Fig. 33.—Section from Cape Breton coal-field, showing four planes of vertical stems, and seven ancient soils with their covering of vegetation.
a, sandstones; b, shales; c, coal; d, fire-clays; e, arenaceous shales.
The warfare seems now pretty nearly at an end, and as often happens in such cases, it has been found that each party was to some extent in the right and to some extent in the wrong. It has been ascertained that some coal-seams must have originated from the deposition of drift-wood in the mud and ooze of the sea-bottom, while others undoubtedly arose from the decay and entombment of vegetation in swampy plains of the land. That the latter mode of formation has been the usual one in most of our coal-fields has been generally acknowledged since Sir William Logan's announcement that each coal-seam, for the most part, rests upon a bed of fire-clay, which, with its embedded roots, marks the site of an ancient soil. This fact has been abundantly confirmed in every part of this country, and indeed wherever an extended series of coal-seams has been examined. Not only have the underlying fire-clays been found, but in not a few instances erect stems of trees, passing down through the coal-seam and spreading out their divergent roots in the clay below, exactly as they must have done when they flourished green and luxuriant in the times of the Carboniferous system. This was especially the case in the Parkfield Colliery, Wolverhampton, where seventy-three trunks were laid bare in the space of about a quarter of an acre, each with its roots attached. The same appearance was observed some years ago in the Dalkeith coal-field, where a group of erect trees was encountered covering a space of several square yards. Some instructive sections of such fossil-forests are given by Mr. Brown from the Cape Breton coal-field.[63] In one of them ([Fig. 33]) no fewer than four planes occur, each supporting its group of erect steins. Now, no one can glance over this and the other sections illustrative of the same paper, or the descriptions given by Sir Charles Lyell and others of the Nova Scotian coal-field, without being compelled to admit that the trees in question grew just where their upright stems can still be seen, and consequently that the accompanying coal-seams originated not from vegetation drifted by river-action, but from vegetation that grew upon the spot. And though erect stems do not exist in every coal-field, we seldom fail to detect the not less important occurrence of the fire-clays and hardened shales that support the coal-seams and prove by their embedded rootlets their identity with ancient soils. Thus we arrive at the inference that while in certain localities coal-seams have resulted from drifted vegetable matter, they have nevertheless for the most part been formed from plants that flourished where the collier now excavates, amid damp and dripping caverns, their carbonized remains.
[63] Quart. Jour. Geol. Soc. vol. vi. pp. 120, 130. The cut given above ([Fig. 33]) is taken from one of these sections as modified by the late Sir Henry de la Beche (Geological Observer, p. 582). In the original the beds are inclined at a considerable angle, but for the sake of clearness they are here reduced to horizontality.
Applying, then, this deduction to the strata occurring on the horizon of the mountain limestone in Mid-Lothian, we are led to believe that the North Greens coal-seam marks the site of a former land-surface. It shows no vertical stems, but has all the other accompaniments of an ordinary seam, such as the underlying fire-clay and shale, with their included stigmariæ. And this conclusion has more than ordinary interest, for if it be true, we have evidence of a terrestrial formation among strata unequivocally marine; in other words, we see proofs either of an elevation or a filling-up[64] of the sea-bottom carried slowly on until land-plants grew up in matted swamps where once there swarmed corals and encrinites, and then of a gradual subsidence, so that marine organisms flourished again in abundance over the site of the submerged vegetation. It is not insisted that each of the thin coal-seams among the limestone strata marks a former terrestrial area. Some of them may possibly have resulted from the transport and deposition of plants borne from the land. Yet there are others of wide extent resting upon beds of fire-clay which contains stigmaria rootlets, &c. These I cannot but regard as the remains of plants that grew upon the spot. And so, while we recognise in the beds of limestone undoubted evidence of a former sea-bottom, I am persuaded we must equally admit that at least several of the coal-seams bear fair evidence of a former land-surface, scarcely raised above the sea-level indeed, but nourishing nevertheless a thickly matted vegetation. In this way we shall see the mountain limestone series of the Lothians to be not a purely marine formation, but one partly marine and partly deltoid, showing in the succession of its strata proofs of a gradual submergence, interrupted by movements of elevation, so that the area which at one period formed the ocean-bed became at a later time low delta-land, and after continuing perhaps for ages to stretch out its verdant surface beneath the open sky, sank again amid the corals of a wide-spread sea.
[64] If it be correct to set down the North Greens coal-seam as really representing a terrestrial surface, that is, of course, a flat delta or plain scarcely raised above the sea-level, we must, I suspect, call in the aid of a slight elevatory process, or else hold that the depth of the sea at the locality where the lower limestone was forming did not exceed 80 or 90 feet, and may have been considerably less, and that this space came to be eventually filled up by the detritus of the river. But the wide extent and sometimes the great thickness of the limestone beds seem to indicate a greater depth, and thus favour the idea of an elevation of the sea-bottom to form the North Greens coal-seam.
