ii. SKYE

All through the Inner Hebrides the base of the basalt-plateaux presents abundant examples of sills. The general parallelism of these intrusive sheets to the bedding of the Jurassic strata among which they lie has been above referred to as having given rise to the erroneous conclusion that in Skye and elsewhere the basalts are interstratified with Jurassic rocks, and are consequently of Jurassic age. It was Macculloch who first described and figured in detail the proofs of their intrusive nature. His well-known sections in plate xvii. of the illustrations to his work on the _Western Islands_ have been repeatedly copied, and have served as typical figures of intrusive igneous rocks.

Nowhere in North-Western Europe can the phenomena of sills be studied so fully and with such exuberance and variety of detail as in the island of Skye and its surrounding islets. On the western coast the greater subsidence of the basaltic plateau has for the most part submerged the platform of intrusive sheets, though wherever the base of the bedded lavas is brought up to the surface the accompanying sills are exposed to view. The east coast of the island has been classic ground for this part of volcanic geology since it supplied the materials for Macculloch's descriptions and diagrams. From the mouth of Loch Sligachan to Rudha Hunish, at the north end of Skye, a series of sills may be traced, sometimes crowning the cliffs as a columnar mural escarpment, sometimes burrowing in endless veins and threads through the Jurassic rocks. The horizontal distance to which this continuous band of sills extends in Skye is not far short of 30 miles. But it stretches beyond the limits of the island. It forms the group of islets which prolongs the geological structure and topographical features of Trotternish for 4 miles further to the north-west. It reappears 10 miles still further on in the Shiant Isles. Thus its total visible length is fully 40 miles, or if we include some outlying sills near the Point of Sleat, to be afterwards described, it extends over a distance of not less than 60 miles. From the last outlier in Skye to the sills of the Isle of Eigg is a distance of only 8 miles, thence to those of Ardnamurchan 17 miles, and to those of the south coast of Mull 25 miles. Thus this platform of intrusive sheets of the Inner Hebrides can be interruptedly followed for a space of not less than 110 miles.

Though none of the sills in Skye itself attain the dimensions of the Fair Head sheet, they present a greater variety of rock and of geological structure than is to be found in Antrim. They are specially developed at the base of the thick, overlying, basalt-plateau--a platform on which such a prodigious quantity of eruptive material has been injected. Part of this material consists of basic rocks in the form of dykes, veins, or sills; part of it is included in the intermediate and acid groups, and comprises veins, sheets, and bosses of granitoid, felsitic, rhyolitic, trachytic, and pitchstone rocks. One of the peculiarities of the Skye sills is the occurrence among them of compound examples, where sheets of basic and acid material have been injected along the same general platform. These will be more specially referred to in Chapter xlviii. With regard to the basic sills (dolerites, basalts, etc.), I would remark that while in Western Scotland the Antrim type of short, thick intrusions, or laccolites, is also found, the vast majority of the sheets are much thinner, more persistent, and less easily distinguishable from the bedded basalts.

In describing the sills of Skye I shall take first those of the eastern and then those of the western side of the island. Along the east coast, from Loch Sligachan to the most northerly headlands and islets the sills play a notable part in the scenery, inasmuch as they cap the great sea-cliff of Trotternish and run as a line of ridges parallel to the trend of the coast, while the plateau-basalts rise above them further inland as a lofty escarpment, which includes the picturesque landslips of the Storr Rock and Quiraing (Figs. 318, 319). Beneath the thick sills, the Jurassic sandstones form a range of pale yellow precipices, along which many thinner sheets of eruptive material have been intruded. As Macculloch well showed, many of these sheets, if seen only at one point, might readily be taken for regularly interstratified beds, but perhaps only a few yards distant they may be found to break across the strata and to resume their course on a different level.

The sills of this Trotternish coast may be distinguished even at some distance from the bedded basalts by the regular prismatic jointing, already referred to, and by their frequently greater thickness, while on closer inspection they are characterized by their much coarser texture. They are generally somewhat largely crystalline ophitic dolerites, gabbros or diabases, and exhibit the persistent uniformity of composition and structure so characteristic of intrusive sheets and dykes. These characters are well exhibited in the Kilt Rock, a columnar sill capping the cliffs to the south of Loch Staffin (Fig. 319).

