Nowhere are the distinctive topographical features and geological structure of the basalt-plateaux better displayed than in the northern half of the island of Skye. The green terraced slopes, with their parallel bands of brown rock formed by the outcrop of the nearly flat basalt-beds, rise from the bottoms of the valleys into flat-topped ridges and truncated cones (Fig. 283). The hills everywhere present a curiously tabular form that bears witness to the horizontal sheets of rock of which they are composed.[262] And along the sea-precipices, each excessive sheet of basalt can be counted from base to summit, and followed from promontory to promontory (Figs. [284], [286]). In the district of Trotternish, the basalt hills reach a height of 2360 feet. Further west, the singular flat-topped eminences, called "Macleod's Tables" ([Fig. 283]) ascend to 1600 feet.
[262] These features are more fully described in my Scenery of Scotland, 2nd edit (1887), pp. 74, 145, 216.
Fig. 283.—Terraced Hills of Basalt Plateau (Macleod's Tables), Skye.
Along the western side of Skye, the basalts descend beneath the level of the Atlantic, save at Eist in Duirinish, where the Secondary strata, with their belt of intrusive sills, rise from underneath them, and at the Sound of Soa, where they rest on the Torridon Sandstone. Along the eastern side, their base runs on the top of the great Jurassic escarpment, whose white and yellow sandstones rise there, and on the east side of Raasay, into long lines of pale cliffs. To the south-east, the regularity of the volcanic plateau is effaced, as in Mull and Ardnamurchan, by the protrusion of extensive masses of eruptive rocks constituting the Cuillin and Red Hills, east of which the basalts have been almost entirely removed by denudation, so as to expose the older rocks which they once covered, and through which the younger eruptive bosses made their way. This is undoubtedly the most instructive district for the study of that late phase in the volcanic history of Britain comprised in the eruptive bosses of basic and acid rocks.
The magnificent plateau of this island has been so profoundly cut down into glens and arms of the sea, and its component layers are exposed along so many leagues of precipice, that its structure is perhaps more completely laid open than that of any of the other Tertiary volcanic areas in Britain. It is built up of a succession of basalts and dolerites of the usual types, which still reach a thickness of more than 2000 feet, though in this instance, also, denudation has left only a portion of them, without any evidence by which to reckon what their total original depth may have been. In rambling over Skye, the geologist is more than ever struck with the remarkable scarcity and insignificance of the interstratifications of tuff or of any other kind of sedimentary deposit between the successive lava-sheets. One of the thickest accumulations of volcanic tuff and conglomerate has already been referred to as occurring on the south side of Portree Harbour, where it attains a depth of about 200 feet. As it is in immediate connection with its parent vent, it will be more fully alluded to in Chapter xli. Here, as is so generally observable among the basalt-plateaux, traces of vegetation are plentiful among the stratified intercalations, even forming thin seams of lignite and coal, one of which was formerly worked. That volcanic eruptions, though possibly of a feebler kind, continued during the interval between the basalt-outflows at this locality, is shown by the thick accumulation of tuff and by the occurrence of abundant lapilli of fine basic pumice among the shales, even to a distance of several miles from the vent.
Fig. 284.—"Macleod's Maidens" and part of Basalt Cliffs of Skye.
Another conspicuous intercalation of sedimentary materials in the Skye plateau occurs on the Talisker cliffs at the mouth of Loch Bracadale, where, on the face of the great precipice of Rudha nan Clach, some conspicuous bands of lilac and red are interspersed among the basalts. These bands were noticed by Macculloch, who described them as varieties of "iron-clay."[263] I have not had an opportunity of examining them except from the sea at a little distance. But they suggest a similarity to some of the variegated clays between the upper and lower basalt series of Antrim.
[263] Western Islands, vol. i. p. 376.