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.
CHAPTER XII.
Trap-pebbles of the boulder—Thickness of the earth's crust unknown—Not of much consequence to the practical geologist—Interior of the earth in a highly heated condition—Proofs of this—Granite and hypogene rocks—Trap-rocks; their identity with lavas and ashes—Scenery of a trappean country—Subdivisions of the trap-rocks—Intrusive traps—Trap-dykes-intrusive sheets—Salisbury Crags—Traps of the neighbourhood of Edinburgh—Amorphous masses—Contemporaneous trap-rocks of two kinds—Contemporaneous melted rocks—Tests for their age and origin—Examples from neighbourhood of Edinburgh—Tufas or volcanic ashes—Their structure and origin—Example of contemporaneous trap-rocks—Mode of interpreting them—Volcanoes of Carboniferous times—Conclusion.
In the previous pages, allusion has been made to the trap-pebbles imbedded in the boulder, to the various forms of decay exhibited by granitic and trappean rocks, and to the elevation and depression of the solid crust of the earth. Will the reader bear with me for but a few pages more, while I seek to indicate one or two points of interest in a branch of geology that would abundantly reward a diligent observer? Since the days of Hutton, the investigation of what are called igneous rocks has fallen somewhat into the background, and geologists have given themselves, perhaps too exclusively, to the study of organic remains, so that while the palæontology of the British islands has enjoyed an extensive exploration, but little has been done towards the elucidation of our igneous formations and their accompanying phenomena. Much remains to be accomplished, even in those districts usually regarded as in a manner thread-bare, and he must be but an indifferent observer who cannot add a few gleanings to the general stock of information upon this branch of British geology.
Many conjectures have been formed, and many theories propounded, as to the nature of the internal parts of our globe. Some have supposed that there is an outer solid film or crust, some ten or twenty miles thick, enveloping a vast ball of intensely heated matter; others have attempted to show that the interior must be nearly solid throughout, with, however, great lakes, or vesicles of gas and melted rock, somewhat after the fashion, we may suppose, of the oil-holes in a Gruyère cheese. But whether the heated material occupy the whole or only parts of the internal area, is not of much consequence to the practical geologist; he is content to believe that it exists, and in sufficient quantity, too, to produce the most momentous changes on the surface of the earth. We see the effects of this subterraneous agent in earthquakes and volcanoes, and the geologist can tell us of similar, as well as of other changes, effected by it during past ages. Granite hills, and mountainous districts of mica-slate and gneiss, bear evidence of what is termed metamorphism—a change in the mineral structure of rocks, believed to have taken place through the agency of heat deep in the interior of the earth; for no analogous appearances have been detected in progress at the surface. Such rocks, known as metamorphic, or hypogene, still form a difficult problem, not likely to be satisfactorily solved until the chemist shall have thoroughly investigated the subject; for it seems likely to be found, after all, that long-continued chemical action, without a very alarming degree of heat, may have produced even the most intense metamorphism. But dropping this part of the subject, in which so much yet remains to be discovered, let us look for a little at another branch of the geologist's evidence, where we meet with no such hampering hypotheses and doubtful conjectures, namely, the trap-rocks.
Every one knows that basalt, lava, pumice, scoriæ, and ashes, are the various matters ejected from volcanoes. When these materials are found interstratified among the various geological formations, they are termed trap-rocks,—a name derived from the Swedish trappa, a stair, in allusion to the step-like or terraced appearance which they often present. They are of all ages, having been detected in the lower Silurians of Wales, and in the deposits of all subsequent periods up to the volcanic eruptions of the present day; thus evidencing, that from the remotest times there have been Ætnas and Vesuvii slumbering perhaps for ages, and then awakening to lay the surrounding districts in ruins. I have already said that the rocks from which the geologist has to compile his history, are mostly relics of the sea; hence most of the trap-rocks which he meets with in his explorations are the products of submarine eruptions. Far away down among the Silurian rocks, he can trace the floor of a primeval ocean thickly covered with stone-lilies, trilobites, and molluscs, and in following it out he marks how ashes and lapilli, ejected from some submarine orifice, settled down amid the organisms and well-nigh destroyed them, while at other times streams of molten matter were poured out along the sea-bottom, and hardened into masses of solid rock. He sometimes even encounters what seems the vent whence these eruptions proceeded, filled up now by a boss or plug of hardened trap, but he never can detect any trace of land. Some of these oceanic volcanoes may, like Graham's Island in the Mediterranean, have raised their tops above water, sending clouds of steam and cinders far and wide through the air, but the waves would eventually wear down the new-born land, and scatter its broken fragments along the floor of the sea. Among the carboniferous rocks of Scotland, however, we meet with a different condition of things. There, too, we can trace out submarine lava-streams, and mark how showers of ashes destroyed the delicate organisms of the deep; but we encounter, besides, undoubted traces of a land, not parched and ruinous as though the igneous forces had laid it waste for ever, but thickly clothed with vegetation of a more luxuriant type than that which clusters over Vesuvius and Calabria, or lies spread out across the "level plains of fruit-teeming Sicily."[74] We have looked at the plants and animals of the Carboniferous era; its rivers and deltas; its slow elevations and depressions of the ground. It may, perhaps, complete the picture of that ancient period, if we examine, though but briefly, its igneous eruptions, the more especially since these may be regarded as, to a considerable extent, typical of trap-rocks belonging to every age and every country.
[74] Της καλλικἁρπον Σικελἱας λενροὑς γὑας. Æsch. Prom. Vinct. 369—a passage graphically descriptive of an ancient eruption of Ætna.
Unless when deeply buried beneath drift-sand and clay, trappean regions usually possess scenery of a marked kind. A green undulating country stretches out as far as the eye can reach, diversified here and there with bold abrupt crags and conical hills. The lower grounds show in the winter season their rich brown loam, that waxes green as the spring comes on, and ere summer's close spreads out its heavy crops of golden grain. The higher ridges are for the most part thickly wooded, yet the soil is often scanty, and, among the white stems of the beech, or the matted roots of the fir and the elm, we may not unfrequently see the rock protruding its lichen-crusted face, mottled with mosses and liverworts, while some sluggish runnel collects in stagnant pools, or trickles over the blocks with a thick green scum. Sometimes the hill has never been planted, but stands up now, as it has done for centuries; its western face craggy and precipitous, with bushes of sloe-thorn and furze, and stray saplings of mountain-ash clinging to the crevices, while its eastern slope sinks down into the rolling country around with a green lumpy surface, through which, at many a point, the grey time-stained rock may be seen. The whole district suggests to the fancy a billowy sea, and, as one casts his eye from some commanding hill-top athwart the wide expanse of hill and valley, sweeping away in endless undulations, he is apt to bethink him of some day far back in the past, when the verdant landscape around lay barren and desolate, while the solid earth rocked and heaved in vast ground-swells like a wide tempested ocean. Such is the aspect presented by some of the more trappean regions of Scotland. But the origin of this kind of scenery must be ascribed to the effects of denuding currents in scooping out the softer strata into clefts and valleys, and leaving the harder trap-rocks in prominent relief, rather than to any great inequality of surface produced by the eruption of igneous matter; for we shall find that the throwing out of sheets of lava and showers of volcanic ashes was often a very quiet process after all.