, flourished in Madeira. These
lavas are inclined at an angle of about 15° to the north, and came down
from the great central region of eruption. Their accumulation implies a
long period of intermittent volcanic action, subsequently to which the
ravine of San Jorge was hollowed out. Secondly—Some few of the plants,
though perhaps all of living species, are supposed to be of genera not
now existing in the island. They have been described by Sir Charles
Bunbury and Professor Heer, and the former first pointed out that many
of the leaves are of the laurel type, and analogous to those now
flourishing in the modern forests of Madeira. He also recognised among
them the leaves of _Woodwardia radicans_, _and Davallia Canariensis,_
ferns now abundant in Madeira. Thirdly—the great age of this leaf-bed
of San Jorge, which was perhaps originally formed in the crater of some
ancient volcanic cone afterwards buried under lava, is proved by its
belonging to a part of the eastern extremity of Madeira, which, after
the close of the igneous eruptions, became covered in the adjoining
district of Caniçal with blown sand in which a vast number of
land-shells were buried. These fossil shells belonged to no less than
36 species, among which are many now extremely rare in the island, and
others, about five per cent, extinct or unknown in any part of the
world. Several of these of the genus _Helix_ are conspicuous from the
peculiarity of their forms, others from their large dimensions. The
geographical configuration of the country shows that this shell-bed is
considerably more modern than the leaf-bed; it must therefore be
referred to the Newer Pliocene, according to the definition of this
period given in a former chapter (p. 143).

Older Pliocene Period.—_Italy._—In Tuscany, as at Radicofani, Viterbo,
and Aquapendente, and in the Campagna di Roma, submarine volcanic tuffs
are interstratified with the Older Pliocene strata of the Sub-apennine
hills in such a manner as to leave no doubt that they were the products
of eruptions which occurred when the shelly marls and sands of the
Sub-appenine hills were in the course of deposition. This opinion I
expressed[5] after my visit to Italy in 1828 and it has recently (1850)
been confirmed by the argument adduced by Sir R. Murchison in favour of
the submarine origin of the tertiary volcanic rocks of Italy.[6] These
rocks are well-known to rest conformably on the Sub-apennine marls,
even as far south as Monte Mario, in the suburbs of Rome. On the exact
age of the deposits of Monte Mario new light has recently been thrown
by a careful study of their marine fossil shells, undertaken by MM.
Rayneval, Van den Hecke, and Ponzi. They have compared no less than 160
species with the shells of the Coralline Crag of Suffolk, so well
described by Mr. Searles Wood; and the specific agreement between the
British and Italian fossils is so great, if we make due allowance for
geographical distance and the difference of latitude, that we can have
little hesitation in referring both to the same period, or to the Older
Pliocene of this work. It is highly probable that, between the oldest
trachytes of Tuscany and the newest rocks in the neighbourhood of
Naples, a series of volcanic products might be detected of every age
from the Older Pliocene to the historical epoch.

_Pliocene Volcanoes of the Eifel._—Some of the most perfect cones and
craters in Europe, not even excepting those of the district round
Vesuvius, may be seen on the left or west bank of the Rhine, near Bonn
and Andernach. They exhibit characters distinct from any which I have
observed elsewhere, owing to the large part which the escape of aqueous
vapour has played in the eruptions and the small quantities of lava
emitted. The fundamental rocks of the district are grey and red
sandstones and shales, with some associated limestones, replete with
fossils of the Devonian or Old Red Sandstone group. The volcanoes broke
out in the midst of these inclined strata, and when the present systems
of hills and valleys had already been formed. The eruptions occurred
sometimes at the bottom of deep valleys, sometimes on the summit of
hills, and frequently on intervening platforms. In travelling through
this district we often come upon them most unexpectedly, and may find
ourselves on the very edge of a crater before we had been led to
suspect that we were approaching the site of any igneous outburst.
Thus, for example, on arriving at the village of Gemund, immediately
south of Daun, we leave the stream, which flows at the bottom of a deep
valley in which strata of sandstone and shale crop out. We then climb a
steep hill, on the surface of which we see the edges of the same strata
dipping inward towards the mountain. When we have ascended to a
considerable height, we see fragments of scoriæ sparingly scattered
over the surface; until at length, on reaching the summit, we find
ourselves suddenly on the edge of a _tarn,_ or deep circular lake-basin
called the Gemunder Maar. In it we recognise the ordinary form of a
crater, for which we have been prepared by the occurrence of scoriæ
scattered over the surface of the soil. But on examining the walls of
the crater we find precipices of sandstone and shale which exhibit no
signs of the action of heat; and we look in vain for those beds of lava
and scoriæ, dipping outward on every side, which we have been
accustomed to consider as characteristic of volcanic vents. As we
proceed, however, to the opposite side of the lake, we find a
considerable quantity of scoriæ and some lava, and see the whole
surface of the soil sparkling with volcanic sand, and strewed with
ejected fragments of half-fused shale, which preserves its laminated
texture in the interior, while it has a vitrified or scoriform coating.

Other crater lakes of circular or oval form, and hollowed out of
similar ancient strata, occur in the Upper Eifel, where copious
aëriform discharges have taken place, throwing out vast heaps of
pulverized shale into the air. I know of no other extinct volcanoes
where gaseous explosions of such magnitude have been attended by the
emission of so small a quantity of lava. Yet I looked in vain in the
Eifel for any appearances which could lend support to the hypothesis
that the sudden rushing out of such enormous volumes of gas had ever
lifted up the stratified rocks immediately around the vent so as to
form conical masses, having their strata dipping outward on all sides
from a central axis, as is assumed in the theory of elevation craters,
alluded to in the last chapter.

I have already given (Fig. 590) an example in the Eifel of a small
stream of lava which issued from one of the craters of that district at
Bertrich-Baden. It shows that when some of these volcanoes were in
action the valleys had already been eroded to their present depth.

_Trass._—The tufaceous alluvium called _trass,_ which has covered large
areas in the Eifel, and choked up some valleys now partially
re-excavated, is unstratified. Its base consists almost entirely of
pumice, in which are included fragments of basalt and other lavas,
pieces of burnt shale, slate, and sandstone, and numerous trunks and
branches of trees. If, as is probable, this trass was formed during the
period of volcanic eruptions, it may have originated in the manner of
the moya of the Andes.

We may easily conceive that a similar mass might now be produced, if a
copious evolution of gases should occur in one of the lake-basins. If a
breach should be made in the side of the cone, the flood would sweep
away great heaps of ejected fragments of shale and sandstone, which
would be borne down into the adjoining valleys. Forests might be torn
up by such a flood, and thus the occurrence of the numerous trunks of
trees dispersed irregularly through the trass can be explained. The
manner in which this trass conforms to the shape of the present valleys
implies its comparatively modern origin, probably not dating farther
back than the Pliocene Period.

 [1] Caldcleugh, Phil. Trans., 1836, p. 27.

 [2] See Principles, _Index,_ “Skaptar Jokul.”

 [3] Principles of Geology, vol. i, p. 626.

 [4] Scrope’s Central France, p. 60, and plate.

 [5] See 1st edit. of Principles of Geology, vol. iii, chaps. xiii and
 xiv, 1833; and former editions of this work, chap. xxxi.

 [6] Quart. Geol. Journ., vol. vi, p. 281.




CHAPTER XXX.
AGE OF VOLCANIC ROCKS—_continued._


Volcanic Rocks of the Upper Miocene Period. — Madeira. — Grand Canary.
— Azores. — Lower Miocene Volcanic Rocks. — Isle of Mull. — Staffa and
Antrim. — The Eifel. — Upper and Lower Miocene Volcanic Rocks of
Auvergne. — Hill of Gergovia. — Eocene Volcanic Rocks of Monte Bolca. —
Trap of Cretaceous Period. — Oolitic Period. — Triassic Period. —
Permian Period. — Carboniferous Period. — Erect Trees buried in
Volcanic Ash in the Island of Arran. — Old Red Sandstone Period. —
Silurian Period. — Cambrian Period. — Laurentian Volcanic Rocks.

Volcanic Rocks of the Upper Miocene Period.—_Madeira._—The greater part
of the volcanic eruptions of Madeira, as we have already seen (p. 532),
belong to the Pliocene Period, but the most ancient of them are of
Upper Miocene date, as shown by the fossil shells included in the
marine tuffs which have been upraised at San Vicente, in the northern
part of the island, to the height of 1300 feet above the level of the
sea. A similar marine and volcanic formation constitutes the
fundamental portion of the neighbouring island of Porto Santo, forty
miles distant from Madeira, and is there elevated to an equal height,
and covered, as in Madeira, with lavas of supra-marine origin.

The largest number of fossils have been collected from the tuffs and
conglomerates and some beds of limestone in the island of Baixo, off
the southern extremity of Porto Santo. They amount in this single
locality to more than sixty in number, of which about fifty are
mollusca, but many of these are only casts. Some of the shells probably
lived on the spot during the intervals between eruptions, and some may
have been cast up into the water or air together with muddy ejections,
and, falling down again, have been deposited on the bottom of the sea.
The hollows in some of the fragments of vesicular lava of which the
breccias and conglomerates are composed are partially filled with
calc-sinter, being thus half converted into amygdaloids. Among the
fossil shells common to Madeira and Porto Santo, large cones, strombs,
and cowries are conspicuous among the univalves, and _Cardium,
Spondylus,_ and _Lithodomus_ among the lamellibranchiate bivalves, and
among the _Echinoderms_ the large Clypeaster called _C. altus,_ an
extinct European Miocene fossil.

The largest list of fossils has been published by Mr. Karl Meyer, in
Hartung’s “Madeira;” but in the collection made by myself, and in a
still larger one formed by Mr. J. Yate Johnson, several remarkable
forms not in Meyer’s list occur, as, for example, _Pholadomya,_ and a
large _ Terebra._ Mr. Johnson also found a fine specimen of _Nautilus
(Atruria) ziczac_ (Fig. 211), a well-known Falunian fossil of Europe;
and in the same volcanic tuff of Baixo, the Echinoderm _Brissus
Scillæ,_ a living Mediterranean species, found fossil in the Miocene
strata of Malta. Mr. Meyer identifies one-third of the Madeira shells
with known European Miocene (or Falunian) forms. The huge Strombus of
San Vicente and Porto Santo, _S. Italicus,_ is an extinct shell of the
Sub-apennine or Older Pliocene formations. The mollusca already
obtained from various localities of Madeira and Porto Santo are not
less than one hundred in number, and, according to the late Dr. S. P.
Woodward, rather more than a third are of species still living, but
many of these are not now inhabitants of the neighbouring sea.

It has been remarked (p. 212), that in the Older Pliocene and Upper
Miocene deposits of Europe many forms occur of a more southern aspect
than those now inhabiting the nearest sea. In like manner the fossil
corals, or Zoantharia, six in number, which I obtained from Madeira, of
the genera _Astræa, Sarcinula, Hydnophora,_ were pronounced by Mr.
Lonsdale to be forms foreign to the adjacent coasts, and agreeing with
the fauna of a sea warmer than that now separating Madeira from the
nearest part of the African coast. We learn, indeed, from the
observations made in 1859, by the Reverend R. T. Lowe, that more than
one-half, or fifty-three in ninety, of the marine mollusks collected by
him from the sandy beach of Mogador are common British species,
although Mogador is 18½ degrees south of the nearest shores of England.
The living shells of Madeira and Porto Santo are in like manner those
of a temperate climate, although in great part differing specifically
from those of Mogador.[1]

_Grand Canary._—In the Canaries, especially in the Grand Canary, the
same marine Upper Miocene formation is found. Stratified tuffs, with
intercalated conglomerates and lavas, are there seen in nearly
horizontal layers in sea-cliffs about 300 feet high, near Las Palmas.
Mr. Hartung and I were unable to find marine shells in these tuffs at a
greater elevation than 400 feet above the sea; but as the deposit to
which they belong reaches to the height of 1100 feet or more in the
interior, we conceive that an upheaval of at least that amount has
taken place. The _Clypeaster altus, Spondylus gæderopus, Pectunculus
pilosus, Cardita calyculata,_ and several other shells, serve to
identify this formation with that of the Madeiras, and _Ancillaria
glandiformis,_ which is not rare, and some other fossils, remind us of
the faluns of Touraine.

The sixty-two Miocene species which I collected in the Grand Canary
were referred by the late Dr. S. P. Woodward to forty-seven genera, ten
of which are no longer represented in the neighbouring sea, namely
_Corbis,_ an African form, Hinnites, now living in Oregon, _Thecidium_
(_T. Mediterranean,_ identical with the Miocene fossil of St. Juvat, in
Brittany), _Calyptræa, Hipponyx, Nerita, Erato, Oliva, Ancillaria,_ and
_ Fasciolaria._

These tuffs of the southern shores of the Grand Canary, containing the
Upper Miocene shells, appear to be about the same age as the most
ancient volcanic rocks of the island, composed of slaty diabase,
phonolite, and trachyte. Over the marine lavas and tuffs trachytic and
basaltic products of subaërial volcanic origin, between 4000 and 5000
feet in thickness, have been piled, the central parts of the Grand
Canary reaching the height of about 6000 feet above the level of the
sea. A large portion of this mass is of Pliocene date, and some of the
latest lavas have been poured out since the time when the valleys were
already excavated to within a few feet of their present depth.

On the whole, the rocks of the Grand Canary, an island of a nearly
circular shape, and 6½ geographical miles diameter, exhibit proofs of a
long series of eruptions beginning like those of Madeira, Porto Santo,
and the Azores, in the Upper Miocene period, and continued to the
Post-Pliocene. The building up of the Grand Canary by subaërial
eruptions, several thousand feet thick, went on simultaneously with the
gradual upheaval of the earliest products of submarine eruptions, in
the same manner as the Pliocene marine strata of the oldest parts of
Vesuvius and Etna have been upraised during eruptions of Post-tertiary
date.

In proof that movements of elevation have actually continued down to
Post-tertiary times, I may remark that I found raised beaches
containing shells of the Recent Period in the Grand Canary, Teneriffe,
and Porto Santo. The most remarkable raised beach which I observed in
the Grand Canary, in the study of which I was assisted by Don Pedro
Maffiotte, is situated in the north-eastern part of the island at San
Catalina, about a quarter of a mile north of Las Palmas. It intervenes
between the base of the high cliff formed of the tuffs with Miocene
shells and the sea-shore. From this beach, at an elevation of
twenty-five feet above high-water mark, and at a distance of about 150
feet from the present shore, I obtained more than fifty species of
living marine shells. Many of them, according to Dr. S. P. Woodward,
are no longer inhabitants of the contiguous sea, as, for example,
_Strombus bubonius,_ which is still living on the West Coast of Africa,
and _Cerithium procerum,_ found at Mozambique; others are Mediterranean
species, as _Pecten Jacobæus_ and _P. polymorphus._ Some of these
testacea, such as _Cardita squamosa,_ are inhabitants of deep water,
and the deposit on the whole seems to indicate a depth of water
exceeding a hundred feet.

_Azores._—In the island of St. Mary’s, one of the Azores, marine fossil
shells have long been known. They are found on the north-east coast on
a small projecting promontory called Ponta do Papagaio (or
Point-Parrot), chiefly in a limestone about twenty feet thick, which
rests upon, and is again covered by, basaltic lavas, scoriæ, and
conglomerates. The pebbles in the conglomerate are cemented together
with carbonate of lime.

Mr. Hartung, in his account of the Azores, published in 1860, describes
twenty-three shells from St. Mary’s,[2] of which eight perhaps are
identical with living species, and twelve are with more or less
certainty referred to European Tertiary forms, chiefly Upper Miocene.
One of the most characteristic and abundant of the new species,
_Cardium Hartungi,_ not known as fossil in Europe, is very common in
Porto Santo and Baixo, and serves to connect the Miocene fauna of the
Azores and the Madeiras. In some of the Azores, as well as in the
Canary islands, the volcanic fires are not yet extinct, as the recorded
eruptions of Lanzerote, Teneriffe, Palma, St. Michael’s, and others,
attest.

Lower Miocene Volcanic Rocks.—_Isle of Mull and Antrim._—I may refer
the reader to the account already given (p. 247) of leaf-beds at
Ardtun, in the Isle of Mull in the Hebrides, which bear a relation to
the associated volcanic rocks of Lower Miocene date analogous to that
which the Madeira leaf-bed, above described (p. 532), bears to the
Pliocene lavas of that island. Mr. Geikie has shown that the volcanic
rocks in Mull are above 3000 feet in thickness. There seems little
doubt that the well-known columnar basalt of Staffa, as well as that of
Antrim in Ireland, are of the same age, and not of higher antiquity, as
once suspected.

_The Eifel._—A large portion of the volcanic rocks of the Lower Rhine
and the Eifel are coeval with the Lower Miocene deposits to which most
of the “Brown-Coal” of Germany belongs. The Tertiary strata of that age
are seen on both sides of the Rhine, in the neighbourhood of Bonn,
resting unconformably on highly inclined and vertical strata of
Silurian and Devonian rocks. The Brown-Coal formation of that region
consists of beds of loose sand, sandstone, and conglomerate, clay with
nodules of clay-iron-stone, and occasionally silex. Layers of light
brown and sometimes black lignite are interstratified with the clays
and sands, and often irregularly diffused through them. They contain
numerous impressions of leaves and stems of trees, and are extensively
worked for fuel, whence the name of the formation. In several places
layers of trachytic tuff are interstratified, and in these tuffs are
leaves of plants identical with those found in the brown-coal, showing
that, during the period of the accumulation of the latter, some
volcanic products were ejected. The igneous rocks of the Westerwald,
and of the mountains called the Siebengebirge, consist partly of
basaltic and partly of trachytic lavas, the latter being in general the
more ancient of the two. There are many varieties of trachyte, some of
which are highly crystalline, resembling a coarse-grained granite, with
large separate crystals of feldspar. Trachytic tuff is also very
abundant.

M. Von Dechen, in his work on the Siebengebirge,[3] has given a copious
list of the animal and vegetable remains of the fresh-water strata
associated with the brown-coal of that part of Germany. Plants of the
genera _Flabellaria, Ceanothus,_ and _ Daphnogene,_ including _D.
cinnamomifolia_ (Fig. 155), occur in these beds, with nearly 150 other
plants. The fishes of the brown-coal near Bonn are found in a
bituminous shale, called paper-coal, from being divisible into
extremely thin leaves. The individuals are very numerous; but they
appear to belong to a small number of species, some of which were
referred by Agassiz to the genera _Leuciscus, Aspius,_ and _Perca._ The
remains of frogs also, of extinct species, have been discovered in the
paper-coal; and a complete series may be seen in the museum at Bonn,
from the most imperfect state of the tadpole to that of the full-grown
animal. With these a salamander, scarcely distinguishable from the
recent species, has been found, and the remains of many insects.

Upper and Lower Miocene Volcanic Rocks of Auvergne.—The extinct
volcanoes of Auvergne and Cantal, in central France, seem to have
commenced their eruptions in the Lower Miocene period, but to have been
most active during the Upper Miocene and Pliocene eras. I have already
alluded to the grand succession of events of which there is evidence in
Auvergne since the last retreat of the sea (see p. 527).

The earliest monuments of the Tertiary Period in that region are
lacustrine deposits of great thickness, in the lowest conglomerates of
which are rounded pebbles of quartz, mica-schist, granite, and other
non-volcanic rocks, without the slightest intermixture of igneous
products. To these conglomerates succeed argillaceous and calcareous
marls and limestones, containing Lower Miocene shells and bones of
mammalia, the higher beds of which sometimes alternate with volcanic
tuff of contemporaneous origin. After the filling up or drainage of the
ancient lakes, huge piles of trachytic and basaltic rocks, with
volcanic breccias, accumulated to a thickness of several thousand feet,
and were superimposed upon granite, or the contiguous lacustrine
strata. The greater portion of these igneous rocks appear to have
originated during the Upper Miocene and Pliocene periods; and extinct
quadrupeds of those eras, belonging to the genera Mastodon, Rhinoceros,
and others, were buried in ashes and beds of alluvial sand and gravel,
which owe their preservation to overspreading sheets of lava.

In Auvergne, the most ancient and conspicuous of the volcanic masses is
Mont Dor, which rests immediately on the granitic rocks standing apart
from the fresh-water strata. This great mountain rises suddenly to the
height of several thousand feet above the surrounding platform, and
retains the shape of a flattened and somewhat irregular cone, the slope
of which is gradually lost in the high plain around. This cone is
composed of layers of scoriæ, pumice-stones, and their fine detritus,
with interposed beds of trachyte and basalt, which descend often in
uninterrupted sheets until they reach and spread themselves round the
base of the mountain.[4] Conglomerates, also, composed of angular and
rounded fragments of igneous rocks, are observed to alternate with the
above; and the various masses are seen to dip off from the central
axis, and to lie parallel to the sloping flanks of the mountain. The
summit of Mont Dor terminates in seven or eight rocky peaks, where no
regular crater can now be traced, but where we may easily imagine one
to have existed, which may have been shattered by earthquakes, and have
suffered degradation by aqueous agents. Originally, perhaps, like the
highest crater of Etna, it may have formed an insignificant feature in
the great pile, and, like it, may frequently have been destroyed and
renovated.

Respecting the age of the great mass of Mont Dor, we cannot come at
present to any positive decision, because no organic remains have yet
been found in the tuffs, except impressions of the leaves of trees of
species not yet determined. It has already been stated (p. 234) that
the earliest eruptions must have been posterior in origin to those
grits and conglomerates of the fresh-water formation of the Limagne
which contain no pebbles of volcanic rocks. But there is evidence at a
few points, as in the hill of Gergovia, presently to be mentioned, that
some eruptions took place before the great lakes were drained, while
others occurred after the desiccation of those lakes, and when deep
valleys had already been excavated through fresh-water strata.

The valley in which the cone of Tartaret, above-mentioned (p. 527), is
situated affords an impressive monument of the very different dates at
which the igneous eruptions of Auvergne have happened; for while the
cone itself is of Post-Pliocene date, the valley is bounded by lofty
precipices composed of sheets of ancient columnar trachyte and basalt,
which once flowed from the summit of Mont Dor in some part of the
Miocene period. These Miocene lavas had accumulated to a thickness of
nearly 1000 feet before the ravine was cut down to the level of the
river Couze, a river which was at length dammed up by the modern cone
and the upper part of its course transformed into a lake.

_Gergovia._—It has been supposed by some observers that there is an
alternation of a contemporaneous sheet of lava with fresh-water strata
in the hill of Gergovia, near Clermont. But this idea has arisen from
the intrusion of the dike represented in Fig. 604, which has altered
the green and white marls both above and below. Nevertheless, there is
a real alternation of volcanic tuff with strata containing Lower
Miocene fresh-water shells, among others a Melania allied to _M.
inquinata_ (Fig. 217), with a Melanopsis and a Unio; there can,
therefore, be no doubt that in Auvergne some volcanic explosions took
place before the drainage of the lakes, and at a time when the Lower
Miocene species of animals and plants still flourished.

Fig. 604: Hill of Gergovia.

Eocene Volcanic Rocks.—_Monte Bolca._—The fissile limestone of Monte
Bolca, near Verona, has for many centuries been celebrated in Italy for
the number of perfect Ichthyolites which it contains. Agassiz has
described no less than 133 species of fossil fish from this single
deposit, and the multitude of individuals by which many of the species
are represented is attested by the variety of specimens treasured up in
the principal museums of Europe. They have been all obtained from
quarries worked exclusively by lovers of natural history, for the sake
of the fossils. Had the lithographic stone of Solenhofen, now regarded
as so rich in fossils, been in like manner quarried solely for
scientific objects, it would have remained almost a sealed book to
palæontologists, so sparsely are the organic remains scattered through
it. When I visited Monte Bolca, in company with Sir Roderick Murchison,
in 1828, we ascertained that the fish-bearing beds were of Eocene date,
containing well-known species of Nummulites, and that a long series of
submarine volcanic eruptions, evidently contemporaneous, had produced
beds of tuff, which are cut through by dikes of basalt. There is
evidence here of a long series of submarine volcanic eruptions of
Eocene date, and during some of them, as Sir R. Murchison has
suggested, shoals of fish were probably destroyed by the evolution of
heat, noxious gases, and tufaceous mud, just as happened when Graham’s
Island was thrown up between Sicily and Africa in 1831, at which time
the waters of the Mediterranean were seen to be charged with red mud,
and covered with dead fish over a wide area.[5]

Associated with the marls and limestones of Monte Bolca are beds
containing lignite and shale with numerous plants, which have been
described by Unger and Massalongo, and referred by them to the Eocene
period. I have already cited (p. 263) Professor Heer’s remark, that
several of the species are common to Monte Bolca and the white clay of
Alum Bay, a Middle Eocene deposit; and the same botanist dwells on the
tropical character of the flora of Monte Bolca and its distinctness
from the sub-tropical flora of the Lower Miocene of Switzerland and
Italy, in which last there is a far more considerable mixture of forms
of a temperate climate, such as the willow, poplar, birch, elm, and
others. That scarcely any one of the Monte Bolca fish should have been
found in any other locality in Europe, is a striking illustration of
the extreme imperfection of the palæontological record. We are in the
habit of imagining that our insight into the geology of the Eocene
period is more than usually perfect, and we are certainly acquainted
with an almost unbroken succession of assemblages of shells passing one
into the other from the era of the Thanet sands to that of the
Bembridge beds or Paris gypsum. The general dearth, therefore, of fish
in the different members of the Eocene series, Upper, Middle, and
Lower, might induce a hasty reasoner to conclude that there was a
poverty of ichthyic forms during this period; but when a local
accident, like the volcanic eruptions of Monte Bolca, occurs, proofs
are suddenly revealed to us of the richness and variety of this great
class of vertebrata in the Eocene sea. The number of genera of Monte
Bolca fish is, according to Agassiz, no less than seventy-five, twenty
of them peculiar to that locality, and only eight common to the
antecedent Cretaceous period. No less than forty-seven out of the
seventy-five genera make their appearance for the first time in the
Monte Bolca rocks, none of them having been met with as yet in the
antecedent formations. They form a great contrast to the fish of the
secondary strata, as, with the exception of the Placoids, they are all
Teleosteans, only one genus, _Pycnodus,_ belonging to the order of
Ganoids, which form, as before stated, the vast majority of the
ichthyolites entombed in the secondary are Mesozoic rocks.

Cretaceous Period.—M. Virlet, in his account of the geology of the
Morea, p. 205, has clearly shown that certain traps in Greece are of
Cretaceous date; as those, for example, which alternate conformably
with cretaceous limestone and greensand between Kastri and Damala, in
the Morea. They consist in great part of diallage rocks and serpentine,
and of an amygdaloid with calcareous kernels, and a base of serpentine.
In certain parts of the Morea, the age of these volcanic rocks is
established by the following proofs: first, the lithographic limestones
of the Cretaceous era are cut through by trap, and then a conglomerate
occurs, at Nauplia and other places, containing in its calcareous
cement many well-known fossils of the chalk and greensand, together
with pebbles formed of rolled pieces of the same serpentinous trap,
which appear in the dikes above alluded to.

Period of Oolite and Lias.—Although the green and serpentinous
trap-rocks of the Morea belong chiefly to the Cretaceous era, as before
mentioned, yet it seems that some eruptions of similar rocks began
during the Oolitic period;[6] and it is probable that a large part of
the trappean masses, called ophiolites in the Apennines, and associated
with the limestone of that chain, are of corresponding age.

Trap of the New Red Sandstone Period.—In the southern part of
Devonshire, trappean rocks are associated with New Red Sandstone, and,
according to Sir H. De la Beche, have not been intruded subsequently
into the sandstone, but were produced by contemporaneous volcanic
action. Some beds of grit, mingled with ordinary red marl, resemble
sands ejected from a crater; and in the stratified conglomerates
occurring near Tiverton are many angular fragments of trap porphyry,
some of them one or two tons in weight, intermingled with pebbles of
other rocks. These angular fragments were probably thrown out from
volcanic vents, and fell upon sedimentary matter then in the course of
deposition.[7]

Trap of the Permian Period.—The recent investigations of Mr. Archibald
Geikie in Ayrshire have shown that some of the volcanic rocks in that
county are of Permian age, and it appears highly probable that the
uppermost portion of Arthur’s Seat in the suburbs of Edinburgh marks
the site of an eruption of the same era.

Trap of the Carboniferous Period.—Two classes of contemporaneous
trap-rocks occur in the coal-field of the Forth, in Scotland. The
newest of these, connected with the higher series of coal-measures, is
well exhibited along the shores of the Forth, in Fifeshire, where they
consist of basalt with olivine, amygdaloid, greenstone, wacke, and
tuff. They appear to have been erupted while the sedimentary strata
were in a horizontal position, and to have suffered the same
dislocations which those strata have subsequently undergone. In the
volcanic tuffs of this age are found not only fragments of limestone,
shale, flinty slate, and sandstone, but also pieces of coal. The other
or older class of carboniferous traps are traced along the south margin
of Stratheden, and constitute a ridge parallel with the Ochils, and
extending from Stirling to near St. Andrews. They consist almost
exclusively of greenstone, becoming, in a few instances, earthy and
amygdaloidal. They are regularly interstratified with the sandstone,
shale, and iron-stone of the lower coal-measures, and, on the East
Lomond, with Mountain Limestone. I examined these trap-rocks in 1838,
in the cliffs south of St. Andrews, where they consist in great part of
stratified tuffs, which are curved, vertical, and contorted, like the
associated coal-measures. In the tuff I found fragments of
carboniferous shale and limestone, and intersecting veins of
greenstone.

_Fife—Flisk Dike._—A trap dike was pointed out to me by Dr. Fleming, in
the parish of Flisk, in the northern part of the county of Fife, which
cuts through the grey sandstone and shale, forming the lowest part of
the Old Red Sandstone, but which may probably be of carboniferous date.
It may be traced for many miles, passing through the amygdaloidal and
other traps of the hill called Norman’s Law in that parish. In its
course it affords a good exemplification of the passage from the
trappean into the Plutonic, or highly crystalline texture. Professor
Gustavus Rose, to whom I submitted specimens of this dike, found it to
be dolerite, and composed of greenish black augite and Labrador
feldspar, the latter being the most abundant ingredient. A small
quantity of magnetic iron, perhaps titaniferous, is also present. The
result of this analysis is interesting, because both the ancient and
modern lavas of Etna consist in like manner of augite, Labradorite, and
titaniferous iron.

_Erect Trees buried in Volcanic Ash at Arran._—An interesting discovery
was made in 1867 by Mr. E. A. Wünsch in the carboniferous strata of the
north-eastern part of the island of Arran. In the sea-cliff about five
miles north of Corrie, near the village of Laggan, strata of volcanic
ash occur, forming a solid rock cemented by carbonate of lime and
enveloping trunks of trees, determined by Mr. Binney to belong to the
genera Sigillaria and Lepidodendron. Some of these trees are at right
angles to the planes of stratification, while others are prostrate and
accompanied by leaves and fruits of the same genera. I visited the spot
in company with Mr. Wünsch in 1870, and saw that the trees with their
roots, of which about fourteen had been observed, occur at two distinct
levels in volcanic tuffs parallel to each other, and inclined at an
angle of about 40°, having between them beds of shale and coaly matter
seven feet thick. It is evident that the trees were overwhelmed by a
shower of ashes from some neighbouring volcanic vent, as Pompeii was
buried by matter ejected from Vesuvius. The trunks, several of them
from three to five feet in circumference, remained with their
Stigmarian roots spreading through the stratum below, which had served
as a soil. The trees must have continued for years in an upright
position after they were killed by the shower of burning ashes, giving
time for a partial decay of the interior, so as to afford hollow
cylinders into which the spores of plants were wafted. These spores
germinated and grew, until finally their stems were petrified by
carbonate of lime like some of the remaining portions of the wood of
the containing Sigillaria. Mr. Carruthers has discovered that sometimes
the plants which had thus grown and become fossil in the inside of a
single trunk belonged to several distinct genera. The fact that the
tree-bearing deposits now dip at an angle of 40° is the more striking,
as they must clearly have remained horizontal and undisturbed during a
long period of intermittent and contemporaneous volcanic action.

In some of the associated carboniferous shales, ferns and calamites
occur, and all the phenomena of the successive buried forests remind us
of the sections in pp. 410 and 411 of the Nova Scotia coal-measures,
with this difference only, that in the case of the South Joggins the
fossilisation of the trees was effected without the eruption of
volcanic matter.

Trap of the Old Red Sandstone Period.—By referring to the section
explanatory of the structure of Forfarshire, already given (p. 74), the
reader will perceive that beds of conglomerate, No. 3, occur in the
middle of the Old Red Sandstone system, 1, 2, 3, 4. The pebbles in
these conglomerates are sometimes composed of granitic and quartzose
rocks, sometimes exclusively of different varieties of trap, which
last, although purposely omitted in the section referred to, is often
found either intruding itself in amorphous masses and dikes into the
old fossiliferous tilestones, No. 4, or alternating with them in
conformable beds. All the different divisions of the red sandstone, 1,
2, 3, 4, are occasionally intersected by dikes, but they are very rare
in Nos. 1 and 2, the upper members of the group consisting of red shale
and red sandstone. These phenomena, which occur at the foot of the
Grampians, are repeated in the Sidlaw Hills; and it appears that in
this part of Scotland volcanic eruptions were most frequent in the
earlier part of the Old Red Sandstone period. The trap-rocks alluded to
consist chiefly of feldspathic porphyry and amygdaloid, the kernels of
the latter being sometimes calcareous, often chalcedonic, and forming
beautiful agates. We meet also with claystone, greenstone, compact
feldspar, and tuff. Some of these rocks look as if they had flowed as
lavas over the bottom of the sea, and enveloped quartz pebbles which
were lying there, so as to form conglomerates with a base of
greenstone, as is seen in Lumley Den, in the Sidlaw Hills. On either
side of the axis of this chain of hills (see Fig. 55), the beds of
massive trap, and the tuffs composed of volcanic sand and ashes, dip
regularly to the south-east or north-west, conformably with the shales
and sandstones.

