274 DISCUSSIONS IN COSMOLOGY. 



Carboniferous period. And again, in the case of the 

 Lancashire coal-fields, to which reference has been 

 made, nearly two miles in thickness of strata had been 

 removed in the interval which elapsed between the 

 Millstone Grit and the Permian periods. 



Rate of Denudation. — As we are enabled, from 

 geological evidence, to form some rough estimate of 

 the extent to which the country in various places has 

 been lowered by sub-aerial denudation during a given 

 epoch, it is evident that we should have a means of 

 arriving at some idea of the length of that epoch, did 

 we know the probable rate at which the denudation 

 took place. If we had a means of forming even the 

 roughest estimate of the probable average rate of sub- 

 aerial denudation during past ages, we should be 

 enabled thereby to assign approximately an inferior 

 limit to the age of the stratified rocks. We could then 

 tell, at least, whether the amount of sub-aerial 

 denudation known to have been effected during past 

 geological ages could have been accomplished within 

 20 million years or not, and this is about all with 

 which we are at present concerned. And if it can be 

 proved that a period of 20 millions of years is much 

 too short to account for the amount of denudation 

 known to have taken place, then it is certain that the 

 gravitation theory cannot explain the origin and 

 source of the sun's heat. 



A very simple and obvious method of determining 

 the present mean rate of sub-aerial denudation was 

 pointed out by me several years ago,* viz., that the 

 rate of denudation must be equal to the rate at which 

 the materials are carried off the land into the sea. 

 But the rate at which the materials are thus abstracted 



* "Phil. Mag.," May, 1868; Feb., 1867. 'Climate and Time,' 

 Chapter XX. 



