That such has really been the chief source of the blocks in the conglomerates I cannot doubt. At the east end of Canna we actually detect a volcanic cone, partly washed down and overlain by a pile of river-shingle. There were probably many such mounds of slag and stones along lines of fissure all over the lava-fields. The river in its winding course might come upon one cone after another, and during times of flood, or when its waters burst through any temporary barrier created by volcanic operations it would attack the slopes of loose material and sweep their detritus onward. At the same time, the current would carry forward its own natural burden of far-transported sediment, and hence on its flood-plains, buried and preserved under sheets of basalt, we find abundant pebbles of the old Highland rocks which it had borne across the whole breadth of the basaltic lowland.

But the destruction of volcanic cones was probably not the only source of the basaltic detritus in the conglomerates of Canna and Sanday. I have shown that these conglomerates pass laterally into tuffs, and are sometimes underlain, sometimes overlain, with similar material. It is quite obvious that their deposition was contemporaneous with volcanic action in the immediate neighbourhood, and that at least part of their finer sediment was obtained directly from volcanic explosions. In wandering over the coast-sections of these coarse deposits, I have been impressed with the enormous size of many of the stones, their resemblance to the ejected blocks of the agglomerate, and the distinction that may sometimes be made with more or less clearness between their rather angular forms and the more rounded and somewhat water-worn aspect of the other boulders. It seems to me not improbable that some of the remarkably coarse masses of unstratified conglomerate in Canna Harbour consist largely of ejected blocks from the adjacent vent.

The only instance which I have observed of erosion of the basalt contemporaneous with the operations of the river that spread out this conglomerate is to be found in the striking stack of Dùn Beag already alluded to. [251] This extraordinary monument of geological history forms an outlying obelisk which rises from the platform of the shore to a height of about 70 feet. Seen from the south-west, it appears to consist entirely of bedded basalt resting on some stratified tuff and shale which intervene between these lavas and that of the broad platform of basalt on which the obelisk stands. On that side it presents no essential difference from the structure of the Dùn Mòr to the west, save that the lower conglomerate of that outlier is here represented by fine sediment, and the upper conglomerate is wanting. The general aspect of this south-western front of the stack is shown in Fig. 272. If, however, we approach the rock from the coast-gully to the north, we form a very different impression of its structure. It then appears to consist chiefly of conglomerate with a capping of basalt on the top (Fig. 273). Not until a close scrutiny is made of the eastern and western faces of the column do the true structure and history of this singular piece of topography become apparent.

On the eastern front, the section represented in Fig. 274 is exposed. At the bottom, forming the pediment of the column, lies a sheet of slaggy and vesicular or amygdaloidal basalt (a), which shelves gently in a south-westerly direction into the sea. The lowest band (b) in the structure of the stack is a thin group of lilac, brown, and green shale and volcanic mudstone or tuff, which encloses pieces of coniferous wood, and becomes markedly carbonaceous in its uppermost layers. Above these strata on the south front comes the pile of bedded basalts (c) with their slaggy lower and upper surfaces. But as we follow them round the east side, we find them to be abruptly cut off by a mass of conglomerate (d). That the vertical junction-line is not a fault is speedily ascertained. The lower platform of slaggy basalt runs on unbroken under both shales and conglomerate. Moreover, the line of meeting of this conglomerate with the basalts that overlie the shales is not a clean-cut straight wall, but displays projections and recesses of the igneous rocks, round and into which the materials of the conglomerate have been deposited. The pebbles may be seen filling up little crevices, passing under overhanging ledges of the basalts, and sharply truncating lines of scoriaceous structure in these rocks. The same relations may be observed on the west front of the stack. There the ashy shales and tuffs are sharply cut out by the conglomerate, which wraps round and underlies a projecting cornice of the slaggy bottom of the basalt that rests on the stratified band (Fig. 275).

The conglomerate is rudely stratified horizontally, its bedding being best shown by occasional partings of greenish sandstone. It consists of well-rounded, polished, and water-worn stones, chiefly of members of the volcanic series—basalts, and dolerites, both compact and amygdaloidal or slaggy—but with a conspicuous admixture of Torridon Sandstone, gneiss, grey granite, grit and different schists. The coarsest part of the deposit lies toward the bottom where the volcanic blocks, some of them being six and eight feet in diameter, may have originally fallen from the basalts against which the conglomerate now reposes. The far-transported stones are also of considerable size, pieces of granite and gneiss frequently exceeding a foot in length. The well-rounded pebbles of foreign materials have been washed into the interstices between the large volcanic blocks.

