A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

We have seen that, in the Appalachian coal-field, there is an intimate connection between the extent to which the coal has parted with its gaseous contents, and the amount of disturbance which the strata have undergone. The coincidence of these phenomena may be attributed partly to the greater facility afforded for the escape of volatile matter, where the fracturing of the rocks had produced an infinite number of cracks and crevices, and also to the heat of the gases and water penetrating these cracks, when the great movements took place, which have rent and folded the Appalachian strata. It is well known that, at the present period, thermal waters and hot vapours burst out from the earth during earthquakes, and these would not fail to promote the disengagement of volatile matter from the carboniferous rocks.

Continuity of seams of coal.—As single seams of coal are continuous over very wide areas, it has been asked, how forests could have prevailed uninterruptedly over such wide spaces, without being oftener flooded by turbid rivers, or, when submerged, denuded by marine currents. It appears, from the description of the Cape Breton coal-field, by Mr. Richard Brown, that false stratification is common in the beds of sand, and some partial denudation of these, at least, must often have taken place during the accumulation of the carboniferous series.

In the Forest of Dean, ancient river-channels are found, which pass through beds of coal, and in which rounded pebbles of coal occur. They are of older date than the overlying and undisturbed coal-measures. The late Mr. Buddle, who described them to me, told me he had seen similar phenomena in the Newcastle coal-field. Nevertheless, instances of these channels are much more rare than we might have anticipated, especially when we remember how often the roots of trees (Stigmariæ) have been torn up, and drifted in broken fragments into the grits and sandstones. The prevalence of a downward movement is, no doubt, the principal cause which has saved so many extensive seams of coal from destruction by fluviatile action.

The purity of the coal, or its non-intermixture with earthy matter, presents another theoretical difficulty to many geologists, who are inclined to believe that the trees and smaller plants of the carboniferous period grew in extensive swamps, rather than on land not liable to be inundated. It appears, however, that in the alluvial plain and delta of the Mississippi, extensive "cypress swamps," as they are called, densely covered with various trees, occur, into which no matter held in mechanical suspension is ever introduced during the greatest inundations, inasmuch as they are all surrounded by a dense marginal belt of reeds, canes, and brushwood. Through this thick barrier the river-water must pass, so that it is invariably well filtered before it can reach the interior of the forest-covered area, within which, vegetable matter is continually accumulating from the decay of trees and semi-aquatic plants. In proof of this, I may observe, that whenever any part of a swamp is dried up, during an unusually hot season, and the wood set on fire, pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left.[334-A] At the bottom of these "cypress swamps" of the Mississippi, a bed of clay is found, with roots of the tall cypress (Taxodium distichum), just as the underclays of the coal are filled with Stigmaria.

Climate of Coal Period.—So long as the botanist taught that a tropical climate was implied by the carboniferous flora, geologists might well be at a loss to reconcile the preservation of so much vegetable matter with a high temperature; for heat hastens the decomposition of fallen leaves and trunks of trees, whether in the atmosphere or in water.[335-A] It is well known that peat, so abundant in the bogs of high latitudes, ceases to grow in the swamps of warmer regions. It seems, however, to have become a more and more received opinion, that the coal-plants do not, on the whole, indicate a climate resembling that now enjoyed in the equatorial zone. Tree-ferns range as far south as the southern part of New Zealand, and Araucarian pines occur in Norfolk Island. A great predominance of ferns and lycopodiums indicates warmth, moisture, equability of temperature, and freedom from frost, rather than intense heat; and we know too little of the sigillariæ, calamites, asterophyllites, and other peculiar forms of the carboniferous period, to be able to speculate with confidence on the kind of climate they may have required.

No doubt, we are entitled to presume, from the corals and cephalopoda of the mountain limestone, that a warm temperature characterized the northern seas in the carboniferous era; but the absence of cold may have given rise (as at present in the seas of the Bermudas, under the influence of the gulf stream) to a very wide geographical range of stone-building corals and shell-bearing cuttle-fish, without its being necessary to call in the aid of tropical heat.[335-B]

CARBONIFEROUS REPTILES.

Where we have evidence in a single coal-field, as in that of Nova Scotia, or South Wales, of fifty or even a hundred ancient forests buried one above the other, with the roots of trees still in their original position, and with some of the trunks still remaining erect, we are apt to wonder that until the year 1844 no remains of contemporaneous air-breathing creatures, except a few insects, had been discovered. No vertebrated animals more highly organized than fish, no mammalia or birds, no saurians, frogs, tortoises, or snakes, were yet known in rocks of such high antiquity. In the coal-field of Coalbrook Dale mention had been made of two species of beetles of the family Curculionidæ, and of a neuropterous insect resembling the genus Corydalis, with another related to the Phasmidæ.[335-C] In other coal-measures in Europe we find notice of a scorpion and of a moth allied to Tinea, also of one air-breathing crustacean, or land-crab. Yet Agassiz had already described in his great work on fossil fishes more than one hundred and fifty species of ichthyolites from the coal strata, ninety-four belonging to the families of shark and ray, and fifty-eight to the class of ganoids. Some of these fish are very remote in their organization from any now living, especially those of the family called Sauroid by Agassiz; as Megalichthys, Holoptychius, and others, which are often of great size, and all predaceous. Their osteology, says M. Agassiz, reminds us in many respects of the skeletons of saurian reptiles, both by the close sutures of the bones of the skull, their large conical teeth striated longitudinally (see [fig. 383.]), the articulations of the spinous processes with the vertebræ, and other characters. Yet they do not form a family intermediate between fish and reptiles, but are true fish, though doubtless more highly organized than any living fish.[336-A]

Fig. 383.