Produced by Juliet Sutherland, Tom Cosmas and the Online Distributed Proofreading Team at http://www.pgdp.net Transcriber's Note Italic text is denoted by _underscores_ and bold text by =equal signs=. Whole and partial numbers are displayed as 2-1/2. [Illustration: Photo, Ridge Studio; Courtesy Ogden Chamber of Commerce] _A great ledge in Ogden Canyon near Ogden, Utah. The rock, still retaining its stratification, was deposited layer upon layer horizontally mostly as sand upon the floor of a sea which covered the region fully 25,000,000 years ago. That the sea was of very early Paleozoic (i.e., Cambrian) age has been proved by fossils in associated strata. Long after their deep burial and consolidation within the earth, the strata were subjected to tremendous mountain-making pressure, notably altered to a rock called "Quartzite," raised high above sea level, and tilted almost vertically. Then through long ages (millions of years) overlying rocks of great thickness have been cut away (eroded) by weathering and stream action, laying bare the ledge as we see it to-day._ Popular Science Library EDITOR-IN-CHIEF GARRETT P. SERVISS AUTHORS WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER ERNEST INGERSOLL WILFRED MASON BARTON WILLIAM B. SCOTT ERNEST J. STREUBEL NORMAN TAYLOR DAVID TODD CHARLES FITZHUGH TALMAN ROBIN BEACH ARRANGED IN SIXTEEN VOLUMES WITH A HISTORY OF SCIENCE, GLOSSARIES AND A GENERAL INDEX _ILLUSTRATED_ [Illustration] VOLUME THREE P. F. COLLIER & SON COMPANY NEW YORK Copyright 1922 By P. F. Collier & Son Company MANUFACTURED IN U. S. A. GEOLOGY The Science of the Earth's Crust BY WILLIAM J. MILLER Professor of Geology, Smith College [Illustration] P. F. COLLIER & SON COMPANY NEW YORK PREFACE In the preparation of this book the author has attempted to present, in popular form, the salient points of a general survey of the whole great science of geology, the science which deals with the history of the earth and its inhabitants as revealed in the rocks. The use of technical and unusual terms has been reduced to a minimum compatible with a reasonable understanding of the subject by the layman. Each of the relatively few scientific terms is explained where first used in the text, and a glossary of common geological terms has been appended. The matter of illustrations has received very careful attention, and only pictures, maps, and diagrams are used which actually illustrate important features of the text. A special point has been made to introduce only cuts of simple construction comparatively free from technicalities. Nearly every illustration is accompanied by a really explanatory title. A number of the pictures are from the author's collection of photographs, and many of the line-cuts have either been made or considerably modified by the author. Among the numerous sources of illustrations, special mention should be made of the United States Geological Survey, the New York State Museum, the American Museum of Natural History, the University of Chicago Press, and various individuals, full credit being given wherever due. William J. Miller. Northampton, Mass. CONTENTS CHAPTER PAGE I. Introduction 9 II. Weathering and Erosion 22 III. Stream Work 30 IV. The Sea and Its Work 51 V. Glaciers and Their Work 59 VI. The Action of Wind 71 VII. Instability of the Earth's Crust 76 VIII. Volcanoes and Igneous Rocks 99 IX. Waters Within the Earth 113 X. How Mountains Come and Go 130 XI. A Study of Lakes 142 XII. How the Earth May Have Originated 158 XIII. Very Ancient Earth History 164 XIV. Ancient Earth History 179 XV. Medieval Earth History 208 XVI. Modern Earth History 221 XVII. Evolution of Plants 249 XVIII. Geological History of Animals (Excluding Vertebrates) 259 XIX. Geological History of Vertebrate Animals (Including Man) 281 XX. Mineralogy 307 XXI. Economic Geology 342 Glossary of Common Geological Terms 377 LIST OF ILLUSTRATIONS Ledge of Rock with Nearly Vertical Strata _Frontispiece_ PLATE FACING PAGE 1 Granite Weathering to Soil Leaving Residual Cores of Joint Blocks (_Upper picture_) 32 Looking-Glass Rock, Utah. Stratified Sandstone Sculptured by Wind Erosion (_Lower picture_) 32 2 Grand Canyon of the Yellowstone River. A Channel Worn by Erosion 33 3 Gorge of the Niagara River Below the Falls. A Sample of Recent Erosion 64 4 Winding Stream in the St. Lawrence Valley with Flood Plain (_Upper picture_) 65 Davidson Glacier, Alaska, Showing Action on the Valley's Walls and Floor (_Lower picture_) 65 5 Swift Current Valley in Glacier National Park, Formed by Stream and Glacier Action 96 6 Yosemite Valley from Western Entrance. The Result of Glacial Action 97 7 An Upbend Fold or Anticline in Maryland 128 8 Ledge of Igneous Rock Showing Joints (_Upper picture_) 129 Fault Fracture in Limestone Formations (_Lower pictures_) 129 9 Molten Lava Flowing Over a Cliff in Hawaii (_Upper picture_) 160 Dikes of Granite Cutting Old Rock (_Lower picture_) 160 10 Lassen Peak, California, in Eruption (_Left picture_) 161 Devil's Tower, Wyoming, Once a Mass of Molten Rock Forced Through Strata (_Right picture_) 161 11 Mammoth Hot Springs Terrace in Yellowstone National Park (_Upper picture_) 224 Crater Lake, Oregon, Formed by the Subsidence of a Volcano (_Lower picture_) 224 12 Archeozoic Rock, Oldest Known Rock Formation on Earth (_Upper picture_) 225 Twisted Mass of Stratified Limestone, Surrounded by Granite (_Lower picture_) 225 13 Paleozoic Rock, Covered with Oldest Known Animal Remains 256 14 "Stone-Lily" Stems in Ordovician Strata (_Upper left picture_) 257 Various Brachiopods in Ordovician Strata (_Upper right picture_) 257 Stratified Limestone of Middle Ordovician Era Containing Fossils (_Lower picture_) 257 15 A Landscape of the Coal Age (_Upper picture_) 288 Fossil Fern on a Piece of Shale (_Lower picture_) 288 16 Restoration of Huge Animals of the Mesozoic Era 289 17 Skeleton of Two-Legged Carnivorous Dinosaur (_Large picture_) 320 Earliest Known Bird Form (_Insert_) 320 18 Skeleton of Large Flying Reptile (_Upper picture_) 321 Skeleton of a Swimming Reptile of the Mesozoic Era (_Lower picture_) 321 19 Appalachians Along New River, Peneplain Upraised Again 352 20 Glacial Bowlder Left by Ice Sheet (_Upper picture_) 353 Esker, Deposited