CHAP. X.] 
THE EARTH’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 denudation 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. 1 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. 
Hoiv 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 Lyell’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 
1 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 would 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 oh in one part or other of the surface. 
P 
