But to return. We have seen that the long-continued action of the sea has been sufficient to breach and waste away the existing coast-line of western Scotland. When, therefore, such results are produced by so ordinary a cause, need we go to seek the agency of great debacles to explain the denudation of other parts of the country? It is known that at great depths currents have little effect upon the rocks which they traverse, and that their action is greater as it nears the surface. To account for the phenomena of crag and tail, and the general denudation of the country, we may suppose the land to have been often submerged and re-elevated. As hill after hill rose towards or sank below the sea-level, it would be assailed by a strong current that flowed from the west and north-west, until, in its slow upward or downward progress, it got beyond the reach of the denuding agencies. In this way the general contour of the land would be greatly though very gradually changed. Hills of sandstone, or other material of feeble resistance, would be swept away, the harder trap-rocks would stand up bared of the strata which once covered them, deep hollows would be excavated in front of all the more prominent eminences, and long declivities would be left behind them.—(See [Fig. 2].)
If my reader has ever visited the channel of a mountain-torrent—
"Imbres
Quern super notas aluere ripas"—
he must have noticed an exact counterpart to these appearances. When the waters have subsided, the overflowed parts are seen to be covered in many places with sand. Wherever a pebble occurs along the surface of this sand, it has invariably a hollow before it on the side facing the direction whence the stream is flowing, and a long tail of sand pointing down the channel. If we watch the motion of the water along its bed, the denuding agency may be seen actively at work. Every pebble that protrudes above the shallow streamlet arrests the course of the current, which is then diverted in three directions. One part turns off to the right hand of the pebble, and cuts away the sand from its flank; another part strikes off to the left, and removes the sand from that side; while a middle part descends in front of the pebble, and, by a kind of circular or gyratory movement, scoops out a hollow in the sand in front. Behind the pebble the water is pretty still, so that the sand remains undisturbed, and is further increased by the accumulation above it of sediment swept round by the lateral currents. Now, in place of the supposed stream, let us substitute the ocean with its westerly current—for the pebble, a great trap-hill—for the sand, easily friable shales and sandstones, and we have exactly the condition of things which produced crag and tail.
This process of destruction must have been in progress during many geological ages. We may suppose, that in that time the land often changed level, sometimes rising far above the sea, and sometimes sinking deep below it. We can well believe that the surface would often be covered with vegetation; that plants, widely differing from those which are now indigenous, clothed its hill-sides and shaded its valleys; and that animals of long extinct forms roamed over its plains or prowled amid its forests. When the country, in the lapse of centuries, sank beneath the sea-level, all trace of these scenes would eventually be effaced. The westerly currents would soon recommence the process of degradation, uprooting the forests, devastating the plains, wearing down the hills, and scooping out the valleys; and so, when the ocean-bed, in the course of ages, became again dry land, it would arise "another and yet the same." The little valley, where once, perchance, the mastodon used to rest his massive bulk amid a rich growth of ferns, shaded by the thick umbrage of coniferous trees, would emerge a deep glen with bare and barren rocks on either side; the site of the hill whereon herds of the gazelle-like anoplothere were wont to browse, might reappear a level plain; the low-browed rock, under whose shadow the ungraceful palæothere used of old to rest from the heat of the noon-tide sun, might emerge a beetling crag shooting up several hundred feet over the valley. It is by this repeated elevation and submergence, carried on for many ages, that our country has acquired its present configuration.
We can easily picture to ourselves the appearance which the British Islands would thus at different periods present. At one time, nearly the whole of England would be under water, with, however, a few islands representing the higher peaks of Cornwall; others scattered over the site of the West Riding of Yorkshire; and a hilly tract of land over what is now Wales. Scotland must have existed in a sorely mutilated state. A thick-set archipelago would represent the Cheviot Hills, and the country south of the Forth and the Clyde; north of which there would intervene a broad strait, with a comparatively large area of undulating land beyond, stretching across what is now the area of the Grampian Hills. A narrow fiord would run along the site of the Caledonian Canal, cutting the country into two parts, and running far into it on either side as deep lochs and bays. I have had such a condition of things vividly recalled when on the summit of a lofty hill in early morning, while the mists were still floating over the lower grounds, and only the higher hill-tops, like so many islands, rose above the sea of cloud. It was not a little interesting to cast the eye athwart this changing scene, and mark how each well-known peak and eminence looked when deprived of its broad sweep of base. What before had always seemed an abrupt precipitous summit, now took the form of a lonely rock or deep-sea stack, that might have served as a haunt for the gull and the gannet. The long swelling hill rose above the mist as a low undulating island, treeless and barren. It was easy to think of that wide expanse of mist as the veritable domain of ocean, to picture the time when these were veritable islands lashed by the surge, and to conjure up visions of ice-floes drifting through the narrows, or stranding on the rocks, amid a scene of wide-spread nakedness and desolation.
CHAPTER IV.
Interior of the boulder Wide intervals of Geology—Illustration—Long interval between the formation of the boulder as part of a sand-bed, and its striation by glacial action—Sketch of the intervening ages—The boulder a Lower Carboniferous rock—Cycles of the astronomer and the geologist contrasted—Illustration—Plants shown by the boulder once grew green on land—Traces of that ancient land—Its seas, shores, forests, and lakes, all productive of material aids to our comfort and power—Plants of the Carboniferous era—Ferns—Tree-ferns—Calamites—Asterophyllites—Lepidodendron—Lepidostrobus—Stigmaria—Scene in a ruined palace—Sigillaria—Coniferæ, Cycadeæ—Antholites, the oldest known flower—Grade of the Carboniferous flora—Its resemblance to that of New Zealand.
I have likened the boulder to an old volume of the middle ages encased in a modern binding. We have looked a little into the mechanism and history of the boards; in other words, we have gone over the history of the scratched surface of the boulder, of the clays and sands around it, and of that still earlier cycle of denudation whereof the rock itself is probably a relic. Before proceeding to open the volume itself, it will be well that we clearly mark the wide interval in time between the ages represented by the surface-striation and those indicated by the interior of the boulder. When we proceed from the groovings on the outside to the plants within, we pass, to be sure, over scarcely an inch of space, but we make a leap over untold millenniums in point of time. It is as if we had laid our hands on a volume of history which had by some misfortune found its way into the nursery. The first page that catches our eye relates the battle of the Reform Bill, and, on turning the previous leaf, we find ourselves with Boadicea and her woad-coloured soldiery. Now, if one utterly ignorant of the chronology of the country were to be told that the volume related solely to one people, he would at once see from the manners and customs delineated, that the two pages referred to very different states of civilisation, and consequently to widely-separated periods. But he could give no account of how long an interval might have elapsed between the time when London had its inhabitants massacred by Boadicea, and the time when another generation of them was excited by the tardiness of King William iv. He could form no conjecture as to what events might have happened in the meanwhile. The interval might be a century or twenty centuries, wherein the city might have been burnt down fifty times. Clearly, if he wished to make himself acquainted with the intervening history, he would have to betake himself to an unmutilated volume.