THE ELEMENTS OF GEOLOGY
BY WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE
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
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 "authorities" are reached by a private road that is closed to him. The further extension of this method of presentation is urged upon the teacher, so that the young geologist may always learn the evidence that leads to a conclusion, and not only the conclusion itself.
W. M. DAVIS
HARVARD UNIVERSITY, CAMBRIDGE, MASS.
INTRODUCTION.—THE SCOPE AND AIM OF GEOLOGY
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
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
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
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 rocks the history of our planet and the successive forms of life which have dwelt upon it.
EXTERNAL GEOLOGICAL AGENCIES
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 granite bowlders and pebbles more or less decayed may be found in a surface sheet of stony clay called the drift. Of the different minerals composing granite, quartz alone remains unaltered. Mica weathers to detached flakes which have lost their elasticity. The feldspar crystals have lost their luster and hardness, and even have decayed to clay. Where long- weathered granite forms the country rock, it often may be cut with spade or trowel for several feet from the surface, so rotten is the feldspar, and here the rock is seen to break down to a clayey soil containing grains of quartz and flakes of mica.
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 rocks of that arid region throw off scales from a fraction of an inch to four inches in thickness, and loud reports are made as huge bowlders split apart. Desert pebbles weakened by long exposure to heat and cold have been shivered to fine sharp-pointed fragments on being placed in sand heated to 180 degrees F. Beds half a foot thick, forming the floor of limestone quarries in Wisconsin, have been known to buckle and arch and break to fragments under the heat of the summer sun.
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.
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 rot down to a residual clay gritty with particles of quartz and colored red or yellow with iron oxide.
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 which are downhill and retards those which are uphill. On the whole, therefore, the downhill movements prevail, and the mantle of waste, block by block and grain by grain, creeps along the downhill path.
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.
ROCK SCULPTURE BY WEATHERING
We are now to consider a few of the forms into which rock masses are carved by the weather.
BOWLDERS OF WEATHERING. In many quarries and outcrops we may see that the blocks into which one or more of the uppermost layers have been broken along their joints and bedding planes are no longer angular, as are those of the layers below. The edges and corners of these blocks have been worn away by the weather. Such rounded cores, known as bowlders of weathering, are often left to strew the surface.
DIFFERENTIAL WEATHERING. This term covers all cases in which a rock mass weathers differently in different portions. Any weaker spots or layers are etched out on the surface, leaving the more resistant in relief. Thus massive limestones become pitted where the weather drills out the weaker portions. In these pits, when once they are formed, moisture gathers, a little soil collects, vegetation takes root, and thus they are further enlarged until the limestone may be deeply honeycombed.
On the sides of canyons, and elsewhere where the edges of strata are exposed, the harder layers project as cliffs, while the softer weather back to slopes covered with the talus of the harder layers above them. It is convenient to call the former cliff makers and the latter slope makers.
Differential weathering plays a large part in the sculpture of the land. Areas of weak rock are wasted to plains, while areas of hard rock adjacent are still left as hills and mountain ridges, as in the valleys and mountains of eastern Pennsylvania. But in such instances the lowering of the surface of the weaker rock is also due to the wear of streams, and especially to the removal by them from the land of the waste which covers and protects the rocks beneath.
Rocks owe their weakness to several different causes. Some, such as beds of loose sand, are soft and easily worn by rains; some, as limestone and gypsum for example, are soluble. Even hard insoluble rocks are weak under the attack of the weather when they are closely divided by joints and bedding planes and are thus readily broken up into blocks by mechanical agencies.
OUTLIERS AND MONUMENTS. As cliffs retreat under the attack of the weather, portions are left behind where the rock is more resistant or where the attack for any reason is less severe. Such remnant masses, if large, are known as outliers. When
Note the rain furrows on the slope at the foot of the monuments. In the foreground are seen fragments of petrified trunks of trees, composed of silica and extremely resistant to the weather. On the removal of the rock layers in which these fragments were imbedded they are left to strew the surface in the same way as are the residual flints of southern Missouri. flat-topped, because of the protection of a resistant horizontal capping layer, they are termed mesas,—a term applied also to the flat-topped portions of dissected plateaus (Fig. 129). Retreating cliffs may fall back a number of miles behind their outliers before the latter are finally consumed.
