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                       Common Minerals and Rocks

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                   Boston Society of Natural History

                      GUIDES FOR SCIENCE-TEACHING

                                NO. XII

                       COMMON MINERALS AND ROCKS

                          BY WILLIAM O. CROSBY

                     D. C. HEATH & CO., PUBLISHERS

                       BOSTON  NEW YORK  CHICAGO




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                              _Copyright_

                BY THE BOSTON SOCIETY OF NATURAL HISTORY

                                  1881


                                 I D 2




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                             INTRODUCTION.

                             [Illustration]


Minerals and rocks, or the inorganic portions of the earth, constitute
the proper field or subject-matter of the science of Geology. Now the
inorganic earth, like an animal or plant, may be and is studied in three
quite distinct ways, giving rise to three great divisions of geology,
which, as will be seen, correspond closely to the main divisions of
Biology.

First, we may study the forces now operating upon and in the earth—the
geological agencies—such as the ocean and atmosphere, rivers, rain and
frosts, earthquakes, volcanoes, hot springs, etc., and observe the
various effects which they produce. We are concerned here with the
dynamics of the earth; and this is the great division of _dynamical
geology_, corresponding to physiology among the biological sciences.

Or, second, instead of geological causes, we may study more particularly
geological effects, observing the different kinds of rocks and of
rock-structure produced by the geological agencies, not only at the
present time, but also during past ages. This method of study gives us
the important division of _structural geology_, corresponding to anatomy
and morphology.

All phenomena present two distinct and opposite aspects or phases which
we call _cause_ and _effect_; and so in dynamical and structural geology
we are really studying the opposite sides of essentially the same
classes of phenomena. In the first division we study the causes now in
operation and observe their effects; and then, guided by the light of
the experience thus gained, we turn to the effects produced in the past
and seek to refer them to their causes.

These two divisions together constitute what is properly known as
physiography; and they are both subordinate to the third great division
of geology,—_historical geology_,—which corresponds to embryology.

The great object of the geologist is, by studying the geological
formations in regular order, from the oldest up to the newest, to work
out, in their proper sequence, the events which constitute the earth’s
history; and dynamical and structural geology are merely introductory
chapters, the alphabet, as it were, which must be learned before we are
prepared to read understandingly the grand story of the geological
record.

Our work in this short course will be limited to the first two
divisions,—_i.e._, to dynamical and structural geology. We will attempt,
first, a general sketch of the forces now concerned in the formation of
rocks and rock-structures; and after that we will study the composition
and other characteristics of the common minerals and rocks.

The scope of this work, and its relations to the whole field of geology,
are more clearly indicated by the following classification of the
geological sciences:—

                {DYNAMICAL GEOLOGY    {_Physical Geology._
                                      {_Chemical Geology._

       GEOLOGY  {STRUCTURAL GEOLOGY   {_Mineralogy._
                                      {_Petrography_ {Lithology.
                                                     {Petrology.
                {HISTORICAL GEOLOGY.

Many teachers will desire to fill in some of the details of the outline
sketch presented in this Guide, and for this purpose the following works
are especially recommended:—

    ELEMENTS OF GEOLOGY. By Prof. Joseph Le Conte. 1882. D.
        Appleton & Co., New York. Nearly 600 pages.

    MANUAL OF GEOLOGY. By Prof. J. D. Dana. Third edition. 1880.
        800 pages.

    TEXT-BOOK OF GEOLOGY. By Prof. A. Geikie. 1882. Macmillan &
        Co., London. Nearly 1000 pages.

As a reference-book for mineralogy, the following treatise is
unsurpassed:—

    TEXT-BOOK OF MINERALOGY. By Edward S. Dana. 1883. John Wiley
        & Sons, New York.

And, as an introduction to the study of minerals, and, through these, to
the study of rocks,—

    FIRST LESSONS IN MINERALS. Science Guide No. XIII. By Mrs.
        E. H. Richards.

cannot be too highly recommended. Teachers will find this little primer
of 46 pages invaluable with young children, and with all who have had no
previous training in chemistry.

As an admirable continuation of the work begun in these pages, teachers
are referred to Professor Shaler’s “First Book in Geology.” In this our
brief sketch of the geological agencies is amplified and beautifully
illustrated; and rarely have the wonderful stories of the river,
ocean-beach, glacier, and volcano been told so effectively. In the
chapter on the history of life on the globe the main outlines of
historical geology are skillfully brought within the comprehension of
beginners. The directions to teachers are fully in accord with the
modern methods and ideas, and are a very valuable feature of the book.

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                           DYNAMICAL GEOLOGY.

                             [Illustration]


When we think of the ocean with its waves, tides, and currents, of the
winds, and of the rain and snow, and the vast net-work of rivers to
which they give rise, we realize that the energy or force manifested
upon the earth’s surface resides chiefly in the _air_ and _water_—in the
earth’s fluid envelope and not in its solid crust. And it would be an
easy matter to show that, with the exception of the tidal waves and
currents, which of course are due chiefly to the attraction of the moon,
nearly all this energy is merely the transformed heat of the sun. Now
the air and water are two great geological agencies, and therefore the
geological effects which they produce are traceable back to the sun.

Organic matter is another important geological agent; but all are
familiar with the generalization that connects the energy exhibited by
every form of life with the sun; and, besides, it is scarcely necessary
to allude to the obvious fact that all animals and plants, so far at
least as any display of energy is concerned, are merely differentiated
portions of the earth’s fluid envelope. And so, if space permitted, it
might be shown that, with the exception of the tides, nearly every form
of force manifested upon the earth’s surface has its origin in the sun.

Of this trio of geological agencies operating upon the earth’s surface
and vitalized by the sun—_water_, _air_, and _organic matter_—the water
is by far the most important, and so it is common to call these
collectively the aqueous agencies. Hence we have _solar agencies_ and
_aqueous agencies_ as synonymous terms.

The aqueous agencies include, on one side, _air_ and _water_, or
_inorganic_ agencies; and, on the other, _animals_ and _plants_, or
_organic_ agencies.

Let us notice briefly the operation of these, beginning with the air and
water.




                          I. AQUEOUS AGENCIES.


               1. _Air and Water, or Inorganic Agencies._

CHEMICAL EROSION.—Attention is invited first to the specimens numbered
1, 2, 3, and 4. No. 1 is a sound, fresh piece of the rather common rock,
diabase; and those who are acquainted with minerals will recognize that
the light-colored grains in the rock are feldspar, and the dark, augite.
This specimen came from a depth in the quarry, and has not been exposed
to the action of the weather.

The second specimen differs from the first, apparently, as much as
possible; and yet, except in being somewhat finer grained, it was
originally of precisely similar composition and appearance. In fact, it
is a portion of the same rock, but a _weathered_ portion. In this we can
no longer recognize the feldspar and augite as such, but both these
minerals are very much changed, while in the place of a strong, hard
rock we have an incoherent friable mass, which is, externally at least,
easily crushed to powder; and with the next step in the weathering, as
we may readily observe in the natural ledges, the rock is completely
disintegrated, forming a loose earth or soil.

We have two examples of such natural powders in the specimens numbered 3
and 4; and by washing these (especially the finer one, No. 4) with
water, we can prove that they consist of an impalpable substance which
we may call clay, and angular grains which we may call sand. The
sand-grains are really portions of the feldspar not yet entirely changed
to clay.

Thus we learn that the result of the exposure of this hard rock to the
weather is that it is reduced to the condition of sand and clay. What we
mean especially by the weather are _moisture_ and certain constituents
of the air, particularly _carbon dioxide_.

The action of the weather on the rocks is almost entirely chemical. With
a very few exceptions, the principal minerals of which rocks are
composed, such as feldspar, hornblende, augite, and mica, are silicates,
_i.e._, consist of silicic acid or silica combined with various bases,
especially aluminum, magnesium, iron, calcium, potassium, and sodium.

Now the silica does not hold all these bases with equal strength; but
carbon dioxide, in the presence of moisture, is able to take the sodium,
potassium, calcium, and magnesium away from the silica in the form of
carbonates, which, being soluble, are carried away by the rain-water.

The silicate of aluminum, with more or less iron, takes on water at the
same time, and remains behind as a soft, impalpable powder, which is
common clay.

In the case of our diabase, continued exposure to the weather would
reduce the whole mass to clay. But other rocks contain grains of quartz,
a hard mineral which cannot be decomposed, and it always forms sand.
Certain classes of rocks, too, such as the limestones and some
iron-ores, are completely dissolved by water holding carbon dioxide in
solution, and nothing is left to form soil, except usually a small
proportion of insoluble impurities like sand or clay.

Let us see next how these agents of decay get at the rocks. Neither
water nor air can penetrate the solid rock or mineral to any
considerable extent, so that practically the action is limited to
surfaces, and whatever multiplies surfaces must favor decomposition.

First, we have the upper surface of the rock where it is bare, but more
especially where it is covered with soil, for there it is always wet.

All rocks are naturally divided by joints into blocks, which are
frequently more or less regular, and often of quite small size. Water
and air penetrate into these cracks and decompose the surfaces of the
blocks, and thus the field of their operations is enormously extended.
These rock-blocks sometimes show very beautifully the progress of the
decomposing agents from the outside inward by concentric layers or
shells of rotten material, which, in the larger blocks, often envelop a
nucleus of the unaltered rock.

It is interesting to observe, too, that these concentric lines of decay
cut off the angles of the original blocks, so that the undecomposed
nucleus, when it is found, is approximately spherical instead of
cuboidal. Both these points are well illustrated by specimen No. 2; for
although now nearly spherical, it was originally perfectly angular, and
has become rounded by the peeling off, in concentric layers, of the
decomposed material, and in most cases several of these layers are
distinctly visible, like the coats of an onion. But by stripping these
off we should discover, in all the larger balls at least, a solid,
spheroidal nucleus, while in the smaller balls the decomposition has
penetrated to the centre.

In the rocks also we find many imperfect joints and minute cracks. In
cold countries these are extended and widened by the expansive power of
freezing water, and thus the surfaces of decomposition become constantly
greater.

Nearly all rocks suffer this chemical decomposition when exposed to the
weather, but in some the decay goes on much faster than in others.
Diabase is one of the rocks which decay most readily; while granite is,
among common rocks, one of those that resist decay most effectually.

The caverns which are so large and numerous in most limestone countries
are a splendid example of the solvent action of meteoric waters, being
formed entirely by the dissolving out of the limestone by the water
circulating through the joint cracks. The process must go on with
extreme slowness at first, when the joints are narrow, and more rapidly
as they are widened and more water is admitted. We get some idea, too,
of the magnitude of the results accomplished by these silent and
unobtrusive agencies when we reflect that almost all the loose earth and
soil covering the solid rocks are simply the insoluble residue which
carbon dioxide and water cannot remove. In low latitudes, where a warm
climate accelerates the decay of the rocks, the soil is usually from 50
to 300 feet deep.

MECHANICAL EROSION.—_On the edge of the land._—Let us trace next the
_mechanical_ action of water and air upon the land. First we will
consider the _edge_ of the land, where it is washed by the waves of the
sea. Whoever has been on the shore must have noticed that the sand along
the water’s edge is kept in constant motion by the ebb and flow of the
surf.

Where the beach is composed of gravel or shingle the motion is evident
to the _ear_ as well as the eye; and when the surf is strong, the
rattling and grinding of the pebbles as they are rolled up and down the
beach develops into a roar.

The constant shifting of the grains of sand, pebbles, and stones is, of
course, attended by innumerable collisions, which are the cause of the
noise. Now it is practically impossible, as we may easily prove by
experiment, to knock or rub two pieces of stone together, at least so as
to produce much noise, without abrading their surfaces; small particles
are detached, and sand and dust are formed.

That this abrasion actually occurs in the case of the moving sand is
most beautifully shown by the sandblast. We are to conclude, then, that
every time a pebble, large or small, is rolled up or down the beach it
becomes smaller, and some sand and dust or clay are formed which are
carried off by the water.

But what are the pebbles originally? This question is not difficult. A
little observation on the beach shows us that the pebbles are not all
equally round and smooth, but many are more or less angular. And we soon
see that it is possible to select a series showing all gradations
between the most perfectly rounded forms and angular fragments of rock
that are only slightly abraded on the corners. The three principal
members of such a series are shown in specimens 5, 6, and 7 from the
beach on Marblehead Neck; but equally instructive specimens can be
obtained at many other points on our coast. It is also observable that
the well-rounded pebbles are much smaller on the average than the
angular blocks.

From these facts we draw the legitimate inference that the pebbles were
all originally angular, and that the same abrasion which diminishes
their size makes them round and smooth.

A little reflection, too, shows that the rounding of the angular
fragments is a natural and necessary result of their mutual collisions;
for the angles are at the same time their weakest and most exposed
points, and must wear off faster than the flat or concave surfaces.

Having traced each pebble back to a larger angular rock-fragment, the
question arises, Whence come these angular blocks?

Behind our gravel-beach, or at its end, we have usually a cliff of
rocks. As we approach this it is distinctly observable that the angular
pebbles are more numerous, larger, and more angular; and a little
observation shows that these are simply the blocks produced by jointing,
and that the cliff is entirely composed of them. In other words, our
cliff is a mass of natural masonry, which chemical agencies, the frost,
and the sea are gradually disintegrating and removing. As soon as the
blocks are brought within reach of the surf their mutual collisions make
them rounder and smaller; and small round pebbles, sand, and clay are
the final result.

For a more complete account of the formation of pebbles, teachers are
referred to the first or introductory number of this series of guides,
by Prof. Hyatt, “About Pebbles.”

Where the waves can drive the shingle directly against the base of the
cliff, this is gradually ground away in the same manner as the loose
stones themselves, sometimes forming a cavern of considerable depth, but
always leaving a smooth, hard surface, which is very characteristic, and
contrasts strongly with the upper portion of the cliff, which is acted
on only by the rain and frost. A good example of such a pebble-carved
cliff may be seen behind the beach on the sea-ward side of Marblehead
Neck.

The sea acts within very narrow limits vertically, a few feet or a few
yards at most; but the coast-lines of the globe (including inland lakes
and seas) have an aggregate length of more than 150,000 miles. Hence it
is easy to see that the amount of solid rock ground to powder in the
mill of the ocean-beach annually must be very considerable.

MECHANICAL EROSION.—_On the surface of the land._—I next ask attention
to the _mechanical_ action of water upon the _surface_ of the land.

It is a familiar fact that after heavy rains the roadside rills carry
along much sand and clay (which we know have been produced by the
previous action of chemical forces), and also frequently small pebbles
or gravel. It is easy to show that in all important respects the rill
differs in size only from brooks and rivers; and the former afford us
fine models of the systems of valleys worn out during the lapse of ages
by rivers. The turbidity of rivers is often very evident, and in shallow
streams we can sometimes see the pebbles rolled along by the current.

Now here, just as on the beach, the collisions of rock-fragments are
attended by mutual abrasion, sand and clay are formed, and the fragments
become smaller and rounder. Our series of pebbles from the beach might
be matched perfectly among the river-gravel. In mountain streams
especially we may often observe that pebbles of a particular kind of
rock become more numerous, larger, and more angular as we proceed up
stream, until we reach the solid ledge from which they were derived,
showing the same gradation as the beach pebbles when followed back to
the parent cliff.

The pebbles, however, not only grind each other, but also the solid
rocks which form the bed of the streams in many places, and these are
gradually worn away. When the rocky bed is uneven and the water is
swift, pebbles collect in hollows where eddies are formed, by which they
are kept whirling and turning, and the hollow is deepened to a pot-hole,
while the pebbles, the river’s tools, are worn out at the same time.

By these observations we learn not only that running water carries away
sand and clay already formed, but that it also has great power of
grinding down hard rocks to sand and clay. Of course the pulverized rock
always moves in the same direction as the stream which carries it; and,
in a certain sense, all streams run in one direction, viz., toward the
sea. Therefore the constant tendency of the rain falling upon the land
is to break up the rocks by chemical and mechanical action and transport
the débris to the sea.

Rivers, as we all know, are continually uniting to form larger and
larger streams; and thus the drainage of a wide area sometimes, as in
the case of the Mississippi Valley, reaches the sea through a single
mouth. By careful measurements made at the mouth of the Mississippi it
has been shown that the 20,000,000,000,000 cubic feet of water
discharged into the Gulf of Mexico annually carries with it no less than
7,500,000,000 cubic feet of sand, clay, and dissolved mineral matter;
and this, spread over the whole Mississippi basin, would form a layer a
little more than 1/5000 of a foot in thickness. So that we may conclude
that the surface of the continent is being cut down on the average about
_one foot_ in _five thousand_ years.

We can only allude in passing to the very important geological action of
water in the solid state, as in glaciers and icebergs. The moisture
precipitated from the atmosphere, and falling as rain, makes ordinary
rivers; but falling in the form of snow in cold regions, where more snow
falls than is melted, the excess accumulates and is gradually compacted
to ice, which, like water, yields to the enormous pressure of its own
mass and flows toward lower levels. When the ice-river reaches the sea
it breaks off in huge blocks, which float away as icebergs. Moving ice,
like moving water, is a powerful agent of erosion; and the glacial marks
or scratches observable upon the ledges everywhere in the Northern
States and Canada attest the magnitude of the ice-action at a
comparatively recent period.

We have already noticed incidentally the powerful disintegrating action
of water where it freezes in the joints and pores of the rocks; and it
is probable that it thus facilitates the destruction of the rocks in
cold countries nearly as much as the higher temperature and greater
rain-fall do in warm countries.

Our observations up to this point show us that _erosion_, by which we
mean the breaking up by chemical and mechanical action of the rocks of
the land and the transportation of the débris into the sea, is one great
result accomplished by the inorganic aqueous agencies.

MECHANICAL DEPOSITION.—Next let us notice what becomes of all this vast
amount of clay, sand, and gravel after it is washed into the ocean. By
taking up a glass of turbid water from our roadside rill, and observing
that as soon as the water is undisturbed the sand and clay begin to
settle, we learn that the solid matter is held in suspension by the
motion of the water. But it does not remain in suspension long after
being washed into the sea, for otherwise the sea would, in the course of
time, become turbid for long distances from shore; and it is a
well-known fact that the sea-water is usually clear and free from
sensible turbidity close along shore and even near the mouths of large
rivers, while at a distance of only 50 or 100 miles we find the
transparency of the central ocean.

Putting these facts together, we see that the ocean, nothwithstanding
the ceaseless and often violent undulations of its surface, must be as a
whole a vast body of still water; and to the reflecting mind the almost
perfect tranquillity of the ocean is one of its most impressive
features. For it is in striking contrast, in this respect, with the more
mobile aerial ocean above it.

We have got hold, now, of two facts of great geological importance: (1)
The débris washed off the land by waves and rivers into the still water
of the ocean very soon settles to the bottom; and (2) it nearly all
settles on that part of the ocean-floor near the land.

And now we have in view the second great office of the inorganic aqueous
agencies,—deposition, the counterpart or complement of erosion.

The land is the great theatre of erosion and the sea of deposition; the
rocks which are constantly wasting away on the former are as constantly
renewed in the latter.

We will now observe the process of deposition a little more closely.
Each of these two bottles contains the same amount of fine yellow clay,
but in one the water is fresh, and in the other it is salt. At the
beginning of the lesson, as you may have observed, I brought the clay in
both bottles into suspension by violent agitation, and since then they
have remained undisturbed. The main point is that the salt water has
become quite clear, while the fresh water is still distinctly turbid,
showing that the salt favors the rapid deposition of the clay. At the
second lecture, a week later, these two bottles, yet undisturbed, were
exhibited, and the fresh water seen to be still sensibly turbid. The
fact is, the clay is not held in suspension wholly by the _motion_ of
the water; but, just as in the case of dust in the atmosphere, a small
portion of the medium is condensed around or adheres to each solid
particle, _i.e._, each clay particle in our experiment has an atmosphere
of water which moves with it and buoys it up. Now the effect of the salt
is to diminish the adhesion of the water to the particles, _i.e._, to
diminish their atmospheres, and consequently their buoyancy. The
diminished adhesion of the salt water is well shown by the smaller drops
which it forms on a glass rod.

The geological importance of this principle is very great; for it is
undoubtedly largely to the saltness of the sea that we owe its
transparency, and the fact that the fine, clayey sediment from the land,
like the coarse, is deposited near the shore.

This bottle of fresh water contains some fine gravel, coarse sand, fine
sand, and clay. By agitating the water, all this material is brought
into suspension. Now, suddenly placing the bottle in a state of rest, we
observe that the gravel falls to the bottom almost instantly, followed
quickly by the coarse sand, and very soon afterward by the fine sand;
and then there appears to be a pause, the fine particles of clay all
remain in suspension; but finally, when the water is quite motionless,
they begin to settle; they fall very slowly, however, and the water will
not be clear for hours.

This is a very instructive experiment. We learn from it:

First, that the power of the water to hold particles in suspension is
inversely proportional to the size of the particles;

Second, that all materials deposited in water are assorted according to
size;

Third, and this is one of the most important facts in geology, all
water-deposited sediments are arranged in horizontal layers, _i.e._, are
stratified. And we have now traced to its conclusion, though very
briefly, the process of the formation of one great division of
_stratified_ rocks,—the _mechanically-formed_ or _fragmental_ rocks.
These are so called because the clay, sand, and gravel are, in every
instance, fragments of pre-existing rocks; and because the formation,
transportation, and especially the _deposition_ of these fragments, are
the work chiefly or entirely of mechanical forces.

CHEMICAL DEPOSITION.—It is a well-known fact that the sea holds in
solution vast amounts of common salt as well as many other substances;
and analyses of river-waters show that dissolved minerals derived from
the chemical decomposition of the rocks of the land are being constantly
carried into the sea.

Portions of the sea which are cut off from the main body, and which are
gradually drying up, like the Great Salt Lake, Dead Sea, and Caspian
Sea, become saturated solutions of the various dissolved minerals, and
these are slowly deposited. This process is very nicely illustrated
along our shores in summer, where, during storms, salt-water spray is
thrown above the reach of the tides, and, collecting in hollows in the
rocks, gradually dries up, leaving behind a crust of salt.

When the sea lays down matter which it held in _suspension_, we call the
process _mechanical_ deposition, and the result is _mechanically_-formed
rocks.

But when it lays down matter which it held in _solution_, we call the
process _chemical_ deposition, and the result is _chemically_-formed
rocks.

The principal substances which the sea deposits chemically are common
salt, forming beds of rock-salt; sulphate of calcium, forming beds of
gypsum; carbonate of calcium, forming beds of limestone; and the double
carbonate of calcium and magnesium, forming beds of dolomite.

Inorganic deposition, like inorganic erosion, is both chemical and
mechanical.


             2. _Animals and Plants, or Organic Agencies._

We turn now to the consideration of the _organic_ agencies. And I will
merely allude in passing to the vast importance of the fossil organic
remains found in the stratified rocks as marks by which to determine the
relative ages of the formations.

As regards the _destruction_ of rocks—_erosion_—plants and animals are
almost powerless; but in the role of _rock-makers_ they play a very
important part, being very efficient agents of _deposition_.

FORMATION OF COALS AND BITUMENS.—Specimen No. 8 is an example of peat
from the vicinity of Boston; but just as good specimens may be obtained
in thousands of places in this and other States.

The general physical conditions under which peat is formed are familiar
facts. We require simply low, level land, covered with a thin sheet of
water and abundant vegetation; in other words, a marsh or swamp. If
plants decay on the dry land, the decomposition is complete; they are
burned up by the oxygen of the air to _carbon dioxide_ and _water_ just
as surely as if they had been thrown into a furnace, though less
rapidly, and nothing is returned to the soil but what had been taken
from it by the plants during their growth. But if the plants decay under
water, as in a peat-marsh or bog, the decay is incomplete, and most of
the carbon of the wood is left behind. Now, if this incomplete
combustion of vegetable tissues takes place in a charcoal-pit, where the
wood is out of contact with air from being covered with earth, we call
the carbonaceous product charcoal; but if under the water of a marsh, in
Nature’s laboratory, we call the product peat. Peat is simply a natural
charcoal; and, just as in ordinary charcoal, its vegetable origin is
always perfectly evident. But when the deposit becomes thicker, and
especially when it is buried under thick formations of other rocks, like
sand and clay, the great pressure consolidates the peat; it becomes
gradually more mineralized and shining, shows the vegetable tissues less
distinctly, becomes more nearly pure carbon, and we call it in
succession lignite, bituminous coal, and anthracite.

This is, briefly, the way in which all varieties of coal, as well as the
more solid kinds of bitumen, like asphaltum, are formed. But the lighter
forms of bitumen, such as petroleum and naphtha, are derived mainly, if
not entirely, from the partial decomposition of animal tissues. These,
it is well known, decay much more readily than vegetable tissues; and
the water of an ordinary marsh or lake contains sufficient oxygen for
their complete and rapid decomposition. In the deeper parts of the
ocean, however, the conditions are very different, for recent researches
have shown, contrary to the old idea, that the deep sea holds an
abundant fauna. All grades of animal life, from the highest to the
lowest, have need of a constant supply of oxygen. On the land vegetation
is constantly returning to the air the oxygen consumed by animals, but
in the abysses of the ocean vegetable life is scarce or wanting; and
hence it must result that over these greater than continental areas
countless myriads of animals are living habitually on short rations of
oxygen, and in water well charged with carbon dioxide, the product of
animal respiration. As a consequence, when these animals die their
tissues do not find the oxygen essential for their perfect
decomposition, and in the course of time become buried, in a
half-decayed state, in the ever-increasing sediments of the ocean-floor.

It is important to observe that an abundance of organic matter decaying
under water is not the only condition essential to the formation of beds
of coal and bitumen; for this condition is realized in the luxuriant
growth of sea-weeds fringing the coast in every quarter of the globe;
and yet coals and bitumens are rarely of sea-shore origin. These organic
products, even under the most favorable circumstances, accumulate with
extreme slowness; far more slowly, as a rule, than the ordinary
mechanical sediments, like sand and clay, with which they are mixed, and
in which they are often completely lost. Consequently, although the
deposition of the carbonized remains of plants and animals is taking
place in nearly all seas, lakes, and marshes, it is only in those places
where there is little or no mechanical sediment that they can
predominate so as to build up beds pure enough to be called coal or
bitumen. In all other cases we get merely more or less carbonaceous sand
or clay. Now these especially favorable localities will manifestly not
be often found along the seashore, where we have strewn the sand and
clay brought down by rivers or washed off the land directly by the
ever-active surf; but they must exist in the central portions of the
ocean, where there is almost no mechanical sediment and yet an abundance
of life, and in swamps and marshes, where there is scarcely sufficient
water to cover the vegetation, and no waves or currents to wash down the
soil from the surrounding hills.

FORMATION OF IRON-ORES.—The iron-ores are another class of rocks which
are formed only through the agency of organic matter. Iron is an
abundant and wide-spread element in the earth’s crust, and, but for the
intervention of life, we might say that, while there is iron everywhere,
there is not much of it in any one place, since it is originally very
thinly diffused. All rocks and soils contain iron, but it is mainly in
the form of the peroxide, in which state it is entirely insoluble, and
hence cannot be soaked out of the soil by the rain-water and
concentrated by the evaporation of the water at lower levels in ponds
and marshes, as a soluble substance like salt would be. If carried off
with the sand and clay, by the mechanical action of water, it remains
uniformly mixed with them, and there is no tendency to its separation
and concentration so as to form a true iron-ore.

But what water cannot do alone is accomplished very readily when the
water is aided by decaying organic matter, which is always hungry for
oxygen, being, in the language of the chemist, a powerful reducing
agent. The soil, in most places, has a superficial stratum of vegetable
mould or half-decayed vegetation. The rainwater percolates through this
and dissolves more or less of the organic matter, which is thus carried
down into the sand and clay beneath and brought in contact with the
ferric oxide, from which it takes a certain proportion of oxygen,
reducing the ferric to the ferrous oxide. At the same time the
vegetation is burned up by the oxygen thus obtained, forming carbon
dioxide, which immediately combines with the ferrous oxide, forming
carbonate of iron, which, being soluble under these conditions, is
carried along by the water as it gradually finds its way by subterranean
drainage to the bottom of the valley and emerges in a swamp or marsh.

Here one of two things will happen: If the marsh contains little or no
decaying vegetation, then as soon as the ferrous carbonate brought down
from the hills is exposed to the air it is decomposed, the carbon
dioxide escapes, and the iron, taking on oxygen from the air, returns to
its original ferric condition; and being then quite insoluble, it is
deposited as a loose, porous, earthy mass, commonly known as
bog-iron-ore, which becomes gradually more solid and finally even
crystalline through the subsequent action of heat and pressure. When
first deposited, the ferric oxide is combined with water or hydrated,
and is then known as limonite (specimen No. 12); at a later period the
water is expelled, and we call the ore hematite (specimen No. 13); and
at a still later age it loses part of its oxygen, becomes magnetic and
more crystalline, and is then known as magnetite (specimen No. 14). Thus
it is seen that the iron-ores, as we pass from bog-limonite to
magnetite, form a natural series similar to and parallel with that
afforded by the coals as we pass from peat to graphite.

If the drainage from the hills is into a marsh containing an abundance
of decaying vegetation, _i.e._, if peat is forming there, the ferrous
carbonate, in the presence of the more greedy organic matter, will be
unable to obtain oxygen from the air; and as the evaporation of the
water goes on, it will sooner or later become saturated with this salt,
and the latter will be deposited. Here we find an explanation of a fact
often observed by geologists, viz., that the carbonate iron-ores are
usually associated with beds of coal.

The formation of the iron-ores, like that of the coals and bitumens, is
a slow process; and the ores, like the coals, etc., will be pure only
where there is a complete absence of mechanical sediment, a condition
that is realized most nearly in marshes.

FORMATION OF LIMESTONE, DIATOMACEOUS EARTH, ETC.—Marine animals take
from the sea-water certain mineral substances, especially silica and
carbonate of calcium, to form their skeletons. Silica is used only by
the lowest organisms, such as Radiolaria, Sponges, and the minute
unicellular plants, Diatoms. The principal animals secreting carbonate
of calcium are Corals and Mollusks. These hard parts of the organisms
remain undissolved after death; and over portions of the ocean-floor
where there is but little of other kinds of sediment they form the main
part of the deposits, and in the course of ages build up very extensive
formations which we call diatomaceous earth or tripolite, if the
organisms are siliceous, or limestone if they are calcareous. A very
satisfactory account of the formation of limestone on a stupendous scale
by the polyps in coral reefs and islands is contained in No. IV. of this
series of guides.

The rocks here considered may be, and, as we have already seen,
sometimes are, deposited in a purely chemical way, without the aid of
life; and it is important to observe that in no case do the organisms
make the silica and carbonate of calcium of their skeletons, but they
simply appropriate and reduce to the solid state what exists ready made
in solution in the sea-water. These minerals, and others, as we know,
are produced by the decomposition of the rocks of the land, and are
being constantly carried into the sea by rivers; and, if there were no
animals in the sea, these processes would still go on until the
sea-water became saturated with these substances, when their
precipitation as limestone, etc., would necessarily follow. Hence it is
clear that all the animals do is to effect the precipitation of certain
minerals somewhat sooner than it would otherwise occur; so that from a
geological standpoint the differences between chemical and organic
deposition are not great.

This section of our subject may be summarized as follows: Animals and
plants contribute to the formation of rocks in three distinct ways:—

1. During their growth they deoxidize carbon dioxide and water,
and reduce to the solid state in their tissues carbon and the
permanent gases oxygen, hydrogen, and nitrogen; and after death,
through the accumulation of the half-decayed tissues in favorable
localities,—marshes, etc.,—these elements are added to the solid
crust of the earth in the form of coal and bitumen.

2. During the decomposition, _i.e._, oxidation, of the organic tissues,
the iron existing everywhere in the soil is partially deoxidized, and,
being thus rendered soluble, is removed by rain-water and concentrated
in low places, forming beds of iron-ore.

3. Through the agency of marine organisms, certain mineral substances
are being constantly removed from the sea-water and deposited upon the
ocean floor, forming various calcareous and siliceous rocks.

