Tufas and contemporaneous melted traps are very generally found together interstratified regularly with each other, and the inference to be drawn from their juxtaposition is of course simply this, that at one time liquid lava rolled along the bottom of the water, while at another showers of volcanic dust and cinders settled down in successive beds. This active play of the igneous forces took place at the mouths of estuaries or farther to sea; and it is accordingly sometimes not a little interesting to trace, amid the sediment that accumulated below the water during the pauses between the eruptions, well-preserved remains now of plants that had come drifting from the land, anon of slim spirifers, and producti that swarmed upon the hardened lava-streams, and amid the thickening volcanic mud that slowly sank to the sea-bottom. Such a sequence of events will be made plain from the following section, the materials of which are derived from different parts of the trappean region of Linlithgowshire. The undermost bed here shown (1) is one of marine limestone, abounding with encrinal joints, corals, spirifers, and other undoubtedly marine organisms. Above it comes a layer of tufa or volcanic ash (2) of a dull green aspect, the boundary line between the two rocks lying as clear as if the quarryman had marked it off with his foot-rule. The upper part of the ash, however, does not show an equally clear line of demarcation with the stratum above. On the contrary, it gradually changes its character, becomes more calcareous as it goes up, with here and there a stone-lily joint or a stray productus, until these organisms increase so much in number that the rock insensibly passes into an ordinary limestone (3) like that below. Next succeeds a thin seam of ash (4) resting sharply on the limestone and overlaid by a bed of shale (5) containing the same marine organisms. Another stratum of ash (6) resembling those below follows the shale, and is surmounted by a close compact greenstone (7) that hardens the ash on which it rests, but produces no apparent alteration on the soft fissile shale (8) above it. Next is a fourth seam of volcanic ash (9) resembling those below it, but without any shells or crinoidal joints, the only fossils observable being a few carbonized stems apparently of calamites and lepidodendra. Above it comes a bed of white quartzy sandstone (10) with similar vegetable remains, and then a layer of white stiff fire-clay (11) with rootlets of stigmaria, above which lies a seam of coal (12). A thin layer of soft blue shale (13) here intervenes, somewhat baked along its upper portions by another bed of compact vesicular greenstone (14), which displays in places a well-marked columnar structure. It is surmounted by a highly characteristic ash (15) in which there occur numerous large bombs chiefly of trap of different kinds, some of them highly vesicular. Fragments of shale also occur, mingled here and there with black carbonized fragments of coal-measure plants, but without any of the shells and other marine organisms so abundant below. The topmost bed is a grey carbonaceous sandstone (16), underlying a thin covering of vegetable mould.
Fig. 35.—Contemporaneous Trap.
Such is the skeleton, as it were, of the section; the mere dry bones which remain to the geologist, and which he must study closely to be able to give them life again. The lowest bed visible, with its stone-lilies and molluscs, we readily recognise as marking an old ocean-bed, so that the little episode in the primeval records of our planet here presented to us opens, like the two great epics of antiquity, within sound of the wide-roaring sea. The seam of ash which follows shows, from the sharpness of its line of demarcation with the limestone, how the denizens of the sea-bottom were suddenly destroyed by a thick shower of volcanic dust that settled down over their remains. The waters, however, soon cleared, and ere long stone-lilies and producti were plentiful as ever, mingling their remains among the upper layers of the soft muddy ash, and giving rise therefrom to a sort of calcareous ash or ashy limestone, until in the course of time the volcanic matter became wholly covered over by a seam of ordinary limestone. The corals and stone-lilies were, however, anew destroyed by the deposition of volcanic dust that settled over them as a seam of ash, after which the water was again rendered turbid and muddy by the inroad of foreign matter, which, brought down by rivers or by the changing currents of the ocean, sank to the bottom and eventually consolidated into a seam of shale. Thereafter the volcanic forces began once more to eject a quantity of dust and scoriæ that fell into the water and spread along the bottom as a stratum of ash, and to pour out a current of lava which hardened into a great sheet visible now as the undermost greenstone of the section. The emission of the lava seems to have terminated the eruption, for the next stratum is one of shale like that below the ash, so that the muddy sediment, the deposition of which was interrupted for a while by the volcanic products, began afresh to settle down along the sea-bottom. This last condition of things seems to have continued for a considerable period, seeing that the shale bed is relatively thick, and from its fissile laminated structure indicates a slow and tranquil deposition. Another eruption of volcanic dust and ashes again interrupted the detrital deposits, and gave rise to another seam of tufa. This last subterranean movement seems to have considerably altered the general contour of the sea-bottom, and so elevated it, at least at one part, that a thick accumulation of sand, and subsequently of clay, filled it up to the level of the water or nearly so, giving rise to a dense growth of the stigmaria and other coal-measure plants whose roots are still seen imbedded in the fire-clay on which, as a soft muddy soil, they originally grew. It is probable, however, that, notwithstanding such elevations of the sea-bed, there was a general subsidence of the ground during the accumulation of these strata, for we see that the peaty morass, represented now by the coal-seam, ere long sank beneath the waters, with the inroads of which it was unable to keep pace, while there slowly silted over it a muddy sediment that hardened at length into what is now a seam of shale. But this order of things had been in existence for but a comparatively short period when the igneous forces broke out again, ejecting a stream, of molten lava that spread along the bottom of the shallow waters and hardened as before into a sheet of greenstone. This was followed by an abundant shower of dust and lapilli, along with numerous large masses of greenstone and basalt. These falling into the water accumulated on the upper surface of the lava-stream, then somewhat cooled, and formed in the end a stratum of ash of a rubbly conglomeritic aspect. That the sheet of greenstone really spread out along the sea-bottom before the ejection of the ash, and was not intruded among the beds at a later period,—that, in short, it must be regarded as a contemporaneous and not as an intrusive rock, seems sufficiently shown by its great regularity and evenness, and by the unaltered condition of the fine soft felspathic matter which covers its upper surface. It was assuredly in a highly-heated condition when poured out, as may be gathered from the baked aspect of the mud over which it rolled; but it had cooled and solidified, at least along its upper surface, ere buried beneath the shower of ashes. The last bed exhibited in the section is a grey sandstone, with many carbonaceous streaks and traces of land-plants, showing a pause in the volcanic activity of the district, during which the streams from the land brought down sandy sediment, with an abundant admixture of macerated leaves, branches, and other drift-wood.