Monuments are smaller masses and may be but partially detached from the cliff face. In the breaking down of sheets of horizontal strata, outliers grow smaller and smaller and are reduced to massive rectangular monuments resembling castles (Fig. 17). The rock castle falls into ruin, leaving here and there an isolated tower; the tower crumbles to a lonely pillar, soon to be overthrown. The various and often picturesque shapes of monuments depend on the kind of rock, the attitude of the strata, and the agent by which they are chiefly carved. Thus pillars may have a capital formed of a resistant stratum. Monuments may be undercut and come to rest on narrow pedestals, wherever they weather more rapidly near the ground, either because of the greater moisture there, or—in arid climates—because worn at their base by drifting sands.
Stony clays disintegrating under the rain often contain bowlders which protect the softer material beneath from the vertical blows of raindrops, and thus come to stand on pedestals of some height. One may sometimes see on the ground beneath dripping eaves pebbles left in the same way, protecting tiny pedestals of sand.
MOUNTAIN PEAKS AND RIDGES. Most mountains have been carved out of great broadly uplifted folds and blocks of the earth's crust. Running water and glacier ice have cut these folds and blocks into masses divided by deep valleys; but it is by the weather, for the most part, that the masses thus separated have been sculptured to the present forms of the individual peaks and ridges.
Frost and heat and cold sculpture high mountains to sharp, tusklike peaks and ragged, serrate crests, where their waste is readily removed.
The Matterhorn of the Alps is a famous example of a mountain peak whose carving by the frost and other agents is in active progress. On its face "scarcely a rock anywhere is firmly attached," and the fall of loosened stones is incessant. Mountain climbers who have camped at its base tell how huge rocks from time to time come leaping down its precipices, followed by trains of dislodged smaller fragments and rock dust; and how at night one may trace the course of the bowlders by the sparks which they strike from the mountain walls. Mount Assiniboine, Canada (Fig. 20), resembles the Matterhorn in form and has been carved by the same agencies.
"The Needles" of Arizona are examples of sharp mountain peaks in a warm arid region sculptured chiefly by temperature changes.
Chemical decay, especially when carried on beneath a cover of waste and vegetation, favors the production of rounded knobs and dome-shaped mountains.
THE WEATHER CURVE. We have seen that weathering reduces the angular block quarried by the frost to a rounded bowlder by chipping off its corners and smoothing away its edges. In much the same way weathering at last reduces to rounded hills the earth blocks cut by streams or formed in any other way. High mountains may at first be sculptured by the weather to savage peaks (Fig. 181), but toward the end of their life history they wear down to rounded hills (Fig. 182). The weather curve, which may be seen on the summits of low hills (Fig. 21), is convex upward.
In Figure 22, representing a cubic block of stone whose faces are a yard square, how many square feet of surface are exposed to the weather by a cubic foot at a corner a; by one situated in the middle of an edge b; by one in the center of a side c? How much faster will a and b weather than c, and what will be the effect on the shape of the block?
THE COOPERATION OF VARIOUS AGENCIES IN ROCK SCULPTURE. For the sake of clearness it is necessary to describe the work of each geological agent separately. We must not forget, however, that in Nature no agent works independently and alone; that every result is the outcome of a long chain of causes. Thus, in order that the mountain peak may be carved by the agents of disintegration, the waste must be rapidly removed,—a work done by many agents, including some which we are yet to study; and in order that the waste may be removed as fast as formed, the region must first have been raised well above the level of the sea, so that the agents of transportation could do their work effectively. The sculpture of the rocks is accomplished only by the cooperation of many forces.
The constant removal of waste from the surface by creep and wash and carriage by streams is of the highest importance, because it allows the destruction of the land by means of weathering to go on as long as any land remains above sea level. If waste were not removed, it would grow to be so thick as to protect the rock beneath from further weathering, and the processes of destruction which we have studied would be brought to an end. The very presence of the mantle of waste over the land proves that on the whole rocks weather more rapidly than their waste is removed. The destruction of the land is going on as fast as the waste can be carried away.
We have now learned to see in the mantle of waste the record of the destructive action of the agencies of weathering on the rocks of the land surface. Similar records we shall find buried deeply among the rocks of the crust in old soils and in rocks pitted and decayed, telling of old land surfaces long wasted by the weather. Ever since the dry land appeared these agencies have been as now quietly and unceasingly at work upon it, and have ever been the chief means of the destruction of its rocks. The vast bulk of the stratified rocks of the earth's crust is made up almost wholly of the waste thus worn from ancient lands.
