Common Minerals and Rocks

Part 1

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

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INTRODUCTION.

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.

Part 2

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.

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.

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