PHENOMENA OF ORGANIC NATURE 337 
crust is a comparatively simple matter. Take a 
broad average, ascertain how fast the mud is 
d ge upon the bottom of the sea, or in the 
stuary of rivers ; take it to be an inch, or two, or 
ti three inches a wear: or whatever you may roughly 
estimate it at; then take the total thickness of 
the whole ios 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 
qu — deposited i in one year, and the result will, 
“of course, give you the number of years which the 
( 7 st has taken to form. 
3 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 
“sed SS ies are deposited; but the main difficulty 
fF =a difficulty which renders any certain calcula- 
ions of such a matter out of the question—is 
his, the sea-bottom on which the deposit takes 
place is continually shifting. 
Instead of the surface of the earth bemg that 
ti table, fixed thing that it is popularly believed to 
, being, in common parlance, the very emblem 
Basi, itself, it is incessantly moving, and is, 
i n fact, as unstable as the surface of the sea, 
; Xce ept that its undulations are infinitely stones 
4 nd enormously higher and deeper. 
_ Now, what is the effect of this oscillation ? 
ake the case to which I have previously 
‘vou. u Z 
