Chapter xlvii

.]

From the specimens collected by me among the Inner Hebrides up to the year 1888, I selected two dozen which seemed to be fairly typical of these altered rocks, and placed thin slices of them for microscopic examination in Dr. Hatch's hands. His notes may be condensed into the following summary. One of the most frequent features in the slides is the tendency in the component minerals to assume granular forms. In one specimen from Loch Spelve, Mull, the rock, probably originally a dolerite, shows only a few isolated recognizable crystals of plagioclase and augite, the whole of the rest of the rock consisting of roundish granules embedded in a felspathic matrix. The felspar crystals are sometimes broken up into a mosaic, though retaining their external contours. Besides the granules, which are no doubt augite, a few grains of magnetite are scattered through the rock, aggregated here and there into little groups. In another specimen, taken from the junction with the granophyre in Glenmore in the same island, parts of the augite crystals are converted into granular aggregates associated with large grains and patches of magnetite. The latter mineral also assumes in some of the rocks granular and even globular shapes suggestive of fusion.

The felspars, which in most of the basic rocks are usually remarkably clear and fresh, show marked kaolinization in some of these altered masses. Minute dusky scales of kaolin are developed, sometimes also with the separation of minute grains of quartz. The augite shows frequent alteration to hornblende, proceeding as usual from the exterior inward. In some cases only an envelope of uralite appears round the augite, while in others only a kernel of the original mineral is left, or the whole crystal has been changed. In many cases the altered substance appears as minute needles, blades and fibres of

## actinolite. Occasionally, besides the green hornblende, shred-like

pieces of a strongly pleochroic brown hornblende make their appearance. Serpentinous and chloritic substances are not infrequent. Epidote is sometimes abundant. The titaniferous iron has commonly passed more or less completely into leucoxene. Here and there a dark mica may be detected.

Since the year 1888 I have continued the investigation of this subject, and have especially studied the metamorphism of the bedded basalts on the western shores of Loch Scavaig, where, as already described, they are truncated by vertical beds of gabbro, and are traversed by basalt-dykes and by abundant veins of fine-grained granophyre. The alteration here effected affords excellent materials for study, as the very same sheets of basalt can be followed from the normal conditions outside to the altered state within the influence of the metamorphic agent. The alternations of amygdaloidal and more compact sheets can still be recognized, although their enclosed amygdales have in places been almost effaced. They show the dull, indurated, splintery character, with the white weathered crust, so distinctive of this type of contact-metamorphism. They are traversed by numerous sills and veins of gabbro. As has been already suggested, although no large mass of granophyre appears here at the surface, the alteration of the basalts is probably to be attributed not so much to the influence of the gabbro, as to the abundant acid sills, dykes and veins, for there may be a considerable body of granophyre underneath the locality, the dykes and veins being indications of its vicinity.

In the summer of 1895 I examined the locality with much care, and collected some typical specimens illustrative of the conditions of metamorphism presented by different varieties of the bedded basalts. Thin slices cut from these specimens were placed in Mr. Harker's hands for microscopical examination, and he furnished me with the following notes regarding them.

"In hand-specimens the bedded basalts from the neighbourhood of the gabbro of Loch Scavaig [6613-6618] do not appear very different from the normal basalts of this region. The most conspicuous secondary mineral is yellowish-green epidote in patches, and especially in the amygdales.

"The texture of the rocks varies, and the slices show that the micro-structure also varies, the augite occurring sometimes in small ophitic plates, sometimes in small rounded granules. The chief secondary change in the body of the rock is shown by the augite, which is seen in various stages of conversion to greenish fibrous hornblende. Some round patches seem also to consist mainly of the latter mineral, and are probably pseudomorphs after olivine. Here the little fibres are confusedly matted together, without the parallelism proper to uralite derived from augite. No fresh olivine has been observed. The felspar and magnetite of the basalts show little or no sign of metamorphic processes, unless a rather unusual degree of clearness in the felspar crystals is to be regarded in that light.

"The contents of the metamorphosed amygdales are not always the same. Epidote is usually present in some abundance, and in well-shaped crystals. It has a pale citron tint in the slices, with marked pleochroism; but a given crystal is not always uniform in its optical characters. Frequently the interior is pale, and has a quite low birefringence. This is probably to be regarded as an intergrowth of zoisite in the epidote, and there are a few distinct crystals of zoisite seen in some places.

