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Title: The Elements of Geology
Author: William Harmon Norton
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THE ELEMENTS OF GEOLOGY
BY WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE
PREFACE
Geology is a science of such rapid growth that no apology is expected when from time to time a new text-book is added to those already in the field. The present work, however, is the outcome of the need of a text-book of very simple outline, in which causes and their consequences should be knit together as closely as possible,—a need long felt by the author in his teaching, and perhaps by other teachers also. The author has ventured, therefore, to depart from the common usage which subdivides geology into a number of departments,—dynamical, structural, physiographic, and historical,—and to treat in immediate connection with each geological process the land forms and the rock structures which it has produced.
It is hoped that the facts of geology and the inferences drawn from them have been so presented as to afford an efficient discipline in inductive reasoning. Typical examples have been used to introduce many topics, and it has been the author's aim to give due proportion to both the wide generalizations of our science and to the concrete facts on which they rest.
There have been added a number of practical exercises such as the author has used for several years in the class room. These are not made so numerous as to displace the problems which no doubt many teachers prefer to have their pupils solve impromptu during the recitation, but may, it is hoped, suggest their use.
In historical geology a broad view is given of the development of the North American continent and the evolution of life upon the planet. Only the leading types of plants and animals are mentioned, and special attention is given to those which mark the lines of descent of forms now living.
By omitting much technical detail of a mineralogical and paleontological nature, and by confining the field of view almost wholly to our own continent, space has been obtained to give to what are deemed for beginners the essentials of the science a fuller treatment than perhaps is common.
It is assumed that field work will be introduced with the commencement of the study. The common rocks are therefore briefly described in the opening chapters. The drift also receives early mention, and teachers in the northern states who begin geology in the fall may prefer to take up the chapter on the Pleistocene immediately after the chapter on glaciers.
Simple diagrams have been used freely, not only because they are often clearer than any verbal statement, but also because they readily lend themselves to reproduction on the blackboard by the pupil. The text will suggest others which the pupil may invent. It is hoped that the photographic views may also be used for exercises in the class room.
The generous aid of many friends is recognized with special pleasure. To Professor W. M. Davis of Harvard University there is owing a large obligation for the broad conceptions and luminous statements of geologic facts and principles with which he has enriched the literature of our science, and for his stimulating influence in education. It is hoped that both in subject-matter and in method the book itself makes evident this debt. But besides a general obligation shared by geologists everywhere, and in varying degrees by perhaps all authors of recent American text- books in earth science, there is owing a debt direct and personal. The plan of the book, with its use of problems and treatment of land forms and rock structures in immediate connection with the processes which produce them, was submitted to Professor Davis, and, receiving his approval, was carried into effect, although without the sanction of precedent at the time. Professor Davis also kindly consented to read the manuscript throughout, and his many helpful criticisms and suggestions are acknowledged with sincere gratitude.
Parts of the manuscript have been reviewed by Dr. Samuel Calvin and Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W. Beyer of the Iowa College of Agriculture and Mechanic Arts; Dr. U. S. Grant of Northwestern University; Professor J. A. Udden of Augustana College, Illinois; Dr. C. H. Gordon of the New Mexico State School of Mines; Principal Maurice Ricker of the High School, Burlington, Iowa; and the following former students of the author who are engaged in the earth sciences: Dr. W. C. Alden of the United States Geological Survey and the University of Chicago; Mr. Joseph Sniffen, instructor in the Academy of the University of Chicago, Morgan Park; Professor Martin Iorns, Fort Worth University, Texas; Professor A. M. Jayne, Dakota University; Professor G. H. Bretnall, Monmouth College, Illinois; Professor Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable, instructor in the Iowa State Normal College; Principal C. C. Gray of the High School, Fargo, North Dakota; and Mr. Charles Persons of the High School, Hannibal, Missouri. A large number of the diagrams of the book were drawn by Mr. W. W. White of the Art School of Cornell College. To all these friends, and to the many who have kindly supplied the illustrations of the text, whose names are mentioned in an appended list, the writer returns his heartfelt thanks.
WILLIAM HARMON NORTON
CORNELL COLLEGE, MOUNT VERNON, IOWA
JULY, 1905
INTRODUCTORY NOTE
During the preparation of this book Professor Norton has frequently discussed its plan with me by correspondence, and we have considered together the matters of scope, arrangement, and presentation.
As to scope, the needs of the young student and not of the expert have been our guide; the book is therefore a text-book, not a reference volume.
In arrangement, the twofold division of the subject was chosen because of its simplicity and effectiveness. The principles of physical geology come first; the several chapters are arranged in what is believed to be a natural order, appropriate to the greatest part of our country, so that from a simple beginning a logical sequence of topics leads through the whole subject. The historical view of the science comes second, with many specific illustrations of the physical processes previously studied, but now set forth as part of the story of the earth, with its many changes of aspect and its succession of inhabitants. Special attention is here given to North America, and care is taken to avoid overloading with details.
