GEOLOGICAL RELATIONS. 177 
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 reservoirs 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 which 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 climate. 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. 46 