In studying the various geological agencies we must remember the almost inconceivable times in which they work. The slowest process when multiplied by the immense time in which it is carried on produces great results. The geologist looks upon the land forms of the earth's surface as monuments which record the slow action of weathering and other agents during the ages of the past. The mountain peak, the rounded hill, the wide plain which lies where hills and mountains once stood, tell clearly of the great results which slow processes will reach when given long time in which to do their work. We should accustom ourselves also to think of the results which weathering will sooner or later bring to pass. The tombstone and the bowlder of the field, which each year lose from their surfaces a few crystalline grains, must in time be wholly destroyed. The hill whose rocks are slowly rotting underneath a cover of waste must become lower and lower as the centuries and millenniums come and go, and will finally disappear. Even the mountains are crumbling away continually, and therefore are but fleeting features of the landscape.
THE WORK OF GROUND WATER
LAND WATERS. We have seen how large is the part that water plays at and near the surface of the land in the processes of weathering and in the slow movement of waste down all slopes to the stream ways. We now take up the work of water as it descends beneath the ground,—a corrosive agent still, and carrying in solution as its load the invisible waste of rocks derived from their soluble parts.
Land waters have their immediate source in the rainfall. By the heat of the sun water is evaporated from the reservoir of the ocean and from moist surfaces everywhere. Mingled as vapor with the air, it is carried by the winds over sea and land, and condensed it returns to the earth as rain or snow. That part of the rainfall which descends on the ocean does not concern us, but that which falls on the land accomplishes, as it returns to the sea, the most important work of all surface geological agencies.
The rainfall may be divided into three parts: the first DRIES UP, being discharged into the air by evaporation either directly from the soil or through vegetation; the second RUNS OFF over the surface to flood the streams; the third SOAKS IN the ground and is henceforth known as GROUND or UNDERGROUND WATER.
THE DESCENT OF GROUND WATER. Seeping through the mantle of waste, ground water soaks into the pores and crevices of the underlying rock. All rocks of the upper crust of the earth are more or less porous, and all drink in water. IMPERVIOUS ROCKS, such as granite, clay, and shale, have pores so minute that the water which they take in is held fast within them by capillary attraction, and none drains through. PERVIOUS ROCKS, on the other hand, such as many sandstones, have pore spaces so large that water filters through them more or less freely. Besides its seepage through the pores of pervious rocks, water passes to lower levels through the joints and cracks by which all rocks, near the surface are broken.
Even the closest-grained granite has a pore space of 1 in 400, while sandstone may have a pore space of 1 in 4. Sand is so porous that it may absorb a third of its volume of water, and a loose loam even as much as one half.
THE GROUND-WATER SURFACE is the name given the upper surface of ground water, the level below which all rocks are saturated. In dry seasons the ground-water surface sinks. For ground water is constantly seeping downward under gravity, it is evaporated in the waste and its moisture is carried upward by capillarity and the roots of plants to the surface to be evaporated in the air. In wet seasons these constant losses are more than made good by fresh supplies from that part of the rainfall which soaks into the ground, and the ground-water surface rises.
In moist climates the ground-water surface (Fig. 24) lies, as a rule, within a few feet of the land surface and conforms to it in a general way, although with slopes of less inclination than those of the hills and valleys. In dry climates permanent ground water may be found only at depths of hundreds of feet. Ground water is held at its height by the fact that its circulation is constantly impeded by capillarity and friction. If it were as free to drain away as are surface streams, it would sink soon after a rain to the level of the deepest valleys of the region.
WELLS AND SPRINGS. Excavations made in permeable rocks below the ground-water surface fill to its level and are known as wells. Where valleys cut this surface permanent streams are formed, the water either oozing forth along ill-defined areas or issuing at definite points called springs, where it is concentrated by the structure of the rocks. A level tract where the ground-water surface coincides with the surface of the ground is a swamp or marsh.
By studying a spring one may learn much of the ways and work of ground water. Spring water differs from that of the stream into which it flows in several respects. If we test the spring with a thermometer during successive months, we shall find that its temperature remains much the same the year round. In summer it is markedly cooler than the stream; in winter it is warmer and remains unfrozen while the latter perhaps is locked in ice. This means that its underground path must lie at such a distance from the surface that it is little affected by summer's heat and winter's cold.
