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]
[350] Western Islands, i. p. 486.
[351] Quart. Journ. Geol. Soc. xxx. p. 253.
[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.
Fig. 339.—Outline of the Hills of the Island of Rum, sketched from near the Isle of Eigg.
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.
Fig. 340.—View of Allival, Rum, sketched from the base of the north-east side of the cone.
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.
Fig. 341.—Section of foliated gabbros in the Tertiary volcanic series of Allival, Rum.
a, massive gabbro with rude lamination parallel to bedding, only seen in some weathered surfaces; b, laminated troctolite; c, massive coarsely crystalline gabbro rudely laminated.
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]
[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]
[354] Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871) p. 58.
[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.
Fig. 342.—Altered Plateau-Basalts invaded by Gabbro, and with a Dyke of prismatic Basalt cutting both rocks, north slope of Ben Buy, Mull.
a a, amygdaloidal basalt, much altered; b, gabbro; c, finely prismatic basalt.
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]
[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.
Fig. 343.—Theoretical representation of the structure of one of the Gabbro Bosses of the Inner Hebrides.
a a, platform of older rock on which the bedded basalts (b b) have been poured out; c, gabbro.
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]
[357] See the remarks and diagram, ante, p. 86.
[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.
CHAPTER XLV
THE ACID ROCKS
Their Petrography—Their Stratigraphical Position and its Analogies in Central France
We now come to the examination of another distinct phase of volcanic action during Tertiary time in Britain. The igneous rocks that have been under consideration in the foregoing chapters, whether poured out at the surface or injected below ground, have been chiefly of basic, partly indeed, like the peridotites, of ultra-basic character. Some, however, have shown an andesitic or intermediate composition. Reference has also been made to the probable eruption of acid rhyolites in the long interval between the outflow of the lower and the upper basalts in Antrim. But we now encounter a great series, decidedly acid in composition, in the more largely crystalline members of which the excess of silica is visible to the eye in the form of free quartz. While there is a strong contrast in chemical composition between this series and the rocks hitherto under discussion, there are also marked differences in structure and mode of occurrence. Like the gabbros, all the masses of acid rock now visible appear to be intrusive. They have been injected beneath the surface, and therefore record for us subterranean rather than superficial manifestations of volcanic action.
The existence of rocks of this class in the midst of the basic masses has long been recognized. They were noticed by Jameson, who described the hills between Loch Sligachan and Broadford as composed of "a compound of felspar and quartz, or what may be called a granitel, with occasional veins of pitchstone."[359] Macculloch gave a fuller account of the same region, and classed the rocks as chiefly "syenite" and "porphyry."[360] In Antrim, also, even in the midst of the basalt-tableland, masses of "pitchstone-porphyry "pearlstone-porphyry," "clay-porphyry," and "greystone" were observed and described.[361] In more recent years Professor Zirkel has given a brief account of the so-called "syenite and porphyry" of Mull and Skye,[362] and the late Professor Von Lasaulx fully described the "trachyte" or rhyolite of Antrim.[363]
[359] Mineralogical Travels, ii. 90.
[360] Western Isles, see the descriptions of Skye, Mull and Rum.
[361] Berger, Trans. Geol Soc. iii. (1816), p. 190; Portlock, Journ. Geol. Soc. Ireland, vol. i. (1834), p. 9.
[362] Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871), pp. 54, 77, 84, 88.
[363] Tschermak's Min. und Petrog. Mittheilungen, 1878, p. 412. The chemical composition of this rock and its place among the rhyolites had already been determined by E. T. Hardman from analysis, Journ. Geol. Soc. Ireland, vol. iii. (1871), p. 32.
This interesting series of rocks embraces a greater variety of petrographical characters than any other portion of the British Tertiary volcanic rocks. On the one hand, it presents thoroughly vitreous masses, some of which in their colour, lustre and microscopic structure remind us of recent obsidians. On the other hand, it affords coarsely crystalline compounds, to which no other name than granite can be assigned, and which, did we not know their geological position, might almost be classed with some of the most ancient eruptive rocks. Between these two extremes abundant gradations may be found, including beautiful spherulitic rocks, felsites and rhyolites.
In dealing with such a series of intrusive rocks, we again encounter the difficulty of reaching certainty as to their relative dates of eruption, since in each case all that can usually be affirmed is that the intrusive mass is younger than that into which it is injected. It is quite possible that protrusions of acid rocks occurred at intervals during the accumulation of the basic masses, as may perhaps be inferred from the rhyolite-tuffs and conglomerates of Antrim and from the occurrence of fragments of siliceous lavas in the gravels near the base of the basalt-plateau of Mull, and in the agglomerates of that island as well as of other districts.[364] It is probable, therefore, that at the time when the basalts of the plateaux were emitted, there existed, within reach of volcanic explosions, masses of granophyric, felsitic or rhyolitic rocks, fragments from which were shot up the funnels of discharge. That portions of these rocks were actually intruded into the basalt-sheets before the building up of the plateaux was completed appears to be proved in Antrim. Elsewhere, however, no evidence has yet been obtained of any such intrusion until after the close of the plateau-period. On the contrary, in every case where the relative ages of the rocks can be fixed, the acid are younger than the basic protrusions.
[364] Reference may also again be made to the agglomerates of Strath, Skye, which contain in some parts abundant fragments of acid rocks that closely resemble some of the masses of granophyre which disrupt these agglomerates.
The only known exceptions to this rule are the latest basalt-dykes. Hence, while amid the large and varied series of acid rocks, which no doubt represents a wide interval of time, some may belong to comparatively early epochs in the protracted volcanic period, the actual available evidence places the emission of these rocks, as a whole, towards the end of the volcanic history. This evidence I shall bring, forward in full detail, since it necessitates an abandonment of what has been the general belief in regard to the relative ages of the rocks.
i. PETROGRAPHY OF THE ACID ROCKS
The classification of the rocks which best harmonizes the field-evidence and the detailed study of their mineralogical composition, is one that arranges these volcanic protrusions into two series. In the one, the orthoclase is sanidine, and the rocks range from the most vitreous pitchstone through perlitic and spherulitic varieties to rhyolite ("quartz-trachyte"). In the other series, which embraces by far the largest proportion of the whole, the orthoclase is always turbid, and in this respect as well as in many others the rocks remind us rather of ancient eruptive masses than of those which have appeared in Tertiary time. They range from flinty felsitic varieties, which are obviously devitrified glasses, through different textures of quartz-porphyry into granophyre, and finally into granite. As I have been unable to recognize any essential difference of structure and composition between these acid Tertiary rocks and those of far earlier geological time, I give them the names which no petrographer would hesitate to apply to them if they were of Palæozoic age. It has long appeared to me that these rocks furnish conclusive evidence of the misleading artificiality of any petrographical nomenclature in which relative antiquity is made an essential element of discrimination.
Granite.—That true granites form part of the Tertiary volcanic series of the British Isles has now been completely established. They occur as bosses and sills which have been intruded into the gabbros and all older rocks. They are thus proved not only to belong to the Tertiary period, but to one of the latest phases of its volcanic history. But besides these granites, the relative age of which can be definitely fixed, there occur others which, standing alone and at some distance from the basaltic plateaux, can only be inferentially classed in the Tertiary series. To this group belong the granite masses of the Isle of Arran and the Mourne Mountains in north-eastern Ireland.
Taking first the unquestionably Tertiary granites which occur as bosses and intrusive sheets, we have to note that the more coarsely crystalline granophyres are hardly to be distinguished externally from granite. As the dark ferro-magnesian constituent of these rocks was generally believed to be hornblende, they were called by the older petrographers "syenite"; that is, granite with hornblende instead of mica. The peculiar micropegmatitic groundmass, which constitutes the distinguishing feature of the granophyres, may occasionally be observed so reduced in amount as only to appear here and there between the other minerals, which are grouped in a granitic structure. From this condition, one step further carries us into a true granite, from which all trace of the granophyric character has disappeared. Such gradations may be traced even within short distances in the same boss of rock. Thus, in the hornblende-biotite-granite boss of Beinn-an-Dubhaich, Skye, a thoroughly granitic arrangement of the component minerals is observable in the centre, while a specimen taken from near the edge on the shore of Camas Malag shows the development of a granophyric groundmass. But, though the large bosses are usually somewhat coarsely crystalline in the centre, and tend to assume finer felsitic textures around their borders, as was observed long ago by Oeynhausen and Von Dechen,[365] the granitic structure is sometimes exhibited even at the very edge, and not only so, but in the dykes that protrude from the bosses into the surrounding rocks. Thus the Beinn-an-Dubhaich mass, at its margin in Camas Malag, sends a vein into the surrounding limestone, but though more close-grained than the main body of the rock, this vein is neither felsitic nor granophyric, but truly granitic in structure.
[365] Karsten's Archiv, i. p. 89.
So far as I have observed, the true granites contain a brown mica and also a little hornblende, both visible to the naked eye, but generally somewhat decomposed. These rocks are thus hornblende-biotite-granites (amphibole-granitites of Rosenbusch). They may be defined as medium-grained aggregates of quartz, orthoclase (also plagioclase), biotite and hornblende, with sometimes magnetite, apatite, epidote and zircon. Dr. Hatch found that in some instances (Beinn-an-Dubhaich) the quartz contains minute inclusions (glass?), bearing immovable bubbles with strongly-marked contours; while in others (Beinn-na-Chro, Skye) this mineral is full of liquid inclusions with bubbles, sometimes vibratile, sometimes fixed. He remarked that the quartz and felspar have consolidated almost simultaneously, but that in some instances (Marsco, Glen Sligachan) there are isolated roughly idiomorphic crystals, of a white, less turbid orthoclase, which belong to a slightly earlier consolidation than that of the more kaolinized felspar of the rest of the rock.
The granite of the island of Arran, in the Birth of Clyde, which is here included in the Tertiary volcanic series, has long been recognized as consisting of two distinct portions, an eastern or coarse-grained, and a western or fine-grained variety. The latter sends veins into the former. These granites contain orthoclase, plagioclase, quartz and dark mica, the quartz being often idiomorphic with respect to the felspar, and a tendency towards a micropegmatitic structure being sometimes observable. A distinguishing characteristic of the Arran granite is the cavernous or drusy structure which it presents, the cavities being often lined with well-crystallized orthoclase and smoky quartz.[366] The granite of the Mourne Mountains in Ireland closely resembles that of Arran. Its druses, with their beautifully terminated minerals, have long been well known.
[366] See Mr. Teall's British Petrography, p. 328.
Microgranite.—This term is applied to certain intrusive masses, which megascopically may be classed with the quartz-porphyries and felsites, but which microscopically are found to possess a holocrystalline granitic groundmass of quartz and orthoclase, through which are scattered porphyritic crystals of the same two minerals, sometimes also with plagioclase, augite, magnetite or apatite. Rocks of this type do not appear to be abundant. They occur as dykes and bosses, but occasionally also as sheets. I have collected them from Skye, Rum and Ardnamurchan.
Granophyre.—Under this name may be grouped the large majority of the acid rocks which play an important part in the geology of the West of Scotland. They are typically developed in the islands of Mull and Skye. Generally pale grey or buff in colour, they range in texture from the true granites, into which, as above stated, they graduate, to exceedingly close-grained varieties like the felsites of Palæozoic formations. In the great majority of them the micrographic intergrowth of quartz and felspar, known as micropegmatite, is their conspicuous structure, and even constitutes most of their substance. They may thus be classed generally as granophyres, in the sense in which this term is employed by Rosenbusch, but without his limitation of it to pre-Tertiary rocks.
The specific gravity of these rocks has been determined from a series of specimens by Mr. A. Harker to range from about 2·3 among the felsites to 2·7 among the granites. No chemical analyses of these rocks have yet been made, but they have been subjected to microscopical examination, and their general structure and composition are now known.
The typical granophyre of the Inner Hebrides outwardly closely resembles an ordinary granite of medium grain, in which the component dull felspar and clear quartz can be readily distinguished by the naked eye. Throughout all the varieties of texture there is a strong tendency to the development of minute irregularly-shaped drusy (miarolitic) cavities, which here and there give a carious aspect to the rock. That these cavities, however, are part of the original structure of the rock, and are not due to mere weathering, is shown by the well-terminated crystals of quartz and felspar which project into them. On a small scale, it is the same structure so characteristic of the granite of the Mourne Mountains and of parts of that of Arran.
Examined under the microscope, a normal specimen of the granophyre of the Western Isles presents a holocrystalline groundmass, which fills all the interspaces between the crystals of earlier consolidation. This groundmass consists of an aggregate of clear quartz and turbid orthoclase, arranged as micropegmatite, but also in more or less idiomorphic crystals. In some parts, the two dominant minerals are grouped in alternate parallel fibres, diverging from the surface of the enclosed crystals, which are thus more or less completely surrounded by a radially fibrous mass. The felspathic portion of the micropegmatite which usually surrounds the orthoclase crystals, when viewed between crossed Nicols, is found to extinguish simultaneously with the central crystal.[367] In other parts, the felspar forms a kind of network, the meshes of which are filled up with quartz. Through the groundmass, besides the clear quartz and dull orthoclase, some ferro-magnesian or other additional constituent is generally distributed, but usually somewhat decomposed. In certain varieties Dr. Hatch found an abundant brown mica, as in the rock at Camas Malag, Skye. In others, a pyroxene occurs, which he observed in minute greenish grains, sometimes completely enclosed in the quartz. In a third variety, the dark constituent is hornblende, the most remarkable example of which is one to be seen at Ishriff, in the Glen More of Mull, where the ferro-magnesian mineral takes the form of long dirty-green needles, conspicuous on a weathered surface of the rock. A fourth variety is distinguished by containing plagioclase in addition to or instead of orthoclase. In the rock of the sheet forming Cnoc Carnach, near Heast, in Skye, Dr. Hatch observed both orthoclase and plagioclase scattered through a fine micropegmatitic groundmass, and in a part of the boss at Ishriff he found the rock to be composed mainly of plagioclase, in a micropegmatitic groundmass of quartz and felspar, with a few scattered grains of a pale brown augite and grains of magnetite. A fifth variety is marked by the prominence of the crystals of quartz and felspar of earlier consolidation, and by the fineness of grain in the surrounding micropegmatitic groundmass, whereby a distinct porphyritic structure is developed. Rocks of this kind are megascopically like ordinary quartz-porphyries. Still another variety has been detected by Mr. Teall in the rock of Meall Dearg, at the head of Glen Sligachan, Skye, in which, besides irregular patches which may represent decayed biotite, and others which are possibly ilmenite, the rare mineral riebeckite is present.[368]
[367] Mr. Teall, Quart. Journ. Geol. Soc. vol. 1. (1894) p. 219. See also his British Petrography, p. 327.
[368] Quart. Journ. Geol. Soc. vol. 1. (1894), p. 219.
Felsite.—The close-grained rocks into which the ordinary granophyres frequently graduate may be conveniently grouped under the general name of Felsite. They differ in no essential feature from the felsites of the Palæozoic formations. They are more particularly developed, as might be expected, in those places where the conditions have been most favourable for rapid cooling, while the more coarsely crystalline granophyres occur where the material may be supposed to have consolidated most slowly. Where the acid magma has been injected into chinks and fissures so as to take the form of veins or dykes, it is sometimes felsitic, sometimes granophyric, in texture. Along the margin of large bosses, like those of Mull and Skye, it frequently though not invariably has assumed a fine texture, with even spherulitic and flow-structures. But in the centre of large bosses it usually appears as coarse granophyre or as granite.
The felsites vary in texture from flinty or horny to dull finely-granular, and in colour from white through shades of grey, buff and lilac, to black, generally with porphyritic felspars and blebs of quartz. Where these porphyritic enclosures increase in size and number, the rocks cannot be distinguished externally from ancient quartz-porphyries. In general the groundmass of these rocks has been completely devitrified. But in some dykes enough of the glassy base remains to show their original vitreous condition. A gradation can thus be traced from thoroughly glassy pitchstone into completely lithoid felsites and crystalline granophyres.
A characteristic feature of the felsitic varieties of acid rock is their flow-structure, which they often display in great perfection. Sometimes, indeed, this structure has been so strongly developed as to cause the rock to weather along the planes of flow and to break up into thin slabs.
Many of these rocks also present admirably developed spherulitic structures, varying from microscopic minuteness up to large round or egg-shaped balls nearly two inches in diameter, and often distributed in lines along those of flow-structure. They likewise exhibit a frequent development of micropegmatite. No line indeed can be drawn between these felsites and the granitoid varieties, for the same characteristic granophyric intergrowth of felspar and quartz runs through them all.
Pitchstone.—This name is applied to the glassy varieties apart from their chemical composition, and specially denotes the possession of a vitreous structure. Some of the rocks to which it has been applied are probably glassy varieties of andesite, others are dacites, while some may be as acid as the most acid felsites and granophyres. The pitchstones are found in veins or dykes which traverse different geological formations up to and including the great granophyre bosses of the Inner Hebrides. They vary in colour from a deep jet-black or raven-black to a pale bottle-green, and in lustre from an almost glassy obsidian-like to a dull resinous aspect. Occasionally they assume a felsitic texture, owing to devitrification, and also a finely spherulitic structure. Some varieties appear to the naked eye to be perfectly homogeneous, others become porphyritic by the appearance of abundant sanidine crystals.
The microscopic structure of the British pitchstones has not yet been fully worked out. The beautiful feathery microlites of the Arran dykes, first made known by David Forbes, and subsequently described by Zirkel, Allport and others, are well known objects to geological collectors. Dr. Hatch, in whose hands I placed my tolerably large collection of specimens and their thin slides, furnished me with some preliminary notes on the slides, from which the following generalized summary is compiled.
At the one end of the pitchstone group we have a nearly pure glass, with no microlites, and only a few scattered crystals of sanidine, quartz, augite or magetite. The glass in thin slices is almost colourless, but generally inclines to yellow, sometimes to dark-grey. Some varieties of the rock are crowded with microlites, in others these bodies are gathered into groups, the glass between which is nearly free from them. Among the minerals that have been observed in this microlitic form are sanidine, augite, hornblende (forming the beautiful green feathery or fern-like aggregates in the Arran pitchstones, [Fig. 3]) and magnetite. Sometimes the rudimentary forms appear as globulites, or as belonites, but more commonly as dark trichites. Among the more definite mineral forms are grains of sanidine, quartz and augite. The porphyritic crystals are chiefly sanidine, augite and magnetite, but plagioclase occasionally occurs. The development of spherulites is well seen in a few of the slides, and occasionally perlitic structure makes its appearance.
The interesting rhyolitic areas of Antrim include several varieties of pitchstone. One of these is described by Professor Cole as "a glassy pyroxene-rhyolite, on the verge of the rhyolitic andesites." Another is a blue-black porphyritic obsidian.[369]
[369] Scientif. Trans. Roy. Dublin Soc. vol. vi. (ser. ii.) 1896, p. 77.
Rhyolite (Quartz-Trachyte).—This rock has been abundantly erupted in north-east Ireland, where it rises in occasional bosses among the plateau-basalts.[370] It is best exposed at the Tardree and Carnearny Hills, where it has long been quarried. Its petrographical characters at that locality were described by Von Lasaulx as those of a typical quartz-trachyte rich in tridymite, and containing large crystals of glassy sanidine, isolated narrow laths of plagioclase (probably andesine), grains of smoky-grey quartz, partly bounded by dihexahedral faces, and a few scattered flakes of a dark-coloured mica. The groundmass is microgranitic, and under a high power is resolvable into a confused aggregate of minute microlites of felspar, with interstitial quartz-granules.[371] More recently a detailed investigation of the petrography of the Antrim rhyolites has been conducted by Professor Cole, who has called attention to their remarkable varieties of structure, ranging from perfect volcanic glass to a thoroughly lithoidal texture, and exhibiting flow, perlitic and spherulitic structures.[372]
[370] Fragments of acid rock were detected by Prof. Cole in the gravel among the Ardtun basalt of Mull, as already noticed on p. 212.
[371] Tschermak's Min. und Pet. Mittheil. 1878, p. 412.
[372] Scientif. Trans. Roy. Dublin Soc. vol. vi. (ser. ii.) 1896, p. 77. This paper gives an excellent account of the microscopical character and mineralogical and chemical compositions of these rocks.
Intrusive masses of rhyolite are also found in the Carlingford region. One of these, seen at Forkhill, is a velvet-black almost resinous rock with abundant quartz and felspar, and sometimes displaying beautiful flow-structure. It will be more particularly described in Chapter xlvii. Some of the acid dykes and sills of the Inner Hebrides are varieties of rhyolite. No undoubted example has yet been observed of a superficial rhyolite-lava, though such not improbably appeared in the interval between the lower and upper basalts of Antrim.
ii. STRATIGRAPHICAL POSITION.—ANALOGIES FROM CENTRAL FRANCE
In the history of opinion regarding the relative position of the Tertiary eruptive rocks, no feature is so remarkable as the universal acceptance of the misconception regarding the place of the acid protrusions. In tracing this mistake to its source, we find that it probably arose from the fact that along their line of junction the granitoid masses generally underlie the basic. This order of superposition, which would usually suffice to fix the age of two groups of stratified rocks, is obviously not of itself enough to settle the relative epochs of two groups of intrusive rocks. Yet it has been assumed as adequate for this purpose, and hence what can be proved to be really the youngest has been placed as the oldest part of the Tertiary volcanic series.