It is, I think, tolerably clear that the wall of basalt against which this conglomerate has been laid down is one of erosion. The beds of basalt have here been trenched by some agent which has likewise scooped out the soft underlying shales, and even cut them away from under their protecting cover of basalt. There can be little hesitation in regarding this agent as a water-course, which for some considerable interval of time continued to dig its channel through the hard basalts. There is not room enough between the basalt-wall of Dùn Beag and the opposite cliffs of the shore (where no trace of this conglomerate is to be seen) for any large stream to have found its way. I do not therefore seek to identify this relic of an ancient waterway with the channel of the main river which deposited the conglomerate bands of Canna and Sanday. More probably it was either a mere torrential chasm, or a tributary stream draining a certain part of the volcanic plateau and allowed to retain its channel long enough to be able to erode it to a depth of nearly 50 feet. Erosion had reached down through the underlying tuffs to the slaggy basalt below, but before it had made any progress in that sheet its operations were brought to an end at this locality by the floods that swept in the coarse shingle, and by the subsequent stream of basalt of which a mere outlying fragment now forms the upper third of the stack (e, Fig. 274).

That the ravine or gully of Dùn Beag probably lay within the reach of the floods of the main river, may be inferred from the number and size of the far-transported rocks in its conglomerate. It was filled up gradually, but the conditions of deposition remained little changed during the process, except that the largest blocks of rock were swept into the chasm in the earlier part of its history, while much smaller and more water-worn shingle were introduced towards the close.

Denudation, which has performed such marvels in the topography of the West of Scotland since older Tertiary time, has here obliterated every trace of this ancient gully, save the little fragment of one of the walls which survives in the stack of Dùn Beag. When in the course of centuries this picturesque obelisk shall have yielded to the action of the elements, the last leaflet of one of the most interesting chapters in the geological history of the Inner Hebrides will have been destroyed.

The question naturally arises—What was the subsequent history of the river which has left so many records of its floods entombed among the basalts of Canna and Sanday? In particular, can any connection be traced or plausibly conjectured between it and the river-bed preserved under the Scuir of Eigg? To this question I shall return after the evidence for the existence and date of the latter stream has been laid before the reader.

In the chain of the Inner Hebrides, broken as it is in outline and varied in its types of scenery, there is no object more striking than the island of Eigg. Though only about five miles long and from a mile and a half to three miles and a half broad, and nowhere reaching a height of so much as 1300 feet, this little island, from the singularity of one feature of its surface, forms a conspicuous and familiar landmark. Viewed in the simplest way, Eigg may be regarded as consisting of an isolated part of the basaltic plateau which, instead of forming a rolling tableland or a chain of hills with terraced sides, as in Antrim, Mull and Skye, has been so tilted that, while it caps a lofty cliff about 1000 feet above the waves at the north end, it slopes gently along the length of the island to the south end. In its southern half, however, the ground rises, owing to the preservation of an upper mass of lavas, which denudation has removed from the northern half. On this thicker part of the plateau stands the distinguishing feature of the island, the strange fantastic ridge of the Scuir, which, seen from the north or south, looks like a long steep hill-crest, ending in a sharp precipice on the east. Viewed from the east, this precipice is seen to be the end of a huge mountain-wall, which rises vertically above the basalt-plateau to a height of more than 350 feet. The accompanying map (Fig. 276) shows that the ridge of the Scuir corresponds with the area occupied by a mass of pitchstone, and that while the basaltic rocks cover the whole of the rest of the southern half of the island, they gradually rise towards the north, successive members of the Jurassic series making their appearance until, at the cliffs of Dunan Thalasgair, the latter cover the greater part of the surface, and leave the volcanic rocks as a mere stripe capping the cliffs. In the section (Fig. 277) the general structure of the island is represented.