But the geological structure of the Pentland Hills, near Edinburgh,
shows that igneous rocks were there formed during the newer part of the
Devonian or “Old Red” period. These hills are 1900 feet high above the
sea, and consist of conglomerates and sandstones of Upper Devonian age,
resting on the inclined edges of grits and slates of Lower Devonian and
Upper Silurian date. The contemporaneous volcanic rocks intercalated in
this Upper Old Red consist of feldspathic lavas, or feldstones, with
associated tuffs or ashy beds. The lavas were some of them originally
compact, others vesicular, and these last have been converted into
amygdaloids. They consist chiefly of feldstone or compact feldspar. The
Pentland Hills, say Messrs. Maclaren and Geikie, afford evidence that
at the time of the Upper Old Red Sandstone, the district to the
south-west of Edinburgh was for a long while the seat of a powerful
volcano, which sent out massive streams of lava and showers of ash, and
continued active until well-nigh the dawn of the Carboniferous
period.[8]

Silurian Volcanic Rocks.—It appears from the investigations of Sir R.
Murchison in Shropshire, that when the Lower Silurian strata of that
country were accumulating, there were frequent volcanic eruptions
beneath the sea; and the ashes and scoriæ then ejected gave rise to a
peculiar kind of tufaceous sandstone or grit, dissimilar to the other
rocks of the Silurian series, and only observable in places where
syenitic and other trap-rocks protrude. These tuffs occur on the flanks
of the Wrekin and Caer Caradoc, and contain Silurian fossils, such as
casts of encrinites, trilobites, and mollusca. Although fossiliferous,
the stone resembles a sandy claystone of the trap family.[9]

Thin layers of trap, only a few inches thick, alternate in some parts
of Shropshire and Montgomeryshire with sedimentary strata of the Lower
Silurian system. This trap consists of slaty porphyry and granular
feldspar rock, the beds being traversed by joints like those in the
associated sandstone, limestone, and shale, and having the same strike
and dip.[10]

In Radnorshire there is an example of twelve bands of stratified trap,
alternating with Silurian schists and flagstones, in a thickness of 350
feet. The bedded traps consist of feldspar porphyry, and other
varieties; and the interposed Llandeilo flags are of sandstone and
shale, with trilobites and graptolites.[11]

The Snowdonian hills in Carnarvonshire consist in great part of
volcanic tuffs, the oldest of which are interstratified with the Bala
and Llandeilo beds. There are some contemporaneous feldspathic lavas of
this era, which, says Professor Ramsay, alter the slates on which they
repose, having doubtless been poured out over them, in a melted state,
whereas the slates which overlie them having been subsequently
deposited after the lava had cooled and consolidated, have entirely
escaped alteration. But there are greenstones associated with the same
formation, which, although they are often conformable to the slates,
are in reality intrusive rocks. They alter the stratified deposits both
above and below them, and when traced to great distances are sometimes
seen to cut through the slates, and to send off branches. Nevertheless,
these greenstones appear to belong, like the lavas, to the Lower
Silurian period.

Cambrian Volcanic Rocks.—The Lingula beds in North Wales have been
described as 5000 feet in thickness. In the upper portion of these
deposits volcanic tuffs or ashy materials are interstratified with
ordinary muddy sediment, and here and there associated with thick beds
of feldspathic lava. These rocks form the mountains called the Arans
and the Arenigs; numerous greenstones are associated with them, which
are intrusive, although they often run in the lines of bedding for a
space. “Much of the ash,” says Professor Ramsay, “seems to have been
subaërial. Islands, like Graham’s Island, may have sometimes raised
their craters for various periods above the water, and by the waste of
such islands some of the ashy matter became waterworn, whence the ashy
conglomerate. Viscous matter seems also to have been shot into the air
as volcanic bombs, which fell among the dust and broken crystals (that
often form the ashes) before perfect cooling and consolidation had
taken place.”[12]

Laurentian Volcanic Rocks.—The Laurentian rocks in Canada, especially
in Ottawa and Argenteuil, are the oldest intrusive masses yet known.
They form a set of dikes of a fine-grained dark greenstone or dolerite,
composed of feldspar and pyroxene, with occasional scales of mica and
grains of pyrites. Their width varies from a few feet to a hundred
yards, and they have a columnar structure, the columns being truly at
right angles to the plane of the dike. Some of the dikes send off
branches. These dolerites are cut through by intrusive syenite, and
this syenite, in its turn, is again cut and penetrated by feldspar
porphyry, the base of which consists of petrosilex, or a mixture of
orthoclase and quartz. All these trap-rocks appear to be of Laurentian
date, as the Cambrian and Huronian rocks rest unconformably upon
them.[13] Whether some of the various conformable crystalline rocks of
the Laurentian series, such as the coarse-grained granitoid and
porphyritic varieties of gneiss, exhibiting scarcely any signs of
stratification, and some of the serpentines, may not also be of
volcanic origin, is a point very difficult to determine in a region
which has undergone so much metamorphic action.

 [1] Linnean Proceedings; Zoology, 1860.

 [2] Hartung, Die Azoren, 1860; also Insel Gran Canaria, Madeira und
 Porto Santo, 1864, Leipsig.

 [3] Geognost. Beschreib. des Siebengebirges am Rhein. Bonn, 1852.

 [4] Scrope’s Central France, p. 98.

 [5] Principles of Geology, chap. xxvi, 9th ed., p. 432.

 [6] Boblaye and Virlet, Morea, p. 23.

 [7] De la Beche, Geol. Proceedings, vol. ii, p. 198.

 [8] Maclaren, Geology of Fife and Lothians. Geikie, Trans. Royal Soc.
 Edinburgh, 1860-1861.

 [9] Murchison, Silurian System, etc., p. 230.

 [10] Ibid., p. 212.

 [11] Murchison, Silurian System, etc., p. 325.

 [12] Quart. Geol. Journ., vol. ix, p. 170, 1852.

 [13] Logan, Geology of Canada, 1863.




CHAPTER XXXI.
PLUTONIC ROCKS.


General Aspect of Plutonic Rocks. — Granite and its Varieties. —
Decomposing into Spherical Masses. — Rude columnar Structure. — Graphic
Granite. — Mutual Penetration of Crystals of Quartz and Feldspar. —
Glass Cavities in Quartz of Granite. — Porphyritic, talcose, and
syenitic Granite. — Schorlrock and Eurite. — Syenite. — Connection of
the Granites and Syenites with the Volcanic Rocks. — Analogy in
Composition of Trachyte and Granite. — Granite Veins in Glen Tilt, Cape
of Good Hope, and Cornwall. — Metalliferous Veins in Strata near their
Junction with Granite. — Quartz Veins. — Exposure of Plutonic Rocks at
the surface due to Denudation.

The Plutonic rocks may be treated of next in order, as they are most
nearly allied to the volcanic class already considered. I have
described, in the first chapter, these Plutonic rocks as the
unstratified division of the crystalline or hypogene formations, and
have stated that they differ from the volcanic rocks, not only by their
more crystalline texture, but also by the absence of tuffs and
breccias, which are the products of eruptions at the earth’s surface,
whether thrown up into the air or the sea. They differ also by the
absence of pores or cellular cavities, to which the expansion of the
entangled gases gives rise in ordinary lava, never being scoriaceous or
amygdaloidal, and never forming a porphyry with an uncrystalline base,
nor alternating with tuffs.

From these and other peculiarities it has been inferred that the
granites have been formed at considerable depths in the earth, and have
cooled and crystallised slowly under great pressure, where the
contained gases could not expand. The volcanic rocks, on the contrary,
although they also have risen up from below, have cooled from a melted
state more rapidly upon or near the surface. From this hypothesis of
the great depth at which the granites originated, has been derived the
name of “Plutonic rocks.” The beginner will easily conceive that the
influence of subterranean heat may extend downward from the crater of
every active volcano to a great depth below, perhaps several miles or
leagues, and the effects which are produced deep in the bowels of the
earth may, or rather must, be distinct; so that volcanic and Plutonic
rocks, each different in texture, and sometimes even in composition,
may originate simultaneously, the one at the surface, the other far
beneath it. The Plutonic formations also agree with the volcanic in
having veins or ramifications proceeding from central masses into the
adjoining rocks, and causing alterations in these last, which will be
presently described. They also resemble trap in containing no organic
remains; but they differ in being more uniform in texture, whole
mountain masses of indefinite extent appearing to have originated under
conditions precisely similar.

The two principal members of the Plutonic family of rocks are Granite
and Syenite, each of which, with their varieties, bear very much the
same relation to each other as the trachytes bear to the basalts.
Granite is a compound of feldspar, quartz, and mica, the feldspars
being rich in silica, which forms from 60 to 70 per cent of the whole
aggregate. In Syenite quartz is rare or wanting, hornblende taking the
place of mica, and the proportion of silica not exceeding 50 to 60 per
cent.

Fig. 605: Mass of granite near the Sharp Tor, Cornwall.

Granite and its Varieties.—Granite often preserves a very uniform
character throughout a wide range of territory, forming hills of a
peculiar rounded form, usually clad with a scanty vegetation. The
surface of the rock is for the most part in a crumbling state, and the
hills are often surmounted by piles of stones like the remains of a
stratified mass, as in Figure 605, and sometimes like heaps of
boulders, for which they have been mistaken. The exterior of these
stones, originally quadrangular, acquires a rounded form by the action
of air and water, for the edges and angles waste away more rapidly than
the sides. A similar spherical structure has already been described as
characteristic of basalt and other volcanic formations, and it must be
referred to analogous causes, as yet but imperfectly understood.
Although it is the general peculiarity of granite to assume no definite
shapes, it is nevertheless occasionally subdivided by fissures, so as
to assume a cuboidal, and even a columnar, structure. Examples of these
appearances may be seen near the Land’s End, in Cornwall. (See Fig.
606.)

Feldspar, quartz, and mica are usually considered as the minerals
essential to granite, the feldspar being most abundant in quantity, and
the proportion of quartz exceeding that of mica. These minerals are
united in what is termed a confused crystallisation; that is to say,
there is no regular arrangement of the crystals in granite, as in
gneiss (see Fig. 622), except in the variety termed graphic granite,
which occurs mostly in granitic veins. This variety is a compound of
feldspar and quartz, so arranged as to produce an imperfect laminar
structure. The crystals of feldspar appear to have been first formed,
leaving between them the space now occupied by the darker-coloured
quartz. This mineral, when a section is made at right angles to the
alternate plates of feldspar and quartz, presents broken lines, which
have been compared to Hebrew characters. (See Fig. 608.) The variety of
granite called by the French _Pegmatite,_ which is a mixture of quartz
and common feldspar, usually with some small admixture of white silvery
mica, often passes into graphic granite.

Fig. 606: Granite having a cuboidal and rude columnar structure, Land’s
End, Cornwall.

Ordinary granite, as well as syenite and eurite, usually contains two
kinds of feldspar: First, the common, or orthoclase, in which potash is
the prevailing alkali, and this generally occurs in large crystals of a
white or flesh colour; and secondly, feldspar in smaller crystals, in
which soda predominates, usually of a dead white or spotted, and
striated like albite, but not the same in composition.[1]

Graphic granite. Fig. 607: Section parallel to the laminæ. Fig. 608:
Section transverse to the laminæ.

As a general rule, quartz, in a compact or amorphous state, forms a
vitreous mass, serving as the base in which feldspar and mica have
crystallised; for although these minerals are much more fusible than
silex, they have often imprinted their shapes upon the quartz. This
fact, apparently so paradoxical, has given rise to much ingenious
speculation. We should naturally have anticipated that, during the
cooling of the mass, the flinty portion would be the first to
consolidate; and that the different varieties of feldspar, as well as
garnets and tourmalines, being more easily liquefied by heat, would be
the last. Precisely the reverse has taken place in the passage of most
granite aggregates from a fluid to a solid state, crystals of the more
fusible minerals being found enveloped in hard, transparent, glassy
quartz, which has often taken very faithful casts of each, so as to
preserve even the microscopically minute striations on the surface of
prisms of tourmaline. Various explanations of this phenomenon have been
proposed by MM. de Beaumont, Fournet, and Durocher. They refer to M.
Gaudin’s experiments on the fusion of quartz, which show that silex, as
it cools, has the property of remaining in a viscous state, whereas
alumina never does. This “gelatinous flint” is supposed to retain a
considerable degree of plasticity long after the granitic mixture has
acquired a low temperature. Occasionally we find the quartz and
feldspar mutually imprinting their forms on each other, affording
evidence of the simultaneous crystallisation of both.[2]

According to the experiments and observations of Gustavus Rose, the
quartz of granite has the specific gravity of 2·6, which characterises
silica when it is precipitated from a liquid solvent, and not that
inferior density, namely, 2·3, which belongs to it when it cools in the
laboratory from a state of fusion in what is called the dry way. By
some it had been rashly inferred that the manner in which the
consolidation of granite takes place is exceedingly different from the
cooling of lavas, and that the intense heat supposed to be necessary
for the production of mountain masses of Plutonic rocks might be
dispensed with. But Mr. David Forbes informs me that silica can
crystallise in the dry way, and he has found in quartz forming a
constituent part of some trachytes, both from Guadeloupe and Iceland,
glass cavities quite similar to those met with in genuine volcanic
minerals.

These “glass cavities,” which with many other kindred phenomena have
been carefully studied by Mr. Sorby, are those in which a liquid, on
cooling, has become first viscous and then solid without crystallising
or undergoing a definite change in its physical structure. Other
cavities which, like those just mentioned, are frequently discernible
under the microscope in the minerals composing granitic rocks, are
filled, some of them with gas or vapour, others with liquid, and by the
movements of the bubbles thus included the distinctness of such
cavities from those filled with a glassy substance can be tested. Mr.
Sorby admits that the frequent occurrence of fluid cavities in the
quartz of granite implies that water was almost always present in the
formation of this rock; but the same may be said of almost all lavas,
and it is now more than forty years since Mr. Scrope insisted on the
important part which water plays in volcanic eruptions, being so
intimately mixed up with the materials of the lava that he supposed it
to aid in giving mobility to the fluid mass. It is well known that
steam escapes for months, sometimes for years, from the cavities of
lava when it is cooling and consolidating. As to the result of Mr.
Sorby’s experiments and speculations on this difficult subject, they
may be stated in a few words. He concludes that the physical conditions
under which the volcanic and granitic rocks originate are so far
similar that in both cases they combine igneous fusion, aqueous
solution, and gaseous sublimation—the proof, he says, of the operation
of water in the formation of granite being quite as strong as of that
of heat.[3]

When rocks are melted at great depths water must be present, for two
reasons—First, because rainwater and seawater are always descending
through fissured and porous rocks, and must at length find their way
into the regions of subterranean heat; and secondly, because in a state
of combination water enters largely into the composition of some of the
most common minerals, especially those of the aluminous class. But the
existence of water under great pressure affords no argument against our
attributing an excessively high temperature to the mass with which it
is mixed up. Bunsen, indeed, imagines that in Iceland water attains a
white heat at a very moderate depth. To what extent some of the
metamorphic rocks containing the same minerals as the granites may have
been formed by hydrothermal action without the intervention of intense
heat comparable to that brought into play in a volcanic eruption, will
be considered when we treat of the metamorphic rocks in the
thirty-third chapter.

Fig. 609: Porphyritic granite. Land’s End, Cornwall.

_Porphyritic Granite._—This name has been sometimes given to that
variety in which large crystals of common feldspar, sometimes more than
three inches in length, are scattered through an ordinary base of
granite. An example of this texture may be seen in the granite of the
Land’s End, in Cornwall (Fig. 609). The two larger prismatic crystals
in this drawing represent feldspar, smaller crystals of which are also
seen, similar in form, scattered through the base. In this base also
appear black specks of mica, the crystals of which have a more or less
perfect hexagonal outline. The remainder of the mass is quartz, the
translucency of which is strongly contrasted to the opaqueness of the
white feldspar and black mica. But neither the transparency of the
quartz nor the silvery lustre of the mica can be expressed in the
engraving.

The uniform mineral character of large masses of granite seems to
indicate that large quantities of the component elements were
thoroughly mixed up together, and then crystallised under precisely
similar conditions. There are, however, many accidental, or
“occasional,” minerals, as they are termed, which belong to granite.
Among these black schorl or tourmaline, actinolite, zircon, garnet, and
fluor spar are not uncommon; but they are too sparingly dispersed to
modify the general aspect of the rock. They show, nevertheless, that
the ingredients were not everywhere exactly the same; and a still
greater difference may be traced in the ever-varying proportions of the
feldspar, quartz, and mica.

_Talcose Granite,_ or Protogine of the French, is a mixture of
feldspar, quartz, and talc. It abounds in the Alps, and in some parts
of Cornwall, producing by its decomposition the kaolin or china clay,
more than 12,000 tons of which are annually exported from that country
for the potteries.

_Schorl-rock, and Schorly Granite._—The former of these is an aggregate
of schorl, or tourmaline, and quartz. When feldspar and mica are also
present, it may be called schorly granite. This kind of granite is
comparatively rare.

_Eurite, Feldstone._—Eurite is a rock in which the ingredients of
granite are blended into a finely granular mass, mica being usually
absent, and, when present, in such minute flakes as to be invisible to
the naked eye. It is sometimes called _Feldstone,_ and when the
crystals of feldspar are conspicuous it becomes _Feldspar porphyry._
All these and other varieties of granite pass into certain kinds of
trap—a circumstance which affords one of many arguments in favour of
what is now the prevailing opinion, that the granites are also of
igneous origin. The contrast of the most crystalline form of granite to
that of the most common and earthy trap is undoubtedly great; but each
member of the volcanic class is capable of becoming porphyritic, and
the base of the porphyry may be more and more crystalline, until the
mass passes to the kind of granite most nearly allied in mineral
composition.

_Syenitic Granite._—The quadruple compound of quartz, feldspar, mica,
and hornblende, may be termed Syenitic Granite, and forms a passage
between the granites and the syenites. This rock occurs in Scotland and
in Guernsey.

Syenite.—We now come to the second division of the Plutonic rocks, or
those having less than 60 per cent of silica, and which, as before
stated (p. 552), are usually called syenitic. Syenite originally
received its name from the celebrated ancient quarries of Syene, in
Egypt. It differs from granite in having hornblende as a substitute for
mica, and being without quartz. Werner at least considered syenite as a
binary compound of feldspar and hornblende, and regarded quartz as
merely one of its occasional minerals.

_Miascite._—Miascite is one of the varieties of syenite most frequently
spoken of; it is composed chiefly of orthoclase and nepheline, with
hornblende and quartz as occasional accessory minerals. It derives its
name from Miask, in the Ural Mountains, where it was first discovered
by Gustavus Rose. _Zircon-syenite_ is another variety closely allied to
Miascite, but containing crystals of Zircon.

Connection of the Granites and Syenites with the Volcanic Rocks.—The
minerals which constitute alike the Plutonic and volcanic rocks
consist, almost exclusively, of seven elements, namely, silica,
alumina, magnesia, lime, soda, potash, and iron (see Table p. 449); and
these may sometimes exist in about the same proportions in a porous
lava, a compact trap, and a crystalline granite. The same lava, for
example, may be glassy, or scoriaceous, or stony, or porphyritic,
according to the more or less rapid rate at which it cools.

It would be easy to multiply examples and authorities to prove the
gradation of the Plutonic into the trap rocks. On the western side of
the Fiord of Christiania, in Norway, there is a large district of trap,
chiefly greenstone-porphyry and syenitic-greenstone, resting on
fossiliferous strata. To this, on its southern limit, succeeds a region
equally extensive of syenite, the passage from the trappean to the
crystalline Plutonic rock being so gradual that it is impossible to
draw a line of demarkation between them.

“The ordinary granite of Aberdeenshire,” says Dr. MacCulloch, “is the
usual ternary compound of quartz, feldspar, and mica; though sometimes
hornblende is substituted for the mica. But in many places a variety
occurs which is composed simply of feldspar and hornblende; and in
examining more minutely this duplicate compound, it is observed in some
places to assume a fine grain, and at length to become
undistinguishable from the greenstones of the trap family. It also
passes in the same uninterrupted manner into a basalt, and at length
into a soft claystone, with a schistose tendency on exposure, in no
respect differing from those of the trap islands of the western coast.”
The same author mentions, that in Shetland a granite composed of
hornblende, mica, feldspar, and quartz graduates in an equally perfect
manner into basalt.[4] In Hungary there are varieties of trachyte,
which, geologically speaking, are of modern origin, in which crystals,
not only of mica, but of quartz, are common, together with feldspar and
hornblende. It is easy to conceive how such volcanic masses may, at a
certain depth from the surface, pass downward into granite.

Granitic Veins.—I have already hinted at the close analogy in the forms
of certain granitic and trappean veins; and it will be found that
strata penetrated by Plutonic rocks have suffered changes very similar
to those exhibited near the contact of volcanic dikes. Thus, in Glen
Tilt, in Scotland, alternating strata of limestone and argillaceous
schist come in contact with a mass of granite. The contact does not
take place as might have been looked for if the granite had been formed
there before the strata were deposited, in which case the section would
have appeared as in Fig. 610; but the union is as represented in Fig.
611, the undulating outline of the granite intersecting different
strata, and occasionally intruding itself in torturous veins into the
beds of clay-slate and limestone, from which it differs so remarkably
in composition. The limestone is sometimes changed in character by the
proximity of the granitic mass or its veins, and acquires a more
compact texture, like that of hornstone or chert, with a splintery
fracture, and effervescing freely with acids.

Fig. 610 and Fig. 611: Junction of granite and arbillaceous schist in
Glen Tilt. (MacCulloch.) Fig. 610 and Fig. 611: Junction of granite and
arbillaceous schist in Glen Tilt. (MacCulloch.)[5]

The conversion of the limestone and these and many other instances into
a siliceous rock, effervescing slowly with acids, would be difficult of
explanation, were it not ascertained that such limestones are always
impure, containing grains of quartz, mica, or feldspar disseminated
through them. The elements of these minerals, when the rock has been
subjected to great heat, may have been fused, and so spread more
uniformly through the whole mass.

In the Plutonic, as in the volcanic rocks, there is every gradation
from a tortuous vein to the most regular form of a dike, such as
intersect the tuffs and lavas of Vesuvius and Etna. Dikes of granite
may be seen, among other places, on the southern flank of Mount
Battock, one of the Grampians, the opposite walls sometimes preserving
an exact parallelism for a considerable distance. As a general rule,
however, granite veins in all quarters of the globe are more sinuous in
their course than those of trap. They present similar shapes at the
most northern point of Scotland, and the southernmost extremity of
Africa, as Figs. 612 and 613 will show.

Fig. 612: Granite veins traversing clay slate, Table Mountain, Cape of
Good Hope. Fig. 612: Granite veins traversing clay slate, Table
Mountain, Cape of Good Hope.[6]

Fig. 613: Granite veins traversing gneiss, Cape Wrath. Fig. 613:
Granite veins traversing gneiss, Cape Wrath.[7]

It is not uncommon for one set of granite veins to intersect another;
and sometimes there are three sets, as in the environs of Heidelberg,
where the granite on the banks of the river Necker is seen to consist
of three varieties, differing in colour, grain, and various
peculiarities of mineral composition. One of these, which is evidently
the second in age, is seen to cut through an older granite; and
another, still newer, traverses both the second and the first. In
Shetland there are two kinds of granite. One of them, composed of
hornblende, mica, feldspar, and quartz, is of a dark colour, and is
seen underlying gneiss. The other is a red granite, which penetrates
the dark variety everywhere in veins.[8]

Fig. 614 is a sketch of a group of granite veins in Cornwall, given by
Messrs. Von Oeynhausen and Von Dechen.[9] The main body of the granite
here is of a porphyritic appearance, with large crystals of feldspar;
but in the veins it is fine-grained, and without these large crystals.
The general height of the veins is from 16 to 20 feet, but some are
much higher.

Fig. 614: Granite veins passing through hornblende slate, Carnsilver
Cove, Cornwall.

Granite, syenite, and those porphyries which have a granitiform
structure, in short all Plutonic rocks, are frequently observed to
contain metals, at or near their junction with stratified formations.
On the other hand, the veins which traverse stratified rocks are, as a
general law, more metalliferous near such junctions than in other
positions. Hence it has been inferred that these metals may have been
spread in a gaseous form through the fused mass, and that the contact
of another rock, in a different state of temperature, or sometimes the
existence of rents in other rocks in the vicinity, may have caused the
sublimation of the metals.[10]

Fig. 615: a, b. Quartz vein passing through gneiss and greenstone.
Tronstad Strand, near Christiania.

Veins of pure quartz are often found in granite as in many stratified
rocks, but they are not traceable, like veins of granite or trap, to
large bodies of rock of similar composition. They appear to have been
cracks, into which siliceous matter was infiltered. Such segregation,
as it is called, can sometimes clearly be shown to have taken place
long subsequently to the original consolidation of the containing rock.
Thus, for example, I observed in the gneiss of Tronstad Strand, near
Drammen, in Norway, the section on the beach shown in Figure 615. It
appears that the alternating strata of whitish granitiform gneiss and
black hornblende-schist were first cut by a greenstone dike, about 2½
feet wide; then the crack _a, b,_ passed through all these rocks, and
was filled up with quartz. The opposite walls of the vein are in some
parts incrusted with transparent crystals of quartz, the middle of the
vein being filled up with common opaque white quartz.

Fig. 616: Euritic porphyry alternating with primary fossiliferous
strata, near Christiania.

We have seen that the volcanic formations have been called overlying,
because they not only penetrate others but spread over them. M. Necker
has proposed to call the granites the underlying igneous rocks, and the
distinction here indicated is highly characteristic. It was, indeed,
supposed by some of the earlier observers that the granite of
Christiania, in Norway, was intercalated in mountain masses between the
primary or palæozoic strata of that country, so as to overlie
fossiliferous shale and limestone. But although the granite sends veins
into these fossiliferous rocks, and is decidedly posterior in origin,
its actual superposition in mass has been disproved by Professor
Keilhau, whose observations on this controverted point I had
opportunities, in 1837, of verifying. There are, however, on a smaller
scale, certain beds of euritic porphyry, some a few feet, others many
yards in thickness, which pass into granite, and deserve, perhaps, to
be classed as Plutonic rather than trappean rocks, which may truly be
described as interposed conformably between fossiliferous strata, as
the porphyries (_a, c,_ Fig. 616) which divide the bituminous shales
and argillaceous limestones, _f, f._ But some of these same porphyries
are partially unconformable, as _b,_ and may lead us to suspect that
the others also, notwithstanding their appearance of
interstratification, have been forcibly injected. Some of the
porphyritic rocks above mentioned are highly quartzose, others very
feldspathic. In proportion as the masses are more voluminous, they
become more granitic in their texture, less conformable, and even begin
to send forth veins into contiguous strata. In a word, we have here a
beautiful illustration of the intermediate gradations between volcanic
and Plutonic rocks, not only in their mineralogical composition and
structure, but also in their relations of position to associated
formations. If the term “overlying” can in this instance be applied to
a Plutonic rock, it is only in proportion as that rock begins to
acquire a trappean aspect.

It has been already hinted that the heat which in every active volcano
extends downward to indefinite depths must produce simultaneously very
different effects near the surface and far below it; and we cannot
suppose that rocks resulting from the crystallising of fused matter
under a pressure of several thousand feet, much less several miles, of
the earth’s crust can exactly resemble those formed at or near the
surface. Hence the production at great depths of a class of rocks
analogous to the volcanic, and yet differing in many particulars, might
have been predicted, even had we no Plutonic formations to account for.
How well these agree, both in their positive and negative characters,
with the theory of their deep subterranean origin, the student will be
able to judge by considering the descriptions already given.

It has, however, been objected, that if the granitic and volcanic rocks
were simply different parts of one great series, we ought to find in
mountain chains volcanic dikes passing upward into lava and downward
into granite. But we may answer that our vertical sections are usually
of small extent; and if we find in certain places a transition from
trap to porous lava, and in others a passage from granite to trap, it
is as much as could be expected of this evidence.

The prodigious extent of denudation which has been already demonstrated
to have occurred at former periods, will reconcile the student to the
belief that crystalline rocks of high antiquity, although deep in the
earth’s crust when originally formed, may have become uncovered and
exposed at the surface. Their actual elevation above the sea may be
referred to the same causes to which we have attributed the upheaval of
marine strata, even to the summits of some mountain chains.

 [1] Delesse, Ann. des Mines, 1852, tome iii, p. 409, and 1848, tome
 xiii, p. 675.

 [2] Bulletin, 2e série, iv, 1304; and D’Archiac, Hist. des Progrès de
 la Géol., i, 38.

 [3] See Quart. Geol. Journ., vol. xiv, pp. 465, 488.

 [4] Syst. of Geol., vol. i, pp. 157 and 158.

 [5] Geol. Trans., First Series, vol. iii, pl. 21.

 [6] Captain B. Hall, Trans. Roy. Soc. Edinburgh, vol. vii.

 [7] Western Islands, pl. 31.

 [8] MacCulloch, Syst. of Geol., vol. ii, p. 58.

 [9] Phil. Mag. and Annals, No. 27, New Series, March, 1829.

 [10] Necker, Proceedings of the Geol. Soc., No. 26, p. 392.




CHAPTER XXXII.
ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.


Difficulty in ascertaining the precise Age of a Plutonic Rock. — Test
of Age by Relative Position. — Test by Intrusion and Alteration. — Test
by Mineral Composition. — Test by included Fragments. — Recent and
Pliocene Plutonic Rocks, why invisible. — Miocene Syenite of the Isle
of Skye. — Eocene Plutonic Rocks in the Andes. — Granite altering
Cretaceous Rocks. — Granite altering Lias in the Alps and in Skye. —
Granite of Dartmoor altering Carboniferous Strata. — Granite of the Old
Red Sandstone Period. — Syenite altering Silurian Strata in Norway. —
Blending of the same with Gneiss. — Most ancient Plutonic Rocks. —
Granite protruded in a solid Form.

When we adopt the igneous theory of granite, as explained in the last
chapter, and believe that different Plutonic rocks have originated at
successive periods beneath the surface of the planet, we must be
prepared to encounter greater difficulty in ascertaining the precise
age of such rocks than in the case of volcanic and fossiliferous
formations. We must bear in mind that the evidence of the age of each
contemporaneous volcanic rock was derived either from lavas poured out
upon the ancient surface, whether in the sea or in the atmosphere, or
from tuffs and conglomerates, also deposited at the surface, and either
containing organic remains themselves or intercalated between strata
containing fossils. But the same tests entirely fail, or are only
applicable in a modified degree, when we endeavour to fix the
chronology of a rock which has crystallised from a state of fusion in
the bowels of the earth. In that case we are reduced to the tests of
relative position, intrusion, alteration of the rocks in contact,
included fragments, and mineral character; but all these may yield at
best a somewhat ambiguous result.

Test of Age by Relative Position.—Unaltered fossiliferous strata of
every age are met with reposing immediately on Plutonic rocks; as at
Christiania, in Norway, where the Post-pliocene deposits rest on
granite; in Auvergne, where the fresh-water Miocene strata, and at
Heidelberg, on the Rhine, where the New Red sandstone occupy a similar
place. In all these, and similar instances, inferiority in position is
connected with the superior antiquity of granite. The crystalline rock
was solid before the sedimentary beds were superimposed, and the latter
usually contain in them rounded pebbles of the subjacent granite.

Test by Intrusion and Alteration.—But when Plutonic rocks send veins
into strata, and alter them near the point of contact, in the manner
before described (p. 559), it is clear that, like intrusive traps, they
are newer than the strata which they invade and alter. Examples of the
application of this test will be given in the sequel.

Test by Mineral Composition.—Notwithstanding a general uniformity in
the aspect of Plutonic rocks, we have seen in the last chapter that
there are many varieties, such as syenite, talcose granite, and others.
One of these varieties is sometimes found exclusively prevailing
throughout an extensive region, where it preserves a homogeneous
character; so that, having ascertained its relative age in one place,
we can recognise its identity in others, and thus determine from a
single section the chronological relations of large mountain masses.
Having observed, for example, that the syenitic granite of Norway, in
which the mineral called zircon abounds, has altered the Silurian
strata wherever it is in contact, we do not hesitate to refer other
masses of the same zircon-syenite in the south of Norway to a
post-Silurian date. Some have imagined that the age of different
granites might, to a great extent, be determined by their mineral
characters alone; syenite, for instance, or granite with hornblende,
being more modern than common or micaceous granite. But modern
investigations have proved these generalisations to have been
premature.

Test by Included Fragments.—This criterion can rarely be of much
importance, because the fragments involved in granite are usually so
much altered that they cannot be referred with certainty to the rocks
whence they were derived. In the White Mountains, in North America,
according to Professor Hubbard, a granite vein, traversing granite,
contains fragments of slate and trap which must have fallen into the
fissure when the fused materials of the vein were injected from
below,[1] and thus the granite is shown to be newer than those slaty
and trappean formations from which the fragments were derived.

Recent and Pliocene Plutonic Rocks, why invisible.—The explanations
already given in the 28th and in the last chapter of the probable
relation of the Plutonic to the volcanic formations, will naturally
lead the reader to infer that rocks of the one class can never be
produced at or near the surface without some members of the other being
formed below. It is not uncommon for lava-streams to require more than
ten years to cool in the open air; and where they are of great

depth, a much longer period. The melted matter poured from Jorullo, in
Mexico, in the year 1759, which accumulated in some places to the
height of 550 feet, was found to retain a high temperature half a
century after the eruption.[2] We may conceive, therefore, that great
masses of subterranean lava may remain in a red-hot or incandescent
state in the volcanic foci for immense periods, and the process of
refrigeration may be extremely gradual. Sometimes, indeed, this process
may be retarded for an indefinite period by the accession of fresh
supplies of heat; for we find that the lava in the crater of Stromboli,
one of the Lipari Islands, has been in a state of constant ebullition
for the last two thousand years; and we may suppose this fluid mass to
communicate with some caldron or reservoir of fused matter below. In
the Isle of Bourbon, also, where there has been an emission of lava
once in every two years for a long period, the lava below can scarcely
fail to have been permanently in a state of liquefaction. If then it be
a reasonable conjecture, that about 2000 volcanic eruptions occur in
the course of every century, either above the waters of the sea or
beneath them,[3] it will follow that the quantity of Plutonic rock
generated or in progress during the Recent epoch must already have been
considerable.

But as the Plutonic rocks originate at some depth in the earth’s crust,
they can only be rendered accessible to human observation by subsequent
upheaval and denudation. Between the period when a Plutonic rock
crystallises in the subterranean regions and the era of its protrusion
at any single point of the surface, one or two geological periods must
usually intervene. Hence, we must not expect to find the Recent or even
the Pliocene granites laid open to view, unless we are prepared to
assume that sufficient time has elapsed since the commencement of the
Pliocene period for great upheaval and denudation. A Plutonic rock,
therefore, must, in general, be of considerable antiquity relatively to
the fossiliferous and volcanic formations, before it becomes
extensively visible. As we know that the upheaval of land has been
sometimes accompanied in South America by volcanic eruptions and the
emission of lava, we may conceive the more ancient Plutonic rocks to be
forced upward to the surface by the newer rocks of the same class
formed successively below—subterposition in the Plutonic, like
superposition in the sedimentary rocks, being usually characteristic of
a newer origin.

Fig. 617: Diagram showing the relative position which the Plutonic and
sedimentary formations of different ages may occupy.

In Fig. 617 an attempt is made to show the inverted order in which
sedimentary and Plutonic formations may occur in the earth’s crust. The
oldest Plutonic rock, No. I, has been upheaved at successive periods
until it has become exposed to view in a mountain-chain. This
protrusion of No. I has been caused by the igneous agency which
produced the newer Plutonic rocks Nos. II, III and IV. Part of the
primary fossiliferous strata, No. I, have also been raised to the
surface by the same gradual process. It will be observed that the
Recent _strata_ No. 4 and the Recent _ granite_ or Plutonic rock No. IV
are the most remote from each other in position, although of
contemporaneous date. According to this hypothesis, the convulsions of
many periods will be required before Recent or Post-tertiary granite
will be upraised so as to form the highest ridges and central axes of
mountain-chains. During that time the _recent_ strata No. 4 might be
covered by a great many newer sedimentary formations.

