GEOLOGICAL RELATIONS. J 77 



decomposes and destroys them ; hence, in a short time 

 after the death of the animals, scarcely a vestige of 

 their shells is to be found. The formations which con- 

 tain them, and in which alone they have been found 

 thus far, are those which have been deposited in the 

 estuaries and bays at the mouths of ancient rivers, or 

 in the beds of lakes. The streams and rivers which 

 discharged their waters into those reservou'S were filled 

 with the washings of the countries through which they 

 flowed. The shells and other substances borne along 

 by their currents, on reaching the sea or lake in which 

 they terminated, were deposited in still water, and being 

 immediately covered by succeeding deposits, remained 

 protected from disturbing causes until the beds and 

 strata Avhich contained them underwent the fossilizing 

 process. But it may have happened that a river, rising 

 in a high latitude, and flowing towards the equator, 

 like the present Mississippi, may only have reached its 

 outlet within or near the tropics. It may have re- 

 ceived tributaries through its whole course, some of 

 which, uniting with it near its mouth, may have passed 

 through regions enjoying a tropical cUmate. The 

 waters of such a river would of course be freighted 

 with the animal and vegetable productions of regions 

 very remote from each other, and of very different 

 climes, and they would be deposited promiscuously 

 together. Genera and species peculiar to mountainous 

 countries would be found side by side with those belong- 

 voL. I. 22 



