CHAP. X.] 



THE EAKTH'S AGE. 



209 



follows: Europe 671 feet, Asia 1,132 feet, Africa 900 feet, 

 North America 748 feet, and South America 1.151 feet. At 

 the rate of demidation above given, it results that, were no 

 other forces at work, Europe would be planed down to the sea- 

 level in about two million years ; while if we take a somewhat 

 slower rate for North America, that continent might last about 

 three million years. ^ This also implies that the mean height of 

 these continents would have been double what it is now two 

 million and three million years ago respectively : and as we have 

 no reason to suppose this to have been the case, we are led to 

 infer the constant action of that upheaving force which the 

 presence of sedimentary formations even on the highest 

 mountains also demonstrates. 



We have already discussed the unequal rate of denudation on 

 hills, valleys, and lowlands, in connection with the evidence of 

 remote glacial epochs (p. 166) ; what we have now to consider 

 is, what becomes of all this denuded matter, and how far the 

 known rate of denudation affords us a measure of the rate of 

 deposition, and thus gives us some indication of the lapse of 

 geological time from a comparison of this rate with the observed 

 thickness of stratified rocks on the earth's surface. 



How to estimate the Thickness of the Sedimentary Rocks. — 

 The sedimentary rocks of which the earth's crust is mainly 

 composed consist, according to Sir Charles Ly ell's classification, 

 of fourteen great formations, of which the most ancient is the 

 Laurentian, and the most recent the Post-Tertiary ; with thirty 

 important sub-divisions, each of which again consists of a more 

 or less considerable number of distinct beds or strata. Thus, the 



^ These figures are merely used to give an idea of the rate at which de- 

 nudation is actually going on now ; but if no elevatory forces were at 

 work, the rate of denudation would certainly diminish as the mountains 

 were lowered and the slope of the ground everywhere rendered flatter. 

 This would follow not only from the diminished power of rain and rivers, 

 but because the climate would become more uniform, the rainfall probably 

 less, and no rocky peaks w^ould be left to be fractured and broken up by 

 the action of frosts. It is certain, however, that no continent has ever 

 remained long subject to the influences of denudation alone, for, as we 

 have seen in our sixth chapter, elevation and depression have always been 

 going on in one part or other of the surface. 



P 



