i6 THE AGE OF THE EARTH 



effective ; and we are free to proceed, and to look for 

 the conclusions warranted by our own evidence. In this 

 matter we are at one with the geologists ; for, as has been 

 already pointed out, we rely on them for an estimate of 

 the time occupied by the deposition of the stratified 

 rocks, while they rely on us for a conclusion as to how far 

 this period is sufficient for the whole of organic evolution. 



First, then, we must briefly consider the geological 

 argument, and I cannot do better than take the case as 

 put by Sir Archibald Geikie in his Presidential Address 

 to this Association at Edinburgh in 1892. 



Arguing from the amount of material removed from the 

 land by denuding agencies, and carried down to the sea 

 by rivers, he showed that the time required to reduce 

 the height of the land by one foot varies, according to 

 the activity of the agencies at work, from 730 years to 

 6,800 years. But this also supplies a measure of the rate 

 of deposition of rock ; for the same material is laid down 

 elsewhere, and would of course add the same height of 

 one foot to some other area equal in size to that from 

 which it was removed. 



The next datum to be obtained is the total thickness 

 of the stratified rocks from the Cambrian system to the 

 present day. ' On a reasonable computation these stratified 

 masses, where most fully developed, attain a united 

 thickness of not less than 100,000 feet. If they were all 

 laid down at the most rapid recorded rate of denudation, 

 they would require a period of seventy-three millions of 

 years for their completion. If they were laid down at 

 the slowest rate, they would demand a period of not less 

 than 680 millions.' 



The argument that geological agencies acted much 

 more vigorously in past times he entirely refuted by 

 pointing to the character of the deposits of which the 

 stratified series is composed. 'We can see no proof 

 whatever, nor even any evidence which suggests that on 

 the whole the rate of waste and sedimentation was more 

 rapid during Mesozoic and Palaeozoic time than it is 

 to-day. Had there been any marked difference in this 



