Following still the western seaboard of Skye, we meet with other striking examples of sills at a distance of some eight miles in a straight line eastward where, between Lochs Slapin and Eishort, the prominent headland of Suisnish juts out into the sea. This promontory has long been known to geologists from the section of it given by Macculloch as an instance of the connection between overlying rocks and dykes. I have already alluded to it in that relation, and refer to it again as an example of one of the thicker intrusive sheets of the Inner Hebrides. Denudation has here also proceeded so far that the whole of the volcanic plateau has been stripped off, only some of the underlying sills being left, together with the platform of older rocks between which and the vanished basalts they were injected. Most of these sills consist of granophyres belonging to the acid group of rocks to be afterwards described. But basic sheets occur not infrequently interposed between the granophyres and the subjacent Lias, and sometimes even intercalated in the former rock. Though at first sight it might be thought that these sills had insinuated themselves after the eruption of the granophyre, and there are instances where this cannot be shown not to be the case, I have obtained so many proofs of the invasion of the basic by the acid rock that I have no doubt the former is, as a general rule, the older of the two.
The Suisnish headland exhibits the structure represented in [Fig. 249]. For about 300 feet above the sea-level the steep grassy slope shows outcrops of the dark, sandy shales and yellowish brown, shaly sandstones of the Lias which form the range of cliffs to the eastward. These gently inclined strata are cut through by many vertical basalt-dykes, some of which intersect each other, but among which by far the largest is the mass shown in the figure. This broad dyke consists of a dolerite or gabbro the largely crystalline texture of which marks it off at once from the others, which are of the usual dark, heavy, fine-grained type, with an occasional less basic and porphyritic variety. Traced up from the sea-margin, the dyke loses itself in a talus of blocks from the cliff above, so that its actual junction with the mural front of the sill cannot be seen. But that it joins that mass, with which it agrees in petrographical characters, hardly admits of question. The cliff consists of a thick sheet of coarsely crystalline dolerite or gabbro (d in [Fig. 249]), which in its general aspect at once recalls the rock of Fair Head. It varies considerably in texture, some parts of the mass are exceedingly coarse, like the Skye gabbros, and present a fibrous structure in their augite resembling that of the diallage in these rocks; other portions assume the compactness of basalt. A specimen of medium grain under the microscope shows the typical ophitic structure so generally found among the dolerites both of the plateaux and of the intrusive sheets. This sill must be about 200 feet thick, and like the rock at Fair Head is traversed from top to bottom by joints that divide it into prisms. It appears to bifurcate eastward, one portion running with a tolerably uniform thickness of a few feet as a prominent band at the top of the shales and sandstones, the other slanting upwards and gradually thinning away in the granophyre.
Towards its base, near the contact with the underlying shales, the rock as usual becomes finer grained, and the thin band just referred to resembles in texture one of the wider basalt-dykes. Westwards the rock can be followed round the top of the grassy slopes formed by the decay of the shales. Though concealed by intervals of moorland and peat, it is visible in the stream sections, and I think must be continuous, as a band only a few yards thick, round the northern side of the hills as far as Beinn Bhuidhe, where a similar sill makes a prominent crag. Its total area measures a mile and a quarter in length by half a mile in breadth. The granophyre which overlies it forms part of an interesting series of sheets which I have traced all the way from Suisnish to the braes above Skulamus.
Whether or not the whole sheet of basic rock is continuous, and whether it all proceeded from the great Suisnish dyke, cannot be confidently decided until the ground is mapped in detail, though from the great thickness of the sill at the dyke, its attenuation outwards from that centre and its uniformity of petrographical character, I am disposed to answer affirmatively. There is no other probable vent to be seen in the neighbourhood, unless a massive dyke that runs from Loch Fada north-westwards into Glen Boreraig can be so regarded.
Not far from the extreme southern point of Skye a singularly interesting example of a sill remains as a detached survival of the basaltic plateau and its accompaniments. In his map of Skye, Macculloch showed the position of this outlier, which he classed with the general "trap" formation of the island. The locality was visited by Professor Judd, who regarded the intrusive rock as a "phonolite"[318] In 1894, during an excursion with my colleague Mr. C. T. Clough, I had an opportunity of examining the rocks and collecting notes for the following account of them.
[318] Quart. Jour. Geol. Soc. vol. xxxiv. (1878) p. 692.
At Rudh' an Iasgaich, about two miles from the Point of Sleat, a small outlier of conglomerate lies on the edges of the Torridon Sandstone. This deposit has been correctly identified by Professor Judd with the similar strata which, in Skye and elsewhere on the west coast of Scotland, underlie the Liassic series. It is here about 10 or 12 feet thick, reddish and yellowish in colour, and distinctly calcareous. Its component pebbles consist largely of Cambrian (Durness) limestone, quartzite, and Torridon Sandstone—rocks which all occur in situ in Sleat. It may be compared with the limestone conglomerates of Strath and those which underlie the Lias at Heast on Loch Eishort.[319] That here, as elsewhere in this region, the basement conglomerate was followed by the rest of the Lias and Oolites may be inferred with some confidence from the copious development of the Jurassic series a few miles off, both to north and south. But the whole of this overlying succession of formations has here been swept away, and, but for the protection afforded by the eruptive rocks of Rudh' an Iasgaich, the conglomerate would likewise have disappeared.
[319] Op. cit. vol. xiv. (1857), p. 9; vol. xliv. (1888), p. 71.
Fig. 324.—Section of Dolerite Sill cut by another sill, both being traversed by dykes, Rudh' an Iasgaich, western side of Sleat, Skye.