In the islands of Eigg and Rum, excellent examples occur of the tendency which the sheets of porphyry or granophyre manifest to appear at or about the base of the bedded basalts. I have already alluded to the boss or sheet at the north end of the former island. A still more striking illustration occurs in Rum. All along the base of the great mass of gabbro, protrusions of various kinds of acid rock have taken place. The great mass of Orval, already described, is one of these. Below Barkeval and round the foot of the hills to the south-east of that eminence an interrupted band of quartz-porphyry may be traced, from which veins proceed into the gabbros and dolerites.
But it is in Skye and Raasay that the intrusive sheets of the acid group of rocks reach their chief development. They have been most abundantly injected underneath the bedded basalts, particularly among the Jurassic strata. A band or belt of them, though not continuous, can be traced round the east side of the main body of granophyre, at a distance of from a mile and a half to about three miles. Beginning near the point of Suisnish, this belt curves through the hilly ground for some five miles, until it dies out on the slopes above Skulamus. It may be found again on the west side of the ridge of Beinn Suardal, and on the moors above Corry, till it reaches the shore at the Rudh' an Eireannich (Irishman's Point). It skirts the west side of Scalpa Island, and runs for some miles through Raasay. Another series of sills occurs below the basalts and gabbros in the Blaven group of hills.
Over a large part of their course, the rocks of the eastern belt rest in great overlying sheets upon the Jurassic strata, which may almost everywhere be seen dipping under them. From the analogy of other districts, we may, I think, infer that the position of these sills here points to their having been intruded at the base of the plateau-basalts which have since been removed from almost the whole tract. Fortunately, a portion of the basalts remains in Raasay, and enables us to connect that island with the great plateau of Skye of which it once formed a part. There can be no doubt that the basalts of the Dùn Caan ridge once extended westwards across the tract of granophyre which now forms most of the surface between that ridge and the Sound of Raasay. A thin sheet of quartz-porphyry, interposed among the Oolitic strata, may be seen a little inland from the top of the great eastern cliff and below the position of the bedded basalts.
The great sheet, or rather series of sheets, which stretches north-eastwards from Suisnish at the mouth of Loch Eishort in Skye, consists of a rock which for the most part may readily be distinguished in the field from the granitoid material of the bosses. It appears to the naked eye to be a rather close-grained or finely crystalline-granular quartz-porphyry, with scattered blebs or bi-pyramidal crystals of quartz and crystals of orthoclase. At the contact with adjacent rocks, the texture becomes more felsitic, sometimes distinctly spherulitic (west side of Carn Nathragh, next Lias shale). Under the microscope the rock is seen to be a fine-grained granophyric porphyry or porphyritic granophyre. It caps Carn Dearg (636 feet) above Suisnish, where it covers a space of nearly a square mile, and reaches at its eastern extremity (Beinn Bhuidhe), a height of 908 feet above the sea (Fig. 249). This rock rests upon a sill of dolerite, and is apparently split up by it. But, as I have already stated, the basic rock is probably the older of the two, and the granophyre seems to have wedged itself between two earlier doleritic sheets. To the north-west of Carn Dearg, above the northern end of the crofts of Suisnish, the same sill, or one occupying a similar position, crops out between masses of granophyre, and is intersected by narrow veins from that rock.
Though severed by denudation, the large sheets of granophyre to the east of Beinn Bhuidhe are no doubt continuations of the Carn Dearg mass, or at least occupy a similar position. That they are completely unconformable to the Jurassic strata is shown by the fact, that while at Suisnish they lie on sandstones which must be fully 1000 feet above the bottom of the Lias, only two miles to the east they are found resting on the very basement limestones, within a few yards from the underlying quartzite and Torridon sandstone. I do not think that this transgression can be accounted for by intrusion obliquely across the stratification. I regard it as arising from the eruptive rock having forced its way between the bottom of the now vanished basalt-plateau and the denuded surface of Jurassic rocks, over which the basalts were poured. The platform underneath these granophyre sills thus represents, in my opinion, the terrestrial surface before the beginning of the volcanic period.
But there is abundant proof that though the intruded granophyre sills followed generally this plane of separation, they did not rigidly adhere to it, but burrowed, as it were, along lower horizons. Thus on the south-east front of Beinn a' Chàirn, which forms so fine an escarpment above the valley of Heast, the base of the granophyre, after creeping upward across successive beds of limestone, sends out a narrow tongue into these strata, and continues its course a little higher up in the Lias. The same rock, after spreading out into the broad flat tableland of Beinn a' Chàirn (983 feet), rapidly contracts north-eastwards into a narrow strip which forms the crest of the ridge, and at once suggests a much-weathered lava-stream. The resemblance to a coulée is heightened by the curious thinning off of the rocks where the two streams emerge from the Heast lochs; it looks as if the igneous mass were a mere superficial ridge which had been cut down by erosion, so as to expose the shales beneath it. But that the granophyre is really a sill becomes abundantly clear at its eastern end, where we find that it consists of two separate sheets with intervening Liassic shales. The structure of this interesting locality is shown in [Fig. 372]. In this instance also, there is evidence that the acid sills are younger than the basic, for the upper sheet of granophyre sends up into the overlying dark basaltic rock narrow vertical felsitic veins, a quarter of an inch to an inch in width, which being more durable, stand out above the decomposable surface of the containing rock, and show their quartz-blebs and felspar crystals on the weathered surface.
Perhaps the most striking feature of the granophyre sills of Skye is their general association with thinner basic intrusive sheets between which they have insinuated themselves. This characteristic structure, pointed out by me in 1888, has recently been more minutely mapped in the progress of the Geological Survey. Mr. Harker has found the typical arrangement to be the occurrence of a thick sill of granophyre interposed between two sills of basalt, each of which is usually not more than six or eight feet thick. Where the granophyre has been intruded independently among the Lias formations, it does not assume the regularity and persistence which mark it where it has followed the course of basic sills.
Fig. 372.—Section across the Granophyre Sills at Loch a' Mhullaich, above Skulamus, Skye.
a, Jurassic sandstones and shales; b, Jurassic dark brown sandy shales; c, sills of basalt, some bands highly cellular; c′, basalt-sill with veins of felsite rising into it from the granophyre below; d d, intrusive sheets or sills of granophyre.
"The acid rock," Mr. Harker observes, "is invariably the later intrusion, for it sends narrow veins into the basalts, metamorphosing them to some extent and frequently enclosing fragments of them. These fragments are always rounded by corrosion, and show various stages of dissolution down to mere darker patches as seen by the naked eye. Such inclusions and patches are found in the marginal part of a granophyre, where no continuous basalt occurs, but where the acid magma has evidently in places completely destroyed the earlier basic sheets between which it was forced. It seems probable that in all cases a certain amount of solution of the basalt by the granophyre magma took place at their contact, facilitating the injection of the later intrusion and accounting for its persistent choice of the contact-plane of two basalt-sills as the surface offering least resistance to its injection."