Fig. 2 "Crag and tail."
But when we come to measure the actual amount of material that has been carried away, we are lost in conjecture as to the vastness of the time which such a process must have occupied. For instance, the coal-bearing strata of Mid-Lothian must at one period have been connected with those of Linlithgow and Stirling. At a subsequent date, the western area subsided to form the Stirlingshire coal-basin, and the eastern area, in like manner, sank down to form the coal-basin of Mid-Lothian, while the intermediate portion stretched from east to west as a great arch, or, as it is termed geologically, an anticlinal axis. Now, the whole of this arch has been worn away, not a vestige of it remains, and yet its upper or coal-bearing part was fully 3000 feet thick.[12]
[12] This remarkable example of denudation was first described by Mr. M'Laren, in his Sketch of the Geology of Fife and the Lothians, a work in which the author showed himself to be in advance of the science of his time.
Let us take a small portion of this district, and endeavour to calculate the amount of matter thus removed. The Pentland hills form a chain stretching from near Edinburgh for some fourteen miles southward, and having an average breadth of about two miles and a half. They are formed chiefly of felspathic trap-rocks, resting upon and interstratified with conglomerate apparently of Old Red age, which in turn lies upon vertical Silurian slates. Before the Carboniferous strata were thrown down by successive faults, they must have covered these hills completely to a depth of not less than 6000 feet.[13] From this small area, therefore, stratified sandstones, shales, limestones, and coal, must have been removed to the enormous extent of one billion, eight hundred and fifty-four thousand, four hundred and sixty-four millions of cubic feet.
[13] The actual depth of the Mid-Lothian coal-field, to the base of the carboniferous limestone, is rather more than 3000 feet. It is, perhaps, rather under than over the truth to allow 3000 feet for the total thickness of beds from the limestone to the conglomerate of Liberton, though, owing to the curved and contorted position of the strata from Edinburgh to Stirlingshire, it is impossible to obtain a measurement of their real thickness. I have attributed the isolation of the Falkirk and Mid-Lothian coal-fields to the effect of faults and general depressions of their areas. This was assuredly the case in the latter coal-field, and probably in the former also. The trap which occurs between them, though in great abundance, has certainly not acted as an elevating agent. It occurs in beds among the strata, and, judging from the number of associated tufas, appears to have been to a considerable extent erupted while the lower carboniferous series was forming. Mr. M'Laren, in his excellent work, p. 100, states his opinion that the traps may have materially contributed to push up the coal strata. A careful and extended examination of the district has convinced me that this view is incorrect.
But, perhaps, the most striking instances of denudation in the British Islands are the three famous Ross-shire hills—Suil Veinn, Coul Mor, and Coul Bheig. They are formed of piles of sandstone beds like tiers of regular masonry, and reach a height of 3000 feet over the sea. The sandstone of which they are composed must once have formed a bed or set of beds fully 2000 feet thick, that covered the whole district for many miles around. Yet of this extensive deposit there now exist only a few isolated fragments. I have watched the sunshine and shadow of an autumn sky resting alternately on these strange pyramidal hills, as they towered in their giant proportions like the last remnants of a mighty rampart that had stood the brunt of a long siege, and, breached at last in many places, had been all but levelled to the ground. How long-continued and how potent must that agency have been which could cut down and disperse the massive barrier that flanked the western coast of Ross-shire to a height of 2000 feet!