These massive sills are prolonged in a series of picturesque flat tabular islets beyond the most northerly headlands of Skye. They probably continue northwards under the sea at least 12 miles further, for sills of the same type rise there in the singularly striking group of the Shiant Isles (Fig. 320). These lonely islets, extending in an east and west direction for about three miles, display in great perfection most of the chief characters of the Skye sills. They are especially noteworthy for including the thickest intrusive sheet and the noblest columnar cliff in the whole of the Tertiary volcanic series of Britain. The larger of the two chief islands consists of two masses of rock connected by a strip of shingle-beach, and having a united length from north to south of about two miles. The northern half, or Garbh Eilean, presents towards the north a sheer precipice 500 feet high. This magnificent face of rock consists of one single sill, but as its original upper limit has been removed by denudation and its base, where it is thickest, is concealed under the sea, the sill may exceed 500 feet in thickness. The rock has the usual prismatic structure, which imparts to it an impressive appearance of regularity. The columns retain their individuality to a great height, and though none of them perhaps can be followed from base to crest of the cliff, many of them are evidently at least 300 or 400 feet long.

Macculloch, who gave the first geological description of the Shiant Isles, showed the intrusive nature of the igneous rocks, and described the remarkable globular or botryoidal structure of the Jurassic shales between which they have been injected.[315] Professor Heddle has published a brief account of the geology of the islands.[316] Professor Judd visited the group and brought away a series of specimens of their eruptive rocks, which he found to include basic and ultra-basic varieties.[317]

[Footnote 315: _Western Islands_, vol. i. p. 441.]

[Footnote 316: _Trans. Norfolk Nat. Hist. Soc._ vol. iii. (1880) p. 61.]

[Footnote 317: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878) p. 677, and xli. (1885) p. 393. My description in the text is the result of three successive visits to the islands.]

In Garbh Eilean, where the thickest mass of erupted material presents itself, at least three sills may be observed. Some low reefs that run parallel with the northern coast of the island consist of coarse ophitic gabbro in two or more sheets which have been intruded between the Jurassic shales. Above these strata comes the great columnar sill, its base gradually sinking towards the west until it passes under the sea, and the vertical columns then plunge abruptly into the water. The rock of which this massive sill consists is another large-grained gabbro or dolerite, with an ophitic structure. Owing to the form of the ground it cannot be so satisfactorily examined as the neighbouring island of Eilean Mhuire, which, though less lofty and rather smaller than Garbh Eilean, affords a succession of admirable and easily examined sections along its precipitous shores.

Professor Judd found that while the rocks are mainly ophitic gabbros and dolerites, they include such highly basic compounds as dunite. An examination of the Eilean Mhuire cliffs enables the observer to ascertain that the sills display considerable variety in texture and in the character and arrangement of their component minerals. They are marked by a persistent, more or less distinct disposition in rude beds, and these again often display a banding of their constituents in lines parallel with the general bedding. Some of these bands are largely felspathic, and are thus paler in colour. Others, where the ferro-magnesian minerals and ores are more specially aggregated, are dark in colour. In some layers the long black prisms of augite are ranged in a general parallelism with the banding.

A specimen selected as typical of the ordinary coarse-grained amorphous rock was sliced and placed in Mr. Harker's for microscopic examination, and he has supplied the following observations regarding it: "The gabbro from Eilean Mhuire [7110] is a crystalline rock showing to the eye lustrous black augites, half an inch long, and (predominating) felspar. The microscope reveals, in addition, irregular grains of black iron-ore and little hexagonal prisms of apatite. No olivine is to be detected. As regards structure, the augite has tended to crystallise out in advance of the felspar, but this relation is not constant.

