CHAPTER XI.
Intercalation of coal seams among mountain limestone beds of Mid-Loihian—North Greens seam—Most of our coal seams indicate former land-surfaces—Origin of coal a debated question—Erect fossil trees in coal-measures—Deductions to be drawn therefrom—Difference between the mountain limestone of Scotland and that of England—Coal-bearing character of the northern series—Divisions of the Mid-Lothian coal-field—The Edge coals—Their origin illustrated by the growth of modern deltas—Delta of the Nile—Of the Mississippi—Of the Ganges—Progress of formation of the Edge coals—Scenery of the period like that of modern deltas—Calculations of the time required for the growth of a coal-field—Why of doubtful value—Roslyn Sandstone group—Affords proofs of a general and more rapid subsidence beneath the sea—Its great continuity—Probable origin—Flat coals—Similar in origin to the Edge coals below—Their series not now complete—Recapitulation of the general changes indicated by the Mid-Lothian coal-field.
Among the old quarries of Roman Camp Hill and down the course of several streams in the same county, the limestone beds of the mountain limestone series are seen to be associated with strata of shale, some of which are highly calcareous, and charged with the same organic remains that occur in the limestones. Such shaly intercalations mark as before the transport and deposition of muddy sediment around and above the corals and stone-lilies of the sea-bottom. All these beds must undoubtedly be regarded as marine. But there occur, besides, seams of sandstone and black partially-bituminous shale, with layers of coal and fire-clay. To this singular intermixture it may be well to advert more particularly, since it forms one of the distinguishing features of these northern rocks, as contrasted with those of central and south-western England, and more especially since it will lead us to mark again the value of fossil remains as evidence of the ancient changes of land and sea.
The southern part of Mid-Lothian consists of a broad heathy moorland, that slopes northward into the more cultivated country, and swells upward to the south into the series of undulating ridges that form the Moorfoot Hills. It is traversed by several streams which rise high among the pasture grounds of the south, and flow some into the valley of the Esk, and thence into the sea at Musselburgh; others past the ancient fortalices of Borthwick and Crichton, and so by the valley of the Tyne into the sea at Tyningham. In their upper course they traverse a broad belt of the mountain limestone that stretches across this part of the country from east to west, and dips away north under the coal-field. Where the streams have been able to cut through the thick mantle of heath, sand, gravel, and clay, by which these higher grounds are covered, we sometimes obtain a complete section of the strata displayed in regular sequence along the bottom of the channels. Thus, one of the rivulets that trickles slowly through the swampy ground of Middleton Muir, on approaching the line of limestone begins to descend more rapidly, and has excavated its course through several feet of the rock below. The limestones are well exposed along each side of the stream, forming in some places steep walls tapestried with moss and overhung with scraggy furze, and offering to the student an instructive series of sections. Near the farm of Esperston, where the stream flows through a narrow secluded valley, the limestones form a floor which the water in the course of centuries has worn smooth, so that the rock with its included encrinal stems and shells, polished by the ceaseless flow of the current, shows like a sheet of variegated marble. At one point on the side of the water-course the observer may notice a low ledge of rock jutting out for a short way along the edge of the stream. The upper part is a hard compact limestone, full of small crinoidal joints. The bed underneath it has been greatly eroded by the rivulet, but enough remains to show that the stratum is one of coal. It rests upon the series of limestones and sandstones seen in the upper part of the water-course, and is surmounted by the thick limestones of Arniston and Middleton. A similar seam nineteen inches thick has been worked among the limestone about three miles to the west at Fountain. The same bed occurs among the quarries on Roman Camp Hill already mentioned, and I have seen an equivalent stratum intercalated among sheets of cup-corals and stone-lilies on the shore at Aberlady, where the waves have laid open perhaps the finest section of Carboniferous limestone strata in Scotland. In West-Lothian, too, the same intercalation of coal-seams among the mountain limestone beds can be seen in many places. Thus, in the bed of the River Almond, near Blackburn, the following section is laid bare:—
Calcareous shale.
