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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- 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