I now bring our study of the aqueous or superficial agencies to a
conclusion by noting once more that the great geological results
accomplished by _air_, _water_, and _organic matter_ or _life_ are: (1)
_Erosion_, or the wearing away of the surface of the land; and (2)
_Deposition_, or the formation from the débris of the eroded land of two
great classes of stratified rocks,—the mechanically formed or fragmental
rocks, and the chemically and organically formed rocks.




                         II. IGNEOUS AGENCIES.


We pass next to a very brief consideration of operations that originate
below the earth’s surface. The records of deep mines and artesian wells
show that the temperature of the ground always increases downwards from
the surface; and the much higher temperatures of hot springs and
volcanoes show that the heat continues to increase to a great depth, and
is not a merely superficial phenomenon. The observed rate of increase is
not uniform, but it seldom varies far from the average, which is about
1° Fahr. per 53 feet of vertical descent, or, in round numbers, 100° per
mile. This rate, if continued, would give a very high temperature at
points only a few miles below the surface; and until within a few years
the idea was generally accepted by geologists that the increase of
temperature is sensibly uniform for an indefinite distance downward;
that in the central regions of the earth the temperature is far higher
than anything we can conceive, and that everywhere below a depth of 20
to 40 miles the temperature is above the fusing-point of all rocks; and
hence that the earth is an incandescent liquid globe covered by a thin
shell or crust of cold, solid rock.

Our limited space will not permit us to enter into a discussion of the
condition of the earth’s interior, and I will merely point out in a few
sentences the position occupied by geologists at the present time. The
reasoning of Thompson has shown that the temperature cannot increase
downward at a uniform rate, but at a constantly and rapidly diminishing
rate; and that everywhere below a depth of 300 miles the temperature is
probably sensibly the same, and nowhere, probably, above 8000° to
10,000° Fahr.

Unlike water, all rocks contract on solidifying and expand on melting,
and consequently the high pressures to which they are subjected in the
earth’s interior—10,000,000 to 20,000,000 pounds per square inch—must
raise their fusing-points enormously, and the probabilities are that
they are solid, in spite of the high temperature. But Thompson and
Darwin have shown us farther that the phenomena of the oceanic tides
could not be what they are known to be if the earth were any less rigid
than a globe of solid steel; while Hopkins has proved that the
astronomical phenomena of precession and nutation could not be what they
are if the earth’s crust were less than 800 or 1000 miles thick. Putting
these considerations together, geologists are almost universally agreed
that, while the earth has an incandescent interior, it is still
continuously solid from centre to circumference, with the exception of a
thin plastic stratum at a depth not exceeding 40 or 50 miles, which
forms the seat of volcanic action.

The earth is not only a very hot body, but it is rotating through almost
absolutely cold space, and therefore must be a cooling body. But, except
at the very beginning of the cooling, the loss of heat has gone on
almost entirely from the interior; and since cooling means contraction,
the heated interior must be constantly tending to shrink away from the
cold external crust.

Of course no actual separation between the crust and interior or nucleus
can take place, but there is no doubt that the crust is left unsupported
to a certain extent, and it must then behave like an arch with a radius
of 4000 miles, and the result is an enormous horizontal or tangential
pressure.

This lateral pressure in the earth’s crust is one of the most important
and most generally accepted facts in geology, and lies at the bottom of
many geological theories. According to what seems to me to be the most
probable theory of the origin of continents and ocean-basins, they are
broad upward and downward bendings or arches into which the crust is
thrown by the tangential pressure. Finally, the strain becomes great
enough to crush the crust along those lines where it is weakest. When
the crust is thus mashed up by horizontal pressure, a mountain range is
formed, the crust becomes enormously thicker, and a weak place becomes a
strong one.

During the formation of mountains the stratified rocks, which were
originally horizontal, are thrown into folds or arches, and tipped up at
all possible angles; they are fractured and faults produced; and by the
immense pressure the structure known as slaty cleavage is developed. In
fact, a vast amount and variety of structures are produced during the
growth of a mountain range.

These great earth-movements are not always perfectly smooth and steady,
but they are accompanied by slipping or crushing now and then; and, as a
result of the shock thus produced, a swift vibratory movement or jar,
which we know as an _earthquake_, runs through the earth’s crust.

Extensive fissures are also formed, opening down to the regions where
the rocks are liquid or plastic, and through these the melted rocks flow
up to or toward the surface. That portion which flows out on the surface
builds up a volcanic cone, while that which cools and solidifies below
the surface, in the fissures, forms dikes. Thus among the igneous or
eruptive rocks we have two great classes,—the _dike_ rocks and the
_volcanic_ rocks.

It is important to observe that all these subterranean operations—the
formation of continents, of mountain-ranges with all their attendant
phenomena of folds, faults and cleavage, and every form and phase of
earthquake and volcanic activity—depend upon or originate in the
interior heat of the earth. And over against the _superficial_ or
_aqueous_ agencies, originating in the _solar_ heat and producing the
_stratified_ or _sedimentary_ rocks, we set the _subterranean_ or
_igneous_ agencies originating in the _central_ heat, and producing the
_unstratified_ or _eruptive_ rocks.

------------------------------------------------------------------------




                          STRUCTURAL GEOLOGY.

                             [Illustration]


In geology, just as in biology, there are two ways of studying
structure,—the small way and the large way. In the case of an organism,
we may select a single part or organ, and, disregarding its external
form and relations to other parts, observe its composition and minute
structure, the various forms and arrangements of the cells, etc. This is
histology, and it is the complement of that larger method of studying
structure which is ordinarily understood by anatomy.

The divisions of structural geology corresponding to histology and
anatomy are _lithology_ and _petrology_. Lithology is an in-door
science; we use the microscope largely, and work with hand specimens or
thin sections of the rocks, observing the composition and those small
structural features which go under the general name of texture.

In petrology, on the other hand, we consider the larger kinds of
rock-structure, such as stratification, jointing, folds, faults,
cleavage, etc.; and it is essentially an out-door science, since to
study it to the best advantage we must have, not hand specimens, but
ledges, cliffs, railway-cuttings, gorges, and mountains.




                               LITHOLOGY.


A _rock_ is any mineral, or mixture of minerals, occurring in masses of
considerable size. This distinction of size is the only one that can be
made between rocks and minerals, and that is very indefinite. A rock,
whether composed of one mineral or several, is always an aggregate; and
therefore no single crystal or mineral-grain can properly be called a
rock.

Before proceeding to study particularly the various kinds of rocks, a
little more preliminary work should be done. As already intimated, the
more important characteristics of rocks may be grouped under two general
heads,—_composition_ and _texture_.


                        _Composition of Rocks._

Rocks are properly defined as large masses or aggregates of mineral
matter, consisting in some cases of one and in other cases of several
mineral species. Hence it is clear that the composition of rocks is of
two kinds: chemical and mineralogical; for the various chemical elements
are first combined to form minerals, and then the minerals are combined
to form rocks.

Of course those minerals and elements which can be described as
principal or important rock-constituents must be the common minerals and
elements. Now it is a very important and convenient fact that although
chemists recognize about sixty-five elementary substances, and these are
combined to form nearly one thousand mineral species, yet both the
_common_ elements and the _common_ minerals are few in number.

So that, although it is very desirable and even necessary for the
student of lithology to know something of chemistry and mineralogy, it
by no means follows that he or she must be master of those sciences. A
knowledge of the chemical and physical characteristics of a few common
minerals is all that is absolutely essential, though it may be added
that an excess of wisdom in these directions is no disadvantage.


                     Chemical Composition of Rocks.

The elementary substances of which rocks are chiefly composed, which
make up the main mass of the earth so far as we are acquainted with it,
number only fourteen:—

_Non-Metallic or Acidic Elements._—Oxygen, silicon, carbon, sulphur,
chlorine, phosphorus, and fluorine.

_Metallic or Basic Elements._—Aluminum, magnesium, calcium, iron,
sodium, potassium, and hydrogen.

The elements are named in each group in about the order of their
relative abundance; and to give some idea of the enormous differences in
this respect it may be stated that two of the elements—oxygen and
silicon—form more than half of the earth’s crust.

Silicon, calcium, and fluorine, although exceedingly abundant, are also
very difficult to obtain in the free or uncombined state, and specimens
large enough to exhibit to a class would be very expensive. With these
exceptions, however, examples of these common rock-forming elements are
easily obtained.

My purpose in calling attention to this point is simply to suggest that
the proper way to begin the study of minerals and rocks with children is
to first familiarize them with the elements of which they are composed.
The most important thing to be known about any mineral is its chemical
composition; and when a child is told that a mineral—corundum, for
example—is composed of oxygen and aluminum, he should have a distinct
conception of the properties of each of those elements, for otherwise
corundum is for him a mere compound of names.

It is very important, too, if the pupil has not already studied
chemistry, that he should be led to some comprehension of the nature of
chemical union and of the difference between a chemical compound and a
mechanical mixture. For this purpose a few simple experiments (the
details of which would be out of place here) with the more common and
familiar elements will be sufficient. Mrs. Richard’s “First Lessons in
Minerals” should be introduced here.


                  Mineralogical Composition of Rocks.

The fourteen elements named above are combined to form about fifty
minerals with which the student of geology should be acquainted; but not
more than one-half of these are of the first importance. It is desired
to lay especial emphasis upon the importance of a perfect familiarity
with these few common minerals. There is nothing else in the whole range
of geology so easily acquired which is at the same time so valuable; for
it is entirely impossible to comprehend the definitions of rocks, or to
recognize rocks certainly and scientifically, unless we are acquainted
with their constituent minerals.

With one or two exceptions, these common rock-forming minerals may be
easily distinguished by their physical characters alone, so that their
certain recognition is a matter of the simplest observation, and
entirely within the capacity of young children. Furthermore, being
common, specimens of these minerals are very easily obtained, so that
there is no reason why teachers should not here adopt the best method
and place a specimen of each mineral in the hands of each pupil. Typical
examples, large enough to show the characteristics well, ought not to
cost, on the average, over two cents apiece.

A MINERAL is an inorganic body having theoretically a definite chemical
composition, and usually a regular geometric form.

THE PRINCIPAL CHARACTERISTICS OF MINERALS.—These may be grouped under
the following general heads:—

(1) _Composition_, (2) _Crystalline form_, (3) _Hardness_, (4) _Specific
gravity_, (5) _Lustre_, (6) _Color and Streak_.

1. _Composition._—This, according to the definition of a mineral, ought
to be _definite_, and expressible by a chemical formula. When it is not
so, we usually consider that the mineral is partially decomposed, or
that we are dealing with a mixture of minerals. It is well to impress
upon the mind of the pupil the important fact that the more fundamental
properties of the elements, such as specific gravity and lustre, are not
lost when they combine, but may be traced in the compounds. In other
words, the properties of minerals are, in a very large degree, the
average of the properties of the elements of which they are composed;
minerals in which heavy metallic elements predominate being heavy and
metallic, and _vice versa_.

To fully appreciate this point it is only necessary to compare a mineral
like galenite—a common ore of lead, and containing nearly 87 per cent.
of that heavy metal; or hematite (specimen 13), containing 70 per cent.
of another heavy metal, iron—with quartz (specimen 15), which is
composed in nearly equal parts of oxygen and silicon, two typical
non-metallic elements. Many minerals contain water, _i.e._, are
hydrated. Now water, whether we consider the liquid or solid state, is
one of the lightest and softest of mineral constituents; and it is a
very important fact that hydrated minerals are invariably lighter and
usually softer than anhydrous species of otherwise similar composition.
Other striking illustrations of this principle will be pointed out in
the descriptions of the minerals which follow.

2. _Crystalline form._—A crystal is bounded by plane surfaces
symmetrically arranged with reference to certain imaginary lines passing
through its centre and called axes. Crystals of the same species are
always constant in the angles between like planes, while similar angles
usually vary in different species; so that each species has its own
peculiar form.

“Besides external symmetry of form, crystallization produces also
regularity of internal structure, and often of fracture. This regularity
of fracture, or tendency to break or cleave along certain planes, is
called cleavage. The surface afforded by cleavage is often smooth and
brilliant (see specimens 17, 18, and 21), and is always parallel with
some external plane of the crystal. It should be understood that the
cleavage lamellæ are not in any sense present before they are made to
appear by fracture.”—(Dana.)

Crystals are arranged in six systems, based upon the number and
relations of the axes, as follows:—

Isometric System.—Three equal axes crossing at right angles. Example,
cube.

Tetragonal System.—Two axes equal, third unequal, all crossing at right
angles. Example, square prism.

Orthorhombic System.—Three unequal axes, but intersections all at right
angles. Example, rhombic prism.

Monoclinic System.—Three unequal axes, one intersection oblique.
Example, oblique rhombic prism.

Triclinic System.—Three unequal axes, all crossing obliquely. Example,
oblique rhomboidal prism.

Hexagonal System.—Three equal axes lying in one plane and intersecting
at angles of 60°, and a fourth axis crossing each of these at right
angles and longer or shorter. Example, hexagonal prism.

By the truncation and bevelment of the angles and edges of these
fundamental forms a vast variety of secondary forms are produced. The
limits of the guide will not permit us to follow this topic farther; but
it may be added that for the proper elucidation of even the simpler
crystalline forms the teacher should be provided with a set of wooden
crystal models and Dana’s “Text-Book of Mineralogy.”

The crystallization of a mineral may be manifested in two ways: first,
by the regularity of its internal structure or molecular arrangement, as
shown by cleavage and the polarization of transmitted light; and,
second, by the regularity of external form which follows, _under
favorable conditions_, as a necessary consequence of symmetry in the
arrangement of the molecules.

When a mineral is entirely devoid of crystalline structure, both
externally and internally, it is said to be _amorphous_.

Perfect and distinct crystals are the rare exception, most mineral
specimens being simply aggregates of imperfect crystals. In such cases,
and when the mineral is amorphous, the _structure_ of the _mass_ may
be:—

Columnar or fibrous.

Lamellar, foliaceous, or micaceous.

Granular.—When the grains or crystalline particles are invisible to the
naked eye the mineral is called _impalpable_, _compact_, or _massive_.

And the _external form_ of the mass may be:—

Botryoidal, having grape-like surfaces.

Stalactitic, forming stalactites or pendant columns.

Amygdaloidal or Concretionary, forming separate globular masses in the
enclosing rock.

Dendritic, branching or arborescent.

3. _Hardness._—By the hardness of a mineral we mean the resistance which
it offers to abrasion. But hardness is a purely relative term, calcite,
for example, being hard compared with talc, but very soft compared with
quartz. Hence mineralogists have found it necessary to select certain
minerals to be used as a standard of comparison for all others, and
known as the _scale of hardness_. These are arranged at nearly equal
intervals all the way from the softest mineral to the hardest, as
follows:—


                          _Scale of Hardness._

                          (1) Talc.
                          (2) Gypsum.
                          (3) Calcite.
                          (4) Fluorite.
                          (5) Apatite.
                          (6) Orthoclase.
                          (7) Quartz.
                          (8) Topaz or Beryl.
                          (9) Corundum.
                          (10) Diamond.

If a mineral scratches calcite and is scratched by fluorite, we say its
hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is
scratched by orthoclase, its hardness is 6; and so on. There are very
few minerals harder than quartz, and hence the first seven members of
the scale are sufficient for all ordinary purposes; and these are all
included in the series of specimens accompanying this Guide.

Although it is desirable to be acquainted with the scale of hardness,
and to understand how to use it, still the student will learn, after a
little practice, that almost as good results may be obtained much more
conveniently by the use of his thumb-nail and a good knife-blade or
file. Talc and gypsum are easily scratched with the nail; calcite and
fluorite yield easily to the knife or file, apatite with more
difficulty; while orthoclase is near the limit of the hardness of
ordinary steel, and quartz is entirely beyond it.

4. _Specific Gravity._—The specific gravity of a mineral, by which we
mean its weight as compared with the weight of an equal volume of water,
is determined by weighing it first in air and then in water, and
dividing the weight in air by the difference of the two weights.
Minerals exhibit a wide range in specific gravity; from petroleum, which
floats on water, to gold, which is nearly twenty times heavier than
water. Although this is one of the most important properties of
minerals, yet, being more difficult to measure than hardness, it is less
valuable as an aid in distinguishing species. One can with practice,
however, estimate the density of a mineral pretty closely by lifting it
in the hand.

5. _Lustre._—Of all the properties of minerals depending on their
relations to light the most important is lustre, by which we mean the
quality of the light reflected by a mineral as determined by the
character or minute structure of its surface. Two kinds of lustre, the
_metallic_ and _vitreous_, are of especial importance; in fact all other
kinds are merely varieties of these.

The metallic lustre is the lustre of all true metals, as copper and tin,
and characterizes nearly all minerals in which metallic elements
predominate. The vitreous lustre is best exemplified in glass, but
belongs to most minerals composed chiefly of non-metallic elements.
Metallic minerals are always opaque, but vitreous minerals are often
transparent.

Other kinds of lustre are the _adamantine_ (the lustre of diamond),
_resinous_, _pearly_, and _silky_. When a mineral has no lustre, like
chalk, it is said to be dull.

It should be made clear to children that lustre and color are entirely
distinct and independent. Thus, iron, copper, gold, silver, and lead are
all metallic; while white or colorless quartz, black tourmaline, green
beryl, red garnet, etc., are all vitreous. Generally speaking, any color
may occur with any lustre.

6. _Color and Streak._—The colors of minerals are of two
kinds,—_essential_ and _non-essential_. By the essential color in any
case we mean the color of the mineral itself in its purest state. The
non-essential colors, on the other hand, are chiefly the colors of the
impurities contained in the minerals.

Metallic minerals, which are always opaque, usually have essential
colors; but vitreous minerals, which are always more or less
transparent, often have non-essential colors. The explanation is this:
In opaque minerals we can only see the impurities immediately on the
surface, and these are, as a rule, not enough to affect its color; but
in diaphanous minerals we look _into_ the specimen and see impurities
below the surface, and thus bring into view, in many cases, sufficient
impurity so that its color drowns that of the mineral.

To prove this we have only to take any mineral (serpentine is a good
example in our series) having a non-essential color, and make it opaque
by pulverizing it or abrading its surface, when the non-essential color,
the color of the impurity, immediately disappears; just as water, yellow
with suspended clay, becomes white when whipped into foam, and thus made
opaque.

What we understand by the _streak_ of a mineral is its essential color,
the color of its powder; and it is so called because the powder is most
readily observed by scratching the surface of the mineral, and thereby
pulverizing a minute portion of it. The streak and hardness are thus
determined at the same time. The streak of soft minerals is easily
determined by rubbing them on any white surface of suitable hardness, as
paper, porcelain, or Arkansas stone.

ESSENTIAL AND ACCESSORY MINERALS.—Lithologists, regarding minerals as
constituents of rocks, divide them into two great classes: the
_essential_ and the _accessory_. The essential constituents of a rock
are those minerals which are essential to the definition of the rock.
For example, we cannot properly define granite without naming quartz and
orthoclase; hence these are essential constituents of granite; and if
either of these minerals were removed from granite it would not be
granite any longer, but some other rock. But other minerals, like
tourmaline and garnet, may be indifferently present or absent; it is
granite still; hence they are merely accidental or accessory
constituents. They determine the different _varieties_ of granite, while
the essential minerals make the _species_.

This classification, of course, is not absolute, for in many cases the
same mineral forms an essential constituent of one rock and an accessory
constituent of another. Thus, quartz is essential in granite, but
accessory in diorite.

PRINCIPAL MINERALS CONSTITUTING ROCKS.—Having studied in a general way
the more important characteristics of minerals, brief descriptions of
the chief rock-forming species are next in order. We will notice first
and principally those minerals occurring chiefly as _essential_
constituents of rocks.

1. Graphite.—Essentially pure carbon, though often mixed with a little
iron oxide. Crystallizes in hexagonal system, but usually foliated,
granular, or massive. Hardness, 1-2, being easily scratched with the
nail. Sp. gr., 2.1-2.3. Lustre, metallic; an exception to the rule that
acidic elements have non-metallic or vitreous lustres. Streak, black and
shining (see pencil-mark on white paper). Color, iron-black. Slippery or
greasy feel. Every _black-lead_ pencil is a specimen of graphite.
Specimen 9.

The different kinds of mineral coal are, geologically, as we have seen,
closely related to graphite, but they are such familiar substances that
they need not be described here.

2. Halite (common salt).—Chloride of sodium: chlorine, 60.7; sodium,
39.3; = 100. Isometric system, usually forming cubes. Hardness, 2.5, a
little harder than the nail. Sp. gr., 2.1-2.6. Lustre, vitreous. Streak
and color both white, and hence color is essential. Often transparent.
Soluble; taste, purely saline. In specific gravity and lustre it is a
good example of a mineral in which an acidic element predominates.
Specimen 11.

3. Limonite.—Hydrous sesquioxide of iron: oxygen, 25; iron, 60; water,
15; = 100. Usually amorphous; occurring in stalactitic and botryoidal
forms, having a fibrous structure; and also concretionary, massive, and
earthy (yellow ochre). Hardness, 5-5.5. Sp. gr., 3.6-4. Lustre, vitreous
or silky, inclining to metallic, and sometimes dull. Color, various
shades of black, brown, and yellow. Streak, ochre-yellow; hence color
partly non-essential. Specimen 12.

4. Hematite.—Sesquioxide of iron: oxygen, 30; iron, 70; = 100. Hexagonal
system, in distinct crystals, but usually lamellar, granular, or
compact,—columnar, botryoidal, and stalactitic forms being common.
Hardness, 5.5-6.5; good crystals are harder than steel. Sp. gr.,
4.5-5.3. Lustre, metallic, sometimes dull. Color, iron-black, but red
when earthy or pulverized (red ochre). Streak, red, and color,
therefore, mainly non-essential; sometimes attracted by the magnet.
Specimen 13.

Hematite has the same composition as limonite, minus the water; and by
comparing the hardness and specific gravity of these two minerals we see
that they are a good illustration of the principle that hydrous minerals
are softer and lighter than anhydrous minerals of analogous composition.
Limonite and hematite are two great natural coloring agents, and almost
all yellow, brown, and red colors in rocks and soils are due to their
presence.

5. Magnetite.—Protoxide and sesquioxide of iron: oxygen, 27.6; iron,
72.4; = 100. Isometric system, usually in octahedrons or dodecahedrons.
Most abundant variety is coarsely to finely granular, sometimes
dendritic. Hardness, 5.5-6.5, same as hematite. Sp. gr., 4.9-5.2.
Lustre, metallic. Color and streak, iron-black, and hence color
essential. Strongly magnetic; some specimens have distinct polarity, and
are called loadstones. Specimen 14.

The three iron-oxides just described—limonite, hematite, and
magnetite—are all important ores of iron, and form a well-marked natural
series. Thus limonite is never, hematite is usually, and magnetite is
always, crystalline. Again, limonite with 60 per cent. of iron is never
magnetic, hematite with 70 per cent. is sometimes magnetic, while
magnetite with 72.4 per cent. is always magnetic. As the iron increases
so does the magnetism. We have here an excellent illustration of the
principle that the properties of the elements can be traced in those
minerals in which they predominate. Iron is the only strongly magnetic
element: magnetite contains more iron than any other mineral, and it is
the only strongly magnetic mineral.

These three iron-ores are easily distinguished from each other by the
color of their powders or streak,—limonite yellow, hematite red, and
magnetite black,—and from all other common minerals by their high
specific gravity.

6. Quartz.—Oxide of silicon or silica: oxygen, 53.33; silicon, 46.67; =
100. Hexagonal system. The most common form is a hexagonal prism
terminated by a hexagonal pyramid. Also coarsely and finely granular to
perfectly compact, like flint; the compact or cryptocrystalline
varieties often assuming botryoidal, stalactitic, and concretionary
forms. It has no cleavage, but usually breaks with an irregular,
conchoidal fracture like glass. Hardness, 7, being No. 7 of the scale;
scratches glass easily. Sp. gr., 2.5-2.8. Lustre, vitreous. Pure quartz
is colorless or white, but by admixture of impurities it may be of
almost any color. Streak always white or light colored. Quartz is
usually, as in specimen 15, transparent and glassy, but may be
translucent or opaque. It is almost absolutely infusible and insoluble.

The varieties of quartz are very numerous, but they may be arranged in
two great groups:—

1. _Phenocrystalline_ or _vitreous_ varieties, including rock-crystal,
amethyst, rose quartz, yellow quartz, smoky quartz, milky quartz,
ferruginous quartz, etc.

2. _Cryptocrystalline_ or _compact_ varieties, including chalcedony,
carnelian, agate, onyx, jasper, flint, chert, etc. Only three varieties,
however, are of any great geological importance; these are: common
glassy quartz (spec. 15), flint (spec. 16), and chert.

Quartz is one of the most important constituents of the earth’s crust,
and it is also the hardest and most durable of all common minerals. We
have already observed (p. 12) that it is entirely unaltered by exposure
to the weather; _i.e._, it cannot be decomposed; and, being very hard,
the same mechanical wear which, assisted by more or less chemical
decomposition, reduces softer minerals to an impalpable powder or clay,
must leave the quartz chiefly in the form of sand and gravel. This
agrees with our observation that sand (spec. 30), especially, is usually
merely pulverized quartz.

_Opal_ is a mineral closely allied to quartz, and may be mentioned in
this connection. It is of similar composition, but contains from 5 to 20
per cent. of water, and is decidedly softer and lighter. Hardness,
5.5-6.5; sp. gr., 1.9-2.3.

7. Gypsum.—Hydrous sulphate of calcium: sulphur trioxide (SO₃), 46.5;
lime (CaO), 32.6; water (H₂O), 20.9; = 100. Monoclinic system. Often in
distinct rhombic crystals; also foliated, fibrous, and finely granular.
Hardness, 1.5-2; the hardest varieties being No. 2 of the scale of
hardness. Sp. gr., 2.3. Lustre, pearly, vitreous, or dull. Color and
streak usually white or gray. The principal varieties of gypsum are
(_a_) _selenite_, which includes all distinctly crystallized or
transparent gypsum; (_b_) _fibrous gypsum_ or _satin-spar_; (_c_)
_alabaster_, fine-grained, light-colored, and translucent. Gypsum is
easily distinguished from all common minerals resembling it by its
softness and the fact that it is not affected by acids. Specimen 17.

8. Calcite.—Carbonate of calcium: carbon dioxide (CO₂), 44; lime (CaO),
56; = 100. Hexagonal system, usually in rhombohedrons, scalenohedrons,
or hexagonal prisms. Cleavage rhombohedral and highly perfect (specimen
18). Also fibrous and compact to coarsely granular, in stalactitic,
concretionary, and other forms. Hardness, 2.5-3.5, usually 3 (see scale
of hardness). Sp. gr., 2.5-2.75. Lustre, vitreous. Color and streak
usually white. Transparent crystallized calcite is known as
_Iceland-spar_, and is remarkable for its strong double refraction. When
finely fibrous it makes a _satin-spar_ similar to gypsum. Geologically
speaking, calcite is a mineral of the first importance, being the sole
essential constituent of all limestones. It is readily distinguished
from allied species by its perfect rhombohedral cleavage; by its
softness, being easily scratched with a knife; and above all by its
lively effervescence with acids, for it is the _only common_ mineral
effervescing _freely_ with _cold dilute_ acid. To apply this test it is
only necessary to touch the specimen with a drop of dilute chlorohydric
acid. The effervescence, of course, is due to the escape of the carbon
dioxide in a gaseous form. Specimen 18.

9. Dolomite.—Carbonate of calcium and magnesium: carbonate of calcium
(CaCO₃), 54.35; carbonate of magnesium (MgCO₃), 45.65; = 100. Hexagonal
system, being nearly isomorphous with calcite. Rhombohedral cleavage
perfect. Hardness, 3.5-4; sp. gr., 2.8-2.9, being harder and heavier
than calcite. Lustre, color, and streak same as for calcite, from which
it is most easily distinguished by its non-effervescence or only feeble
effervescence with cold dilute acid, though effervescing freely with
strong or hot acid. Spec. 19.

10. Siderite.—Carbonate of iron: carbon dioxide (CO₂), 37.9; protoxide
of iron (FeO), 62.1; = 100. Crystallization and cleavage essentially the
same as for calcite and dolomite. Hardness, 3.5-4.5, and sp. gr.,
3.7-3.9. Lustre, vitreous. Color, white, gray, and brown. Streak, white.
With acid, siderite behaves like dolomite. It is distinguished from both
calcite and dolomite by its high specific gravity, which is easily
explained by the fact that it is largely composed of the heavy element,
iron.

With one exception, the fifteen minerals which we have yet to study
belong to the class of silicates, which includes more than one-fourth of
the known species of minerals, and, omitting quartz and calcite, all of
the really important rock-constituents. The silicate minerals may be
very conveniently divided into two great groups, the _basic_ and
_acidic_. This is not a sharp division; on the contrary, there is a
perfectly gradual passage from one group to the other; and yet this is,
for geological purposes at least, a very natural classification. The
dividing line falls in the neighborhood of 60 per cent. of silica;
_i.e._, all species containing this proportion of silica or _less_ are
classed as basic, since in them the basic elements predominate; while
those containing _more_ than 60 per cent. of silica are classed as
acidic, because their characteristics are determined chiefly by the acid
element or silica. The principal bases occurring in the silicates, named
in the order of their relative importance, are aluminum, magnesium,
calcium, iron, sodium, and potassium; and of these, magnesium, calcium,
iron, and usually sodium, are especially characteristic of basic
species.

Iron is the heaviest base; but all the bases, except sodium and
potassium, are heavier than the acid—silica; consequently basic minerals
must be, as a rule, heavier than acidic minerals. And since basic
minerals contain more iron than acidic, they must be darker colored. In
general, we say, _dark, heavy_ silicates are _basic_, and _vice versa_.
All this is of especial importance because in the rocks nature keeps
these two classes separate in a great degree.

11. Amphibole.—Silicate of aluminum, magnesium, calcium, iron, and
sodium. The bases occur in very various proportions, forming many
varieties; but the only variety of especial geological interest is
_hornblende_, the average percentage composition of which is as follows:
silica (SiO₂), 50; alumina (Al₂O₃), 10; magnesia (MgO), 18; lime (CaO),
12; iron oxide (FeO and Fe₂O₃), 8; and soda (Na₂O), 2; = 100. Monoclinic
system: usually in rhombic or six-sided prisms which may be short and
thick, but are more often acicular or bladed. Hardness, 5-6; sp. gr.,
2.9-3.4. Lustre, vitreous; color, black and greenish black; and streak
similar to color, but much paler. Compare with quartz, and observe the
strong contrast in color possible with minerals having the same lustre.
Specimen 20.

12. Pyroxene.—Like amphibole, this species embraces many varieties, and
these exhibit a wide range in composition; but of these _augite_ alone
is an important rock-constituent. Hence in lithology we practically
substitute for amphibole and pyroxene, hornblende, and augite
respectively.

Augite is very similar in composition to hornblende, but contains
usually more lime and less alumina and alkali. Physically, too, these
minerals are almost identical, crystallizing in the same system and in
very similar forms, and agreeing in hardness, color, lustre, and streak.
Augite is heavier than hornblende, sp. gr., 3.2-3.5. A certain prismatic
angle, which in augite is 87°5´, is 124°30´ in hornblende. Slender,
bladed crystals are more common with hornblende than augite. When
examined in thin sections with the polarizer, augite does not afford the
phenomenon of dichroism, which is strongly marked in hornblende.
However, as these minerals commonly occur in the rocks, in small and
imperfect crystals, these distinctions can only be observed in thin
sections under the microscope; so that, as regards the naked eye, they
are practically indistinguishable.

It might appear at first that the distinction of minerals so nearly
identical is not an important matter; but nature has decreed otherwise.
Augite and hornblende are typical examples of basic minerals; but augite
is, both in its composition and associations, the more basic of the two.
In proof of this we need only to know that it very rarely occurs in the
same rock with quartz, while hornblende is found very commonly in that
association. Quartz in a rock means an excess of acid or silica, and
almost necessarily implies the absence of highly basic minerals. In
other words, hornblende is often, and augite very rarely, found in
connection with acidic minerals; and it is this difference of
association chiefly that makes their distinction essential to the proper
recognition of rocks; while at the same time it affords an easy, though
of course not absolutely certain, means of determining whether the black
constituent of any particular rock is hornblende or augite.

