ORIGIN AND AGE OF THE SUNS HEAT. 277 



assume the rate to have been double the above, or 

 equal to one foot in 3000 years. 



Age of the Stratified Rocks, as determined by the 

 Rate of Denudation. — To lower the country one mile 

 by denudation would therefore require, according to 

 the above rate, about 1 5 million years ; but we have 

 seen that a thickness of rock more than equal to that 

 must have been swept away since the Carboniferous 

 period. For even during the Carboniferous period 

 itself more than a mile in thickness of strata in many 

 places was removed. Again, there can be no doubt 

 whatever that the amount of rock removed during the 

 Old Red Sandstone period was much greater than one 

 mile ; for we know perfectly well that over large tracts 

 of country nearly a mile in thickness of rock was 

 carried away between the period of the Lower Old Red 

 Sandstone and the Carboniferous epoch. Further, all 

 geological facts go to show that the time represented 

 by the Lower Old Red Sandstone itself must have 

 been enormous. 



Now, three miles of rock removed since the com- 

 mencement of the Old Red Sandstone period (which 

 in all probability is an under-estimate) would give us 

 45 million years. 



Again, going further back, we find the lapse of time 

 represented by the Silurian period to be even more 

 striking than that of the Old Red Sandstone. The 

 unconformities in the Silurian series indicate that 

 many thousands of feet of these strata were denuded 

 before overlying members of the same great formations 

 were deposited. And again, this immense formation 

 was formed in the ocean by the slow denudation of pre- 

 existing Cambrian continents, just as these had been 

 built up out of the ruins of the still prior Laurentian 

 land. And even here we do not reach the end of the 



