In the groups of extensive islands enjoying a subtropical climate.* paucity of the gramineæ or grasses, which form so large a proportion of the existing floras, and the predominance of ferns, the vegetation of the coal-measures approached in some respects that of New Zealand, in which the cellulosæ form one-third of the whole, while the grasses are very few in number.† From the researches of Elie de Beaumont ‡ and Godwin-Austen,§ who have constructed approximative charts of the carboniferous area of Western Europe, we know that the coal-measures and the mountain-limestone were deposited for the most part amongst an extensive archipelago, of which some districts of Scandinavia, Scotland, Ireland, Wales, Cornwall, France, and Northern Italy formed parts. || Considerable, but isolated, areas of coaldeposits were formed in lakes on the table-land of what is now Central France; and, according to Mr. Godwin-Austen, equally extensive lakesystems gave origin to the old-red sandstones and conglomerates ¶ of Scotland and Ireland, and probably of that of South Wales also 41. ATMOSPHERIC CONDITIONS DURING THE CARBONIFEROUS EPOCH. It appears remarkable that amidst the luxariant vegetation which prevailed on the dry lands during * See chap. vii. of Lyell's "Principles of Geology," for the full consideration of probable climatal conditions with given proportions of land and sea. + See Medals of Creation, vol. i. p. 210; and Lyell's "Principles," See Beudant's Géologie. p. 87. A very important feature of this old archipelago was a narrow tract of land, or of shoals, reaching from the Franco-Belgian frontier to Somersetshire and the south of Ireland; now recognised chiefly in the Ardennes, on the east, and the Mendips on the west, and by the presence of the elevated and denuded valley of the Weald, and by other geological features along its line. As the coal-measures and the coalbearing mountain-limestone lie against the flanks of this old transverse ridge, and as it is not covered up by the whole series of the secondary strata, but only by the cretaceous and tertiary series, it is considered probable that coal-beds might be found at much less than 2000 feet below the surface in the neighbourhood of London; especially as coal has already been met with in a deep boring at Calais, on the same line of old rocks. At Harwich, a deep boring has reached this subterranean ridge at about 1000 feet in depth; but it is there composed apparently of rocks older than those of the coal-series. ¶ Quart. Journ. Geol. Soc. vol. xii. p. 51, &c. the Carboniferous epoch, there should not have existed contemporaneous herbivorous quadrupeds; but not a relic of any animal of this kind has been discovered in the coal-strata. Indeed, with the exception of the Iguanodon of the Wealden (see p. 436), no remains of large vegetable feeders * have been found in any of the deposits anterior to the eocene, in which first appear relics of the herbivorous pachyderms. The coalmeasures of Nova Scotia alone present vestiges of phytophagous terrestrial mollusca. It was an opinion once very generally entertained, and the idea still seems to find favour, that previously to and during the Carboniferous period the atmosphere was so charged with carbonic-acid gas as to be unfitted for the respiration of animals of a higher order than reptiles; and that the dense and luxuriant vegetation of that epoch was designed to purify the air, by elaborating coal, and thus abstracting the superabundance of irrespirable gas, and setting free a corresponding proportion of oxygen thus rendering the surface of the earth suitable for the existence of terrestrial reptiles, and ultimately of birds and mammalia. But Sir C. Lyell has argued that, so far as we know, if any change were induced in the constitution of the atmosphere by such an agency, it would be the reverse of that assumed; for an excess of vegetation would tend to diminish the average amount of carbonic acid, and consequently the air must have been purer than in the succeeding epochs.† It is therefore probable that the absence of herbivorous animals cannot be explained by unfavourable atmospheric conditions: and again we may point to New Zealand as a country having a luxuriant vegetation, yet without herbivorous quadrupeds.‡ 42. FORMATION OF COAL-MEASURES. From the facts which have passed under our examination,§ we may now advantageously consider what were the circumstances which gave rise to these prodigious layers of carbonized matter *The Plagiaulax of Purbeck and the Stereognathus of Stonesfield were smaller herbivores. The other known mammals of the secondary period were probably insectivorous. + Lyell's "Travels in North America, vol. i. p. 152; and "Principles," p. 248. The only indigenous mammals in New Zealand are one species of Rat and another of Bat; and there are no large reptiles whatever. § See especially p. 681, &c. unmixed with other materials,—these immense beds of vegetables, from which animal remains are often almost wholly excluded; and whether accumulations of trees and plants, which in after-ages shall present phenomena of a like nature, are in progress at the present time? The manner in which the carboniferous strata have been deposited has been a fruitful source of discussion among geologists. Some have contended that the coal-measures were originally peat-bogs, and that the successive layers were occasioned by repeated subsidences of the land; -others, that the vegetable matter originated from rafts, or masses of drifted forest-trees, like those of the Mississippi,* which floated out to sea, and there became ingulfed ;-others suppose that they were formed in inland seas or lakes, the materials of the successive beds being brought down by periodical land-floods: and the supporters of each hypothesis adduce numerous facts in corroboration of their respective opinions. There can be no doubt that coal may be, and has been, formed under each of these conditions; and that at different periods, and in different localities, all these causes have been in operation; in some instances singly, and in others in combination. An interesting example of the latter conditions is described by Prof. Dawson, in the Canadian Naturalist (October, 1857), as occurring at Pictou, where some portions of the coal-measures present appearances