420 GEOLOGICAL TIME. 



ones ; thus all the flint gravels in the south-east of England 

 have been produced by the destruction of chalk. This again 

 is a very slow process. It has been estimated that a cliff 500 

 feet high will be worn away at the rate of an inch in a century. 

 This may seem a low rate, but we must bear in mind that 

 along any line of coast there are comparatively few points 

 which are suffering at one time, and that even on those, when 

 a fall of cliff has taken place, the fragments serve as a pro- 

 tection to the coast until they have been gradually removed 

 by the waves. The Wealden Valley is twenty-two miles in 

 breadth, and on these data it has been calculated that the 

 denudation of the Weald must have required more than 

 150,000,000 of years. 



There can be no doubt about the interest of these calcula- 

 tions, and they have also the great merit of giving some defi- 

 niteness to our ideas. We must not, however, attribute to 

 them a value which has been distinctly disclaimed even by 

 their authors. Moreover, we must remember that these esti- 

 mates are brought forward not as a proof, but as a measure, 

 of antiquity. Our belief in the antiquity of man rests not on 

 any isolated calculations, but on the changes which have taken 

 place since his appearance ; changes in the geography, in the 

 fauna, and in the climate of Europe. Valleys have been 

 deepened, widened, and partially filled up again; caves through 

 which subterranean rivers once ran are now left dry ; even 

 the configuration of laud has been materially altered, and 

 Africa finally separated from Europe. 



Our climate has greatly changed for the better, and with it 

 the fauna has materially altered. In some cases for instance, 

 in that of the hippopotamus and of the African elephant we 

 may probably look to the diminution of food and the presence 

 of man as the main cause of their disappearance ; the extinc- 

 tion of the mammoth, the Eleplias antiquus, and the Rhinoceros 

 tichorhinus, may possibly be due to the same influences ; but 



