iii. PARISH OF SMALL ISLES PLATEAU

The parish of Small Isles includes the islands of Eigg, Rum, Canna, Sanday and Muck (Map VI.). The fragmentary basalt-plateau which it contains, although the smallest of the whole series, is surpassed by none in the variety and interest of its geology. It contains by far the most complete records of the rivers which, during the volcanic period, flowed across the lava plains. And it alone has preserved a relic of the latest lava which, after the basalt-plateau had been built up and had been greatly eroded, flowed over the denuded surface in streams of volcanic-glass that found their way into a river-channel and sealed it up.

That the fragments of the basaltic plateau preserved in each member of the group of the Small Isles were once connected as a continuous volcanic plain can hardly be doubted. Indeed, as already stated, they were not improbably united with the plateau of Skye on the north, and with that of Mull, Morven and Ardnamurchan oh the south. Taking the whole space of land and sea within which the basalt of Small Isles is now confined, we may compute it at not much less than 200 square miles. In Eigg, Muck, Canna and Sanday the basalts retain their almost horizontal position, and from underneath them the Jurassic strata emerge in the first of these islands. The central part of the plateau in the island of Rum has suffered greatly from denudation. It now consists of four small outliers of basalt, which lie at levels of 1200 feet and upwards, on the western slope. The basalt is underlain by a thick mass of red Torridon Sandstone, which, with some gneisses and schists, forms the general underlying platform of this island. These rocks are doubtless a continuation of the red sandstone and schists of Sleat, in Skye, and like them have been subjected to those post-Cambrian convolutions and metamorphism whereby the Lewisian Gneiss and Torridon Sandstone have been brought above younger rocks, and have been crushed and rolled out so as to assume a new schistose arrangement. Before the time when volcanic action began, a mass of high ground, consisting of these ancient rocks, stood where the island of Rum is now situated. The streams of basalt spread around it, not only covering the surrounding low tracts of Jurassic rocks, but gradually accumulating against the hills, and thus reducing them both in area and in height above the plain.[246] Viewed from Canna the western coast of Rum presents a striking picture of the general relations of the volcanic masses of the Inner Hebrides and of the enormous denudation which they have undergone (Fig. 267). The Torridon Sandstones are there seen to mount into ranges of hills, capped with outliers of the basalt-plateau, while behind rise the great eruptive bosses of gabbro and granophyre. The edges of the sheets that form the outliers would, if prolonged, cover the northern or lower half of the island, where pre-Cambrian rocks form the surface. In the southern half, the continuity of the basalt has been partly obscured and partly destroyed by the protrusion of the great masses of gabbro that form the singularly picturesque mountain group to which this island owes its prominence as a landmark far and wide along the West Coast of Scotland.

[Footnote 246: That the lava-fields did not completely bury this nucleus of older rocks has been supposed to be shown by the fragments of red sandstone found in the ancient river-bed of Eigg, which was scooped out of the basalt-plateau and sealed up under pitchstone. But I am disposed to think that these fragments, together with those of Jurassic sandstone, came, not from Rum, but from some district more to the north and east, as will be explained in a later page. At Canna, a few miles to the west, fragments of red sandstone not improbably derived from Rum are abundant in the conglomerates between the basalts.]

The most varied and interesting of the fragments of the basaltic plateau in the area of the Small Isles is that which forms the island of Canna, with its appendage Sanday. Canna measures five miles in length by from half a mile to a mile in breadth, and consists entirely of the rocks of the plateau and their accompaniments. The basalts are exposed along the north coast in a range of mural precipices rising to a height of about 600 feet above the sea. From the top of that escarpment the ground falls by successive rocky terraces and grassy slopes to the southern shore-line. Sanday, connected with the large island by a shoal and foot-bridge, is two miles long and 220 to about 1200 yards broad. Its highest cliffs range along its southern shore to a height of 193 feet, whence they slope gently northward into the hollow between the two islands. This peculiar topography accounts for the manner in which the geological sections of most interest are distributed.

The first, and still the best, account of the geology of these islands is that of Macculloch. He showed that the rocks all belong to the series of the plateau-basalts, and he described the presence among them of a "trap-conglomerate." He noticed the occurrence also of trap-tuff and the occasional appearance of carbonized wood in these deposits. Reasoning upon these observations in his characteristically vague and verbose manner, "bewildered in the regions of conjecture," he concludes that the basalts instead of belonging to "one general formation" have been successively deposited on the same spot, "since lapse of time is evidently implied in the formation of a conglomerate." He inclines to believe that they have been discharged by ancient volcanoes from which in the course of time all traces of their original outline have been more or less completely removed, the existing basalts being merely fragments of once more extensive masses.[247]

[Footnote 247: _Western Isles_, vol. i. pp. 448-459, and pl. xix. Figs. 2, 3 and 4. See also Jameson's _Mineralogy of the Scottish Isles_.]

Macculloch regarded the intercalated-conglomerates as having been arranged under water and as marking pauses in the deposition of the sheets of "trap." He gave two diagrams in illustration of the relations of these detrital deposits, but he expressed no definite opinion as to their origin, though from one passage it would seem that he inclined towards the belief that they were formed in the sea.[248] Since his time, so far as I am aware, no fresh light has been thrown upon the subject.

