XLI. THE ERUPTIVE VENTS OF THE BASALT-PLATEAUX
Vents filled with Basalt or other Lava-form Rock--Vents filled with Agglomerate
It is one of the most interesting points in the Tertiary volcanic history that, in spite of the enormous geological revolutions that have passed since they became extinct, the sites of many scattered vents can still be recognized. A far greater number must lie buried under the basalts, and of others the positions are concealed by the sea, which now covers so large an area of the old lava-fields. Nevertheless, partly within the area of the plateaux, but still more on the surrounding tracts from which the basalts have been removed by denudation, the traces of unmistakable vents of discharge may be recognized amid the general wreck.
In Britain and the Faroe Isles, it is chiefly along the coast-line that the process of denudation has revealed the volcanic vents of Tertiary time. The interior of the country is often loaded with peat, covered with herbage, or strewn with glacial detritus: and even where indications of the vents are to be detected, it is not always possible to ascertain their true limits and connections. But where the structure of the plateaux has been laid bare along ranges of rocky precipice, the vents have sometimes been so admirably dissected by the sea that every feature of their arrangements can be satisfactorily determined.
As the actual physical connexion of these volcanic orifices with the plateaux has been in most cases removed by denudation, we can usually only by inference place them in what was probably their true relation to the plateau-eruptions. Those which project from the surface of the plateaux must, of course, be younger than the basalts through which they rise; how much younger we cannot tell. They may possibly be later than any of the plateau-sheets; they may even belong to a subsequent and waning condition of volcanic action. On the other hand, the vents which can now be traced outside of the present limits of the edges of the plateaux may, like those just mentioned, be younger than the basalt-sheets, or, on the contrary, they may be records of a period of eruptivity anterior to the emission of any of the rocks of the plateaux, and may have been deeply buried under a mass of basalt-beds subsequently removed. Positive demonstration is, from the nature of the case, impossible in these instances. But examples will be cited from the Western Isles and from Faroe, where the vents can be proved to belong to the time of the plateau-eruptions, for they are seen to have broken through some of the basalt-sheets and to have been buried under others. With this clear evidence of relationship in some cases, there need be little hesitation in believing that in other instances where no such positive connexion can be found, but where the vents are obviously such as the general structure of the plateaux would have led us to expect, they may be confidently regarded as part of the phenomena of the plateau-eruptions.
Sometimes the vents can be linked with lines of fissures or dykes. This is especially the case where they are small in size. More usually, however, no such relation can be demonstrated. It will be remembered that among the modern Icelandic eruptions, some eruptive vents, like the later cinder-cones of Laki, are ranged in a linear direction along the great fissure, while others, of an older series in the same district, almost engulphed amidst the more recent lavas, are clustered irregularly in groups. A similar diversity of arrangement has been observed among the volcanic cones of the Velay in Central France.
Considering as a whole the volcanic necks or eruptive vents which rise from the older rocks around the Tertiary basalt-plateaux, and sometimes even from the surface of these plateaux themselves, we may conveniently follow the same classification as was adopted in dealing with those of Palæozoic age, and, according to the nature of the material that now fills them, arrange them in two series: (1) Those occupied by some form of crystalline eruptive rock, and (2) those filled with volcanic agglomerate.
i. VENTS FILLED WITH DOLERITE, BASALT, ETC.
These, as the composition of the plateaux would lead us to anticipate, are numerous. They perhaps attain their most conspicuous development in Antrim, either on the tableland or among the underlying rocks round its edges. The finest example in that district is undoubtedly furnished by the lofty eminence called Slemish, which rises above the surrounding basalt-terrace, to a height of 1437 feet above the sea (Fig. 294). It is elliptical in ground-plan, measuring some 4000 feet in length by 1000 in breadth. Seen from the north, it appears as a nearly perfect cone. The material of which it consists is a coarsely crystalline olivine-dolerite, presenting under the microscope a nearly holocrystalline aggregate, in which the lath-shaped felspars penetrate the augite, with abundant fresh olivine, and wedge-shaped patches of interstitial matter. The rock is massive and amorphous, except that it is divided by parallel joints into large quadrangular blocks like a granitic rock, and wholly different from the character of the surrounding basalts. The latter, which possess the ordinary characters of the rocks of the plateaux, can be followed to within 80 yards of this neck, which rises steeply from them, but their actual junction with it is concealed under the depth of talus.
At the nearest point to which the two rocks are traceable, the basalts appear somewhat indurated, break with a peculiar splintery fracture, and weather with a white crust. These characters are still better shown on abundant fragments which may be picked up among the debris further up the slope. There can be no doubt, I think, that a ring of flinty basalt, differing considerably in texture from the usual aspect of that rock in the district, surrounds the neck. The meaning of this ring will be more clearly seen from the description of another example in Mull. About four miles to the north-east of Slemish, a smaller and less conspicuous neck rises out of the plateau-basalts. The rock of which it consists is less coarsely crystalline than that of Slemish, but its relations to the surrounding volcanic rocks are obviously the same. On the west side of Belfast Lough a boss of similar rock, about 1200 feet in diameter, rises at the very edge of the basalt escarpment into the eminence known as Carnmony Hill (Fig. 295). On its northern side it presents along its wall a mass of interposed volcanic agglomerate.[295] On visiting with Mr. M'Henry the quarry opened on the eastern face of this vent, I was much struck with the remarkable cellular structure of some parts of the dolerite. Many of the vesicles are lined with a thin pellicle of black glass, and the same substance occurs in minute patches in the body of the rock. A thin slice exhibiting this structure was found by Mr. Watts to possess the following characters:--"The rock is an ophitic dolerite consisting of plagioclase, augite, and iron ores, without olivine, enclosing one or two patches of finer basalt. The vesicles in the latter, and certain angular spaces between the crystals of the former, have been wholly or partially filled with brown glass, the outer part of which has been converted into radiating crystals of a brown mineral." The occurrence of patches of glass which seem to have been squeezed into vesicles or cracks in the body of a dolerite or andesite has been noticed in some of the Tertiary dykes. But in the present case the glass occurs as a mere coating on the walls of the larger spheroidal vesicles, the interior of which generally remains empty.
[Footnote 295: This neck was recognised by Du Noyer in 1868 as "one of the great pipes or feeders of the basaltic flows." See Prof. Hull, Explanation of Sheets 21, 28 and 29, Geol. Survey of Ireland (1876), p. 30.]
Of the other doleritic necks scattered over the surface of the Antrim plateau, I will refer to only one which occurs on the hillslopes between Glenarm and Larne. It forms a prominence known as the Scawt Hill, and consists of a boss of basalt, which, in rising through a vent in the plateau-sheets, has carried up with it and converted into marble a large mass of chalk which is now exposed along its eastern wall (Fig. 296).
As examples of similar necks which have been exposed by denudation outside the present limits of the same plateau, I may allude to those which rise through the Cretaceous and other Secondary strata on the northern coast near Ballintoy. One of the most striking of these may be seen at Bendoo, where a plug of basalt, measuring about 1400 feet in one diameter and 800 feet in another, rises through the Chalk, and alters it around the line of contact (Fig. 297). Another remarkably picturesque example is to be seen near Cushendall, where a prominent doleritic cone rises out of the platform of Old Red Sandstone, some distance to the north of the present edge of the volcanic escarpment (Fig. 298).
The greater coarseness of grain of the material filling these pipes, compared with that of the sheets in the terraces, is only what the very different conditions of cooling and consolidation would lead us to expect. There is no essential difference of composition between the two rocks. Where the erupted material has been poured out at the surface, it has assumed a finely crystalline texture, while, where it has slowly solidified within a volcanic pipe at some depth beneath the surface, and where consequently its component crystals have had more time for development, the resulting structure is much more largely crystalline, with a more or less complete development of the ophitic structure.
In the island of Mull, another instance of the same kind of vent has been observed and described by Professor Judd.[296] It rises in the conspicuous hill, 'S Airde Beinne (Sarta Beinn), about two miles south-west from Tobermory, and consists of a coarsely crystalline dolerite, which becomes finer in grain towards the outer margin (Fig. 299). No bedding, or structure of any kind beyond jointing, is perceptible in it. Examined in thin sections under the microscope, this rock is found to be another typical ophitic dolerite, consisting of lath-shaped felspars embedded in augite, with here and there wedge-shaped portions of interstitial matter and grains of olivine. Dr. Hatch found the felspars to contain spherical inclusions of devitrified glass, filled with black granules and trichites, and he observed that, under a high power, the interstitial matter is seen to consist mainly of a greenish-brown isotropic substance, in which are inclosed small crystals of augite, skeleton-forms and microlites of felspar, sometimes in stellate aggregates, as well as club-shaped, cruciform, arrow-headed and often crested microlites of magnetite.
[Footnote 296: Quart. Jour. Geol. Soc., xxx. (1874), p. 264.]
Towering prominently above the flat basalt sheets, this neck has an oval form, measuring about half a mile in length by a quarter of a mile in breadth. Its central portion, however, instead of rising into a rugged hill-top, as is usually the case, sinks into a deep hollow, which is filled with water, and reminds one of a true crater-lake (Figs. 299, 300). The middle of the neck is thus concealed from view, and we can only examine the hard prominent ring of dolerite that surrounds the tarn. The material occupying the hollow may be softer than that of the ring, and may have been scooped out by denudation. What we now see may not be the original surface, but may have been exposed after the removal of possibly hundreds of feet of overlying material. On the other hand, it is conceivable that the hollow is really a crater-lake which was filled up with detritus and may have been overspread with basalt, since removed. It may be suggestively compared with the crater-hollows revealed by denudation on the cliffs of Stromö and Portree Harbour, which will be described in a later part of this chapter. Possibly some more easily removable agglomerate, representing an eruption later than that of the dolerite, may occupy the centre of the volcanic pipe.
One of the most interesting features of this vent is to be found in its relation to the surrounding basalts. The marginal parts of the rock along the line of contact are much finer in grain than the rest, and have obviously cooled more rapidly. The contrast between them and the ordinary dolerite nearer the centre, however, cannot be properly understood, except in thin sections under the microscope. Dr. Hatch, to whom I submitted my specimens, observed that, in place of the structure above described, the marginal parts show an absence of the ophitic grouping except in small isolated patches. Instead of occurring in large grains or plates enveloping the felspars, the augite is found in numerous small roundish grains, together with grains of magnetite, in equal abundance and of similar size. The felspars are speckled over with opaque particles; olivine has not been detected.
For miles around the vent, the plateau-rocks are of the usual type--black, compact, sometimes amygdaloidal, alternating with more coarsely crystalline decomposing bands, the separation between different sheets being often marked by the ordinary red ferruginous partings. But around the margin of the neck, they have undergone a remarkable metamorphism. The portions of them which adhere to the outer wall of the neck have lost their distinct bedding, and have been, as it were, welded together into an indurated compact, black to dull-grey rock, so shattery and jointed that fresh hand-specimens, three or four inches in length, are not easily obtainable. Especially marked is one set of joints which, running approximately parallel, cause the rock to split into plates or slabs. These joints are sometimes curved. Yet, in spite of the alteration from its normal character, the basalt retains in places some of its more usual external features, such, for instance, as its amygdaloidal structure, the amygdales consisting of calcite, finely acicular mesotype, and other minerals.
Examined under the microscope, this altered basalt presents "a confused aggregate of colourless microlites (felspar?) and innumerable minute granules of magnetite, these two constituents being very unequally distributed. Sometimes the colourless portions preponderate, in other places the opaque granules are heaped together in black patches, which may possibly mark the position of fused augites."[297]
[Footnote 297: Notes by Dr. Hatch.]
In the zone of contact-metamorphism around some of the volcanic pipes in the plateaux, we see changes analogous to, but less developed than, those which have been superinduced on so large a scale round the great eruptive bosses of gabbro, granophyre, etc., that have broken up the terraced basalts along the west coast of Scotland. I shall accordingly return to this subject in connection with phenomena presented by these younger rocks (p. 386).
ii. VENTS FILLED WITH AGGLOMERATE
While the necks of dolerite or basalt cannot always be satisfactorily discriminated from bosses which may never have established a connection with the surface, there is no room for any doubt in this respect in the case of those filled with fragmentary materials. As has been already pointed out, the occurrence of true volcanic agglomerate may be accepted as evidence of the existence of an eruptive vent communicating with the surface of the earth. The agglomerate in the vents associated with the basalt-plateaux, like that of the Palæozoic vents, is generally exceedingly coarse, and without any trace of structure. Blocks of all sizes up to masses some yards in length, and of the most diversified materials, both volcanic and non-volcanic, are dispersed confusedly through a granular paste of similar miscellaneous composition.
An instructive example of the general characteristics of agglomerate-vents, and of the relation of these vents to the surrounding tuffs and basalts, is to be found at the island of Carrick-a-raide, on the north coast of Antrim, and on the opposite mainland. The visible mass of this neck is about 1000 feet in diameter, but the boundaries, except on the land side, are concealed by the sea. The material filling up the vent is a coarse agglomerate, in which blocks and bombs of basalt, with pieces of chalk and flint, are stuck at all angles in a dull dirty-green granular tuff. Some large and small intrusions of basalt rise through it. Owing partly to these intrusions, and partly to the grass-covered slope that separates it from the line of cliff, the actual contact of this neck with the volcanic beds of the escarpment cannot be seen. I have no doubt, however, that the tuff, which has already been referred to as so conspicuous a member of the series here, was discharged from this vent.[298] The materials are as usual coarser in the pipe than beyond it, but the finer portion or matrix of the agglomerate is similar to many bands of the tuff. The structure of the locality may be diagrammatically represented as in Fig. 301. The bedded tuff is thickest in the neighbourhood of the vent, and gradually dies away on either side of it.
[Footnote 298: See Explanation of Sheets 7 and 8, Geol. Survey of Ireland (1888), p. 31.]
But another important inference may be drawn from this locality. I have already pointed out that the lower basalts here reach their minimum thickness. Their basement beds thin away towards the vent as markedly as the tuff thickens. Obviously they cannot have proceeded from that point of eruption. Yet, that they had begun to be poured out before the discharge of the tuff is shown by their underlying as well as overlying that rock, though westward, owing to the thinning away of the undermost basalts, the tuff comes to lie directly on the Chalk. Hence, we may legitimately infer that in this neighbourhood one or more other vents supplied the sheets of the lower basalts.
In the island of Mull a number of detached bosses or patches of agglomerate much obscured by invasions of granophyre probably mark the sites of volcanic vents. They will be more particularly noticed in Chapter xlvii. One of their most interesting features is the large number of fragments of felsitic or rhyolitic rocks which they contain.
In the promontory of Ardnamurchan, where the basalt-plateau has been invaded and displaced by later intrusions of crystalline rocks, and has likewise been reduced to such a fragmentary condition by denudation, some interesting examples of agglomerate necks have been laid bare. One of the largest of these occurs on the north shore at Faskadale. Cut open by the sea for more than a quarter of a mile, this neck is seen to be filled with a coarse agglomerate, composed mainly of basalt-blocks and debris, but crowded also with angular and subangular pieces of different close-grained andesitic, felsitic and porphyritic rocks belonging to the acid series to be afterwards described.[299] Some of these stones exhibit a very perfect flow-structure, and closely resemble certain fine-grained, flinty, intrusive rocks in Mull, to which allusion will subsequently be made. The matrix of the agglomerate is of the usual dull dirty-green colour, but is so intensely indurated that on a fresh fracture it can hardly be distinguished from some of the crystalline rocks of the locality. The neck is pierced in all directions with dykes and veins of basalt, dolerite, andesite, gabbro, and felsitic rocks. Similar intrusions continue and increase in numbers farther west until the cliffs become a labyrinth of dykes and veins running through a mass of rocks which appears to consist mainly of dull dolerites and fine gabbros. Though the relations of this vent to the plateau-basalts are not quite plain, the agglomerate seemed to me to rise out of these rocks. At least the basalts extend from Achateny to Faskadale, but, as they are followed westwards, they are more and more invaded by eruptive sheets, and assume the indurated character to which I have already referred.
[Footnote 299: One of these felsites when viewed under a high magnifying power is seen to present an abundant development of exceedingly minute micropegmatite arranged in patches and streaks parallel with the lines of flow-structure in the general cryptocrystalline groundmass. The close relationship between the felsites, quartz-porphyries, and granophyres will be afterwards pointed out in the description of the acid rocks. It is remarkable that, though these rocks occur abundantly in fragments in the volcanic necks and agglomerates of the plateaux, not a single instance has been observed of their intercalation as contemporaneous sheets among the basic lavas. The analogous case of the interstratification of felsitic tuffs among basic lavas in the volcanic series of the Old Red Sandstone of Central Scotland has been described (vol. i. p. 279). It is interesting to note that liparitic pumice and dykes have been erupted by some of the basaltic craters of Iceland, for example at Askja, Öræfajökull and Snaefellsjökull. (Mr. Thoroddsen, Dansk. Geograf. Tidsskrift, vol. xiii. 7th and 8th parts.)]
On the south side of the peninsula of Ardnamurchan, another agglomerate, noticed by Professor Judd,[300] rises into the bold headland of Maclean's Nose, at the mouth of Loch Sunart, and affords better evidence of its relation to the bedded basalts. It measures about 1000 yards in length by 300 in breadth, and its summit rises more that 900 feet above the sea, which washes the base of its southern front. It is filled with an agglomerate even coarser than that on the northern coast. The blocks are of all sizes, up to eight or ten feet in diameter. By far the largest proportion of them consists of varieties of basalt and andesite, slaggy and vesicular structures being especially conspicuous. There are also large blocks of different andesitic porphyries and felsitic rocks like those just referred to, a porphyry with felspar crystals two inches long being particularly abundant. All the stones are more or less rounded, and are wrapped up in a dull-green compact matrix of basalt-debris. There is no stratification or structure of any kind in the mass. Numerous dykes or veins of basalt, of andesite, and of a porphyry, resembling that of Craignure, in Mull, traverse the agglomerate. Some of the narrow basalt-dykes cut through the others.
[Footnote 300: Quart. Journ. Geol. Soc. xxx. (1874), p. 261. Professor Judd has subsequently (op. cit. xlvi. 1890, pp. 374 et seq.) given a map, section and description of what he believes to be the structure of this ground, with numerous details as to the petrography of the rocks. The geological structure of this area is more fully referred to on pp. 318 et seq.]
The position of the vent, with reference to the surrounding rocks, will be understood from the accompanying section (Fig. 302). On the eastern side, the agglomerate can be seen to abut against the truncated ends of the flat beds of the plateau-basalts, which are of the usual bedded compact and amygdaloidal character. There can be no doubt, therefore, that the vent has been opened through these basalts. But it will be observed that the latter belong to the lower part of the volcanic series. These lowest sheets are exposed on the slope, resting upon yellowish and spotted grey sandstone, with seams of jet and a reddish breccia, which, lying in hollows of the quartzites, quartz-schists, and mica-schists, form no doubt the local base of the Jurassic rocks of the district. Hence, the vent, though younger than the older sheets of the plateau, may quite well be contemporaneous with some of the later sheets.[301]
[Footnote 301: It may here be remarked that there is evidence of great differences in the level of the base of the Jurassic series and the bottom of the volcanic plateau in this district. On the south and west sides of Ben Hiant the Jurassic conglomerates may be seen lying on the edges of the crystalline schists only a little above high-water mark, while on the north side, the schists, with their overlying unconformable cake of limestones, rise several hundred feet above sea-level. The surface on which the basalts were poured out was probably very uneven, but there may also have been some considerable displacements of these basalts either before or during the injection of the dolerite sills of Ben Hiant.]
An interesting feature at this locality is the peculiar grouping of some of the large dykes in the area around the agglomerate. They run in the direction of the vent, and one or other of them may represent the fissure or fissures on which the volcanic orifice was blown open to the surface. Another notable element in the geological structure of the ground is the vast amount of intrusive material, both in dykes and sheets, which has been erupted. The intrusive sheets of Ben Hiant form the most prominent eminence in this part of Ardnamurchan. Reserving them for description in the following Chapter (p. 318), I will only remark here that they partly overlie the agglomerate, and are therefore, to some extent at least, younger than the vent. They belong to that late stage in the history of the basalt-plateaux when the molten material, no longer getting ready egress to the surface, forced its way among the rocks about the base of the bedded basalts, and more especially on the sites of older vents, which were doubtless weak places, where it could more easily find relief.
The large neck now described is only one of a group scattered around it in the ground to the north. Two of these may be seen rising through a detached area of Jurassic limestones and shales at the northern base of Ben Hiant. A third, almost obliterated by the intrusive sheets, may be traced at the western end of that mountain above Coiremhuilinn. Two others rising through the schists on either side of Beinn na h-Urchrach, have been much invaded by the sills of that eminence (Fig. 326). It is doubtless owing to the extensive denudation of the basalt-plateau, and the consequent uncovering of the rocks underneath it, that this series of vents has been laid bare.[302]
[Footnote 302: Professor Judd has united these scattered vents into a continuous platform of volcanic agglomerates, which he represents as underlying the supposed lavas of Ben Hiant. Since the publication of his map and description, I have re-examined the ground without being able to discover any trace of this platform. All the visible agglomerates are separate necks, their actual walls being sometimes exposed, as in the neck immediately north of the base of Ben Hiant, where the limestone in contact is marmorised, though twelve yards of it is an ordinary dull blue rock.]
