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.
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.
The edge series, which forms the lowest, and of course oldest of the above groups, averages from 800 to 900 feet in thickness. It contains about thirty seams of coal above a foot thick, and many more of less size. They occur irregularly, some lying only a few inches apart, others from eighty to ninety feet, the intervening space being occupied by sandstone or shale.
Now as each coal-seam, with its associated under-clay, appears to mark a former land surface, it will follow that there must be as many old land surfaces in this series of strata as there are such coal-seams, and that for every intervening mass of sandstone or shale, the area of vegetation must have been submerged. This conclusion would have been violently resisted by the supporters of the "drift" theory. They would have roundly asserted that such an unsteady surface was a mere supposition to suit a hypothesis, unsupported by fact, and contrary to the analogy of existing nature; and they would not perhaps have hesitated to maintain, that such an oscillating land could be little fitted to nourish so rich and luxuriant a vegetation as that of the Carboniferous period. But it will not be difficult to show that our conclusion, so far from being contrary to analogy, is amply borne out by the processes of existing nature, and that its opponents, and even its original asserters, failed to perceive that what it demands is not a rapidly oscillating crust, but one as steady and uniform as that of many of the least disturbed countries at the present day; and that we do not require to call in the aid of a special elevation and submergence for every coal-seam, but that for the most part the hypothesis of a steady sinking of the area of a coal-field, interrupted perhaps by occasional elevatory movements, along with an active and constant deposition of sediment by the varying currents of a large river, is sufficient, if not thoroughly to explain, at least to throw great light upon the origin of those enormous masses of strata composing our present coal-basins. The oft-recurring variations in the nature of the strata that form our coal-measures, sandstones alternating with shales, these again with coals and fire-clays, together also with the terrestrial origin of the coal-seams, and the occasional presence of true marine organisms, make it evident that, to obtain any modern analogue to such a condition of things, we must examine those localities where large bodies of fresh water, carrying sediment and vegetation from the land, mingle with the sea. Let us then look for a little at the operations now in progress at the mouths of the larger rivers, and mark how far they elucidate the structure and history of a coal-field.
"Egypt is the gift of the Nile." Such was the conclusion arrived at by one of the most diligent observers of ancient Greece—the venerable Herodotus.[67] He sailed up the river marking all the leading features in its scenery, and noting the more apparent evidences of ancient physical changes. His remarks on these subjects form one of the earliest specimens of scientific reasoning that have come down to us, and are remarkable for their correctness and the truly inductive mode of thought which they evince. Modern travellers have amply confirmed the opinions of the father of history, and we now know that but for its central river, Egypt would be a vast dreary expanse of arid sand like the neighbouring deserts of Lybia. The Nile, by annually inundating the country, deposits over it a stratum of rich loam, and thus not only waters the land, but continually renews the soil. The sediment in this way brought down has gradually encroached upon the waters of the Mediterranean, being heaped up at the river mouth into shifting sand-banks, islets, and great tracts of low, swampy ground, slightly raised above the sea-level. Through this series of silting deposits, the river sends a number of branches, often winding in labyrinthine convolutions, and ever changing their course, by wearing away the silt at one place, and throwing it down at another. The area traversed by the mouths of the Nile was called by the Greeks the Delta, from its similarity in form to the Greek letter, and the name has since been given to all such fluviatile deposits, whether they have this general form or not.
[67] Euterpe, 5.—His words are very emphatic. "To one of ordinary intelligence, who has not heard of it before, but sees it, Egypt is manifestly land acquired by the inhabitants, and a gift from the river—δωρον τον ποταμον." The 10th and 12th chapters of the same book deserve especial study for the admirable inductive style in which the historian compares the phenomena observable in Egypt with what were well known as the results of river action in other lands. The passages might be quoted word for word in the most rigid scientific argument of any modern geologist.
