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 eff"ect 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 glacialion; 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- 