By far the largest mass of agglomerate in any of the Tertiary volcanic areas of Britain is that which occurs on the north side of the main valley of Strath, in Skye.[303] Unfortunately, it has been so seriously invaded by the eruptive rocks of the Red Hills, that its original dimensions and its relations to the surrounding rocks, especially to the bedded basalts, are much obscured (see Fig. 348). It can be followed continuously from the lower end of Loch Kilchrist along the southern slopes of Beinn Dearg Bheag round to the western roots of Beinn Dearg Mhor--a distance of more than two miles in a straight line, and from Kilbride to the flank of Beinn na Caillich above Coire-chat-achan--a direct distance of two miles and a quarter. A similar rock, possibly a portion of the same mass, appears in Creagan Dubha, on the north side of the Red Hills. If the whole of this agglomerate forms part of one originally continuous mass, it must have been upwards of two miles in diameter. There may, however, have been two or three closely adjacent vents. The Beinn na Caillich patch, for example, appears to belong to a different area, and that of Creagan Dubha is also probably distinct. But there seems no reason to doubt that the mass which forms Cnoc nam Fitheach, and all the long declivity on the southern flank of Beinn Dearg Bheag, occupies part of the site of a single volcano. Owing to the absence of sufficient sections, it is hardly possible to determine how much of this fragmentary material should be assigned to the actual chimney. The diameter of the whole mass is almost two miles. But possibly a considerable proportion of this accumulation belongs to the external cone which gathered round the vent, so that the eruptive pipe might thus be of much smaller dimensions than the superficial area of the agglomerate. The subsequent invasion of so much granophyre, not only that of the Red Hills, but that of numerous smaller intrusions, has indurated the agglomerate and made the investigation of its structure somewhat unsatisfactory.
[Footnote 303: This extensive mass was not separated from the "syenite" of the Red Hills by Macculloch. Von Oeynhausen and Von Dechen noticed it as a conglomerate with quartz pebbles, but did not realise its volcanic nature (Karsten's Archiv, i. p. 90). In my map of Strath (Quart. Jour. Geol. Soc. xiv. plate i.) I distinguished it from the rock of the Red Hills, but no name for it appears in the legend of the map, nor is it referred to in the text. Its character as a true volcanic agglomerate was recognised by Professor Judd, op. cit. p. 255. See postea, pp. 384 et seq.]
It might be supposed that the mere existence of intrusive bosses and veins rather furnishes an argument in favour of considering the visible agglomerate to belong to a deeper-seated part of the erupted material than the external cone. But, as will be afterwards shown, there is some reason to regard the present conical or dome-shaped outlines of the granophyre hills as not far from their original forms, and to believe that, like the trachytic Puys of Auvergne, they were much more superficial than plutonic eruptions. A study of the cinder cones of Central France shows that even these superficial accumulations have been invaded not only by bosses but by dykes.[304]
[Footnote 304: The existence of a small dyke of andesite on the northern rim of the well-known crater of the Puy Parion has already been noticed.]
The agglomerate of the great Strath vent is a coarse tumultuous assemblage of blocks and bombs, imbedded in the usual dull, dirty-green matrix. Among the stones, grit and sandstone, together with scoriaceous, vesicular and amygdaloidal basalts are specially abundant; also pieces of various quartz-porphyries and granophyres, among which a black felsite like that of Mull may often be recognised. In some places, large masses of altered limestone and quartzite (Cambrian) are included; in others, pieces of yellow sandstone and dark shale (Jurassic), or of the bedded lavas. Some of these masses may be 100 yards or more in length. Occasionally a breccia, mainly made up of acid materials--granophyre or granite,--has been noticed by Mr. Harker along the north side of the Red Hills, which he thinks may rather be of the nature of a crush-breccia than a part of the true agglomerate.
The agglomerate of this district is wholly without stratification or structure of any kind. On the north-west side of Loch Kilchrist, indeed, it weathers into large tabular forms, the parallel surfaces of which dip to south-west; but this is probably due only to jointing. Here and there, dykes of basalt cut the rock in a general north-westerly direction, but their number is remarkably small when compared with the prodigious quantity of them in the limestone at the bottom and opposite side of the valley, some of which may possibly mark the fissure on which the vent was placed. More abundant and extensive are the masses of granophyre that rise particularly along the outer margin of the agglomerate near Loch Kilchrist. These may be connected with the great boss that forms the Red Hills, of which further details will be given in Chapter xlvi.[305]
[Footnote 305: The granophyre intrusions in this agglomerate have been found by Mr. Harker to have taken up and dissolved a considerable proportion of fragments of gabbro, Chapter xlvi. p. 392.]
The important question of the relation of this agglomerate to the plateau-basalts does not admit of satisfactory treatment, owing to destruction of the evidence by the intrusion of the granophyre, and likewise to enormous denudation. Nevertheless, some traces still remain to indicate that the basalts once stretched over the site of the vent, which probably rose through them. Looking westward from the Hanks of Beinn Dearg Bheag to the other side of Loch Slapin, the geologist sees the bold basalt-escarpment of Strathaird presenting its truncated beds to him at a distance of only two miles. That these lavas were once prolonged eastwards beyond their present limits is obvious, and that they stretched at least over these two intervening miles can hardly be doubted. But we can still detect relics of them on the flanks of Beinn Dearg. As we follow the agglomerate round the margin of the granophyre that mounts steeply from it, we lose it here and there under beds of amygdaloidal basalt. The rocks next the great eruptive mass of the mountain are so indurated and shattered that it is difficult to separate them from each other and determine their relative positions. But, so far as I could ascertain, these basalts are fragments of beds that overlie the agglomerate (Fig. 303). This is not the only place along the flanks of the Red Hills where portions of the bedded basalts have survived. Other localities will be subsequently alluded to.
The Strath vent has been drilled through the Cambrian limestone, and as the result of protracted denudation it now towers steeply 500 or 600 feet above that formation on the floor of the valley. Of the material discharged from it over the surrounding country no certain trace now remains. We may infer from the nature of the rock which fills it that towards the end, if not from the beginning of its activity, its discharges consisted mainly of dust and stones. A cone, of which the remains are two miles in diameter, must surely have sent its fragmentary materials far and wide over the surrounding region. But on the bare platform of older rocks to the south, beyond the bottom of the agglomerate declivities, not a vestige of these erupted materials can now be found. Westward the escarpment of Strathaird remains to assure us that no thick showers of ashes fell at even so short a distance as two miles, either before or during the outpouring of the successive basalt sheets still remaining there. We may therefore conclude with some confidence that here, as at Ardnamurchan, the vent is younger than at least the older parts of the basalt-plateau. Unfortunately the uprise of the large bosses of granophyre that stretch from the Red Hills to Loch Sligachan has entirely destroyed the vent and its connections in that direction. There is no certain proof that any molten rock ever issued from this orifice, unless we suppose the fragmentary patches of amygdaloid on the southern flank of Beinn Dearg Bheag to be portions of flows that proceeded from this centre of eruption. The basalt-plateau which still remains in Strathaird no doubt formerly extended eastwards over Strath and northwards across the site of the Red Hills and Cuillins, joining on to the continuous tableland north of Lochs Brittle and Sligachan. How much of the plateau had been built up here before the outburst of the vent cannot be ascertained. The agglomerate may possibly, of course, belong to the very latest period of the plateau-eruptions, or even to a still younger phase of Tertiary volcanic history. The impression, however, made on my mind by a study of the evidence from the Western and Faroe Isles is that the necks of agglomerate, like those of dolerite and basalt, really belong to different epochs of the plateau period itself; and mark some of the vents from which the materials of the plateaux were successively emitted.
The example of Carrick-a-raide (p. 277) is peculiarly suggestive when we regard it in connexion with the great Strath vent. Already the progress of denudation has removed at least half of the layer of dust and stones which, thrown out from that little orifice, fell over the bare chalk-wolds and black basalt-fields of Antrim. The neck that marks the position of the volcanic funnel has been largely cut away by the waves, and is almost entirely isolated among them. The vents at Canna, Portree and the Faroe Isles, to be afterwards described, unquestionably belong to the eruptions of the plateau-period, for their connection with the basalts can be clearly established. At the Strath vent, however, the march of destruction has been greater. The connexion between this vent and the materials ejected from it has been entirely removed, and we can only guess from the size of the remaining neck what may have been the area covered by the discharges from this largest of all the volcanic cones of the Inner Hebrides.
Other masses of similar agglomerate are observable in the same region of Skye, where they not improbably mark the sites of other vents. Unfortunately their original limits and relations to the rocks through which the eruptive orifices were drilled have been much obscured by the uprise of the great masses of gabbro and granophyre of the Cuillin Hills. Several of these isolated intrusions occur in the midst of the gabbro, as in Harta Corry and on the west side of the Blaven ridge. Another mass is interposed between the gabbro and granophyre on Druim an Eidhne and at the base of the lavas between Druim an Eidhne and the Camasunary valley. Mr. Harker has found a huge mass of agglomerate underlying the bedded basalts to the north and west of Belig, one of the hills on the west side of the large valley that runs from the head of Loch Slapin to Loch Aynort. This mass has its bottom concealed by the granophyre which underlies it; but it reaches a maximum thickness of perhaps 1000 feet, rapidly thinning out and disappearing. It generally resembles the Strath agglomerate, but is distinguished by including a large proportion of fragments of gabbro. Mr. Harker remarks that "a study of these agglomerates points to the existence of both gabbros and granophyres older than the volcanic series, and therefore distinct from the gabbros and granophyres now exposed at the surface."
It is a suggestive fact that so many detached masses of agglomerate should occur around and within the areas of the great eruptive bosses of gabbro and granophyre. They seem to indicate the former existence of groups of volcanic vents in these tracts, and may thus account for the uprise of such large bodies of intrusive material through what must have been a weakened part of the terrestrial crust.
Further north in Skye a much smaller but more perfectly preserved vent has been laid open by denudation on the south side of Portree Bay--a deep inlet which has been cut out of the plateau-basalts and their underlying platform of Jurassic sandstones and shales. The great escarpment of the basalts has, at the recess of Camas Garbh, been trenched by a small rivulet, aided by the presence of two dykes. The gully thus formed exposes a section of a neck of agglomerate that underlies the basalts of the upper half of the cliff. This neck is connected with a thick deposit of volcanic conglomerate and tuff which, lying between the basalts, extends from the neck to a considerable distance on either hand. The general relations of the rocks at this locality are represented in Fig. 304.
The agglomerate (b) is quite tumultuous, and here and there strikingly coarse. Some of its included blocks measure five feet in length. These fragments represent most of the varieties of the lavas of the district. Large slaggy masses are abundant among them, and sometimes exhibit the annelide-like elongation of the vesicles which I have referred to as occasionally displayed by the plateau-basalts. More than 60 feet of agglomerate are visible in vertical height from where its base is concealed by debris and vegetation to where its upper surface passes under a banded rock to be afterwards described. That this unstratified mass of volcanic detritus marks the site of a vent can hardly be doubted, although denudation has not revealed the actual walls of the chimney. The steep grassy slopes do not permit the relations of the rocks to be everywhere seen, but the agglomerate appears to pass laterally into finer, rudely-stratified material of a similar kind, which extends towards east and west as a thick deposit between the bedded basalts. Possibly denudation has only advanced far enough to lay bare the crater and its surrounding sheets of fragmentary material, while the chimney lies still buried underneath.
To the east or left of the agglomerate the detritus becomes less coarse, and shows increasing indications of a bedded arrangement. Close to the agglomerate the dip of the coarse tuff is towards that rock at about 10°. A few yards further east a sheet of very slaggy basalt is seen to lie against the tuff, which it does not pierce. The vesicles in this adhering cake of lava have been pulled out in the direction of the slope till they have become narrow tubes four or five inches long and parallel to each other. Some parts of this rock have a curved ropy surface, like that of well-known Vesuvian lavas, suggestive of the molten rock having flowed in successive thin viscous sheets down the slope, which has a declivity of about 30°. This part of the section may possibly preserve a fragment of the actual inner slope of the crater formed of rudely-bedded tuffs.
Continuing still eastward, we find the feebly stratified tuff (a) to be perhaps 200 feet thick. It forms a grassy declivity that descends from the basalt-escarpment above to the grass-covered platform which overlies a lower group of basalts. The visible portion of this tuff presents a thoroughly volcanic character, being made up of the usual dull dirty-green granular paste, through which are dispersed angular and rough lumps of slag and pieces of more solid basalt, varying up to a foot or two feet in length. These stones are generally disposed parallel to the indistinct bedding, but are sometimes placed on end, as if they had assumed that position on falling from an explosive shower. Among the smaller stones, pieces of a finely vesicular basic pumice are frequent and are among the most strikingly volcanic products of the deposit. From a characteristic sample of these stones, a thin slice was prepared and placed in Mr. Harker's hands. The following are his observations on it:--"A very compact dark grey rock, amygdaloidal on a minute scale. The lighter grey crust is probably due merely to weathering, and the specimen seems to be a distinct fragment, not a true bomb. The slice shows it to be essentially a brown glass with only occasional microscopic crystals of a basic plagioclase. It has been highly vesicular, and the vesicles are now filled by various secondary products, including a chloritic mineral, nearly colourless and singly refracting in thin section, and a zeolite."
Tracing now the tuff from the west or right side of the vent, we can follow it to a greater distance. No abrupt line can be detected here, any more than on the other side, between the agglomerate and the tuff. The latter rock extends under the overlying plateau of basalt, at least as far west as Portree Loch, a distance of fully a mile, but rapidly diminishes in thickness in that direction. Traces of what is probably the same tuff can be detected between the basalts at Ach na Hannait, more than three miles to the south (Fig. 305). It is thus probable that from the Portree vent fragmentary discharges took place over an area of several square miles.
Above the agglomerate of this vent two lavas may be seen to start towards opposite directions. One of these (c), already referred to, is a dull prismatic basalt with a slaggy bottom, its vesicles being pulled out in the direction of the general bedding of the section. It descends by a twist or step, and then lies on the inclined surface of the tuff which dips towards the agglomerate and seems to pass into that rock. Further east this basalt increases in thickness and forms the lowest of the basalt-sheets of the cliff. The lava that commences on the west side of the agglomerate (d) is a massive jointed basalt, which, though not seen at the vent, appears immediately to the west of it and rapidly swells out so as to become one of the thickest sheets of the locality. It lies upon the rudely-bedded tuff, and is covered by the other basalts of the cliff.
That these two basalts came out of this vent cannot be affirmed. If they did so at different times, their emission must have been followed by the explosion which cleared the funnel and left the central mass of agglomerate there. But that some kind of saucer-shaped depression was still left above the site of the vent is indicated by the curious elliptical mass of rock (e) that lies immediately above the agglomerate, from which it is sharply marked off. This is one of the most puzzling rocks in the district, probably in large measure owing to its advanced state of decay. It is dull-red in colour, and decomposes into roughly parallel layers, so that at a short distance it looks like a bedded tuff, or like some of the crumbling varieties of banded lavas. I could not obtain specimens fresh enough to put its nature and origin beyond dispute. Whatever may have been its history, this ferruginous rock rests in a flat basin-shaped hollow directly above the agglomerate of the vent. The form of this depression corresponds fairly well with what we may suppose to have been the final position and shape of the crater of the little volcano. The rock that occupies the bowl dies out towards the east on the face of the cliff, and the prismatic basalt (c) is then immediately covered by the rest of the basalt-sheets of the plateau (f). On the west side its precise termination is concealed by grass. But it must rapidly dwindle in that direction also, for not many yards away it is found to have disappeared, and the basalts (d and f) come together.
Though the decayed state of this rock does not warrant any very confident opinion regarding its history, I am inclined to look upon it as a deposit of much disintegrated volcanic detritus washed into the hollow of the old crater when it had become filled with water, and had passed into the condition of a maar. The peculiarly oxidized condition of its materials points probably to long atmospheric exposure, and an examination of the surrounding parts of the district furnishes more or less distinct evidence that a considerable lapse of time did actually intervene between the cessation of the eruptions of the Portree volcano and the next great basalt-floods of this part of Skye.
That volcanic eruptions from other vents continued after the Portree vent had become extinct is proved by the great sheets of basalt (f) that overspread it, and still bury a large tract of the fragmentary material which it discharged. At a later time a fissure that was opened across the vent, allowed the uprise of a basalt dyke (g), and subsequently another injection of similar material took place along the same line of weakness (h).
Before leaving this interesting locality we may briefly take note of the distribution of the ashes and stones ejected by the volcano, and the evidence for the relative length of the interval between the outflow of the lavas below and that of those above the tuff and volcanic conglomerate. These deposits may be traced in clear sections along the base of the cliffs for a mile to the west of the vent. They thin away so rapidly in that direction that at a distance of three-quarters of a mile they do not much exceed fifty feet in thickness. At Camas Bàn they consist mainly of a fine, dull-green, granular, rudely-stratified basalt-tuff, through which occasional angular pieces of different lavas and rough slags are irregularly dispersed. These stones occur here and there in rows, suggestive of more vigorous discharges, the layers between the platforms of coarser detritus being occupied by fine tuff. Some of the ejected blocks are imbedded on end--an indication of the force with which they were projected so as to fall nearly a mile from the crater.
The upper parts of the tuff pass upward into fine yellow, brown, and black clays a few feet in thickness, the darker layers being full of carbonaceous streaks. On this horizon the coal of Portree was formerly mined. The workings, however, have long been abandoned, and, owing to the fall of large blocks from the basalt-cliff overhead, the entrance to the mine is almost completely blocked up. One wooden prop may still be seen keeping up the roof of the adit, which is here a slaggy basalt.
To the east and south-east of the Portree vent, extensive landslips of the volcanic series and of the underlying Jurassic formations make it hardly possible to trace the continuation of the tuff-zone in that direction. To the south, however, at a distance of rather more than three miles, what is probably the same stratigraphical horizon may be conveniently examined from Ach na Hannait for some way to the north of Tianavaig Bay. At the former locality the calcareous sandstones of the Inferior Oolite are unconformably covered by the group of rocks represented in Fig. 305. At the bottom of the volcanic series lies a sheet of nodular dolerite with a slaggy upper surface (a). Wrapping round the projections and filling up the depressions of this lava comes a thin group of sedimentary strata from an inch or two to eighteen inches or more in thickness (b). These deposits consist of hardened shale charged with macerated fragments of linear leaves and other plant-remains, including and passing into streaks of coal, which may be looked upon as probably occupying the same horizon with the coal of Portree. But here, instead of reposing on a mass of stratified tuff, the carbonaceous layers lie on one of the bedded lavas. The tuff has died out in the intervening three miles, yet that some of the discharges of volcanic detritus reached even to this distance, and that they took place during the accumulation of these layers of mud and vegetation, is shown by the occurrence in the shales of pieces of finely amygdaloidal basalt, from less than an inch to six inches in length, likewise lapilli of a fine minutely cellular basic pumice, like some varieties of palagonite. The overlying dolerite (c) becomes finely prismatic at its junction with the sedimentary layers and has probably indurated them.
This intercalation of a shaly and coaly band among the lavas can be followed northward along the coast. In some places it has been invaded by dykes, sills, and threads of basalt on the most remarkably minute scale, of which I shall give some account in Chapter xlii. (see Fig. 321). North of Tianavaig Bay--that is, about three-quarters of a mile nearer to the Portree vent--a perceptible increase in the amount of volcanic material is observable among the shales and leaf-beds. Not only are lapilli of basic pumice abundant, but the volcanic detritus has accumulated here and there in sufficient amount to form a band of dull greenish-brown tuff.
These coast-sections in the neighbourhood of Portree afford additional illustrations of the characteristic fact, on which I have already insisted, that the interstratifications of sedimentary material in the basalt-plateaux frequently terminate upward in leaf-beds, thin coals, or layers of shale, full of indistinctly preserved remains of plants. As I have endeavoured to show, this vegetation, which was undoubtedly terrestrial, probably grew not far from the sites where its remains have been preserved. Leaves and seeds would naturally be blown or washed into pools on the lava-fields, and would gather there among the mud and sand carried by rain from the surrounding ground. Such a topography and such a sequence of events point to intervals of longer or shorter duration between the successive outpourings of basalt. It was probably during one of these intervals of quietude that the crater of the Portree volcano became a maar and was finally silted up.
Reference has already been made to a conspicuous mass of agglomerate which occurs at the east end of the island of Canna, and marks the site of an important volcanic vent belonging to the Small Isles plateau. A portion of it projects from the grassy slopes, and rises vertically above the beach as a picturesque crag, in front of the precipice of Compass Hill (Fig. 306). But the same rock may be traced southward to the Coroghon Mòr, and north-westward in the lower part of the cliffs to a little beyond the sea-stack of An Stòll. It has thus a diameter of at least 3000 feet. Westward it passes under the conglomerate described in Chapter xxxviii. Its eastern extension has been concealed by the sea.