With respect to method of presentation, it must not be forgotten that the text-book is only one factor in good teaching, and that in geology, as in other sciences, the teacher, the laboratory, and the local field are other factors, each of which should play an appropriate part. The text suggests observational methods, but it cannot replace observation in field or laboratory; it offers certain exercises, but space cannot be taken to make it a laboratory manual as well as a book for study; it explains many problems, but its statements are necessarily more terse than the illustrative descriptions that a good and experienced teacher should supply. Frequent use is made of induction and inference in order that the student may come to see how reasonable a science is geology, and that he may avoid the too common error of thinking that the opinions of
W. M. DAVIS
HARVARD UNIVERSITY, CAMBRIDGE, MASS.
JULY, 1905
CONTENTS
INTRODUCTION.—THE SCOPE AND AIM OF GEOLOGY
PART I
EXTERNAL GEOLOGICAL AGENCIES
I. THE WORK OF THE WEATHER II. THE WORK OF GROUND WATER III. RIVERS AND VALLEYS IV. RIVER DEPOSITS V. THE WORK OF GLACIERS VI. THE WORK OF THE WIND VII. THE SEA AND ITS SHORES VIII. OFFSHORE AND DEEP-SEA DEPOSITS
PART II
INTERNAL GEOLOGICAL AGENCIES
IX. MOVEMENTS OF THE EARTH'S CRUST X. EARTHQUAKES XI. VOLCANOES XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN XIII. METAMORPHISM AND MINERAL VEINS
PART III
HISTORICAL GEOLOGY
XIV. THE GEOLOGICAL RECORD XV. THE PRE-CAMBRIAN SYSTEMS XVI. THE CAMBRIAN XVII. THE ORDOVICIAN AND SILURIAN XVIII. THE DEVONIAN XIX. THE CARBONIFEROUS XX. THE MESOZOIC XXI. THE TERTIARY XXII. THE QUATERNARY INDEX
THE ELEMENTS OF GEOLOGY
INTRODUCTION
THE SCOPE AND AIM OF GEOLOGY
Geology deals with the rocks of the earth's crust. It learns from their composition and structure how the rocks were made and how they have been modified. It ascertains how they have been brought to their present places and wrought to their various topographic forms, such as hills and valleys, plains and mountains. It studies the vestiges which the rocks preserve of ancient organisms which once inhabited our planet. Geology is the history of the earth and its inhabitants, as read in the rocks of the earth's crust.
To obtain a general idea of the nature and method of our science before beginning its study in detail, we may visit some valley, such as that illustrated in the frontispiece, on whose sides are rocky ledges. Here the rocks lie in horizontal layers. Although only their edges are exposed, we may infer that these layers run into the upland on either side and underlie the entire district; they are part of the foundation of solid rock which everywhere is found beneath the loose materials of the surface.
The ledges of the valley of our illustration are of sandstone. Looking closely at the rock we see that it is composed of myriads of grains of sand cemented together. These grains have been worn and rounded. They are sorted also, those of each layer being about of a size. By some means they have been brought hither from some more ancient source. Surely these grains have had a history before they here found a resting place,—a history which we are to learn to read.
The successive layers of the rock suggest that they were built one after another from the bottom upward. We may be as sure that each layer was formed before those above it as that the bottom courses of stone in a wall were laid before the courses which rest upon them.
We have no reason to believe that the lowest layers which we see here were the earliest ever formed. Indeed, some deep boring in the vicinity may prove that the ledges rest upon other layers of rock which extend downward for many hundreds of feet below the valley floor. Nor may we conclude that the highest layers here were the latest ever laid; for elsewhere we may find still later layers lying upon them.
A short search may find in the rock relics of animals, such as the imprints of shells, which lived when it was deposited; and as these are of kinds whose nearest living relatives now have their home in the sea, we infer that it was on the flat sea floor that the sandstone was laid. Its present position hundreds of feet above sea level proves that it has since emerged to form part of the land; while the flatness of the beds shows that the movement was so uniform and gentle as not to break or strongly bend them from their original attitude.
The surface of some of these layers is ripple-marked. Hence the sand must once have been as loose as that of shallow sea bottoms and sea beaches to-day, which is thrown into similar ripples by movements of the water. In some way the grains have since become cemented into firm rock.
Note that the layers on one side of the valley agree with those on the other, each matching the one opposite at the same level. Once they were continuous across the valley. Where the valley now is was once a continuous upland built of horizontal layers; the layers now show their edges, or OUTCROP, on the valley sides because they have been cut by the valley trench.
The rock of the ledges is crumbling away. At the foot of each step of rock lie fragments which have fallen. Thus the valley is slowly widening. It has been narrower in the past; it will be wider in the future.
Through the valley runs a stream. The waters of rains which have fallen on the upper parts of the stream's basin are now on their way to the river and the sea. Rock fragments and grains of sand creeping down the valley slopes come within reach of the stream and are washed along by the running water. Here and there they lodge for a time in banks of sand and gravel, but sooner or later they are taken up again and carried on. The grains of sand which were brought from some ancient source to form these rocks are on their way to some new goal. As they are washed along the rocky bed of the stream they slowly rasp and wear it deeper. The valley will be deeper in the future; it has been less deep in the past.