The sediment annually deposited by the Nile varies in thickness in different years. The mean thickness of the annual layers at Cairo has been calculated not to exceed that of a sheet of thin pasteboard, so that "a stratum of two or three feet must represent the accumulation of a thousand years."[68] Such thin laminæ must resemble greatly some of the more fissile shales in the Carboniferous system, which were, perhaps, formed by as slow a process, and in their aggregate depth probably took many thousand years to accumulate. But those fluviatile depositions of the Nile vary little in kind, for when cut through they are found regularly stratified down to their base, which rests upon the great underlying sand. They show us how the argillaceous seams of the coal-measures may have originated; but the diversity of character in these Carboniferous rocks indicates a more varied kind of sediment, and probably more rapid and active transporting currents. A closer analogy to such a condition of things meets us on the shores of the New World.
[68] Lyell's Principles, p. 262.
The Mississippi, so magnificent in all its proportions, has raised a delta which covers a tract of about 14,000 square miles, equal to almost half the area of Ireland. The lower parts of this delta are formed of low, shifting banks, traversed by innumerable streams that diverge from the main river, and alternately throw down and remove vast quantities of earthy sediment, intermingled with rafts of drift-wood. These swamps are covered with a rank growth of long grass and reeds, and for about six months of the year are more or less submerged below the waters of the river, while liable at the same time to continual inundation and encroachment from the sea. The higher parts of the delta, though also subject to a similar periodical submergence, nourish a more luxuriant vegetation. Vast tracts of level sandy soil are densely overgrown with pine, which is used extensively for making pitch. Large districts of the swampy ground are covered with willows, poplars, and thickets of the deciduous cypress, an elegant tree that rises more than 100 feet above the soil. When in hot seasons these swamps get dried up, "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. At the bottom of all these 'cypress swamps' a bed of clay is found, with roots of the tall cypress, just as the underclays of the coal are filled with stigmaria."[69] In this way a thick accumulation of vegetable matter goes on forming for years, until either the river changes its course, and inundating the swamp gradually covers it over with sand and mud, or until, owing to oscillations of the earth's crust, the district is either permanently submerged, so as to be silted over, or elevated to nourish a new and different kind of vegetation.
[69] Lyell's Elements, p. 386.
That such changes have taken place in the past history of the river we have several interesting proofs. Thus, owing to the great earthquakes of 1811, 1812, an area of more than 2000 square miles was permanently submerged.[70] Since then it has gone under the name of the "Sunk Country;" and Sir Charles Lyell, who visited the locality in 1846, that is, thirty-four years afterwards, tells us that he saw innumerable submerged trees, some erect, others prostrate. Now, it is easy to see how such an area may, when the climate suits, become the receptacle of vast accumulations of peat, which, by pressure and chemical action, will ultimately pass into coal. If we suppose the submergence carried on more rapidly at some periods, the plants might have been unable to keep pace with the ever-increasing inroads of sand and mud. In such cases the layer of vegetation would become eventually entombed beneath succeeding deposits of earthy matter. Were the amount of sediment thus thrown down sufficient in the end to counteract the downward motion of the earth's crust, and so raise the bottom of the river or lake to the level of the water, vegetation would spring up afresh and clothe the new raised surface as densely as in former years. This alternation, according as the amount of sinking or the amount of sediment predominated, might go on for thousands of years, until a series of strata many thousand feet thick were accumulated, and tranquilly carried down bed after bed below the level of the waters.
[70] See Sir Charles Lyell's Second Visit to United Stales, chap, xxxiii.
It is interesting to know that the case supposed here has actually been realized in the delta of the Ganges. Some years ago an Artesian well was attempted to be made near Calcutta, and the auger was sunk to a depth of 481 feet.[71] The material passed through consisted of sand, clay, and nodules of argillaceous limestone, and at various depths, from 50 to 380 feet, several seams of decaying wood and peat were found, along with bones of various animals, such as deer and fresh-water tortoises, and fragments of lacustrine shells. Each of these vegetable layers evidently formed at one time a forest-covered swamp like those of the surrounding delta at the present day; and hence it follows, that during the accumulation of the Gangetic delta, the ground in that locality must have undergone a depression of more than 300 feet, and that this sinking has been interrupted by slight elevations, or by periods when the ground remained stationary, so as to admit of a dense and prolonged growth of vegetation, at successive intervals, upon the swampy flats and shifting islands. The general appearance of these old forests is pretty well shown by the mangrove swamps along the mouths of the river. These trees flourish in dense jungles on the banks, and extend even below high water mark, being covered in places by shell fish. So that were these maritime parts of the delta inundated by the ocean, and buried beneath a mass of mud and silt, the peaty layer that would be formed would display trunks of trees still occupying their original erect position, and spreading out their roots in the clay below, exactly as the sigillaria is found to do in the coal-seams of the carboniferous rocks, while clustered round the carbonized stems, or scattered among the decayed leaves and branches, there might be detected limpets and barnacles (as lingulæ and pectens occur in the coal-seams), showing, by their mode of occurrence, that they lived and died upon the spot.