_Mica Family._—Mica is not the name of a single mineral, but of a whole
family of minerals, including some half-dozen species. Only two,
however,—muscovite and biotite,—are sufficiently abundant to engage our
attention. These are complex, basic silicates of aluminum, magnesium,
iron, potassium, and sodium. The crystallization of biotite is
hexagonal, and of muscovite monoclinic; but both occur commonly in flat
six-sided forms. Undoubtedly the most important and striking
characteristic of the whole mica family is the remarkably perfect
cleavage parallel with the basal planes of the crystals, and the
wonderful _thinness_, and above all the _elasticity_, of the cleavage
lamellæ. The cleavage contrasts the micas with all other common
minerals, and makes their certain identification one of the easiest
things in lithology. The micas are soft minerals, the hardness ranging
from 2 to 3, and being usually easily scratched with the nail. Sp. gr.
varies from 2.7-3.1. Lustre, pearly; and streak, white or uncolored.

The distinguishing features of muscovite and biotite are as follows:—

13. Muscovite.—Contains 47 per cent. of silica, 3 per cent. of
sesquioxide of iron, and 10 per cent. of alkalies, chiefly potash; and
the characteristic colors are white, gray, and, more rarely, brown and
yellow. Non-dichroic. Usually found in association with acidic minerals.
The mica used in the arts is muscovite. Specimen 21.

14. Biotite.—Contains only 36 per cent. of silica, 20 per cent. of oxide
of iron, and 17 per cent. of magnesia; colors, deep black to green.
Strongly dichroic. Commonly occurs with other basic minerals. Compare
color with per cent. of iron.

These differences are tabulated below:—

            _Muscovite_ =              _Biotite_ =
                Acidic mica.               Basic mica.
                Non-ferruginous mica.      Ferruginous mica.
                Potash mica.               Magnesian mica.
                White mica.                Black mica.
                Non-dichroic mica.         Dichroic mica.

_Feldspar Family._—Like mica, _feldspar_ is the name of a family of
minerals; and these are, geologically, the most important of all
minerals. They are, above all others, the minerals of which rocks are
made, and their abundance is well expressed in the name,—feldspar being
simply the German for field-spar, implying that it is the common spar or
mineral of the fields.

Chemically, the feldspars are silicates of aluminum and potassium,
sodium or calcium. They crystallize in the monoclinic and triclinic
systems; and all possess easy cleavage in two directions at right angles
to each other, or nearly so. The general physical characters, including
the cleavage, are well exhibited in the common species, orthoclase
(specimen 22).

In hardness the feldspars range from 5 to 7, being usually near 6, and
almost always distinctly softer than quartz. Sp. gr. varies from
2.5-2.75; lustre, from vitreous to pearly; color, from white and gray to
red, brown, green, etc., but usually light. Streak, always white; rarely
transparent. By exposure to the weather, feldspars gradually lose their
alkalies and lime, become hydrated, and are changed to kaolin or common
clay. A similar change takes place with the micas, augite, and
hornblende; but these species, being usually rich in iron, make clays
which are much darker colored than those derived from feldspars. The
fact that the feldspars contain little or no iron undoubtedly explains
their low specific gravity and light colors, as compared with the other
minerals just named. The only common minerals for which the feldspars
are liable to be mistaken are quartz and the carbonates. From the latter
they are easily distinguished by their superior hardness and
non-effervescence with acids; and from the former, by possessing
distinct cleavage, by being rarely transparent, by being somewhat
softer, and by changing to clay on exposure to the weather.

The feldspars of greatest geological interest are five in number, and
may be classified chemically as follows:—

     Orthoclase,—silicate of aluminum and potassium, or
                                                potash feldspar.

     Albite,—silicate of aluminum and sodium, or
                                                soda feldspar.

     Anorthite,—silicate of aluminum and calcium, or
                                                lime feldspar.

     Oligoclase,—silicate of aluminum and sodium, and calcium, or
                                                soda-lime feldspar.

     Labradorite,—silicate of aluminum and calcium, and sodium, or
                                                lime-soda feldspar.

This appears like a complex arrangement, but it can be simplified.
Orthoclase crystallizes in the monoclinic system, and all the other
feldspars in the triclinic system. With the exception of albite, which
is a comparatively rare species, the triclinic feldspars all contain
less silica than orthoclase; _i.e._, are more basic. This is shown by
the subjoined table giving the average composition of each of the
feldspars:—

                 SiO₂     Al₂O₃      K₂O      Na₂O      CaO   Total.
   Orthoclase,    65        18        17       --        --  =  100
   Albite,        68        20        --       12        --  =  100
   Oligoclase,    62        24        --        9         5  =  100
   Labradorite,   53        30        --        4        13  =  100
   Anorthite,     43        37        --       --        20  =  100

As we should naturally expect, the triclinic feldspars occur usually
with other basic minerals, while the monoclinic species, orthoclase, is
acidic in its associations; furthermore, the triclinic feldspars are
often intimately associated with each other, but are rarely important
constituents of rocks containing much orthoclase. In other words, the
distinction of orthoclase from the basic or triclinic feldspars is
important and comparatively easy, while the distinction of the different
basic feldspars from each other is both unimportant and difficult.
Hence, in lithology, we find it best to put all these basic feldspars
together, as if they were one species, under the name _plagioclase_,
which refers to the oblique cleavage of all these feldspars, and
contrasts with _orthoclase_, which refers to the right-angled cleavage
of that species.

This statement of the relations of the feldspars is, of course, beyond
the comprehension of many children, and yet it should be understood by
the teacher who would lead the children to any but the most superficial
views.

15. Orthoclase.—This is the common feldspar, and the most abundant of
all minerals, being the principal constituent of granite, gneiss, and
many other important rocks. The most characteristic colors are white,
gray, pinkish, and flesh-red. Specimen 22.

16. Plagioclase.—Like orthoclase, these species may be of almost any
color; yet these two great divisions of the feldspars are usually
contrasted in this respect. Thus, bluish and grayish colors are most
common with plagioclase, and white or reddish colors with orthoclase.
Specimen 23 is labradorite, and, in every respect, a typical example of
plagioclase. On certain faces and cleavage-surfaces of the plagioclase
crystals we may often observe a series of straight parallel lines or
bands which are often very fine,—fifty to a hundred in a single crystal.
These striæ are due to the mode of twinning, and are of especial
importance, since, while they are very characteristic of plagioclase,
they never occur in orthoclase. As stated, these twinning striæ in
plagioclase are often visible to the naked eye; and when they are not,
they may usually be revealed by examining a thin section under the
microscope with polarized light. Plagioclase decays much more rapidly
when exposed to the weather than orthoclase. This point becomes
perfectly clear when we compare weathered ledges of diabase (or any
trap-rock, see specimen 2) and granite; for plagioclase is the principal
constituent of the former rock, and orthoclase of the latter.

_Hydrous Silicates._—Many silicates contain water, and some of these are
of great geological importance. What has been stated on a preceding page
concerning the softness and lightness of hydrated minerals is especially
applicable here; for all the geologically important hydrous silicates
are distinctly softer and lighter than anhydrous minerals of otherwise
similar composition. Furthermore, they usually have an unctuous or
slippery feel; and, with one exception (kaolin), are of a green or
greenish color.

17. Kaolinite (Kaolin).—Hydrous silicate of aluminum: silica (SiO₂), 46;
alumina (Al₂O₃), 40; and water (H₂O), 14; = 100. Orthorhombic system, in
rhombic or hexagonal scales or plates, but usually earthy or clay-like.
Hardness, 1-2.5; sp. gr., 2.4-2.65. The pure mineral is white; but it is
usually colored by impurities, the principal of which are iron oxides
and carbonaceous matter. Kaolin is the most abundant of all the hydrous
silicates, and it is the basis and often the sole constituent of common
clay,—a very common mineral, but rarely pure. We have already (p. 11)
noticed the mode of origin of kaolin or clay. It results from the
decomposition of various aluminous silicate minerals, especially the
feldspars. Under the combined influence of carbon dioxide and moisture,
feldspars give up their potassium, sodium, and calcium, and take on
water, and the result is kaolin. This mineral is believed to be always a
decomposition product. Perhaps the best, or at least the most
convenient, test for kaolin is the argillaceous odor, the odor of
moistened clay. Specimen 24.

18. Talc.—Hydrous silicate of magnesium: silica (SiO₂), 63 (acidic);
magnesia (MgO), 32; water (H₂O), 5; = 100. Orthorhombic system, but
rarely in distinct crystals. Cleavage in one direction very perfect; the
cleavage lamellæ are flexible, but not elastic, as in mica. Hardness, 1;
see scale. Sp. gr., 2.55-2.8. Lustre, pearly. Color, apple-green to
white; and streak, white. The feel is very smooth and greasy; and, in
connection with the color and foliation, affords the best means of
distinguishing talc from allied minerals. Talc sometimes results from
the alteration of augite, hornblende, and other minerals, but it is not
always nor usually an alteration product.

19. Serpentine.—Hydrous silicate of magnesium: silica (SiO₂), 44
(basic); magnesia (MgO), 44; water (H₂O), 12; = 100. Essentially
amorphous. Hardness, 2.5-4; sp. gr., 2.5-2.65. Lustre, greasy, waxy, or
earthy. Color, various shades of green and usually darker than talc, but
streak always white. Feel, smooth, sometimes greasy. Distinguished from
talc by its hardness, compactness, and darker green. Sometimes results
from the alteration of olivine and other magnesian minerals, but usually
we are to regard it as an original mineral. Specimen 25.

20. Chlorite.—This is, properly, the name of a group of highly basic
minerals of very variable composition, but they are all essentially
hydrous silicates of aluminum, magnesium, and iron; and the average
composition of the most abundant species, prochlorite, is as follows:
silica (SiO₂), 30; alumina (Al₂O₃), 18; magnesia (MgO), 15; protoxide of
iron (FeO), 26; and water (H₂O), 11; = 100. The chlorites crystallize in
several different systems, but in all there is a highly perfect cleavage
in one direction, giving, as in talc, a foliated structure with flexible
but inelastic laminæ. The cleavage scales, however, are sometimes
minute, and the structure massive or granular. Hardness of prochlorite,
1-2; between talc and serpentine. Sp. gr., 2.78-2.96. All the chlorites
have a pearly to vitreous lustre. Color usually some shade of green; in
prochlorite a dark or blackish green, darker than serpentine, as that is
darker than talc. Streak, a lighter, whitish green. Less unctuous than
talc, but more so than serpentine. The chlorites are produced very
commonly, but not generally, by the alteration of basic anhydrous
silicates, like augite and hornblende. Specimen 26.

21. Hydro-mica.—This, too, is properly the name of a group of minerals;
but for geological purposes they may be regarded as one species. Taking
a general view of the composition, these are simply the anhydrous or
ordinary micas, which we have already studied, with from 5 to 10 per
cent. of water added. In crystallization and structure they are
essentially mica-like. Although not distinctly softer than the common
micas, they are lighter, always more unctuous and slippery, and usually
of a greenish color. The micaceous structure with _elastic_ laminæ
serves to distinguish the hydro-micas from other hydrous silicates.

22. Glauconite.—Hydrous silicate of aluminum, iron, and potassium:
silica (SiO₂), 50; alumina (Al₂O₃), protoxide of iron (FeO), and potash
(K₂O), together, 41; and water (H₂O), 9; = 100. Amorphous, forming
rounded and generally loose grains, which often have a microscopic
organic nucleus. It is dull and earthy, like chalk, and always soft,
green, and light, but not particularly unctuous. Glauconite is the
principal, often the sole, constituent of the rock greensand, which
occurs abundantly in the newer geological formations, and is now forming
in the deep water of the Gulf of Mexico and along our Atlantic
sea-board. Specimen 27.

This completes our list of minerals occurring chiefly as _essential_
constituents of rocks; and following are three of the more common and
important minerals occurring chiefly as _accessory_, rarely as
essential, rock-constituents.

23. Chrysolite (Olivine).—Silicate of magnesium and iron: silica (SiO₂),
41; magnesia (MgO), 51; protoxide of iron (Fe₂O₃), 8; = 100.
Orthorhombic system; but usually in irregular glassy grains. Hardness,
6-7. Sp. gr., 3.3-3.5. Lustre, vitreous; color, usually some shade of
green; and streak, white. Chrysolite sometimes closely resembles quartz,
but its green color usually suffices to distinguish it. It is a common
constituent of basalt and allied rocks. By absorption of water it is
changed into serpentine and talc. See examples in specimen.

24. Garnet.—The composition of this mineral is extremely variable; but
the most important variety is a basic silicate of aluminum and iron:
silica (SiO₂), 37; alumina (Al₂O₃), 20; and protoxide of iron (FeO), 43;
= 100. Isometric system, usually in distinct crystals, twelve-sided
(dodecahedrons) and twenty-four-sided (trapezohedrons) forms being most
common. Hardness, 6.5-7.5; average as hard as quartz. Sp. gr., 3.15-4.3;
compare with the high percentage of iron. Lustre, vitreous; colors,
various, usually some shade of red or brown; and streak, white. Some
varieties contain iron enough to make them magnetic. Garnet is easily
distinguished by its form, color, and hardness from all other minerals.
It is a common but not an abundant mineral, occurring most frequently in
gneiss, mica schist, and other stratified crystalline rocks. See
examples in specimen.

25. Pyrite.—Sulphide of iron: sulphur, 53.3; iron, 46.7; = 100.
Isometric system, occurring usually in distinct crystals, the cube and
the twelve-sided form known as the pyritohedron being the most common.
Hardness, 6-6.5, striking fire with steel. Sp. gr., 4.8-5.2; heavy
because rich in iron. Lustre, metallic and splendent. Color, pale,
brass-yellow, and streak, greenish or brownish. Pyrite is sometimes
mistaken for gold, but it is not malleable; while its color, hardness,
and specific gravity, combined, easily distinguish it from all common
minerals. As an accessory rock-constituent, pyrite occurs usually in
isolated cubes or pyritohedrons. Specimen 10.


                          _Textures of Rocks._

_Texture_ is a general name for those smaller structural features of
rocks which can be studied in _hand specimens_, and which depend upon
the _forms_ and _sizes_ of the _constituent particles_ of the rocks, and
the _ways_ in which these are _united_.

By “constituent particles” we mean, not the atoms or molecules of matter
composing the rocks, but the _pebbles_ in conglomerate, _grains of sand_
in sandstone, _crystals of quartz_, _feldspar_, and _mica_ in granite,
etc. The four most important textures are:—

(1) _Fragmental texture._—The rock is composed of mere irregular,
angular, or rounded, but visible, fragments. Examples: sand, sandstone,
gravel, conglomerate, etc. Specimens 30, 31, 28, 29.

(2) _Crystalline texture._—The constituent particles are chiefly, at
least, distinctly crystalline, as shown either by external form, or
cleavage, or both. Examples: granite, diabase, gneiss, etc. Specimens
45, 1, 41.

(3) _Compact texture._—The constituent particles are indistinguishable
by the naked eye, but become visible under the microscope, appearing as
separate crystalline grains or as irregular fragments. In other words,
if, in the case of either the granular or crystalline textures, we
conceive the particles to become microscopically small, then we have the
compact texture. Examples: clay, slate, many limestones, basalt, etc.
Specimens 34, 35, 39.

(4) _Vitreous texture._—The texture of glass, in which the constituent
particles are absolutely invisible even with the highest powers of the
microscope, and may be nothing more than the _molecules_ of the
substance, which thus, so far as our powers of observation are
concerned, presents a perfectly continuous surface. Examples: obsidian,
glassy quartz, and some kinds of coal. Specimens 47, 15.

These four textures, which, it will be observed, are determined by the
forms and sizes of the constituent particles, may be called the
_primary_ textures, because every rock _must_ possess one of them. We
cannot conceive of a rock which is neither fragmental, crystalline,
compact, nor vitreous. But in addition to one of the primary textures, a
rock may or may not have one or more of what may be called _secondary_
textures. These are determined by the way in which the particles are
united, the mode or pattern of the arrangement, etc. Following are
definitions of the principal secondary textures:—

(1) _Laminated texture._—This exists where the particles are arranged in
thin, parallel layers, which may be marked simply by planes of division,
or the alternate layers may be composed of particles differing in
composition, form, size, or color, etc. Among the laminated textures we
thus distinguish: (_a_) the _banded_ texture, where the layers are
contrasted in color, texture, or composition, but cohere, so that there
is no cleavage or easy splitting parallel with the stratification; and
(_b_) the _schistose_ or _shaly_ texture, where such fissility or
stratification-cleavage exists. If a fragmental, compact, or vitreous
rock is fissile, we use the term _shaly_; but a fissile, crystalline
rock is described as _schistose_. The banded texture may occur with the
fragmental,—banded sandstones, etc.; with the crystalline,—many
gneisses, etc. (specimen 41); with the compact,—many slates, limestones,
felsites, etc. (specimens 34, 42); with the vitreous,—banded obsidian,
furnace slags, and some coal. The schistose texture may occur with the
crystalline,—mica schist, etc. (specimen 43); and the shaly texture with
the compact and fragmental, but rarely with the vitreous.

(2) _Porphyritic texture._—We have this texture when _separate_ and
_distinct crystals_ of _any_ mineral, but most commonly of feldspar, are
enclosed in a _relatively_ fine-grained base or matrix, which may be
either crystalline, compact, or vitreous, but rarely fragmental.
Specimens 5, 6, 7 are examples of the porphyritic compact texture.

(3) _Concretionary texture._—When one or more constituents of a rock
have the form, in whole or in part, not of distinct angular crystals,
but of rounded concretions, the texture is described as concretionary,
the concretions taking the place in this texture of the isolated
crystals in the porphyritic texture. This texture occurs in connection
with all the primary textures, but the most familiar example is oölitic
limestone.

(4) _Vesicular texture._—A rock has this texture when it contains
numerous small cavities or vesicles. These are most commonly produced by
the expansion of steam and other vapors when the rock is in a plastic
state; and hence the vesicular texture is found chiefly in volcanic
rocks. Except rarely, it is associated only with the compact
texture,—ordinary stony lavas (specimen 49); and with the vitreous
texture,—pumice (specimen 48).

(5) _Amygdaloidal texture._—In the course of time the vesicles of common
lava are often filled with various minerals deposited by infiltrating
waters, giving rise to the amygdaloidal texture, from the Latin
_amygdalum_, an almond, in allusion to a common form of the vesicles, or
amygdules, as they are called, after being filled. The amygdaloidal
texture is thus necessarily preceded by the vesicular, and is limited to
the same classes of rocks. Specimen 50.

Besides the foregoing, there are many minor secondary textures. The
rocks known as tufas have what may be called the _tufaceous_ texture.
Then we have kinds of texture depending on the _strength_ of the union
of the particles, as _strong_, _weak_, _friable_, _earthy_, etc.


                       _Classification of Rocks._

Having finished our preliminary observations on the characteristics of
rocks, we are now about ready to begin a systematic study of the rocks
themselves; but it is needful first to say a few words about the
classification of rocks, since upon this depends not only the order in
which we shall take the rocks up, but also the ideas that will be
imparted concerning their relations and affinities. The classifications
which have been proposed at different times are almost as numerous as
the rocks themselves. Some of these are confessedly, and even
designedly, artificial, as when we classify stones according to their
uses in the arts, etc. But we want something more scientific, a
_natural_ classification; that is, one based upon the natural and
permanent characteristics of rocks. Rocks have been classified according
to chemical composition, mineralogical composition, texture, color,
density, hardness, etc.; but these arrangements, taken singly or all
combined, are inadequate.

A _natural_ classification may be defined as a concise and systematic
statement of the natural relations existing among the objects
classified. Now the most important relations existing among rocks are
those due to their different origins. We must not forget that lithology
is a branch of geology, and that geology is first of all a _dynamical_
science. The most important question that can be asked about any rock
is, not What is it made of? but _How_ was it made? What were the general
forces or agencies concerned in its formation? Rocks are the material in
which the earth’s history is written, and what we want to know first
concerning any rock is what it can tell us of the condition of that part
of the earth at the time it was made and subsequently.


                       _Classification of Rocks._

 +--------------------------------------------------------------------------+
 |                      Sedimentary or Stratified Rocks.                    |
 +--------------------------------------------------------------------------+
 |                         MECHANICALLY FORMED.                             |
 +-----------------------+-------------------------+------------------------+
 |                       |   _Unconsolidated._     |   _Consolidated._      |
 +-----------------------+-------------------------+------------------------+
 |     _Conglomerate     |        Gravel.          |    Conglomerate.       |
 |        group._        |                         |                        |
 +-----------------------+-------------------------+------------------------+
 |      _Arenaceous      |         Sand.           |      Sandstone.        |
 |        group._        |                         |                        |
 +-----------------------+-------------------------+------------------------+
 |     _Argillaceous     |         Clay.           |        Slate.          |
 |        group._        |                         |                        |
 +-----------------------+-------------------------+------------------------+
 |                  CHEMICALLY AND ORGANICALLY FORMED.                      |
 +-----------------------+-------------------------+------------------------+
 |   _Coal   | _Iron-ore |_Calcareous |  _Metamorphic group (Silicates)._   |
 |  group._  | group._   |  group._   |   Acidic.              Basic.       |
 |           |           |            |/----/\----\      /-------/\-------\ |
 |           |           |            |85   80    70    60    50    40    30|
 +-----------+-----------+------------+-------------------------------------+
 |                                                   Feldspathic.           |
 +-----------+-----------+------------+-------------------------------------+
 |   Peat.   | Limonite. | Limestone. |    Gneiss.     Diorite.     :       |
 | Lignite.  | Hematite. | Dolomite.  |...........................  :       |
 |Bit. Coal. |Magnetite. |  Gypsum.   |     :     :     :     :     :       |
 |Anthracite.| Siderite. | Rock-salt. |     :     :Syenite. Norite. :       |
 | Graphite. |           |  Phosphate |     :     :  .............. :       |
 |Asphaltum. |           |   Rock.    |     :     :     :     :     :       |
 +-----------+-----------+------------+-------------------------------------+
 |                                                  Non-Feldspathic.        |
 +-----------+-----------+------------+-------------------------------------+
 |           |_Siliceous |            |     :   Mica Schist.  :     :       |
 |           | group._   |            |...........................  :       |
 |           +-----------+            |     :     :     :     :     :       |
 |           |Tripolite. |            |     :Hornbl. Schist.  Amphibolite.  |
 |           |  Flint.   |            |     :.............. ........:       |
 |           | Siliceous |            |     :     :     :     :     :       |
 |           |   Tufa.   |            |     :  Talc Schist.   :Chl. Schist. |
 |           |Novaculite.|            |     :     :........   :   ..........|
 |           |           |            |     :     :     :     :     :       |
 |           |           |            |     :     :  Greensand. Serpentine. |
 |           |           |            |     :     :  ........... ........   |
 |           |           |            |     :     :     :     :     :       |
 +-----------+-----------+------------+-------------------------------------+
 |                      Eruptive or Unstratified Rocks.                     |
 +------------------------------------+-------------------------------------+
 |                                    |              PLUTONIC.              |
 |                                    |             Feldspathic.            |
 |                                    +-------------------------------------+
 |                                    |     : Granite.  : Diorite.  :       |
 |                                    |.................. ........  :       |
 |                                    |     :     :     :     :     :       |
 |                                    |     :     :  Syenite. Diabase.      |
 |                                    |     :     :  ....... .......:       |
 |                                    |     :     :     :     :     :       |
 |                                    +-------------------------------------+
 |                                    |              VOLCANIC.              |
 |This part of the classification     |             Feldspathic.            |
 |is a blank, for the reason that     +-------------------------------------+
 |no eruptive rocks are known         |     :Rhyolite.  : Andesite. :       |
 |which are chiefly composed of       |...........................  :       |
 |minerals belonging to the classes   |     :     :     :     :     :       |
 |of Native Elements, Chlorides,      |     :     :  Trachyte. Basalt.      |
 |Oxides, Sulphates, or Carbonates;   |              ...............:       |
 |_i.e._, all eruptive rocks, so      |     :     :     :     :     :       |
 |far as known, are principally       |     :Obsidian.  : Tachylite.:       |
 |composed of minerals belonging      |............................ :       |
 |to the class of Silicates.          |     :     :     :     :     :       |
 |                                    |     Petrosilex. :Porphyrite.:       |
 |                                    |............................ :       |
 |                                    |     :     :     :     :     :       |
 |                                    |     :     : Felsite. Melaphyr.      |
 |                                    |     :     :  ....... .......:       |
 |                                    |     :     :     :     :     :       |
 +------------------------------------+-------------------------------------+

The geological agencies, as we have already learned, may be arranged in
two great classes: first, the aqueous or superficial agencies
originating in the solar heat, and producing the sedimentary or
stratified rocks; and, second, the igneous or subterranean agencies
originating in the central or interior heat, and producing the eruptive
or unstratified rocks. Hence, we want to know first of any rock whether
it is of aqueous or igneous origin. Then, if it is a sedimentary rock,
whether it has been formed by the action chiefly of mechanical forces,
or of chemical and organic forces. And, if it is an eruptive rock,
whether it has cooled and solidified below the earth’s surface in a
fissure, and is a dike or trappean rock, or has flowed out on the
surface and cooled in contact with the air, and thus become an ordinary
lava or volcanic rock.

Here we have the outlines of our classification, and it will be observed
that we have simply reached the conclusion, in a somewhat roundabout
manner, that there should always be a general correspondence between the
classification of rocks and the classification of the forces that
produce them. The general plan of the preceding scheme of the
classification must now be clear, and the details will be explained as
we go along.


                        _Descriptions of Rocks._


                  1.—Sedimentary or Stratified Rocks.

1. MECHANICALLY FORMED OR FRAGMENTAL ROCKS.—These consist of materials
deposited from _suspension_ in water, and the process of their formation
is throughout chiefly mechanical. The materials deposited are mere
fragments of older rocks; and, if the fragments are large, we call the
newly deposited sediment gravel; if finer, sand; and, if impalpably
fine, clay. These fragmental rocks cannot be classified chemically,
since the same handful of gravel, for instance, may contain pebbles of
many different kinds of rocks, and thus be of almost any and very
variable composition. Such chemical distinctions as can be established
are only partial, and the classification, like the origin, must be
mechanical. Accordingly, as just shown, we recognize three principal
groups based upon the size of the fragments; viz.:—

                        (1) Conglomerate group.
                        (2) Arenaceous group.
                        (3) Argillaceous group.

This mode of division is possible and natural, simply because, as we
observed in an early experiment, materials arranged by the mechanical
action of water are always assorted according to size. When first
deposited, the gravel, sand, and clay are, of course, perfectly loose
and unconsolidated; but in the course of time they may, under the
influence of pressure, heat, and chemical action, attain almost any
degree of consolidation, becoming _conglomerate_, _sandstone_, and
_slate_, respectively. The pressure may be vertical where it is due to
the weight of newer deposits, or horizontal where it results from the
cooling and shrinking of the earth’s interior. The heat may result from
mechanical movements, or contact with eruptive rocks; or it may be due
simply to the burial of the sediments, which, it will be seen, must
virtually bring them nearer the great source of heat in the earth’s
interior, on the same principle that the temperature of a man’s coat, on
a cold day, is raised by putting on an overcoat. The effect of the heat,
ordinarily, is simply drying, coöperating with the pressure to expel the
water from the sediments; but, if the temperature is high, it may bake
or vitrify them, just as in brick-making. Sediments are consolidated by
chemical action when mineral substances, especially calcium carbonate,
the iron oxides, and silica are deposited between the particles by
infiltrating waters, cementing the particles together. This principle is
easily demonstrated experimentally by taking some loose sand and wetting
it repeatedly with a saturated solution of some soluble mineral, like
salt or alum, allowing the water to evaporate each time before making a
fresh application. The interstices between the grains are gradually
filled up, and the sand soon becomes a firm rock. But the student should
clearly understand that, in geology, gravel, sand, and clay are just as
truly _rocks_ before their consolidation as after. It is plain then that
in each of the principal groups of fragmental rocks we must recognize an
unconsolidated division and a consolidated division.

(1) _Conglomerate group._—The rocks belonging in this group we know
before consolidation as _gravel_, and after consolidation as
_conglomerate_.

Gravel.—The pebbles, as we have already seen, are usually, though not
always, well rounded or water-worn; and they may be of any size from
coarse grains of sand to boulders. As a rule, however, the larger
pebbles, especially, are of approximately uniform size in the same bed
or layer of gravel, with, of course, sufficient fine material to fill
the interstices. Although the same limited mass of gravel may show the
widest possible range in chemical and mineralogical composition, yet
hard rocks are evidently more likely than soft rocks to form pebbles;
and hence quartz and quartz-bearing rocks usually predominate in
gravels. Specimen 28.

Conglomerate.—Consolidated gravel. Children should be led to an
appreciation of this point by a careful comparison of the forms of the
pebbles in the gravel and conglomerate. The conglomerate seems to
contain a larger proportion of fine material than ordinary gravel. But
this is because the gravel is usually, as with our specimen, taken from
the _surface_ of the beach, where, of course, the pebbles are clean and
separate; but if it had remained there to be covered by a subsequently
deposited layer, enough fine stuff would have been sifted into the holes
to fill them. And in the finished gravel, just as in the conglomerate,
the pebbles are usually closely packed, with just sufficient sand and
clay, or _paste_, as the material in which the pebbles are imbedded is
called, to fill the interstices. The paste is usually similar in
composition to the pebbles, with this difference: hard materials
predominate in the pebbles and soft in the paste.

Stratified rocks generally show the stratification in parallel lines or
bands differing in color, composition, etc.; but nothing like this can
be detected in our specimens of conglomerate; and the question might be
asked, How do we know that this is a stratified rock? In answer, it can
be said that our hand-specimens appear unstratified simply because the
rock is so coarse; but when we look at large masses, and especially when
we see it in place in the quarry, that parallel arrangement of the
material which we call stratification is usually very evident; and we
often see precisely the same thing in gravel banks. It is, however,
wholly unnecessary that we should _see_ the stratification in order to
know certainly that this is a stratified or aqueous rock, because the
forms of the pebbles show very plainly that they have been fashioned and
deposited by moving water; and we have in the smallest specimen proof
positive that our conglomerate is a consolidated sea-beach.

Conglomerate shows the same variations in composition and texture as
gravel; it may be composed of almost any kind of material in pebbles of
almost any size. We recognize two principal varieties of conglomerate
based on the forms of the pebbles; if, as is usual, these are well
rounded and water-worn, the rock is true _pudding-stone_ (specimen 29);
but, if they are angular, or show but little wear, it is called
_breccia_.

(2) _Arenaceous Group._—The conglomerate group passes insensibly into
the arenaceous group; for, from the coarsest gravel to the finest sand,
the gradation is unbroken, and every sandstone is merely a conglomerate
on a small scale.

Sand.—Like gravel, sand may be of almost any composition, but as a rule
it is quartzose; quartz, on account of its hardness and the absence of
cleavage, being better adapted than any other common mineral to form
sand. Where the composition of a sand is not specified, a quartzose sand
is always understood. By examining a typical sand with a lens, and
noting the glassy appearance of the grains, and then testing their
hardness on a piece of glass, which they will scratch as easily as
quartz, the pupil is readily convinced that each grain is simply an
angular fragment of quartz. Specimen 30.

Sandstone.—Consolidated sand. In proving this, children will notice
first the granular or sandy appearance of the sandstone; and then, with
the lens, that the grains in the sandstone have the same forms as the
sand-grains. The stratification cannot be seen very distinctly in our
hand-specimens, but in larger masses it is usually very plain, as may be
observed in the blocks used for building, and still better in the
quarries. However, even if the stratification were not visible to the
eye, we could have no doubt that sandstone is a mechanically formed
stratified rock; because the form of the grains, just as in the
conglomerate, tells us that. Many sandstones, too, contain the fossil
remains of plants and animals, and these are always regarded as
affording positive proof that the rocks containing them belong to the
aqueous or stratified series.

