424 Darwin, and after Darwin. 
as far as shells and bones of all kinds are concerned— 
and this to an extent of which we have probably no adequate 
conception. 
Of still greater destructive influence, however, than these 
solvent agencies in earth and sea, are the erosive agencies of 
both. Any one who watches the pounding of the waves 
upon the shore; who then observes the effect of it upon the 
rocks broken into shingle, and on the shingle reduced to 
sand; who, looking behind him at the cliffs, sees there the 
evidence of the gradual advance of this all-pulverising power 
—an advance so gradual that no yard of it is accomplished 
until within that yard the “ white teeth” have eaten well into 
the “ bowels of the earth” ; who then reflects that this process 
is going on simultaneously over hundreds of thousands of miles 
of coast-lines throughout the world; and who finally extends 
his mental vision from space to time, by trying dimly to 
imagine what this ever-roaring monster must have consumed 
during the hundreds of millions of years that slowly rising 
and slowly sinking continents have exposed their whole areas 
to her jaws; whoever thus observes and thus reflects must be 
a dull man, if he does not begin to feel that in the presence 
of such a destroyer as this we have no reason to wonder at a 
frequent silence in the testimony of the rocks. 
But although the erosive agency of the sea is thus so 
inconceivably great, it is positively small if compared with 
erosive agencies on land. The constant action of rain, wind, 
and running water, in wearing down the surfaces of all lands 
into “the dust of continents to be”; the disintegrating 
effects on all but the very hardest rocks of winter frosts 
alternating with summer heats; the grinding power of ice 
in periods of glaciation; and last, but not least, the whole- 
sale melting up of sedimentary formations whenever these 
have sunk for any considerable distance beneath the earth’s 
surface :—all these agencies taken together constitute so 
prodigious a sum of energies combined through immeasure- 
