[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.
[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.
[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 canbe 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 squaremiles, 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 limestonesare 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 thecoast, 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 swarmedwith 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 theFlat Coals. Theirtotal 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) atp. 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 carriedto 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.