[71] See Lyell's Principles, p. 280.
If my reader will now suppose this sand of the Indian river to be hardened into sandstone, the mud in like manner compressed into shale, and the peat beds chemically altered into coal, can he fail to perceive the striking analogy between the section thus displayed and those already given from the Mid-Lothian and Cape Breton coal-fields? The differences between the ancient and modern strata are not in kind but in degree. The Scottish series reaches to more than six times the thickness of the Indian one, and the coal-seams in the one exceed in individual thickness the peat-beds in the other. We must remember, however, that the climate of Hindustan is not remarkably favourable to the accumulation of vegetable matter, the heat being so great that the plants decay almost as rapidly as they grow. And it should likewise be borne in mind, that were the conditions of subsidence and of the gradual accumulation of sedimentary matter to continue even in the same ratio as heretofore, the Ganges might, in the course of ages, heap up a series of stratified sands, clays, and peat-beds, many thousand feet in thickness, and many thousand square miles in extent, rivalling, or perhaps surpassing in depth, the largest coal-field in the world. The parallelism between this delta and an ordinary coal-field holds singularly close, not merely as regards the nature of the stratified deposits. The alluvial plain of Bengal has undergone a process of subsidence to an unknown depth, whereby successive areas of terrestrial vegetation have been carried down to be entombed beneath fluviatile sand and mud. It is likewise subject to the more sudden operation of earthquakes, whereby large tracts of country become permanently altered, and changes are effected on the direction, rapidity, and detritus of the streams. It is, moreover, liable to wide-spread inroads of the sea, which sometimes covers cultivated districts to a depth of several feet, laying waste the fields and destroying the inhabitants. These and other features help us to understand the origin of such vast masses of sedimentary strata as those of our coal-fields, where terrestrial, fluviatile, and marine remains alternate in rapid sequence, or sometimes occur together.
The origin of the constant succession of coal seams, sandstones, and shales, of the Edge series may be thus accounted for. The area of Mid-Lothian formed part of a great delta, which, like that of the Ganges, was undergoing a gradual subsidence during the Carboniferous era. The rate of this movement probably varied at different times, and might even be occasionally interrupted by short periods of elevation. When the ever-increasing accumulations of silt brought down by the river reached or nearly reached the surface of the water, they would become the site of wide tracts of swampy vegetation that flourished for hundreds or thousands of years. Eventually, however, these jungles, invaded by the changing currents of the river, were buried beneath a thick deposit of fluviatile sediment, or more probably the vegetation might become unable to keep pace with an accelerated rate of submergence, and the forests would then be tranquilly carried down beneath the water, and soon covered over with sand and mud. The detrital matter might in like manner continue to be deposited over the sunk forest for many years, perhaps centuries, until the muddy bottom again reached the surface, and once more waved green with sigillariæ, calamites, and lepidodendra. Another long interval might here elapse, in which a thick bed of vegetable matter might accumulate, much after the manner of the formation of peat among the bogs and mosses of our own country. The periodical inundations of the river probably gave rise to wide marshes and lagoons, often tenanted by lacustrine shells, and thickly overgrown with aquatic vegetation. The decaying plants decomposed the red ochreous matter with which the water was charged, and re-deposited it among the mud and rotting leaves at the bottom as a carbonate of iron. Such ferruginous accumulations, often entombing fern-stems and other plants, with scales and teeth of ganoidal fishes, sometimes conulariæ and lingulæ, and, in certain localities, whole acres and miles of fresh-water shells, are known now as our clay-band and black-band ironstones. We can easily conceive that, in shallower parts of the lagoons, a dense growth of marshy plants might spring up, preventing any deposition of iron, and when the whole came to be covered over with later accumulations of sand or mud, the deeper parts of the old lake would be covered with a seam of ironstone, and the shallower portions would display a bed of coal. In some such way we may account for the frequent passage of ironstone into coal, and coal into ironstone in many of our coal-fields. If undisturbed by the ever changing currents of the river, these wide expanses of marsh and lake might continue for many long years, the constant evaporation being counterbalanced by continual supplies of water from the main stream. Eventually, however, owing