An introduction to physical geology : with special reference to North America

Front matter

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Preface

Tus “Introduction to Physical Geology” has been prepared as a companion volume to the author’s ‘Introduction to Historical Geology.”’ The two books together are intended to serve as a text for a one-year college course in general geology. For such a purpose, the two books may be procured bound in a single volume called ‘‘An Introduction to Geology.” The present book may be used, however, as a text for special courses in physical geology. It may be adapted readily to shorter courses by the omission of certain portions, according to the judgment of the instructor.

The book has been so written that a formal knowledge of neither chemistry nor physics is a prerequisite for a reasonable understanding of its contents. The instructor ordinarily can supply the most needed information along these lines in the class-room and laboratory lectures and discussions.

It is assumed that at least some laboratory and field work will be carried on in connection with the course. Field trips should be made to illustrate as many of the principles of the science as time and conditions will permit. Such out-door work greatly aids in making the study more realistic and interesting. Laboratory work should be, as far as possible, directly correlated with the class-room and text-book work. The student should study specimens of minerals and rocks, and also models, maps, and diagrams. Many laboratory problems and exercises may be devised along the general lines of United States Geological Survey Professional Paper No. 60, entitled “Interpretation of Topographic Maps.” The Geological Survey has a wealth of topographic maps, any of which may be purchased singly or in duplicate at very small cost.

Concrete examples are freely used to illustrate important facts and principles, and, since geology is essentially an_ historical science, the historical order has been emphasized in the treatment of the special topics and concrete examples.

Careful attention has been given to the arrangement of the subject-matter. The very nature of the subject is such, however,

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Iv PREFACE that some repetition and anticipation are unavoidable no matter what the arrangement. The purpose has been not only to make the order of treatment logical, but also to avoid repetition and anticipation as far as possible. With these ends in view, the materials of the earth — minerals and rocks — are first considered after the introductory chapter. This is because the subject of study is the earth, and a knowledge of the nature of its materials is fundamental to all that follows. In the writer’s experience, it is well to begin the laboratory work in geology with a study of minerals and rocks, using the data in Chapters III and IV as a laboratory guide. Then the manner in which rocks weather, the movements to which they are subjected in the earth’s crust, and their structural arrangement are taken up in regular order. The important chapter on the work of streams is then taken up much more satisfactorily, it is believed, than in most text-books because the knowledge of earth materials, weathering, earth-crust movements, and earth-crust structures already gained greatly aids in the understanding of certain important phases of river work. Glaciers, wind, sea, volcanoes, and underground waters are next discussed as geological agencies. Mountains, lakes, and economic geology are taken up last because a knowledge of so many principles of the science is necessary to a proper understanding of these subjects.

Much time and thought have been devoted to the gathering of the illustrations which form an essential part of the book. Most of them have never before appeared in any text-book. Many of the half-tones have been made from pictures taken by the author for the express purpose of illustrating the book. All of the views and diagrams illustrate important facts and principles of the science, and they should be, therefore, carefully studied in connection with the text.

Among the numerous original sources of photographs, special mention should be made of the United States Geological Survey, the United States Reclamation Service, the United States Forestry Bureau, the National Park Service, and the American Museum of Natural History. The Macmillan Company has generously permitted the use of several cuts.

The most important physiographic features and places referred to in the text are shown on the photographed relief map of North America, and on the map of the United States, in Chapter I. In

PREFACE V addition to these maps, the student should have at hand some good geography or atlas in order that other places and features mentioned and described can be definitely and readily located.

The author is under particular obligation to Professor J. A. Bownocker of Ohio State University for valuable suggestions, nearly all of which have been incorporated in the book. The author gratefully acknowledges his indebtedness to Associate Professor Aida A. Heine and Dr. Isabel F. Smith of the Department of Geology of Smith College for reading manuscript and offering suggestions. Conversations with various experienced teachers of geology also have been very helpful.