"In the slide which best exhibits these features [6613] the crystals of epidote are in part enwrapped and enclosed by what are doubtless zeolitic minerals. At least two of these are to be distinguished. One, very nearly isotropic, and with a pale-brownish tint, is probably analcime. Associated with this is a colourless mineral with partial radiate arrangement and with twin lamellation; the birefringence is somewhat higher than that of quartz, and the γ-axis of optic elasticity makes a small angle with the twin-line. These characters agree with those of epistilbite. In other parts of the same large amygdale, the epidote crystals are embedded in what seems to be a felspar. This latter mineral is rather obscure, and twin-lamellation is rarely to be detected; but it seems highly probable that felspar has here been developed by metamorphic agency at the expense of zeolites which once occupied the amygdale. I have observed undoubted examples of this in metamorphosed basalts from other parts of Skye, _e.g._ from Creagan Dubha, near the granophyre mass of Beinn Dearg.[396] The felspar occurs there in the same fashion, and in the same relation to epidote [2700, 2701]. In the specimens now described the chief minerals in the metamorphosed amygdales are those already named: others occur more sparingly, associated with them. In some cases there is a grass-green, strongly pleochroic, actinolitic hornblende, accompanied by a little iron pyrites [6615].

[Footnote 396: Compare _Trans. Roy. Soc. Edin._ vol. xxxv. p. 166.]

"Epidote and various hornblendic and augitic minerals are characteristic products in the metamorphism of amygdaloidal basalts in other regions: felspar with this mode of occurrence I have not seen except in Skye, where it seems to connect itself naturally with the abundance of zeolites in the amygdales of the non-metamorphosed lavas. It is to be observed that in these basalts from Loch Scavaig the alteration is shown especially in the amygdales, the body of the rock not being greatly affected: this indicates a not very advanced stage of metamorphism. The production of uralitic hornblende, rather than brown mica, from the augite and its decomposition-products, seems to be characteristic of the metamorphism of basaltic as distinguished from andesitic rocks, and is well illustrated by a comparison of the two sets of lavas near the Shap granite."[397]

[Footnote 397: _Quart. Journ. Geol. Soc._ vol. xlix. (1893) p. 361.]

Mr. Harker, who is at present engaged in mapping the central region of Skye, has had occasion to go over a number of the localities (Creagan Dubha, etc.) originally cited by me, and, while corroborating my general conclusions regarding them, has been able to obtain much fresh evidence regarding the nature and extent of the metamorphism which the bedded basalts have undergone. The results of his investigations will be published when the Geological Survey of Skye is further advanced.

(3) _Relation of the Granophyre to the Gabbros._--That the granophyres invade the gabbros has been incidentally illustrated in the foregoing pages. But as the mutual relations of the two rocks in the island of Skye have been the subject of frequent reference in previous writings of geologists, it is desirable to adduce some detailed evidence from a region which has been regarded as the typical one for this feature in the geological structure of the Inner Hebrides. No geological boundary is more easily traced than that between the pale reddish granophyre and the dark gabbro. It can be followed with the eye up a whole mountain side, and can be examined so closely that again and again the observer can walk or climb for some distance with one foot on each rock. That there should ever have been any doubt about the relations of the two eruptive masses is possibly explicable by the very facility with which their junction can be observed. Their contrasts of form and colour make their boundary over crag and ridge so clear that geologists do not seem to have taken the trouble to follow it out in detail. And as the pale rock undoubtedly often underlies the dark, they have assumed this infraposition to mark its earlier appearance.

I will only cite one part of the junction line, which is easily accessible, for it lies in Glen Sligachan immediately to the south of the mouth of Harta Corry. The rounded eminence of Meall Dearg, which rises to the south of the two Black Lochs, belongs to the granophyre, while the rugged ground to the west of it lies in the gabbro. The actual contact between the two rocks can be followed from the side of Harta Corry over the ridge and down into Strath na Creitheach, whence it sweeps northward between the red cone of Ruadh Stac and the black rugged declivities of Garbh Beinn. There is no more singular scene in Skye than the lonely tract on the south side of Meall Dearg. The ground for some way is nearly level, and strewn with red shingle from the decomposing granophyre underneath. It reminds one of some parts of the desert "Bad lands" of Western America. Grim dark crags of gabbro, with veins from the granophyre, rise along its western border, beyond which tower the black precipices of the Cuillins, while the flaming reddish-yellow cones of Glen Sligachan stand out against the northern sky.

Having recently described in some detail the relations of the boss of granophyre at this interesting locality, I will only here offer a brief summary of the chief features.[398] The granophyre of Meall Dearg forms a marginal portion of the great mass of the Red Hills. It has broken across the banded gabbros, and also cuts an isolated boss of agglomerate in the ridge of Druim an Eidhne. Its line of junction is nearly vertical, but along part of its course the wall of gabbro rises higher than that of the more decomposable granophyre. Hence the origin of the black crags that crown the red slopes of granophyre debris. Seen from a distance the basic rock seems to rest as a great bed upon the acid mass.

[Footnote 398: See _Quart. Journ. Geol. Soc._ vol. 1. (1894) p. 212.]

The younger date and intrusive nature of the granophyre are well shown by the change in the texture of the mass as it approaches the rocks against which it has cooled. The ordinary granophyric characters rapidly pass into a fine-grained felsitic texture, and this change is accompanied with the development of a remarkably well-defined flow-structure and of rows of spherulites which run parallel to the boundary wall. In a ravine on the west side of Meall Dearg, the lines of flow-structure and rows of large spherulites are seen to be arranged vertically against the face of gabbro.