In this little valley we see slow changes now in progress. We find also in the composition, the structure, and the attitude of the rocks, and the land forms to which they have been sculptured, the record of a long succession of past changes involving the origin of sand grains and their gathering and deposit upon the bottom of some ancient sea, the cementation of their layers into solid rock, the uplift of the rocks to form a land surface, and, last of all, the carving of a valley in the upland. Everywhere, in the fields, along the river, among the mountains, by the seashore, and in the desert, we may discover slow changes now in progress and the record of similar changes in the past. Everywhere we may catch glimpses of a process of gradual change, which stretches backward into the past and forward into the future, by which the forms and structures of the face of the earth are continually built and continually destroyed. The science which deals with this long process is geology. Geology treats of the natural changes now taking place upon the earth and within it, the agencies which produce them, and the land forms and rock structures which result. It studies the changes of the present in order to be able to read the history of the earth's changes in the past.
The various agencies which have fashioned the face of the earth may. be divided into two general classes. In Part I we shall consider those which work upon the earth from without, such as the weather, running water, glaciers, the wind, and the sea. In Part II we shall treat of those agencies whose sources are within the earth, and among whose manifestations are volcanoes and earthquakes and the various movements of the earth's crust. As we study each agency we shall notice not only how it does its work, but also the records which it leaves in the rock structures and the land forms which it produces. With this preparation we shall be able in Part III to read in the records of the
PART I
EXTERNAL GEOLOGICAL AGENCIES
CHAPTER I
THE WORK OF THE WEATHER
In our excursion to the valley with sandstone ledges we witnessed a process which is going forward in all lands. Everywhere the rocks are crumbling away; their fragments are creeping down hillsides to the stream ways and are carried by the streams to the sea, where they are rebuilt into rocky layers. When again the rocks are lifted to form land the process will begin anew; again they will crumble and creep down slopes and be washed by streams to the sea. Let us begin our study of this long cycle of change at the point where rocks disintegrate and decay under the action of the weather. In studying now a few outcrops and quarries we shall learn a little of some common rocks and how they weather away.
STRATIFICATION AND JOINTING. At the sandstone ledges we saw that the rock was divided into parallel layers. The thicker layers are known as STRATA, and the thin leaves into which each stratum may sometimes be split are termed LAMINAE. To a greater or less degree these layers differ from each other in fineness of grain, showing that the material has been sorted. The planes which divide them are called BEDDING PLANES.
Besides the bedding planes there are other division planes, which cut across the strata from top to bottom. These are found in all rocks and are known as joints. Two sets of joints, running at about right angles to each other, together with the bedding planes, divide the sandstone into quadrangular blocks.
SANDSTONE. Examining a piece of sandstone we find it composed of grains quite like those of river sand or of sea beaches. Most of the grains are of a clear glassy mineral called quartz. These quartz grains are very hard and will scratch the steel of a knife blade. They are not affected by acid, and their broken surfaces are irregular like those of broken glass.
The grains of sandstone are held together by some cement. This may be calcareous, consisting of soluble carbonate of lime. In brown sandstones the cement is commonly ferruginous,—hydrated iron oxide, or iron rust, forming the bond, somewhat as in the case of iron nails which have rusted together. The strongest and most lasting cement is siliceous, and sand rocks whose grains are closely cemented by silica, the chemical substance of which quartz is made, are known as quartzites.
We are now prepared to understand how sandstone is affected by the action of the weather. On ledges where the rock is exposed to view its surface is more or less discolored and the grains are loose and may be rubbed off with the finger. On gentle slopes the rock is covered with a soil composed of sand, which evidently is crumbled sandstone, and dark carbonaceous matter derived from the decay of vegetation. Clearly it is by the dissolving of the cement that the rock thus breaks down to loose sand. A piece of sandstone with calcareous cement, or a bit of old mortar, which is really an artificial stone also made of sand cemented by lime, may be treated in a test tube with hydrochloric acid to illustrate the process.
A LIMESTONE QUARRY. Here also we find the rock stratified and jointed (Fig. 2). On the quarry face the rock is distinctly seen to be altered for some distance from its upper surface. Below the altered zone the rock is sound and is quarried for building; but the altered upper layers are too soft and broken to be used for this purpose. If the limestone is laminated, the laminae here have split apart, although below they hold fast together. Near the surface the stone has become rotten and crumbles at the touch, while on the top it has completely broken down to a thin layer of limestone meal, on which rests a fine reddish clay.
Limestone is made of minute grains of carbonate of lime all firmly held together by a calcareous cement. A piece of the stone placed in a test tube with hydrochloric acid dissolves with brisk effervescence, leaving the insoluble impurities, which were disseminated through it, at the bottom of the tube as a little clay.
We can now understand the changes in the upper layers of the quarry. At the surface of the rock the limestone has completely dissolved, leaving the insoluble residue as a layer of reddish clay. Immediately below the clay the rock has disintegrated into meal where the cement between the limestone grains has been removed, while beneath this the laminae are split apart where the cement has been dissolved only along the planes of lamination where the stone is more porous. As these changes in the rock are greatest at the surface and diminish downward, we infer that they have been caused by agents working downward from the surface.
At certain points these agencies have been more effective than elsewhere. The upper rock surface is pitted. Joints are widened as they approach the surface, and along these seams we may find that the rock is altered even down to the quarry floor.
