ii. SKYE
All through the Inner Hebrides the base of the basalt-plateaux presents abundant examples of sills. The general parallelism of these intrusive sheets to the bedding of the Jurassic strata among which they lie has been above referred to as having given rise to the erroneous conclusion that in Skye and elsewhere the basalts are interstratified with Jurassic rocks, and are consequently of Jurassic age. It was Macculloch who first described and figured in detail the proofs of their intrusive nature. His well-known sections in plate xvii. of the illustrations to his work on the Western Islands have been repeatedly copied, and have served as typical figures of intrusive igneous rocks.
Nowhere in North-Western Europe can the phenomena of sills be studied so fully and with such exuberance and variety of detail as in the island of Skye and its surrounding islets. On the western coast the greater subsidence of the basaltic plateau has for the most part submerged the platform of intrusive sheets, though wherever the base of the bedded lavas is brought up to the surface the accompanying sills are exposed to view. The east coast of the island has been classic ground for this part of volcanic geology since it supplied the materials for Macculloch's descriptions and diagrams. From the mouth of Loch Sligachan to Rudha Hunish, at the north end of Skye, a series of sills may be traced, sometimes crowning the cliffs as a columnar mural escarpment, sometimes burrowing in endless veins and threads through the Jurassic rocks. The horizontal distance to which this continuous band of sills extends in Skye is not far short of 30 miles. But it stretches beyond the limits of the island. It forms the group of islets which prolongs the geological structure and topographical features of Trotternish for 4 miles further to the north-west. It reappears 10 miles still further on in the Shiant Isles. Thus its total visible length is fully 40 miles, or if we include some outlying sills near the Point of Sleat, to be afterwards described, it extends over a distance of not less than 60 miles. From the last outlier in Skye to the sills of the Isle of Eigg is a distance of only 8 miles, thence to those of Ardnamurchan 17 miles, and to those of the south coast of Mull 25 miles. Thus this platform of intrusive sheets of the Inner Hebrides can be interruptedly followed for a space of not less than 110 miles.
Fig. 318.—View of the Trotternish Coast, showing the position of the band of Sills.
The dark band crowning the first slope above sea-level marks a conspicuous band of sills which towards the right descends to the beach and is prolonged seaward in the group of islands. The Storr Rock appears as a slanting obelisk of rock on the hill to the left.
Though none of the sills in Skye itself attain the dimensions of the Fair Head sheet, they present a greater variety of rock and of geological structure than is to be found in Antrim. They are specially developed at the base of the thick, overlying, basalt-plateau—a platform on which such a prodigious quantity of eruptive material has been injected. Part of this material consists of basic rocks in the form of dykes, veins, or sills; part of it is included in the intermediate and acid groups, and comprises veins, sheets, and bosses of granitoid, felsitic, rhyolitic, trachytic, and pitchstone rocks. One of the peculiarities of the Skye sills is the occurrence among them of compound examples, where sheets of basic and acid material have been injected along the same general platform. These will be more specially referred to in Chapter xlviii. With regard to the basic sills (dolerites, basalts, etc.), I would remark that while in Western Scotland the Antrim type of short, thick intrusions, or laccolites, is also found, the vast majority of the sheets are much thinner, more persistent, and less easily distinguishable from the bedded basalts.
Fig. 319.—Columnar Sill intrusive in Jurassic Strata east of Kilmartin, Trotternish, Skye.
[The high ground to the left is a portion of the basalt-plateau to the north of the well-known Quiraing.]
In describing the sills of Skye I shall take first those of the eastern and then those of the western side of the island. Along the east coast, from Loch Sligachan to the most northerly headlands and islets the sills play a notable part in the scenery, inasmuch as they cap the great sea-cliff of Trotternish and run as a line of ridges parallel to the trend of the coast, while the plateau-basalts rise above them further inland as a lofty escarpment, which includes the picturesque landslips of the Storr Rock and Quiraing (Figs. 318, 319). Beneath the thick sills, the Jurassic sandstones form a range of pale yellow precipices, along which many thinner sheets of eruptive material have been intruded. As Macculloch well showed, many of these sheets, if seen only at one point, might readily be taken for regularly interstratified beds, but perhaps only a few yards distant they may be found to break across the strata and to resume their course on a different level.
The sills of this Trotternish coast may be distinguished even at some distance from the bedded basalts by the regular prismatic jointing, already referred to, and by their frequently greater thickness, while on closer inspection they are characterized by their much coarser texture. They are generally somewhat largely crystalline ophitic dolerites, gabbros or diabases, and exhibit the persistent uniformity of composition and structure so characteristic of intrusive sheets and dykes. These characters are well exhibited in the Kilt Rock, a columnar sill capping the cliffs to the south of Loch Staffin ([Fig. 319]).