Due acknowledgment is here made for the help obtained from the various manuals, text-books, and special treatises on geology, and also from many publications of the United States Geological Survey.

Corrections and suggestions for the improvement of the book will be heartily welcomed. ; WILLIAM J. MILLER UNIVERSITY OF CALIFORNIA, SOUTHERN BRANCH, Los Angeles, California, July, 1914.

CONTENTS CHAPTER I. INTRODUCTION . Pie oat A Il. MATERIALS OF THE EARTH — MINERALS .

Il]. Mareriats oF THE EARTH — Rocks IV. Rock WEATHERING. tal V. INSTABILITY OF THE EaARTH’s CRUST VI. SrructuRE oF THE EARTH’S CRUST .

VII. Tae Work or STREAMS VIII. GuAcIERS AND THEIR WorK IX. Gero.tocicaL AcTION oF WIND X. THe SEA AND ITs WoRK .

XI. VOLCANOES XII. SuspsurFAcE WATER XIII. Mountains, PLATEAUS AND PLAINS .

XIV. Orictn AND History oF LAKEs .

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WITH SPECIAL REFERENCE TO NORTH AMERICA

Introduction

DEFINITION OF GEOLOGY

GEOLOGY (meaning literally “ earth science ’’) deals with the history of the earth and its inhabitants as recorded in the rocks. Broadly considered, the science may be divided into physical geology and historical geology. Physical geology deals with the materials of the earth; earth-crust movements; the structure of the earth; and the processes and agencies by which the earth has been _ for many millions of years, and is being, modified, including such agencies as weather, wind, streams, glaciers, sea, organisms, volcanoes, subterranean waters, and lakes. Historical geology deals with the records of the successive events of earth history, and with the history and evolutionary changes of the organisms which have lived upon the earth. This book deals with physical geology only. Geography deals with the distribution of the earth’s physical features, in their relation to each other, and to the life of sea and land, especially human life and activity. Geography may, therefore, be regarded as the outward and present-day expression of geological effects. Geology includes geography as cause includes effect.

THe EARTH AS A PLANET

Since the earth is our subject for study, it is important that the reader have clearly in mind certain well-known facts in regard to it as a planet. The earth is a member of the so-called solar system of which the sun, whose diameter is about 866,000 miles, is the center. Eight planets, including the earth, revolve around the sun. The earth, whose diameter is nearly 8000 miles, rotates on its axis once in 24 hours. Its average distance from the sun is nearly 93,000,000 miles, and it revolves around the sun once in a little over 365 days. Certain planets are much larger and farther away from the sun than the earth, and some others are smaller and nearer the sun.

One satellite, called the moon, revolves around the earth once in about 28 days. Although the moon is much smaller than the earth, nevertheless it has a considerable indirect geological influence upon the earth because it is the principal cause of ocean tides which latter have been of some importance for many millions of years of the earth’s history.

The geological influence of the sun upon the earth is far greater than that of the moon because it is the chief source of the earth’s light, heat, and energy which have made largely or wholly possible not only the work of rock weathering, streams, glaciers, and winds, but also plant and animal growth and progressive development.

The earth seems large, and it is very important to us as human beings, but it is very small as compared to the size of the solar system, and almost infinitesimal within the vast universe of which even the solar system is only a very small part.

THE GREAT PARTS OF THE EARTH

The three great parts of the earth are the lithosphere, hydrosphere, and atmosphere. The lithosphere (meaning ‘“ rock sphere ’’), which consists of solid, rocky material, constitutes by far the greater portion of the earth, including not only the lands, but also the rocky materials under the oceans. The lithosphere is of supreme importance to the geologist because the records of the wonderful events of the earth’s history are found in the rocks.

The partial envelope comprising all the waters on and near the earth’s surface is called the hydrosphere. Most of the waters by far are in the oceans, but streams, lakes, and underground waters are also important. Water is one of the greatest of all geological agencies which, for countless ages, have been modifying the earth. Its greatest function is the wearing down (erosion) of the higher portions of the lithosphere, and the transportation and deposition

INTRODUCTION 3 of the resulting sediment in the various lower portions (basins) of the lithosphere.