A SHALE PIT. Let us now visit some pit where shale—a laminated and somewhat hardened clay—is quarried for the manufacture of brick. The laminae of this fine-grained rock may be as thin as cardboard in places, and close joints may break the rock into small rhombic blocks. On the upper surface we note that the shale has weathered to a clayey soil in which all traces of structure have been destroyed. The clay and the upper layers of the shale beneath it are reddish or yellow, while in many cases the color of the unaltered rock beneath is blue.
THE SEDIMENTARY ROCKS. The three kinds of layered rocks whose acquaintance we have made—sandstone, limestone, and shale—are the leading types of the great group of stratified, or sedimentary, rocks. This group includes all rocks made of sediments, their materials having settled either in water upon the bottoms of rivers, lakes, or seas, or on dry land, as in the case of deposits made by the wind and by glaciers. Sedimentary rocks are divided into the fragmental rocks—which are made of fragments, either coarse or fine—and the far less common rocks which are constituted of chemical precipitates.
The sedimentary rocks are divided according to their composition into the following classes:
1. The arenaceous, or quartz rocks, including beds of loose sand and gravel, sandstone, quartzite, and conglomerate (a rock made of cemented rounded gravel or pebbles).
2. The calcareous, or lime rocks, including limestone and a soft white rock formed of calcareous powder known as chalk.
3. The argillaceous, or clay rocks, including muds, clays, and shales. These three classes pass by mixture into one another. Thus there are limy and clayey sandstones, sandy and clayey limestones, and sandy and limy shales.
GRANITE. This familiar rock may be studied as an example of the second great group of rocks,—the unstratified, or igneous rocks. These are not made of cemented sedimentary grains, but of interlocking crystals which have crystallized from a molten mass. Examining a piece of granite, the most conspicuous crystals which meet the eye are those of feldspar. They are commonly pink, white, or yellow, and break along smooth cleavage planes which reflect the light like tiny panes of glass. Mica may be recognized by its glittering plates, which split into thin elastic scales. A third mineral, harder than steel, breaking along irregular surfaces like broken glass, we identify as quartz.
How granite alters under the action of the weather may be seen in outcrops where it forms the bed rock, or country rock, underlying the loose formations of the surface, and in many parts of the northern states where
These are a few simple illustrations of the surface changes which some of the common kinds of rocks undergo. The agencies by which these changes are brought about we will now take up under two divisions,—CHEMICAL AGENCIES producing rock decay and MECHANICAL AGENCIES producing rock disintegration.
THE CHEMICAL WORK OF WATER
As water falls on the earth in rain it has already absorbed from the air carbon dioxide (carbonic acid gas) and oxygen. As it sinks into the ground and becomes what is termed ground water, it takes into solution from the soil humus acids and carbon dioxide, both of which are constantly being generated there by the decay of organic matter. So both rain and ground water are charged with active chemical agents, by the help of which they corrode and rust and decompose all rocks to a greater or less degree. We notice now three of the chief chemical processes concerned in weathering,— solution, the formation of carbonates, and oxidation.
SOLUTION. Limestone, although so little affected by pure water that five thousand gallons would be needed to dissolve a single pound, is easily dissolved in water charged with carbon dioxide. In limestone regions well water is therefore "hard." On boiling the water for some time the carbon dioxide gas is expelled, the whole of the lime carbonate can no longer be held in solution, and much of it is thrown down to form a crust or "scale" in the kettle or in the tubes of the steam boiler. All waters which flow over limestone rocks or soak through them are constantly engaged in dissolving them away, and in the course of time destroy beds of vast extent and great thickness.
The upper surface of limestone rocks becomes deeply pitted, as we saw in the limestone quarry, and where the mantle of waste has been removed it may be found so intricately furrowed that it is difficult to traverse.
Beds of rock salt buried among the strata are dissolved by seeping water, which issues in salt springs. Gypsum, a mineral composed of hydrated sulphate of lime, and so soft that it may be scratched with the finger nail, is readily taken up by water, giving to the water of wells and springs a peculiar hardness difficult to remove.
The dissolving action of moisture may be noted on marble tombstones of some age, marble being a limestone altered by heat and pressure and composed of crystalline grains. By assuming that the date on each monument marks the year of its erection, one may estimate how many years on the average it has taken for weathering to loosen fine grains on the polished surface, so that they may be rubbed off with the finger, to destroy the polish, to round the sharp edges of tool marks in the lettering, and at last to open cracks and seams and break down the stone. We may notice also whether the gravestones weather more rapidly on the sunny or the shady side, and on the sides or on the top.
The weathered surface of granular limestone containing shells shows them standing in relief. As the shells are made of crystalline carbonate of lime, we may infer whether the carbonate of lime is less soluble in its granular or in its crystalline condition.
THE FORMATION OF CARBONATES. In attacking minerals water does more than merely take them into solution. It decomposes them, forming new chemical compounds of which the carbonates are among the most important. Thus feldspar consists of the insoluble silicate of alumina, together with certain alkaline silicates which are broken up by the action of water containing carbon dioxide, forming alkaline carbonates. These carbonates are freely soluble and contribute potash and soda to soils and river waters. By the removal of the soluble ingredients of feldspar there is left the silicate of alumina, united with water or hydrated, in the condition of a fine plastic clay which, when white and pure, is known as KAOLIN and is used in the manufacture of porcelain. Feldspathic rocks which contain no iron compounds thus weather to whitish crusts, and even apparently sound crystals of feldspar, when ground to thin slices and placed under the microscope, may be seen to be milky in color throughout because an internal change to kaolin has begun.
