Chapter IV: The Earth

In beginning the study of the earth it is important that the student should at once form the habit of keeping in mind the spherical form of the planet. Many persons, while they may blindly accept the fact that the earth is a sphere, do not think of it as having that form. Perhaps the simplest way of securing the correct image of the shape is to imagine how the earth would appear as seen from the moon. In its full condition the moon is apt to appear as a disk. When it is new, and also when in its waning stages it is visible in the daytime, the spherical form is very apparent. Imagining himself on the surface of the moon, the student can well perceive how the earth would appear as a vast body in the heavens; its eight thousand miles of diameter, about four times that of the satellite, would give an area sixteen times the size which the moon presents to us. On this scale the continents and oceans would appear very much more plain than do the relatively slight irregularities on the lunar surface.

With the terrestrial globe in hand, the student can readily construct an image which will represent, at least in outline, the appearance which the sphere he inhabits would present when seen from a distance of about a quarter of a million miles away. The continent of Europe-Asia would of itself appear larger than all the lunar surface which is visible to us. Every continent and all the greater islands would be clearly indicated. The snow covering which in the winter of the northern hemisphere wraps so much of the land would be seen to come and go in the changes of the seasons; even the permanent ice about either pole, and the greater regions of glaciers, such as those of the Alps and the Himalayas, would appear as brilliant patches of white amid fields of darker hue. Even the changes in the aspect of the vegetation which at one season clothes the wide land with a green mantle, and at another assumes the dun hue of winter, would be, to the unaided eye, very distinct. It is probable that all the greater rivers would be traceable as lines of light across the relatively dark surface of the continents. By such exercises of the constructive imagination--indeed, in no other way--the student can acquire the habit of considering the earth as a vast whole. From time to time as he studies the earth from near by he should endeavour to assemble the phenomena in the general way which we have indicated.

The reader has doubtless already learned that the earth is a slightly flattened sphere, having an average diameter of about eight thousand miles, the average section at the equator being about twenty-six miles greater than that from pole to pole. In a body of such large proportions this difference in measurement appears not important; it is, however, most significant, for it throws light upon the history of the earth's mass. Computation shows that the measure of flattening at the poles is just what would occur if the earth were or had been at the time when it assumed its present form in a fluid condition. We readily conceive that a soft body revolving in space, while all its particles by gravitation tended to the centre, would in turning around, as our earth does upon its axis, tend to bulge out in those parts which were remote from the line upon which the turning took place. Thus the flattening of our sphere at the poles corroborates the opinion that its mass was once molten--in a word, that its ancient history was such as the nebular theory suggests.

Although we have for convenience termed the earth a flattened spheroid, it is only such in a very general sense. It has an infinite number of minor irregularities which it is the province of the geographer to trace and that of the geologist to account for. In the first place, its surface is occupied by a great array of ridges and hollows. The larger of these, the oceans and continents, first deserve our attention. The difference in altitude of the earth's surface from the height of the continents to the deepest part of the sea is probably between ten and eleven miles, thus amounting to about two fifths of the polar flattening before noted. The average difference between the ocean floor and the summits of the neighbouring continents is probably rather less than four miles. It happens, most fortunately for the history of the earth, that the water upon its surface fills its great concavities on the average to about four fifths of their total depth, leaving only about one fifth of the relief projecting above the ocean level. We have termed this arrangement fortunate, for it insures that rainfall visits almost all the land areas, and thereby makes those realms fit for the uses of life. If the ocean had only half its existing area, the lands would be so wide that only their fringes would be fertile. If it were one fifth greater than it is, the dry areas would be reduced to a few scattered islands.

From all points of view the most important feature of the earth's surface arises from its division into land and water areas, and this for the reason that the physical and vital work of our sphere is inevitably determined by this distribution. The shape of the seas and lands is fixed by the positions at which the upper level of the great water comes against the ridges which fret the earth's surface. These elevations are so disposed that about two thirds of the hard mass is at the present time covered with water, and only one third exposed to the atmosphere. This proportion is inconstant. Owing to the endless up-and-down goings of the earth's surface, the place of the shore lines varies from year to year, and in the geological ages great revolutions in the forms and relative area of water and land are brought about.

