Chapter xlvi. in connection with the action of the granophyre. Whether

this alteration has been produced by the intrusion of the gabbro or of some concealed mass of granophyre underneath, of which only projecting dykes and veins reach the surface, must remain a matter of doubt. On the whole, as the gabbro is here undoubtedly thrown against the basalts and Torridon sandstone by a fault, it seems most probable that the change has been mainly due to the influence of the acid rock.

In the Blath Bheinn group of hills the relations of the gabbro to the bedded basalts have recently been mapped in detail by Mr. Harker during the progress of the Geological Survey of Skye. He has observed that, allowing for irregularities of form, the mass of gabbro obliquely overlies the basalts as a great sheet, not necessarily due to a single intrusion, which dips towards the west. He has found the rock to vary from a coarse gabbro to a diabasic type, and to vary also in mineralogical constitution, becoming in places very rich in olivine, though the banded structure is here only exceptionally developed. North of Garbh Bheinn the gabbro is much crushed and the outlying patch to the north of Belig is in part a crush-breccia. Mr. Harker remarks that similar brecciated structures are common among the granophyres of the Red Hills, and that it is sometimes difficult to distinguish their structure from that of the true volcanic agglomerates.

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.