"The augite is of a light-brown tint in slices, and has an unusual kind of pleochroism. The colour for vibrations parallel to the [Greek: b]-axis is of the purplish-brown tone seen in some soda-bearing augites; parallel to [Greek: g] and [Greek: a] it has a yellow or citron tint. The colour and pleochroism are more marked in the interior of a crystal than towards the margin, but some crystals pass at the margin into a slightly pleochroic, pale-green, recalling ægerine-augite. The felspar tends to build elongated crystals. It is a rather finely lamellated labradorite, sometimes showing pericline- as well as albite-lamellae."

Another specimen from one of the black bands in the same island, with a linear arrangement of its component minerals, is thus described by the same petrographer: "This rock [7111] is of darker appearance than the preceding, and contains abundant black iron-ore, besides some pyrites. It also differs in having a marked parallel disposition of its crystals.

"Except for the greater prominence of large irregular grains of iron-ore, this rock under the microscope closely resembles the last described, the parallel structure not being conspicuous in the slice. The augite has the peculiar colour and pleochroism already noted, and the felspar is of the same kind as before."

I did not succeed in finding in place any bands of dunite, but this basic material probably occurs at the base of some of the sills where it has segregated from the rest of the mass, like the picrite at the bottom of the Bathgate diabase.

The amount of contact-metamorphism effected even by such thick sills as those of Trotternish and Shiant is much less than might be expected. It seldom goes beyond a mere induration of the strata for a few yards, often only for a few inches from the surface of junction. In the Shiant Isles, however, the shales between the sills have undergone a more remarkable alteration. They have not only been greatly indurated, but have acquired the globular or botryoidal structure so fully described by Macculloch. The spheroidal aggregates vary from not more than a line to more than half an inch in diameter, and appear on the surface as dark, irregularly grouped, pea-like aggregates. This structure is perhaps best developed immediately under the thick sill on the west side of Eilean Mhuire.

The massive sills are not the only evidence of the injection of igneous material on the Shiant Isles. The sill, or more probably group of sills, forming Eilean Mhuire is traversed by a number of sheets of basalt varying from only two or three inches to 20 feet in thickness. These black fine-grained rocks invariably present chilled selvages next the coarse gabbro, and though they have been on the whole injected parallel to the general bedding or banding, they here and there break across it as veins. The most important of these later intrusions forms a columnar sill on the eastern side of the island, and can be followed for several hundred yards. It consists of a dark finely crystalline olivine-basalt, which towards the margin assumes a dense black texture. Under the microscope Mr. Harker found a thin slice of this rock to be "an olivine-basalt of semi-ophitic, semi-granulitic structure [7112]. The olivine is mostly fresh, but part of it is converted into a yellowish-brown pseudomorph like iddingsite. Magnetite occurs chiefly in imperfect octohedra. The felspar is in little lath-shaped sections, many of which are finely striated, and give extinction-angles indicating a labradorite. The augite, light brown in the slice, never has crystal-boundaries, and often enwraps the felspars."

The narrow veins are composed of a much closer-grained basalt in which a few scattered felspars are visible. Mr. Harker remarks, with regard to a thin slice of one of these rocks [7113], that "the microscope shows this, too, to be an olivine-basalt. The porphyritic felspars are twinned on the Carlsbad and albite laws. Olivine and pseudomorphs after it are well represented. Magnetite is only sparingly present. The general mass of the rock consists of very small striated prisms of labradorite, granules of augite, and interstitial matter which must be partly glassy."

This is perhaps the most striking of all the examples known to me where an older sill has been split open to receive a subsequent injection of molten material. The Eilean Mhuire gabbro must be at least 200 feet thick, and it not impossibly passes under the still thicker pile of Garbh Eilean. Yet it has been horizontally ruptured near its base, and into the rent thus produced another mass of molten matter has been thrust. This subject will be again referred to in connection with another remarkable example on the west coast of Skye.