Macculloch, who showed that his "syenites" and "porphyries" had invaded the Secondary strata of the Inner Hebrides, and must therefore be of younger date than these, left their relations to the other igneous rocks of the region in a curiously indefinite position. He was disposed to regard them all as merely parts of one great series; and seems to have thought that they graduate into each other, and that any attempt to discriminate between them as to relative age is superfluous. Yet he evidently felt that the contrasts of topography which he described could hardly fail to raise the question of whether rocks so distinct in outward form did not differ also in relative antiquity. But he dismissed the question without answering it, remarking that if there is any difference of age between the two kinds of rock, "there appears no great prospect of discovering it."[373] He records an instance of a vein of "syenite" traversing the "hypersthene rock" in the valley of Coruisk. "If this vein," he says, "could be traced to the mass of syenite, it might be held a sufficient ground of judgment, but under the present circumstances it is incapable of affording any assistance in solving the difficulty."[374] Instead, however, of being a solitary instance, it is only one of hundreds of similar intrusions which can be connected with the general body of granitic and granophyric masses, and which put the relative ages of the several groups of rock beyond any further doubt.
[373] Western Islands, i. p. 368; see also pp. 488, 575, 578.
[374] Op. cit. p. 370.
Boué, who knew the geology of some of the extinct volcanic regions of Europe, recognized the similarity of the Scottish masses to those of the Continent, and classed the acid rocks as "trachytes." He saw in each of the volcanic areas of the West of Scotland a trachytic centre, and supposed that the more granitoid parts might represent the centres in the European trachytic masses. He traced in imagination the flow of the lava-streams from these foci of volcanic activity, distinguishing them as products of different epochs of eruption, among the last of which he thought that the trachytic porphyries might have been discharged. He admitted, however, that his restoration could not be based on the few available data without recourse to theoretical notions drawn from the analogy of other regions.[375]
[375] Essai Géologique sur l'Écosse, pp. 291, 322, 327.
In the careful exploration of the central region of Skye made by Von Oeynhausen and Von Dechen, these able observers traced the boundary between the "syenite" and the "hypersthene rock"; and as they found the former lying underneath the latter, they seem naturally to have considered it to be the older protrusion of the two.[376] Principal Forbes came to a similar conclusion from the fact that he found the dark gabbro always overlying the light-coloured felspathic masses.[377] Professor Zirkel also observed the same relative position, and adopted the same inference as to the relative age of the rocks.[378] Professor Judd followed these writers in placing the acid rocks before the basic. He supposed the granitoid masses to form the cores of volcanic piles probably of Eocene age, through and over which the protrusions of gabbro and the eruptions of the plateau-basalts took place.[379]
[376] Karsten's Archiv, i. p. 82. It will be shown in later pages that the apparent infraposition of the granophyre is often deceptive, the real junction being vertical.
[377] Edin. New Phil. Jour. xl. (1846) p. 84.
[378] Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871) pp. 90, 95. He says that the gabbro seems to be the younger rock, so far as their relations to each other can be seen.
[379] Quart. Jour. Geol. Soc. xxx. (1874) p. 255.
The evidence for the posteriority of the acid rocks will be fully detailed in later pages. Before entering upon its consideration, however, I would remark that the uprise of the British granophyres presents so many points of resemblance to that of the trachytes and phonolites among the basalt-plateaux of Auvergne and the Velay in Central France, that a brief account of the acid protrusions of these regions may be suitably given here as an introduction to the account of those of the Inner Hebrides. A succession of stages in the progress of denudation allows us to follow the gradual isolation and dissection of the French volcanic groups. The youngest examples occur in the chain of cones and craters, in the region of the Puy de Dôme. These may be of Pleistocene, or even of more recent date. Older and more deeply eroded than these are the numerous domes and cones in the territory of Haute Loire. Yet more ancient and still more stupendously denuded come the bosses, sills and dykes of Britain. Nevertheless, the geologist, by the methods so admirably devised by Desmarest, may follow the chain of relationship through these different regions and trace a remarkable continuity of structure. The younger rocks serve to illustrate the original condition of the more ancient, while the latter, by their extensive denudation, permit points of structure to be seen which in the former are still concealed.
No feature in the interesting volcanic district of Auvergne has attracted more attention than the trachytic protrusions.[380] Rising conspicuously along the chain of puys, they claim notice even from a distance owing to the topographical contrast which their pale rounded domes offer to the truncated, crater-bearing cones of dark cinders around them. They consist of masses of a pale variety of trachyte (domite), which in ground-plan present a circular or somewhat elliptical outline. They vary in size from the nearly circular dome of the Grand Sarcoui, which measures about 400 yards in diameter, to the largest mass of all—that of the Puy de Dôme, which extends for some 1500 yards from north to south with a breadth varying from 500 to 800 yards. They are likewise prominent from their height; in the Puy de Dôme they form the highest elevation of the whole region (1465 metres), and even in the less conspicuous hills they rise from 500 to 600 feet above the surrounding plateau.
[380] The admirable Map and Memoirs of Desmarest on Auvergne are classics in geology. Scrope's work, vol. i. p. 45, gives still the best published account of this district. See also the work of Lecoq (ibid.). The results of more detailed petrographical research regarding the rocks will be found in the essays of M. Michel Lévy (Bull. Soc. Géol. France, 1890, p. 688) and in the Clermont sheet of the Geological Survey Map of France (Feuille, 166). A bibliography of the district up to the year 1890 is given in the volume of the Bull. Soc. Géol. France just cited, p. 674.
Five such dome-shaped protrusions of trachyte have made their appearance among the cinder-cones in a space of about five English miles in length by about two miles in extreme breadth. Though opinions have varied as to the mode of formation of these domes, there has been a general agreement that their present topographic contours cannot be far from the original outlines assumed by the masses at the time of their production. The position of the trachyte bosses among the puys serves to show that they were not deep-seated masses which have been entirely uncovered by denudation, but were essentially superficial, and were protruded to the surface at various points along the plateau in the midst of already existing cinder-cones. In some cases, they have risen on or near the position of the vents of these cones. Thus the Puy de Chopine is half encircled by the crater of the Puy de la Goutte, and the Grand Sarcoui stands in a similar relation to the fragmentary crater-wall of the Petit Sarcoui.
M. Michel Lévy, in pointing out the superficial character of the domitic protrusions, has forcibly dwelt on the evidence that these rocks have undergone a comparatively trifling denudation, and that they could never have extended much beyond their present limits.[381] As Scrope pointed out, they were obviously protruded in a pasty condition, not flowing out in streams like the other lavas of the district, but consolidating within their chimneys and rising from these in rounded domes.
[381] Op. cit. p. 711.
Fig. 344.—Section through the Puy de la Goutte and Puy de Chopine.
1, Mica-schist; 2 2, Granite; 3 3, Tuffs; 4, Trachyte; 5, Basalt dyke.
Undoubtedly denudation, cannot have left them altogether unaffected, but must have removed some amount of material from their surface. There is reason to believe that the material so removed may have been in large part of a fragmental character, and that it was under a covering of loose pyroclastic debris that the upward termination of the trachyte column assumed its typical dome-form. Thus in the crater-wall of the Puy de la Goutte, layers of buff-coloured trachytic tuff dip gently away from the central domite mass of the Puy de Chopine. That this material was thrown out from the vent previous to the uprise of the domite may be inferred from the way in which the latter rock has obliterated the northern half of the crater. The relations of the rocks are somewhat obscured by talus and herbage, but when I last visited the locality in the spring of 1895 the structure seemed to me to be as expressed in the accompanying diagram ([Fig. 344]).[382]
[382] Compare M. Michel Lévy, ibid.
The relative date of the protrusion of the trachytic domes cannot be very precisely defined. There can, indeed, be no doubt that it belongs to a late phase of the volcanic history. It came long after the outpouring of the older basaltic plateaux, of which large fragments emerge from beyond the limits of the younger lavas on both sides of the great ridge of the puys, and not only long after that outpouring, but even after the widespread sheets of basalt had been deeply trenched by valleys and isolated into outliers capping the hill-tops. Yet there is good evidence also that the uprise of the comparatively acid trachytes was not the last volcanic episode of the district. The abundance of dark slags and fragments of basalt lying on the domite hills shows that discharges of more basic detritus occurred after these hills had taken their place in the landscape.
Since the latest eruptions, a gradual alteration of the topographical features by denudation has been slowly but continuously going on. The Grand Sarcoui, possibly from having originally had a considerable covering of fragmentary material, shows least the effects of this waste. Its remarkably regular form, like that of an inverted cauldron (the "Chaudron," as it is called in the district), presents, in a distant view, a smooth grassy surface which slopes steeply down into the great volcanic plain. But on a nearer examination these declivities are found to be seamed with trenches which the rain-storms of centuries have dug out. The covering of loose debris has been largely washed away, though many fragments of dark slag are still strewn over the slopes, and the scars are now being cut into the domite below. A more advanced stage of decay may be seen on the Puy de Dôme, where, from greater elevation and exposure, the domite is already deeply gashed by gullies and ravines, while the slopes below are strewn with its detritus.
The region of the Velay displays on a far more extensive scale the protrusion of trachytic and phonolitic bosses, but as its volcanic history goes back beyond the time of the Puys of Auvergne, its volcanic monuments have consequently been more extensively affected by denudation.[383] A series of basaltic eruptions forming extensive sheets can there be traced, the oldest dating from Miocene time, the youngest coming down to the age of the mammoth, cave-bear and early man. During this prolonged outpouring of basic lavas there were several intervals during which materials of a more acid nature—trachytes and phonolites—were erupted. These rocks occur partly as extensive tracts, covering five or six square miles, like those of the Mezenc, the Megal, the Pic de Lizieux, and the Rand, and partly in isolated conical or dome-shaped prominences, sometimes only a few hundred feet in diameter. Upwards of one hundred distinct eruptions of phonolite have been observed in the Velay. Even in the tracts where they cover the largest space, several prominent eminences may usually be observed, not unlike in general shape the isolated cones and domes of Auvergne. In these wider areas there appears to be evidence of the outcome of the lava from one or more vents, either as superficial streams or as underground intrusive sheets. M. Boule has expressed his opinion that most of the masses of trachyte and phonolite have been the result of local and limited eruptions, the pasty rock having risen in and accumulated around its pipe, without flowing far in any direction. A section across one of these masses would present a somewhat mushroom-shaped form.[384]
[383] In addition to the work of Scrope, the student of this important volcanic district will find an invaluable guide in the Le Puy Sheet (No. 186) of the Geological Survey Map of France, and in the Bulletins of the Survey, particularly those by MM. Termier and Boule, No. 13 (1890) and No. 28 (1892).
[384] Bull. Carte. Géol. France, No. 28 (tome iv.) p. 125.
That fragmentary ejections accompanied the protrusion of these rocks, though probably on a very limited scale, is shown by the occasional survival of portions of trachyte tuff around them. One of the most notable of these deposits occurs in the hollow between the Suc du Pertuis and the next dome to the south. It consists of fine and coarse, trachytic detritus, which in one place is rudely bedded and appears to dip away from the phonolite dome behind it at an angle of 30°. This material and its inclination are what might be expected to occur round an eruptive vent, and may be compared with those of the crater-wall of the Puy de la Goutte in relation to the domite boss of the Puy de Chopine.
The denudation of Velay has undoubtedly advanced considerably further than that of the Puys of Auvergne. The pyroclastic material which may have originally covered the domes of trachyte and phonolite has been in great part swept away. The surrounding rocks, too, both aqueous and igneous, have been extensively removed from around the necks of more enduring material. Hence the trachyte and phonolite bosses stand out with so striking a prominence as to arrest the eye even for a distance of many miles.
Fig. 345.—View of the Huche Pointue and Huche Platte west of Le Pertuis.
The cone is one of the trachytic domes, while the flat plateau to the left is a denuded outlier of the basalt sheets.
There cannot be any doubt that these necks have pierced the older basalts, and therefore belong to a later epoch in the volcanic history. The approximately horizontal sheets of basalt have been deeply eroded and reduced to mere fragments, and in some instances their existing portions owe their survival to the protection afforded to them by the immense protrusions of more acid material. But there is here, as well as in Auvergne, evidence of the uprise of a later more basic magma, for sheets of basalt are found overlying some parts of the trachytes and phonolites.
While the external forms of these Velay necks recall with singular vividness the features of many more ancient necks in Britain, an examination of the internal structure of some of them affords some further interesting points of resemblance. The slabs into which, by means of weathering along the joints, the rock is apt to split up are sometimes arranged with a general dip outwards from the centre of the hill, so that their flat surfaces roughly coincide with the hillslopes. In other cases the peculiar platy structure, so characteristic of phonolite, is disposed vertically or dips at a steep angle into the hill, so that the edges of the slabs are presented to the declivities, which consequently become more abrupt and rugged.
Though none of the volcanic series in Auvergne or the Velay is so acid in composition as the more acid members of the Tertiary volcanic series of Britain, the manner in which the trachytes and phonolites of the French region make their appearance presents some suggestive analogies to that of the corresponding rocks in this country. We see that they were erupted long after the outpouring of extensive basaltic plateaux, that they belonged to successive epochs of volcanic activity, that they were protruded in a pasty condition to the surface, where, more or less covered with fragmentary ejections, they terminated in dome-shaped hills or spread out to a limited distance around the vents, and lastly, that they were succeeded by a still later series of more basic eruptions, which completed the long volcanic history. We shall see in the following pages how closely the various stages in this complex record of volcanic activity may be paralleled in the geological records of Tertiary time in Britain.[385]
[385] The phonolite necks of Bohemia, which form so prominent a feature in the Tertiary geology of that country, might likewise be cited here in illustration of the acid domes and bosses of the British Isles.
CHAPTER XLVI
TYPES OF STRUCTURE IN THE ACID ROCKS—BOSSES
Returning now to the consideration of the acid rocks as these manifest themselves in the volcanic areas of Britain, I would remark that three distinct types of structure may be noted among them, viz. (1) bosses, (2) sills or intrusive sheets, (3) veins and dykes. These types, as above remarked, belong entirely to the underground operations of volcanism, for though the rhyolitic fragments in the tuffs and agglomerates of the plateaux prove that acid lavas existed near the surface, no undoubted case of superficial lava belonging to the acid series has yet been observed.[386]
[386] The rhyolites of Tardree in Antrim have recently been claimed by Professor Cole as true lavas grouped round an eruptive vent. For reasons to be given in the next chapter I regard them as intrusive masses, though they may not improbably have been connected with streams of lava now entirely removed.
The bosses of acid material in the British Tertiary volcanic series are irregular protrusions, varying in size from knobs only a few square yards in area up to huge masses many square miles in extent, and comprising groups of lofty hills. As a rule, their outlines are markedly irregular. Beneath the surface they plunge down almost vertically through the rocks which they traverse, but in not a few instances their boundaries are inclined to the horizon, so that the contiguous rocks seem to rest against them, and sometimes lie in outliers on their sides and summits. From the margins of these bosses apophyses are given off into the surrounding rocks, sometimes only rarely and at wide intervals, in other places in prodigious numbers. Sometimes the acid material has been injected in thousands of veins and minute threads, which completely enclose fragments of the surrounding rock.
The rock of which the bosses consist is generally granophyric in texture, passing on the one hand, particularly in the central parts, into granite, and on the other, and especially towards the margin, into various more compact felsitic varieties, and sometimes exhibiting along the outer edge more or less developed spherulitic and flow-structures.
Decided contact metamorphism is traceable round the bosses, but is by no means uniform even in the same rock, some parts being highly altered, while others, exposed apparently to the same influences, have undergone little change. The most marked examples of this metamorphism are those in which the Cambrian limestone of Skye has been converted into a pure white saccharoid marble. But the most interesting to the student of volcanic action are those where the altered rocks are older parts of the volcanic series. As the bosses of each volcanic area offer distinctive peculiarities they will here be described geographically.
i. THE ACID BOSSES OF SKYE
It is in the island of Skye that the granophyre and granite bosses attain their largest dimensions and afford, on the whole, the most complete evidence of their structures and their relations to the other parts of the volcanic series (Map VI.). They cover there a total area of about 25 square miles, and form characteristic groups of hills from 2000 to 2500 feet in height. On the south-east side, three conspicuous cones (the Red Hills) rise from the valley of Strath (Beinn Dearg Mhor, Beinn Dearg Bheag and Beinn na Caillich). A solitary graceful pointed cone (Beinn na Cro) stands between Strathmore and Strathbeg, while to the north-west a continuous chain of connected cones runs from Loch Sligachan up into the heart of the Cuillin Hills. Their conical outlines, their smooth declivities, marked with long diverging lines of screes, and their pale reddish or reddish-yellow hue, that deepens after a shower into glowing orange, mark off these hills from all the surrounding eminences, and form in especial a singular contrast to the black, spiry, and rugged contours of the gabbro heights to the west of them.
Besides this large continuous mass, a number of minor bosses are scattered over the district. Of these the largest forms the ridge of Beinn an Dubhaich, south of Loch Kilchrist. Several minor protrusions lie between that ridge and the flank of Beinn Dearg. Others protrude through the moory ground above Corry; several occur on the side of the Sound of Scalpa, about Strollamus; and one, already referred to, lies at the eastern base of Blath Bheinn. In the neighbouring island of Raasay, a large area of granophyre likewise occurs, which will be described with the Sills in later pages.
In so extensive a district there is room for considerable diversity of composition and texture among the rocks. As already stated, in some places, more particularly in the central parts of the hills, the acid material assumes the character of a granite, being made up of a holocrystalline aggregate of quartz, orthoclase, plagioclase, hornblende and biotite, without granophyric structure, and thus becomes a hornblende-biotite-granite (quartz-syenite, granite-syenite of Zirkel, or amphibole-granitite of Rosenbusch). By the development of the micropegmatitic structure and radiated spherical concretions, it passes into granophyre. By the appearance of a felsitic groundmass, it shades off into different varieties of quartz-porphyry or rhyolite, sometimes with distinct bi-pyramidal crystals of quartz.[387] This change, which here and there is observable along the edge of a boss, is sometimes accompanied with an ample development of spherulitic and flow-structures. As it is convenient to adopt some general term to express the whole series of varieties, I have used the word granophyre for this purpose.
[387] The best account yet published of these varieties in Skye is that by Prof. Zirkel, Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871) p. 88.
Fig. 346.—View of Glamich, 2537 feet, Glen Sligachan. (From a photograph by R. J. A. Berry, M.D., lent by the Scottish Mountaineering Club).
That the large area of these rocks in Skye was the result of many separate protrusions from distinct centres of emission may be inferred, I think, not only from the varieties of petrographical character in the material, but also from the peculiar topography of the ground, and perhaps from the curious relation which seems, in some instances at least, to be traceable between the external features and apparent internal structure of the hills. It will be seen from the Map (No. VI.) that in the area lying to the east of Strath More the granophyre is broken up into nearly detached portions by intervening patches of older rocks. There can be little doubt that the mass of Beinn na Caillich and the two Beinn Deargs is the product of a distinct orifice, if not of more than one. Beinn na Cro, lying between its two deep bounding glens, is another protrusion. The western cones stand so closely together that their screes meet at the bottoms of the intervening valleys. Yet each group is not improbably the result of emission from an independent funnel, like the separate domite puys of Auvergne.
But, though I believe this large area of granitoid rock to have proceeded not from one but from many orifices, I have only here and there obtained, from the individual hills themselves, indications of an internal structure suggestive of distinct and successive protrusions of material from the same vent of discharge. On the outer declivities of some of the cones we may detect a rudely bedded structure, which will be subsequently referred to as well displayed in Rum (p. 403). This structure is specially observable along the east side of Glen Sligachan. Down the northern slopes of Marsco the granophyre (here in part a hornblende-biotite-granite) is disposed in massive sheets or beds that plunge outwards from the centre of the hill at angles of 30° to 40°. On the southern front of the same graceful cone, as well as on the flanks of its neighbour, Ruadh Stac, still plainer indications of a definite arrangement of the mass of the rock in irregular lenticular beds may be noticed. These beds, folding over the axis of the hill, dip steeply down as concentric coats of rock. The external resemblance of the red conical mountains of Skye to the trachyte puys of Auvergne was long ago remarked by J. D. Forbes,[388] and in this internal arrangement of their materials, indefinite though it may be, there is a further resemblance to the onion-like coatings which Von Buch and Scrope remarked in the structure of the interior of the Grand Sarcoui.[389]
[388] Edin. New Phil. Jour. xl. p. 78.
[389] Von Buch, Geognostische Beobachtungen auf Reisen durch Deutschland und Italien, vol. ii. (1809) p. 245; Scrope, Geology and Extinct Volcanoes of Central France, 2nd edit. p. 68. Von Buch regarded the external form of this Puy as having been determined by its internal structure.