OXIDATION. Rocks containing compounds of iron weather to reddish crusts, and the seams of these rocks are often lined with rusty films. Oxygen and water have here united with the iron, forming hydrated iron oxide. The effects of oxidation may be seen in the alteration of many kinds of rocks and in red and yellow colors of soils and subsoils.
Pyrite is a very hard mineral of a pale brass color, found in scattered crystals in many rocks, and is composed of iron and sulphur (iron sulphide). Under the attack of the weather it takes up oxygen, forming iron sulphate (green vitriol), a soluble compound, and insoluble hydrated iron oxide, which as a mineral is known as limonite. Several large masses of iron sulphide were placed some years ago on the lawn in front of the National Museum at Washington. The mineral changed so rapidly to green vitriol that enough of this poisonous compound was washed into the ground to kill the roots of the surrounding grass.
AGENTS OF MECHANICAL DISINTEGRATION
HEAT AND COLD. Rocks exposed to the direct rays of the sun become strongly heated by day and expand. After sunset they rapidly cool and contract. When the difference in temperature between day and night is considerable, the repeated strains of sudden expansion and contraction at last become greater than the rocks can bear, and they break, for the same reason that a glass cracks when plunged into boiling water (Fig. 5).
Rocks are poor conductors of heat, and hence their surfaces may become painfully hot under the full blaze of the sun, while the interior remains comparatively cool. By day the surface shell expands and tends to break loose from the mass of the stone. In cooling in the evening the surface shell suddenly contracts on the unyielding interior and in time is forced off in scales.
Many rocks, such as granite, are made up of grains of various minerals which differ in color and in their capacity to absorb heat, and which therefore contract and expand in different ratios. In heating and cooling these grains crowd against their neighbors and tear loose from them, so that finally the rock disintegrates into sand.
The conditions for the destructive action of heat and cold are most fully met in arid regions when vegetation is wanting for lack of sufficient rain. The soil not being held together by the roots of plants is blown away over large areas, leaving the rocks bare to the blazing sun in a cloudless sky. The air is dry, and the heat received by the earth by day is therefore rapidly radiated at night into space. There is a sharp and sudden fall of temperature after sunset, and the rocks, strongly heated by day, are now chilled perhaps even to the freezing point.
In the Sahara the thermometer has been known to fall 131 degrees F. within a few hours. In the light air of the Pamir plateau in central Asia a rise of 90 degrees F. has been recorded from seven o'clock in the morning to one o'clock in the afternoon. On the mountains of southwestern Texas there are frequently heard crackling noises as the
FROST. By this term is meant the freezing and thawing of water contained in the pores and crevices of rocks. All rocks are more or less porous and all contain more or less water in their pores. Workers in stone call this "quarry water," and speak of a stone as "green" before the quarry water has dried out. Water also seeps along joints and bedding planes and gathers in all seams and crevices. Water expands in freezing, ten cubic inches of water freezing to about eleven cubic inches of ice. As water freezes in the rifts and pores of rocks it expands with the irresistible force illustrated in the freezing and breaking of water pipes in winter. The first rift in the rock, perhaps too narrow to be seen, is widened little by little by the wedges of successive frosts, and finally the rock is broken into detached blocks, and these into angular chip-stone by the same process.
It is on mountain tops and in high latitudes that the effects of frost are most plainly seen. "Every summit" says Whymper, "amongst the rock summits upon which I have stood has been nothing but a piled-up heap of fragments" (Fig. 7). In Iceland, in Spitsbergen, in Kamchatka, and in other frigid lands large areas are thickly strewn with sharp-edged fragments into which the rock has been shattered by frost.
ORGANIC AGENTS
We must reckon the roots of plants and trees among the agents which break rocks into pieces. The tiny rootlet in its search for food and moisture inserts itself into some minute rift, and as it grows slowly wedges the rock apart. Moreover, the acids of the root corrode the rocks with which they are in contact. One may sometimes find in the soil a block of limestone wrapped in a mesh of roots, each of which lies in a little furrow where it has eaten into the stone.
Rootless plants called lichens often cover and corrode rocks as yet bare of soil; but where lichens are destroying the rock less rapidly than does the weather, they serve in a way as a protection.
CONDITIONS FAVORING DISINTEGRATION AND DECAY. The disintegration of rocks under frost and temperature changes goes on most rapidly in cold and arid climates, and where vegetation is scant or absent. On the contrary, the decay of rocks under the chemical action of water is favored by a warm, moist climate and abundant vegetation. Frost and heat and cold can only act within the few feet from the surface to which the necessary temperature changes are limited, while water penetrates and alters the rocks to great depths.
The pupil may explain.
In what ways the presence of joints and bedding planes assists in the breaking up and decay of rocks under the action of the weather.
Why it is a good rule of stone masons never to lay stones on edge, but always on their natural bedding planes.
Why stones fresh from the quarry sometimes go to pieces in early winter, when stones which have been quarried for some months remain uninjured.