In Eigg the fragment of the basalt-plateau which has been preserved, rests unconformably on successive platforms of the Jurassic formations. Its component sheets of lava rise in cliffs around the greater part of the island. As they dip gently southwards their lower members are seen along the northern and eastern shores, while on the south-west side their higher portions are exposed in the lofty precipices which there plunge vertically into the sea. The total thickness of the volcanic series may here be about 1100 feet. The rocks consist of the usual types—black, fine-grained, columnar and amorphous basalts, more coarsely crystalline dolerites, dull earthy amygdaloids with red partings, and occasional thin bands of basalt-conglomerate or tuff. The individual beds range in thickness from 20 to 50 or 60 feet. Though they seem quite continuous when looked at from the sea, yet, on closer examination, they are found not unfrequently to die out, the place of one bed being taken by another, or even by more than one, in continuation of the same horizon. The only marked petrographical variety which occurs among them is a light-coloured band which stands out conspicuously among the darker ordinary sheets of the escarpment on the east side of the island. The microscopic characters of this rock show it to belong to the same series of highly felspathic, andesitic, or trachitic lavas as the "pale group" of Ben More, in Mull. It is strongly vesicular, and the cells are in some parts so flattened and elongated as to impart a kind of fissile texture to the rock. There can be no doubt that this band is a true lava, and that it was poured out during the accumulation of the basalt-plateau. It supplies an interesting example of the intercalation of a lighter and less basic lava among the ordinary heavy basic basalts and dolerites.

That feature of the island of Eigg which renders it so remarkable and conspicuous an object on the west coast is the long ridge of the Scuir. Rising gently from the valley which crosses the island from Laig Bay to the Harbour, the basaltic plateau ascends south-westwards in a succession of terraces, until along its upper part it forms a long crest, from 900 to 1000 feet above the sea, to which it descends on the other or south-west side, first by a sharp slope, and then by a range of precipices. Along the watershed of this crest runs, in a graceful double curve, the abrupt ridge of the Scuir, terminating on the north-west at the edge of the great sea-cliff (975 feet), and ending off on the south-east in that strange well-known mountain-wall (1272 feet high) which rises in a sheer cliff nearly 300 feet above the basalt-plateau on the one side and more than 400 feet on the other (Fig. 278). The total length of the Scuir ridge is two miles and a quarter, its greatest breadth 1520, its least breadth 350 feet. Its surface is very irregular, rising into minor hills and sinking into rock-basins, of which nine are small tarns, besides still smaller pools, while six others, also filled with water, lie partly on the ridge and partly on the basaltic plateau. No one, indeed, who looks on the Scuir from below, and notes how evenly it rests upon the basalt-plateau, would be prepared for so rugged a landscape as that which meets his eye everywhere along the top of the ridge. Two minor arms project from the east side of the ridge; one of these forms the rounded hill called Beinn Tighe (968 feet), the other the hill of A chor Bheinn.

Singular as the Scuir of Eigg is, regarded merely as one of the landmarks of the Hebrides, its geological history is not less peculiar. The natural impression which arises in the mind when this mountain comes into view for the first time is, that the huge wall is part of a great dyke or intrusive mass which has been thrust through the older rocks.[252] It was not until after some time that the influence of this first impression passed off my own mind, and the true structure of the mass became apparent.

The ridge of the Scuir, presenting as it does so strong a topographical contrast to the green terraced slopes of the plateau-basalts on which it rests, consists of some very distinct bands of black and grey lava, long known as "pitchstone-porphyry." To the nature and history of these rocks I shall return after we have considered a remarkable bed of conglomerate which lies below them. On the lower or southern side of the ridge the bottom of the pitchstone, dipping into the hill, is exposed on the roof of a small cave where the ends of its columns form a polygonal reticulation. It is there seen to repose upon a bed of breccia or conglomerate, having a pale-yellow or grey felspathic matrix like the more decomposing parts of the grey devitrified parts of the pitchstone. Through this deposit are dispersed great numbers of angular and subangular pieces of pitchstone, some of which have a striped texture. Fragments of basalt, red (Torridon) sandstone, and other rocks are rare; and the bed suggests the idea that it is a kind of brecciated base or floor of the main pitchstone mass. A similar rock is found along the bottom of the pitchstone on both sides of the ridge (c, in Fig. 279). Here and there where this breccia is only a yard or two in thickness, it consists of subangular fragments of the various dolerites and basalts of the neighbourhood, together with pieces of red sandstone, quartzite, clay-slate, etc. The matrix is in some places a mass of hard basalt debris; in others it becomes more calcareous, passing into a sandstone or grit in which chips and angular or irregular-shaped pieces of coniferous wood are abundant.[253] A little further east, beyond the base of the Scuir, a patch of similar breccia is seen, but with the stones much more rounded and smoothed. This outlier rests against the denuded ends of the basalt-beds forming the side of the hill. Its interest arises from the evidence it affords of the prolongation of the deposit eastward, and consequently of the former extension of the precipice of the Scuir considerably beyond its present front.

