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the hill-tops at the south end of the island of Eigg, and forms the well-known Scuir. Seen from the sea on the east side, this ridge rises as a lofty massive column, towering to the height of some 400 feet above the high ground on which it stands, and 1289 feet above the sea. Its sides are quite vertical, so much so, that if one has a steady enough head to stoop over the edge of the precipice he may see its base 400 feet below. What seems a broad and lofty tower, when looked at from the east, is really the

FIG. 36.-Section of west end of Scuir of Eigg, showing the basalt sheets traversed by dykes and covered with an old river gravel which is buried under pitchstone.

abrupt end of a long narrow ridge, which widens out westward until it loses itself in a mass of rugged ground, abounding in little rock-basins filled with water. The Scuir itself, with these broken heights into which it merges, consists of a black glassy rock known as pitchstone, almost everywhere columnar, the columns being sometimes piled up horizontally with their weathered ends exposed, sometimes slanting inwards or outwards, like a chevaux de frise, and often built round a hummock of rock, very much in the way

the peasants stack their peats. It is a thoroughly volcanic rock, having been poured forth as a molten lava and having assumed these columnar forms as it cooled and consolidated.

At either end of the long ridge, this pitchstone is seen to lie upon a hollow eroded out of the underlying level sheets of basalt and filled up with compacted shingle. Among the rounded stones of this shingle-bed there is an abundance of coniferous wood, in chips and broken branches, yet so well preserved that, when newly taken out and still damp, it might be taken, but for its weight, for the relics of some old pine-forest buried in a peat-bog.

The hollow in which the shingle lies is evidently the channel of an ancient stream, which had eroded the older plateau-basalts. At the time when this stream was flowing, the island of Eigg must have been joined to some higher land, probably to the west or north-west, for the stream brought down with it blocks of hard Cambrian sandstone —a rock not found in Eigg, but abundant on the opposite island of Rum. Where now is the ridge of the Scuir there was therefore a valley, watered by a stream that flowed with considerable volume to be able to carry along the blocks, sometimes two or three feet in diameter, which are found in its shingle. A long interval had passed away since the eruption of the basalts, and these rocks had been much abraded by atmospheric waste and running water. But the volcanic eruptions had not finally died out in the west of Scotland. Eventually an eruption of black glassy pitchstone took place. The stream of molten rock rolled along the river-channel, and ascended for a short way the courses of some tributary streams, burying the whole under a mass of solid rock.

But denudation was not arrested by this renewed manifestation of volcanic activity. The process of disinte

gration at once began upon the congealed pitchstone, and has been continued from that time till now. The result of the prolonged waste may be briefly summed up. The land that united the basalt plateaux to mountains of Cambrian sandstone has been destroyed, and Eigg has become an island. The basalts forming the higher grounds that bounded the old river-valley, have been worn down and reduced to slopes that shelve into the sea. So complete has been the change that the buried valley, under protection of the singularly indestructible pitchstone, now runs along the top of a ridge. What were once hills have disappeared, and what used to be a valley is now the crest of a lofty hill. The pitchstone which, when it rolled down that ancient water-course, sought the lowest level it could find, rises to-day into one of the most conspicuous landmarks in the west of Scotland. Yet even of this firm rock only a fragment now remains, which is mouldering into ruins, and the debris of which is strewn thickly along the base of the cliffs. Every century must certainly, though perhaps to human eye imperceptibly, lessen the size of the Scuir, and we can look to a distant time when the last remains of it shall have disappeared. The lowering of the basalt-ground, no longer protected by the more durable pitchstone, will then proceed apace.

From all the evidence which has now been adduced it will, I think, be manifest to the reader that a comparison of the external configuration of the Highlands with their geological structure, inevitably leads us to the conclusion that of the original surface of this part of the globe, as it was left after the crumpling and dislocation of the schists, not a vestige can possibly now remain; that thousands of feet of solid rock have since then been worn away from it, and that the present inequalities of the ground, instead of

being memorials of primeval convulsions, are monuments of prolonged denudation. But while these deductions compel our assent, they by no means exclude all influence of subterranean movement upon external features. It will be my aim in subsequent chapters to bring forward abundant proof of that influence, and to show how it can be traced even where the proofs of stupendous denudation are clearest. The positions into which the rocks were thrown by contortions and dislocations have, in many cases, materially guided the powers of waste in the long process of superficial degradation. Larger features, such as hill ranges and lines of valley, have had their general trend determined by that of the anticlinal and synclinal foldings of the strata. Minor details, which give individuality to the forms of cliff, crag, and mountain, have been largely dependent upon the several structures superinduced by underground movements upon rocks.

But alike in the greater and the lesser elements of the scenery, there has been a presiding power of erosion, which, though its working has been modified by local circumstances, has laid its finger on every rood of the surface, and has carved out for itself the present system of glen and mountain, valley and hill.

The levelling down of the ancient table-land of the Highlands is shown to be of high geological antiquity. The process was begun before the Lower Old Red Sandstone, and has been continued with many interruptions from that time until to-day. That the table-land should now be traceable only in fragments, that it is cut down by wide straths and deep glens, and that its general surface has been most unequally eroded, need not be matter for surprise. When we reflect on its extreme age and on the long cycle of geological revolutions that have befallen it, the wonder rather is that any trace of it should now remain.

CHAPTER VIII

THE HIGHLAND VALLEYS

THOUGH an eminently hilly country, Scotland is not dominated by any leading mountain chain, on which all the other topographical features are dependent. Even in the Highlands, where the highest elevations are reached, ridge succeeds ridge in endless succession, not one of them ever attaining such an altitude as to mark it out as a great central axis of upheaval. Nor is there any more evidence of a dominant line of elevation among the Southern Uplands. Seen, indeed, either from a distance or from any commanding summit in their midst, the high grounds of Scotland, as I have already remarked, seem to undulate up to a common average level, and are to be considered rather as a broken and sorely wasted table-land than as a series of true mountains. Careful examination soon shows that the dominant features are not the monotonous ridges, but the valleys that have been opened through them. If these valleys were filled up, the high grounds, as we have now seen, would once more become what they probably were at first, elevated plains or plateaux, with no strongly-marked features, —no eminences rising much above nor hollows sinking much below the general surface.

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