The Hebrides are but the shattered relics of an old land that had its mountain-peaks and its glens, its streams and lakes, and may have nursed in its solitude the red-deer and the eagle, but was never trodden by the foot of man. A glance at the map is enough to convince us of this. We there see islands, and peninsulas, and promontories, and deep bays, and long-retiring inlets, as though the country had been submerged and only its higher points remained above water. The conviction is impressed more strongly upon us by a visit to these shores. We sail through the windings of one of the "sounds," and can scarcely believe that we are on the bosom of the salt sea. Hills rise on all sides, and the water, smooth as a polished mirror, shows so pure and limpid that in the sunshine we can see the white pebbles that strew its bed many fathoms down. The eastern shore is often abruptly interrupted by long-receding lochs edged round with lofty mountains, and thus, where we had looked to see a deep heathy glen, with, perchance, a white tree-shaded mansion in the far distance, and a few dun smoking cottages in front, we are surprised to catch a glimpse of the white sails of a yacht, or the darker canvas of the herring-boats. We sail on, and soon a sudden turn brings us abruptly to the mouth of the sound. A bold headland, studded around with rocky islets, rises perpendicularly from the sea, bleak and bare, without a bush or tree, or the faintest trace of the proximity of man. The broad swell of the Atlantic comes rolling in among these rocks, and breaks in foam against the grey cliffs overhead. In tempests, such a scene must be of the most terrific kind. Wo to the hapless vessel that is sucked into the vortex of these breakers, whose roar is sometimes heard at the distance of miles! Even in the calmest weather the white surf comes surging in, and a low sullen boom is ever reverberating along the shore. We see the harder rocks protruding far into the sea, and often pierced with long twilight caves, while the softer ones are worn into deep clefts, or hollowed out into open bays strewed over with shingle. The sunken rocks and islets, scarcely showing their tops above water, were all evidently at one time connected, for, as we recede from the shore, we can mark how the process of demolition goes on. There is first the projecting ness or promontory, well-nigh severed from the mainland, but still connected by a rude arch, through which the swell ever gurgles to and fro. Then, a little farther from the shore, a huge isolated crag, washed on all sides by the surge, raises its grey lichen-clothed summit. A short way beyond, there is the well-worn islet whose surface shelters neither lichen nor sea-weed, but is ever wet with the dash of the waves. Further to the sea, the white gleam of the breakers marks the site of the sunken rock. Thus, in the space of a hundred yards, we may sometimes behold the progress of change from land to sea, and see before us a specimen of that action which slowly but yet steadily has narrowed and breached the outline of our western shores.[14]
[14] I have endeavoured to illustrate the process of denudation by a reference to breaker-action on the existing coast-line of the Hebrides; but a strong current must have materially increased the force of the ancient waves, and produced abrasion to some depth below them.
If we attempt to trace the connexions of strata among the Hebrides, we shall be more fully impressed with the magnitude of the changes which have been effected. Thus the Lias and Oolite occur in patches along the shores of Mull, Morven, Ardnamurchan, Eigg, Skye, Raasay, and Applecross. But though now only in patches, these formations must once have extended over a considerable area, for they seem to form the under-rock of the whole of the northern part of Skye, and are seen in almost every lone island from Ardnamurchan Point to the Shiant Isles. These scattered portions, often many miles distant from each other, are the remnants of a great sheet of liassic and oolitic strata, now almost entirely swept away, and are extant from having been covered over with hard trap-rocks. But for these it may be doubted whether we should ever have known that corals once gleamed white along the shores of Skye, that the many-chambered ammonite swam over the site of the Coolin Hills, that the huge reptilian monsters of these ancient times, icthyosaurs and plesiosaurs, careered through the waters that laved the grey hills of Sleat, and that forests of zamia and cycas, and many other plants indicative of a warm climate, bloomed green and luxuriant along the site of that strange mist-clad cliff-line, that shoots up into the pinnacles of the Storr and Quiraing. It is curious to reflect, that the records of these peaceful scenes have been preserved to us by the devastating eruptions of volcanic forces; that the old lava-streams which spread death through the waters along whose bed they travelled, have yet been the means of protecting the districts which they wasted, while those parts where they did not reach have been long since swept away. It is allowable to believe, that in the portions of liassic strata which have been destroyed there existed the remains of not a few species, perhaps some genera, to be found nowhere else, and of whose former existence there is now, by consequence, no trace. In the small island of Pabba—a relic of the Scottish Lias—I found thirty-one species, of which Dr. Wright has pronounced four to be new.[15] A subsequent visit to the adjacent island of Raasay has increased the list. In short, every patch of these Secondary rocks, if thoroughly explored, might be found to yield its peculiar organisms. And in the far larger area that has been carried away there existed, doubtless, many more. We are accustomed to see individuals perish and their remains crumble away, but the species still holds on. In the stratified portion of the earth's crust, however, we mark how not merely individuals have perished, but whole genera and species; but of these the remains are still before us in the rocks; we can study their forms, and, from a comparison with recent species and genera, can arrive at some idea of their nature and functions. In this way, we are able to picture the various conditions of the earth when these organisms lived in succession upon its surface. Yet, we may readily conjecture, that in ancient eras many tribes and genera of plants and animals lived for ages, and then passed away without leaving any record of their existence. Many circumstances might concur to prevent the preservation of their remains. The species of the Hebrides were preserved in the usual manner, but the cemetery in which their remains were entombed has been washed away, and they can be seen nowhere else. It is as if on some isolated country there had lived a race of men, tall Patagonians, or swarthy Hottentots, or diminutive Laplanders, with a civilisation of their own; owing to some change of climate the race gradually dwindled down until it died out; eventually, too, the land settled down beneath the sea with all its ruined cities and villages, which, as they reached in succession the level of the waves, were torn up and dispersed, and other races at last voyaged over the site of that old land, dreaming not, that in bygone years fellow-mortals of an extinct type had pastured their herds where now there rolled a widespread sea.
[15] Quart. Jour. Geol. Soc., vol. xiv. p. 26.