Miocene Plutonic Rocks.—A considerable mass of syenite, in the Isle of
Skye, is described by Dr. MacCulloch as intersecting limestone and
shale, which are of the age of the lias. The limestone, which at a
greater distance from the granite contains shells, exhibits no traces
of them near its junction, where it has been converted into a pure
crystalline marble.[4] MacCulloch pointed out that the syenite here, as
in Raasay, was newer than the secondary rocks, and Mr. Geikie has since
shown that there is a strong probability that this Plutonic rock may be
of Miocene age, because a similar Syenite having a true granitic
character in its crystallisation has modified the Tertiary volcanic
rocks of Ben More, in Mull, some of which have undergone considerable
metamorphism.

Eocene Plutonic Rocks.—In a former part of this volume (Chapter 16),
the great nummulitic formation of the Alps and Pyrenees was referred to
the Eocene period, and it follows that vast movements which have raised
those fossiliferous rocks from the level of the sea to the height of
more than 10,000 feet above its level have taken place since the
commencement of the Tertiary epoch. Here, therefore, if anywhere, we
might expect to find hypogene formations of Eocene date breaking out in
the central axis or most disturbed region of the loftiest chain in
Europe. Accordingly, in the Swiss Alps, even the _flysch,_ or upper
portion of the nummulitic series, has been occasionally invaded by
Plutonic rocks, and converted into crystalline schists of the hypogene
class. There can be little doubt that even the talcose granite or
gneiss of Mont Blanc itself has been in a fused or pasty state since
the _flysch_ was deposited at the bottom of the sea; and the question
as to its age is not so much whether it be a secondary or tertiary
granite or gneiss, as whether it should be assigned to the Eocene or
Miocene epoch.

Great upheaving movements have been experienced in the region of the
Andes, during the Post-tertiary period. In some part, therefore, of
this chain, we may expect to discover tertiary Plutonic rocks laid open
to view; and Mr. Darwin’s account of the Chilian Andes, to which the
reader may refer, fully realises this expectation: for he shows that we
have strong ground to presume that Plutonic rocks there exposed on a
large scale are of later date than certain Secondary and Tertiary
formations.

But the theory adopted in this work of the subterranean origin of the
hypogene formations would be untenable, if the supposed fact here
alluded to, of the appearance of tertiary granite at the surface, was
not a rare exception to the general rule. A considerable lapse of time
must intervene between the formation of Plutonic and metamorphic rocks
in the nether regions and their emergence at the surface. For a long
series of subterranean movements must occur before such rocks can be
uplifted into the atmosphere or the ocean; and, before they can be
rendered visible to man, some strata which previously covered them must
have been stripped off by denudation.

We know that in the Bay of Baiæ in 1538, in Cutch in 1819, and on
several occasions in Peru and Chili, since the commencement of the
present century, the permanent upheaval or subsidence of land has been
accompanied by the simultaneous emission of lava at one or more points
in the same volcanic region. From these and other examples it may be
inferred that the rising or sinking of the earth’s crust, operations by
which sea is converted into land, and land into sea, are a part only of
the consequences of subterranean igneous action. It can scarcely be
doubted that this action consists, in a great degree, of the baking,
and occasionally the liquefaction, of rocks, causing them to assume, in
some cases a larger, in others a smaller volume than before the
application of heat. It consists also in the generation of gases, and
their expansion by heat, and the injection of liquid matter into rents
formed in superincumbent rocks. The prodigious scale on which these
subterranean causes have operated in Sicily since the deposition of the
Newer Pliocene strata will be appreciated when we remember that
throughout half the surface of that island such strata are met with,
raised to the height of from 50 to that of 2000 and even 3000 feet
above the level of the sea. In the same island also the older rocks
which are contiguous to these marine tertiary strata must have
undergone, within the same period, a similar amount of upheaval.

The like observations may be extended to nearly the whole of Europe,
for, since the commencement of the Eocene Period, the entire European
area, including some of the central and very lofty portions of the Alps
themselves, as I have elsewhere shown,[5] has, with the exception of a
few districts, emerged from the deep to its present altitude. There
must, therefore, have been at great depths in the earth’s crust, within
the same period, an amount of subterranean change corresponding to this
vast alteration of level affecting a whole continent.

The principal effect of subterranean movements during the Tertiary
Period seems to have consisted in the upheaval of hypogene formations
of an age anterior to the Carboniferous. The repetition of another
series of movements, of equal violence, might upraise the Plutonic and
metamorphic rocks of many secondary periods; and, if the same force
should still continue to act, the next convulsions might bring up to
the day the _tertiary_ and _ recent_ hypogene rocks. In the course of
such changes many of the existing sedimentary strata would suffer
greatly by denudation, others might assume a metamorphic structure, or
become melted down into Plutonic and volcanic rocks. Meanwhile the
deposition of a great thickness of new strata would not fail to take
place during the upheaval and partial destruction of the older rocks.
But I must refer the reader to the last chapter but one of this volume
for a fuller explanation of these views.

Fig. 618: Section through three layers (b, c, d) of the Cretaceous
series over granite (A).
Plutonic Rocks of Cretaceous Period.—It will be shown in the next
chapter that chalk, as well as lias, has been altered by granite in the
eastern Pyrenees. Whether such granite be cretaceous or tertiary,
cannot easily be decided. Suppose _b, c, d,_ Fig. 618, to be three
members of the Cretaceous series, the lowest of which, _b,_ has been
altered by the granite A, the modifying influence not having extended
so far as _c,_ or having but slightly affected its lowest beds. Now it
can rarely be possible for the geologist to decide whether the beds _d_
existed at the time of the intrusion of A, and alteration of _ b_ and
_c,_ or whether they were subsequently thrown down upon _c._ But as
some Cretaceous and even Tertiary rocks have been raised to the height
of more than 9000 feet in the Pyrenees, we must not assume that
plutonic formations of the same periods may not have been brought up
and exposed by denudation, at the height of 2000 or 3000 feet on the
flanks of that chain.

Fig. 619: Junction of granite with Jurassic or Oolite strata in the
Alps, near Champoleon.

Plutonic Rocks of the Oolite and Lias.—In the Department of the Hautes
Alpes, in France, M. Élie de Beaumont traced a black argillaceous
limestone, charged with belemnites, to within a few yards of a mass of
granite. Here the limestone begins to put on a granular texture, but is
extremely fine-grained. When nearer the junction it becomes grey, and
has a saccharoid structure. In another locality, near Champoleon, a
granite composed of quartz, black mica, and rose-coloured feldspar is
observed partly to overlie the secondary rocks, producing an alteration
which extends for about 30 feet downward, diminishing in the beds which
lie farthest from the granite. (See Fig. 619.) In the altered mass the
argillaceous beds are hardened, the limestone is saccharoid, the grits
quartzose, and in the midst of them is a thin layer of an imperfect
granite. It is also an important circumstance that near the point of
contact, both the granite and the secondary rocks become metalliferous,
and contain nests and small veins of blende, galena, iron, and copper
pyrites. The stratified rocks become harder and more crystalline, but
the granite, on the contrary, softer and less perfectly crystallised
near the junction.[6] Although the granite is incumbent in the section
(Fig. 619), we cannot assume that it overflowed the strata, for the
disturbances of the rocks are so great in this part of the Alps that
their original position is often inverted.

At Predazzo, in the Tyrol, secondary strata, some of which are
limestones of the Oolitic period, have been traversed and altered by
Plutonic rocks, one portion of which is an augitic porphyry, which
passes insensibly into granite. The limestone is changed into granular
marble, with a band of serpentine at the junction.[7]

Plutonic Rocks of Carboniferous Period.—The granite of Dartmoor, in
Devonshire, was formerly supposed to be one of the most ancient of the
Plutonic rocks, but is now ascertained to be posterior in date to the
culm-measures of that county, which from their position, and, as
containing true coal-plants, are now known to be members of the true
Carboniferous series. This granite, like the syenitic granite of
Christiania, has broken through the stratified formations, on the
north-west side of Dartmoor, the successive members of the
culm-measures abutting against the granite, and becoming metamorphic as
they approach. These strata are also penetrated by granite veins, and
Plutonic dikes, called “elvans.”[8] The granite of Cornwall is probably
of the same date, and, therefore, as modern as the Carboniferous
strata, if not newer.

Fig. 620: Section through Silurian strata and Granite.

Plutonic Rocks of Silurian Period.—It has long been known that a very
ancient granite near Christiania, in Norway, is posterior in date to
the Lower Silurian strata of that region, although its exact position
in the Palæozoic series cannot be defined. Von Buch first announced, in
1813, that it was of newer origin than certain limestones containing
orthocerata and trilobites. The proofs consist in the penetration of
granite veins into the shale and limestone, and the alteration of the
strata, for a considerable distance from the point of contact, both of
these veins and the central mass from which they emanate. (See p. 562)
Von Buch supposed that the Plutonic rock alternated with the
fossiliferous strata, and that large masses of granite were sometimes
incumbent upon the strata; but this idea was erroneous, and arose from
the fact that the beds of shale and limestone often dip towards the
granite up to the point of contact, appearing as if they would pass
under it in mass, as at _a,_ Fig. 620, and then again on the opposite
side of the same mountain, as at _b,_ dip away from the same granite.
When the junctions, however, are carefully examined, it is found that
the Plutonic rock intrudes itself in veins, and nowhere covers the
fossiliferous strata in large overlying masses, as is so commonly the
case with trappean formations.[9]

Now this granite, which is more modern than the Silurian strata of
Norway, also sends veins in the same country into an ancient formation
of gneiss; and the relations of the Plutonic rock and the gneiss, at
their junction, are full of interest when we duly consider the wide
difference of epoch which must have separated their origin.

Fig. 621: Granite sending veins into Silurian strata and gneiss.
Christiania, Norway.

The length of this interval of time is attested by the following facts:
The fossiliferous, or Silurian, beds rest unconformably upon the
truncated edges of the gneiss, the inclined strata of which had been
denuded before the sedimentary beds were superimposed (see Figure 621).
The signs of denudation are twofold; first, the surface of the gneiss
is seen occasionally, on the removal of the newer beds containing
organic remains, to be worn and smoothed; secondly, pebbles of gneiss
have been found in some of these Silurian strata. Between the origin,
therefore, of the gneiss and the granite there intervened, first, the
period when the strata of gneiss were denuded; secondly, the period of
the deposition of the Silurian deposits upon the denuded and inclined
gneiss, a. Yet the granite produced after this long interval is often
so intimately blended with the ancient gneiss, at the point of
junction, that it is impossible to draw any other than an arbitrary
line of separation between them; and where this is not the case,
tortuous veins of granite pass freely through gneiss, ending sometimes
in threads, as if the older rock had offered no resistance to their
passage. These appearances may probably be due to hydrothermal action
(see p. 584). I shall merely observe in this place that had such
junctions alone been visible, and had we not learnt, from other
sections, how long a period elapsed between the consolidation of the
gneiss and the injection of this granite, we might have suspected that
the gneiss was scarcely solidified, or had not yet assumed its complete
metamorphic character when invaded by the Plutonic rock. From this
example we may learn how impossible it is to conjecture whether certain
granites in Scotland, and other countries, which send veins into gneiss
and other metamorphic rocks, are primary, or whether they may not
belong to some secondary or tertiary period.

Oldest Granites.—It is not half a century since the doctrine was very
general that all granitic rocks were _ primitive,_ that is to say, that
they originated before the deposition of the first sedimentary strata,
and before the creation of organic beings (see p. 34). But so greatly
are our views now changed, that we find it no easy task to point out a
single mass of granite demonstrably more ancient than known
fossiliferous deposits. Could we discover some Laurentian strata
resting immediately on granite, there being no alterations at the point
of contact, nor any intersecting granitic veins, we might then affirm
the Plutonic rock to have originated before the oldest known
fossiliferous strata. Still it would be presumptuous, as we have
already pointed out (p. 464), to suppose that when a small part only of
the globe has been investigated, we are acquainted with the oldest
fossiliferous strata in the crust of our planet. Even when these are
found, we cannot assume that there never were any antecedent strata
containing organic remains, which may have become metamorphic. If we
find pebbles of granite in a conglomerate of the Lower Laurentian
system, we may then feel assured that the parent granite was formed
before the Laurentian formation. But if the incumbent strata be merely
Cambrian or Silurian, the fundamental granite, although of high
antiquity, may be posterior in date to _known_ fossiliferous
formations.

Protrusion of Solid Granite.—In part of Sutherlandshire, near Brora,
common granite, composed of feldspar, quartz, and mica is in immediate
contact with Oolitic strata, and has clearly been elevated to the
surface at a period subsequent to the deposition of those strata.[10]
Professor Sedgwick and Sir R. Murchison conceive that this granite has
been upheaved in a solid form; and that in breaking through the
submarine deposits, with which it was not perhaps originally in
contact, it has fractured them so as to form a breccia along the line
of junction. This breccia consists of fragments of shale, sandstone,
and limestone, with fossils of the oolite, all united together by a
calcareous cement. The secondary strata at some distance from the
granite are but slightly disturbed, but in proportion to their
proximity the amount of dislocation becomes greater.

Mr. T. McKenney Hughes has suggested to me in explanation of these
phenomena that they may be the effect of the association of more pliant
strata with hard unyielding rocks, the whole of which were subjected
simultaneously to great movements, whether of elevation or subsidence,
and of lateral pressure, during which the more solid granite, being
incapable of compression, was forced through the softer beds of shale,
sandstone, and limestone. He remarks that similar breccias with
slickensides are observed on a minor scale where rocks of different
composition and rigidity are contorted together. Such protrusion may
have been brought about by degrees by innumerable shocks of earthquakes
repeated after long intervals of time along the same tract of country.
The opening of new fissures in the hardest rocks is a frequent
accompaniment of such convulsions, and during the consequent
vibrations, breccias must often be caused. But these catastrophes, as
we well know, do not imply that the land or sea of the disturbed region
are rendered uninhabitable by living beings, and by no means indicate a
state of things different from that witnessed in the ordinary course of
nature.

 [1] Silliman’s Journ., No. 69, p. 123.

 [2] See “Principles,” _Index,_ “Jorullo.”

 [3] Ibid., “Volcanic Eruptions.”

 [4] “Western Islands,” vol. i, p. 330.

 [5] See map of Europe, and explanation, in Principles, book i.

 [6] Élie de Beaumont sur les Montagnes de l’Oisans, etc. Mém. de la
 Soc. d’Hist. Nat. de Paris, tome v.

 [7] Von Buch, Annales de Chimie, etc.

 [8] Proceed. Geol. Soc., vol. ii, p. 562; and Trans., 2nd series, vol.
 v, p. 686.

 [9] See the Gæa Norvegica and other works of Keilhau, with whom I
 examined this country.

 [10] Murchison, Geol. Trans., 2nd series, vol. ii, p. 307.




CHAPTER XXXIII.
METAMORPHIC ROCKS.


General Character of Metamorphic Rocks. — Gneiss. — Hornblende-schist.
— Serpentine. — Mica-schist. — Clay-slate. — Quartzite. —
Chlorite-schist. — Metamorphic Limestone. — Origin of the metamorphic
Strata. — Their Stratification. — Fossiliferous Strata near intrusive
Masses of Granite converted into Rocks identical with different Members
of the metamorphic Series. — Arguments hence derived as to the Nature
of Plutonic Action. — Hydrothermal Action, or the Influence of Steam
and Gases in producing Metamorphism. — Objections to the metamorphic
Theory considered.

We have now considered three distinct classes of rocks: first, the
aqueous, or fossiliferous; secondly, the volcanic; and, thirdly, the
Plutonic; and it remains for us to examine those crystalline (or
hypogene) strata to which the name of _ metamorphic_ has been assigned.
The last-mentioned term expresses, as before explained, a theoretical
opinion that such strata, after having been deposited from water,
acquired, by the influence of heat and other causes, a highly
crystalline texture. They who still question this opinion may call the
rocks under consideration the stratified hypogene formations or
crystalline schists.

These rocks, when in their characteristic or normal state, are wholly
devoid of organic remains, and contain no distinct fragments of other
rocks, whether rounded or angular. They sometimes break out in the
central parts of mountain chains, but in other cases extend over areas
of vast dimensions, occupying, for example, nearly the whole of Norway
and Sweden, where, as in Brazil, they appear alike in the lower and
higher grounds. However crystalline these rocks may become in certain
regions, they never, like granite or trap, send veins into contiguous
formations. In Great Britain, those members of the series which
approach most nearly to granite in their composition, as gneiss,
mica-schist, and hornblende-schist, are confined to the country north
of the rivers Forth and Clyde.

Many attempts have been made to trace a general order of succession or
superposition in the members of this family; clay-slate, for example,
having been often supposed to hold invariably a higher geological
position than mica-schist, and mica-schist to overlie gneiss. But
although such an order may prevail throughout limited districts, it is
by no means universal. To this subject, however, I shall again revert,
in Chapter XXXV, where the chronological relations of the metamorphic
rocks are pointed out.

Principal Metamorphic Rocks.—The following may be enumerated as the
principal members of the metamorphic class:—gneiss, mica-schist,
hornblende-schist, clay-slate, chlorite-schist, hypogene or metamorphic
limestone, and certain kinds of quartz-rock or quartzite.

Fig. 622: Fragment of gneiss; section made at right angles to the
planes of foliation.

_Gneiss._—The first of these, gneiss, may be called stratified—or by
those who object to that term, foliated—granite, being formed of the
same materials as granite, namely, feldspar, quartz, and mica. In the
specimen in Fig. 622, the white layers consist almost exclusively of
granular feldspar, with here and there a speck of mica and grain of
quartz. The dark layers are composed of grey quartz and black mica,
with occasionally a grain of feldspar intermixed. The rock splits most
easily in the plane of these darker layers, and the surface thus
exposed is almost entirely covered with shining spangles of mica. The
accompanying quartz, however, greatly predominates in quantity, but the
most ready cleavage is determined by the abundance of mica in certain
parts of the dark layer. Instead of consisting of these thin laminæ,
gneiss is sometimes simply divided into thick beds, in which the mica
has only a slight degree of parallelism to the planes of
stratification.

Hand specimens may often be obtained from such gneiss which are
undistinguishable from granite, affording an argument to which we shall
allude in the concluding part of this chapter, in favour of those who
regard all granite and syenite not as igneous rocks, but as aqueous
formations so altered as to have lost all signs of their original
stratified arrangement. Gneiss in geology is commonly used to designate
not merely stratified and foliated rocks having the same component
materials as granite or syenite, but also in a wider sense to embrace
the formation with which other members of the metamorphic series, such
as hornblende-schist, may alternate, and which are then considered
subordinate to the true gneiss.

The different varieties of rock allied to gneiss, into which feldspar
enters as an essential ingredient, will be understood by referring to
what was said of granite. Thus, for example, hornblende may be
superadded to mica, quartz, and feldspar, forming a hornblendic or
syenitic gneiss; or talc may be substituted for mica, constituting
talcose gneiss (called stratified protogine by the French), a rock
composed of feldspar, quartz, and talc, in distinct crystals or grains.

_Eurite,_ which has already been mentioned as a Plutonic rock, occurs
also with precisely the same composition in beds subordinate to gneiss
or mica-slate.

_Hornblende-schist_ is usually black, and composed principally of
hornblende, with a variable quantity of feldspar, and sometimes grains
of quartz. When the hornblende and feldspar are in nearly equal
quantities, and the rock is not slaty, it corresponds in character with
the greenstones of the trap family, and has been called “primitive
greenstone.” It may be termed hornblende rock, or amphibolite. Some of
these hornblendic masses may really have been volcanic rocks, which
have since assumed a more crystalline or metamorphic texture.

_Serpentine_ is a greenish rock, a silicate of magnesia, in which there
is sometimes from 30 to 40 per cent of magnesia. It enters largely into
the composition of a trap dike cutting through Old Red Sandstone in
Forfarshire, and in that case is probably an altered basaltic dike
which had contained much olivine. The theory of its having been
originally a volcanic product subsequently altered by metamorphism may
at first sight seem inconsistent with its occurrence in large and
regularly stratified masses in the metamorphic series in Scotland, as
in Aberdeenshire. But it has been suggested in explanation that such
serpentine may have been originally regularly-bedded trap tuff, and
volcanic breccia, with much olivine, which would still retain a
stratified appearance after their conversion into a metamorphic rock.

_Actinolite Schist_ is a slaty foliated rock, composed chiefly of
actinolite, an emerald-green mineral, allied to hornblende, with some
admixture of garnet, mica, and quartz.

_Mica-schist_ or _Micaceous Schist_ is, next to gneiss, one of the most
abundant rocks of the metamorphic series. It is slaty, essentially
composed of mica and quartz, the mica sometimes appearing to constitute
the whole mass. Beds of pure quartz also occur in this formation. In
some districts, garnets in regular twelve-sided crystals form an
integrant part of mica-schist. This rock passes by insensible
gradations into clay-slate.

_Clay-slate—Argillaceous Schist—Argillite._—This rock sometimes
resembles an indurated clay or shale. It is for the most part extremely
fissile, often affording good roofing-slate. Occasionally it derives a
shining and silky lustre from the minute particles of mica or talc
which it contains. It varies from greenish or bluish-grey to a lead
colour; and it may be said of this, more than of any other schist, that
it is common to the metamorphic and fossiliferous series, for some
clay-slates taken from each division would not be distinguishable by
mineral characters alone. It is not uncommon to meet with an
argillaceous rock having the same composition, without the slaty
cleavage, which may be called argillite.

_Chlorite Schist_ is a green slaty rock, in which chlorite is abundant
in foliated plates, usually blended with minute grains of quartz, or
sometimes with feldspar or mica; often associated with, and graduating
into, gneiss and clay-slate.

_Quartzite,_ or _Quartz Rock,_ is an aggregate of grains of quartz
which are either in minute crystals, or in many cases slightly rounded,
occurring in regular strata, associated with gneiss or other
metamorphic rocks. Compact quartz, like that so frequently found in
veins, is also found together with granular quartzite. Both of these
alternate with gneiss or mica-schist, or pass into those rocks by the
addition of mica, or of feldspar and mica.

_Crystalline,_ or _Metamorphic Limestone._—This hypogene rock, called
by the earlier geologists _primary limestone,_ is sometimes a white
crystalline granular marble, which when in thick beds can be used in
sculpture; but more frequently it occurs in thin beds, forming a
foliated schist much resembling in colour and arrangement certain
varieties of gneiss and mica-schist. When it alternates with these
rocks, it often contains some crystals of mica, and occasionally
quartz, feldspar, hornblende, talc, chlorite, garnet, and other
minerals. It enters sparingly into the structure of the hypogene
districts of Norway, Sweden, and Scotland, but is largely developed in
the Alps.

Origin of the Metamorphic Strata.—Having said thus much of the mineral
composition of the metamorphic rocks, I may combine what remains to be
said of their structure and history with an account of the opinions
entertained of their probable origin. At the same time, it may be well
to forewarn the reader that we are here entering upon ground of
controversy, and soon reach the limits where positive induction ends,
and beyond which we can only indulge in speculations. It was once a
favourite doctrine, and is still maintained by many, that these rocks
owe their crystalline texture, their want of all signs of a mechanical
origin, or of fossil contents, to a peculiar and nascent condition of
the planet at the period of their formation. The arguments in
refutation of this hypothesis will be more fully considered when I
show, in Chapter XXXV, to how many different ages the metamorphic
formations are referable, and how gneiss, mica-schist, clay-slate, and
hypogene limestone (that of Carrara, for example) have been formed, not
only since the first introduction of organic beings into this planet,
but even long after many distinct races of plants and animals had
flourished and passed away in succession.

The doctrine respecting the crystalline strata implied in the name
metamorphic may properly be treated of in this place; and we must first
inquire whether these rocks are really entitled to be called stratified
in the strict sense of having been originally deposited as sediment
from water. The general adoption by geologists of the term stratified,
as applied to these rocks, sufficiently attests their division into
beds very analogous, at least in form, to ordinary fossiliferous
strata. This resemblance is by no means confined to the existence in
both occasionally of a laminated structure, but extends to every kind
of arrangement which is compatible with the absence of fossils, and of
sand, pebbles, ripple-mark, and other characters which the metamorphic
theory supposes to have been obliterated by Plutonic action. Thus, for
example, we behold alike in the crystalline and fossiliferous
formations an alternation of beds varying greatly in composition,
colour, and thickness. We observe, for instance, gneiss alternating
with layers of black hornblende-schist or of green chlorite-schist, or
with granular quartz or limestone; and the interchange of these
different strata may be repeated for an indefinite number of times. In
the like manner, mica-schist alternates with chlorite-schist, and with
beds of pure quartz or of granular limestone. We have already seen
that, near the immediate contact of granitic veins and volcanic dikes,
very extraordinary alterations in rocks have taken place, more
especially in the neighbourhood of granite. It will be useful here to
add other illustrations, showing that a texture undistinguishable from
that which characterises the more crystalline metamorphic formations
has actually been superinduced in strata once fossiliferous.

Fossiliferous Strata rendered metamorphic by intrusive Masses of
Granite.—In the southern extremity of Norway there is a large district,
on the west side of the fiord of Christiania, which I visited in 1837
with the late Professor Keilhau, in which syenitic granite protrudes in
mountain masses through fossiliferous strata, and usually sends veins
into them at the point of contact. The stratified rocks, replete with
shells and zoophytes, consist chiefly of shale, limestone, and some
sandstone, and all these are invariably altered near the granite for a
distance of from 50 to 400 yards. The aluminous shales are hardened,
and have become flinty. Sometimes they resemble jasper. Ribboned jasper
is produced by the hardening of alternate layers of green and
chocolate-coloured schist, each stripe faithfully representing the
original lines of stratification. Nearer the granite the schist often
contains crystals of hornblende, which are even met with in some places
for a distance of several hundred yards from the junction; and this
black hornblende is so abundant that eminent geologists, when passing
through the country, have confounded it with the ancient
hornblende-schist, subordinate to the great gneiss formation of Norway.
Frequently, between the granite and the hornblende-slate
above-mentioned, grains of mica and crystalline feldspar appear in the
schist, so that rocks resembling gneiss and mica-schist are produced.
Fossils can rarely be detected in these schists, and they are more
completely effaced in proportion to the more crystalline texture of the
beds, and their vicinity to the granite.

Fig. 623: Ground-plan of altered slate and limestone near granite.
Christiania. The arrows indicate the dip, and the oblique lines the
strike of the beds.

In some places the siliceous matter of the schist becomes a granular
quartz; and when hornblende and mica are added, the altered rock loses
its stratification, and passes into a kind of granite. The limestone,
which at points remote from the granite is of an earthy texture and
blue colour, and often abounds in corals, becomes a white granular
marble near the granite, sometimes siliceous, the granular structure
extending occasionally upward of 400 yards from the junction; the
corals being for the most part obliterated, though sometimes preserved,
even in the white marble. Both the altered limestone and hardened slate
contain garnets in many places, also ores of iron, lead, and copper,
with some silver. These alterations occur equally whether the granite
invades the strata in a line parallel to the general strike of the
fossiliferous beds, or in a line at right angles to their strike, both
of which modes of junction will be seen by the ground-plan in Fig.
623.[1]

The granite of Cornwall sends forth veins into a coarse
argillaceous-schist, provincially termed killas. This killas is
converted into hornblende-schist near the contact with the veins. These
appearances are well seen at the junction of the granite and killas, in
St. Michael’s Mount, a small island nearly 300 feet high, situated in
the bay, at a distance of about three miles from Penzance. The granite
of Dartmoor, in Devonshire, says Sir H. De la Beche, has intruded
itself into the Carboniferous slate and slaty sandstone, twisting and
contorting the strata, and sending veins into them. Hence some of the
slate rocks have become “micaceous; others more indurated, and with the
characters of mica-slate and gneiss; while others again appear
converted into a hard zoned rock strongly impregnated with
feldspar.”[2]

We learn from the investigation of M. Dufrenoy that in the eastern
Pyrenees there are mountain masses of granite posterior in date to the
formations called lias and chalk of that district, and that these
fossiliferous rocks are greatly altered in texture, and often charged
with iron-ore, in the neighbourhood of the granite. Thus in the
environs of St. Martin, near St. Paul de Fenouillet, the chalky
limestone becomes more crystalline and saccharoid as it approaches the
granite, and loses all trace of the fossils which it previously
contained in abundance. At some points, also, it becomes dolomitic, and
filled with small veins of carbonate of iron, and spots of red
iron-ore. At Rancie the lias nearest the granite is not only filled
with iron-ore, but charged with pyrites, tremolite, garnet, and a new
mineral somewhat allied to feldspar, called, from the place in the
Pyrenees where it occurs, “couzeranite.”

“Hornblende-schist,” says Dr. MacCulloch, “may at first have been mere
clay; for clay or shale is found altered by trap into Lydian stone, a
substance differing from hornblende-schist almost solely in compactness
and uniformity of texture.”[3] “In Shetland,” remarks the same author,
“argillaceous-schist (or clay-slate), when in contact with granite, is
sometimes converted into hornblende-schist, the schist becoming first
siliceous, and ultimately, at the contact, hornblende-schist.” In like
manner gneiss and mica-schist may be nothing more than altered
micaceous and argillaceous sandstones, granular quartz may have been
derived from siliceous sandstone, and compact quartz from the same
materials. Clay-slate may be altered shale, and granular marble may
have originated in the form of ordinary limestone, replete with shells
and corals, which have since been obliterated; and, lastly, calcareous
sands and marls may have been changed into impure crystalline
limestones.

The anthracite and plumbago associated with hypogene rocks may have
been coal; for not only is coal converted into anthracite in the
vicinity of some trap dikes, but we have seen that a like change has
taken place generally even far from the contact of igneous rocks, in
the disturbed region of the Appalachians. At Worcester, in the State of
Massachusetts, 45 miles due west of Boston, a bed of plumbago and
impure anthracite occurs, interstratified with mica-schist. It is about
two feet in thickness, and has been made use of both as fuel, and in
the manufacture of lead pencils. At the distance of 30 miles from the
plumbago, there occurs, on the borders of Rhode Island, an impure
anthracite in slates containing impressions of coal-plants of the
genera _Pecopteris, Neuropteris, Calamites,_ etc. This anthracite is
intermediate in character between that of Pennsylvania and the plumbago
of Worcester, in which last the gaseous or volatile matter (hydrogen,
oxygen, and nitrogen) is to the carbon only in the proportion of three
per cent. After traversing the country in various directions, I came to
the conclusion that the carboniferous shales or slates with anthracite
and plants, which in Rhode Island often pass into mica-schists, have at
Worcester assumed a perfectly crystalline and metamorphic texture; the
anthracite having been nearly transmuted into that state of pure carbon
which is called plumbago or graphite.[4]

Now the alterations above described as superinduced in rocks by
volcanic dikes and granite veins prove incontestably that powers exist
in nature capable of transforming fossiliferous into crystalline
strata, a very few simple elements constituting the component materials
common to both classes of rocks. These elements, which are enumerated
in the table at p. 499, may be made to form new combinations by what
has been termed Plutonic action, or those chemical changes which are no
doubt connected with the passage of heat, unusually heated steam and
waters, through the strata.

Hydrothermal Action, or the Influence of Steam and Gases in producing
Metamorphism.—The experiments of Gregory Watt, in fusing rocks in the
laboratory, and allowing them to consolidate by slow cooling, prove
distinctly that a rock need not be perfectly melted in order that a
re-arrangement of its component particles should take place, and a
partial crystallisation ensue.[5] We may easily suppose, therefore,
that all traces of shells and other organic remains may be destroyed,
and that new chemical combinations may arise, without the mass being so
fused as that the lines of stratification should be wholly obliterated.
We must not, however, imagine that heat alone, such as may be applied
to a stone in the open air, can constitute all that is comprised in
Plutonic action. We know that volcanoes in eruption not only emit fluid
lava, but give off steam and other heated gases, which rush out in
enormous volume, for days, weeks, or years continuously, and are even
disengaged from lava during its consolidation.

We also know that long after volcanoes have spent their force, hot
springs continue for ages to flow out at various points in the same
area. In regions, also, subject to violent earthquakes such springs are
frequently observed issuing from rents, usually along lines of fault or
displacement of the rocks. These thermal waters are most commonly
charged with a variety of mineral ingredients, and they retain a
remarkable uniformity of temperature from century to century. A like
uniformity is also persistent in the nature of the earthy, metallic,
and gaseous substances with which they are impregnated. It is well
ascertained that springs, whether hot or cold, charged with carbonic
acid, especially with hydrofluoric acid, which is often present in
small quantities, are powerful causes of decomposition and chemical
reaction in rocks through which they percolate.

The changes which Daubrée has shown to have been produced by the
alkaline waters of Plombières in the Vosges, are more especially
instructive.[6] These waters have a heat of 160° F., or an excess of
109° above the average temperature of ordinary springs in that
district. They were conveyed by the Romans to baths through long
conduits or aqueducts. The foundations of some of their works consisted
of a bed of concrete made of lime, fragments of brick, and sandstone.
Through this and other masonry the hot waters have been percolating for
centuries, and have given rise to various zeolites—apophyllite and
chabazite among others; also to calcareous spar, arragonite, and fluor
spar, together with siliceous minerals, such as opal—all found in the
inter-spaces of the bricks and mortar, or constituting part of their
re-arranged materials. The quantity of heat brought into action in this
instance in the course of 2000 years has, no doubt, been enormous, but
the intensity of it developed at any one moment has been always
inconsiderable.

From these facts and from the experiments and observations of
Sénarmont, Daubrée, Delesse, Scheerer, Sorby, Sterry Hunt, and others,
we are led to infer that when in the bowels of the earth there are
large volumes of matter containing water and various acids intensely
heated under enormous pressure, these subterranean fluid masses will
gradually part with their heat by the escape of steam and various gases
through fissures, producing hot springs; or by the passage of the same
through the pores of the overlying and injected rocks. Even the most
compact rocks may be regarded, before they have been exposed to the air
and dried, in the light of sponges filled with water. According to the
experiments of Henry, water, under a hydrostatic pressure of 96 feet,
will absorb three times as much carbonic acid gas as it can under the
ordinary pressure of the atmosphere. There are other gases, as well as
the carbonic acid, which water absorbs, and more rapidly in proportion
to the amount of pressure. Although the gaseous matter first absorbed
would soon be condensed, and part with its heat, yet the continual
arrival of fresh supplies from below might, in the course of ages,
cause the temperature of the water, and with it that of the containing
rock, to be materially raised; the water acts not only as a vehicle of
heat, but also by its affinity for various silicates, which, when some
of the materials of the invaded rocks are decomposed, form quartz,
feldspar, mica, and other minerals. As for quartz, it can be produced
under the influence of heat by water holding alkaline silicates in
solution, as in the case of the Plombières springs. The quantity of
water required, according to Daubrée, to produce great transformations
in the mineral structure of rocks, is very small. As to the heat
required, silicates may be produced in the moist way at about incipient
red heat, whereas to form the same in the dry way would require a much
higher temperature.