Noting the greater divisions of land and water as they are shown on a globe, we readily perceive that those parts of the continental ridges which rise above the sea level are mainly accumulated in the northern hemisphere--in fact, far more than half the dry realm is in that part of the world. We furthermore perceive that all the continents more or less distinctly point to the southward; they are, in a word, triangles, with their bases to the northward, and their apices, usually rather acute, directed to the southward. This form is very well indicated in three of the great lands, North and South America and Africa; it is more indistinctly shown in Asia and in Australia. As yet we do not clearly understand the reason why the continents are triangular, why they point toward the south pole, or why they are mainly accumulated in the northern hemisphere. As stated in the chapter on astronomy, some trace of the triangular form appears in the land masses of the planet Mars. There, too, these triangles appear to point toward one pole.

Besides the greater lands, the seas are fretted by a host of smaller dry areas, termed islands. These, as inquiry has shown, are of two very diverse natures. Near the continents, practically never more than a thousand miles from their shores, we find isles, often of great size, such as Madagascar, which in their structure are essentially like the continents--that is, they are built in part or in whole of non-volcanic rocks, sandstones, limestones, etc. In most cases these islands, to which we may apply the term continental, have at some time been connected with the neighbouring mainland, and afterward separated from it by a depression of the surface which permitted the sea to flow over the lowlands. Geologists have traced many cases where in the past elevations which are now parts of a continent were once islands next its shore. In the deeper seas far removed from the margins of the continents the islands are made up of volcanic ejections of lava, pumice, and dust, which has been thrown up from craters and fallen around their margin or are formed of coral and other organic remains.

Next after this general statement as to the division of sea and land we should note the peculiarities which the earth's surface exhibits where it is bathed by the air, and where it is covered by the water. Beginning with the best-known region, that of the dry land, we observe that the surface is normally made up of continuous slopes of varying declivity, which lead down from the high points to the sea. Here and there, though rarely, these slopes centre in a basin which is occupied by a lake or a dead sea. On the deeper ocean floors, so far as we may judge with the defective information which the plumb line gives us, there is no such continuity in the downward sloping of the surface, the area being cast into numerous basins, each of great extent.

When we examine in some detail the shape of the land surface, we readily perceive that the continuous down slopes are due to the cutting action of rivers. In the basin of a stream the waters act to wear away the original heights, filling them into the hollows, until the whole area has a continuous down grade to the point where the waters discharge into the ocean or perhaps into a lake. On the bottom of the sea, except near the margin of the continent, where the floor may in recent geological times have been elevated into the air, and thus exposed to river action, there is no such agent working to produce continuous down grades.

Looking upon a map of a continent which shows the differences in altitude of the land, we readily perceive that the area is rather clearly divided into two kinds of surface, mountains and plains, each kind being sharply distinguished from the other by many important peculiarities. Mountains are characteristically made up of distinct, more or less parallel ridges and valleys, which are grouped in very elongated belts, which, in the case of the American Cordilleras, extend from the Arctic to the Antarctic Circle. Only in rare instances do we find mountains occupying an area which is not very distinctly elongated, and in such cases the elevations are usually of no great height. Plains, on the other hand, commonly occupy the larger part of the continent, and are distributed around the flanks of the mountain systems. There is no rule as to their shape; they normally grade away from the bases of the mountains toward the sea, and are often prolonged below the level of the water for a considerable distance beyond the shore, forming what is commonly known as the continental shelf or belt of shallows along the coast line. We will now consider some details concerning the form and structure of mountains.