It is at the extreme north-western extremity of the pitchstone ridge, however, that the most remarkable exposure of this intercalated detrital band is now to be seen. Sweeping along the crest of the plateau the ridge reaches the edge of the great precipice of Bideann Boidheach, by which its end is truncated, so as to lay open a section of the gravelly deposit along which the pitchstone flowed.

The accompanying diagram (Fig. 279) represents the natural section there exposed. Rising over each other in successive beds, with a hardly perceptible southerly dip of 2°, the sheets of basalt form a mural cliff about 700 feet high. The bedded character of these rocks and their alternations of compact, columnar, amorphous and amygdaloidal beds are here strikingly seen. They are traversed by veins and dykes of an exceedingly close-grained, sometimes almost flinty, basalt. But the conspicuous feature of the cliff is the hollow which has been worn out of these rocks, and which, after being partially filled with coarse conglomerate, has been buried under the huge pitchstone mass of the Scuir. The conglomerate consists of water-worn fragments, chiefly of dolerite and basalt, but with some also of the white Jurassic sandstones, imbedded in a compacted sand derived from the waste of the older volcanic rocks. The grey devitrified bands in the pitchstone, so conspicuous at the east end of the Scuir, here disappear and leave the conglomerate covered by one huge overlying mass of glassy pitchstone.

If any doubt could arise as to the origin of the mass of detritus exposed under the pitchstone at the east end of the Scuir it would be dispelled by the section at the west end, which shows with unmistakable clearness that the conglomerate is a fluviatile deposit and lies in the actual channel of the ancient river which was eroded out of the basalt plateau, and was subsequently sealed up by streams of pitchstone-lava.

An examination of the fragments of rock found in the conglomerate affords here, as in Canna and Sanday, some indication of the direction in which the river flowed. The occurrence of pieces of red sandstone, which no one who knows West-Highland geology can fail to recognize as of Torridonian derivation, at once makes it clear that the higher grounds from which they were borne probably lay to the north or north-east. The fragments of white sandstone may also have been derived from the same quarter, for the thick Jurassic series of Eigg once extended further in that direction. The pieces of quartzite and clay-slate bear similar testimony to an eastern or north-eastern source. In short, there seems every probability that this old Tertiary river flowed through a forest-clad region, of which the red Torridon mountains of Ross-shire, the white sandstone cliffs of Raasay and Skye, and the quartzite and schist uplands of Western Inverness-shire are but fragments, that it passed over a wide and long tract of the volcanic plateau, and continued to flow long enough to be able to carve out for itself a channel on the surface of the basalt. Its course across what is now the island of Eigg took a somewhat north-westerly direction, probably guided by inequalities on the surface of the lava-plain. It is there marked by the winding ridge of the Scuir, the pitchstone of which flowed into the river-bed and sealed it up. Several minor spurs, which project from the eastern side of the main ridge, show the positions of small tributary rivulets that entered the principal channel from the slopes of the basaltic tableland. One of these, on the south-east side of the hill called Corven, must have been a gully in the basalt with a rapid or waterfall. The pitchstone has flowed into it, and some of the rounded pebbles that lay in the channel of this vanished brook may still be gathered where the degradation of the pitchstone has once more exposed them to the light. That the Eigg river here flowed in a westerly direction may be inferred from the angle at which the beds of the small tributaries meet the main stream, and also from the fact that the old river-bed at the east end of the Scuir is considerably higher than at the west end.

Several features in the geological structure of this locality serve to impress on the mind the great lapse of time represented by the erosion of the river-channel of Eigg. Thus at the narrowest point of the pitchstone ridge, near the little Loch a' Bhealaich, the bottom of the glassy lava is about 200 feet above its base on the south side, so that the valley cut out of the plateau-basalts must have been more than 200 feet deep. Even the little tributaries had cut ravines or cañons in the basalts before the ground was buried under the floods of pitchstone. In the most northerly spur of the ridge, for example, the hill of Beinn Tighe, which represents one of these tributaries, shows a considerable difference between the level of the bottom of the pitchstone on the east and west sides.