M. Fournet, in his description of the metalliferous gneiss near
Clermont, in Auvergne, states that all the minute fissures of the rock
are quite saturated with free carbonic acid gas; which gas rises
plentifully from the soil there and in many parts of the surrounding
country. The various elements of the gneiss, with the exception of the
quartz, are all softened; and new combinations of the acid with lime,
iron, and manganese are continually in progress.[7]

The power of subterranean gases is well illustrated by the stufas of
St. Calogero in the Lipari Islands, where the horizontal strata of
tuffs, forming cliffs 200 feet high, have been discoloured in places by
the jets of steam often above the boiling point, called “stufas,”
issuing from the fissures; and similar instances are recorded by M.
Virlet of corrosion of rocks near Corinth, and by Dr. Daubeny of
decomposition of trachytic rocks by sulphureted hydrogen and muriatic
acid gases in the Solfatara, near Naples. In all these instances it is
clear that the gaseous fluids must have made their way through vast
thicknesses of porous or fissured rocks, and their modifying influence
may spread through the crust for thousands of yards in thickness.

It has been urged as an argument against the metamorphic theory, that
rocks have a small power of conducting heat, and it is true that when
dry, and in the air, they differ remarkably from metals in this
respect. The syenite of Norway, as we have seen (p. 558), has sometimes
altered fossiliferous strata both in the direction of their dip and
strike for a distance of a quarter of a mile, but the theory of gneiss
and mica-schist above proposed requires us to imagine that the same
influence has extended through strata miles in thickness. Professor
Bischof has shown what changes may be superinduced, on black marble and
other rocks, by the steam of a hot spring having a temperature of no
more than 133° to 167° Fahrenheit, and we are becoming more and more
acquainted with the prominent part which water is playing in
distributing the heat of the interior through mountain masses of
incumbent strata, and of introducing into them various mineral elements
in a fluid or gaseous state. Such facts may induce us to consider
whether many granites and other rocks of that class may not sometimes
represent merely the extreme of a similar slow metamorphism. But, on
the other hand, the heat of lava in a volcanic crater when it is white
and glowing like the sun must convince us that the temperature of a
column of such a fluid at the depth of many miles exceeds any heat
which can ever be witnessed at the surface. That large portions of the
Plutonic rocks had been formed under the influence of such intense heat
is in perfect accordance with their great volume, uniform composition,
and absence of stratification. The forcing also of veins into
contiguous stratified or schistose rocks is a natural consequence of
the hydrostatic pressure to which columns of molten matter many miles
in height must give rise.

Objections to the Metamorphic Theory considered.—It has been objected
to the metamorphic theory that the crystalline schists contain a
considerable proportion of potash and soda, whilst the sedimentary
strata out of which they are supposed to have been formed are usually
wanting in alkaline matter. But this reasoning proceeds on mistaken
data, for clay, marl, shale, and slate often contain a considerable
proportion of alkali, so much so as to make them frequently unfit to be
burnt into bricks or pottery, and the Old Red Sandstone in Forfarshire
and other parts of Scotland, derived from disintegration of granite,
contains much triturated feldspar rich in potash. In the common salt by
which strata are often largely impregnated, as in Patagonia, much soda
is present, and potash enters largely into the composition of fossil
sea-weeds, and recent analysis has also shown that the carboniferous
strata in England, the Upper and Lower Silurian in East Canada, and the
oldest clay-slates in Norway, all contain as much alkali as is
generally present in metamorphic rocks.

Another objection has been derived from the alternation of highly
crystalline strata with others less crystalline. The heat, it is said,
in its ascent from below, must have traversed the less altered schists
before it reached a higher and more crystalline bed. In answer to this,
it may be observed, that if a number of strata differing greatly in
composition from each other be subjected to equal quantities of heat,
or hydrothermal action, there is every probability that some will be
much more fusible or soluble than others. Some, for example, will
contain soda, potash, lime, or some other ingredient capable of acting
as a flux or solvent; while others may be destitute of the same
elements, and so refractory as to be very slightly affected by the same
causes. Nor should it be forgotten that, as a general rule, the less
crystalline rocks do really occur in the upper, and the more
crystalline in the lower part of each metamorphic series.

 [1] Keilhau, Gæa Norvegica, pp. 61-63.

 [2] Geol. Manual, p. 479.

 [3] Syst. of Geol., vol. i, pp. 210, 211.

 [4] See Lyell, Quart. Geol. Journ., vol. i, p. 199.

 [5] Phil. Trans., 1804.

 [6] Daubrée, Sur le Métamorphisme. Paris, 1860.

 [7] See Principles, _Index,_ “Carbonated Springs,” etc.




CHAPTER XXXIV.
METAMORPHIC ROCKS—_continued._


Definition of slaty Cleavage and Joints. — Supposed Causes of these
Structures. — Crystalline Theory of Cleavage. — Mechanical Theory of
Cleavage. — Condensation and Elongation of slate Rocks by lateral
Pressure. — Lamination of some volcanic Rocks due to Motion. — Whether
the Foliation of the crystalline Schists be usually parallel with the
original Planes of Stratification. — Examples in Norway and Scotland. —
Causes of Irregularity in the Planes of Foliation.

We have already seen that chemical forces of great intensity have
frequently acted upon sedimentary and fossiliferous strata long
subsequently to their consolidation, and we may next inquire whether
the component minerals of the altered rocks usually arrange themselves
in planes parallel to the original planes of stratification, or
whether, after crystallisation, they more commonly take up a different
position.

In order to estimate fairly the merits of this question, we must first
define what is meant by the terms cleavage and foliation. There are
four distinct forms of structure exhibited in rocks, namely,
stratification, joints, slaty cleavage, and foliation; and all these
must have different names, even though there be cases where it is
impossible, after carefully studying the appearances, to decide upon
the class to which they belong.

Slaty Cleavage.—Professor Sedgwick, whose essay “On the Structure of
large Mineral Masses” first cleared the way towards a better
understanding of this difficult subject, observes, that joints are
distinguishable from lines of slaty cleavage in this, that the rock
intervening between two joints has no tendency to cleave in a direction
parallel to the planes of the joints, whereas a rock is capable of
indefinite subdivision in the direction of its slaty cleavage. In cases
where the strata are curved, the planes of cleavage are still perfectly
parallel. This has been observed in the slate rocks of part of Wales
(see Fig. 624), which consists of a hard greenish slate. The true
bedding is there indicated by a number of parallel stripes, some of a
lighter and some of a darker colour than the general mass. Such stripes
are found to be parallel to the true planes of stratification, wherever
these are manifested by ripple-mark or by beds containing peculiar
organic remains. Some of the contorted strata are of a coarse
mechanical structure, alternating with fine-grained crystalline
chloritic slates, in which case the same slaty cleavage extends through
the coarser and finer beds, though it is brought out in greater
perfection in proportion as the materials of the rock are fine and
homogeneous. It is only when these are very coarse that the cleavage
planes entirely vanish. In the Welsh hills these planes are usually
inclined at a very considerable angle to the planes of the strata, the
average angle being as much as from 30° to 40°. Sometimes the cleavage
planes dip towards the same point of the compass as those of
stratification, but often to opposite points.[1] The cleavage, as
represented in Fig. 624, is generally constant over the whole of any
area affected by one great set of disturbances, as if the same lateral
pressure which caused the crumpling up of the rock along parallel,
anticlinal, and synclinal axes caused also the cleavage.

Fig. 624: Parallel planes of cleavage intersecting curved strata.

Fig. 625: Section in Lower Silurian slates of Cardiganshire, showing
the cleavage planes bent along the junction of the beds.
Mr. T. McK. Hughes remarks, that where a rough cleavage cuts
flag-stones at a considerable angle to the planes of stratification,
the rock often splits into large slabs, across which the lines of
bedding are frequently seen, but when the cleavage planes approach
within about 15° of stratification, the rock is apt to split along the
lines of bedding. He has also called my attention to the fact that
subsequent movements in a cleaved rock sometimes drag and bend the
cleavage planes along the junction of the beds in the manner indicated
in Fig. 625.

Jointed Structure.—In regard to joints, they are natural fissures which
often traverse rocks in straight and well-determined lines. They afford
to the quarryman, as Sir R. Murchison observes, when speaking of the
phenomenon, as exhibited in Shropshire and the neighbouring counties,
the greatest aid in the extraction of blocks of stone; and, if a
sufficient number cross each other, the whole mass of rock is split
into symmetrical blocks. The faces of the joints are for the most part
smoother and more regular than the surfaces of true strata. The joints
are straight-cut chinks, sometimes slightly open, and often passing,
not only through layers of successive deposition, but also through
balls of limestone or other matter which have been formed by
concretionary action since the original accumulation of the strata.
Such joints, therefore, must often have resulted from one of the last
changes superinduced upon sedimentary deposits.[2]

Fig. 626: Stratification, joints, and cleavage.

In Fig. 626 the flat-surfaces of rock, A, B, C, represent exposed faces
of joints, to which the walls of other joints, J J, are parallel. S S
are the lines of stratification; D D are lines of slaty cleavage, which
intersect the rock at a considerable angle to the planes of
stratification.

In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous
masses of limestone are cut through so regularly by nearly vertical
partings, and these joints are often so much more conspicuous than the
seams of stratification, that an inexperienced observer will almost
inevitably confound them, and suppose the strata to be perpendicular in
places where in fact they are almost horizontal.[3]

Now such joints are supposed to be analogous to the partings which
separate volcanic and Plutonic rocks into cuboidal and prismatic
masses. On a small scale we see clay and starch when dry split into
similar shapes; this is often caused by simple contraction, whether the
shrinking be due to the evaporation of water, or to a change of
temperature. It is well known that many sandstones and other rocks
expand by the application of moderate degrees of heat, and then
contract again on cooling; and there can be no doubt that large
portions of the earth’s crust have, in the course of past ages, been
subjected again and again to very different degrees of heat and cold.
These alternations of temperature have probably contributed largely to
the production of joints in rocks.

In many countries where masses of basalt rest on sandstone, the aqueous
rock has, for the distance of several feet from the point of junction,
assumed a columnar structure similar to that of the trap. In like
manner some hearth-stones, after exposure to the heat of a furnace
without being melted, have become prismatic. Certain crystals also
acquire by the application of heat a new internal arrangement, so as to
break in a new direction, their external form remaining unaltered.

Crystalline Theory of Cleavage.—Professor Sedgwick, speaking of the
planes of slaty cleavage, where they are decidedly distinct from those
of sedimentary deposition, declared, in the essay before alluded to,
his opinion that no retreat of parts, no contraction in the dimensions
of rocks in passing to a solid state, can account for the phenomenon.
He accordingly referred it to crystalline or polar forces acting
simultaneously, and somewhat uniformly, in given directions, on large
masses having a homogeneous composition.

Sir John Herschel, in allusion to slaty cleavage, has suggested that
“if rocks have been so heated as to allow a commencement of
crystallisation—that is to say, if they have been heated to a point at
which the particles can begin to move among themselves, or at least on
their own axes, some general law must then determine the position in
which these particles will rest on cooling. Probably, that position
will have some relation to the direction in which the heat escapes.
Now, when all, or a majority of particles of the same nature have a
general tendency to one position, that must of course determine a
cleavage-plane. Thus we see the infinitesimal crystals of
fresh-precipitated sulphate of barytes, and some other such bodies,
arrange themselves alike in the fluid in which they float; so as, when
stirred, all to glance with one light, and give the appearance of silky
filaments. Some sorts of soap, in which insoluble margarates[4] exist,
exhibit the same phenomenon when mixed with water; and what occurs in
our experiments on a minute scale may occur in nature on a great
one.”[5]

Mechanical Theory of Cleavage.—Professor Phillips has remarked that in
some slaty rocks the form of the outline of fossil shells and
trilobites has been much changed by distortion, which has taken place
in a longitudinal, transverse, or oblique direction. This change, he
adds, seems to be the result of a “creeping movement” of the particles
of the rock along the planes of cleavage, its direction being always
uniform over the same tract of country, and its amount in space being
sometimes measurable, and being as much as a quarter or even half an
inch. The hard shells are not affected, but only those which are
thin.[6] Mr. D. Sharpe, following up the same line of inquiry, came to
the conclusion that the present distorted forms of the shells in
certain British slate rocks may be accounted for by supposing that the
rocks in which they are imbedded have undergone compression in a
direction perpendicular to the planes of cleavage, and a corresponding
expansion in the direction of the dip of the cleavage.[7]

Subsequently (1853) Mr. Sorby demonstrated the great extent to which
this mechanical theory is applicable to the slate rocks of North Wales
and Devonshire,[8] districts where the amount of change in dimensions
can be tested and measured by comparing the different effects exerted
by lateral pressure on alternating beds of finer and coarser materials.
Thus, for example, in Fig. 627 it will be seen that the sandy bed _d
f,_ which has offered greater resistance, has been sharply contorted,
while the fine-grained strata, _a, b, c,_ have remained comparatively
unbent. The points _d_ and _f_ in the stratum _d f_ must have been
originally four times as far apart as they are now. They have been
forced so much nearer to each other, partly by bending, and partly by
becoming elongated in the direction of what may be called the longer
axes of their contortions, and lastly, to a certain small amount, by
condensation. The chief result has obviously been due to the bending;
but, in proof of elongation, it will be observed that the thickness of
the bed _d f_ is now about four times greater in those parts lying in
the main direction of the flexures than in a plane perpendicular to
them; and the same bed exhibits cleavage planes in the direction of the
greatest movement, although they are much fewer than in the slaty
strata above and below.

Fig. 627: Vertical section of slate rock in the cliffs near Ilfracombe,
North Devon.
Above the sandy bed _d f,_ the stratum _c_ is somewhat disturbed, while
the next bed, _b,_ is much less so, and a not at all; yet all these
beds, _c, b,_ and _a,_ must have undergone an equal amount of pressure
with _d,_ the points a and g having approximated as much towards each
other as have _ d_ and _f._ The same phenomena are also repeated in the
beds below _d,_ and might have been shown, had the section been
extended downward. Hence it appears that the finer beds have been
squeezed into a fourth of the space they previously occupied, partly by
condensation, or the closer packing of their ultimate particles (which
has given rise to the great specific gravity of such slates), and
partly by elongation in the line of the dip of the cleavage, of which
the general direction is perpendicular to that of the pressure. “These
and numerous other cases in North Devon are analogous,” says Mr. Sorby,
“to what would occur if a strip of paper were included in a mass of
some soft plastic material which would readily change its dimensions.
If the whole were then compressed in the direction of the length of the
strip of paper, it would be bent and puckered up into contortions,
while the plastic material would readily change its dimensions without
undergoing such contortions; and the difference in distance of the ends
of the paper, as measured in a direct line or along it, would indicate
the change in the dimensions of the plastic material.”

By microscopic examination of minute crystals, and by other
observations, Mr. Sorby has come to the conclusion that the absolute
condensation of the slate rocks amounts upon an average to about one
half their original volume. Most of the scales of mica occurring in
certain slates examined by Mr. Sorby lie in the plane of cleavage;
whereas in a similar rock not exhibiting cleavage they lie with their
longer axes in all directions. May not their position in the slates
have been determined by the movement of elongation before alluded to?
To illustrate this theory some scales of oxide of iron were mixed with
soft pipe-clay in such a manner that they inclined in all directions.
The dimensions of the mass were then changed artificially to a similar
extent to what has occurred in slate rocks, and the pipe-clay was then
dried and baked. When it was afterwards rubbed to a flat surface
perpendicular to the pressure and in the line of elongation, or in a
plane corresponding to that of the dip of cleavage, the particles were
found to have become arranged in the same manner as in natural slates,
and the mass admitted of easy fracture into thin flat pieces in the
plane alluded to, whereas it would not yield in that perpendicular to
the cleavage.[9]

Dr. Tyndall, when commenting in 1856 on Mr. Sorby’s experiments,
observed that pressure alone is sufficient to produce cleavage, and
that the intervention of plates of mica or scales of oxide of iron, or
any other substances having flat surfaces, is quite unnecessary. In
proof of this he showed experimentally that a mass of “pure white wax,
after having been submitted to great pressure, exhibited a cleavage
more clean than that of any slate-rock, splitting into laminæ of
surpassing tenuity.”[10] He remarks that every mass of clay or mud is
divided and subdivided by surfaces among which the cohesion is
comparatively small. On being subjected to pressure, such masses yield
and spread out in the direction of least resistance, small nodules
become converted into laminæ separated from each other by surfaces of
weak cohesion, and the result is that the mass cleaves at right angles
to the line in which the pressure is exerted. In further illustration
of this, Mr. Hughes remarks that “concretions which in the undisturbed
beds have their longer axes parallel to the bedding are, where the rock
is much cleaved, frequently found flattened laterally, so as to have
their longer axes parallel to the cleavage planes, and at a
considerable angle, even right angles, to their former position.”

Mr. Darwin attributes the lamination and fissile structure of volcanic
rocks of the trachytic series, including some obsidians in Ascension,
Mexico, and elsewhere, to their having moved when liquid in the
direction of the laminæ. The zones consist sometimes of layers of
air-cells drawn out and lengthened in the supposed direction of the
moving mass.[11]

Foliation of Crystalline Schists.—After studying, in 1835, the
crystalline rocks of South America, Mr. Darwin proposed the term
_foliation_ for the laminæ or plates into which gneiss, mica-schist,
and other crystalline rocks are divided. Cleavage, he observes, may be
applied to those divisional planes which render a rock fissile,
although it may appear to the eye quite or nearly homogeneous.
Foliation may be used for those alternating layers or plates of
different mineralogical nature of which gneiss and other metamorphic
schists are composed.

That the planes of foliation of the crystalline schists in Norway
accord very generally with those of original stratification is a
conclusion long since espoused by Keilhau.[12] Numerous observations
made by Mr. David Forbes in the same country (the best probably in
Europe for studying such phenomena on a grand scale) confirm Keilhau’s
opinion. In Scotland, also, Mr. D. Forbes has pointed out a striking
case where the foliation is identical with the lines of stratification
in rocks well seen near Crianlorich on the road to Tyndrum, about eight
miles from Inverarnon, in Perthshire. There is in that locality a blue
limestone foliated by the intercalation of small plates of white mica,
so that the rock is often scarcely distinguishable in aspect from
gneiss or mica-schist. The stratification is shown by the large beds
and coloured bands of limestone all dipping, like the folia, at an
angle of 32° N.E.[13] In stratified formations of every age we see
layers of siliceous sand with or without mica, alternating with clay,
with fragments of shells or corals, or with seams of vegetable matter,
and we should expect the mutual attraction of like particles to favour
the crystallisation of the quartz, or mica, or feldspar, or carbonate
of lime, along the planes of original deposition, rather than in planes
placed at angles of 20 or 40 degrees to those of stratification.

We have seen how much the original planes of stratification may be
interfered with or even obliterated by concretionary action in deposits
still retaining their fossils, as in the case of the magnesian
limestone (see p. 63). Hence we must expect to be frequently baffled
when we attempt to decide whether the foliation does or does not accord
with that arrangement which gravitation, combined with current-action,
imparted to a deposit from water. Moreover, when we look for
stratification in crystalline rocks, we must be on our guard not to
expect too much regularity. The occurrence of wedge-shaped masses, such
as belong to coarse sand and pebbles—diagonal lamination (p.
42)—ripple-marked, unconformable stratification,—the fantastic folds
produced by lateral pressure—faults of various width—intrusive dikes of
trap—organic bodies of diversified shapes, and other causes of
unevenness in the planes of deposition, both on the small and on the
large scale, will interfere with parallelism. If complex and
enigmatical appearances did not present themselves, it would be a
serious objection to the metamorphic theory. Mr. Sorby has shown that
the peculiar structure belonging to ripple-marked sands, or that which
is generated when ripples are formed during the deposition of the
materials, is distinctly recognisable in many varieties of mica-schists
in Scotland.[14]

Fig. 628: Lamination of clay-stone. Montagne de Seguinat, near
Gavarnie, in the Pyrenees.
In Fig. 628 I have represented carefully the lamination of a coarse
argillaceous schist which I examined in 1830 in the Pyrenees. In part
it approaches in character to a green and blue roofing-slate, while
part is extremely quartzose, the whole mass passing downward into
micaceous schist. The vertical section here exhibited is about three
feet in height, and the layers are sometimes so thin that fifty may be
counted in the thickness of an inch. Some of them consist of pure
quartz. There is a resemblance in such cases to the diagonal lamination
which we see in sedimentary rocks, even though the layers of quartz and
of mica, or of feldspar and other minerals, may be more distinct in
alternating folia than they were originally.

 [1] Geol. Trans., 2nd series, vol. iii, p. 461.

 [2] Silurian System, p. 246.

 [3] Introduction to Geology, chap. iv.

 [4] Margaric acid is an oleaginous acid, formed from different animal
 and vegetable fatty substances. A margarate is a compound of this acid
 with soda, potash, or some other base, and is so named from its pearly
 lustre.

 [5] Letter to the author, dated Cape of Good Hope, Feb. 20, 1836.

 [6] Report, Brit. Assoc., Cork, 1843, Sect. p. 60.

 [7] Quart. Geol. Journ., vol. iii, p. 87, 1847.

 [8] On the Origin of Slaty Cleavage, by H. C. Sorby, Edin. New Phil.
 Journ., 1853, vol. lv, p. 137.

 [9] Sorby, as cited above, p. 741, note.

 [10] Tyndall, View of the Cleavage of Crystals and Slate rocks.

 [11] Darwin, Volcanic Islands, pp. 69, 70.

 [12] Norske Mag. Naturvidsk., vol. i, p. 71.

 [13] Memoir read before the Geol. Soc. London, Jan. 31, 1855.

 [14] H. C. Sorby, Quart. Geol. Journ., vol. xix., p. 401.




CHAPTER XXXV.
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.


Difficulty of ascertaining the Age of metamorphic Strata. — Metamorphic
Strata of Eocene date in the Alps of Switzerland and Savoy. — Limestone
and Shale of Carrara. — Metamorphic Strata of older date than the
Silurian and Cambrian Rocks. — Order of Succession in metamorphic
Rocks. — Uniformity of mineral Character. — Supposed Azoic Period. —
Connection between the Absence of Organic Remains and the Scarcity of
calcareous Matter in metamorphic Rocks.

According to the theory adopted in the last chapter, the metamorphic
strata have been deposited at one period, and have become crystalline
at another. We can rarely hope to define with exactness the date of
both these periods, the fossils having been destroyed by Plutonic
action, and the mineral characters being the same, whatever the age.
Superposition itself is an ambiguous test, especially when we desire to
determine the period of crystallisation. Suppose, for example, we are
convinced that certain metamorphic strata in the Alps, which are
covered by cretaceous beds, are altered lias; this lias may have
assumed its crystalline texture in the cretaceous or in some tertiary
period, the Eocene for example.

When discussing the ages of the Plutonic rocks, we have seen that
examples occur of various primary, secondary, and tertiary deposits
converted into metamorphic strata near their contact with granite.
There can be no doubt in these cases that strata once composed of mud,
sand, and gravel, or of clay, marl, and shelly limestone, have for the
distance of several yards, and in some instances several hundred feet,
been turned into gneiss, mica-schist, hornblende-schist,
chlorite-schist, quartz rock, statuary marble, and the rest. (See the
two preceding chapters.) It may be easy to prove the identity of two
different parts of the same stratum; one, where the rock has been in
contact with a volcanic or Plutonic mass, and has been changed into
marble or hornblende-schist, and another not far distant, where the
same bed remains unaltered and fossiliferous; but when hydrothermal
action, as described in Chapter XXXIII, has operated gradually on a
more extensive scale, it may have finally destroyed all monuments of
the date of its development throughout a whole mountain chain, and all
the labour and skill of the most practised observers are required, and
may sometimes be at fault. I shall mention one or two examples of
alteration on a grand scale, in order to explain to the student the
kind of reasoning by which we are led to infer that dense masses of
fossiliferous strata have been converted into crystalline rocks.

Eocene Strata rendered metamorphic in the Alps.—In the eastern part of
the Alps, some of the Palæozoic strata, as well as the older Mesozoic
formations, including the oolitic and cretaceous rocks, are distinctly
recognisable. Tertiary deposits also appear in a less elevated position
on the flanks of the Eastern Alps; but in the Central or Swiss Alps,
the Palæozoic and older Mesozoic formations disappear, and the
Cretaceous, Oolitic, Liassic, and at some points even the Eocene
strata, graduate insensibly into metamorphic rocks, consisting of
granular limestone, talc-schist, talcose-gneiss, micaceous schist, and
other varieties.

As an illustration of the partial conversion into gneiss of portions of
a highly inclined set of beds, I may cite Sir R. Murchison’s memoir on
the structure of the Alps. Slates provincially termed “flysch” (see p.
278), overlying the nummulite limestone of Eocene date, and comprising
some arenaceous and some calcareous layers, are seen to alternate
several times with bands of granitoid rock, answering in character to
gneiss. In this case heat, vapour, or water at a high temperature may
have traversed the more permeable beds, and altered them so far as to
admit of an internal movement and re-arrangement of the molecules,
while the adjoining strata did not give passage to the same heated
gases or water, or, if so, remained unchanged because they were
composed of less fusible or decomposable materials. Whatever hypothesis
we adopt, the phenomena establish beyond a doubt the possibility of the
development of the metamorphic structure in a tertiary deposit in
planes parallel to those of stratification. The strata appear clearly
to have been affected, though in a less intense degree, by that same
Plutonic action which has entirely altered and rendered metamorphic so
many of the subjacent formations; for in the Alps this action has by no
means been confined to the immediate vicinity of granite. Granite,
indeed, and other Plutonic rocks, rarely make their appearance at the
surface, notwithstanding the deep ravines which lay open to view the
internal structure of these mountains. That they exist below at no
great depth we cannot doubt, for at some points, as in the Valorsine,
near Mont Blanc, granite and granitic veins are observable, piercing
through talcose gneiss, which passes insensibly upward into secondary
strata.

It is certainly in the Alps of Switzerland and Savoy, more than in any
other district in Europe, that the geologist is prepared to meet with
the signs of an intense development of Plutonic action; for here strata
thousands of feet thick have been bent, folded, and overturned, and
marine secondary formations of a comparatively modern date, such as the
Oolitic and Cretaceous, have been upheaved to the height of 12,000, and
some Eocene strata to elevations of 10,000 feet above the level of the
sea; and even deposits of the Miocene era have been raised 4000 or 5000
feet, so as to rival in height the loftiest mountains in Great Britain.
In one of the sections described by M. Studer in the highest of the
Bernese Alps, namely in the Roththal, a valley bordering the line of
perpetual snow on the northern side of the Jungfrau, there occurs a
mass of gneiss 1000 feet thick, and 15,000 feet long, which I examined,
not only resting upon, but also again covered by strata containing
oolitic fossils. These anomalous appearances may partly be explained by
supposing great solid wedges of intrusive gneiss to have been forced in
laterally between strata to which I found them to be in many sections
unconformable. The superposition, also, of the gneiss to the oolite
may, in some cases, be due to a reversal of the original position of
the beds in a region where the convulsions have been on so stupendous a
scale.

Northern Apennines.—Carrara.—The celebrated marble of Carrara, used in
sculpture, was once regarded as a type of primitive limestone. It
abounds in the mountains of Massa Carrara, or the “Apuan Alps,” as they
have been called, the highest peaks of which are nearly 6000 feet high.
Its great antiquity was inferred from its mineral texture, from the
absence of fossils, and its passage downward into talc-schist and
garnetiferous mica-schist; these rocks again graduating downward into
gneiss, which is penetrated, at Forno, by granite veins. But the
researches of MM. Savi, Boué, Pareto, Guidoni, De la Beche, Hoffman,
and Pilla demonstrated that this marble, once supposed to be formed
before the existence of organic beings, is, in fact, an altered
limestone of the Oolitic period, and the underlying crystalline schists
are secondary sandstones and shales, modified by Plutonic action. In
order to establish these conclusions it was first pointed out that the
calcareous rocks bordering the Gulf of Spezia, and abounding in Oolitic
fossils, assume a texture like that of Carrara marble, in proportion as
they are more and more invaded by certain trappean and Plutonic rocks,
such as diorite, serpentine, and granite, occurring in the same
country.

It was then observed that, in places where the secondary formations are
unaltered, the uppermost consist of common Apennine limestone with
nodules of flint, below which are shales, and at the base of all,
argillaceous and siliceous sandstones. In the limestone fossils are
frequent, but very rare in the underlying shale and sandstone. Then a
gradation was traced laterally from these rocks into another and
corresponding series, which is completely metamorphic; for at the top
of this we find a white granular marble, wholly devoid of fossils, and
almost without stratification, in which there are no nodules of flint,
but in its place siliceous matter disseminated through the mass in the
form of prisms of quartz. Below this, and in place of the shales, are
talc-schists, jasper, and hornstone; and at the bottom, instead of the
siliceous and argillaceous sandstones, are quartzite and gneiss.[1] Had
these secondary strata of the Apennines undergone universally as great
an amount of transmutation, it would have been impossible to form a
conjecture respecting their true age; and then, according to the method
of classification adopted by the earlier geologists, they would have
ranked as primary rocks. In that case the date of their origin would
have been thrown back to an era antecedent to the deposition of the
Lower Silurian or Cambrian strata, although in reality they were formed
in the Oolitic period, and altered at some subsequent and perhaps much
later epoch.

Metamorphic Strata of older date than the Silurian and Cambrian
Rocks.—It was remarked (Fig. 617) that as the hypogene rocks, both
stratified and unstratified, crystallise originally at a certain depth
beneath the surface, they must always, before they are upraised and
exposed at the surface, be of considerable antiquity, relatively to a
large portion of the fossiliferous and volcanic rocks. They may be
forming at all periods; but before any of them can become visible, they
must be raised above the level of the sea, and some of the rocks which
previously concealed them must have been removed by denudation.

In Canada, as we have seen (p. 491), the Lower Laurentian gneiss,
quartzite, and limestone may be regarded as metamorphic, because, among
other reasons, organic remains (_Eozoon Canadense_) have been detected
in a part of one of the calcareous masses. The Upper Laurentian or
Labrador series lies unconformably upon the Lower, and differs from it
chiefly in having as yet yielded no fossils. It consists of gneiss with
Labrador-feldspar and feldstones, in all 10,000 feet thick, and both
its composition and structure lead us to suppose that, like the Lower
Laurentian, it was originally of sedimentary origin and owes its
crystalline condition to metamorphic action. The remote date of the
period when some of these old Laurentian strata of Canada were
converted into gneiss may be inferred from the fact that pebbles of
that rock are found in the overlying Huronian formation, which is
probably of Cambrian age (p. 490).

The oldest stratified rock of Scotland is the hornblendic gneiss of
Lewis, in the Hebrides, and that of the north-west coast of Ross-shire,
represented at the base of the section given at Fig. 82. It is the same
as that intersected by numerous granite veins which forms the cliffs of
Cape Wrath, in Sutherlandshire (see Fig. 613), and is conjectured to be
of Laurentian age. Above it, as shown in the section (Fig. 82), lie
unconformable beds of a reddish or purple sandstone and conglomerate,
nearly horizontal, and between 3000 and 4000 feet thick. In these
ancient grits no fossils have been found, but they are supposed to be
of Cambrian date, for Sir R. Murchison found Lower Silurian strata
resting unconformably upon them. These strata consist of quartzite with
annelid burrows already alluded to (p. 112), and limestone in which Mr.
Charles Peach was the first to find, in 1854, three or four species of
_Orthoceras,_ also the genera _Cyrtoceras_ and _ Lituites,_ two species
of _Murchisonia,_ a _ Pleurotomaria,_ a species of _Maclurea,_ one of _
Euomphalus,_ and an _Orthis._ Several of the species are believed by
Mr. Salter to be identical with Lower Silurian fossils of Canada and
the United States.

The discovery of the true age of these fossiliferous rocks was one of
the most important steps made of late years in the progress of British
Geology, for it led to the unexpected conclusion that all the Scotch
crystalline strata to the eastward, once called primitive, which
overlie the limestone and quartzite in question, are referable to some
part of the Silurian series.

These Scotch metamorphic strata are of gneiss, mica-schist, and
clay-slate of vast thickness, and having a strike from north-east to
south-west almost at right angles to that of the older Laurentian
gneiss before mentioned. The newer crystalline series, comprising the
crystalline rocks of Aberdeenshire, Perthshire, and Forfarshire, were
inferred by Sir R. Murchison to be altered Silurian strata; and his
opinion has been since confirmed by the observations of three able
geologists, Messrs. Ramsay, Harkness, and Geikie. The newest of the
series is a clay-slate, on which, along the southern borders of the
Grampians, the Lower Old Red, containing _Cephalaspis Lyelli,
Pterygotus Anglicus,_ and _Parka decipiens,_ rests unconformably.

Order of Succession in Metamorphic Rocks.—There is no universal and
invariable order of superposition in metamorphic rocks, although a
particular arrangement may prevail throughout countries of great
extent, for the same reason that it is traceable in those sedimentary
formations from which crystalline strata are derived. Thus, for
example, we have seen that in the Apennines, near Carrara, the
descending series, where it is metamorphic, consists of, first,
saccharine marble; second, talcose-schist; and third, of quartz-rock
and gneiss: where unaltered, of, first, fossiliferous limestone;
second, shale; and third, sandstone.

But if we investigate different mountain chains, we find gneiss,
mica-schist, hornblende-schist, chlorite-schist, hypogene limestone,
and other rocks, succeeding each other, and alternating with each other
in every possible order. It is, indeed, more common to meet with some
variety of clay-slate forming the uppermost member of a metamorphic
series than any other rock; but this fact by no means implies, as some
have imagined, that all clay-slates were formed at the close of an
imaginary period when the deposition of the crystalline strata gave way
to that of ordinary sedimentary deposits. Such clay-slates, in fact,
are variable in composition, and sometimes alternate with fossiliferous
strata, so that they may be said to belong almost equally to the
sedimentary and metamorphic order of rocks. It is probable that, had
they been subjected to more intense Plutonic action, they would have
been transformed into hornblende-schist, foliated chlorite-schist,
scaly talcose-schist, mica-schist, or other more perfectly crystalline
rocks, such as are usually associated with gneiss.

_Uniformity of Mineral Character in Hypogene Rocks._—It is true, as
Humboldt has happily remarked, that when we pass to another hemisphere,
we see new forms of animals and plants, and even new constellations in
the heavens; but in the rocks we still recognise our old
acquaintances—the same granite, the same gneiss, the same micaceous
schist, quartz-rock, and the rest. There is certainly a great and
striking general resemblance in the principal kinds of hypogene rocks
in all countries, however different their ages; but each of them, as we
have seen, must be regarded as geological families of rocks, and not as
definite mineral compounds. They are more uniform in aspect than
sedimentary strata, because these last are often composed of fragments
varying greatly in form, size, and colour, and contain fossils of
different shapes and mineral composition, and acquire a variety of
tints from the mixture of various kinds of sediment. The materials of
such strata, if they underwent metamorphism, would be subject to
chemical laws, simple and uniform in their action, the same in every
climate, and wholly undisturbed by mechanical and organic causes. It
would, however, be a great error to assume, as some have done, that the
hypogene rocks, considered as aggregates of simple minerals, are really
more homogeneous in their composition than the several members of the
sedimentary series. Not only do the proportional quantities of
feldspar, quartz, mica, hornblende, and other minerals, vary in
hypogene rocks bearing the same name; but what is still more important,
the ingredients, as we have seen, of the same simple mineral are not
always constant (see p. 503 and table, p. 499).