The materials that fill this vent consist of a typical agglomerate composed entirely, or almost entirely, of volcanic detritus. The embedded blocks vary up to eight feet in diameter or even more. They are chiefly fragments of various basalts and andesites, generally vesicular or amygdaloidal. Some of these, which have evidently been broken off from already consolidated lavas, are angular or subangular in shape, and their steam-holes are cut across by the outer surfaces of the stones. Where they consist of calcite, zeolite, etc., the amygdales so exactly resemble those of the bedded basalts of the plateaux that, as already remarked, we must believe them to have been already filled by infiltration before the disruption of the rocks by volcanic explosions. Other blocks are true bombs, with a fine-grained crust outside and a more cellular texture inside, the vesicles of the outer crust being sometimes dragged round the surface of the stone. The variety of materials included among the ejected blocks and the abundance of pieces of the red bole which so generally separates the plateau-basalts indicate that a considerable thickness of bedded lavas has probably been broken through by the vent.
Beside the volcanic materials, occasional angular pieces of red (Torridon) sandstone may be observed in the agglomerate. The paste is a comminuted mass of the same material as the blocks, tolerably compact, and entirely without any trace of stratification.
The actual margin of this vent has nowhere been detected by me. We never reach here the base of the volcanic series, for it is sunk under the sea-level. On the other hand, the upper limits of the agglomerate have been partially effaced or obscured by the conglomerates which overlie it. From the breadth of ground across which the agglomerate can be followed along the shore, the vent might be regarded as having been perhaps not less than three-quarters of a mile in diameter. But there is the same difficulty here as at the Strath vent in Skye in determining the actual limits of the volcanic funnel. Possibly there may have been more than one vent in close proximity. Even if there was only one, the existing agglomerate may include not only what filled the chimney, but also a portion of what had accumulated round the orifice and formed the external cone. That the volcano continued for some time in vigorous eruption may be judged from the amount of material ejected from it, the large size of its blocks, and the distance to which they were sometimes thrown.
The pieces of Torridon Sandstone were no doubt derived from the extension of that formation underneath Canna. On the opposite island of Rum, where these pre-Cambrian red sandstones are copiously developed, they form the platform through which the Tertiary volcanic series has been erupted. The several remaining outliers of the bedded basalts, referred to in a previous chapter (p. 215 and Fig. 267) as visible on the west side of this island, show that the basalt-plateau of Small Isles, which once covered that area, rested immediately on the inclined edges of the Torridon Sandstones. Probably the same structure stretches westward under Canna and Sanday. No traces of any Jurassic strata have been detected beneath the volcanic rocks of Rum, though they are so well developed a few miles to the east in the island of Eigg. Either they were not deposited over the pre-Cambrian rocks of Rum, or they had been removed from that ancient ridge before the beginning of the Tertiary volcanic period. Certainly I have not detected a single recognizable fragment of any Jurassic sedimentary rock in the agglomerate of Canna.
This Canna vent exhibits, better than is usually shown, the occurrence of dykes and irregular injections of lava through the agglomerate. A large mass of a finely columnar basalt runs up from the beach at Garbh Asgarnish. A similar rock forms several detached crags a little further south, particularly in the headland of Coroghon Mòr and the island of Alman. Here the basalt is beautifully columnar, its slender prisms curving from a central line until their ends abut against the agglomerate. The truly intrusive character of this basalt is well shown on the southern front of Coroghon Mòr, and on the northern face of Alman, as represented in the accompanying diagrams (Figs. 307 and 308).
Although there is no conclusive evidence that these intrusions belong to the time of the activity of the vent, yet they differ so much from the ordinary dykes (one of which also cuts the agglomerate and ascends through the conglomerates and basalts above), are confined so markedly to the vent and its immediate proximity, and resemble so closely the basalt-injections of other vents, such as those of the Carboniferous and Permian necks of Scotland, that they may with probability be regarded as part of the mechanism of the Canna volcano.
Though the form and size of the vent of this volcano cannot be precisely defined, the upper part of its agglomerate, as we have seen (ante, p. 219), is dovetailed in the most interesting way with the series of coarse conglomerates, which indicate strong river-action in this part of the volcanic area during the time of the eruption of the plateau-basalts.
The agglomerate vents described in the foregoing pages as occurring in Antrim and among the Inner Hebrides all appear either in the midst of the plateau-basalts or in close proximity to them. Before quitting the Scottish examples, I may refer to some that rise through much more ancient formations at a distance from any portion of the volcanic plateaux, and yet may with probability be assigned to the Tertiary volcanic period.
During the progress of the Geological Survey through the district of Applecross, in the western part of the mainland of Ross-shire, and far away from the basalt-plateau of Skye, Mr. John Horne[306] has found two small necks rising on each side of a line of fracture, through gently inclined Torridon Sandstones. They are conspicuous from a distance by the verdure of their slopes, in contrast with the brown tints of the surrounding moorland. The larger of the two necks measures about 180 by 150 feet, and abruptly truncates the beds of Torridon Sandstone, which as they approach it assume a bleached aspect and become indurated. The material filling this vent is an agglomerate made up mainly of pieces of Torridon Sandstone and grit which, though generally small, occasionally measure a foot across, and in one case were found to reach a length of four feet. They are not as a rule markedly altered, but some of them have acquired a glazed or vitreous texture. Besides these fragments of the general rock of the district, there occur abundant lapilli of a basic volcanic rock, found by Mr. Teall to consist of porphyritic felspar, extremely minute acicular microlites of felspar, somewhat irregular transparent spaces now occupied by a yellowish-green substance, and interstitial matter. At the south end of the vent a small mass of decayed basalt appears to pierce the agglomerate.
[Footnote 306: Trans. Geol. Soc. Edin. vii. (1894), p. 35.]
Though there is no indication of the age of these necks, they agree so closely in general character with known vents of the Tertiary volcanic plateaux that there cannot be much hesitation in regarding them as dating from the same great period of basalt-eruption. But no relic now exists anywhere around of lavas or tuffs ejected from them. They rise on the bare Applecross hills, 1000 feet above sea-level, two miles from the shore, and about ten miles from the nearest outlier of the basalt-plateau in the Dùn Can of Raasay. If they once discharged streams of lava that united with the rest of the plateau, the total destruction of this lava affords another impressive picture of the waste which the volcanic rocks of the Inner Hebrides have undergone.
The large proportion of Torridon Sandstone blocks in these two Applecross necks suggests, however, that the orifices never became active volcanic vents. They may have been mere spiracles, or blow-holes, where the funnels drilled by explosive vapours were filled up with the debris of the rocks that were blown out. But that lava did rise within them is shown by the basic lapilli in the agglomerate, and by the basalt which in both vents has found its way up the chimney.
In the island of Raasay Mr. Teall, during the summer of 1894, observed a group of curious neck-like masses of breccia which pierce the Torridon Sandstone near Brochel (Fig. 309). The blocks in them are large angular unaltered pieces of the surrounding sandstones and shales, sometimes ten feet or more in length, and the matrix is sometimes pure crystalline calcite like Iceland spar. The breccia is generally coarsest towards the outer margin. But though the Lewisian gneiss exists immediately below the thin cake of Torridonian strata, not a fragment of it could either Mr. Teall or I, when I visited the locality with him, find among the components of the breccia. Nor did we detect any trace of volcanic material. The general ground-plan of these masses is elliptical, the most northerly measuring 30 yards in diameter. Where the junction of the breccia with the Torridon strata can be seen it is a nearly vertical one, the sandstones and shales being much jumbled and broken, but not sensibly indurated. This little cluster of patches of breccia can hardly be due to local crushing of the rocks. Their definite outlines and composition seem rather to indicate spiracles of Tertiary time, which never became vents erupting lava or ashes. The absence of fragments of the underlying gneiss may be accounted for if we suppose that the orifices were completely cleared out by the violence of the explosions and were afterwards filled up by the falling in of the walls of the higher parts now removed by denudation, which consisted of Torridon Sandstone and shale.[307]
[Footnote 307: It is on one of these neck-like patches of breccia that Brochel Castle stands, of which Macculloch gave so sensational a picture in one of the plates of his Western Isles.]
Further research may detect at still greater distances from the basalt-plateaux ancient volcanic necks that might, with more or less probability, be referred to the Tertiary period. As an instance of this kind, I refer to the neck at Bunowen, County Galway, recently described by Mr. M'Henry and Professor Sollas. Though so remote from the Tertiary basalt-plateaux, the rock of this boss is an olivine-basalt presenting a close resemblance to some of the rocks of Antrim.[308]
[Footnote 308: Trans. Roy. Irish Acad., 1896].
As a final illustration of Tertiary volcanic vents I will now describe the Faroe group already alluded to (vol. i. p. 63, vol. ii. p. 256). It was almost by a kind of happy accident that these vents were discovered. Noticing at a distance of a mile or more from the deck of a steam-yacht that the base of the great basalt cliffs on the west side of Stromö were varied by what looked like agglomerate, I steamed inshore, and was delighted to find, as the vessel drew near to the cliff, that the agglomerate assumed definite boundaries and occurred in several distinct patches, until at last it presented the unmistakable outlines of a group of vents underlying and overspread by the bedded basalts of the plateau. Favoured by an unusually calm sea, I was enabled to boat into every nook and round every buttress and islet of this part of the coast-line.
The basalt-plateau here presents to the western ocean a nearly vertical escarpment which must reach a height of at least 1000 feet (see Fig. 328), and displays a magnificent section of the bedded lavas. The lower part of this section shows chiefly the banded structure already described, the layers of different consistency being etched out by the weather in such a way as to give them the look of stratified rocks. In the upper part of the precipice columnar and jointed or prismatic sheets are more common, but the most prominent band is the great sill, to which further reference will be made in the next Chapter.
In the course of the gradual retreat of the cliff, as the waves tunnel its base, and slice after slice is detached from its vertical front, a group of at least five small vents has been uncovered lying along a nearly north and south line. Of two of these a segment remains still on the cliff-wall and passes under the basalts; the others have been dissected and half cut away from the cliff, while groups of stacks and rocky islets of agglomerate may mark the position of others almost effaced. The horizontal distance within which the vents are crowded is probably less than half a mile, but the lofty proportions of the precipice tend to lead the eye to underestimate both heights and distances.
The agglomerate is a thoroughly volcanic rock, consisting of large and small blocks of various basalts, among which large slags are specially conspicuous, the whole being wrapped in a granular matrix of comminuted volcanic detritus. The arrangement of this material is best seen in the fourth vent (Figs. 310 and 311). In this characteristic volcanic neck (b in Fig. 311) the boundary walls, as laid bare on the face of the precipice, are vertical, and are formed of the truncated ends of the banded lavas (a a) which have been blown out at the time of the formation of the orifice. The visible diameter of the vent was roughly estimated by me to be about 100 yards. No appreciable alteration was observed in the ends of the lavas next the vent.
The agglomerate is coarsest in the centre, where huge blocks of slaggy lava lie imbedded in the amorphous mass of compacted debris. On either side of this structureless central portion the agglomerate is distinctly stratified from the walls towards the middle, at angles of 30° to 35°. Even from a distance it can be observed that the upper limit of the agglomerate is saucer-shaped, the sloping sides of the depression dipping towards the centre of the neck at about the same angle as the rudely-stratified agglomerate underneath. From the bottom of this basin to the sea-level may be a vertical distance of some 30 yards. The basin itself has been filled up by three successive flows of basalt, of which the first (c) has merely overflowed the bottom, the second (d), entering from the northern rim of the basin, extends across to the southern slope, while the third (e), also flowing from the north, has filled up the remainder of the hollow and extended completely across it. The next succeeding lava (f) stretched over the site in such a way as to bury it entirely, and to provide a level floor for the piling up of the succeeding sheets of basalt.
The second vent, which is represented in Fig 312, exhibits the same features, but with some additional points of interest. It measures roughly about 20 yards in diameter at the sea-level, rises through the same group of banded basalt (a a), and is filled with a similar agglomerate (b). Its more northerly wall is now coincident with a line of fault (h) which ascends the cliff, and probably marks some subsidence after the eruptions had ceased. The southern wall shows that a dyke of basalt (g) has risen between the agglomerate and the banded basalts, and that a second dyke (g´) traverses the latter at a distance of a few feet. In this instance, also, the upper surface of the agglomerate forms a cup-shaped depression which has been filled in by two successive streams of lava (c, d). Among the succeeding lavas (e) the prominent sill (f) has been intruded, to which further allusion is made on p. 323.
These necks are obviously volcanic vents belonging to the time of the basaltic eruptions. They have been drilled through the basalts of the lower part of the cliff, but have been buried under those of the central and higher parts. The arrangement of their component materials in rude beds dipping towards the middle of each vent shows that the ejected dust and stones must have fallen back into the orifice so as to be rudely stratified towards the centre of the chimney, which was finally closed by its own last discharges of coarse detritus. The saucer-shaped upper limit of the agglomerate seems to indicate, as has been suggested above in the case of the Portree volcano, that after the eruptions ceased each vent remained as a hollow or maar on the surface of the lava-fields. And the manner in which they are filled with successive sheets of basalt shows that in course of time other eruptions from neighbouring orifices gave forth streams of lava which, in flowing over the volcanic fields, eventually buried and obliterated each of the vents.
In the destruction of the precipice some of the vents have been so much cut away that only a small part of the wall is left, with a portion of the agglomerate adhering to it. The third neck, for instance, affords the section represented in Fig. 313, where the horizontal sheets of basalt (a) have still a kind of thick pellicle of the volcanic detritus (b) adhering to what must have been part of the side of the orifice of eruption. The waves have cut out a cave at the base, so that we can, by boat, get behind the agglomerate and see the margin of the volcanic funnel in the roof overhead.
The fragment of geological history so picturesquely laid bare on the Stromö cliffs presents a significant illustration of what seems to have been a frequent, if not the normal type of volcanic vent in the Tertiary basalt-plateaux. By the fortunate accident that denudation has not proceeded too far, we are able to observe the original tops of at least two of the vents, and to see how such volcanic orifices, which were doubtless abundant all over these plateaux, came to be entombed under the ever-increasing pile of accumulating basalt.
There is still one feature of interest in these cliff-sections which deserves notice here. Every geologist who has studied the composition of the basalt-plateaux has remarked the comparatively insignificant part played by tuffs in these volcanic accumulations. Hundreds of feet of successive basalt-sheets may often be examined without the discovery of any intercalation of fragmental materials, and even where such intercalations do occur they are for the most part quite thin and extremely local. I found it impossible to scale the precipice for the purpose of ascertaining whether around the Stromö vents, and connected with them, there might not be some beds of tuff interstratified between the basalts. If such beds exist, they can only be of trifling thickness and extent. Here, then, are examples of once active vents, the funnels of which are still choked up with coarse fragmentary ejections, yet from which little or no discharge of ashes and stones took place over the surrounding ground. They seem to have been left as crater-like hollows on the bare surface of the lava-fields.
XLII. THE BASIC SILLS OF THE BASALT-PLATEAUX
We have now followed the distribution of the basalt-plateaux, the arrangement of their component materials which were erupted at the surface, and the character of the dyke-fissures and vents from which these materials were ejected. But there remains to be considered an extensive series of rocks which display some of the underground phenomena of the Tertiary volcanoes. The injection of many basaltic sheets had been clearly enforced by Macculloch. In 1871 I pointed out that at different horizons in the plateau-basalts, but especially at their base and among the stratified rocks underneath them, sheets of basalt and dolerite occur which, though lying parallel with the stratification of the volcanic series, are not truly bedded, but intrusive, and therefore younger than the rocks between which they lie.[309] The non-recognition of their true nature had led to their being regarded as proofs of volcanic intercalations in the Jurassic series of Scotland. There is, however, no trace of the true interstratification of a volcanic band in that series, every apparent example being due to the way in which intrusive sheets simulate the characters of contemporaneous flows.
[Footnote 309: Quart. Jour. Geol. Soc. xxvii. (1871), p. 296.]
If such sheets had been met with only at one or two localities, we might regard them as due to some mere local accident of structure in the overlying crust through which the erupted material had to make its way. But when we find them everywhere from the cliffs of Antrim to the far headlands of Skye and the Shiant Isles, and see them reappear among the Faroe Islands, it is obvious that, like those of Palæozoic time, they must be due to some general cause, and that they contain the record of a special period or phase in the building up of the Tertiary volcanic tablelands. I will first describe some typical examples of them from different districts, and then discuss their probable relations with the other portions of the plateaux.
i. ANTRIM
First to be examined, and now most familiar to geologists, are the remarkable sheets that underlie the plateau of Antrim, and project at various parts of the picturesque line of coast between Portrush and Fair Head. From the shore at Portrush, as I have already remarked, came the evidence that was supposed to prove basalt to be a rock of aqueous origin, inasmuch as shells were obtained there from what was believed to be basalt. The long controversy to which this supposed discovery gave rise is one of the most curious in the history of geology.[310] It continued even after the illustrious Playfair had shown that the pretended basalt was in reality highly indurated shale, and hence that, instead of furnishing proof of the aqueous formation of basalt, the Portrush sections only contributed another strong confirmation of the Huttonian theory, which claimed basalt to be a rock of igneous origin.
[Footnote 310: For an excellent summary of this controversy and an epitome of the descriptions of the Portrush section, see the Report on the Geology of Londonderry, etc. (Mem. Geol. Survey), by J. E. Portlock (1843), p. 37.]
It is now well known that the rock which yielded the fossils is a Liassic shale, that it is traversed by several sheets of eruptive rock, and that by contact-metamorphism it has been changed into a highly indurated substance, breaking with a splintery, conchoidal fracture, but still retaining its ammonites and other fossils. The eruptive material is a coarse, distinctly crystalline dolerite, in some parts of which the augite, penetrated by lath-shaped crystals of plagioclase, is remarkably fresh, while the olivine has begun to show the serpentinous change along its cracks.[311] This rock has been thrust between the bedding planes of the shales, but also breaks across them, and occurs in several sheets, though these may all be portions of one subterranean mass. Some of the sheets are only a few inches thick, and might at first be mistaken for sedimentary alternations in the shale. But their mode of weathering soon enables the observer readily to distinguish them. It is to be noticed that these thin layers of eruptive material assume a fine grain, and resemble the ordinary dykes of the district. This closeness of texture, as Griffith long ago pointed out,[312] is also to be noticed along the marginal portions of the thicker sheets where they lie upon or are covered by the shales. But away from the surfaces of contact, the rock assumes a coarser grain, insomuch that in its thickest mass it presents crystals measuring sometimes an inch in length, and then externally resembles a gabbro. A more curious structure is shown in one of these coarsely crystalline portions by the occurrence of a band a few inches broad which is strongly amygdaloidal, the cells, sometimes three inches or more in diameter, being filled with zeolites.[313] The general dip of the shales and of the intrusive sheets which have been injected between them is towards the east. From underneath them a thick mass of dolerite rises up to form the long promontory that here projects northwards from the coast-line, and is prolonged seawards in the chain of the Skerries.
[Footnote 311: Dr. F. Hatch, Explanation of Sheets 7 and 8, Geol. Survey of Ireland, p. 40.]
[Footnote 312: "Address to Geological Society of Dublin, 1835," p. 13, Jour. Geol. Soc. Dublin, vol. i. The varieties of the Portrush rock were described by the late Dr. Oldham, in Portlock's Report on the Geology of Londonderry, p. 150; see also the same work for Portlock's own remarks, p. 97.]
[Footnote 313: For a list of the minerals in this rock, see Oldham, op. cit. p. 151.]
An interesting feature of the Portrush sections is the clear way in which they exhibit the phenomena of "segregation-veins"--so characteristic of the thicker and more coarsely crystalline sills. These veins or seams here differ from the rest of the rock mainly in the much larger size and more definitely crystalline form of their component minerals. Though sharply defined, when looked at from a little distance, they are found on closer inspection to shade into the surrounding rock by a complete interlacing of crystals. On the shore, they can be seen to lie, on the whole, parallel with the bedding of the sheets in which they occur, but without rigidly following it, since they undulate and even ramify. A good section across their dip has been exposed in a quarry near the end of the promontory, and shows that they are considerably less regular than the plan of their outcrop on the shore would have led us to anticipate. The accompanying drawing (Fig. 314) represents the veins laid bare on a face of rock nine feet in length by five feet in height. It will be seen that while there is a general tendency to conform to the dip-slope, which is here from right to left, the seams or layers unite into a large rudely-bedded mass, which sends out processes at different angles. The peculiar aggregation of minerals which distinguishes such veins is perhaps best seen at Fair Head, and I reserve for the description of that locality what I have to say on the subject, only remarking with regard to the Portrush rock that the felspar shows a disposition to collect in the centre of the veins with the augite and the other dark minerals at the outer margins.
The contact-metamorphism at this locality is of more historical interest in connection with the progress of geological theory than of scientific importance. It consists mainly in an intense induration of the argillaceous strata. These pass here from their usual condition of fissile, laminar, dull, dark shales into an exceedingly compact, black, flinty substance, which in its fracture, colour and hardness reminds one of Lydian stone. Yet the ammonites and other organic remains have not been destroyed. They are preserved in pyrites.