by a Stream in the Great Glacier (_Lower picture_) 353 CHAPTER I INTRODUCTION Earth features are not fixed. The person of ordinary intelligence, surrounded as he is by a great variety of physical features, is, unless he has devoted some study to the subject, very likely to regard those features as practically unchangeable, and to think that they are now essentially as they were in the beginning of the earth's history. Some of the most fundamental ideas taught in this book are that the physical features of the earth, as we behold them to-day, represent but a single phase of a very long-continued history; that significant changes are now going on all around us; and that we are able to interpret present-day earth features only by an understanding of earth changes in the past. _Geology_, meaning literally "earth science," deals with the history of the earth and its inhabitants as revealed in the rocks. The science is very broad in its scope. It treats of the processes by which the earth has been, and is now being, changed; the structure of the earth; the stages through which it has passed; and the evolution of the organisms which have lived upon it. _Geography_ deals with the distribution of the earth's physical features, in their relation to one another, to the life of sea and land, and human life and culture. It is the present and outward expression of geological effects. As a result of the work of many able students of geology during the past century and a quarter, it is now well established that our planet has a definitely recorded history of many millions of years, and that during the lapse of those eons, revolutionary changes in earth features have occurred, and also that there has been a vast succession of living things which, from very early times, have gradually passed from simple into more and more complex forms. The physical changes and the organisms of past ages have left abundant evidence of their character, and the study of the rock formations has shown that within them we have a fairly complete record of the earth's history. Although very much yet remains to be learned about this old earth, it is a remarkable fact that man, through the exercise of his highest faculty, has come to know so much concerning it. The following words, by the late Professor Barrell, admirably summarize the significance of geological history. "The great lesson taught by the study of the outer crust is that the earth mother, like her children, has attained her present form through ceaseless change, which marks the pulse of life and which shall cease only when her internal forces slumber and the cloudy air and surf-bound ocean no more are moving garments. The flowing landscapes of geologic time may be likened to a kinetoscopic panorama. The scenes transform from age to age, as from act to act; 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. Science demonstrates that mountains are transitory forms, but the eye of man through all his lifetime sees no change, and his reason is appalled at the conception of a duration so vast that the milleniums of written history have not accomplished the shifting of even one of the fleeting views which blend into the moving picture."[A] [A] Central Connecticut in the Geologic Past, pp. 1-2. Or in the words of Tennyson: There rolls the deep where grew the tree. O, earth, what changes hast thou seen! There where the long street roars, hath been The stillness of the central sea. The hills are shadows, and they flow From form to form, and nothing stands; They melt like mist, the solid lands, Like clouds they shape themselves and go. The following statement of some of the more definite important conclusions regarding earth changes may serve to make still clearer the general scope of the science of geology. The evidences upon which these conclusions are based are discussed in various parts of this book. For untold millions of years the rocks at and near the earth's surface have been crumbling; streams have been incessantly sawing into the lands; the sea has been eating into continental masses; the winds have been sculpturing desert lands; and, more intermittently and locally, glaciers have plowed through mountain valleys, and even great sheets of ice have spread over considerable portions of continents. Throughout geologic time, the crust of the earth has shown marked instability. Slow upward and downward movements of the lands relative to sea level have been very common, in many cases amounting to even thousands of feet. Various parts of the earth have been notably affected by sudden movements (resulting in earthquakes) along fractures in the outer crust. During millions of years molten materials have, at various times, been forced into the earth's crust, and in many cases to its surface. Mountain ranges have been brought forth and cut down. The site of the Appalachian Mountains was, millions of years ago, the bottom of a shallow sea. Lakes have come and gone. The Great Lakes have come into existence very recently (geologically), that is to say, since the great Ice Age. A study of stratified rocks of marine origin shows that all, or nearly all, of the earth's surface has at some time, or times, been covered by sea water. Over certain districts the sea has transgressed and retrogressed repeatedly. Organisms have inhabited the earth for many millions of years. In earlier known geologic time, the plants and animals were comparatively simple and low in the scale of organization, and through the succeeding ages higher and more complex types were gradually evolved until the highly organized forms of the present time, including the human race, were produced. The rocks of the earth constitute the special field of study for the geologist because they contain the records of events through which the earth and its inhabitants have passed during the millions of years of time until their present conditions have been reached. All the rocks of the earth's crust may be divided into three great classes: _igneous_, _sedimentary_, and _metamorphic_. _Igneous_ rocks comprise all those which have ever been in a molten condition, and of these we have the _volcanic_ rocks (for example, lavas), which have cooled at or near the