Again, all along the ridge of the Scuir, the basalt-dykes are abruptly cut off at the denuded surface on which the pitchstone rests. This feature is conspicuously displayed on the great sea-wall at the west end (Fig. 279). The truncation of the dykes demonstrates that a considerable mass of material must have been eroded before these lava-filled fissures could be laid bare at the surface. And the removal of this material shows that the denudation must have been continued for a long period of time.

The river-channel of Eigg, since it was eroded long after the cessation of the outflows of basalt in the plateau of Small Isles, must be much later in origin than those of Canna and Sanday which, as we have seen, were contemporaneous with the basalt-eruptions. But the river that excavated the channels and deposited the gravels may have been the same in both areas.

In dealing with this subject, though the evidence is admittedly scanty, we are not left wholly to conjecture. A consideration of the general topographical features of the wide region of the Inner Hebrides, from the beginning of the volcanic period onward, will convince us that, in spite of the effects of prolonged basalt-eruptions, the persistent flow of the drainage of the Western Highlands must have taken a westerly direction. It was towards the west that the low grounds lay. Though the long and broad valley which stretched northwards from Antrim, between the line of the Outer Hebrides and the West of Scotland, was gradually buried under a depth of two or three thousand feet of lava, the volcanic plain that overspread it probably remained even to the end lower than the mountainous Western Highlands. Hence the rivers, no matter how constantly they may have had their beds filled up and may have been driven into new channels, would nevertheless always seek their way westwards into the Atlantic.

On Canna and Sanday the traces of a river are preserved which poured its flood-waters across the lava-fields in that part of the volcanic region, while streams of basalt were still from time to time issuing from vents and fissures. Not more than fourteen miles to south-east stands the Scuir of Eigg, with its buried river-channel and its striking evidence that there, also, a river flowed westwards, but at a far later time, when the basalt-eruptions had ceased and the volcanic plain had been already deeply trenched by erosion, yet before the subterranean fires were finally quenched, as the pitchstone of the Scuir abundantly proves.

When one reflects upon the enormous denudation of this region, to which more special reference will be made in the sequel, one is not surprised that many connecting links should have been effaced. The astonishment rather arises that so continuous a story can still be deciphered. Even, however, had the original record been left complete, it would have been exceedingly difficult to trace the successive mutations of a river-channel during long ages of volcanic eruptions. Such a channel would have been concealed from view by each lava-stream that poured into it, and would not have been again exposed save by the very process of erosion that destroys while it reveals.

While, therefore, there is not and can never be any positive proof that in the fluviatile records of Canna, Sanday and Eigg successive phases are registered in the history of one single stream, I believe that this identity is highly probable. It was a river which seems to have risen among the mountains of Western Inverness-shire, and it had doubtless already taken its course to the sea before any volcanic eruptions began. It continued to flow westwards across the lava-floor that gradually spread over the plains. Its channel was constantly being filled up by fresh streams of basalt or deflected by the uprise of new cinder-cones. But, fed by the Atlantic rains, it maintained its seaward flow until the general subsidence which carried so much of the volcanic plain below the sea. Yet the higher part of this ancient water-course is no doubt unsubmerged, still traversing the schists of the Western Highlands as it has done since older Tertiary time. It may, perhaps, be recognized in one of the glens which carry seaward the drainage of the districts of Morar, Arisaig, or Moidart.

Let us now turn to the remarkable lava which has sealed up the river-channel of Eigg, and of which the remaining fragment stands up as the great ridge of the Scuir. This rock presents characters that strongly distinguish it from the surrounding basalts. It is not one single uniform mass, but consists of a number of distinct varieties, some of which are a volcanic glass, while others are a grey "porphyry," or devitrified pitchstone. These are arranged in somewhat irregular, but well-marked, and, in a general sense, horizontal sheets. On the great eastern terminal gable of the Scuir this bedded structure is not clearly displayed, for the cliff seems there to be built up of one homogeneous mass, save a markedly columnar band that runs obliquely up the base of the precipice (Fig. 278). If, however, the ridge is looked at from the south, the truly bedded character of its materials becomes a conspicuous feature. Along the cliffs on that side the two varieties of rock are strongly distinguished by their contrasting colour and mode of weathering, the sombre-hued pitchstone standing up in a huge precipice striped with columns, and barred horizontally with bands of the pale-grey "porphyry," which, from its greater proneness to decay, seems sunk into the face of the cliff. At the south-east end of the ridge the bedding is especially distinct. West of the precipices, to the south of the Loch a' Bhealaich, the dark pitchstone which forms the main mass is divided by two long parallel intercalations of grey rock, and two other short lenticular seams of the same material (see Figs. 280, 281). It is clear from these features, which are not seen by most travellers who pass Eigg in the tourist-steamer that the Scuir is in no sense of the word a dyke.