There are many varieties of sandstone depending upon differences in
composition, texture, etc., but we have not space to notice them in
detail. In sandstone, just as in sand, quartz is the predominant
constituent, although we sometimes find varieties composed largely or
entirely of feldspar, mica, calcite, or other minerals. Specimen 31 is
an example of the architectural variety known as freestone, which is
merely a fine-grained, light-colored, uniform sandstone, not very hard,
and breaking with about equal freedom in all directions. The
consolidation of sandstones is due chiefly to chemical action. The
cementing materials are commonly either: _ferruginous_ (iron oxides),
giving red or brown sandstones; _calcareous_, forming soft sandstones,
which effervesce with acid if the cement is abundant; or _siliceous_,
making very strong, light-colored sandstones. Ferruginous sandstones are
the most valuable for architectural purposes; for, while not excessively
hard, they have a very durable cement. Siliceous sandstones are too
hard; and the calcareous varieties crumble when exposed to the weather
because the cement is soluble in water containing carbon dioxide, as all
rain-water does. Specimen 32 is a good example of a ferruginous
sandstone, and it is coarse enough so that we can see that each grain of
quartz is coated with the red oxide of iron. The mica scales visible
here and there in this specimen are interesting as showing that the
grains are not necessarily all quartz; and it is important to observe
that the mica was not made in the sandstone, but, like the quartz, has
come from some older rock.

Quartzite.—This rock is simply an unusually hard sandstone. Now the
hardness of any rock depends upon two things: (1) the hardness of the
individual grains or particles; and (2) the firmness with which they are
united one to another. Therefore, the hardest sandstones must be those
in which grains of quartz are combined with an abundant siliceous
cement; and that is precisely what we have in a typical quartzite, such
as specimen 33. Quartzite is distinguished, in the hand-specimen, from
ordinary quartz by its granular texture (compare specimens 15 and 33);
and of course in large masses the stratification is an important
distinguishing feature.

3. _Argillaceous group._—Just as the conglomerate group shades off
gradually into the arenaceous group, so we find it difficult to draw any
sharp line of division between the arenaceous group and the
argillaceous, but we pass from the largest pebble to the most minute
clay-particle by an insensible gradation. For the sake of convenience,
however, we draw the line at the limit of visibility, and say that in
the true clay and slate the individual particles are invisible to the
naked eye; in other words, these rocks have a perfectly compact texture,
while the two preceding groups are characterized by a granular texture.
Although clay, like sand and gravel, may be of almost any composition,
yet it usually consists chiefly, often entirely, of the mineral kaolin.
The reason for this is easily found. Quartz resists both mechanical and
chemical forces, and is rarely reduced to an impalpable fineness; but
all the other common minerals, such as feldspar, hornblende, mica, and
calcite, on account of their cleavage and inferior hardness, are easily
pulverized; but it is practically impossible that this should happen
without their being broken up chemically at the same time. Decomposition
follows disintegration; and, while calcite is completely dissolved and
carried away, the other minerals are reduced, as we have seen, to
impalpable hydrous silicates of aluminum, _i.e._, to kaolin. Hence, we
find that the fragmental rocks are composed principally of two minerals,
quartz and kaolin,—the former predominating in the conglomerate and
arenaceous groups, and the latter in the argillaceous group.

Clay.—That kaolin is the basis of common clay is proved by the
argillaceous odor, which is so characteristic of moist clay. Pure kaolin
clay is white and impalpable, like China clay; but pure clays are the
exception. They often become coarse and gritty by admixture with sand,
forming _loam_; and they also usually contain more or less carbonaceous
matter, which makes black clays; or more or less _ferrous_ oxide, which
makes blue clays; or more or less _ferric_ oxide, which makes red,
brown, and yellow clays. By mixing these coloring materials in various
proportions, almost any tint may be explained. Clays are sometimes
calcareous, from the presence of shells and shell-fragments or of
pulverized limestone. These usually effervesce with acid, and are
commonly known as _marl_. It is the calcareous material in a pulverulent
and easily soluble condition that makes the marls valuable as soils.

Slate.—Consolidated clay. The compact texture and argillaceous odor are
usually sufficient to prove this. To get the odor we need simply to
breathe upon the specimen, and then smell of it. We find all degrees of
induration in clay. It sometimes, as every one knows, becomes very hard
by simple drying; but this is not slate, and no amount of mere drying
will change clay into slate; for, when moistened with water, the dried
clay is easily brought back to the plastic state. To make a good slate,
the induration must be the result of pressure, aided probably to some
extent by heat. True slate, then, is a permanently indurated clay which
will not soak up and become soft when wet.

Slate is usually easily scratched with a knife, and it is distinguished
from limestone by its non-effervescence with acid. As we should expect,
it shows precisely the same varieties in color and composition as clay.
A good assortment of colors is afforded by the roofing-slates. Specimen
34 is a typical slate, for it not only has a compact texture and
argillaceous odor, but it is very distinctly stratified. The
stratification is marked by alternating bands of slightly different
colors, and is much finer and more regular than we usually observe in
sandstone, and of course entirely unlike the stratification of
conglomerate. These differences are characteristic. Some slates,
however, are so homogeneous that the stratification is scarcely visible
in small pieces. Thus the roofing-slates (specimen 35) rarely show the
stratification; for it is an important fact that the thin layers into
which this variety splits are entirely independent of the
stratification. This is the structure known as slaty cleavage; it is not
due to the stratification, but is developed in the slate subsequently to
its deposition, by pressure. Some roofing-slates, known as
ribbon-slates, show bands of color across the flat surfaces. These bands
are the true bedding, and indicate the absolute want of conformity
between this structure and the cleavage. Few rocks are richer in fossils
than slate, and these prove that it is a stratified rock. Slate which
splits easily into thin layers _parallel with the bedding_ is known as
_shale_.

Porcelainite.—This is clay or slate which has been baked or partially
vitrified by heat so as to have the hardness and texture of porcelain.

2. CHEMICALLY AND ORGANICALLY FORMED ROCKS.—We have already learned that
from a geological point of view the differences between chemical and
organic deposition are not great, the process being essentially chemical
in each case; and since the limestones and some other important rocks
are deposited in both ways, it is evidently not only unnatural, but
frequently impossible, to separate the chemically from the organically
formed rocks. Unlike the fragmental rocks, the rocks of this division
not only admit, but require, a chemical classification. This is natural
because they are of chemical origin; and it is practicable because, with
few exceptions, only one class of minerals is deposited at the same time
in the same place,—a very convenient and important fact. Therefore our
arrangement will be mineralogical, thus:—

                   (1) Coal Group.
                   (2) Iron-ore Group.
                   (3) Siliceous Group.
                   (4) Calcareous Group.
                   (5) Metamorphic Group (Silicates).

Most of the silicate rocks are mixed, _i.e._, are each composed of
several minerals; but some silicate rocks and all the rocks of the other
divisions are simple, each species consisting of a single mineral only.

(1) _Coal Group._—These are entirely of organic origin, and include two
allied series, which are always merely the more or less extensively
transformed tissues of plants or animals; viz.:—

Coals and Bitumens.—At the first lesson we examined a sample of peat
(specimen 8), and considered the general conditions of its formation,
peat being in every instance simply partially decayed marsh
vegetation. It was also stated that, as during the lapse of time the
transformation becomes more complete, the peat is changed in
succession to _lignite_, _bituminous coal_, _anthracite_, and
_graphite_. The coals, indeed, make a very beautiful and perfect
series, whether we consider the composition—there being a gradual,
progressive change from the composition of ordinary woody fibre in the
newest peat to the pure carbon in graphite,—or the degree of
consolidation and mineralization—since there is a gradual passage from
the light, porous peat, showing distinctly the vegetable forms, to the
heavy crystalline graphite, bearing no trace of its vegetable origin.
This relation is easily appreciated by a child, if a proper series of
specimens is presented. The coals also make a chronological series,
graphite and anthracite occurring only in the older formations, and
lignite and peat in the newer, while bituminous coal is found in
formations of intermediate age.

Bituminous coal is the typical, the representative coal; and from a good
specimen of this variety we may learn two important facts:—

(1) That true coals, no less than peat, are of vegetable origin. To see
this we must look at the flat or charcoal surfaces of the coal. These
soil the fingers like charcoal, and usually show the vegetable forms
distinctly.

(2) That coals are stratified rocks. These dirty charcoal surfaces
always coincide with the stratification, being merely the successive
layers of vegetation deposited and pressed together to build up the
coal; and when we look at the edge of the specimen the stratification
shows plainly enough.

The bitumens form a similar though less perfect series, beginning with
the organic tissues, and ending, in the opinion of some of the best
chemists and mineralogists, with diamond. In fact the coals and bitumens
form two distinct but parallel series. The coals are exclusively of
vegetable origin, while the bitumens are largely of animal origin. The
organic tissues in which the two series originate are chemically
similar,—the animal tissues, which produce the lighter forms of bitumen,
however, containing more hydrogen and less carbon and oxygen than
vegetable tissues; while the final terms, as just shown, are probably
chemically identical, being pure carbon,—graphite for the coals and
diamond for the bitumens; so that the entire process of change in each
series is essentially carbonization, a gradual elimination of the
gaseous elements, oxygen and hydrogen, until pure solid carbon alone
remains.

The principal differences between the coals and bitumens are the
following:—

    Coals are rich in carbon, with some oxygen and little
        hydrogen.

    Bitumens are rich in hydrogen, with some carbon and little
        or no oxygen.

    Coals are entirely insoluble.

    Bitumens are soluble in ether, benzole, turpentine, etc.,
        and the solid forms are soluble in the more fluid,
        naphtha-like varieties.

    Coals are never liquid, and cannot be melted or, with
        trifling exceptions, even softened by heat.

    Many bitumens are naturally liquid, and all become so on the
        application of heat.

    The coals partake of the characteristics of their chief
        constituent element, carbon, the most thoroughly solid
        substance known; while the bitumens similarly show the
        influence of hydrogen, the most perfectly fluid
        substance known.

The two bitumens of the greatest geological importance are asphaltum or
mineral pitch and petroleum; but these substances are too familiar to
require any farther description here.

(2) _Iron-ore Group._—These interesting and important stratified rocks
include the three principal oxides of iron,—limonite, hematite, and
magnetite,—as well as the carbonate of iron, siderite; and the rocks
have essentially the same characteristics as the minerals. In economical
importance they are second only to the coals; and the history of their
formation through the agency of organic matter is one of the most
interesting chapters in chemical geology (see page 26). The three oxides
are easily distinguished from each other by the colors of their powders
or streaks, and the magnetism of magnetite, and from all other common
rocks by their high specific gravity. Magnetite is the richest in iron,
and limonite the poorest. As regards the degree of crystallization and
order of occurrence in the formations, they form a series parallel with
the coal series, thus:—

    Limonite, never crystalline, and found in recent formations.

    Hematite, often crystalline, and found in older formations.

    Magnetite, always crystalline, and found in oldest
        formations.

Siderite effervesces with strong acid; and this separates it from all
other rocks, except limestone and dolomite; and from these it is
distinguished by its high specific gravity. As a mineral, siderite is
often light colored; but as a rock it is always dark, and usually black,
from admixture chiefly of carbonaceous matter. In studying dynamical
geology, we have learned (page 28) the reason for the intimate
association of siderite with beds of coal, and this accounts equally for
the carbon contained in the rock itself. The connection of this rock
with the coal-formations adds much to its value as an ore of iron.

Finally, the iron-ores, at least where of much economical importance,
are truly stratified. This can often be seen in hand-specimens; and is
well shown by their relations to other rocks, in quarries and mines; and
in many cases, for limonite and hematite, by the fossils which they
contain.

(3) _Siliceous Group._—These rocks are composed of pure silica in the
forms of quartz and opal. When first deposited, whether organically,
like tripolite, or chemically, like siliceous tufa, the siliceous rocks
are soft and light, and the silica is in the form of opal. Subsequently
it changes to quartz, and the rocks assume the much harder and denser
forms of chert and novaculite, respectively.

Tripolite or Diatomaceous Earth.—This interesting rock is soft, light,
and looks like clay; but it is lighter, and the argillaceous odor is
faint or wanting. It does not effervesce with acid. Hence, it is neither
clay nor chalk. Notwithstanding its softness, it is really composed of a
hard substance, viz., silica, in the form known as opal. By rubbing off
a little of the dust, and examining it under the microscope, we easily
prove that the silica is mainly or entirely of organic origin; for the
dust is seen to be simply a mass of more or less fragmentary organic
remains, occurring in great variety, and of wonderful beauty and
minuteness. There are few rocks so unpromising on the exterior, and yet
so beautiful within. We have already learned that these organic bodies
are principally Diatom cases, Radiolaria shells, and Sponge spicules. We
can form some idea of their minuteness from Ehrenberg’s estimate that a
single cubic inch of pure tripolite contained no less than
41,000,000,000 organisms.

The lightness of tripolite (sp. gr., 1-1.5) is due to the facts that
opal is a light mineral (sp. gr., 1.9-2.2), and that many of the shells
are hollow. Tripolite is a good example of a soft rock composed of a
hard mineral; and it owes its value as a polishing material to the fact
that it consists of a hard mineral in an exceedingly fine state of
division. Tripolite, when pure, is snow-white; but it is rarely pure,
being commonly either argillaceous or calcareous. This rock is now
forming in thousands of places, in both fresh water and the ocean.

Flint and Chert.—During the course of geological time, beds of tripolite
are gradually consolidated, chiefly by percolating waters, which are
constantly dissolving and re-depositing the silica; and, finally, in the
place of a soft, earthy rock, we get a hard, flinty one, which we call
_flint_ if it occurs in the newer, or _chert_ if it occurs in the older,
geological formations. Besides forming beds of nearly pure silica, which
we call tripolite, the microscopic siliceous organisms are diffused more
or less abundantly through other rocks, especially chalk and limestone.
In such cases the consolidation of the silica implies its segregation
also; _i.e._, the silica dissolved by percolating water is deposited
only about certain points in the rock, building up rounded concretions
or nodules. Thus, a siliceous limestone becomes, by the segregation of
the silica, a pure limestone containing nodules of chert, which are
usually arranged in lines parallel with the stratification. Specimen 16
is a fragment of a flint-nodule from the chalk-formation of England.

Siliceous Tufa.—Hot water, and especially hot alkaline water,
circulating through the earth’s crust, is always charged with silica
dissolved out of the rocks; and when such water issues on the surface in
a hot spring or geyser, it is cooled by contact with the air, its
solvent power is diminished thereby, and a large part of the silica is
deposited around the outlet as a snowy-white porous material called
_siliceous tufa_. Silica deposited in this way is, like organic silica,
always in the form of opal. Siliceous tufa is distinguished from clay,
slate, chalk, and limestone by the same tests as tripolite, and from
tripolite itself by the absence of microscopic organisms.

Novaculite.—Through the action of percolating water and pressure,
siliceous tufa, like tripolite, becomes harder and denser and is thus
changed to _novaculite_, which holds the same relation to chemically
deposited silica that chert and flint do to organically deposited
silica. The white novaculite obtained at the Hot Springs of Arkansas,
and commonly known as Arkansas stone, is a typical example of this rock.
The rock which, on account of the use to which it is put, is known as
buhr-stone, is also an excellent example of chemically deposited silica.
It is usually somewhat porous and fossiliferous.

(4) _Calcareous Group._—These are the lime-rocks, including the
carbonate of lime, in limestone and dolomite, the sulphate of lime, in
gypsum, and the phosphate of lime, in phosphate rock. These rocks are
even more closely connected in origin than in composition; and it is for
this reason that rock-salt, which of course contains no lime, is also
included in this group. Limestones are formed abundantly in the open
sea, through the accumulation of shells and corals; but when portions of
the sea become detached from the main body and gradually dry up, like
the Dead Sea and Great Salt Lake, dolomite, gypsum, and rock-salt are
deposited in succession as chemical precipitates. Phosphate rock may be
regarded as a variety of limestone, resulting from the accumulation of
the skeletons and excrement of the higher animals.

Limestone.—This is the lithologic or rock form of carbonate of lime or
calcite, and one of the most important, interesting, and useful of all
rocks. Although so simple in composition,—calcite being the only
essential constituent,—limestone embraces many distinct varieties, and
is really equivalent to a whole family of rocks. A highly fossiliferous
limestone, such as specimen 38, is, perhaps, the best variety with which
to begin the study of the species. The softness of the fossil shells of
which the rock is so largely composed, and the fact that they effervesce
readily with dilute acid, proves that they are still carbonate of lime;
and by applying the acid more carefully, it can be seen that the compact
matrix of the rock also effervesces, consisting of shells more finely
broken or comminuted and mixed with more or less clay and other
impurity, almost the entire rock being of organic origin.

On the coast of Florida, and in many other places, we find beautiful
examples of shell-limestone now in process of formation. These are at
first very open and porous, because the interstices between the nearly
entire shells are not yet filled up with smaller fragments and sand. But
when that is done, we shall have a rock similar to the old fossiliferous
limestone. Specimen 37.

The shells and fragments, and the grains of calcareous sand, are, as a
rule, quickly cemented together by the deposition of carbonate of lime
between them; so that limestone is nowhere observed occurring abundantly
in an unconsolidated form.

Limestone, as a rule, is not distinctly stratified in hand-specimens,
but of course that it is a true sedimentary rock is abundantly proved by
the fossils; and it goes almost without saying that limestone, being
necessarily mainly composed of organic remains, must be to a greater
extent than any other rock the great store-house of fossils; and in no
other rock are the fossils so well preserved and perfect as in
limestone.

Nevertheless, there are extensive formations of limestone containing no
discernible traces of fossils. And some of these non-fossiliferous
limestones, too, are of very recent formation. Some of the modern
coral-reefs, for example, are composed of limestone which was formed
only yesterday, as it were, and which, from its mode of formation, must
consist entirely of corals; and yet it shows no trace of its organic
origin, but is perfectly compact, or, possibly, crystalline. This
frequent obliteration of the organic remains, as well as the perfect
consolidation of the rock, is attributed to its solubility. The calcium
carbonate is gradually dissolved by the water, and then deposited in the
interstices in other parts of the rock.

Specimen 39 is that variety of limestone known as _chalk_. It is soft
and earthy, resembling both clay and tripolite, but differing from the
former in lacking the distinct argillaceous odor, and from both by its
lively effervescence with acids. It appears to be entirely destitute of
organic remains, but this is a defect of our vision and not of the rock;
for, like the tripolite, it often appears under the microscope to be
little else than a mass of shells. Tripolite is a deposit built up of
the siliceous shells of Diatoms and Radiolaria, while chalk is chiefly
composed of the similar but calcareous shells of Foraminifera. Our
specimen is from the Cretaceous formation of England; but we have good
reason to believe that chalk is _now forming_ on a very extensive scale.
There are millions of square miles in the deeper parts of the ocean
where the dredge brings up little else but a perfectly impalpable, gray,
calcareous slime or ooze. When examined microscopically, this is seen to
be composed chiefly of Foraminifera shells, and among these the genus
Globigerina predominates; so that the deposit is frequently called
Globigerina ooze. Now this gray, calcareous ooze, when dried and
compacted by pressure, makes a soft, _white_ rock which can scarcely be
distinguished from chalk; in fact, it is a modern chalk. And there seems
no good reason to doubt that the deposition of chalk has gone on
continuously since Cretaceous time—for several millions of years at
least.

Specimen 40 is also a white rock, easily scratched with the knife, and
effervescing freely with acid, and therefore a variety of limestone. But
its texture is very different from the other varieties we have studied.
It has a sparkling surface, which we explain by saying that the rock is
crystalline. It is, in fact, a mass of minute crystals of calcite. The
crystalline limestones have not always been crystalline, but it is safe
to assume that they were originally entirely uncrystalline, and in many
cases rich in fossils; but the fossils have been mainly obliterated by
the crystallization.

Crystallization generally in rocks is an indication of great age, so
that we usually say crystalline rocks must be older than uncrystalline
rocks of the same composition; and this is mainly true with the
limestones. When the crystallization is rather fine, as in our specimen,
resembling granulated sugar, we have what is commonly called
saccharoidal limestone. This is the typical marble. Marble is not a
scientific name, and the term is usually applied to any calcareous rock
which will take a polish, and sometimes even to rocks which are not
calcareous at all.

In the section on dynamical geology, we learned that the carbonate of
calcium or calcite is deposited from the sea-water, and limestones
formed, in two ways: first, in a purely chemical way, where the water
becomes saturated with calcite; and, second, organically, where the
calcium carbonate is taken from the water by marine organisms to form
their shells and skeletons, and the gradual accumulation of these on the
ocean-floor builds up a limestone. As before stated, the difference
between these two methods of deposition is not so great as it often
seems, because we know that the animals never make the carbonate of
calcium which they secrete, but it comes into the sea ready made with
the drainage from the land.

The limestones forming at the present time are almost wholly organic;
but the rock known as _calcareous tufa_ is an exception. This is formed
under the same general conditions as siliceous tufa, but much more
abundantly, and in cold water as well as warm; because calcite is far
more soluble (especially in water containing carbon dioxide) than opal
or quartz. It is deposited, not only around the mouths of springs, but
also along the beds of the streams which they form, enveloping stones,
roots, grasses, etc., and building up usually a loose, spongy mass
having a very characteristic turfaceous texture.

The principal accessory minerals occurring in limestone are: (1)
_kaolin_, forming argillaceous or slaty limestone, which may be
recognized by the argillaceous odor and dark color; (2) _quartz_,
forming siliceous or cherty limestone, known by its hardness or by the
nodules of flint or chert; (3) _dolomite_, forming dolomitic or
magnesian limestone, which effervesces less freely with acid; and (4)
_serpentine_, forming serpentinic limestone, which is sharply
distinguished by the green grains of serpentine mingled with the white
calcite. A concretionary texture is common with limestone. If the
concretions are small, like mustard-seed, we call the rock _oölite_; if
larger, like peas, _pisolite_.

Dolomite.—If for calcite, which is the sole essential constituent of all
limestone, we substitute the allied mineral dolomite, we have the rock
dolomite. As might be inferred from its composition, dolomite is very
closely related to limestone, although there are some important
differences. Physically, the two rocks differ about as the two minerals
do. Dolomite is harder than limestone, and being also less soluble, it
resists the action of the weather more. Dolomite, if pure, effervesces
feebly, or not at all, with cold dilute acid. Here, however, we have to
recognize the fact that dolomite is rarely pure; but there exists, in
consequence of the admixture of calcite, a perfectly gradual passage
from pure dolomite to pure limestone, and parallel with this every
degree of vigor in the reaction with acid. Hence, it is entirely an
arbitrary matter as to where we shall draw the line between dolomitic
limestone and calcareous dolomite. Dolomite is a very much less abundant
rock than limestone, and, unlike limestone, it rarely contains many
fossils, and is never of organic origin; _i.e._, there are no organisms
which secrete the mineral dolomite to form their hard parts or
skeletons. Like gypsum and rock-salt, dolomite is probably never
deposited in the open ocean, but only in closed basins. Like limestone,
it occurs with both the compact and the crystalline textures.

Gypsum.—When pure, this rock (specimen 36) is identical with the mineral
gypsum (specimen 17), except that it is rarely crystalline. It is
usually, however, not only perfectly compact, but more or less
dark-colored from the admixture of clay and other impurities. Its most
notable characteristics are its softness, the absence of the
argillaceous odor, except where it contains much clayey impurity, and
its non-effervescence with acids. The first two usually serve to
distinguish it from slate, while the acid test separates it readily from
limestone and all other carbonate rocks. The deposition of gypsum is
purely chemical, and it occurs under about the same physical conditions
as the deposition of salt; _i.e._, in drying-up portions of the sea.
Hence we usually find gypsum associated with beds of rock-salt; and,
since drying-up seas are few in number, and small compared with the
whole extent of the ocean, we can easily understand why neither
rock-salt nor gypsum are abundant rocks, except in a few localities.

Rock-Salt.—This interesting and useful rock, as we have already learned,
is deposited in a purely chemical way, and only in drying-up portions of
the sea, like the Dead Sea, Great Salt Lake, etc. In some parts of
Europe there are beds of solid rock-salt over a hundred feet thick.

Phosphate Rock.—Although not specially abundant or attractive, this rock
is of great economic interest and importance on account of its extensive
use as a fertilizer. Under the general head of phosphate rock are
included: (1) the typical guano, which is the consolidated excrement of
certain marine birds inhabiting in great numbers small coral islands in
the dry or rainless regions of the tropics; (2) the underlying coral
rock, which is often changed to phosphate rock through the percolation
of the rain-water falling on the guano; (3) accumulations of the bones
and coprolites of the higher animals; (4) phosphatic limestones from
which the carbonate of lime has been largely dissolved away, leaving the
more insoluble phosphate of lime.

(5) _Metamorphic Group_ (stratified silicates).—All the chemically and
organically formed rocks which we have studied up to this point are
simple, _i.e._, they consist each of only one essential mineral; but
most of the rocks in this great group of silicates are mixed, or consist
each of several essential minerals. Quartz is the only important
constituent of these rocks which is not, strictly speaking, a silicate,
but in a certain sense it is also not an exception, since it may always
be regarded as an excess of acid in the rock.

This group of stratified rocks composed of silicate minerals is of
exceptional importance, first, on account of the large number of species
which it includes, and, second, on account of the vast abundance of some
of the species. These are, above all others, the rocks of which the
earth’s crust is composed. With unimportant exceptions, all the rocks of
this group are crystalline; and they constitute the principal part of
what is generally included under the term _metamorphic rocks_—a general
name for all stratified rocks which have been so acted upon by heat,
pressure, or chemical forces as to make them crystalline. Although the
crystalline limestone, dolomite, iron-ores, etc., show us that
metamorphic rocks are not wanting in the other groups.

As already explained, the metamorphic or crystalline stratified rocks
are usually older than the corresponding uncrystalline rocks; but a
point of greater importance here is this: the development in the
silicate rocks of crystalline characters has usually made it impossible
to determine the method of their deposition, whether mechanical or
chemical. In a few cases, as with the rock greensand, we know that the
deposition is chemical; while it is equally certain that such common
silicate rocks as gneiss, mica schist, and many others, often result
from the crystallization of ordinary mechanical sediments, like
sandstone and conglomerate. We classify all these rocks as of chemical
origin, however, without considering the mode of their deposition,
because the subsequent crystallization is itself essentially a chemical
process; and that justifies us in saying that these rocks are made what
they now are chiefly by the action of chemical forces. Whatever they
were originally, they have become, through their crystallization, rocks
having a definite mineral composition which can be classified
chemically.

Some of the details of the classification of this group, as shown in the
table, require explanation. In studying the silicate minerals it was
stated to be important to recognize two classes—the _acidic_ and the
_basic_—the dividing line falling in the neighborhood of 60 per cent. of
silica. This division is important simply because Nature has in a great
degree kept the acidic and basic minerals separate in the rocks; and few
things in lithology are more important than the distinction of the
silicate rocks in which acidic minerals predominate from those in which
basic minerals predominate. The amount of silica which any rock of this
group contains is shown at a glance by the chart. The vertical broken
lines, with the figures at the top, indicate the proportion of silica,
which increases from 30 per cent. on the right to 85 per cent. on the
left; so that the percentage of silica which a rock contains determines
its position, the acidic species being on the left, and the basic on the
right. As most of these rocks are composed of two or more minerals mixed
in very various proportions, there is usually a wide range in the
percentage of silica which the same species may contain; and this is
expressed in each case by the length of the dotted line under the name
of the rock. Thus, in syenite, the silica ranges from 55 per cent. to 65
per cent. The horizontal line in the chart separates the gneisses,
containing feldspar as an essential constituent, from the schists, in
which feldspar is wanting, except as an accessory constituent. We will
take up the gneisses first.

Gneiss.—This is the most important of all rocks. It forms not far from
one-half of New England, and a very large proportion of the earth’s
crust. The name (pronounced same as _nice_) is known to have originated
among the Saxon miners, but its precise derivation is lost in obscurity.
To find out what this very important rock is, we will consult specimen
41. The first glance shows us that it is not, like the rocks we have
just been studying, composed of a single mineral, but of several
minerals, the most conspicuous of which is the pink feldspar—orthoclase.
This we recognize as a feldspar: (1) by its hardness, which is a little
less than that of quartz, and distinguishes it from calcite, a mineral
having the general appearance of feldspar; (2) by its color, which
separates it from hornblende and augite; and (3) by its cleavage, which
distinguishes it easily from quartz. Finally, we know it is orthoclase,
and not plagioclase, by its general aspect, and by its association with
an abundance of quartz, which is the next most important constituent of
the rock. The quartz is less abundant than the orthoclase, and more
easily overlooked, yet anyone familiar with the mineral will not fail to
recognize it. It forms small, irregular, glassy grains, entirely devoid
of cleavage, and scratching glass easily. On weathered surfaces of the
rock the orthoclase becomes soft and chalky, while the quartz remains
clear and hard, and then the two minerals are very easily distinguished.
Besides these, there are numerous black, thin, glistening scales, which
we can easily prove to be elastic, and recognize as mica.

In most books on the subject, these three minerals—orthoclase, quartz,
and mica—are set down as the normal or essential constituents of gneiss.
But it is now recognized by the best lithologists that we may have true
gneiss without any mica; or we may have hornblende in the place of mica.
Quartz and orthoclase are the only essential constituents of gneiss; and
when these alone are present, we have the variety known as binary
gneiss. The addition to these essential constituents of mica, gives
micaceous gneiss; and of hornblende, hornblendic gneiss. Of these three
principal varieties, the micaceous gneiss is by far the most common and
important. The mica may be either the white species, muscovite, or the
black species, biotite; but it is usually the former.

Orthoclase is the predominant constituent in all typical gneiss, usually
forming at least one-half of the rock. The orthoclase may, however, be
replaced to a greater or less extent by albite, or even by oligoclase.
But we frequently see the term _gneiss_ carelessly, or ignorantly,
applied to rocks which are destitute of feldspar, though having the
general aspect of gneiss.

Augite rarely occurs in gneiss; and hence, when we observe a gneiss
containing a black mineral which we know is either augite or hornblende,
it is pretty safe to call it the latter.

Mica and hornblende, although the principal, are not the only, accessory
minerals in gneiss; but the following species are also of common
occurrence: garnet, cyanite, tourmaline, fibrolite, epidote, and
chlorite. Gneisses, as the table indicates, exhibit a wide range in the
proportion of silica which they contain, varying from 60 to 85 per
cent.; and there is a concomitant variation in specific gravity, from
about 2.5 in the most acidic to 2.8 in the most basic varieties.

That gneiss is a true, stratified rock is very clearly shown in specimen
41; but, unfortunately, the stratification is not always so evident as
in this case. The mica-scales, it will be observed, lie parallel with
the stratification, and assist very materially to make it visible; and
gneisses containing little or no mica, as well as some that are rich in
mica, frequently appear almost or quite unstratified. These obscurely
stratified varieties are commonly known as granitoid gneiss, having the
texture and general aspect of granite. The sedimentary origin of gneiss
is also clearly proved by its interstratification with undoubted
sedimentary rocks, such as limestone, iron-ores, graphite, quartzite,
etc.

Syenite.—This is a much abused term, but, as now employed by the best
lithologists, it is the name of a rock having a single essential
constituent, viz., orthoclase. Syenite in its simplest variety contains
nothing but orthoclase; but in addition we usually have either
hornblende, forming hornblendic syenite, or mica, forming micaceous
syenite.

Syenite, it will be observed, is equivalent to gneiss with the quartz
removed; but, while gneiss is the most abundant of all rocks, syenite is
a comparatively rare rock; and this is simply another way of saying that
nearly all orthoclase is associated with quartz. By admixture of quartz
we get a perfectly gradual passage from syenite to gneiss. The
orthoclase in syenite is more frequently replaced by plagioclase than it
is in gneiss. In syenite, too, hornblende is much more abundant than
mica; although just the opposite is true in gneiss. And, again, in
gneiss the mica is principally muscovite; but in syenite it is almost
exclusively biotite. Augite is a common accessory in the more basic
syenite; but garnet, tourmaline, and the other accessory minerals,
occurring so frequently in gneiss, are almost unknown in syenite. The
specific gravity of syenite varies from 2.7 to 2.9.

