70 THEORY OF THE EARTH. 



respecting them, that we have been enabled to 

 discover the little that we yet know concerning 

 the revolutions of our globe. From them we have 

 learned that the strata, or at least those which 

 contain their remains, have been quietly deposited 

 in a fluid ; that the variations of the several strata 

 must have corresponded with the variations in the 

 nature of the fluid ; that they have been left bare 

 by the transportation of this fluid to some other 

 place ; and that this fact must have happened 

 more than once. Nothing of all this could have 

 been known with certainty, without the aid of ex- 

 traneous fossils. 



The study of the mineralogical part of geo- 

 logy, though not less necessary, and even a great 

 deal more useful to the practical arts, is yet much 

 less instructive so far as respects the objects of 

 our present inquiry. We remain in utter igno- 

 rance respecting the causes which have given rise 

 to the variety in the mineral substances of which 

 strata are composed. We are ignorant even of 

 the agents which may have held some of these sub- 

 stances in a state of solution ; and it is still dispu- 

 ted respecting several of them, whether they have 

 owed their origin to the agency of water or fire. 

 After all, philosophers are only agreed on one point, 

 which is, that the sea has changed its place ; and 

 this could never have been certainly known, but 

 for the existence of extraneous fossils. These 

 fossils, then, which have given rise to the theory 