[Footnote 248: _Op. cit._ pp. 449, 457, pl. xix. Figs. 2 and 3.]

During a yachting cruise in the summer of 1894 I visited Canna for the first time and found so much that was new to me in regard to the history of Tertiary volcanic action, and which demanded a careful survey, that I returned to the locality the following summer and remained in the island until I had mapped it and its dependencies upon the Ordnance Survey sheets on the scale of six inches to a mile. The following narrative is the result of the observations then made.

As far back as the year 1865 I published an account of an ancient river-channel which, during the volcanic period, had been eroded on the surface of the basalt-plateau, and of which a small portion had been preserved under a stream of pitchstone-lava that had flowed into and buried it.[249] This water-course, now marked by the picturesque ridge of the Scuir of Eigg, was shown to have been excavated by a stream which came from the north-east or east, and to be younger, not only than the plateau-basalts of the district, but than even the dykes which cut these basalts. Yet that it belonged to the volcanic period was proved by the manner in which it had been sealed up and preserved under the black glassy lava of the Scuir. Its history and the data from which this history is compiled will be narrated in a later part of this chapter.

[Footnote 249: _Scenery of Scotland_ (1865); _Quart. Journ. Geo. Soc._ vol. xxvii. (1871), p. 303.]

My examination of the islands of Canna and Sanday, however, brought to light other and more abundant evidence of river-action in the same region of the Inner Hebrides, but belonging to an earlier part of the volcanic period. This evidence reveals that a powerful river, flowing westwards from the Highland mountains, swept over the volcanic plain, while the sheets of basalt were still being poured forth, and while volcanic eruptions were taking place from cones of slag.

The basalt-plateau of Canna resembles in all essential particulars those of the other Western Isles. Its base is everywhere concealed under the sea, but from the fragments of Torridon Sandstone in its agglomerates we may infer that it probably rests on that formation, like the volcanic outliers in Rum. It is formed of successive sheets of different basalts including the usual banded, amygdaloidal and columnar forms. Some of them towards the west are specially marked by the great abundance and large size of their porphyritic felspars. The magnetic properties of the basalts at the east end of the island have long been known, and have given rise to various modern myths regarding their influence on the compasses of passing vessels.

But it is in its conglomerates, tuffs and agglomerates and the light they cast on some aspects of the volcanic period, elsewhere hardly recorded, that the geology of Canna possesses a special importance. To these, therefore, we may at once turn.

The conglomerates are best developed at the eastern end of the island, where the cliffs present the structure represented in Fig. 268. At the base, and passing under the level of the sea, lies the agglomerate (_a_) of a vent which will be described in Chapter xli., together with other eruptive orifices of the various plateaux (p. 288). This rock has a somewhat uneven upper surface which rises in places about 150 feet above high tide-mark. Here and there it shades off upward into the conglomerate that overlies it; water-worn pebbles appear among its contents, and rude traces of bedding begin to show themselves, until, within the course of a few feet, we pass upward into an undoubted conglomerate. Elsewhere, however, and particularly along the precipices west of Compass Hill, the two deposits are more distinctly marked off from each other. The agglomerate has there a hummocky, irregular upper surface, as if it had been thrown down in heaps. The hollows between these protuberances have been filled up with conglomerate and sandstone, forming the base of the thick overlying deposit.

It is thus clear that the loose materials of the vent were directly exposed at the surface when the conglomerate was accumulated, and, indeed, that these materials served to supply some of the detritus of which the conglomerate consists. The absence of any trace of a cone and crater at the vent may perhaps be explicable on the supposition that their incoherent material was washed down by the currents that swept along and deposited the conglomerate.

The mass of sedimentary material (_b_) which overlies the agglomerate of the vent forms a conspicuous feature along the lower half of the precipices at the eastern end of Canna. It rises to a height of 250 to 300 feet above sea-level, and must reach a maximum thickness of probably not less than 100 to 150 feet. It gradually descends in a westward direction, both along the northern cliffs and in the lower ground round Canna Harbour, insomuch that in about a mile, owing to the gentle westerly dip of the whole volcanic series, combined with the effect of a number of small faults, it passes under the level of the sea.

Great variation in the character of the detritus composing this thick group of strata may be observed as it is followed westward. On the cliffs below Compass Hill, as represented in Fig. 268, the coarse conglomerate with water-worn stones, hardly to be distinguished from the volcanic agglomerate of the vent, shows more or less distinct bedding, or at least a succession of coarser and finer bands. Towards its base it encloses numerous pieces of Torridon Sandstone, sometimes subangular, but often so well and smoothly rounded as to show that they must have been long subjected to the action of moving water. It is further observable that, while in the agglomerate the volcanic stones have rough surfaces, those in the conglomerate begin to show increasing evidence of attrition, until, as the deposit is traced upwards, they become almost as well rounded and water-worn as the non-volcanic stones.

