Fig. 378.—Plan of pale granophyric dyke, with spherulitic and flow-structure, cutting and enclosing dark gabbro, Druim an Eidhne.
Fig. 379.—Dyke (six to ten feet broad) proceeding from a large body of granophyre and traversing gabbro, from the same locality as Figs. [375] and [377].
A remarkable feature has been recently observed by Mr. Harker among the abundant granophyre dykes and veins which intersect the gabbros and older rocks, along the eastern flanks of the Red Hills of Skye between Broadford and the Sound of Scalpa. Broad dykes of granophyre which traverse the Cambrian limestone of that district might be supposed at first sight to be cut off by the intrusions of gabbro. But closer examination proves that their apparent truncation arises from their suddenly breaking up into a network of small veins where they abut against the basic rock. This structure evidently belongs to the same type as that of the St. Kilda granophyre.
Fig. 380.—Section of intruded veins of various acid rocks above River Clachaig, Mull.
a a, basalt, dolerite, etc.; b b, granophyre.
Compound dykes and sills, where one or more of the injections has consisted of acid material, have been already noticed as intimately associated together in Skye (p. 162). Dykes of this nature are more particularly abundant in Strath, especially along its eastern side. In addition to the examples cited already from that district, I may refer to other two which intersect the Middle Lias shales and limestones in the island of Scalpa. They are both compound dykes, but the more basic marginal bands are not always continuous, having possibly been here and there dissolved by the acid invasion. Though they do not show any distinct spherulitic forms, the presence of flow-structure is indicated by the thin slabs into which the rocks weather parallel to the dyke-walls. The rock in each case is a fine-grained felsitic mass, with bi-pyramidal crystals of quartz. It is observable that where these dykes come directly against the Liassic strata, the latter are more seriously indurated than where they are traversed by the ordinary basic dykes.
In the central mountainous tract of the island of Mull veins of acid material are extraordinarily abundant. They probably proceed from a much larger subterranean body of granophyre than any of the comparatively small bosses of this rock which appear at the present surface of the ground. They show themselves partly at the margins of the visible bosses, but much more profusely in that tract of altered basalt, with intrusive sheets and dykes of basalt, dolerite and gabbro, which lies within the great ring of heights between Loch na Keal and Loch Spelve. In some areas, the amount of injected material appears to equal the mass of more basic rock into which it has been thrust. Pale grey and yellowish porphyries and granophyres, varying from thick dykes down to the merest threads, ramify in an intricate network through the dark rocks of the hills, as shown in the accompanying illustration ([Fig. 380]), which represents a portion of the hillside between Beinn Fhada and the Clachaig River. Such a profusion of veins probably indicates the existence here of some large mass of granophyre or granite, at no great depth beneath the surface.
In Mull, as in the other islands of the Inner Hebrides, two horizons on which protrusions of acid materials have been specially abundant, are the base of the bedded basalts of the plateau and the bottom of the thick sheets of gabbro. Dykes and veins of granophyre, quartz-porphyry, felsite and other allied rocks are sometimes crowded together along these two horizons, though they may be infrequent above or below them.