But although the Scuir is thus a bedded mass, the bedding is far different from the regularity and parallelism of that which obtains among the bedded basalt-rocks below. Even where no intervening "porphyry" occurs, the pitchstone can be recognized as made up of many beds, each marked by the different angle at which its columns lie. And when the "porphyry" does occur and forms so striking a division in the pitchstone, its beds die out rapidly, appearing now on one horizon, now on another, along the face of the cliffs, and thickening and thinning abruptly in short distances along the line of the same bed. Perhaps the best place for examining these features is at the Bhealaich, the only gully practicable for ascent or descent, at the south-eastern face of the ridge.

By much the larger part of the mass of the Scuir consists of vitreous material. As a rule this rock is columnar, the columns being much slimmer and shorter than those of the basalt-rocks. They rise sometimes vertically, and often obliquely, or project even horizontally from the face of the cliff. They are seldom quite straight, but have a wavy outline; and when grouped in knolls here and there along the top of the ridge they remind one of gigantic bunches of some of the Palæozoic corals, such as Lithostrotion. In other cases they slope out from a common centre, and show an arrangement not very unlike that of a Highland peat-stack.

The pitchstone of the Scuir differs considerably in petrographical character from other pitchstones of the island which occur in dykes and veins. Its base is of a velvet-black colour, and is so much less vitreous in aspect than ordinary pitchstone as to have been described by Jameson and later writers as intermediate between pitchstone and basalt.[254] A chemical analysis of the rock by Mr. Barker North,[255] gave the following composition:—

The grey devitrified bands, which occur as a subordinate part of the mass of the Scuir ridge, are usually somewhat decomposed. Where a fresh fracture is obtained, the material shows a fine-grained, sometimes almost flinty, grey felsitic base, containing clear granules of quartz, and facets of glassy felspar. In some places the rock is strongly porphyritic. Examined under the microscope it presents a more thoroughly devitrified groundmass, with the minutest depolarizing microlites, large porphyritic crystals of plagioclase and sanidine, grains of augite, and sometimes exceedingly abundant particles of magnetite.[256]

Although the line of separation between the grey dull felsitic sheets and the more ordinary glassy pitchstone is usually well defined, the two rocks may be observed to shade into each other in such a manner as to show that the lithoid material is only a devitrified and somewhat decomposed condition of the glassy rock. This connection is particularly to be observed under the precipice at the east end of the Scuir. At that locality the pitchstone is underlain by a very hard flinty band, varying in colour from white through various shades of flesh-colour and brown into black, containing a little free quartz and crystals of glassy felspar. Where it becomes black it passes into a rock like that of the main mass of the Scuir. Such vitreous parts of the bed lie as kernels in the midst of the more lithoid and decomposed rock. The lower six feet of the "porphyry" are white and still more decomposed. The relations of this mass are represented in Fig. 282, where the basalt-rocks of the plateau (a) are shown to be cut through by basalt dykes (b b), and overlain by the "porphyry" (c) and the pitchstone (d). In the porphyry are shown several pitchstone kernels (p, p). It is deserving of remark also that in different parts of the Scuir, particularly along the north side, the bottom of the pitchstone beds passes into a dull grey earthy lithoid substance, like that now under description.

The bedded character of the rock of the Scuir and the well-marked lithological distinction between its several component sheets show the lava to have been the product of a number of separate outflows that found their way one after another into the river-valley, which was the lowest ground in the vicinity of the active vent. There can be little doubt, I think, that the lava flowed down the valley. Its successive streams are still inclined from east to west. The vent of eruption, therefore, ought to be looked for towards the east. Nowhere within the Tertiary volcanic region is there any boss of pitchstone or any mass the shape or size of which is suggestive of this vent. In the island of Eigg no boss of any kind exists, save those of granophyric porphyry to be afterwards referred to. But none of these affords any satisfactory links of connection with the rock of the Scuir. More probably the vent lay somewhere to the east on ground now overflowed by the sea. The pitchstone veins of Eigg may represent some of the subterranean extrusions from the same volcanic pipe, and if so, its site could not be far off.