Where the contour of the cones is regular, and the declivities are not marked by prominent scars and ribs of rock, this monotony of feature betokens a corresponding uniformity of petrographical character. But where, on the other hand, the slopes are diversified by projecting crags and other varieties of outline, a greater range of texture and composition in the material of the hills is indicated. This relation is well brought out on the western front of Marsco, where numerous alternations of granitoid and felsitic textures occur. On many declivities also, which at a distance look quite smooth, but which are really rough with angular blocks detached from the parent mass underneath, an occasional basalt-dyke will be observed to rise as a prominent dark rib. A good example of this structure is to be seen on the south front of Beinn na Caillich. Where a group of dark parallel dykes runs along the sides of one of these pale cones, it sometimes produces a curiously deceptive appearance of bedding. A conspicuous illustration may be noticed on the southern front of Beinn Dearg Meadhonach, north from Marsco. When I first saw that hillside I could not realize that the parallel bars were actually dykes until I had crossed the valley and climbed the slopes of the hill.[390]
[390] The difference of contour and colour between the ordinary reddish smooth-sloped "syenite" and the black craggy "hypersthene rock" and "greenstone" in the Glamaig group of hills caught the eyes of Von Oeynhausen and Von Dechen (Karsten's Archiv, i. p. 83).
Good evidence of successive protrusions of the acid rock within the great area of the Red Hills may be found on the south side of Meall Dearg at the head of Glen Sligachan, where the granophyre is traversed by a younger band or dyke of fine-grained spherulitic material about ten feet broad. The rock exhibits there the same beautiful flow-structure with rows of spherulites as is to be seen along the contact of the main granophyre mass with the gabbro on the same hill, which will be afterwards described. This dyke, vein or band, though possibly belonging to the same epoch of protrusion as the surrounding granophyre, must obviously be later than the consolidation of the rock which it traverses.
Occasionally round the margin of the granophyre a singular brecciated structure is to be seen. I have found it well marked on weathered faces, along the flanks of Glamaig and of Marsco, and Mr. Harker has observed many examples of it on the north side of the granophyre mass of the Red Hills. When the rock is broken open, it is less easy to detect the angular and subangular fragments from the surrounding matrix, which is finely crystalline or felsitic.
The actual junction of the eruptive mass with the surrounding rocks through which it has ascended is generally a nearly vertical boundary, but the granophyre sometimes plunges at a greater or less angle under the rocks that lie against or upon it. On the north side of Glamaig, for instance, the prophyritic and felsitic margin of the great body of eruptive rock descends as a steeply inclined wall, against which the red sandstones and marls at the base of the Secondary formations are sharply tilted. On the south side of the area a similar steep face of fine-grained rock forms the edge of the granophyre of the great southern cones, and plunges down behind Lias limestone and shale, Cambrian limestone and quartzite, or portions of the Tertiary volcanic series. Where the granophyre cuts vertically through the gabbro, the latter rock being more durable is apt to rise above the more decomposable granophyre as a crag or wall, and thus the deceptive appearance arises of the basic overlying the acid rock. As above mentioned, there seems every reason to believe that this peculiarity of weathering has given rise to or confirmed the mistaken impression that the granophyre is older than the gabbro.
There can be no doubt, however, that along many parts of the boundary-line the acid eruptive mass extends underneath the surface far beyond the actual base of the cones, for projecting knobs as well as veins and dykes of it rise up among the surrounding rocks. This is well seen along the northern foot of Beinn na Caillich. But of all the Skye bosses none exhibits its line of junction with the surrounding rocks so well and continuously as Beinn an Dubhaich. This isolated tract of eruptive material lies entirely within the area of the Cambrian limestone, and its actual contact with that rock, and with the basalt-dykes that traverse it, can be examined almost everywhere. The junction is usually vertical or nearly so, sometimes inclining outwards, sometimes inwards. It is notched and wavy, the granite sending out projecting spurs or veins, and retiring into little bays, which are occupied by the limestone. The subdivisions of the latter rock have recently been traced by Mr. Harker up to one side of the granite and recognized again on the other side, with no apparent displacement, as if so much limestone had been punched out to make way for the uprise of the acid boss. The older dykes, too, are continuous on either side of the ridge. The granite is massive and jointed, splitting up into great quadrangular blocks like an ancient granite, and weathering into rounded boulders. Its granitic composition and texture are best seen where the mass is broadest, south of Kilbride. Towards its margin, on the shore of Camas Malag, the granophyric structure appears, especially in narrow ribbons or veins that run through the more granitic parts of the rock. These may be compared with the much larger dyke of spherulitic rock above noticed as traversing the granophyre of Meall Dearg.
Fig. 347.—Section across the north slope of Beinn an Dubhaich, Skye.
a a, Cambrian limestone; b b, basalt dykes; c, granite.
Immediately to the south of Camas Malag the junction with the limestone is well displayed, and the eruptive rock, which is there granitic in character, sends out into the limestone a vein or dyke about two feet broad, of closer grain than the main body of the boss, but still distinctly granitic in structure. The junction on the north side is equally well seen below the crofts of Torran. Here the rock of the boss, for a few yards from its margin, assumes a fine-grained felsitic aspect, and under the microscope presents a curious brecciated appearance, suggestive of its having broken up at the margin before final consolidation. Portions of the already crystallized granite seem to be involved in a microgranitic base. The rock has here truncated a number of basalt-dykes which intersect the Cambrian limestone. To one of these further reference will be made in the sequel.
On the surface of the mass of Beinn an Dubhaich, a few little patches of limestone occur to the south of Kilchrist Loch. Considering the nearly vertical wall which the granophyre presents to the adjacent rock all round its margin, we may perhaps reasonably infer that these outliers of limestone are remnants of a once continuous limestone sheet that overlay the eruptive rock, and hence that, with due allowance for considerable denudation, the present surface of the boss represents approximately the upper limit to which the granophyre ascended through the limestone. The actual facts are shown in [Fig. 347].
All round the margin of this boss, the limestone has been converted for a variable distance of a few feet or many yards into a granular crystalline marble. The lighter portions of the limestone have become snowy white; but some of the darker carbonaceous beds retain their dark tint. The nodules of chert, abundant in many of the limestones, project from the weathered faces of the marble. The dolomitic portions of the series have likewise undergone alteration into a thoroughly crystalline-granular or saccharoid rock. The most thorough metamorphism is exhibited by portions of the limestone which are completely surrounded by and rest upon the granite. The largest of these overlying patches was many years ago quarried for white marble above the old Manse of Kilchrist. I have shown by lithological, stratigraphical and palæontological evidence that this limestone, instead of belonging to the Lias, as was formerly believed, forms a part of the Cambrian or possibly the very lowest Silurian series, being a continuation of the fossiliferous limestone of western Sutherland and Ross-shire.[391] Mr. Clough and Mr. Harker, in the progress of the Geological Survey in Skye, have ascertained that the distinctive characters of the three groups of strata into which the limestone can be divided may be recognized even through the midst of the metamorphism.[392]
[391] Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 62.
[392] Annual Report of Director-General of the Geological Survey for 1895.
The generally vertical line of separation between the rock of Beinn an Dubhaich and the contiguous limestone has been taken advantage of for the segregation of mineral veins. On the southern boundary at Camas Malag, a greenish flinty layer, from less than an inch to two or three inches in width, consisting of a finely-granular aggregate of some nearly colourless mineral, which polarizes brilliantly, coats the wall of the granophyre, and also both sides of the vein which proceeds from that rock into the limestone. But the most abundant and interesting deposits are metalliferous. Fragments of a kind of "gossan" may be noticed all along the boundary-line of the boss, and among these are pieces of magnetic iron-ore and sulphides of iron and copper. The magnetite may be seen in place immediately to the south of Kilbride. A mass of this ore several feet in diameter sends strings and disseminated particles through the surrounding granophyre, and is partially coated along its joints with green carbonate of copper.
From the Skye area important evidence is obtainable in regard to the relation of the acid eruptions to (1) earlier eruptive vents filled with agglomerate; (2) the bedded basalts of the plateaux; (3) the bosses, sills and dykes of gabbro and dolerite; and (4) the great system of basic dykes.
(1) Relation of the Granophyre to older Eruptive Vents.—The granophyre of Beinn na Caillich and the two Beinn Deargs has invaded on its north-eastern side the Cambrian limestone and quartzite, and has truncated the sheets of intrusive dolerite and gabbro that have there been injected into them. But to the south-west it rises through the great Strath agglomerate already described, and continues in that rock round to the entrance into Strath Beg. The eruptive mass is in great part surrounded with a ring of agglomerate, as if it had risen up a huge volcanic chimney and solidified there, though probably there were more than one vent in this agglomerate area. Again the thick mass of agglomerate north of Belig is interposed between the bedded lavas and the great granophyre mass which extends northwards to Loch Sligachan. On the west side of the Blaven ridge, a number of masses of agglomerate are found on both sides of Glen Sligachan, along the border of the same great tract of acid rock.
Fig. 348.—Section from Beinn Dearg to Beinn an Dubhaich, Skye.
a a, Cambrian limestone; b b, volcanic agglomerate; c c c, basalt-dykes older than granophyre; d1, granophyre of Beinn Dearg; d2, granophyre in the agglomerate neck; d3, granite of Beinn an Dubhaich; e, basalt-dyke younger than granite.
With regard to the relation of the granophyre of the Red Hills to the great agglomerate of Strath, we may infer that the granophyre has not risen exactly in the centre of the old funnel, but rather to the north of it, unless we suppose, as already suggested, that some of the agglomerate belongs to the cone that gathered round the eruptive orifice. It is interesting to observe, however, that granophyre, from the same or from another centre of protrusion, has likewise risen along the outer or southern margin of the agglomerate, generally between that rock and the limestone, but sometimes entirely within the agglomerate. The distance between the nearest part of this ring of eruptive rock and the edge of the boss of Beinn an Dubhaich is under 400 yards, the intervening space being occupied by limestone (or marble), much traversed by north-west basalt-dykes. Most of these dykes do not enter the rocks of the vent, and are abruptly truncated by the mass of Beinn an Dubhaich. The probable structure of this locality is shown in Fig 348.
The masses of agglomerate which further westward so curiously follow the margin of the great granophyre bosses, and those which are entangled in that rock and in the gabbro, probably indicate, as already suggested, the position of a group of older volcanic funnels which provided facilities for the uprise of the basic and acid magmas. The group of vents which, as we have seen, probably rose out of the plateau-basalts, and first served for the rise of the masses of gabbro, has by the subsequent protrusion of the granophyres been still further destroyed and concealed.
The granophyre intrusions in the great Strath agglomerate have lately been mapped and described by Mr. Harker. As regards their internal structure and composition, this observer remarks that compared with the normal granophyres of the Red Hills and other bosses of the district, these smaller intrusive masses are darker and manifestly richer in the iron-bearing minerals, and have a slightly higher specific gravity. But in their general characters they agree with the other granophyres. The most interesting feature in them is the evidence they afford that they have enclosed and partially dissolved fragments of basic rocks. To this evidence further reference will be made on a later page (see [p. 392]).
(2) Relation of the Granophyre to the Bedded Basalts of the Plateaux. Metamorphism of the Basalts.—On the north-west side, the granophyre of Glamaig and Glen Sligachan mounts directly out of the bedded basalts. These latter rocks, which rise into characteristic terraced slopes on the north side of Loch Sligachan, appear on the south side immediately to the west of Sconser, and stretch westwards round the roots of Glamaig into the Coire na Sgairde. As they approach that hill they assume the usual dull, indurated, splintery, veined character of their contact metamorphism, and weather with a pale crust. Some of them are highly amygdaloidal, and between their successive beds thin bands of basalt-breccia, also much hardened, occasionally appear. Veins of granophyre become more numerous nearer the main mass of that rock. The actual line of junction runs into the Coire na Sgairde and slants up the Druim na Ruaige, ascending to within a few feet of the top of that ridge. A dark basic rock lies on the granophyre, the latter being here finer grained and greenish in colour, and projecting up into the former.[393] There is so much detritus along the sides and floor of Glen Sligachan that the relations of the two groups of rock cannot be well examined there. But the basalts, which present their ordinary characters to the north of the Inn, are observed to become more and more indurated, close-grained, dull and splintery, as they draw nearer to the granophyre of Marsco. This part of the district furnishes the clearest evidence of the posteriority of the great cones of Glamaig and its neighbours to the plateau-basalts which come up to the very base of these hills.[394]
[393] I think it probable that some of the greenish portions of the granophyre along this part of the junction-line will be found to have had their structure and composition altered by having incorporated into their substance a proportion of the bedded basalts through which they have been disrupted.
[394] The dykes of granophyre in these basalts are referred to at p. 444.
Round the eastern group of cones some interesting fragments of the once continuous sheet of plateau-basalts remain, and show the same relation of the acid protrusions on that side. One of these lies on the granophyre of the flanks of Beinn na Caillich, a little to the west of the loch at the northern base of that hill. Another of larger size forms a prominent knob about three-quarters of a mile further west, and is prolonged into the huge dark excrescence of Creagan Dubha, which rises in such striking contrast to the smooth red declivities of the granophyre cones around it. This prominence at its eastern and northern parts consists of highly indurated splintery basalt in distinct beds, some of which are strongly amygdaloidal. The bedding is nearly vertical, but with an inclination inwards to the hill. Towards the south-west end a thin band of basalt-breccia makes its appearance between two beds of basalt. Its thickness rapidly increases southward until it is the only rock adhering to the granophyre. Beyond the foot of the hill, limestone and quartzite occupy for some distance the bottom of Strath Beg, much invaded by masses of quartz-porphyry. At the summit of Creagan Dubha abundant veins run into the basic rocks from the granophyre, which is here finer grained towards the margin; and there are likewise veins of quartz-porphyry which, though their actual connection with the main mass of granophyre cannot be seen, are no doubt apophyses from it.
This outlier of altered basalt and breccia appears to me to be a fragment of the plateau-basalts which once overlay the Cambrian and Jurassic rocks of Strath Beg, and were disrupted by the uprise of the granophyre. It continues to adhere to the wall of the eruptive mass that broke up and baked its rocks. Its breccia, passing southward into a coarse agglomerate, may be a product of the same vent or group of vents that discharged the great agglomerate mass above Kilbride and Kilchrist. I have already (p. 282) referred to what appears to be another outlier of the basalts on the south side of Beinn Dearg.
On the northern and southern flanks of Beinn na Cro, similar evidence may be observed of the posteriority of the granophyre to the basic rocks. Round the northern base of the hill a continuous tract of plateau-basalts, dolerites and gabbros forms the ridge between Strathmore and Strathbeg. There is an admirable section of the relation of the two groups of rock on the eastern side of the western glen. Along the lower part of the declivity, coarsely-crystalline gabbros, like some of those in the Cuillin Hills, are succeeded by sheets of dolerite and basalt, the whole forming an ascending succession of beds to the summit of the ridge. The edges of these beds are obliquely truncated by the body of granophyre, which slants up the hill across them and sends veins into them. They are further traversed by basalt dykes, which here, as almost everywhere, abound ([Fig. 349]). On the south side of Beinn na Cro, highly indurated black and grey Lias shales and sandstones have been tilted up steeply and indurated by the eruptive rock of the hill; and at one place some 800 feet above the sea, a little patch of altered basalt, lying on the shale, but close up against the steep declivity of granophyre, forms a conspicuous prominence on the otherwise featureless slope.
Reference has already been made to the mass of fine-grained hornblende-granite which runs for several miles at the base of the volcanic series on the eastern side of the Blaven group of hills. Mr. Harker has traced a great development of granophyre on the west side of these hills, where the acid rock sends apophyses both into the bedded basalts and into the gabbros.
Fig. 349.—Section at north end of Beinn na Cro, Skye.
a, basalt, dolerite and gabbro; b, granophyre of Beinn na Cro; b′, dyke of granophyre; c c, basalt dykes.
Combining the results of observations made not only in Skye but in Mull, Rum and Ardnamurchan, I shall here give a fuller account of the metamorphism of the basalts, to which frequent allusion has been made as one of the evidences of the posteriority of the eruptive bosses of rock round which it occurs.[395] The field-geologist observes that the basalts, as they are traced towards these bosses, lose their usual external characters. They no longer weather into spheroidal blocks with a rich brown loam, but project in much jointed crags, and their hard rugged surface shows when broken a thin white crust, beneath which the rock appears black or dark bluish-grey, dull and splintery. They are generally veined with minute threads or strings of calcite, epidote and quartz, which form a yellowish-brown network that projects above the rest of the weathered surface. Where they are amygdaloidal, the kernels no longer decay away or drop out, leaving the empty smooth-surfaced cells, but remain as if they graduated into the surrounding rock by an interlacing of their crystalline constituents. They then look at a distance more like spots of decoloration, and even when seen close at hand would hardly at first betray their real nature.
[395] Many years ago I was much struck with the evidence of alteration in the igneous rocks of Mull, and referred to it in several papers, Proc. Roy. Soc. Edin. (1866-67) vol. vi. p. 73; Quart. Journ. Geol. Soc. xxvii. (1871) p. 282, note. The subject was more fully discussed in my memoir in the Trans. Roy. Soc. Edin. vol. xxxv. (1888) p. 167, from which the account in the text is taken. Prof. Judd has more recently referred the alteration to solfataric action (Quart. Journ. Geol. Soc. xlvi. 1890, p. 341). As already mentioned, I have been unable to detect evidence of such action. The alteration is always intimately connected with the presence of intrusive masses, and it affects indifferently any part of the basalt-plateaux which may chance to lie next to these masses. The bedded lavas can be traced step by step from their usual unaltered condition in the plateaux to their metamorphosed state next to the eruptive rocks. The nature or degree of the metamorphism has doubtless somewhat varied with the composition and structure of the rocks affected, and with the character and mass of the eruptive material; but it is certainly not confined to the older parts of the plateaux, nor to any supposed pre-basaltic group of andesites. I have found no evidence that such a group anywhere preceded the plateau-basalts. The andesites, so far at least as my observations go, were erupted at intervals during the plateau period, and alternate with the true basalts. The greatest accumulation of them lies not below but above the general body of the basalts, in the "pale group" of Mull. Nor even if the term "propylite" be adopted for these altered rocks, can it be applied to any special horizon in the volcanic series. The alteration of the basic rocks by the granophyre of St. Kilda will be described in the account of that island in Chapter xlvii.
From the specimens collected by me among the Inner Hebrides up to the year 1888, I selected two dozen which seemed to be fairly typical of these altered rocks, and placed thin slices of them for microscopic examination in Dr. Hatch's hands. His notes may be condensed into the following summary. One of the most frequent features in the slides is the tendency in the component minerals to assume granular forms. In one specimen from Loch Spelve, Mull, the rock, probably originally a dolerite, shows only a few isolated recognizable crystals of plagioclase and augite, the whole of the rest of the rock consisting of roundish granules embedded in a felspathic matrix. The felspar crystals are sometimes broken up into a mosaic, though retaining their external contours. Besides the granules, which are no doubt augite, a few grains of magnetite are scattered through the rock, aggregated here and there into little groups. In another specimen, taken from the junction with the granophyre in Glenmore in the same island, parts of the augite crystals are converted into granular aggregates associated with large grains and patches of magnetite. The latter mineral also assumes in some of the rocks granular and even globular shapes suggestive of fusion.
The felspars, which in most of the basic rocks are usually remarkably clear and fresh, show marked kaolinization in some of these altered masses. Minute dusky scales of kaolin are developed, sometimes also with the separation of minute grains of quartz. The augite shows frequent alteration to hornblende, proceeding as usual from the exterior inward. In some cases only an envelope of uralite appears round the augite, while in others only a kernel of the original mineral is left, or the whole crystal has been changed. In many cases the altered substance appears as minute needles, blades and fibres of actinolite. Occasionally, besides the green hornblende, shred-like pieces of a strongly pleochroic brown hornblende make their appearance. Serpentinous and chloritic substances are not infrequent. Epidote is sometimes abundant. The titaniferous iron has commonly passed more or less completely into leucoxene. Here and there a dark mica may be detected.
Since the year 1888 I have continued the investigation of this subject, and have especially studied the metamorphism of the bedded basalts on the western shores of Loch Scavaig, where, as already described, they are truncated by vertical beds of gabbro, and are traversed by basalt-dykes and by abundant veins of fine-grained granophyre. The alteration here effected affords excellent materials for study, as the very same sheets of basalt can be followed from the normal conditions outside to the altered state within the influence of the metamorphic agent. The alternations of amygdaloidal and more compact sheets can still be recognized, although their enclosed amygdales have in places been almost effaced. They show the dull, indurated, splintery character, with the white weathered crust, so distinctive of this type of contact-metamorphism. They are traversed by numerous sills and veins of gabbro. As has been already suggested, although no large mass of granophyre appears here at the surface, the alteration of the basalts is probably to be attributed not so much to the influence of the gabbro, as to the abundant acid sills, dykes and veins, for there may be a considerable body of granophyre underneath the locality, the dykes and veins being indications of its vicinity.
In the summer of 1895 I examined the locality with much care, and collected some typical specimens illustrative of the conditions of metamorphism presented by different varieties of the bedded basalts. Thin slices cut from these specimens were placed in Mr. Harker's hands for microscopical examination, and he furnished me with the following notes regarding them.
"In hand-specimens the bedded basalts from the neighbourhood of the gabbro of Loch Scavaig [6613-6618] do not appear very different from the normal basalts of this region. The most conspicuous secondary mineral is yellowish-green epidote in patches, and especially in the amygdales.