Diorite.—This is a more important rock than syenite; but it is of
analogous, though more basic, composition, containing a single essential
constituent, viz., plagioclase. Any of the triclinic feldspars may occur
in this rock, but oligoclase is most common. Like syenite, diorite
usually contains hornblende, often in large proportion, forming
hornblendic diorite, which sometimes passes into rocks composed entirely
of hornblende. It also, but less frequently, contains mica, forming
micaceous diorite. The mica is usually biotite, rarely muscovite. Mica
and hornblende also often occur together in diorite, and the same is
true of syenite and gneiss. Quartz is of common occurrence in the more
acidic varieties of diorite, and augite in the more basic.

This is a good example of a basic rock, for all its normal constituents
are basic; but the percentage of silica varies from 45 in those
varieties richest in labradorite and augite to 60 or more in those
containing more or less quartz and orthoclase. There is a corresponding
change of color from dark to light, and of specific gravity from 2.7 to
3.1.

Diorite is not rich in accessory minerals; besides those already
mentioned, the most important are chlorite, epidote, pyrite, and
magnetite.

Few rocks are more clearly stratified than diorite, whether we consider
the hand-specimen, or its relations to other formations. It is an
abundant rock in New England.

Norite.—Like diorite, this is essentially a plagioclase rock; but there
are, nevertheless, important differences. The plagioclase in diorite is
mainly the more acidic species, like oligoclase; while in norite the
more basic species, such as labradorite and anorthite, predominate.
Hornblende, which we have observed to be an important and rather
constant constituent of diorite and syenite, is much less abundant in
norite; but its place is taken by augite and the allied minerals,
hypersthene and enstatite. Black mica is common in norite; but white
mica, orthoclase, and quartz rarely occur.

Norite is the most basic of all the feldspathic rocks, as gneiss is the
most acidic; while syenite and diorite stand as connecting links,
forming a gradual passage between the two extremes. Thus, in passing
from gneiss to norite, we have observed a gradual diminution of the
quartz, a gradual change in feldspar from orthoclase to the most basic
plagioclase; at first a gradual increase in hornblende, and then a
gradual change from hornblende to augite; and, finally, a gradual
substitution of black mica for white. The amount of silica has decreased
over 40 per cent.; and the specific gravity has increased from 2.5 in
the lightest gneiss to at least 3.2 in the heaviest norite. We have also
passed from light colored rocks to dark; and from those resisting
atmospheric action to those easily decomposed.

The most characteristic accessory constituents of norite, besides those
already mentioned, are magnetite and chrysolite; though garnet,
serpentine, and pyrite often occur. In texture, this rock varies from
compact to very coarsely crystalline. The specimen of labradorite (No.
23), from the norite of Labrador, affords some idea of the coarseness of
the crystallization in much of this rock. It is not a common rock,
except in certain regions, the best known of which in eastern North
America are the coast of Labrador, various points in Canada north of the
St. Lawrence, and the eastern border of the Adirondack Mountains. In
hand-specimens, norite rarely appears stratified; but in the solid
ledges the stratification is often as distinct as could be desired.

Many lithologists call the rocks here designated norite _gabbro_, and
class them all in the eruptive division as essentially a coarse variety
of diabase. In a similar manner, diorite and syenite are denied a place
in the sedimentary series. But the stratified plagioclase rocks seem to
have as strong a claim to recognition as gneiss.

We turn now to the important and interesting division of the
non-feldspathic rocks or schists.

Mica Schist.—This is, next to gneiss, the most abundant rock in New
England. Specimen 43 is a typical example, and from it we can readily
learn what mica schist is. A glance suffices to show that it is chiefly
composed of mica, but not entirely; for, on carefully examining the
edges of the specimen, we cannot fail to see thin layers of hard, glassy
quartz interwoven with the mica. The quartz layers are short and
overlapping, and we have here a good illustration of the schistose
texture; this is, in fact, a typical schist.

Mica schist usually consists, as in this instance, of mica and quartz;
but it may be composed of mica alone; and sometimes kaolin or clay takes
the place of the quartz, forming argillaceous mica schist. The mica in
the latter is usually in very fine scales and rather inconspicuous, and
the rock often passes into ordinary clay slate. Similarly, when the mica
becomes deficient in the quartzose mica schist, a passage into ordinary
quartzite is the result. A little feldspar is sometimes present in the
rock, which thus passes into micaceous gneiss. Specimen 43 contains
several crystals of red garnet, giving the variety garnetiferous mica
schist. There is no other rock that contains such a large variety of
beautiful accessory minerals as mica schist; and for the mineralogist it
is one of the most attractive rocks. Few rocks are more distinctly
stratified; and the stratification can usually be observed in
hand-specimens. The mica in these rocks may be either muscovite or
biotite, or both; but the former is most common. No rock shows a greater
variation in the percentage of silica which it contains than mica
schist, as we pass from varieties which are nearly all quartz to those
which are nearly all mica.

Closely related to mica schist is the rock now known as hydromica
schist, in which the ordinary anhydrous micas are replaced by hydromica.
It is distinguished from mica schist by being somewhat softer, less
harsh to the touch, and less lustrous. It is to be regarded usually as
an incipient mica schist, which has not yet become anhydrous; though it
may sometimes be just the reverse; viz.: an old mica schist which has
become hydrated through the action of meteoric waters. It contains fewer
accessory minerals than mica schist.

Hornblende Schist.—This is a stratified aggregate of hornblende and
quartz. The quartz is granular and in thin layers, as in mica schist;
but the micaceous structure is wanting, and consequently the rock does
not cleave readily in the direction of the bedding. The hornblende is
mostly finely crystalline, but sometimes occurs in large, bladed
crystals. Garnet and some other minerals are of common occurrence in the
rock; but it is not rich in accessories like mica schist. The chief
difficulty in recognizing this rock consists in determining whether the
white mineral is all quartz or partly feldspar. In the latter case, of
course, it becomes a hornblendic gneiss.

Amphibolite (Hornblende Rock).—This is the name applied to a rock having
hornblende as its sole essential constituent. Hornblende schist
sometimes passes into amphibolite, through the absence of quartz; and so
does diorite, when the feldspar is deficient or wanting. Specimen 20,
though small, is a typical example of this rock. The physical and
chemical characteristics are essentially the same as for the mineral
hornblende. The texture varies from coarsely to finely crystalline. The
crystals are usually short and thick, and lie in all directions in the
rock, which is thus very massive, the schistose texture being entirely
wanting, and the stratification rarely showing in small masses. Biotite
is a common accessory in amphibolite, and garnet and magnetite
frequently occur.

By the substitution of augite for hornblende, in the description of
amphibolite, we get the much rarer, but otherwise very similar, rock,
_pyroxenite_.

Talc Schist (Steatite or Soapstone).—Although not abundant, this is a
useful and familiar rock. The composition is implied in the name; and by
comparing it with the specimen of talc (No. 58) we can readily see that
they are essentially identical. Typical talc schist is pure talc; but
the talc is often mixed with more or less quartz or feldspar; and mica,
chlorite, hornblende, garnet, and other minerals are of common
occurrence.

This rock embraces two distinct varieties, the massive and the
schistose, or foliated. The former is the common soapstone (specimen
71), which is a confused mass of crystals lying in all directions, and
with no visible stratification in the small mass. In the latter, as in
specimen     , the talc scales lie in parallel planes, giving the rock a
micaceous structure, and causing it to split easily in the direction of
the stratification. The cleavage surfaces are often wavy or corrugated;
and the same is true of all schistose rocks. Talc schist is easily
distinguished from all other rocks by its light-grayish or greenish
color, combined with its extreme softness, and its smooth, slippery
feel.

Chlorite Schist.—The one essential constituent of this rock is chlorite,
and the mineral specimen (No. 26) answers equally well as an example of
the rock. As with talc schist, quartz, feldspar, and hydromica are
rarely entirely absent. Besides these, the principal accessories are
hornblende, magnetite, garnet, and epidote. This rock also agrees with
talc schist in presenting two principal varieties, the massive and the
schistose. It is easily distinguished from talc schist by its darker
color and streak, which are very characteristic; while its green color,
softness, and unctuous feel separate it from all other rocks.

This is the most basic of all the silicate rocks; but, in consequence of
containing a large proportion of water, it is not the heaviest. It is,
in fact, interesting and important to observe that all these hydrous
silicate rocks—talc schist, chlorite schist, greensand, and
serpentine—are distinctly lighter in each case than anhydrous rocks
containing the same proportion of silica. They are also notable, as a
class, for their softness, smooth feel, and green color.

Serpentine.—As the name implies, this rock is simply the mineral
serpentine occurring in large masses, and its characteristics are
precisely the same. It is fine-grained, massive, compact, rather soft,
but very tough, and varies in color from very dark green to light
greenish-yellow. The dark colors predominate, and specimen 25 is a
typical example.

Serpentine is often intimately associated with limestone and dolomite.
The white veins running irregularly through the variety known as Verd
Antique Marble, however, are not calcite, as commonly supposed, but
magnesite. They do not effervesce freely with cold, dilute acid, for the
entire rock is magnesian, and it is probable have been at one time
simply cracks along which water holding carbon dioxide has penetrated,
changing the magnesia from a silicate to a carbonate.

Geologists were, at one time, almost unanimous in the opinion that all
serpentine is of eruptive origin; but now it is conceded by the great
majority to be in some cases a sedimentary rock. It is found
interstratified with gneiss, limestone, all the schists, and many other
stratified rocks. When occupying the position of an eruptive it is never
an original rock; but has been formed by the alteration, _in situ_, of
some basic anhydrous rock, most commonly olivine basalt.

Greensand.—This rock (specimen 27) consists chiefly of the mineral
glauconite, mingled usually with more or less sand, clay, or calcareous
matter. It is usually very friable, or in an entirely unconsolidated
state. It is most abundant in the newer geological formations,
especially the Cretaceous and Tertiary; and is, perhaps, the only one of
the stratified silicate rocks now forming on an extensive scale in the
ocean. Its value as a fertilizer, for which purpose it is extensively
employed, is due to the potash that it contains.

Following is a systematic summary of the mineralogical composition of
the rocks of this great division of silicates; and this, combined with
the classification on page 69, presents in a condensed form all the more
important facts contained in the preceding descriptions. Only the more
constant and normal constituents of the species are enumerated in each
case:—

       ====================+====================================
        Names of Species.  |       Constituent Minerals.
       --------------------+------------------------------------
                          ⎧| Orthoclase and Quartz.
       Gneiss             ⎨| Orthoclase, Quartz, and Mica.
                          ⎩| Orthoclase, Quartz, and Hornblende.
       --------------------+------------------------------------
                          ⎧| Orthoclase.
       Syenite            ⎨| Orthoclase and Hornblende.
                          ⎩| Orthoclase and Mica.
       --------------------+------------------------------------
                          ⎧| Plagioclase (chiefly Oligoclase).
       Diorite            ⎨| Plagioclase and Hornblende.
                          ⎩| Plagioclase and Mica.
       --------------------+------------------------------------
                          ⎧| Plagioclase (chiefly Labradorite).
       Norite             ⎨| Plagioclase and Augite (Diallage).
                          ⎩| Plagioclase and Mica.
       --------------------+------------------------------------
                          ⎧| Mica.
       Mica Schist        ⎨| Mica and Quartz.
                          ⎩| Mica and Kaolin.
       --------------------+------------------------------------
       Hornblende Schist   | Hornblende and Quartz.
       --------------------+------------------------------------
       Amphibolite         | Hornblende.
       Pyroxenite          | Pyroxene.
       --------------------+------------------------------------
       Talc Schist         | Talc.
       --------------------+------------------------------------
       Chlorite Schist     | Chlorite.
       --------------------+------------------------------------
       Serpentine          | Serpentine.
       --------------------+------------------------------------
       Greensand           | Glauconite.
       ====================+====================================


                   2. Eruptive or Unstratified Rocks.

The rocks of this great class are formed by the cooling and
solidification of materials that have come up from a great depth in the
earth’s crust in a melted and highly heated condition. When the fissures
in the earth’s crust reach down to the great reservoirs of liquid rock,
and the latter wells up and overflows on the surface, forming a volcano,
then we may, as was pointed out on page 33, divide the eruptive mass
into two parts: first, that which has actually flowed out on the
surface, and cooled and solidified in contact with the air, forming a
lava flow; second, that which has failed to reach the surface, but
cooled and solidified in the fissure, forming a dike.

Lava flows or volcanic rocks and dikes or plutonic rocks are identical
in composition; but there is a vast difference in texture, due to the
widely different conditions under which the rocks have solidified. The
dike or fissure rocks solidify under enormous pressure, and this makes
them heavy and solid—free from pores. They are surrounded on all sides
by warm rocks: this causes them to cool very slowly, and allows the
various minerals time to crystallize. Other things being equal, the
slower the cooling the coarser the crystallization; and hence, the
greater the depth below the surface at which the cooling takes place,
the coarser the crystallization.

The volcanic rock, on the other hand, cools under very slight pressure;
and the steam, which exists abundantly in nearly all igneous rocks at
the time of their eruption, is able to expand, forming innumerable small
vesicles or bubbles in the liquid lava; and these remain when it has
become solid. Cooling in contact with the air, the lava cools quickly,
and has but little chance for crystallization. Hence, to sum up the
matter, we say: plutonic rocks are solid and crystalline; and volcanic
rocks are usually porous or vesicular, and uncrystalline.

As we descend into the earth’s crust, it is perfectly manifest that the
volcanic must shade off insensibly into the dike rocks, and we find it
impossible to draw any but an arbitrary plane of division between them;
but this is no argument against this classification, for, as already
stated, all is gradation in geology, and we experience just the same
difficulty in drawing a line between conglomerate and sandstone, or
between gneiss and mica schist, as between the dike rocks and volcanic
rocks.

We will now observe to what extent the distinctions between these two
great classes of eruptives can be traced in the rocks themselves,
beginning with the dike rocks. But first it is important to notice the
general fact, clearly expressed in the classification, that, with
perhaps some trifling exceptions which need not be mentioned here, all
eruptive rocks are silicates, and nearly all are feldspathic silicates.
They are of definite mineralogical composition, and, like the chemically
and organically formed stratified rocks, can be classified chemically.
But, although there are eruptives corresponding closely in composition
to the feldspathic silicates, which we have just studied, we find among
them little to represent the non-feldspathic silicates, and nothing
corresponding in composition to the limestones, dolomites, gypsum,
flint, tripolite, siliceous tufa, iron-ores, bitumens or coals.

1. PLUTONIC (DIKE) ROCKS.—These are also known as the _ancient_ eruptive
rocks, and for this reason: It is impossible, of course, for us to
observe them except where they occur on or near the earth’s surface.
But, since they are formed wholly below the surface, and usually at
great depths in the earth, it is evident that they can appear on the
surface only as the result of enormous erosion; and erosion is a slow
process, demanding, in these cases, many thousands or millions of years.
Therefore, the more ancient dike rocks alone are within our reach; those
of recent formation being still deeply buried in the earth’s crust. It
follows, as a corollary to this explanation, that the coarseness of the
crystallization of any dike rock must be a rough measure of its age and
of the amount of erosion which the region has suffered since its
eruption.

As regards composition, the dike rocks present, as already stated,
essentially the same combinations of minerals as the feldspathic
silicates of the stratified series, but occurring under different
physical conditions and having a widely different origin. The only
important difference in texture between the two classes of rocks is that
the sedimentary rocks are stratified and the dike rocks are not; and
when we consider that the dike rocks sometimes present a laminated
structure that resembles stratification, while the sedimentary rocks
frequently appear unstratified, it is easy to understand why, in the
absence of any marked difference in composition, geologists have often
found it difficult to distinguish the two classes of rocks. We also find
here the explanation and the justification of the fact that the names of
the dike rocks are in most cases the same as those of the sedimentary
rocks of similar composition.

Granite.—Granite (from the Latin _granum_, a grain) is a
crystalline-granular rock, agreeing in composition with gneiss. The
essential constituents are quartz and orthoclase; and when they alone
are present we have the variety _binary granite_. Mica, however
(commonly muscovite, sometimes biotite, and frequently both) is usually
added to these, forming _micaceous granite_ (specimen 44); and often
hornblende, forming _hornblendic granite_ (specimen 45). The orthoclase
is sometimes replaced in part by triclinic species, especially albite
and oligoclase. Accessory minerals are not so abundant in granite as in
gneiss; but, besides those named, garnet, tourmaline, pyrite, apatite,
and chlorite are most common. Orthoclase is always the predominant
ingredient; and, except when there is much hornblende present, usually
determines the color of the granite. Thus, specimens 44 and 45 are gray
because they contain gray orthoclase; while all red granites contain red
or pink orthoclase. The quartz has usually been the last of the
constituents to crystallize or solidify; and, having been thus obliged
to adapt itself to the contours of the orthoclase and mica, it is rarely
observed in distinct crystals.

In texture, the granites vary from perfectly compact varieties,
approaching petrosilex, to those which are so coarsely crystalline that
single crystals of orthoclase measure several inches in length. Of
course one of the most important things to be observed about granite,
especially in comparing it with gneiss, is the complete absence of
anything like stratification; that, as before stated, being the only
important distinction between the two rocks. Gneiss is the most abundant
of all stratified rocks, and granite stands in the same relation to the
eruptive series.

Syenite.—This is an instance where stratified and eruptive rocks,
agreeing in composition, have the same name. That rocks consisting of
orthoclase, of orthoclase and hornblende, or of orthoclase and mica,
_i.e._, having the composition of syenite, do occur in both the eruptive
and stratified series there can be no doubt. They should, however, have
distinct names on account of their unlike origins; and would have but
for the practical difficulty in determining, in many cases, whether the
rock is stratified or not. The best that we can do now, when we desire
to be specific, and have the necessary information, is to say stratified
syenite or eruptive syenite, as the case may be.

Diorite.—Here, again, we find identity of names, as well as of
composition, between the two great series. Eruptive diorite is an
abundant and well known rock, and consists of the same minerals as
stratified diorite combined in the same proportions. Diorite includes a
large part of the dike rocks commonly known as “trap” and “greenstone.”
The principal accessories are chlorite, epidote, pyrite, magnetite,
apatite, and quartz. The texture varies from perfectly compact or
felsitic to coarsely crystalline; averaging, however, less coarse than
syenite and granite.

Diabase.—By referring to the classification it will be seen that diabase
occupies the same position among the dike rocks as norite among the
stratified rocks. Like norite it consists usually of the more basic
varieties of plagioclase with or without augite, diallage, or
hypersthene. Augite, or one of its representatives, is usually present,
and is often the principal constituent. Specimen 1 shows a somewhat
equal development of the feldspar and augite. The name _gabbro_ is
sometimes applied to the coarser and more feldspathic diabases, and
especially to those containing diallage or hypersthene in the place of
common augite. In the opinion of some high authorities, however, it is
unnecessary to recognize two species here; and it makes the
classification more simple and symmetrical not to do it. The principal
accessories in diabase are biotite, chlorite, magnetite, pyrite,
calcite, and olivine. Chlorite is often an important constituent, giving
the rock a greenish aspect; but here, as well as in diorite, the
chlorite is due chiefly or entirely to the alteration of the augite and
feldspar; and the chloritic varieties of diorite and diabase together
make up the old species “greenstone.” Similarly, the more compact and
darker varieties of these two rocks, forming regular, wall-like dikes,
are known as “trap.” Specimen 46.

In consequence of their more basic composition, diabase and diorite are
usually strongly contrasted with granite and syenite in color and
specific gravity, being darker and heavier. The basic rocks, too, decay
much more readily than the acidic.

2. VOLCANIC ROCKS.—As regards composition, we shall find nothing new in
the volcanic series; for the rocks of this group present essentially the
same combination of minerals as the dike rocks. In composition, the dike
and volcanic rocks are identical; but in texture, as already explained,
there is a vast difference. The volcanic rocks differ so widely in
texture from both the dike and stratified species, that there is rarely
any difficulty in distinguishing them; and hence they have in every
instance distinct names.

Volcanic rocks are rarely found in this part of the world; and specimens
of most of them are difficult to obtain. For this reason they can only
be noticed briefly here, since it is the plan of this Guide to give
especial attention only to those portions of the subject which can be
illustrated by material within easy reach of teachers.

Rhyolite.—This rock corresponds in composition with granite and gneiss,
but is less frequently micaceous. The orthoclase in rhyolite, and
generally in volcanic rocks, is the clear, pellucid variety—_sanidine_.
It is more difficult to separate from quartz than ordinary orthoclase,
the chief distinguishing feature being its cleavage. Plagioclase and
hornblende are common, but not abundant, constituents. The mica, when
present, is usually biotite. The texture of rhyolite is often more or
less distinctly porphyritic, having a finely crystalline or granular
matrix, with interspersed crystals of sanidine and quartz. The rock has
usually a rough, harsh feel; and while the coarser varieties have the
aspect of granite, the finer approach petrosilex; but all are somewhat
porous, which is seen in the lower specific gravity of rhyolite as
compared with granite and gneiss.

Trachyte.—In texture and general aspect rhyolite and trachyte are nearly
identical. Trachyte, however, is darker, contains little or no quartz,
and more hornblende and plagioclase. In fact, it agrees in composition
with syenite. This is one of the most important of the volcanic rocks.

Obsidian.—Obsidian is sharply distinguished from all other rocks by its
perfect vitreous texture; it is a true volcanic glass. Its surface
(specimen 47) is smooth and glassy, and its fracture eminently
conchoidal. To the naked eye, and usually under the microscope, the
typical variety is perfectly homogeneous; chemical analysis, however,
shows that it has the composition, commonly of rhyolite, but sometimes
of trachyte. Obsidian is, in fact, simply rhyolite or trachyte which,
cooling quickly, has not had time to crystallize, but has remained
permanently in the amorphous or glassy state. The composition is
sometimes partially revealed where a portion of the sanidine comes out
in distinct crystals porphyritically interspersed through the glass. The
homogeneity of the texture is sometimes disturbed: by numerous minute
concentric cracks, forming what is known as perlitic structure and the
variety perlite; by numerous small spherical concretions, forming the
spherulitic structure and the variety spherulite; and also by the
banding, which is the result of flowing while in a plastic state,
whereby portions of the glass of slightly different colors are drawn out
into layers and interlaminated. The bands are rarely continuous for any
distance, being usually merely elongated lenticular streaks. The glassy
state is generally one of inferior density, and hence we find that
obsidian is lighter than the crystalline rocks of the same composition.
Obsidian is a good illustration of a non-essential color, for its
capacity and jet-black color are due entirely to impurities. In very
thin flakes it is transparent and white. It also forms a white powder
when crushed, _i.e._, it has a white streak.

Obsidian is often vesicular, from the expansion of the steam and other
gases which it contained when liquid. The most thoroughly vesicular
varieties are known as _pumice_ (specimen 48). The vesicular texture, by
rendering the rock impervious to light, conceals the impurities, and
thus we get a snow-white pumice from black obsidian. The vesicles are
frequently elongated, sometimes in a definite direction, though often
forming an irregular net-work of glassy fibres. Pumice is often light
enough to float on water, and it is transported thousands of miles by
the oceanic currents. It is employed in the arts, and good specimens can
be obtained at almost any drug-store.

Petrosilex and Felsite.—Sharply defined groups are unknown in lithology,
but all is gradation; and between rhyolite and trachyte, which are
always more or less distinctly crystalline, and obsidian, which is a
true glass and perfectly amorphous, there is no break. It is impossible
to draw a sharp line and say, Here the vitreous texture ends and the
crystalline begins; for the transition is not abrupt, but gradual. We
recognize, really, in these feldspathic rocks, an intermediate state,
which is neither crystalline nor colloid, but both; and this
lithologists have designated the _felsitic_ texture. Felsitic matter
cannot, even with the highest powers of the microscope, be resolved into
separate grains or particles; and it does not exhibit, except perhaps
very indistinctly, the phenomenon of double refraction. In other words,
it is not truly crystalline or stony, and yet it is just as clearly not
amorphous or glassy.

Feldspathic rocks exhibiting the felsitic texture in whole or in part
are known as _felsites_. Many high authorities hold that true felsites
are found only among the eruptive rocks; while others claim that they
are in part, or wholly, of sedimentary origin. The writer accepts the
former view. The felsites are in part acid lavas which have cooled too
slowly to form a true glass, like obsidian, and yet too quickly to
become truly crystalline, like rhyolite and trachyte. But they are also
in large part simply devitrified obsidian. Glass is an unstable form of
mineral matter; and every species of glass, including obsidian, tends
with the lapse of time to become crystalline or stony, the amorphous
changing to the felsitic structure. Thus, in many cases or usually, what
we now call felsites were originally true glassy obsidian. Being
perfectly intimate mixtures of the component minerals, the composition
of felsites can usually be determined with certainty only by means of
chemical analysis. By this means chiefly, it has been proved that there
are felsites agreeing in composition with both rhyolite and trachyte.
There is this general difference in composition, however, between these
crystalline rocks and the felsites; viz.: mica, hornblende, and augite
are generally wanting in the latter. From this it follows that the
felsites are, with unimportant exceptions, composed either of quartz and
feldspar or of feldspar alone.

The physical differences between the felsites of unlike composition are
not great; but they are sufficient to warrant the division of the
felsites into two species: a basic species, to which the term _felsites_
may properly be restricted; and an acidic species, for which
_petrosilex_ is a very appropriate name. According to this arrangement,
felsite is composed chiefly of orthoclase, and, as the table shows,
agrees in composition with trachyte; while petrosilex consists mainly of
orthoclase and quartz, agreeing in composition with rhyolite. We find
here nothing new in composition; but petrosilex and felsite are simply
the crystalline rocks which we have already studied, repeated under a
different texture.

The typical felsite or petrosilex is composed entirely of felsitic
matter, and is perfectly homogeneous, like flint or jasper, which it
closely resembles in hardness and other physical characteristics. As a
rule, however, the rock is not entirely homogeneous, but there is a
manifest tendency in the component minerals, and especially in the
feldspar, to separate out, usually in the form of crystals. In the
banded variety (specimen 42) the rock is built up of thin layers, which
are often alternately quartzose and feldspathic. There is not a perfect
separation of the minerals; but that the quartz is chiefly in the dark
layers, and the feldspar in the light, is shown by the way in which the
layers are affected by the weather.

One of the most common varieties is where a portion, frequently a large
portion, of the feldspar comes out in the form of distinct, separate
crystals, producing a porphyritic texture. Specimens 5, 6, and 7 are
examples of porphyritic felsite; and after examining these we can no
longer doubt that feldspar is an important constituent of the rock.
Petrosilex and felsite are more generally porphyritic than any other
rocks; and they are commonly called porphyry. It is better, however,
since almost any rock may be porphyritic, and since this texture cannot
be correlated with any particular composition, not to use porphyry as a
rock-name, but simply as the name of a very important rock-texture. The
banded and porphyritic textures are about equally characteristic of
petrosilex and felsite. In petrosilex, quartz, as well as feldspar, is
sometimes porphyritically developed, forming the variety known as
quartz-porphyry. There is no limit to the proportion of the quartz and
feldspar which may crystallize out in this way, and thus we find a
perfectly gradual passage from normal petrosilex or felsite to
thoroughly crystalline granite and syenite.

Andesite.—This rock has nearly the texture of rhyolite and trachyte, but
is darker and heavier, and corresponds in composition to diorite,
consisting of plagioclase and hornblende, with usually more or less
sanidine, quartz, augite, biotite, and magnetite.

Basalt.—The rock bearing this familiar name represents diabase among the
dike rocks. It is the most basic of the volcanic rocks, and consists of
the more basic varieties of plagioclase, especially labradorite, with
augite, magnetite, and titanic iron. Olivine is a very common and
characteristic constituent, and the plagioclase is often replaced in
part by leucite and nephelite. The basalts are usually black, and of
high specific gravity; and vary in texture from compact to coarsely
crystalline. The contraction due to cooling frequently results in the
development of a columnar structure of remarkable regularity, the
columns being normally hexagonal and standing perpendicularly to the
cooling surfaces of the mass. This structure occurs in other eruptive
rocks, but is most characteristic of basalt.

Tachylite.—Tachylite is a highly basic volcanic glass, standing in the
same relation to basalt and andesite that obsidian does to trachyte and
rhyolite. It is much heavier than obsidian, and is perfectly black and
opaque, except in the finest fibres. It is a comparatively rare rock,
for the reason that basalt and andesite crystallize more readily than
the acidic rocks on passing from the liquid to the solid state. On the
surface of the basic lava, however, where it is in contact with the air,
and congeals almost instantly, a film of glass is formed; but this may
not be more than a small fraction of an inch in thickness. Like
obsidian, tachylite is often vesicular; but the vesicular basic rocks,
as well as the solid, are usually stony. They occur in vast abundance in
many volcanic regions, and may be considered the typical lava (specimen
49).

In the more ancient lavas, the vesicles are frequently filled by various
minerals—chlorite, epidote, quartz, calcite, etc.—deposited by
infiltrating waters, and derived in most cases from the decomposition of
the original constituents of the rock. Thus the vesicular is changed to
the amygdaloidal texture, and the lava becomes an amygdaloid (specimen
50). The amygdaloidal texture is common in the basic lavas and rare in
pumice, simply because the former are more readily decomposed and
contain a greater variety of bases from which secondary minerals can be
formed.

Porphyrite and Melaphyr.—These two rocks hold essentially the same
relation as regards origin and structure to the basic lavas that
petrosilex and felsite do to the acidic lavas. Porphyrite agrees in
composition with andesite, and melaphyr with basalt. They are usually
dark-colored rocks having a compact or felsitic texture. Porphyrite is,
as the name implies, very commonly porphyritic; while melaphyr is often
vesicular or brecciated, exhibiting all the structural features of
tachylite and basalt, and being in its older forms very generally
amygdaloidal.

Volcanic Tuff and Agglomerate.—Besides the crystalline, glassy, and
felsitic lavas, already described, and due chiefly to the rate of
cooling of the liquid rock, we may recognize a fourth class to include
the very abundant lavas which, during explosive eruptions, are ejected
in the solid state, being violently blown out of the crater in the form
of dust and fragments. Falling on the slopes of the volcano or over the
surrounding country, as in the case of the buried city of Pompeii, the
fragmental lavas remain largely unstratified. But when, as frequently
happens, they fall into the sea, they are assorted by the waves and
currents and arranged in layers after the manner of ordinary sediments,
with which they are often more or less mixed. Before they become
consolidated the finer fragmental lava, of whatever composition, is
called volcanic dust, and the coarser lapilli or volcanic sand; while
the consolidated materials are known as tuff and agglomerate
respectively.

------------------------------------------------------------------------




                        SUPPLEMENT TO LITHOLOGY.

                             [Illustration]




                              VEIN ROCKS.


All rocks are not embraced in the sedimentary and eruptive divisions,
but there is a third grand division, which, although rarely mentioned or
recognized in the more comprehensive works on geology, it is deemed best
not to leave entirely unnoticed here. These are the _vein_ rocks. They
present an immense number of varieties, and yet, taken altogether, form
but a small fraction of the earth’s crust. They are, however, the great
repositories of the precious and other metals, and hence are objects of
far greater interest to the miner and practical man than the eruptive
rocks, or, in some parts of the world, even than the sedimentary rocks.

The vein rocks, like the eruptive rocks, occupy fissures in the earth’s
crust intersecting the stratified formations; but the fissures filled
with vein rocks are called veins, and not dikes. We will first notice
the mode of formation of a typical vein, and then examine its contents.
Geologists are agreed that water penetrates to a very great depth in the
earth’s crust. All minerals are more or less soluble in water; and we
may consider the water circulating through the rocks, especially at
considerable depths, as, in most cases, a saturated solution of the
various minerals of which they are composed. Very slight changes in the
conditions will cause saturated solutions to deposit part of their
mineral load. The water at great depths has a high temperature, and is
subjected to an enormous pressure; and both of these circumstances favor
solution. Suppose, now, that these hot subterranean waters enter a
fissure in the crust and flow upwards, perhaps issuing on the surface as
a warm mineral spring; as they approach the surface, the temperature and
pressure, and consequently their solvent power, are diminished; and a
portion of the dissolved minerals must be deposited on the walls of the
fissure, which thus becomes narrower, and in the course of time is
gradually filled up. The vein is then complete; and the mineral waters
are forced to seek a new outlet.