Of all the examples of Tertiary sills in Britain few are more imposing than that of the noble range of precipices which form the promontory of Fair Head. Leaving out of account the minor masses of eruptive rock which occur underneath it, we find the main sheet to extend along the coast for nearly four miles, to rise to a height of 636 feet above the sea, and to attain a maximum thickness of 250 feet. This enormous bed dies out rapidly both to the east and west, and seems also to thin away inland. Seen from the north, it stands upon a talus of blocks as a sheer vertical wall, 250 feet high, and the rude prisms into which it is divided are continuous from top to bottom (Fig. 315). So regular is this prismatic structure, and so much does it recall the more minute columnar grouping of the bedded basalts, that at a little distance we can hardly realize the true scale of the structure. It is only when we stand at the base of the cliff or scramble down its one accessible gully, the "Grey Man's Path," that we appreciate how long and thick each of the prisms actually is (Fig. 316). It may here be remarked that this regular prismatic jointing is one of the distinguishing features of the large sills, and serves to mark them off from the bedded basalts, even when these have assumed a columnar structure. The prisms are much larger than the basalt-columns, and never display the irregular starch-like arrangement so common among the plateau-basalts.
The rock composing this magnificent sheet is a coarsely crystalline, ophitic, olivine-dolerite.[314] The same diminution of the component crystals, which is so marked along the margins of the eruptive masses at Portrush, is strikingly exhibited at the borders of the Fair Head sill. For about 18 or 20 inches upward from the bottom, where the bed rests on the black, Carboniferous shales, the dolerite is dark and finely crystalline, weathering spheroidally in the usual manner. But immediately above that bottom layer of closer grain, the normal coarsely crystalline texture rapidly supervenes. A similar closeness of grain is observable at the surfaces of contact where the sheet splits up on its western border.
[Footnote 314: Professor Judd has described what he calls a "glomero-porphyritic structure" in this rock (_Quart. Journ. Geol. Soc._ xlii. (1886), p. 71).]
Nowhere, so far as I know, can the phenomena of "segregation-veins" be so instructively studied as along the abundant exposures of this great sheet. The veins are most conspicuous where the rock occurs in thickest mass. They vary up to three or four feet in thickness, and, as at Portrush and elsewhere, lie on the whole parallel to the upper and under surfaces of the sheet. An erroneous impression may be conveyed by the term "veins" applied to them. They are quite as much layers, parallel on the whole with the bedding of the sheet, yet not adhering rigidly to one plane, but passing across here and there from one horizon to another. That they are not due to any long subsequent protrusion of younger material through the main sheet is made manifest by the thorough interlocking of their component crystals with those of the body of the rock in which they lie. The material that fills these veins has obviously been introduced into them while there was still some freedom of movement among the crystals of the surrounding rock, which must thus have been still not quite consolidated and therefore intensely hot. Both crystallized slowly, and in so doing their component minerals dovetailed with each other. The constituents of the veins consist of an exceedingly coarse aggregate of crystals, or rather of crystalline lumps of the same minerals that constitute the general mass of the rock, the felspar and augite showing the ophitic intergrowth of the main rock, but on a far larger scale. Some of the pieces of augite measure two inches or more in diameter. The conditions under which these veins were produced must have differed in some essential respects from those that prevailed during the formation of the fine-grained, highly siliceous veins already described as occurring in some dykes and sills.
This great Fair Head sill lies upon Carboniferous strata, but that it is to be classed with the Tertiary volcanic series is, I think, demonstrated by its relations to the Chalk at its eastern end. It has there broken through that rock, and converted it for a short distance into a white, granular marble. But it is at the western side that the most interesting sections occur to show the truly intrusive nature of the mass. The rock there splits up into about a dozen sheets, which, keeping generally parallel with each other, have forced their way between and partly across the bedding planes of the Carboniferous shales (Fig. 317). In this way the huge, unbroken mass, 250 feet thick, subdivides itself and disappears in a few hundred yards, though it continues a little further inland, and approaches the shore again half a mile to the south-west. Further evidence of the intrusive nature of this rock may be observed along the base of the precipice, where at least one sheet 70 feet thick diverges from the main mass and runs eastwards between the Carboniferous shales (Fig. 315). At the contact with the eruptive rock the shales are everywhere much indurated.
ii. SKYE
All through the Inner Hebrides the base of the basalt-plateaux presents abundant examples of sills. The general parallelism of these intrusive sheets to the bedding of the Jurassic strata among which they lie has been above referred to as having given rise to the erroneous conclusion that in Skye and elsewhere the basalts are interstratified with Jurassic rocks, and are consequently of Jurassic age. It was Macculloch who first described and figured in detail the proofs of their intrusive nature. His well-known sections in plate xvii. of the illustrations to his work on the Western Islands have been repeatedly copied, and have served as typical figures of intrusive igneous rocks.
Nowhere in North-Western Europe can the phenomena of sills be studied so fully and with such exuberance and variety of detail as in the island of Skye and its surrounding islets. On the western coast the greater subsidence of the basaltic plateau has for the most part submerged the platform of intrusive sheets, though wherever the base of the bedded lavas is brought up to the surface the accompanying sills are exposed to view. The east coast of the island has been classic ground for this part of volcanic geology since it supplied the materials for Macculloch's descriptions and diagrams. From the mouth of Loch Sligachan to Rudha Hunish, at the north end of Skye, a series of sills may be traced, sometimes crowning the cliffs as a columnar mural escarpment, sometimes burrowing in endless veins and threads through the Jurassic rocks. The horizontal distance to which this continuous band of sills extends in Skye is not far short of 30 miles. But it stretches beyond the limits of the island. It forms the group of islets which prolongs the geological structure and topographical features of Trotternish for 4 miles further to the north-west. It reappears 10 miles still further on in the Shiant Isles. Thus its total visible length is fully 40 miles, or if we include some outlying sills near the Point of Sleat, to be afterwards described, it extends over a distance of not less than 60 miles. From the last outlier in Skye to the sills of the Isle of Eigg is a distance of only 8 miles, thence to those of Ardnamurchan 17 miles, and to those of the south coast of Mull 25 miles. Thus this platform of intrusive sheets of the Inner Hebrides can be interruptedly followed for a space of not less than 110 miles.
Though none of the sills in Skye itself attain the dimensions of the Fair Head sheet, they present a greater variety of rock and of geological structure than is to be found in Antrim. They are specially developed at the base of the thick, overlying, basalt-plateau--a platform on which such a prodigious quantity of eruptive material has been injected. Part of this material consists of basic rocks in the form of dykes, veins, or sills; part of it is included in the intermediate and acid groups, and comprises veins, sheets, and bosses of granitoid, felsitic, rhyolitic, trachytic, and pitchstone rocks. One of the peculiarities of the Skye sills is the occurrence among them of compound examples, where sheets of basic and acid material have been injected along the same general platform. These will be more specially referred to in Chapter xlviii. With regard to the basic sills (dolerites, basalts, etc.), I would remark that while in Western Scotland the Antrim type of short, thick intrusions, or laccolites, is also found, the vast majority of the sheets are much thinner, more persistent, and less easily distinguishable from the bedded basalts.
In describing the sills of Skye I shall take first those of the eastern and then those of the western side of the island. Along the east coast, from Loch Sligachan to the most northerly headlands and islets the sills play a notable part in the scenery, inasmuch as they cap the great sea-cliff of Trotternish and run as a line of ridges parallel to the trend of the coast, while the plateau-basalts rise above them further inland as a lofty escarpment, which includes the picturesque landslips of the Storr Rock and Quiraing (Figs. 318, 319). Beneath the thick sills, the Jurassic sandstones form a range of pale yellow precipices, along which many thinner sheets of eruptive material have been intruded. As Macculloch well showed, many of these sheets, if seen only at one point, might readily be taken for regularly interstratified beds, but perhaps only a few yards distant they may be found to break across the strata and to resume their course on a different level.
The sills of this Trotternish coast may be distinguished even at some distance from the bedded basalts by the regular prismatic jointing, already referred to, and by their frequently greater thickness, while on closer inspection they are characterized by their much coarser texture. They are generally somewhat largely crystalline ophitic dolerites, gabbros or diabases, and exhibit the persistent uniformity of composition and structure so characteristic of intrusive sheets and dykes. These characters are well exhibited in the Kilt Rock, a columnar sill capping the cliffs to the south of Loch Staffin (Fig. 319).
These massive sills are prolonged in a series of picturesque flat tabular islets beyond the most northerly headlands of Skye. They probably continue northwards under the sea at least 12 miles further, for sills of the same type rise there in the singularly striking group of the Shiant Isles (Fig. 320). These lonely islets, extending in an east and west direction for about three miles, display in great perfection most of the chief characters of the Skye sills. They are especially noteworthy for including the thickest intrusive sheet and the noblest columnar cliff in the whole of the Tertiary volcanic series of Britain. The larger of the two chief islands consists of two masses of rock connected by a strip of shingle-beach, and having a united length from north to south of about two miles. The northern half, or Garbh Eilean, presents towards the north a sheer precipice 500 feet high. This magnificent face of rock consists of one single sill, but as its original upper limit has been removed by denudation and its base, where it is thickest, is concealed under the sea, the sill may exceed 500 feet in thickness. The rock has the usual prismatic structure, which imparts to it an impressive appearance of regularity. The columns retain their individuality to a great height, and though none of them perhaps can be followed from base to crest of the cliff, many of them are evidently at least 300 or 400 feet long.
Macculloch, who gave the first geological description of the Shiant Isles, showed the intrusive nature of the igneous rocks, and described the remarkable globular or botryoidal structure of the Jurassic shales between which they have been injected.[315] Professor Heddle has published a brief account of the geology of the islands.[316] Professor Judd visited the group and brought away a series of specimens of their eruptive rocks, which he found to include basic and ultra-basic varieties.[317]
[Footnote 315: Western Islands, vol. i. p. 441.]
[Footnote 316: Trans. Norfolk Nat. Hist. Soc. vol. iii. (1880) p. 61.]
[Footnote 317: Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 677, and xli. (1885) p. 393. My description in the text is the result of three successive visits to the islands.]
In Garbh Eilean, where the thickest mass of erupted material presents itself, at least three sills may be observed. Some low reefs that run parallel with the northern coast of the island consist of coarse ophitic gabbro in two or more sheets which have been intruded between the Jurassic shales. Above these strata comes the great columnar sill, its base gradually sinking towards the west until it passes under the sea, and the vertical columns then plunge abruptly into the water. The rock of which this massive sill consists is another large-grained gabbro or dolerite, with an ophitic structure. Owing to the form of the ground it cannot be so satisfactorily examined as the neighbouring island of Eilean Mhuire, which, though less lofty and rather smaller than Garbh Eilean, affords a succession of admirable and easily examined sections along its precipitous shores.
Professor Judd found that while the rocks are mainly ophitic gabbros and dolerites, they include such highly basic compounds as dunite. An examination of the Eilean Mhuire cliffs enables the observer to ascertain that the sills display considerable variety in texture and in the character and arrangement of their component minerals. They are marked by a persistent, more or less distinct disposition in rude beds, and these again often display a banding of their constituents in lines parallel with the general bedding. Some of these bands are largely felspathic, and are thus paler in colour. Others, where the ferro-magnesian minerals and ores are more specially aggregated, are dark in colour. In some layers the long black prisms of augite are ranged in a general parallelism with the banding.
A specimen selected as typical of the ordinary coarse-grained amorphous rock was sliced and placed in Mr. Harker's for microscopic examination, and he has supplied the following observations regarding it: "The gabbro from Eilean Mhuire [7110] is a crystalline rock showing to the eye lustrous black augites, half an inch long, and (predominating) felspar. The microscope reveals, in addition, irregular grains of black iron-ore and little hexagonal prisms of apatite. No olivine is to be detected. As regards structure, the augite has tended to crystallise out in advance of the felspar, but this relation is not constant.
"The augite is of a light-brown tint in slices, and has an unusual kind of pleochroism. The colour for vibrations parallel to the [Greek: b]-axis is of the purplish-brown tone seen in some soda-bearing augites; parallel to [Greek: g] and [Greek: a] it has a yellow or citron tint. The colour and pleochroism are more marked in the interior of a crystal than towards the margin, but some crystals pass at the margin into a slightly pleochroic, pale-green, recalling ægerine-augite. The felspar tends to build elongated crystals. It is a rather finely lamellated labradorite, sometimes showing pericline- as well as albite-lamellae."
Another specimen from one of the black bands in the same island, with a linear arrangement of its component minerals, is thus described by the same petrographer: "This rock [7111] is of darker appearance than the preceding, and contains abundant black iron-ore, besides some pyrites. It also differs in having a marked parallel disposition of its crystals.
"Except for the greater prominence of large irregular grains of iron-ore, this rock under the microscope closely resembles the last described, the parallel structure not being conspicuous in the slice. The augite has the peculiar colour and pleochroism already noted, and the felspar is of the same kind as before."
I did not succeed in finding in place any bands of dunite, but this basic material probably occurs at the base of some of the sills where it has segregated from the rest of the mass, like the picrite at the bottom of the Bathgate diabase.
The amount of contact-metamorphism effected even by such thick sills as those of Trotternish and Shiant is much less than might be expected. It seldom goes beyond a mere induration of the strata for a few yards, often only for a few inches from the surface of junction. In the Shiant Isles, however, the shales between the sills have undergone a more remarkable alteration. They have not only been greatly indurated, but have acquired the globular or botryoidal structure so fully described by Macculloch. The spheroidal aggregates vary from not more than a line to more than half an inch in diameter, and appear on the surface as dark, irregularly grouped, pea-like aggregates. This structure is perhaps best developed immediately under the thick sill on the west side of Eilean Mhuire.
The massive sills are not the only evidence of the injection of igneous material on the Shiant Isles. The sill, or more probably group of sills, forming Eilean Mhuire is traversed by a number of sheets of basalt varying from only two or three inches to 20 feet in thickness. These black fine-grained rocks invariably present chilled selvages next the coarse gabbro, and though they have been on the whole injected parallel to the general bedding or banding, they here and there break across it as veins. The most important of these later intrusions forms a columnar sill on the eastern side of the island, and can be followed for several hundred yards. It consists of a dark finely crystalline olivine-basalt, which towards the margin assumes a dense black texture. Under the microscope Mr. Harker found a thin slice of this rock to be "an olivine-basalt of semi-ophitic, semi-granulitic structure [7112]. The olivine is mostly fresh, but part of it is converted into a yellowish-brown pseudomorph like iddingsite. Magnetite occurs chiefly in imperfect octohedra. The felspar is in little lath-shaped sections, many of which are finely striated, and give extinction-angles indicating a labradorite. The augite, light brown in the slice, never has crystal-boundaries, and often enwraps the felspars."
The narrow veins are composed of a much closer-grained basalt in which a few scattered felspars are visible. Mr. Harker remarks, with regard to a thin slice of one of these rocks [7113], that "the microscope shows this, too, to be an olivine-basalt. The porphyritic felspars are twinned on the Carlsbad and albite laws. Olivine and pseudomorphs after it are well represented. Magnetite is only sparingly present. The general mass of the rock consists of very small striated prisms of labradorite, granules of augite, and interstitial matter which must be partly glassy."
This is perhaps the most striking of all the examples known to me where an older sill has been split open to receive a subsequent injection of molten material. The Eilean Mhuire gabbro must be at least 200 feet thick, and it not impossibly passes under the still thicker pile of Garbh Eilean. Yet it has been horizontally ruptured near its base, and into the rent thus produced another mass of molten matter has been thrust. This subject will be again referred to in connection with another remarkable example on the west coast of Skye.
In contrast to such enormous thicknesses of intrusive material as those of Trotternish and the Shiant Isles, instances may be culled from the same belt of sills where the molten rock has been injected in thin leaves and mere threads into the Jurassic sandstones and shales, or into the shales and coals intercalated among the plateau-basalts. Thus, on the cliff immediately to the north of Ach na Hannait, between Loch Sligachan and Portree Bay, the section may be seen which is represented in Fig. 321. At the base lies a vesicular dolerite with a slaggy upper surface (a). Next comes a zone of sedimentary material about five or six feet thick, the lower portion consisting of an impure coal, which passes towards the right hand into brown and grey carbonaceous shale with plant-remains (b). This coaly layer has been already alluded to as probably lying on the same horizon with the coal of Portree (p. 288). Traced northward, it is found to have a bed of fine tuff beneath it, and sometimes a volcanic breccia or conglomerate. It fills up rents in the underlying slaggy lava, and was undoubtedly deposited upon the cooled surface of that rock. Immediately above this lower band the black carbonaceous shale which follows has been invaded by an extraordinary number of thin cakes or sills and also by veins or threads of basalt. For a thickness of two or three feet the band (d) consists mainly of these intrusions, which, in the form of a fine grey basalt, vary from less than an inch to three or four inches in thickness. They are separated by thin partings of coaly shale, and as they tend to break up into detached nodule-like portions, especially towards the right hand of the section, they might, on casual inspection, be easily mistaken for nodules in the dark shales. Somewhat later in the time of intrusion are veins of basalt which, as at c, break across the nodular sills, and sometimes expand into thicker beds (c´).
I have never seen such a congeries of minute sills among the Tertiary basalt-plateaux as that here exhibited. In a space of about three feet of vertical height there must be more than a dozen of roughly parallel leaves of intrusive rock. Veins (e) run up from the chief band of eruptive material into the overlying finely vesicular basalt (f). The dyke (g) is probably the youngest rock in the section.
The more general and extensive submergence of the base of the basalt-plateau on the west side of Skye has for the most part carried the platform of sills below sea-level, so that it is only exceptionally where, owing to local irregularities, that base has been brought up to the air, that the intrusive sheets show themselves. Yet the persistence of the platform on that side is indicated by its extension even as far as the southern promontory of the island.
The Trotternish type of sill extends down the west coast under the headlands of Duirinish. Thus at the mouth of Dunvegan Loch, where the underlying Jurassic platform has been ridged up above the surface of the sea, it has carried with it the marked sill which forms the islets of Mingay and Clett that lie as a protecting breakwater across the entrance of the inlet. The intrusive rock rests on shell-limestones full of oysters (Ostrea hebridica), and referable to the Loch Staffin group of the Great Oolite Series. This sill, when observed from a little distance, presents the usual regularly prismatic or columnar structure so well developed among the Trotternish examples, but on a closer view shows this structure less distinctly. It is an olivine-dolerite of medium and fine texture, which in thin slices displays under the microscope a distinctly ophitic structure, the abundant light-brown augite enclosing the striated felspars. Its lowest portion, from three to seven or eight inches upward from the bottom, is much closer-textured than the rest of the rock and is finely amygdaloidal. Its vesicles are in many cases drawn out to a length of three or four inches, and the zeolites which now fill them look like parallel annelid tubes or stems of Lithostrotion. It is noteworthy also that the elongation of the vesicles has sometimes taken place at a right angle to the surface of contact with the underlying strata. But the most remarkable feature in this sill is the surface which it presents to the oyster-beds on which it rests. The fine-grained dark dolerite has there assumed the aspect of a sheet of iron-slag, with a smooth or wrinkled, twisted, ropy surface, which displays fine curving flow-lines. No one looking at a detached specimen of this surface would be ready to admit that it could possibly have come from anything but a true lava-stream that flowed out at the surface. The contours of a viscous lava are here precisely reproduced on the under surface of a massive sill.
A little further south, the promontory of Eist, forming the western breakwater of Moonen Bay, consists of an important sill or group of sills which has insinuated itself among shales, shell-limestones, and shaly sandstones, full of Ostrea hebridica, Cyrena aurata, etc., and belonging to the Loch Staffin group of the Great Oolite Series. The shore-cliff below the waterfall affords the section given in Fig. 322, illustrating the manner in which a thick intrusive sheet may sometimes give off thin veins from its mass. The rock attains on the Eist promontory a thickness of probably at least 100 feet, where it is thickest and undivided. But the two main sheets, or branches of one great sheet, on this peninsula have probably a united depth of more than 300 feet. Landwards the rock splits up and encloses cakes of the Jurassic strata. It possesses the usual prismatic structure and doleritic composition. In Moonen Bay, as shown in Fig. 322, it presents a banded structure, marked especially by an alternation of lines of amygdales and layers of more compact and solid dolerite, with occasional enclosed cakes of baked shale or sandstone. Its upper surface is somewhat uneven, and from it are given off narrow, wavy, ribbon-like veins (d), from less than an inch to three inches or more in width, which keep in a general sense parallel to the top of the sill, but at a distance of a few inches or feet from it. The sill becomes as usual fine-grained towards the contact, the shales and sandstones being indurated and the limestone marmorized.
