FOSSIL SHELLS. 191 
level of the sea; and in places to which the sea 
could not be conveyed by any existing cause. They 
are not only enclosed in loose sand, but also in- 
crusted by hard stones. In short, both hemispheres, 
every continent, and every island of any size, exhibit 
the same phenomenon. 
The traces of revolutions become still more ap- 
parent and decisive, when we ascend a little higher, 
and approach nearer to the foot of the great chain of 
mountains. These contain many beds of shells, 
some of which are even larger and more solid, and 
as well preserved, though not of the same species 
with those of less elevated regions. And, whereas, 
in the plains, low hills, and valleys, it is necessary 
to dig deep, in order to detect the succession of the 
strata; here we perceive them by means of the rents, 
and excavations, which time, or violence, has oc- 
casioned, and which disclose their edges to the view 
of the observer. At the bottom of these declivities, 
huge masses of their wrecks are frequently collected, 
and form round hills, the height of which is conti- 
nually augmented by the operation of every thaw, 
and every storm. 
In Chili, some of the loftiest mountains are formed 
entirely of shells; and on the Descaheyado, whose 
towering summits aspire nearly to the height of the 
magnificent Chimborazo, Oysters and Periwinkles are 
