OF GEOLOGICAL DYNAMICS. 85 



' which the human mind is capable of con- 

 " ceiving. 



" Mr. Darwin, in his admirably-reasoned book 

 " on the origin of species, so full of information 

 ''and suggestion on all geological subjects, 

 "estimates the time required for the denudation 

 " of the rocks of the weald of Kent, or the erosion 

 " of space between the ranges of chalk hills, known 

 " as the North and South Downs, at three hundred 

 " millions of years. 1 The grounds for forming this 

 " estimate arc of course of the vaguest description. 

 " It may be possible, perhaps, that the estimate 

 " is a hundred times too great, and that the real 

 " time elapsed did not exceed three million years ; 

 " but, on the other hand, // is just as likely that tJ e 

 " time which actually elapsed since the first cou:- 

 " mencement of the erosion till it was nearly as 

 " complete as it now ts, was really a hundred times 



1 Prof. Phillips refers to this estimate of Mr. Darwin's ; prefers 

 one inch per annum to one inch per century as the rate of erosion ; 

 and says that most observers would consider even the one inch per 

 annum too small for all but the most invincible coasts ! He thus, 

 on purely geological grounds, reduces Mr. Darwin's estimate of the 

 time to less than one one-hundredth. PIIILUPS'S Life on the Earth. 

 Cambridge, 1860 (Rede Lecture). 