The next uprise of the base of the basalt-plateau on the west side of Skye lies about 25 miles to the south-east, where it emerges from the sea in the Sound of Soa (Fig. 323). A vast volcanic pile has there been heaped up on the Torridon sandstone, the whole of the thick Jurassic series, which is found in force only three miles distant in Strathaird, having been removed by denudation from this area before the beginning of the Tertiary volcanic period. The plateau-basalts rests on the upturned edges of the Torridonian sandstones and shales, and are accompanied as usual by their underlying network of intrusive rocks. It is hardly possible to exaggerate the wild confusion of sills, dykes and veins which have been injected among the rocks, at and on both sides of the unconformability. Endless sheets of basalt and dolerite have forced their way between the bedded basalts and the sandstones, while across the whole rise vast numbers of dykes and veins. Narrow, black, wavy ribbons of basic material cross many of these veins, while the later north-west dykes cut sharply through everything older than themselves. As a natural section for the study of the phenomena of intrusion in many of their most characteristic phases, I know no locality equal to the northern coast-line of the Sound of Soa, unless it be the cliffs of Ardnamurchan. But the Skye cliffs, though less imposing than those of the great Argyllshire headland, have this advantage, that instead of being exposed to the full roll of the open Atlantic, they form the margin of a comparatively sheltered strait, and can thus be conveniently examined.
Following still the western seaboard of Skye, we meet with other striking examples of sills at a distance of some eight miles in a straight line eastward where, between Lochs Slapin and Eishort, the prominent headland of Suisnish juts out into the sea. This promontory has long been known to geologists from the section of it given by Macculloch as an instance of the connection between overlying rocks and dykes. I have already alluded to it in that relation, and refer to it again as an example of one of the thicker intrusive sheets of the Inner Hebrides. Denudation has here also proceeded so far that the whole of the volcanic plateau has been stripped off, only some of the underlying sills being left, together with the platform of older rocks between which and the vanished basalts they were injected. Most of these sills consist of granophyres belonging to the acid group of rocks to be afterwards described. But basic sheets occur not infrequently interposed between the granophyres and the subjacent Lias, and sometimes even intercalated in the former rock. Though at first sight it might be thought that these sills had insinuated themselves after the eruption of the granophyre, and there are instances where this cannot be shown not to be the case, I have obtained so many proofs of the invasion of the basic by the acid rock that I have no doubt the former is, as a general rule, the older of the two.
The Suisnish headland exhibits the structure represented in Fig. 249. For about 300 feet above the sea-level the steep grassy slope shows outcrops of the dark, sandy shales and yellowish brown, shaly sandstones of the Lias which form the range of cliffs to the eastward. These gently inclined strata are cut through by many vertical basalt-dykes, some of which intersect each other, but among which by far the largest is the mass shown in the figure. This broad dyke consists of a dolerite or gabbro the largely crystalline texture of which marks it off at once from the others, which are of the usual dark, heavy, fine-grained type, with an occasional less basic and porphyritic variety. Traced up from the sea-margin, the dyke loses itself in a talus of blocks from the cliff above, so that its actual junction with the mural front of the sill cannot be seen. But that it joins that mass, with which it agrees in petrographical characters, hardly admits of question. The cliff consists of a thick sheet of coarsely crystalline dolerite or gabbro (d in Fig. 249), which in its general aspect at once recalls the rock of Fair Head. It varies considerably in texture, some parts of the mass are exceedingly coarse, like the Skye gabbros, and present a fibrous structure in their augite resembling that of the diallage in these rocks; other portions assume the compactness of basalt. A specimen of medium grain under the microscope shows the typical ophitic structure so generally found among the dolerites both of the plateaux and of the intrusive sheets. This sill must be about 200 feet thick, and like the rock at Fair Head is traversed from top to bottom by joints that divide it into prisms. It appears to bifurcate eastward, one portion running with a tolerably uniform thickness of a few feet as a prominent band at the top of the shales and sandstones, the other slanting upwards and gradually thinning away in the granophyre.
Towards its base, near the contact with the underlying shales, the rock as usual becomes finer grained, and the thin band just referred to resembles in texture one of the wider basalt-dykes. Westwards the rock can be followed round the top of the grassy slopes formed by the decay of the shales. Though concealed by intervals of moorland and peat, it is visible in the stream sections, and I think must be continuous, as a band only a few yards thick, round the northern side of the hills as far as Beinn Bhuidhe, where a similar sill makes a prominent crag. Its total area measures a mile and a quarter in length by half a mile in breadth. The granophyre which overlies it forms part of an interesting series of sheets which I have traced all the way from Suisnish to the braes above Skulamus.
Whether or not the whole sheet of basic rock is continuous, and whether it all proceeded from the great Suisnish dyke, cannot be confidently decided until the ground is mapped in detail, though from the great thickness of the sill at the dyke, its attenuation outwards from that centre and its uniformity of petrographical character, I am disposed to answer affirmatively. There is no other probable vent to be seen in the neighbourhood, unless a massive dyke that runs from Loch Fada north-westwards into Glen Boreraig can be so regarded.
Not far from the extreme southern point of Skye a singularly interesting example of a sill remains as a detached survival of the basaltic plateau and its accompaniments. In his map of Skye, Macculloch showed the position of this outlier, which he classed with the general "trap" formation of the island. The locality was visited by Professor Judd, who regarded the intrusive rock as a "phonolite"[318] In 1894, during an excursion with my colleague Mr. C. T. Clough, I had an opportunity of examining the rocks and collecting notes for the following account of them.
[Footnote 318: Quart. Jour. Geol. Soc. vol. xxxiv. (1878) p. 692.]
At Rudh' an Iasgaich, about two miles from the Point of Sleat, a small outlier of conglomerate lies on the edges of the Torridon Sandstone. This deposit has been correctly identified by Professor Judd with the similar strata which, in Skye and elsewhere on the west coast of Scotland, underlie the Liassic series. It is here about 10 or 12 feet thick, reddish and yellowish in colour, and distinctly calcareous. Its component pebbles consist largely of Cambrian (Durness) limestone, quartzite, and Torridon Sandstone--rocks which all occur in situ in Sleat. It may be compared with the limestone conglomerates of Strath and those which underlie the Lias at Heast on Loch Eishort.[319] That here, as elsewhere in this region, the basement conglomerate was followed by the rest of the Lias and Oolites may be inferred with some confidence from the copious development of the Jurassic series a few miles off, both to north and south. But the whole of this overlying succession of formations has here been swept away, and, but for the protection afforded by the eruptive rocks of Rudh' an Iasgaich, the conglomerate would likewise have disappeared.
[Footnote 319: Op. cit. vol. xiv. (1857), p. 9; vol. xliv. (1888), p. 71.]
Above the conglomeratic band lies a sheet of intrusive rock, which in one place has apparently cut it out, so as to rest directly upon the Torridon Sandstone (a, Fig. 324). The decay of the softer detrital rock underneath has caused the sill to break off in slices, which have left behind them a bold mural escarpment (b b).
The rock of this sill is a rather coarsely crystalline porphyritic olivine-dolerite, which towards the north attains a thickness of about 70 feet. It exhibits the usual prismatic jointing, but less perfectly than some of the Trotternish sills already referred to. Besides these vertical joints, it is also traversed by a system of horizontal divisional planes which, though somewhat irregular in their course, run, in a general sense, parallel to the upper and under surfaces of the sill.
It seems to have been along this transverse series of joints that a second sill (c), five or six feet thick, has been injected. The material of this younger intrusion is a black, finely crystalline dolerite or basalt, with rudely prismatic jointing. Its most striking feature, besides its regularity of position and persistency for several hundred yards as a platform along the shore, is the basalt-glass which marks both its under and upper surfaces of contact, and which is here developed upon a scale to which I have not met with an equal among the Tertiary sills of this country.
The selvage of glass appears as a black tar-like layer, varying from a mere film to two or three inches in thickness. It is found not only on the upper and under surfaces, but descends along abrupt step-like interruptions of the upper surface, a foot or more in height, as if the sill had been broken by a series of subsidences. The apparent fracture, however, is probably due to the irregularities of the passage forced for itself by the molten rock as it passed from one line of horizontal joint to another through the heart of the older sheet.
The exposed surface of black glass on the top of the younger sill exhibits long parallel lines, probably marking flow-structure, which are made conspicuous by a pale yellow ferruginous weathered crust. Portions of the larger intrusive sheet have been broken off and involved in the later rock. The younger sill disappears to the north, and is not found in the cliff of Rudha Chàrn nan Cearc, where the thick sill, lying once more on the band of conglomerate, forms a fine escarpment above the shore. Dykes of fine-grained basalt (d d) with compact chilled margins rise through both sills, together with veins which pursue a wavy upward path like strips of black ribbon.
This example, and that of the Shiant Isles already described, cannot but impress the observer with the prodigious force with which the material of the sills was injected. In these instances solid sheets of intrusive rock have subsequently been rent open, doubtless under a superincumbent pressure of many hundreds of feet of the terrestrial crust, and a new injection of molten magma has made its way into the rents thus caused. In each case, the position of the rents was obviously determined by structural lines in the older sills, but we are lost in astonishment at the energy required to split open, even along these lines, such solid crystalline masses as the thick sills, and to overcome the superincumbent pressure of so deep a pile of rock.
The isolation of a relic of the Tertiary sills on the west side of the promontory of Sleat presents some interesting problems to the mind of the geologist. The locality lies about midway between the basalt-plateau of Strathaird and that of Eigg, and some eight or nine miles in a direct line from either. The basalts cannot be proved to have once stretched continuously between Eigg and Strathaird, and to have covered this part of Sleat; but the position of the Sleat sills makes it probable that this continuation did formerly exist. The denudation of the West of Scotland since early Tertiary time has been so stupendous that I am prepared for almost any seemingly incredible evidence of its effects. There can hardly be any doubt, however, that the sills here described belong to the great platform of intrusive sheets, and that they were injected under a pile of Secondary strata, if not also of Tertiary basalts, which has here been entirely removed.
Reference may be made, in conclusion, to a not infrequent feature of the Skye sills. Like the dykes, they are often double or multiple, molten material having been successively injected along the same plane. The example just cited from the west side of Sleat illustrates one type of such compound sills. More frequently, however, the subsequent injections have been made along the floor or roof of the first sheet. Mr. Harker has found numerous cases of this structure in the Strath district. They are recognizable even from a distance by their terraced contours when seen in profile. They often vary considerably in thickness owing to the dying out or coming-in of their separate bands; while, on the other hand, single sills tend to maintain a uniform thickness for long distances, or taper away gradually. The compound arrangement of the basic sills is well brought out where acid material has been injected between the sheets, as will be more fully described in Chapter xlviii.
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.
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.
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.]
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.]
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.
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.
* * * * *
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.
XLIII. THE BOSSES AND SHEETS OF GABBRO
Petrography of the Rocks--Relations of the Gabbros to the other members of the Volcanic series--Description of the Gabbro districts--Skye
In singular contrast to the nearly flat basalts of the plateaux, another series of rocks rises high and abruptly above these tablelands into groups of dome-shaped, conical, spiry, and rugged hills. It is these heights which, more than any other feature, relieve the monotony of the wide areas of almost horizontal stratification so characteristic of the volcanic region of the north-west. Their geological structure and history are much less obvious than those of the bedded basalts. Their mountainous forms at once suggest a wholly different origin. Some portions of them have even been compared with the oldest or Archæan rocks.[325] That they are really portions of the Tertiary volcanic series, and that they reveal a wholly distinct phase in the history of volcanic action, is now frankly admitted. Whether we regard them from the petrographical or structural point of view, they naturally arrange themselves into two well-defined groups. Of these one consists of highly basic compounds, of which olivine-gabbro is the most prominent. The other comprises numerous varieties--granite, granophyre, felsite, quartz-porphyry, pitchstone and others--all of them being more or less decidedly acid, and some of them markedly so. For reasons which will appear in the sequel, the former group must be considered as the older of the two, and it will therefore be described first.
[Footnote 325: This was my own first impression, when I began, as a boy, to ramble among them. The remarkable resemblance of some parts of them to ancient gneisses will be afterwards dwelt upon. Macculloch had correctly grouped them with the other overlying rocks, and this conclusion was afterwards confirmed by Prof. Zirkel.]
i. PETROGRAPHY OF THE GABBRO AREAS
Since the publications of Macculloch, the occurrence of beautiful varieties of highly basic rocks among the igneous masses of the Western Isles has been familiar to geologists. They were named by him "hypersthene rock" and "augite rock,"[326] names which continued in use until 1871, when my friend Professor Zirkel published the results of his tour through the West of Scotland, and showed that the rocks in question were mostly true gabbros.[327] Since his observations were published some of these rocks have formed the subject of important papers by Professor Judd.[328]
[Footnote 326: Western Islands, vol. i. pp. 385, 484.]
[Footnote 327: Zeitschrift. Deutsch. Geol. Gesellsch. xxiii. (1871), p. 1.]
[Footnote 328: Quart. Jour. Geol. Soc. xli. (1885), p. 354; xlii. (1886), p. 49.]
The general petrographical characters of the gabbro areas of Western Scotland may be summarized as follows:--A very considerable variety of petrological structure and chemical composition is observable among the rocks. At the one end of the series are compounds of plagioclase and augite, which, though wanting in olivine, have the general structure and habit of dolerites. At the other end are mixtures wherein felspar is scarce or absent, and where olivine becomes the chief constituent. Between these two extremes are many intermediate grades, of which the most important are those containing the variety of augite known as diallage and also olivine. These are the olivine-gabbros, which form so marked a feature in the central parts of the great basic bosses. That some of these varieties of rock pass into each other cannot be doubted. Their distinctive composition and structure appear to have been largely determined by their position in the eruptive mass. The outer and thinner sheets are in great measure dolerites, with little or no olivine. Coarse gabbros are abundant in the inner portions. Rocks rich in olivine, however, occur at the outer and especially the lower part of the gabbro masses of Rum and in some parts of Skye. The following leading varieties may be enumerated:--
Dolerite.--This rock varies from an exceedingly close grain (when it approaches and graduates into basalt) up to a coarse granular crystalline texture, in which the component minerals are distinctly visible to the naked eye. An average sample is found to consist of plagioclase, usually lath-shaped, and crystals or grains of augite with or without olivine. Under the microscope, the different varieties are distinguished by the presence of more or less distinct ophitic structure, the felspar being enveloped in the augite. For the most part they are holocrystalline, but occasionally show traces of a glassy base. Ilmenite is not infrequent, with its characteristic turbid decomposition product (leucoxene). In other cases, the iron-ore is probably magnetite. Between the dolerites and gabbros no line of demarcation can be drawn in the field, nor can a much more satisfactory limitation be made even with the aid of the microscope. As a rule, the thickest and largest intrusive masses or bosses are gabbro, those of less size are dolerite, while the smallest (and sometimes the edges of the others) assume externally the aspect of basalts.
Gabbro.--Under this term I arrange, as proposed by Professor Judd, all the coarse-grained granitoid basic rocks of the region without reference to the variety of augite present in them. Under the microscope, they are found to be holocrystalline, but with a granitic or granulitic rather than an ophitic structure, though traces of the latter are by no means rare. To the naked eye their component minerals are usually recognizable. Professor Zirkel, from his examination of the Mull gabbros, believed them to consist of three parts of plagioclase, two parts of olivine, and one part of diallage.[329] Olivine, however, is not invariably present.[330] The pyroxene also does not always show the peculiar fibrous structure of diallage. Professor Judd, indeed, maintains that the diallagic form is due to a deep-seated process of alteration (schillerization), and that the same crystal may consist partly of ordinary augite and partly of diallage.[331] Ilmenite (with leucoxene), magnetite, apatite, biotite, and epidote are not infrequent constituents.
[Footnote 329: Zeitschr. Deutsch. Geol. Gesellsch. xxiii. (1871), p. 59.]
[Footnote 330: Professor Judd (Quart. Jour. Geol. Soc. xlii. p. 62) believes that originally all the gabbros contained olivine, and that where it is now absent, it has been altered into magnetite or serpentine. But in some coarse massive gabbros this mineral does not appear to have been an essential constituent. See op. cit. vol. l. p. 654.]
[Footnote 331: Op. cit. xli. In a later paper he insists on the gradation of the coarse granitoid varieties (gabbros) into holocrystalline compounds, where the felspar appears in lath-shapes with crystals or rounded grains of augite and olivine (dolerites), and thence into true basalts, magma-basalts, and tachylytes (op. cit. xlii. p. 62).]
In a recent study of the gabbros of the Cuillin Hills of Skye by Mr. J. J. H. Teall and myself, four characteristic types have been recognized.[332]
[Footnote 332: Quart. Journ. Geol. Soc. vol. 1. (1894), pp. 645-659, and Plates xiii. xxvi.-xxviii. See also Prof. Judd's paper, op. cit. (1886), p. 49.]
(1) Granulitic Gabbros.--These are dark, fine-grained rocks which externally resemble some of the altered basalts of the plateau-series. They occur in bands or sheets which, so far as can be made out, are the oldest portions of the whole gabbro mass. Under the microscope they are found to possess a finely granulitic structure, and to consist of grains of pyroxene (augite, but more usually with the inclusions characteristic of diallage and pseudo-hypersthene), and of felspar allied to labradorite, with green pseudomorphs agreeing in form and size with the pyroxene-grains, but made of minute prisms and fibres of green hornblende and a little chlorite.
(2) Banded Gabbros.--These are characterized by a remarkable arrangement in parallel bands of different mineral composition like the banding of ancient gneisses. This structure will be more particularly described in later pages. They are coarse-grained rocks composed of pyroxene, plagioclase, olivine and magnetite. But these minerals are not distributed equally through the mass. The pale bands contain much felspar; the dark bands are largely composed of the ferro-magnesian minerals and magnetite. The pyroxene, occurring as ordinary augite, not uncommonly shows a tendency to ophitic structure. The felspar, a variety closely allied to labradorite, occurs as grains, as irregular ophitic patches, and also in forms that give broad rectangular sections. Olivine in an unaltered condition has been detected by Mr. Teall in only one specimen, and he thinks that this mineral probably never played an important part in the original constitution of these rocks. Its rounded grains may be observed to have the other minerals moulded round them, whence it may be inferred to be of older consolidation. Magnetite is generally present, either in rounded grains or in large irregular masses. Though it occurs also in strings traversing the other minerals as a secondary product, it must undoubtedly have entered largely into the original composition of these rocks. It is found enclosing the augite grains and behaving like a groundmass between the felspars. Among the dark bands there occur narrow lenticular black layers ('schlieren') composed entirely of augite and iron-ore.
The extraordinary differences between the composition of the pale felspathic and the dark ultra-basic bands are well brought out in the following analyses by Mr. J. Hort Player, No. 1 being from a light-coloured band consisting mainly of labradorite with some augite, uralitic hornblende and magnetite; No. 2 from a dark band composed of augite, magnetite and labradorite; and No. 3 from a thin ultra-basic layer mainly formed of augite and magnetite. All these specimens were taken from the ridge of Druim an Eidhne, on the eastern side of the Cuillin Hills, Skye.[333]
I. II. III.
Silica 52·8 40·2 29·5 Titanic acid ·5 4·7 9·2 Alumina 17·8 9·5 3·8 Ferric oxide 1·2 9·7 17·8 Ferrous oxide 4·8 12·2 18·2 Ferric sulphide ··· ·4 ·4 Oxide of manganese ··· ·4 ·3 Lime 12·9 13·1 10·0 Magnesia 4·8 8·0 8·7 Soda 3·0 ·8 ·2 Potash ·5 ·2 ·1 Loss by ignition 1·2 ·5 1·0 ---- ---- ---- 99·5 99·7 99·2 ---- ---- ---- Spec. grav. 2·91 3·36 3·87 ==== ==== ====
[Footnote 333: Quart. Journ. Geol. Soc. vol. 1. (1894), p. 653. Banded structures have been recognized in many gabbros of different ages. See the references in this paper; also Mr. W. S. Bayley, Journ. Geol. Chicago, ii. (1895), p. 814, and vol. iii. p. 1.]
(3) Coarse-grained massive Gabbros.--These rocks, so abundant among the great basic bosses of the Inner Hebrides, are characterized by their coarse granitic structure, their component crystals being sometimes more than an inch long. They occur as sheets, veins and irregular masses traversing the varieties of gabbro already mentioned. They consist of the same minerals as the banded forms, and indeed are themselves sometimes banded. They are more uniform in composition than the typical banded gabbros, though showing also some variation in the relative proportions of their constituents. The specific gravity of three specimens was found to be 2·82, 2·97, and 3·06.
(4) Pale Gabbros of the Veins.--These occur abundantly as irregular branching veins, from less than an inch to several yards in width, and cross all the other varieties (Fig. 330[334]). Their whiteness on weathered surfaces makes them conspicuous by contrast with the dark brown or black hue of the rocks which they traverse, and shows at once that they must be poorer in bases than these. They are found on microscopic examination to consist of the same minerals as the more coarsely crystalline gabbros, but with a much greater abundance of the felspar. They contain also apatite, and hornblende appears to predominate in them over augite. They are to be distinguished from the pale veins that form apophyses from the intrusive granophyres.
[Footnote 334: Figs. 330, 336 and 337 are from photographs taken for the Geological Survey by Mr. R. Lunn.]
