"The augite is of a light-brown tint in slices, and has an unusual kind of pleochroism. The colour for vibrations parallel to the β-axis is of the purplish-brown tone seen in some soda-bearing augites; parallel to γ and α it has a yellow or citron tint. The colour and pleochroism are more marked in the interior of a crystal than towards the margin, but some crystals pass at the margin into a slightly pleochroic, pale-green, recalling ægerine-augite. The felspar tends to build elongated crystals. It is a rather finely lamellated labradorite, sometimes showing pericline- as well as albite-lamellae."
Another specimen from one of the black bands in the same island, with a linear arrangement of its component minerals, is thus described by the same petrographer: "This rock [7111] is of darker appearance than the preceding, and contains abundant black iron-ore, besides some pyrites. It also differs in having a marked parallel disposition of its crystals.
"Except for the greater prominence of large irregular grains of iron-ore, this rock under the microscope closely resembles the last described, the parallel structure not being conspicuous in the slice. The augite has the peculiar colour and pleochroism already noted, and the felspar is of the same kind as before."
I did not succeed in finding in place any bands of dunite, but this basic material probably occurs at the base of some of the sills where it has segregated from the rest of the mass, like the picrite at the bottom of the Bathgate diabase.
The amount of contact-metamorphism effected even by such thick sills as those of Trotternish and Shiant is much less than might be expected. It seldom goes beyond a mere induration of the strata for a few yards, often only for a few inches from the surface of junction. In the Shiant Isles, however, the shales between the sills have undergone a more remarkable alteration. They have not only been greatly indurated, but have acquired the globular or botryoidal structure so fully described by Macculloch. The spheroidal aggregates vary from not more than a line to more than half an inch in diameter, and appear on the surface as dark, irregularly grouped, pea-like aggregates. This structure is perhaps best developed immediately under the thick sill on the west side of Eilean Mhuire.
The massive sills are not the only evidence of the injection of igneous material on the Shiant Isles. The sill, or more probably group of sills, forming Eilean Mhuire is traversed by a number of sheets of basalt varying from only two or three inches to 20 feet in thickness. These black fine-grained rocks invariably present chilled selvages next the coarse gabbro, and though they have been on the whole injected parallel to the general bedding or banding, they here and there break across it as veins. The most important of these later intrusions forms a columnar sill on the eastern side of the island, and can be followed for several hundred yards. It consists of a dark finely crystalline olivine-basalt, which towards the margin assumes a dense black texture. Under the microscope Mr. Harker found a thin slice of this rock to be "an olivine-basalt of semi-ophitic, semi-granulitic structure [7112]. The olivine is mostly fresh, but part of it is converted into a yellowish-brown pseudomorph like iddingsite. Magnetite occurs chiefly in imperfect octohedra. The felspar is in little lath-shaped sections, many of which are finely striated, and give extinction-angles indicating a labradorite. The augite, light brown in the slice, never has crystal-boundaries, and often enwraps the felspars."
The narrow veins are composed of a much closer-grained basalt in which a few scattered felspars are visible. Mr. Harker remarks, with regard to a thin slice of one of these rocks [7113], that "the microscope shows this, too, to be an olivine-basalt. The porphyritic felspars are twinned on the Carlsbad and albite laws. Olivine and pseudomorphs after it are well represented. Magnetite is only sparingly present. The general mass of the rock consists of very small striated prisms of labradorite, granules of augite, and interstitial matter which must be partly glassy."
This is perhaps the most striking of all the examples known to me where an older sill has been split open to receive a subsequent injection of molten material. The Eilean Mhuire gabbro must be at least 200 feet thick, and it not impossibly passes under the still thicker pile of Garbh Eilean. Yet it has been horizontally ruptured near its base, and into the rent thus produced another mass of molten matter has been thrust. This subject will be again referred to in connection with another remarkable example on the west coast of Skye.
Fig. 321.—Section of thin Intrusive Sheets and Veins in carbonaceous shales lying among the Plateau-basalts, cliffs north of Ach na Hannait, between Portree Bay and Lock Sligachan.