perhaps to another period of more rapid submergence, the water gained the ascendency, and once more rolled over prostrate stems and matted thickets of ferns, that sank slowly down beneath a deepening sheet of sand and mud. Often, too, the sea must have flooded, perhaps for years, the flat delta-lands, carrying with it its own productions, such as the lingulæ and cardiniæ, which we find among the coal seams. And thus the process went on during the long ages of the Carboniferous system. Forest after forest spread its continuous mantle of green athwart the low swampy lands of that old delta, and each in succession foundered amid the muddy waters, now of the ocean and now of the river, that strewed over its site a rich detritus which went to form the soil of new jungles and forests.
The Edge series measures from 800 to 900 feet in depth, so that the depression must have been carried on till the forest that once grew nearly on the sea-level had sunk 800 feet below it This process was undoubtedly a very slow and tranquil one. Yet geologists used to regard these frequent changes of sedimentary matter as so many proofs of repeated catastrophic submergences, when the ocean came rolling over the land, prostrating forests, uprooting the hugest trees, and leaving the scattered bones and scales of fishes amid vast accumulations of mud and sand, where but lately there had bloomed a luxuriant vegetation. But the sober and diligent student of geologic fact will read in these rocks no such record of cataclysms. He will see in them evidences of the same gradual and sure operation which marks the processes of Nature at the present day. He will note how during a tranquil and probably imperceptible submergence of the river-bottom, forest after forest sprang up, flourished perhaps for ages, and eventually settled down beneath the waters of the river and sometimes of the ocean, amid ever increasing accumulations of mud and sand. Musing on these ancient changes he will be lost in wonder at the immense duration of the period during which they were in progress; and he will try in some measure to realize the features of their scenery. He will picture the delta with its ever-varying islets and sand-banks, its lakes and submerged forests, its leafless trunks peering above the water and sticking along the shoaling mud, and its crowded jungles that cover every drier spot. He will cast his eyes to where the delta opens out into the ocean, and mark how the waves encroach upon the mud-banks, cutting away what the river has piled up, and washing the roots of gigantic trees that wave their green coronal of fronds above, and overshadow the rippling of the green sea below. He will try to thread the windings of the stately river through brakes of ferns and calamites, and banks richly hung with tree-ferns and sigillariæ, and then upward through dark shaggy pine-woods, silent and gloomy, with the water creeping lazily through the shade or dashing in white cascades over dripping rocks, and onward still, far away among the distant hills till the fountainhead of the great stream is reached, gushing from the splintered sides of some lone rock, or pouring perchance out of the glimmering caverns of some massive glacier high amid the regions of perpetual snow.
Many attempts have been made to estimate the amount of time which some of our coal-fields may have required for their accumulation. But so large a number of conjectural elements must necessarily enter into such calculations, that the results come to be of very doubtful value. By estimating the amount of sediment annually transported by such rivers as the Ganges or Mississippi, we may ascertain how long a mass of similar sedimentary strata would take to form under similar conditions. And if our calculation had to do merely with such detrital accumulations, we might hope to arrive at some approach to accuracy. But besides these sedimentary strata, the formation of which must have been wholly analogous to that of similar deposits at the present day, we have to deal with the problems suggested by the coal-seams. We know nothing of the climate of the Carboniferous period save what may be conjectured from the analogy of existing climates; and in a question regarding the accumulation of decaying vegetable matter climate is a subject of the first importance. We are ignorant, too, of the rate of growth peculiar to the carboniferous flora; and even if we hold that it was probably rapid, the process of decay may have been equally speedy, and so a forest might go on shooting up fresh trees as the old ones rotted away, yet at the end of a thousand years there might be a scarcely greater thickness of vegetable matter on the ground than at the commencement. A seam of coal two feet thick might thus represent, say the accumulation of a hundred years, and another of exactly the same thickness might stand as the accumulation of a thousand years. Until we know more of the vegetation and climate of the coal period, the thickness of a coal-seam can hardly be held as a certain guide to the lapse of time required for its formation.
