1870.J 



The Bate of Geological Change. 



329 



consider a million years to represent the twentieth part of a com- 

 plete revolution in species, and we might thus estimate the number 

 of years required for the elaboration of the successive Tertiary for- 

 mations." Proceeding thus, Sir Charles Lyell calculates that two 

 hundred and forty millions of years have elapsed since the beginning 

 of the Cambrian period. Mr. Wallace, however, by taking the same 

 facts and figures, manipulates them differently, and comes to the 

 conclusion that the lapse of time is exactly one-tenth of that esti- 

 mated by Sir Charles Lyell. This difference of result is of very 

 little consequence, as both calculations are equally speculative, and 

 it is chiefly to a point of resemblance that I wish to draw attention. 

 Sir Charles Lyell and Mr. Wallace, however much they may 

 differ in other matters, whether they regard the revolution of species 

 as having been accomplished in no less a period than twenty millions, 

 or in one no greater than two millions, are agreed in using the same 

 rate of change for the Palaeozoic as for the more recent periods. 

 Taking the figures of the latter author, and calculating the time 

 required for the deposition of strata on his hypothetical rate of 

 change in species, we get at the following results : — 



Periods. 



Cainozoic 

 Mesozoic . 

 Palaeozoic 



Duration 

 (Wallace). 



6,000,000 



8,000,000 



10,000,000 



Thickness of 



Rocks 



(England). 



2,240 

 23,190 

 57,124 



Rate of 



Deposit; years 



per foot. 



2,678 

 345 

 175 



It may be urged that the Tertiary rocks are comparatively thin 

 in England, owing to the absence of Miocene deposits ; but the 

 same argument may be applied to each of the three periods with 

 greater or less force ; and at any rate the relative rapidity of deposit 

 Would not be very much disturbed if we took the maximum thick- 

 ness all over the world instead of those occurring in the British 

 Islands. Leaving denudation entirely out of the question, it does 

 not seem at all probable that the Palaeozoic rocks should have been 

 deposited at the swift rate of one foot in 175 years; and if we 

 deduct from the calculation the period represented by "breaks," 

 which Professor Eamsay regards as longer even than that repre- 

 sented by strata, we shall have the conclusion drawn that the 

 Palaeozoic rocks were deposited at the rate of one foot in less than 

 a century ! At the present day there are many localities peculiarly 

 favourable to the rapid accumulation of deposits over limited areas, 

 as, for instance, the deltas of the great rivers ; but even in such 

 cases the rate of deposition is probably not more than what, accord- 

 ing to Mr. Wallace's estimate, must have been general all over the 

 aquiferous surface of the earth during Palaeozoic times. 



