30 The Founders of Geology LECT. 



like the Devil's Gap in Normandy. 1 He combats the 

 notion that landslips, such as had occurred at Issoire in 

 Auvergne in the year 1733, were caused by internal fires or 

 subterranean winds, and agrees with a previous writer in 

 regarding them as the result of the penetration of water 

 from the surface into the interior of the hill. He thus 

 recognized the efficacy of subterranean as well as superficial 

 water, in changing the face of a country. 



He believed the sea to be the most potent destroyer of 

 the land, and as an instance of its power he was accustomed 

 to regard the chalk cliffs of the north-west of France as 

 the relics of a great chain of hills, of which the greater part 

 had been swept away by the sea. 2 He shows, further, 

 that while the hills are worn down by the waves, by the 

 rains, and by the inundations to which the rains give rise, 

 the materials removed from them are not destroyed, but 

 are deposited either on the land or along the shores of the 

 sea. 3 He further points out that the detritus of separate 

 river-basins may greatly differ, and that materials may be 

 carried into districts where the rocks are entirely distinct 

 from those in the areas whence the transport has taken 

 place. He refers to the practical value of this observation 

 in questions regarding the source of minerals, ores and 

 useful stones. 4 



He is thus led to give, from his wide knowledge of 

 France, a sketch of the character of the rocks in the 

 different river-basins of the country, and the nature of the 

 materials which the rivers have in each case to transport. 

 He passes in review all the large streams that enter the 

 Atlantic from the Khine to the shores of Gascony, and 



1 P. 214. 2 Pp. 220, 222. 3 P. 222. 4 P. 223. 