In contrast to such enormous thicknesses of intrusive material as those of Trotternish and the Shiant Isles, instances may be culled from the same belt of sills where the molten rock has been injected in thin leaves and mere threads into the Jurassic sandstones and shales, or into the shales and coals intercalated among the plateau-basalts. Thus, on the cliff immediately to the north of Ach na Hannait, between Loch Sligachan and Portree Bay, the section may be seen which is represented in Fig. 321. At the base lies a vesicular dolerite with a slaggy upper surface (_a_). Next comes a zone of sedimentary material about five or six feet thick, the lower portion consisting of an impure coal, which passes towards the right hand into brown and grey carbonaceous shale with plant-remains (_b_). This coaly layer has been already alluded to as probably lying on the same horizon with the coal of Portree (p. 288). Traced northward, it is found to have a bed of fine tuff beneath it, and sometimes a volcanic breccia or conglomerate. It fills up rents in the underlying slaggy lava, and was undoubtedly deposited upon the cooled surface of that rock. Immediately above this lower band the black carbonaceous shale which follows has been invaded by an extraordinary number of thin cakes or sills and also by veins or threads of basalt. For a thickness of two or three feet the band (_d_) consists mainly of these intrusions, which, in the form of a fine grey basalt, vary from less than an inch to three or four inches in thickness. They are separated by thin partings of coaly shale, and as they tend to break up into detached nodule-like portions, especially towards the right hand of the section, they might, on casual inspection, be easily mistaken for nodules in the dark shales. Somewhat later in the time of intrusion are veins of basalt which, as at _c_, break across the nodular sills, and sometimes expand into thicker beds (_c´_).

I have never seen such a congeries of minute sills among the Tertiary basalt-plateaux as that here exhibited. In a space of about three feet of vertical height there must be more than a dozen of roughly parallel leaves of intrusive rock. Veins (_e_) run up from the chief band of eruptive material into the overlying finely vesicular basalt (_f_). The dyke (_g_) is probably the youngest rock in the section.

The more general and extensive submergence of the base of the basalt-plateau on the west side of Skye has for the most part carried the platform of sills below sea-level, so that it is only exceptionally where, owing to local irregularities, that base has been brought up to the air, that the intrusive sheets show themselves. Yet the persistence of the platform on that side is indicated by its extension even as far as the southern promontory of the island.

The Trotternish type of sill extends down the west coast under the headlands of Duirinish. Thus at the mouth of Dunvegan Loch, where the underlying Jurassic platform has been ridged up above the surface of the sea, it has carried with it the marked sill which forms the islets of Mingay and Clett that lie as a protecting breakwater across the entrance of the inlet. The intrusive rock rests on shell-limestones full of oysters (_Ostrea hebridica_), and referable to the Loch Staffin group of the Great Oolite Series. This sill, when observed from a little distance, presents the usual regularly prismatic or columnar structure so well developed among the Trotternish examples, but on a closer view shows this structure less distinctly. It is an olivine-dolerite of medium and fine texture, which in thin slices displays under the microscope a distinctly ophitic structure, the abundant light-brown augite enclosing the striated felspars. Its lowest portion, from three to seven or eight inches upward from the bottom, is much closer-textured than the rest of the rock and is finely amygdaloidal. Its vesicles are in many cases drawn out to a length of three or four inches, and the zeolites which now fill them look like parallel annelid tubes or stems of _Lithostrotion_. It is noteworthy also that the elongation of the vesicles has sometimes taken place at a right angle to the surface of contact with the underlying strata. But the most remarkable feature in this sill is the surface which it presents to the oyster-beds on which it rests. The fine-grained dark dolerite has there assumed the aspect of a sheet of iron-slag, with a smooth or wrinkled, twisted, ropy surface, which displays fine curving flow-lines. No one looking at a detached specimen of this surface would be ready to admit that it could possibly have come from anything but a true lava-stream that flowed out at the surface. The contours of a viscous lava are here precisely reproduced on the under surface of a massive sill.