Supposed Azoic Period.—The total absence of any trace of fossils has
inclined many geologists to attribute the origin of the most ancient
strata to an azoic period, or one antecedent to the existence of
organic beings. Admitting, they say, the obliteration, in some cases,
of fossils by Plutonic action, we might still expect that traces of
them would oftener be found in certain ancient systems of slate which
can scarcely be said to have assumed a crystalline structure. But in
urging this argument it seems to have been forgotten that there are
stratified formations of enormous thickness, and of various ages, some
of them even of Tertiary date, and which we know were formed after the
earth had become the abode of living creatures, which are,
nevertheless, in some districts, entirely destitute of all vestiges of
organic bodies. In some, the traces of fossils may have been effaced by
water and acids, at many successive periods; indeed the removal of the
calcareous matter of fossil shells is proved by the fact of such
organic remains being often replaced by silex or other minerals, and
sometimes by the space once occupied by the fossil being left empty, or
only marked by a faint impression.

Those who believed the hypogene rocks to have originated antecedently
to the creation of organic beings, imputed the absence of lime, so
remarkable in metamorphic strata, to the non-existence of those
mollusca and zoophytes by which shells and corals are secreted; but
when we ascribe the crystalline formations to Plutonic action, it is
natural to inquire whether this action itself may not tend to expel
carbonic acid and lime from the materials which it reduces to fusion or
semi-fusion. Not only carbonate of lime, but also free carbonic acid
gas, is given off plentifully from the soil and crevices of rocks in
regions of active and spent volcanoes, as near Naples and in Auvergne.
By this process, fossil shells or corals may often lose their carbonic
acid, and the residual lime may enter into the composition of augite,
hornblende, garnet, and other hypogene minerals. Although we cannot
descend into the subterranean regions where volcanic heat is developed,
we can observe in regions of extinct volcanoes, such as Auvergne and
Tuscany, hundreds of springs, both cold and thermal, flowing out from
granite and other rocks, and having their waters plentifully charged
with carbonate of lime.

If all the calcareous matter transferred in the course of ages by these
and thousands of other springs from the lower part of the earth’s crust
to the atmosphere could be presented to us in a solid form, we should
find that its volume was comparable to that of many a chain of hills.
Calcareous matter is poured into lakes and the ocean by a thousand
springs and rivers; so that part of almost every new calcareous rock
chemically precipitated, and of many reefs of shelly and coralline
stone, must be derived from mineral matter subtracted by Plutonic
agency, and driven up by gas and steam from fused and heated rocks in
the bowels of the earth.

The scarcity of limestone in many extensive regions of metamorphic
rocks, as in the Eastern and Southern Grampians of Scotland, may have
been the result of some action of this kind; and if the limestones of
the Lower Laurentian in Canada afford a remarkable exception to the
general rule, we must not forget that it is precisely in this most
ancient formation that the _Eozoon Canadense_ has been found. The fact
that some distinct bands of limestone from 700 to 1500 feet thick occur
here, may be connected with the escape from destruction of some few
traces of organic life, even in a rock in which metamorphic action has
gone so far as to produce serpentine, augite, and other minerals found
largely intermixed with the carbonate of lime.

 [1] See notices of Savi, Hoffman, and others, referred to by Boué,
 Bull. de la Soc. Géol. de France, tome v, p. 317 and tome iii, p. 44;
 also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v, p. 266.




CHAPTER XXXVI.
MINERAL VEINS.


Different Kinds of mineral Veins. — Ordinary metalliferous Veins or
Lodes. — Their frequent Coincidence with Faults. — Proofs that they
originated in Fissures in solid Rock. — Veins shifting other Veins. —
Polishing of their Walls or “Slicken sides.” Shells and Pebbles in
Lodes. — Evidence of the successive Enlargement and Reopening of veins.
— Examples in Cornwall and in Auvergne. — Dimensions of Veins. — Why
some alternately swell out and contract. — Filling of Lodes by
Sublimation from below. — Supposed relative Age of the precious Metals.
— Copper and lead Veins in Ireland older than Cornish Tin. — Lead Vein
in Lias, Glamorganshire. — Gold in Russia, California, and Australia. —
Connection of hot Springs and mineral Veins.

The manner in which metallic substances are distributed through the
earth’s crust, and more especially the phenomena of those more or less
connected masses of ore called mineral veins, from which the larger
part of the precious metals used by man are obtained, are subjects of
the highest practical importance to the miner, and of no less
theoretical interest to the geologist.

On different Kinds of Mineral Veins.—The mineral veins with which we
are most familiarly acquainted are those of quartz and carbonate of
lime, which are often observed to form lenticular masses of limited
extent traversing both hypogene strata and fossiliferous rocks. Such
veins appear to have once been chinks or small cavities, caused, like
cracks in clay, by the shrinking of the mass, during desiccation, or in
passing from a higher to a lower temperature. Siliceous, calcareous,
and occasionally metallic matters have sometimes found their way
simultaneously into such empty spaces, by infiltration from the
surrounding rocks. Mixed with hot water and steam, metallic ores may
have permeated the mass until they reached those receptacles formed by
shrinkage, and thus gave rise to that irregular assemblage of veins,
called by the Germans a “stockwerk,” in allusion to the different
floors on which the mining operations are in such cases carried on.

The more ordinary or regular veins are usually worked in vertical
shafts, and have evidently been fissures produced by mechanical
violence. They traverse all kinds of rocks, both hypogene and
fossiliferous, and extend downward to indefinite or unknown depths. We
may assume that they correspond with such rents as we see caused from
time to time by the shock of an earthquake. Metalliferous veins
referable to such agency are occasionally a few inches wide, but more
commonly three or four feet. They hold their course continuously in a
certain prevailing direction for miles or leagues, passing through
rocks varying in mineral composition.

That Metalliferous Veins were Fissures.—As some intelligent miners,
after an attentive study of metalliferous veins, have been unable to
reconcile many of their characteristics with the hypothesis of
fissures, I shall begin by stating the evidence in its favour. The most
striking fact, perhaps, which can be adduced in its support is, the
coincidence of a considerable proportion of mineral veins with
_faults,_ or those dislocations of rocks which are indisputably due to
mechanical force, as above explained (p. 87). There are even proofs in
almost every mining district of a succession of faults, by which the
opposite walls of rents, now the receptacles of metallic substances,
have suffered displacement. Thus, for example, suppose _a a,_ Fig. 629,
to be a tin lode in Cornwall, the term _lode_ being applied to veins
containing metallic ores. This lode, running east and west, is a yard
wide, and is shifted by a copper lode (_b b_) of similar width. The
first fissure (_a a_) has been filled with various materials, partly of
chemical origin, such as quartz, fluor-spar, peroxide of tin, sulphuret
of copper, arsenical pyrites, bismuth, and sulphuret of nickel, and
partly of mechanical origin, comprising clay and angular fragments or
detritus of the intersected rocks. The plates of quartz and the ores
are, in some places, parallel to the vertical sides or walls of the
vein, being divided from each other by alternating layers of clay or
other earthy matter. Occasionally the metallic ores are disseminated in
detached masses among the vein-stones.

It is clear that, after the gradual introduction of the tin and other
substances, the second rent (_b b_) was produced by another fracture
accompanied by a displacement of the rocks along the plane of _b b._
This new opening was then filled with minerals, some of them resembling
those in _a a,_ as fluor-spar (or fluate of lime) and quartz; others
different, the copper being plentiful and the tin wanting or very
scarce. We must next suppose a third movement to occur, breaking
asunder all the rocks along the line _c c,_ Fig. 630; the fissure, in
this instance, being only six inches wide, and simply filled with clay,
derived, probably, from the friction of the walls of the rent, or
partly, perhaps, washed in from above. This new movement has displaced
the rock in such a manner as to interrupt the continuity of the copper
vein (_b b_), and, at the same time, to shift or heave laterally in the
same direction a portion of the tin vein which had not previously been
broken.

Vertical sections of the mine at Huel Peever, Redruth, Cornwall. Fig.
629: Tin; Fig. 630: Copper; Fig. 631: Clay.

Again, in Fig. 631 we see evidence of a fourth fissure (_d d_), also
filled with clay, which has cut through the tin vein (_a a_), and has
lifted it slightly upward towards the south. The various changes here
represented are not ideal, but are exhibited in a section obtained in
working an old Cornish mine, long since abandoned, in the parish of
Redruth, called Huel Peever, and described both by Mr. Williams and Mr.
Carne.[1] The principal movement here referred to, or that of _c c,_
Fig. 631, extends through a space of no less than 84 feet; but in this,
as in the case of the other three, it will be seen that the outline of
the country above, _d, c, b, a,_ etc., or the geographical features of
Cornwall, are not affected by any of the dislocations, a powerful
denuding force having clearly been exerted subsequently to all the
faults. (See p. 93.) It is commonly said in Cornwall, that there are
eight distinct systems of veins, which can in like manner be referred
to as many successive movements or fractures; and the German miners of
the Hartz Mountains speak also of eight systems of veins, referable to
as many periods.

Besides the proofs of mechanical action already explained, the opposite
walls of veins are often beautifully polished, as if glazed, and are
not unfrequently striated or scored with parallel furrows and ridges,
such as would be produced by the continued rubbing together of surfaces
of unequal hardness. These smoothed surfaces resemble the rocky floor
over which a glacier has passed (see Fig. 106). They are common even in
cases where there has been no shift, and occur equally in
non-metalliferous fissures. They are called by miners “slicken-sides,”
from the German _schlichten,_ to plane, and _seite,_ side. It is
supposed that the lines of the striæ indicate the direction in which
the rocks were moved.

In some of the veins in the mountain limestone of Derbyshire,
containing lead, the vein-stuff, which is nearly compact, is
occasionally traversed by what may be called a vertical crack passing
down the middle of the vein. The two faces in contact are
slicken-sides, well polished and fluted, and sometimes covered by a
thin coating of lead-ore. When one side of the vein-stuff is removed,
the other side cracks, especially if small holes be made in it, and
fragments fly off with loud explosions, and continue to do so for some
days. The miner, availing himself of this circumstance, makes with his
pick small holes about six inches apart, and four inches deep, and on
his return in a few hours finds every part ready broken to his hand.[2]

That a great many veins communicated originally with the surface of the
country above, or with the bed of the sea, is proved by the occurrence
in them of well-rounded pebbles, agreeing with those in superficial
alluviums, as in Auvergne and Saxony. Marine fossil shells, also, have
been found at great depths, having probably been ingulfed during
submarine earthquakes. Thus, a gryphæa is stated by M. Virlet to have
been met with in a lead-mine near Semur, in France, and a madrepore in
a compact vein of cinnabar in Hungary.[3] In Bohemia, similar pebbles
have been met with at the depth of 180 fathoms; and in Cornwall, Mr.
Carne mentions true pebbles of quartz and slate in a tin lode of the
Relistran Mine, at the depth of 600 feet below the surface. They were
cemented by oxide of tin and bisulphuret of copper, and were traced
over a space more than twelve feet long and as many wide.[4] When
different sets or systems of veins occur in the same country, those
which are supposed to be of contemporaneous origin, and which are
filled with the same kind of metals, often maintain a general
parallelism of direction. Thus, for example, both the tin and copper
veins in Cornwall run nearly east and west, while the lead veins run
north and south; but there is no general law of direction common to
different mining districts. The parallelism of the veins is another
reason for regarding them as ordinary fissures, for we observe that
faults and trap dikes, admitted by all to be masses of melted matter
which have filled rents, are often parallel.

_Fracture, Re-opening and Successive Formation of Veins._—Assuming,
then, that veins are simply fissures in which chemical and mechanical
deposits have accumulated, we may next consider the proofs of their
having been filled gradually and often during successive enlargements.

Werner observed, in a vein near Gersdorff, in Saxony, no less than
thirteen beds of different minerals, arranged with the utmost
regularity on each side of the central layer. This layer was formed of
two plates of calcareous spar, which had evidently lined the opposite
walls of a vertical cavity. The thirteen beds followed each other in
corresponding order, consisting of fluor-spar, heavy spar, galena, etc.
In these cases the central mass has been last formed, and the two
plates which coat the walls of the rent on each side are the oldest of
all. If they consist of crystalline precipitates, they may be explained
by supposing the fissure to have remained unaltered in its dimensions,
while a series of changes occurred in the nature of the solutions which
rose up from below: but such a mode of deposition, in the case of many
successive and parallel layers, appears to be exceptional.

If a vein-stone consist of crystalline matter, the points of the
crystals are always turned inward, or towards the centre of the vein;
in other words, they point in the direction where there was space for
the development of the crystals. Thus each new layer receives the
impression of the crystals of the preceding layer, and imprints its
crystals on the one which follows, until at length the whole of the
vein is filled: the two layers which meet dovetail the points of their
crystals the one into the other. But in Cornwall, some lodes occur
where the vertical plates, or _combs,_ as they are there called,
exhibit crystals so dovetailed as to prove that the same fissure has
been often enlarged. Sir H. De la Beche gives the following curious and
instructive example (Fig. 632), from a copper-mine in granite, near
Redruth.[5] Each of the plates or combs (_a, b, c, d, e, f_) is double,
having the points of their crystals turned inward along the axis of the
comb. The sides or walls (2, 3, 4, 5 and 6) are parted by a thin
covering of ochreous clay, so that each comb is readily separable from
another by a moderate blow of the hammer. The breadth of each
represents the whole width of the fissure at six successive periods,
and the outer walls of the vein, where the first narrow rent was
formed, consisted of the granitic surfaces 1 and 7.

Fig. 632: Copper lode, near Redruth, enlarged at six successive
periods.

A somewhat analogous interpretation is applicable to many other cases,
where clay, sand, or angular detritus, alternate with ores and
vein-stones. Thus, we may imagine the sides of a fissure to be
incrusted with siliceous matter, as Von Buch observed, in Lancerote,
the walls of a volcanic crater formed in 1731 to be traversed by an
open rent in which hot vapours had deposited hydrate of silica, the
incrustation nearly extending to the middle.[6] Such a vein may then be
filled with clay or sand, and afterwards re-opened, the new rent
dividing the argillaceous deposit, and allowing a quantity of rubbish
to fall down. Various metals and spars may then be precipitated from
aqueous solutions among the interstices of this heterogeneous mass.

That such changes have repeatedly occurred, is demonstrated by
occasional cross-veins, implying the oblique fracture of previously
formed chemical and mechanical deposits. Thus, for example, M. Fournet,
in his description of some mines in Auvergne worked under his
superintendence, observes that the granite of that country was first
penetrated by veins of granite, and then dislocated, so that open rents
crossed both the granite and the granitic veins. Into such openings,
quartz, accompanied by sulphurets of iron and arsenical pyrites, was
introduced. Another convulsion then burst open the rocks along the old
line of fracture, and the first set of deposits were cracked and often
shattered, so that the new rent was filled, not only with angular
fragments of the adjoining rocks, but with pieces of the older
vein-stones. Polished and striated surfaces on the sides or in the
contents of the vein also attest the reality of these movements. A new
period of repose then ensued, during which various sulphurets were
introduced, together with hornstone quartz, by which angular fragments
of the older quartz before mentioned were cemented into a breccia. This
period was followed by other dilatations of the same veins, and the
introduction of other sets of mineral deposits, as well as of pebbles
of the basaltic lavas of Auvergne, derived from superficial alluviums,
probably of Miocene or even Older Pliocene date. Such repeated
enlargement and re-opening of veins might have been anticipated, if we
adopt the theory of fissures, and reflect how few of them have ever
been sealed up entirely, and that a country with fissures only
partially filled must naturally offer much feebler resistance along the
old lines of fracture than anywhere else.

Cause of alternate Contraction and Swelling of Veins.—A large
proportion of metalliferous veins have their opposite walls nearly
parallel, and sometimes over a wide extent of country. There is a fine
example of this in the celebrated vein of Andreasburg in the Hartz,
which has been worked for a depth of 500 yards perpendicularly, and 200
horizontally, retaining almost everywhere a width of three feet. But
many lodes in Cornwall and elsewhere are extremely variable in size,
being one or two inches in one part, and then eight or ten feet in
another, at the distance of a few fathoms, and then again narrowing as
before. Such alternate swelling and contraction is so often
characteristic as to require explanation. The walls of fissures in
general, observes Sir H. De la Beche, are rarely perfect planes
throughout their entire course, nor could we well expect them to be so,
since they commonly pass through rocks of unequal hardness and
different mineral composition. If, therefore, the opposite sides of
such irregular fissures slide upon each other, that is to say, if there
be a fault, as in the case of so many mineral veins, the parallelism of
the opposite walls is at once entirely destroyed, as will be readily
seen by studying Figs. 633 to 635.

Let _a b,_ Fig. 633, be a line of fracture traversing a rock, and let
_a b,_ Fig. 634, represent the same line. Now, if we cut in two a piece
of paper representing this line, and then move the lower portion of
this cut paper sideways from _a_ to _a′_, taking care that the two
pieces of paper still touch each other at the points 1, 2, 3, 4, 5, we
obtain an irregular aperture at _c,_ and isolated cavities at _d, d,
d,_ and when we compare such figures with nature we find that, with
certain modifications, they represent the interior of faults and
mineral veins. If, instead of sliding the cut paper to the right hand,
we move the lower part towards the left, about the same distance that
it was previously slid to the right, we obtain considerable variation
in the cavities so produced, two long irregular open spaces, _f, f,_
Fig. 635, being then formed. This will serve to show to what slight
circumstances considerable variations in the character of the openings
between unevenly fractured surfaces may be due, such surfaces being
moved upon each other, so as to have numerous points of contact.

Figs. 633, 634, 635: Lines of fracture traversing a rock.

Fig. 636: Nipped ores where the course of a vein departs from
verticality.

Most lodes are perpendicular to the horizon, or nearly so; but some of
them have a considerable inclination or “hade,” as it is termed, the
angles of dip being very various. The course of a vein is frequently
very straight; but if tortuous, it is found to be choked up with clay,
stones, and pebbles, at points where it departs most widely from
verticality. Hence at places, such as _a,_ Fig. 636, the miner
complains that the ores are “nipped,” or greatly reduced in quantity,
the space for their free deposition having been interfered with in
consequence of the pre-occupancy of the lode by earthy materials. When
lodes are many fathoms wide, they are usually filled for the most part
with earthy matter, and fragments of rock, through which the ores are
disseminated. The metallic substances frequently coat or encircle
detached pieces of rock, which our miners call “horses” or “riders.”
That we should find some mineral veins which split into branches is
also natural, for we observe the same in regard to open fissures.

Chemical Deposits in Veins.—If we now turn from the mechanical to the
chemical agencies which have been instrumental in the production of
mineral veins, it may be remarked that those parts of fissures which
were choked up with the ruins of fractured rocks must always have been
filled with water; and almost every vein has probably been the channel
by which hot springs, so common in countries of volcanoes and
earthquakes, have made their way to the surface. For we know that the
rents in which ores abound extend downward to vast depths, where the
temperature of the interior of the earth is more elevated. We also know
that mineral veins are most metalliferous near the contact of Plutonic
and stratified formations, especially where the former send veins into
the latter, a circumstance which indicates an original proximity of
veins at their inferior extremity to igneous and heated rocks. It is
moreover acknowledged that even those mineral and thermal springs
which, in the present state of the globe, are far from volcanoes, are
nevertheless observed to burst out along great lines of upheaval and
dislocation of rocks.[7] It is also ascertained that all the substances
with which hot springs are impregnated agree with those discharged in a
gaseous form from volcanoes. Many of these bodies occur as vein-stones;
such as silex, carbonate of lime, sulphur, fluor-spar, sulphate of
barytes, magnesia, oxide of iron, and others. I may add that, if veins
have been filled with gaseous emanations from masses of melted matter,
slowly cooling in the subterranean regions, the contraction of such
masses as they pass from a plastic to a solid state would, according to
the experiments of Deville on granite (a rock which may be taken as a
standard), produce a reduction in volume amounting to 10 per cent. The
slow crystallisation, therefore, of such Plutonic rocks supplies us
with a force not only capable of rending open the incumbent rocks by
causing a failure of support, but also of giving rise to faults
whenever one portion of the earth’s crust subsides slowly while another
contiguous to it happens to rest on a different foundation, so as to
remain unmoved.

Although we are led to infer, from the foregoing reasoning, that there
has often been an intimate connection between metalliferous veins and
hot springs holding mineral matter in solution, yet we must not on that
account expect that the contents of hot springs and mineral veins would
be identical. On the contrary, M. E. de Beaumont has judiciously
observed that we ought to find in veins those substances which, being
least soluble, are not discharged by hot springs—or that class of
simple and compound bodies which the thermal waters ascending from
below would first precipitate on the walls of a fissure, as soon as
their temperature began slightly to diminish. The higher they mount
towards the surface, the more will they cool, till they acquire the
average temperature of springs, being in that case chiefly charged with
the most soluble substances, such as the alkalies, soda and potash.
These are not met with in veins, although they enter so largely into
the composition of granitic rocks.[8]

To a certain extent, therefore, the arrangement and distribution of
metallic matter in veins may be referred to ordinary chemical action,
or to those variations in temperature which waters holding the ores in
solution must undergo, as they rise upward from great depths in the
earth. But there are other phenomena which do not admit of the same
simple explanation. Thus, for example, in Derbyshire, veins containing
ores of lead, zinc, and copper, but chiefly lead, traverse alternate
beds of limestone and greenstone. The ore is plentiful where the walls
of the rent consist of limestone, but is reduced to a mere string when
they are formed of greenstone, or “toad-stone,” as it is called
provincially. Not that the original fissure is narrower where the
greenstone occurs, but because more of the space is there filled with
vein-stones, and the waters at such points have not parted so freely
with their metallic contents.

“Lodes in Cornwall,” says Mr. Robert W. Fox, “are very much influenced
in their metallic riches by the nature of the rock which they traverse,
and they often change in this respect very suddenly, in passing from
one rock to another. Thus many lodes which yield abundance of ore in
granite, are unproductive in clay-slate, or killas and _vice versa._

Supposed relative Age of the different Metals.—After duly reflecting on
the facts above described, we cannot doubt that mineral veins, like
eruptions of granite or trap, are referable to many distinct periods of
the earth’s history, although it may be more difficult to determine the
precise age of veins; because they have often remained open for ages,
and because, as we have seen, the same fissure, after having been once
filled, has frequently been re-opened or enlarged. But besides this
diversity of age, it has been supposed by some geologists that certain
metals have been produced exclusively in earlier, others in more modern
times; that tin, for example, is of higher antiquity than copper,
copper than lead or silver, and all of them more ancient than gold. I
shall first point out that the facts once relied upon in support of
some of these views are contradicted by later experience, and then
consider how far any chronological order of arrangement can be
recognised in the position of the precious and other metals in the
earth’s crust.

In the first place, it is not true that veins in which tin abounds are
the oldest lodes worked in Great Britain. The government survey of
Ireland has demonstrated that in Wexford veins of copper and lead (the
latter as usual being argentiferous) are much older than the tin of
Cornwall. In each of the two countries a very similar series of
geological changes has occurred at two distinct epochs—in Wexford,
before the Devonian strata were deposited; in Cornwall, after the
Carboniferous epoch. To begin with the Irish mining district: We have
granite in Wexford traversed by granite veins, which veins also intrude
themselves into the Silurian strata, the same Silurian rocks as well as
the veins having been denuded before the Devonian beds were
superimposed. Next we find, in the same county, that elvans, or
straight dikes of porphyritic granite, have cut through the granite and
the veins before mentioned, but have not penetrated the Devonian rocks.
Subsequently to these elvans, veins of copper and lead were produced,
being of a date certainly posterior to the Silurian, and anterior to
the Devonian; for they do not enter the latter, and, what is still more
decisive, streaks or layers of derivative copper have been found near
Wexford in the Devonian, not far from points where mines of copper are
worked in the Silurian strata.

Although the precise age of such copper lodes cannot be defined, we may
safely affirm that they were either filled at the close of the Silurian
or commencement of the Devonian period. Besides copper, lead, and
silver, there is some gold in these ancient or primary metalliferous
veins. A few fragments also of tin found in Wicklow in the drift are
supposed to have been derived from veins of the same age.[9]

Next, if we turn to Cornwall, we find there also the monuments of a
very analogous sequence of events. First, the granite was formed; then,
about the same period, veins of fine-grained granite, often tortuous
(see Fig. 614), penetrating both the outer crust of granite and the
adjoining fossiliferous or primary rocks, including the coal-measures;
thirdly, elvans, holding their course straight through granite,
granitic veins, and fossiliferous slates; fourthly, veins of tin also
containing copper, the first of those eight systems of fissures of
different ages already alluded to, p. 607. Here, then, the tin lodes
are newer than the elvans. It has, indeed, been stated by some Cornish
miners that the elvans are in some instances posterior to the oldest
tin-bearing lodes, but the observations of Sir H. de la Beche during
the survey led him to an opposite conclusion, and he has shown how the
cases referred to in corroboration can be otherwise interpreted.[10] We
may, therefore, assert that the most ancient Cornish lodes are younger
than the coal-measures of that part of England, and it follows that
they are of a much later date than the Irish copper and lead of Wexford
and some adjoining counties. How much later, it is not so easy to
declare, although probably they are not newer than the beginning of the
Permian period, as no tin lodes have been discovered in any red
sandstone which overlies the coal in the south-west of England.

There are lead veins in Glamorganshire which enter the lias, and others
near Frome, in Somersetshire, which have been traced into the Inferior
Oolite. In Bohemia, the rich veins of silver of Joachimsthal cut
through basalt containing olivine, which overlies tertiary lignite, in
which are leaves of dicotyledonous trees. This silver, therefore, is
decidedly a tertiary formation. In regard to the age of the gold of the
Ural mountains, in Russia, which, like that of California, is obtained
chiefly from auriferous alluvium, it occurs in veins of quartz in the
schistose and granitic rocks of that chain, and is supposed by Sir R.
Murchison, MM. Deverneuil and Keyserling to be newer than the syenitic
granite of the Ural—perhaps of tertiary date. They observe that no gold
has yet been found in the Permian conglomerates which lie at the base
of the Ural Mountains, although large quantities of iron and copper
detritus are mixed with the pebbles of those Permian strata. Hence it
seems that the Uralian quartz veins, containing gold and platinum, were
not formed, or certainly not exposed to aqueous denudation, during the
Permian era.

In the auriferous alluvium of Russia, California, and Australia, the
bones of extinct land-quadrupeds have been met with, those of the
mammoth being common in the gravel at the foot of the Ural Mountains,
while in Australia they consist of huge marsupials, some of them of the
size of the rhinoceros and allied to the living wombat. They belong to
the genera Diprotodon and Nototherium of Professor Owen. The gold of
Northern Chili is associated in the mines of Los Hornos with copper
pyrites, in veins traversing the cretaceo-oolitic formations, so-called
because its fossils have the character partly of the cretaceous and
partly of the oolitic fauna of Europe.[11] The gold found in the United
States, in the mountainous parts of Virginia, North and South Carolina,
and Georgia, occurs in metamorphic Silurian strata, as well as in
auriferous gravel derived from the same.

Gold has now been detected in almost every kind of rock, in slate,
quartzite, sandstone, limestone, granite, and serpentine, both in veins
and in the rocks themselves at short distances from the veins. In
Australia it has been worked successfully not only in alluvium, but in
vein-stones in the native rock, generally consisting of Silurian shales
and slates. It has been traced on that continent over more than nine
degrees of latitude (between the parallels of 30° and 39° S.), and over
twelve of longitude, and yielded in 1853 an annual supply equal, if not
superior, to that of California; nor is there any apparent prospect of
this supply diminishing, still less of the exhaustion of the
gold-fields.

_Origin of Gold in California._—Mr. J. Arthur Phillips,[12] in his
treatise “On the Gold Fields of California,” has shown that the ore in
the gold workings is derived from drifts, or gravel clay, and sand, of
two distinct geological ages, both comparatively modern, but belonging
to different river-systems, the older of which is so ancient as to be
capped by a thick sheet of lava divided by basaltic columns. The
auriferous quartz of these drifts is derived from veins apparently due
to hydrothermal agency, proceeding from granite and penetrating strata
supposed to be of Jurassic and Triassic date. The fossil wood of the
drift is sometimes beautifully silicified, and occasionally the trunks
of trees are replaced by iron pyrites, but gold seems not to have been
found as in the pyrites of similarly petrified trees in the drift of
Australia.

The formation of recent metalliferous veins is now going on, according
to Mr. Phillips, in various parts of the Pacific coast. Thus, for
example, there are fissures at the foot of the eastern declivity of the
Sierra Nevada in the state of that name, from which boiling water and
steam escape, forming siliceous incrustations on the sides of the
fissures. In one case, where the fissure is partially filled up with
silica inclosing iron and copper pyrites, gold has also been found in
the vein-stone.

It has been remarked by M. de Beaumont, that lead and some other metals
are found in dikes of basalt and greenstone, as well as in mineral
veins connected with trap-rock, whereas tin is met with in granite and
in veins associated with the Plutonic series. If this rule hold true
generally, the geological position of tin accessible to the miner will
belong, for the most part, to rocks older than those bearing lead. The
tin veins will be of higher relative antiquity for the same reason that
the “underlying” igneous formations or granites which are visible to
man are older, on the whole, than the overlying or trappean formations.

If different sets of fissures, originating simultaneously at different
levels in the earth’s crust, and communicating, some of them with
volcanic, others with heated Plutonic masses, be filled with different
metals, it will follow that those formed farthest from the surface will
usually require the longest time before they can be exposed
superficially. In order to bring them into view, or within reach of the
miner, a greater amount of upheaval and denudation must take place in
proportion as they have lain deeper when first formed and filled. A
considerable series of geological revolutions must intervene before any
part of the fissure which has been for ages in the proximity of the
Plutonic rock, so as to receive the gases discharged from it when it
was cooling, can emerge into the atmosphere. But I need not enlarge on
this subject, as the reader will remember what was said in the 30th,
32nd, and 35th chapters on the chronology of the volcanic and hypogene
formations.

 [1] Geol. Trans., vol. iv, p. 139; Trans. Royal Geol. Society,
 Cornwall, vol. ii, p. 90

 [2] Conybeare and Phil. Geol., p. 401, and Farey’s Derbyshire, p. 243.

 [3] Fournet, Études sur les Dépôts Métallifères.

 [4] Carne, Trans. Geol. Soc., Cornwall, vol. iii, p. 238.

 [5] Geol. Rep. on Cornwall, p. 340.

 [6] Principles, chap. xxvii, 8th edit., p. 422.

 [7] See Dr. Daubeny’s Volcanoes.

 [8] Bulletin, iv, p. 1278.

 [9] Sir H. De la Beche, MS. Notes on Irish Survey.

 [10] Report on Geology of Cornwall, p. 310.

 [11] Darwin’s South America, p. 209, etc.

 [12] Proc. Royal Soc., 1868, p. 294.


INDEX.

