(1) The occurrence of Stigmaria under nearly every bed of coal proves, beyond question, that the material was accumulated by growth in situ, while the character of the sediments intervening between the beds of coal proves with equal certainty the abundant transport of mud and sand by water. In other words, conditions similar to those of the swampy deltas of great rivers, or the swampy flats of the interiors of great continents, are implied.
(2) The true coal consists principally of the flattened bark of sigillaroid and other trees, intermixed with leaves of ferns and Cordaites, and other herbaceous débris, including vast numbers of spores and spore cases, and with fragments of decayed wood constituting "mineral charcoal," all their materials having manifestly alike grown and accumulated where we find them.
(3) The microscopical structure and chemical composition of the beds of cannel coal and earthy bitumen, and of the more highly bituminous and carbonaceous shales, show them to have been of the nature of the fine vegetable mud which accumulates in the ponds and shallow lakes of modern swamps. These beds are always distinct from true subaërial coal. When such fine vegetable sediment is mixed, as is often the case, with mud, it becomes similar to the bituminous limestone and calcareo-bituminous shales of the coal measures.
(4) A few of the underclays which support beds of coal are of the nature of the vegetable mud above referred to; but the greater part are argillo-arenaceous in composition, with little vegetable matter, and bleached by the drainage from them of water containing the products of vegetable decay. They are, in short, loamy or clay soils in the chemical condition in which we find such soils under modern bogs, and must have been sufficiently above water to admit of drainage. The absence, or small quantity of sulphides, and the occurrence of carbonate of iron in connection with them, prove that when they existed as soils, rain water, and not sea water, percolated them.
(5) The coal and the fossil trees present many evidences of subaërial conditions. Most of the erect and prostrate trees had become hollow shells of bark before they were finally imbedded, and their wood had broken into cubical pieces of mineral charcoal. Land snails and galley worms (Xylobius) crept into them, and they became dens or traps for reptiles. Large quantities of mineral charcoal occur on the surfaces of all the larger beds of coal. None of these appearances could have been produced by subaqueous action.
(6) Though the roots of Sigillaria bear some resemblance to the rhizomes of certain aquatic plants, yet structurally they have much resemblance to the roots of Cycads, which the stems also resemble. Further, the Sigillariæ grew on the same soils which supported conifers, Lepidodendra, Cordaites, and ferns, plants which could not have grown in water. Again, with the exception, perhaps, of some Pinnulariæ and Asterophyllites, and Rhizocarpean spores, there is a remarkable absence from the coal measures of any form of properly aquatic vegetation.
(7) The occasional occurrence of marine or brackish-water animals in the roofs of coal beds, or even in the coal itself, affords no evidence of subaqueous accumulation, since the same thing occurs in the case of modern submarine forests. Such facts merely imply that portions of the areas of coal accumulation were liable to inundation of a character so temporary as not finally to close the process, as happened when at last a roof shale was deposited by water over the coal. Cannel coals and bituminous shales holding mussel-like shells, fish scales, etc., imply the existence sometimes for long periods of ponds, lakes or lagoons in the coal swamps, but ordinary coal did not accumulate in these. It is in the cannels and similar subaqueous coals that the macrospores which I attribute in great part to aquatic plants, allied to modern Salvinia, etc., are chiefly found.[115]
[115] "Geological History of Plants," Bulletin Chicago Academy of Sciences, 1886.
For these and other reasons, some of which are more fully stated in the papers referred to, while I admit that the areas of coal accumulation were frequently submerged, I must maintain that the true coal is a subaërial accumulation by vegetable growth on soils wet and swampy, it is true, but not submerged. I would add the further consideration, already urged elsewhere, that in the case of the fossil forests associated with the coal, the conditions of submergence and silting-up which have preserved the trees as fossils, must have been precisely those which were fatal to their existence as living plants, a fact sufficiently evident to us in the case of modern submarine forests, but often overlooked by the framers of theories of the accumulation of coal.
It seems strange that the occasional inequalities of the floors of the coal beds, the sand or gravel ridges which traverse them, the channels cut through the coal, the occurrence of patches of sand, and the insertion of wedges of such material splitting the beds, have been regarded by some able geologists as evidences of the aqueous origin of coal. In truth, these appearances are of constant occurrence in modern swamps and marshes, more especially near their margins, or where they are exposed to the effects of ocean storms or river inundations. The lamination of the coal has also been adduced as a proof of aqueous deposition; but the miscroscope shows, as I have elsewhere pointed out, that this is entirely different from aqueous lamination, and depends on the superposition of successive generations of more or less decayed trunks of trees and beds of leaves. The lamination in the truly aqueous cannels and carbonaceous shales is of a very different character.