"The texture of the rocks varies, and the slices show that the micro-structure also varies, the augite occurring sometimes in small ophitic plates, sometimes in small rounded granules. The chief secondary change in the body of the rock is shown by the augite, which is seen in various stages of conversion to greenish fibrous hornblende. Some round patches seem also to consist mainly of the latter mineral, and are probably pseudomorphs after olivine. Here the little fibres are confusedly matted together, without the parallelism proper to uralite derived from augite. No fresh olivine has been observed. The felspar and magnetite of the basalts show little or no sign of metamorphic processes, unless a rather unusual degree of clearness in the felspar crystals is to be regarded in that light.
"The contents of the metamorphosed amygdales are not always the same. Epidote is usually present in some abundance, and in well-shaped crystals. It has a pale citron tint in the slices, with marked pleochroism; but a given crystal is not always uniform in its optical characters. Frequently the interior is pale, and has a quite low birefringence. This is probably to be regarded as an intergrowth of zoisite in the epidote, and there are a few distinct crystals of zoisite seen in some places.
"In the slide which best exhibits these features [6613] the crystals of epidote are in part enwrapped and enclosed by what are doubtless zeolitic minerals. At least two of these are to be distinguished. One, very nearly isotropic, and with a pale-brownish tint, is probably analcime. Associated with this is a colourless mineral with partial radiate arrangement and with twin lamellation; the birefringence is somewhat higher than that of quartz, and the γ-axis of optic elasticity makes a small angle with the twin-line. These characters agree with those of epistilbite. In other parts of the same large amygdale, the epidote crystals are embedded in what seems to be a felspar. This latter mineral is rather obscure, and twin-lamellation is rarely to be detected; but it seems highly probable that felspar has here been developed by metamorphic agency at the expense of zeolites which once occupied the amygdale. I have observed undoubted examples of this in metamorphosed basalts from other parts of Skye, e.g. from Creagan Dubha, near the granophyre mass of Beinn Dearg.[396] The felspar occurs there in the same fashion, and in the same relation to epidote [2700, 2701]. In the specimens now described the chief minerals in the metamorphosed amygdales are those already named: others occur more sparingly, associated with them. In some cases there is a grass-green, strongly pleochroic, actinolitic hornblende, accompanied by a little iron pyrites [6615].
[396] Compare Trans. Roy. Soc. Edin. vol. xxxv. p. 166.
"Epidote and various hornblendic and augitic minerals are characteristic products in the metamorphism of amygdaloidal basalts in other regions: felspar with this mode of occurrence I have not seen except in Skye, where it seems to connect itself naturally with the abundance of zeolites in the amygdales of the non-metamorphosed lavas. It is to be observed that in these basalts from Loch Scavaig the alteration is shown especially in the amygdales, the body of the rock not being greatly affected: this indicates a not very advanced stage of metamorphism. The production of uralitic hornblende, rather than brown mica, from the augite and its decomposition-products, seems to be characteristic of the metamorphism of basaltic as distinguished from andesitic rocks, and is well illustrated by a comparison of the two sets of lavas near the Shap granite."[397]
[397] Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 361.
Mr. Harker, who is at present engaged in mapping the central region of Skye, has had occasion to go over a number of the localities (Creagan Dubha, etc.) originally cited by me, and, while corroborating my general conclusions regarding them, has been able to obtain much fresh evidence regarding the nature and extent of the metamorphism which the bedded basalts have undergone. The results of his investigations will be published when the Geological Survey of Skye is further advanced.
(3) Relation of the Granophyre to the Gabbros.—That the granophyres invade the gabbros has been incidentally illustrated in the foregoing pages. But as the mutual relations of the two rocks in the island of Skye have been the subject of frequent reference in previous writings of geologists, it is desirable to adduce some detailed evidence from a region which has been regarded as the typical one for this feature in the geological structure of the Inner Hebrides. No geological boundary is more easily traced than that between the pale reddish granophyre and the dark gabbro. It can be followed with the eye up a whole mountain side, and can be examined so closely that again and again the observer can walk or climb for some distance with one foot on each rock. That there should ever have been any doubt about the relations of the two eruptive masses is possibly explicable by the very facility with which their junction can be observed. Their contrasts of form and colour make their boundary over crag and ridge so clear that geologists do not seem to have taken the trouble to follow it out in detail. And as the pale rock undoubtedly often underlies the dark, they have assumed this infraposition to mark its earlier appearance.
I will only cite one part of the junction line, which is easily accessible, for it lies in Glen Sligachan immediately to the south of the mouth of Harta Corry. The rounded eminence of Meall Dearg, which rises to the south of the two Black Lochs, belongs to the granophyre, while the rugged ground to the west of it lies in the gabbro. The actual contact between the two rocks can be followed from the side of Harta Corry over the ridge and down into Strath na Creitheach, whence it sweeps northward between the red cone of Ruadh Stac and the black rugged declivities of Garbh Beinn. There is no more singular scene in Skye than the lonely tract on the south side of Meall Dearg. The ground for some way is nearly level, and strewn with red shingle from the decomposing granophyre underneath. It reminds one of some parts of the desert "Bad lands" of Western America. Grim dark crags of gabbro, with veins from the granophyre, rise along its western border, beyond which tower the black precipices of the Cuillins, while the flaming reddish-yellow cones of Glen Sligachan stand out against the northern sky.
Having recently described in some detail the relations of the boss of granophyre at this interesting locality, I will only here offer a brief summary of the chief features.[398] The granophyre of Meall Dearg forms a marginal portion of the great mass of the Red Hills. It has broken across the banded gabbros, and also cuts an isolated boss of agglomerate in the ridge of Druim an Eidhne. Its line of junction is nearly vertical, but along part of its course the wall of gabbro rises higher than that of the more decomposable granophyre. Hence the origin of the black crags that crown the red slopes of granophyre debris. Seen from a distance the basic rock seems to rest as a great bed upon the acid mass.
[398] See Quart. Journ. Geol. Soc. vol. 1. (1894) p. 212.
The younger date and intrusive nature of the granophyre are well shown by the change in the texture of the mass as it approaches the rocks against which it has cooled. The ordinary granophyric characters rapidly pass into a fine-grained felsitic texture, and this change is accompanied with the development of a remarkably well-defined flow-structure and of rows of spherulites which run parallel to the boundary wall. In a ravine on the west side of Meall Dearg, the lines of flow-structure and rows of large spherulites are seen to be arranged vertically against the face of gabbro.
Further proof of the later date of the protrusion of the granophyre is supplied by abundant felsitic dykes and veins which traverse the gabbro, and some of which can be seen to proceed from the main body of granophyre. These intrusions will be described in the next chapter, in connection with the dykes and veins of the acid rocks.
Additional evidence as to the posteriority of the granophyre to the gabbro has recently been obtained by Mr. Harker from a study of the internal structure and composition of the masses of these rocks which have been intruded into the agglomerate above Loch Kilchrist in Strath. He has found that the granophyre has there caught up from some subterranean depth portions of gabbro, and has partially dissolved them, thereby undergoing a modification of its own composition. "The gabbro-debris," he remarks, "has been for the most part completely disintegrated by the caustic or solvent action of the acid magma on some of its minerals. Those constituents which resisted such action have been set free and now figure as xenocrysts [foreign crystals], either intact or more or less perfectly transformed into other substances. At the same time the material absorbed has modified the composition of the magma, in the general sense of rendering it less acid." Mr. Harker has traced the fate of each of the minerals of the gabbro in the process of solution and isolation in the acid magma, which, where this process has been most developed, is believed by him to have taken up foreign material amounting to fully one-fourth of its own bulk, derived not from the rocks immediately around, but from a gabbro probably at a considerable depth beneath.[399]
[399] Quart. Journ. Geol. Soc. vol. lii. (1896) p. 320.
Fig. 350.—Ground-plan of basic dyke in Cambrian Limestones truncated by granophyre which encloses large blocks of the dyke, Torrin, Skye.
(4) Relation of the Granophyre to the Basic Dykes and Veins.—Reference has already been made to the fact that the "syenite" bosses of Skye cut off most of the basalt-dykes, but are themselves traversed by a few others.[400] The locality that furnished me with the evidence on which this statement was originally made nearly forty years ago affords in small compass a clearer presentation of the facts than I have elsewhere met with. The sections described by me are visible at the eastern end of the boss of Beinn an Dubhaich, Strath; but similar and even better examples may be cited from the whole northern and southern margins of that eruptive mass. On the north side an extraordinary number of dykes may be traced in the Cambrian limestone from the shores of Loch Slapin eastwards. They have a general north-westerly trend, but one after another, as I have already remarked, is abruptly cut off by the granophyre. As an example of the way in which this truncation takes place, I may site a single illustration from the northern margin of the eruptive mass, near Torrin. It might perhaps be contended that the numerous dykes which traverse the limestone and stop short at the edge of the acid rock, are not necessarily older than the granophyre, but may actually be younger, their sudden termination at the edge of the acid boss being due to their inability to traverse that rock. That this explanation is untenable is readily proved by such sections as that given in [Fig. 350], where a basic dyke (b) 9 or 10 feet broad running through the Cambrian Limestone (a a) is abruptly cut off by the edge of the great granophyre boss. Not only is the dyke sharply truncated, but numerous pieces of it, from an inch to more than a foot in length, are enclosed in the granophyre. The latter is well exposed along the shore of Loch Slapin in an almost continuous section of nearly a mile in length. The contrast therefore between the development of dykes within and beyond its area cannot but arrest the attention of the observer. Though I was on the outlook for dykes in the granophyre, I found only one. Yet immediately beyond the eruptive boss they at once appear on either side up to its very edge, where they suddenly cease. The conclusion cannot be resisted that the protrusion of the acid rock took place after most of the dykes of the district had been formed, but before the emission of the very latest dykes, which pursue a north-west course across the boss ([Fig. 348]).
[400] Ante, p. 173, and Quart. Journ. Geol. Soc. vol. xiv. (1857) p. 16.
Some sections on the southern margin of Beinn an Dubhaich complete the demonstration that such has been the order of appearance of the rocks. Near the head of the Allt Lèth Slighe (or Half-way Burn), where the granite has pushed a long tongue into the limestone, a north-west basalt-dyke is abruptly cut off by the main body of the boss and by the protruded vein ([Fig. 351]). Besides this truncation, the acid rock sends out strings and threads of its own substance into and across the dyke, these injected portions being as usual of an exceedingly fine felsitic texture.
Fig. 351.—Section on south side of Beinn an Dubhaich, Skye, showing the truncation of a basalt-dyke (b), in Cambrian Limestone (a), by the granite (c) of Beinn an Dubhaich, Skye.
Similar evidence may be gathered from the area of the great granophyre cones further north. The profusion of basalt-dykes in the surrounding rocks stops short at the margin of that area. The comparatively few dykes which cross the boundary pursue a general north-west course through the granophyre, and, as already remarked, from their dark colour, greater durability and straightness of direction, stand out as prominent ribs on the flanks of the pale cones which they traverse.
CHAPTER XLVII
THE ACID BOSSES OF MULL, SMALL ISLES, ST. KILDA, ARRAN AND THE NORTH-EAST OF IRELAND
ii. THE ACID BOSSES OF MULL
Though of comparatively small extent, the granophyre bosses of the island of Mull afford to the geologist a large amount of instruction in regard to the relations of the different members of the volcanic series to each other. Especially important is the evidence which they contain of the connection between the acid and basic groups of rocks. They have been laid bare in many natural sections, some of which, forming entire hillsides, are among the most astonishing in the whole wonderful series which, dissected by denudation, reveal to us the structure of these volcanic regions. They lie in two chief areas. One of these extends along the northern flanks of the mountainous tract from the western side of Beinn Fhada across Loch Ba' to the west side of Glen Forsa. The other occupies for over three miles the bottom of Glen More, the deep valley which, skirting the southern side of the chief group of hills, connects the east side of the island by road with the head of the great western inlet of Loch Scridain. There are other minor areas. One of these extends for about a mile along the declivities to the south of Salen, across the valley of the Allt na Searmoin; another occurs at Salen; a third runs along the shore at Craignure. In the interior also, many isolated areas of similar rocks, besides thousands of veins, occur in the central group of hills and valleys which form the basins of the Glencannel and Forsa rivers (Map VI.).
The chief northern boss, which for the sake of convenience of reference may be called that of Loch Ba', has a length of nearly six miles, with a breadth varying from a quarter of a mile to about a mile and a quarter. It descends to within 50 feet of the sea-level, and is exposed along the crest of Beinn Fhada at a height of more than 1800 feet. It chiefly consists of a grey crystalline rock which might readily be identified as a granite, but which when examined microscopically is found to possess the granophyric structure. With this distinctly granular-crystalline rock are associated various porphyritic and felsitic masses, which pass into it, and are more specially observable along its border. An exceedingly compact black quartz-felsite or rhyolite forms its southern boundary, runs as a broad dyke-like ridge from the head of the Scarrisdale Water north-eastward across Loch Ba' ([Fig. 352]), and spreads out eastward into a mass more than a mile broad on the heights above Kilbeg in Glen Forsa. The sharp line of demarcation of this felsite, and its mass and extent, point to a different period of extravasation.
Fig. 352.—View of the hills on the south side of the head of Loch na Keal, showing the junction of the granophyre and the bedded basalts.
One bird, the bedded basalts of the Gribon plateau; two birds, the bedded dolerites and basalts of Beinn a' Chraig adhering to the northern slope and capping the hill; three birds, summit of Ben More, with A'Chioch to the left and the top of Beinn Fhada appearing in the middle distance between them; four birds, the granophyre slopes of Beinn a' Chraig with the great dyke-like mass of felsite on the left.
The geologist, who approaches this district from the north-east, has his attention arrested, even at a distance of several miles, by the contrast between the outer and inner parts of the hills that lie to the south-west of Loch Ba'. He can readily trace from afar the dark bedded basic rocks rising terrace above terrace, from the shores of Loch na Keal, to form the seaward faces of the hills along the southern side of that fjord. But he observes that immediately behind these terraces the mass of the rising ground obviously consists of some amorphous rock, which weathers into white debris. Nothing can be sharper than the contrast of colour and form between the two parts of the hills. The bedded plateau-rocks lie as a kind of wall or veneer against a steep face of the structureless interior ([Fig. 352]). Seen from the other or hilly side, the contrast is perhaps even more striking. But the astonishment with which it is beheld at a distance becomes intensified when one climbs the slopes, and finds that the sheets of dolerite and basalt (which from some points of view look quite level, yet dip towards the north-east at a gentle angle) are immediately behind the declivity abruptly truncated by a mass of granophyre. Of all the junction-lines between the acid bosses and the lavas of the plateaux, those exposed on these Mull hillsides are certainly the most extraordinary. So little disturbed are the lavas, that one's first impulse is to search for pebbles of the granophyre between the basalts, for it seems incredible that the inner rock should be anything but a central core of older eruptive material, against and round which the younger basic rocks have flowed. But, though the granophyre is so decomposing and covers its slopes with such "screes" of debris, that had the basalts been poured round it, they must infallibly have had some of its fragments washed down between their successive flows, not a single pebble of it is there to be found. This might not be considered decisive evidence, but it is extended and confirmed by the fact that the acid rock gives off veins which ramify through the basalts.
Before examining the actual contact of the two rocks, however, the geologist will not fail to observe here an admirable example of the gradual change which was described in the foregoing chapter as coming over the bedded basalts near the acid bosses. As he approaches the nucleus of white rock, the basalts assume the usual hard indurated character, not decaying into brown sand as on the plateaux, but often standing out as massive crags with vertical clean-cut joint-faces. This metamorphosed condition extends in some cases to a considerable distance from the main body of acid rock, especially where knobs of that material, protruding through the more basic lavas, show that it must extend in some mass underneath. Thus along the shore at Saline the bedded basalts succeed each other in well-defined sheets, some being solid, massive and non-amygdaloidal, others quite vesicular, and recalling the black scoriform surfaces of recent Vesuvian lavas; yet they are all more indurated than in the normal plateau-country, and they break with a hard splintery fracture. Immense numbers of dykes cut these rocks, and they are likewise pierced by occasional felsitic intrusions.
If we cross to the other side of the island and trace the bedded basalts away from the central masses of acid rock we meet with so gradual a diminution of the induration that no definite boundary-line for the metamorphism can be drawn. As we recede from the centre of alteration, the rocks insensibly begin to show brown weathered crusts, with spheroidal exfoliation, the reticulations of epidote and calcite become much less abundant, the amygdaloids gradually assume their normal earthy character, and eventually we find ourselves on the familiar types of the plateau. This transition is well seen along the shores of Loch na Keal.[401]
[401] Some of the thick massive sheets of basic rock along the south side of this inlet may possibly be altered sills.
These proofs of the alteration of the plateau-basalts are accompanied in Mull as in Skye by further abundant evidence that the acid rocks are of younger date than the basic. In particular, dykes and veins may be traced proceeding from the former and intersecting the latter. Thus, in the bed of the south fork of the Scarrisdale stream, a separate mass of granophyre (which under the microscope exhibits in perfection the characteristic structure of this rock) protrudes through the basalts in advance of the main mass, and a little higher up on the outskirts of that mass narrow ribbons of the granophyre run through the basic rocks. The contrast of colour between the pale veins of the intrusive rock and the dark tint of the basalts is well shown in the channel of the water. Similar sections may be seen on the flanks of Beinn Fhada, especially in the great corry north of Ben More, where the granophyre sends a tongue of finer grain between the beds of basalt. On the east side of Loch Ba' numerous proofs of similar intrusion may be observed. Thus at the east end of Loch na Dàiridh, where the granophyre has been intruded into the basalts, hand-specimens may be obtained showing the two rocks welded together. On the slopes of Cruach Tòrr an Lochain, where the granophyre has a felsitic selvage, the bedded basalts are traversed by veins of the latter material ([Fig. 353]). A little further east, at the head of the Allt na Searmoin, the bedded basalts, some of which are separated by slaggy scoriaceous surfaces, are intersected by another protrusion from the compact felsitic porphyry ([Fig. 354]).[402] A mile lower down the same valley a separate mass of granophyre sends out veins into the basalt, which as usual is dark bluish-grey in colour, indurated and splintery.
[402] This rock appears to the eye as a black finely crystalline-granular felsite. Under the microscope, it was found by Dr. Hatch to "present a markedly granulitic structure, consisting mainly of small rounded grains of dirty brown turbid felspar, with isolated granules of colourless quartz. Scattered through the rock, or accumulated in patches, are small spherical or drop-like granules of a bright green augite (coccolite)."
Fig. 353.—Section on south side of Cruach Tòrr an Lochain, Mull.
a, bedded basalts and dolerites; b, granophyre; c, marginal finer-grained band; d d, veins from the granophyre traversing the basic rocks.
Fig. 354.—Section at head of Allt na Searmoin, Mull.
a, basalts and dolerites, with slaggy upper surfaces; b, felsite.
As the posteriority of the Mull granophyre and felsites to the basalts is thus proved, the further question remains as to their manner of intrusion. Here and there, especially on the south-eastern side, between the head of the Scarrisdale river and Loch Ba', the line of junction between the two rocks is nearly vertical, but a body of black felsite intervenes as a huge wall between the ordinary granophyre and the basalt. On Beinn Fhada and Beinn a' Chraig the line of separation, as I have above remarked, is inclined outwards, and plunges under the basalts at an angle of 30° to 40°. The terraced basalts and dolerites are not sensibly disturbed, but end off abruptly against the steep face of intrusive rock. We might suppose that in this case the younger rock had merely carried upward the continuation of the beds that are truncated by it, as if an orifice had been punched out for its ascent. But on the top of the ridge of Beinn a' Chraig we find that the outliers which there remain are not portions of the lower basalts, but of the upper "pale group" of Ben More. The same rocks are prolonged on the other side of the Scarrisdale Glen, sweep over the summit of Beinn Fhada, and run on continuously into the crest of A'Chioch and the upper part of Ben More. The granophyre has usurped the place of the lower dolerites and basalts, but has left the more felspathic lavas of the "pale group" in their proper position. And to make this remarkable structure still more clear, sections may be seen on the southern flanks of Beinn Fhada, where the upper surface of the granophyre comes down obliquely across the edges of the lavas, and allows the junction of the basalts and the "pale group" to be seen above it ([Fig. 355]). As in the case of Beinn an Dubhaich, it is as if the granophyre had eaten its way upward and dissolved the rocks which it has replaced.
Fig. 355.—Section on south side of Beinn Fhada, Mull.
a, bedded basalts and dolerites; b, "pale group" of Ben More; c, granophyre.
The usual kind of contact-metamorphism has been produced around this intrusive boss. It is most marked in the outliers that cap Beinn a' Chraig and on the two ridges to the south-west, where it is seen to consist in a high degree of induration, the production of a shattery, irregularly-jointed structure, and the effacement of the obvious bedding which characterizes the unaltered rocks.