Yet amidst and overlying these proofs of transport from some little distance lie abundant huge slags and blocks of amygdaloidal lava, sometimes closely aggregated, sometimes scattered through a volcanic tuff or ashy sandstone. The composition and structure of these stones, and the manner of their dispersion through the deposit, leave little doubt that they were ejected from the vent. We are thus confronted with the interesting fact that, while the materials of the volcanic cone were being washed down by running water, eruptions were still taking place. But by degrees these indications of contemporaneous volcanic activity diminish. The detrital materials become coarser and more distinctly water-rolled until they pass into greenish sandstones and fine conglomerates. Yet the matrix even of these higher sediments is largely composed of fine volcanic detritus, and probably points to occasional discharges of dust and ashes.

Various sills or intrusive sheets have been injected into this sedimentary group along the precipices at the east end of Canna, and form there lenticular bands. One of these (_c_) is shown in Fig. 268.

Immediately above the massive greenish pebbly sandstone (_d_) which caps the stratified series lies a group of basalts (_e_), composed of several distinct beds, having a united thickness of from 80 to 100 feet. The lowest of these has a regular columnar structure, while those overlying it exhibit the confused starch-like grouping of curved and rather indistinctly-formed prisms.

The next band in upward succession is one of conglomerate (_f_), which runs as a continuous and conspicuous feature along the upper part of the cliff. This rock presents in many respects a strong contrast to the conglomerates underneath. It is dull-green to yellow in colour, and is well stratified, being marked by the interstratification of finer layers, and passing down into a band of pebbly sandstone, which rests immediately on the basalt (_e_). Its component stones are thoroughly water-worn, ranging up to six inches or even more in length. But its most distinctive character lies in the nature of its pebbles. Instead of consisting mainly of volcanic materials, these stones have almost all been transported for some distance. They include abundant fragments of Torridon Sandstone, gneiss, schists, grits, and other rocks like those in Rum and Western Inverness-shire. No such rocks exist _in situ_ in Canna. The nearest tract of Torridon Sandstone is in Rum, about four miles to the eastward. But the pieces of schist and epidotic grit like the rocks of the Western Highlands, have probably travelled at least 30 miles.

It is important to observe that all these transported stones indicate a derivation from some source lying to the eastward of Canna. The evidence in this respect agrees with that furnished by the ancient river-gravel under the pitchstone of the Scuir of Eigg. It is clear that the waters which found their way across the lava-fields of this part of the Inner Hebrides took their rise somewhere to the eastward, probably among the mountains of Inverness-shire.

The conglomerate now described is from 40 to 50 feet thick. It can be followed along the face of the cliffs for more than a mile on the north side of Canna. Less persistent on the south side, its outcrop strikes from the edge of the precipice inland, keeping to the south of the top of Compass Hill. It is well seen in the ravine above the Coroghon, but cannot be followed further westward among the basalt-terraces. Yet, though this stratified intercalation is not traceable far as a band of conglomerate, the same stratigraphical horizon is probably indicated elsewhere by other kinds of sedimentary deposits, to which further reference will be made in the sequel.

The section now described establishes the existence of at least two successive platforms of conglomerate in the volcanic series. Following these platforms along their outcrop, we obtain additional light on their origin, and on the topographical conditions under which they were deposited, and we learn further that other prolonged intervals, which were likewise marked by intercalations of sedimentary material, occurred in the outpouring of the basalts.

Taking first the lower conglomerate of Compass Hill and tracing it westward, we find it to form the depression in which the sheltered inlet of Canna Harbour lies. It is exposed along the shores and also in the islands enclosed within the same bay. But it is not traceable further west, possibly because it seems to sink beneath the level of the sea. To the south-east, though it is there likewise for the most part concealed under the waves, it rises above them in one or two parts of the coast-line of Sanday, particularly at the Uamh Ruadh or Red Cave, and likewise on a surf-beaten skerry off Ceann an Eilein, the highest part of the Sanday cliffs--a distance of about a mile and a half from Compass Hill. Throughout this space it retains its remarkably coarse character and is mainly made up of volcanic material.

The numerous sections exposed in Canna Harbour enable us to study the composition and local variations of this curious deposit. On the north side of the basin, while the lower part of the sedimentary series continues to be an exceedingly coarse volcanic conglomerate, it passes upward into finer conglomerates, tuffs, and shales. In front of Canna House the imbedded blocks are of large size, occasionally as much as three or four feet in diameter. They are still more gigantic on the island of Eilean a' Bhaird, where I found one to contain 150 cubic feet in the exposed part, the rest being still concealed in the matrix. As they are generally somewhat rounded, here and there markedly so, most of these stones have probably undergone a certain amount of attrition in water. The great majority of them, and certainly all those of larger size, are pieces of basalt, dolerite, andesite, etc. Among them huge blocks of amygdaloid and coarsely vesicular lava are specially abundant. Some of these look like pieces of slag torn from the upper surface of lava-streams. Others, displaying a highly vesicular centre and a close-grained outer crust, are suggestive of bombs. It is interesting to note here again that the amygdaloidal blocks present their zeolitic infiltrations so precisely like those of the amygdaloids of the plateau that it seems reasonable to suppose the carbonate of lime, zeolites, etc. to have been introduced before the blocks were imbedded in the conglomerate.