Now this condition of things differs entirely from what is presented by the Mountain Limestone group of England. That formation, when typically developed, attains a thickness of from 1000 to 2000 feet, and gives rise to that green hilly kind of scenery whence it has derived its name. It is unequivocally a marine deposit, since it abounds in corals, echinoderms, brachiopodous molluscs, and other productions of the deep. Northward, however, it undergoes a gradual change, getting greatly thinner, and split up by a series of intercalations of shale and sandstone. This alteration goes on until, on the border-land between the two countries, the massive limestone of Derbyshire has dwindled down into a series of thin beds, often widely separated by intervening strata, which contain many seams of coal. After crossing the Silurian district, and descending the northern slopes of the Lammermuir Hills, we get into the Carboniferous system again, and find its limestone series still farther diminished. With this decrease of marine formations, we can detect an augmentation of coal-bearing strata. Thus the Berwickshire coal-field lies in this lower set of beds, far under the coal-measures of Newcastle. In the Lothians, too, as has been shown, coal is extensively worked in the same series, and these seams also find their representatives in Fife and Lanarkshire. The gradual change from the kind of strata found on the horizon of the Burdiehouse limestone, to those occurring on the horizon of the Mountain limestone, indicates, as we saw, a gradual change of the conditions of deposition; and the nature of this alteration is shown by the difference in the character of the imbedded fossils. The passage of the massive Derbyshire limestone into the thin limestones and coal-bearing sandstones of the north, as decidedly marks another change in the relative position of sea and land. The former was a succession in time, the latter was one in space, but the mode of reasoning remains the same for both. In the former case, we saw estuarine strata passing upward into others wholly marine, and the order of superposition told us that the locality was first an estuary, and then slowly became open sea. In the latter case, we see marine beds not succeeded by estuarine strata, but becoming estuarine strata themselves. The thick limestones gradually thin out horizontally into a great series of sandstones and shales, with interbedded coal-seams, so that what bears evidence of a deep sea at the one end, gives proof of a muddy and sandy delta at the other. In other words, during the ages represented by what we call the Mountain Limestone, the central and south-western portions of England lay far below a wide breadth of ocean, and nourished a luxuriant crop of stone-lilies, mingled with the other denizens of the deep, while the Border district, and the whole of central Scotland, exhibited all the conditions of a vast delta, sometimes spreading out as broad verdant jungles, anon showing only scattered irregular groups of low, bare mud-banks and sand-spits, which at other times disappeared altogether beneath the dun discoloured waves. Now the reader will not fail to mark that this curious and interesting fact in the past history of our country, is ascertained solely from a comparison of fossil remains. The stone-lilies and shells of Derbyshire, and the lepidodendra and land-plants of the Lothians, form our sole basis of evidence, and we may rest on them with as perfect certainty as if they were so many duly attested documents deposited among the archives of our State-Paper Office.
In our survey of the coal-field of Mid-Lothian, we have passed from the Lower Carboniferous estuary beds of Burdiehouse to the Middle Carboniferous marine beds of Roman Camp Hill, and their associated terrestrial strata,—the coal-seams and fire-clays. We come now, in our upward progress, to the Upper Carboniferous group, or Coal-measures proper.[65] These strata rest immediately upon the limestones, and attain a depth here of over three thousand feet. They consist of a great series of sandstones, shales, coals, and fire-clays, that vary in thickness from less than an inch to many feet, or even yards. The coal-seams are especially variable, many of them existing as mere films of carbonaceous matter; others varying up to a depth of fourteen feet. There are from fifty to sixty that exceed a foot, but the average thickness throughout the whole series is about three and a half feet.[66] They are nearly all underlaid by fire-clay or shale, and very generally have a roof of the latter material.
[65] These terms—Lower, Middle, and Upper Carboniferous, are used for want of others, and for the sake of clearness. They must not be regarded, however, as equivalent to similar groupings of the English carboniferous rocks, for the Scottish series is probably much older than the greater part of the English, and coeval, to a considerable extent, with the mountain limestone and millstone grit of the latter country.
[66] See Milne on Mid-Lothian Coal-field. Trans. Royal Soc. Edin. vol. xiv. p. 256, whence the above details are taken.
By referring to the diagram of this coal-field, given above at [p. 196], the reader will notice that the series is divisible into three groups:—1st, and undermost, a considerable depth of coal bearing strata known as the edge series, because they lie along the western limits of the coal-basin at a high angle, and sometimes even on edge; 2d, A great thickness of sandstones nearly barren of coal, but containing at least three beds of limestone this may be termed the Roslyn sandstone group; 3d, and highest, another series of coal-bearing strata, commonly called the flat coals, because they occupy the centre of the basin where the beds repose at a low angle, and are in places quite flat. It will be convenient to keep in mind this three-fold division, for it will point us to some important changes in the ancient conditions of this coal-field.