Some Salient Points in the Science of the Earth

by Sir John William Dawson

[143] Journal of Geological Society, vol. xi.

[144] In the marshes at the mouth of Scarborough River, in Maine, channels not more than a foot wide, and far from the sea, are full of Mussels and Myæ; and in little pools communicating with these channels there are often many young Limuli, which seem to prefer such places, and the cast-off shells and other remains of which may become imbedded in mud and mixed with land plants, just as in the shales of the coal measures.

These considerations serve, I think, to explain all the apparently anomalous associations of coal plants with marine fossils; and I do not know any other arguments of apparent weight that can be adduced in favour of the marine or even aquatic origin of coal, except such as are based on misconceptions of the structure and mode of growth of sigillaroid trees and of the stratigraphical relations of the coal itself.[145] It is to be observed, however, that while I must maintain the essentially terrestrial character of the ordinary coal and of its plants, I have elsewhere admitted that cannel coals and earthy bitumen present evidences of subaquatic deposition; and have also abundantly illustrated the facts that the coal plants grew on swampy flats, liable not only to river inundations, but also to subsidence and submergence.[146] In the oscillation of these conditions it is evident that Sigillariæ and their contemporaries must often have been placed in conditions unfavourable or fatal to them, and when their remains are preserved to us in these conditions, we may form very incorrect inferences as to their mode of life. Further, it is to be observed that the conditions of submergence and silting up which were favourable to the preservation of specimens of Sigillariæ as fossils, must have been precisely those which were destructive to them as living plants; and on the contrary, that the conditions in which these forests may have flourished for centuries must have been those in which there was little chance of their remains being preserved to us, in any other condition at least than that of coal, which reveals only to careful microscopic examination the circumstances, whether aërial or aquatic, under which it was formed.

[145] It is unfortunate that few writers on this subject have combined with the knowledge of the geological features of the coal a sufficient acquaintance with the phenomena of modern marshes and swamps, and with the conditions necessary for the growth of plants such as those of the coal. It would be easy to show, were this a proper place to do so, that the "swells," "rock faults," splitting of beds, and other appearances of coal seams quite accord with the theory of swamp accumulation; that the plants associated with Sigillariæ could not have lived with their roots immersed in salt water; that the chemical character of the underclays implies drainage and other conditions impossible under the sea; that the composition and minute structure of the coal are incompatible with the supposition that it is a deposit from water, and especially from salt water; and that it would be more natural to invoke wind driftage as a mode of accumulation for some of the sandstones, than water driftage for the formation of the coal. At the same time it is pretty certain that such beds as the cannels and earthy bitumens which appear to consist of finely comminuted vegetable matter, without mineral charcoal, may have been deposits of muck in shallow lakes or lagoons.

[146] Journal of Geol. Socy., vols. x. and xv., and "Acadian Geology."

It is also noticeable that, in conditions such as those of the coal formation, it would be likely that some plants would be specially adapted to occupy newly emerged flats and places liable to inundation and silting up. I believe that many of the Sigillariæ, and still more eminently the Calamites, were suitable to such stations. There is direct evidence that the nuts named Trigonocarpa were drifted extensively by water over submerged flats of mud. Many Cardiocarpa were winged seeds which may have drifted in the air. The Calamites may, like modern Equiseta, have produced spores with elaters capable of floating them in the wind. One of the thinner coals at the Joggins is filled with spores or spore cases that seem to have carried hairs on their surfaces, and may have been suited to such a mode of dissemination. I have elsewhere proved[147] that at least some species of Calamites were, by their mode of growth, admirably fitted for growing amid accumulating sediment, and for promoting its accumulation.

[147] "Acadian Geology," chapter on Coal Plants.

