Fig. 32.—Section from Gilmerton to Crichton; a, Lower Carboniferous; b, Mountain Limestone; c, Edge Series; d, Roslyn Sandstone Group; e, Flat Coals; y, Drift.
The neighbourhood of Edinburgh affords many facilities for the study of these rocks. They can be seen, for instance, at many points along the ridge of the Roman Camp Hill, near Dalkeith, exposed in the operations of quarrying. That ridge is formed by what is known technically as an anticlinal axis ([Fig. 32]); in other words, the lower beds of the coal-measures rise up here into a sort of broad wave-like undulation, round the sides of which the higher parts of the series are folded. The elevated area has either been pushed up from below, or the more level country around has subsided into two trough-like hollows, so that now the strata, which geologically speaking are lowest, come to occupy the highest ground in the district. Seated on some of the opener spots of this woody eminence the observer has a noble prospect on which to expatiate. The ground around him is rich in historic associations, and links itself to many a varied page in the annals of Scotland. The hill on which he rests is crowned by the mouldering mounds of what tradition reports to have been a Roman station, but which may perhaps belong to a still earlier era. A few hundred yards north rise the wooded slopes of Carberry Hill, where the hapless Mary surrendered to her rebel lords, and whence she was led into her own capital amid the insults of an infuriate rabble. Northward, too, lies the fatal field of Pinkie, and eastward the less deadly but not less decisive field of Prestonpans. To the west the eye can mark the grey smoke of the Scottish metropolis, with its dusky towers and its lion-shaped hill, and then the blue waving outline of the Pentlands that sweep away south and lose themselves among the distant hills which girdle in the coal-basin of Edinburgh and Haddington. The course of the Esk—that fabulosus amnis—passes by many a time-honoured spot, from Habbie's Howe and the scene of the Gentle Shepherd down by the haunted scenery of Roslyn, the cliffs of Hawthornden, the grounds of Newbattle, and the old Roman station of Inveresk. East, west, and south, the broad expanse of green field and clustering wood swells upward to the distant hills that encircle the landscape with a wavy line of softest blue. Northward the eye rests on the Firth of Forth with its solitary sails, bounded by the bosky heights of Fife, and opening outwards by the May Island and the Bass Rock into the far-off hazy ocean. On every side objects of historic interest lie crowded together, about which many pleasant volumes have been and might still be written. If the observer be a lover of geological science he will find an examination of the structure of the hill to impart an additional interest to the scene. From the wide panorama of hill and dale, river and sea, with all its battle-fields, castles, and abbeys, and all its memories of the olden time, let him turn into one of the quarries that indent the flanks of the hill, and try to decipher there the records of a still older history. An hour or two thus spent will pass swiftly and pleasantly away, and on quitting the quarry he will have gained a new light in which to look on the landscape that lies spread out below.
The mountain limestone of Mid-Lothian consists, as has been mentioned, of several seams interbedded with black and calcareous shales. The quarries on Roman Camp Hill have been opened in several of the thickest of these seams. Let us enter one of the excavations. A vertical face of rock forms the background, overhung above by long dangling tufts of withered grass, and washed below by a pool of water having that milky green tint peculiar to old lime-quarries. The lowest rock visible is a dull grey limestone with a yellowish weathered surface. Above it rests a mass of hard yellow calcareous shale, known to the workmen as "bands." This rock is worthless as a source of lime, nor from its irregular laminations and shivery structure has it much value in any other way. A few inches of surface-soil form the upper part of the section. It requires but a glance over the weathered surface of the limestone to mark that the rock abounds in fossils. Of these by far the most numerous are the joints of the stone-lily, for the most part of small size, and when broken across, with their minute central apertures, looking like so many fractured stems of tobacco-pipes. Other organisms also occur, such as a small delicately-plaited productus, a larger and more boldly-ribbed spirifer, a small cyathophyllum or cup-coral, and the fragile interlacing meshes of one of the net-like bryozoa—the fenestella. Of rarer occurrence are the whorled shells called bellerophon, the long chambered shells of orthoceratites, and the grooved tapering shells of pinnæ. Many of the same fossils can be detected in the beds above, which thus evidently all form part of one series with the rock below. What, then, were the circumstances under which these strata originated?
