It is
enough that we find the substance of which we treat delivered into
the sea, and regularly deposited at the bottom, after having been
transported by the currents of the ocean. Now the currents of the ocean,
however regular they may be for a certain period of time, and however
long this period may be protracted, naturally change; and then the
currents, which had given birth to one species of stratum in one place,
will carry it to another; and the sediment which the moment before
had formed a coal stratum, or a bed of that bituminous matter, may
be succeeded either with the sediment of an argillaceous stratum, or
covered over with a bed of sand, brought by the changed current of the
sea.
We have now considered all the appearances of coal strata, so far as
these depend upon the materials, and their original collection. But,
as those bituminous strata have been changed in their substance by the
operation of subterranean heat and inspissation, we are now to look for
the necessary consequences of this change in the body of the stratum;
and also for other mineral operations common to fossil coal with
consolidated strata of whatever species.
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