Fig. 338.—Sketch of Banded Structure in the Gabbros of the hills at the head of Loch Scavaig.
Besides the main area of gabbro in Skye, a great many small detached bosses, sills and dykes lie further east on the flanks of the Red Hills. One of the best marked of these detached areas forms a conspicuous crag on the east side of Strath More, immediately to the north of Beinn na Cro. It consists of beds of coarse gabbro, with others of dolerite intercalated in an outlier of the plateau-basalts, and is traversed by veins from the granophyre of the glen, as well as by the usual north-west basalt dykes ([Fig. 349]). It appears to be a marginal portion of the main gabbro area separated by the intrusion of the great granitoid boss of the Red Hills. On the north-eastern side of Beinn na Caillich numerous intrusive sheets of gabbro and dolerite traverse the quartzite and limestone, and extend down to the sea-margin in the Sound of Scalpa.
There is an important feature in the main gabbro area of Skye not yet clearly understood, and which only a minute and patient survey can elucidate. Though I have found among the Cuillin Hills no distinct proof that the mass of gabbro ever gave rise to discharges of material, either lava-form or fragmentary, which reached the surface, the gabbro area, as already remarked, contains unquestionable evidence of explosions and the production of pyroclastic masses. Among the moraine-mounds of Harta Corry, blocks of basalt-agglomerate are strewn about, full of angular fragments of altered basalt, sometimes highly amygdaloidal, and also boulders in which lumps of coarse gabbro are enveloped in a matrix of finer material. I did not find the parent rocks from which these glacier-borne masses had been derived, but there can be no doubt that they exist among the gabbro crags that surround that deep glen. Reference has already been made to the similar rock found in situ on the opposite side of the Cuillin ridge at the head of the great cauldron of Corry na Creich; likewise to the mass of coarse agglomerate which forms a group of knolls and crags on the east side of Druim an Eidhne above the head of Glen Sligachan. This rock contains abundant blocks of various slaggy lavas like those of the basalt-plateau, and runs for some distance along the eastern limit of the gabbro, between that rock and the granophyre. It is intersected by numerous basalt-veins. Mr. Harker, as above mentioned, has recently found some considerable strips of agglomerate which, like that which I traced round the west side of Beinn Dearg, are interposed between the gabbro and the bosses of granophyre, or lie at the base of the volcanic series (p. 284).
There does not, however, appear to be any evidence to connect these isolated masses of agglomerate with the phenomena attending the uprise of the gabbro. They seem to be more probably related to the plateau eruptions, and may be compared with those of Strath, Ardnamurchan and Mull (pp. 278, 280, 384). That the huge gabbro mass of Skye, besides invading and altering the bedded basalts, may have communicated eventually with the surface, and have given rise to superficial discharges, is not at all improbable, but of any such outflows not a vestige appears now to remain. We must remember, however, that the gabbro no doubt in many places found its readiest upward ascent in vents belonging to the plateau-period, and that portions of the agglomerates of these earlier vents may be expected to be found involved in it, as the agglomerate of the great vent of Strath has been invaded by the granophyre.
CHAPTER XLIV
THE BOSSES AND SHEETS OF GABBRO IN THE DISTRICTS OF RUM, ARDNAMURCHAN, MULL, ST. KILDA AND NORTH-EAST IRELAND. HISTORY OF THE GABBRO INTRUSIONS
2. The Island of Rum
The mountains of the island of Rum, rising as they do from a wide expanse of open sea, present one of the most prominent and picturesque outlines in the West Highlands (Map VI.). More inaccessible than most of the other parts of the volcanic region, they have been less visited by geologists. They were described by Macculloch as composed of varieties of "augite rock." He noticed in this rock "a tendency to the same obscurely bedded disposition as is observed in other rocks of the trap family," and found at one place that it assumed "a regularly bedded form, being disposed in thin horizontal strata, among which are interposed equally thin beds of a rock resembling basalt in its general characters."[350] Professor Judd repeats Macculloch's observation, that "the great masses of gabbro in Rum often exhibit that pseudo-stratification so often observed in igneous rocks." He regards these masses, like those of Skye and Mull, as representing the core of a volcano from which the superficial discharges have been entirely removed, and he gives a section of the island in which the gabbro is represented as an amorphous boss sending veins into a surrounding mass of granite.[351] In a subsequent paper he gave an excellent detailed account of the mineralogical composition of some of the remarkably varied and beautiful basic rocks constituting the hills of Rum, but added no further information regarding the geological structure of the island.[352]
[350] Western Islands, i. p. 486.
[351] Quart. Journ. Geol. Soc. xxx. p. 253.