C 674 ] 
fhclls ought never to be found at lefs depths in the 
earth than from 1 50 to 3000 fathoms. 
II. Though foil'll fhells are to be found in almofl 
all the plainer parts of the furface of the earth, yet 
there are certain very large tradts, where fuch bodies 
are never found, viz. the mountains, which feem to 
be the remains of the original flrata of the earth. It 
is true indeed, that there are many eminences, which 
have been by our modern theories taken for moun- 
tains, where fea-fhells, and calcareous matter, of every 
kind, are to be found in great abundance : but thel'e 
are very inconfiderable, and only appear as little hil- 
locks, compared with the large mountains, which 
contain mines, veins of metal, and precious ftones, 
and may be traced in immenfe chains, without al- 
moft any difeontinuity from one continent to another; 
and from continents to neighbouring and oppofite 
iilands, &c. infomuch that ail thefe chains not only 
of the old, but li'kewi'fe of the new world feem con- 
nected one with another ; an obfervation, which alone 
would indicate the importance of diligently inquiring 
into their ftruCture, in order to form a true theory of 
the earth. Monf. de Buffon and the author of Tel- 
liamed, who endeavour to prove, that all mountains 
have been formed by fea-currents, and bring one of 
their principal arguments in proof of this opinion 
from marine bodies being found in great quantities 
in the flrata, of which they are compofed, feem never 
to have made obfervations on mountains ; elfe they 
■might have obferved this remarkable difference be- 
tween them and the calcareous flrata of the plains, 
that the former contain none of thofe marine bodies, 
-though the latter are almofl intirely made up of them. 
In 