——::——

_The Fossils, the names of which appear in Italics, are figured in the
Text._

ABBEVILLE, flint tools of, 152
Aberdeenshire, granite of, 558
Abich, M., on trachytic rocks, 504
_Acer trilobatum,_ Miocene, 220, 221
_Acrodus nobilis,_ Lias, 359
Acrogens, term explained, 303
_Acrolepis Sedgwickii,_ Permian, 390
_Actæon acutus,_ Great Oolite, 345
_Actinocyclas,_ in Atlantic mud, 288
Actinolite, 499, 502
—— schist, 578
_Æchmodus Leachii,_ Lias, 358
_Adiantites Hibernica,_ Old Red, 441
Agassiz on fish of Sheppey, 267
—— on fish of the Brown-Coal, 540
—— on fish of Monte Bolca, 544
—— on Old Red fossil fish, 443, 447
—— on Silurian fish, 460
Age of metamorphic rocks, 597
—— of Plutonic rocks, 564
—— of strata, tests of, 123
—— of volcanic rocks, 520
Agglomerate described, 509
_Agnostus integer. A. Rex_, 488
Air-breathers of the Coal, 413
Aix-la-Chapelle, Cretaceous flora of, 302
Alabaster defined, 39
Alberti on Keuper, 376
Albite, 499, 500
Aldeby and Chillesford beds, 192
Alkali, present in the Palæozoic strata, 587
Alpine blocks on the Jura, 169
Alps, age of metamorphic rocks in, 599
——, nummulitic limestone and flysch of, 77
Alum schists of Norway and Sweden, 489
Alluvial deposits, Recent and Post-pliocene, 151
Alluvium, term explained, 99
—— in Auvergne, 100
Alternations of marine and fresh-water strata, 72
Alum Bay beds, plants of the, 262
Amblyrhynchus cristatus, a living marine saurian, 362
America. _See_ United States, Canada, Nova Scotia.
——, North, Glacial formations of, 182
——, South, gradual rise of land in, 72
——, Silurian strata of, 478
American character of Lower Miocene flora, 238
—— forms in Swiss Miocene flora, 223
Amiens, flint tools of, 152
_Ammonites bifrons,_ Lias, 356
—— _Braikenridgii,_ Oolite, 351
—— _Bucklandi,_ Lias, 356
—— _Deshayesii,_ Neocomian, 311
—— _Humphresianus,_ Inferior Oolite, 351
—— _Jason,_ Oxford Clay, 340
—— _Noricus,_ Speeton, 312
—— _macrocephalus,_ Oolite, 352
—— _margaritatus,_ Lias, 357
—— _planorbis,_ Lias, 356
—— _rhotomagensis,_ Chalk marl, 298
Amphibole group of minerals, 499, 502
_Amphistegina Hauerina,_ Vienna basin, 225
_Amphitherium Broderipii,_ in Stonesfield, 348
—— _Prevostii,_ Stonesfield slate, 347
_Ampullaria glauca_, 56
_Amygdaloid_, 507
Analcime, 500
Anamesite, a variety of basalt, 504
_Ananchytes ovatus,_ White chalk, 293
——, with crania attached, 49
_Ancillaria subulata,_ Eocene, 57
_Ancyloceras gigas_, 309
—— _spinigerum,_ Gault, 301
—— _Duvallei,_ Neocomian, 312
_Ancylus velletia (A. elegans)_, 55
Andalusite, 500
Andes, Plutonic rocks of the, 569
Andreasburg, metalliferous vein of, 611
Angelin, on Cambrian of Sweden, 489
Angiosperms, 303
—— of the Coal, 429
Anglesea, dike cutting through shale in, 514
_Anodonta Cordierii_, 54
—— _Jukesii,_ Upper Old Red, 441
—— _latimarginata_, 54
_Anoplotherium commune,_ Binstead, 254
—— _gracile,_ Paris basin, 271
Anorthite, 499, 501
_Annularia sphenophylloides,_ Coal, 425
_Antholithes,_ coal-measures, 429
Anthracite, conversion of coal into, 408
Anticlinal and synclinal curves, 74, 85
Antrim, Chalk altered by a dike in, 516
——, Lower Miocene, volcanic rocks of, 539
Antwerp Crag, 204
Apateon pedestris, a carboniferous reptile, 406
Apatite, 500
Apennines, Northern, metamorphic rocks of, 599
Apes, fossil of the Upper Miocene, 215
_Apiocrinites rotundus,_ Bradford, 343
Appalachians, long lines of flexures in, 92, 93
——, vast thickness of successive strata in, 110
_Aptychus,_ part of ammonite, 336
Aqueous rocks defined, 27, 35
_Araucaria sphærocarpa,_ Inferior Oolite, 348
Arbroath, section of Old Red at, 74
_Archæopteryx macrura,_ Solenhofen, 338
_Archegosaurus minor and A. medius,_ coal measures, 406, 407
Archiac, M. de, on nummulites, 277
——, on chalk of France, 306
Arctic Miocene Flora, 239
Area of the Wealden, 319
Areas, permanence of continental, 117
Arenaceous rocks described, 35
_Arenicolites linearis,_ Arenig beds, 475
Arenig or Stiper-Stones group, 474
——, volcanic formations of, 549
Argile plastique, 276
Argillaceous rocks described, 36
Argillite, Argillaceous schist, 579
Argyll, Duke of, on Isle of Mull leaf-beds, 247
Armagh, bone-beds in Mountain Limestone at, 437
Arran, amygdaloid filled with spar near, 518
——, erect trees in volcanic ash of, 546
——, Greenstone dike in, 514
Arthur’s seat, trap rocks of, 545
_Arvicola,_ tooth of, 165
_Asaphus caudatus,_ Silurian, 467
—— _tyrannus, A. Buchii_, 474
Ascension, lamination of volcanic rocks in, 595
Ash, Mr., on fossils of Tremadoc beds, 483
Ashby-de-la-Zouch, fault in coal field of, 91
_Aspidura loricata,_ Muschelkalk, 379
_Astarte borealis_ (=_A. arctica=A. compressa_), 176
—— _Omalii,_ Crag, 199
_Asterophyllites foliosus,_ Coal, 425
_Astrangia lineata (Anthophyllum lineatum)_, 229
_Astræa basaltiforme,_ Carboniferous, 432
_Astropecten crispatus,_ London clay, 266
Atherfield clay, 309
Atlantic mud, composition of, 287
_Atrypa reticularis,_ Aymestry, 462
_Aturia ziczac (Nautilus ziczac)_, 266
Augite, 499, 502
_Auricula,_ recent, 55
Austen, Mr. Godwin, on marine deposit of Selsea Bill, 182
——, on boulders in chalk, 292
Australian cave breccias, 158
Australia, auriferous gravel of, 617
Auvergne, alluvium in, 100
——, chain of extinct volcanoes in, 495
——, granite veins in, 610
——, Lower Miocene of, 233
——, Miocene volcanic rocks of, 540
——, Post-pliocene volcanic eruptions in, 527
——, springs from spent volcanoes in, 604
Aveline Mr., on Tarannon shales, 468
_Avicula contorta,_ Rhætic beds, 366
—— _cygnipes,_ Lias, 355
—— _inæquivalvis,_ Lias, 355
—— _socialis,_ Muschelkalk, 379
_Aviculopecten papyraceus,_ coal measures, 405
—— _sublobatus,_ mountain limestone, 434
Aymestry Limestone, 461
Azoic period, supposed, 603
Azores, Miocene lavas with shells, 539

_BACILLARIA paradoxa_, 51
_Baculites anceps,_ Lower Chalk, 298
—— _Fauiasii,_ chalk, 286
Baffin’s Bay, formation of drift in, 171, 173
Bagshot sands, 258, 259, 262
Baiæ, Bay of, subterranean igneous action in, 569
Bakewell, Mr., on cleavage in Swiss Alps, 590
Bala and Caradoc beds, 470
_Balistidæ,_ defensive spine of, 261
Bangor, or Longmynd group, 485
_Banksia,_ seed and fruit of, Lower Miocene, 238
Barmouth sandstones, 486
Barnes, Mr. J., on insects in American coal, 416
Barnstaple, Upper Devonian of, 450
Barrande, M. Joachim, his “Primordial Zone,” 471, 482, 487
——, on metamorphosis of trilobites, 471
Barrett, Mr., on bird in Blackdown beds, 299
Barton series sands and clays, 258
—— shells, percentage of, common to London clay, 258
Basalt, columnar, 511
——, composition of, 504
Basaltic rocks, poor in silica, 504
——, specific gravity of minerals in, 504
_Basilosaurus,_ Eocene, United States, 280
Basset, term explained, 83
Basterot, M. de, on Bordeaux tertiary strata, 141
Bath Oolite, 342
Batrachian reptiles in coal, 406
Bay of Fundy, denudation in coalfield in, 418
Bean, Mr., on Yorkshire Oolite, 350
Bear Island carboniferous flora, 441
Beaumont, M. E. de, on island in Cretaceous sea, 305
——, on mineral veins, 613
——, on Jurassic plutonic rocks, 571
——, on formation of granite, 553
Beckles, Mr. S. H., on footprints in Hastings sands, 315, 330
—— on Mammalia of Purbeck, 326
_Belemnitella mucronata,_ Chalk, 283
_Belemnites hastatus,_ Oxford clay, 340
—— _Puzosianus,_ Oxford clay, 341
Belgium, Lower Miocene of, 241
_Bellerophon costatus,_ Mountain Limestone, 436
_Belosepia sepioidea,_ Sheppey, 266
Belt, Mr., on subdivision of Lingula Flags, 484
Bembridge beds, Yarmouth, 252
Berger, Dr., on rocks altered by dikes, 515
Berlin, Miocene strata near, 242
Bernese Alps, gneiss in the, 599
Berthier on isomorphism, 502
Bertrich-Baden, columnar basalt of, 512
Beyrich on term Oligocene for Lower Miocene, 244
Billings, Mr., on trilobites, 471
Binney, Mr., on Sigillariæ in volcanic ash, 546
——, on Stigmaria, the root of Sigillaria, 426
Biotite, 499, 501
Bird in argile plastique, 276
Bischoff, Professor, on Nile and Rhine mud, 154
——, on conversion of coal into anthracite, 403
——, on hydrothermal action, 586
Blackdown beds, 301
Blacklead of Borrowdale, 65
Bog-iron-ore, 52
Bohemia, Cambrian rocks of, 487
——, silver veins in, 616
Bolderberg, in Belgium, Upper Miocene of, 224
Bone-bed of fish remains, Armagh, 437
—— of Upper Ludlow, 450
—— of the Trias, 367
Boom, Lower Miocene of, 241
Bordeaux, Upper Miocene of, 214
Borrowdale, blacklead of, 65
Bosquet, M. on chalk fossils, 283
——, on Maestricht beds, 283
Botanical nomenclature, 303
Boucher de Perthes on Abbeville alluvium, 152
Boulder-clay, whether formed by icebergs or land-ice, 166-73, 178
Boulder-clay of Canada, 182
—— fauna of, 176, 189
Boulders and pebbles in chalk, 292
Bournemouth beds (Lower Bagshot), 262
Bovey Tracey, lignites and clays of, 246
Bowerbank, Mr., on fossil fruits of London Clay, 265
——, on fossil fruits of Sheppey, 265
Bowman, Mr., on uniting of distinct coal-seams, 401
Brachiopoda, preponderance of, in older rocks, 470
——, mode of recognising shells of, 471
Bracklesham beds and Bagshot Sands, 259
Bradford encrinites, 342
Breccias of Lower Permian, 391
Brick-earth or fluviatile loam, 153
Bridlington drift, 189
Bristol, dolomitic conglomerate of, 373
Bristow, Mr., on volcanic minerals, 500
Brixham cave near Torquay, 158
Brocchi on Italian tertiary strata, 141
—— on subapennine strata, 208
Brockenhurst, corals and shells of, 257
Brodie, Rev. P. B., on Lias insects, 363
Brodie, Mr. W. R., on Purbeck mammalia, 326
Brongniart, M. Adolphe, on botanical nomenclature, 303
——, on Lias plants, 364
——, on flora of the Bunter, 380
——, on flora of the coal, 420
——, on fruit of Lepidodendron, 424
——, M. Alex., on Tertiary series, 141
_Bronteus flabellifer_, Devonian, 453
Brora, oolitic coal formation of, 350
Brown, Mr. Richard, on Stigmaria, 426
——, on carboniferous rain-prints, 416
Brown, Robert, on Eocene protaceous fruit, 264
Brown, Reverend T., on marine shells in Scotch drift, 177
Brown-coal of Germany, 540
Bryce, Mr., on Scotch till, 176
Bryozoa of Mountain Limestone, 433
—— and polyzoa, terms explained, 197
Buch, von. _See_ Von Buch.
Buckland, Dr., on Kirkdale cave, 158
——, on violent death of saurians, 362
——, on spines of fish, 359
——, on Eocene oysters, 268
——, on pot-stones in chalk, 291
Buddle, Mr., on creeps in coal-mines, 78
_Bulimus ellipticus_, Bembridge, 253
—— _lubricus_, Loess, 56
Bullock, Capt., R.N., on Atlantic mud, 287
Bunbury, Sir C., on leaf-bed of Madeira, 532
——, on ferns of the Maryland coal, 421
Bunter of Germany, 380
—— or Lower Trias of England, 372
_Buprestis? Elytron of_, Stonesfield, 346
Burmeister on trilobites, 471

CAINOZOIC, term defined, 123
Caithness, fish beds of, 443
_Calamite_, root of, 425
_Calamites Sucowii_, coal, and restored stem, 424
_Calamophyllia radiata_, Bath Oolite, 342
Calcaire de la Beauce, age of the, 230
—— grossier, fossils of the, 274
—— siliceux of France, 273
Calcareous matter poured out by springs, 604
—— rocks described, 36
—— nodules in Lias, 63
_Calcarina rarispina_, Eocene, 275
_Calceola sandalina_, Devonian, 453
——, schiefer of Germany, 453
California, aurifrous gravel of, 617
——, gold in petrified wood of age of alluvium, 601
_Calymene Blumenbachii_, Silurian, 466
Cambrian Group, classification of the, 481
Cambrian, Upper, 482
——, Lower, 484
——, of Sweden and Norway, 489
——, strata of Bohemia, 487
——, of North America, 489
——, volcanic rocks, 549
_Campophyllum flexuosum_, 431
Canada, Cambrian of, 489
——, Devonian of, 455
——, trap-rocks of, 549
Canadian drift, 182
Canary, Grand, shelly tuffs of, 538
Cantal, Lower Miocene of the, 231
Cape Breton, rain-prints in coal-measures of, 416
Cape Wrath, granite veins in gneiss at, 560
Caradoc and Bala beds, 470
Carbonate of lime in rocks, how tested, 37
Carboniferous Group, subdivisions of the, 394
—— flora, 420-30
—— limestone, thickness of, 396
——, marine fauna of the, 432
—— Period, trap-rocks of, 545
—— plutonic rocks, 572
—— reptiles, 406
—— insects, 405
_Carcharodon angustidens_, Bracklesham, 262
Cardiganshire, section of slaty cleavage in, 589
_Cardiocarpon Ottonis_, Permian, 393
_Cardita (Venericardia) planicosta_, 260
—— _sulcata_, Barton, 259
_Cardium dissimile_, Portland Stone, 336
—— _rhæticum_, Rhætic Beds, 366
—— _striatulum_, Kimmeridge clay, 336
Carne, Mr. N., on Cornish lodes, 607
Carpenter, Dr., on Atlantic mud, 288
——, on Eozoon Canadense, 491
Carrara, marble of, 599
Carruthers, Mr., on Eocene proteaceous fruit, 265
——, on cycads of the Purbeck, 332
——, on leaves of calamite, 425
——, on spores of carboniferous Lycopodiaceæ, 422
——, on structure of sigillaria, 426
——, on trees in volcanic ash, 547
Cashmere, recent formations in, 146
Cassian, St., Triassic strata of, 376
Castrogiovanni, curved strata near, 86
Catania, laterite formed in, 510
——, Tertiary beds in, 206
_Catillus Lamarckii_, White Chalk, 295
Caucasus, absence of lakes in the, 187
_Caulopteris primæva_, Coal, 421
Cave-breccias of Australia, 158
Cavern deposits with human and animal remains, 156
Caves of Kirkdale and Brixham, 157
Celts described, 152
Cementing of strata, 61
_Cephalaspis Lyelli_, Old Red, 446
_Ceratites nodosus_, Muschelkalk, 379
_Cerithium concavum_, Headon, 256
—— _elegans_, Hempstead beds, 245
—— (_Terebra_) Portlandicum, 335
—— _plicatum_, Hempstead beds, 245
—— _melanoides_, 268
_Cervus alces_, tooth of, 164
_Cestracion Phillippi_, Recent, 297
Chabasite, 500
Chalk, composition, extent, and origin of, 286
—— of Faxoe, 286
—— flints, origin of, 290
—— fossils of the White, 293-6
——, iceborne boulders in the, 292
—— of North and South Europe, 305
——, Lower White, without flints, 298
—— marl, fossils of the, 298
—— Period, popular error concerning, 288
Chalk-pit with pot-stones, view of, 291
_Chama squamosa_, Barton, 258
Champoleon, junction of granite with Jurassic strata near, 571
_Chara elastica, C. medicaginula_, 58
—— _tuberculata_, Bembridge, 253
Charpentier, M., on Alpine glaciers, 170
——, on depression of Alps in Glacial Period, 185
Chatham coal-field, 383
_Cheirotherium_, footprints of, 372
Chemical deposits in veins, 612
—— and mechanical deposits, 60
Chiapa, fall of volcanic dust at, 523
Chichester, erratics near, 181
Chili, copper pyrites with gold in, 616
——, walls cracked by earthquake in, 87
Chillesford and Aldeby beds, 192
_Chimæra monstrosa_, Lias, 359
Chlorite-schist, 579
Chloritic series, or Upper Greensand, 298
Christiania, Euritic porphyry at, 562
——, granite veins in Silurian strata of, 572
——, quartz vein in gneiss at, 561
Chronological groups of formations, 129
Chronology, test of, in rocks, 121
Cinder-bed of the Purbeck, 325
_Cinnamomum polymorphum_, Miocene, 219
—— _Rossmässleri_, Miocene, 239
Claiborne beds, Eocene fossils of, 279
Clarke County, United States, Zeuglodon of, 279
Classification of Tertiary formations, 137, 143
——, value of shells in, 142
_Clausilia bidens_, Loess, 56
Clay defined, 36
—— iron-stone defined, 404
——, plastic, 267
—— slate, 579
——, Weald, 313
Cleavage explained, 502
——, crystalline theory of, 591
——, mechanical theory of, 592
—— of metamorphic rocks, 588
_Cleidotheca operculata_, 483
Clermont, metalliferous gneiss near, 586
Climate of the Crags, 200
—— of the Coal, 430
—— of the Miocene in the Arctic regions, 240
—— of the Post-pliocene period, 161
Clinkstone, 506
Clinton group, fossils of the, 479
Clyde, buried canoes in estuary of, 146
——, arctic marine shells in drifts of, 176
_Clymenia linearis_, Devonian, 451
Clymenien-Kalk of Germany, 450
Coal, conversion into anthracite of, 403
—— a land and swamp formation, 397
——, cause of the purity of, 402
——, conversion of lignite into, 403
——, erect trees in, 411
——, structure of the, 412
——, vegetation of the, 420
——, air-breathers in the, 405, 413
Coal Period, climate of the, 430
—— field of Virginia, 382
—— measures of Nova Scotia, 408
—— measures, thickness of, in Wales, 397
—— pipes, danger of, 390
——, rainprints in, 416
—— seams, uniting of, 400
Coalbrook-Dale, faults in, 88
_Cochliodus contortus_, 437
Cockfield Fell rocks, altered by dikes, 516
_Coelacanthus granulatus,_ Permian, 390
Coleoptera of Œningen beds, 223
_Collyrites ringens,_ Inferior Oolite, 351
Columnar structure of volcanic rocks, 510
—— basalt in the Vicentin, 511
Compact feldspar, 501
Concretionary structure, 63
Cone of Tartaret, 527, 542
—— of Côme, 28
Cones and craters described, 495
——, absence of, in England, 30
Conformable stratification, 39
Conglomerate or pudding-stone, 36
——, Dolomitic, of Bristol, 373
Coniferæ of the coal-measures, 427
Connecticut Valley, New Red Sandstone of, 381
_Conocephalus striatus_, 488
_Conocoryphe striata_, 488
Conrad, Mr., on age of American cretaceous rocks, 307
Consolidation of strata, 61
Continents and oceans, permanence of, 117
Contorted strata, in drift, 178
_Conularia ornata,_ Devonian, 453
_Conulus priscus,_ Coal, 415
_Conus deperditus,_ Bracklesham, 262
Conybeare and Phillips on ninety-fathom dike, 90
Conybeare, Mr., on reptiles of the Lias, 360
Copper lode near Redruth, 607
Coprolite bed of Chloritic Series, 299
—— beds of Red and Coralline crags, 197, 198
_Coprolites of fish from the chalk_, 298
Coral Rag, fossils of the, 339
Coralline of White Crag, 197
Corals of the Devonian, 451
—— of the Mountain Limestone, 433
——, _Neozoic type of_, 431
——, _Palæozoic type of_, 431
_Corbicella (Cyrena) fluminalis_, 54
_Corbula pisum,_ Hempstead beds, 245
Corinth, corrosion of rocks by gases near, 586
Cornbrash or Forest Marble, 341
Cornwall, granite veins in, 561, 582
——, lodes in, 615
——, mass of granite in, 552
——, vertical sections of veins in mine, 607
Cosequina volcano, burying of organic remains by, 523
Crag, term defined, 192
—— of Antwerp, 204
——, fauna of, its relation to that of present seas, 201
——, Norwich, 193
——, Coralline or White, 197
——, Red, 194
——, tables of marine testacea in, 202
—— deposits, climate of, 200
_Crania_ attached to a sea-urchin, 49
—— _Parisiensis,_ White Chalk, 294
_Crassatella sulcata,_ Barton, 259
Craters and cones described, 495
——, Theory of Elevation, 496
Craven fault, 90
Creeps in coal-mines, 78
Cretaceous rocks of United States, 307
—— Period, error as to continuity of, 288
——, flora of the Upper, 302
—— volcanic rocks, 544
—— plutonic rocks, 570
—— Period, distinct mineral character of rocks in, 292
—— rocks, classification of, 282
—— strata, connection between Upper and Lower, 301
Crinoidea of Mountain Limestone, 433
Croatia, Lower Miocene beds of, 242
Croll, Mr., on amount of subaërial denudation, 114
Cromer forest-bed, 191
Crop out, term explained, 83
Crossopterygidæ, or fringe-finned fish, 443
Crowfoot, Mr., on shells of Aldeby beds, 192
Crust of the earth defined, 26
Crustaceans of Old Red Sandstone, 446
_Cryptodon angulatum,_ London Clay, 266
Crystalline Limestone, 579
—— rocks defined, 32
—— schists, much alkali in the, 587
—— theory of cleavage, 591
Cup and Star corals, 431
Curved strata, 73-76
Cutch, salt-layers in the Runn of, 375
Cuvier, M., on fauna of the Paris basin, 271
——, on Mammalia of Paris gypsum, 231
——, on Tertiary series, 141
_Cyathocrinus caryocrinoides_, 433
—— _planus_, 433
_Cyathophyllum cæspitosum_, 451
Cyclopean isles, beds of tuff and clay in, 529
——, contorted strata in, 530
_Cyclopteris Hibernica,_ Old Red, 441
_Cyclostigma (Lepidodendron),_ Old Red, 441
_Cyclostoma elegans,_ Loess, 56
_Cylindrites acutus,_ Great Oolite, 345
Cypress swamps of the Mississippi, 402
Cyprides in the Weald Clay, 315
_Cypridina serrato-striata_, 451
Cypris in fresh-water deposits, 57
—— _gibbosa, C. tuberculata, C. leguminella_, 324
—— _striato-punctata, C. fasciculata, C. granulata_, 325
—— _Purbeckensis, Cypris punctata_, 331
—— _spinigera,_ Weald Clay, 315
_Cyrena (Corbicella) fluminalis_, 54
—— _cuneiformis,_ Woolwich Clays, 268
—— _obovata_, 54
—— _semistriata,_ Hempstead beds, 245
Cystideæ of Silurian rocks, 466
_Cythere inflata,_ coal-measures, 405

DADOXYLON, fragment of coniferous wood, 428
Dana, on volcanic minerals, 500
Danish kitchen-middens, 146
_Dapedius monilifer_, Lias, 358
Darbishire on shells of Moel Tryfaen, 180
Dartmoor, post-carboniferous granite of, 572
—— intrusive granite at, 572
Darwin, Mr., on foliation and lamination, 595
——, on mammalia of South America, 160
——, on marine saurian, 362
——, on rise of part of South America, 72
——, on sinking of coral reefs, 72
——, on plutonic rocks of the Andes, 569
——, on relationship of extinct to living types, 160
Dates of discovery of fossil vertebrata, 464
Daubeny, Dr., on decomposition of trachytic rocks, 586
Daubrée, on formation of zeolites, 521
——, on alkaline waters of Plombières, 584
Davidson, Mr., on Spiriferina, 355
Davis, Mr. E., on fossils of Lingula Flags, 484
Dawkins, Mr. Boyd, on Hyæna spelæa, 158
——, on mammalia of Cromer Forest-bed, 191
——, on Triassic mammifer, 369
Dawson, Dr., on Devonian flora and insects, 456, 457
——, on Eozoon Canadense, 491
——, on Nova Scotia coal-measures, 409
——, on Nova Scotia coal-plants, 410, 412
——, on Pupa vetusta, 415
——, on reptiles and shells in Nova Scotia coal, 413
——, on structure of calamite, 425
——, on structure of sigillaria, 426
Deane, Dr., on footprints in Trias, 382
Debey, Dr., on flora and fauna of Aix, 302-04
Dechen, M. von, on organic remains of the brown coal, 540
——, on Cornish granite veins, 560
De la Beche, Sir H., on granite of Dartmoor, 582
——, on Carrara marble, 599
——, on mineral veins, 616
——, on Redruth copper-mine, 610
——, on saurians of the Lias, 362
——, on trap-rocks of New Red, 545
——, on Welsh coal-measures, 397
Delesse, on action of water in metamorphism, 585
Deltas, strata accumulated in, 28
Dendrerpeton in Coal, 413
Denudation defined, 96
——, subaërial, 97
——, littoral, 102
——, submarine, 105
——, average annual amount of subaërial, 113
—— of carboniferous strata, 396
—— counteracting upheaval, 106-15, 108-15
—— a means of exposing crystalline rocks, 563
——, trap-dikes cut off by, 518
—— and volcanic force antagonistic powers, 115
Deposition, rate of, shown by fossils, 47
Derbyshire, veins in Mountain Limestone, 608
Derivative shells of the Red Crag, 195-203
Desnoyers, M., on age of Faluns, 142
——, on Eocene fossil footprints, 272
Desor, M., on Celtic coins in lake-dwellings, 149
Devonian Period, Upper, 450
Middle, 450
Lower, 453
—— fossils of the Eifel, 534
—— of Russia, 454
—— of United States and Canada, 455
—— insects of Canada, 457
—— strata, classification of, 439-50
Devonshire, cleavage of slate rocks in, 593
Diabase, 505
Diagonal, or cross-stratification, 42
Diagram of fossiliferous rocks, 137
—— of plutonic and sedimentary formations, 567
Diallage, 500, 502
_Diastopora diluviana_, Bath Oolite, 343
Diatomaceæ forming tripoli, 51
_Diceras Lonsdalii_, Neocomian, 310
_Didelphys Azaræ_, Recent, 347
_Didymograpsus geminus_, 476
—— _Murchisonii_, 473
Dike cutting through shale, Anglesea, 515
—— cutting through chalk, Antrim, 515, 516
_Dikelocephalus Minnesotensis_, 490
Dikes defined, 30
—— of Monte Somma, 526
—— in Palagonia, ground-plan of, 532
——, volcanic or trap, 513-7
Diluvium, origin of term, 167
Dinornis Palapteryx, of New Zealand, 160
_Dinotherium giganteum_, 212
Diorite, 505
Dip and strike, terms explained, 80
_Diplograpsus folium_, Llandeilo Flags, 474
—— _pristis_, Llandeilo beds, 473
Dirt-bed of the Purbeck, 331
Dogger-bank described, 105
Dolerite, a variety of basalt, 504
Dolomite defined, 38
Dolomitic conglomerate of Bristol, 373
Downs, escarpments of North and South, 104
Downton Sandstone, 459
Dowson, Mr., on shells of Aldeby beds, 192
Drew, Mr., on Hastings Sands, 316
Drift of Ireland, 190
—— of Norfolk cliffs, 190
—— of Scandinavia, 174
—— of Bridlington, 189
—— carried by icebergs, 172
—— shells in Canada, 183
——, contorted strata in, 178
——, marine shells in Scotch, 175
Dudley Limestone, 465
Dufrenoy, M., on granite of Pyrenees, 582
Dumont, Professor, on Belgian Lower Eocene, 282
Duncan, Dr., on Neozoic corals passing down to Devonian, 432
Dundry Hill, near Bristol, section of, 130
Dunker, Dr., on wealden of Germany, 319
Dura Den, yellow sandstone of, 440

EARTH’S crust defined, 26
Echinoderms of Suffolk Crag, 200
_Echinosphæronites balticus_, 472
Egerton, Sir P., on fish of Headon series, 256
——, on fish of the Permian, 389
——, on fish of Penarth beds, 366
Ehrenberg, Professor, on term Bryozoum, 197
——, on Silurian foraminifera, 478
——, on infusoria, 51
Eifel Limestone, 453
——, Lake-craters of, 534
—— Miocene, volcanic rocks of, 539
—— Pliocene, volcanoes of the, 534
——, trass of the, 535
_Elephas antiquus_, molar of, 163
—— _meridionalis_, molar of, 163
—— _primigenius_, molar of, 162
Elevation craters, theory of, 496
Elvans, term explained, 572
—— of Ireland and Cornwall, 615
_Elytron of Buprestis?_ Stonesfield, 346
Emmons, Professor, on jaws of Triassic quadruped, 383
——, on Dromatherium, 383
Encrinites of Bradford, 342
_Encrinus liliiformis_, Muschelkalk, 379
Endogens, term explained, 303
Engihoul cave, human and animal remains in, 157
England and Wales, glaciation of, 180
Enstatite, 501
Eocene areas of Europe, map of, 250
—— foraminifera, 274
—— formations of France, 270-6
—— of England, 252
—— period, volcanic rocks of, 543
——, plutonic rocks of the, 568
——, metamorphic rocks of the, 598
—— of France, footprints in, 272
—— and Miocene, line between the, 230, 250
——, term defined, 143
—— of the United States, 278
_Eozoon Canadense_, oldest known fossil, 492
Epidote, 500
Eppelsheim, Dinotherium of, 225
Equisetaceæ of the Coal, 424
_Equisetites columnaris_, Keuper, 376
_Equus caballus_, tooth of, 164
Erratic blocks, nature of, 167
—— of Greenland, 171
—— near Chichester, 181
—— in the Red Crag, 201
Erratics, Alpine, 169
Escarpments explained, 104
_Eschara disticha_, White Chalk, 296
_Escharina oceani_, White Chalk, 296
_Estheria minuta_, Trias, 370
—— _ovata_, Richmond, Virginia, 383
Ethridge, Mr., on Atlantic mud, 288
——, on Devonian series, in Devon, 450
——, on Devonian fauna, 451, 454
——, on mollusca of Bracklesham, 260
——, on St. Cassian fossils, 377
Etna, built up since Newer Pliocene, 204
——, Pliocene lavas of, 529
Ettingshausen on Sheppey Eocene fruit, 265
_Eunomia radiata_, Bath Oolite, 342
_Eunotia bidens_, Atlantic mud, 288
_Euomphalus pentangulatus_, 435
Eurite, 557, 578
Euritic porphyry of Norway, 562
Evans, Mr., on Archæopteryx, 337
Exogens, 297
_Exogyra virgula_, Kimmeridge Clay, 336
_Extracrinus (Pentacrinus) Briareus_, Lias, 357

FALCONER, Dr., on Miocene fauna of Siwalik Hills, 226
——, on Brixham Cave flint knives, 157
——, on Purbeck mammalia, 326
Faluns of Loire, recent shells in, 214
—— of Touraine, 211
Farnham, phosphate of lime near, 299
_Fascicularia aurantium_, Coralline crag, 199
Faults in coal-measures of Coalbrook Dale, 88
—— described, 87-92
—— often the result of repeated movements, 90
Fauna of the crag, its relation to that of our present seas, 201
—— of the Mountain Limestone, 430
—— of the Paris basin, 271
_Favosites cervicornis_, Devonian, 451
—— _Gothlandica_, Silurian, 465
Favre, M. E., on glaciers and moraines of the Caucasus, 187
Faxoe, chalk of, 285
Feldspar-porphyry, 557
Feldspar, varieties of, 499, 500
Feldstone, 557
_Felis tigris_, tooth of, 166
_Fenestella retiformis_, Magnesian Limestone, 388
Ferns of the coal, 421
Fife, trap-dike in, 543
Fish, fossil of the Carboniferous, 436
——, Eocene of Monte Bolca, 544
——, oldest known fossil, 463
——, number of living, 445
——, fresh-water and marine, 58
—— of the Upper Ludlow, 459
—— of the Old Red Sandstone, 443-5
—— of the Permian marl slate, 389
—— of the brown coal, 540
—— of the Lias, 358
Fisherton, Greenland lemming in drift of, 161
Fissures, filled with metallic matter, 606
Fitton, Dr., on the Neocomian strata, 314
Fleming, Dr., on Parka decipiens, 448
——, on trap-dike in Fife, 546
Flints in the Chalk, 290
Flisk dike of Fife, 546
Flora of the Carboniferous, 420
——, Devonian, compared to Carboniferous, 457
—— of the Subapennines, 208
——, Lower Miocene of Switzerland, 235
——, Miocene of the Arctic Regions, 239
——, Older Pliocene of Italy, 208
—— of the Permian, 392
—— of the Upper Cretaceous, 302
——, Upper Miocene of Switzerland, 215-22
—— of the Wealden, 320
Fluvio-marine or Norwich Crag, 193
Flysch of the Alps, 278
——, plutonic rocks invading, 568
Folding and denudation of Nova Scotia Carboniferous rocks, 417
Folds of parallel strata, arrangement and direction of, 93
Foliation of crystalline rocks, 595
——, irregularities in, 596
Folkestone and Hythe beds, 308
Fontainebleau, Gres de, 230
Footprints in Potsdam sandstone, 490
—— _of reptiles in Coal-measures_, 408
——, _fossil in New red_, 381
—— in Paris gypsum, 272
Foraminifera, Eocene, 275
—— of Mountain Limestone, 437
—— of the Chalk, 287
Forbes, Mr. David, on glass cavities in quartz, 555
——, on planes of foliation, 595
——, on specific gravity of quartz, 500
——, on volcanic minerals, 498
Forbes, Professor E., on fossils of Bembridge beds, 252
——, on Hempstead beds, 244
——, on shells of the crag, 200
——, on sphæronites, 472
——, on subdivisions of the Purbeck, 333
——, on testacea of the Faluns, 212
——, on thickness of Upper Neocomian, 309
Forest-bed at Cromer, 191
—— marble or cornbrash, 341
——, submerged, 103, 104
——, fossil in Coal, 400
——, fossil of Isle of Portland, 332
Forfarshire, Cephalaspis beds of, 446
——, contorted strata in, 178
Formation, term defined, 27
Fossil, term defined, 29
—— trees erect in coal, 410
—— Fish of Old Red Sandstone, 442
Fossiliferous groups, table of succession of, 131
Fossils, arrangement of, in strata, 47
——, destruction of, in older formations, 139
——, fresh-water and marine, 52
—— obliterated by metamorphic action, 603
——, recent, and Post-pliocene, 154-65
—— of the drift, 176, 180, 192
—— of the Crags, 193-203
——, Upper Miocene, 214-29
——, Lower Miocene of Switzerland, 236
—— of the Hempstead Beds, 244
——, Eocene, 253
—— of the Barton Clay, 259
—— of the White Chalk, 293
—— of the Neocomian, 309
—— of the Oolite, 324
—— of the Stonesfield Slate, 347
—— of the Lias, 354
—— of the Trias, 370
—— of the Magnesian Limestone, 387
—— of the Coal, 405
—— plants of the Coal, 421
—— of the Mountain Limestone, 430
——, Devonian, 449
——, Silurian, 460
——, Cambrian, 484
—— Laurentian, 492
Fournet, M. on metalliferous gneiss, 586
——, on veins in granite, 610
Fox, Rev. D., on Isle of Wight Eocene fossils, 254
Fox, Mr. R., on lodes in Cornwall, 614
Fractures of strata, and faults, 87
Fragments, included, a test of age of plutonic rocks, 565
——, included, a test of age of strata, 129
—— a test of age in volcanic rocks, 524
France, Eocene formations of, 270-6
——, Lower Miocene of, 231
——, Upper Miocene of, 211
Freshfield, Mr., on absence of lakes in the Caucasus, 187
Fresh-water strata, how distinguished from marine, 53-9
—— formation of Auvergne, 233
Fucoid sandstones of Sweden, 489
_Fulgur canaliculatus_, Maryland, 228
Fuller’s earth, fossils of the, 348
Fundy, Bay of, fossil trees exposed in cliffs at, 412
_Fusilina cylindrica_, 438
Fusion of quartz, 500
_Fusus contrarius (Trophon antiquum)_, 196
—— _quadricostatus_, Maryland, 228