Troctolite (Forellenstein).--This beautiful variety of plagioclase-olivine rock occurs as a conspicuous feature on the east side of the gabbro-area of the island of Rum. It forms a sill on the side of the mountain Allival, in which the component minerals are drawn out parallel with the upper and under surfaces of the bed (Fig. 341). So marked is this flow-structure that hand-specimens might readily be taken at the first glance for ancient schistose limestone. "The felspathic ingredient (probably labradorite or anorthite) is white, and its lath-shaped crystals have ranged themselves with their long axes parallel to the line of flow. The olivine occurs in perfectly fresh grains, which in hand-specimens have a delicate green tint. Under the microscope they appear colourless, and are penetrated by the felspar prisms in ophitic intergrowth. There is a small quantity of a pale brownish augite, which not only occurs in wedge-shaped portions between the felspars, but also as a narrow zone round the olivines."[335] Considerable differences are visible in the development of the flow-structure, and with these there appear to be accompanying variations in the microscopic structure. Dr. Hatch, to whom I submitted my specimens, informed me that in one of them, where the flow-structure is so marked as to give a finely schistose aspect to the rock, "there is a larger proportion of augite, some of which exhibits a distinct diallagic striping; the olivine grains show no ophitic structure, but are sometimes completely embedded in the augite." To this remarkable flow-structure I shall again refer in connection with the light it throws on the bedded character of much of the gabbro bosses.
[Footnote 335: MS. of Dr. Hatch.]
Between the different basic intrusive igneous rocks of the Inner Hebrides, as Professor Judd has shown, there are many gradations according to the varying proportions of the chief component minerals. Thus from the olivine-gabbros, by the diminution or disappearance of the augite we get such rocks as troctolite; where the plagioclase diminishes or vanishes, we have different forms of picrite; where the olivine is left out, we come to compounds, like eucrite; while by the lessening or disappearance of the felspar and augite, we are led to ultra basic compounds, consisting in greatest part of olivine, like lherzolite, dunite and serpentine. To some of the features and probable origin of these chemical and mineralogical diversities in the same great eruptive mass further reference will be made in later pages.
ii. RELATIONS OF THE GABBROS TO THE OTHER MEMBERS OF THE VOLCANIC SERIES
Various opinions have been expressed regarding the connection between the amorphous eruptive rocks of the hill-groups and the level basalt-sheets of the plateaux. Jameson, though he landed at Rudh' an Dunain, in Skye, where this connection can readily be found, does not seem to have made any attempt to ascertain it. He noticed that the lower grounds were formed of basalt, and that the mountains "appeared to be wholly composed of syenite and hornblende rock, traversed by basalt veins."[336] Macculloch, in many passages of his Western Islands, alludes to the subject as one which he knew would interest geologists, but about which he felt that he could give no satisfactory information, and with characteristic verbiage he refers to the impossibility of determining boundaries, to the transition from one rock into another, to the inaccessible nature of the ground, to the almost insuperable obstacles that impede examination, to the distance from human habitation, and to the stormy climate,--a formidable list of barriers, in presence of which he leaves the relative position and age of the rocks unsettled.[337]
[Footnote 336: Mineralogical Travels (1813), vol. ii. p. 72.]
[Footnote 337: See his Western Islands, vol. i. pp. 368, 374, 385, 386. With much admiration for the insight and zeal, amounting almost to genius, which Macculloch displayed in his work among the Western Islands, at a time when, with poor maps and inadequate means of locomotion, geological surveying was a more difficult task than it is now, I have found it impossible to follow in his footsteps with his descriptions in hand, and not to wish that for his own fame he had been content to claim credit only for what he had seen. His actual achievements were enough to make the reputation of half a dozen good geologists. It was unfortunate that he did not realize how inexhaustible nature is, how impossible it is for one man to see and understand every fact even in the little corner of nature which he may claim to have explored. He seems to have had a morbid fear lest any one should afterwards discover something he had missed; he writes as if with the object of dissuading men from travelling over his ground, and he indeed tacitly lays claim to anything they may ascertain by averring that those who may follow him "will find a great deal that is not here described, although little that has not been examined" (p. 373). Principal Forbes long ago exposed this weak side of Macculloch and his work (Edin. New Phil. Journ. xl. 1846, p. 82).]
Von Oyenhausen and Von Dechen, who wrote so excellent an account of their visit to Skye, and who traced much of the boundary-line between the gabbros and the other mountainous eruptive masses ("syenite"), seem to have made no attempt to work out the connection between the former and the rest of the volcanic rocks.[338]
[Footnote 338: Karsten's Archiv, i. p. 99. They frankly admit that "the relation of the hypersthene rock to the other trap rocks was not ascertained."]
J. D. Forbes, in his able sketch of the Topography and Geology of the Cuchullin Hills, was the first to recognize the superposition of the "hypersthene rock" upon the "common trap rocks"--that is, the plateau-basalts. He was disposed to consider the "hypersthene mass as a vast bed, thinning out both ways, and inclined at a moderate angle towards the S.E."[339]
[Footnote 339: Edin. New Phil. Journ. xl. (1846), pp. 85, 86.]
Professor Judd regarded the bosses of basic and acid rocks that rise out of the bedded basalts as the basal cores of enormously denuded volcanic cones. He believed the granitoid rocks to have been first erupted, and that after a long interval the basic masses were forced through them, partly consolidating underneath and partly appearing at the surface as the plateau-basalts.[340] That the order of appearance of the several rocks has been exactly the reverse of this supposed sequence was fully established by me in the year 1888, and has since been amply confirmed.[341] Professor Zirkel recognized that the gabbros are a dependence of the basalts, that they overlie them, and that on the naked flanks of the mountains they are regularly bedded with them.[342]
[Footnote 340: Quart. Journ. Geol. Soc. xxx. (1874), p. 249.]
[Footnote 341: Trans. Roy. Soc. Edin. xxxv. (1888), pp. 122 et seq.; Quart. Journ. Geol. Soc. vol. 1. (1894), pp. 216, 645; vol. lii. (1896), p. 384, and Mr. Harker, ibid. p. 320.]
[Footnote 342: Zeitschrift. Deutsch. Geol. Gesellsch. xxiii. (1871), pp. 58, 92.]
Up to the time of the publication of my memoir in 1888 no one had traced out in more detail the actual boundaries of the several rocks on the ground, so as to obtain evidence of their true relations to each other as regards structure and age. Some of the numerous impediments recorded by Macculloch no doubt retarded the investigation. But, as Forbes so well pointed out, there is really no serious difficulty in determining the true structural connection of the amorphous rocks with each other and with the bedded basalts of the plateaux. I have ascertained them in each of the districts,[343] and have found that there cannot be the least doubt that the amorphous bosses, both basic and acid, are younger than the surrounding bedded basalts, and that the acid protrusions are on the whole younger than the basic, I shall now proceed to show how these conclusions are established by the evidence of each of the areas where the several kinds of rock occur.
[Footnote 343: In two of my excursions in Mull, and once in Skye, I was accompanied by my former colleague Mr. H. M. Cadell, who rendered me great assistance in mapping those regions.]
iii. DESCRIPTIONS OF THE SEVERAL GABBRO-DISTRICTS
1. The Gabbro of Skye
The largest, most picturesque, and to the geologist most important area of Tertiary gabbro in Britain, is that of Skye (Map. VI.). Though, like every other portion of the Tertiary volcanic districts, it has suffered enormous denudation, and has thereby been trenched to the very core, it reveals, more conspicuously and clearly than can be seen anywhere else, the relation of the gabbro to the bedded basalts on the one hand, and to the acid protrusions on the other. Its chief portion is that which rises into the group of the Cuillin Hills, which for blackness of hue, ruggedness of surface, jaggedness of crest, and general grimness of aspect, have certainly no rivals within the limits of the British Isles (Fig. 331). It has long been known to extend eastwards into Blath Bheinn (Blaven) and its immediate northern neighbours. There is, indeed, no break whatever between the rock of the Cuillins and that of the hills on the east side of Strath na Creitheach. In Strath More the gabbro is interrupted by the granitoid mass of the Red Hills. Patches of it, however, occur further to the east, even as far as the Sound of Scalpa.
If we throw out of account the invading granitoid rocks, and look upon the whole tract within which the gabbro occurs as originally one connected area, we find that it covered an elliptical space measuring about nine miles from south-west to north-east and six miles from north-west to south-east, and embracing at least 40 square miles.[344] But that its original size was greater is strikingly shown more particularly on the western margin, which like that of the basalt-escarpments, has obviously been determined by denudation, for its separate beds present their truncated ends to the horizon all along the flanks of the Cuillins, from the head of Glen Brittle round to Loch Scavaig (Fig. 332), and from Strath na Creitheach round the southern flanks of Blath Bheinn to Loch Slapin and Strath More.
[Footnote 344: Though this and the other bosses are here spoken of as consisting of gabbro, it will be understood that this rock only constitutes the larger portion of their mass, which includes also dolerites and other more basic compounds, together with involved portions of the plateau-basalts and masses of agglomerate which probably mark the position of older vents.]
The first point to be ascertained in regard to the gabbro and associated basic rocks of the mountainous tract is their connection in geological structure and age with the bedded basalts of the plateau. This initial and fundamental relation, as Forbes long ago said, can be examined along the whole western and southern flank of the Cuillin Hills, from the foot of Glen Sligachan round to the mouth of Loch Scavaig. Even from a distance, the observer, who is favoured with clear weather, can readily trace the almost level sheets of basalt till they dip gently under the darker, more massive rock of the hills. Tourists, who approach Skye by way of Loch Coruisk, have an opportunity, as the steamer nears the island of Soay, of following with the eye the basalt-terraces of the promontory of Rudh' an Dunain until they disappear under the gabbro of the last spur of the Cuillins that guards the western entrance to Loch Scavaig.
What is so evident at a distance becomes still more striking when viewed from nearer ground. Nowhere can it be more impressively seen than at the head of Glen Brittle. Looking westwards, the traveller sees in front of him only the familiar level terraces and green slopes of the basalt-plateau, rising platform above platform to a height of nearly 1500 feet above the sea. But turning to the east, he beholds the dark, gloomy, cauldron-like Corry na Creiche, from which rise some of the ruggedest and loftiest crests of the Cuillins. On the hills that project from either side of this recess and half enclose it, the bedded basalts mount from the bottom of the valley, with their lines of parallel terrace dipping gently inward below the black rugged gabbro that crowns them and sweeps round to form the back or head of the corry. Down the whole length of Glen Brittle the same structure conspicuously governs the topographical features. On the right hand, the ordinary terraced basalts form the slopes; and they rise for some 500 or 600 feet up the eastern side, until they pass under the darker, more rugged, and less distinctly bedded rocks of the mountains (Fig. 332). The dip of the whole series is here at a gentle angle towards south-east, that is, into or under the main mass of the Cuillin group.
When, however, we proceed to examine the junction between the two rocks we find it to be less simple than it appears. It is not an instance of mere superposition. The gabbro unquestionably overlies the basalts, and is therefore of younger date. But it overlies them, not as they rest on each other, in regular conformable sequence of eruption, but intrusively, as a sill does upon the rocks on which it appears to follow in the unbroken order of accumulation. This important structure may be ascertained in almost any of the many sections cut by the torrents which have so deeply trenched with gullies the flanks of the hills. Starting from the ordinary bedded basalts, we observe, in mounting the slopes and approaching the gabbro, that the rocks insensibly assume that indurated shattery character, which has been referred to as characteristic of them round the margins of vents, and which will be shown to be not less so in contact with large eruptive masses of basic or acid rock.[345] Beds of dolerite make their appearance among the basalts, so distinctly crystalline, and so similar in character to the rocks of the sills, that there can be little hesitation in regarding them as intrusive. These sills increase in size and number as we ascend, though hardened amygdaloidal basalts may still be observed. True gabbros then supervene in massive beds, and at last we find ourselves entirely within the gabbro area, where, however, thin bands of highly altered basalt may still for some distance appear. One further fact will generally be noticed, viz. that before reaching the main mass of gabbro, veins and sills of basalt, as well as of various felsitic and porphyritic members of the acid group, come in abundantly, crossing and recrossing each other in the most intricate network. The base of the thick gabbro-sheets is thus another horizon on which, as on that below the plateau-basalts, intrusive masses have been especially developed. Through all these rocks numerous parallel basalt-dykes, running in a general persistent N.N.W. direction, with a later N.E. series, rise from below the sea-level up even to the very crests of the Cuillins (Fig. 333).
[Footnote 345: This indurated, altered character of the bedded basalts near the intrusive bosses and sills will be more particularly described in a later chapter in connection with the granophyre intrusions (see p. 386). The metamorphism induced by the basic rocks has generally been less pronounced than that effected by the acid masses.]
The sections on the western side of the gabbro area of Skye thus prove that this rock inosculates with the bedded basalts by sending into them, between their bedding planes, sheets which vary in texture from fine dolerites at the outside into coarse gabbros further towards the central mass, and that this intrusion has been accompanied by a certain amount of induration of the older rocks.
On the eastern side, the same structure can be even more distinctly seen, for it is not only exposed in gullies and steep declivities, but can be traced outward into the basalt-plateau. In the promontory of Strathaird, Jurassic sandstones and shales, which form the coast-line and lower grounds, are surmounted by the bedded basalts. Denudation has cut the plateau into two parts. The smaller of these makes the outlier that rises into Ben Meabost (1128 feet). The larger stretches continuously from Glen Scaladal and Strathaird House northward into Blath Bheinn. Hence from the ordinary terraced basalts, with their amygdaloids, thin tuffs, red partings, and seams of lignite, every step can be followed into the huge gabbro mountain. Starting from the black Jurassic shales on which the lowest basalt lies, we walk over the successive terraces up into the projecting ridge of An da Bheinn. But as we ascend, sheets of dolerite and gabbro make their appearance between the basalts, which gradually assume the altered aspect already noticed. The dip of the whole series is at a low angle northwards, and the beds can be followed round the head of the Glen nan Leac into the southern slopes of Blath Bheinn. Seen from the eastern side of this valley, the bedded character of that mountain is remarkably distinct, but it becomes less marked towards the upper part of the ridge where the gabbros preponderate. One of the most striking features of the locality is the number and persistence of the dykes, which strike across from the ordinary unaltered basalts of the plateau up into the highest gabbros of the range. Where less durable than the intractable gabbro, they have weathered out on the face of the precipices, thereby causing the vertical rifts and gashes and the deep notches on the crest that form so marked a feature in the scenery. On the other hand, they are often less destructible than the plateau-basalts, and hence in the Glen nan Leac they may be seen projecting as low dams across the stream which throws itself over them in picturesque waterfalls. The youngest dykes in the Blath Bheinn group of hills, have been found by Mr. Harker to have a north-easterly trend, and a north-westerly hade of about 40°, and to give a stratified appearance to the gabbro when viewed from a distance.
The deep dark hollow of the Coire Uaigneich has been cut out of the very core of Blath Bheinn, and lays bare the structure of the east part of the mountain in the most impressive as well as instructive way (Fig. 334). By ascending into this recess from Loch Slapin, we pass over the whole series of rocks, and can examine them in an almost continuous section in the bed of the stream and on the bare rocky slopes on either side. Sandstones and shales of the Jurassic series extend up the Allt na Dunaiche for nearly a mile, much veined with basalt and quartz-porphyry, by which the sandstones are locally indurated into quartzite. At last these strata are overlapped by the basalts of the Strathaird plateau, which with a marked inclination to N.N.W., here dip towards the mountains. But by the time these rocks have reached the valley, they have already lost their usual brown colour and crumbling surfaces, and have assumed the indurated splintery character, though still showing their amygdaloidal structure. They are much traversed by felsitic veins and strings which proceed from a broad band of fine-grained hornblende-granite that runs up the bottom of the Coire Uaigneich and, ascending the col, crosses it south-westwards into the Glen nan Leac. On the left or south-eastern side of this intrusive mass, a portion of Lias shales and limestone (here and there altered into white marble) is traceable for several hundred yards up the stream.[346]
[Footnote 346: This limestone was formerly identified by me with the Cambrian strata of the district. It was noticed by Von Oyenhausen and Von Dechen, who, as Mr. Harker has recently ascertained, correctly believed it to be a portion of the Lias torn off and carried upward by the eruptive rocks (Karsten's Archiv, i. p. 79).]
The bedded basalts of Strathaird, after dipping down towards the N.N.W., bend up where they are interbanded with dolerites and gabbros, and form the prominence called An Stac, which rises as the eastern boundary of the Coire Uaigneich. Their steep dip away from the mountain is well seen from the east side, and their outward inclination is continued into the ridge to the southward. Similar rocks appear on the other flank of the band of granite, and form the base of Blath Bheinn. They are likewise continued in the mountains further north called Sgurr nan Each and Belig, where they dip in a northerly direction away from Blath Bheinn, which seems to be the centre of uprise, with the gabbro-sheets dipping away from it. The bedded basalts have been traced by Mr. Harker up to a height of well over 2000 feet on the Blath Bheinn range. They are of the usual altered, indurated, and splintery character. The intrusive sheets interposed between them become thicker and more abundant higher up, until they constitute the main mass of the mountain. But that they are in separate sheets, and not in one amorphous mass, can be recognized by the parallel lines that mark their boundaries. The junction of the gabbro sills and the lavas is a very irregular one, portions of the latter rocks being enveloped in the intrusive sills.
The granite which sends out veins into the surrounding rocks is obviously the youngest protrusion of the locality, except of course the basalt-dykes which cross it, and which are nowhere seen in a more imposing display than round the flanks of Blath Bheinn. A section across the corry shows the structure represented in Fig. 334.
It is thus demonstrable that when its line of junction with the surrounding plateau-basalts is traced in some detail, the gabbro is found to overlie them as a whole, but also to be intercalated with them in innumerable beds, bands, or veins which rapidly die out as they recede outwards from the main central mass; that these interposed beds are intrusive sheets or sills from that mass which have cut off and enveloped portions of the basalts, and that the contiguous bedded basalts show more or less marked metamorphism.
We have now to consider the structure of the interior of the gabbro area of the Cuillin Hills. The first impression of the geologist who visits that wild district is that the main mass of rock is as thoroughly amorphous as a core of granite. Yet a little further examination will reveal to him many varieties of texture, sometimes graduating into, sometimes sharply marked off from, each other, and suggesting that the rock is not the product of one single protrusion. He will notice further indications of successive discharges or extravasations of crystalline material during probably a protracted period of time, and in the intricate network of veins crossing each other and the general body of the rock in every direction, as well as in the system of basalt-dykes that traverse all the other rocks, he will recognize the completion of the evidence of repeated renewals of subterranean energy.
But the observer will be struck with the absence of the more usual proofs of volcanic activity in such forms as vesicular lavas and abundant masses of slag, bombs and tuffs, which are commonly associated with the idea of the centre of a volcanic orifice, though he will meet with isolated masses of coarse volcanic agglomerate within the gabbro area and along some parts of its junction with the granophyre. The general characters of the rocks around him suggest that he stands, as it were, far beneath that upper part of the earth's crust which is familiar to us in the phenomena of modern volcanoes; that he has been admitted into the heart of one of the deeper layers, where he can study the operations that go on at the very roots of an active vent.
When the geologist begins a more leisurely and systematic examination of the interior of the gabbro area of Skye he soon sees reason to modify the impression he may at first have received that this rugged region presents the characters of one single eruptive mass. The more he climbs among the hills the more will he meet with evidence of long-continued and oft-repeated extravasation, one portion having solidified before another broke through it, and both having been subsequently disrupted by still later protrusions.
But if by chance he should begin his examination of the ground upon some of the more typically banded varieties of rock, he may for a time almost refuse to admit that these can be either of volcanic origin or of Tertiary age.[347] He will find among them such startling counterparts of the structure of the ancient Lewisian gneiss of the North-West of Scotland that he may well be pardoned if for a time he seeks for evidence that they really do belong to that primeval formation, and have only been accidentally involved among the Tertiary volcanic rocks. If, for instance, he should land in Loch Scavaig, and first set foot upon the gabbros as they appear around Loch Coruisk, he would find himself upon masses of grey coarsely crystalline, rudely banded rock, like much of the old gneiss of Sutherland and Ross. Ascending over the ice-worn domes, he would notice that the banding becomes here and there more definitely marked by strong differences in texture and colour, while elsewhere it disappears and is replaced by a granitoid arrangement of the crystals, which are often as large as walnuts.
[Footnote 347: See Quart. Journ. Geol. Soc. vol. l. pp. 217, 657, and a paper by the author, "Sur la Structure rubannée des plus anciens Gneiss et des Gabbros Tertiaires," Compt. rend. Cong. Géol. Internat. 1894, p. 139.]
Nowhere is the gneissoid banding more beautifully developed than on the east side of the Cuillin group near the head of Glen Sligachan along the ridge of Druim an Eidhne. It was at this locality that the four typical structures were observed which have already been referred to (p. 329). The varieties of colour and composition depend upon the exceedingly irregular distribution of the component minerals. The paler bands, rich in felspar, lie parallel with dark brown bands full of pyroxene, olivine and magnetite, in which, moreover, thin ribs of glistening black consist in large part of the iron ore. These layers vary in thickness from mere pasteboard-like laminæ to beds a yard or more in thickness. Within a space of a few square yards their parallelism reminds one of stratified deposits (Fig. 336), but traced over a wider space they are found to be more or less irregular in thickness and lenticular in form.
