216 



ISLAND LIFE 



PART I 



continents has been recently estimated by Mr. John 

 Murray ,1 to be as follows : Europe 939 feet, Asia 3,189 

 feet, Africa 2020 feet, North America 1,888 feet, and South 

 America 2,078 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 eight 

 hundred thousand years ; while if we take a somewhat 

 slower rate for North America, that continent might last 

 about four or five million years.^ This also implies that 

 the mean height of these continents would have been 

 about double what it is now three million and five 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 denuda- 

 tion on hills, valleys, and lowlands, in connection with the 

 evidence of remote glacial epochs (p. 173) ; 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. 



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. 



^ " On the Height of the Land and the Depth of the Ocean," in the 

 Scottish Geographical Magazine, 1888. 



- 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 on in one part or other of the surface. 



