Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/387

Rh DnxUn.u'1:::I..] This degradation of the surface may be aptly co111pared to a process of sculpturing, which begins as soon as the land emerges from the sea, and never ceases-so long as any por- tion of the land remains above water. The implements employed by nature in this great work are those epigene forces whose operations have already been described. Each of them, like a special kind of graving tool, produces its own characteristic impress 011 the land. The work of rain, of frost, of rivers, of glaciers, can be readily discriminated, though they all combine harmoniously towards the achieve- ment of their one common task. Hence the present con- tours of the land must depend partly (1) on the vigour with which the several epigene agents perform their work of erosion, on the original conﬁguration of the ground, and the influence it may have had i11 guiding the opera- tions of these agents, and on the varying structure and powers of resistance possessed by the rocks. 1. Taking a broad view of denudation, we may conveni- ently group together the action of air, frost, springs, rivers, glaciers, and the other agents which wear down the surface of the land, under the one common designation of subaerial, and that of the sea as marine. The general results of sub- aerial action are—to furrow and channel the land, to erode valleys, to sharpen and splinter the ridges of mountains, and thus, while roughening, to lower the general surface and carry out the detritus to the sea. The action of the sea, on the other hand, is to plane down the land to the level at which the inﬂuence of breakers and ground-swell ceases to have any erosive effect ; the flat platform, so often visible between tide—n1arks on a rocky exposed coast-line, is an impressive illustration of the tendency of marine denudation. The combined result of subaerial a11d marine action, if unimpeded by any subterranean movement, would evidently be to reduce the land to one general level u11der the sea. For, except in that upper marginal zone where the waves and tidal currents play, the waters of the ocean protect the solid rocks which they cover. And the rocks indeed can ﬁnd no permanent protection anywhere else. But to reduce a large area of land such as a continent to the condition of a submarine plain, would require a longer period of time than seems to have elapsed between two epochs of upheaval. Traces of ancient plains of marine de- nudation are to be met with in Scandinavia and in Scotland, on b11t a comparatively small scale, as if there had been time for only a narrow platform to be formed before the next paroxysm of contraction and uplift completely renovated the geography of the region. Instead of trying to estimate how much work is done by each of the subaerial agents in eroding the land, we gain a much more impressive idea of the reality and magnitude of their work as a whole by treating their operations as one great process, the effects of which can be actually measured. The true gauge of the present yearly waste of the surface of the land is furnished by the amount of mineral matter carried every year into the sea by rivers. This mineral matter is partly in mechanical suspension,partly in chemical solution, and is to no small extent pushed in the form of shingle and sand along the bottoms of the streams. Some data respecting its amount have been already given (anle, pp. 274, 278). If we take the ratios furnished by the Mississippi as a fair average, which, from the vast area and varied climatal a11d geographical characters of the region drained by that river, they probably are, then we learn that Wlﬁtli of a foot is worn away from the general surface of the land every year. At this rate, if the present erosion could be sustained, the whole American continent, of which, according to Humboldt, the mean height is 748 feet, would be worn down to the sea-level in about 4% millions of years —a comparatively short period in geological chronology. It is obvious, however, that the denudation is not equally GEOLOGY 373 distributed over the whole surface of the land. If nlﬁtlr of a foot is the mean rate from the whole surface, then some parts, including the more level grounds, must lose very much less than that amount, while other parts, such as the slopes and valleys, must lose very much more. The pro- portions between these extremes must continually vary throughout every country, according to angle of declivity, nature of surface, amount and distribution of rainfall, and whether the rain is spread over the year or concentrated into a short period. The proportion between the area covered by the more level ground of a country, where the rate of denudation is least, and that of the declivities, valleys, and stream chan- nels, where that rate is greatest, may be assumed as nine to one. The extent of the annual waste may be further taken to be nine times greater over the latter than over the former, so that, while the more level parts of the surface have been lowered 1 foot, the valleys l1ave lost 9 feet. Taking the mean rates of waste over the whole area to be 3—o%m,tl1 of a foot per annum we find that on these data the annual loss amounts to gths of a foot from the ﬂatter grounds and 5 feet from the valleys in 6000 years. This is equal to a loss of 1 foot from the former in 10,800 years and from the later in 1200 years, or to ‘1'1’2't1l of an inch from the one in 75 and from the other in 8% years. At this rate of erosion, a valley 1000 feet deep may be excavated in 1,200,000 years. These estimates are only approximations to the truth, but they are valuable in directing attention to the real efficacy of the apparently insigniﬁcant subaerial denuda- tion now in progress. Any other estimates of the relative amount of material worn away from the different parts of the surface may be taken, but the mean annual loss from the whole area, as ascertained by the river discharge, re- mains unaffected. If we represent too large an amount as removed from the valleys we diminish the loss from the open country, or if we make the contingent derived from the latter too great we lessen that from the former. 2. From this reasoning it follows that, apart altogether from irregularities of surface due to inequalities of upheaval, every area of land exposed to ordinary subaerial action must, in the end, be channeled into a system of valleys. Even a smooth featureless tract elevated uniformly above the sea would eventually be widely and deeply eroded. N or would this require a long geological period, for, at the present rate of waste iii the Mississippi basin, valleys 800 feet might be carved out in a million years. Un- doubtedly the original features snperindnced by subter- ranean action would guide and modify the operations of running water, though their inﬂuence would certainly wane as the features themselves slowly disappeared. In no case probably would the aboriginal contour remain through a succession of geological periods. Traces of it might still be discernible, but they would be well—nigh effaced by the new outlines produced by the superﬁcial agents. In the vast tablelands of Colorado and the other western regions of the United States an impressive picture is visible of the results of mere subaerial erosion on undisturbed and nearly level strata. Systems of stream—courses and valleys, river gorges unexampled elsewhere in the world for depth and length, vast winding lines of escarpment, like ranges of sea- cliffs, terraced slopes rising from plateau to plateau, huge buttresses and solitary stacks standing like islands out of the plains, great mountain masses towering into picturesque peaks and pinnacles cleft by innumerable gullies, yet every- where marked by the parallel bars of the horizontal strata out of which they l1ave been carved—these are the orderly symmetrical characteristics of a country where the scenery is due entirely to the action of subaerial agents on the one hand and the varying resistance of perfectly regular stratified rocks on the other. The Alps, on the contrary, present an