Veins have the same general forms as dikes, since the fissures are
formed in the same way for both; but the vein is of slow growth, and may
require ages for its completion, while the dike is formed in an hour or
a day. It is now generally believed that water is an important agent in
the formation of eruptive rocks; since they all contain water at the
time of their eruption; and since it has been demonstrated that, while
ordinary rocks require a temperature of 2000° to 3000° for their fusion
in the absence of water, they are liquified at temperatures below 1000°
in the presence of water. In other words, common rocks are very
infusible and insoluble bodies, and heat and water acting independently
have little effect upon them; but when fire and water are combined in
what is now known as aqueo-igneous fusion, they prove very efficient
agents of liquefaction.

If we adopt these views, then it can be shown that, in origin, veins and
dikes differ in degree only, and are not fundamentally unlike; and the
formation and relations of the three great classes of rocks may be
summarized as follows:—

The ocean and atmosphere, operating on the earth’s surface, have worked
over and stratified the crust, until the sedimentary rocks have now an
average thickness variously estimated at from ten to thirty or forty
miles. This entire thickness of stratified rocks, and a considerable
depth of the underlying unstratified crust, must be saturated with
water; and all but the more superficial portions of this water-soaked
crust must be very hot, the temperature increasing steadily downwards
from the surface. Both eruptive and vein rocks originate in this highly
heated, hydrated crust. Eruptive rocks are formed when the heat, aided
by more or less water, softens the rocks, either stratified or
unstratified, by aqueo-igneous fusion, and the plastic materials are
forced up through fissures to or toward the surface. Vein rocks are
formed when the water, aided by more or less heat, dissolves the rocks,
either stratified or unstratified, by what may be called igneo-aqueous
solution, and subsequently deposits the mineral matter in, _i.e._, on
the walls of, fissures leading up to or toward the surface. In the case
of the dike rocks, heat is the chief agent, and water merely an
auxiliary; while with the vein rocks it is just the reverse. But between
the two it is probably impossible to draw any sharp line.

The water circulating through the crust, and saturated with its various
mineral constituents, has been called the “juice” of the crust; and
veins are formed by the concentration of this earth-juice in fissures.
One of the most important characteristics of the vein rocks, as a class,
is the immense variety which they present; for nearly every known
mineral is embraced among their constituents; and these are combined in
all possible ways and proportions, so that the number of combinations is
almost endless. The solvent power of the subterranean waters varies for
different minerals; and appears often to be greatest for the rarer
species. In other words, there is a sort of selective action, whereby
many minerals which exist in stratified and eruptive rocks, so thinly
diffused as to entirely escape the most refined observation, are
concentrated in veins in masses of sensible size; and our lists of known
minerals and chemical elements are undoubtedly much longer than they
would be if these wonderful storehouses of fine minerals which we call
veins had never been explored. As a rule, the minerals in veins form
larger and more perfect crystals than we find in either of the other
great classes of rocks. This is simply because the conditions are more
favorable for crystallization in veins than in dikes or sedimentary
strata. In both dike and stratified rocks, the growing crystals are
surrounded on all sides by solid or semi-solid matter; and, being thus
hampered, it is simply impossible that they should become either large
or perfect. In the vein, on the other hand, there are usually no such
obstacles to be overcome; but the crystals, starting from the walls of
the fissure, grow toward its centre, their growing ends projecting into
a free space, where they have freedom to develop their normal forms and
to attain a size limited only, in many cases, by the breadth of the
fissure. With, possibly, some rare exceptions, all the large and perfect
crystals of quartz, feldspar, mica, beryl, apatite, fluorite, and of
minerals generally, which we see in mineralogical cabinets, have
originated in veins. Those fissures which become the seats of mineral
veins are really Nature’s laboratories for the production of rare and
beautiful mineral specimens; and hence the vein rocks are the chief
resort of the mineralogist, to whom they are of far greater interest
than all the eruptive and stratified rocks combined.

The leading characteristics, then, of the vein rocks may be summarized
as follows: (1) They contain nearly all known minerals, including many
rare species and elements which are unknown outside of this class of
rocks. (2) These mineral constituents, occurring singly and combined,
give rise to a number of varieties of rocks so vast as to baffle
detailed description. (3) They exceed all other rocks in the coarseness
of their crystallization, and in the perfection and beauty of the single
crystals which they afford.




                               PETROLOGY.


In lithology we investigate the nature of the materials composing the
earth’s crust—the various minerals and aggregates of minerals, or rocks;
while in petrology we consider the forms and modes of arrangement of the
rock-masses,—in other words, the architecture of the earth.

Petrology is the complement of lithology, and in many respects it is the
most fascinating division of geology, since in no other direction in
this science are we brought constantly into such intimate relations with
the beautiful and sublime in nature. The structures of rocks are the
basis of nearly all natural scenery; for what we call scenery is usually
merely the external expression, as developed by the powerful but
delicate sculpture of the agents of erosion—rain and frost, rivers and
glaciers, etc.—of the geological structure of the country. And to the
practised eye of the geologist, a fine landscape is not simply a
pleasantly or grandly diversified _surface_, but it has _depth_; for he
reads in the superficial lineaments the structure of the rocks out of
which they are carved.

But, while the magnitude of the phenomena adds greatly to the charm of
the study, it also increases the difficulties and taxes the ingenuity of
the teacher whose work must be done indoors. According to our ideal
method, natural science ought to be taught with natural specimens; and
yet here our main reliance must be upon pictures and diagrams.

Nature, however, has not been wholly unmindful of our needs; for she has
worked often upon a very small as well as a very large scale; many of
the grandest phenomena being repeated in miniature. Thus we observe
rock-folds or arches miles in breadth and forming mountain masses, and
of all sizes from that down to the minutest wrinkle. So with veins,
faults, etc. And the wonderful thing is that these small examples, which
may be brought into the class-room, are usually, except in size, exactly
like the large. Now the aim of every teacher in this department should
be to secure a collection of these natural models. It is not an easy
thing to do, except one has plenty of time; for they can rarely be
purchased of dealers, but must usually come as the choicest fruit of
repeated excursions to the natural ledges and quarries, the seashore and
the mountains. But for the difficulty of getting the specimens there is
some compensation, since it may be truly said that for the _collector_
specimens obtained in this way have an interest, a value, and a power of
instruction beyond what they would otherwise possess.


                    _Classification of Structures._

The structures of rock divide, at the outset, into two classes:—(1) the
_original structures_, or those produced at the same time and by the
same forces as the rocks themselves, and which are, therefore, peculiar
to the class of rocks in which they occur (_e.g._ stratification,
ripple-marks, fossils, etc.); and (2) the _subsequent structures_, or
those developed in rocks subsequently to their formation, and by forces
that act more or less uniformly upon all classes of rocks, and which
are, therefore, in a large degree, common to all kinds of rocks (_e.g._
folds, faults, joints, etc.).

The original structures are conveniently and naturally classified in
accordance with the three great classes of rocks: (1) stratified rocks,
(2) eruptive rocks, and (3) vein rocks; while the subsequent structures,
not being peculiar to particular classes of rocks, are properly divided
into those produced by (1) the subterranean or igneous agencies, and (2)
the superficial or aqueous agencies.

    [Illustration: Fig. 1.—Section through sediment deposited by
        rain in a roadside pool: _a._ surface of roadway; _b._
        layer of small pebbles and coarse sand; _c._ fine sand
        passing into _d_; _d._ the finest sand and mud.]


                Original Structures of Stratified Rocks.

STRATIFICATION.—All rocks formed by strewing materials in water, and
their deposition in successive, parallel, horizontal layers, are
_stratified_; and this structure is their _stratification_. It is the
most important of all rock structures; and there is no kind of structure
the origin of which is more fully or certainly known. The deposition of
sediment in carefully assorted horizontal layers is readily brought
within the comprehension of children by simple experiments with sand and
clay in water; and still better by the examination of the deposits
formed in roadside pools during heavy rains (Fig. 1), and by digging
into beaches and sandbars, which every child will recognize as formed of
materials arranged by water. Great stress should be laid upon the fact
that a lake like Erie or Champlain is simply a large pool with several
more or less turbid streams flowing into it, while the single stream
flowing out is clear, the sediment having evidently been deposited in
the lake; and that every lake is, like the roadside pool, being
gradually filled up with sedimentary or stratified rocks. But the ocean
is a still larger pool, receiving mud and sand from many streams; and
since we know that nothing escapes from the ocean but invisible vapor,
it is plain that the mud and sand and all other kinds of sediment
carried into the ocean must be deposited on its floor, and chiefly, as
we have seen, on that part nearest the land. The consolidation of
beaches, bars, and mud-flats is all that is necessary to convert them
into stratified formations of conglomerate sandstone and slate.

Let us notice now, more particularly, the causes of visible
stratification. As we can easily prove by an experiment with clay in a
bottle of water, if the same kind of material is deposited continuously
there will be no visible stratification in the deposit. It will be as
truly stratified as any formation, but not visibly so; because there is
nothing in the nature of the material or the way in which it is laid
down to bring out distinct lines of stratification. Continuous and
uniform deposition obtains very frequently in nature, but rarely
continues long enough to permit the formation of thick beds or strata.
Hence, while the stratification is almost always visible on the large
surfaces of sandstone, slate, etc., exposed in quarries and railway
cuttings, and may usually be seen in the quarried blocks, it is often
not apparent in hand specimens, which may represent a single homogeneous
layer. There is one important exception, and that is where the
particles, although of the same kind, are flat or elongated. Pebbles of
these forms are common on many beaches; and since they are necessarily
arranged horizontally by the action of the water, they will, by their
parallelism, make the stratification of the pudding-stone visible. The
same result is accomplished still more distinctly by the mica scales,
etc., in sandstone and slates, the leaves and flattened stems of
vegetation in bituminous coal, and the flat shells in limestone.

In all other cases, visible stratification implies some change in the
conditions; either the deposition was interrupted, or different kinds of
material were deposited at different times. The first cause produces
planes of easy splitting, or fissility, especially in fine-grained
rocks, like shale. This shaly structure or lamination-cleavage may be
due, in some cases, to pressure, but it is commonly understood to mean
that each thin layer of clay became partially consolidated before the
next one was deposited upon it, so that the two could not perfectly
cohere. Parallel planes of easy splitting are, however, by themselves,
of little value as indications of stratification, since the
lamination-cleavage is not easily distinguished from slaty-cleavage
(roofing slate) and parallel jointing, structures developed subsequently
to the deposition of sediments and quite independent of the
stratification. The second cause, or variations in the kind of sediment,
gives alternating layers differing in color, texture, or composition, as
is seen frequently in sandstone, slate, gneiss, etc.; and of all the
indications of stratification these are the most important and reliable.

    [Illustration: Fig. 2.—Section showing strata and laminæ:
        _a._ conglomerate; _b._ sandstone; _c._ shale; _d._
        limestone.]

A layer composed throughout of essentially the same kind of rock, as
conglomerate or sandstone, and showing no marked planes of division, is
usually regarded as one _bed_ or _stratum_, although it may vary
considerably in texture or color; while the thinner portions composing
the stratum and differing slightly in color, texture, and composition,
and the thin sheets into which shaly rocks split, are the _laminæ_ or
_leaves_. In Fig. 2 the strata are designated by letters, and the fine
lines and rows of dots show the constituent laminæ, while the whole
section may be regarded as a small part of a great geological formation.
The geological record is written chiefly in the sedimentary rocks; and
the formations, strata, and laminæ may be regarded as the volumes,
chapters, and pages in the history of the earth. Now every feature of a
rock, lithological or petrological, finds its highest interest in the
light which it throws upon the history of the rock, _i.e._, upon the
conditions of its formation. Observe what the section in Fig. 2 teaches
concerning the geological history of that locality; premising that any
chapter of geological history written in the stratified rocks should be
read from the bottom upwards, since the lowest strata must have been
formed first and the highest last. The lowest stratum exposed is
conglomerate, indicating a shingle beach swept by strong currents which
carried away the finer material. Upwards, the conglomerate becomes finer
and shades off into sandstone, and finally into shale, showing that the
water has become gradually deeper and more tranquil, the shore having,
in consequence of the subsidence, advanced toward the land. The next two
strata show that this movement is probably reversed; at any rate, the
currents become stronger again, and the shale passes gradually into
sandstone and conglomerate. The beach condition prevails now for a long
time, and thick beds of sand and gravel are formed. The sea then deepens
again, and we observe a third passage from coarse to fine sediment. This
locality is now remote from the shore, the gentle currents bringing only
the finest mud, which slowly builds up the thick bed of shale, in the
upper part of which shells are abundant, indicating that the deposition
of mechanical sediment has almost ceased, and that the shale is changing
to limestone. The purity of the limestone, and the crinoids and other
marine organisms which it contains, prove that this has now become the
deep, clear sea; and this condition is maintained for a long period, for
the limestone is very thick, and this rock is formed with extreme
slowness.

The most important point to be gained here is that every line of
stratification and every change in the character of the sediments is due
to some change of corresponding magnitude in the conditions under which
the rock was formed. The slight and local changes in the conditions
occur frequently and mark off the individual laminæ and strata, while
the more important and wide-spread changes determine the boundaries of
the groups of strata and the formations.

Strata are subject to constant lateral changes in texture and
composition, _i.e._, a bed or formation rarely holds the same
lithological characteristics over an extended area. There are some
striking exceptions, especially among the finer-grained rocks, like
slate, limestone, and coal, which have been deposited under uniform
conditions over wide areas. It is the general rule, however,
particularly with the coarse-grained rocks, which have been deposited in
shallow water near the land, that the same continuous stratum undergoes
great changes in thickness and lithological character when followed
horizontally. A stratum of conglomerate becomes finer grained and
gradually changes into sandstone, which shades off imperceptibly into
slate, and slate into limestone, etc. Where the stratum is conglomerate,
its thickness will usually be much greater and more variable than where
it is composed of the finer sediments. The rapidity of these changes in
certain cases is well shown by the parallel sections in Fig. 3. These
represent precisely the same beds, as the connecting lines indicate, at
points only twenty feet apart.

    [Illustration: Fig. 3.—Parallel sections showing rapid
        lateral changes in strata: _c._ clay; _s._ sand; _ss._
        sandstone; _l._ lignite; _f._ fireclay.]

When we glance at the conditions under which stratified rocks are now
being formed, it is plain that all strata must terminate at the margin
of the sea in which they were deposited, and in the marginal portions of
that sea, especially, must exhibit frequent and rapid changes in
composition, etc. The sediments forming the surface of the sea-bottom at
the present time may be regarded as belonging to one continuous stratum;
and it is instructive to examine a chart of any part of our coast, such
as Massachusetts Bay, on which the nature of the bottom is indicated for
each sounding, and observe the distribution of the different kinds of
sediment. On an irregular coast like this, especially, the gravel, sand,
and mud of different colors and textures, and the different kinds of
shelly bottom, form a patchwork, the patches being, for the most part,
of limited extent and shading off gradually into each other.

On a more regular coast, like that of New Jersey, the sediments are
distributed with corresponding uniformity, the changes are less frequent
and more gradual, and we have here a better chance to observe the normal
arrangement of the sediments along a line from the shore
seawards—gravel, sand, mud, and shells. On the beach we find the shingle
and coarse pebbles, shading off rapidly into fine pebbles and sand. The
zone or belt of sandy bottom may vary in width from a mile or two to
twenty miles or more, becoming gradually finer and changing into clay or
mud, which covers, usually, a much broader zone, sometimes extending
into the deeper parts of the sea, but gradually giving way to calcareous
sediments. Hence we may say that the finer the sediment the greater the
area over which it is spread; but, on the other hand, the coarser the
sediment the more rapidly it increases in thickness. In other words, the
horizontal extent of a formation deposited in any given period of time
is inversely, and the vertical extent or thickness is directly,
proportional to the size of the particles.

Observations made in deep wells and mines, and where, by upturning and
erosion, the edges of the strata are exposed on the surface, show that
the vertical order of the different kinds of sedimentary rocks in the
earth’s crust is extremely variable. But when we take a general view of
a great formation, it is often apparent that it consists chiefly of
coarse-grained rocks in the lower part and fine-grained rocks in the
upper part. This is, in general, a necessary consequence of the fact
that a great thickness of sediments can only be formed on a subsiding
sea-floor. Such a formation must consist chiefly of shore deposits, and
be deposited near the shore where the sea is shallow. Hence, 10,000 feet
of sediments implies nearly that amount of subsidence. In consequence,
the shore line and the several zones of sediment advance towards the
land; and sand is deposited where gravel was at first, and as the
subsidence continues, both clay and limestone are finally deposited over
the original beach. When the sea-floor rises, the order of the sediments
is reversed; and it will be observed that in consequence of the advance
and retreat of the shore-line, the formations grow edgewise to a
considerable extent.

    [Illustration: Fig. 4.—Overlap and unconformability.]

OVERLAP AND INTERPOSITION OF STRATA.—Another consequence of the constant
oscillation of the shoreline is that successive deposits in the same sea
will often cover different and unequal areas. When, in consequence of
subsidence, one formation extends beyond and covers the edge of another,
as shown in Fig. 4, we have the phenomenon described as overlap.
Interposition is similar, being the case where a formation (Fig. 5,
_c._) does not, in certain directions, cover so wide an area as the
strata (_b. d._) above and below it, which are thus sometimes found in
contact, although normally separated by the entire thickness of the
intermediate and, seemingly, interposed stratum.

    [Illustration: Fig. 5.—Interposition of strata.]

UNCONFORMABILITY.—We have already seen that the rocks on the land are
being constantly worn away by the agents of erosion; and it is also a
matter of common observation that the strata thus exposed are often not
horizontal, but highly inclined, having been greatly disturbed and
crumpled during their elevation. Now, when such a land-surface subsides
to form the sea-bottom, and new strata are spread horizontally over it,
they will lie across the upturned and eroded edges of the older rocks,
and fill the hollows worn out of the latter, as shown in Fig. 6; and the
new formation is then said to rest unconformably upon the older. Two
strata or formations are unconformable when the older has suffered
erosion (Fig. 6), or both disturbance and erosion (Fig. 4) before the
deposition of the newer.

    [Illustration: Fig. 6.—Unconformability.]

When strata are conformable, the deposition may be presumed to have been
nearly or quite continuous; but unconformability clearly proves a
prolonged interruption of the deposition during which the elevation,
erosion, and subsidence of the sea-bottom took place. The section in
Fig. 7 shows a second unconformability, proving that the sea-bottom has
here been lifted three times to form dry land. An unconformability may
sometimes be clearly established when the actual contact of the two
formations cannot be seen, as where the new formation is a conglomerate
containing fragments of the older.

IRREGULARITIES OF STRATIFICATION.—These are especially noticeable in
sandstone and conglomerate, which have been deposited chiefly by strong,
local, and variable currents; the kind and quantity of sediment, of
course, varying with the strength and direction of the current. Two
kinds of irregularity only may be specially noticed here: (1)
contemporaneous erosion and deposit, where, in consequence of a change
in the currents, fine material recently deposited is partially swept
away and its place taken by coarser sediments; and (2) oblique
lamination, or current-bedding, where the strata are horizontal as
usual, but the component laminæ are inclined at various angles. This
structure is characteristic of sediments swept along by strong currents,
especially when deposited in shallow basins or depressions.

    [Illustration: Fig. 7.—Double Unconformability: _q._
        quartzite; _s._ sandstone; _d._ drift.]

RIPPLE-MARKS.—All who have been on a beach or sand-bar must have noticed
the lines of wavy ridges and hollows, or ripples, on the surface of the
sand. These are sand-waves, produced by water moving over the sand, or
by air moving over dry sand, as ordinary waves are formed by air moving
over water. Each tide usually effaces the ripple-marks made by its
predecessor and leaves a new series, to be obliterated by the next tide.
But where sediment is constantly accumulating, a rippled surface may be
gently overspread by a new layer, and thus preserved. Other series of
ripples may, in like manner, be formed and preserved in overlying
layers; and when the beach becomes a firm sandstone, a section of it
will show the rippled surfaces almost as distinctly as when they were
first formed (Fig. 8). Ripple-marks are most perfect in fine sand. They
are not formed in gravel, because it is too coarse; nor in clay, because
it is too tenacious. They are usually limited to shallow water; and are
always regarded as proving that the rocks in which they occur are
shallow-water or beach deposits. They are normally at right angles to
the current that produces them, and where this changes with the
direction of the wind, cross-ripples and other irregularities are
introduced. Ripple-marks are also usually parallel with the beach, and
when they are found in the rocks they give us the direction, as well as
the position, of the ancient shore-line.

Again, the friction of the water pushes the sand-grains along, rolling
them up on one side of the ripple and letting them fall down on the
other. Hence ripples, formed by a current are always moving and are
unsymmetrical on the cross-section, presenting a long, gentle slope
toward the current, and a short, steep slope away from it, the arrow in
the figure indicating the direction of the current, or of the sea in the
case of a beach. And we may thus learn from the fossil ripples, in some
cases, not only the position and direction of the ancient shore, but
also on which side the land lay, and on which side the sea. When the
water is in a state of oscillation, without any distinct current, more
symmetrical ripples are produced.

    [Illustration: Fig. 8.—Ripple-marks in sandstone.]

RILL-MARKS, RAIN-PRINTS, AND SUN-CRACKS.—“One of the most fascinating
parts of the work of a field-geologist consists in tracing the shores of
former seas and lakes, and thus reconstructing the geography of
successive geological periods.” His conclusions, as we have already
seen, are based largely upon the nature of the sediments; but still more
convincing is the evidence afforded by those superficial features of the
strata, which, like ripple-marks, seem, by themselves, quite
insignificant. And among these he lays special emphasis upon those which
show that during their deposition strata have at intervals been laid
bare to sun and air.

During ebb tide water which has been left at the upper edge of the beach
runs down across the beach in small rills, which excavate miniature
channels; and when these are preserved in the hard rocks, they prove
that the latter are beach deposits, and, like the ripple-marks, show the
direction of the old shore.

If a heavy shower of rain falls on a muddy beach or flat, the sediment
deposited by the returning tide may cover, without obliterating, the
small but characteristic impressions of the individual drops; and these
markings are frequently found well preserved in the hardest slates and
sandstones, testifying unequivocally to the conditions under which the
rocks were formed. In some cases the rain-prints are found to be ridged
up on one side only, in such a manner as to indicate that the drops as
they fell were driven aslant by the wind. The prominent side of the
marking, therefore, indicates the side towards which the wind blew.

Muddy sediments, especially in lakes and rivers, are often exposed to
the air and sun during periods of drouth, and as they gradually dry up,
polygonal cracks are formed. The sediment of the next layer will fill
these sun-cracks; and when, as often happens, it is slightly different
from the dessicated layer, they may still be traced. Sun-cracks
preserved in this way are very characteristic of argillaceous rocks,
and, of course, prove that in early times, as at the present day,
sediments of this class were exposed by the temporary retreat of the
water. The foot-prints or trails of land-animals are often, as in the
sandstones and shales of the Connecticut Valley, associated with, and of
course strongly corroborate, all these other evidences of shore
deposits. From the foot-prints preserved in the rocks we pass naturally
to the consideration of the fossil remains of plants and animals found
entombed in the strata.

FOSSILS.—Although fossils find their highest interest in the light which
they throw upon the succession of life on the globe, they may also be
properly regarded as structural features of stratified rocks; and any
one who has seen the dead shells, crabs, fishes, etc., on the beach will
readily understand how fossils get into the rocks. It is not our
province here to study the structure of the fossils themselves, for that
would involve us in a course in paleontology, a task belonging to the
biologist rather than the geologist; but we will merely observe the
three principal degrees in the preservation of fossils:—

1. _Original composition not completely changed._—Extinct elephants have
been found frozen in the river-bluffs of Siberia so perfectly preserved
that dogs and wolves ate their flesh. The bodies of animals are also
found well preserved in peat-bogs. All coal is simply fossil vegetation
retaining in a large degree the original composition; and the same is
true of ferns, etc., preserved as black impressions in the rocks. All
bones and shells consist of mineral matter which makes them hard, and
animal matter which makes them tough and strong. In very many cases,
especially in the newer formations, the animal matter is still
partially, and the mineral matter almost wholly, intact.

2. _Original composition completely changed, but form and structure
preserved._—All kinds of fossils are commonly called petrifactions, but
only those preserved in this second way are truly petrified, _i.e._,
turned to stone. “Petrified wood is the best illustration, and in a good
specimen not only the external form of the wood, not only its general
structure—bark, wood, radiating silver-grain, and concentric rings of
growth—are discernible, but even the microscopic cellular structure of
the wood, and the exquisite sculpturing of the cell-walls, are perfectly
preserved, so that the kind of wood may often be determined by the
microscope with the utmost certainty. Yet not one particle of the
organic matter of the wood remains. It has been entirely replaced by
mineral matter; usually by some form of silica. The same is true of the
shells and bones of animals.”—LE CONTE.

3. _Original composition and structure both obliterated, and form alone
preserved._—This occurs most commonly with shells, although fossil trees
are also often good illustrations. The general result is accomplished in
several ways: (_a_) The shell after being buried in the sediment may be
removed by solution, leaving a _mould_ of its external form, (_b_) This
mould may subsequently be filled by the infiltration of finer sediment,
forming a _cast_ of the exterior of the shell. (_c_) The shell, before
its solution, may have been filled with mud; and if the shell itself is
then dissolved, we have a cast of its interior in a mould of its
exterior.

TIME REQUIRED FOR THE FORMATION OF STRATIFIED ROCKS.—Many attempts have
been made to determine the time required for the deposition of any given
thickness of stratified rocks. Of course, only roughly approximate
results can be hoped for in most cases; but these are at least
sufficient to make it certain that geological time is very long. The
average relative rate of growth of different kinds of sediment is,
however, less open to doubt, for we have already seen that coarse
sediments like gravel and sand accumulate much more rapidly than finer
sediments like clay and limestone; and we are sometimes able to compare
these two classes of rocks on a very large scale.

Thus, during what is known as the Paleozoic era, a sea extended from the
Blue Ridge to the Rocky Mountains. Along the eastern margin of this sea,
where the Alleghany Mountains now stand, sediments—chiefly conglomerate
and sandstone, with some slate and less limestone—accumulated to a
thickness of nearly 40,000 feet. Toward the west, away from the old
shore-line, the coarse sediments gradually die out, and the formations
become finer and thinner. In western Ohio and Indiana, slate and
limestone predominate; while in the central part of the ancient sea, in
Illinois and Missouri, the paleozoic sediments are almost wholly
limestones, and have a thickness of only 4000 to 5000 feet. In other
words, while one foot of limestone was forming in the Mississippi
Valley, eight to ten feet of coarser sediments were deposited in
Pennsylvania.

The best estimates show that coral-reefs rise—_i.e._, limestones are
formed on them—at the rate of about one foot in two hundred years. But
coral limestones grow much more rapidly than limestones in general.
Sandstones sometimes accumulate so rapidly that trees are buried before
they have time to decay and fall (Fig. 9). Such a buried forest, like a
coal-bed, represents a land surface, and proves a subsidence of the
land; and in some cases, as indicated by the section, repeated
oscillations of the crust may be proved in this way.

The mud deposited by the annual overflow of the Nile is forty feet thick
near the ancient city of Memphis; and the pedestal of the statue of
Rameses II., believed to have been erected B.C. 1361, is buried to a
depth of nine feet, four inches, indicating that 13,500 years have
elapsed since the Nile began to spread its mud over the sands of the
desert.

    [Illustration: Fig. 9.—Erect fossil trees.]

But the greatest difficulty in estimating the time required for the
formation of any series of strata arises from the fact that we cannot
usually even guess at the length of the periods when the deposition has
been partially or wholly interrupted. Now and then, however, we find
evidence that these periods may be very long. A layer of fossil shells
in sandstone or slate proves an interruption of mechanical deposition.
Beds of coal, fossil forests, and other indications of land surfaces are
still more conclusive. The interposition of strata (Fig. 5) proves a
prolonged interruption of deposition over the area not covered by the
interposed bed. But the most important of all evidence is that afforded
by unconformability; and the length of the lost interval between the two
formations is measured approximately by the erosion of the older.


                 Original Structures of Eruptive Rocks.

The structures of this class are divisible into those pertaining to the
volcanic rocks and those pertaining to the fissure or dike rocks. But
since volcanoes are rare in this part of the world, while dikes are well
developed in many sections of our country, it seems best to give our
attention chiefly to the latter.

    [Illustration: Fig. 10.—Typical dikes.]

    [Illustration: Fig. 11.—Section of a granite mass.]

The term _dike_ is a general name for all masses of eruptive rocks that
have cooled and solidified in fissures or cavities in the earth’s crust.
But the name is commonly restricted to the more regular, wall-like
masses (Fig. 10), those having extremely irregular outlines, like most
masses of granite (Fig. 11), being known simply as _eruptive masses_.
The propriety of this distinction is apparent when we consider the
origin of _dike_ as a geological term. It was first used in this sense
in southern Scotland, where almost any kind of a wall or barrier is
called a dike. The dikes traverse the different stratified formations
like gigantic walls, which are often encountered by the coal-miners, and
on the surface are frequently left in relief by the erosion of the
softer enclosing rock, so that in the west of Scotland, especially, they
are actually made use of for enclosures. In other cases the dike has
decayed faster than the enclosing rock, and its position is marked by a
ditch-like depression. The narrow, straight, and perpendicular clefts or
chasms observed on many coasts are usually due to the removal of the
wall-like dikes by the action of the waves. Dikes are sometimes mere
plates of rock, traceable for a few yards only; and they range in size
from that up to those a hundred feet or more in width, and traceable for
scores of miles across the country, their outcrops forming prominent
ridges. The sides of dikes are often as parallel and straight of those
of built walls, the resemblance to human workmanship being heightened by
the numerous joints which, intersecting each other along the face of a
dike, remind us of well-fitted masonry.

FORMS OF DIKES.—A dike is essentially a casting. Melted rock is forced
up from the heated interior into a cavity or crack in the earth’s crust,
cools and solidifies there, and, like a metallic casting, assumes the
form of the fissure or mould. In other words, the form of the dike is
exactly that of the fissure into which the lava was injected. Now the
forms of fissures depend partly upon the nature of the force that
produces them, but very largely upon the structure—and especially the
joint-structure—of the enclosing rocks. Nearly all rocks are traversed
by planes of division or cracks called joints, which usually run in
several directions, dividing the rock into blocks. And it is probable
that dike-fissures are most commonly produced, not by breaking the rocks
anew, but by widening or opening the pre-existing joint-cracks. Hence
the straight and regular jointing of slate, limestone and most
sedimentary rocks is accompanied by wall-like dikes—the typical dikes
(Fig. 10); while the more irregular jointing of granite and other
massive rocks gives rise to sinuous, branching, variable dikes. The
general dependence of dikes upon the joint-structure of the rocks is
proved by the facts that dikes, like joints, are normally vertical or
highly inclined, and that they are usually parallel with the principal
systems of joints in the same district. The wall-like dikes also give
off branches, but usually in a regular manner, as shown in Fig. 12.

    [Illustration: Fig. 12.—Dike with regular branches.]

STRUCTURE OF DIKES.—The rock traversed by a dike is called the _country_
or _wall_ rock. Fragments of this are often torn off by the igneous
material, and become enclosed in the latter. Such enclosed fragments may
sometimes form the main part of the dike, which then, since they are
necessarily angular, often assumes the aspect of a breccia. This is the
only important exception to the rule that dikes are homogeneous in
composition; _i.e._, in the same dike we can usually find—from end to
end, from side to side, and probably from top to bottom—no essential
difference in composition. But there is often a marked contrast in
_texture_ between different parts of a dike, and especially between the
sides and central portion. The liquid rock loses heat most rapidly where
it is in contact with the cold walls of the fissure, and solidifies
before it has time to crystallize, remaining compact and sometimes even
glassy; while in the middle of the dike, unless it is very narrow, it
cools so slowly as to develop a distinctly crystalline texture. There is
no abrupt change in texture, but a gradual passage from the compact
border to the coarsely crystalline or porphyritic middle portion. It is
obvious that a similar gradation in texture must exist between the top
and bottom of a dike.

