[141] Mazonia Acadica, and a second species of Mazonia, with fragments of a third species, generally distinct. Proceedings Royal Society of London, 1892.
In the previous paper we have considered the mode of accumulation of Coal, and it may be useful here to note the light thrown on this subject by the Air-breathers of the coal formation and their mode of occurrence.
In no part of the world are the coal measures better developed, or more fully exposed, than in the coast sections of Nova Scotia and Cape Breton; and in these, throughout their whole thickness, no indication has been found of any of the marine fossils of the Lower Carboniferous Limestone. Abundant remains of fishes occur, but these may have frequented estuaries, streams and ponds, and the greater part of them are small ganoids which, like the modern Lepidosteus and Amia, may have been specially fitted by their semi-reptilian respiration, for the impure waters of swampy districts. Bivalve mollusks also abound; but these are all of the kinds to which I have given the generic name Naiadites, and Mr. Salter those of Anthracomya and Anthracoptera. These shells are all distinct from any known in the marine limestones. Their thin edentulous valves, their structure consisting of a wrinkled epidermis, a thin layer of prismatic shell and an inner layer of imperfectly pearly shell, all remind us of the Anodons and Unios. A slight notch in front concurs with their mode of occurrence in rendering it probable that, like mussels in modern estuaries, they attached themselves to floating or sunken timber. They are thus removed, both in structure and habit, from truly marine species; and may have been fresh-water or brackish-water mussels closely allied to modern Unios.
Carboniferous Millipedes, Xylobius Sigillariæ, Dawson (a, c), and Archiulus xylobioides, Scudder (b).
The crustaceans (Eurypterus, Diplostylus, Cyprids), and the worm shell (Spirorbis) found with them, are not necessarily marine, though some of them belonged probably to brackish water, and they have not yet been found in those carboniferous beds deposited in the open sea. There is thus in the whole thickness of the middle coal measures of Nova Scotia a remarkable absence at least of open sea animals; and if, as is quite probable, the sea inundated at intervals the areas of coal accumulation, the waters must have been shallow, and to a great extent land-locked, so that brackish-water rather than marine animals inhabited them.
On the other hand, there are in these coal measures abundant evidences of land surfaces; and subaërial decay of vegetable matter in large quantity is proved by the occurrence of the mineral charcoal of the coal itself, as I have elsewhere shown.[142] The erect trees which occur at so many levels also imply subaërial decay. A tree imbedded in sediment and remaining under water, could not decay so as to become hollow and deposit the remains of its wood in the state of mineral charcoal within the hollow bark. Yet this is the case with the greater part of the erect Sigillariæ which occur at more than twenty levels in the Joggins section. Nor could such hollow trunks become repositories for millipedes, snails and reptiles, if under water. On the other hand, if, as seems necessary to explain the character of the reptiliferous erect trees, these remained dry, or nearly so, in the interior, this would imply not merely a soil out of water, but comparatively well drained; as would indeed always be the case, when a flat resting on a sandy subsoil was raised several feet above the level of the water. Further, though the peculiar character of the roots of Sigillariæ and Calamites may lend some countenance to the supposition that they could grow under water, or in water-soaked soils, this will not apply to coniferous trees, to ferns, and other plants, which are found under circumstances which show that they grew with the Sigillariæ.
[142] Journal of Geological Survey, vol. xv.
In the coal measures of Nova Scotia, therefore, while marine conditions are absent, there are ample evidences of fresh-water or brackish-water conditions, and of land surfaces, suitable for the air-breathing animals of the period. Nor do I believe that the coal measures of Nova Scotia were exceptional in this respect. It is true that in Great Britain evidences of marine life do occur in the coal measures; but not, so far as I am aware, in circumstances which justify the inference that the coal is of marine origin. Alternations of marine and land remains, and even mixtures of these, are frequent in modern submarine forests. When we find, as at Fort Lawrence in Nova Scotia, a modern forest rooted in upland soil forty feet below high-water mark,[143] and covered with mud containing living Tellinas and Myas, we are not justified in inferring that this forest grew in the sea. We rather infer that subsidence has occurred. In modern salt marshes it is not unusual to find every little runnel or pool full of marine shell fish, while in the higher parts of the marsh land plants are growing; and in such places the deposit formed must contain a mixture of land plants and marine animals with salt grasses and herbage—the whole in situ.[144]