We have been hitherto considering fossil coal as formed of the impalpable parts of inflammable bodies, united together by pressure, and made to approach in various degrees to the nature of a chemical coal, by means of subterranean heat; because, from the examination of those strata, many of them have evidently been formed in this manner. But vegetable bodies macerated in water, and then consolidated by compression, form a substance of the same kind, almost undistinguishable from some species of fossil coal. We have an example of this in our turf pits or peat mosses; when this vegetable substance has been compressed under a great load of earth, which sometimes happens, it is much consolidated, and hardens, by drying, into a black body, not afterwards dilutable or penetrated by water, and almost undistinguishable in burning from mineralised bodies of the same kind.
Also, when fossil wood has been condensed by compression and changed by the operation of heat, as it is frequently found in argillaceous strata, particularly in the aluminous rock upon the coast of Yorkshire, it becomes a jet almost undistinguishable from some species of fossil coal.
There cannot therefore be a doubt, that if this vegetable substance, which is formed by the collection of wood and plants in water upon the surface of the earth, were to be found in the place of fossil coal, and to undergo the mineral operations of the globe, it must at least augment the quantity of those strata, though it should not form distinct strata by itself.
It may perhaps be thought that vegetable bodies and their impalpable parts are things too far distant in the scale of magnitude to be supposed as subsiding together in the ocean; and this would certainly be a just observation with regard to any other species of bodies: But the nature of vegetable bodies is to be floatant in water; so that we may suppose them carried at any distance from the shore; consequently, the size of the body here makes no difference with regard to the place or order in which these are to be deposited.
The examination of fossil coal fully confirms those reasonable suppositions. For, first, The strata that attend coal, whether the sandstone or the argillaceous strata, commonly, almost universally, abound with the most distinct evidence of vegetable substances; this is the impressions of plants which are found in their composition. Secondly, There is much fossil coal, particularly that termed in England clod coal, and employed in the iron foundry, that shows abundance of vegetable bodies in its composition. The strata of this coal have many horizontal interstices, at which the more solid shining coal is easily separated; here the fibrous structure of the compressed vegetable bodies is extremely visible; and thus no manner of doubt remains, that at least a part of this coal had been composed of the vegetable bodies themselves, whatever may have been the origin of the more compact parts where nothing is to be distinguished.
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.