In contrast to such enormous thicknesses of intrusive material as those of Trotternish and the Shiant Isles, instances may be culled from the same belt of sills where the molten rock has been injected in thin leaves and mere threads into the Jurassic sandstones and shales, or into the shales and coals intercalated among the plateau-basalts. Thus, on the cliff immediately to the north of Ach na Hannait, between Loch Sligachan and Portree Bay, the section may be seen which is represented in [Fig. 321]. At the base lies a vesicular dolerite with a slaggy upper surface (a). Next comes a zone of sedimentary material about five or six feet thick, the lower portion consisting of an impure coal, which passes towards the right hand into brown and grey carbonaceous shale with plant-remains (b). This coaly layer has been already alluded to as probably lying on the same horizon with the coal of Portree (p. 288). Traced northward, it is found to have a bed of fine tuff beneath it, and sometimes a volcanic breccia or conglomerate. It fills up rents in the underlying slaggy lava, and was undoubtedly deposited upon the cooled surface of that rock. Immediately above this lower band the black carbonaceous shale which follows has been invaded by an extraordinary number of thin cakes or sills and also by veins or threads of basalt. For a thickness of two or three feet the band (d) consists mainly of these intrusions, which, in the form of a fine grey basalt, vary from less than an inch to three or four inches in thickness. They are separated by thin partings of coaly shale, and as they tend to break up into detached nodule-like portions, especially towards the right hand of the section, they might, on casual inspection, be easily mistaken for nodules in the dark shales. Somewhat later in the time of intrusion are veins of basalt which, as at c, break across the nodular sills, and sometimes expand into thicker beds (c′).
I have never seen such a congeries of minute sills among the Tertiary basalt-plateaux as that here exhibited. In a space of about three feet of vertical height there must be more than a dozen of roughly parallel leaves of intrusive rock. Veins (e) run up from the chief band of eruptive material into the overlying finely vesicular basalt (f). The dyke (g) is probably the youngest rock in the section.
The more general and extensive submergence of the base of the basalt-plateau on the west side of Skye has for the most part carried the platform of sills below sea-level, so that it is only exceptionally where, owing to local irregularities, that base has been brought up to the air, that the intrusive sheets show themselves. Yet the persistence of the platform on that side is indicated by its extension even as far as the southern promontory of the island.
The Trotternish type of sill extends down the west coast under the headlands of Duirinish. Thus at the mouth of Dunvegan Loch, where the underlying Jurassic platform has been ridged up above the surface of the sea, it has carried with it the marked sill which forms the islets of Mingay and Clett that lie as a protecting breakwater across the entrance of the inlet. The intrusive rock rests on shell-limestones full of oysters (Ostrea hebridica), and referable to the Loch Staffin group of the Great Oolite Series. This sill, when observed from a little distance, presents the usual regularly prismatic or columnar structure so well developed among the Trotternish examples, but on a closer view shows this structure less distinctly. It is an olivine-dolerite of medium and fine texture, which in thin slices displays under the microscope a distinctly ophitic structure, the abundant light-brown augite enclosing the striated felspars. Its lowest portion, from three to seven or eight inches upward from the bottom, is much closer-textured than the rest of the rock and is finely amygdaloidal. Its vesicles are in many cases drawn out to a length of three or four inches, and the zeolites which now fill them look like parallel annelid tubes or stems of Lithostrotion. It is noteworthy also that the elongation of the vesicles has sometimes taken place at a right angle to the surface of contact with the underlying strata. But the most remarkable feature in this sill is the surface which it presents to the oyster-beds on which it rests. The fine-grained dark dolerite has there assumed the aspect of a sheet of iron-slag, with a smooth or wrinkled, twisted, ropy surface, which displays fine curving flow-lines. No one looking at a detached specimen of this surface would be ready to admit that it could possibly have come from anything but a true lava-stream that flowed out at the surface. The contours of a viscous lava are here precisely reproduced on the under surface of a massive sill.
A little further south, the promontory of Eist, forming the western breakwater of Moonen Bay, consists of an important sill or group of sills which has insinuated itself among shales, shell-limestones, and shaly sandstones, full of Ostrea hebridica, Cyrena aurata, etc., and belonging to the Loch Staffin group of the Great Oolite Series. The shore-cliff below the waterfall affords the section given in [Fig. 322], illustrating the manner in which a thick intrusive sheet may sometimes give off thin veins from its mass. The rock attains on the Eist promontory a thickness of probably at least 100 feet, where it is thickest and undivided. But the two main sheets, or branches of one great sheet, on this peninsula have probably a united depth of more than 300 feet. Landwards the rock splits up and encloses cakes of the Jurassic strata. It possesses the usual prismatic structure and doleritic composition. In Moonen Bay, as shown in [Fig. 322], it presents a banded structure, marked especially by an alternation of lines of amygdales and layers of more compact and solid dolerite, with occasional enclosed cakes of baked shale or sandstone. Its upper surface is somewhat uneven, and from it are given off narrow, wavy, ribbon-like veins (d), from less than an inch to three inches or more in width, which keep in a general sense parallel to the top of the sill, but at a distance of a few inches or feet from it. The sill becomes as usual fine-grained towards the contact, the shales and sandstones being indurated and the limestone marmorized.
Fig. 322.—Upper part of Sill, Moonen Bay, Waternish, Skye, showing the divergence of veins.
a, false-bedded shaly sandstone; b, shell-limestone; c, dolerite sill; d, veins proceeding from the sill.
Length of section about five yards.
The next uprise of the base of the basalt-plateau on the west side of Skye lies about 25 miles to the south-east, where it emerges from the sea in the Sound of Soa ([Fig. 323]). A vast volcanic pile has there been heaped up on the Torridon sandstone, the whole of the thick Jurassic series, which is found in force only three miles distant in Strathaird, having been removed by denudation from this area before the beginning of the Tertiary volcanic period. The plateau-basalts rests on the upturned edges of the Torridonian sandstones and shales, and are accompanied as usual by their underlying network of intrusive rocks. It is hardly possible to exaggerate the wild confusion of sills, dykes and veins which have been injected among the rocks, at and on both sides of the unconformability. Endless sheets of basalt and dolerite have forced their way between the bedded basalts and the sandstones, while across the whole rise vast numbers of dykes and veins. Narrow, black, wavy ribbons of basic material cross many of these veins, while the later north-west dykes cut sharply through everything older than themselves. As a natural section for the study of the phenomena of intrusion in many of their most characteristic phases, I know no locality equal to the northern coast-line of the Sound of Soa, unless it be the cliffs of Ardnamurchan. But the Skye cliffs, though less imposing than those of the great Argyllshire headland, have this advantage, that instead of being exposed to the full roll of the open Atlantic, they form the margin of a comparatively sheltered strait, and can thus be conveniently examined.
Fig. 323.—Section of the base of the Basalt-plateau with sill and dykes, Sound of Soa, Skye.
a a, Torridon Sandstone; b, Bedded basalts; c, Sill; d d, Dykes.