CONTACT PHENOMENA.—Under this head are grouped the interesting and
important phenomena observable along the contact between the dike and
wall-rock. These throw light upon the conditions of formation of dikes,
and are often depended upon to show whether a rock mass is a dike or
not. The student will observe here:—

1. The detailed form of the contact. It may be straight and simple, or
exceedingly irregular, the dike penetrating the wall, and enclosing
fragments of it, as in Fig. 11, which is a typically igneous contact.

2. The alteration of the wall-rock by heat. This may consist in: (_a_)
_coloration_, shales and sandstones being reddened in the same way as
when clay is burnt for bricks; (_b_) _baking and induration_, sandstone
being converted into quartzite and even jasper; clay, slate, etc., being
not only baked to a flinty hardness, but actually vitrified, as in
porcelainite; and bituminous coal being converted into natural coke or
anthracite; and (_c_) _crystallization_, chalk, and other limestones
being changed to marble, and crystals of pyrite, calcite, quartz, etc.,
being developed in slate, sandstone, and other rocks.

3. The alteration of the dike-rock by (_a_) more rapid cooling, and
(_b_) the access of thermal waters.

The alteration of the wall-rock may extend only a few inches or many
yards from the dike, gradually diminishing with the distance; and the
cases are surprisingly numerous where there is no perceptible
alteration; and, again, the alteration is usually mutual, the dike-rock
being altered in texture, color, and composition.

    [Illustration: Fig. 13.—Ideal cross-section of a laccolite.]

    [Illustration: Fig. 14.—Ideal cross-section of a volcano.]

INTRUSIVE BEDS.—We commonly think of dikes as cutting across the strata,
but they often lie in planes parallel with them; and the same dike may
run across the beds in some parts of its course and between them in
others (Fig. 12), or the conformable dike maybe simply a lateral branch
of a main vertical dike, as shown in the same figure. All dikes or
portions of dikes lying conformably between the strata are called
_intrusive beds_ or _sheets_.

When a dike fails to reach the surface, but spreads out horizontally
between the strata, forming a thick dome or oven-shaped intrusive bed,
the latter is called a _laccolite_ (Fig. 13). Laccolites are sometimes
of immense volume, containing several cubic miles of rock. Fig. 14
enables us to compare the laccolite with the volcano.

In the one case a large mound of eruptive material accumulates between
the strata, the overlying beds being lifted into a dome; while in the
other case the fissure or vent reaches the surface, and the mound of
lava is built up on top of the ground.

COTEMPORANEOUS BEDS.—When the lava emitted by a crater is sufficiently
liquid, it spreads out horizontally, forming a volcanic sheet or bed. If
such an eruption is submarine, or the lava flow is subsequently covered
by the sea, sedimentary deposits are formed over it; and beds of lava
which thus come to lie conformably between sedimentary strata are known
as _cotemporaneous sheets_ or _beds_, because they belong, in order of
time, in the position in which we find them, being, like any member of a
stratified series, newer than the underlying and older than the
overlying strata. Cotemporaneous lava-flows are sometimes repeated again
and again in the same district, and thus important formations are built
up of alternating igneous and aqueous deposits. Evidently, the student
who would read correctly the record of igneous activity in the past must
be able to distinguish intrusive and cotemporaneous beds. The principal
points to be considered in making this distinction are: (1) The
intrusive bed is essentially a dike, dense and more or less crystalline
in texture, altering, and often enclosing fragments of, both the
underlying and overlying strata, and frequently jogging across or
penetrating the sediments. (2) The cotemporaneous bed, on the other
hand, being essentially a lava-flow, is much less dense and crystalline,
being usually distinctly scoriaceous or amygdaloidal, especially at the
borders, and the underlying strata alone showing heat action, or
occurring as enclosures in the lava; for the overlying strata are newer
than the lava, and often consist largely, at the base, of water-worn
fragments of the lava.

AGES OF DIKES.—The ages of dikes may be estimated in several ways. They
are necessarily newer than any stratified formation which they intersect
or of which they enclose fragments; but any formation crossing the top
of a dike must usually be regarded as newer than the dike, especially if
it contains water-worn fragments of the dike rock.

The relative ages of different dikes are determined by their relations
to the stratified formations; and still more easily by their mutual
intersections, on the principle that when two dikes cross each other,
the intersecting must be newer than the intersected dike. It is
sometimes possible, in this way, to prove several distinct periods of
eruption in the same limited district. The textures of dikes also often
afford reliable indications of their ages; for, as we have already seen,
the upper part of a dike, cooling rapidly and under little pressure,
must be less dense and crystalline than the deep-seated portion, which
cools slowly and under great pressure.

Now, the lower, coarsely crystalline part of a dike can usually be
exposed on the surface only as the result of enormous erosion; and
erosion is a slow process, requiring vast periods of time. Hence, when
we see a coarse-grained dike outcropping on the surface, we are
justified in regarding it as very old, for all the fine-grained upper
part has been gradually worn away by the action of the rain, frost, etc.
Other things being equal, coarse-grained must be older than fine-grained
dikes; and the texture of a dike is at once a measure of its age and of
the amount of erosion which the region has suffered since it was formed.

ERUPTIVE MASSES.—In striking contrast with the more or less wall-like
dikes are the highly irregular, and even ragged, outlines of the
eruptive masses; and it is worth while to notice the probable cause of
this contrast. The true dikes are formed, for the most part, of
comparatively fine-grained rocks—the typical “traps”; while the eruptive
masses consist chiefly of the coarse-grained or granitic varieties. Now
we have just seen that the coarse-grained rocks have been formed at
great depths in the earth’s crust, while the fine-grained are
comparatively superficial. But we have good reason for believing that
the joint-structure, upon which the forms of dikes so largely depend, is
not well developed at great depths, where the rocks are toughened, if
not softened, by the high temperature. In other words, trap dikes are
formed in the jointed formations, which break regularly; while the
granitic masses are formed where the absence of joint-structure and a
high temperature combine to cause extremely irregular rifts and cavities
when the crust is broken.

VOLCANIC PIPES OR NECKS.—Every volcano and every lava-flow or volcanic
sheet must be connected with the earth’s interior by a channel or
fissure, which becomes a dike when the lava ceases to flow. But the
converse proposition is not true, for it is probable that many dikes did
not originally reach the surface, but have been exposed by subsequent
denudation. This is conspicuously the case with laccolites and other
forms of intrusive sheets. Volcanic sheets or beds have probably often
resulted from the overflow of the lava at all points of an extensive
fissure or system of fissures; but the vent of the true volcano must be
more circumscribed, an approximately circular opening in the earth’s
crust, although doubtless originating in a fissure or at the
intersection of two or more fissures, the lava continuing to flow at the
widest part of the wound in the crust long after it has congealed in the
narrower parts. Such a tube is known as the neck or pipe of the volcano;
and volcanic necks are a highly interesting class of dikes, since they
determine the exact location of many an ancient volcano, where the
volcanic pile itself has long since been swept away. Necks and dikes are
the downward prolongations or roots of the volcanic cone or sheet, and
cannot be exposed on the surface until the volcanic fires have gone out
and the agents of erosion have removed the greater part of the ejected
materials.

Hence, equally with the dikes which originally failed to reach the
surface, they, wherever open to our observation, testify to extensive
erosion and a vast antiquity.


                   Original Structures of Vein Rocks.

Many things called veins are improperly so called, such as dikes of
granite and trap, and beds of coal and iron-ore. The smaller, more
irregular, branching dikes, especially, are very commonly called veins,
and to distinguish the true veins from these eruptive masses, they are
designated as _mineral veins_ or _lodes_, although the term _lode_ is
usually restricted to the metalliferous veins.

ORIGIN OF VEINS.—Various theories of the formation of veins have been
proposed, but the most of these are of historic interest merely, for
geologists are now well agreed that nearly all true veins have been
formed by the deposition of minerals from solution in fissures or
cavities in the earth’s crust. In many cases, especially where the veins
are of limited extent, it seems probable that a part or all of the
mineral matter was derived from the immediately enclosing rocks, being
dissolved out by percolating water; and these are known as segregation
or lateral secretion veins. But it is quite certain that as a general
rule the mineral solutions have come chiefly from below, the deep-seated
thermal waters welling up through any channel opened to them, and
gradually depositing the dissolved minerals on the walls of the fissure
as the temperature and pressure are diminished. This case, however,
differs from the first only in deriving the vein-forming minerals from
more remote and deeper portions of the enclosing rocks; and thus we see
that vein-formation, whether on a large or a small scale, is always
essentially a process of segregation.

We know that every volcano and every lava flow must be connected below
the surface with a dike; and it is almost equally certain that the
waters of mineral springs forming tufaceous mineral deposits on the
surface, as in the geyser districts, also deposit a portion of the
dissolved minerals on the walls of the subterranean channels, which are
thus being gradually filled up and converted into mineral veins, which
will be exposed on the surface when erosion has removed the tufaceous
overflow. This connection of vein-formation with the superficial
deposits of existing springs has been clearly proved in several
important instances in Nevada and California.

Veins occur chiefly in old, metamorphic, and highly disturbed
formations, where there is abundant evidence of the former existence of
profound fissures, and in regions similar to those in which thermal
springs occur to-day.

In the supplement to the lithological section the student will find the
formation of a typical vein briefly described and contrasted with that
of a typical dike; also a brief account of the lithological
peculiarities of vein rocks, and general statements concerning their
relative abundance and vast economic importance.

EXTERNAL CHARACTERISTICS OF VEINS.—The typical vein may be described as
a fissure of indefinite length and depth, filled with mineral substances
deposited from solution. Externally, it is very similar to the typical
dike, for the fissures are made in the same way for both. Veins are
normally highly inclined to the horizon; they exhibit in nearly every
respect the same general relations to the structure of the country rock
as dikes; and the ages of veins are determined in the same way as the
ages of dikes.

The extensive mining operations to which veins have been subjected in
all parts of the world, have made our knowledge of their forms below the
surface very full and accurate. It has been learned in this way that
very often the corresponding portions of the walls of a vein do not
coincide in position, but one side is higher or lower than the other,
showing that the walls slipped over each other when the fissure was
formed or subsequently; and this faulting or displacement of the walls
appears to be much more common with veins than with dikes, perhaps
because the fissures remained open much longer. This slipping of the
walls is the principal cause of the almost constant changes in the width
of veins. For, since the walls are never true planes, and are often
highly irregular any unequal movements must bring them nearer together
at some points than at others. As a rule, the enormous friction
accompanying the faulting, either crushes the wall-rock, or polishes and
striates it, producing the highly characteristic surfaces known as
_slicken-sides_. Where the wall is finely pulverized in this way, or is
partially decomposed before or after the filling of the fissure, a thin
layer of soft, argillaceous material is formed, separating the vein
proper from the wall-rock. The miners call this the _selvage_; and it is
a very characteristic feature of the true fissure veins.

Fragments of the wall-rock are frequently enclosed in veins, and the
latter sometimes branch or divide in such a way as to surround a large
mass of the wall, which is known as a “horse.” A similar result is
accomplished when a fissure is re-opened after being filled, if the new
fissure does not coincide exactly with the old. It has been proved that
veins have thus been re-opened and filled several times in succession;
and in this way fragments of the older vein material become enclosed in
the newer.

Although usually determined in direction by the joint-structure of the
country rock, veins are often parallel with the bedding, especially in
highly inclined, schistose formations. Such interbedded veins are
commonly distinctly lenticular in form, occupying rifts in the strata
which thin out in all directions and are often very limited in extent.

Whether conforming with the joint-structure or bedding, veins are
commonly arranged in systems by their parallelism, those of different
systems or directions usually differing in age and composition, and the
older veins being generally faulted or displaced when intersected by the
newer.

INTERNAL CHARACTERISTICS OF VEINS.—Internally, veins and dikes are
strongly contrasted; and it is upon the internal features, chiefly, as
previously explained, that we must depend for their distinction. In
metalliferous veins the minerals containing the metal sought for (the
galenite, sphalerite, etc.) are the _ore_; while the non-metalliferous
minerals (the quartz, feldspar, calcite, etc.) are called the _gangue_
or vein-stone proper. Although the combinations of minerals in veins are
almost endless, yet certain associations of ores with each other and
with different gangue minerals are tolerably constant, and constitute an
important subject for the student of metallurgy and mining.

When a vein is composed of a single mineral, as quartz, it may rival a
dike in its homogeneity. Most important veins, however, are composed of
several or a large number of minerals, which may be sometimes more or
less uniformly mixed with each other, but are usually distributed in the
fissure in a very irregular manner. The great granite veins which are
worked for mica, feldspar and quartz, are good illustrations, on a large
scale, of the structure of veins in which several minerals have been
deposited cotemporaneously. The individual minerals are found to a large
extent, in great, irregular masses, with no order observable in their
arrangement.

When a mineral is deposited from solution, it crystallizes by preference
on a surface of similar composition, thus quartz on quartz, feldspar on
feldspar, and so on; and it seems probable that this selective action of
the wall-rock may be a principal cause of the irregular distribution of
minerals in veins. It has often been observed in metalliferous veins
that the richness varies with the nature of the adjacent country rock.
This dependence of the contents of a fissure upon the wall-rock may be
due in part to the selective deposition of the minerals, and in part to
their derivation from the contiguous portions of the country or
wall-rock, as in the so-called segregated veins. Temperature and
pressure exert an important influence upon chemical precipitation, and
it is, therefore, probable that the composition of many veins varies
with the depth.

    [Illustration: Fig. 15.—Ideal section of a vein.]

Frequently, perhaps usually, the minerals of composite veins are
deposited in succession, instead of cotemporaneously, giving rise to the
remarkable banded structure so characteristic of this class of veins.
The first mineral deposited in the fissure forms a layer covering each
wall, and is in turn covered by layers of the second mineral, and that
by the third, and so on, until the fissure is filled, or the solution
exhausted. The distinguishing features of this structure are shown in
Fig. 15, in which _w w_ represents the wall-rock, _a a_, _b b_, _c c_
are successive layers of quartz, fluorite and barite, and the central
band, _d_, is galenite. Since the vein grows from the outside inward,
the outer layers are the oldest, and the central layers are the newest;
again, the layers are symmetrically arranged, being repeated in the
reverse order on opposite sides of the middle of the vein; and, lastly,
in layers composed of prismatic crystals, as quartz (see the figure);
the crystals are perpendicular to the wall and often project into, and
even through, the succeeding layers. Such a crystalline layer is called
a “_comb_” and the interlocking of the layers in this way is described
as the _comb-structure_ of the vein. The banding of veins is thus
strongly contrasted with stratification, and with the structure in dikes
due to the more rapid cooling along the walls. The duplicate layers are
often discontinuous and of unequal thickness, on account of the strong
tendency to segregation in the materials. This is clearly shown in Fig.
16, drawn on a reduced scale from a polished section of a lead vein in
Cumberland, England, contained in the Museum of the Boston Society of
Natural History. In this the gangue minerals are fluorite (_f_) and
barite (_b_). The central band (_f g_) is a darker fluorite containing
irregular masses of galenite. The banded structure of veins is exactly
reproduced in miniature in the banding of agates, geodes, and the
amygdules formed in old lavas. Unfilled cavities frequently remain along
the middle of the vein. When small, these are known as “pockets.” They
are commonly lined with crystals; and when the latter are minute, the
pockets are called druses. In metalliferous veins, the ore is much more
abundant in some parts than in others, and these ore-bodies, especially
when somewhat definite in outline, are known in their different forms
and in different localities, as _courses_, _slants_, _shoots_,
_chimneys_, and _bonanzas_ of ore. The intersections and junctions of
veins are often among the richest parts, as if the meeting of dissimilar
solutions had determined the precipitation of the ore.

    [Illustration: Fig. 16.—Section of a lead vein, one-fifth
        natural size.]

Metalliferous veins, especially, are usually deeply decomposed along the
outcrop by the action of atmospheric agencies. The ore is oxidized, and
to a large extent removed by solution, leaving the quartz and other
gangue minerals in a porous state, stained by oxides of iron, copper,
and other metals, forming the _gossan_ or _blossom-rock_ of the vein.

PECULIAR TYPES OF VEINS.—In calcareous or limestone formations,
especially, the joint-cracks and bedding-cracks are often widened
through the solution of the rock by infiltrating water, and thus become
the channels of a more or less extensive subterranean drainage, by which
they are more rapidly enlarged to a system of galleries and chambers,
and, in some cases, large limestone caverns. The water dripping into the
cavern from the overlying limestone is highly charged with carbonate of
lime, which is largely deposited on the ceiling and floor of the cavern,
forming stalactitic and stalagmitic deposits. These are masses of
mineral matter deposited from solution in cavities in the earth’s crust,
and are essentially vein-formations. Portions of caverns deserted by the
flowing streams by which they were excavated, are often filled up in
this way, being converted into irregular veins of calcite. But calcite
is not the only mineral found in these cavern deposits, for barite and
fluorite, and various lead and zinc ores, especially the sulphides of
these metals—galenite and sphalerite—have also been leached out of the
surrounding limestone and concentrated in the caverns. The celebrated
lead mines of the Mississippi Valley, and some of the richest
silver-lead mines of Utah and Nevada are of this character. The forms of
these cavern-deposits vary almost indefinitely, and are often highly
irregular. The principal types are known as _gash-veins_, _flats_ and
_sheets_ (Fig. 17), _chambers_ and _pockets_.

Where joints and other cracks have opened slightly in different
directions and become filled with infiltrated ores, we have what the
German miners call a _stock-work_,—an irregular network of small and
interlacing veins.

    [Illustration: Fig. 17.—Gash-veins and sheets.]

An _impregnation_ is an irregular segregation of metalliferous minerals
in the mass of some eruptive or crystalline rock. Its outlines are not
sharply defined, but it shades off gradually into the enclosing rock.

_Fahlbands_ are similar ill-defined deposits or segregations in
stratified rocks. An impregnation or vein occurring along the contact
between two dissimilar rocks is called a _contact deposit_. These are
usually found between formations of different geological ages, and
especially between eruptive and sedimentary rocks.


        Subsequent Structures produced by Subterranean Agencies.

The subterranean forces concerned in the formation of rocks are chiefly
various manifestations of that enormous tangential pressure developed in
the earth’s crust, partly by the cooling and shrinkage of its interior,
but largely, it is probable, by the diminution of the velocity of the
earth’s rotation by tidal friction, and the consequent diminution of the
oblateness of its form. It is well known that the centrifugal force
arising from the earth’s rotation is sufficient to change the otherwise
spherical form of the earth to an oblate spheroid, with a difference of
twenty-six miles between the equatorial and polar diameters. It is also
well known that while the earth turns from west to east on its axis, the
tidal wave moves around the globe from east to west, thus acting like a
powerful friction-brake to stop the earth’s rotation. Our day is
consequently lengthening, and the earth’s form as gradually approaching
the perfect sphere. This means a very decided shortening and consequent
crumpling of the equatorial circumference, and is equivalent to a marked
shrinkage of the earth’s interior, so far as the equatorial regions are
concerned.

The most important and direct result of the horizontal thrust, whether
due to cooling or tidal friction, is the corrugation or wrinkling of
the crust; and the earth-wrinkles are of three orders of
magnitude,—continents, mountain-ranges, and rock-folds or arches.

Continents and ocean-basins, although the most important and permanent
structural features of the earth’s crust, do not demand further
consideration here, since their forms and relations are adequately
described in the better text-books of physical geography. The forms and
distribution of mountain-ranges might be dismissed in the same way; but,
unlike continents, the structure of mountains, upon which their reliefs
mainly depend, is quite fully exposed to our observation, and is one of
the most important fields of the student of structural geology.
Mountains, however, as previously explained, combine nearly all the
kinds of structure produced by the subterranean agencies, and their
consideration, therefore, belongs at the end rather than the beginning
of this section.

INCLINED OR FOLDED STRATA.—Normally, strata are horizontal, and dikes
and veins are vertical or nearly so. Hence the stratified rocks are more
exposed to the crumpling action of the tangential pressure in the
earth’s crust than the eruptive and vein rocks; and it is for this
reason and partly because the stratified rocks are vastly more abundant
than the other kinds, that the effects of the corrugation of the crust
are studied chiefly in the former. But it should be understood that
folded dikes and veins are not uncommon.

That the stratified rocks have, in many instances, suffered great
disturbance subsequent to their deposition, is very evident; for, while
the strata must have been originally approximately straight and
horizontal, they are now often curved, or sharply bent and contorted,
and highly inclined or even vertical. All inclined beds or strata are
portions of great folds or arches. Thus we may feel sure when we see a
stratum sloping downward into the ground, that its inclination or dip
does not continue at the same angle, but that at some moderate depth it
gradually changes and the bed rises to the surface again. Similarly, if
we look in the opposite direction and think of the bed as sloping
upward—we know that the surface of the ground is being constantly
lowered by erosion, and consequently that the inclined stratum formerly
extended higher than it does now, but not indefinitely higher; for, in
imagination, we see it curving and descending to the level of the
present surface again. Hence it forms, at the same time, part of one
side of a great concave arch, and of a great convex arch, just as every
inclined surface on the ground indicates both a hill and a valley. And
guided by this principle we can often reconstruct with much probability
folds that have been more or less completely swept away by erosion, or
that are buried beyond our sight in the earth’s crust.

The arches of the strata are rarely distinctly indicated in the
topography, but must be studied where the ground has been partly
dissected, as in cliffs, gorges, quarries, etc. They are also, as a
rule, far more irregular and complex than they are usually conceived or
represented. The wrinkles of our clothing are often better illustrations
of rock-folds than the models and diagrams used for that purpose. This
becomes self-evident when we reflect that the earth’s crust is
exceedingly heterogeneous in composition and structure, and must,
therefore, yield unequally to the unequal strains imposed upon it.

The folds or undulations of the strata may be profitably compared with
water-waves. In fact, the comparison is so close that they have been not
inaptly called rock-waves. Folds, like waves, unless very large, rarely
continue for any great distance, but die out and are replaced by others,
giving rise to the _en echelon_ or step-like arrangement. The plan of
both a wave and a fold is a more or less elongated ellipse, each stratum
in a fold being semi-ellipsoidal or boat-shaped. In other words, a
normal fold is an elongated mound of concentric strata, being highest at
the centre, sloping very gradually toward the ends, and much more
abruptly toward the sides.

    [Illustration: Fig. 18.—Anticlinal and synclinal folds.]

The imaginary line passing longitudinally through a fold, about which
the strata appear to be bent, is the _axis_; and the plane lying midway
between the two sides of a fold and including the axis is the _axial
plane_. The two principal kinds of folds are the _anticline_ (Fig. 18,
_A_), where the strata dip away from the axis; and the _syncline_ (Fig.
18, _B_), where they dip toward the axis. They are commonly, but not
always, correlative, like hill and valley.

Rock-folds are of all sizes, from almost microscopic wrinkles to great
arches miles in length and breadth, and thousands of feet in height. The
smaller folds, or such as may be seen in hand specimens and even in
considerable blocks of stone, are commonly called contortions, and it is
interesting to observe that they are, in nearly everything except size,
precisely like the large folds, so that they answer admirably as
geological models. Large folds, however, are almost necessarily curves,
but contortions are frequently angular (Fig. 19). With folds, as with
waves, the small undulations are borne upon the large ones; but the
contortions are not uniformly distributed. An inspection of Fig. 18
shows that when the rocks are folded they must be in a state of tension
on the anticlines (_A_), and in a state of compression in the synclines
(_B_), and the latter is evidently the normal position of the puckerings
or contortions of the strata, as shown in Fig. 20. Contortions are also
most commonly found in thin-bedded, flexible rocks, such as shales and
schists. And when we find them in hard, rigid rocks, like gneiss and
limestone, it must mean either that the structure was developed with
extreme slowness, or that the rock was more flexible then and possibly
plastic.

    [Illustration: Fig. 19.—Contorted strata.]

    [Illustration: Fig. 20.—Contorted syncline.]

    [Illustration: Fig. 21.—Section of anticlinal mountains.]

It is very interesting to notice the relations of anticlinal and
synclinal folds to the agents of erosion. At the time the folds are
made, the anticlinals, of course, are ridges, and the synclinals,
valleys, and this relation sometimes continues, as shown in Fig. 21; but
we have seen that the rocks in the trough of the synclinal are
compressed and compacted, _i.e._, made more capable of resisting
erosion, while those on the crest of the anticlinal are stretched and
broken, _i.e._, made more susceptible of erosion. The consequence is
that the anticlinals are usually worn away very much faster than the
synclinals; so much faster that in many cases the topographic features
are completely transposed, and in place of anticlinal ridges and
synclinal valleys (Fig. 21) we find synclinal ridges and anticlinal
valleys (Fig. 22).

    [Illustration: Fig. 22.—Section of synclinal mountains.]

    [Illustration: Fig. 23.—Monoclinal fold.]

    [Illustration: Fig. 24.—Unsymmetrical and inverted folds.]

Besides the anticlinal and synclinal folds already explained, there are
folds that slope in only one direction, one-sided or _monoclinal_ folds
(Fig. 23). Anticlinal and synclinal folds are _symmetrical_ when the dip
or slope of the strata is the same on both sides and the axial plane is
vertical. The great majority of folds, however, are _unsymmetrical_, the
opposite slopes being unequal, and the axial planes inclined to the
vertical (Fig. 24, _A_). This means that the compressing or plicating
force has been greater from one side than from the other, as indicated
by the arrows. It acted with the greatest intensity on the side of the
gentler slope, the tendency evidently having been to crowd or tip the
fold over in the direction of the steep slope. When the steep slope
approaches the vertical, this tendency is almost unresisted, and when it
passes the vertical, gravitation assists in overturning the fold (Fig.
24, _B_). Such highly unsymmetrical folds, including all cases where the
two sides of the fold slope in the same direction, are described as
_overturned_ or _inverted_, although the latter term is not strictly
applicable to the entire fold, but only to the strata composing the
under or lee side of it. Fig. 24, _B_, shows that these beds are
completely inverted, the older, as the figures indicate, lying
conformably upon the newer. This inversion is one of the most important
features of folded strata, and it has led to many mistakes in
determining their order of succession. In the great mountain-chains,
especially, it is exhibited on the grandest scale, great groups of
strata being folded over and over each other as we might fold carpets.
An inverted stratum is like a flattened S or Z, and may be pierced by a
vertical shaft three times, as has actually happened in some coal mines.
Folds are _open_ when the sides are not parallel, and _closed_ when they
are parallel, the former being represented by a half-open, and the
latter by a closed, book. Closed folds are usually inverted, and when
the tops have been removed by erosion (Fig. 25), the repetition of the
strata may escape detection, and the thickness of the section be, in
consequence, greatly overestimated. Thus, a geologist traversing the
section in Fig. 25 would see thirty-two strata, all inclined to the left
at the same angle, those on the right apparently passing below those on
the left, and all forming part of one great fold. The repetition of the
strata in reverse order, as indicated by the numbers, and the structure
below the surface, show, however, that the section really consists of
only four beds involved in a series of four closed folds, the true
thickness of the beds in this section being only one-eighth as great as
the apparent thickness.

    [Illustration: Fig. 25.—Series of closed folds.]

The most important features to be noted in observing and describing
inclined or folded strata are the _strike_ and _dip_. The strike is the
compass bearing or horizontal direction of the strata. It is the
direction of the outcrop of the strata where the ground is level. It may
also be defined as the direction of a level line on the surface of a
stratum, and is usually parallel with the axis of the fold.

    [Illustration: Fig. 26.—Dip and strike.]

The dip is the inclination of the beds to the plane of the horizon, and
embraces two elements: (_a_) the direction of the dip, which is always
at right angles to the strike, being the line of steepest descent on the
surface of the stratum, and (_b_) the amount of the dip, which is the
value of the angle between the line of steepest descent and the horizon.

In Fig. 26, _s t_ is the direction of the strike, and _d p_ that of the
dip. The strike and direction of the dip are determined with the
compass, and the amount of the dip with the clinometer, an instrument
for measuring vertical angles.

The strike is much less variable than the dip, being often essentially
constant over extensive districts; while the dip, except in very large
or closed folds, is constantly changing in direction and amount.

When the dip and surface breadth of a series of strata have been
measured, it is a simple problem in trigonometry to determine the true
thickness, and the depth below the surface of any particular stratum at
any given distance from its outcrop. When the strata are vertical, the
surface breadth or traverse measure is equal to the thickness.

By the _outcrop_ of a stratum or formation we ordinarily understand its
actual exposure on the surface, where it projects through the soil in
ledges or quarries. But the term is also more broadly defined to mean
the exposure of the stratum as it would appear if the soil were entirely
removed. It is instructive to observe the relations of the outcrop to
the form of the surface. Its breadth varies with its inclination to the
surface, appearing narrow and showing its true thickness where it is
perpendicular to the surface, and broadening out rapidly where the
surface cuts it obliquely. The outcrops of horizontal strata form level
lines or bands along the sides of hills and valleys, essentially contour
lines in the topography; and appear as irregular, sinuous bands
bordering the streams and valleys in the map-view of the country. The
outcrops of vertical strata, dikes, or veins, on the other hand, are
represented by straight lines and bands on the map. While the outcrops
of inclined strata are deflected to the right or left in crossing ridges
and valleys, according to the direction and amount of their inclination.

A geological map shows the surface distribution of the rocks, _i.e._,
gives in one view the forms and arrangement of the outcrops of all the
rocks in the district mapped, including the trend or strike of the
folded strata. The map may be lithological, each kind of rock, as
granite, sandstone, limestone, etc., being represented by a different
color; or, it may be historical, each color representing one geological
formation, _i.e._, the rocks formed during one period of geological
time, without reference to their lithological character. But in the best
maps these two methods are combined. The geological section shows the
arrangement of the rocks below the surface, revealing the dip of the
strata and supplementing the map, both modes of representation, the
horizontal and vertical, being required to give a complete idea of the
geological structure of a country. For a detailed and satisfactory
explanation of the construction and use of geological maps and sections,
students are referred to Prof. Geikie’s “Outlines of Field Geology.”

CLEAVAGE STRUCTURE.—This important structure is now known to be, like
rock-folds, a direct result of the great horizontal pressure in the
earth’s crust. It is entirely distinct in its nature and origin from
crystalline cleavage, and may properly be called lithologic cleavage. It
is also essentially unlike stratification and joint-structure. It agrees
with stratification in dividing the rocks into thin parallel layers, but
the cleavage planes are normally vertical instead of horizontal. And the
cleavage planes differ from joints in running in only one direction,
dividing the rock into layers; while joints, as we shall see, traverse
the same mass of rock in various directions, dividing it into blocks.

    [Illustration: Fig. 27.—Slaty cleavage in contorted strata.]