The gaseous envelope of the earth is called the atmosphere (air). To some extent it penetrates the outer portion of the earth’s i

Biot 1 Map of North America showing the relations of land and sea during part of Cretaceous time, millions of years ago. (After C. Schuchert.)

crust through openings in the rocks, and to some extent it is dissolved in the waters. The chief constituents of the air are nitrogen, oxygen, water vapor, and carbon dioxide, all of which, excepting the first, effect important changes in the rocks and minerals of the earth. Movements of the atmosphere (winds) cause important modifications of the lands, especially in arid regions. Probably the greatest geological function of the atmosphere is its making possible precipitation (rainfall and snowfall), which in turn makes possible the work of running water and glaciers.

THE SCOPE AND SIGNIFICANCE OF GEOLOGY

The person of ordinary intelligence is, unless he has devoted some study to the matter, very likely to regard the great variety of physical features and life of the earth as practically unchangeable, and to think that they were essentially the same in the beginning of the earth’s history as they are now. But the study of geology has firmly established the great fact that the face of the earth, and the life upon it, represent merely a single phase of a tremendously long history which has involved many profound and far-reaching changes.

‘ The following concise statements of some of the more definite and important conclusions regarding earth changes may serve to give a fair con- | ception of the general scope

Fig. 2 and significance of geology.

A fossil sea-animal (Trilobite) at For untold millions of years

least 25,000,000 years old found rocks at and near the surface

9000 feet above sea level in the of the earth have been erum-

Rocky Mountains. (Photo by the bling under the weather; author.) 3

streams have been sawing

incessantly into the lands; the sea has been eating into con-

tinental masses; the winds have been sculpturing desert lands;

and, more locally and intermittently, glaciers have plowed

INTRODUCTION 5 through mountain valleys, and even vast sheets of ice have — spread over considerable portions of continents. The outer shell . (so-called “ crust ’’) of the earth has shown marked instability throughout geologic time. Slow upward and downward movements of the lands relative to sea level have been very common, in many cases amounting to thousands of feet. Various parts of the earth have been, and are being, affected by sudden movements

Fig. 3 The skeleton of a great carnivorous reptile unearthed from rocks millions of years old. (Courtesy of the American Museum of Natural History.)

(resulting in earthquakes) along fractures in the outer crust. During the eons of geological time, vast quantities of molten materials have, at intervals, been forced not only into the earth’s crust, but also often out upon the surface. Mountain ranges have been brought forth and cut down, and sometimes rejuvenated. Sea waters have spread over many parts of what are now continental areas. There have been repeated advances and retreats of the sea over many districts. Lakes have come and gone. Plants and animals have inhabited the earth for many millions of years. In earlier known geological time the organisms were comparatively simple and low in the scale of organization. Through the succeeding ages higher and more complex types were gradually evolved until the highly organized forms of the present time, including human beings, were produced.

GEOLOGICAL TIME

The great importance of the time element in the study of geology cannot be too strongly impressed upon the reader. The length of time of known human history is, indeed, very short as compared to that of known geological time. The one is to be measured by thousands of years, and the other by tens, or possibly hundreds, of millions of years. To the geologist a lapse of hundreds of thousands of years is a ‘‘short”’ time. ‘“ The flowing landscapes of geologic time may be likened to a kinetoscopic panorama. The scenes transform from age to age; seas and plains and mountains of different types follow and replace each other through time, as the traveler sees them succeed each other in space. At times the drama hastens, and unusual rapidity of geologic action has, in fact, marked those epochs since man has been a spectator upon the earth. (Geological) science demonstrates that mountains are transitory forms, but the eye of man through all his lifetime sees no (important) change, and his reason is appalled at the conception of a duration so vast that the millenniums of human history have not accomplished the shifting of even one of the fleeting views which blend into the moving picture ”’ (J. Barrell).