While the stream is often turbid with surface waste washed into it by rains, the spring remains clear; its water has been filtered during its slow movement through many small underground passages and the pores of rocks. Commonly the spring differs from the stream in that it carries a far larger load of dissolved rock. Chemical analysis proves that streams contain various minerals in solution, but these are usually in quantities so small that they are not perceptible to the taste or feel. But the water of springs is often well charged with soluble minerals; in its slow, long journey underground it has searched out the soluble parts of the rocks through which it seeps and has dissolved as much of them as it could. When spring water is boiled away, the invisible load which it has carried is left behind, and in composition is found to be practically identical with that of the soluble ingredients of the country rock. Although to some extent the soluble waste of rocks is washed down surface slopes by the rain, by far the larger part is carried downward by ground water and is delivered to streams by springs.
In limestone regions springs are charged with calcium carbonate (the carbonate of lime), and where the limestone is magnesian they contain magnesium carbonate also. Such waters are "hard"; when used in washing, the minerals which they contain combine with the fatty acids of soap to form insoluble curdy compounds. When springs rise from rocks containing gypsum they are hard with calcium sulphate. In granite regions they contain more or less soda and potash from the decay of feldspar.
The flow of springs varies much less during the different seasons of the year than does that of surface streams. So slow is the movement of ground water through the rocks that even during long droughts large amounts remain stored above the levels of surface drainage.
MOVEMENTS OF GROUND WATER. Ground water is in constant movement toward its outlets. Its rate varies according to many conditions, but always is extremely slow. Even through loose sands beneath the beds of rivers it sometimes does not exceed a fifth of a mile a year.
In any region two zones of flow may be distinguished. The UPPER ZONE OF FLOW extends from the ground-water surface downward through the waste mantle and any permeable rocks on which the mantle rests, as far as the first impermeable layer, where the descending movement of the water is stopped. The DEEP ZONES OF FLOW occupy any pervious rocks which may be found below the impervious layer which lies nearest to the surface. The upper zone is a vast sheet of water saturating the soil and rocks and slowly seeping downward through their pores and interstices along the slopes to the valleys, where in part it discharges in springs and often unites also in a wide underflowing stream which supports and feeds the river (Fig. 24).
A city in a region of copious rains, built on the narrow flood plain of a river, overlooked by hills, depends for its water supply on driven wells, within the city limits, sunk in the sand a few yards from the edge of the stream. Are these wells fed by water from the river percolating through the sand, or by ground water on its way to the stream and possibly contaminated with the sewage of the town?
At what height does underground water stand in the wells of your region? Does it vary with the season? Have you ever known wells to go dry? It may be possible to get data from different wells and to draw a diagram showing the ground-water surface as compared with the surface of the ground.
FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in pervious strata which are overlain by some impervious stratum. Such layers are often carried by their dip to great depths, and water may circulate in them to far below the level of the surface streams and even of the sea. When a fissure crosses a water- bearing stratum, or AQUIFIER, water is forced upward by the pressure of the weight of the water contained in the higher parts of the stratum, and may reach the surface as a fissure spring. A boring which taps such an aquifer is known as an artesian well, a name derived from a province in France where wells of this kind have been long in use. The rise of the water in artesian wells, and in fissure springs also, depends on the following conditions illustrated in Figure 29. The aquifer dips toward the region of the wells from higher ground, where it outcrops and receives its water. It is inclosed between an impervious layer above and water- tight or water-logged layers beneath. The weight of the column of water thus inclosed in the aquifer causes water to rise in the well, precisely as the weight of the water in a standpipe forces it in connected pipes to the upper stories of buildings.
Which will supply the larger region with artesian wells, an aquifer whose dip is steep or one whose dip is gentle? Which of the two aquifers, their thickness being equal, will have the larger outcrop and therefore be able to draw upon the larger amount of water from the rainfall? Illustrate with diagrams.
THE ZONE OF SOLUTION. Near the surface, where the circulation of ground water is most active, it oxidizes, corrodes, and dissolves the rocks through which it passes. It leaches soils and subsoils of their lime and other soluble minerals upon which plants depend for their food. It takes away the soluble cements of rocks; it widens fissures and joints and opens winding passages along the bedding planes; it may even remove whole beds of soluble rocks, such as rock salt, limestone, or gypsum. The work of ground water in producing landslides has already been noticed. The zone in which the work of ground water is thus for the most part destructive we may call the zone of solution.