surface; _plutonic_ rocks (for example, granites), which have cooled in great masses at considerable depths below the surface; and the _dike_ rocks which, when molten, have been forced into fissures in the earth's crust and there cooled. _Sedimentary_ rocks comprise all those which have been deposited under water, except some wind-blown deposits, and they are nearly always arranged in layers (stratified). Such rocks are called strata. They may be of mechanical origin such as clay or mud which hardens to _shale_; sand, which consolidates into _sandstone_; and gravel, which when cemented becomes _conglomerate_. They may be of organic origin such as limestone, most of which is formed by the accumulation of calcareous shells; _flint_ and _chert_, which are accumulations of siliceous shells; or _coal_, which is formed by the accumulation of partly decayed organic matter. Or, finally, they may be formed by chemical precipitation, as beds of _salt_, _gypsum_, _bog iron ore_, etc. _Metamorphic_ rocks include both sedimentary and igneous rocks which have been notably changed from their original condition. Traces or remains of plants and animals preserved in the rocks are known as fossils. The term originally meant anything dug out of the earth, whether organic or inorganic, but for many years it has been strictly applied to organic remains. Many thousands of species of fossils are known from rocks of all ages except the oldest, and more are constantly being brought to light, but these represent only a small part of the life of past ages because relatively few organic remains were deposited under conditions favorable for preservation in fossil form. The fossils in the rocks are, however, a fair average of the groups of organisms to which they belong. It is really remarkable that such a vast number of fossils are imbedded in the rocks, and from a study of these many fundamental conclusions regarding the history of life on our planet may be drawn. As early as the fifth century B. C., Xenophanes is said to have observed fossil shells and plants in the rocks of Paros, and to have attributed their presence to incursions of the sea over the land. Herodotus, about a century later, came to a similar conclusion regarding fossil shells in the mountains of Egypt. None of the ancients, however, seemed to have the slightest conception of the significance of fossils as time markers in the history of the earth. (See discussion below.) In the Middle Ages, distinguished writers held curious views regarding fossils. Thus Avicenna (980-1037) believed that fossils represented unsuccessful attempts on the part of nature to change inorganic materials into organisms within the earth by a peculiar creative force (_vis plastica_). About two centuries later, Albertus Magnus held a somewhat similar view. Leonardo da Vinci (1452-1519), the famous artist, architect, and engineer, while engaged in canal building in northern Italy, saw fossils imbedded in the rocks, and concluded that these were the remains of organisms which actually lived in sea water which spread over the region. During the seventeenth and eighteenth centuries, many correctly held that fossils were really of organic origin, but it was commonly taught that all fossils represented remains of organisms of an earlier creation which were buried in the rocks during the great Deluge (Noah's Flood). William Smith (1769-1839), of England, was, however, the first to recognize the fundamental significance of fossils for determining the relative ages of sedimentary rocks. This discovery laid the foundation for the determination of earth chronology which is of great importance in the study of the history of the earth. (See discussions below.) Organic remains, dating as far back as tens of millions of years, have been preserved in the rocks of the earth in various ways. A very common kind of fossilization is the preservation of only the hard parts of organisms. Thus the soft parts have disappeared by decomposition, while the hard parts, such as bones, shells, etc., remain. In many cases practically complete skeletons of large and small animals which lived millions of years ago have been found intact in the rocks. Fossils which show none of the original material, but only the shape or form, are also very abundant. When sediment hardens around an imbedded organism, and the organism then decomposes or dissolves away, a cavity or fossil mold only is left. Casts of organisms or parts of them are formed by filling shells or molds with sediment or with mineral matter carried in solution by underground water. Only rarely have casts of wholly soft animals been found in ancient rocks. In other cases both original form and structure are preserved, but none of the original material. This is known as petrifaction which takes place when a plant or hard part of an animal has been replaced, particle by particle, by mineral matter from solution in underground water. Not uncommonly organic matter, such as wood, or inorganic matter, such as carbonate of lime shells, has been so perfectly replaced that the original structures are preserved almost as in life. The popular idea that petrified wood is wood which has been changed into stone is, of course, incorrect. It is doubtful if flesh has ever been truly petrified. In many cases mainly the carbon only of organisms has been preserved. This is also true of plants where, under conditions of slow chemical change or decomposition, the hydrogen and oxygen mostly disappear, leaving much of the carbon with original structures often remarkably preserved. Fine examples are fossil plants in the great coal-bearing strata. Much more rarely entire organisms have been preserved either by freezing or by natural embalmment. Most remarkable are the species of mammoths and rhinoceroses, extinct for thousands of years, bodies of which, with flesh, hide, and hair still intact, have been held in cold storage in the frozen soils of Siberia, or other cases. Insects have been perfectly preserved in amber, as, for example, in the Baltic region. This amber is a hardened resin in which the insects were caught while it was still soft and exuding from the trees. Finally, we should mention the preservation of tracks