GABBRO, 505
_Gaillonella ferruginea_, and _G. distans,_ 52
Galapagos Islands, living marine saurian in, 362
_Galeocerdo latidens_, Bracklesham, 262
_Galerites albogalerus_, White Chalk, 294
Galestes in Middle Purbeck, 328
Ganoids, the type of Old Red Sandstone fish, 443
—— of the Wealden, 316
—— of the Trias, 383
Gaps in the sequence of fossil remains, 138
Garnet, 500
Gases, corrosion of rocks by, 586
Gaudin on Lower Miocene of Switzerland, 236
—— on Pliocene flora of Italy, 209
—— on Proteaceæ in Bournemouth Eocene, 263
Gault, thickness and fossils of, 300
Geikie, Mr. A., on Ayrshire Permian trap-rocks, 545
——, on subaërial denudation, 115
——, on ice erosion of lake-basins, 187
——, on Isle of Mull volcanic rocks, 539
——, on Pentland Old Red volcanic rocks, 548
——, on Silurian metamorphic rocks, 602
——, on syenite of Skye, 568
Geinitz, M., on Permian flora, 393
Gemunder Maar, volcanic rocks of, 534
Geneva, Lower Miocene of, 236
Geology defined, 25
Gergovia, tuffs and associated lacustrine strata of, 542
Germany, Lower Miocene of, 242
——, Triassic fauna of, 375
Gers, Upper Miocene of, 215
_Gervillia anceps_, Neocomian, 310
—— _socialis_, Muschelkalk, 379
Giant’s Causeway basalt, age of, 248
——, laterite of the, 509
——, columnar basalt of, 510
Girgenti, Newer Pliocene of, 207
Glacial drift, distribution and nature of, 166
—— epoch in the Post-pliocene, 166
—— formations of Pliocene age, 189-92
Glaciation of Russia and Scandinavia, 174
—— of Scotland, 175
—— of Wales and England, 180
—— of North America, 182
Glaciers, transporting and abrading power of, 168
Glasgow, marine strata near, 146
Glauconie grossiere, 275
Glen Tilt, junction of granite and schist at, 559
Globiform pitchstone, 512
_Globigerina bulloides_, 288
Globular structure of volcanic rocks, 510
_Glyptostrobus, Europæus,_ Œningen, 223
Gneiss, granite veins traversing, 560
—— defined and figured, 577
——, fundamental, of Scotland, 493
Gold mines of Australia and Chili, 616
—— veins of Russia, 616
—— of California, of age of alluvium, 617
Goldenberg, Professor, on Saarbrück coal insects, 406
Goldfuss, Professor, on reptiles in coal, 406
_Goniatites crenistria_, 436
—— _Listeri,_ coal-measures, 405
Göppert, on American forms in Swiss Miocene flora, 223
—— on petrification, 68
—— on plants of coal-measures, 398
_Gorgonia infundibuliformis,_ Permian, 388
Graham’s Island, forming ashy conglomerate, 549
Grampians, Old Red conglomerates of, 73
——, trap-rocks of the, 547
——, former glaciers in the, 175
Grand Canary, Upper Miocene, shelly tuffs of, 558
Granite, composition of, 552
——, graphic and columnar, 553, 554
——, how far connected with trap-rocks, 558
——, hydrothermal action in formation of, 555
—— metamorphosing fossiliferous strata, 581
——, porphyritic, 556
——, oldest, 574
——, protrusion of solid, 574
——, passage of, into trap, 558
——, schorly, 557
—— veins, 559
—— veins in talcose gneiss, 560
Granton, angiosperm found in coal at, 429
Graptolites of Llandeilo flags, 474
_Graptolites Murchisonii._ Llandeilo flags, 473
_Graptolithus priodon,_ Silurian, 467
Gray’s, Essex, pachyderms found at, 161
Great (or Bath) Oolite, 342
Greece, Upper Miocene formations of, 226
Greenland, continental ice of, 170
——, gradual sinking of, 72
Greenstone, 505
Gres de Beauchamp, Paris basin, 273
Gres de Fontainebleau, age of the, 230
Griffiths, Sir R., on yellow sandstone of Ireland, 441
Grit defined, 36
Groups, older, rise highest above the sea, 139
—— why the newest to be studied first, 140
_Gryllacris lithanthraca,_ coal, 405
_Gryphæa_ coated with _serpulæ_, 48
—— _columba,_ Chloritic Sand, 300
—— _convexa,_ Chalk, 295
—— _incurva (G. arcuata)_, 54, 354
—— _virgula,_ Kimmeridge clay, 336
Gryphite Limestone, 354
Guadaloupe, glass cavities in quartz of, 555
Gulf-Stream, probable abrading power of, 105
Gümbel, M., on Rhætic beds, 366
Gunn, Mrs., on pot-stones in the chalk, 291
Gutbier, Colonel, on Permian flora, 393
Gymnogens, term explained, 303
Gypseous marls of Auvergne, 233
Gypsum and gypseous marl defined, 38, 39
_Gyrolepis tenuistriatus,_ Rhætic beds, 367

HAIME, Mr., on palæozoic corals, 431
_Hakea silicina_ and _Hakea saligna,_ Œningen, 222
Hall, Captain Basil, on Cyclopean Isles, 530
Hall, Sir James, on curved strata, 75
Hall, Mr. J., on Appalachian palæozoic rocks, 110
Hallstadt and St. Cassian beds, 376
_Halysites catenularis,_ Silurian, 465
Hamilton, Sir W., on eruption of Vesuvius, 1779, 526
_Hamites spiniger,_ Gault, 301
Hancock, Mr., on Protosaurus in Permian, 390
Harkness, Professor, on Silurian metamorphic rocks, 602
Harlech grits, fossils of the, 486
Harris, Major, on the Salt Lakes, 374
_Harpactor maculipes,_ Œningen, 224
Harpe, M. de la, on Bournemouth Eocene flora, 263
Hartung, Mr., cited, 496
Hartz mountains, mineral veins of, 608
——, Bunter Sandstein of, 380
Hastings Sands, subdivisions of the, 316
Hautes Alpes, granite of the, 571
Hauy on isomorphism, 502
Headon series, fossils of the, 255
Heat, powerful in consolidating rocks, 65
——, rocks upraised and folded by, 92
Hébert, M., on age of Sables de Bracheux, 330
——, comparison of Sables Moyens and Barton shells, 258
——, on pisolitic limestone, 285
Hebrides, dikes in the, 514
Heer, Professor, on American genera in Swiss Miocene, 239
——, on age of Madeira leaf-bed, 532
——, on Arctic Miocene flora, 239
——, on Bear Island flora, 441
——, on Bovey Tracey Miocene flora, 247
——, on fossil plants of Switzerland, 215, 219, 221, 224, 236
——, on Lower Miocene plants of Mull, 248
——, on Monte Bolca Eocene plants, 263, 543
——, on Proteas of Lower Miocene, 237
——, on plants of Hempstead beds, 246
——, on plants of coal-field, Virginia, 383
——, on Swiss Miocene insects, 223
——, on supposed Proteaceæ of Œningen beds, 221
——, on Superga fossil plants, 244
Heidelberg, varieties of granite near, 560
_Heliolites porosa_, Devonian, 451
_Helix hispida (plebeia)_, 155
—— _labyrinthica,_ Headon, 255
—— _occlusa,_ Bembridge, 253
—— _Turonensis,_ faluns, 56
_Hemicidaris Purbeckensis,_ Purbeck, 324
_Hemipneustes radiatus,_ Chalk, 284
_Hemitelites Brownii,_ Inferior Oolite, 350
Hempstead beds, subdivisions of the, 244
Henry, on absorption of carbonic acid gas in water, 585
Henslow, Professor, on dike in Anglesea, 515
——, on Red Crag coprolite bed, 197
Herschel, Sir J., on slaty cleavage, 590
Hertfordshire pudding-stone, 62
_Heterocercal tail of fish_, 389
Hicks, Dr., on fossils of Arenig beds, 476
——, on fossils of Harlech grits, 486
——, on Menevian beds, 485
Himalaya, shells 18,000 feet high in, 29
——, Upper Miocene of, 226
_Hippopodium ponderosum,_ Lias, 355
_Hippopotamus, tooth of_, 164
Hippurite Limestone, 304
_Hippurites organisans,_ Chalk, 306
_Histioderma hibernica_, 486
Hitchcock, Professor, on Trias footprints, 381
_Holoptychius nobilissimus,_ scale of, and restoration, 442
_Homalonotus Delphinocephalus_, 467
—— _armatus,_ Devonian, 454
Homfray, Mr., on fossils of Tremadoc beds, 483
_Homocercal tail of fish_, 389
Hooghly River, analysis of water, 69
Hooker, Dr., on coniferæ, 429, 430
——, on structure of sigillaria, 426
——, on sporangia of Silurian plant, 460
Horizontality of strata, 40
Horizontal strata, upheaval of, 71
Hornblende, 499, 502
Hornblende-schist, 578
Hörnes, Dr., on fossil mollusca of Vienna basin, 225
Horstead, pot-stones at, 291
Hour-glass illustrating the destruction and renovation of land, 119
Howse, Mr., on Protosaurus in Permian, 390
Hubbard, Professor, on granite of White Mountains, 565
Hudson River Group, fossils of the, 479
Hughes, Mr. T. McKenny, cited, 450
——, on slaty cleavage, 589
——, on protrusion of solid granite, 575
Hull, Mr. E., on breccias in Permian, 391
——, on carboniferous of Lancashire, 395
——, on carboniferous rocks of north of England, 111
——, on faults in Lancashire coal-field, 91
——, on anticlinals and synclinals, Lancashire, 85
——, on thickness of the Upper Trias, 369
——, on thickness of Permian, 386
——, on three lines of flexure since the coal in Lancashire, 94
Human remains of Recent Period, 157
—— in cavern deposits, 156
Humboldt, on mineral character of rocks, 602
Humphrey and Abbot on Mississippi denudation, 114
Hungary, trachyte of, 558
Hunt, Sterry, on action of water in metamorphism, 585
Huronian series, thickness of the, 490
Huxley, Professor, on Atlantic chalk-mud, 287
——, on affinity between reptiles and birds, 338
——, on batrachians of the coal, 407
——, on fish of Old Red Sandstone, 443-5
——, on Pteraspis, 463
Hyæna den of Kirkdale cave, 157
_Hyæna spelæa,_ tooth of, 165
_Hybodus plicatilis,_ Rhætic beds, 367
—— _reticulatus,_ Lias, 359
Hydrothermal action producing metamorphism, 584
—— in formation of granite, 555
—— forming granite veins, 573
_Hymenocaris vermicauda_, 484
_Hyperodapedon Gordoni,_ Trias, 370
Hypersthene, 499, 502
—— rock, 505
—— rocks of Skye, 491
Hypogene rocks, uniformity of mineral character in, 602
—— rocks, term defined, 26
_Hypsiprymnus Gaimardi,_ molar of recent, 327
Hythe, Neocomian beds of, 308

ICE, erosion of lake-basins considered, 184, 188
——, abrading power of, 168
——, continental, of Greenland, 170
Icebergs, drift carried by, 172
—— stranded in Baffin’s Bay, 173
Ice-borne erratics at Chichester, 181
Iceland, glass cavities in quartz of, 555
——, flow of lava in, 523
_Ichthyosaurus communis,_ Lias, 361
Idocrase, 500
Ichthyodorulite of the Lias, 359
_Iguanodon Mantelli,_ Weald Clay, 315
Ilfracombe Group of Devon, 449
Inclined strata, 73
India, Miocene formations of, 226
India, Upper Miocene of, 226
Inferior Oolite, thickness and fossils of, 349
Infusoria in tripoli, 51
Inland sea-cliffs, 103
_Inoceramus Lamarckii,_ White Chalk, 295
Insect in American coal, 416
—— beds of the Lias, 363
_Insect wing of neuropterous_, 363
Insects, Devonian, of Canada, 457
—— in European coal, 405
——, Miocene, of Croatia, 243
——, Upper Miocene, at Œningen, 223
Intrusion, a test of age of Plutonic rocks, 565
——, a test of age of volcanic rocks, 521
Inundation mud of rivers, 153
Ireland, glacial drift of, 190
——, yellow sandstone of, 441
Iron pyrites, 500
—— weapons of Swiss lake-dwellings, 148
_Isastræa oblonga,_ Portland Sand, 335
Isle of Bourbon, lava current of the, 566
—— Wight, Hempstead beds, 244
—— Wight, Eocene beds, 255
—— Mull, Miocene leaf-bed of, 247
—— Mull, volcanic rocks, 248
Isomorphism, theory of, 502
Italy, Lower Miocene of, 244
——, Older Pliocene volcanoes of, 523
——, Pliocene of, 207
——, Older Pliocene flora of, 208
——, Upper Miocene strata of, 226

JAMIESON, Mr. T. F., on Scotch glacial drift, 175
Jaws of mammalia in Purbeck, 327
Jeffreys, Mr. Gwyn, on Atlantic mud, 288
Jointed structure of metamorphic rocks, 589
Jones, Dr. Rupert, on Eozoon Canadense, 491
Jorullo, lava stream of, 566
Judd, Mr., on Speeton clay, 311
Jukes, Mr., on Tarannon shales, 468
Jura, erratic blocks on the, 169
——, structure of the, 82

KANGAROO, jaws of, 159
Käsegrotte, Bertrich Baden, Basaltic pillars of, 512
Kaup, Professor, on footprints of the Trias, 373
Keilhau, Professor, on granite veins, 562
——, on planes of foliation, 595
——, on Silurian granite of Norway, 573
——, on protrusion of granite, 581
Keller, Dr. F., on lake-dwellings, 148
Kelloway Rock, percentage of Oxford clay fossils in, 341
Kentish Rag, 308
Keuper, of Germany, 375
—— or Upper Trias of England, 369
Kilkenny, fossil plants of, 441
Killas, altered by granite in Cornwall, 582
Kiltorkan, yellow sandstone of, with Anodonta, 441
Kimmeridge Clay, 335
King, Dr., on reptile footprints in coal, 407
King, Mr., on Permian fossils, 388
Kirkdale cave, hyæna’s den of, 157
Kitchen-middens of Denmark, 146
Kleyn Spawen beds, 242
Könen, Baron von, on Brockenhurst shells, 257
Koninck, M. de, on Mountain Limestone fish, 436
——, on shells of Mayence basin, 242
_Koninckia Leonhardi,_ Hallstadt, 377

LABRADOR rock, 505
—— series, 490
Labradorite, 499, 501
_Labyrinthodon Jægeri, section of tooth_, 371
——, _tooth of_, 370
Labyrinthodonts of Coal, 407
Lake-craters of the Eifel, 534
Lake districts, southern limits of the, 184
Lake-dwellings, scarcity of human remains in, 149
—— of Switzerland, 148
Lakes, deposits in, 27
——, connection of, with glacial action, 184-8
Lamarck on bivalve mollusca, 54
Lamination of clay slate, 594
_Lamna elegans,_ Bracklesham, 262
Lancashire, vast thickness of rocks without corresponding altitude in,
111
Land, balance of dry, how preserved, 116, 118
—— has been raised, not the sea lowered, 70
——, mean height of, above the sea, 115
——, rise of, in Sweden, 72
——, rise and fall of, affecting denudation, 101
Land-ice, action of, in Greenland, 171
Land’s End, columnar granite at, 553
——, porphyritic granite at, 556
La Roche, recent deposits in estuary of, 40
Lartet, M., on mammalia of Faluns, 214
——, on Gastornis Parisiensis, 276
——, on reindeer period, 150
_Lastræa stiriaca,_ Monod, 239
Lateral compression causing curved strata, 75
Laterite of Giant’s Causeway, 509
Laurentian gneiss of Scotland, 493
—— Group, Upper and Lower, 491
—— metamorphic rocks, 601
—— volcanic rocks, 549
Lava, 507
—— consolidating on slopes, 496
—— currents of Auvergne, 541
—— streams, effect of, 30
—— of La Coupe d’Ayzac, 511
—— of Jorullo, 566
Lead veins, age of, 616
Leaf-bed of Madeira in basalt and scoriæ, 532
——, Isle of Mull Miocene, 248
_Leda amygdaloides,_ London Clay, 266
—— _Deshayesiana (Nucula Deshayesiana)_, 241
—— _lanceolata (L. oblonga),_ Scotch drift, 176
—— _truncata,_ Scotch drift, 177
Lee, Mr. J.E., on Pteraspis of Lower Ludlow, 463
Leidy, Dr., on fossil quadrupeds of Nebraska, 249
_Leperditia inflata,_ coal-measures, 405
_Lepidodendron,_ Griffithsii, 441
—— _corrugatum,_ carboniferous., 417
—— _Sternbergii,_ coal-measures, 423
Lepidolite, 499, 501
_Lepidostrobus ornatus,_ Coal, 424
_Lepidotus gigas,_ Lias, 358
—— _Mantelli,_ Wealden, 317
_Leptæna depressa,_ Wenlock, 466
—— _Moorei,_ Lias, 355
Level of surface altered by change of subterranean heat, 119
Lewis, hornblendic gneiss of, 601
Lias, fishes of the, 358
——, fossils of the, 354
—— and Oolite, origin of the, 364
——, reptiles of the, 360
——, insects of the, 363
——, plants of the, 364
——, plutonic rocks of the, 571
——, subdivisions of the, 353
——, volcanic rocks of the, 544
Liebig, on conversion of coal into anthracite, 403
——, on origin of stalactite, 156
Liége, limestone caverns at, 156
Lightbody, Mr., on Lower Ludlow shales, 461
Lignite, conversion of into coal, 403
_Lima giganteum_, 354
—— _Hoperi,_ Chalk, 300
—— _spinosa,_ White Chalk, 294
Limagne d’Auvergne, Lower Miocene mammalia of the, 234
Limburg beds, 242
Lime, scarcity of, in metamorphic rocks, 604
—— in solution, source of, 69
Limestone, block of striated, 168
——, brecciated, 387
—— of chemical and organic origin, 61
——, compact, 501
——, Hippurite, 304
——, magnesian, 387
——, metamorphic or crystalline, 579
——, Mountain, and its fossils, 430-8
——, striated, 168
_Limnæa longiscata_, 55
Lingula beds, volcanic tuffs of the, 549
_Lingula Credneri_, Permian, 388
Lingula Flags, fossils of the, 484
_Lingula Dumortieri,_ Crag, 200
—— _Lewisii,_ Ludlow, 462
_Lingulella Davisii_, 484
Lipari Isles, tufas in, 586
_Liquidambar europæum_, 209
_Lithrostrotion basaltiforme,_ Carboniferous, 432
Lits coquilliers, 275
Littoral denudation defined, 102
_Lituites giganteus,_ Ludlow, 463
Llanberis slates, 486
Llandeilo Flags, fossils of the, 473-5
Llandeilo formation, thickness of the, 475
——, Lower, 475
Llandovery Group, classification of the, 468
—— Rocks, thickness of the Lower, 469
Loam defined, 38, 153
Lodes, shells and pebbles in, 608
—— _See_ Mineral Veins.
Loess of fluviatile loam described, 153
——, fossil shells of the, 154
Logan, Sir W., on Eozoon Canadense, 490
——, on Gaspe sandstones, 455
——, on Huronian and Laurentian, 490
——, on stigmaria in under-clays, 398
——, on thickness of Nova Scotia coal, 409
——, on thickness of Laurentian in Canada, 113
Loire, faluns of the, 211
London Clay, fossils of the, 264, 266
Longevity, relative, of mammalia and testacea, 162
Longmynd Group, fauna of the, 486
Lonsdale, Mr., on corals of America, 229
——, on Devonian fossils, 449
——, on Stonesfield slate, 345
——, on United States Miocene corals, 229
_Lonsdaleia floriformis,_ Carboniferous, 432
Lowe, Reverend R. T., on Mogador shells, 537
Lubbock, Sir J., on the two stone-periods, 147
_Lucina serrata,_ Bracklesham, 262
Ludlow formation, Upper, 459; Lower, 461
——, bone-bed of the Upper, 459
Lulworth Cove, dirt-bed of, 333
Lycett, Mr., on fossils of the Great Oolite, 344
Lycopodiaceæ of Coal, 422
_Lycopodium densum,_ living species, 423
Lym-fiord, mingled fresh-water and marine strata of, 59
_Lymnea caudata,_ Headon, 256
—— _longiscata,_ Bembridge, 253
Lynton Group of Devon, 454

MACLAREN, Mr., on Pentland Hills, volcanic rocks, 548
Macclesfield, marine shells 1,200 feet high at, 181
MacClintock, Sir L., on Atlantic mud, 287
MacCulloch, Dr., on Aberdeenshire granite, 558
——, on basaltic columns in Skye, 510
——, on formation of hornblende-schist, 582
——, on trap, 519
MacMullen, Mr. J., on Eozoon Canadense, 491
_Macropus atlas,_ lower jaw of, 158
—— _major_ (living), lower jaw of, 159
Madeira, beds of laterite in, 509
——, dike in valley in, 513
——, Pliocene leaf-bed and shells in lavas of, 532
——, Miocene volcanic rocks of, 536
——, wind, removing scoriæ in, 97
Maestricht beds and their fossils, 283
Maffiotte, Don Pedro, cited, 538
_Magas pumila,_ White Chalk, 294
Magnesian Limestone defined, 38
—— and marl-slate, 387
Magnetite, 500
Maidstone, Upper Cretaceous fossils of, 297
Malacolite, 502
Malaise, Professor, on Engihoul cave, 157
Mammalia, anterior to Paris gypsum, table of, 329
——, extinct, coeval with man, 152, 157
——, fossil, of Middle Purbeck, 325
——, fossil, in Pliocene in Val d’Arno, 208
——, fossil, in the Crag, 193, 197
——, fossil, of Vienna basin, 225
—— of the Limagne d’Auvergne, 234
—— of Siwalik Hills, 227
—— of the Stonesfield slate, 345
——, _teeth of Post-pliocene_, 165
Mammalia and testacea, comparative longevity of, 162
Mammoth, rude carving of in Perigord cave, 150
—— in Scotch till, 175
—— _See_ Elephas primigenius.
Man, antiquity of, 152
Manfredi on amount of subaërial denudation, 114
Mantell, Dr., on iguanodon of Wealden, 313
——, on Oxford Clay belemnites, 340
——, on Wealden fossils, 316
_Mantellia nidiformis,_ Purbeck, 331
Map of Chalk formation in France, 305
—— of Eocene tertiary basins, 250
—— of Hallstadt and St. Cassian beds, 376
Marble defined, 37
—— of Carrara, metamorphic, 599
Marcou, M., on age of Wealden beds, 319
Margaric acid, 591
Marine fauna of the Carboniferous, 432
—— beds underlying the London Clay, 269
—— and brackish-water strata in coal, 404
—— strata, how distinguished from fresh-water, 53-59
Marl from Lake Superior, 63
—— and marl-slate defined, 38
——, red, green, and white, of Auvergne, 233
—— slate of Middle Permian, 387
Marsupials, extinct, of Australia, 159
_Marsupites Milleri,_ White Chalk, 294
Massachusetts, plumbago of, 583
_Mastodon arvernensis,_ molar of, Norwich crag, 193
—— _giganteus,_ in United States after the drift, 183
Mayence basin tertiaries, 242
May-Hill Sandstone, 468
Mechanical and chemical deposits, 60
—— theory of cleavage, 592
Mediterranean, one zoological province, 127
_Megalodon cucullatus,_ Devonian, 452
_Melania inquinata (Cerithium melanoides)_, 55, 268
_Melania turritissima,_ Bembridge, 253
_Melanopsis buccinoidea_, 55
Melaphyre, a variety of basalt, 504
Menevian beds and their fossils, 484
Mesozoic, term explained, 123
—— and Cainozoic periods, gap between the, 282
—— and Palæozoic rocks, limits of the, 385
Metals, relative age of different, 614
Metamorphic limestone, 579
—— strata, origin of, 579
—— theory, objections to, considered, 587
—— rocks defined, 32
Metamorphic rocks, 576
——, cleavage of, 588
——, scarcity of lime in, 604
——, ages of, 597
——, order of succession of, 602
——, uniformity of mineral character in, 602
Metamorphism, hydrothermal action producing, 584
Metamorphosis of trilobites, 471, 487
Meteorites, minerals in, 501
Mexico, Gulf of, terrestrial remains washed into, 128
Meyer, Mr. Karl, on fossil shells of Madeira, 537
——, M. H. von, on reptiles in coal, 407
——, on Wealden of Germany, 319
Miascite, 558
Mica and its varieties, 499, 501
——, how deposited, 40
—— schist or micaceous schist, 578
Micaceous sandstone, origin of, 36
_Micraster cor-anguinum_, 294
_Microconchus carbonarius,_ coal-measures, 405
_Microlestes antiquus,_ Upper Trias, 368
Migrations of quadrupeds, 161
Miliolite limestone, 274
Miller, Hugh, on Old Red Sandstone fish, 443
——, on salt lakes, 375
Milne Edwards, Mr., on Palæozoic corals, 432
Minchinhampton, Great Oolite of, 344
Mineral composition a test of age of volcanic rocks, 523
—— a test of age of plutonic rocks, 565
—— a test of age of strata, 124
—— character of hypogene rocks, 602
—— springs of Auvergne, 604
Mineral veins, 605
—— formed in fissures, 606
——, successive formation of, 609
——, swelling and contraction of, 611
——, relative age of, 614
——, pebbles in, 608
Mineralisation of organic remains, 65
Minerals in meteorites, 501
——, table of the most abundant in hypogene rocks, 499
Miocene of Bordeaux and south of France, 214
—— and Eocene, line between the, 230, 251
——, Lower, of England, 244
——, Lower, of Germany and Croatia, 242
——, Lower, of Central France, 231
——, Lower, of Italy, 244
——, Lower, of Nebraska, United States, 248
——, term defined, 143
——, Upper, of the Bolderberg, 224
——, Upper, of France, 211
——, Upper, of Italy, 226
——, Upper, of Greece, 226
——, Upper, of India, 226
——, Upper, of Vienna basin, 224
Mississippi, sediment of, used as a test of denudation by rivers, 114
—— valley, deposition and denudation in the, 102
Mitchell, Mr., on Aralia fruit in Alum Bay, Eocene, 263
Mitchell, Sir T., on Wellington caves, 158
Mitchell, Rev. Hugh, on Pteraspis, 446
_Mitra Scabra,_ Barton clay, 259
Mitscherlich, on Isomorphism, 502
_Modiola acuminata,_ Permian, 387
Moel Tryfaen, shells found at, 181
Mohs on isomorphism, 502
Molasse, Lower, of Switzerland, 235
——, Middle, or Marine, of Switzerland, 223
——, Upper, fresh-water, of Switzerland, 217
——, term explained, 217
Mollusca. _See_ Shells.
——, longevity of species of, 162
—— of Hallstadt beds, 377
——, value of, in classification, 142
—— of the Carboniferous, 435
Monitor of Thuringia, 463
Monoclinic feldspars, 501
Monod, flora of the Lower Molasse at, 236
Mons, unconformable strata near, 95
Montblanc, talcose granite of, 568
—— Dor, Auvergne, extinct volcanoes of, 232
——, age of volcano of, 541
Monte Bolca, fossil fish of, 543
—— Calvo, section of cross stratification, 44
—— Mario, age of volcanic deposits of, 533
—— Nuovo, formed 1538, 525
Montmartre, gypseous series of, 270
Monts Dome, Auvergne, extinct volcanoes, 495
Moore, Mr. C., on Rhætic beds, 366
——, on Upper Trias quadrupeds, 369
Moraines described, 169
Morea, cretaceous volcanic rocks of, 544
Mortillet, M. de, on ice-erosion of lake-basins, 184
Morton, Dr., on age of American cretaceous rocks, 307
_Mosasaurus Camperi,_ Chalk, 284
Mountain Limestone, fossils of the, 433-8
Mull, Isle of, leaf-bed, 247
Münster, Count, on fossils of Solenhofen, 337
Murchison, Sir R., on brackish-water strata in coal, 404
——, on Devonian series, 439, 449, 454
——, on Devonian ichthyolites, 453
——, on Eocene igneous rocks, 278
——, on Llandovery beds, 468
——, on Laurentian gneiss of Scotland, 492
——, on metamorphic rocks of North Highlands, 601
——, on Monte Bolca fish-beds, 543
——, on name Permian, 385
——, on Old Red Sandstone, 449
——, on Palæozoic strata, Queenaig, 112, 113
——, on protrusion of solid granite, 574
——, on Silurian, 458, 459, 461, 467, 470, 473, 475
——, on Tertiary volcanic rocks of Italy, 533
——, on thickness of chalk in Russia, 287
——, on thickness of the Trias, 369
——, on the Upper “Old Red”, 468
_Murchisonia gracilis_, 479
_Murex vaginatus_, 204
Muschelkalk, fossils of the, 378
Muscovite, or common mica, 499, 501
Musk-ox, fossil, in Thames valley, 161
_Myliobates Edwardsi,_ Bracklesham, 261
_Mytilus septifer,_ Permian, 387

NAPLES, Post-pliocene volcanic rocks of, 525
——, escape of carbonic acid near, 604
_Natica clausa,_ Scotch drift, 176
—— _helicoides,_ Chillesford beds, 192
Natrolite, 500
_Nautilus centralis,_ London Clay, 266
—— _Danicus,_ Faxoe Chalk, 286
—— _plicatus,_ Hythe beds, 309
—— _truncatus,_ Lias, 356
—— _ziczac (Aturia ziczac)_, 266
Nebraska, Miocene strata of, 248
Necker, M., on “underlying” igneous rocks, 562
——, on dikes in Vesuvius, 526
Neocomian, Upper, 308
——, Middle, 312
——, Lower, 312
——, use of the term, 282
Neolithic era, 147
Neozoic type of corals, 431
_Nerinæa Goodhallii,_ Coral Rag, 339
Nerinæan limestone, 340
_Nerita conoidea (N. Schmidelliana)_, 275
—— _costulata,_ Great Oolite, 345
—— _granulosa_, 55
_Neritina concava,_ Headon, 255
—— _globulus_, 55
Neufchâtel, coins and iron tools in lake of, 149
Newberry, Dr., on flora of American cretaceous rocks, 307
Newcastle coal-field, faults in, 90
Newfoundland bank described, 106
New Jersey, mastodon in, 183
New Madrid, “Sunk Country” in, 402
New Red sandstone of Connecticut Valley, 381
——, trappean rocks of the, 545
New York, Devonian strata of, 456
——, Cambrian strata of, 490
——, Silurian strata of, 478
——, Laurentian strata of, 491
Niagara Limestone, fossils of the, 479
Nidau, iron tools in lake of, 148
Nile, homogeneous mud of the, 154
Ninety-fathom dike in coal, 90
_Nipadites ellipticus,_ Sheppey, 264
Nodules in strata, how formed, 63
_Noeggerathia cuneifolia,_ Permian, 393
Nomenclature of rocks, 140
—— of volcanic minerals, 499
Norfolk cliffs, drift of, 190
North America. _See_ America.
Norway, Cambrian of, 489
——, foliation of crystalline schists in, 595
——, granite veins in gneiss of, 573
——, granite altering fossiliferous strata in, 581
Norwich, or Fluvio-marine crag, 193
Nova Scotia coal-measures, 409
—— coal, reptiles and shells in, 414
——, folding and denudation of beds in, 417
_Nucula Cobboldiæ,_ Crag, 194
_Nummulites lævigata,_ Bracklesham, 260
—— _Puschi,_ Pyrenees, 278
—— _variolaria,_ Bracklesham, 259
Nummulitic formations, 277

_OBOLUS APOLLINIS,_ in Russian grit, 478
Obsidian, 505
Oceanic areas, permanence of, 117
Œningen, Upper Miocene beds of, 215
Oeynhausen, M. von, on Cornish granite veins, 560
_Ogygia Buchii_, 474
_Oldhamia radiata: O. antiqua_, 487
Old Red Sandstone, Upper, 440
——, Middle, with fish, 443
——, Lower, 446
——, trap of the, 547
——, classification of, 439
_Olenus micrurus_, 484
Oligocene, term for Lower Miocene, 230, 244
Oligoclase, 499, 500
_Oliva Dufresnii,_ Bolderberg, Belgium, 224
Olivine, 499
_Omphyma turbinatum,_ Silurian, 466
_Onchus tenuistriatus,_ Silurian, 460
Oolite, classification and physical geography of the, 321
——, defined, 37
——, Inferior, fossils of the, 349, 350
—— and Lias, origin of the, 364
—— and Chalk, Palæontological break between, 338
Oolitic strata, palæontological relations of, 351
—— volcanic rocks, 545
_Ophioderma tenuibrachiata,_ Lias, 357
Oppel on zones of Lias, 353
Orbigny, Alcide de, on foraminifera of Vienna basin, 225
——, on orbitoidal limestone, 279
——, on Pisolitic limestone, 285
——, on Sénonian, 302
_Oreodaphne Heerii,_ Italian Pliocene, 209
Organic remains, mineralisation of, 65
——, tests of age of strata, 125
——, tests of age of volcanic rocks, 522
——, geological provinces of, 127
Oriskany Sandstone, 478
_Orthis elegantula,_ Ludlow, 46
—— _grandis,_ Caradoc beds, 470
—— _tricenaria,_ Bala beds, 470
—— _vespertilio,_ Bala beds, 470
_Orthoceras duplex,_ 474
—— _Ludense,_ Silurian, 463
—— _laterale_, 436
—— _ventricosum,_ Silurian, 462
Orthoclase, 499, 500
Orthoclastic feldspars, 501
Osborne or St. Helen’s series, Eocene, 255
_Osteolepis,_ Old Red Sandstone, 444
_Ostraceon,_ spine of, Bracklesham, 261
_Ostrea acuminata,_ Fuller’s earth, 349
—— _carinata,_ Chalk marl, 300
—— _columba,_ Chloritic sand, 300
—— _gregarea,_ Coral Rag, 339
—— _deltoidea,_ Kimmeridge clay, 336
—— _distorta,_ Middle Purbeck, 324
—— _expansa,_ Portland sand, 336
—— _Marshii,_ Oolite, 351
—— _vesicularis,_ Chalk, 295
_Otodus obliquus,_ Bracklesham, 262
Outcrop of strata, 83
Overlapping strata, 95
Owen, Professor on Archæopteryx, 337
——, on Eocene Zeuglodon, 279
——, on footprints in Trias, 382
——, on fauna of Sheppey, 265, 267
——, on Gastornis Parisiensis, 276
——, on Labyrinthodon, 370
——, on mammalia of Stonesfield, 347
——, on Purbeck mammalia, 326, 328
——, on reptiles of coal, 407, 414
——, on zoological provinces of extinct animals, 160
_Ox, tooth of_ (recent), 165
Oxford Clay, thickness and fossils of, 340

