240 THEORY OF THE EARTH. 



at least by land animals, and, therefore, one pre- 

 ceding revolution at least had put them under 

 water ; and if we may judge by the different or- 

 ders of animals the remains of which are observed 

 in them, they had perhaps been subjected to two 

 or three irruptions of the sea. 



Further Researches to be made in Geology. 



These alternations now appear to me to form 

 the problem in geology that it is of most import- 

 ance to solve, or rather to define and circumscribe 

 within due limits ; for, in order to resolve it satis- 

 factorily, it would be necessary to discover the 

 cause of these events, an undertaking which 

 presents a difficulty of quite a different kind. 



I repeat it, we see pretty clearly what is going 

 on at the surface of the continents in their pre- 

 sent state ; we have formed a tolerable conception 

 of the uniform progress and regular succession of 

 the primitive formations, but the study of the se- 

 condary formations has been little more than 

 merely commenced. That wonderful series of un- 

 known zoophytes and marine mollusca, succeeded 

 by reptiles and fresh-water fishes equally un- 

 known ; and these again replaced, in their turn, 

 by other zoophytes and mollusca, more nearly re- 

 lated to those of the present day ; those land ani- 

 mals, and those equally unknown fresh water inoj- 

 lusca and other animals which next occupied 



