ORIGIN AND AGE OF THE SUNS HEAT. 279 



exposes it the more to denudation, and it is certainly 

 not an exaggerated assumption to suppose that whilst 

 the general surface of the country was being lowered 

 6000 feet by denudation, the mountain itself was at 

 least lowered by 3000 feet. 



The very common existence of mountains two or 

 three thousand feet in height formed by sub-aerial 

 denudation, proves that at least one mile must have 

 been worn off the general surface of the country. It 

 does not, of course, follow that the general surface ever 

 stood at an elevation of one mile above the sea-level, 

 since denudation would take place as the land gradually 

 rose. We know that the land was once under the sea, 

 for it was there that it was formed. It is built up out 

 of the materials resulting from the carving out into 

 hill and dale, through countless ages, of a previously 

 existing land, just as this latter had resulted from the 

 destruction of a still older land, and so on in like 

 manner back into the unknown past. We have no 

 means of knowing how often the materials composing 

 the sedimentary rocks may have passed through the 

 process of denudation* 



It has now been proved, by the foregoing very simple 

 and obvious method, that the age of the earth must be 

 far more than 20 or 30 million years. This method, it 

 is true, does not enable us to determine with anything 

 like accuracy the actual age of the globe, but it enables 

 us to determine with absolute certainty that it must 

 be far greater than 20 million years. We have not 

 sufficient data to determine how many years have 



* The overlooking of this important point by Mr. Alfred R.Wallace 

 in his determination of the age of stratified rocks ("Island Life," 

 chap, x.) appears to me to vitiate his whole argument. This, I 

 think, has been clearly shown by Mr. T. Mellard Reade, "Geological 

 Magazine," Decade ii. vol. x. pp. 309, 571. 