The position of this eruptive mass, quite a mile broad, breaking through, without violently tilting, more than 1800 feet of the bedded basalts, and then stopping short about the base of the "pale group," presents a curious problem to the student of geological physics. It at once reminds him of many sections among Palæozoic granites where an eruptive boss has ascended and taken the place of an equivalent volume of the surrounding rocks, which, though more or less metamorphosed, are not made to dip away from it as from a solid wedge driven upwards through them. In this Mull case, however, there are some peculiar features that deserve consideration, for they seem to show that here, as elsewhere, passages for the uprise of the intrusive rock were already provided by the presence of volcanic pipes, which, even if filled up with fragmentary materials, would no doubt continue to be points of weakness. Round the flanks of the Loch Ba' boss, and here and there on its surface, patches of intensely indurated volcanic agglomerate may be detected. A little to the south of the tarn called Loch na Dàiridh, the granophyre is succeeded by the black, flinty felsite or rhyolite already referred to. This rock in some places exhibits a beautiful flow-structure, with large porphyritic felspars, and encloses a great many fragments of dolerite and gabbro, varying from the size of a pea up to blocks several inches in diameter. Lying on its surface are detached knolls of much altered dolerite, basalt, and coarse breccia or agglomerate. On its southern margin one of these patches of agglomerate contains abundant fragments of various felsitic rocks, among which are pieces of a compact rock with flow-structure like that found in place immediately to the north; also rounded pieces of quartzite, and of compact and amygdaloidal basalt wrapped up in a very hard matrix which seems to consist largely of basalt-dust. No bedding can be made out in this rock, and the mass looks like part of a true neck. Further down the slope the bedded basalts appear. The actual junctions of the different rocks cannot be satisfactorily traced, but the structure of the ground appears to me to be as shown in [Fig. 356]. A patch of similar agglomerate appears a little to the south-west of the last section in front of a cliff of the felsite, and seems to be enclosed in the latter rock, and other exposures of agglomerate, underlain and intensely indurated by the felsite, may be noticed on the ground that slopes towards Loch Ba'.
Fig. 356.—Section to south of Loch na Dàiridh, Mull.
a, basalts; b, dolerite; c, volcanic agglomerate; d, black felsite; e, granophyre.
That these agglomerates do not belong to the period of the eruption of the granophyre and felsite, but to that of the bedded basalts, may be inferred from their intense induration next the acid rocks, and also from the fact that similar breccias are actually found here interposed between the bedded basalts. This is well shown on the hill above the Coille na Sròine, where the accompanying section can be seen ([Fig. 357]). The broad dyke-like mass of black flinty felsite already referred to runs as a prominent rib over the southern end of Beinn a' Chraig into the head of the Scarrisdale glen (see [Fig. 352]). It cuts across the bedded basalts, and immediately to the south of where these appear, a thin intercalated bed of breccia crops out, of the usual dull-green colour, with abundant fragments of basalt and many of yellow and grey felsite.
From these various facts we may, I think, conclude that along the strip of ground now occupied by the Loch Ba' boss of granophyre and felsite, there once stood a line or group of vents, from which, besides the usual basalt-debris, there were ejected many pieces of different felsitic or rhyolitic rocks, and that these eruptions of fragmentary material took place during the accumulation of the plateau-basalts. These volcanic funnels occasioned a series of points or a line of weakness of which, in a long subsequent episode of the protracted volcanic period, the acid rocks took advantage, forcing themselves upwards therein, and leaving only slight traces of the vents which assisted their ascent. The mingling of acid and basic fragments in the material ejected from these vents is another proof of the existence of acid rocks in the volcanic reservoirs before the advent of the great granophyre intrusions. The evidence thus entirely confirms the conclusions deduced from the Skye area.
Fig. 357.—Section of junction of south side of Loch Ba' granophyre boss, with the bedded basalts, Mull.
a, bedded basalts; b b, basalt-tuff and breccia; c, granophyre; d, black felsite; e, coarse dolerite dyke, 30 or 40 feet wide.
The second or Glen More boss, instead of rising into hilly ground, is confined to the bottom of the main and tributary valleys, and has only been revealed by the extensive denudation to which these hollows owe their origin. It begins nearly a mile below Torness and extends up to Loch Airdeglais—a distance of almost four miles. Though singularly devoid of topographical feature, it exhibits with admirable clearness the relation of the granophyres to the gabbros, and thus deserves an important place among the tracts of acid rocks in the Western Islands. Its petrographical characters change considerably from one part of its body to another. For the most part, it is a true granophyre, sometimes with orthoclase, sometimes with plagioclase as its predominant felspar. At Ishriff, as already stated, it is sprinkled with long acicular decayed crystals of hornblende; but at the watershed the ferro-magnesian mineral is augite. The surrounding rocks are mainly the plateau-basalts, with their sills of dolerite and gabbro.
Fig. 358.—Mass of dark gabbro about two feet in diameter traversed by pale veins of granophyre, lying on north slope of Creag na h-Iolaire, Mull.
Fig. 359.—Section at Creag na h-Iolaire, Glen More, Mull, showing basalts and gabbros resting on and pierced by granophyre.
a, much indurated and altered basalts and dolerites; b b, gabbro; c, granophyre; d d, basalt dykes.
This strip of granophyre sends abundant apophyses from its mass into the dark basic rocks around it. Some of the best sections to show the nature of these offshoots are to be found on the steep hillslope which mounts from the watershed in Glen More southward into the Creag na h-Iolaire (Eagle's Crag), and thence up into the great gabbro ridge of Ben Buy. From the main body of granophyre a multitude of veins ascends through the basalts and gabbros from two feet or more in breadth down to mere filaments ([Fig. 358]). Even at a height of 300 feet up the hill some of these veins are still three inches broad, and present the usual granophyric structure, though rather finer in grain than the general mass of the boss, and sometimes assuming a compact felsitic and spherulitic texture at the immediate contact with the surrounding rock. One of the most striking proofs of the posteriority of these veins is furnished by the perfect flow-structure they not infrequently exhibit along their margins, their long felspar crystals being arranged parallel to the walls in lines that follow the sinuosities of the boundary between the two rocks. Patches of gabbro and of the indurated basalts may be seen lying on the granophyre, from which veins and strings ramify through them ([Fig. 359]). Similar veins can be traced upward into the main body of coarse gabbro, forming the ridge of Ben Buy. Some of them are of the usual granular granophyric texture, others are dull and fine-grained (claystones of the older authors).
Hence it is evident that the granophyres of Mull have been protruded not only after the accumulation of the plateau-basalts, but after these were traversed by the sheets and veins of gabbro. The amount of acid rock injected into these older rocks over the mountainous part of the island is enormous; but I reserve further reference to it for the section on acid Dykes and Veins, for these are the forms in which it chiefly occurs in that region. It should be added, that in the localities here referred to basalt-veins and dykes are generally abundant, cutting through all the other rocks ([Fig. 359]). So numerous are they that the geologist ceases to take note of them when his thoughts are engaged upon the problems presented by the masses through which they rise.
iii. THE ACID BOSSES OF SMALL ISLES
In the island of Eigg three small bosses or sheets of acid rock occur. That at the northern end rises through the Jurassic sedimentary rocks, and forms a bold cliff from 150 to 200 feet high. It is a light grey granophyric porphyry, with rounded blebs of quartz in a micropegmatic base of quartz and felspar. The other two masses, of smaller size, cut through the bedded basalts[403] (Map VI.).
[403] Quart. Journ. Geol. Soc. xxvii. (1871) p. 294.
In the opposite island of Rum, the acid protrusions play a much more important part. On the east side of the hills, they occur in sheets at the base of the gabbros; on the west side, they form a large tract of hilly ground, which, stretching along the coast-line for about three and a half miles from the headland of A' Bhrideanach to Harris, forms there a range of shattered sea-cliffs, that tower for 1000 feet above the Atlantic breakers that beat about their base. The area extends inland to the slopes on the west side of Loch Sgathaig, a distance of about three and a half miles, descending in a range of precipices along its northern front, and reaching in its culminating summit, Orval, a height of 1868 feet above the sea. The rocks of which this triangular area consists resemble those of the Mull bosses. They are chiefly quartz-porphyries, becoming felsitic in texture towards their contact with adjacent rocks. In some places, as was noticed by Macculloch on the sea-cliffs,[404] they have a rudely bedded structure. Thus on the north-west front of Orval, this structure is shown by parallel planes that dip outwards or north-west at 30° to 40°, and which are made still more distinct by an occasional intrusive dyke or sheet of basalt between their surfaces. I have already alluded to indications of an internal arrangement in the granitoid bosses of Skye (p. 381).
[404] Western Islands, vol. i. p. 487.
Fig. 360.—Section on north side of Orval, Rum.
a, Torridon sandstones; b, bedded basalts of Fionn Chro; c, dolerite; d, quartz-porphyry.
Fig. 361.—Junction of Quartz-Porphyry (Microgranite) and Basic Rocks, south-east side of Orval, Rum.
a, basalts and dolerites; b, dolerite and gabbro veins; c, quartz-porphyry cutting a and b.
As in the other islands, the granophyres, porphyries and felsites of Rum have been intruded at the base of the volcanic series. Over much, if not all, of their area they lie directly on the red Torridon sandstone. That the bedded basalts once covered them is indicated by the position of the three outliers of the basalt-plateau already noticed. But a fourth outlier still lies upon the porphyry of Orval as a cake that dips gently northward. It consists of a bedded, dark, finely-crystalline, ophitic dolerite, porphyritic in places, with a rudely prismatic or columnar structure ([Fig. 360]). It has undergone contact-metamorphism, and tongues from the underlying rock project up into it. On the south-eastern side of the same hill, still more striking evidence is presented of the posteriority of the acid to the basic rocks. The porphyry shows here the same tendency to assume a bedded structure, the parallel "beds" again dipping outward or south-east at 40°. They plunge under the body of gabbro, dolerite and other intrusive masses which from this point stretch eastward into the great cones of Allival and its neighbours. The rock at the junction is a fine microgranite with traces of micropegmatite. It is composed of a holocrystalline base of quartz and orthoclase, with porphyritic crystals of microcline, blebs of quartz and scattered granules of augite. The rocks that rest immediately next it are basalt and dolerite, into which it has sent an intricate network of veins ([Fig. 361]).[405] It has also pushed long tongues down the slope into them, which may be seen traversing the dolerite and gabbro veins that cut the basalts. The basic rocks next the porphyry have been intensely altered. They seem in places as if they have been shattered by some explosive force, and had then been invaded by the mass that rushed into all the rents thus caused. This remarkable structure is still better displayed on St. Kilda, and is more fully described in the following account of the geology of that island.
[405] In a thin slice cut from a specimen showing the junction, there is a minute vein of the porphyry penetrating the basalt which is much altered, while the porphyry becomes much finer in grain than at a distance from the contact.
iv. THE ROCKS OF ST. KILDA
Brief allusions to St. Kilda and its rocks have already been made (pp. 173, 358). We may now enter more fully upon the consideration of its geological structure and history.
When the weather is clear there may be seen from the western headlands of the Outer Hebrides a small blue cone rising above the Atlantic horizon at a distance of about 60 miles. As the voyager approaches this distant land it gradually shapes itself into a group of islets of which St. Kilda, the largest and only inhabited, has an extreme length of about four miles, a breadth of less than two miles, and a height of 1262 feet above the sea. Four miles to the north-east Borrera, about one square mile in extent, rises with precipitous sides to a height of 1000 feet. Off the north-western promontory of St. Kilda the huge rock of Soay, half a square mile in area, towers from 600 to 800 feet above the waves. Borrera has two attendant rocks—Stack Li and Stack an Armin—huge pyramidal masses several hundred feet high, and the home of thousands of gannets. St. Kilda possesses two less imposing islets between its north-western headland and Soay, and a third to the south-east known as Levenish.
The scenery of this picturesque group affords a good indication of its geological structure. It displays two distinct types of topographical form. In Borrera the marvellous combination of spiry ridges, deep gullies and clefts, notched crests and splintered pinnacles, at once reminds the visitor of the outlines of the Cuillin Hills of Skye. The same features are repeated on a less magnificent scale in Soay and along the whole of the south-western precipitous coast-line of St. Kilda.
In marked contrast to these varied outlines, the eastern half of St. Kilda rises with a smooth green surface, varied with sheets of grey screes, up to the rounded summit of Conagher, the highest point in the island. If the dark crags of the rest of the island group remind one of the Cuillins, this eastern tract recalls at once the form and colour of the Red Hills of Skye. A closer examination shows that in each case the topography arises from the influence of the very same rocks and geological structure as in that island.
There is, however, one aspect in which St. Kilda has no rival throughout the Western Isles. Its russet-coloured cone, though rising on the west side with gentle green slopes from the central valley, plunges on the eastern side in one vast precipice from a height of 1000 feet or more into the surge at its base. Nowhere among the Inner Hebrides, not even on the south-western side of Rum, is there any such display of the capacity of the youngest granite to assume the most rugged and picturesque forms. It is hardly possible to exaggerate the variety of outline assumed by the rock as it yields along its system of joints to the influence of a tempestuous climate. It has been carved into huge projecting buttresses and deep alcoves, the naked stone glowing with tints of orange and fawn colour, veiled here and there with patches of bright green slope, or edged with fringes of sea-pink and camomile. Every outstanding bastion is rent with chasms and split into blocks, which accumulate on the ledges like piles of ruined walls. To one who boats underneath these cliffs the scene of ceaseless destruction which they present is vividly impressive.
The geology of St. Kilda was sketched by Macculloch, who recognized the close resemblance of its two groups of rock to the "augite-rock" (gabbro) and "syenite" (granophyre) of Skye and other islands of the Inner Hebrides. But he left the relations of the two groups to each other undetermined.[406] Professor Heddle has published a brief reference to the rocks of St. Kilda, without, however, offering any definite opinion as to the geological structure of the islands.[407] The best account of the geology has been given by Mr. Alexander Ross, who obtained evidence that the acid sends veins into the basic rock. He brought away specimens clearly showing this relation, but in his description left the question open for further inquiry.[408] To some of the observations in these papers reference will be made in the sequel. The following account is based on the results of two visits paid by me to St. Kilda in the summers of 1895 and 1896, during which I was enabled to examine the rocks on land, and to sail several times round the islands, boating along those parts of the cliffs which presented features of special geological importance.
[406] Description of the Western Isles, vol. ii. p. 54.
[407] In an article on the general geological features of the Outer Hebrides contributed to A Vertebrate Fauna of the Outer Hebrides, by J. A. Harvie-Brown and T. E. Buckley, 1888.
[408] British Association Report, 1885, p. 1040, and a much fuller paper in the Proceedings of the Inverness Field Club, vol. iii. (1884), p. 72.
In the St. Kilda islets three groups of rock differing from each other in age may be recognized. 1st, A series of gabbros, dolerites and basalts which have been intruded through and between each other as sills; 2nd, a mass of granophyre which invades these sills; and 3rd, abundant dykes and veins of basalt which occur both in the basic and acid masses.
From the extension of the basalt-dykes across the Outer Hebrides it is clear that the Tertiary volcanic region reached at least to within 60 miles of St. Kilda. Whether or not it stretched over the intervening space now overflowed by the Atlantic must be matter for conjecture. There can be no doubt that the intrusive rocks of St. Kilda are in age and origin the equivalents of those of the Inner Hebrides. The remnants left of them were assuredly not superficial extrusions, but are characteristic examples of the more deep-seated intrusions of the Tertiary volcanic period. Down to the most minute details of structure they reproduce the features so well displayed by the gabbros and granophyres of Skye, Rum and Mull. If it is demonstrable in the case of these islands that the intrusions have taken place under a deep cover of basalt-sheets, now in large part removed, the inference may legitimately be drawn that at St. Kilda a basalt-plateau once existed which has been more completely destroyed than in the other regions. Not a fragment of such a plateau has survived, unless we may perhaps be allowed to recognize it in some of the basalts enclosed among the gabbro-sills. Placed far amid the melancholy main and exposed to the full fury of the Atlantic gales, these islets must be regarded as the mere fragmentary cores of a once much more extensive volcanic area. The geologist who visits them is deeply impressed at every turn by the evidence of the active and unceasing destruction which their cliffs are undergoing. Nothing now remains save the deep-seated nucleus of intrusive sills, bosses and dykes.
1. The Gabbro Sills.—The rudely-bedded arrangement of these rocks is conspicuous along the west side of St. Kilda, in Soay and in Borrera. They consist of coarse and fine varieties disposed in successive sheets which dip at angles varying from as little as 15° up to as much as 60° or even more. In St. Kilda they form the picturesque promontory of the Dune, and extend thence along the western side of the island to its extreme northern end. Their escarpments face the ocean, and their dip-slopes descend towards the north-east in grassy declivities to the south bay and the long verdant glen which runs thence across to the north bay. The same strike is prolonged into Soay, but further east in Borrera the direction curves so as to present vast escarpments towards the west and shelving sheets of rock towards the east.
None of the gabbros seen by me are as coarse as the large-grained varieties of Skye, nor does there appear ever to be such a marked banded structure among them as that displayed by the Cuillin rocks. Faint banding, however, may be noticed. A series of specimens which I collected from the west side of the island has been sliced for microscopic examination, and Mr. Harker has furnished me with the following notes regarding them.
"An olivine-gabbro from the west side of St. Kilda [7107] is a dark, heavy, medium-grained rock, in which augite and felspar are conspicuous. The microscope shows, in addition, plentiful grains of olivine, with but little original iron-ore, and some apatite-needles. The structure is ophitic, the plates of pale-brown augite enveloping both olivine and felspar. A little brown hornblende and red-brown mica are probably original, the rock showing little sign of alteration. The felspar is labradorite, with albite- and Carlsbad-twinning, and forms elongated rectangular crystals.
"Another specimen [7108] is a rock of similar appearance but somewhat coarser texture, and structurally is a more typical gabbro than the preceding, the felspar having little of the 'lath' shape, while the augite, though still moulded on the felspar, scarcely assumes an ophitic habit. A striking feature in this rock is the way in which the augite is crowded with 'schiller'-inclusions, in places so closely as to be almost opaque. A high magnification shows that these inclusions are dark, linear in form, and disposed along two directions intersecting at a high angle. The labradorite has unusually close twin-lamellation on both albite and pericline laws, and it is possible that this is a strain-effect.
"A third specimen [7109] is from a rock in every respect identical with the preceding, except that the olivine is rather more plentiful, and in some grains is partially serpentinized."
While the gabbros of St. Kilda are not a mere uniform boss, but a series of sills and irregular masses which have been successively injected into each other, they have subsequently been cut through by many basalt-dykes and veins. These, which are sometimes as abundant as in the gabbro of the Cuillin Hills, traverse the rocks both in the line of bedding and also at many different angles across it. As they generally weather faster than the gabbros, they give rise to deep narrow clefts which may be traced up the whole height of the precipices, occasioning sea-caves below and sharp notches on the crests above.
These scenic features, so indicative of the geological structure that causes them, are specially well seen on the western face of the Dune or south-western promontory of the island, and likewise in the strangely rifted precipices further north and in Soay. They are, however, most impressively displayed around the naked walls of Borrera, which in their marvellous combination of spiry ridges, deep straight gullies, and splintered crests, remind one at every turn of the scenery of Blaven and the Cuillin Hills.
2. The Granophyre Boss and its Apophyses.—The eastern half of the island of St. Kilda consists of a pale rock which Macculloch long ago identified with the granophyre of Skye, and which, as he pointed out, has much resemblance to parts of the granite of Arran.[409] Not only does it give rise to topographical forms like those of the Red Hills, but it weathers, like the Skye granophyre and the Arran granite, into thick bed-like sheets divided by transverse joints into large quadrangular blocks. On closer inspection it is found to resemble still more precisely the acid rocks of the Inner Hebrides. It possesses the same drusy micropegmatitic structure as the granophyres of Skye, Rum and Mull. The ferro-magnesian constituents are present in small quantity, hence the pale hue of the stone. The quartz and felspar project in well-terminated crystals into the drusy cavities, which are sometimes further adorned with delicate tufts of clear crystallized epidote. In these and other respects the rock displays the familiar external forms of the younger or Tertiary granites of Britain.
[409] Description, vol. ii. p. 54.
Mr. Harker's notes on the microscopic structure of this granophyre are as follows:—"The prevailing felspar is orthoclase, often very turbid from secondary products. Even what appear to be distinct crystals are sometimes seen in the slices to be invaded on the margin by quartz in rough micrographic intergrowths, and much of the finer intergrowth occurs as a fringe to the crystals. In this case the felspar of the micropegmatite can often be verified to be in crystalline continuity with the crystal which has served as a nucleus [6624]. Quartz occurs in distinct crystals and grains as well as in the micropegmatite. There is a more granitoid variety of the rock, in which only a very rude approach to micrographic intergrowths is seen [6623]. In both varieties there is but little trace of any ferro-magnesian mineral; the more typical granophyre has what seems to be destroyed augite, while the granitoid rock contains a little deep-brown biotite. Scattered crystal-grains of magnetite occur in both."
