no respect from sand, except in point of
size. Next after gravel, in the order of ascent, come stones; and these
bear nearly the same relation to gravel as gravel does to sand. Now,
by stones is to be understood the fragments of rocks or solid mineral
bodies; and there is a perfect gradation from those stones to sand.
I have already endeavoured to explain the formation of those stony
substances; and now I am treating of a certain system of circulation,
which is to be found among minerals.

M. de Luc censures me for not giving the origin of sand, of which I form
the strata of the earth. He seems to have misunderstood my treatise. I
do not pretend, as he does in his theory, to describe the beginning of
things; I take things such as I find them at present, and from these
I reason with regard to that which must have been. When, from a thing
which is well known, we explain another which is less so, we then
investigate nature; but when we imagine things without a pattern or
example in nature, then, instead of natural history, we write only
fable.

M. de Luc, in the letter already mentioned, says, "that sand may be, and
I think it is, a substance which has formed _strata_ by _precipitation
in a liquid_." This is but an opinion, which may be either true or
false. If it be true, it is an operation of the mineral kingdom of which
I am ignorant. In all the sand which I have ever examined, I have never
seen any that might not be referred to the species of mineral substance
from which it had been formed. When this author shall have given us
any kind of information with regard to the production of sand _by
precipitation in a liquid_, it will then be time enough to think of
forming the strata of the earth with that sand.]

Clay is now to be considered as the last of those materials of which
our strata are composed; but, in order to understand the nature of this
ingredient, something must be premised.

Clay is a mixture of different earths or hard substances, in an
impalpable state. Those substances are chiefly the siliceous and
aluminous earths. Other earths are occasionally mixed in clays, or
perhaps always to be found in some small portion. But this does not
affect the general character of clay; it only forms a special variety in
the subject. A sensible or considerable portion of calcareous earth, in
the composition of clay, constitutes a marl, and a sufficient admixture
of sand, a loam.

An indefinite variety of those composi