[349] Consult the Memoirs cited in the footnote on p. 342.
Similar structures are found to be widely developed through the gabbros of the Cuillin Hills. Not only are these rocks disposed in distinct beds, but many of the beds display the most perfect banding. Thus the mountains that surround the head of Loch Scavaig and sweep round Loch Coruisk up to the great splintered crests of Sgurr na Banachdich display on their bare black crags a distinct bedded structure. On the east side of Loch Scavaig the rock presents a rudely-banded character, the bands or beds being piled over each other from the sea-level up to the summits of the rugged precipices, and dipping into the hill at angles of 25° to 35°. Abundant dykes and veins of various basic, intermediate and acid rocks cut this structure. The individual layers here show sometimes the wavy and puckered condition already referred to.
Even from a distance the alternating lighter and darker bands can readily be seen, so that this structure, with the variations in its inclination, can be followed from hill to hill ([Fig. 338]). The regularity of the arrangement, however, is often less pronounced on closer inspection. While the gabbro is rudely disposed in thick beds, indicative of different intrusive sheets or sills, with which the banding is generally parallel, considerable irregularities may be observed in the arrangement of the structure of individual sheets. These sheets may be parallel to each other, and yet, while in some the banding is tolerably regular in the direction of the planes of the sheets, in others it is much twisted or inclined at various angles.
Fig. 337.—Banded and doubly-folded Gabbro, Druim an Eidhne, 10 feet broad.
On the west side of the Coruisk river the banding is vertical; southward from that stream it inclines slightly towards the south, but soon again becomes vertical, and continues conspicuously so at the junction of the gabbro with the Torridon sandstones and plateau-basalts on the west side of Loch Scavaig.
Thus, instead of being one great eruptive boss, the gabbro of this district is in reality an exceedingly complicated network of sills, veins and dykes. While the general inclination of the bedding sometimes continues uniform in direction and amount from one ridge to another, it is apt to change rapidly, as if the complex assemblage of intruded masses had been disrupted and had subsided in different directions. For example, after overlying the bedded basalts of the plateau all the way from Glen Brittle to the west side of Loch Scavaig, the gabbro descends abruptly across these basalts and also across the Torridon sandstones, on which they unconformably rest. These two groups of rocks are not only truncated by the gabbro, but are traversed by the intricate system of sills, dykes and veins already referred to. Where it abuts against the sandstones and basalts in Loch Scavaig, the gabbro is arranged in vertical bands of different mineral composition and texture. Much of it is remarkably coarse, some bands displaying pyroxene crystals more than an inch in length. There is no fine-grained selvage here, indicative of more rapid cooling. So coarse, indeed, is the rock close up against the sandstone, that the junction-line can hardly be supposed to be the normal contact of the intrusive rock. This inference is confirmed by the existence of a singular kind of breccia between the gabbro and the sandstones. It is a tumultuous mass of fragments of coarse and fine gabbro, Torridon sandstone and shale, and plateau-basalts, embedded in a pale crystalline matrix of fine granular granophyre; veins from this acid intrusion run off into the gabbro on the one side as well as into the Torridon sandstones on the other. It would seem that this junction-line has been one of great movement, that the gabbro-sheets have subsided against a fault-wall of plateau-basalt and Torridon sandstone, and that subsequently an intrusion of finely granular granophyre has come up the fissure, involving in its ascent fragments of all the materials around.
The rocks for a considerable distance to the south of the gabbro are intensely altered. The Torridon sandstone has been so indurated as to pass into a bleached white quartzite, while the shales interstratified with it have been converted into a kind of porcellanite. But the most interesting alterations are those to be observed in the plateau-basalts, which at a height of about 300 feet above the sea, are to be seen in nearly horizontal sheets that lie immediately on the upturned edges of the Torridon sandstones. These lavas have suffered great metamorphism, to which more particular reference will be made in 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.