The rock of the Scuir of Eigg has a special importance in the history of the volcanic plateaux. It is, so far as we know, the latest of all the superficial lavas of Britain.[257] From the basalts on which it rests it was separated by an enormous interval of time, during which these older lavas were traversed by dykes and were worn down into valleys. Its presence shows that long after the basalts of Small Isles had ceased to be erupted, a new outbreak of volcanic activity took place in this district, when lavas of a more acid composition flowed out at the surface. Whether this outburst was synchronous with the appearance of the great granophyric protrusions of the Inner Hebrides, or with the still later extravasation of pitchstone dykes, can only be surmised.

When one scans the great precipice on the west side of Eigg, with its transverse section of the pitchstone-lava, buried river-bed and basalt-plateau underneath, there seems no chance of any further westward trace of the pitchstone being ever found. The truncated end of the Scuir looks from the top of the cliff out to sea, and the progress of denudation might have been supposed to have effectually destroyed all evidence of the continuation of the rock in a westerly direction. Some years ago, however, my friend Prof. Heddle, while cruising among the Inner Hebrides, landed upon the little uninhabited islet of Hysgeir, which, some eighteen miles to the westward of Eigg, rises out of the open sea. He at once recognized the identity of the rock composing this islet with that of the Scuir, and in the year 1892 published a brief account of this interesting discovery.[258]

I have myself been able to land on Hysgeir in two successive summers, and can entirely confirm Prof. Heddle's identification. The islet stands on the eastern edge of the submarine ridge which, running in a north-easterly direction, culminates in the island of Canna. Hysgeir is a mere reef or skerry, of which the top rises only 38 feet above the Ordnance datum-level. Its surface is one of bare rock, save where a short but luxuriant growth of grasses has found root on the higher parts of two or three of its ridges, and on the old storm-beach of shingle which remains on the summit. The rock undulates in long low swells, that run in a general direction 20° to 45° west of north, and are separated by narrow channels or hollows. The place is a favourite haunt of gulls, terns, eider-ducks and grey seals, and is used by the proprietor of Canna for the occasional pasturage of sheep or cattle. So numerous are the sea-fowl during the breeding-season that the geologist, intent upon his own pursuits, may often tread on their nests unawares, while he is the centre of a restless circle of white wings and anxious cries.

The pitchstone of Hysgeir, like that of Eigg, is columnar, the columns being irregularly polygonal and varying from three to ten inches in diameter. They are packed so close together that the domes of rock on which their ends appear look like rounded masses of honeycomb. They may here and there be observed to be arranged radially with their ends at right angles to the curved exterior of the ridges, as if this external surface represented the original form of the cooled pitchstone, and were not due to mere denudation. There can be no doubt, however, that the island has been well ice-worn.

At the north-west promontory a beautiful example of fan-shaped grouping of columns may be observed on a face of rock which descends vertically into the sea. Here, too, is almost the only section on which the sides of the columns may be examined, for, as a rule, it is merely their ends on the rounded domes which are to be observed, and which everywhere slip under the waves. The columns in a cliff from 15 to 20 feet high show the slightly wavy, starch-like arrangement so often to be met with among the plateau-basalts.

The rock presents a tolerably uniform texture throughout, though in some parts it is blacker, more resinous, and less charged with porphyritic enclosures than in the general body of the rock. Large fresh felspars are generally scattered through it. To the naked eye it reproduces every feature of the pitchstone of the Scuir of Eigg.

A microscopic examination completes our recognition of the identity of these two rocks. Mr. Harker has examined a thin slice prepared from the Hysgeir pitchstone, and remarks regarding it that "the large felspars are not the only porphyritic element. The microscope shows the presence also of smaller imperfect crystals of augite, very faint green in the slice, and small grains of magnetite. The felspars have been deeply corroded by the enveloping magma, and irregular included patches of the groundmass occupy nearly half the bulk of some of the crystals. This latter feature is seen especially in some of the larger crystals, which seem to be sanidine. They are, for the most part, apparently simple crystals, but in places there is a scarcely defined lamellar twinning, or, again, small patches not extinguishing with the rest; so that we are probably dealing with some perthitic intergrowth on a minute scale.[259]