It thus appears that not only were the plains and hills of the Carboniferous era richly clothed with vegetation, and its waters crowded with animals, but that then, as now, subterranean forces were at work, sometimes elevating, sometimes depressing the area alike of the land and of the sea; while, not unfrequently, melted lava rose from below, rolling along the bottom of the waters, and showers of ashes were flung far and wide through the air, settling at last as a thickening sediment along the floor of the sea, or athwart the marshy swamps of the delta. Whether the interior of the land had burning cones among its pine-covered hills we know not yet. Such, however, probably existed; nay, there may have been among the higher peaks some "snowy pillar of heaven," like the Ætna of Pindar, raising its smoking summit among everlasting crags of ice in solitudes lifeless and bare.[77]
[77] The highest points of New Zealand, nearly 10,000 feet above the sea, are said to be clothed for two-thirds of their height with ice and snow. If, therefore, during Carboniferous times, there existed somewhere to the west of what is now central Scotland, a chain cf hills 5000 or 6000 feet high, their summits might perhaps have been as wintry us that of Mont Blanc.
Our boulder has served us like the minstrels in modern Gothic poetry, who appear between the cantos, and give an air of unity and completeness to what would otherwise be often rambling and unconnected. And now, at the close, it comes again before us, lying in its bed of clay, clustered with mosses of brightest green, and overshadowed by its flickering canopy of beechen leaves. Silent and senseless, the emblem, seemingly, of calm repose and unchanging durability, what could we have conceived it should have to chronicle, save the passing, perchance, of many a wintry December and many a sultry June. Such, indeed, would be the character of its records of the centuries that have passed away since the birth of man, did any such record survive in its keeping. But it rests there as the memorial of far earlier centuries, and of an older creation; and though now surrounded with all that is lovely or picturesque—the twinkling flowers on every side, the wide arch of boughs overhead, and the murmuring streamlet in the dell below—and though forming itself no unimpressive object in the scene, the boulder looks out upon us unconnected with anything around. Like a sculptured obelisk transported from the plains of Assyria to the streets of London, it offers no link of association with the order of things around it; its inscriptions are written in hieroglyphics long since extinct, but of which the key yet remains to show us that the rocks of our planet are not masses of dead, shapeless matter, but chronicles of the past; and that all the varied beauty of green field and waving wood is but a thin veil of gossamer spread out over the countless monuments of the dead. We have raised one little corner of this gauze-like covering, and tried to decipher the memorials of bygone creations, traced in clear and legible characters on the boulder. First, there lies spread out before us a wide arctic sea, studded with icebergs that come drifting from the north. Here and there a bare barren islet rises above the waste of waters, and the packed ice-floes often strand along its shores, while at other parts great towering bergs, aground in mid-ocean, keep rising and falling with the heavings of the surge, and seem ever on the verge of toppling into the deep. But this scene, so bleak and lifeless, erelong fades away, and we can descry a wide archipelago of islands, green well-nigh to the water's edge, and looking like the higher hill-tops of some foundered continent. The waves are actively at work wearing down the shores, which present for the most part an abrupt cliff-line to the west. This picture, too, gets gradually dim, and when the darkness and haze have cleared away, the scene is wholly new. For miles around there spreads out an expanse of water, like a wide lake, thickly dotted with islets of every form and size, clothed with a rich vegetation. Here a jungle of tall reeds shoots out of the water, clustering with star-like leaves; there a group of graceful trees, fluted like the columns of an ancient temple, and crowned by a coronal of sweeping fronds, spread out their roots amid the soft mud. Yonder lies a drier islet, rolling with ferns of every shape and size, with here and there a lofty tree-fern, waving its massive boughs high overhead. The vegetation, rank and luxuriant in the extreme, strikes us as different from anything visible at the present day, though, as our eyes rest on the muddy discoloured current, we can mark, now and then, huge trunks, branchless and bare, that recall some of the living pine-trees. The denizens of the water seem to be equally strange. Occasionally a massive head, with sharp formidable tusks, peers above the surface, and then the gleam of fins and scales reveals a creature some twenty or thirty feet long. Glancing down into the clearer spots, we can detect many other forms of the finny tribes, all cased in a strong glistening armature of scales, and darting about with ceaseless activity. Beyond this scene of almost tropical luxuriance, on the one side, lies the blue ocean, with its countless shells and corals, its stone-lilies and sea-urchins, and its large predaceous fish; on the other side stretches a far-off chain of hills, whose nether slopes, dark with pine-woods, sweep down into the rich alluvial plains. And then this landscape, too, fades slowly away, and thick darkness descends upon us. Yet through the gloom we feel ever and anon the rambling earthquake, and see in the distance the glare of some active volcano that throws a ruddy gleam amid the pumice and ashes, ever dancing along the surface of the sea. And now this last scene melts away like the rest, and dark night comes down in which we can detect no ray of light, and beyond which we cannot go. The record of the boulder can conduct us no further into the history of the past.
