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 wholesale 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 immeasureable ages in their common work of destruction, that when we try to realise what it must amount to, we can scarcely fail to wonder, not that the geological record is highly imperfect, but that so much of the record has survived as we find to have been the case. And, if we add to these erosive and solvent agencies on land the erosive and solvent agencies of the sea, we may almost begin to wonder that anything deserving the name of a geological record is in existence at all.