For the sake of illustration, let me take the following fragment of a coal-measure section:—
| Shale, | 20 | feet. |
| Coal, | 4 | " |
| Fire-clay, | 6 | " |
| Sandstone, | 40 | " |
Beginning at the bottom, we may compute the period of the forty feet of sandstone variously, according to the river selected as the type of a transporting agent. Tried by the standard of the Nile, all other conditions being similar, such a deposit would require perhaps not less than 14,000 years; by that of the Mississippi, 5000; and by that of the Ganges, nearly 2000.[72] We come, then, to the superincumbent fire-clay and coal, representing an ancient soil and the forest that grew on it. The occurrence of these seams shows us that the river-bed had become a swampy tract clothed with vegetation; but who shall say how long it may have continued so? Like the sunk country of the Mississippi, it may have been submerged, and to some extent cut off from the sediment-transporting channels of the river, and thus, as a vast lake, have nourished a prolific growth of marshy and aquatic plants. If the temperature resembled that of our own country, the growth of peaty matter, other circumstances being favourable, might be comparatively rapid. If, however, as seems probable, the climate were more warm and humid, giving rise to a more luxuriant vegetation, and at the same time to a more rapid decay, a long interval might have elapsed without adding materially to the thickness of the vegetable accumulations, and the eventual entombment of peaty matter sufficient to consolidate into four feet of coal, might be owing in some measure to the submergence of the swamp beneath the waters of the river, whereby a quantity of detrital matter was deposited that arrested the process of putrefaction, and entombed the thickly matted plants which were growing on the spot at the time. Hence, until we know more of the conditions under which vegetation may accumulate at river-mouths in such a climate as the coal plants are conjectured to have enjoyed, calculations of the amount of time required for the formation of a great series of coal-bearing strata must be regarded as premature. In the present instance, we can but affirm that the growth of the four-foot coal-seam probably occupied many long years, even at the most rapid rate of accumulation known to us. The forest-covered swamp on which the plants grew was eventually invaded by muddy detritus brought down by the river; and during another period of indefinite extent—five hundred years or five thousand years—fine mud continued to settle down over the foundered forest, hardening eventually into twenty feet of shale.
[72] Some observers have pointed to the occurrence of vertical and inclined trunks of trees in the Carboniferous sandstones, and deduced therefrom what has seemed to them a triumphant argument in favour of the rapidity wherewith our coal-fields must have formed. A foundered tree, they say, sank with its heavy-laden roots among the sand at the bottom, its stem pointing up into the water like the snags of the Mississippi, so that the sand must have come rapidly down to entomb the whole before it had time to decay, and thus thirty or forty feet of sediment must have been deposited in a few years, perhaps even months. But this is somewhat like a begging of the question. We have yet to learn how long a water-logged trunk will resist decomposition.
The Edge coals of the Mid-Lothian coal-field are succeeded by a group of sandstones and thin shales, with three or more seams of limestone. This group of strata, which we may call the Roslyn Sandstone Series, reaches a thickness of from 1200 to 1500 feet, and serves as a middle zone to divide the Edge coals below from the Flat coals above. It contains only a few thin laminations of coal, and these chiefly at its upper and under portions. Such a great intercalation of beds, without coal-seams, points, we might readily conjecture, to some change in the physical conditions of the ancient delta. The nature of this change can be easily made out from an examination of the rocks, and the reader will see that here again we are indebted to fossil remains for the most conclusive and satisfactory evidence of these old physical revolutions.
