There can be no doubt as to the true nature of the Carboniferous forests; for the structural character of the trees is as strongly marked in their fossil remains as in any living plants of the same character. We distinguish the Ferns not only by the peculiar form of their leaves, often perfectly preserved, but also by the fructification on the lower surface of the leaves, and by the distinct marks made on the stem at their point of juncture with it. The leaf of the Fern, when falling, leaves a scar on the stem varying in shape and size according to the kind of Fern, so that the botanist readily distinguishes any particular species of Fern by this means,--a birth-mark, as it were, by which he detects the parentage of the individual. Another indication, equally significant, is found in the tubular structure of the wood in Ferns. On a vertical section of any well-preserved Fern-trunk from the old forests the little tubes may be seen very distinctly running up its length; or, if it be cut through transversely, they may be traced by the little pores like dots on the surface. Trees of this description are found in the Carboniferous marshes, standing erect and perfectly preserved, with trunks a foot and a half in diameter, rising to a height of many feet. Plants so strongly bituminous as the Ferns, when they equalled in size many of our present forest-trees, naturally made coal deposits of the most combustible quality. It is true that we find the anthracite coal of the same period with comparatively little bituminous matter; but this is where the bitumen has been destroyed by the action of the internal heat of the earth. Next to the Ferns, the Club-Mosses (_Lycopodiacae_) seem to have contributed most largely to the marsh-forests. They were characterized, then, as now, by the small size of the leaves growing close against the stem, so that the stem itself, though covered with leaves, looks almost naked, like the stem of the Cactus. Beside these, there are the tree-like Equiseta, in which we find the articulations on the trunk corresponding exactly to those now so characteristic of those marsh-grasses which are the modern representatives of this family of plants, with cone-like fructifications on the summit of the stem.