The known history of the earth has been more or less definitely divided into great eras, and these in turn into periods and epochs. In the accompanying table, the era and period names, except those representing the earlier times, are mostly world-wide in their usage. Epoch names are too numerous, and usually too local in application, to be included in the table for our general use in this book. It will be well for the reader to learn at least the names of the eras, and to refer to the table whenever, in the study of the text, he is in doubt concerning any part of it.

How does the geologist determine the geological age of a given rock formation? This very important question is solved by methods of historical geology. Very briefly stated, however, two fundamental principles are involved. First, the order of

INTRODUCTION ie succession of stratified rock formations is determined, the older strata underlying the younger because they were first deposited. Second, each period and epoch of geologic time (except the very oldest) is known to have had a characteristic assemblage of organisms. A consideration of the characteristic fossil content of any set of strata, therefore, in direct connection with order of succession (superposition) of the larger pile of strata to which the set belongs, serves to determine the relative position of the set of strata (or formation), and hence its geological age.

TABLE OF MAIN

GEOLOGICAL DIVISIONS

Estimated Minimum Eras Periods ‘Characteristic Life Duration 3 to 5 Age of highest million Quaternary. Age of man. order of years Cenozoic plants and Tertiary. Age of mammals, animals.

Cretaceous. First high order flowering| Age of reptiles 5 to 10 plants. and million Mesozoic. Jurassic. First birds and modern fishes. ceycad plants. years. Triassic. First mammals (very primitive).

Permian. Age of amphibians: first insects; and _ first Pennsylvanian. reptiles. Great coal age (Pennsylvanian) Mississippian. with large, non-flowering plants, 15 to 25 million years. Paleozoic. Devonian. Age of primitive fishes, and first-known (very primitive) land plants.

Silurian. Age of invertebrate animals; first (very primi- Ordovician. tive) vertebrates in Ordovician; and no Cambrian. known land animals.

Many millions Meager records of relatively simple inverteof years. | Proterozoic. | Algonkian. brate animals, and very simple plants. Many millions of years. Archean. Primitive life; no determinable fossils.

Archeozoic.

BRANCHES OF GEOLOGICAL SCIENCE

It has already been suggested plainly that geology is very broad in its scope, and so with this general science, as with most

Fig. 4 Relief Map of North America. (Courtesy of the United States Geological Survey.)

other great departments of human knowledge, a number of more or less separate branches of geology have come into existence as special fields of study. Most of the principal branches are as follows:

INTRODUCTION 9

Mineralogy is the study of minerals which are natural, homogeneous substances of definite (chemical) composition. With the exception of a relatively very slight amount of organic material, minerals constitute the whole lithosphere as far as it is known.

Petrology is the study of rocks, which are more or less extensive constituents (or formations) of the earth’s crust, and which are nearly always made up of mixtures of minerals, or, more rarely, of masses of single minerals.

Dynamical geology is the study of the agencies and processes whereby the outer portion of the earth has been, and is being, modified. Important dynamical agents are weather, wind, running water, the sea, glaciers, igneous actions, and earth-crust movements.

Physiography, sometimes called physical geography, deals with the topography of the earth’s surface and the manner of its origin. It is closely related to dynamical geology because it involves a consideration of the same modifying forces.

Structural geology is the study of the arrangement or architecture of the materials of the earth. In a real sense it includes a study of the materials themselves, especially the rocks, and so it may be regarded as including petrology, and possibly mineralogy.

Paleontology deals with the plant and animal life of the geolo- ,gical ages as shown by the fossil remains of organisms found in the rocks.

Stratigraphy deals with the arrangement and succession of the strata of the earth.

Paleogeography is the study of the geographic conditions of the earth during former (geologic) ages, especially with the relations of lands and seas. Paleontology, stratigraphy, and paleogeography are really subdivisions of hzstorical geology, which, as already defined, deals with the successive events of earth history, including the history of organisms.

