Chapter xlviii

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iii. EIGG, ARDNAMURCHAN

The phenomena of the coasts of Skye are repeated on the east side of Raasay, in Eigg, and still more magnificently along the south coast of Mull. A single example is here given (Fig. 325) from the east side of Eigg. Over the Jurassic sandstones (_a_ _a_) a sill of basalt (1) four to six feet thick has been injected between the stratification, and another (2) two to four feet thick has forced its way across the middle of one of the bedded basalts (_b_ _b_) in which it bifurcates, and above which comes the thick series of lavas of the plateau (_c_, _d_). In one of the streamlets, which exposes a section of the Jurassic strata below the volcanic escarpment, more than twenty intrusive sheets may be counted among the shales and limestones. They are sometimes not six inches thick, and seldom exceed six or eight feet.

[Illustration: Fig. 325.--Section to show Bedded and Intrusive Sheets, Eigg.]

I will conclude this account of the Tertiary basic sills of Britain by referring to one further district in the West of Scotland, where they are well displayed on bare hillslopes and also along a picturesque sea-coast. In the promontory of Ardnamurchan in the west of Argyleshire, one of the most conspicuous eminences, known as Ben Hiant, affords a striking mass of intrusive material, which, extending along a rugged shore for three-quarters of a mile, mounts thence inland in a series of rocky knolls, and in rather less than a mile culminates in a summit, 1729 feet above sea-level.[320] The rocks which cover this large space are disposed in numerous rude beds, which have a seaward dip of perhaps 15° to 20°, and are sometimes distinctly prismatic, the prisms being not infrequently grouped in fan-shape. They are evidently due to successive intrusions. Although generally coarsely crystalline in texture, they include also intermediate and fine-grained sheets. They are never, so far as I have been able to discover, amygdaloidal,[321] nor do they present the ordinary external characters of the beds of the plateaux, though here and there they appear to have caught up portions of the plateau-series. They distinctly overlie the bedded basalts on their eastern and southern margins; but westwards they appear to lie transgressively across the edges of these rocks, while to the north-west and north they rest on quartzites and schists and on Jurassic limestones. An outlier from the main mass forms the prominent hill of Sròn Mhor, and can be seen distinctly overlying the bedded basalts as well as the neck of agglomerate already described (Fig. 302).

[Footnote 320: This locality has been described by Professor Judd (_Quart. Jour. Geol. Soc._ xxx. (1874), p. 261; and xlvi. (1890), p. 373).]

[Footnote 321: As amygdaloidal structure is occasionally to be found among both dykes and sills its presence in the Ben Hiant rocks would not be inconsistent with their intrusive origin.]

The prevalent rocks of Ben Hiant are well crystallized, ophitic olivine-dolerites and gabbros. A specimen taken from the shore on the west side of the mass was found by Dr. Hatch to present under the microscope its augite in large plates, which enclose narrow laths and needles of plagioclase felspar as well as grains of olivine. All the felspars are in lath-shapes, sometimes extremely long and narrow. The iron-ore likewise assumes an ophitic character, enclosing rectangular portions of felspar. Dr. Hatch observed in another specimen, taken from the south-east side of the hill, "a curious intermixture of two different structures. Scattered portions which show the usual ophitic structure, their felspar and augite occurring in large crystals, are, so to speak, imbedded in a groundmass which presents rather a basaltic type, its felspar, augite, and magnetite, in long thin needles, microlites, and other skeleton forms, being enclosed in a dark devitrified base." A third specimen, selected from one of the columnar sheets near the top of Ben Hiant, is "a fine-grained dolerite (or gabbro) showing little ophitic structure, the augite occurring in roundish grains, and only slightly intergrown with the felspars, which are more or less lath-shaped. The rock contains a considerable quantity of black iron-ore in irregular grains and some dirty-green viridite." Still another variety of structure occurs in a specimen which I broke from one of the shore crags on the south-west side of the hill. Under the microscope, Dr. Hatch found in it a beautiful aggregate of "skeleton crystals and microlites of plagioclase, with here and there a rectangular crystal, long slender microlites of augite, and short serrated microlites of magnetite, the whole being confusedly imbedded in a dark glassy base powdered over with a fine magnetite dust."[322] A sill of pitchstone lies among the bedded basalts on the east side of the hill.

[Footnote 322: Professor Judd has called the rocks of Beinn Hiant augite-andesites, and has given descriptions and figures of their structure, and analyses of their chemical composition (_op. cit._).]