and trails of land and water animals. Thousands of tracks of long-extinct great reptiles occur in the sandstones and shales of the Connecticut Valley of Massachusetts. The footprints were made in soft sandy mud which hardened and then became covered with more sediment. Few fossils occur in other than the sedimentary rocks. Most numerous, by far, are fossils in rocks of marine origin, because on relatively shallow sea bottoms, where sediments of the geologic ages have largely accumulated, the conditions for fossilization have been most favorable. Among the many conditions which have produced great diversity in numbers and distribution of marine organisms during geologic time are temperature, depth of water, clearness of water, nature of sea bottom, degree of salinity, and food supply. River and lake deposits also not uncommonly contain remains of organisms which inhabited the waters, but also others which were carried in. "Surrounding trees drop their leaves, flowers, and fruit upon the mud flats, insects fall into the quiet waters, while quadrupeds are mired in mud or quicksand and soon buried out of sight. Flooded streams bring in quantities of vegetable debris, together with carcasses of land animals drowned by the sudden rise of the flood" (W. B. Scott). In the study of the many changes which have taken place in the history of the earth, a fundamental consideration is the determination of the relative ages of the rocks, especially the strata. How can the geologist assign a rock formation of any part of the earth to a particular age in the history of the earth? How can it be proved that certain rock formations in various parts of the earth originated practically at the same time? There are two important criteria. First, in any region where the strata have not been disturbed from their normal order, the older strata underlie the younger because the underlying sediments must have been deposited first. Now, the total thickness of the stratified series of the earth has been estimated to be no less than 200,000 feet and only a small part of this is actually present in any given locality or region. It is, therefore, evident that the order of superposition of strata is in itself not sufficient for the determination of the relative ages of all the strata in even a considerable portion of a single continent, not to mention its utter inadequacy in building up the geological column of the whole earth. When, however, the second criterion, namely, the fossil content of the strata, is used in direct connection with the order of superposition, we have the real basis for determining the relative ages of strata for all parts of the earth. The discovery of this method was very largely due to the painstaking field work in England by William Smith about the beginning of the nineteenth century. It is a well-established fact that organisms have inhabited the earth for many millions of years and that, through the geologic ages, they have continuously changed, with gradual development of higher and higher types. Tens of thousands of species have come and gone. Accepting this fact, it is then clear that strata which were formed at notably different times must contain notably different fossils, while strata which accumulated at practically the same time contain similar fossils, allowing, of course, for reasonable differences in geographical distribution of organisms as at the present time. Each epoch of earth history or series of strata has its characteristic assemblage of organisms. In short, "a geological chronology is constructed by carefully determining, first of all, the order of superposition of the stratified rocks, and next by learning the fossils characteristic of each group of strata.... The order of succession among the fossils is determined from the order of superposition of the strata in which they occur. When that succession has been thus established, it may be employed as a general standard" (W. B. Scott). It should, however, be borne in mind that precise contemporaneity of strata in widely separated districts can rarely, if ever, be determined because of the very great length of geologic time and the general slowness of the evolution of organisms. Rocks carrying remarkably similar fossils may really be several thousand years different in age; but this is, indeed, a very small limit of error when one considers the vast antiquity of the earth. Much very accurate and satisfactory work has been done, especially in Europe and North America, in correlating strata and assigning them to their places in the geological time table (see below), but a vast amount of work yet remains to be done before the task is complete. Certain types or species of organisms are much more useful than others in the determination of earth chronology. Best of all for world-wide correlations are species which were widely distributed and which persisted for relatively short times. Thus any species which lived in the surface waters of the ocean and was easily distributed over wide areas, while, at the same time, it existed as such only a short time, is the best type of chronologic indicator. The known history of the earth has been more or less definitely divided into great eras and lesser periods and epochs, constituting what may be called the geologic time scale. In the accompanying table the era and period names, except those representing earlier time, are mostly world-wide in their usage. Epoch names, being more or less locally applied, are omitted from the table. Very conservative estimates of the length of time represented by the eras and the most characteristic general features of the life of the main divisions are also given. PRINCIPAL DIVISIONS OF GEOLOGIC TIME (Modified after U. S. Geological Survey.) ========================================================================= | | |Millions Era. | Period. | Characteristic life. |of years | | |estimated -----------+--------------+------------------------------------+--------- | |"Age of man." Animals and plants | |Quaternary. |of modern types. | +--------------+------------------------------------+ Cenozoic | |"Age of mammals." Rise of highest |3 to 5. |Tertiary. |animals except man. Rise and | | |development of highest orders of | | |plants. | -----------+--------------+------------------------------------+--------- | |"Age of reptiles." Rise and | |Cretaceous. |culmination of huge land reptiles | | |(dinosaurs), of shellfish with | | |complexly partitioned coiled | +--------------+shells (ammonites), and of great | | |flying reptiles. First appearance | Mesozoic |Jurassic. |(in Jurassic) of birds and mammals; |5 to 10. | |of cycads, an order of palm-like | +--------------|plants (in Triassic); and of | | |angiospermous plants, among | |Triassic. |which are palms and hardwood | | |trees (in Cretaceous). | -----------+--------------+------------------------------------+--------- | |"Age of amphibians." Dominance of | |Permian. |club mosses (lycopods) and plants | +--------------+of horsetail and fern types. | | |Primitive flowering plants and | |Pennsylvanian.