In almost any mountain region a glance over the surface of the country will give the reader a clew to the principal factor which has determined the existence of these elevations. Wherever the bed rocks are revealed he will recognise the fact that they have been much disturbed. Almost everywhere the strata are turned at high angles; often their slopes are steeper than those of house roofs, and not infrequently they stand in attitudes where they appear vertical. Under the surface of plains bedded rocks generally retain the nearly horizontal position in which all such deposits are most likely to be found. If the observer will attentively study the details of position of these tilted rocks of mountainous districts, he will in most cases be able to perceive that the beds have been flexed or folded in the manner indicated by the diagram. Sometimes, though rarely, the tops of these foldings or arches have been preserved, so that the nature of the movement can be clearly discerned. More commonly the upper parts of the upward-arching strata have been cut off by the action of the decay-bringing forces--frost, flowing water, or creeping ice in glaciers--so that only the downward pointing folds which were formed in the mountain-making are well preserved, and these are almost invariably hidden within the earth.

By walking across any considerable mountain chain, as, for instance, that of the Alleghanies, it is generally possible to trace a number of these parallel up-and-down folds of the strata, so that we readily perceive that the original beds had been packed together into a much less space than they at first occupied. In some cases we could prove that the shortening of the line has amounted to a hundred miles or more--in other words, points on the plain lands on either side of the mountain range which now exists may have been brought a hundred miles or so nearer together than they were before the elevations were produced. The reader can make for himself a convenient diagram showing what occurred by pressing a number of leaves of this book so that the sheets of paper are thrown into ridges and furrows. By this experiment he also will see that the easiest way to account for such foldings as we observe in mountains is by the supposition that some force residing in the earth tends to shove the beds into a smaller space than they originally occupied. Not only are the rocks composing the mountains much folded, but they are often broken through after the manner of masonry which has been subjected to earthquake shocks, or of ice which has been strained by the expansion that affects it as it becomes warmed before it is melted. In fact, many of our small lakes in New England and in other countries of a long winter show in a miniature way during times of thawing ice folds which much resemble mountain arches.

At first geologists were disposed to attribute all the phenomena of mountain-folding to the progressive cooling of the earth. Although this sphere has already lost a large part of the heat with which it was in the beginning endowed, it is still very hot in its deeper parts, as is shown by the phenomena of volcanoes. This internal heat, which to the present day at the depth of a hundred miles below the surface is probably greater than that of molten iron, is constantly flowing away into space; probably enough of it goes away on the average each day to melt a hundred cubic miles or more of ice, or, in more scientific phrase, the amount of heat rendered latent by melting that volume of frozen water. J.R. Meyer, an eminent physicist, estimated the quantity of heat so escaping each day of the year to be sufficient to melt two hundred and forty cubic miles of ice. The effect of this loss of heat is constantly to shrink the volume of the earth; it has, indeed, been estimated that the sphere on this account contracts on the average to the amount of some inches each thousand years. For the reason that almost all this heat goes from the depths of the earth, the cool outer portion losing no considerable part of it, the contraction that is brought about affects the interior portions of the sphere alone. The inner mass constantly shrinking as it loses heat, the outer, cold part is by its weight forced to settle down, and can only accomplish this result by wrinkling. An analogous action may be seen where an apple or a potato becomes dried; in this case the hard outer rind is forced to wrinkle, because, losing no water, it does not diminish in its extent, and can only accommodate itself to the interior by a wrinkling process. In one case it is water which escapes, in the other heat; but in both contraction of the part which suffers the loss leads to the folding of the outside of the spheroid.

Although this loss of heat on the part of the earth accounts in some measure for the development of mountains, it is not of itself sufficient to explain the phenomena, and this for the reason that mountains appear in no case to develop on the floors of the wide sea. The average depth of the ocean is only fifteen thousand feet, while there are hundreds, if not thousands, of mountain crests which exceed that height above the sea. Therefore if mountains grew on the sea floor as they do upon the land, there should be thousands of peaks rising above the plain of the waters, while, in fact, all of the islands except those near the shores of continents are of volcanic origin--that is, are lands of totally different nature.

