xi.J (Mogkal |k&rrm. 245 



must be limited within some sucli period of time as one 

 hundred million years" (loc. cit. p. 25). 



The first inquiry which arises plainly is, has it ever 

 been denied that this period may be enough for the 

 purposes of geology? 



The discussion of this question is greatly embarrassed 

 by the vagueness with which the assumed limit is, I 

 will not say defined, but indicated, "some such period 

 of past time as one hundred million years." Now 

 does this mean that it may have been two, or three, or 

 four hundred million years ? Because this really makes 

 all the difference. 1 



I presume that 100,000 feet may be taken as a full 

 allowance for the total thickness of stratified rocks con- 

 taining traces of life ; 100,000 divided by 100,000,000 

 =0'001. Consequently, the deposit of 100,000 feet of 

 stratified rock in 100,000,000 years means that the 

 deposit has taken place at the rate of y^Vo of a foot, or, 

 say, -rar of an inch, per annum. 



Well, I do not know that any one is prepared to main- 

 tain that, even making all needful allowances, the 

 stratified rocks may not have been formed, on the 

 average, at the rate of ^V of an inch per annum. 

 I suppose that if such could be shown to be the 

 limit of world-growth, we could put up with the 

 allowance without feeling that our speculations had 

 undergone any revolution. And perhaps, after all, the 

 qualifying phrase " some such period " may not neces- 

 sitate the assumption of more than -ri* or rf-g- or -5?? of 

 an inch of deposit per year, which, of course, would 

 give us still more ease and comfort. 



But, it may be said, that it is biology, and not geology, 



1 Sir William Thomson implies (loc. cit. p. 16), that the precise time is of 

 no consequence : " the principle is the same ;" but, as the principle is 

 admitted, the whole discussion turns on its practical results. 



