Fig. 342.—Altered Plateau-Basalts invaded by Gabbro, and with a Dyke of prismatic Basalt cutting both rocks, north slope of Ben Buy, Mull.
a a, amygdaloidal basalt, much altered; b, gabbro; c, finely prismatic basalt.
The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge overlying sheet, which probably reaches a thickness of at least 800 feet. It seems to descend rather across the bedding into the hollow of Glen More, and possibly its main pipe of supply lay in that direction. Being enormously thicker than any other sheet in the island, it exhibits the crystalline peculiarities which are so well developed in the central portions of the larger bosses of gabbro. It presents more coarsely crystalline varieties than appear in the thinner sheets, some portions showing crystals of diallage and felspar upwards of an inch in length. It likewise contains admirable examples of banded structure, which, as in Skye and elsewhere, is best developed where the texture becomes especially coarse. Veins or bands, in which the constituent minerals have crystallized out in more definite and conspicuous forms, here and there succeed each other so quickly as to impart a bedded or foliated look to the body of rock, recalling, as in Skye, the aspect of some coarsely crystalline granitoid gneiss. In these respects the Mull gabbro closely resembles that of the Cuillin Hills. Occasionally, on the exposed faces of crags, portions of such bands or veins are seen to be detached and enveloped in a finer surrounding matrix. The thick belts or bands of coarser and finer texture alternate, and give an appearance of bedding to the mass. Nevertheless they are really intrusive sills, which run generally parallel with beds of finer gabbro or with sheets of highly indurated basalt, that may be detached portions of the ordinary rocks of the plateau. The thick sheet of Ben Buy, like the mass of the Cuillin Hills, is thus the result not of one but of many uprises of gabbro.
Of the thinner sheets of dolerite and gabbro in Mull little need here be said. I have referred to their great abundance in the range of eastern hills that rise from the Sound of Mull between Loch Spelve and Fishnish Bay. Though obviously intrusive, they lie on the whole parallel to the bedding of the basalts. The latter rocks exhibit the usual dull indurated shattery character which they assume where large bosses of gabbro have invaded them, and which gradually disappears as we follow them down hill away from the intrusive sheets to the shores of the Sound. They dip towards the centre of the hill group, that is, to south-west in the ridge of Mainnir nam Fiadh, Dùn da Ghaoithe, and Beinn Meadhon, the angle increasing southwards to 15°-20°, and at the south end reaching as much as 35°-40°. Some fine crags of gabbro and dolerite form a prominent spur on the east side of the ridge of Ben Talaidh, in the upper part of Glen Forsa. These consist of successive sheets bedded with the basalts, and dipping south-west. A large sheet stands out conspicuously on the north front of Ben More, lying at the base of the "pale lavas," and immediately above the ordinary basalts. It circles round the fine corry between Ben More and A'Chioch, some of its domes being there beautifully ice-worn. This is the highest platform to which I have satisfactorily traced any of the intrusive sheets of Mull. Another dyke-like mass emerges from beneath the talus slopes of A'Chioch, on the southern side, and runs eastward across the col between the Clachaig Glen and Loch Scridain.
5. The Gabbros of St. Kilda and North-east Ireland
Sixty miles to the westward of the Outer Hebrides lies the lonely group of islets of which St. Kilda is the chief. As the main feature of geological interest in this group is the relation of the acid protrusions to the other rocks, the account of the geology will be more appropriately given as a whole in Chapter xlvii. I need only remark here that the predominant rocks of these islands are dark basic masses, chiefly varieties of gabbro, but including also dolerites and basalts. Reasons will be afterwards brought forward for regarding these rocks as parts of the Tertiary volcanic series. They present a close parallel to the gabbros and associated rocks of Skye. But in one important respect they stand alone. No certain trace remains of any basalt-plateau at St. Kilda such as those through which the gabbros of Skye, Mull and Ardnamurchan have been injected. In regard to their mode of production they have doubtless been intruded at some considerable depth beneath the surface. But no relic appears to have survived of the overlying cover of rock under which they consolidated, and into which they were injected.
In the remarkable volcanic district of the north-east of Ireland a series of basic rocks appears, which in its mode of occurrence and its relation to the other members of the series presents many points of resemblance to the gabbros of the Inner Hebrides. The Irish gabbros are well developed in the Carlingford district, where they form intrusive bosses and sheets which have been erupted through the Palæozoic rocks (Map VII.). They are themselves pierced by later masses of granophyre and other acid rocks. Further reference will be made to these gabbros in later pages, where an account will be given of the granite masses of Mourne, Barnavave and Slieve Gullion.
It is interesting to observe that, while in St. Kilda no relic of any basaltic plateau has been preserved, in the Faroe Islands, on the other hand, no sign has been revealed by denudation that the volcanic plateau of that region is pierced by any eruptive core of gabbro or of granophyre. During my cruises round these islands and through their channels, I was ever on the outlook for any difference in topography that might indicate the presence of some eruptive boss like the gabbro and granophyre masses of the Inner Hebrides. But nothing of that nature could be discerned. Everywhere the long level lines of the bedded basalts were seen mounting up to the crests of the ridges and the tops of the highest peaks. Though I cannot assert that no intrusions of gabbro or of granophyre exist among the Faroe Islands, I feel confident that any such masses which may appear at the surface must be of quite insignificant dimensions, and do not make the important feature in geology and topography which they do among the Inner Hebrides. It is, of course, possible that, vast as the denudation of these islands has undoubtedly been, it has not yet trenched the plateau deeply enough to expose any great intrusive bosses and sills which may underlie and invade the basalts.
iv. HISTORY OF THE GABBRO INTRUSIONS
We are now in a position to draw, from the observations which have been given in this and the preceding chapter regarding the different areas of gabbro in the Tertiary volcanic region of Britain, some general conclusions with respect to the type of geological structure and the phases of volcanic energy which they illustrate.