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.
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