The absence of coal-seams suffices to indicate that during the formation of the middle group that part of the delta occupying the site of Mid-Lothian was continually submerged, and never rose to the surface so as to allow a covering of vegetation to form upon it.[73] The large beds of sandstone prove a continued transport and deposition of detritus during undisturbed periods of considerable length. The intercalations of shale, pointing to local changes in the currents or other modifying causes, are usually of small thickness and extent, while the sandstone beds sometimes attain a depth of 150 or 200 feet, and extend over wide areas of country. So far these mechanical rocks indicate the deposition of sand and mud under water, but whether at river-mouth or sea-bottom is left uncertain. From the fossil remains, however, we learn that the deposition took place in the sea, but at no great distance from land; in other words, the area of Mid-Lothian, which, during the accumulation of the edge coals, had been alternately clothed with vegetation and inundated by the river, sank down many fathoms, so that the sea rolled over it and all its submerged forests. The proof is two-fold, first, from the character of the organic remains in the limestones; and second, from that of those in the sandstones and shales.
[73] Of course, this deduction is founded, as the reader will notice, on the assumption that we have now the series, as it was deposited, and that no peaty swamp or forest was denuded away, and its site occupied by sand and silt. But the assumption is rendered probable from the conditions of formation indicated by the Roslyn group.
In some of the streamlets that flow into the beautifully wooded vale of the Esk, south of Penicuik, these limestones can be well seen, worn in the water-channel, or crusted over with moss along the banks. Their organisms are singularly abundant, and consist of cyathophylla, encrinites, spirifers, producti, &c., all exclusively marine. In a picturesque brook that falls into the Esk near a saw-mill in the grounds of Penicuik House, I have seen the little cup-corals clustered by dozens on the weathered rock, showing their delicate striated wrinkles in high relief among the scattered valves of productus and innumerable joints of the stone-lily. They were all well preserved, and in their grouping and general appearance differed in no respect from similar organisms in the mountain limestone of Roman Camp Hill. The inference to be drawn from them must accordingly correspond with what has been deduced from the mountain limestone fossils, viz., that they mark the site of a sea-bottom which remained free from mud and sand for considerable periods, during each of which there abounded corals and shells, whose exuviæ went to form several seams of limestone. But that this sea-bottom was at no period very far distant from land, is proved by the drifted plants that occur in the sandstones and shales both below and above, and which often show so little trace of maceration, that we can hardly believe they were carried far, or floated for a long while previous to being enveloped in the sand or mud at the bottom. I have never detected vegetable remains in the limestones themselves, but there seems no reason why they should not be found there.
One of the most remarkable and difficult phenomena presented by these limestones is their great persistency. I have traced them over a large part of Mid-Lothian, from the highly inclined beds at Joppa to the contorted and faulted strata near Carlops. I have found them, too, in many parts of West-Lothian and Stirlingshire, from the sea at Borrowstounness southwards into Lanarkshire. They likewise occur in Fife, and seem to sweep away through Lanark and Ayrshire. The area in which I have found them cannot be much under 700 square miles, yet they are probably spread over a much greater extent of country. Throughout this region they appear to continue on the whole at pretty much the same vertical distance from each other, and average three or four feet thick each. They vary in number, three being found in parts of Mid-Lothian, in other parts only two. Throughout West-Lothian there seem to be but two seams in the middle or moor-rock series, and the same two seams are found passing over into Perth near Culross. There are differences, too, in the structure and composition of the seams, one running sometimes as a single bed of dull blue limestone, and then gradually splitting up into three layers of a greyer and more earthy texture, with soft shale between them. But making all these abatements, the observer cannot fail to be struck with the general regularity and continuity of these limestones. And the fact becomes all the more remarkable when we consider the great irregularity, and continual intercalations, and repetitions of the strata, both above and below. Marine beds are usually persistent over large areas, especially where extensively developed. As they decrease in thickness, their continuity for the most part lessens, so that the rule is, on the whole, a safe one, the thinner any particular stratum, the less likely are we to trace it to a considerable distance. Yet, not only are these Mid-Lothian limestones thin, but they occur in regular sequence among a set of continually alternating and very irregular beds, and extend over several hundred square miles of country. And this, too, not in a single seam, but in two, three, or even more, so that the difficulty of accounting for such intercalations is proportionately increased.