A little further south, the promontory of Eist, forming the western breakwater of Moonen Bay, consists of an important sill or group of sills which has insinuated itself among shales, shell-limestones, and shaly sandstones, full of _Ostrea hebridica_, _Cyrena aurata_, etc., and belonging to the Loch Staffin group of the Great Oolite Series. The shore-cliff below the waterfall affords the section given in Fig. 322, illustrating the manner in which a thick intrusive sheet may sometimes give off thin veins from its mass. The rock attains on the Eist promontory a thickness of probably at least 100 feet, where it is thickest and undivided. But the two main sheets, or branches of one great sheet, on this peninsula have probably a united depth of more than 300 feet. Landwards the rock splits up and encloses cakes of the Jurassic strata. It possesses the usual prismatic structure and doleritic composition. In Moonen Bay, as shown in Fig. 322, it presents a banded structure, marked especially by an alternation of lines of amygdales and layers of more compact and solid dolerite, with occasional enclosed cakes of baked shale or sandstone. Its upper surface is somewhat uneven, and from it are given off narrow, wavy, ribbon-like veins (_d_), from less than an inch to three inches or more in width, which keep in a general sense parallel to the top of the sill, but at a distance of a few inches or feet from it. The sill becomes as usual fine-grained towards the contact, the shales and sandstones being indurated and the limestone marmorized.

The next uprise of the base of the basalt-plateau on the west side of Skye lies about 25 miles to the south-east, where it emerges from the sea in the Sound of Soa (Fig. 323). A vast volcanic pile has there been heaped up on the Torridon sandstone, the whole of the thick Jurassic series, which is found in force only three miles distant in Strathaird, having been removed by denudation from this area before the beginning of the Tertiary volcanic period. The plateau-basalts rests on the upturned edges of the Torridonian sandstones and shales, and are accompanied as usual by their underlying network of intrusive rocks. It is hardly possible to exaggerate the wild confusion of sills, dykes and veins which have been injected among the rocks, at and on both sides of the unconformability. Endless sheets of basalt and dolerite have forced their way between the bedded basalts and the sandstones, while across the whole rise vast numbers of dykes and veins. Narrow, black, wavy ribbons of basic material cross many of these veins, while the later north-west dykes cut sharply through everything older than themselves. As a natural section for the study of the phenomena of intrusion in many of their most characteristic phases, I know no locality equal to the northern coast-line of the Sound of Soa, unless it be the cliffs of Ardnamurchan. But the Skye cliffs, though less imposing than those of the great Argyllshire headland, have this advantage, that instead of being exposed to the full roll of the open Atlantic, they form the margin of a comparatively sheltered strait, and can thus be conveniently examined.

Following still the western seaboard of Skye, we meet with other striking examples of sills at a distance of some eight miles in a straight line eastward where, between Lochs Slapin and Eishort, the prominent headland of Suisnish juts out into the sea. This promontory has long been known to geologists from the section of it given by Macculloch as an instance of the connection between overlying rocks and dykes. I have already alluded to it in that relation, and refer to it again as an example of one of the thicker intrusive sheets of the Inner Hebrides. Denudation has here also proceeded so far that the whole of the volcanic plateau has been stripped off, only some of the underlying sills being left, together with the platform of older rocks between which and the vanished basalts they were injected. Most of these sills consist of granophyres belonging to the acid group of rocks to be afterwards described. But basic sheets occur not infrequently interposed between the granophyres and the subjacent Lias, and sometimes even intercalated in the former rock. Though at first sight it might be thought that these sills had insinuated themselves after the eruption of the granophyre, and there are instances where this cannot be shown not to be the case, I have obtained so many proofs of the invasion of the basic by the acid rock that I have no doubt the former is, as a general rule, the older of the two.