Why quarrymen in the northern states often keep their quarry floors flooded during winter.
Why laminated limestone should not be used for curbstone.
Why rocks composed of layers differing in fineness of grain and in ratios of expansion do not make good building stone.
Fine-grained rocks with pores so small that capillary attraction keeps the water which they contain from readily draining away are more apt to hold their pores ten elevenths full of water than are rocks whose pores are larger. Which, therefore, are more likely to be injured by frost?
Which is subject to greater temperature changes, a dark rock or one of a light color? the north side or the south side of a valley?
THE MANTLE OF ROCK WASTE
We have seen that rocks are everywhere slowly wasting away. They are broken in pieces by frost, by tree roots, and by heat and cold. They dissolve and decompose under the chemical action of water and the various corrosive substances which it contains, leaving their insoluble residues as residual clays and sands upon the surface. As a result there is everywhere forming a mantle of rock waste which covers the land. It is well to imagine how the country would appear were this mantle with its soil and vegetation all scraped away or had it never been formed. The surface of the land would then be everywhere of bare rock as unbroken as a quarry floor.
THE THICKNESS OF THE MANTLE. In any locality the thickness of the mantle of rock waste depends as much on the rate at which it is constantly being removed as on the rate at which it is forming. On the face of cliffs it is absent, for here waste is removed as fast as it is made. Where waste is carried away more slowly than it is produced, it accumulates in time to great depth.
The granite of Pikes Peak is disintegrated to a depth of twenty feet. In the city of Washington granite rock is so softened to a depth of eighty feet that it can be removed with pick and shovel. About Atlanta, Georgia, the rocks are completely rotted for one hundred feet from the surface, while the beginnings of decay may be noticed at thrice that depth. In places in southern Brazil the rock is decomposed to a depth of four hundred feet.
In southwestern Wisconsin a reddish residual clay has an average depth of thirteen feet on broad uplands, where it has been removed to the least extent. The country rock on which it rests is a limestone with about ten per cent of insoluble impurities. At least how thick, then, was that portion of the limestone which has rotted down to the clay?
DISTINGUISHING CHARACTERISTICS OF RESIDUAL WASTE. We must learn to distinguish waste formed in place by the action of the weather from the products of other geological agencies. Residual waste is unstratified. It contains no substances which have not been derived from the weathering of the parent rock. There is a gradual transition from residual waste into the unweathered rock beneath. Waste resting on sound rock evidently has been shifted and was not formed in place.
In certain regions of southern Missouri the land is covered with a layer of broken flints and red clay, while the country rock is limestone. The limestone contains nodules of flint, and we may infer that it has been by the decay and removal of thick masses of limestone that the residual layer of clay and flints has been left upon the surface. Flint is a form of quartz, dull-lustered, usually gray or blackish in color, and opaque except on thinnest edges, where it is translucent.
Over much of the northern states there is spread an unstratified stony clay called the drift. It often rests on sound rocks. It contains grains of sand, pebbles, and bowlders composed of many different minerals and rocks that the country rock cannot furnish. Hence the drift cannot have been formed by the decay of the rock of the region. A shale or limestone, for example, cannot waste to a clay containing granite pebbles. The origin of the drift will be explained in subsequent chapters.
The differences in rocks are due more to their soluble than to their insoluble constituents. The latter are few in number and are much the same in rocks of widely different nature, being chiefly quartz, silicate of alumina, and iron oxide. By the removal of their soluble parts very many and widely different rocks
In a broad way the changes which rocks undergo in weathering are an adaptation to the environment in which they find themselves at the earth's surface,—an environment different from that in which they were formed under sea or under ground. In open air, where they are attacked by various destructive agents, few of the rock- making minerals are stable compounds except quartz, the iron oxides, and the silicate of alumina; and so it is to one or more of these comparatively insoluble substances that most rocks are reduced by long decay.
Which produces a mantle of finer waste, frost or chemical decay? which a thicker mantle? In what respects would you expect that the mantle of waste would differ in warm humid lands like India, in frozen countries like Alaska, and in deserts such as the Sahara?
THE SOIL. The same agencies which produce the mantle of waste are continually at work upon it, breaking it up into finer and finer particles and causing its more complete decay. Thus on the surface, where the waste has weathered longest, it is gradually made fine enough to support the growth of plants, and is then known as soil. The coarser waste beneath is sometimes spoken of as subsoil. Soil usually contains more or less dark, carbonaceous, decaying organic matter, called humus, and is then often termed the humus layer. Soil forms not only on waste produced in place from the rock beneath, but also on materials which have been transported, such as sheets of glacial drift and river deposits. Until rocks are reduced to residual clays the work of the weather is more rapid and effective on the fragments of the mantle of waste than on the rocks from which waste is being formed. Why?
Any fresh excavation of cellar or cistern, or cut for road or railway, will show the characteristics of the humus layer. It may form only a gray film on the surface, or we may find it a layer a foot or more thick, dark, or even black, above, and growing gradually lighter in color as it passes by insensible gradations into the subsoil. In some way the decaying vegetable matter continually forming on the surface has become mingled with the material beneath it.