The resemblance to gneisses, and sometimes to the flow-structure of coarse rhyolites, is still further sustained by occasional undulations or minute puckerings (Fig. 335). Still more extraordinary are the examples of the actual plication of a group of successive bands, as shown in Fig. 337, wherein such a group about ten feet thick is shown to have been doubly folded between parallel bands above and below. This structure is not due to any deformation of the gabbro long subsequent to the consolidation of the mass. It belongs to the phenomena of protrusion and solidification. An examination of thin slices of these rocks under the microscope reveals no evidence of crushing. On the contrary, the minerals of one band interlock with those of the band adjoining, in such a manner as to prove that the differences of composition cannot be due to crushing and shearing or to successive intrusion, but must have been present before the final consolidation of the whole rock.[348]
[Footnote 348: Mr. J. J. H. Teall and A. G., Quart. Journ. Geol. Soc. vol. 1. (1894), p. 652.]
The conclusion which seems most consonant with the facts is that the magma which supplied the visible masses of gabbro in Skye existed below in a heterogeneous condition, that portions of it, differing considerably from each other in composition, were simultaneously intruded, and that by the deformation of these portions during their intrusion their present plicated structures were produced. A careful study of these banded gabbros offers many suggestive points of comparison with the gneisses and anorthosite (Norian) rocks of pre-Cambrian age. It seems in the highest degree probable that the banded structures and peculiar mineral aggregation in these ancient rocks arose under conditions closely analogous to, if not identical with, those in which the Tertiary gabbros of Skye originated.[349]
[Footnote 349: Consult the Memoirs cited in the footnote on p. 342.]
Similar structures are found to be widely developed through the gabbros of the Cuillin Hills. Not only are these rocks disposed in distinct beds, but many of the beds display the most perfect banding. Thus the mountains that surround the head of Loch Scavaig and sweep round Loch Coruisk up to the great splintered crests of Sgurr na Banachdich display on their bare black crags a distinct bedded structure. On the east side of Loch Scavaig the rock presents a rudely-banded character, the bands or beds being piled over each other from the sea-level up to the summits of the rugged precipices, and dipping into the hill at angles of 25° to 35°. Abundant dykes and veins of various basic, intermediate and acid rocks cut this structure. The individual layers here show sometimes the wavy and puckered condition already referred to.
Even from a distance the alternating lighter and darker bands can readily be seen, so that this structure, with the variations in its inclination, can be followed from hill to hill (Fig. 338). The regularity of the arrangement, however, is often less pronounced on closer inspection. While the gabbro is rudely disposed in thick beds, indicative of different intrusive sheets or sills, with which the banding is generally parallel, considerable irregularities may be observed in the arrangement of the structure of individual sheets. These sheets may be parallel to each other, and yet, while in some the banding is tolerably regular in the direction of the planes of the sheets, in others it is much twisted or inclined at various angles.
On the west side of the Coruisk river the banding is vertical; southward from that stream it inclines slightly towards the south, but soon again becomes vertical, and continues conspicuously so at the junction of the gabbro with the Torridon sandstones and plateau-basalts on the west side of Loch Scavaig.
Thus, instead of being one great eruptive boss, the gabbro of this district is in reality an exceedingly complicated network of sills, veins and dykes. While the general inclination of the bedding sometimes continues uniform in direction and amount from one ridge to another, it is apt to change rapidly, as if the complex assemblage of intruded masses had been disrupted and had subsided in different directions. For example, after overlying the bedded basalts of the plateau all the way from Glen Brittle to the west side of Loch Scavaig, the gabbro descends abruptly across these basalts and also across the Torridon sandstones, on which they unconformably rest. These two groups of rocks are not only truncated by the gabbro, but are traversed by the intricate system of sills, dykes and veins already referred to. Where it abuts against the sandstones and basalts in Loch Scavaig, the gabbro is arranged in vertical bands of different mineral composition and texture. Much of it is remarkably coarse, some bands displaying pyroxene crystals more than an inch in length. There is no fine-grained selvage here, indicative of more rapid cooling. So coarse, indeed, is the rock close up against the sandstone, that the junction-line can hardly be supposed to be the normal contact of the intrusive rock. This inference is confirmed by the existence of a singular kind of breccia between the gabbro and the sandstones. It is a tumultuous mass of fragments of coarse and fine gabbro, Torridon sandstone and shale, and plateau-basalts, embedded in a pale crystalline matrix of fine granular granophyre; veins from this acid intrusion run off into the gabbro on the one side as well as into the Torridon sandstones on the other. It would seem that this junction-line has been one of great movement, that the gabbro-sheets have subsided against a fault-wall of plateau-basalt and Torridon sandstone, and that subsequently an intrusion of finely granular granophyre has come up the fissure, involving in its ascent fragments of all the materials around.
The rocks for a considerable distance to the south of the gabbro are intensely altered. The Torridon sandstone has been so indurated as to pass into a bleached white quartzite, while the shales interstratified with it have been converted into a kind of porcellanite. But the most interesting alterations are those to be observed in the plateau-basalts, which at a height of about 300 feet above the sea, are to be seen in nearly horizontal sheets that lie immediately on the upturned edges of the Torridon sandstones. These lavas have suffered great metamorphism, to which more particular reference will be made in Chapter xlvi. in connection with the action of the granophyre. Whether this alteration has been produced by the intrusion of the gabbro or of some concealed mass of granophyre underneath, of which only projecting dykes and veins reach the surface, must remain a matter of doubt. On the whole, as the gabbro is here undoubtedly thrown against the basalts and Torridon sandstone by a fault, it seems most probable that the change has been mainly due to the influence of the acid rock.
In the Blath Bheinn group of hills the relations of the gabbro to the bedded basalts have recently been mapped in detail by Mr. Harker during the progress of the Geological Survey of Skye. He has observed that, allowing for irregularities of form, the mass of gabbro obliquely overlies the basalts as a great sheet, not necessarily due to a single intrusion, which dips towards the west. He has found the rock to vary from a coarse gabbro to a diabasic type, and to vary also in mineralogical constitution, becoming in places very rich in olivine, though the banded structure is here only exceptionally developed. North of Garbh Bheinn the gabbro is much crushed and the outlying patch to the north of Belig is in part a crush-breccia. Mr. Harker remarks that similar brecciated structures are common among the granophyres of the Red Hills, and that it is sometimes difficult to distinguish their structure from that of the true volcanic agglomerates.
Besides the main area of gabbro in Skye, a great many small detached bosses, sills and dykes lie further east on the flanks of the Red Hills. One of the best marked of these detached areas forms a conspicuous crag on the east side of Strath More, immediately to the north of Beinn na Cro. It consists of beds of coarse gabbro, with others of dolerite intercalated in an outlier of the plateau-basalts, and is traversed by veins from the granophyre of the glen, as well as by the usual north-west basalt dykes (Fig. 349). It appears to be a marginal portion of the main gabbro area separated by the intrusion of the great granitoid boss of the Red Hills. On the north-eastern side of Beinn na Caillich numerous intrusive sheets of gabbro and dolerite traverse the quartzite and limestone, and extend down to the sea-margin in the Sound of Scalpa.
There is an important feature in the main gabbro area of Skye not yet clearly understood, and which only a minute and patient survey can elucidate. Though I have found among the Cuillin Hills no distinct proof that the mass of gabbro ever gave rise to discharges of material, either lava-form or fragmentary, which reached the surface, the gabbro area, as already remarked, contains unquestionable evidence of explosions and the production of pyroclastic masses. Among the moraine-mounds of Harta Corry, blocks of basalt-agglomerate are strewn about, full of angular fragments of altered basalt, sometimes highly amygdaloidal, and also boulders in which lumps of coarse gabbro are enveloped in a matrix of finer material. I did not find the parent rocks from which these glacier-borne masses had been derived, but there can be no doubt that they exist among the gabbro crags that surround that deep glen. Reference has already been made to the similar rock found in situ on the opposite side of the Cuillin ridge at the head of the great cauldron of Corry na Creich; likewise to the mass of coarse agglomerate which forms a group of knolls and crags on the east side of Druim an Eidhne above the head of Glen Sligachan. This rock contains abundant blocks of various slaggy lavas like those of the basalt-plateau, and runs for some distance along the eastern limit of the gabbro, between that rock and the granophyre. It is intersected by numerous basalt-veins. Mr. Harker, as above mentioned, has recently found some considerable strips of agglomerate which, like that which I traced round the west side of Beinn Dearg, are interposed between the gabbro and the bosses of granophyre, or lie at the base of the volcanic series (p. 284).
There does not, however, appear to be any evidence to connect these isolated masses of agglomerate with the phenomena attending the uprise of the gabbro. They seem to be more probably related to the plateau eruptions, and may be compared with those of Strath, Ardnamurchan and Mull (pp. 278, 280, 384). That the huge gabbro mass of Skye, besides invading and altering the bedded basalts, may have communicated eventually with the surface, and have given rise to superficial discharges, is not at all improbable, but of any such outflows not a vestige appears now to remain. We must remember, however, that the gabbro no doubt in many places found its readiest upward ascent in vents belonging to the plateau-period, and that portions of the agglomerates of these earlier vents may be expected to be found involved in it, as the agglomerate of the great vent of Strath has been invaded by the granophyre.
Chapter XLIV
THE BOSSES AND SHEETS OF GABBRO IN THE DISTRICTS OF RUM, ARDNAMURCHAN, MULL, ST. KILDA AND NORTH-EAST IRELAND. HISTORY OF THE GABBRO INTRUSIONS
2. The Island of Rum
The mountains of the island of Rum, rising as they do from a wide expanse of open sea, present one of the most prominent and picturesque outlines in the West Highlands (Map VI.). More inaccessible than most of the other parts of the volcanic region, they have been less visited by geologists. They were described by Macculloch as composed of varieties of "augite rock." He noticed in this rock "a tendency to the same obscurely bedded disposition as is observed in other rocks of the trap family," and found at one place that it assumed "a regularly bedded form, being disposed in thin horizontal strata, among which are interposed equally thin beds of a rock resembling basalt in its general characters."[350] Professor Judd repeats Macculloch's observation, that "the great masses of gabbro in Rum often exhibit that pseudo-stratification so often observed in igneous rocks." He regards these masses, like those of Skye and Mull, as representing the core of a volcano from which the superficial discharges have been entirely removed, and he gives a section of the island in which the gabbro is represented as an amorphous boss sending veins into a surrounding mass of granite.[351] In a subsequent paper he gave an excellent detailed account of the mineralogical composition of some of the remarkably varied and beautiful basic rocks constituting the hills of Rum, but added no further information regarding the geological structure of the island.[352]
[Footnote 350: Western Islands, i. p. 486.]
[Footnote 351: Quart. Journ. Geol. Soc. xxx. p. 253.]
[Footnote 352: Op. cit. xli. (1885) p. 354. See also his paper in vol. xlii. of the same Journal.]
Even from a distance of eight or ten miles, the hills of Rum are seen to be obviously built up of successive nearly horizontal tiers of rock. As the summer tourist is carried past the island, in that wonderful moving panorama revealed to him by the "swift steamer" of modern days, these great dark cones remind him of colossal pyramids, and as the ever-varying lights and shadows reveal more prominently the alternate nearly level bars of crag and stripes of slope, the resemblance to architectural forms stamps these hills with an individuality which strikes his imagination and fixes itself in his memory. If choice or chance should give him a nearer view of the scene, he would not fail to notice that it is among the northern hills of the island that the bedded character is so conspicuous, and that it ceases to be prominent in the southern heights, though here and there, as in the upper part of Scuir na Gillean, it may in certain lights be detected even from a distance. Crossing over from Eigg, he would recognize each of the features represented in the sketch reproduced in Fig. 339. Along the shore, red sandstones rise in naked cliffs, from the top of which the ground slopes upward in brown moors to the bare rocky declivities. A deep valley (Glen Dibidil) is seen to run into the heart of the hills, between the bedded group to the north and the structureless group to the south. If the weather is favourable, some eight or more prominent parallel bars of rock may be counted on the two higher cones to the right. These bars are not quite level, but slope gently from right to left. They remind one of the terraced basalts of the plateaux, but present a massiveness and a breadth of intervening bare talus-slope such as are not usual among those rocks.
Nor is this impression of regularity and bedded arrangement lessened when we actually climb the slopes of the hills. I had for years been familiar with the outlines of Rum as seen from a distance, and had sketched them from every side, but I shall never forget the surprise and pleasure when my first ascent of the cones revealed to me the meaning of these parallel tiers of rock. I found it to be the structure of the Cuillin Hills repeated, but with some minor differences which are of interest, inasmuch as they enlarge our conceptions of the process by which the gabbro-bosses were formed.
The northern half of the island of Rum consists almost entirely of red sandstone, which, as already stated, is a continuation of the same formation (Torridonian) so well developed in the south-east of Skye, Applecross and Loch Torridon, and traceable between the Archæan gneiss and the Cambrian strata up as far as Cape Wrath. The sandstones, though full of false bedding, show quite distinctly their true stratification, which is inclined with singular persistence towards W.N.W., at angles averaging from 15° to 20°. If they are not repeated by folds or faults, they must reach in this island a thickness of some 10,000 feet. Their red or rather pinkish tint seems mainly to arise from the pink felspar so abundant in them, for in many places they really consist of a kind of arkose. Pebbly bands with rounded pieces of quartz are of common occurrence throughout the whole formation. Dykes and veins of basalt are profusely abundant. Sometimes these run with the bedding, and might at a distance be taken for dark layers among the pink sandstones. They often also strike obliquely up the face of the cliffs like ribbons.
But, notwithstanding their apparent continuity, there can be no doubt that these sandstones have suffered from those powerful terrestrial disturbances which have affected all the older rocks of the North-West Highlands. On the west side, where they plunge steeply into the sea, they have undergone a change into fine laminated rocks, which might at first be mistaken for shales, but which owe their fissility to shearing movements. Along their southern border, from a point on the east coast near Bagh-na-h-Uamha, south of Loch Scresort, to the head of Kilmory Glen, they are abruptly truncated against a group of dark, flaggy and fissile schists and fine quartzites or grits, which in some places are black and massive like basalt, and in others are associated with coarse grey gneiss. That some of these rocks are portions of the Lewisian series can hardly be doubted, and their structure and relations are probably repetitions of those between the Lewisian gneiss and Torridon sandstone of Sleat in Skye. I found also on the northern slopes of Glen Dibidil a patch of much altered grey and white limestone or marble, which reminded me of the Cambrian limestone of Skye. The red sandstones in a more or less altered condition are prolonged to the south-east promontory of the island.
In passing over the zone of these more ancient rocks, we find them to present increasing signs of alteration as they are traced up the slopes towards the great central mass of erupted material. The pink sandstones gradually lose their characteristic tint, and grow much harder and more compact, while the veins and dykes of basalt and sheets of dolerite intersecting them increase in number. The zone of black compact quartzite, which lies to the south of the sandstones, and which at one point reminds us of basalt, at another of the flinty slate of the schistose series, likewise displays increasing induration. Its bedding, not always to be detected, is often vertical and crumpled. But the most remarkable point in its structure is the intercalation in it of bands of breccia. These vary from less than an inch to several yards in diameter; they run mostly with the bedding, but occasionally across it. The stones in them are fragments of the surrounding rock embedded in a matrix of the same material, but also with pieces of a somewhat coarser grit or quartzite. A band of coarse breccia forms the southern limit of this zone along the northern base of Barkeval and Allival. In general character it resembles the thinner seams of the same material just referred to. The matrix so closely agrees with the black flinty quartzite, that but for the included stones it could hardly be distinguished; so greatly has the mass been indurated that the stones seem to shade off into the rest of the rock. But here and there its true brecciated nature is conspicuously revealed by prominent blocks of hardened sandstone. This band of breccia must in some places be 150 or 200 feet broad. It has no distinct bedding, but seems to lie as a highly inclined bed dipping into the hill. It may possibly be a crush-breccia belonging to a period earlier than the volcanic eruptions. It is at once succeeded by a black flinty felsite like that of Mull. The groundmass of this rock, so thickly powdered with magnetite grains as to be almost opaque under the microscope, displays good flow-structure round the turbid crystals of orthoclase and the clear granules of quartz. Further up the hill, the rock becomes lighter in colour and less flinty in texture--a change which is found to arise from more complete devitrification, the groundmass having become a crystalline granular aggregate of quartz and felspar with scattered porphyritic crystals of these minerals (microgranite). In some places, the felsite incloses fragments of other rocks. A specimen of this kind, taken from the head of Coire Dubh, shows under the microscope a brown micro-felsitic groundmass, with crystals of felspar and augite, inclosing a piece of basalt, composed of fine laths of plagioclase, abundant magnetite and a smaller proportion of granules of augite.
This band of felsite and microgranite may be traced continuously from Loch Gainmich along the base of Barkeval and Allival, and similar rocks appear at intervals on the same line round the eastern base of the hills. Immediately above this belt of felsitic protrusions comes the great body of gabbro. It will be observed that here, as in Skye, the base of the gabbro mass presents a horizon on which injections of acid rocks have been particularly abundant. Whether the breccias be regarded as the result of earlier rock-crushings, or as due to volcanic explosions during the Tertiary period, they are evidently older than the eruption of the gabbros. In that respect they may be compared with the agglomerates through which the youngest eruptive bosses of Skye have made their way; but their component materials have been derived from the surrounding platform of ancient rocks, and not from subterranean lavas.
For my present purpose, however, the chief point of importance is the structure of the gabbro mass that springs from that platform into the great conical hills of Rum. The accompanying sketch (Fig. 340) will convey a better idea of this structure than a mere description. At the base, immediately above the felsite just referred to, bedded dolerites make their appearance, much intersected with veins from the siliceous rock. Veins and dykes of basalt also cut all the rocks here, the newest being those which run in a north-west direction. The lowest sheets of dolerite are succeeded by overlying sills of coarser dolerites, gabbros, troctolites, etc., which are as regular in their thickness and continuity as the ordinary basalts of the plateaux. The band of light-coloured troctolite, in particular (Fig. 341), about 20 to 30 feet thick, which has been already referred to for its remarkable laminar structure, can be followed for some distance along the base of the hill as a marked projecting escarpment. This rock at once arrests attention by its platy or fissile structure, parallel to the bedding-surfaces of the sheet. Indeed hand-specimens of it, as I have said, might readily pass for pieces of schistose limestone, especially if taken from the upper part. It consists of successive layers, which on the weathered surface divide it into beds almost as regular as those of a flagstone, each bed being further separated into laminæ marked off by the darker and lighter tints of their mineral constituents. The darker layers consist of olivine, and the lighter of plagioclase. This segregation here and there takes the form of rounded masses, where the minerals are more indefinitely gathered together. The affinity of the rock with intrusive sheets is further displayed by the occurrence of abundant nut-like aggregates of pale green olivine. Examined under the microscope, flow-structure is admirably seen, the lath-shaped felspars being drawn out parallel to the planes of movement, and giving thereby the peculiarly schistose structure which is so deceptive.
The massive and coarsely crystalline gabbros below and above this troctolite are all more or less affected by the same laminar structure. Some of those in higher parts of the mountain are quite massive in part, but also include bands of lamination. Banding like that of the Skye gabbros is generally developed among them, the individual bands varying from less than an inch to a foot or more in thickness. This structure, like the lamination, is parallel to the general bedding of the sheets. As in the Cuillin Hills, the bands differ from each other in the relative proportions of the constituent minerals, especially the predominant pyroxene and olivine. The crystals or crystalline aggregates are often from a quarter of an inch to an inch in diameter, and in these large forms are crowded together in certain bands. Magnetite, on the whole, is rather less conspicuous than in the Cuillin gabbro: at least, it is not so prominently aggregated in special layers. In one or two instances I have observed curvature of the banding, but no example so striking as that cited from the Cuillin area (Fig. 337).
On weathered surfaces, where the felspars decay into a creamy white and the ferro-magnesian minerals assume tints of green, brown and red, the resemblance of the rocks to schists is striking. This external likeness is combined with a tendency to split into thin plates parallel to the lamination, which still further increases their schistose appearance. Though less developed than in Skye, the banding appears to be of the same kind and origin; but in Rum it is combined with the remarkable lamination above mentioned, produced by the arrangement of the component minerals with their longer axes parallel to the planes of bedding, as in flow-structure--a combination which I have not yet observed in Skye.
The bedded arrangement of the gabbros of Rum, so conspicuous in the great eastern cones (Figs. 339 and 340), is emphasized by the fact that some sheets, of a more durable kind, stand out boldly as prominent ribs, while the softer crumble into a kind of sand, which forms talus-slopes between the others. Alternations of this nature are continued up to the very top of the mountains. The beds are nearly flat, but dip slightly into the interior or towards the south-west. On the west side of the island also, beyond Loch Sgathaig, a distinct bedding may be traced, the inclination being here once more inwards or to the east. But from Glen Harris and the base of Askival this structure becomes less marked, and gradually disappears. There is thus a central or southern more amorphous region, while round the margin towards the north and east the rock appears in frequent alternating beds.
