8 THEORY OF THE EARTH. 



detrition nor fracture in them, nothing, in short, 

 that announces a violent removal from their 

 original places ; the smallest of them retain 

 their sharpest ridges, and their most delicate 

 spines. They have, therefore, not only lived in 

 the sea, hut they have also been deposited by it. 

 It is the sea which has left them in the places 

 where they are now found. But this sea has 

 remained for a certain period in those places ; 

 it has covered them long enough, and with suffi- 

 cient tranquillity to form those deposits, so re- 

 gular, so thick, so extensive, and partly also so 

 solid, which contain those remains of aquatic ani- 

 mals. The basin of the sea has therefore under- 

 gone one change at least, either in extqnt, or in 

 situation. 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 high- 

 er, and approach nearer to the foot of the great 

 chains. There are still found many beds of shells ; 

 some of these are even thicker and more solid ; 

 the shells are quite as numerous, and as well pre- 

 served, but they are no longer pf the same spe- 

 cies. The strata which contain them are not so 

 generally horizontal ; they assume an oblique po- 

 sition, and are sometimes almost vertical. While 

 in the plains and low hills it was necessary to 

 dig deep, in order to discover the succession of the 