The whole aspect of this deposit is eminently volcanic. It looks like a vast sheet of lava-fragments swept away from one or more cones of slags and cinders, or from the scoriaceous surface of a lava-stream. Where the vesicles were still empty, the large boulders could be more easily swept along by moving water. But a powerful current must have been needed to transport and wear down into more or less rounded forms blocks of basic lava, many of which must weigh several tons. The large block on Eilean a' Bhaird probably exceeds 12 tons in weight.

Besides the obviously volcanic contents of the conglomerate there occur here also, as in the Compass Hill cliffs, abundant pieces of Torridon Sandstone. These stones are notably smaller in size and more perfectly water-worn and even polished than the blocks of lava. Obviously they have travelled further and have undergone more prolonged attrition.

The matrix of the rock consists essentially of the fine detritus of basic lavas, probably mingled with true volcanic dust. The coarser parts display only the feeblest indication of stratification; indeed, in a limited exposure the rock might be regarded as a tumultuous agglomerate. But the manner in which the deposit is intercalated with, and sometimes overlies, green tuffs and shales, together with the water-worn condition of its stones, shows that it has not been accumulated in a volcanic chimney, but has been thrown down by some powerful body of water, with probably the co-operation of volcanic discharges.

While the composition of the conglomerate suffices to indicate that this deposit was formed at a time when some volcano was active in the immediate neighbourhood, singularly convincing proofs of the work of this vent are to be seen in the form of intercalated sheets of lava. Thus on Eilean a' Bhaird the boulders of the conglomerate are overlain and wrapped round by a sheet of rudely prismatic basalt, with lines of vesicles arranged in the direction of the bedding. A similar relation can be traced along the beach between Canna House and the wooden pier, where successive sheets of basalt have flowed over the conglomerate (Fig. 269).

But, besides coarse volcanic detritus, the sedimentary platform represented by the lower conglomerate of Compass Hill includes other deposits of which good sections may be examined all round Canna Harbour. Beds of fine well-stratified dull-green tuff pass by an admixture of pebbles into fine ashy conglomerate or pebbly sandstone, and by an increase in the proportion of their fine detritus into volcanic mudstone and fine shales. The shales vary from a pale grey or white tone into blackish grey, brown, and black. They are well stratified and are frequently interleaved with layers of fine tuff. The darker bands are carbonaceous, and are not infrequently full of ill-preserved vegetation. Indeed, leaves and stems in a rather macerated condition are of common occurrence in all the shaly layers. Here and there, especially in some ashy shales in front of Canna House, I observed a recognisable _Sequoia_. The mudstones are dull green, close-grained shattery rocks composed of fine volcanic detritus, and pass both laterally and vertically into shales, tuffs, and conglomerates. They suggest showers of fine dust or streams of volcanic mud. They, too, contain fragmentary plants.

It is a noteworthy fact that the sedimentary intercalations among the Canna basalts generally end upward in carbonaceous shales or coaly layers. The strong currents and overflows of water, which rolled and spread out the coarse materials of the conglomerates, gave way to quieter conditions that allowed silt and mud to gather over the water-bottom, while leaves and other fragments of vegetation, blown or washed into these quiet reaches, were the last of the suspended materials to sink to the bottom. Good illustrations of this sequence in the case of the lower conglomerate zone of Canna may be studied along the shores of Sanday, from the Catholic Chapel eastwards. The fine pebbly sandstones, tuffs, and shales, which there overlie the coarse conglomerate, are surmounted by dark brown or black carbonaceous shale, with lenticles of matted vegetation that pass into impure coal. Immediately overlying this coaly layer comes a sheet of prismatic vesicular basalt, followed by another with an exceedingly slaggy texture.

Lenticles of shale and mudstone likewise occur in the heart of the finer parts of the conglomerate, especially towards the top, as may be seen in the section exposed beneath the basalt behind the first cottage west from Canna House. One of the most interesting layers in this section is a seam of tuff, varying up to about two inches in thickness, which lies at the top of the lenticular band of tuffs and shales, and immediately beneath the band of basalt-conglomerate, on which a basalt, carrying a vesicular band near its bottom, rests. Traced laterally, the dark brown tuff of this seam gradually passes into a series of rounded bodies and flattened shells composed of a colourless mineral which has evidently been developed _in situ_ after the deposition of the tuff. Mr. Harker's notes on thin slices made from this band are as follows:--

"This is a rusty-brown, dull-looking rock, rather soft and seemingly light, but too absorbent to permit of its specific gravity being tested. The dark brown mass is in great part studded with little spheroidal bodies, 1/50 to 1/10 inch in diameter, of paler colour, but the larger ones having a dark nucleus. In other parts larger flat bodies have been formed, as if by the coalescence of the spheroids, extending as inconstant bands in the direction of lamination for perhaps 1/2 inch, with a thickness of 1/10 inch or less. The appearance is that of a spherulitic rather than an oolitic structure.

"A slice [6658 A] shows the general mass of the rock to be of an extremely finely divided but coherent substance of brown colour, which can scarcely be other than a fine volcanic dust, composed of minute particles of basic glass or 'palagonite' compacted together. Scattered through this are fragments of crystals recognizable as triclinic and perhaps monoclinic felspars, green hornblende, augite, olivine (?), and magnetite, usually quite fresh.

