The state in which we often find fossil wood in strata gives reason to conclude that this body of vegetable production, in its condensed state, is in appearance undistinguishable from fossil coal, and may be also in great quantity; as, for example, the Bovey coal in Devonshire.

Thus the strata of fossil coal would appear to be formed by the subsidence of inflammable matter of every species at the bottom of the sea, in places distant from the shore, or where there had been much repose, and where the lightest and most floatant bodies have been deposited together.  This is confirmed in examining those bodies of fossil coal; for, though there are often found beds of sand-stone immediately above and below the stratum of the coal, we do not find any sand mixed in the strata of the coal itself.

Having found the composition of coal to be various, but all included within certain rules which have been investigated, we may perceive in this an explanation of that diversity which is often observed among the various strata of one bed of coal.  Even the most opposite species of composition may be found in the thickness of one bed, although of very little depth, that is to say, the purest bituminous coal may, in the same bed, be conjoined with that which is most earthy.

Fossil coal is commonly alternated with regular sand-stone and argillaceous strata; but these are very different bodies; therefore, it may perhaps be inquired how such different substances came to be deposited in the same place of the ocean.  The answer to this is easy; we do not pretend to trace things from their original to the place in which they had been ultimately deposited at the bottom of the sea.  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.