CHAPTER XIV.
On the Formation of Coal.
Coal is universally allowed to be of vegetable origin; it is a mass of the débris of trees and plants, that having been buried under mud and sand, has been subsequently converted into coal, the muds and sands being similarly converted into shales, and clays, and sandstones. There are, however, two opinions as to how the vegetables got into the situation we now find them in. The first, and at one time the more general opinion was, that trees and plants were drifted into large lakes, estuaries, and shallow seas, and there becoming water-logged, sank to the bottom, and were subsequently covered up there by the other accumulations. The second opinion, and perhaps the most generally entertained now, is, that the plants entering into the composition of the coal were not drifted, but grew and perished on the very spot we now find them forming coal, just as our own peat bogs at the present day would form coal if buried for a vast series of years under a great accumulation of earthy matters.
Botanists tell us that all the plants entering into the composition of coal, so far as they have been able to trace and verify any part of their structure, appear to have been not aquatic but terrestrial plants. For the formation of the beds of coal, therefore, in situ, it is necessary to suppose that the water in which the shales and sandstones were deposited became filled up, and the space converted into dry land, or, at all events, into a marsh at or above the level of the water; that on this dry land or marsh, plants accumulated in sufficient quantity to form a bed of coal; that then a depression took place, and the space became again covered by water, in which more shale and sandstone materials accumulated, again filling it up to the level of the water, and then another marsh, and so on.
I by no means intend to range myself among the advocates of either one or the other opinion, but I think there are certain difficulties in the way of the latter, which in spite of all the evidence as to the roots[1] and upright stems of trees, (or whatever the plants called Sigillaria, Stigmaria, &c, may have been,) would make me hesitate to embrace it exclusively. Some of these difficulties arise from facts observed in the South Staffordshire coal-field, and these it is now proposed to lay before the reader.
1st. The "rolls," "swells," or "horse's backs."—We have seen, page 52, Fig. 9, that these are long ridge-like accumulations
- ↑ Among other evidences is the case described by my friend Mr. Beckett, of Wolverhampton, in the first vol. of the Geological Journal. I afterwards accompanied him to the open work at Parkfields, where these stumps of plants occurred in the coal. They certainly looked as if they had grown there, and perhaps they may have done so, but even so, it by no means settles the whole question.
