Fig. 325.—Section to show Bedded and Intrusive Sheets, Eigg.
I will conclude this account of the Tertiary basic sills of Britain by referring to one further district in the West of Scotland, where they are well displayed on bare hillslopes and also along a picturesque sea-coast. In the promontory of Ardnamurchan in the west of Argyleshire, one of the most conspicuous eminences, known as Ben Hiant, affords a striking mass of intrusive material, which, extending along a rugged shore for three-quarters of a mile, mounts thence inland in a series of rocky knolls, and in rather less than a mile culminates in a summit, 1729 feet above sea-level.[320] The rocks which cover this large space are disposed in numerous rude beds, which have a seaward dip of perhaps 15° to 20°, and are sometimes distinctly prismatic, the prisms being not infrequently grouped in fan-shape. They are evidently due to successive intrusions. Although generally coarsely crystalline in texture, they include also intermediate and fine-grained sheets. They are never, so far as I have been able to discover, amygdaloidal,[321] nor do they present the ordinary external characters of the beds of the plateaux, though here and there they appear to have caught up portions of the plateau-series. They distinctly overlie the bedded basalts on their eastern and southern margins; but westwards they appear to lie transgressively across the edges of these rocks, while to the north-west and north they rest on quartzites and schists and on Jurassic limestones. An outlier from the main mass forms the prominent hill of Sròn Mhor, and can be seen distinctly overlying the bedded basalts as well as the neck of agglomerate already described ([Fig. 302]).
[320] This locality has been described by Professor Judd (Quart. Jour. Geol. Soc. xxx. (1874), p. 261; and xlvi. (1890), p. 373).
[321] As amygdaloidal structure is occasionally to be found among both dykes and sills its presence in the Ben Hiant rocks would not be inconsistent with their intrusive origin.
The prevalent rocks of Ben Hiant are well crystallized, ophitic olivine-dolerites and gabbros. A specimen taken from the shore on the west side of the mass was found by Dr. Hatch to present under the microscope its augite in large plates, which enclose narrow laths and needles of plagioclase felspar as well as grains of olivine. All the felspars are in lath-shapes, sometimes extremely long and narrow. The iron-ore likewise assumes an ophitic character, enclosing rectangular portions of felspar. Dr. Hatch observed in another specimen, taken from the south-east side of the hill, "a curious intermixture of two different structures. Scattered portions which show the usual ophitic structure, their felspar and augite occurring in large crystals, are, so to speak, imbedded in a groundmass which presents rather a basaltic type, its felspar, augite, and magnetite, in long thin needles, microlites, and other skeleton forms, being enclosed in a dark devitrified base." A third specimen, selected from one of the columnar sheets near the top of Ben Hiant, is "a fine-grained dolerite (or gabbro) showing little ophitic structure, the augite occurring in roundish grains, and only slightly intergrown with the felspars, which are more or less lath-shaped. The rock contains a considerable quantity of black iron-ore in irregular grains and some dirty-green viridite." Still another variety of structure occurs in a specimen which I broke from one of the shore crags on the south-west side of the hill. Under the microscope, Dr. Hatch found in it a beautiful aggregate of "skeleton crystals and microlites of plagioclase, with here and there a rectangular crystal, long slender microlites of augite, and short serrated microlites of magnetite, the whole being confusedly imbedded in a dark glassy base powdered over with a fine magnetite dust."[322] A sill of pitchstone lies among the bedded basalts on the east side of the hill.
[322] Professor Judd has called the rocks of Beinn Hiant augite-andesites, and has given descriptions and figures of their structure, and analyses of their chemical composition (op. cit.).
From a number of specimens collected by me during a second visit to this district in the summer of 1896, I selected some for microscopic examination and submitted them to Mr. Harker, who has furnished me with the following descriptions of them: "The sill at the north end of Camas na Cloiche, Ben Hiant [7114] is an olivine-gabbro of medium grain and fresh appearance. Olivine, fresh or partly serpentinized, is plentiful. The felspar is a labradorite with Carlsbad- and albite- (rarely pericline-) twinning, and some of it has zonary banding. It is for the most part in crystals giving rectangular sections, but there are some of allotriomorphic form. Magnetite occurs chiefly in shapeless grains of later crystallization than the felspar, but sometimes presenting crystal-faces to the augite. The augite is light-brown in the slice, without any true diallage-structure, and tends to enwrap the earlier minerals in ophitic patches.
"The sill south of Uamh na Creadha, on the west side of Ben Hiant [7115], is a rock of different type, having porphyritic crystals of felspar, up to an inch or more in length, in a rather finely-crystalline groundmass. The microscope shows it to be a dolerite of granulitic structure, the main mass of the rock consisting of little striated labradorite-crystals, grains of pyroxene, and rather abundant crystal-grains of magnetite. The pyroxene seems to be chiefly augite, but hypersthene is also present, and builds rather larger and more idiomorphic crystals with characteristic pleochroism."
In rambling over this Ardnamurchan district I have often been reminded of the great intrusive sheets of Fair Head. One of the features in which the rocks of the two localities resemble each other is their tendency to assume a coarsely crystalline texture. In some parts of Ben Hiant the individual crystals reach an inch or more in length. These more largely crystalline portions, however, do not form distinct bands so much as patches in the midst of the general mass; at least I have not noticed any examples of such veins of segregation as are so prominent in Antrim.