The reptiles of the coal formation are probably the oldest known to us, and possibly, though this we cannot affirm, the highest products of creation in this period. Supposing, for the moment, that they are the highest animals of their time, and, what is perhaps less likely, that those which we know are a fair average of the rest, we have the curious fact that they are all carnivorous, and the greater part of them fitted to find food in the water as well as on the land. The plant feeders of the period, on the land at least, are all invertebrates, as snails, millipedes, and perhaps insects. The air-breathing vertebrates are not intended to consume the exuberant vegetable growth, but to check the increase of its animal enemies. Plant life would thus seem to have had in every way the advantage. The millipedes probably fed only on roots and decaying substances, the snails on the more juicy and succulent plants growing in the shadow of the woods, and the great predominance of the family of cockroaches among carboniferous insects points to similar conclusions as to that class. While, moreover, the vegetation of the coal swamps was most abundant, it was not, on the whole, of a character to lead us to suppose that it supported many animals. Our knowledge of the flora of the coal swamps is sufficiently complete to exclude from them any abundance of the higher phænogamous plants. We know little, it is true, of the flora of the uplands of the period; but when we speak of the coal-formation land, it is to the flats only that we refer. The foliage of the plants on these flats with the exception of that of the ferns, was harsh and meagre, and there seem to have been no grasses or other nutritious herbaceous plants. These are wants of themselves likely to exclude many of the higher forms of herbivorous life. On the other hand, there was a profusion of large nut-like seeds, which in a modern forest would probably have afforded subsistence to squirrels and similar animals. The pith and thick soft bark of many of the trees must at certain seasons have contained much nutritive matter, while there was certainly sufficient material for all those insects whose larvæ feed on living and dead timber, as well as for the creatures that in turn prey on them. It is remarkable that there seem to have been no vertebrate animals fitted to avail themselves of these vast stores of food. The question: "What may have fed on all this vegetation?" was never absent from my mind in all my explorations of the Nova Scotia coal sections; but no trace of any creature other than those already mentioned has ever rewarded my search. In Nova Scotia it would seem that a few snails, gally-worms, and insects were the sole links of connection between the plant creation and air-breathing vertebrates. Is this due to the paucity of the fauna, or the imperfection of the record? The fact that a few erect stumps have revealed nearly all the air-breathers yet found, argues strongly for the latter cause; but there are some facts bearing on the other side.

A gally-worm, if, like its modern relatives, hiding in crevices of wood in forests, was one of the least likely animals to be found in aqueous deposits. The erect trees gave it its almost sole chance of preservation. Pupa vetusta is a small species, and its shell very thin and fragile, while it probably lived among thick vegetation. Further, the measures 2,000 feet thick, separating the lowest and highest beds in which it occurs, include twenty-one coal seams, having an aggregate thickness of about twenty feet, three beds of bituminous limestone of animal origin, and perhaps twenty beds holding Stigmaria in situ, or erect Sigillariæ and Calamites. The lapse of time implied by this succession of beds, many of them necessarily of very slow deposition, must be very great, though it would be mere guess work to attempt to resolve it into years. Yet long though this interval must have been, Pupa vetusta lasted without one iota of change through it all; and, more remarkable still, was not accompanied by more than two other species of its family. Where so many specimens occur, and in situations so diverse, without any additional species, the inference is strong that no other of similar habits existed. If in any of those subtropical islands, whose climate and productions somewhat resemble those of the coal period, after searching in and about decaying trees, and also on the bars upon which rivers and lakes drifted their burdens of shells, we should find only three species, but one of these in very great numbers, we would surely conclude that other species, if present, were very rare.

Again, footprints referable to Dendrerpeton, or similar animals, occur in the lower Carboniferous beds below the marine limestones, in the middle coal measures, and in the upper coal formation, separated by a thickness of beds which may be estimated at 15,000 feet, and certainly representing a vast lapse of time. Did we know the creature by these impressions alone, we might infer its continued existence for all this great length of time; but when we also find its bones in the principal repositories of reptile remains, and in company with the other creatures found with it, we satisfy ourselves that of them all it was the most likely to have left its trail in the mud flats. We thus have reason to conclude that it existed alone during this period, in so far as its especial kind of habitat was concerned; though there lived with it other reptiles, some of which, haunting principally the woods, and others the water, were less likely to leave impressions of their footprints. These may be but slight indications of truth, but they convey strong impressions of the persistence of species, and also of the paucity of species belonging to these tribes at the time.