|earliest cone-bearing trees. | | |Beginnings of back-boned land | +--------------+animals (land vertebrates). Insects.| |Mississippian.|Animals with nautiluslike coiled | | |shells (ammonites) and sharks | | |abundant. | +--------------+------------------------------------+ | |"Age of fishes." Shellfish | |Devonian. |(mollusks) also abundant. Rise of | | |amphibians and land plants. | +--------------+------------------------------------| | A {|Shell-forming sea animals dominant, | | g {|especially those related to | | e {|the nautilus (cephalopods). Rise | Paleozoic | {|and culmination of the marine |17 to 25. |Silurian. o {|animals sometimes known as sea | | f {|lilies (crinoids) and of giant | | {|scorpionlike crustaceans | | I {|(eurypterids). Rise of fishes and | | n {|of reef-building corals. | +-----------n-{+------------------------------------+ | v {|Shell-forming sea animals, | | e {|especially cephalopods and | |Ordovician.r {|mollusk-like brachiopods, abundant. | | t {|Culmination of the buglike marine | | e {|crustaceans known as trilobites. | +-----------b-{+------------------------------------+ | r {|Trilobites and brachiopods most | |Cambrian. a {|characteristic animals. Seaweeds | | t {|(algæ) abundant. No trace of | | e {|land animals found. | -----------+-----------s-{+------------------------------------+--------- | |First life that has left distinct | Proterozoic|Algonkian. |record. Crustaceans, brachiopods, | | |and seaweeds. | -----------+--------------+------------------------------------+ | |Organic matter in form of graphite |25 to 50+ Archeozoic |Archean. |(black lead), but no determinable | | |fossils found. | -----------+--------------+------------------------------------+--------- The length of time represented by human history is very short compared to the vast time of known geological history. The one is measured by thousands of years, while the other must be measured by tens of millions of years. Just as we may roughly divide human history into certain ages according to some notable person, nation, principle, or force as, for example, the "Age of Pericles," the "Roman Period," the "Age of the French Revolution," or the "Age of Electricity," so geologic history may be subdivided according to great predominant physical or organic phenomena, such as "the Appalachian Mountain Revolution" (toward the end of the Paleozoic era), the "Age of Fishes" (Devonian period), or the "Age of Reptiles" (Mesozoic era). In the study of earth history, as in the study of human history, it is important to distinguish between events and records of events. Historical events are continuous, but they are by no means all recorded. Records of events are often interrupted and seemingly sharply separated from each other. CHAPTER II WEATHERING AND EROSION All rocks at and near the surface of the earth crumble or decay. The term "weathering" includes all the processes whereby rocks are broken up, decomposed, or dissolved. A mass of very hard and seemingly indestructible granite, taken from a quarry, will, in a very short time, geologically considered, crumble (Plate 1). During the short span of the ordinary human life weathering effects are generally of very little consequence, but during the long ages of geologic time the various processes of weathering have been slowly and ceaselessly at work upon the outer crust of the earth, and such tremendous quantities of rock material have been broken up that the lands of the earth have everywhere been profoundly affected. Most of us have noticed buildings and monuments in which the stones show marked effects of weathering. A good case in point is Westminster Abbey, London, in which many of the stones are badly weathering, some of the more ornamental parts having crumbled beyond recognition since the building was erected in the thirteenth century. In many countries, tombstones and monuments only one or two centuries old are so badly weathered that the inscriptions are scarcely if at all legible. What are some of the processes of nature whereby rocks are weathered? In cold countries, and often in mountains of generally mild climate regions, the alternate freezing and thawing of water is a potent agency in breaking up rocks where the soils are thin or absent. On freezing, water expands about one-tenth of its volume and exerts the enormous pressure of over 2,000 pounds per square inch. Nearly all relatively hard rock formations are separated into more or less distinct blocks by natural cracks called "joints" (Plate 8). Very commonly the rocks also contain minute crevices, fissures, and pores. Repeated freezing and thawing of water which finds its way into such openings finally causes even the most resistant rocks to break up into smaller and smaller fragments. A very striking example of difference in climatic effect upon a given rock mass is the obelisk in Central Park, New York. For many centuries this famous monument stood practically without change in the dry, frostless climate of Egypt, but very soon after its removal to the moist, frosty climate of New York, it began to crumble so rapidly that it was necessary to cover it with a coating of glaze to protect it from the atmosphere. Temperature change, especially in dry regions, is also an important agency for mechanical breaking up of rocks. On high mountains and on deserts, a daily range