34 THE PAST CONDITION 



is a comparatively simple matter. Take a broad aver- 

 age, ascertain how fast the mud is deposited upon the 

 bottom of the sea, or in the estuary of rivers ; take it 

 to be an inch, or two, or three inches a year, or what- 

 ever you may roughly estimate it at ; then take the 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 quantity 

 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 

 are deposited; but the main difficulty — a difficulty 

 which renders any certain calculations of such 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 ? 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 depo- 

 sited at the bottom. 