"The curious spheroidal and elongated growths already mentioned are better seen in another slide [6658 B], where they occupy the larger part of the field, leaving only an interstitial framework of the brown matrix. The substance of the little spheroids is clear, colourless, and apparently structureless. The centre is often occupied by an irregularly stellate patch of brown colour, and sometimes cracks tend to run in radiating fashion, but these are the only indications of radial structure. The outer boundary is sharply defined, and where the slice is shattered the spheroids have separated from the matrix. The matrix is darker than in the normal rock, being obscured by iron-oxide which we may conceive as having been expelled from the spaces occupied by the spheroids. The little crystal-fragments are enclosed in the spheroids as well as in the matrix, but there is no appearance of their having served as starting-points for radiate growths. The flat elongated bodies are like the spheroids, with merely the modifications implied in their different shape.

"The identity of the clear colourless substance seems to be rather doubtful. It is sensibly isotropic and of refractive power distinctly lower than that of felspar. These characters would agree with analcime, which is not unknown as a contact-mineral; but it is difficult to understand how analcime, even a lime-bearing variety like that of Plas Newydd,[250] could be formed in abundance from palagonitic material. An alternative supposition, perhaps more probable, is that the clear substance is a glass, modified from its former nature, especially by the expulsion of the iron-oxide into the remaining matrix. A comparison is at once suggested with certain types of 'Knotenschiefer,' but respecting the thermal metamorphism of fine volcanic tuffs there seems to be little or no direct information."

[Footnote 250: Henslow, _Trans. Camb. Phil. Soc._ (1821), vol. i. p. 408; Mr. Harker, _Geol. Mag._ (1887), p. 414. Mr. W. W. Watts suggests a comparison with the hexagonal bodies figured by Mr. Monckton in an altered limestone from Stirlingshire: _Quart. Journ. Geol. Soc._, vol. li. p. 487.]

Lenticular interstratifications of shale and mudstone make their appearance even in the coarser parts of the conglomerate, as may be observed on the beach below Canna House where, as shown in Fig. 269, some shales and tuffs (_a_) full of ill-defined leaves are surmounted by a conglomerate (_b_). The deposition of this overlying bed of boulders has given rise to some scooping-out of the finer strata underneath. Subsequently both the conglomerate and shales have been overspread by a stream of dolerite (_c_), the slaggy bottom of which has ploughed its way through them.

Before discussing the probable conditions under which the group of sedimentary deposits now described was formed, we may conveniently follow the upper conglomerate band of the Compass Hill, and note the variations in structure and composition which its outcrop presents.

This yellowish conglomerate can be traced along the cliffs for more than a mile, when it descends below the sea-level at the solitary stack of Bod an Stòl. A few hundred yards further west, what is probably the same band appears again at the base of the precipice overlain by prismatic basalts. But the conglomerate, here only 12 feet thick, is made of much finer detritus which, largely composed of volcanic material, includes small well-rounded and polished pebbles of Torridon Sandstone. Beneath it lies a bed of dark shale, with remains of plants, resting immediately on a zeolitic amygdaloid which plunges into the sea. The chief interest of this locality is to be found in the shale which, instead of being at the top of the sedimentary group, lies at the bottom. I was informed by Mr. A. Thom that leaves had been obtained from this shale; but I was not successful in my search for them. The locality is only accessible by boat, and, as the coast is fully exposed to the Atlantic swell, landing at the place is usually difficult and often impossible.

About a mile and a half still further west, where a foreshore fronts the precipice of Earnagream at the Camas Tharbernish, a band of intercalated sedimentary material underlies the great escarpment of basalts and rests upon the slaggy sheet with the singular surface already referred to (p. 187). This band not improbably occupies the same platform as the upper conglomerate of Compass Hill. It is only about seven feet thick, the lower four feet consisting of a dull green pebbly tuff or ashy sandstone, with small rounded pieces of Torridon Sandstone, while the upper three feet are formed of dark shale with crowded but indistinct remains of plants. Here the more usual order in the sequence of deposition is restored. The shale is indurated and shattery, so that no slabs can be extracted without the use of quarrying tools.

Rather less than half a mile towards the south, on the roadside at the gully of Cùl nam Marbh, the basalts enclose a sedimentary interstratification which not improbably lies on the same horizon as those just described along the northern shore. The relations of the rocks at this locality are shown in Fig. 270. A remarkable slaggy basalt (_a_) rises into a hummock, against which have been deposited some fine granular tuffs (_b_) whereof only a few inches are visible, that pass up into a thin band of dark shale (_c_), including a layer of pebbly ferruginous tuff, with small rounded pea-like pieces of basalt, basic pumice, bole, limonite, etc. At the top of this shale an irregular parting of coaly material (_d_) lies immediately under the slaggy base of the succeeding basalt (_e_). It will be observed that this upper lava cuts out the shale and thus comes to rest directly upon the lower sheet. At the point where it begins to descend it has caught up and enclosed a small tree-stump (_d´_) which stands upright on the coaly parting and shale. This stump, at the time of my visit, measured five inches in height by three inches in breadth; it had been thoroughly charred and was crumbling away on exposure, but among the pieces which I took from it sufficient trace of structure can be detected with the microscope to show the tree to have been a conifer.