suggestive of " the idea of patches of grey sand rising from a bottom of red mud, with clumps of growing Calamites, which arrested quantities of drift-plants, consisting principally of Sternbergia and fragments of much-decayed wood and bark, now in the state of coaly matter too much penetrated by iron-pyrites to show its structure distinctly. We thus probably have the fresh-growing Calamites, entombed along with the debris of the old decaying Conifers of some neighbouring shore; furnishing an illustration of the truth, that the most ephemeral and perishable forms may be fossilized and preserved contemporaneously with the decay of the most durable tissue. The Rush of a single summer may be preserved with its minutest striæ unharmed, when the giant Pine of centuries has crumbled into mould. It is so now, and it was so equally in the Carboniferous period." That some of the isolated basins of coal may be carbonized peat-bogs is not improbable, considering that peat often occurs in beds including trees in an erect position, and extending over extensive tracts of country; *On this river and its branches accumulations of timber, forming rafts 10 miles long, are said to have occurred.-Lyell's " Principles," p. 267. and in modern peat-bogs (see p. 66) layers are occasionally found having the conchoidal fracture and lustrous appearance of coal. Other coal-measures may have been accumulated in fresh-v -water lakes, in estuaries, or in lagoons of fresh or of brackish water, into which the sea made occasional irruptions. To this class would belong the greater proportion of the known coal-measures. Other carboniferous strata, intercalated with sandstones and limestones containing marine remains in abundance, like those of Russia (see p. 684), must have been deposited in the sea. The occasional erect position of the stems and the preservation of delicate leaves do not invalidate this inference; for in the rafts formed by the entangled floating forests of the American rivers, trunks of trees frequently occur upright; and in the interior of these rafts grasses and tender plants are often found entire. Such masses, therefore, might be drifted many hundred miles, and yet the imbedded fragile species, protected by the external network of entangled branches, remain uninjured; and, undergoing bituminization, while enveloped by the soft mud permeating the mass, might become changed into durable forms, like those which abound in the natural herbaria of the coal-measures. The verticality of tree-trunks in some of the strata of the carboniferous series-as, for instance, in the Sandstone of Cragleith, near Edinburghmay have originated simply from the trees having floated, and been ultimately moored, with their stems upright, or nearly so, in consequence of their roots being loaded with soil. This is a constant occurrence in the great floods of the American rivers; the snags, as they are termed, which often render the navigation up the stream difficult and perilous, being formed by drifted trees fixed by their stumps in the river-bottom, and forced into an oblique direction by the current. Trees in an upright position are often carried out to sea, and have been seen far from land, floating with their topmost branches above the water. Under these circumstances they have been instrumental (like sea-weeds and icebergs, in different degrees) in carrying pebbles and blocks of stone from the land into the deep abysses of the ocean, or to far distant coasts 43. COAL-MEASURES ORIGINATING IN SUBMERGED LANDS. -The theory, so ably advocated by Sir C. Lyell and other eminent geologists, of the formation of coal-measures from repeated submergences and elevations of lands covered with dense forests (see p. 682, 684), seems to be especially applicable to those carboniferous formations which are made up of regular alternations of coal with beds composed of such earthy materials as render it probable they were once capable of supporting a luxuriant vegetation. And the fact that the roots (Stigmaria) of the forest-trees (Sigillaria) of the carboniferous period are generally present, and for the most part in their natural position, in the underclay beneath each bed of pure coal, proves that those trees grew on the areas now occupied by their carbonized remains. Without, however, having recourse to the hypothesis of alternate subsidence and elevation of the land, may we not suppose that there were extensive inland areas, * depressed, like the basin of the Caspian, many hundred feet below the level of the sea, affording the shelter, warmth, and moisture required by a subtropical flora, and subjected to * The most remarkable known instance of an area of land depressed far below the sea-level is that mentioned by Humboldt (Cosmos). In descending eastward from Jerusalem to the Dead Sea and the Valley of Jordan, a view is enjoyed which, according to our present hypsometric knowledge of the earth's surface, has no parallel in any other region. The rocks on which the traveller treads, with the open sky over his head, are 1312 feet below the level of the Mediterranean. Though from the revolutions which have swept over the earth's surface, and the displacements and mutations which its crust has undergone since the carboniferous period, there is but little probability that any of the coal-fields are now in the same position in relation to the sea-level as at the period of their formation, I would, nevertheless, direct attention to the following statements and remarks of the illustrious philosopher just cited:-The depth of the Coal-basin at Liege is estimated by Von Dechen at 3809 feet below the surface, and 3464 feet beneath the level of the sea; and that of Mons, at 5329: while the lowermost coal-strata of the Saar-Rivier (Saarbrück) are computed by the same eminent observer to descend to a depth of 21,358 feet below the sea-level, or 3.6 geographical miles. This is a depth below the sea equal to that of Chimborazo above it; and the temperature would be 467° of Fahrenheit if the increase be in the supposed ratio of 1° for every 54 feet of vertical depth. We have, therefore, from the highest summits of the Himalayahs to the lowest portions of the basins which contain the fossil flora of the carboniferous epoch, a vertical distance of about 48,000 feet, or 5th of the earth's semi-diameter. |