CAVES. In massive limestone rocks, ground water dissolves channels which sometimes form large caves (Fig. 30). The necessary conditions for the excavation of caves of great size are well shown in central Kentucky, where an upland is built throughout of thick horizontal beds of limestone. The absence of layers of insoluble or impervious rock in its structure allows a free circulation of ground water within it by the way of all natural openings in the rock. These water ways have been gradually enlarged by solution and wear until the upland is honeycombed with caves. Five hundred open caverns are known in one county.
Mammoth Cave, the largest of these caverns, consists of a labyrinth of chambers and winding galleries whose total length is said to be as much as thirty miles. One passage four miles long has an average width of about sixty feet and an average height of forty feet. One of the great halls is three hundred feet in width and is overhung by a solid arch of limestone one hundred feet above the floor. Galleries at different levels are connected by well-like pits, some of which measure two hundred and twenty-five feet from top to bottom. Through some of the lowest of these tunnels flows Echo River, still at work dissolving and wearing away the rock while on its dark way to appear at the surface as a great spring.
NATURAL BRIDGES. As a cavern enlarges and the surface of the land above it is lowered by weathering, the roof at last breaks down and the cave becomes an open ravine. A portion of the roof may for a while remain, forming a "natural bridge."
SINK HOLES. In limestone regions channels under ground may become so well developed that the water of rains rapidly drains away through them. Ground water stands low and wells must be sunk deep to find it. Little or no surface water is left to form brooks.
Thus across the limestone upland of central Kentucky one meets but three surface streams in a hundred miles. Between their valleys surface water finds its way underground by means of sink holes. These are pits, commonly funnel shaped, formed by the enlargement of crevice or joint by percolating water, or by the breakdown of some portion of the roof of a cave. By clogging of the outlet a sink hole may come to be filled by a pond.
Central Florida is a limestone region with its drainage largely subterranean and in part below the level even of the sea. Sink holes are common, and many of them are occupied by lakelets. Great springs mark the point of issue of underground streams, while some rise from beneath the sea. Silver Spring, one of the largest, discharges from a basin eight hundred feet wide and thirty feet deep a little river navigable for small steamers to its source. About the spring there are no surface streams for sixty miles.
THE KARST. Along the eastern coast of the Adriatic, as far south as Montenegro, lies a belt of limestone mountains singularly worn and honeycombed by the solvent action of water. Where forests have been cut from the mountain sides and the red soil has washed away, the surface of the white limestone forms a pathless desert of rock where each square rod has been corroded into an intricate branch work of shallow furrows and sharp ridges. Great sink holes, some of them six hundred feet deep and more, pockmark the surface of the land. The drainage is chiefly subterranean. Surface streams are rare and a portion of their courses is often under ground. Fragmentary valleys come suddenly to an end at walls of rock where the rivers which occupy the valleys plunge into dark tunnels to reappear some miles away. Ground water stands so far below the surface that it cannot be reached by wells, and the inhabitants depend on rain water stored for household uses. The finest cavern of Europe, the Adelsberg Grotto, is in this region. Karst, the name of a part of this country, is now used to designate any region or landscape thus sculptured by the chemical action of surface and ground water. We must remember that Karst regions are rare, and striking as is the work of their subterranean streams, it is far less important than the work done by the sheets of underground water slowly seeping through all subsoils and porous rocks in other regions.
Even when gathered into definite channels, ground water does not have the erosive power of surface streams, since it carries with it little or no rock waste. Regions whose underground drainage is so perfect that the development of surface streams has been retarded or prevented escape to a large extent the leveling action of surface running waters, and may therefore stand higher than the surrounding country. The hill honeycombed by Luray Cavern, Virginia, has been attributed to this cause.
CAVERN DEPOSITS. Even in the zone of solution water may under certain circumstances deposit as well as erode. As it trickles from the roof of caverns, the lime carbonate which it has taken into solution from the layers of limestone above is deposited by evaporation in the air in icicle-like pendants called STALACTITES. As the drops splash on the floor there are built up in the same way thicker masses called STALAGMITES, which may grow to join the stalactites above, forming pillars. A stalagmitic crust often seals with rock the earth which accumulates in caverns, together with whatever relics of cave dwellers, either animals or men, it may contain.
Can you explain why slender stalactites formed by the drip of single drops are often hollow pipes?