Whenever a considerable mountain chain is formed, although the actual folding of the beds is limited to the usually narrow field occupied by these disturbances, the elevation takes place over a wide belt of country on one or both sides of the range. Thus if we approach the Rocky Mountains from the Mississippi Valley, we begin to mount up an inclined plane from the time we pass westward from the Mississippi River. The beds of rock as well as the surface rises gradually until at the foot of the mountain; though the rocks are still without foldings, they are at a height of four or five thousand feet above the sea. It seems probable--indeed, we may say almost certain--that when the crust is broken, as it is in mountain-building, by extensive folds and faults, the matter which lies a few score miles below the crust creeps in toward those fractures, and so lifts up the country on which they lie. When we examine the forms of any of our continents, we find that these elevated portions of the earth's crust appear to be made up of mountains and the table-lands which fringe those elevations. There is not, as some of our writers suppose, two different kinds of elevation in our great lands--the continents and the mountains which they bear--but one process of elevation by which the foldings and the massive uplifts which constitute the table-lands are simultaneously and by one process formed.

Looking upon continents as the result of mountain growth, we may say that here and there on the earth's crust these dislocations have occurred in such association and of such magnitude that great areas have been uplifted above the plain of the sea. In general, we find these groups of elevations so arranged that they produce the triangular form which is characteristic of the great lands. It will be observed, for instance, that the form of North America is in general determined by the position of the Appalachian and Cordilleran systems on its eastern and western margins, though there are a number of smaller chains, such as the Laurentians in Canada and the ice-covered mountains of Greenland, which have a measure of influence in fixing its shore lines.

The history of plains, as well as that of mountains, will have further light thrown upon it when in the next chapter we come to consider the effect of rain water on the land. We may here note the fact that the level surfaces which are above the seashores are divisible into two main groups--those which have been recently lifted above the sea level, composed of materials laid down in the shallows next the shore, and which have not yet shared in mountain-building disturbances, and those which have been slightly tilted in the manner before indicated in the case of the plains which border the Rocky Mountains on the east. The great southern plain of eastern and southern United States, extending from near New York to Mexico, is a good specimen of the level lands common on all the continents which have recently emerged from the sea. The table-lands on either side of the Mississippi Valley, sloping from the Alleghanies and the Cordilleras, represent the more ancient type of plain which has already shared in the elevation which mountain-building brings about. In rarer cases plains of small area are formed where mountains formerly existed by the complete moving down of the original ridges.

There is a common opinion that the continents are liable in the course of the geologic ages to very great changes of position; that what is now sea may give place to new great lands, and that those already existing may utterly disappear. This opinion was indeed generally held by geologists not more than thirty years ago. Further study of the problem has shown us that while parts of each continent may at any time be depressed beneath the sea, the whole of its surface rarely if ever goes below the water level. Thus, in the case of North America, we can readily note very great changes in its form since the land began to rise above the water. But always, from that ancient day to our own, some portion of the area has been above the level of the sea, thus providing an ark of refuge for the land life when it was disturbed by inundations. The strongest evidence in favour of the opinion that the existing continents have endured for many million years is found in the fact that each of the great lands preserves many distinct groups of animals and plants which have descended from ancient forms dwelling upon the same territory. If at any time the relatively small continent of Australia had gone beneath the sea, all of the curious pouched animals akin to the opossum and kangaroo which abound in that country--creatures belonging in the ancient life of the world--would have been overwhelmed.

We have already noted the fact that the uplifting of mountains and of the table-lands about them, which appears to have been the basis of continental growth, has been due to strains in the rocks sufficiently strong to disturb the beds. At each stage of the mountain-building movement these compressive strains have had to contend with the very great weight of the rocks which they had to move. These lands are not to be regarded as firm set or rigid arches, but as highly elastic structures, the shapes of which may be determined by any actions which put on or take off burden. We see a proof of this fact from numerous observations which geologists are now engaged in making. Thus during the last ice epoch, when almost all the northern part of this continent, as well as the northern part of Europe, was covered by an ice sheet several thousand feet thick, the lands sank down under their load, and to an extent roughly proportional to the depth of the icy covering. While the northern regions were thus tilted down by the weight which was upon them, the southern section of this land, the region about the Gulf of Mexico, was elevated much above its present level; it seems likely, indeed, that the peninsula of Florida rose to the height of several hundred feet above its present shore line. After the ice passed away the movements were reversed, the northern region rising and the southern sinking down. These movements are attested by the position of the old shore lines formed during the later stages of the Glacial epoch. Thus around Lake Ontario, as well as the other Great Lakes, the beaches which mark the higher positions of those inland seas during the closing stages of the ice time, and which, of course, were when formed horizontal, now rise to the northward at the rate of from two to five feet for each mile of distance. Recent studies by Mr. G.K. Gilbert show that this movement is still in progress.