The Suisnish headland exhibits the structure represented in Fig. 249. For about 300 feet above the sea-level the steep grassy slope shows outcrops of the dark, sandy shales and yellowish brown, shaly sandstones of the Lias which form the range of cliffs to the eastward. These gently inclined strata are cut through by many vertical basalt-dykes, some of which intersect each other, but among which by far the largest is the mass shown in the figure. This broad dyke consists of a dolerite or gabbro the largely crystalline texture of which marks it off at once from the others, which are of the usual dark, heavy, fine-grained type, with an occasional less basic and porphyritic variety. Traced up from the sea-margin, the dyke loses itself in a talus of blocks from the cliff above, so that its actual junction with the mural front of the sill cannot be seen. But that it joins that mass, with which it agrees in petrographical characters, hardly admits of question. The cliff consists of a thick sheet of coarsely crystalline dolerite or gabbro (_d_ in Fig. 249), which in its general aspect at once recalls the rock of Fair Head. It varies considerably in texture, some parts of the mass are exceedingly coarse, like the Skye gabbros, and present a fibrous structure in their augite resembling that of the diallage in these rocks; other portions assume the compactness of basalt. A specimen of medium grain under the microscope shows the typical ophitic structure so generally found among the dolerites both of the plateaux and of the intrusive sheets. This sill must be about 200 feet thick, and like the rock at Fair Head is traversed from top to bottom by joints that divide it into prisms. It appears to bifurcate eastward, one portion running with a tolerably uniform thickness of a few feet as a prominent band at the top of the shales and sandstones, the other slanting upwards and gradually thinning away in the granophyre.

Towards its base, near the contact with the underlying shales, the rock as usual becomes finer grained, and the thin band just referred to resembles in texture one of the wider basalt-dykes. Westwards the rock can be followed round the top of the grassy slopes formed by the decay of the shales. Though concealed by intervals of moorland and peat, it is visible in the stream sections, and I think must be continuous, as a band only a few yards thick, round the northern side of the hills as far as Beinn Bhuidhe, where a similar sill makes a prominent crag. Its total area measures a mile and a quarter in length by half a mile in breadth. The granophyre which overlies it forms part of an interesting series of sheets which I have traced all the way from Suisnish to the braes above Skulamus.

Whether or not the whole sheet of basic rock is continuous, and whether it all proceeded from the great Suisnish dyke, cannot be confidently decided until the ground is mapped in detail, though from the great thickness of the sill at the dyke, its attenuation outwards from that centre and its uniformity of petrographical character, I am disposed to answer affirmatively. There is no other probable vent to be seen in the neighbourhood, unless a massive dyke that runs from Loch Fada north-westwards into Glen Boreraig can be so regarded.

Not far from the extreme southern point of Skye a singularly interesting example of a sill remains as a detached survival of the basaltic plateau and its accompaniments. In his map of Skye, Macculloch showed the position of this outlier, which he classed with the general "trap" formation of the island. The locality was visited by Professor Judd, who regarded the intrusive rock as a "phonolite"[318] In 1894, during an excursion with my colleague Mr. C. T. Clough, I had an opportunity of examining the rocks and collecting notes for the following account of them.

[Footnote 318: _Quart. Jour. Geol. Soc._ vol. xxxiv. (1878) p. 692.]

At Rudh' an Iasgaich, about two miles from the Point of Sleat, a small outlier of conglomerate lies on the edges of the Torridon Sandstone. This deposit has been correctly identified by Professor Judd with the similar strata which, in Skye and elsewhere on the west coast of Scotland, underlie the Liassic series. It is here about 10 or 12 feet thick, reddish and yellowish in colour, and distinctly calcareous. Its component pebbles consist largely of Cambrian (Durness) limestone, quartzite, and Torridon Sandstone--rocks which all occur _in situ_ in Sleat. It may be compared with the limestone conglomerates of Strath and those which underlie the Lias at Heast on Loch Eishort.[319] That here, as elsewhere in this region, the basement conglomerate was followed by the rest of the Lias and Oolites may be inferred with some confidence from the copious development of the Jurassic series a few miles off, both to north and south. But the whole of this overlying succession of formations has here been swept away, and, but for the protection afforded by the eruptive rocks of Rudh' an Iasgaich, the conglomerate would likewise have disappeared.

[Footnote 319: _Op. cit._ vol. xiv. (1857), p. 9; vol. xliv. (1888), p. 71.]

