i. THE SILLS

Not only have the acid rocks been protruded in small and large bosses, they have also been injected as sills between the bedding-planes of stratified rocks, between the surfaces of the basalt-beds, and between the bottom of the plateau-basalts or of the gabbros and the platform of older rock on which the volcanic series has been piled up. Every gradation of size may be observed, from mere partings not more than an inch or two in thickness, up to massive sheets, which now, owing to the removal of their original covering of rock by denudation, form minor groups and ranges of hills. Where the sheets are numerous, they are usually small in size; where, on the other hand, they are few in number, they reach their greatest dimensions.

It is not always possible to discriminate between bosses and large irregular sills. A good illustration of the connection between these two forms of intrusion will be cited from the island of Raasay, where a widespread intrusive sheet is in part connected with a true boss.

In Mull, sills of acid eruptive rocks are profusely abundant throughout the central mountainous tract between Loch na Keal and Loch Spelve. If we ascend the slopes from the Sound of Mull, for instance, we have not gone far before some of these sheets make their appearance. They are usually dull granular quartz-porphyries, or granophyres, often only two or three feet in thickness, and interposed between the beds of basalt that form the mass of the hills. Along the crest of the ridge that stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh they take a prominent place among the ledges of basalt, basalt-conglomerate and dolerite. The largest sheet in Mull is probably that which has thrust itself between the base of the basalts and the underlying Jurassic strata and crystalline-schists on the shore of the Sound of Mull at Craignure. The porphyry of this sheet is referred to by Professor Zirkel as only a finer-grained variety of the same quartziferous rock, with hornblende and orthoclase crystals, which in Skye breaks through the Lias.[430] On the south coast also, at the base of the thick basalt series, similar porphyries have been injected into the underlying strata; and under the great gabbro mass of Ben Buy similar protrusions occur. But as we retire from the mountainous tract into the undisturbed basalts of the plateau, these acid intercalations gradually disappear.

[Footnote 430: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. p. 54.]

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.

"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."

These observations throw fresh light on the remarkable original regularity and persistence of the basic sills. Where one of these sills disappears above or below a granophyre sheet its probable former presence is often indicated by corroded fragments of the basic in the acid rock. Mr. Harker remarks that the acid magma seems to have been "in itself less adapted than the basic to follow accurately a definite horizon and to maintain a uniform thickness in its intruded sheets, but could do both when guided by a pre-existing basalt-sill, or especially when insinuated between contiguous basalt-sills." The corrosive action of the acid magma on the surface of the basalt, which enabled it to force its way more readily between the basic sills, might proceed so far as partially or wholly to destroy these sills.

This solvent action may serve to explain some of the irregularities of the granophyre intrusions. According to the same observer, such irregularities are found "where the granophyre sheet and its encasing basalt-sills are not co-extensive, or again where the two basalt-sills separate, owing to one of them cutting obliquely across the bedding. In the latter case, which is not common, the granophyre follows one of the basalt-sills, necessarily parting from the other. When one of the two guiding basalt-sills dies out, the granophyre may still continue, following the sill which persists. If the latter also dies out, while the granophyre is still in some force, the acid magma seems to have been reluctant to travel beyond the limit of the basalt, but has drawn towards it, and the granophyre presents a blunt laccolitic form, which contrasts with the acutely tapering edge of a granophyre which dies out before reaching the limit of its basalt-sills. If, on the other hand, on reaching the limit of the basalt, the acid magma has been in such force as to be driven further, it is usually found to lose something of its regularity and to depart from the exact horizon which it has hitherto followed. This seems to happen, for instance, in the Beinn a' Chàirn sheet, which, when traced westward, is found to behave as a 'boss' and is obviously transgressive, having cut across the bedding of the strata so as to enter the limestones, where it no longer behaves in any degree as a sill. The district affords many examples of the tendency of intrusive masses in general to cut sharply across the beds when they enter a group of limestones."

More complex examples of acid sills are to be found where there have been three or more basic sheets together. The great granophyre sheet already referred to at Suisnish affords the best illustration of this structure. Mr. Harker has noticed that "round most of its circumference there is seen merely a single basalt-sill passing under the granophyre. Probably there has been another similar sheet over the acid rock, but if so, it has been removed by erosion, the granophyre itself forming everywhere the surface of the plateau. On the southern side, however, we see that the original basalt must have been at least triple, or counting the uppermost member, now removed, quadruple. The granophyre has forced its way in between the several members of the multiple basalt-sill, the intermediate ones being thus completely enveloped. They are evidently metamorphosed as well as veined by the granophyre, and when traced onward they give place to detached portions which, floating as it were in the acid rock, are soon lost."