HOW HUMUS AND THE SUBSOIL ARE MINGLED. The mingling of humus and the subsoil is brought about by several means. The roots of plants penetrate the waste, and when they die leave their decaying substance to fertilize it. Leaves and stems falling on the surface are turned under by several agents. Earthworms and other animals whose home is in the waste drag them into their burrows either for food or to line their nests. Trees overthrown by the wind, roots and all, turn over the soil and subsoil and mingle them together. Bacteria also work in the waste and contribute to its enrichment. The animals living in the mantle do much in other ways toward the making of soil. They bring the coarser fragments from beneath to the surface, where the waste weathers more rapidly. Their burrows allow air and water to penetrate the waste more freely and to affect it to greater depths.
ANTS. In the tropics the mantle of waste is worked over chiefly by ants. They excavate underground galleries and chambers, extending sometimes as much as fourteen feet below the surface, and build mounds which may reach as high above it. In some parts of Paraguay and southern Brazil these mounds, like gigantic potato hills, cover tracts of considerable area.
In search for its food—the dead wood of trees—the so-called white ant constructs runways of earth about the size of gas pipes, reaching from the base of the tree to the topmost branches. On the plateaus of central Africa explorers have walked for miles through forests every tree of which was plastered with these galleries of mud. Each grain of earth used in their construction is moistened and cemented by slime as it is laid in place by the ant, and is thus acted on by organic chemical agents. Sooner or later these galleries are beaten down by heavy rains, and their fertilizing substances are scattered widely by the winds.
EARTHWORMS. In temperate regions the waste is worked over largely by earthworms. In making their burrows worms swallow earth in order to extract from it any nutritive organic matter which it may contain. They treat it with their digestive acids, grind it in their stony gizzards, and void it in castings on the surface of the ground. It was estimated by Darwin that in many parts of England each year, on every acre, more than ten tons of earth pass through the bodies of earthworms and are brought to the surface, and that every few years the entire soil layer is thus worked over by them.
In all these ways the waste is made fine and stirred and enriched. Grain by grain the subsoil with its fresh mineral ingredients is brought to the surface, and the rich organic matter which plants and animals have taken from the atmosphere is plowed under. Thus Nature plows and harrows on "the great world's farm" to make ready and ever to renew a soil fit for the endless succession of her crops.
The world processes by which rocks are continually wasting away are thus indispensable to the life of plants and animals. The organic world is built on the ruins of the inorganic, and because the solid rocks have been broken down into soil men are able to live upon the earth.
SOLAR ENERGY. The source of the energy which accomplishes all this necessary work is the sun. It is the radiant energy of the sun which causes the disintegration of rocks, which lifts vapor into the atmosphere to fall as rain, which gives life to plants and animals. Considering the earth in a broad way, we may view it as a globe of solid rock,—the lithosphere,—surrounded by two mobile envelopes: the envelope of air,—THE ATMOSPHERE, and the envelope of water,—THE HYDROSPHERE. Under the action of solar energy these envelopes are in constant motion. Water from the hydrosphere is continually rising in vapor into the atmosphere, the air of the atmosphere penetrates the hydrosphere,—for its gases are dissolved in all waters,—and both air and water enter and work upon the solid earth. By their action upon the lithosphere they have produced a third envelope,—the mantle of rock waste.
This envelope also is in movement, not indeed as a whole, but particle by particle. The causes which set its particles in motion, and the different forms which the mantle comes to assume, we will now proceed to study.
MOVEMENTS OF THE MANTLE OF ROCK WASTE
At the sandstone ledges which we first visited we saw not only that the rocks were crumbling away, but also that grains and fragments of them were creeping down the slopes of the valley to the stream and were carried by it onward toward the sea. This process is going on everywhere. Slowly it may be, and with many interruptions, but surely, the waste of the land moves downward to the sea. We may divide its course into two parts,—the path to the stream, which we will now consider, and its carriage onward by the stream, which we will defer to a later chapter.
GRAVITY. The chief agent concerned in the movement of waste is gravity. Each particle of waste feels the unceasing downward pull of the earth's mass and follows it when free to do so. All agencies which produce waste tend to set its particles free and in motion, and therefore cooperate with gravity. On cliffs, rocks fall when wedged off by frost or by roots of trees, and when detached by any other agency. On slopes of waste, water freezes in chinks between stones, and in pores between particles of soil, and wedges them apart. Animals and plants stir the waste, heat expands it, cold contracts it, the strokes of the raindrops drive loose particles down the slope and the wind lifts and lets them fall. Of all these movements, gravity assists those
A slab of sandstone laid on another of the same kind at an angle of 17 degrees and left in the open air was found to creep down the slope at the rate of a little more than a millimeter a month. Explain why it did so.
RAIN. The most efficient agent in the carriage of waste to the streams is the rain. It moves particles of soil by the force of the blows of the falling drops, and washes them down all slopes to within reach of permanent streams. On surfaces unprotected by vegetation, as on plowed fields and in arid regions, the rain wears furrows and gullies both in the mantle of waste and in exposures of unaltered rock (Fig. 17).