From a number of specimens collected by me during a second visit to this district in the summer of 1896, I selected some for microscopic examination and submitted them to Mr. Harker, who has furnished me with the following descriptions of them: "The sill at the north end of Camas na Cloiche, Ben Hiant [7114] is an olivine-gabbro of medium grain and fresh appearance. Olivine, fresh or partly serpentinized, is plentiful. The felspar is a labradorite with Carlsbad- and albite- (rarely pericline-) twinning, and some of it has zonary banding. It is for the most part in crystals giving rectangular sections, but there are some of allotriomorphic form. Magnetite occurs chiefly in shapeless grains of later crystallization than the felspar, but sometimes presenting crystal-faces to the augite. The augite is light-brown in the slice, without any true diallage-structure, and tends to enwrap the earlier minerals in ophitic patches.

"The sill south of Uamh na Creadha, on the west side of Ben Hiant [7115], is a rock of different type, having porphyritic crystals of felspar, up to an inch or more in length, in a rather finely-crystalline groundmass. The microscope shows it to be a dolerite of granulitic structure, the main mass of the rock consisting of little striated labradorite-crystals, grains of pyroxene, and rather abundant crystal-grains of magnetite. The pyroxene seems to be chiefly augite, but hypersthene is also present, and builds rather larger and more idiomorphic crystals with characteristic pleochroism."

In rambling over this Ardnamurchan district I have often been reminded of the great intrusive sheets of Fair Head. One of the features in which the rocks of the two localities resemble each other is their tendency to assume a coarsely crystalline texture. In some parts of Ben Hiant the individual crystals reach an inch or more in length. These more largely crystalline portions, however, do not form distinct bands so much as patches in the midst of the general mass; at least I have not noticed any examples of such veins of segregation as are so prominent in Antrim.

No one familiar with the well-marked distinctions between the lavas of the plateaux and the sills which traverse them can hesitate in which series to place the rocks of Ben Hiant. Since, however, these rocks have been claimed by Professor Judd as the superficial lava-currents of a volcano which broke out after the time of the plateau-basalts, like the Scuir of Eigg, some further details in regard to the geological structure of the district, which would otherwise be superfluous, may here be given.

The number of sills and dykes in Ardnamurchan is astonishingly great. There must be hundreds of them visible, and perhaps as many more concealed under superficial coverings. They are well exposed on the shore traversing the Jurassic strata and the schists. The sills become especially large and abundant in the direction of Ben Hiant, which has evidently been the principal centre from which their materials were injected. The rocks composing these sills are quite similar to those of Ben Hiant, save that, as they occur in thinner sheets than in that mountain, they do not attain the same coarseness of texture which the more massive beds there display. They generally possess fine-grained chilled selvages along their upper and under surfaces.

[Illustration: Fig. 326.--Ground-plan of Sills at Ben Hiant, Ardnamurchan.

_a_ _a_, crystalline schists; _b_ _b_, necks of volcanic agglomerate; _c_ _c_, numerous thin sills; D, massive sill of Beinn na h-Urchrach; E, north side of Ben Hiant; F, sill proceeding from the series forming Ben Hiant and joining that of Beinn na h-Urchrach. The arrows mark the dip.]

These abundant sills may be traced up into the mass of Ben Hiant from which they have issued, and of the individual sheets of which they are a continuation. One of the most striking and easily-followed examples of this connection is to be seen on the north side of the mountain. A thick sheet in the middle of Ben Hiant descends from among its contiguous sheets and, as a prominent rib, runs down the scree-slope into the valley below, where it forms a prominent feature. Crossing the streamlet in the middle of the valley, where a section has been cut through its upper surface, it gradually bends round towards the north-east, mounts the side of Beinn na h-Urchrach until it reaches the crest of the ridge and joins the other sills of which this eminence is built up. The route of this band of rock will be understood from the annexed ground-plan (Fig. 326).

That this prolongation of one of the thick beds of Ben Hiant is in no respect a superficial lava-stream but a true sill, is proved not only by its escarpment and dip-slope, but by its actually passing under and indurating the schistose grits, as may be seen in the stream-section. Again Beinn na h-Urchrach, which is mapped by Professor Judd as a northern expansion of Ben Hiant, is likewise not a lava but a true sill. Not only does it dip northwards at an angle of about 20°, having the schists immediately below its crest on the one side and descending with a long dip-slope on the other, but dwindling down rapidly from a thickness of 100 or 200 feet in the centre to no more than a few feet in a south-westerly direction, it there passes under schistose grits like those on which it lies. The strata that adhere to its upper surface are as usual indurated.

A section drawn across this attenuated development of the Beinn na h-Urchrach sill and that from Ben Hiant shows the structure represented in the accompanying diagram (Fig. 327), which simply gives the facts as exposed on the ground. The lower sill is that which issues from the main body of Ben Hiant, massive at first but diminishing in thickness as it recedes from its source.

Again, among the sheets which descend from the northern face of the summit of Ben Hiant and strike into the Jurassic outlier below, intensely indurated shale may be seen lying between two of the dolerites, which are unquestionably sills that have been injected into the Jurassic series.