of temperature of from 70 degrees to 80 degrees is frequent. Due to heat absorption, rocks in desert regions, during the day, not uncommonly reach temperatures of fully 120 degrees, while during the night, due to heat radiation, their temperature falls greatly. During the heating of the outer portion of the rock, the various minerals each expand differently, thus setting up a series of stresses and strains tending to cause the minerals to pull apart. The outer portions of the rocks which are subjected to unstable and relatively rapid temperature changes, often crack or peel off in slabs or flakes, this process being called exfoliation. Stone Mountain in Georgia, and some of the mountains of the southern Sierra Nevada range in California, are excellent examples of mountains which are being rounded off by exfoliation. The principle is the same as that which causes the "spalling" of stones in buildings during fires. Masses of débris consisting of more or less angular rock fragments of all sizes commonly occur at the bases of cliffs and mountains. They represent materials which have weathered off the ledges mainly by frost action and temperature changes. Where electrical storms are frequent, lightning often shatters portions of rock ledges. Many such cases have come under the writer's observation in the Adirondack Mountains of New York. The total effect of lightning as a weathering agency is, however, relatively small. Another minor weathering effect is the mechanical action of plants. The principle is well illustrated by the breaking or tilting of sidewalks by the wedging action of the growing roots of trees. In many places the roots of plants growing in cracks in rocks, exert powerful pressure causing the rocks or blocks of rocks to wedge apart. Let us now briefly consider some of the chemical processes of weathering. The solvent effect of perfectly pure water upon rocks is very slight and slow. But such water is not found in nature because certain atmospheric gases, especially oxygen and carbonic acid gas, are always present in it, and they notably increase the solvent power of the water. Such water has the power to slowly but completely dissolve the common rock called limestone which consists of carbonate of lime. This material is then carried away by the streams. Rocks, like certain sandstones which contain carbonate of lime cementing material, are caused to crumble due to removal of the cement in solution. Carbonic acid gas in water also has the power to chemically alter various minerals in many common rocks and thus the rocks fall apart and the carbonates which result from the action usually are carried away in solution. One of the most important changes of this kind takes place when the very common mineral feldspar is attacked by water containing carbonic acid gas and the mineral alters to a soluble carbonate, kaolin (or clay) and silica. The oxygen, both of the air and that which is contained in water, is a very important chemical agent of decomposition of many rocks. Water at the surface and the upper part of the crust of the earth as well as moisture in the air are also important chemical agents which bring about rock decay. We are all familiar with the rusting of iron which is due to the chemical union of the iron with oxygen, thus forming an iron oxide which in turn commonly unites with water from air or earth. Now, many rocks contain iron, not as such, but held in combination with other substances in the form of various minerals, and this iron of the rocks, where subjected to the oxygen and moisture of air or water, slowly unites with the oxygen and water to form a hydrated iron oxide which is essentially iron-rust. The minerals containing considerable iron are, therefore, decomposed and the rocks crumble. There are various iron oxides, usually more or less hydrated, ranging in color from red through brown to yellow, and these constitute probably the most common and striking colors of the rocks of the earth. The gorgeously colored Grand Canyon of the Yellowstone River is a very fine example of large scale coloring due to development of much hydrated oxide of iron during the weathering of lava rock, the process having been aided by the action of heated underground waters. Most of the soils of the earth are the direct result of weathering. Important exceptions are soils which have been transported by the action of water, ice, or wind. Although the process of weathering is very slow and relatively superficial, it is, nevertheless, true that in many places, the products of weathering form faster than they can be carried away. Such weathered materials accumulate in their place of origin to form soils. The upper few hundred feet of the earth's crust is everywhere more or less fractured and porous and the rocks are there affected in varying degrees by most of the ordinary agents of weathering. In such cases, outside the areas which were recently covered by ice during the great Ice Age, it is common to find the loose soil grading downward into rotten rock, and this in turn into the fresh practically unaltered bedrock. Soils of this kind are generally not more than ten or twenty feet deep, though under exceptional conditions, as in parts of Brazil, they attain depths of several hundred feet. In order to make still clearer some of the above principles of weathering and also to give the reader some understanding of the most common types of residual soils, we shall consider what happens to a few rather definite types of ordinary rocks when they are subjected to weathering. A very simple case is that of a sandstone, the mineral grains (mostly quartz) of which are held together by carbonate of lime. The lime simply dissolves and is carried away, while many of the mineral grains may remain to form a soil of nearly pure sand. Where oxide of iron forms the cementing material, the rock yields less readily to weathering, and the sandy soil will be yellowish brown or red according to the climate. Another simple case is that of limestone which when perfectly pure yields no soil because it is all soluble. Pure limestone is, however, rare, and the various mineral impurities in it, being to a considerable degree insoluble, tend to remain to