We have seen above that the area of a delta is often partially submerged below the sea, and that such changes may become of the most marked kind where the country is liable to be depressed by earthquakes. There can accordingly be no difficulty in understanding how the ancient carboniferous delta of Mid-Lothian may have likewise subsided. But the limestones are unmistakable evidence that not only was the area of the delta submerged, but that for a while no sediment was deposited over it, and hence marine animals peculiar to clear water flourished so long and so abundantly as to form by their remains several beds of limestone. Had these beds been merely local we might have regarded them as having been deposited in lagoon-like portions of the delta, shut out from the detrital matter of the river on the one side and open to the sea on the other. But their wide extent and nearly uniform thickness preclude such a supposition. The following explanation appears to me the most probable:—
After the series of the Edge coals had been brought to a close, the coal-fields of Scotland underwent a complete submergence below the sea. This depression was probably very gradual, yet more rapid than that long-continued one which had been going on during the earlier part of the Carboniferous series, and the consequence of this greater rapidity was to prevent the growth of stigmaria swamps or reedy jungles, by keeping the alluvial surface continually sunk to some depth below the water. The amount of subsidence until the deposition of the lowest limestone may not have been great, but even a slight depression would tell vastly on an area of flat delta land. Mud banks would be brought down into the region of waves and surface-currents, and speedily be spread out over the floor of the sea. Forest-covered islands would in like manner be levelled down, and their trees sent drifting seaward or submerged amid the re-formed silt. Thus altered, the delta would sink below the sea, and the sediment borne down by the river would be scattered out over the older deposits as a slowly-forming sheet. By degrees this detrital matter must have been carried less and less farther out to sea; in other words, the area of deposit or delta must have crept gradually nearer to the land—a result owing partly to the recession of the ancient coast-line, and partly perhaps to a greater amount of depression inland than at the coast, which would of course lessen the velocity of the streams and cause them to deposit their burden of sediment at higher levels than before. The consequence of this retreat of the delta from the sea would be to purify the water over the site of the old swamps, and render it fitted for the habitation of corals, molluscs, and other marine animals. A medium thus prepared would not be allowed to remain long untenanted, and so we find that it came to be densely peopled with the organisms peculiar to such a station. Stone-lilies, cup-corals, net-like bryozoa, molluscs of many kinds, and large predatory fish, swarmed in these old waters, and their calcareous shells and skeletons are now broken up by the quarryman and the collier as hard compact limestone.
After these animals had lived and died in successive generations, perhaps for thousands of years, the downward movement of the earth's crust seems to have ceased for a while or to have become greatly less. The effect of this would be just to reverse what had been previously done, especially if a slight elevatory movement took place. The streams would in such circumstances descend from the uplifted ground with renewed velocity and transport their detritus to gradually increasing distances. The muddy and sandy sediment thus borne seawards would slowly silt over the coral-banks at the bottom, and in conditions so ungenial the organisms would dwindle down and finally die out. A great thickness of sand and mud would be spread out over their remains so long as the currents from the land continued to carry sediment out to sea, and thus probably originated the sandstones and shales superposed above the lowest limestone.
Eventually the old steady downward movement returned, and with it the corals and stone-lilies. The detritus again sank to the bottom much nearer the land, forming great banks and shoals that choked up the river-mouth. Seaward the water regained its purity, and the bottom once more swarmed with living things. Another lapse of many thousand years may have here intervened during which the marine exuviæ gathered into another seam of limestone, until again the process of subsidence either ceased for a time, or what is perhaps more probable, became considerably feebler. Detrital matter began to creep seaward as before, and eventually entombed the corallines and crinoids to a great depth. The calcareous bed thus formed is the second limestone, and the superincumbent silt-beds represent the sandstones and shales that rest above it.