Economic geology is the practical application of geology to the arts and industries. It deals with geological products of value to mankind, such as coal, petroleum, ores of the metals, building stones, salt, gypsum, etc.

Geography, as already defined, is, in a broad sense, a branch of geology because it is the outward and present-day expression of geological effects. It is, however, often treated as a separate department of knowledge.

SELECTED REFERENCES ON PHYSICAL GEOLOGY

General Works

Chamberlin and Salisbury: Geology, Vol. 1 (Henry Holt & Co., 1903). An elaborate American work.

Grabau: Text-book of Geology, Vol. 1 (Heath & Co., 1920). A comprehensive treatment of physical geology.

Geikie, A.: Text-book of Geology, Vol. 1 (Macmillan Co., 1903). A comprehensive English work with emphasis upon European geology.

Haug: T'raité de Géologie, Vol. 1 (A. Colin, Paris, 1911). A comprehensive French work with emphasis upon European geology.

Kayser: Lehrbuch der Geologie, Part 1 (F. Enke, Stuttgart, 1912). A comprehensive German work with emphasis upon European geology.

Lake and Rastall: A Text-book of Geology (E. Arnold, London, 1910). Contains a fairly comprehensive treatment with emphasis upon European | geology.

Dana, J.: Manual of Geology, Parts 1, 2, and 3 (American Book Co., 1895). An older, fairly comprehensive treatment.

Tarr: Elementary Geology, Parts 1 and 2 (Macmillan Co., 1897). An older, fairly comprehensive treatment.

LeConte: Hlements of Geology, Parts 1 and 2 (Appleton & Co., 1877, 1898, 1907). An older, fairly comprehensive treatment.

Chamberlin & Salisbury: College Geology, Part 1 (Henry Holt & Co., 1909). A fairly comprehensive treatment. :

Scott: An Introduction to Geology, Parts 1, 2, and 3 (Macmillan Co., 1897, 1907). <A fairly comprehensive treatment.

Pirsson & Schuchert: Tezt-book of Geology, Part 1 (John Wiley & Sons, 1915, 1920). A fairly comprehensive treatment.

Cleland: Geology, Physical and Historical, Part 1 (American Book Co., 1916). A fairly comprehensive treatment.

Brigham: A Text-book of Geology (Appleton & Co., 1902). Contains an elementary discussion of physical geology.

Norton: Elements of Geology, Parts 1 and 2 (Ginn & Co., 1905). An elementary discussion.

Blackwelder & Barrows: Elements of Geology, Part 1 (American Book Co., 1911). An elementary discussion.

Chamberlin & Salisbury: Introductory Geology, Part 1 (Henry Holt & Co., 1914). An elementary discussion.

Miller, W. J.: Geology; the Science of the Earth’s Crust (Vol. 3 of Popular Science Library by P. F. Collier & Son Co., 1922), An elementary discussion in popular form.

Dana, J.: Text-book of Geology, Parts 1, 2, and 3 (American Book Co., 1897). A very elementary discussion.

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Special Works

Hobbs: Earth Features and Their Meaning (Macmillan Co., 1912). A fairly comprehensive treatment of physical geology, but with emphasis upon glacial geology.

Tarr & Martin: College Phystography (Macmillan Co., 1914). Contains a comprehensive treatment of physiographic geology.

Salisbury: Phystography (Henry Holt & Co., 1907, 1919). Contains a fairly comprehensive treatment of physiographic geology.

Fairbanks: Practical Physiography (Allyn & Bacon, 1906). <A fairly comprehensive discussion of physiographic geology.

Geikie, J.: Harth Sculpture (Putnam’s Sons, 1898). A fairly comprehensive discussion of the origin of land forms with emphasis upon European examples.

Davis, W. M.: Geographical Essays (Ginn & Co., 1909). Elaborate discussions of principles of land sculpture.

Geikie, J.: Structural and Field Geology (D. Van Nostrand Co., 1908). A fairly comprehensive treatment of the subjects indicated, with emphasis upon European examples.

