The last kind of petrified trees which we shall mention, are those which occur in the coal strata, or, properly speak ing, in the sandstones of this series. As it is not yet agreed to what the squamous trunks belong, whether to large herbaceous, or to ligneous plants, we shall consider them among the former. The very decided trees to which we now allude, are by no means common, but they have been found in our own island, and chiefly in the Scotch coal mines, or in the quarries that have been excavated in the arenaceous beds of these. Habel indeed mentions a vertical stem which he saw, near Nassau, in an erect position, and forty feet long, being, at the same time, a foot in diameter at the base; but, for want of a more par ticular description, we cannot ascertain that it belongs to those here under consideration. Ramifying branches or trunks have been found near Edinburgh ; and at Penny cuick there are the remains, or rather the cast, of the lower part of a large tree, with its roots imbedded in the sandstone and shale, and in its natural erect position. A similar specimen, with parts of the branches extending from it, has lately been found near Glasgow. In these cases it is remarkable, that while the body of the tree has entirely vanished, so as to be replaced by sandstone, the bark is converted into coal. But this also happens some times in the squamous stems : nor, perhaps, is it difficult to explain, when we recollect that, in cases where wood is merely submerged, the wood is frequently destroyed while the bark remains sound. This is particularly re markable with regard to the birch, the bark of which seems to be imperishable, as noticed by M. Maupertuis in Lapland.
On the Lignite& These are properly ranked among fossil woods, and they form a sort of intermediate link between peat and coal, as well in respect to their claims as organic fossils, as in their chemical nature. Under peat we here rank sebmerged or bog wood, which properly belongs to that substance, and cannot rightly be considered a fossil, as not partaking in any respect of a mineral character, though we have described it, following the usual practice. Coal, standing at the very opposite extreme, must be consider ed merely as a mineral, having no claim on the organic character more than limestone from which the shells have disappeared, although both are indebted to organic bodies for their existence. The lignites combine both ; the or ganic structure of submerged or natural wood, with the mineral nature of coal.
The geological distinctions between these three sub stances arc also very characteristic. .Peat is found above the alluvial soils, lignite beneath them, and coal below solid rocks. The alluvia that contain lignite are always too of considerable antiquity ; and, indeed, it is some times found in the rocky strata, and, as we shall afterwards see, under the trap rocks.
As the superior antiquity of lignite to peat is proved by its geological situation ; so it is inferred from its more perfect approaches to bituminization. It is also a pro
cess which appears to have ceased long ago, while the production of peat is still going on. It is therefore essen tial to the lignites, as distinguished from mere submerged wood, either to yield bitumen on distillation, or to be con verted into that peculiar hydrocarbonaceous compound which forms Cologne earth.
Although the lignites may retain the marks of vege table organization, they seldom possess the original form of the wood whence they were derived. They are com monly flattened, as if from the consequences of compres sion, while the forms of submerged wood are not often altered in the same manner. As the boundary of lignite towards peat must, however, be indefinite, in many cases, to the eye, the distinctions must depend on the chemical characters above mentioned. It is equally indefinite at times towards coal ; so much so, indeed, in certain cases, that the distinction can scarcely be made at all. It is, however, best made by mechanical characters in this case, as it is by chemical ones in the other. Coal has the form and character of a rock, and is inorganic ; lignite has not those forms, and always contains the traces, more or less distinct, of its former organization. There are also distinctions to be derived from the nature of the produce Nvhich these respectively yield on distillation. From coal we obtain naphtha ; but the oil procured in the same way from lignite, is a compounded substance, with a peculiar smell; being, in short, a mixture of the volatile oil of wood and of naphtha also.
The different substances that must be ranked under lignite, are brown coal, or Bovey coal, surturbrand, jet, Cologne earth, or pulverulent lignite, and the bituminized wood found under the trap rocks. But between the three first there are gradations, so that the characters are not al ways so definite as the names would lead us to expect. The lignite of trap is the only one which passes into genuine coal. It is conveniently designated by the term basaltic wood.
Among the numbers of submerged forests that have been described by observers in different parts of the world, the discoverers have generally neglected to in quire whether they possessed the characters of lignite or not. In some cases, however, the wood has been found flattened, and, as it is said, bituminous ; so that these should be classed with lignite, and not with sub merged wood. When the distinction shall become more generally known, it is probable that the deposits of this nature will prove more numerous than has yet been supposed. Indeed, as might be expected from the tedious nature of the process of bituminization, unchanged, or common submerged wood, has been found in the same places as brown coal or lignite ; and thus also lignites varying in the degree of bituminization, occur in the same place, as happens at Bovey. The most perfect kinds, it is proper to remark, occupy the lowest positions in these cases.