In some such way as this does the Roslyn sandstone series appear to have originated. I have indicated what seems to have been the main features in the process, but it was probably a very complex one. There may have been a great many oscillations of level of variable effects, some of them raising the disturbed area to a much greater height at one point than at another. This inequality would of course produce marked effects along a low flat country such as that at the mouth of a great river. New currents would be produced and the direction of old ones changed; great shoals and banks of silt would be worn down only to be thrown up again at some new point, where another oscillatory movement would expose them afresh to destructive denudation. The variations in the amount of elevation and depression would likewise modify the transport of detritus to the sea, and give rise to a varied and ever-changing sea-bottom. In short, the alternations and variations must have been endless, for to the ordinary multiplied interchanges of a delta we must add those induced by a constant and unequal oscillation of the earth's crust.
The Roslyn sandstone series comes to a close, and passing onward in ascending scale we meet with another great group of coal-bearing strata. They occupy the central area of the Mid-Lothian coal-field, and from their gentle inclination as compared with the lower strata that rise up from under them on either side of the basin, are known as the Flat Coals. Their total thickness—that is to say, all that has escaped denudation—amounts to a thousand feet or more. They consist chiefly of sandstones, shales, ironstones, and fire-clays, with from twenty to twenty-five seams of coal, of which sixteen are thick enough to be worked. Their similarity to the Edge coals below points to a similarity in the conditions of formation. The frequent alternations of sandstone and shale show how the delta gradually pushed outwards again and re-occupied its ancient site above the successive forests of the Edge series and the successive coral-beds of the Roslyn group. The coal-seams indicate the further progress of the detrital accumulations, and the eventual formation of vast swampy flats that nourished a rank growth of stigmariæ, and tracts of drier ground waving with ferns, and shadowed by the spiky foliage of the club-moss and the broader fronds of the tree-fern.
The Flat coals are not succeeded by any other palæozoic strata. Above them stretches the drift already described: sometimes in the form of a stiff blue clay resting on a striated rock-surface; sometimes as a coarse gravel containing fragments of all the rocks in the neighbourhood; and sometimes as a fine white sand diagonally laminated, and often showing dark partings of coal-fragments. From the section given above ([Fig. 32]) at [p. 196], the reader, will see that as the upper limit of the Flat coals is formed by the drift, a large part of that series may have been borne away by denuding agencies. Had there been even a seam of limestone above them, it would have sufficed to show their true thickness, for we should then have seen, that how much soever had been removed in later times from above the limestone, nothing had been removed from below it; and so it would mark the true original limit of the series. We cannot now tell how much thicker the upper part of the Mid-Lothian carboniferous system may have been. Probably, during the long ages that intervened between palæozoic and post-tertiary times, many hundred feet were borne away and carried to other sites, there to grow up into new islands and continents, clothed with other types of verdure, and peopled by other races of animals, and fitted to become, in a long subsequent period, the dwelling-place of man.
In fine, the evidence of these ancient changes in the history of the Mid-Lothian coal-field is derived, as we have seen, from two sets of facts; first, those of a mechanical, and, second, those of an organic kind—the one class explaining and confirming the other. Beginning our investigation at the horizon of the Burdiehouse limestone, we saw the curtain rise slowly from off a wide estuary, in which there gambolled large bone-covered fishes, while huge pine-trees—branchless and bare, seed-cones, fern-fronds, and twigs of club-moss, floated slowly away out to sea. The panorama moved on, and brought before us the ocean-bed of the Roman Camp limestone, with its groves of stone-lilies and bunches of coral; its tiny shells moored to the bottom, or creeping slowly athwart the limy floor, or spreading out their many arms, and rising or sinking at will. This picture passed slowly away, and then came the delta of the Edge coals, with its sand-banks and ever-shifting currents, its stigmaria swamps, and its forest-covered islets. We saw the delta gradually sink beneath the sea, and corals and stone-lilies cluster thick over its submerged area, to form the limestones of the Roslyn group. Again, the mud-bars of the river crept out to sea, and tangled forests waved green as of old, washed by the sea or inundated by the river. How this last period came to a close, we shall probably never know, and have no possible means of conjecturing. We pass at one step from the ancient era of the coal to the comparatively modern one of the drift—from a verdant palæozoic land, to an icy post-tertiary sea. It is like a leap in history from the days of Pericles and Aspasia to those of King Otho, or from the tents of Runnymede to the Crystal Palace of Sydenham.