The ridge of Ben Hiant is thus found to consist of a thick and complex series of sills, some of which are not traceable beyond the side of the mountain, while others can be followed outwards among the surrounding rocks. The specially marked dyke-like sills diverge from the main mass and run for some distance north-eastward, one of them, fully a mile long, descending among the schists into the valley and ascending into the basalt-plateau on the opposite side.[323]

[Footnote 323: The sills of Ben Hiant descend on the south-west side into the sea, and can be examined along the slopes and the beach, where Professor Judd has mapped a continuous platform of agglomerate. The broad hollow between that mountain and Beinn na h-Urchrach, over which he has spread his "augite-andesite lavas," appears to be underlain mainly by the crystalline schists through which sills from Ben Hiant have been injected. The northern eminence, which he has united with Ben Hiant, is entirely separate and, as above shown, is an obvious sill.]

[Illustration: Fig. 327.--Section of two Sills in schistose grits, west end of Beinn na h-Urchrach, Ardnamurchan.

_a_ _a_, crystalline schists; _b_, neck of volcanic agglomerate; _c_, small sill; D, massive sill of Beinn na h-Urchrach; F, sill proceeding from the series forming Ben Hiant and joining that of Beinn na h-Urchrach.]

On the south-east side of the mountain where the bedded basalts can be traced close up to the intrusive dolerites, they are found to present the usual dull indurated aspect so characteristic of contact alteration among these rocks. There cannot therefore be any doubt that Ben Hiant never was itself a volcano. Its rocks are characteristically those of subterranean intrusions. They seem to have been injected from a line of fissure or from several such lines, running in a general north-easterly direction, at some late part of the volcanic period. The group of agglomerate necks of older date shows that already the ground underneath had been drilled by a number of distinct volcanic funnels, and discloses a weak part in the terrestrial crust.

iv. FAROE ISLES

In the Faroe Islands the actual base of the volcanic series is nowhere visible. Hence, the great lower platform of intrusive sheets being there concealed, this feature of the basalt-plateaux is less conspicuous than it is in the Inner Hebrides. A number of sills, however, have been noticed by previous observers,[324] and I have observed others on the sides of Stromö, Kalsö, Kunö and other islands. In the lofty precipices of the Haraldsfjord, many of the massive light-coloured prismatic sheets are intrusive, for though they preserve their parallelism with the bedded sheets for considerable distances, they may be seen sometimes to break across these, as is strikingly shown in one of the great corries on the east side of Kunö.

[Footnote 324: See in particular Prof. James Geikie and Mr. Lomas, in the papers already cited on p. 191.]

[Illustration: Fig. 328.--Sill traversing bedded Basalts, cliffs of Stromö, at entrance of Vaagöfjord.

The caves and notches shown at the bottom of the precipice mark the position of the vents represented in Figs. 311, 312, 313, 314.]

One of the most remarkable sills in the Faroe Islands is probably that which forms so prominent an object on the western cliffs of Stromö, at the entrance into the Vaagöfjord (Figs. 328, 329). It is prismatic in structure, and where it runs along the face of the cliffs, parallel to the bedded basalts among which it has been intruded, presents the familiar characters of such sheets. The precipice of which it forms a part is that which rises above the row of volcanic vents already described. But it there begins to ascend the cliffs obliquely across the basalts until it reaches the crest of the great wall of volcanic rock at a height of probably about 1000 feet above the waves. From the crest of the precipice the upward course of the sill is continued into the interior of the island. It pursues its way as a line of bold crag along the ridges of the plateau, gradually ascending till it forms the summit of one of the most prominent hills in the district (Fig. 329).

Some further idea of the enormous energy with which the sills were injected may be formed from this example, where the eruptive materials followed neither the line of bedding nor a vertical fissure, but took an oblique course through the plateau-basalts for a vertical distance of probably more than 1500 feet.

V. GENERAL DEDUCTIONS REGARDING THE TERTIARY BASIC SILLS

If we consider the facts which have now been adduced regarding the position and structure of the sills, we are led, I think, to regard these masses as certainly belonging to the history of the basalt-plateaux, but, on the whole, to a comparatively late part of it. They consist of essentially the same materials as the lavas that form these plateaux, though with the differences of structure which the conditions of their production would lead us naturally to expect. Where they occur in thick masses, which must obviously have cooled much more slowly at some depth beneath the surface than the comparatively thin sheets could do that were poured out above ground, they have assumed a far more largely crystalline texture than that of the superficial lavas. As a rule, we may say that the thicker the sill the coarser is its texture, while the thinnest sheets are the most close-grained. Sills are especially abundant about the base of the basaltic-plateaux. We may examine miles of the central and higher parts of the escarpments without detecting a single example of them, but if the escarpment is cut down to the base we seldom need to search far to find them.

