OF ORGANIC NATURE 31 



of rock. And, seeing that every part of the crust of the 

 earth is made up in this way, you might think that the 

 determination of the chronology, the fixing of the time 

 which it has taken to form this crust is a comparatively 

 simple matter. Take a broad average, 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 whatever 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 w T hich 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 deposited at the bottom. 



Let C 37 (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 we have regarded as 

 stationary, goes down; as it does so, both A and B go 

 further out from the shore, which will be at y 1 , x l y 1 

 being the new sea-level. The consequence will be that 

 the layer of mud (A), being now, for the most part, further 