form a residual soil which may vary from sandy to clayey, and usually brown or red due to the setting free of oxides of iron. According to one estimate a thickness of about 100 feet of a certain fairly impure limestone formation in Virginia must weather to yield a layer of soil one foot thick. Soils of this kind, which are usually rich, are common in many limestone valleys of the Appalachian Mountains. In the case of shale rock, which is hardened mud, the cementing materials are removed, some chemical changes in the minerals may take place, and the rock crumbles to a claylike soil. What happens to a very hard, resistant igneous rock like granite when attacked by the weather? Such a rock always consists mainly of the two very common minerals feldspar and quartz, usually with smaller amounts of other minerals such as mica, hornblende, augite, or magnetite. The feldspar, which when fresh is harder than steel, slowly yields when attacked by water containing carbonic acid gas and crumbles or decays to a mixture of kaolin (clay), carbonate of potash, and silica (quartz). Clay is an important constituent of most good soils, while the carbonate of potash is essential as a food for most plants. Due to yielding of the grains or crystals of feldspar, the granite falls apart (see Plate 1). The grains of quartz remain chemically unchanged, though they may be more or less broken by changes of temperature, and the other minerals, which are mostly iron-bearing, yield more or less to weathering, resulting in a variety of products, among which are oxides of iron. A typical granite, therefore, gives rise to a good heavy soil which is yellow, brown or red according to climate. Such granite soils are common in many parts of the Piedmont Plateau from Maryland to Georgia. Most of the dark-colored igneous rocks, like ordinary basaltic lava, contain much feldspar, various iron-bearing minerals, and little or no quartz. Such rocks yield to the weather like granite but, because of lack of quartz, the soils are more clayey. Rich soils of this kind occur in the great lava fields of the northwestern United States and in the Hawaiian Islands. The importance of the breaking down of feldspar under the influence of the weather, as above described, not only from the standpoint of soil development, but also as regards the wearing down of the lands of the earth, is difficult to overemphasize because that mineral is by far the most abundant constituent of the earth's crust. The term "erosion" is one of the most important in geologic science. It comprises all the processes whereby the lands of the earth are worn down. It involves the breaking up of earth material, and its transportation through the agency of water, ice, or wind. Weathering, including the various subprocesses as above described, is a very important process of erosion. By this process much rock material is got into condition for transportation. Another process of erosion, called "corrasion," consists in the rubbing or bumping of rocks fragments of all sizes carried by water, ice, or wind against the general country rock, thus causing the latter to be gradually worn away. A fine illustration of exceedingly rapid corrasion of very hard rock was that of the Sill tunnel in Austria, which was paved with granite blocks several feet thick. Water carrying large quantities of rock fragments over the pavement at high velocity caused the granite blocks to be worn through in only one year. Ordinarily in nature, however, the rate of wear is much slower than this. Pressure exerted upon the country rock by any agency of transportation may cause relatively loose joint blocks, into which most rock formations are separated, to be pushed away. This process, called "plucking," is especially effective in the case of flowing ice. CHAPTER III STREAM WORK Most streams are incessantly at work cutting or eroding their way into the earth's crust and carrying off the products of weathering. By this means the general level of lands is gradually being reduced to nearer and nearer sea level. Base level of erosion is reached when any stream has eroded to its greatest possible depth, and a whole region is said to be base-leveled when, by the action of streams, it has been reduced to a practically flat condition. A region of this kind is known as a "peneplain." To one who has not seriously considered the matter, the power of even moderately swift water to transport rock débris seems incredible. A well-established law of transportation by running water is that the transporting power of a current varies as the sixth power of its velocity. For example, a current which is just able to move a rock fragment of a given size will, when its velocity is merely doubled, be able to move along a piece of similar rock sixty-four times as large! That this must be the case may be readily proved as follows: A current of given velocity is just able to move a block of rock, say, of one cubic inch in the form of a cube. A cubic block sixty-four times as large has a face of sixteen square inches. By doubling the velocity of the current, therefore, twice as much water must strike each of the sixteen square inches of the face of the larger block with twice the force, thus exerting sixty-four times the power against the face of the larger block, or enough to move it along. This surprising law accounts for the fact that in certain floods, like the one which rushed over Johnstown, Pennsylvania, in 1889, locomotives, massive iron bridges, and great bowlders were swept along with great velocity. It is obvious, then, that ordinarily swift rivers in time of flood accomplish far more work of erosion (especially transportation) than during many days or even some months of low water. Few people have the slightest idea as to the enormous amount of earth material which the rivers are carrying into the sea each year. The burden carried by the Mississippi River has been carefully studied for many years. Each year this river discharges about 400,000,000 tons of material in suspension; 120,000,000 tons in solution; and 40,000,000 tons rolled along the bottom. This all represents earth material eroded from the drainage basin of the river. It is sufficient to cover a square