Other evidence going to show the extent to which the movements of the earth's crust are affected by the weight of materials are found in the fact that wherever along the shores thick deposits of sediments are accumulated the tendency of the region where they lie is gradually to sink downward, so that strata having an aggregate thickness of ten thousand feet or more may be accumulated in a sea which was always shallow. The ocean floor, in general, is the part of the earth's surface where strata are constantly being laid down. In the great reservoir of the waters the _débris_ washed from the land, the dust from volcanoes, and that from the stellar spaces, along with the vast accumulation of organic remains, almost everywhere lead to the steadfast accumulation of sedimentary deposits. On the other hand, the realms of the surface above the ocean level are constantly being worn away by the action of the rivers and glaciers, of the waves which beat against the shores, and of the winds which blow over desert regions. The result is that the lands are wearing down at the geologically rapid average rate of somewhere about one foot in five thousand years. All this heavy matter goes to the sea bottoms. Probably to this cause we owe in part the fact that in the wrinklings of the crust due to the contraction of the interior the lands exhibit a prevailing tendency to uprise, while the ocean floors sink down. In this way the continents are maintained above the level of the sea despite the powerful forces which are constantly wearing their substance away, while the seas remain deep, although they are continually being burdened with imported materials.

It is easy to see that if the sea floors tend to sink downward, while the continental lands uprise, the movements which take place may be compared with those which occur in a lever about a fulcrum point. In this case the sea end of the bar is descending and the land end ascending. Now, it is evident that the fulcrum point may fall to the seaward or to the landward of the shore; only by chance and here and there would it lie exactly at the coast line. By reference to the diagram (Fig. 8), it will be seen that, while the point of rotation is just at the shore, a considerable movement may take place without altering the position of the coast line. Where the point of no movement is inland of the coast, the sea will gain on the continent; where, however, the point is to seaward, beneath the water, the land will gain on the ocean. In this way we can, in part at least, account for the endless changes in the attitude of the land along the coastal belt without having to suppose that the continents cease to rise or the sea floors to sink downward. It is evident that the bar or section of the rocks from the interior of the land to the bottoms of the seas is not rigid; it is also probable that the matter in the depths of the earth, which moves with the motions of this bar, would change the position of the fulcrum point from time to time. Thus it may well come about that our coast lines are swaying up and down in ceaseless variation.

In very recent geological times, probably since the beginning of the last Glacial period, the region about the Dismal Swamp in Virginia has swayed up and down through four alternating movements to the extent of from fifty to one hundred feet. The coast of New Jersey is now sinking at the rate of about two feet in a hundred years. The coast of New England, though recently elevated to the extent of a hundred feet or more, at a yet later time sank down, so that at some score of points between New York and Eastport, Me., we find the remains of forests with the roots of their trees still standing below high-tide mark in positions where the trees could not have grown. Along all the marine coasts of the world which have been carefully studied from this point of view there are similar evidences of slight or great modern changes in the level of the lands. At some points, particularly on the coast of Alaska and along the coast of Peru, these uplifts of the land have amounted to a thousand feet or more. In the peninsular district of Scandinavia the swayings, sometimes up and sometimes down, which are now going on have considerably changed the position of the shore lines since the beginning of the historical period.

There are other causes which serve to modify the shapes and sizes of the continents which may best be considered in the sequel; for the present we may pass from this subject with the statement that our great lands are relatively permanent features; their forms change from age to age, but they have remained for millions of years habitable to the hosts of animals and plants which have adapted their life to the conditions which these fields afford them.