It is clear that in the broad features of their architecture the hills of Rum follow closely the plan shown in the Cuillin Hills of Skye. But, unfortunately, in the former island denudation has gone so far that no connection can be traced on the ground between the gabbros and the plateau-basalts. As already stated, the latter rocks have been almost entirely stripped off from the platform of sandstones and schists which they undoubtedly at one time covered, and the few outliers of them that remain lie at some little distance from the margin of the gabbro area (ante, p. 216). Nevertheless, we are not without some indications of them underneath the gabbros. I have alluded to the basalts that lie at the base of the eastern cones. As we follow the bottom of the gabbro southward round the flanks of the hills, dull compact black shattery basalts, with a white crust, appear from under the more crystalline sheets. These at once remind one of the altered basalts of Skye and Mull. On the west side also, beds of basalt emerge from under the gabbro, but they have been so veined and indurated by the granophyre of that district, that their relations to the gabbro are somewhat obscured. If we could restore the lost portions of the plateau, I believe we should find the gabbros of Rum resting on part of the volcanic plateau, and some of the gabbro-beds prolonged as sills between the sheets of basalt.
3. The Gabbro of Ardnamurchan
The promontory of Ardnamurchan reveals as clearly as the flanks of the Cuillin Hills, though in a less imposing way, the relations of the gabbros to the plateau-basalts (Map VI.). From the southern shore at Kilchoan to the northern shore at Kilmory, bedded basalts, of the usual type, amygdaloidal and compact, weathering into brown soil, may be followed along the eastern slopes of the hills, resting upon the schists and Jurassic series of western Argyleshire. These rocks are a continuation of those that cap the ridges further to the south-east and cross Loch Sunart into Morven. They dip westwards, and followed upwards in that direction, they soon present the usual marks of alteration. They weather with a white crust and become indurated and splintery. Sheets of dolerite with many veins and dykes of basalt run between and across them. Bands of gabbro make their appearance, and these, as we advance westwards, increase in number and in coarseness of grain until this rock, in its rudely bedded form, constitutes practically the whole of the promontory from Meall nan Con to the light-house. Many admirable sections may be seen on the coast-cliffs and in the rugged interior, showing the irregular bedding of the gabbro, and how prone this rock is to develop its component minerals in bands or ribbons, sometimes made up of large crystals, as in Skye, Rum and Mull.
4. The Gabbro of Mull
In the island of Mull, the conclusions to which the geology of the other volcanic districts leads us as to the position of the gabbros in the series of volcanic phenomena, are further confirmed. The first geologist who appears to have observed the relation of these rocks in that island was Jameson, who classed them under the old name of "greenstone," including in the same designation rocks now termed dolerites and gabbros. He ascended one of the hills above Loch Don, probably Mainnir nam Fiadh (2483 feet), which he found to consist of "strata of basalt and greenstone," with some basalt-breccia or tuff and a capping of basalt. He speaks of the "singular scorified-like aspect" of the weathered greenstone--a description which applies to some of the coarser gabbro bands of that locality. But he appears to have recognized the general bedded arrangement of the rocks up even to the summit of the hill.[353]
[Footnote 353: Mineralogy of the Scottish Isles i. p. 205.]
It was not, however, until the visit of Professor Zirkel in 1868, that the true petrographical characters of the gabbro of Mull were recognized. This observer remarked that the rock is regularly interstratified with the basalt.[354] Professor Judd, as already stated, has supposed the gabbros to be the deep-seated portion of the masses which when poured out at the surface became the plateau-basalts, and he represents them in his map and sections of Mull as ramifying through the granitic rocks.[355]
[Footnote 354: Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871) p. 58.]
[Footnote 355: Quart. Jour. Geol. Soc. xxx. (1874).]
In Mull the disposition of the gabbro in beds, sheets or sills is well displayed, for there is here no great central complicated mass of interlacing banded and amorphous sheets. We have seen that a higher group of plateau-basalts has survived in this island better than in the other plateaux, and it would seem that denudation has not yet succeeded here in cutting down so deeply into the gabbro core as in Skye, Rum and Ardnamurchan. Only the upper or outer fringe of intrusive sheets among the bedded basalts has been laid bare. The district within which this fringe may be observed is tolerably well-defined by the difference of contour between the long terraced uplands of the ordinary basalts and the more conical forms of the southern group of gabbro hills between Loch na Keal and Loch Spelve. The number and thickness of the gabbro-sheets increase as we proceed inwards from the basalt-plateau. These sheets are specially prominent along the higher parts of the ridge that runs northwards from the northern end of Loch Spelve, and along the west side of Glen Forsa. But they swell out into the thickest mass in the south-western part of the hilly ground, where, from above Craig, in Glenmore, they cross that valley, and form the rugged ridge that rises into Ben Buy (2354 feet), and stretches eastward to near Ardara (Map VI.). It is in this southern mass that the Mull gabbro approaches nearest in general characters to that of Skye. But even here its true intercalation above a great mass of bedded basalt may readily be ascertained in any of the numerous ravines and rocky declivities.
One of the best lines of section for exhibiting the relations of the rocks is the declivity to the west of Ben Buy and Loch Fhuaran. Ascending from the west side, we walk over successive low escarpments and terraces of the plateau-basalts with a gentle inclination towards north-east or east. These rocks weather in the usual way, some into a brown loam, others into spheroidal exfoliating masses. But as we advance uphill they gradually assume the peculiar indurated shattery character already referred to. The soft earthy amygdaloids become dull splintery rocks, in which the amygdales are no longer sharply defined from the matrix, but rather seem to shade off into it, sometimes with a border of interlacing fibres of epidote. The compact basalts have undergone less change, but they too have become indurated, and generally assume a white or grey crust, and none of them weather out into columnar forms. Strings and threads full of epidote run through much of these altered rocks. Abundant granophyric and felsitic veins traverse them. Sheets of dolerite likewise make their appearance between the basalts, followed further up the slope by sheets of gabbro until the latter form the main body of the hill.
On the north side of the same ridge similar evidence is obtainable, though somewhat complicated by the injections of granophyric and felsitic veins and bosses, to which more detailed reference will afterwards be made. But the altered basalts with their amygdaloidal bands and their intercalated basalt-tuffs and breccias, can be followed from the bottom of the glen up to a height of some 1700 feet, above which the main gabbro mass of Ben Buy sets in. Many minor sheets of dolerite and gabbro make their appearance along the side of the hill before the chief overlying body of the rock is reached. Some of these can be distinctly seen breaking across or ending off between the bedded basalts which here dip gently into the hill (Fig. 342). A conspicuous band of coarse basalt-agglomerate, containing blocks of compact and amygdaloidal basalt a yard or more in diameter, shows by the excessive induration of its dull-green matrix the general alteration which the rocks of the basalt-plateau have here undergone. An almost incredible number of veins of fine basalt, porphyry and felsite has been injected into these rocks--a structure which is precisely a counterpart of what occurs under the main body of gabbro in Skye, Ardnamurchan and Rum.
The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge overlying sheet, which probably reaches a thickness of at least 800 feet. It seems to descend rather across the bedding into the hollow of Glen More, and possibly its main pipe of supply lay in that direction. Being enormously thicker than any other sheet in the island, it exhibits the crystalline peculiarities which are so well developed in the central portions of the larger bosses of gabbro. It presents more coarsely crystalline varieties than appear in the thinner sheets, some portions showing crystals of diallage and felspar upwards of an inch in length. It likewise contains admirable examples of banded structure, which, as in Skye and elsewhere, is best developed where the texture becomes especially coarse. Veins or bands, in which the constituent minerals have crystallized out in more definite and conspicuous forms, here and there succeed each other so quickly as to impart a bedded or foliated look to the body of rock, recalling, as in Skye, the aspect of some coarsely crystalline granitoid gneiss. In these respects the Mull gabbro closely resembles that of the Cuillin Hills. Occasionally, on the exposed faces of crags, portions of such bands or veins are seen to be detached and enveloped in a finer surrounding matrix. The thick belts or bands of coarser and finer texture alternate, and give an appearance of bedding to the mass. Nevertheless they are really intrusive sills, which run generally parallel with beds of finer gabbro or with sheets of highly indurated basalt, that may be detached portions of the ordinary rocks of the plateau. The thick sheet of Ben Buy, like the mass of the Cuillin Hills, is thus the result not of one but of many uprises of gabbro.
Of the thinner sheets of dolerite and gabbro in Mull little need here be said. I have referred to their great abundance in the range of eastern hills that rise from the Sound of Mull between Loch Spelve and Fishnish Bay. Though obviously intrusive, they lie on the whole parallel to the bedding of the basalts. The latter rocks exhibit the usual dull indurated shattery character which they assume where large bosses of gabbro have invaded them, and which gradually disappears as we follow them down hill away from the intrusive sheets to the shores of the Sound. They dip towards the centre of the hill group, that is, to south-west in the ridge of Mainnir nam Fiadh, Dùn da Ghaoithe, and Beinn Meadhon, the angle increasing southwards to 15°-20°, and at the south end reaching as much as 35°-40°. Some fine crags of gabbro and dolerite form a prominent spur on the east side of the ridge of Ben Talaidh, in the upper part of Glen Forsa. These consist of successive sheets bedded with the basalts, and dipping south-west. A large sheet stands out conspicuously on the north front of Ben More, lying at the base of the "pale lavas," and immediately above the ordinary basalts. It circles round the fine corry between Ben More and A'Chioch, some of its domes being there beautifully ice-worn. This is the highest platform to which I have satisfactorily traced any of the intrusive sheets of Mull. Another dyke-like mass emerges from beneath the talus slopes of A'Chioch, on the southern side, and runs eastward across the col between the Clachaig Glen and Loch Scridain.
5. The Gabbros of St. Kilda and North-east Ireland
Sixty miles to the westward of the Outer Hebrides lies the lonely group of islets of which St. Kilda is the chief. As the main feature of geological interest in this group is the relation of the acid protrusions to the other rocks, the account of the geology will be more appropriately given as a whole in Chapter xlvii. I need only remark here that the predominant rocks of these islands are dark basic masses, chiefly varieties of gabbro, but including also dolerites and basalts. Reasons will be afterwards brought forward for regarding these rocks as parts of the Tertiary volcanic series. They present a close parallel to the gabbros and associated rocks of Skye. But in one important respect they stand alone. No certain trace remains of any basalt-plateau at St. Kilda such as those through which the gabbros of Skye, Mull and Ardnamurchan have been injected. In regard to their mode of production they have doubtless been intruded at some considerable depth beneath the surface. But no relic appears to have survived of the overlying cover of rock under which they consolidated, and into which they were injected.
In the remarkable volcanic district of the north-east of Ireland a series of basic rocks appears, which in its mode of occurrence and its relation to the other members of the series presents many points of resemblance to the gabbros of the Inner Hebrides. The Irish gabbros are well developed in the Carlingford district, where they form intrusive bosses and sheets which have been erupted through the Palæozoic rocks (Map VII.). They are themselves pierced by later masses of granophyre and other acid rocks. Further reference will be made to these gabbros in later pages, where an account will be given of the granite masses of Mourne, Barnavave and Slieve Gullion.
* * * * *
It is interesting to observe that, while in St. Kilda no relic of any basaltic plateau has been preserved, in the Faroe Islands, on the other hand, no sign has been revealed by denudation that the volcanic plateau of that region is pierced by any eruptive core of gabbro or of granophyre. During my cruises round these islands and through their channels, I was ever on the outlook for any difference in topography that might indicate the presence of some eruptive boss like the gabbro and granophyre masses of the Inner Hebrides. But nothing of that nature could be discerned. Everywhere the long level lines of the bedded basalts were seen mounting up to the crests of the ridges and the tops of the highest peaks. Though I cannot assert that no intrusions of gabbro or of granophyre exist among the Faroe Islands, I feel confident that any such masses which may appear at the surface must be of quite insignificant dimensions, and do not make the important feature in geology and topography which they do among the Inner Hebrides. It is, of course, possible that, vast as the denudation of these islands has undoubtedly been, it has not yet trenched the plateau deeply enough to expose any great intrusive bosses and sills which may underlie and invade the basalts.
iv. HISTORY OF THE GABBRO INTRUSIONS
We are now in a position to draw, from the observations which have been given in this and the preceding chapter regarding the different areas of gabbro in the Tertiary volcanic region of Britain, some general conclusions with respect to the type of geological structure and the phases of volcanic energy which they illustrate.
1. No evidence exists to show that the masses of gabbro ever communicated directly with the surface. They never exhibit the cellular, slaggy and other structures so characteristic of surface-flows. They are, on the whole, free from included pyroclastic material, though masses of agglomerate are enclosed in, and have probably been invaded by, the gabbro of the Cuillin Hills. If the gabbro-bosses ever were continuous with sheets of rock emitted above ground, all such upward continuations have been entirely removed. In any case, we may be quite certain that in an outburst at the surface, the rock would not have appeared in the form of a coarsely crystalline or granitoid gabbro.
2. The crystalline structures of the gabbros point unmistakably to slow cooling and consolidation at some depth beneath the surface. The most coarsely-crystalline varieties, and those with the best developed banded structure, occur in the largest bodies of rock, where the cooling and consolidation would be most prolonged.[356]
[Footnote 356: On this subject, see the papers by Professor Judd already cited.]
3. The remarkable differences in composition between the dark and pale layers in the banded gabbros cannot be accounted for by segregation or successive intrusion, but seem to point to the existence of a heterogeneous magma from which these distinct varieties of material were simultaneously intruded.
4. From the prevalence of a bedded structure and the occurrence of bands and more irregular portions of considerably different texture and even mineralogical composition which intersect each other, it may be confidently inferred that even what appears now as one continuous mass was produced by more than one intrusion.
5. In every case there would necessarily be one or more pipes up which the igneous material rose. These channels might sometimes be wider parts of fissures, such as those filled by the dykes. In other places, they may have been determined by older vents, which had served for the emission of the plateau-basalts and their pyroclastic accompaniments. There can be no doubt that some of these vents afforded egress for the subsequent eruption of granitoid rocks, as will be pointed out in the following chapters. In the case of the gabbros, however, the position of the vents seems to have been generally concealed by the tendency of these rocks to spread out laterally. Denudation has cut deeply into the gabbro-masses, but apparently not deep enough to isolate any of the pipes from the larger bodies of material which issued from them, and thus to leave solitary necks like those in and around the basalt-plateaux. In Skye, where the central core of gabbro is largest and most completely encircled, we cannot tell how much of it lies above the true pipe or pipes, and has spread out on all sides from the centre of eruption. The prevalence of rude bedding and a banded structure indicate that most of the visible rock occurs in the form of sills, successively injected not only into the plateau-basalts, but between and across each other. Round the margin of the gabbro we undoubtedly reach horizons below that rock, and see that it lies as a cake or series of cakes upon the plateau-basalts. The actual pipe or fissure of supply must in each case lie further inward, away from the margin, and may be of comparatively small diameter.
6. From the central pipe or group of pipes or fissures which rose from the platform of older rocks into the thick mass of the basalt-plateaux, successive sheets of dolerite and gabbro were forced outward between the layers of basalt. This took place all round the orifices of supply, on many different horizons, and doubtless at many different times. In some cases, the intrusive sheets were injected into the very bottom of the basalts, and even between these rocks and the older surface on which they rested. This is particularly the case in Rum, where the gabbro-cones spring almost directly from the ancient grits, schists and sandstones on which they rest. The intrusive sheets have likewise found egress at every higher platform in the basalt-series, up at least to the base of the "pale group" in Mull--that is, through a continuous pile of more than 2000 feet of bedded basalt. But the intrusion did not proceed equally all round an orifice. At all events, the progress of denudation has revealed that on one side of a gabbro area the injected portions may occur on a lower stratigraphical level than they do on the opposite side. At the Cuillin Hills, for example, the visible sheets of dolerite and gabbro to the north of Coire na Creiche begin about 1600 feet above the sea, which must be much more than that distance above the bottom of the basalts. On the south-east side, however, they come down to near the base of the basalts at Loch Scavaig; that is to say, their lowest members lie at least 1600 feet below those on the opposite margin.
7. The uprise of so much igneous material in one or more funnels, and its injection between the beds of plateau-basalt, would necessarily elevate the surface of the ground immediately above, even if we believe that surface to have been eventually disrupted and superficial discharges to have been established. If no disruption took place, then the ground would probably be upraised into a smooth dome, the older lavas being bent up over the cone of injected gabbro until the portion of the plateau so pushed upward had risen some hundreds of feet above the surrounding country. The amount of elevation, which would of course be greatest at the centre of the dome, might be far from equable all round, one side being pushed up further or with a steeper slope than another side. But even in the case of the Cuillin Hill area, it is conceivable that the total uplift produced at the surface a gentle inclination of no more than 8° or 10°.
It is along the periphery of a gabbro area that we may most hopefully search for traces of this uplift. But unfortunately it is just there that the work of denudation has been most destructive. There appears also to have been a general tendency to sagging subsequent to the gabbro protrusions, and the inward dip thereby produced has probably been instrumental in effacing at least the more gentle outward inclinations caused by the uprise of the eruptive rock. In one striking locality, however, to which I have already referred, the effects of both movements are, I think, preserved. The basalt-plateau of Strathaird, which in its southern portion exhibits the ordinary nearly level bedding, dips in its northern part at an unusually steep angle to the north-west, towards the gabbro mass of Blath Bheinn. But before reaching that mountain the basalts, much interbanded with sheets of dolerite and gabbro, suddenly bend up to form the prominent eminence of An Stac, where they dip rapidly towards south-east and south (Fig. 334). This steep dip away from the central mass of gabbro, is repeated in the hills to the north, where the beds are inclined to north-east, the angle gradually lessening northwards till they are truncated by the granophyre of Strathmore. The mass of Blath Bheinn thus occupies the centre of the dome or anticline. The theoretical structure of one of the gabbro bosses is represented in Fig. 343. It will be understood, however, that what for the sake of clearness is here represented in one uniform tint of black in reality consists of an exceedingly complex network of sheets and dykes differing from each other in texture and structure, as well as in the relative dates of their intrusion.
8. The injection of so much igneous material among the bedded basalts has induced in these rocks a certain amount of contact metamorphism. I have referred to it as showing itself in the field as a marked induration, the rocks becoming closer grained, dull, splintery, and weathering, with a grey or white crust, while their amygdales lose their definite outlines, and epidote and calcite run in strings, veins and patches through many parts of the rocks. As already remarked, it is difficult to determine how much of this change should be referred to the influence of the gabbro, and how much to that of the numerous intrusions of granophyre which may be apophyses of much larger bodies of that rock lying not far underneath. On account of this difficulty, the more detailed description of the metamorphism of the plateau-basalts is reserved for Chapter xlvi., where it will find a place in connection with the effects produced by the intruded granophyres, which have undoubtedly been more extensive than those effected by the gabbros.
The structure and history of the gabbro bosses of the Inner Hebrides find a close parallel in those of the Henry Mountains of Southern Utah, so well described by Mr. G. K. Gilbert of the United States Geological Survey.[357] In that fine group of mountains, rising to an extreme height of 5000 feet above the surrounding plateau, and 11,000 feet above the level of the sea, masses of trachyte have been injected between sedimentary strata belonging to the Jura-Triassic and Cretaceous systems. These masses, thirty-six in number, have consolidated in dome-shaped bodies, termed by Mr. Gilbert "laccolites," which have arched up the overlying strata, sending sheets, veins and dykes into them, and producing in them the phenomena of contact metamorphism. There is no proof that any of these protrusions communicated with the surface, and there is positive evidence that most if not all of them did not. The progress of denudation has laid bare the inner structure of this remarkable type of hill, and yet has left records of every stage in its sculpture. In one place are seen only arching strata, the process of erosion not having yet cut down through the dome of stratified rocks into the trachyte that was the cause of their uprise. In another place, a few dykes pierce the arch; in a third, where a greater depth has been bared away, a network of dykes and sheets is revealed; in a fourth, the surface of the underlying "laccolite" is exposed; in a fifth, the laccolite, long uncovered, has been carved into picturesque contours by the weather, and its original form is more or less destroyed.[358]
[Footnote 357: See the remarks and diagram, ante, p. 86.]
[Footnote 358: "Geology of the Henry Mountains," by Mr. G. K. Gilbert, U.S. Geographical and Geological Survey of the Rocky Mountain Region, 1877.]
The gabbro "laccolites" of the West of Scotland belong to an older geological period than those of Utah, and have, therefore, been longer subject to the processes of denudation. They have been enormously eroded. The overlying cover of basalt has been stripped off from them, though from the escarpments beyond them it is not difficult in imagination to restore it. In Rum it has been so completely removed, that only a few fragments remain at some distance from the core of gabbro, which now stands isolated. In Ardnamurchan, and still more in Skye, the surrounding plateau of basalt remains in contact with the gabbro bosses. But in Mull, where the plateau-basalts reach now, and perhaps attained originally a greater thickness than anywhere else, they have protected the intrusive sheets, which are therefore less deeply cut away than in any of the other districts, and no great central core of gabbro has yet been uncovered.