The answer to such a question is not far to seek. The corals and crinoids are exclusively marine families, and so any stratum in which their remains occur must have had a submarine origin. It matters not in this case though the specimens be fragmentary, showing a broken and drifted appearance. For even supposing that they did not live at the spot where their petrified relics are now exhumed by the operations of the quarryman, granting that they were drifted from a distance, still they could only have been drifted from one part of the sea-bottom to another. The state of keeping of the specimen often tells vastly on the value of its evidence when it belongs to a land or fresh-water tribe. Thus, in one of the limestones of West-Lothian I have found a black carbonized stem of sigillaria. Now, the sigillaria was a land-plant as much as any of our hazels or willows, and where the evidence from the associated organisms coincides, furnishes its own testimony as to the origin of the rock which imbeds its remains. But the stem in question was a mere fragment, and showed moreover a worn macerated surface. Such a fossil had evidently no value as a test of the origin of the limestone, which might have been elaborated either in an inland lake or in open sea. That it had really a marine origin, and that the sigillaria actually was, as it seemed to have been, a drifted plant, I ascertained beyond a doubt by detecting on the same slab hundreds of encrinal stems along with the shells, and thin, delicate, silvery spines of productus. Thus, then, the organisms of the land may be carried into the sea, and in dealing with their fossilized remains in the deposits of former ages we must be very careful in the use of evidence derived from fragmentary and drifted specimens. But no such caution is needed in regard to the productions of the sea. If they be fragmentary and drifted, we may believe they were rolled about by tides and currents previous to their final entombment; but still they remain as good a test as ever of the marine character of the rock in which they occur.[61]
[61] The exceptional instance, of the accumulation on the land of blown sand imbedding the broken remains of marine shells, needs only to be noticed here.
The fossils of Roman Camp Hill are not drifted specimens. They must have lived and died where the quarryman now finds them. We recognise them as all unequivocally marine; corals, crinoids, and brachiopodous molluscs, are all clearly the denizens of the sea, and hence we conclude that they mark the site of an ancient ocean. The snail-shells that swarm about the fruit-trees of our orchards not more unmistakably indicate a land-surface than do these petrified relics evidence an old sea-bottom. We can argue, too, from the crowded way in which they lie grouped together, that life must have been prolific in these primeval waters. Every fragment of the rock shows its dozens, nay, hundreds, of stone-lily joints, disjointed indeed, yet easily recognisable. They must have swarmed as thickly along the floor of the sea as the strong-stemmed tangle that darkens the bottom of many a picturesque bay along our western coasts, yet with a gracefulness of outline such as none of our larger sea-weeds can boast. Less numerous but not less markedly in situ are the shells of productus and spirifer, the former with its finely-striated surface fresh as if the creature had died but yesterday, while the slender spines with which it was armed lie strewed around. In short, the whole suite of organisms points to a period of tranquil deposition in a sea of probably no great depth, where the lower forms of the animal kingdom flourished in abundance, contributing by their calcareous secretions to form continuous layers of limestone.
