120 
ELEMENTS OF GEOLOGY. 
which beds of ashes full of recent marine shells have been 
uplifted many hundred feet. We need not be surprised, 
therefore, if we learn from geology that the continents and 
oceans were not always placed where they now are, although 
the imagination may well be overpowered when it endeav¬ 
ors to contemplate the quantity of time required for such 
revolutions. 
We shall have gained a great step if we can approximate 
to the number of millions of years in which the average 
aqueous denudation going on upon the land would convey 
seaward a quantity of matter equal to the average volume 
of our continents, and this might give us a gauge of the 
minimum of volcanic force necessary to counteract such lev¬ 
elling power of running water; but to discover a relation 
between these great agencies and the rate at which species 
of organic beings vary, is at present wholly beyond the 
reach of our computation, though perhaps it may not prove 
eventually to transcend the powers of man. 