At the foot of a hill we may find that the soil has accumulated by creep and wash to the depth of several feet; while where the hillside is steepest the soil may be exceedingly thin, or quite absent, because removed about as fast as formed. Against the walls of an abbey built on a slope in Wales seven hundred years ago, the creeping waste has gathered on the uphill side to a depth of seven feet. The slow-flowing sheet of waste is often dammed by fences and walls, whose uphill side gathers waste in a few years so as to show a distinctly higher surface than the downhill side, especially in plowed fields where the movement is least checked by vegetation.
TALUS. At the foot of cliffs there is usually to be found a slope of rock fragments which clearly have fallen from above. Such a heap of waste is known as talus. The amount of talus in any place depends both on the rate of its formation and the rate of its removal. Talus forms rapidly in climates where mechanical disintegration is most effective, where rocks are readily broken into blocks because closely jointed and thinly bedded rather than massive, and where they are firm enough to be detached in fragments of some size instead of in fine grains. Talus is removed slowly where it decays slowly, either because of the climate or the resistance of the rock. It may be rapidly removed by a stream flowing along its base.
In a moist climate a soluble rock, such as massive limestone, may form talus little if any faster than the talus weathers away. A loose-textured sandstone breaks down into incoherent sand grains, which in dry climates, where unprotected by vegetation, may be blown away as fast as they fall, leaving the cliff bare to the base. Cliffs of such slow-decaying rocks as quartzite and granite when closely jointed accumulate talus in large amounts.
Talus slopes may be so steep as to reach THE ANGLE OF REPOSE, i.e. the steepest angle at which the material will lie. This angle varies with different materials, being greater with coarse and angular fragments than with fine rounded grains. Sooner or later a talus reaches that equilibrium where the amount removed from its surface just equals that supplied from the cliff above. As the talus is removed and weathers away its slope retreats together with the retreat of the cliff, as seen in Figure 9.
GRADED SLOPES. Where rocks weather faster than their waste is carried away, the waste comes at last to cover all rocky ledges. On the steeper slopes it is coarser and in more rapid movement than on slopes more gentle, but mountain sides and hills and plains alike come to be mantled with sheets of waste which everywhere is creeping toward the streams. Such unbroken slopes, worn or built to the least inclination at which the waste supplied by weathering can be urged onward, are known as GRADED SLOPES.
Of far less importance than the silent, gradual creep of waste, which is going on at all times everywhere about us, are the startling local and spasmodic movements which we are now to describe.
AVALANCHES. On steep mountain sides the accumulated snows of winter often slip and slide in avalanches to the valleys below. These rushing torrents of snow sweep their tracks clean of waste and are one of Nature's normal methods of moving it along the downhill path.
LANDSLIDES. Another common and abrupt method of delivering waste to streams is by slips of the waste mantle in large masses. After long rains and after winter frosts the cohesion between the waste and the sound rock beneath is loosened by seeping water underground. The waste slips on the rock surface thus lubricated and plunges down the mountain side in a swift roaring torrent of mud and stones.
We may conveniently mention here a second type of landslide, where masses of solid rock as well as the mantle of waste are involved in the sudden movement. Such slips occur when valleys have been rapidly deepened by streams or glaciers and their sides have not yet been graded. A favorable condition is where the strata dip (i.e. incline downwards) towards the valley (Fig. 11), or are broken by joint planes dipping in the same direction. The upper layers, including perhaps the entire mountain side, have been cut across by the valley trench and are left supported only on the inclined surface of the underlying rocks. Water may percolate underground along this surface and loosen the cohesion between the upper and the underlying strata by converting the upper surface of a shale to soft wet clay, by dissolving layers of a limestone, or by removing the cement of a sandstone and converting it into loose sand. When the inclined surface is thus lubricated the overlying masses may be launched into the valley below. The solid rocks are broken and crushed in sliding and converted into waste consisting, like that of talus, of angular unsorted fragments, blocks of all sizes being mingled pellmell with rock meal and dust. The principal effects of landslides may be gathered from the following examples.
At Gohna, India, in 1893, the face of a spur four thousand feet high, of the lower ranges of the Himalayas, slipped into the gorge of the headwaters of the Ganges River in successive rock falls which lasted for three days. Blocks of stone were projected for a mile, and clouds of limestone dust were spread over the surrounding country. The debris formed a dam one thousand feet high, extending for two miles along the valley. A lake gathered behind this barrier, gradually rising until it overtopped it in a little less than a year. The upper portion of the dam then broke, and a terrific rush of water swept down the valley in a wave which, twenty miles away, rose one hundred and sixty feet in height. A narrow lake is still held by the strong base of the dam.
In 1896, after forty days of incessant rain, a cliff of sandstone slipped into the Yangtse River in China, reducing the width of the channel to eighty yards and causing formidable rapids.
At Flims, in Switzerland, a prehistoric landslip flung a dam eighteen hundred feet high across the headwaters of the Rhine. If spread evenly over a surface of twenty-eight square miles, the material would cover it to a depth of six hundred and sixty feet. The barrier is not yet entirely cut away, and several lakes are held in shallow basins on its hummocky surface.
A slide from the precipitous river front of the citadel hill of Quebec, in 1889, dashed across Champlain Street, wrecking a number of houses and causing the death of forty-five persons. The strata here are composed of steeply dipping slate.
In lofty mountain ranges there may not be a single valley without its traces of landslides, so common there is this method of the movement of waste, and of building to grade over-steepened slopes.