Narrow ribbon-like veins of a finer material, sometimes only an inch in breadth, traverse the ordinary granophyre. Similar veins run through the rock of the Red Hills in Skye; they are sharply defined from the enclosing rock, as if the latter had already solidified before their intrusion. With regard to the microscopic structure of some thin slices prepared from these veins, Mr. Harker remarks that "the material of the veins is of a type intermediate between granophyre and microgranite [6622, 6623]. The chief bulk is a finely-granular aggregate of quartz and felspar, the latter very turbid; but in this aggregate are imbedded numerous patches of micropegmatite, often of perfect and delicate structure. These areas of micropegmatite show some approach to a radiate or rudely spherulitic structure, and, in some cases, are clustered round a crystal of felspar or quartz. Some granules of magnetite and rare flakes of brown biotite are the only other constituents of the rock. Although they must be of somewhat later date, there is evidently nothing in the petrographical characters of these fine-textured veins to separate them widely from the ordinary granophyres of the region."
These veins may be compared with the spherulitic dyke that traverses the granophyre of Meall Dearg at the head of Glen Sligachan (described at p. 381), which, though undoubtedly somewhat younger than the rock that contains it, yet presents the very same structures as are visible at the margin of that rock.[410] The material of this dyke and of the finer veins of St. Kilda and the Red Hills probably belongs to a later period of protrusion from a deeper unconsolidated portion of the same acid magma as at first supplied the general body of granophyre.
[410] Quart. Journ. Geol. Soc. vol. 1. (1894), p. 220.
Undoubtedly the most interesting feature in the granophyre of St. Kilda is its junction with the mass of basic rock to the west of it. This junction-line runs from about the middle of the chief or south bay (where, however, its precise position is concealed under detritus) across the island to the north shore, where it descends the face of the precipice and plunges under the sea. Important as the actual contact of the two rocks obviously is in regard to their relative date, it has not hitherto been observed or described. Macculloch noticed "numerous fragments of trap penetrated by veins of syenite," but he did not see these rocks in place, and, in spite of their apparent testimony to the posteriority of the acid intrusions, he was inclined to believe that the veins were not real veins, but that the "trap" and "syenite" had a common origin and would be found to pass into each other, as he thought also occurred in Mull and Rum. In recent years Mr. Alexander Ross, during his visit to St. Kilda, collected specimens illustrating the varieties of gabbro, dolerite and basalt, and showing the intrusion of the acid into the basic rocks. As already stated, he was disposed to regard the "granite" as of younger date than the gabbros, but left the question undecided.[411]
[411] In his paper, Proceed. Inverness Field Club, vol. iii. (1884), p. 78, Mr. Ross quotes a letter from Prof. Judd, who there states that the rock supposed to be granite "is seen under the microscope to be a quite different rock—a quartz-diorite." Some of the specimens from St. Kilda collected by Mr. Ross were exhibited at a meeting of the Geological Society on 25th January 1893. With regard to these Prof. Judd, in the course of the discussion on his paper on "Inclusions of Tertiary Granite in the Gabbro of the Cuillin Hills," remarked:—"They show a dark rock traversed by veins of a light one, but the dark rock is not a gabbro and the light one is not a granite" (Quart. Journ. Geol. Soc. vol. xlix. (1893), p. 198).
The best locality for the examination of the junction of the main granophyre mass with the gabbros is inaccessible save by boat, and only in the calmest weather. It occurs in the great cliff on the northern side of the island between the north bay and the sea-stack known as the Bragstack. The line of contact emerges from below the sea-level, and ascends the cliff with a westward inclination of from 60° to 80°. Here, as in Skye, the acid rock underlies the basic masses, which are rudely bedded and much jointed. About 150 feet above the sea-level, the nearly vertical cliff breaks up into an exceedingly rocky and rugged acclivity, across which the junction seems to slope at a lower angle. But the place is hardly reachable, save perhaps by the intrepid, barefooted cragsmen of St. Kilda.
Fig. 362.—Junction of granophyre and gabbro, north side of St. Kilda.
Along the sharply defined line of contact the granophyre is close-grained, and sends a network of veins into the dark sheets of gabbro. The general features of the junction are represented in [Fig. 362]. The veins are narrow, those nearest the main body of granophyre diverging from it at a still more acute angle than those from the mass of Meall Dearg ([Fig. 376]), and then branching so as to enclose masses of the gabbro and to run across them in long parallel veins. A characteristic feature of many of these veins, besides their narrowness, is their tendency to split up at the ends into mere fingers and threads as represented in [Fig. 363].
Owing to the depth of soil on the cultivated land, and of boulders and sand on the beach, the actual junction of the main body of granophyre with the gabbro is not seen on the southern shore. But a few yards to the westward of where it must lie, the beach is cumbered with large blocks of rock broken up from the mass, which can be seen in situ a little further south forming a line of low cliff with a rugged foreshore. These rocks consist of various gabbros and basalts of rather fine grain, profusely traversed with veins of white granophyre. Some of these veins are two feet or more in breadth, and, when of that size, show the distinctive granular texture and drusy structure of the main part of the acid rock. But from these dimensions they can be traced through every stage of diminution until they become mere threads. When they are only an inch or two broad, they assume a finely granular texture like that of the veins that run through the body of the granophyre.
Fig. 363.—Veins of granophyre traversing gabbro and splitting up into thin threads, north side of St. Kilda.
The amount of injected material in the dark basic rocks is here and there so great as to form a kind of breccia ([Fig. 364]), which, from the contrast of tone between its two constituents, makes a conspicuous object on the shore. Here, as in the example already cited from Rum, the basic rocks seem to have been shattered into fragments, and the acid material to have been injected into the minutest interstices between them. The enclosed fragments are of all sizes from mere grains up to blocks a foot or more in length. They are generally angular, like rock-chips from a quarry. Moreover, they are not all of the same kind of material. While at this locality most of them consist of basalt, they include also a few large and small pieces of rather coarse gabbro. There has evidently been a certain amount of transport of material, as well as an extensive disruption of the rocks in situ. The granophyre here and there assumes a darker or greener tint, as if it had dissolved and absorbed some portion of the older rock.
Still more astonishing are the sections to be seen on the western cliffs and rocky declivities of the ridge to the north of the Dune, at a distance of perhaps 500 or 600 yards westwards from those of the South Bay. Here the gabbro-sheets are traversed by a number of conspicuous white bands, which on examination prove to be veins or dykes of granophyre. As viewed from the sea, the general disposition of the two groups of rocks is represented in [Fig. 366]. The broadest mass of granophyre breaks out towards the bottom of the precipice, and slants upward as a sheet intercalated between the gabbro sills, with a breadth of about 40 or 50 feet, but rapidly thinning away in its ascent. One of the bands below it has a breadth of about 15 feet. The material of these intrusions is a pale fine-grained granophyre like that of the South Bay, I did not detect, either here or anywhere else in St. Kilda, a definite spherulitic structure such as is so common in the granophyre dykes of Skye.
Fig. 364.—Pale granophyre injected into dark basalt, South Bay, St. Kilda.
The crags on the further side of the bay are the gabbro sheets of the Dune. (From a photograph by Colonel Evans.)
Though the acid intrusions are somewhat irregular both in thickness and direction, they lie generally parallel to each other in the line of strike of the bedding of the gabbros. They are no doubt apophyses from the main body of granophyre, which emerges to the surface about a third of a mile to the eastward, but may of course be at no great depth underneath.
Fig. 365.—Veins of granophyre traversing a line-grained gabbro and scarcely entering a coarse-grained sheet, west side of Rueval, St. Kilda.
Besides the broader bands of acid rock, and diverging from them, a complicated network of veins ramifies in all directions through the gabbros, as at the South Bay. The extraordinary degree to which the basic rocks have been shattered into fragments is strikingly displayed here, likewise the extreme liquidity of the acid magma, whereby it was able to insinuate itself into every chink and cranny. But the observer notices that this condition of excessive disruption is not shared by all the basic sills, and is not attendant upon all the acid dykes. As an example of this irregular distribution of the structure, I give the accompanying sketch ([Fig. 365]), where a fine-grained gabbro has been completely broken up and intersected with granophyre veins, while the coarser sheet overlying it has almost entirely escaped. The dark basalt-like sheets appear generally to have been much more disrupted than the more largely-crystalline varieties. It is noticeable here, also, that the fragments entangled in the network of granophyre veinings do not entirely belong to the rock that has been shattered, but sometimes include large and small lumps of different gabbros, showing some transference of material with the inrush of the acid magma.
Though closer in grain where it comes in contact with the gabbro, the granophyre never assumes any vitreous texture along its margin. A series of thin slices, prepared from specimens collected by me in the South Bay in the summer of 1895, was examined by Mr. Harker, who furnished the following notes regarding them:—"The basalt traversed by the granophyre is a fine-textured variety with small porphyritic felspars. These latter seem to be usually unaltered, retaining the glass cavities which in some of the crystals are abundant. The groundmass, however, shows minerals of metamorphic origin which must be derived mainly from the original augite. A brown mica is the most conspicuous; but with it are associated some brownish-green hornblende and certain chloritic and perhaps serpentinous substances. It is chiefly near the margin of a fragment of basalt that the mica gives place to these minerals. The basalt still retains plenty of unaltered granules of augite in the central parts of a fragment. It is not certain that the secondary minerals named come exclusively from the augite of the basalt; from their form and mode of occurrence they may in part have replaced olivine or even rhombic pyroxene.
Fig. 366.—View of sills and veins of pale granophyre traversing dark sheets of gabbro, west side of St. Kilda.
(From a photograph by Colonel Evans.)
"The acid rock, though styled granophyre above, belongs to a granitoid variety of that group of rocks, and has but little indication of micrographic structures. Compared with the other granophyres from St. Kilda, sliced and examined, these examples show a less acid composition. This is expressed mineralogically in the presence of a somewhat larger proportion of ferro-magnesian minerals and of soda-lime felspar. These features might indeed be matched in many normal granophyres among the Western Isles, but in the present case it can hardly be doubted that they are to be explained, at least in some degree, by the acid magma having taken up a certain amount of material from the basalt. Many of these Tertiary granophyres have undoubtedly been modified by the incorporation of pieces of basalt and gabbro, and a collection made in the Strath district of Skye will furnish examples for future study. Professor Sollas's description of similar phenomena in the Carlingford district has already proved the importance of this kind of action.[412] In the present instance, both brown mica and hornblende occur plentifully in the granophyre, and especially round the basalt fragments. This latter point is conclusive as to the derivation of the basic material, and further proves a certain degree of viscosity in the acid magma at the time of its intrusion."
[412] Trans. Roy. Irish Acad. vol. xxx. (1894), pp. 477-572.
Another series of specimens which I collected in the following year was submitted to Mr. Harker for petrographical determination, and his observations on two of the microscopic slices are as follow: "A breccia from the South Bay, St. Kilda [7105], consists of angular fragments up to two inches in diameter set in a matrix of grey granophyre of medium texture. The fragments belong to two types—one of very close texture (basalt), the other more evidently crystalline (diabase). Both are cut by the slice.
"The basalt shows very evident metamorphism, its augite being wholly transformed into greenish-brown hornblende. The little felspar-laths and granules of iron-ore seem to be unaltered, though the latter may perhaps have contributed to the formation of the hornblende. Another fragment of basalt has some larger crystal-grains of augite, and these are not converted into hornblende.
"The diabase shows a less marked boundary under the microscope, but otherwise has similar characters to the preceding. The striated felspar-crystals and grains of iron-ore have not been re-crystallized. A considerable amount of pale augite remains, but there is also plenty of deeply-coloured hornblende, both fibrous and compact. This diabase is certainly an intrusive rock, but the basalt, from its petrographic character, might be from a lava-flow or from a dyke.
"The granophyre is of somewhat coarse texture, the micrographic structure being only of a rude type. It is notably richer in the darker constituents than is usual in such rocks. Further, the hornblende and magnetite tend to cluster in little patches which suggest destroyed fragments of basic rocks. A grain or two of sphene occur, a mineral foreign to the normal granophyres.
"Another similar specimen [7106] from the same locality shows a basic rock of coarser texture, approaching some of the gabbros in appearance and with boundaries in places not very sharply defined. The grey matrix is again relatively rich in the dark elements, and the manner in which they occur in little patches, like nearly obliterated 'xenoliths,' points unmistakably to a certain amount of absorption of basic material by the acid magma, with consequent enrichment in the ferro-magnesian minerals.
"The slice cuts only the acid rock, which is seen to be of granitoid rather than granophyric structure, though the tendency of the felspar to enclose quartz-grains is unlike a typical granite. Oligoclase, with combined albite- and Carlsbad-twinning, is well represented in addition to orthoclase, and some zoned crystals seem to be of albite with a border of oligoclase. Brown hornblende and a little brown mica are the coloured constituents. Magnetite and apatite are also observed."
The testimony of the rocks of St. Kilda to the posteriority of the granophyre to the gabbros and basalts is thus clear and emphatic. It entirely confirms my previous observations regarding the order of sequence of these rocks in Mull, Rum and Skye. But the St. Kilda sections display, even more strikingly than can be usually seen in these islands, the intricate network of veins which proceed from the granophyre, the shattered condition of the basic rocks which these veins penetrate, the remarkable liquidity of the acid magma at the time of its intrusion, and the solvent action of this magma on the basic fragments which it enveloped.
3. The Basic Dykes.—Reference has already been made to the numerous dykes by which the gabbros of the St. Kilda group of islets is traversed. Similar dykes occur also, though less plentifully, in the granophyre. It remains for future observation to determine whether there is one series older and another later than the intrusion of the acid rock. In any case, it is quite certain that the dykes in the gabbro do not all belong to one period of injection, for frequent examples of intersection may be noticed, especially on the cliffs of Borrera, and also cases of double and even treble dykes which have been formed by successive infillings within the same fissure. The remarkably varied precipices of that island are marked by the long narrow rifts left by the weathering of vertical dykes, which, as above remarked, may be followed with the eye from the sea-level to the sky-line, ascending obliquely across the bedding of the gabbro sheets. Another group of dykes may be traced sloping upward at low angles along the face of the cliffs and affording admirable ledges with overarching roofs for innumerable gannets, kittywakes and guillemots. Other dykes and ribbon-like veins may be seen traversing the gabbro in many different directions, precisely as among the Cuillin Hills. As no similar network of dykes and veins is to be observed in the granophyre, I am disposed to regard a large number of these intrusions as older than that rock. But I did not observe any actual example of a basic dyke truncated by the granophyre.
There can be no doubt, however, that an injection of similar dykes and veins took place after the invasion of the granophyre. These later intrusions are conspicuously displayed along the cliffs that extend from the gabbro junction on the north side of St. Kilda round the eastern coast into the South Bay. They maintain a general parallelism and ascend from the sea-level at varying angles of inclination, running up the pale sea-wall as dark bands. They consist of basalt-rocks, and may often be seen to branch and to die out. Like those in the gabbro, they are not infrequently compound, being made up of two or three or even more distinct dykes. This is well seen on the great precipice below Conacher, where the section given in [Fig. 367] is displayed. Here in a vertical height of about 800 or 900 feet, there must be at least seven dykes, simple and compound. A little further south a triple dyke may be seen to be composed of a thick central zone and two thinner marginal bands, of which the lower strikes off from the others and maintains an independent course through the granophyre ([Fig. 368]).
Fig. 367.—Section of the sea-cliff below Conacher, St. Kilda, showing basic dykes in granophyre.
Fig. 368.—Triple basic dyke, sea-cliff, east side of St. Kilda.
V. THE GRANITE OF ARRAN
The northern half of the island of Arran is mainly occupied by one of the most compact and picturesque groups of granite mountains in Scotland.[413] These heights, rising out of the Firth of Clyde to a height of 2866 feet, present, in their spiry and serrated crests, a contrast to the smoother contours of the older granitic elevations of this country. The granite is surrounded by a ring of schistose rocks, belonging to the metamorphic series of the Southern Highlands, save for a short distance on the eastern margin, where it comes in contact with and indurates the Lower Old Red Sandstone. Macculloch long ago pointed out that no pebbles of the granite are to be found in the surrounding conglomerates and red sandstones of Carboniferous and younger age.[414] Geologists accordingly came to the conclusion that the protrusion of the granite took place after Carboniferous time, and hence that it had no connection with the appearance of the far older granites of the Highlands. In the year 1873 I gave reasons for believing the granite to be not only younger than the Carboniferous formations, but to be referable with most probability to the Tertiary volcanic series.[415] The progress of inquiry has tended to confirm this inference, though no direct proof of its correctness has been obtained. Two lines of investigation may be pursued, and each leads to the conclusion of the probability of the Tertiary age of the granite. One of these proceeds on a comparison of the petrographical characters of the Arran rocks with those of undoubted members of the Tertiary series among the Western Isles. The other inquiry deals with the relation of the rocks to each other in the general geological structure of Arran itself.
[413] The rocks of Arran have often been described. Besides the work of Macculloch above quoted, reference may be made to the paper by Sedgwick and Murchison, Trans. Geol. Soc. 2nd Ser. vol. iii. p. 21; A. C. Ramsay's Geology of the Island of Arran, 1841, the paper of Necker de Saussure quoted on p. 412; J. Bryce's Geology of Clydesdale and Arran, 3rd edit. 1865. The island is at present being surveyed for the Geological Survey by Mr. W. Gunn.
[414] Description of the Western Islands of Scotland, vol. ii. p. 388.
[415] Trans. Edin. Geol. Soc. vol. ii. part iii.
Macculloch first remarked the strong lithological resemblance of the Arran granite to the "syenite," or granophyre, of Skye and St. Kilda.[416] More recent petrographical investigation, as already stated, has furnished additional proofs of the connection between the acid rocks of these islands. So closely indeed are these rocks linked by megascopic and microscopic characters, that the petrologist has no hesitation in placing them together as probably products of the same period of igneous activity.
[416] Description, vol. ii. p. 352.
From the general geological structure of Arran, a further strong argument may be deduced in favour of the late date of the eruptions of granite. Good reasons have been given for classing as Permian the bright red sandstones which occupy much of the central and southern parts of this island, and include the little volcanic group already referred to. These sandstones have been invaded by a complex series of eruptive rocks which would thus be later than the Permian period. No igneous masses posterior to this period are certainly known in Britain save those of Tertiary age. The larger body of granite in the northern half of the island nowhere comes into direct contact with the newer red sandstones, but these strata are pierced by smaller bodies of granite. Hence, both by the evidence of their internal structure and by the stratigraphy of the ground, the later igneous rocks of Arran may be reasonably grouped together as one important and consecutive series, comparable in age and general characters with those of Tertiary date in the Inner Hebrides.
Fig. 369.—Jointed structure of the granite near the top of Goatfell Arran.
(From a photograph by Mr. W. Douglas, lent by the Scottish Mountaineering Club.)
The igneous rocks of Arran, later than the probably Permian sandstones, range from acid to basic in composition. Besides the northern granite, there are in the southern part of the island acid rocks that include granite, coarse-grained quartz-porphyry and fine-grained felsite. Where the relations of these rocks to each other can be seen, the felsite is found by Mr. Gunn to be newer than the porphyry, into which it sends sills and dykes.
A feature observed by the same geologist in Arran offers a further point of resemblance to the acid sills and dykes of Skye. He has noticed that accompanying the quartz-porphyry of Drumadoon and Bennan, a mass of basic rock forms a kind of fringe or selvage round it, sometimes with what appears to be a rock of intermediate character between them. Basic sills are abundant south of Glen Ashdale, though to the west of Whiting Bay most of the intrusive sheets are of acid material.
Some of the quartz-porphyry sheets are markedly columnar. One of them, near Corriegills, displays a divergent grouping of the prisms, not unlike parts of the pitchstone sheets of Eigg and Hysgeir, and suggestive of the rock having flowed along a hollow like that of a valley. No certain trace, however, has been found of any Tertiary lava-stream in Arran, nor has evidence of tuffs been detected in any part of the younger igneous series. All the rocks appear to be intrusive, though so abundant and varied are they as to indicate that they belong to a vigorous eruptive centre, which may have poured out at the surface lavas and ashes, since entirely removed by denudation.
The numerous basic dykes for which the south end of Arran has long been celebrated have a general northerly trend, and appear to be all of the same or nearly the same age. They undoubtedly cut through the quartz-porphyries and the coarse-grained basic sills, but are less numerously visible in the finer-grained basic sills, while in the felsitic sheets they are seldom to be seen. In several places dykes running in an E.N.E. direction cut the others, and are therefore of later date.[417] The compound dykes of Tormore on the west side of the island have been already noticed (p. 161).
[417] Ann. Rep. of Geol. Surv. for 1894, p. 286.
VI. THE NORTH-EAST OF IRELAND
In the north-eastern counties of Ireland there are two regions which afford ample material for discussion in connection with the protrusion of acid rocks during the Tertiary volcanic period. One of these, which for distinction may be called the Carlingford region, embraces the tract of country which includes the Mourne Mountains on the north-east side of Carlingford Lough and the ranges of Slieve Foye and Slieve Gullion on the south-west side. The other lies mainly within the basaltic plateau, the largest of its scattered portions forming parts of the hills of Carnearny and Tardree in the county of Antrim (Map VII.).
