In South Wales the coal-measures have been ascertained by actual measurement to attain the extraordinary thickness of 12,000 feet, the beds throughout, with the exception of the coal itself, appearing to have been formed in water of moderate depth, during a slow but perhaps intermittent depression of the ground, in a region to which rivers were bringing a never-failing supply of muddy sediment and sand. The same area was sometimes covered with vast forests, such as we see in the deltas of great rivers in warm climates, which are liable to be submerged beneath fresh or salt water should the ground sink vertically a few feet.
In one section near Swansea, in South Wales, where the total thickness of strata is 3246 feet, we learn from Sir H. De la Beche that there are ten principal masses of sandstone. One of these is 500 feet thick, and the whole of them make together a thickness of 2125 feet. They are separated by masses of shale, varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in number, are generally from 1 to 5 feet thick, one of them, which has two or three layers of clay interposed, attaining 9 feet.[309-B] At other points in the same coal-field the shales predominate over the sandstones. The horizontal extent of some seams of coal is much greater than that of others, but they all present one characteristic feature, in having, each of them, what is called its underclay. These underclays, co-extensive with every layer of coal, consist of arenaceous shale, sometimes called firestone, because it can be made into bricks which stand the fire of a furnace. They vary in thickness from 6 inches to more than 10 feet; and Mr. Logan first announced to the scientific world in 1841 that they were regarded by the colliers in South Wales as an essential accompaniment of each of the one hundred seams of coal met with in their coal-field. They are said to form the floor on which the coal rests; and some of them have a slight admixture of carbonaceous matter, while others are quite blackened by it.
All of them, as Mr. Logan pointed out, are characterized by inclosing a peculiar species of fossil vegetable called Stigmaria, to the exclusion of other plants. It was also observed that, while in the overlying shales or "roof" of the coal, ferns and trunks of trees abound without any Stigmariæ, and are flattened and compressed, those singular plants in the underclays always retain their natural forms, branching freely, and sending out their slender leaves, as they were formerly styled, through the mud in all directions. Several species of Stigmaria had long been known to botanists, and described by them, before their position under each seam of coal was pointed out. It was conjectured that they might be aquatic, perhaps floating plants, which sometimes extended their branches and leaves freely in fluid mud, and which were finally enveloped in the same mud.
CARBONIFEROUS FLORA.
These statements will suffice to convince the reader that we cannot arrive at a satisfactory theory of the origin of coal till we understand the true nature of Stigmaria; and in order to explain what is now known of this plant, and of others which have contributed by their decay to produce coal, it will be necessary to offer a brief preliminary sketch of the whole carboniferous flora, an assemblage of fossil plants, with which we are better acquainted than with any other which flourished antecedently to the tertiary epoch. It should also be remarked that Göppert has ascertained that the remains of every family of plants scattered through the coal-measures are sometimes met with in the pure coal itself, a fact which adds greatly to the geological interest attached to this flora.
Ferns.—The number of species of carboniferous plants hitherto described amounts, according to M. Ad. Brongniart, to about 500. These may perhaps be a fragment only of the entire flora, but they are enough to show that the state of the vegetable world was then extremely different from that now established. We are struck at the first glance with the similarity of many of the ferns to those now living, and the dissimilarity of almost all the other fossils except the coniferæ. Among the ferns, as in the case of Pecopteris for example ([fig. 351.]), it is not always easy to decide whether they should be referred to different genera from those established for the classification of living species; whereas, in regard to most of the other contemporary tribes, with the exception of the coniferæ, it is often difficult to guess the family, or even the class, to which they belong. The ferns of the carboniferous period are generally without organs of fructification, but in some specimens these are well preserved. In the general absence of such characters, they have been divided into genera, distinguished chiefly by the branching of the fronds, and the way in which the veins of the leaves are disposed. The larger portion are supposed to have been of the size of ordinary European ferns, but some were decidedly arborescent, especially the group called Caulopteris, by Lindley, and the Psaronius of the upper or newest coal-measures, before alluded to ([p. 307.]).
Fig. 351.
Pecopteris lonchitica. (Foss. Flo. 153.)