Salisbury & Atwood: Interpretation of Topographic Maps (U. 8. Geological Survey, Professional Paper No. 60, 1908). A study of land forms by the use of topographic maps. :

Leith: Structural Geology (Henry Holt & Co., 1913, 1923). A fairly comprehensive treatment of the subject indicated.

Willis: Geologic Structures (McGraw-Hill Co., 1923). <A fairly comprehensive treatment of the subject indicated.

Dana, E. S.: Minerals and How to Study Them (John Wiley & Sons, 1895). An elementary discussion of minerals.

Dana, E. S.: A Teat-book of Mineralogy (John Wiley & Sons, 1877, 1916). A rather comprehensive study of minerals.

Rogers: Introduction to the Study of Minerals and Rocks (McGraw-Hill Co., 1912, 1921). A fairly comprehensive discussion of minerals.

Ford: Dana’s Manual of Mineralogy (John Wiley & Sons, 1912). A fairly comprehensive treatment of minerals.

Bayley: Descriptive Mineralogy (Appleton & Co., 1917). A rather elaborate treatment of the subject indicated.

Kraus & Hunt: Mineralogy (McGraw-Hill Co., 1920). A rather comprehensive treatment of minerals.

Kemp: A Handbook of Rocks (D. Van Nostrand Co., 1896, 1911). An elementary discussion of rocks, including a glossary of terms.

Pirsson: Rocks and Rock Minerals (John Wiley & Sons, 1908). An elementary account of common rocks and rock-making minerals.

Merrill, G. P.: Rocks, Rock Weathering, and Soils (Macmillan Co., 1897). A fairly elaborate discussion of the subjects indicated.

Ries: Economic Geology (John Wiley & Sons, 1905, 1916). A fairly comprehensive treatment with special reference to the United States.

Emmons, W. H.: General Economic Geology (McGraw-Hill Co., 1922). A fairly comprehensive treatment of the subject indicated.

INTRODUCTION 13

Lindgren: Mineral Deposits (McGraw-Hill Co., 1918, 1919). A fairly comprehensive treatment of minerals of economic value.

Geikie, J.: Mountains; Their Origin, Growth, and Decay (Oliver & Boyd, 1913).

Dutton: Harthquakes (Putnam’s Sons, 1904).

Hobbs: Earthquakes (Appleton & Co., 1907).

Russell: Rivers of North America (Putnam’s Sons, 1898).

Russell: Glaciers of North America (Ginn & Co., 1897).

Hobbs: Characteristics of Existing Glaciers (Macmillan Co., 1911).

Russell: Volcanoes of North America (Macmillan Co., 1897).

Bonney: Volcanoes, Their Structure and Significance (Putnam’s Sons, 1898).

Russell: Lakes of North America (Ginn & Co., 1897).

Chapter II Materials Of The Earth — Minerals !

DEFINITION AND SIGNIFICANCE OF MINERALS

MINERALS are, with slight exceptions, the materials which constitute the known parts of the earth. Mineralogy is, therefore, in a very real sense the most fundamental of the various branches of the great science of geology because the events of earth-history, as interpreted by the geologist, are recorded in the mineral matter (including most rocks) of the earth. When we examine the rocky material or mineral matter of the earth in any region we find that it consists of various kinds of substances, each of which may be recognized by certain characteristics. Each definite substance (barring those of organic origin) is called a mineral. Or, more specifically, a mineral is a homogeneous substance of definite chemical composition found ready-made in nature and not a product of life. According to this definition, a mineral must be a natural, inorganic substance of the same nature throughout, and its composition must be so definite that it can be expressed by a chemical formula.