THE ZONE OF CEMENTATION. With increasing depth subterranean water becomes more and more sluggish in its movements and more and more highly charged with minerals dissolved from the rocks above. At such depths it deposits these minerals in the pores of rocks, cementing their grains together, and in crevices and fissures, forming mineral veins. Thus below the zone of solution where the work of water is to dissolve, lies the zone of cementation where its work is chemical deposit. A part of the invisible load of waste is thus transferred from rocks near the surface to those at greater depths.
As the land surface is gradually lowered by weathering and the work of rain and streams, rocks which have lain deep within the zone of cementation are brought within the zone of solution. Thus there are exposed to view limestones, whose cracks were filled with calcite (crystallized carbonate of lime), with quartz or other minerals, and sandstones whose grains were well cemented many feet below the surface.
CAVITY FILLING. Small cavities in the rocks are often found more or less completely filled with minerals deposited from solution by water in its constant circulation underground. The process may be illustrated by the deposit of salt crystals in a cup of evaporating brine, but in the latter instance the solution is not renewed as in the case of cavities in the rocks. A cavity thus lined with inward-pointing crystals is called a GEODE.
CONCRETIONS. Ground water seeping through the pores of rocks may gather minerals disseminated throughout them into nodular masses called concretions. Thus silica disseminated through limestone is gathered into nodules of flint. While geodes grow from the outside inwards, concretions grow outwards from the center. Nor are they formed in already existing cavities as are geodes. In soft clays concretions may, as they grow, press the clay aside. In many other rocks concretions are made by the process of REPLACEMENT. Molecule by molecule the rock is removed and the mineral of the concretion substituted in its place. The concretion may in this way preserve intact the lamination lines or other structures of the rock. Clays and shales often contain concretions of lime carbonate, of iron carbonate, or of iron sulphide. Some fossil, such as a leaf or shell, frequently forms the nucleus around which the concretion grows.
Why are building stones more easily worked when "green" than after their quarry water has dried out?
DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where ground water is drawn by capillarity to the surface and there evaporates, it leaves as surface incrustations the minerals held in solution. White limy incrustations of this nature cover considerable tracts in northern Mexico. Evaporating beneath the surface, ground water may deposit a limy cement in beds of loose sand and gravel. Such firmly cemented layers are not uncommon in western Kansas and Nebraska, where they are known as "mortar beds."
THERMAL SPRINGS. While the lower limit of surface drainage is sea level, subterranean water circulates much below that depth, and is brought again to the surface by hydrostatic pressure. In many instances springs have a higher temperature than the average annual temperature of the region, and are then known as thermal springs. In regions of present or recent volcanic activity, such as the Yellowstone National Park, we may believe that the heat of thermal springs is derived from uncooled lavas, perhaps not far below the surface. But when hot springs occur at a distance of hundreds of miles from any volcano, as in the case of the hot springs of Bath, England, it is probable that their waters have risen from the heated rocks of the earth's interior. The springs of Bath have a temperature of 120 degrees F., 70 degrees above the average annual temperature of the place. If we assume that the rate of increase in the earth's internal heat is here the average rate, 1 degree F. to every sixty feet of descent, we may conclude that the springs of Bath rise from at least a depth of forty-two hundred feet.
Water may descend to depths from which it can never be brought back by hydrostatic pressure. It is absorbed by highly heated rocks deep below the surface. From time to time some of this deep- seated water may be returned to open air in the steam of volcanic eruptions.
SURFACE DEPOSITS OF SPRINGS. Where subterranean water returns to the surface highly charged with minerals in solution, on exposure to the air it is commonly compelled to lay down much of its invisible load in chemical deposits about the spring. These are thrown down from solution either because of cooling, evaporation, the loss of carbon dioxide, or the work of algae.
Many springs have been charged under pressure with carbon dioxide from subterranean sources and are able therefore to take up large quantities of lime carbonate from the limestone rocks through which they pass. On reaching the surface the pressure is relieved, the gas escapes, and the lime carbonate is thrown down in deposits called TRAVERTINE. The gas is sometimes withdrawn and the deposit produced in large part by the action of algae and other humble forms of plant life.
At the Mammoth Hot Springs in the valley of the Gardiner River, Yellowstone National Park, beautiful terraces and basins of travertine are now building, chiefly by means of algae which cover the bottoms, rims, and sides of the basins and deposit lime carbonate upon them in successive sheets. The rock, snow-white where dry, is coated with red and orange gelatinous mats where the algae thrive in the over-flowing waters.