We have here another instance of the deposition of volcanic dust and fine mud in a pool that filled a hollow in the lava-field. Again we see that the closing act of sedimentation was the subsidence of vegetable matter in the pool, which was finally buried under another outflow of basalt.

It is on the southern coast of the isle of Sanday that the higher intercalations of sedimentary material among the basalts are most instructively displayed. At the eastern end of this island, as already stated, the lowest and coarsest conglomerate is visible on a skerry immediately to the south of the headland Ceann an Eilein. It doubtless underlies the Sanday cliffs, but is not there visible, for the basalts descend below sea-level. These volcanic sheets have a slight inclination westward; hence in that direction we gradually pass into higher parts of the series. In the Creag nam Faoileann (Seamews' Crag) and the gully that cuts its eastern end, likewise in the two singularly picturesque stacks of Dùn Mòr and Dùn Beag (Big and Little Gull Rocks), which here rise from the foreshore, two distinct platforms of detrital material may be noticed among the basalts. Both of these can be well seen on Dùn Mòr, about 100 feet high, which is represented in Fig. 271. The lower band, four or live feet thick, is here a rather coarse conglomerate which lies upon a sheet of scoriaceous basalt that extends up to the base of the Creag nam Faoileann. It is directly overlain by another basalt, about 30 feet thick, which dips seawards and forms a broad shelving platform, whereon the tides rise and fall. On this stack a second coarse conglomerate, about 10 feet thick, forms a conspicuous band about a third of the height from the bottom; it is composed mainly of well-rounded blocks of various lavas up to 18 inches or more in diameter, but it contains also pieces of Torridon Sandstone. It is covered by about 60 feet of basalt, which towards the base is somewhat regularly columnar, but passes upward into the wavy, starch-like, prismatic structure.

If now we trace these two intercalated zones of conglomerate along the shore, we find them both rapidly to change their characters and to disappear. The lower, though formed of coarse detritus under the Dùn Mòr, passes on the opposite cliff in a space of not more than 60 yards, into fine tuff and shale, about six feet thick, which become carbonaceous at the top where they are overlain by the next basalt. A hundred yards to the east, the band likewise consists of tuffs and ashy shales, which underlie the basalts on the Dùn Beag, and again show the usual coaly layers at the top. On the east side of the gully in the coast, about 160 yards to the north-east of Dùn Mòr, the same band is reduced to not more than three feet in thickness, consisting chiefly of fine conglomerate, wherein well water-worn pebbles of Torridon Sandstone and epidotic grit appear among the predominant volcanic detritus. This conglomerate is surmounted by a few inches of dark carbonaceous mudstone or shale. Rough slaggy basalts lie above and below the band.

The upper conglomerate dies out, both towards the east and the west, in the cliff opposite to Dùn Mòr, dwindling down at last to merely a few pebbles between the basalts. It lies in a kind of channel or hollow among these lavas. This depression, in an east and west direction, cannot be more than about 65 yards broad.

Probably still higher in the series of basalts is another intercalation of sedimentary layers which may be seen in the little bay to the east of Tallabric, rather more than a mile to the west of the Creag nam Faoileann. It rests upon a coarsely slaggy amygdaloid, and is from six to ten feet in thickness. The lower and larger part of the deposit consists of greenish pebbly sandstone and fine conglomerate, largely composed of basaltic detritus, but including abundant well-smoothed and polished pebbles of Torridon Sandstone, green grit, quartzite, etc. The stones vary from mere pea-like pebbles up to pieces two or three inches long, the largest being generally fragments of slag and amygdaloid which are less water-worn than the sandstones and other foreign ingredients. The uppermost two or three feet of the intercalation consist of dark carbonaceous mudstone or shale, made up in large measure of volcanic detritus, which may have been derived partly from eruptions of fine dust, partly from subærial disintegration of the basalt-sheets. Some layers of these finer strata are full of remains of much macerated plants.

Other thin coaly intercalations have been observed among the basalts of Canna, some of which may possibly mark still higher horizons than those now described. But, confining our attention to the regular sequence of intercalations exposed along the Sanday coast, we find at least four distinct platforms of interstratified sediment among the plateau-basalts of this district. Each of these marks a longer or shorter interval in the outflow of lava, and points to the action of moving-water over the surface of the lava-fields.

We may now consider the probable conditions under which this intervention of aqueous action took place. The idea that the sea had anything to do with these conglomerates, sandstones, and shales may be summarily dismissed from consideration. The evidence that the basalt-eruptions took place on a terrestrial surface is entirely convincing, and geologists are now agreed upon this question.

Excluding marine action, we have to choose among forms of fresh water--between lakes on the one hand and rivers on the other. That the agency concerned in the transport and deposition of these strata was that of a river may be confidently concluded on the following grounds:--

1. The large size and rolled shape of the boulders in the conglomerates. To move blocks several tons in weight, and not only to move them but to wear them into more or less rounded forms, must have required the operation of strong currents of water. The coarse detritus intercalated among the basalts is quite comparable to the shingle of a modern river, which descends with rapidity and in ample volume from a range of hills.

