32 THEORY OF THE EARTH. 



these shells and the shells which still inhabit the sea. 

 They have therefore once lived in the sea, and been 

 deposited by it; the sea consequently must haverest- 

 ed in the places where the deposition has taken 

 place. Hence it is evident the basin or reservoir 

 containing the sea has undergone some change at 

 least, either in extent, or in situation, or in both. 

 Such is the result of the very first search, and of 

 the most superficial examination. 



The traces of revolutions become still more ap- 

 parent and decisive when we ascend a little higher, 

 and approach nearer to the foot of the great chains 

 of mountains. There are still found many beds of 

 shells; some of these are even larger and more 

 solid ; the shells are quite as numerous and as en- 

 tirely preserved ; but they are not of the same spe- 

 cies with those which were found in the less eleva- 

 ted regions. The strata which contain them are 

 not so generally horizontal ; they have various de- 

 grees of inclination, and are sometimes situated 

 vertically. While in the plains and low hills it was 

 necessary to dig deep in order to detect the suc- 

 cession of the strata, here we perceive them by 

 means of the valleys which time or violence has pro- 

 duced, and which disclose their edges to the eye 

 of the observer. At the bottom of these declivi- 

 ties, huge masses of their debris are collected, and 

 form round hills, the height of which is augmented 

 by the operation of every thaw and of every storm. 



