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Show 208 ISI_JAND LIFE. (PART I. The Mississippi removes one foot in 6,000 years. , Ganges , , 2,358 , , Hoang Ho , , 1,464 , Rhone , , 1,528 , , Danube , , 6,~46 , , Po , , 729 , , Nith 4,723 " " " Here we see an intelljgible relation between the character of the river basin and the amount of denudation. The Mississippi has a large portion of its basin in an arid country, and its sources are either in forest-clad plateaux or in mountains free from glaciers and with a. scanty rainfall. The Danube flows through Eastern Europe where the rainfall is considerably less than in the west, while comparatively few of its tributaries rise among the loftiest Alps. The proportionate amounts of denudation being then what we might expect, and as all are probably under rather than over the truth, we may safely take tbe average of them all as representing an amount of denudation which, if not true for the whole land surface of the globe, will certainly be so for a very considerable proportion of it. This average is almost exactly one foot in three thousand years. 1 The mean altitude of the several continents has been estimated to be as 1 It has usually been the practice to take the amount of denudation in the Mississippi valley, or one foot in six thousand years, as a measure of the rate of denudation in Europe, from an idea apparently of being on the "safe side,'' and of not over-estimating the rate of change. But this appears to me a most unphilosophical mode of proceeding and unworthy of scientific inquiry. What should we think of astronomers if they always took the lowest estimates of planetary or stellar distances, instead of the mean results of observation, "in order to be on the safe side 1" ? As if error in one direction were any worse than error in another. Yet this is what geologists do systematically. Whenever any calculations are made involving the antiquity of man, it is those that give the lowest results that are always taken, for no reason apparently except that there was, for so long a time, a prejudice, both popular and scientific, against the great antiquity of man ; and now that a means has been found of measuring the rate of denudation, they take the slowest rate instead of the mean rate, apparently only because there is now a scientific prejudice in favour of extremely slow geological change. I take the mean of the whole; and as this is almost exactly the same as the mean of the three great European rivers-the Rhone, Danube, and Po-I cannot believe that this will not be nearer the truth for Europe than taking one North American river as the standard. CHAP. X.] THE EARTH'S AGE. 209 fNoollrotwlls .. AE uro.P e 671 £e et, Asia 1,132 feet, Africa 000 feet tb t menc(a t'- -48 f eet, an d South America 1151 feet At' othee ra£ e of denudation ab ove gi·V en, I· t results ' that wer· e no levelr ino racbe s at wo r k '.E ~rope would be planed down t'o the sea-l out two milhon years; while if we take a somewhat tsh orw er ra·nte· for N or· th A men.c a, that continent rojaht last about th ee ml '?n years.' This also implies that the ,:;ean heiaht of ese contments would ha b d b · . 0 milli . . ve een ou le what It IS now two on and three million years ago respectively: and as we have ~o reason to suppose t~is to have been the case, we are led to mfer the constan.t actlOn of that upheaving force which the presenc~ of sedimentary formations even on the hiahest mountams also demonstrates. 0 . We have already discussed the unequal rate of denudation on lulls, valley~, and low lands, in connection with the evidence of ~emote glacial epochs (p. 166); what we have now to consider IS, what becomes of all this denuded matter, and how far the know~ . rate of denudation affords us a measure of the rate of depos1~10n, ~nd thus gives us some indication of the lapse of ge?log1eal time from a comparison of this rate with the observed thickness of stratified rocks on the earth's surface. How ~o estimate the Thickness of the Sedimentary Roclcs.The sediment~ry rocks. of which the earth's crust is mainly composed consist, accordmg to Sir Charles Lyell's classification of fourt~en great formations, of which the most ancient is tb~ ~aurenhan, and. t~~ most recent the Post-Tertiary ; with thirty Important sub-diVISlOns, each of which aaain consists of a roo l 'd o re or ess consi erable number of distinct beds or strata. Thus, the · 1 T~ese. figures are merely used to give an idea of the rate at which denudatwn IS actually going. on now; but if no elevatory forces were at work, the rate of denudatiOn would certainly diminieh as the mountains we~e lowered and the slope of the ground everywhere rendered flatter. This would follow not only from the diminished power of rain and · b u t b ecause the c1 1' mate would become more -uniform, the rainfall pronbvaebrlsy, less, an~ no rocky peaks would be left to be fractured and broken up by the ~ctlon of fros~s. It is certain, however, that no continent has ever remamed long subJeCt to the influences of denudation alone for s 1 . . , , u we 1a;e seen. m our s1xth chapter, elevation and depression have always been gomg on m one part or other of the surface. p |
