1. _Under-clay_; the lowermost stratum. A tough argillaceous substance, which upon drying becomes a grey friable earth: it is occasionally black, from the presence of carbonaceous matter. It contains innumerable stems of stigmariæ, which are generally of considerable length, and have their rootlets or fibres (see _Lign. 38_) attached, and extending in every direction through the clay: these stems commonly lie parallel with the planes of the bed, and nearer to the top than to the bottom. 2. _Coal._ A carbonized mass, in which the external forms of the plants and trees composing it are obliterated, but the internal structure remains; large trunks or stems, and leaves, are rarely distinguishable in it, but the presence of coniferous wood in many beds of coal, proves that this arises, not from the absence of trees, but from their external forms having been obliterated. 3. _The Roof_, or upper bed. This generally consists of slaty clay, abounding in leaves, trunks, stems, branches, and fruits, and contains layers and nodules of ironstone, inclosing leaves, insects, crustaceans, &c. In some localities beds of fresh-water mussels, and in others of marine shells, are intercalated; layers of shale, finely laminated clay, micaceous sand and grit, and pebbles of limestone, granite, sandstone, and other rocks, are often present. The most illustrative examples of the foliage of the carboniferous flora are found in this deposit, which appears to be an accumulation of drifted materials derived from other rocks, and promiscuously intermingled with the dense foliage and stems of a prostrate forest; the whole having been transported from a distance by a powerful current or flood. [Sidenote: ORIGIN AND NATURE OF COAL.] Thus we have, in the first place, spread uniformly over the bottom, and constituting the bed on which the coal reposes, a stratum of clay (_Under-clay_), composed of fine pulverulent materials, which may have once constituted the soil of a vast plain or savannah; the only remains found in it are the roots of gigantic trees (see _Lign. 36_); for such the stigmariæ are now proved to have been, and not aquatic plants, as was formerly supposed (_Bd._ p. 476). Secondly, a bituminous mass (Coal), composed of coniferous wood, gigantic ferns, club-mosses, &c.; occasionally with trunks of trees penetrating vertically through it. Thirdly, a deposit of drift or water-worn materials (_the Roof_), mixed with the foliage and stems of numerous species of terrestrial plants; the whole appearing to have been subjected to the action of currents. The first, or _Under-clay_, may have been the natural soil, in which the stigmariæ grew; the next,--the _Coal_,--the carbonized stems, and other remains of the trees to which the roots belonged: and the last, or uppermost, forming the roof of the coal, may have resulted from the foliage and branches of a prostrate forest, overwhelmed and buried beneath the transported detritus of distant rocks.