Limestone (marine), eight feet.
Calcareous shale, with spirifers, &c.
Coal, six to eight inches.
Fire-clay.
Sandstone.
A short way further down the stream another bed of limestone occurs with several seams of coal below it, one of them attaining a thickness of six feet.
In addition to the thin seam at Esperston, the Mid-Lothian field contains several others. Of these by much the most important is that known as the North Greens Seam. It varies in thickness from only a few inches to fully 5 feet, and has been extensively worked for the parrot or gas-coal which it contains. It rests upon a pavement of shale, sometimes of fire-clay, and occurs about midway between two thick marine limestones, being from 80 to 90 feet distant from each. I have laid open many a block of the parrot-coal at the pit mouth, and marked the well-defined outlines of the stigmaria covered with a yellowish efflorescence of iron pyrites, like gilded figures upon a black velvet ground. The plants lie with their divergent rootlets spread out regularly along the stem like teeth on the back of a comb, thus seeming to indicate no hurried agglomeration by some tidal wave or turbid river, but rather a slow and tranquil deposition.
The fossils of the coal-seams consist for the most part of the plants above described, which we saw to belong to terrestrial species. But the reader will now understand that in dealing with organic remains we cannot infer, because a certain stratum contains nothing but land-plants, that it must necessarily by consequence be a land-formation. For we have seen that the plants of the Burdiehouse limestone, though all terrestrial, gave no support to the idea that the rock had originated on land. In all such cases regard must be had not only to the nature of the imbedded organisms, but their condition and mode of occurrence, and to the character of those associated with them. Especial care must be taken to distinguish what has been transported from what is in situ, otherwise, by attending only to one part of the evidence, we shall miss the import of the whole, and altogether misinterpret the records which we seek to decipher.
For years the subject of the origin of coal formed one of the many battle-fields on which geologists delighted to break lances. They ranged themselves under two banners, the "drift"-theory men and the "growth"-theory men, the former maintaining strenuously that coal was simply vegetation transported from the land and deposited in large troughs at river-mouths or sea-bottoms, the latter as eagerly contending that the vegetation had not been drifted, but grew on the very locality where its remains are now exhumed. Neither party lacked plausible arguments in support of its doctrines. The "drift" combatants stoutly affirmed it to be contrary to all experience that a land-surface should be so oscillating as their opponents required, that in short it was absurd to hold each coal-seam as marking a period of elevation, for there were often dozens of seams in as many yards of strata, some of them scarcely an inch thick, and yet, according to the "growth" theory, each would have required for its accumulation a special uplifting of the land above the sea-level. These and many other difficulties were thought to be triumphantly overcome by the hypothesis of transport and deposition. The vegetation borne down by some ancient Mississippi would collect in vast rafts, and these becoming water-logged would sink to the bottom, where, by getting eventually covered over with silt and sand, they would in process of time be chemically altered into coal. This explanation was, however, vigorously resisted by the opposite side. They alleged that the "drift" theory could account neither for the wide extent of coal-seams nor for their remarkable persistency in thickness. If the vegetation had really been hurried out to sea by river-action, it seemed natural to expect that the coal-seams should occur in sporadic patches of very unequal thicknesses, according as the drifted plants had been more densely or more loosely packed. But this was found not to be the case in point of fact. The coal-seams were ascertained to be generally singularly continuous, and to retain for the most part a pretty uniform thickness over considerable areas. And what was still more worthy of note, they were, as a whole, markedly free from extraneous matter, such as sand and mud. Where these impurities did occur, it was usually in the form of intercalated seams or partings, often quite as regular and extensive as the coal itself. Had the vegetation, therefore, been transported into the sea, it could hardly fail to get mixed up with the fine impalpable mud which, like that of the Ganges or Mississippi, might have discoloured the ocean for leagues from the river-mouth, and settled down as a thickening stratum at the sea-bottom. And many other arguments, derived from the nature and arrangement of the strata interbedded among the coal-seams, were urged to prove that the latter had originated from vegetation which grew on the spot.