PAGHAM, erratic block at, 182
_Palæaster asperimus,_ 472
_Palæchinus gigas,_ Mountain Limestone, 43
_Palæocoma tenuibrachiata,_ Lias, 357
_Palæoniscus,_ Permian fish, 389
—— _comptus, P. elegans, P. glaphyrus_, 390
_Palæotherium magnum_, 254
_Palæophis typhoeus,_ Bracklesham, 261
Palæozoic or Paleozoic, term defined, 123
—— Plutonic rocks, 572
—— rocks, 458
—— type of corals, 431
Palagonia, dikes of lava in, 531
Paleolithic era, 147, 149
——, alluvial deposits of, 150
Palm in Swiss Miocene, 237
Palma, volcanic crater of, 497
_Paludina lenta,_ Hempstead beds, 55
—— _orbicularis,_ Bembridge, 253
_Paradoxides Bohemicus_, 488
—— _Davidis,_ Lower Cambrian, 485
Parallelism of folded strata for long distances, 93
Paris basin, Tertiary group first studied in, 141
——, Tertiaries of the, 270
_Parka decipiens,_ “Old Red,” 448
Parkfield Colliery, ground-plan of, 400
Patagonia, strata of, rich in soda, 587
_Patella rugosa,_ Great Oolite, 345
Paterson, Dr., on angiosperm of the Coal, 429
Peach, Mr. C, cited, 601
——, Pteraspis, found by, 443
Pearlstone, 505
Pebbles in mineral veins, 608
—— in chalk, 292
_Pecopteris elliptica,_ Coal, 421
_Pecten Beaveri,_ White Chalk, 294
—— _cinctus,_ Neocomian, 312
—— _islandicus,_ Scotch Drift, 176
—— _jacobæus,_ in tertiary of Sicily, 206
—— _quinque-costatus_, 300
—— _Valoniensis,_ Rhætic beds, 366
Pegmatite, 553
Penarth beds, 366
Pengelly, Mr., on Bovey Tracey lignite, 246
——, on flint-knives of Brixham Cave, 157
_Pentacrinus Briareus,_ Lias, 357
_Pentamerus Knightii,_ Aymestry, 461
—— _oblongus,_ and _P. lirata_, 469
Pentland Hills, volcanic rocks of the, 548
Perigord cave, carving of mammoth in, 150
Permanence of continents and oceans, 117
Permian Flora, 392
—— of Germany, 393
—— strata, thickness of, in north of England, 386
——, Upper and Middle, 386, 387
——, Lower, 390
_Perna Mulleti,_ Neocomian, 310
Petherwyn, Devonian fossils of, 450
Petrifaction, process of, 67
_Petrophiloides Richardsoni,_ Sheppey, 25
_Pahcops caudatus,_ Silurian, 467
—— _latifrons,_ Devonian, 450
_Phascolotherium Bucklandi_, 348
_Phasianella Heddingtonensis,_ and cast, 66
Phillippi, on tertiary shells of Sicily, 205
Phillips, Professor, on fossils distorted by cleavage, 592
——, on ninety fathom dike, 90
——, on Wenlock limestone and shale, 465, 467
——, on Yoredale series, 395
Phillips, Mr. J. Arthur, on origin of gold of California, 617
_Phlebopteris contigua,_ Inferior Oolite, 350
Phlogopite, 499, 501
_Pholadomya fidicula,_ Inferior Oolite, 350
Phonolite, 506
_Phorus extensus,_ London Clay, 266
_Phragmoceras ventricosum,_ Silurian, 463
_Physa Bristovii,_ Middle Purbeck, 325
—— _columnaris_, 55
—— _hypnorum_, 55
Piedmont, absence of lakes in, 186
Pile dwellings of Switzerland, 148
Pilton, group of, Devon, 449
_Pinnularia in Atlantic mud_, 288
Pinus sylvestris in peat, 147
Pisolitic limestone of France, 285
Pitchstone, 505
_Placodus gigas,_ Muschelkalk, 380
Placoids, rare in Old Red Sandstone, 443
_Plagiaulax Becklesii, jaw and molar of_, 327
Plagioclastic feldspars, 501
_Plagiostoma giganteum,_ Lias, 354
—— _Hoperi,_ Chalk, 300
_Planorbis discus,_ Bembridge, 253
—— _euomphalus_, 55, 255
Plants of Bovey Tracey, Miocene, 247
——, fossil fresh-water, 57
—— of the Coal, 420
—— of the Lias, 364
—— of the Swiss Upper Miocene, 219
Plas Newydd, rock altered by dike near, 515
Plastic Clay, Eocene, 267
_Platanus aceroides,_ Miocene, 221
_Platystoma Suessii,_ Hallstadt, 377
Playfair, on amount of subaërial denudation, 114
—— on faults, 87
_Plectrodus mirabilis,_ Ludlow, 460
_Plesiosaurus dolichodeirus,_ Lias, 361
_Pleurotoma attenuata,_ Bracklesham, 262
—— _exorta,_ Eocene, 57
_Pleurotomaria anglica,_ and cast, 66
—— _carinata (flammigera)_, 434
—— _granulata,_ Inferior Oolite, 351
—— _ornata,_ Inferior Oolite, 351
Plieninger, Professor, on Triassic mammifer, 368
Pliocene glacial formations, 189-92
—— Period, 189
—— plutonic rocks, 565
—— strata of Sicily, 204
——, term defined, 143
—— volcanic rocks, 529
Plombières, alkaline waters of, 585
Plumbago of Massachusetts, 583
Plutonic and sedimentary formations, diagram of, 567
——, origin of the term, 551
—— rocks, Mesozoic, 570
——, Recent and Pliocene, 565
——, Miocene and Eocene, 568
——, uncertain tests of age of, 564
—— defined, 31
_Podocarya Bucklandi,_ Oolite, 348
_Polypterus_ of the Nile, 444
Polyzoa and Bryozoa, terms explained, 197
Pomel, M., on fossil mammalia of the Limagne, 235
Ponza Islands, globiform pitchstone of, 512
_Porites pyriformis,_ Devonian, 451
Porphyritic granite, 556
Porphyry, 506
Portland, Cycads in dirt-bed of, 331
—— oolite and sand, 334
“_Portland screw,_” a cast of a shell, 335
Porto Santo, marine shells in volcanic tuff of, 536
Post-pliocene period, climate of the, 161
—— mammalia, teeth of, 163
——, term defined, 145
—— lakes of Switzerland, 185
—— volcanic rocks, 524
_Potamides cinctus_, 56
_Pothocites Grantonii,_ coal-measures, 429
Potsdam Sandstone, 480, 489
Pot-stones in the Chalk, 290
Pottsville, coal seams of, 400
Powrie, Mr., on Cephalaspis beds, 446
——, on Parka decipiens, 448
Pratt, Mr., on Eocene Isle of Wight mammalia, 254
Predazzo, altered rocks at, 571
Pressure, solidifying rocks, 65
Prestwich, Mr., on age of Sables inferieurs, 276
——, on Chillesford beds, 192
——, on Coalbrook Dale insects, 405
——, on Eocene strata, 267, 269
——, on faults in coal-measure of Coalbrook Dale, 88
——, on shells of London clay, 264
——, on thickness of Coralline Crag, 198
Prévost, M. Constant, on Paris basin, 270
Primary Limestone, 579
—— rocks, 458
——, term defined, 123
“Primordial Zone” of Bohemia, 481, 482
_Productus horridus,_ Permian, 388
—— _semireticulatus (antiquatus)_, 434
Progressive development indicated by low grade of early mammals, 384
Proteaceæ of Aix-la-Chapelle flora, 304
—— of Lower Molasse, Switzerland, 237
—— of Œningen beds, 221
Protogine, 578
Protosaurus of Thuringia, 390, 464
Protrusion of solid granite, 574
Provinces of animals and plants, 126
_Psammodus porosus_, 437
_Pseudocrinites bifasciatus,_ Silurian, 466
_Psilophyton princeps,_ Devonian, 455
Pteraspis in Lower Ludlow shale, 463
_Pterichthys,_ Old Red Sandstone, 445
Pterodactyl of Kentish chalk, 297
_Pterodactylus anglicus,_ Old Red, 447
—— _crassirostris,_ Solenhofen, 337
_Ptychodus decurrens,_ White Chalk, 297
Pudding-stone or conglomerate, 36
——, formation of, 62
Pumice, 508
Punfield beds, brackish and marine, 318
_Pupa muscorum_, 155
—— _tridens,_ Loess, 56
—— _vetusta,_ Coal, 415
Purbeck beds, Upper, Middle, and Lower, 323, 324, 336
——, fossil mammalia of the Middle, 325
—— marble, 324
——, subdivisions of the, 333
Purity of coal, cause of, 402
_Purpura tetragona,_ Red Crag, 196
_Purpuroidea nodulata,_ Great Oolite, 345
Puy de Côme, cone and lava-current of, 528
—— de Tartaret, lava-current and cone of, 527, 542
—— de Pariou, crater of the, 529
Puzzuoli, elevation of land at, 525
_Pygopterus mandibularis,_ Permian, 390
Pyrenees, chalk altered by granite in the, 570
——, curved strata in, 86
——, lamination of clay-slate in, 596
Pyroxene group of minerals, 499, 502
_Pyrula reticulata,_ Crag, 200

QUADER-SANDSTEIN, Cretaceous age of the, 293
Quadrumana of Gers, 215
Quadrupeds, extinct, in Paleolithic gravels, 152
Quartz, specific gravity of, 499, 500, 555
Quartzite or Quartz Rock, 579
Queenaig, unconformable Palæozoic strata at, 112
Quenstedt on zones of Lias, 353

RADABOJ Miocene, brown coal of, 242
_Radiolites foliaceus,_ White Chalk, 306
—— _Mortoni,_ White Chalk, 295
—— _radiosa,_ White Chalk, 306
Radnorshire, stratified trap in, 549
Rain-prints with worm tracks in Coal, 416
——, carboniferous, 416
Ramsay, Professor, on break between Upper and Lower Cretaceous, 301
——, on breccias in Permian, 391
——, on escarpments, 104
——, on denudation, 98
——, on ice-erosion of lake-basins, 184
——, on Lingula Flags, 484
——, on position of Tremadoc beds, 483
——, on Silurian metamorphic rocks, 602
——, on submergence in glacial period, 181
——, on thickness of the Lower Trias, 372
——, on thickness of Llandeilo beds, 475
——, on thickness of the Bala beds, 473
——, on volcanic tuffs of Snowdon, 549
——, on zones of the Lias, 353
_Rastrites peregrinus,_ Llandeilo Flags, 473
Rath, Von, on Tridymite, 500
Recent Period defined, 145
—— volcanic rocks, 524
Record, imperfection of, in the earth’s crust, 138
Red Crag, older Pliocene, 194
—— Sandstone, Origin of, 374
—— Sea and Mediterranean, distinct species in, 127
Redruth, Cornwall, section of veins in mine, 607
Reindeer Period in South of France, 149
Relistran mine, pebbles in tin of, 609
Reptiles of the Coal, 406, 413
Reptiles of the Lias, 360
_Retepora flustracea,_ Permian, 388
Rhætic beds between Lias and Trias, 366
Rhine, fresh-water strata of the, 53
——, loess of the, 154
Rhinoceros in drift of Abbeville, 153
—— _leptorhinus (megarhinus),_ molar of, 164
—— _tichorhinus,_ molar of, 164
Rhode Island, metamorphic rocks of, 583
_Rhynchonella navicula,_ Ludlow, 460
—— _octoplicata,_ White Chalk, 294
—— _spinosa,_ Inferior Oolite, 350
—— _Wilsoni,_ Aymestry, 462
Richmond, Virginia, Triassic coal-field of, 382
Rigi and Speer, Lower Miocene of the, 235
_Rimula clathrata,_ Great Oolite, 345
Rink, Mr., on Greenland land-ice, 171
Ripple-marked sandstone, how formed, 46
Rise and fall of land, 146
_Rissoa Chastelii,_ Hempstead beds, 245
Rivers, denuding powers of, 101, 114
Roches moutonnees described, 169
Rock, term defined, 26
Rocks altered by volcanic dikes, 514
—— altered by subterranean gases, 586
——, analysis of minerals in, 499
——, aqueous or stratified, 27
——, classification of, 121
——, great thickness of palæozoic, 110
——, glacial scorings on, 169
——, metamorphic, age of, 597
——, plutonic age of, 564
——, volcanic, age of, 520
——, trappean, 497
——, metamorphic, defined, 32
——, four classes of contemporaneous, 33
——, plutonic, defined, 31
——, tests of age of, 123, 125, 520, 564, 597
——, four contemporaneous classes of, 122
——, underlying, not always the oldest, 122
——, volcanic, defined, 29
Rock-salt of Trias, 371
——, origin of, 374
Rogers, Mr. H. D., on blending of coal-seams, 400
——, on Virginian fault, 92
Rose, Gustavus, on isomorphism, 502
——, on Fifeshire dike, 546
——, on quartz in granite, 555
Rosso antico, red porphyry of Egypt, 506
_Rostellaria (Hippocrenes) ampla,_ London Clay, 266
Roth, M., on Miocene of Greece, 226
Runn of Cutch, salt of, 375
Rupelian beds of Dumont, 241, 242
Russia, glaciation of, 174
——, Devonian of, 454
——, Silurian strata of, 478

SAARBRUCK, reptiles in coal-field of, 406
_Sabal major,_ Lower Miocene, 237
Sables de Bracheux, 276
—— moyens, Paris basin, 273
Sahlite, 502
St. Abb’s Head, curved strata of, 76
——, unconformable stratification at, 94
St. Andrews, carboniferous trap-rocks of, 545
St. Cassian, fossil mollusca of, 377
—— and Hallstadt beds, 376
St. David’s, Menevian beds of, 485
St. Mary’s, shells of, 539
Salt, rock, origin of, 372
Salter, Mr., on fossils of Arenig group, 476
——, on Menevian beds, 485
——, on Tremadoc fossils, 483
Sandberger, Dr. F., on Mayence basin, 242
Sandstone, New Red, 369
——, Old Red, 439
—— slab with cracks, 317
——, slab of ripple-marked, 45
—— slab with footprints, 408
_Sao hirsuta_, 488
Saurians of the Lias, 361
——, sudden destruction of, 362
_Saurichthys apicalis,_ Rhætic Beds, 367
Saussure, on vertical conglomerates, 73
_Saxicava rugosa,_ Scotch drift, 176
Saxony, beds of minerals in, 609
Scandinavia, glaciation of, 174
_Scaphites æqualis,_ Chloritic marl, 299
Scapolite, 506
Scheerer on action of water in metamorphism, 585
Schist, mica, 578
——, argillaceous, 579
——, hornblende, 578
_Schizodus Schlotheimi,_ Permian, 387
—— _truncatus,_ Permian, 387
Schmerling, Dr., on Liége caverns, 157
Schorl-rock, and schorly granite, 557
Schwab, M., on Celtic coins in lake-dwellings, 149
_Scoliostoma,_ St. Cassian, 377
Scoresby, on Arctic icebergs, 172
Scoriaceous lava, 507
Scoriæ, 508
Scotland, “Fundamental gneiss” of, 493
——, Old Red Sandstone of, 440
——, glaciation of, 175
Screws, fossil, internal casts of shells, 66
Scrope, Mr., on Isle of Ponza, globiform pitchstone, 512
——, on minerals in lava, 524
——, on water in lava, 555
Scudder, Mr., on Devonian insects of Canada, 457
Sea, apparent fall of, caused by rise of land, 70
——, denuding power of the, 105
——, deep soundings in, 287
——, mean depth of the, 118
—— cliffs, inland, 103
Secondary and Tertiary, gap between the, 281
——, term defined, 123
Section of Auvergne alluvium, 100
—— of carboniferous rocks, Lancashire, 85
—— of chalk and greensand, 287
—— of crags near Woodbridge, Suffolk, 198
—— of cross-stratification, 42-44
—— of curved strata of the Jura, 82
—— of dirt-bed in Isle of Portland, 332
—— of Forfarshire, showing curved strata, 74
—— of fossil tree, showing texture, 67
—— of folded and denuded carboniferous beds, Nova Scotia, 418
—— of the Oolitic strata, 322
—— of Recent and Post-pliocene alluvial deposits, 151
—— showing creeps in coal-mines, 79
—— of slaty cleavage, 589
—— showing valleys of denudation, 98
—— showing the Weald formation, 313
—— of strata thinning out, 41
—— of superimposed groups at Dundry Hill, 130
—— of unconformable strata near Mons, 95
Sections illustrating faults, 88, 90, 91
Sedgwick, Professor, on the Cambrian Group, 481, 482, 486
——, on classification of Arenig group, 476
——, on Devonian series, 439, 449
——, on position of the May-Hill beds, 568
——, on protrusion of solid granite, 574
——, on slaty cleavage, 588, 591
——, on garnet in altered rock, 515
——, on concretionary structure, 63
Sediment, accumulation of, causing a shifting of the subterranean, 117
isothermals. Sedimentary beds of the Carboniferous, 396
Selsea Bill, erratics at, 182
Senarmont on action of water in metamorphism, 585
_Sequoia Langsdorfii_, 238
_“Seraphim,” head of Pterygotus anglicus_, 446
Serapis, marine littoral deposits of, 146
Serpentine, 578
_Serpulæ_ attached to _Gryphæa_, 48
—— attached to _Spatangus_, 49
—— attached to _Apiocrinus_, 343
Shale defined, 36
—— of the Lower Ludlow, 461
Sharpe, Mr. D., on American Silurian fossils, 479
——, on fossils distorted by cleavage, 592
Shell-mounds of Denmark, 146
Shells, Arctic, in Scotch drift, 177
——, derivative, in the Crag, 195-203
——, marine, found at great heights above the sea, 29
——, proportion of living, in the Crags, 194, 195, 199
——, value of, in classification, 142
——, fossil, of Virginia, 228
—— of the London clay, 266
—— of the mountain limestone, 433
—— of the Barton clay, 258
—— of the Oolite, 335, 345, 350
——, marine, of Moel Tryfaen, 180
Sheppey, fauna and flora of, 264
——, Eocene fish of, 267
Sherringham, erratic block at, 191
Shetland, granite of, 558
——, hornblende-schist of, 583
Sicily, fauna and flora of, older than the country itself, 207
——, newer Pliocene strata of, 204
——, subterranean igneous action in, 569
——, undulating gypseous marls of, 86
——, volcanic dikes of, 531
Sidlaw Hills, trap of, 548
Sigillaria in coal-measures, 380, 411, 425
_Sigillaria lævigata,_ coal-measures, 426
Siliceous limestone defined, 37
Silurian, derivation of the name, 458
——, granite of Norway, 573
——, metamorphic, of North Highlands, 601
—— rocks, classification of, 458
—— strata of the continent of Europe, 477
—— strata of United States, 478
—— volcanic rocks, 548
_Siphonotreta unguiculata,_ obolus grits, 478
Siwâlik Hills, fresh-water deposits of, 226
Skaptar Jokul, flow of lava from, 523
Skye, hypersthene rocks of, 491
——, Isle of, Miocene syenite of the, 568
——, trap dike in, 514
Slaty cleavage, 588
Slicken-sides, in opposite walls of veins, 608
——, term defined, 87
_Smilax sagittifera,_ Œningen, 222
Smith, Mr. W., on White Lias bed, 366
Snowdon, volcanic tuffs of, 549
Soissonnais sands, 275
_Solenastræa cellulosa,_ Brockenhurst, 257
Solenhofen stone, fossils in the, 337
Solfatara, decomposition of rocks in the, 586
Somma, cone and dikes of, 526
Sopwith, Mr. T., models of outcrop of strata, 85
Sorby, Mr., on action of water in metamorphism, 585
——, on glass cavities in quartz, 555
——, on mechanical theory of cleavage, 592
——, on ripple-marks in mica schist, 596
South Joggins, section of cliffs at, 410
Spalacotherium, Purbeck, 346
_Spatangus radiatus,_ Chalk, 284
—— with serpula attached, 49
Species, gradual change of, 139
—— older than the land they inhabit, 207
——, similarity of conditions causing reappearance of, 311
Specific gravity of basalt and trachyte, 504
Speer and Rigi, Lower Miocene of the, 235
Speeton Clay, 311
_Sphærexochus mirus,_ Silurian, 467
_Sphærulites agariciformis,_ White Chalk, 306
—— of volcanic minerals, 499
_Sphenophyllum erosum,_ Coal, 425
_Sphenopteris gracilis,_ Hastings sands, 318
Spheroidal concretions in limestone, 64
_Spicula of sponge,_ Atlantic mud, 288
_Spirifera disjuncta,_ Devonian, 450
—— _alata,_ Permian, 388
—— _mucronata_, 454
—— _trigonalis,_ and _S. glabra_, 434
_Spiriferina Walcotti,_ Lias, 355
_Spirolina stenostoma,_ Eocene, 275
_Spirorbis carbonarius,_ coal-measures, 405
_Spondylus spinosus,_ White Chalk, 294
_Sponge in flint from White Chalk_, 296
Sponges, vitreous, in the chalk, 291
Springs, mineral of Auvergne, 604
Staffa, age of columnar basalt of, 539
Stalactite, origin of, explained, 156
_Starfish_ in Silurian strata, 472
Stations of species affecting distribution of fossils, 354
_Stauria astræiformis_, 431
Stereognathus of Stonesfield, 348
Sternberg, Count, on insects in coal, 495
_Stigmaria attached to trunk of Sigillaria_, 427
—— in coal-measures, 398, 411, 426
—— _ficoides_ and surface showing tubercles, Coal, 427
Stilbite, 500
Stiper-Stones or Arenig Group, 475
Stockwerk, assemblage of veins, 605
Stonesfield slate, mammalia of the, 345
Strata, term defined, alternations of fresh-water, and shallow and
deep, 27
sea. ——, alternations of marine and fresh-water, 108
——, curved, inclined, and vertical, 73
——, apparent horizontality of inclined, 81
——, contorted in drift, 178
——, contortion of, in Cyclopean Isles, 530
——, general table of fossiliferous, 131
——, horizontality of, 40
——, origin of metamorphic, 83
——, overlapping, 95
—— repeated by being doubled back, 87
——, slow growth of, attested by fossils, 47-50
—— of organic origin, 51
——, tests of age of, 123
——, unconformability of, 94, 138
——, vast thickness of, not forming high mountains, 109-13
Stratification, diagonal or cross, 42, 44
——, different forms described, 39
—— of metamorphic rocks considered, 580
Stratified rocks, composition of, 35
Striæ, production of, 168
Strickland, Mr., on thickness of the Trias, 369
_Stricklandinia lirata_, 469
Strike, term explained, 80
_Stringocephalus Burtini_, 452
Stromboli, lava of, 566
_Strophomena depressa,_ Wenlock, 466
—— _grandis_, 471
Studer, Mr., on gneiss of the Jungfrau, 599
subaërial denudation, average annual amount of, 113
Subapennine beds, proportion of recent species in, 143
—— strata, older Pliocene, 208
Submarine denudation, 105
Subsidence of land must preponderate over upheaval, 116
_Succinea amphibia_, 55
—— _elongata_, 155
Suess, M., on fossils of St. Cassian beds, 376, 377
——, on Vienna basin, 225
Suffolk, Crag of, 195
“Sunk country,” New Madrid, 402
Superga, Lower Miocene of the, 244
Superior, Lake, marl in, 63
Superposition of deposits, a test of age, 124
—— a test of age of volcanic rocks, 521
Sutherlandshire, unconformable Palæozoic strata in, 112
Swanage, fossil mammalia found at, 326
Sweden, Cambrian of, 489
——, slow rise of land in, 72
——, small thickness of Silurian strata in, 477
Switzerland, lake-dwellings of, 148
——, Lower Molasse of, 235
——, Middle or Marine Molasse of, 223
——, Upper Miocene of, at Œningen, 215
Sydney coal-field, rain-prints in, 416
Syenite, composition of, 552, 557
——, how far connected with trap-rocks, 558
Syenitic granite, 557
Symonds, Rev. W. S., on Moel Tryfaen shells, 180
Synclinal and anticlinal curves, 74, 85

TABLE of Botanical Nomenclature, 303
—— of St. Cassian fossil mollusca, 377
—— of Cretaceous formations, 283
—— of Devonian series in Devon, 449
—— of divisions of Hastings Sand, 316
—— of English and French Eocene strata, 252
—— of ages of fossil vertebrata, 464
—— of Neocomian strata, 308
—— of mammalia older than Paris gypsum, 329
—— of marine testacea in the Crag, 202
—— of Oolitic strata, 321
—— of volcanic minerals, 499
—— of Silurian strata of United States, 478
—— of Silurian rocks, 458
—— of Triassic strata, 375
—— of Cambrian strata, 482
—— of Permian of north of England, 386
—— of Welsh coal-measures, 394
—— of thicknesses of Carboniferous limestone, 395
——, general, of fossiliferous strata, 131
Table Mountain, granite veins in clay-slate of, 560
Tails of homocercal and heterocercal fish, 389
Talcose granite, 557
—— gneiss, 578
Tarannon shales, 468
Tartaret cone, and lava of, 527, 542
Tate, Mr., on St Cassian fossils, 377
Tealby series, Middle Neocomian, 312
Teeth of extinct mammalia, 163, 164
_Tellina balthica (T. solidula)_, 190
—— _calcarea (T. proxima)_, 177
—— _obliqua,_ Crag, 194
_Temnechinus excavatus_, 200
_Temnopleurus excavatus_, 200
_Tentaculites annulatus,_ Silurian, 489
_Terebellum fusiforme,_ Barton, 259
—— _sopita,_ Barton, 259
_Terebratula affinis,_ Aymestry, 462
—— _biplicata,_ White Chalk, 294
—— _carnea,_ White Chalk, 294
—— _digona,_ Bradford clay, 345
—— _fimbria,_ Inferior Oolite, 350
—— _hastata,_ Mountain Limestone, 434
—— _sella,_ Neocomian, 310
—— _Wilsoni,_ Aymestry, 462
_Terebratulina striata,_ White Chalk, 294
_Terebrirostra lyra,_ Chloritic Sand, 300
_Teredo navalis,_ boring wood, 50
Tertiary formations, classification of, 137, 143
—— strata, subdivisions of, 143
——, term defined, 123
Testacea. _See_ Shells.
Thallogens, 303
_Thamnastræa,_ Coral Rag, 339
Thanet sands, 269
_Theca operculata,_ Tremadoc beds, 483
_Thecodontosaurus, tooth of,_ 374
_Thecodus parvidens,_ Ludlow, 460
_Thecosmilia annularis,_ Coral Rag, 339
Thirria, M., on Nerinæan limestone, 340
Thompson, Dr., on Nummulites of Thibet, 277
Thomson, Wyville, on Atlantic mud, 288
——, on sponges in chalk mud, 292
Thuringia, monitor of, 390, 463
Thurmann, M., on Bernese Jura Oolite, 344
——, on structure of the Jura, 83
_Thylacotherium Prevostii,_ Stonesfield, 347
Tile-stones of the Upper Ludlow, 459
Tilgate forest, fossil Iguanodon in, 315
Till described, 166
——, mammoth in Scotch, 175
—— of North America, 182
Tin veins, age of, in Cornwall, 615
Titanoferrite, 500
Torell, Dr., on ice-action in Greenland, 172
——, on Swedish Cambrian fossils, 489
Touraine, faluns of, 211
Tourmaline, 500
Trachytic rocks, 505
—— tuff, 506
—— porphyry, 506
—— lava, age of, 523
Trap, term defined, 498
—— dike, intercepting strata, 518
—— dikes, 513-17
——, intrusion of, between strata, 517
—— rocks, ages of, 524-50
—— rocks passing into granite, 559
—— tuff described, 508
Trappean rocks, nomenclature of, 497
—— rocks, their relation to active volcanoes, 517
Trass of Lower Eifel, 535
Travertin, how deposited, 60
——, inférieur of Paris basin, 273
_Tree ferns, living_, 422
Trees erect in coal, Nova Scotia, 411
Tremadoc slates and their fossils, 482
Tremolite, 499, 502
Trenton limestone, fossils of the, 479
Trezza, volcanic rocks of, 529
Trias, beds of passage between Lias and, 366
—— of England, 369-74
—— of Germany, 375
——, Saurians of the, 370
—— of the United States, 381
Triassic mammifer, North Carolina, 383
Triclinic feldspars, 501
Tridymite, crystallised silica, 500
_Trigonellites latus,_ Kimmeridge clay, 336
_Trigonia caudata,_ Neocomian, 310
—— gibbosa, Portland stone, 335
_Trigonocarpum ovatum,_ and _T. olivæforme,_ Coal, 429
_Trigonotreta undulata,_ Permian, 388
Trilobites of Bala and Caradoc beds, 471
——, metamorphosis of, 471, 488
—— of primordial zone, 487
_Triloculina inflata,_ Eocene, 275
Trimmer, Mr., on contorted strata, 179
——, on shells of Moel Tryfaen, 186
_Trinucleus concentricus, T. Caractaci_, 472
_Trionyx, carapace of,_ Bembridge, 253
Tripoli composed of diatomaceæ, 51
_Trochoceras giganteus,_ Ludlow, 463
_Trophon antiquum (Fusus contrarius)_, 196
—— _clathratum,_ Scotch drift, 176
Tuff defined, 30
——, shelly, of the Grand Canary, 538
——, trappean, of Llandeilo rocks, 473
——, shelly, of Gergovia, 542
_Tupaia Tana,_ recent, 347
Turner, Dr., on chemical decomposition, 68
_Turrilites costatus,_ Chalk, 299
_Turritella multisulcata,_ Bracklesham, 262
Tuscany, mineral springs of, 604
Tylor, Mr., on amount of subaërial denudation, 114
Tyndall, Dr., on slaty cleavage, 594
Tynedale fault, 90
Tynemouth cliff, brecciated limestone of, 387
_Typhis pungens,_ Barton clay, 259

_UNCITES Gryphus,_ Devonian, 452
Unconformability of strata, 94, 138
Underlying, term applied to plutonic rocks, 34
Unger on American forms in Swiss Miocene flora, 223, 239
—— on Miocene plants of Croatia, 243
Ungulite, or Obolus grit of Russia, 477
_Unio littoralis_, 54
—— _Valdensis,_ Hastings Sands, 317
United States, Cambrian of the, 489
——, Cretaceous rocks of, 307
——, Devonian of, 455
——, Eocene strata in the, 278
——, footprints in Carboniferous of, 407
——, Lower Miocene of, 248
——, older Pliocene and Miocene formations of, 227
——, Silurian strata of, 478
——, Trias of the, 381
Upheaval of land more than counteracted by subsidence, 116
——, power of denudation to counteract, 105, 115
Upper Greensand, or Chloritic series, 298
Upsala, erratics on modern marine drift near, 174
Ural Mountains, auriferous alluvium of, 616
Uralite, 499
_Ursus spelæus, tooth of_, 165
Urville, Captain de, on size of icebergs, 172

VAL D&RSQUO;ARNO, Newer Pliocene of, 207
Valleys, origin of, 102
Valorsine, granite veins in talcose gneiss in, 599
_Valvata piscinalis_, 55
_Vanessa Pluto,_ Lower Miocene, Croatia, 243
Vegetation of the Coal, 420
—— of the Devonian of America, 455
——. _See_ Plants.
Veins, chemical deposits in, 612
——, granite rocks altered by, 559
——, different kinds of minerals, 605
——. _See_ Mineral veins.
Vein-stones, 610
_Venericardia planicosta_, 260
Venetz, M., on Alpine glaciers, 170
_Ventriculites radiatus,_ Chalk, 292
Verneuil, M. de, on Russian Silurian, 462
——, on Permian flora, 392
Vertebrata, progress of discovery of fossil, 464
Vertical strata, 73
Vesuvius, Recent and Post-pliocene volcanic rocks of, 525
——, basaltic lavas of, 508
——, tufaceous strata of, 522
——, dikes of, 527
_Vicarya Lujani,_ Punfield, 319
Vicentin, columnar basalt of the, 511
Vienna Basin, Upper Miocene beds of, 224
Vine in Upper Miocene beds at Œningen, 221
Virginia, eighty miles of fault in, 92
——, coal-field of, 382
Virlet, M, on corrosion of rocks near Corinth, 586
——, on Cretaceous traps of Greece, 544
——, on fossils in veins, 608
——, on volcanic rocks of the Morea, 544
Volcanic ash or tuff, 508
—— breccia, 509
—— dikes, 513-16
—— force and denudation opposing powers, 117
—— mountains, structure and origin of, 494
Volcanic rocks defined, 29
——, mineral composition of, 498
——, Recent and Post-pliocene, 524
——, Pliocene, 529
——, Miocene, 536-43
——, Eocene, 543
——, Cretaceous and Liassic, 544, 545
——, New Red, Permian and Carboniferous, 545
——, Old Red Sandstone, 547
——, Silurian, Cambrian and Laurentian, 548, 549
—— of Auvergne, 540
——, columnar and globular, structure of, 510
—— of Grand Canary, 528
—— of Silurian age, 477
——, special forms of structure of, 506
——, tests of age of, 520-4
Volcanoes, extinct, 30
—— of Auvergne, 495
_Voltzia heterophylla,_ Bunter, 380
_Voluta ambigua,_ Barton clay, 259
—— _athleta,_ Barton, 259
—— _Lamberti,_ coralline and Red Crag, 196
—— _Lamberti,_ faluns, 214
—— _nodosa,_ London clay, 266
—— _Selseïensis,_ Bracklesham, 262
Von Buch, Leopold, on “elevation craters,” 496
——, on Silurian plutonic rocks, 572

WACKE described, 508
Wagner, M., on Miocene of Greece, 226
_Walchia piniformis,_ Permian, 392
Wales and England, glaciation of, 180
Wallich, Dr., on Atlantic mud, 287
Water, denuding power of running, 98, 115
——, action of, in metamorphism, 584
Watt, Gregory, on fusion of rock, 584
Weald clay and its fossils, 317
Wealden area, thickness of the, 319
—— formation, 313
—— flora, 320
Webster, Mr. T., on Tertiary strata, 141
Wellington Valley caves, 158
Wenlock formation, fossils of the, 465-8
—— limestone, 465
—— shale, 467
Werner on mineral veins in Saxony, 609
—— on isomorphism, 502
Westwood, Mr., on Lias beetles, 363
Wexford, veins of copper at, 615
Whitaker, Mr., on subaërial origin of escarpments, 104
White or coralline crag, 197
—— sand of Alum Bay, 38
Whymper, Mr., on Arctic Miocene plants, 240
Williams, Mr., on Cornish lodes, 607
Williamson, Professor, on Conifers of the Coal, 428
——, on structure of calamite, 425
Wind, denuding action of the, 97
Wood, Mr. Searles, on Bridlington shells, 190
——, on Chillesford and Aldeby beds, 192
——, on shells of the Crags, 194, 195, 199
——, on shells of Crag and faluns compared, 213
——, on fish of Headon series, 255
——, table of marine testacea of the Crag, 202
——, on thickness of coralline crag, 198
Woodward, Dr., on St. Cassian fossils, 377
Woodward, Mr. H., on Pterygotus, 447
Woolhope beds, 467
Woolwich and Reading series, 267
Wright, Dr., on Barton shells, 258
——, on zones of the Lias, 353
Wunsch, Mr. E. A., on trees in volcanic ash, 546
Wyville Thomson. _See_ Thomson.

_XIPHODON gracile,_ Paris basin, 271
_Xylobius Sigillariæ,_ Nova Scotia coal, 415

YOREDALE beds, thickness of the, 395
Yorkshire, Oolite of, 349
Young, Mr., on seeds washed out of mammoth tusks, 176

ZECHSTEIN of Germany, 392
Zeolites, secondary volcanic minerals, 500
_Zeuglodon cetoides,_ Eocene, United States, 280
Zircon-syenite, 558
_Zoantharia rugosa_ and _Z. aporosa_, 431
Zones of the Lias, 353
_Zonites priscus,_ Coal, 415
Zoological provinces, great extent of, 127
Zoophytes, fossil, 48
——. _See_ Corals, Bryozoa, etc.
Zurich, lake-dwellings in Lake of, 148