The same principles which have been pursued in the previous pages in elucidating the history of the Carboniferous system, will conduct the reader to the true origin and age of any group of rocks he may encounter, whatever its nature, and wheresoever its locality. Let him, therefore, in his country rambles, seek to verify them in valley and hill-side, by lake and cataract, and along river-course and sea-shore. Let him not be content with simply admiring the picturesque grouping of rock-masses, but rather seek to interpret their origin and history, tracing them step by step into the past, amid ages long prior to man. Such a process will give him a yet keener relish for the beauties of their scenery, by ever calling up to his mind some of those striking contrasts with which geology abounds. In the stillness of the mountain-glen, he will see on every side traces of the waves of ocean, and when dipping his oars into the unruffled sea among groups of wasted rocks, miles from shore, he will bethink him, perchance, of some old forest-covered land, of which these battered islets are the sole memorials. His enjoyment of the scenery of nature is thus increased manifold, and he carries about with him a power of making even the tamest landscape interesting. Cowper, in one of his exquisite letters, remarks,—"Everything I see in the fields, is to me an object; and I can look at the same rivulet or at a handsome tree, every day of my life, with new pleasure." Had the sweet singer of Olney lived to witness the results obtained by the geologists whom he satirized, he would perhaps have sauntered along the Ouse with a new pleasure, and have felt a yet more intense delight in casting his eyes athwart the breadth of landscape that spreads out around-the "Peasant's nest."
Such, however, are after all only secondary incentives to the study of the rocks. As a mental exercise, geology certainly yields to none of the other sciences, for it addresses itself at once to the reasoning powers and to the imagination, and may thus be made a source both of intellectual training and of delightful recreation. Of none of the sciences is it so easy to get a general smatter, yet none is so difficult thoroughly to master, for geology embraces all the sciences. In so wide a field, the student will therefore find ample room to expatiate. In beginning the study, he may perhaps think it, as Milton pictured the other paths of learning, "laborious, indeed, at the first ascent; but else so smooth, so green, so full of goodly prospects and melodious sounds on every side, that the harp of Orpheus was not more charming." If time and taste disincline him to travel over the whole of the broad field, there are delightful nooks to which he may betake himself, replete with objects of beauty and interest, where he may spend his leisure, and by so doing not merely delight himself, but enlarge the bounds of human knowledge. No part of the domain can be too obscure or remote to reward his attention; no object too trifling or insignificant: for the march of science, though a stately one, proceeds not by strides, but by steps often toilsome and slow; and she stands mainly indebted for her progress not to the genius of a few gigantic intellects, but to the united efforts of many hundred labourers, each working quietly in his own limited sphere.
But the highest inducement to this study must ever be that so quaintly put by old Sir Thomas Browne: "The world was made to be inhabited by beasts, but studied and contemplated by man: 'tis the debt of our reason we owe unto God, and the homage we pay for not being beasts; without this the world is still as though it had not been, or as it was before the sixth day, when as yet there was not a creature that could conceive or say there was a world." Geology lifts off for us the veil that shrouds the past, and lays bare the monuments of successive creations that had come and gone long ere the human race began. She traces out the plan of the Divine working during a vast cycle of ages-, and points out how the past dovetails with the present, and how the existing condition of things comes in as but the last and archetypal economy in a long progressive series. By thus revealing what has gone before, she enables us more fully to understand what we see around us now. Much that is incomplete she restores; much that is enigmatical she explains. She teaches us more fully man's true position in the created universe, by showing that in him all the geologic ages meet that he is the point towards which creation has ever been tending. How far the facts brought to light by geology may bear upon the future, will not, perhaps, be solved until that future shall have come. There is, nevertheless, in the meanwhile, material enough for solemn and earnest reflection, and as years go by the amount will probably be always increasing. For we must ever be only learners here, and when all earthly titles and distinctions have passed away, and we enter amid the realities of another world, we shall carry with us this one common name alone. It will, perhaps, be then as now, that only