These massive sills are prolonged in a series of picturesque flat tabular islets beyond the most northerly headlands of Skye. They probably continue northwards under the sea at least 12 miles further, for sills of the same type rise there in the singularly striking group of the Shiant Isles ([Fig. 320]). These lonely islets, extending in an east and west direction for about three miles, display in great perfection most of the chief characters of the Skye sills. They are especially noteworthy for including the thickest intrusive sheet and the noblest columnar cliff in the whole of the Tertiary volcanic series of Britain. The larger of the two chief islands consists of two masses of rock connected by a strip of shingle-beach, and having a united length from north to south of about two miles. The northern half, or Garbh Eilean, presents towards the north a sheer precipice 500 feet high. This magnificent face of rock consists of one single sill, but as its original upper limit has been removed by denudation and its base, where it is thickest, is concealed under the sea, the sill may exceed 500 feet in thickness. The rock has the usual prismatic structure, which imparts to it an impressive appearance of regularity. The columns retain their individuality to a great height, and though none of them perhaps can be followed from base to crest of the cliff, many of them are evidently at least 300 or 400 feet long.
Macculloch, who gave the first geological description of the Shiant Isles, showed the intrusive nature of the igneous rocks, and described the remarkable globular or botryoidal structure of the Jurassic shales between which they have been injected.[315] Professor Heddle has published a brief account of the geology of the islands.[316] Professor Judd visited the group and brought away a series of specimens of their eruptive rocks, which he found to include basic and ultra-basic varieties.[317]
[315] Western Islands, vol. i. p. 441.
[316] Trans. Norfolk Nat. Hist. Soc. vol. iii. (1880) p. 61.
[317] Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 677, and xli. (1885) p. 393. My description in the text is the result of three successive visits to the islands.
Fig. 320.—View of the northern precipice (500 feet high) of the largest of the Shiant Isles.
(From a Photograph by Colonel Evans.)
In Garbh Eilean, where the thickest mass of erupted material presents itself, at least three sills may be observed. Some low reefs that run parallel with the northern coast of the island consist of coarse ophitic gabbro in two or more sheets which have been intruded between the Jurassic shales. Above these strata comes the great columnar sill, its base gradually sinking towards the west until it passes under the sea, and the vertical columns then plunge abruptly into the water. The rock of which this massive sill consists is another large-grained gabbro or dolerite, with an ophitic structure. Owing to the form of the ground it cannot be so satisfactorily examined as the neighbouring island of Eilean Mhuire, which, though less lofty and rather smaller than Garbh Eilean, affords a succession of admirable and easily examined sections along its precipitous shores.
Professor Judd found that while the rocks are mainly ophitic gabbros and dolerites, they include such highly basic compounds as dunite. An examination of the Eilean Mhuire cliffs enables the observer to ascertain that the sills display considerable variety in texture and in the character and arrangement of their component minerals. They are marked by a persistent, more or less distinct disposition in rude beds, and these again often display a banding of their constituents in lines parallel with the general bedding. Some of these bands are largely felspathic, and are thus paler in colour. Others, where the ferro-magnesian minerals and ores are more specially aggregated, are dark in colour. In some layers the long black prisms of augite are ranged in a general parallelism with the banding.
A specimen selected as typical of the ordinary coarse-grained amorphous rock was sliced and placed in Mr. Harker's for microscopic examination, and he has supplied the following observations regarding it: "The gabbro from Eilean Mhuire [7110] is a crystalline rock showing to the eye lustrous black augites, half an inch long, and (predominating) felspar. The microscope reveals, in addition, irregular grains of black iron-ore and little hexagonal prisms of apatite. No olivine is to be detected. As regards structure, the augite has tended to crystallise out in advance of the felspar, but this relation is not constant.