2. The evidence that the materials of the conglomerates are not entirely local, but include a marked proportion of foreign stones. The proofs of transport are admirably exhibited by pieces of Torridon Sandstone, epidotic grit, quartzite, and other hard rocks none of which occur _in situ_ except at some distance from Canna. These stones are often not merely rounded, but so well smoothed and polished as to show that they must have been rolled along for some considerable time in water.

3. The lenticular character and rapid lithological variations of the strata, both laterally and vertically. The coarse conglomerates die out as they are followed along their outcrop and pass into finer sediment. They seem to occur in irregular banks, which may not be more than 200 feet broad, like the shingle-banks of a river. The coarser sediment generally lies in the lower part of the sedimentary group. But cases may be observed, such as that shown in Fig. 269, where fine sediment, laid down upon the bottom conglomerate, has subsequently been overspread by another inroad of coarse shingle. Such alternations are not difficult to understand if they are looked upon as indicating the successive floods and quieter intervals of a river.

For these reasons I regard the platforms of sedimentary materials intercalated among the basalts of Canna and Sanday as the successive flood-plains of a river which, like the rivers that traverse the lava-deserts of Iceland, flowed perhaps in many separate channels across the basalt-fields of the Inner Hebrides, and was liable to have its course shifted from time to time by fresh volcanic eruptions. That this river came from the east or north-east and had its source among the Western Highlands of Inverness-shire, may be inferred from the nature of the stones which it has carried for 30 miles or more along its bed. And that it crossed in its course the tract of Torridon Sandstone, of which a portion still remains in Rum, is manifest from the abundance of the fragments of that formation in the conglomerates.

With the remarkable exception of the section on Dùn Beag, to be immediately referred to, no trace of any eroded channel of this river through the lavas of the great volcanic plain has been preserved. Possibly frequent invasions of its bed by streams of basalt from different vents hindered it from remaining long enough in one course to erode anything like a gorge or canon. But, in any case, the main channel of the river probably lay rather to the east of the present islands of Canna and Sanday, on ground which is now covered by the sea. The banks or sheets of boulder-conglomerate undoubtedly show where its current swept with great force over the lava-plain, but the manner in which these coarser materials are so often covered with fine silt suggests that the sedimentary materials now visible were rather deposited on the low grounds over which the steam rushed in times of flood. Pools of water would often be left after such inundations, and in these depressions silt would gradually accumulate, partly carried in suspension by the river, partly washed in by rain, while drift-wood that found its way into these eddies, and leaves blown into them from the trees and shrubs of the surrounding country, would remain for some time afloat and would be the last of the detritus to sink to the bottom. Hence, no doubt, the carbonaceous character of the hardened silt in the upper part of each intercalation of sediment.

If we were to look upon the volcanic materials in the conglomerates as derived from the subærial disintegration of the fields of basalt, we should be compelled to admit a very large amount of erosion of the surface of the volcanic plain during the period when the river flowed over that tract. It would be necessary to suppose not only that there was a considerable rainfall, but that the differences of temperature, either from day to night, or from summer to winter, were so great as to split up the lavas at the surface, in order to provide the river with the blocks which it has rolled into rounded boulders. I do not think, however, that such a deduction would be sound. If we compare the materials that have filled up the large eruptive vent at the east end of Canna (to be afterwards described) with the great majority of the blocks in the coarse conglomerates, we cannot fail to note their strong resemblance. The abundance of lumps of slaggy lava in the river-shingle corresponds with their predominance in the agglomerate of the vent. The boulders of basalt, dolerite, and andesite which crowd the conglomerates need not have been derived from the action of atmospheric waste on the lava-fields, but might quite well have been mainly supplied by the demolition of volcanic cones of fragmental materials.

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.

[Footnote 251: This pinnacle of rock is referred to by Macculloch in his account of Canna, and is figured in Plate xix. Fig. 3 of his work already cited. But neither his description nor his drawing conveys any idea of the real structure of the rock.]

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.

[Footnote 252: Hay Cunningham remarks:--"In regard to the relations of the pitchstone-porphyry of the Scuir and the trap-rocks with which it is connected, it can, after a most careful examination around the whole mass, be confidently asserted that it exists as a great vein which has been erupted through the other Plutonic rocks--thus agreeing in age with all the other pitchstones of the island." Macculloch leaves us to infer that he regarded the rock of the Scuir to be regularly interstratified with the highest beds of the dolerite series (_Western Isles_, i. p. 522). Hugh Miller speaks of the Scuir of Eigg as "resting on the remains of a prostrate forest."--_Cruise of the Betsy_, p. 32.]

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.