Such a condition of things finds a parallel in many parts of the globe at the present day. Thus, the shores of the islands of the Pacific are white with fine calcareous mud, that results from the action of breakers on the surrounding coral-reefs. This mud, enveloping fragments of coral, shells, sea-weed, drift-wood, and other extraneous substances, hardens on exposure, and becomes eventually a limestone, travertine, or calc-sinter. We may believe that the same process goes on out at sea, around the edges of atolls or circular coral-reefs, and that the sediment thus thrown down will enclose any zoophytes or molluscan remains that may lie at the sea-bottom, along perhaps with fuci, and occasional water-logged fragments of wood that have been drifted from land. Along the shores of Guadaloupe a bed of this calcareous silt has formed since America was colonized by man, for it has been found to contain fragments of pottery, arrow-heads, and other articles of human workmanship.[62] The same rock has yielded, besides, the partially-petrified bones of several human skeletons, one of which, though without the head, forms a prominent object among the fossil treasures of the British Museum. The rock in which these remains are embedded is described as harder than statuary marble, notwithstanding its recent origin. By supposing the same process to be carried on over a large area and for a long period, we may see how a continuous stratum of limestone could be elaborated, full of fossil relics of corals, molluscs, and other marine productions. And in some such way, we may be permitted to believe, the seams of limestone on Roman Camp Hill were accumulated. The billows of that old carboniferous ocean may not have sent up their white surf against the margin of snowy coral-reefs, but the currents below did their work of demolition as effectually, and by sweeping through the submarine groves of stone-lilies and cup-corals, as the night winds of autumn sweep athwart the heavy-laden fields, would prostrate many a full-grown stem and scatter its loosened joints among the thickening lime that covered the bottom. Stone-lily, cup-coral, net-coral, productus, spirifer, pinna, nautilus, orthoceratite, all would eventually be entombed amid the decaying remains of their congeners, and thus produce a slowly-increasing seam of limestone.
[62] Lyell's Manual of Elementary Geology, p. 121. Fifth edition.
We still linger in the old quarry on Roman Camp Hill, but the day draws rapidly to a close, and the long level beams of the setting sun lighten up the higher grounds with a golden flush, while the valley below lies deep in shade. The rays fall brightly on the abrupt face of limestone at the further end of the quarry, every prominence standing out in bold relief, and casting its shadow far behind. Our eye, in passing over the sunlit rock, can detect the fractured joint of many an encrinite glancing in the light; along, too, with the strongly defined outlines of some of the lesser and more abundant molluscs—spirifers or producti. Some of them, sorely effaced by the rains, have begun to yield a scanty nestling place for creeping fibres of moss; others yet bare, afford a rest to the Vanessa whereon to spread its wings in the mellow sunset ere flitting homewards among the dewy herbage. The bushes overhead scarcely rustle in the light-breathing air that comes fitfully across the land, and the long grass nods dreamily on the margin of the pool below. There rests a calm stillness on all the nearer landscape, and the distant ground blends away into the shades of evening. The scene, in short, has about it that solemn impressive repose which irresistibly arrests the fancy, and sets it to dress up into fantastic shapes the massive clouds that float in the western sky, to picture grim forms amid the misty shadows of the valley, or to dwell half dreaming upon the memories of the past, that come crowding through the mind in quick succession. Our labours among the fossils of the old quarry, however, enable fancy to draw her stores from another source. We muse on these petrified relics, gilded by the last rays of the setting sun, when slowly, like a dissolving view, sunset and herbage melt away, and the bottom of the old carboniferous ocean lies before us with its corals and shells and stone-lilies, stretching out their quivering arms, or expanding and contracting their flower-like petals amid a scene of ceaseless animation and activity. Geology delights in contrasts, and assuredly the contrast presented to us this evening between the present and the past of Roman Camp Hill, will not rank among the least striking of those which she has to reveal. There is now spread over us the blue sky, richly hung with tinted clouds, and melodious with the evening songs of the lark, the blackbird, and the thrush. Not less surely did a wide expanse of sea during the Carboniferous era roll over the hill on which we stand. And yonder silvery moon that mounts up amid the violet twilight of the east, has witnessed each scene and all the countless changes that have intervened between them. The same pale light that now begins to steal through the woods and athwart the fields, must have streamed down upon that old sea and illumined its green depths. Oceans and continents, islands and lakes, hills and valleys, have come and gone with all their successive races of living things, and that same planet has marked them all. She has seen, too, as but a thing of yesterday, the appearance of man upon the scene, with all the successive centuries that have elapsed since then. Truly the "goddess of the silver bow" would have a strange story to tell us could we interrogate her about the past. But the days of Endymion have gone by, and she now no longer visits in a personal form the seat of beings who gaze at her crescent orb and daringly pronounce it a scene of blasted ruin and desolation.