1. The Carlingford Region
a. The Mourne Mountains.—This compact and picturesque group of hills, about twelve miles long and six miles broad, and reaching a height of 2798 feet in Slieve Donard, presents a comparatively simple geological structure, since it consists almost entirely of granitic rocks which pierce, overlie and underlie Upper Silurian grits and shales. So far as regards the contact of these rocks with the disrupted sedimentary formations, all that can be asserted is that the granite must be later than at least the older part of the Upper Silurian period. But for at least two reasons, the eruptive rocks may be regarded with some confidence as part of the Tertiary series. In the first place, there is a strong petrographical resemblance between the Mourne Mountain granite and that of the Island of Arran and the granitic parts of the granophyre of the Western Isles. And this resemblance is so close as to furnish a cogent argument in favour of grouping all these rocks together as parts of one geologically contemporaneous series. In the second place, the Mourne Mountain granite abruptly cuts off a large number of basic dykes which, running in a general N.N.W. direction, may be looked upon as almost certainly members of the Tertiary system of protrusions.
The manner in which the granite of the district behaves towards certain detached areas of Silurian strata with their accompanying dykes is one of the most astonishing features in the whole assemblage of intrusive rocks in Britain. As has been excellently shown in the Geological Survey Map and sections by Mr. W. A. Traill, the granite has carried up on its surface broad cakes of vertical Silurian strata, together with all their network of dykes.[418] A cake of this kind, from 50 to about 200 feet thick and nearly two miles broad, has been bodily uplifted from the rest of the mass and carried upward by the granite, so that the truncated ends of the beds of grit and shale with their system of dykes stand upon a platform of granite, from which also numerous veins penetrate them. There can be little doubt that the basic dykes thus broken through are parts of the great Tertiary system, and if so, the granite which disrupts them cannot be older than Tertiary time.
[418] See Sheets 60, 61 and 71 of the one-inch map of the Geological Survey of Ireland, and Sheets 22, 23 and 24 of the Horizontal Sections. The Explanation to these Sheets of the map was written by Professor Hull, Mr. Traill having previously retired from the service. The Mourne Mountain area is now undergoing critical revision by Prof. Sollas for the Geological Survey, and important additional material for the elucidation of this district may be expected from him.
Besides the older basic dykes disrupted by the granite, a younger but much less abundant series traverses that rock, and also follows a general north-westerly direction. These later dykes in some cases cross more acid dykes which have risen through the granite. There is no trace of any superficial discharge from the Mourne Mountain area. But from the analogy of other districts we may easily conceive that the granite represents the underground parts of volcanic material which has now been entirely removed.
b. Slieve Foye and Barnavave District.—This area embraces the mountainous ground lying between Carlingford Lough and Dundalk Bay, and culminating in Slieve Foye (1935 feet). It measures roughly about six miles in extreme length and four miles in breadth.
The remarkable assemblage of basic and acid materials in this area has received considerable attention from geologists. The relative order of the two groups of rocks was first clearly recognized by Griffith, who showed that the granite (granophyre) is intruded into the gabbro.[419] Professor Haughton subsequently confirmed this observation, and proved the post-Carboniferous date of the intrusive materials, which he compared with those of Skye.[420] The general distribution of the rocks was traced out in some detail by the Geological Survey, and described in the official Memoirs.[421] More recently the district has been examined by Professor Sollas, who, bringing the photographic camera and the microscope to the aid of field-geology, has elucidated the structure and relations of the rocks, and has obtained abundant evidence that the acid and basic rocks maintain there the same relative order as among the Inner Hebrides.[422]
[419] Journ. Geol. Soc. Ireland (1843), p. 113.
[420] Quart. Journ. Geol. Soc. vol. xii. (1856), p. 171; xiv. p. 300; and Journ. Geol. Soc. Ireland (1876), p. 91.
[421] Sheet 71 of the Geol. Surv. Ireland, and accompanying Explanation. These were the work of Mr. W. A. Traill.
[422] Trans. Roy. Irish Acad. vol. xxx. (1894), p. 477. This is part i. of what is intended to be a series of papers.
One of the first features in this tract of country to arrest the eye of the geologist is the situation of this centre of protrusion and that of Slieve Gullion along a north-west line, coincident with the general direction of the numerous basic dykes of the region. Whether or not the successive intrusions took place contemporaneously in the two areas, they have followed each other in the same order. In the Barnavave district the igneous rocks occupy an area of about 20 square miles. They consist of a central and chief mass composed of acid materials, which have risen through the basic rocks now found as an interrupted ring round them.
In his more recent examination, Prof. Sollas has devoted special attention to the influence of the solvent action of the acid magma upon the basic rocks and upon its own composition and structure. Besides confirming the work of previous observers as to the order of appearance of the two kinds of material, he has obtained evidence that the gabbro had not only completely solidified, but was traversed by contraction-joints, possibly even fractured by earth-movements, before the injection of the granophyric material. He found that this material, like that of the Inner Hebrides and St. Kilda, must have been in a state of great fluidity at the time of its intrusion, and made its way into the minutest cracks and crevices. In observing the solvent action of the granophyre, he ascertained that this action took place even in comparatively narrow dykes, which probably consolidated at no great depth beneath the surface.[423]
[423] Op. cit.
c. The Slieve Gullion District.—This area is separated from that just described by a narrow strip of Silurian strata, so that its isolation as a separate igneous district is complete. It will be observed from the map to continue the same north-westerly line as the Slieve Foye tract, the two together running in that direction for a distance of some 16 miles. It is interesting to note the adoption of this predominant north-westerly trend even by eruptive masses which were mainly of acid material.
This district measures about ten miles in length by from one to five miles in breadth. The rocks are, on the whole, similar to those in the area south of Carlingford Lough, and bear the same relation to each other, the acid being intrusive in the basic series. It is worthy of remark that the Tertiary eruptive rocks have made their appearance in the midst of the older granite of Newry. This granite has been already alluded to as disrupting Upper Silurian strata, and being probably of the age of the Lower Old Red Sandstone ([vol. i. p. 290]). In long subsequent ages, after protracted denudation, during which its cover of Silurian and Carboniferous formations was stripped off and it was laid bare, it was broken through by the whole series of basic and acid protrusions of Slieve Gullion.
This district is portrayed on Sheets 59, 60, 70 and 71 of the Geological Survey of Ireland, which show a central core of basic and acid material piercing the Newry granite.[424] Round this core and touching it at its north-western and south-eastern end, but elsewhere separated from it by a space of several miles, runs a curiously continuous band of igneous material which is marked as "quartziferous porphyry" and "felstone-porphyry" on the Survey maps.
[424] The ground was chiefly mapped and described by Mr. Joseph Nolan and Mr. F. W. Egan.
The south-western portion of this elliptical ring possesses a peculiar interest from its including certain remarkable masses of breccia or agglomerate. These rocks have been mapped by Mr. Nolan, and are described by him in the official Explanation, but in more detail in two separate papers.[425] Having had an opportunity of paying a brief visit to the ground, I can confirm the general accuracy of his mapping and description, and am able to add a few further particulars to the facts enumerated by him.
[425] Sheet 70 of the Geol. Surv. Map of Ireland and Explanation thereto; also Journ. Roy. Geol. Soc. Ireland, vol. iv. (1877), p. 233; Geol. Mag. 1878.
The tract of ground where these agglomerates appear forms a prominent ridge which rises several hundred feet above the lower country on either side, and extends in a W.N.W. direction for about seven miles, nearly along the line of junction between the Newry granite and the Silurian strata. The ridge has a breadth varying from a few hundred yards to upwards of a mile. It is separated from the main igneous mass of the Slieve Gullion area by an intervening strip of lower ground from three-quarters of a mile to about a mile and a half in width, which is occupied by the Newry granite. At the north-west end of the ridge the newer eruptive rocks lie within the area of that granite, while at the south-east end they rise entirely amongst the Silurian strata.
Beginning at the south-eastern extremity, we find the agglomerate occupying several detached eminences and surrounded by altered Silurian grits and shales. Further west the rock occurs in larger and more continuous masses, appearing at intervals, especially along the southern borders of the quartz-porphyry which forms by much the greater part of the ridge. Actual junctions of the agglomerate with the older rocks around seem to be seldom visible. I found one, however, above the gamekeeper's house on the southern flanks of the hill called Tievecrom. The Upper Silurian grits and shales, in a much indurated and shattered condition, are there traceable for several hundred feet up the slope, until they are abruptly cut off by the agglomerate. The line of separation appears to be nearly vertical, the truncated ends of the strata being wrapped round by the mass of fragmental material.
The most remarkable features of this agglomerate, which has been well described by Mr. Nolan, are the notable absence of truly volcanic stones in it, and the derivation of its materials from the rocks around it. I found only one piece of amygdaloid, but not a single lump of slag, no bombs, no broken fragments of lava-crusts, and no fine volcanic dust or enclosed lapilli. The rock may be said to consist entirely of fragments of Silurian grits and shales where it lies among these strata, and of granite where it comes through that rock. Blocks of these materials, of all sizes up to two feet in breadth, are confusedly piled together in a matrix made of comminuted debris of the same ingredients.
The agglomerate on the ridge of Carrickbroad has no definite boundary, but seems to graduate into an andesitic rock, and then into a quartz-felsite or rhyolite. This apparent gradation is one of the most singular features of the ridge. The andesite resembles some of the "porphyrites" of the Old Red Sandstone. It is close-grained, with abundant minute felspar-laths, and numerous large porphyritic felspars, which latter are sometimes aggregated in patches, as in the old porphyries of Portraine, Lambay Island and the Chair of Kildare. This rock has undoubtedly been erupted at the time of the formation of the agglomerate, or at least before the loose materials were compacted together; for it is full of separate stones of the same materials, and becomes so charged with them as to become itself a kind of agglomerate, with a small proportion of andesitic matrix cementing the blocks.
A thin slice prepared from one of the specimens obtained by me from this hill has been studied by Mr. Watts, who reports that the fine-grained andesitic matrix in which the stones are imbedded has often been injected into their minute fissures, and that the minute fragments enclosed in this matrix consist here of a trachyte-like porphyry, felsite, andesites, basalts of various degrees of fineness and olivine-basalt, together with isolated grains of felspar, such as might have been derived from the breaking up of some of these fragments.
Westward from Carrickbroad, the chief eruptive rock is a dark, sometimes nearly velvet-black, flinty, occasionally almost resinous, quartz-porphyry or rhyolite, with abundant quartz and large felspars and occasional well-marked flow-structure. This material, near the much smaller protrusion of andesite, is curiously mixed up with that rock, as if the two had come up together. Sometimes they seem to pass into each other, at least the separation between them cannot be sharply drawn. There can be little doubt, however, that the acid magma continued to ascend after the other, for it sends veins and strings into the more basic material, and encloses blocks of it. This thoroughly acid porphyry plays the same part as the andesite in regard to the stones of the agglomerate. Throughout its whole extent, it is found to enclose these stones, which here and there become so numerous as to form the main bulk of the mass, leaving only a limited amount of quartz-porphyry (rhyolite) matrix to bind the whole into an exceedingly compact variety of breccia. Occasionally the acid rock cuts through the ordinary clastic agglomerate, as may be well seen on the southern face of Tievecrom.
A specimen of this porphyry with its enclosed fragments, which was collected by me from above the old tower at Glendovey, Carrickbroad, has been sliced and examined by Mr. Watts under the microscope, and is thus described by him: "The large fragment in this slide consists of ophitic olivine-dolerite full of large phenocrysts of olivine. It is broken up and penetrated by veins of quartz-porphyry, rich in quartz, which exhibits a beautiful flow-structure. The felspars and augite of the dolerite do not appear to have suffered much alteration at the margin of the fragment, but the olivines are much serpentinized, the serpentine passing into a border of actinolite which runs in veins into the neighbouring rock and even passes out into the quartz-porphyry at the junction, impregnating it with actinolite and chlorite for some distance. A few particles of basalt also occur and a portion of a granite-fragment comes into the slide, from the edge of which a piece of biotite has floated off into the quartz-porphyry."
The essentially non-volcanic material of the agglomerate shows, as Mr. Nolan pointed out, that it was produced by æriform explosions, which blew out the Silurian strata and granite in fragments and dust. These discharges probably took place either from a series of vents placed along a line of fissure running in a north-westerly line, or directly from the open fissure itself. Possibly both of these channels of escape were in use; detached vents appearing at the east end and a more continuous discharge from the fissure further west.
After the earliest explosions had thrown out a large amount of granitic and Silurian detritus, andesitic lava rose in the fissure, and solidifying there enclosed a great deal of the loose fragmentary material that fell back into the chasm. Subsequently, and on a more extensive scale, a much more acid magma ascended from below, likewise involving and carrying up a vast quantity of loose stones, among which are pieces of basalt and dolerite.
No evidence remains as to the extent of the material discharged over the surface from this fissure. Denudation has removed all the surrounding fragmental sheets as well as any lava that may have flowed out upon or become intercalated among them. There remains now only the cores of the little necks at the east end, and the indurated agglomerate and lava that consolidated along the mouth of the fissure or vents.
This is the only example of such a line of fissure-eruption which has yet been met with in the British Isles. Its connection with the eruptive masses of Slieve Gullion and Carlingford links it with the Tertiary volcanic series. But no evidence appears to remain regarding the epoch in the long volcanic period when the eruptions from it took place. They may possibly date back to the time of the plateau-basalts; but the abundant acid magma, which constitutes one of their distinguishing characteristics, suggests that they more probably belong to the later time when the main protrusions of acid material took place. They suggest that coeval with the uprise of the great domes of Slieve Gullion, Carlingford and the Mourne Mountains there may have been many superficial eruptions of which, after prolonged denudation, all trace has now been effaced.
2. The Antrim Region
Reference was made in [Chapter xxxvii]. to the occurrence of rhyolitic conglomerate and tuff between the lower and upper series of basalts in the Antrim plateau, and to the evidence furnished by these detrital deposits either that masses of rhyolite appeared at the surface, or that rhyolitic ashes were discharged from volcanic vents in the long interval that elapsed between the two groups of basalt. The further consideration of this question, and an account of the rhyolite bosses, were reserved for the present chapter, that they might be taken in connection with the other acid eruptions of Tertiary time in Britain.[426]
[426] For an early account of the Antrim trachytic rocks, see Berger, Trans. Geol. Soc. iii. (1816), p. 190. Professor Hull has described the Tardree rock in the Explanation to Sheets 21, 28 and 29, Geol. Survey of Ireland (1876), p. 17, and has supposed it to be older than the basalts, referring it to the Eocene period (Physical Geology and Geography of Ireland, 2nd edit. (1891), pp. 87, 95). Duffin (quoted by Mr. Kinahan) believed that "the trachytes occur at the centre of eruption, and were probably poured out at the end of the outburst." Du Noyer also (quoted by the same writer) thought them to be newer than the plateau-basalts, and to have lifted up masses of these rocks. Mr. Kinahan himself (Geology of Ireland, p. 172) has pointed to the absence of any rhyolitic fragments between the basalts as an argument against the supposed antiquity of the acid protrusions. A petrographical account of the Tardree rock is given by Von Lasaulx in the paper already cited, Tschermak's Min. Pet. Mittheil. (1878), p. 412. A more elaborate discussion of the petrography by Prof. Cole will be found in the Memoir above referred to (Scientif. Trans. Roy. Dublin Soc. vol. vi. 1896), and the geological relations of the rocks are discussed by him in another shorter paper, Geol. Mag. (1895), p. 303. See also Mr. M'Henry on the trachytic rocks of Antrim, Geol. Mag. (1895), p. 260, and Proc. Geol. Assoc. vol. xiv. (1895), p. 140.
With one exception, all the known protrusions of acid material in the Antrim area lie within the limits of the basalt-plateau (see Map. No. VII.). They occur along a line at intervals for a distance of about 17 miles, from Templepatrick to a point four miles north of Ballymena. It is worthy of remark that here again the line of protrusion has a north-west trend. It not improbably indicates the position of a fissure up which the acid material rose at various points.
The petrography of the rocks has been frequently discussed. They include several varieties of rhyolite, generally rather coarsely crystalline, but sometimes becoming compact, and even passing into dark obsidian. No undoubted tuff occurs associated with them in any of the exposures, nor do the rhyolites anywhere display structures that point to their having flowed out at the surface.[427] That the masses now visible may have communicated with the surface is quite conceivable, but what we now see appears in every case to be a subterranean and not a superficial part of the protrusion.
[427] At Sandy Braes an exposure is visible of what at first might be thought to be a volcanic conglomerate, but closer examination shows the rock to consist of obsidian, which decomposes into a clay, leaving round sharply-defined glassy cores enclosed in the decayed material. The "banded rhyolites" do not exhibit any kind of flow-structure that may not be met with in dykes and bosses. Nor have any satisfactory traces been found of vesicular or pumiceous bands such as might mark the upper surfaces of true lava-streams. Professor Cole has described what he calls "The Volcanoe of Tardree" (Geol. Mag. July 1895). If the Tardree mass ever was a volcano, which is far from improbable, its superficial ejections have long ago disappeared. At least, after the most diligent search, I have been unable to discover any trace of them, all that now remains appearing to me to be the neck or core of protruded material.
Fig. 370.—Intrusive rhyolite in the Lower Basalt group of Antrim, Templepatrick.
1 1, Chalk; 2 2, Gravel; 3 3, Bedded basalt; 4 4, Rhyolite, intrusive.
Most of the rhyolitic exposures are extremely limited in area—mere little knobs, sometimes rising in the middle of a bog, and never forming conspicuous features in the landscape. The relation of these rocks to the basalts are generally concealed, but the isolation of the small rhyolitic patches leaves no doubt that they are intrusive as regards the surrounding basalts. This relation is well seen at Templepatrick, where it was first observed by Mr. M'Henry of the Geological Survey ([Fig. 370]). The rhyolite there forms a sill which has been thrust between the basalts and the gravel that underlies them, the basalts being bent back and underlain by the acid rock.[428]
[428] The progress of quarrying operations during the last eight years has somewhat destroyed the section as exposed in 1888. But we now see that the basalt has not only been bent back but is underlain by the acid rock.
The largest and most interesting of the Antrim rhyolite tracts covers a space of about ten square miles in the heart of the basalt-plateau to the north-east of the town of Antrim. It rises to about 1000 feet above the sea, and forms a few featureless hills, some of which are capped with basalt. The best known localities in this tract are Tardree and Carnearny. The rock is chiefly a somewhat coarse-textured lithoidal rhyolite, but includes also vitreous portions.
Fig. 371.—Section across the southern slope of Carnearny Hill, Antrim.
a a a, bedded basalts; b, rhyolite.
Owing to the cover of soil and turf, the junction of this mass with the surrounding basalts cannot be so clearly seen as in the sections of the Inner Hebrides, and hence the stratigraphical relations of the two groups are apt to be misunderstood. What is actually seen is represented in [Fig. 371]. The lithoidal rhyolite emerges from underneath the basalts which abut against its sloping surface, forming on the north side of Carnearny Hill a steep bank about 150 feet above the more gently inclined slope below. The basalts consist of successive nearly level sheets of compact and amygdaloidal rock.
It is obvious that only two explanations of this section are possible. Either the rhyolite was in existence before the basalts which flowed round it and gradually covered it, or it has been erupted through these rocks, and is therefore of later date.
The former supposition has been the more usually received. The rhyolite has been supposed to form the summit of an ancient volcanic dome, perhaps of Eocene age, which had been worn down before the outflow of the plateau-basalts under which it was eventually entombed. Had this been the true history of the locality, it is inconceivable that of a rock which decays so rapidly as this rhyolite, and strews its slopes with such abundance of detritus, not a single fragment should occur between the successive beds of basalt which are supposed to have surrounded and buried it. Though the several beds of basalt are well exposed all round, I could not, on my first visit, find a trace of any rhyolitic fragments between them, nor had Mr. Symes, who mapped the ground in detail for the Geological Survey, been more successful. I have since made a second search with Mr. M'Henry, but without detecting a single pebble of the acid rock among the basalts. Yet it is clear from the upper surfaces of some of these lavas that a considerable interval of time separated their successive outflows, so that there was opportunity enough for the scattering of rhyolite-debris had any hill of that rock existed in the vicinity.
Again, little more than a mile to the east of Carnearny Hill, an outlier of the basalts forming the prominent height of the Brown Dod lies upon and is completely surrounded by the rhyolite, which along the east side of the hill can be traced as it passes under the level sheets of basalt. The line of junction ascends and descends on that flank of the outlier, so that successive flows of basalt are truncated by the acid rock. But I could find no rhyolitic debris between them.
It appears to me, therefore, that the relations between the two groups of rock in this area are similar to those between the granophyres and bedded basalts on the south side of Loch na Keal in Mull (p. 396). In other words, the rhyolites have risen through the basalts, and are therefore younger than these lavas. This conclusion is corroborated by the actual proofs of the intrusion of rhyolite into the basalts at Templepatrick.
All the known rhyolitic masses in Antrim are confined to the Lower group of basalts.[429] And as they traverse some of its highest members, they may be regarded as certainly younger than that group. Mr. M'Henry, who first indicated this relation, suggested that the rhyolites were erupted in the interval between the two basaltic series, and he connected with their eruption the rhyolitic detritus found in association with the iron-ore at so many places in Antrim. It appears to me that this suggestion carries with it much probability. The rhyolitic conglomerate of Glenarm proves that, in the long period represented by the iron-ore and its associated group of sedimentary deposits, there were masses of rhyolite at the surface, the waste of which could supply such detritus. The resemblance between the material of that conglomerate and the rhyolites now visible at Tardree and elsewhere is so close that we cannot doubt that, if not derived from some of the known rhyolitic protrusions, this material certainly came from exposed masses that had the same general petrographic characters.