[Footnote 253: The microscopic structure of this wood was briefly described by Witham (_Fossil Vegetables_, p. 37), and two magnified representations were given to show its coniferous character. Lindley and Hutton further described it in their "Fossil Flora," naming it _Pinites eiggensis_, and regarding it as belonging to the Oolitic series of the Hebrides--an inference founded perhaps on the erroneous statement of Witham to that effect. William Nicol corrected that statement by showing that the wood-fragments occurred, not among the "lias rocks," but "among the debris of the pitchstone" (_Edin. New Phil. Journal_, xviii. p. 154). Hay Cunningham, in the paper already cited, states that the fossil wood really lies in the pitchstone itself! The actual position of the wood, however, in the breccia and conglomerates underlying the pitchstone is beyond all dispute. I have myself dug it out of the bed. The geological horizon assigned to this conifer, on account of its supposed occurrence among Oolitic rocks, being founded on error, no greater weight can be attached to the identification of the plant with an Oolitic species. Our knowledge of the specific varieties of the microscopic structure of ancient vegetation is hardly precise enough to warrant us in definitely fixing the horizon of a plant merely from the examination of the minute texture of a fragment of its wood. From the internal organization of the Eigg pine, there is no evidence that the fossil is of Jurassic age. From the position of the wood above the dolerites and underneath the pitchstone of the Scuir it is absolutely certain that the plant is not of Jurassic but of Tertiary date.]

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:--

Silica 65·81 Alumina 14·01 Ferric oxide 4·43 Lime 2·01 Magnesia 0·89 Soda 4·15 Potash 6·08 Loss in ignition 2·70 ------ 100·08

[Footnote 254: _Mineralogy of the Scottish Isles_, vol. ii. p. 47. See also Macculloch, _Western Isles_, vol. i. p. 521, and Hay Cunningham, _Mem. Wern. Soc._ vol. viii. p. 155.]

[Footnote 255: _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 379.]

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]

[Footnote 256: The microscopic structure of the identical pitchstone of Hysgeir is given on p. 247.]

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.

[Footnote 257: The rocks of Beinn Hiant in Ardnamurchan have been claimed by Professor Judd as superficial lavas. For reasons to be afterwards given (p. 318) I regard them as intrusive sheets. Professor Cole believes the rhyolites and pitchstones of Tardree to be probably evidence of a volcano later than the basalts of Antrim. As I have not been able to detect any actual proofs of superficial outflow there, I relegate the description of the rocks to a future chapter, in which the acid protrusions will be discussed (p. 426).]

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]

[Footnote 258: Appendix C to _A Vertebrate Fauna of Argyle and the Inner Hebrides_, by Messrs. J. A. Harvie-Brown and Thomas E. Buckley, p. 248.]

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]

[Footnote 259: Comp. Prof. Judd's remarks on the Scuir of Eigg rock, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 380.]

"Rather smaller felspar-crystals are rounded by corrosion, but lack the inclusions of groundmass; these have albite-and sometimes pericline-lamellation, and may be referred to oligoclase-andesine. The groundmass of the rock is a brown glass with perlitic cracks, enclosing very numerous microlites of felspar about ·001 inch in length [6619]. The rock is probably to be regarded as a dacite rather than a rhyolite, and thus agrees with Mr. Barker North's analysis of the Eigg pitchstone."[260]

[Footnote 260: _Op. cit._ p. 379.]

There is no trace of any conglomerate _in situ_ like that under the Scuir of Eigg, nor of any other rock, aqueous or igneous. As the pitchstone everywhere slips under the sea, its geological relations are entirely concealed.

The great variety of materials met with in the form of boulders on the island is a testimony to the transport of erratics from the neighbouring islands and the mainland during the Glacial Period. The most abundant rock in these boulders is Torridon Sandstone, derived no doubt from the hills of Rum, but there occur also various kinds of schist, gneisses, quartzites, granites, porphyries, probably from the west of Inverness-shire, as well as pieces of white sandstone, probably Jurassic, which may have come from Eigg.

That the pitchstone of Hysgeir is a continuation of that of the Scuir may be regarded as highly probable. If not a continuation, it must be another stream of the same kind, and doubtless of the same date. If it be regarded as probably a westward prolongation of the Eigg rock, and if it be about as thick as that mass at the west end of the Scuir, then its bottom lies 200 or 300 feet under the waves. The river-channel occupied by the Eigg pitchstone undoubtedly sloped from east to west. The position of Hysgeir, 18 miles further west, may indicate a further fall in the same direction at the rate of perhaps as much as 35 feet in the mile.[261] Unfortunately, however, as no trace of the river-bed can now be seen on this island, any statement in regard to it must rest on mere conjecture.

[Footnote 261: _Rep. Brit. Assoc._ 1894, p. 653.]

Although the question of the denudation of the basalt-plateaux since the close of the volcanic period will be the subject of a special chapter in a later part of this volume, I cannot here refrain from calling attention to the pitchstone of Eigg and Hysgeir as one of the most impressive monuments of denudation to be found within the British Isles. Though now so prominent an object in the West Highlands, this rock once occupied the bottom of a valley worn out of the basaltic tableland. Prolonged and stupendous denudation has destroyed the connection with its source, has cut down its ends into beetling precipices, has reduced the former surrounding hills into gentle slopes and undulating lowland, and has turned the bottom of the ancient valley into a long, narrow and high crest. Moreover, we see that the erosion has not been uniform. The great wall of the Scuir does not stand fairly on the crest of the basalt-plateau but on the south side of it, so that the southern half of the old valley, with all its surrounding hills, has been entirely cut away. That subsidence has also come into play in the destruction of even the youngest parts of the volcanic plateaux will be more fully discussed in a later chapter. I need only remark here that the submergence of Hysgeir probably points to extensive depression of the land-surface on which the lavas were poured out.