We have now arrived at a most important and far-reaching generalization so far as the work performed by running water is concerned, and its action in filling our lakes and ponds; and we have learned by observation on a small scale the means by which such deposits may be recognized. Let us apply these means of recognition to the phenomena shown by our large rivers and the more enduring oceans into which they drain. In the same manner that we have studied the little pool and larger lake, we will look into the work done by the great waterways of our continents, selecting as a type of such streams the mighty Mississippi. Careful measurement has shown that this river annually transports two hundred million tons of sediment mechanically suspended. What becomes of this enormous quantity of sand and clay, equal to a cubic mile in a little over a century, as it is swept into the waters of the Gulf of Mexico? For this purpose we have only to visit the region about its mouth to become acquainted with the almost impotent struggles that have been made by our Government during the last fifty years in an effort to keep the river below New Orleans, in part at least, confined to its present channels; and to study the chart of that portion of the Gulf coast prepared by the United States Coast and Geodetic Survey (see Fig. 6). We have not forgotten the little lobes; their method of growth, and the general form of our first-seen delta, shown in Fig. 3. In viewing the phenomena at the mouth of the Mississippi, it is no longer necessary for our present purposes to make a detailed study, since it will become apparent at once that the river is doing the work on a larger scale typified by the performance of the tiny stream flowing into its temporary pool. In place of the little delta with its still smaller lobes, the Mississippi has deposited at its mouth an enormous delta, thousands of square miles in area, and its bifurcating arms may be seen building out several scallops for miles into the waters of the gulf. For centuries these long lobes have been building in advance of the delta front. The arms gradually become clogged with sediment, a new passage to the ocean is opened on the sides, where deposition will begin at a new point, producing a lobe as before. Situated many miles up the river, it is to-day the great fear of New Orleans that its only navigable arm to the sea will thus be closed to that commerce upon which the life of the city depends.
Fig. 6.—The Delta of the Mississippi.
Only a portion of the sediment brought in by the river goes to form its delta; a large part of the finest material, such as clay, is transported by temporary and permanent currents thousands of miles away, where it is deposited in the more quiet waters of the ocean. In this manner the Mississippi has been shown to deposit a cubic mile of mechanically transported material in a little over a century. What shall we say of the effects produced on the continents and oceans by thousands of rivers, each doing its proportionate share of work and acting through millions of years? Two main results must follow, unless interruptions occur: the lower elevations and the magnificent mountain ranges, which rear their lofty heads above the permanent snow line, will be divided into minor peaks; valleys will be carved out; the whole land surface will slowly waste away, at first rapidly, at last slowly, and be transported to the oceans, where it will form great horizontal beds differing in no essential particular, excepting size, from those shown in Fig. 5—great deposits that are merely deltas on a large scale. The geologist, however, finds no evidence to indicate that at any time in the earth's history have these theoretical results taken place. Land masses, of continental dimensions, have not been allowed thus to waste entirely away to a general flatness on account of the interruptions caused by elevation—the bodily lifting of great areas of rock, even out of the ocean floor, to become mountains or plateaus, in some cases higher than any point in this country. If our observations thus far and those yet to be made serve to make this clear, one of the objects of this article will have been accomplished. It is to be hoped that our observations have made plain the processes of rock disintegration and water transportation; that in the oceans all these materials are eventually deposited in beds horizontally arranged, composed of such products of decay in the condition of sand and mud. We have only to point out the proof that great land masses, composed of water-deposited materials, have been lifted from the ocean to become continents and mountain ranges.
As the ocean deposits slowly accumulate in layers to beds of many thousands of feet in thickness, the lower parts are gradually subjected to greatly increased pressure produced by the overlying beds. During this time waters of a varying temperature, carrying, chemically dissolved, great quantities of lime, silica, and iron oxide, are allowed free circulation through them. These conditions promote chemical change: much silica (the mineral quartz), lesser amounts of carbonate of lime (the mineral calcite), and iron oxide are precipitated about the loose sand grains, firmly cementing them together into a solid rock. A cycle has thus been completed; the dense rocks composing a continent have passed by the process of weathering into incoherent sand and clay, which, when transported to the ocean floor, become again converted into solid rock.
Historical records prove that during the last three thousand years there have taken place many changes in the ocean's level. Old islands have disappeared; new ones have emerged above the surface of the water. Great stretches of seacoast exist at the present time which within the historical period have been covered by the ocean. Even at the present writing we are witnessing the gradual submergence of some parts of the earth and the rising of others; terraces on the northern Atlantic coast may be seen along the hillsides many feet above the present level of the ocean—all of which go to show that the relationship of the land to the water is an unstable one. These are the evidences of continental growth and depressions from the historical standpoint, and the validity of the data upon which the belief is founded can not be shaken. The evidence from the geological side is overwhelming, but before we speak of this it will be well once more to say a word as to the causes of continental uplift.
From an original fluid globe possessing a high temperature, the earth has now cooled down to a degree sufficiently low to permit the formation of a thick rock crust. Underneath this crust an approach to the old surface temperatures is still maintained, and the existence of a certain degree of fluidity is demonstrated to us from time to time by the phenomenon of volcanism. Successive zones of cooling took place. The outer part could only conform to a shrinking interior by wrinkling, folding, or bodily lifting considerable areas above the general level. An adjustment of strains thus set up would take place either with or without folding of the strata. These initial wrinkles gave rise to our first mountains, and the continuation of these conditions at the present time is as surely nourishing mountain growth as at any time in the past. In this way the fluctuations of the ocean's level, above referred to, alone are to be explained, and such form but temporary rises and falls in the history of a continent.
Fig. 7.—Mountain showing Rock Folding.
The rate at which an ocean bed is raised to form a mountain range is, no doubt, a variable one; always slow, often interrupted, but seldom or never violent. During this time the strata usually undergo crushing and folding; stretching takes place, and displacements of the rocks, or faulting, are not uncommon. As an example of the wrinkling that the strata may suffer under these conditions, the reader is referred to the beautiful symmetrical fold shown on the side of a mountain in the Appalachians (Fig. 7). Similar folding is the rule, but often immense areas are raised to great heights above the ocean without disturbing the horizontal position of the beds (see Fig. 8). Coincident with the emergence of the rocks from beneath the water, there begin the attacks of the forces operating to destroy them. Hand in hand there go on growth and destruction. The two may keep an even pace; either may obtain the mastery. In the one case, lack of considerable elevation and flatness result; in the other, great altitudes may be attained. The rivers may cut their valleys downward as fast as the land rises, or the down-cutting may be relatively slower. In any case, after a given land mass has attained its greatest height above the sea, the larger rivers soon cut their channels down as far as river cutting is possible—namely, to within a few feet of sea level. With relatively rapid elevation, soft rocks, and large rivers, the resultant valley takes the form of a cañon, examples of which are found along the courses of the Colorado and the Yellowstone Rivers (see Fig. 8).[11] Valleys of this nature soon lose their steep sides by the action of weathering and all that this implies, and pass into a more open state, like that shown in Fig. 9.