THEORY OP THE EARTH. 167 



they had probably experienced two or three irrup- 

 tions of the sea. 



These alternate revolutions form, in my opinion, 

 the problem in geology that is most important to 

 be solved, or rather to be accurately defined and 

 circumscribed ; for, in order to solve it satisfacto- 

 rily and entirely, it were requisite that we should 

 discover the cause of these events, an enterprise 

 involving difficulties of a very different nature. 



We are able to discover with sufficient preci- 

 sion all that takes place on the surface of our 

 world in its present state, and we have sufficiently 

 ascertained the uniform progress and regular suc- 

 cessions of the primitive formations ; but the study 

 of the secondary formations is as yet scarcely 

 commenced. The wonderful series of unknown 

 marine moluscae and zoophites, followed by fossil 

 remains of serpents and of fresh-water fish, equally 

 unknown, which are again succeeded by other 

 moluscse and zoophites more nearly allied to those 

 which exist at present: All these land animals, 

 these moluscae, and other unknown animals of fresh 

 water, which next occupy the formations, and 

 which are finally succeeded by other molusca? and 

 other animals resembling those of our present seas ; 

 the relations between these various animals and 

 the plants whose remains are mixed among them, 

 and the relations of both with the mineral strata 