Similar terraces of travertine are found to a height of fourteen hundred feet up the valley side. We may infer that the springs which formed these ancient deposits discharged near what was then the bottom of the valley, and that as the valley has been deepened by the river the ground water of the region has found lower and lower points of issue.
In many parts of the country calcareous springs occur which coat with lime carbonate mosses, twigs, and other objects over which their waters flow. Such are popularly known as petrifying springs, although they merely incrust the objects and do not convert them into stone.
Silica is soluble in alkaline waters, especially when these are hot. Hot springs rising through alkaline siliceous rocks, such as lavas, often deposit silica in a white spongy formation known as SILICEOUS SINTER, both by evaporation and by the action of algae which secrete silica from the waters. It is in this way that the cones and mounds of the geysers in the Yellowstone National Park and in Iceland have been formed.
Where water oozes from the earth one may sometimes see a rusty deposit on the ground, and perhaps an iridescent scum upon the water. The scum is often mistaken for oil, but at a touch it cracks and breaks, as oil would not do. It is a film of hydrated iron oxide, or LIMONITE, and the spring is an iron, or chalybeate, spring. Compounds of iron have been taken into solution by ground water from soil and rocks, and are now changed to the insoluble oxide on exposure to the oxygen of the air.
In wet ground iron compounds leached by ground water from the soil often collect in reddish deposits a few feet below the surface, where their downward progress is arrested by some impervious clay. At the bottom of bogs and shallow lakes iron ores sometimes accumulate to a depth of several feet.
Decaying organic matter plays a large part in these changes. In its presence the insoluble iron oxides which give color to most red and yellow rocks are decomposed, leaving the rocks of a gray or bluish color, and the soluble iron compounds which result are readily leached out,—effects seen where red or yellow clays have been bleached about some decaying tree root.
The iron thus dissolved is laid down as limonite when oxidized, as about a chalybeate spring; but out of contact with the air and in the presence of carbon dioxide supplied by decaying vegetation, as in a peat bog, it may be deposited as iron carbonate, or SIDERITE.
TOTAL AMOUNT OF UNDERGROUND WATERS. In order to realize the vast work in solution and cementation which underground waters are now doing and have done in all geological ages, we must gain some conception of their amount. At a certain depth, estimated at about six miles, the weight of the crust becomes greater than the rocks can bear, and all cavities and pores in them must be completely closed by the enormous pressure which they sustain. Below a depth of even three or four miles it is believed that ground water cannot circulate. Estimating the average pore spaces of the different rocks of the earth's crust above this depth, and the average per cents of their pore spaces occupied by water, it has been recently computed that the total amount of ground water is equal to a sheet of water one hundred feet deep, covering the entire surface of the earth.
RIVERS AND VALLEYS
THE RUN-OFF. We have traced the history of that portion of the rainfall which soaks into the ground; let us now return to that part which washes along the surface and is known as the RUN-OFF. Fed by rains and melting snows, the run-off gathers into courses, perhaps but faintly marked at first, which join more definite and deeply cut channels, as twigs their stems. In a humid climate the larger ravines through which the run-off flows soon descend below the ground-water surface. Here springs discharge along the sides of the little valleys and permanent streams begin. The water supplied by the run-off here joins that part of the rainfall which had soaked into the soil, and both now proceed together by way of the stream to the sea.
RIVER FLOODS. Streams vary greatly in volume during the year. At stages of flood they fill their immediate banks, or overrun them and inundate any low lands adjacent to the channel; at stages of low water they diminish to but a fraction of their volume when at flood.
At times of flood, rivers are fed chiefly by the run-off; at times of low water, largely or even wholly by springs.
How, then, will the water of streams differ at these times in turbidity and in the relative amount of solids carried in solution?
In parts of England streams have been known to continue flowing after eighteen months of local drought, so great is the volume of water which in humid climates is stored in the rocks above the drainage level, and so slowly is it given off in springs.
In Illinois and the states adjacent, rivers remain low in winter and a "spring freshet" follows the melting of the winter's snows. A "June rise" is produced by the heavy rains of early summer. Low water follows in July and August, and streams are again swollen to a moderate degree under the rains of autumn.
