136 LIFE ON THE EARTH. 



greater frequency, or greater violence, of movements 

 in the crust of the earth: for thus it would yield 

 more easily to the assaults of the waves and atmo- 

 spheric vicissitudes. During the earlier periods, 

 land rising through the waves was subject to more 

 rapid erosion by the sea and the atmosphere; short 

 seras of enormous waste productive of conglomerates, 

 longer periods of strong action yielding large masses 

 of sandstone, still longer periods of more equable 

 decay producing more extended beds of clay since 

 indurated to shale and slate. 



During the same periods land was sunk in one 

 tract as well as raised in another, perhaps to as 

 great a depth here as the elevation there; while 

 it was sinking, fresh edges were exposed to the 

 sea, and thus the waste was quickened. How are 

 these effects to be calculated ? Strictly they cannot 

 be. But, as an illustration, let us suppose that by 

 these operations the resisting power of the rocky 

 masses of the earth's crust was weaker than at pre- 

 sent only one-half as great so that combining this 

 ratio to resist with that already allowed for the 

 power to destroy, we shall have the earlier atmo- 

 spheric waste effectively four times as great as at 

 present. This allowed, we shall find the whole time, 

 as given by the Uniformitarian hypothesis, cannot 

 be reduced to so little as f , or 38,000,000 years. 