Further proof of the later date of the protrusion of the granophyre is supplied by abundant felsitic dykes and veins which traverse the gabbro, and some of which can be seen to proceed from the main body of granophyre. These intrusions will be described in the next chapter, in connection with the dykes and veins of the acid rocks.

Additional evidence as to the posteriority of the granophyre to the gabbro has recently been obtained by Mr. Harker from a study of the internal structure and composition of the masses of these rocks which have been intruded into the agglomerate above Loch Kilchrist in Strath. He has found that the granophyre has there caught up from some subterranean depth portions of gabbro, and has partially dissolved them, thereby undergoing a modification of its own composition. "The gabbro-debris," he remarks, "has been for the most part completely disintegrated by the caustic or solvent action of the acid magma on some of its minerals. Those constituents which resisted such action have been set free and now figure as xenocrysts [foreign crystals], either intact or more or less perfectly transformed into other substances. At the same time the material absorbed has modified the composition of the magma, in the general sense of rendering it less acid." Mr. Harker has traced the fate of each of the minerals of the gabbro in the process of solution and isolation in the acid magma, which, where this process has been most developed, is believed by him to have taken up foreign material amounting to fully one-fourth of its own bulk, derived not from the rocks immediately around, but from a gabbro probably at a considerable depth beneath.[399]

[Footnote 399: _Quart. Journ. Geol. Soc._ vol. lii. (1896) p. 320.]

[Illustration:

Fig. 350.--Ground-plan of basic dyke in Cambrian Limestones truncated by granophyre which encloses large blocks of the dyke, Torrin, Skye. ]

(4) _Relation of the Granophyre to the Basic Dykes and Veins._--Reference has already been made to the fact that the "syenite" bosses of Skye cut off most of the basalt-dykes, but are themselves traversed by a few others.[400] The locality that furnished me with the evidence on which this statement was originally made nearly forty years ago affords in small compass a clearer presentation of the facts than I have elsewhere met with. The sections described by me are visible at the eastern end of the boss of Beinn an Dubhaich, Strath; but similar and even better examples may be cited from the whole northern and southern margins of that eruptive mass. On the north side an extraordinary number of dykes may be traced in the Cambrian limestone from the shores of Loch Slapin eastwards. They have a general north-westerly trend, but one after another, as I have already remarked, is abruptly cut off by the granophyre. As an example of the way in which this truncation takes place, I may site a single illustration from the northern margin of the eruptive mass, near Torrin. It might perhaps be contended that the numerous dykes which traverse the limestone and stop short at the edge of the acid rock, are not necessarily older than the granophyre, but may actually be younger, their sudden termination at the edge of the acid boss being due to their inability to traverse that rock. That this explanation is untenable is readily proved by such sections as that given in Fig. 350, where a basic dyke (_b_) 9 or 10 feet broad running through the Cambrian Limestone (_a_ _a_) is abruptly cut off by the edge of the great granophyre boss. Not only is the dyke sharply truncated, but numerous pieces of it, from an inch to more than a foot in length, are enclosed in the granophyre. The latter is well exposed along the shore of Loch Slapin in an almost continuous section of nearly a mile in length. The contrast therefore between the development of dykes within and beyond its area cannot but arrest the attention of the observer. Though I was on the outlook for dykes in the granophyre, I found only one. Yet immediately beyond the eruptive boss they at once appear on either side up to its very edge, where they suddenly cease. The conclusion cannot be resisted that the protrusion of the acid rock took place after most of the dykes of the district had been formed, but before the emission of the very latest dykes, which pursue a north-west course across the boss (Fig. 348).

[Footnote 400: _Ante_, p. 173, and _Quart. Journ. Geol. Soc._ vol. xiv. (1857) p. 16.]

Some sections on the southern margin of Beinn an Dubhaich complete the demonstration that such has been the order of appearance of the rocks. Near the head of the Allt Lèth Slighe (or Half-way Burn), where the granite has pushed a long tongue into the limestone, a north-west basalt-dyke is abruptly cut off by the main body of the boss and by the protruded vein (Fig. 351). Besides this truncation, the acid rock sends out strings and threads of its own substance into and across the dyke, these injected portions being as usual of an exceedingly fine felsitic texture.

[Illustration: Fig. 351.--Section on south side of Beinn an Dubhaich, Skye, showing the truncation of a basalt-dyke (_b_), in Cambrian Limestone (_a_), by the granite (_c_) of Beinn an Dubhaich, Skye.]

Similar evidence may be gathered from the area of the great granophyre cones further north. The profusion of basalt-dykes in the surrounding rocks stops short at the margin of that area. The comparatively few dykes which cross the boundary pursue a general north-west course through the granophyre, and, as already remarked, from their dark colour, greater durability and straightness of direction, stand out as prominent ribs on the flanks of the pale cones which they traverse.

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