Above the conglomeratic band lies a sheet of intrusive rock, which in one place has apparently cut it out, so as to rest directly upon the Torridon Sandstone (_a_, Fig. 324). The decay of the softer detrital rock underneath has caused the sill to break off in slices, which have left behind them a bold mural escarpment (_b_ _b_).

The rock of this sill is a rather coarsely crystalline porphyritic olivine-dolerite, which towards the north attains a thickness of about 70 feet. It exhibits the usual prismatic jointing, but less perfectly than some of the Trotternish sills already referred to. Besides these vertical joints, it is also traversed by a system of horizontal divisional planes which, though somewhat irregular in their course, run, in a general sense, parallel to the upper and under surfaces of the sill.

It seems to have been along this transverse series of joints that a second sill (_c_), five or six feet thick, has been injected. The material of this younger intrusion is a black, finely crystalline dolerite or basalt, with rudely prismatic jointing. Its most striking feature, besides its regularity of position and persistency for several hundred yards as a platform along the shore, is the basalt-glass which marks both its under and upper surfaces of contact, and which is here developed upon a scale to which I have not met with an equal among the Tertiary sills of this country.

The selvage of glass appears as a black tar-like layer, varying from a mere film to two or three inches in thickness. It is found not only on the upper and under surfaces, but descends along abrupt step-like interruptions of the upper surface, a foot or more in height, as if the sill had been broken by a series of subsidences. The apparent fracture, however, is probably due to the irregularities of the passage forced for itself by the molten rock as it passed from one line of horizontal joint to another through the heart of the older sheet.

The exposed surface of black glass on the top of the younger sill exhibits long parallel lines, probably marking flow-structure, which are made conspicuous by a pale yellow ferruginous weathered crust. Portions of the larger intrusive sheet have been broken off and involved in the later rock. The younger sill disappears to the north, and is not found in the cliff of Rudha Chàrn nan Cearc, where the thick sill, lying once more on the band of conglomerate, forms a fine escarpment above the shore. Dykes of fine-grained basalt (_d_ _d_) with compact chilled margins rise through both sills, together with veins which pursue a wavy upward path like strips of black ribbon.

This example, and that of the Shiant Isles already described, cannot but impress the observer with the prodigious force with which the material of the sills was injected. In these instances solid sheets of intrusive rock have subsequently been rent open, doubtless under a superincumbent pressure of many hundreds of feet of the terrestrial crust, and a new injection of molten magma has made its way into the rents thus caused. In each case, the position of the rents was obviously determined by structural lines in the older sills, but we are lost in astonishment at the energy required to split open, even along these lines, such solid crystalline masses as the thick sills, and to overcome the superincumbent pressure of so deep a pile of rock.

The isolation of a relic of the Tertiary sills on the west side of the promontory of Sleat presents some interesting problems to the mind of the geologist. The locality lies about midway between the basalt-plateau of Strathaird and that of Eigg, and some eight or nine miles in a direct line from either. The basalts cannot be proved to have once stretched continuously between Eigg and Strathaird, and to have covered this part of Sleat; but the position of the Sleat sills makes it probable that this continuation did formerly exist. The denudation of the West of Scotland since early Tertiary time has been so stupendous that I am prepared for almost any seemingly incredible evidence of its effects. There can hardly be any doubt, however, that the sills here described belong to the great platform of intrusive sheets, and that they were injected under a pile of Secondary strata, if not also of Tertiary basalts, which has here been entirely removed.

Reference may be made, in conclusion, to a not infrequent feature of the Skye sills. Like the dykes, they are often double or multiple, molten material having been successively injected along the same plane. The example just cited from the west side of Sleat illustrates one type of such compound sills. More frequently, however, the subsequent injections have been made along the floor or roof of the first sheet. Mr. Harker has found numerous cases of this structure in the Strath district. They are recognizable even from a distance by their terraced contours when seen in profile. They often vary considerably in thickness owing to the dying out or coming-in of their separate bands; while, on the other hand, single sills tend to maintain a uniform thickness for long distances, or taper away gradually. The compound arrangement of the basic sills is well brought out where acid material has been injected between the sheets, as will be more fully described in Chapter xlviii.