278 DISCUSSIONS IN COSMOLOGY. 



series, for the very Laurentians themselves resulted 

 from the denudation, not of the primary rocks of the 

 globe, but of previously existing sedimentary and pro- 

 bably igneous rocks, of which, perhaps, no recognisable 

 portion now remains. 



Few familiar with the facts of geology will consider 

 it too much to assume that the time which had elapsed 

 prior to the Old Red Sandstone was equal to the time 

 which has elapsed since that period. But if we make 

 this assumption, this will give us at least 90 million 

 years as the age of the stratified rocks. 



That the foregoing is not an over-estimate of the 

 probable amount of rock removed by sub-aerial denu- 

 dation during past geological ages will appear further 

 evident from the following considerations : — The 

 mountain ridges of our globe, in most cases, as is well 

 known, have been formed by sub-aerial denudation : 

 they have been carved out of the solid block. They 

 stand two thousand, four thousand, or five thousand 

 feet high, as the case may be, simply because two 

 thousand, four thousand, or five thousand feet of rock 

 have been denuded from the surrounding country. 

 The mountains are high simply because the country 

 has been lowered. But it must be observed that the 

 height which the mountains reach above the surround- 

 ing country does not measure the full extent to which 

 the country has been lowered by denudation, because 

 the mountains themselves have also been lowered. 

 The height of the mountains represents merely the 

 extent to which the country has been lowered in 

 excess of the mountains themselves. In the formation 

 of a mountain by- denudation, say 3000 feet in height, 

 probably more than 6000 feet of strata may have been 

 removed from the surrouning country. The very fact 

 of a mountain standing above the surrounding country 



