XI PHENOMENA OF ORGANIC NATURE 337 



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 geolo- 

 gists 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 calcula- 

 tions 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 



VOL. II Z 



