THEORY OF THE EARTH. 11 



strata, nor to this retreat of the sea after the for- 

 mation of the new strata, that the revolutions 

 and changes which have given rise to the present 

 state of the Earth are limited. 



When we institute a more detailed compari- 

 son between the various strata and those remains 

 of animals which they contain, we presently per- 

 ceive, that this ancient sea has not always depo- 

 sited mineral substances of the same kind, nor re- 

 mains of animals of the same species ; and that 

 each of its deposits has not extended over the 

 whole surface which it covered. There has ex- 

 isted a succession of variations; the former of 

 which alone have been more or less general, while 

 the others appear to have been much less so. The 

 older the strata are, the more uniform is each of 

 them over a great extent ; the newer they are, the 

 more limited are they, and the more subject to 

 vary at small distances. Thus the displacements 

 of the strata were accompanied and followed by 

 changes in the nature of the fluid, and of the 

 matters which it held in solution ; and when cer- 

 tain strata, by making their appearance above the 

 waters, had divided the surface of the seas by 

 islands and projecting ridges, different changes 

 might take place in particular basins. 



Amidst these variations in the nature of the 

 general fluid, it is evident, that the animals which 

 lived in it could not remain the same. Their 