[Illustration: Fig. 329.--View of the same Sill seen from the channel opposite the island of Kolter.]

That the efforts of the internal magma to establish an outlet towards the surface were accompanied by powerful disturbances of the terrestrial crust is shown by the abundant dykes which traverse all the volcanic districts from Antrim to Iceland, and some of which ascend even to the very highest remaining lavas of the basalt-plateaux. The parallel fissures filled by these dykes prove that even after the accumulation of more than 3000 feet of basalt-sheets, the movements continued to be so powerful as to disrupt these vast piles of volcanic material. But undoubtedly the highest parts of the plateau-basalts are less cut by dykes than the lower parts. There would no doubt come a time when the dislocations would more seldom reach the surface, when dykes would not be formed so abundantly or up to such a high level, and when the volcanic energies would more and more sparingly result in the opening of new vents or in the discharge of fresh eruptions from old ones.

It appears to me most probable that the injection of the sills was connected with the same terrestrial disturbances that produced the dykes which traverse the plateaux. Besides being dislocated by parallel fissures, the earth's crust in North-Western Europe seems to have been ruptured internally along lines more or less at right angles to the vertical fissures. The deep accumulation of bedded basalts presented an increasing obstacle to the ascent of the magma to the surface. Unable to gain ample enough egress through such vertical fissures as might be formed in the volcanic pile, the molten rock would find its lines of least resistance along the planes of the strata and the lower basalt-beds, either by the aid of terrestrial ruptures there, or in virtue of its own energy. On these horizons, accordingly, the sills occur in extraordinary profusion throughout the volcanic regions. They are no doubt of all ages in the progress of the building up of the volcanic plateaux, but I am disposed to believe that a large number of them may belong to the very latest period of the uprise of basalt within the area of Britain.

One of the most suggestive features of the abundant Tertiary sills lies in the evidence they furnish of the enormous energy concerned in the ascent and intrusion of volcanic material. The infilling of dykes or the outpouring of successive streams of lava at the surface hardly appeals to our imagination so strikingly as the proof that the sills have been impelled into their places with a vigour which, even when guided and aided by gigantic terrestrial ruptures, was capable of overcoming the vertical pressure of hundreds, or even thousands of feet of overlying rock. Had these intrusive sheets been mere thin layers, their horizontal extent and persistence would still have excited our astonishment, but when we find them sometimes several hundred feet thick, and to extend in a continuous series for horizontal distances of 50 miles or more, we are lost in wonder at the prodigious expansive strength of the volcanic forces. Again, the intrusions have not always taken place between the bedding-planes of the stratified or igneous rocks, but, as we have seen, solid sheets of already deeply buried lavas have sometimes been split open and the intrusive material has forced itself between the disrupted portions. Such subterranean proofs of the vigour of volcanic energy teach some of the most impressive lessons in the chronicles of volcanic action in the British Isles.

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In closing this history of the accumulation of the great Tertiary volcanic plateaux of North-Western Europe, I would remark that as the result of prolonged eruptions from innumerable vents, the depression that extended from the south of Antrim to the Minch was gradually filled up with successive sheets of basalt to a depth of more than 3000 feet. A succession of lava-fields stretched from the North of Ireland across the West of Scotland, and perhaps even to the Faroe Islands, Iceland and Greenland. That the lava spread round the base of the Highland mountains and ran up the Highland glens, much as the sea now does, is made clear from the position of the outliers of it which have been left perched on the ridges of Morven and Ardnamurchan. So far as can now be surmised, these wide Phlegræan fields were only varied by occasional volcanic cones scattered over their surface, marking some of the last vents from which streams of basalt had flowed. But the volcanic energy was still far from exhaustion. After the accumulation of such a deep and far-extended sheet of lava, those underground movements which produced the fissures that served as channels for the uprise of the earliest dykes continued to show their vigour. The pile of bedded lavas was rent open by innumerable long parallel fissures in the prevalent north-westerly direction, up which basic lavas rose to form dykes, while vast numbers of sills were injected underneath. Whether the outflow of basalt at the surface had wholly ceased when the last of these dykes were injected into the plateaux cannot be told. Nor is there any evidence whether it had ended before the next great episode of the volcanic history--the extravasation of the gabbro bosses. All that we can affirm with certainty is, that the formation of north-west fissures and the uprise of basalt in them were again repeated, for we find north-west dykes traversing even the crests of the later eruptive masses of basic and acid rocks. It is difficult to suppose that none of these latest dykes communicated with the surface, and gave rise to cones with the outpouring of basalt and the ejection of dust and stones. But of such later outflows of basic material over the surface of the plateaux no undoubted trace has yet been recognised.

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