The principal characteristics of lithologic cleavage are: (1) It is
rare, except in fine-grained, soft rocks, having its best development in
the slates, roofing slates and school slates affording typical examples.
Hence it is commonly known as _slaty cleavage_. (2) The cleavage planes
are highly inclined or vertical, very constant in dip and strike, and
quite independent of stratification. (3) It is usually associated with
folded strata, and often with distorted nodules or fossils. The more
important of these characteristics are illustrated by Fig. 27. This
represents a block of contorted strata in which the dark layers are
slate with very perfect cleavage parallel to the left-hand shaded side
of the block; while the white layers are sandstone and quite destitute
of cleavage. Many explanations of this interesting structure have been
proposed, but that first advanced by Sharpe may be regarded as fully
established. He said that _slaty cleavage is always due to powerful
pressure at right angles to the planes of cleavage_. All the
characteristics of cleavage noted above are in harmony with this theory.
Cleavage is limited to fine-grained or soft rocks, because these alone
can be modified internally by pressure, without rupture. Harder and more
rigid rocks may be bent or broken, but they appear insusceptible of
minute wrinkling or other change of structure affecting every particle
of the mass. Since the cleavage planes are normally vertical, the
pressure, according to the theory, must be horizontal. That this
horizontal pressure exists and is adequate in direction and amount, is
proved by the folds and contortions of the cleaved strata; for, as shown
in Fig. 27, the cleavage planes coincide with the strike of the
foldings, and are thus perpendicular to the pressure horizontally as
well as vertically. The distortion of the fossils in cleaved slates is
plainly due to pressure at right angles to the cleavage, for they are
compressed or shortened in that direction, and extended or flattened out
in the planes of cleavage. Again, Tyndall has shown that the magnetism
of cleaved slate proves that it has been powerfully compressed
perpendicularly to the cleavage. And, finally, repeated experiments by
Sorby and others have proved that a very perfect cleavage may be
developed in clay (unconsolidated slate) by compression, the planes of
cleavage being at right angles to the line of pressure. When, however,
Sharpe’s theory had been thus fully demonstrated, the question as to
_how_ pressure produces cleavage still remained unanswered. Sorby held
that clay contains foreign particles with unequal axes, such as
mica-scales, etc., and that these are turned by the pressure so as to
lie in parallel planes perpendicular to its line of action, thus
producing easy splitting or cleavage in those planes. And he proved by
experiments that a mixture of clay and mica-scales does behave in this
way. But Tyndall showed that the cleavage is more perfect just in
proportion as the clay is free from foreign particles, and in such a
perfectly homogeneous substance as beeswax, he developed a more perfect
cleavage than is possible in clay. His theory, which is now universally
accepted, is, that the clay itself is composed of grains which are
flattened by pressure, the granular structure with irregular fracture in
all directions, changing to a scaly structure with very easy and plane
fracture or splitting in one definite direction.

Observations on distorted fossils and nodules have shown that when slaty
cleavage is developed, the rock is, on the average, reduced in the
direction of the pressure to two-fifths of its original extent, and
correspondingly extended in the vertical direction. Thus, whether rocks
yield to the horizontal pressure in the earth’s crust, by folding and
corrugation, or by the flattening of their constituent particles, they
are alike shortened horizontally and extended vertically; and it is
impossible to overestimate the importance of these facts in the
formation of mountains.

FAULTS OR DISPLACEMENTS.—We may readily conceive that the forces which
were adequate to elevate, corrugate, and even crush vast masses of solid
rock were also sufficient to crack and break them; and since the
fractures indicate that the strains have been applied unequally, it will
be seen that unequal movements of the parts must often result. If this
unequal movement takes place, _i.e._, if the rocks on opposite sides of
a fracture of the earth’s crust do not move together, but slip over each
other, a _fault_ is produced. The two sides may move in opposite
directions, or in the same direction but unequally, or one side may
remain stationary while the other moves up or down. It is simply
essential that the movement should be unequal in direction, or amount,
or both; that there should be an actual slip, so that strata that were
once continuous no longer correspond in position, but lie at different
levels on opposite sides of the fracture. The vertical difference in
movement is known as the _throw_, _slip_, or _displacement_ of the
fault. Fault-fractures rarely approach the horizontal direction, but are
usually highly inclined or approximately vertical. When the fault is
inclined, as in Fig. 28, the actual slipping in the plane of the fault
exceeds the vertical throw, for the movement is then partly horizontal,
the beds being pulled apart endwise. The inclination of faults, as of
veins and dikes, should be measured from the vertical and called the
_hade_. Faults are sometimes hundreds of miles in length; and the throw
may vary from a fraction of an inch to thousands of feet.

    [Illustration: Fig. 28.—Section of a normal fault.]

    [Illustration: Fig. 29.—Section of a reversed fault.]

Transverse sections, such as are represented by Fig. 28 and many
specimens and models, do not give the complete plan or idea of a fault;
but this is seen more perfectly in Fig. 30. We learn from this that a
typical fault is a fracture along which the strata have _sagged_ or
settled down unequally. The most important point to be observed here is
that the strata do not drop bodily, but are merely bent, the throw being
greatest at the middle of the fault and gradually diminishing toward the
ends. In other words, every simple fault must die out gradually; for we
cannot conceive of a fault as ending abruptly, except where it turns
upon itself so as to completely enclose a block of the strata, which may
drop down bodily; but the fault is then really endless. A fault may be
represented on a map by a line; if a simple fault, by a single straight
line. But faults are often compound, and are represented by branching
lines; that is, the earth’s crust has been broken irregularly, and the
parts adjoining the fracture have sagged or risen unequally.

    [Illustration: Fig. 30.—Ideal view of a complete fault.]

The rock above an inclined fault, vein, or dike (Fig. 28) is called the
_hanging wall_, and that below the _foot wall_. Now inclined faults are
divided into two classes, according to the relative movements of the two
walls. Usually, the hanging wall slips down and the foot wall slips up,
as in Fig. 28. Faults on this plan are so nearly the universal rule that
they are called _normal_ faults. They indicate that the strata were in a
state of tension, for their broken ends are pulled apart horizontally,
so that a vertical line may cross the plane of a stratum without
touching it.

A few important faults have been observed, however, in which the
foot-wall[**no hyphen before] has fallen and the hanging-wall[**] has
risen (Fig. 29). These are known as _reversed_ faults; and they indicate
that the strata were in a state of lateral compression, the broken ends
of the beds having been pushed horizontally past each other, so that a
vertical line or shaft may intersect the same bed twice, as has been
actually demonstrated in the case of some beds of coal.

    [Illustration: Fig. 31.—Explanation of normal faults.]

The usual explanation of normal faults is given in Fig. 31. The inclined
fractures of the earth’s crust must often be converging, bounding, or
enclosing large V-shaped blocks (_A_, _B_). If now, through any cause,
as the folding of the strata, they are brought into a state of tension,
so that the fractures are widened, the V-shaped masses, being
unsupported, settle down, the fractures bounding them becoming normal
faults, as is seen by tracing the bed _X_ through the dislocations. The
single fracture below the block _A_ is inclined, and the stretching has
been accomplished by slipping along it and faulting the bed _Z_ as well
as _X_, the entire section to the right of this fracture being part of a
much larger V-shaped block the right-hand boundary of which is not seen.
But the united fracture below the block _B_ being vertical, any
horizontal movement must widen it into a fissure, which is kept open by
the great wedge above and may become the seat of a dike or mineral vein.
The beds below the V may, in this case, escape dislocation, as is seen
by tracing the bed _Z_ across the fissure. These pairs of converging
normal faults are called _trough_ faults; and this is the only way in
which we can conceive of important faults as terminating at moderate
depths below the surface, and not affecting the entire thickness of the
earth’s crust.

Important reversed faults are believed to occur chiefly along the axes
of overturned anticlines (Fig. 24) where the strata have been broken by
the unequal strains, and those on the upper side shoved bodily over
those on the lower or inverted side.

An extensive displacement of the strata is sometimes accomplished by
short slips along each of a series of parallel fractures, producing a
_step_ fault.

Faults cutting inclined or folded strata are divided into two classes,
according as they are approximately parallel with the direction of the
dip or of the strike. The first are known as _transverse_ or _dip_
faults, and the second as _longitudinal_ or _strike_ faults. The chief
interest of either class consists in their effect upon the outcrops of
the faulted strata, after erosion has removed the escarpment produced by
the dislocation.

    [Illustration: Fig. 32.—Plan of a dip fault.]

Dip faults cause a lateral shift or displacement of the outcrops, as
shown in Fig. 32, which represents a plan or map-view of the strata
traversed by the fault _b b_, the down throw being on the right and the
up throw on the left. The dip of the strata is indicated by the small
arrows and the accompanying figures; and it will be observed on tracing
the outcrop of any stratum, _a a_, across the fault that it is shifted
to the right. If the throw of the fault were reversed, the displacement
of the outcrop would be reversed, also. Strike faults are of two kinds,
according as they incline in the same direction as the strata, or in the
contrary direction. The effect of the first kind is to conceal some of
the beds, as shown in Fig. 33, in which beds 5 and 6 do not outcrop, but
we pass on the surface abruptly from 4 to 7. The apparent thickness of
the section is thus less than the real thickness. When the fault
inclines against the strata, on the other hand (Fig. 34), the outcrops
of certain strata are repeated on the surface; and a number of parallel
faults of this kind, a step fault, will, like a series of closed folds
(Fig. 25), cause the apparent thickness of the section to greatly exceed
the real thickness. Repetition of the strata by faulting is
distinguished from repetition by folding by being in the same instead of
the reverse order.

    [Illustration: Fig. 33.—Strike fault, concealing strata.]

    [Illustration: Fig. 34.—Strike fault, repeating strata.]

Folds and faults are really closely related. In the former the strata
are disturbed and displaced by bending; in the latter by breaking and
slipping; and the displacement which is accomplished by a fold may
gradually change to a fracture and slip. This relation is especially
noticeable with monoclinal folds (Fig. 23), in which the tendency to
shear or break the beds is often very marked.

Important faults are rarely simple, well-defined fractures; but, in
consequence of the enormous friction, the rocks are usually more or less
broken and crushed, sometimes for a breadth of many feet or yards. The
fragments of the various beds are then strung along the fault in the
direction of the slipping, and this circumstance has been made use of in
tracing the continuation of faulted beds of coal. In other cases the
direction of the slip is plainly indicated by the bending of the broken
ends of the strata (Fig. 35), and the beds are sometimes turned up at a
high angle or even overturned in this way.

    [Illustration: Fig. 35.—Section of beds distorted by a
        fault.]

Since faults are not plane, but undulating and often highly irregular,
fractures, the walls will not coincide after slipping; and if the rocks
are hard enough to resist the enormous pressure, the cavities or
fissures produced in this way may remain open. Now faults are continuous
fractures of the earth’s crust, reaching down to an unknown but very
great depth; and hence they afford the best outlets for the heated
subterranean waters; so that it is common to find an important fault
marked on the surface by a line of springs, and these are often thermal.
The warm mineral waters on their way to the surface deposit part of the
dissolved minerals in the irregular fissures along the fault, which are
thus changed to mineral veins. This agrees with the fact that the walls
of veins usually show faulting as well as crushed rock, slickensides,
and other evidences of slipping.

If the earth’s surface were not subject to erosion, every fault would be
marked on the surface by an escarpment equal in height to the throw of
the fault; and, notwithstanding the powerful tendency of erosion to
obliterate them, these escarpments are sometimes observed, although of
diminished height. Thus, according to Gilbert, the Zandia Mountains in
New Mexico are due to a fault of 11,000 feet, leaving an escarpment
still 7000 feet high. But, as a rule, there is no escarpment or marked
inequality of the surface, the fault, like the fold, not being
distinctly indicated in the topography. In all such cases we must
conclude either that the faults were made a very long time ago, or that
they have been formed with extreme slowness, so slowly that erosion has
kept pace with the displacement, the escarpments being worn away as fast
as formed. These and other considerations make it quite certain that
extensive displacements are not produced suddenly, but either grow by a
slow, creeping motion, or by small slips many times repeated at long
intervals of time.

JOINTS AND JOINT-STRUCTURE.—This is the most universal of all
rock-structures, since all hard rocks and many imperfectly consolidated
kinds, like clay, are jointed. Joints are cracks or planes of division
which are usually approximately vertical and traverse the same mass of
rock in several different directions. They are distinguished from
stratification planes by being rarely horizontal, and from both
stratification and cleavage planes by being actual cracks or fractures,
and by dividing the rock into blocks instead of sheets or layers. The
art of quarrying consists in removing these natural blocks; and most of
the broad flat surfaces of rock exposed in quarries, are the
joint-planes (Fig. 36). Some of the most familiar features of
rock-scenery are also due to this structure, cliffs, ravines, etc.,
being largely determined in form and direction by the principal systems
of joints; and we have already seen that the same is true of veins and
dikes.

Joints are divided by their characteristics and modes of origin into
three classes as follows:—

    [Illustration: Fig. 36.—Quarry showing two systems of
        parallel joints.]

1. _The parallel and intersecting joints._—This is by far the most
important class, and has its best development in stratified rocks, such
as sandstone, slate, limestone, etc. These joints are straight and
continuous cracks which may often be traced for considerable distances
on the surface. They usually run in several definite directions, being
arranged in sets or systems by their parallelism. Thus in Fig. 36 one
set of joints is represented by the broad, flat surfaces in light, and a
second set crossing the first nearly at right angles, by the narrower
faces in shadow. By the intersections of the different sets of joints
the rock is divided into angular blocks.

Although many explanations of this class of joints have been proposed,
it has long been the general opinion of geologists that they are due to
the contraction of the rocks, _i.e._, that they are shrinkage cracks. We
shall soon see, however, that they lack the most important characters of
cracks known to be due to shrinkage; and the present writer has advanced
the view that movements of the earth’s crust, and especially the swift,
vibratory movements known as earthquakes, are a far more adequate and
probable cause. It is well known that earthquakes break the rocks; and,
if space permitted, it could be shown that the earthquake-fractures must
possess all the essential features of parallel and intersecting joints.

    [Illustration: Fig. 37.—Columnar dike.]

2. _Contraction joints or shrinkage cracks._—That many cracks in rocks
are due to shrinkage, there can be no doubt. The shrinkage may result
from the drying of sedimentary rocks; but more generally from the
cooling of eruptive rocks. Every one has noticed in warm weather, the
cracks in layers of mud or clay on the shore, or where pools of water
have dried up; and we have already seen that these sun-cracks are often
preserved in the hard rocks. They have certain characteristic features
by which they may be distinguished from the joints of the first class.
They divide the clay into irregular, angular blocks, which often show a
tendency to be hexagonal instead of quadrangular. The cracks are
continually uniting and dividing, but are not parallel, and rarely cross
each other. Sun-cracks never affect more than a few feet in thickness of
clay, and are an insignificant structural feature of sedimentary rocks.
In eruptive rocks, on the other hand, the contraction joints have a very
extensive, and, in some cases, a very perfect development, culminating
in the prismatic or columnar jointing of the basaltic rocks. This
remarkable structure has long excited the interest of geologists, and,
although the basalt columns were once regarded as crystals, and later as
a species of concretionary structure, it is now generally recognized as
the normal result of slow cooling in a homogeneous, brittle mass. The
columns are normally hexagonal, and perpendicular to the cooling
surface, being vertical in horizontal sheets and lava flows, as in the
classic examples of the Giant’s Causeway and Fingal’s Cave, and
horizontal in vertical dikes (Fig. 37). They begin to grow on the
cooling surface of the mass and gradually extend toward the centre, so
that dikes frequently show two independent sets of columns.

3. _The concentric joints of granitic rocks._—In quarries of granite and
other massive crystalline rocks, it is often very noticeable that the
rock is divided into more or less regular layers by cracks which are
approximately parallel with the surface of the ground, some of the
granite hills having thus a structure resembling that of an onion. The
layers are thin near the surface, become thicker and less distinct
downwards, and cannot usually be traced below a depth of fifty or sixty
feet. These concentric cracks are of great assistance in quarrying, and
are now regarded as due to the expansion of the superficial portions of
the granite caused by the heat of the sun. In reference to this view of
their origin these may be properly called _expansion joints_.

STRUCTURE OF MOUNTAIN-CHAINS.—Mountains are primarily of two
kinds,—volcanic and non-volcanic. The structure of the former belongs
properly with the original structures of the volcanic rocks; but the
latter—the true mountains—owe their internal structure and altitude or
relief almost wholly to the crumpling and mashing together of great
zones of the earth’s crust, being, as already pointed out, the
culminating points of the plication, cleavage, and faulting of the
strata. “A mountain-_chain_ consists of a great plateau or bulge of the
earth’s surface, often hundreds of miles wide and thousands of miles
long. This is usually more or less distinctly divided by great
longitudinal valleys into parallel _ranges_ and _ridges_; and these,
again, are serrated along their crests, or divided into _peaks_ by
transverse valleys. In many cases this ideal chain is far from realized,
but we have instead, a great bulging of the earth’s crust composed on
the surface of an inextricable tangle of ridges and valleys of erosion,
running in all directions. In all cases, however, the erosion has been
immense; for the mountain-chains are the great theatres of erosion as
well as of igneous action. As a general fact, all that we see, when we
stand on a mountain-chain—every peak and valley, every ridge and cañon,
all that constitutes scenery—is wholly due to erosion.”—LE CONTE.

The structure of mountains thus fells under two heads: (1) The internal
structure and altitude, which are due to the action of the subterranean
agencies. (2) The external forms, the actual relief, which are the
product chiefly of the superficial agencies or erosion. The study of
mountains has shown that: (1) They are composed of very thick
sedimentary formations. Thus the sedimentary rocks have a thickness of
40,000 feet in the Alleghanies; of 50,000 feet in the Alps; and of two
to ten miles in all important mountain-chains. Such thick deposits of
sediments, as we have already seen, must be formed on a subsiding
sea-floor, and in many mountain-chains, as in the Alleghanies, the great
bulk of these sediments are still below the level of the sea. Again,
thick sedimentary deposits can only be formed in the shallow, marginal
portions of the sea; and when such a belt of thick shore deposits yields
to the powerful horizontal thrust, and is crumpled and mashed up, it is
greatly shortened in the direction of the pressure and thickened
vertically, so that its upper surface is lifted high above the level of
the sea, and a mountain-chain is formed and added to the edge of the
continent. We thus find an explanation of the important fact that on the
several continents, but notably on the two Americas, the principal
mountain-ranges are near to and parallel with the coast lines.

2. The mountain-forming sediments are usually strongly folded and
faulted, and exhibit slaty cleavage wherever they are susceptible of
that structure; and the older rocks, especially, in mountains are often
highly metamorphosed, and are traversed by numerous veins and dikes, the
infallible signs of intense igneous activity.

“In other words, mountain regions have been the great theatres—(1) of
sedimentation before the mountains were formed; (2) of plication and
upheaval in the formation of the range; and (3) of erosion which
determined the present outline. Add to these the metamorphism, the
faults, veins, dikes, and volcanic outbursts, and it is seen that all
geological agencies concentrate there.”—LE CONTE.

Since mountain-ranges are great up-swellings or bulgings of the strata,
their structure is always essentially anticlinal; and they sometimes
consist of a single more or less denuded anticline (Fig. 38), the oldest
and lowest strata exposed forming the summit of the range. More
commonly, however, the single great arch or uplift is modified by a
series of longitudinal folds, as shown in the section of the Jura
Mountains (Fig. 21). Still more commonly the folds are closely pressed
together, overturned, broken, and almost inextricably complicated by
smaller folds, contortions, and slips.

    [Illustration: Fig. 38.—Anticlinal mountain.]

The strata on the flanks of the mountains are usually less disturbed
than those near the axis of the range, and are sometimes seen to rest
unconformably against the latter. In this way it is proved that some
ranges are formed by successive upheavals. But we have still more
conclusive evidence that mountains are formed with extreme slowness in
the fact that rivers sometimes cut directly through important ranges.
This proves, first, that the river is older than the mountains; second,
that the deepening of its channel has always kept pace with the
elevation of the range.

CONCRETIONS AND CONCRETIONARY STRUCTURE.—Folds, cleavage, faults, and
joints—all the subsequent structures considered up to this point—are the
product of mechanical forces. Chemical agencies, although very efficient
in altering the composition and texture of rocks, are almost powerless
as regards the development of rock-structures; and the only important
structure having a chemical origin is that named above.

Concretions are formed by the segregation of one or more of the
constituents of a rock. But there are three distinct kinds of
segregation. If the water percolating through or pervading a rock,
dissolves a certain mineral and afterwards deposits it in cavities or
fissures, _amygdules_, _geodes_, or _veins_ are the result. If the
mineral is deposited about particular points in the mass of the rock, it
may form _crystals_, the rock becoming _porphyritic_; or it may not
crystallize, but build up instead the rounded forms called
_concretions_, the texture or structure of the rock becoming
_concretionary_. A great variety of minerals occur in the form of
concretions, but this mode of occurrence is especially characteristic of
certain constituents of rocks, such as calcite, siderite, limonite,
hematite, and quartz. Concretions may be classified according to the
nature of the segregating minerals; and in each class we may distinguish
the _pure_ from the _impure_ concretions. A pure concretion is one
entirely composed of the segregating mineral. Most nodules of flint and
chert, quartz, geodes, concretions of pyrite, and many hollow iron-balls
are good illustrations of this class. In all these cases the segregating
mineral has been able in some way to remove the other constituents of
the rock, and make room for itself. But in other cases it has lacked
this power, and has been deposited between and around the grains of
sand, clay, etc.; and the concretions are consequently _impure_, being
composed partly of the segregating mineral, and partly of the other
constituents of the rock. The calcareous concretions known as
clay-stones are a good example of this class, being simply discs of
clay, all the minute interstices of which have been filled with
segregated calcite. The solid iron-balls are masses of sand filled in a
similar manner with iron oxides.

Concretions are of all sizes, from those of microscopic smallness in
some oölitic limestones up to those twenty-five feet or more in diameter
in some sandstones.

The point of deposition, when a concretion begins to grow, is often
determined by some concrete particle, as a grain or crystal of the same
or a different mineral, a fragment of a shell, or a bit of vegetation,
which thus becomes the nucleus of the concretion. The ideal or typical
concretion is spherical; but the form is influenced largely by the
structure of the rock. In porous rocks, like sandstone, they are
frequently very perfect spheres; but in impervious rocks, like clay,
they are flat or disc-shaped, because the water passes much more freely
in the direction of the bedding than across it; while the concretions in
limestones, the nodules of flint and chert, are often remarkable for the
irregularity of their forms. In all sedimentary rocks the concretions
are arranged more or less distinctly in layers parallel with the
stratification, which usually passes undisturbed through the impure
concretions. Many silicious and ferruginous concretions are hollow,
apparently in consequence of the contraction of the substance after its
segregation; and the shrinkage due to drying is still further indicated
by the cracks in the septaria stones. The hollow, silicious concretions
are usually lined with crystals (geodes), while the hollow iron-balls
frequently enclose a smaller concretion. Rocks often have a
concretionary structure when there are no distinct or separable
concretions. And the appearance of a concretionary structure
(pseudo-concretions) is often the result of the concentric decomposition
of the rocks by weathering, as explained on page 13.

SUBSEQUENT STRUCTURES PRODUCED BY THE SUPERFICIAL OR AQUEOUS
AGENCIES.—The superficial agencies, as we have seen in the section on
dynamical geology, are, in general terms, water, air, and organic
matter. Geologically considered, the results which they accomplish, may
be summed up under the two heads of deposition and erosion—the formation
of new rocks in the sea, and the destruction of old rocks on the land.
In the rôle of rock-makers they produce the very important original
structures of the stratified rocks; while as agents of erosion they
develop the most salient of the subsequent structures of the earth’s
crust—the infinitely varied relief of its surface. As a general rule, to
which recent volcanoes are one important exception, the original and
subterranean structures of rocks are only indirectly, and often very
slightly, represented in the topography; for this, as we have seen, is
almost wholly the product of erosion. Therefore, what we have chiefly to
consider in this section is to what extent and how erosion is influenced
by the pre-existing structures of rocks.

Horizontal or very slightly undulating strata, especially if the upper
beds are harder than those below, give rise by erosion to flat-topped
ridges or table-mountains (Fig. 39). But if the strata be softer and of
more uniform texture, erosion yields rounded hills, often very steep,
and sometimes passing into pinnacles, as in the Bad Lands of the west.
Broad, open folds, as we have seen, give, normally, synclinal hills and
anticlinal valleys (Fig. 22), when the erosion is well advanced. But in
more strongly, closely folded rocks the ridges and valleys are
determined chiefly by the outcrops of harder and softer strata, as shown
in Fig. 40, the symmetry of the reliefs depending upon the dip of the
strata. This principle of unequal hardness or durability also determines
most of the topographic features in regions of metamorphic and
crystalline rocks, in which the stratification is obscure or wanting.

    [Illustration: Fig. 39.—Horizontal strata and
        table-mountains.]

    [Illustration: Fig. 40.—Ridges due to the outcrops of hard
        strata.]

The boldness of the topography, and the relation of depth to width in
valleys, depends largely upon the altitude above the sea; but partly,
also, upon the distribution of the rainfall, the drainage channels or
valleys being narrowest and most sharply defined in arid regions
traversed by rivers deriving their waters from distant mountains. That
these are the conditions most favorable for the formation of cañons is
proved by the fact that they are fully realized in the great plateau
country traversed by the Colorado and its tributaries, a district which
leads the world in the magnitude and grandeur of its cañons. But deep
gorges and cañons will be formed wherever a considerable altitude, by
increasing the erosive power of the streams, enables them to deepen
their channels much more rapidly than the general face of the country is
lowered by rain and frost. This is the secret of such cañons as the
Yosemite Valley, and the gorge of the Columbia River, and probably of
the fiords which fret the north-west coasts of this continent and
Europe. For a full description and illustration of the topographic types
developed by the action of water and ice upon the surface of the land,
and of the various characteristic forms of marine erosion, teachers are
referred to the larger works named in the introduction, especially Le
Conte’s Elements of Geology, and to the better works on physical
geography. We will, in closing this section, merely glance at some of
the minor erosion-forms, which are not properly topographic, but may be
often illustrated by class-room and museum specimens. Mere weathering,
the action of rain and frost, develops very characteristic surfaces upon
different classes of rocks, delicately and accurately expressing in
relief those slight differences in texture, hardness, and solubility,
which must exist even in the most homogeneous rocks. Every one
recognizes on sight the hard, smooth surfaces of water-worn rocks. They
are exemplified in beach and river pebbles, in sea-worn cliffs, and
where rivers flow over the solid ledges. The pot-hole (page 17) is a
well-marked and specially interesting rock-form, due to current or river
erosion.

Ice has also left highly characteristic traces upon the rocks in all
latitudes covered by the great ice-sheet. These consist chiefly of
polished, grooved, and scratched or striated surfaces, the grooves and
scratches showing the direction in which the ice moved.

The organic agencies, as already noted, accomplish very little in the
way of erosion, especially in the hard rocks, but the rock-borings made
by certain mollusks and echinoderms may be mentioned as one unimportant
but characteristic form due to organic erosion.

------------------------------------------------------------------------




                                APPENDIX


The following collections are especially prepared and arranged for use
with this text:

                  _Weathering_

                     1 Diabase
                    *2   “      weathered
                    *3   “      disintegrated
                    °4 Felsite: Angular fragment
                    °5   “      Water rounded pebble

                  _Formation of Coals_

                    *6 Peat
                    °7 Lignite
                     8 Bituminous
                    *9 Cannel coal
                   °10 Anthracite
                   °11 Native coke

                  _Rock-forming Minerals_

                   *12 Graphite
                   °13 Halite
                   *14 Limonite
                   *15 Hematite
                   *16 Magnetite
                    17    “      Lodestone
                   *18 Quartz: Glassy
                    19   “     Flint
                    20   “     Chert
                    21 Opalized wood
                   *22 Gypsum
                   *23 Calcite
                   °24 Dolomite
                    25 Siderite
                   *26 Hornblende
                   °27 Pyroxene
                   *28 Muscovite
                    29 Biotite
                   *30 Orthoclase
                   °31 Albite
                   *32 Labradorite
                   *33 Kaolinite
                    34 Talc
                   *35 Serpentine
                   °36 Chlorite
                    37 Glauconite (Green Sand)
                    38 Chrysolite
                    39 Garnet
                    40 Pyrite

                  _Sedimentary and Metamorphic Rocks_

                   *41 Conglomerate: Breccia
                   *42     “         Pudding-stone
                   *43 Sand: Quartz
                   °44   “   Magnetite
                   *45 Sandstone: Ferruginous
                    46     “      Calcareous
                    47     “      Arkose
                   *48 Quartzite
                    49 Clay: Boulder
                   °50  “    Fire
                   *51 Shale
                   *52   “   Carbonaceous
                    53 Slate: Roofing
                    54   “    Flagstone
                    55 Porcelainite
                    56 Tripolite
                   °57 Siliceous Tufa
                    58 Novaculite
                   °59 Asphaltum
                   °60 Oil Sand
                   *61 Limestone: Fossiliferous
                   *62     “      Coquina
                   *63     “      Chalk
                    64     “      Crystalline
                   °65     “      Compact
                    66     “      Hydraulic
                    67 Calcareous Tufa
                    68 Dolomite
                    69 Rock Salt
                   °70 Phosphate Nodule
                   *71 Gneiss: Granitoid
                   *72    “    Micaceous
                    73    “    Hornblendic
                   °74 Norite: Hypersthenite
                   *75 Schist: Mica
                    76    “    Hornblende
                    77    “    Talc
                    78    “    Chlorite
                    79 Amphibolite
                    80 Soapstone
                    81 Verd Antique (Serpentine)

                  _Igneous Rocks_

                   *82 Granite: Binary
                    83   “      Muscovite
                   *84   “      Biotite
                    85   “      Hornblendic
                    86   “      Red
                   *87 Syenite
                    88   “     Elæolite
                   *89 Diorite
                   *90 Diabase: Trap
                   *91 Rhyolite
                    92 Trachyte
                   *93 Obsidian
                    94 Pumice
                   °95 Petrosilex
                   *96 Andesite
                   *97 Basalt
                    98   “    vesicular Lava
                   *99 Melaphyr: Amygdaloidal
                  °100 Volcanic Tuff

     _Collection No. F1._ Entire list of 100 museum size
       specimens (3¼ × 4¼), numbered, labelled and mounted
       on blocks or in improved trays, for museum display
       and laboratory work                                   $40.00

               (The same, labelled but unmounted, $30.00)

     _Collection No. F2._ Same as above, but small museum
       size, mounted in improved trays (2½ × 3½)             $25.00

     _Collection No. F3._ Same as F2, but hand size
       specimens (2 × 2)                                      12.50

     _Collection No. F4._ 80 specimens, omitting those
       marked (°), in individual trays (2½ × 1¾) and two
       cloth-board cases, numbered to correspond with
       accompanying printed list (no labels)                   5.00

     _Collection No. F5._ 40 specimens marked (*), mounted
       as collection F4                                        2.50

     _Collection No. F6._ 100 pupils’ fragments (1 × 1),
       numbered, in paper bags. (Single collection $1.25.)
       In lots of 5 or more, each                              1.00

     _Collection No. F7._ 80 pupils’ fragments (like F6).
       (Single $1.00.) In lots of 5 or more, each               .75

     _Collection No. F8._ 40 pupils’ fragments (like F6).
       (Single 50c.) In lots of 5 or more, each                 .40

     _Collection No. F9._ 25 museum size specimens,
       illustrating structure, faults, stratification, etc.
       Mounted and labelled                                   10.00


           _For further information or in ordering, address_

                  WARD’S NATURAL SCIENCE ESTABLISHMENT
               84-102 College Ave.,      Rochester, N. Y.

------------------------------------------------------------------------

                          Transcriber’s note:

Specimen numbers have been regularised as medium weight.

Page 7, ‘LeConte’ changed to ‘Le Conte,’ “By Prof. Joseph Le Conte.”

Page 12, ‘contined’ changed to ‘continued,’ “continued exposure to the”

Page 28, comma changed to full stop, “associated with beds of coal.”

Page 34, ‘or’ changed to upright, “_superficial_ or _aqueous_ agencies”

Page 96, all instances of ‘per cent’ changed to ‘per cent.’: “60 per
cent.”, “30 per cent.”, “85 per cent.”

Page 97, full stop inserted after ‘crust,’ “of the earth’s crust. The
name”

Page 105, specimen number absent in original.

Page 106, ‘green sand’ changed to ‘greensand,’ “greensand, and
serpentine”

Page 107, ‘magnesion’ changed to ‘magnesian,’ “the entire rock is
magnesian,”

Page 108, ‘70’ changed to ‘69,’ “classification on page 69”

Page 114, full stop inserted after ‘rocks,’ “the stratified rocks. Like”

Page 126, full stop inserted after ‘veins,’ “in veins. Those fissures”

Page 146, instance of thousands formatted without comma delimiter: “4000
to 5000 feet”

Page 188, instance of thousands formatted without comma delimiter: “7000
feet high”





End of Project Gutenberg's Common Minerals and Rocks, by William O. Crosby