OF ORGANIC NATUBE. 60 



total thickness of the whole series of stratified rocks, 

 which geologists estimate at twelve or thirteen miles, 

 or about seventy thousand feet, make a sum in short 

 division, divide the total thickness by that of the quan- 

 tity deposited in one year, and the result will, of course, 

 give you the number of years which the crust has taken 

 to form. 



Truly, that looks a very simple process ! It would 

 be so except for certain difficulties, the very first of 

 which is that of finding how rapidly sediments an 

 posited ; but the main difficulty — a difficulty which 

 renders any certain calculations of sucli a matter out 

 of the question — is this, the sea-bottom on which the 

 deposit takes place is continually shifting. 



Instead of the surface of the earth being that stable, 

 fixed thing that it is popularly believed to be, being, 

 in common parlance, the very emblem of fixity itself, 

 it is incessantly moving, and is, in fact, as unstable as 

 the surface of the sea, except that its undulations are 

 infinitely slower and enormously higher and deeper. 



Now, what is the effect of this oscillation I Take 

 the case to which I have previously referred. The finer 

 or coarser sediments that are carried down by the 

 current of the river, will only be carried out a certain 

 distance, and eventually, as we have already seen, on 

 reaching the stiller part of the ocean, will be deposited 

 at the bottom. 



Let C y (Fig. 4) be the sea-bottom, y D the shore, 

 x y the sea-level, then the coarser deposit will subside 

 over the region B, the finer over A, while beyond A 

 there will be no deposit at all ; and, consequently, no 

 record will be kept, simply because no deposit is going 

 on. Now, suppose that the whole land, C, D, which 