mile 325 feet deep, or if placed in ordinary freight cars it would require a train reaching around the earth several times to contain it. Since the drainage basin of the Mississippi covers about 1,250,000 square miles, it is, therefore, evident that this drainage area is being worn down at the average rate of about one foot in 3,840 years, and this is perhaps, a fair average for the rivers of the earth. The Ganges River, being unusually favorably situated for rapid erosion, wears down its drainage basin about one foot in 1,750 years. It has been estimated that nearly 800,000,000 tons of material are annually carried into the sea by the rivers of the United States. According to this the country, as a whole, is being cut down at the rate of about one foot in 9,000 years. In arriving at this figure it should, of course, be borne in mind that the average level of hundreds of thousands of square miles of the western United States, particularly the so-called Great Basin, is practically not being reduced at all because none of the streams there reach the sea. Deposition of sediment is an important natural consequence of erosion. The destination of most streams is the sea, and where tides are relatively slight the sediments discharged mostly accumulate relatively near the mouths of the rivers in the form of flat, fan-shaped delta deposits. Some rivers, like the Ganges, which carry such unusual quantities of sediment, are able to construct deltas in spite of considerable tides. Deltas also form in lakes. In most cases, however, rivers enter the sea where there are considerable tides and their loads are more widely spread over the marginal sea bottom. But in many cases some of the sediment does not reach the mouth of the stream. It is, instead, deposited along its course either where the velocity is sufficiently checked, as is the case over many flood-plain areas of rivers, or where a heavily loaded, relatively swift stream has its general velocity notably diminished. An excellent example of the latter type of stream is the Platte River, which is swift and loaded with sediment in its descent from the Rocky Mountains, but, on reaching the relatively more nearly level Nebraska country, it has its current sufficiently checked to force it to deposit sediment and build up its channel along many miles of its course, and this in spite of the fact that it still maintains a considerable current. In a mountainous arid region a more or less intermittent stream at times of flood becomes heavily loaded with rock débris and rushes down the mountain side. On reaching the valley floor the velocity is greatly checked and most of the load is deposited at the base of the mountain, successive accumulations of such materials, called alluvial cones or fans, having not uncommonly built up to depths of hundreds, or even several thousand feet. [Illustration: Plate 1.--(_a_) Granite Weathering to Soil near Northampton, Mass. Under the action of weathering all of the once hard, fresh, mass of granite has crumbled to soil except the fairly fresh rounded masses which are residual cores of "joint blocks." (_Photo by the author._)] [Illustration: Plate 1.--(_b_) Looking-Glass Rock, Utah. The rock is stratified sandstone sculptured mainly by wind erosion, that is, by the wind driving particles of sand against it. (_Photo by Cross, U. S. Geological Survey._)] [Illustration: Plate 2.--Grand Canyon of the Yellowstone River in Yellowstone National Park. The great waterfall 308 feet high is shown. The large swift river has here sunk its channel (by erosion) to a maximum depth of 1,200 feet during very recent geological time, and the process is still going on. The wonderful coloring is due to iron oxides set free during weathering of the lava rock. (_Photo by Hillers, U. S. Geological Survey._)] Any newly formed land surface, like a recently drained lake bed or part of the marginal sea bottom which has been raised into land, has a drainage system developed upon it. In the early or youthful stage of such a new land area lying well above sea level, under ordinary climatic conditions a few streams only form and these tend to follow the natural or initial slope of the land. These streams carve out narrow, steep-sided valleys, and all of them are actively engaged in cutting down their channels, or, in other words, none of them have reached base level, and flood plains and meandering curves are therefore lacking. During this youthful stage there are no sharp drainage divides; gorges and waterfalls are not uncommonly present; and the relief of the land in general is not rugged. A good example of youthful topography is the region around Fargo, North Dakota, which is part of the bed of a great recently drained lake. The Grand Canyon of the Yellowstone River is an excellent illustration of a youthful valley cut in a high plateau of geologically recent origin. (Plate 2.) As time goes on, a region in youth gradually gives way to a region in maturity, during which stage the maximum number (usually a network) of streams in broader V-shaped valleys have developed; divisions of drainage are sharp; the maximum ruggedness of relief has developed; the larger streams only have cut down so near base level that winding (meandering) courses and flood plains are well developed along them; and waterfalls and gorges are rarely present. An almost perfect example of a region in maturity is that around Charleston, West Virginia. The old-age stage develops next in the history of the region, during which only a moderate number of streams remain, most of these being at or close to base level so that sweeping curves or meanders (Plate 4) and cut-off meanders or "ox bows" and wide flood plains are characteristic and common. The relief is greatly subdued and the term "rolling country" might be applied to the moderately hilly region. Divisions of drainage are, of course, not at all sharp and the valleys are wide and shallow. Oxbow lakes are common, but gorges and waterfalls are absent. A region typical of old-age topography is that around Caldwell, Kansas. Finally, after the remaining low hills
{'title': "Geology: The Science of the Earth's Crust by William J. Miller", 'source': 'Project Gutenberg', 'lang': 'en'}