All artificial substances, such as laboratory and furnace products, are excluded from the category of minerals because they have taken no part in the history of the earth. Coal is not a mineral both because of its variable composition and its organic origin. A few examples of very common substances which satisfy perfectly the definition of a mineral are quartz, feldspar, mica, calcite, and magnetite. More than a thousand mineral species are known. To these, and their varieties, several thousand names have been given. Not more than forty or fifty of the many minerals are, however, of great geological importance, and of these only six or eight make up more than ninety per cent of the outer

1 Considerable portions of this chapter are taken by permission from Chapter XX of the present author’s Geology: The Science of the Earth’s Crust, which forms Volume 3 of Popular Science Library published by P. F. Collier & Son Company.

or crustal portion of the earth. Only two minerals — water and mercury — ordinarily exist in liquid form.

CHEMICAL MAKE-UP OF MINERALS

It is a surprising fact that of the ninety or more chemical elements, that is, substances which cannot be subdivided into simpler ones, only eight make up more than ninety-eight per cent of the weight of the earth’s crust. It is important to note, however, that, with one very slight exception, none of the eight exists as such in mineral form. These eight elements are oxygen (nearly fifty per cent), silicon (over twenty-five per cent), aluminum (over seven per cent), iron (over five per cent), calcium (or “lime ’’?), magnesium (or ‘‘ magnesia’), sodium (or “ soda ”’), and potassium (or ‘“‘ potash’). Among other elements found in useful or common minerals are carbon, hydrogen, sulphur, chlorine, fluorine, phosphorus, barium, copper, gold, lead, mercury, platinum, silver, tin, and zinc. Some of the elements last named may exist as such in nature, as for example, gold, copper, silver, carbon (in form of graphite and diamond), sulphur, and platinum.

In most cases by far two or more of the chemical elements are variously combined in such a manner (chemically) as to lose their identities as such. Thus the two vicious substances sodium and chlorine are combined to form the beneficial mineral called halite or common salt (composition, chloride of sodium). Oxygen and silicon (a gas and a solid) may be united to form the very hard, common mineral called quartz (composition, oxide of silicon). Three elements — calcium, carbon, and oxygen — are united in the common mineral known as calcite (composition, carbonate of lime). Four elements — potassium, aluminum, silicon, and oxygen — are chemically combined in the exceedingly common mineral known as orthoclase feldspar (composition, potassium aluminum silicate). Some other minerals are still more complicated in composition.

GEOLOGICAL IMPORTANCE OF MINERALS

Certain rock formations are made up essentially of but one mineral in the form of numerous individual grains, as for example pure limestone which may consist wholly of calcite (carbonate of lime), or pure sandstone which may contain only grains of quartz

(oxide of silicon). Most of the ordinary rocks are, however, made up of two or more minerals mechanically bound together. Thus, in a specimen of granite on the author’s desk, several distinct mineral species are distinguishable by the naked eye. _These mineral grains are from one to five millimeters across. Most common among them are hard, clear, glassy grains, called quartz; nearly white, hard grains, often with smooth faces, called feldspar;

Fig. 6 A Group of Quartz Crystals. (Courtesy of the American Museum ‘of Natural History.)

small, silvery white flakes, called mica; and small, hard, black grains, called magnetite.

It is the business of the mineralogist to learn the characters of each mineral, how they may be distinguished from each other, how they may be classified, how they are found in nature, how they originate, and what economic value they may have. It is an important part of the business of the geologist to learn what individual minerals combine to form the various kinds of rocks (described in Chapter II), how such rocks originate, what changes they have undergone, and what geological history they record. It is thus clear that mineralogy is an important part of geology, which latter is essentially the science of rocks.

CrystaL Forms oF MINERALS

One of the most remarkable facts about minerals is that most of them by far have a crystalline structure, that is they are built up of definitely arranged tiny particles known as molecules. Crystalline minerals are often more or less regular, solid forms bounded by plane faces and sharp angles, such forms being known as crystals (Fig. 6). How do crystals develop such regularity of form? Any solid is considered to be made up of many very tiny (sub-microscopic) molecules held together by an attractive force called cohesion. In liquids the molecules may more or less freely roll over each other, thus altering the shape of the mass without disrupting it. In gases the molecules are considered to be rela-