It is seldom easy to determine where lay the vent or vents from which the granophyre sills proceeded. Those of the Skye platform just described may be chiefly concealed under some of the larger areas of the rock, such as the sheets of Carn Dearg or Beinn a' Chàirn. But in several places, in close association with the compound sills of granophyre and basalt, Mr. Harker has found large dyke-like bodies of the acid rock, which may with considerable probability be regarded as marking the position of the channels by which the material of the sills ascended. "These bodies," he remarks, "either occur isolated by erosion, the sills or the parts of the sills presumed to have been in connection with the dykes having been removed, or are only very partially exhibited in direct connection with sills still remaining. Where they can be examined in detail they are seen to be dykes varying up to about 100 feet in width, but of no great longitudinal extent. Between Suisnish and Cnoc Carnach they bear E.N.E., that is, at right angles to the ordinary basic dykes of the district and parallel to the general direction of the axes of folding, though further north they change this trend, but still remain parallel to the strike of the Lias.

"These dykes are composed essentially of granophyre, identical with that of the sills. In some cases, they are flanked with basalt-dykes on one or both sides, or the former existence of such lateral dykes is indicated by partly-destroyed inclusions of the basic rock in the granophyre. The basalt found in these cases is identical with that of the basic sills, and shows the same relation to the granophyre. Discontinuity and failure of the basalt are commoner, however, in the dykes than in the sills--a difference presumably attributable to more energetic destructive action of the acid magma when it was hotter and fresher. These supposed feeders of the granophyre sills are certainly in some cases, and have possibly been in all, double or triple dykes. The acid magma thus appears not only to have spread laterally along the same platforms as the earlier basalts, but to have reached these levels by rising through the same fissures which had already given passage to the basic magma."[431]

[Footnote 431: MS. notes supplied by Mr. Harker.]

The granophyre sills which, as already stated, can be followed as an interrupted band from Suisnish Point to the Sound of Scalpa, emerge again beyond Loch Sligachan and also in the island of Raasay, where a great sheet of the acid rock covers an area of about five square miles. This tract has recently been mapped for the Geological Survey by Mr. H. B. Woodward, who has found it to have been intruded across the Jurassic series, a large part of its mass coming in irregularly about the top of the thick white sandstones of the Inferior Oolite. But it descends beneath the Secondary rocks altogether, and in some places intervenes between the base of the Infra-liassic conglomerates and the Torridon sandstone. Its irregular course transgressively across the Mesozoic formations is probably to be regarded as another example of the intrusion of the acid material preferentially along the line of unconformability between the older rocks and the Tertiary basalts, now nearly all removed from Raasay by denudation, though the intrusion does not rigidly follow that line of division, but sometimes descends below it.

The central portions of this Raasay granophyre possess the ordinary structures of the corresponding rocks in Skye. They show a finely crystalline-granular, micropegmatitic base, through which large felspars and quartzes are dispersed. But at the upper and under junction with the sedimentary rocks, beautiful spherulitic structures are developed. This is well seen on the shore near the Point of Suisnish (Raasay), where, below the Lias Limestones, the top of the granophyre appears, and where its bottom is seen to lie on the Torridon sandstone.

This granophyre sheet presents a further point of interest inasmuch as it appears to have preserved one of the dyke-like masses which may mark channels of escape from the general body of the acid magma below. Near the Manse the section represented in Fig. 373 may be observed. Owing to great denudation, the massive sheet of granophyre has been cut into isolated outliers which cap the low hills, and the rock may be seen descending through the Jurassic sandstones, which in places are much indurated. It is observable that the amount of contact-metamorphism induced by the granophyre sills upon the rocks between which they have been injected is, in general, comparatively trifling. It is for the most part a mere induration, sometimes accompanied with distortion and fracture.

Although the intrusion of the granophyre sills has been subsequent to that of the basalt-sheets with which they are so generally associated, we may expect that as there is a series of post-granophyre basic dykes, so there may be some basic sills later than the injections of the acid sheets. The Raasay granophyre appears to furnish an example of such a later basic intrusion. At the Point of Suisnish on that island I have observed the relations shown in Fig. 374. There the dark shales of the Lower Lias (_a_ _a_) are immediately overlain by the granophyre sill (_b_), and are cut by a basalt-dyke which, when it rises to the base of the granophyre, turns abruptly to one side, and then pursues its course as a sill (_c_) between the granophyre and the shales. There can be little doubt that this intrusion is later than the granophyre. Here a basic sill is interposed at the bottom of the acid sheet; and is visibly connected with the actual fissure up which its molten material was impelled.