THE DISCHARGE OF STREAMS. The per cent of rainfall discharged by rivers varies with the amount of rainfall, the slope of the drainage area, the texture of the rocks, and other factors. With an annual rainfall of fifty inches in an open country, about fifty per cent is discharged; while with a rainfall of twenty inches only fifteen per cent is discharged, part of the remainder being evaporated and part passing underground beyond the drainage area. Thus the Ohio discharges thirty per cent of the rainfall of its basin, while the Missouri carries away but fifteen per cent. A number of the streams of the semi-arid lands of the West do not discharge more than five per cent of the rainfall.
Other things being equal, which will afford the larger proportion of run-off, a region underlain with granite rock or with coarse sandstone? grass land or forest? steep slopes or level land? a well-drained region or one abounding in marshes and ponds? frozen or unfrozen ground? Will there be a larger proportion of run-off after long rains or after a season of drought? after long and gentle rains, or after the same amount of precipitation in a violent rain? during the months of growing vegetation, from June to August, or during the autumn months?
DESERT STREAMS. In arid regions the ground-water surface lies so low that for the most part stream ways do not intersect it. Streams therefore are not fed by springs, but instead lose volume as their waters soak into the thirsty rocks over which they flow. They contribute to the ground water of the region instead of being increased by it. Being supplied chiefly by the run-off, they wither at times of drought to a mere trickle of water, to a chain of pools, or go wholly dry, while at long intervals rains fill their dusty beds with sudden raging torrents. Desert rivers therefore periodically shorten and lengthen their courses, withering back at times of drought for scores of miles, or even for a hundred miles from the point reached by their waters during seasons of rain.
THE GEOLOGICAL WORK OF STREAMS. The work of streams is of three kinds,—transportation, erosion, and deposition. Streams TRANSPORT the waste of the land; they wear, or ERODE, their channels both on bed and banks; and they DEPOSIT portions of their load from time to time along their courses, finally laying it down in the sea. Most of the work of streams is done at times of flood.
THE INVISIBLE LOAD OF STREAMS. Of the waste which a river transports we may consider first the invisible load which it carries in solution, supplied chiefly by springs but also in part by the run-off and from the solution of the rocks of its bed. More than half the dissolved solids in the water of the average river consists of the carbonates of lime and magnesia; other substances are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate (Epsom salts), sodium chloride (common salt), and even silica, the least soluble of the common rock-making minerals. The amount of this invisible load is surprisingly large. The Mississippi, for example, transports each year 113,000,000 tons of dissolved rock to the Gulf.
THE VISIBLE LOAD OF STREAMS. This consists of the silt which the stream carries in suspension, and the sand and gravel and larger stones which it pushes along its bed. Especially in times of flood one may note the muddy water, its silt being kept from settling by the rolling, eddying currents; and often by placing his ear close to the bottom of a boat one may hear the clatter of pebbles as they are hurried along. In mountain torrents the rumble of bowlders as they clash together may be heard some distance away. The amount of the load which a stream can transport depends on its velocity. A current of two thirds of a mile per hour can move fine sand, while one of four miles per hour sweeps along pebbles as large as hen's eggs. The transporting power of a stream varies as the sixth power of its velocity. If its velocity is multiplied by two, its transporting power is multiplied by the sixth power of two: it can now move stones sixty-four times as large as it could before.
Stones weigh from two to three times as much as water, and in water lose the weight of the volume of water which they displace. What proportion, then, of their weight in air do stones lose when submerged?
MEASUREMENT OF STREAM LOADS. To obtain the total amount of waste transported by a river is an important but difficult matter. The amount of water discharged must first be found by multiplying the number of square feet in the average cross section of the stream by its velocity per second, giving the discharge per second in cubic feet. The amount of silt to a cubic foot of water is found by filtering samples of the water taken from different parts of the stream and at different times in the year, and drying and weighing the residues. The average amount of silt to the cubic foot of water, multiplied by the number of cubic feet of water discharged per year, gives the total load carried in suspension during that time. Adding to this the estimated amount of sand and gravel rolled along the bed, which in many swift rivers greatly exceeds the lighter material held in suspension, and adding also the total amount of dissolved solids, we reach the exceedingly important result of the total load of waste discharged by the river. Dividing the volume of this load by the area of the river basin gives another result of the greatest geological interest,— the rate at which the region is being lowered by the combined action of weathering and erosion, or the rate of denudation.