[429] The only exception to this rule was believed to be that of the mass at Eslerstown, four miles east of Ballymena, which, as originally mapped, was shown as crossing from the Lower into the Upper basalts. Mr. M'Henry, however, has recently ascertained that the acid rock is entirely restricted to the area of the older group.
While the rhyolite pebbles in the Glenarm conglomerate are distinctly rounded and water-worn, showing that some prominences of acid rock were undergoing active denudation at the time when this conglomerate was laid down, the finer rhyolitic detritus in the tuffs of Ballypallidy rather suggests the actual discharge of rhyolitic ashes during the same period. But it would appear that the superficial outbursts of rhyolitic material, whether in the form of lava or of tuff, were only of trifling extent, or else that the interval between the eruption of the two basalt-groups was so prolonged that any such superficial material was then removed by denudation. The varieties of lithological character to be met with among the acid protrusions of Antrim suggests a succession of uprises of rhyolites differing from each other more or less in composition and structure. Unfortunately the ground is generally so covered with superficial accumulations, and the exposures of rock are so poor and limited, that no sequence has yet been determined among the several kinds of acid rock. The only locality where I have observed clear evidence of such a sequence is on the old quarries half a mile west of Shankerburn Bridge, and three miles north-west of Dromore, County Down. A small boss of rhyolite there rises through the Silurian strata. It consists partly of a coarse-grained lithoidal rhyolite, with large smoky quartzes and felspars, and partly of a much finer textured variety. The latter, on the south side of the small brook which separates the quarries, can be seen to ascend vertically through the coarse-grained rock into which it sends a projecting vein. Its margin shows a streaky flow-structure parallel with its vertical wall and is in places spherulitic. Here the closer-grained rock is certainly later than the rest of the mass.
CHAPTER XLVIII
THE ACID SILLS, DYKES AND VEINS
i. THE SILLS
Not only have the acid rocks been protruded in small and large bosses, they have also been injected as sills between the bedding-planes of stratified rocks, between the surfaces of the basalt-beds, and between the bottom of the plateau-basalts or of the gabbros and the platform of older rock on which the volcanic series has been piled up. Every gradation of size may be observed, from mere partings not more than an inch or two in thickness, up to massive sheets, which now, owing to the removal of their original covering of rock by denudation, form minor groups and ranges of hills. Where the sheets are numerous, they are usually small in size; where, on the other hand, they are few in number, they reach their greatest dimensions.
It is not always possible to discriminate between bosses and large irregular sills. A good illustration of the connection between these two forms of intrusion will be cited from the island of Raasay, where a widespread intrusive sheet is in part connected with a true boss.
In Mull, sills of acid eruptive rocks are profusely abundant throughout the central mountainous tract between Loch na Keal and Loch Spelve. If we ascend the slopes from the Sound of Mull, for instance, we have not gone far before some of these sheets make their appearance. They are usually dull granular quartz-porphyries, or granophyres, often only two or three feet in thickness, and interposed between the beds of basalt that form the mass of the hills. Along the crest of the ridge that stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh they take a prominent place among the ledges of basalt, basalt-conglomerate and dolerite. The largest sheet in Mull is probably that which has thrust itself between the base of the basalts and the underlying Jurassic strata and crystalline-schists on the shore of the Sound of Mull at Craignure. The porphyry of this sheet is referred to by Professor Zirkel as only a finer-grained variety of the same quartziferous rock, with hornblende and orthoclase crystals, which in Skye breaks through the Lias.[430] On the south coast also, at the base of the thick basalt series, similar porphyries have been injected into the underlying strata; and under the great gabbro mass of Ben Buy similar protrusions occur. But as we retire from the mountainous tract into the undisturbed basalts of the plateau, these acid intercalations gradually disappear.
[430] Zeitsch. Deutsch. Geol. Gesellsch. xxiii. p. 54.
In the islands of Eigg and Rum, excellent examples occur of the tendency which the sheets of porphyry or granophyre manifest to appear at or about the base of the bedded basalts. I have already alluded to the boss or sheet at the north end of the former island. A still more striking illustration occurs in Rum. All along the base of the great mass of gabbro, protrusions of various kinds of acid rock have taken place. The great mass of Orval, already described, is one of these. Below Barkeval and round the foot of the hills to the south-east of that eminence an interrupted band of quartz-porphyry may be traced, from which veins proceed into the gabbros and dolerites.
But it is in Skye and Raasay that the intrusive sheets of the acid group of rocks reach their chief development. They have been most abundantly injected underneath the bedded basalts, particularly among the Jurassic strata. A band or belt of them, though not continuous, can be traced round the east side of the main body of granophyre, at a distance of from a mile and a half to about three miles. Beginning near the point of Suisnish, this belt curves through the hilly ground for some five miles, until it dies out on the slopes above Skulamus. It may be found again on the west side of the ridge of Beinn Suardal, and on the moors above Corry, till it reaches the shore at the Rudh' an Eireannich (Irishman's Point). It skirts the west side of Scalpa Island, and runs for some miles through Raasay. Another series of sills occurs below the basalts and gabbros in the Blaven group of hills.
Over a large part of their course, the rocks of the eastern belt rest in great overlying sheets upon the Jurassic strata, which may almost everywhere be seen dipping under them. From the analogy of other districts, we may, I think, infer that the position of these sills here points to their having been intruded at the base of the plateau-basalts which have since been removed from almost the whole tract. Fortunately, a portion of the basalts remains in Raasay, and enables us to connect that island with the great plateau of Skye of which it once formed a part. There can be no doubt that the basalts of the Dùn Caan ridge once extended westwards across the tract of granophyre which now forms most of the surface between that ridge and the Sound of Raasay. A thin sheet of quartz-porphyry, interposed among the Oolitic strata, may be seen a little inland from the top of the great eastern cliff and below the position of the bedded basalts.
The great sheet, or rather series of sheets, which stretches north-eastwards from Suisnish at the mouth of Loch Eishort in Skye, consists of a rock which for the most part may readily be distinguished in the field from the granitoid material of the bosses. It appears to the naked eye to be a rather close-grained or finely crystalline-granular quartz-porphyry, with scattered blebs or bi-pyramidal crystals of quartz and crystals of orthoclase. At the contact with adjacent rocks, the texture becomes more felsitic, sometimes distinctly spherulitic (west side of Carn Nathragh, next Lias shale). Under the microscope the rock is seen to be a fine-grained granophyric porphyry or porphyritic granophyre. It caps Carn Dearg (636 feet) above Suisnish, where it covers a space of nearly a square mile, and reaches at its eastern extremity (Beinn Bhuidhe), a height of 908 feet above the sea (Fig. 249). This rock rests upon a sill of dolerite, and is apparently split up by it. But, as I have already stated, the basic rock is probably the older of the two, and the granophyre seems to have wedged itself between two earlier doleritic sheets. To the north-west of Carn Dearg, above the northern end of the crofts of Suisnish, the same sill, or one occupying a similar position, crops out between masses of granophyre, and is intersected by narrow veins from that rock.
Though severed by denudation, the large sheets of granophyre to the east of Beinn Bhuidhe are no doubt continuations of the Carn Dearg mass, or at least occupy a similar position. That they are completely unconformable to the Jurassic strata is shown by the fact, that while at Suisnish they lie on sandstones which must be fully 1000 feet above the bottom of the Lias, only two miles to the east they are found resting on the very basement limestones, within a few yards from the underlying quartzite and Torridon sandstone. I do not think that this transgression can be accounted for by intrusion obliquely across the stratification. I regard it as arising from the eruptive rock having forced its way between the bottom of the now vanished basalt-plateau and the denuded surface of Jurassic rocks, over which the basalts were poured. The platform underneath these granophyre sills thus represents, in my opinion, the terrestrial surface before the beginning of the volcanic period.
But there is abundant proof that though the intruded granophyre sills followed generally this plane of separation, they did not rigidly adhere to it, but burrowed, as it were, along lower horizons. Thus on the south-east front of Beinn a' Chàirn, which forms so fine an escarpment above the valley of Heast, the base of the granophyre, after creeping upward across successive beds of limestone, sends out a narrow tongue into these strata, and continues its course a little higher up in the Lias. The same rock, after spreading out into the broad flat tableland of Beinn a' Chàirn (983 feet), rapidly contracts north-eastwards into a narrow strip which forms the crest of the ridge, and at once suggests a much-weathered lava-stream. The resemblance to a coulée is heightened by the curious thinning off of the rocks where the two streams emerge from the Heast lochs; it looks as if the igneous mass were a mere superficial ridge which had been cut down by erosion, so as to expose the shales beneath it. But that the granophyre is really a sill becomes abundantly clear at its eastern end, where we find that it consists of two separate sheets with intervening Liassic shales. The structure of this interesting locality is shown in [Fig. 372]. In this instance also, there is evidence that the acid sills are younger than the basic, for the upper sheet of granophyre sends up into the overlying dark basaltic rock narrow vertical felsitic veins, a quarter of an inch to an inch in width, which being more durable, stand out above the decomposable surface of the containing rock, and show their quartz-blebs and felspar crystals on the weathered surface.
Perhaps the most striking feature of the granophyre sills of Skye is their general association with thinner basic intrusive sheets between which they have insinuated themselves. This characteristic structure, pointed out by me in 1888, has recently been more minutely mapped in the progress of the Geological Survey. Mr. Harker has found the typical arrangement to be the occurrence of a thick sill of granophyre interposed between two sills of basalt, each of which is usually not more than six or eight feet thick. Where the granophyre has been intruded independently among the Lias formations, it does not assume the regularity and persistence which mark it where it has followed the course of basic sills.
Fig. 372.—Section across the Granophyre Sills at Loch a' Mhullaich, above Skulamus, Skye.
a, Jurassic sandstones and shales; b, Jurassic dark brown sandy shales; c, sills of basalt, some bands highly cellular; c′, basalt-sill with veins of felsite rising into it from the granophyre below; d d, intrusive sheets or sills of granophyre.
"The acid rock," Mr. Harker observes, "is invariably the later intrusion, for it sends narrow veins into the basalts, metamorphosing them to some extent and frequently enclosing fragments of them. These fragments are always rounded by corrosion, and show various stages of dissolution down to mere darker patches as seen by the naked eye. Such inclusions and patches are found in the marginal part of a granophyre, where no continuous basalt occurs, but where the acid magma has evidently in places completely destroyed the earlier basic sheets between which it was forced. It seems probable that in all cases a certain amount of solution of the basalt by the granophyre magma took place at their contact, facilitating the injection of the later intrusion and accounting for its persistent choice of the contact-plane of two basalt-sills as the surface offering least resistance to its injection."
These observations throw fresh light on the remarkable original regularity and persistence of the basic sills. Where one of these sills disappears above or below a granophyre sheet its probable former presence is often indicated by corroded fragments of the basic in the acid rock. Mr. Harker remarks that the acid magma seems to have been "in itself less adapted than the basic to follow accurately a definite horizon and to maintain a uniform thickness in its intruded sheets, but could do both when guided by a pre-existing basalt-sill, or especially when insinuated between contiguous basalt-sills." The corrosive action of the acid magma on the surface of the basalt, which enabled it to force its way more readily between the basic sills, might proceed so far as partially or wholly to destroy these sills.
This solvent action may serve to explain some of the irregularities of the granophyre intrusions. According to the same observer, such irregularities are found "where the granophyre sheet and its encasing basalt-sills are not co-extensive, or again where the two basalt-sills separate, owing to one of them cutting obliquely across the bedding. In the latter case, which is not common, the granophyre follows one of the basalt-sills, necessarily parting from the other. When one of the two guiding basalt-sills dies out, the granophyre may still continue, following the sill which persists. If the latter also dies out, while the granophyre is still in some force, the acid magma seems to have been reluctant to travel beyond the limit of the basalt, but has drawn towards it, and the granophyre presents a blunt laccolitic form, which contrasts with the acutely tapering edge of a granophyre which dies out before reaching the limit of its basalt-sills. If, on the other hand, on reaching the limit of the basalt, the acid magma has been in such force as to be driven further, it is usually found to lose something of its regularity and to depart from the exact horizon which it has hitherto followed. This seems to happen, for instance, in the Beinn a' Chàirn sheet, which, when traced westward, is found to behave as a 'boss' and is obviously transgressive, having cut across the bedding of the strata so as to enter the limestones, where it no longer behaves in any degree as a sill. The district affords many examples of the tendency of intrusive masses in general to cut sharply across the beds when they enter a group of limestones."
More complex examples of acid sills are to be found where there have been three or more basic sheets together. The great granophyre sheet already referred to at Suisnish affords the best illustration of this structure. Mr. Harker has noticed that "round most of its circumference there is seen merely a single basalt-sill passing under the granophyre. Probably there has been another similar sheet over the acid rock, but if so, it has been removed by erosion, the granophyre itself forming everywhere the surface of the plateau. On the southern side, however, we see that the original basalt must have been at least triple, or counting the uppermost member, now removed, quadruple. The granophyre has forced its way in between the several members of the multiple basalt-sill, the intermediate ones being thus completely enveloped. They are evidently metamorphosed as well as veined by the granophyre, and when traced onward they give place to detached portions which, floating as it were in the acid rock, are soon lost."
It is seldom easy to determine where lay the vent or vents from which the granophyre sills proceeded. Those of the Skye platform just described may be chiefly concealed under some of the larger areas of the rock, such as the sheets of Carn Dearg or Beinn a' Chàirn. But in several places, in close association with the compound sills of granophyre and basalt, Mr. Harker has found large dyke-like bodies of the acid rock, which may with considerable probability be regarded as marking the position of the channels by which the material of the sills ascended. "These bodies," he remarks, "either occur isolated by erosion, the sills or the parts of the sills presumed to have been in connection with the dykes having been removed, or are only very partially exhibited in direct connection with sills still remaining. Where they can be examined in detail they are seen to be dykes varying up to about 100 feet in width, but of no great longitudinal extent. Between Suisnish and Cnoc Carnach they bear E.N.E., that is, at right angles to the ordinary basic dykes of the district and parallel to the general direction of the axes of folding, though further north they change this trend, but still remain parallel to the strike of the Lias.
"These dykes are composed essentially of granophyre, identical with that of the sills. In some cases, they are flanked with basalt-dykes on one or both sides, or the former existence of such lateral dykes is indicated by partly-destroyed inclusions of the basic rock in the granophyre. The basalt found in these cases is identical with that of the basic sills, and shows the same relation to the granophyre. Discontinuity and failure of the basalt are commoner, however, in the dykes than in the sills—a difference presumably attributable to more energetic destructive action of the acid magma when it was hotter and fresher. These supposed feeders of the granophyre sills are certainly in some cases, and have possibly been in all, double or triple dykes. The acid magma thus appears not only to have spread laterally along the same platforms as the earlier basalts, but to have reached these levels by rising through the same fissures which had already given passage to the basic magma."[431]
[431] MS. notes supplied by Mr. Harker.
The granophyre sills which, as already stated, can be followed as an interrupted band from Suisnish Point to the Sound of Scalpa, emerge again beyond Loch Sligachan and also in the island of Raasay, where a great sheet of the acid rock covers an area of about five square miles. This tract has recently been mapped for the Geological Survey by Mr. H. B. Woodward, who has found it to have been intruded across the Jurassic series, a large part of its mass coming in irregularly about the top of the thick white sandstones of the Inferior Oolite. But it descends beneath the Secondary rocks altogether, and in some places intervenes between the base of the Infra-liassic conglomerates and the Torridon sandstone. Its irregular course transgressively across the Mesozoic formations is probably to be regarded as another example of the intrusion of the acid material preferentially along the line of unconformability between the older rocks and the Tertiary basalts, now nearly all removed from Raasay by denudation, though the intrusion does not rigidly follow that line of division, but sometimes descends below it.
The central portions of this Raasay granophyre possess the ordinary structures of the corresponding rocks in Skye. They show a finely crystalline-granular, micropegmatitic base, through which large felspars and quartzes are dispersed. But at the upper and under junction with the sedimentary rocks, beautiful spherulitic structures are developed. This is well seen on the shore near the Point of Suisnish (Raasay), where, below the Lias Limestones, the top of the granophyre appears, and where its bottom is seen to lie on the Torridon sandstone.
This granophyre sheet presents a further point of interest inasmuch as it appears to have preserved one of the dyke-like masses which may mark channels of escape from the general body of the acid magma below. Near the Manse the section represented in [Fig. 373] may be observed. Owing to great denudation, the massive sheet of granophyre has been cut into isolated outliers which cap the low hills, and the rock may be seen descending through the Jurassic sandstones, which in places are much indurated. It is observable that the amount of contact-metamorphism induced by the granophyre sills upon the rocks between which they have been injected is, in general, comparatively trifling. It is for the most part a mere induration, sometimes accompanied with distortion and fracture.
Fig. 373.—Section to show the connection of a sill of Granophyre with its probable funnel of supply, Raasay.
a a, Jurassic sandstones; b, granophyre.
Fig. 374.—Granophyre sill resting on Lower Lias shales with a dyke of basalt passing laterally into a sill, Suisnish Point, Isle of Raasay.
Although the intrusion of the granophyre sills has been subsequent to that of the basalt-sheets with which they are so generally associated, we may expect that as there is a series of post-granophyre basic dykes, so there may be some basic sills later than the injections of the acid sheets. The Raasay granophyre appears to furnish an example of such a later basic intrusion. At the Point of Suisnish on that island I have observed the relations shown in [Fig. 374]. There the dark shales of the Lower Lias (a a) are immediately overlain by the granophyre sill (b), and are cut by a basalt-dyke which, when it rises to the base of the granophyre, turns abruptly to one side, and then pursues its course as a sill (c) between the granophyre and the shales. There can be little doubt that this intrusion is later than the granophyre. Here a basic sill is interposed at the bottom of the acid sheet; and is visibly connected with the actual fissure up which its molten material was impelled.
ii. THE ACID DYKES AND VEINS
Besides bosses and sills, the acid rocks of the Inner Hebrides take the form of Dykes and Veins which have invaded the other members of the volcanic series. Some of these have already been referred to; but a more particular description of the venous development of the acid rocks as a whole is now required.
As regards their occurrence and distribution, they present two phases, which, however, cannot always be distinguished from each other. On the one hand, they are found abundantly either directly proceeding from the bosses (more rarely from the sills), or in such immediate proximity and close relationship to these as to indicate that they must be regarded as apophyses from the larger bodies of eruptive material. On the other hand, they present themselves as solitary individuals, or in groups at a distance of sometimes several miles from any visible boss of granophyre. In such cases, it is of course obvious that though not exposed at the surface, there may be a large mass of the acid magma at no great distance beneath, and that these isolated dykes and veins do not essentially differ in origin from those of which the relations to eruptive bosses can be satisfactorily observed or inferred.
Considered as a petrographical group, these Dykes and Veins are marked by the following characters. At the one extreme, we have thoroughly vitreous rocks in the pitchstones. From these, through various degrees of devitrification, we are led to completely lithoid felsites, quartz-porphyries or rhyolites. Micropegmatitic structure is commonly present, and as it increases in development, the rocks assume the ordinary characters of granophyre. Occasionally the structure becomes microgranitic in the immediate periphery of a boss wherein a granitic character has been assumed. Viewed as a whole, however, it may be said that the dull lithoid rocks of the dykes and veins can generally be resolved under the microscope into some variety of granophyric porphyry or granophyre.
A characteristic feature in the granophyric, felsitic or rhyolitic dykes and veins is the presence of spherulitic structure (Figs. [375], [377]). In some cases this structure is hardly traceable save with the aid of the microscope, but from these minute proportions it may be followed up to such a strong development that the individual spherulites may be an inch or two in diameter, and lie crowded together, like the round pebbles of a conglomerate. The structure is a contact phenomenon, being specially marked along the margin of the dykes, as it is on the edge of sills and bosses. In the Strath district of Skye, Mr. Clough and Mr. Harker have observed that the spherulites are apt to be grouped in parallel lines so as to form rod-like aggregates along the walls, and that where the rock is fairly fresh the centre of the dyke sometimes consists of glassy pitchstone, so that the spherulitic felsite or granophyre is probably devitrified pitchstone. Frequently flow-structure is admirably developed in these dykes, the streaky layers of devitrification flowing round the spherulites and any enclosed fragments as perfectly as in any rhyolitic lava ([Fig. 378]).
Fig. 375.—Weathered surface of spherulitic granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size.
In regard to their modes of occurrence, the dykes of acid material differ in some important respects from those of basic composition. More especially they are apt to assume the irregular venous form, rather than the vertical wall-like character of ordinary dykes. They take the form of dykes, particularly where their material has been guided in its uprise by one or more already existent basic or intermediate dykes, as in the compound dykes, already described. The conditions for their production must thus have been essentially different from those of the great body of the basic dykes. Their intrusion was not marked by any general and widespread fissuring of the earth's crust, such as prepared rents for the reception of the basalt and andesite dykes. They were rather accompaniments of the protrusion of large masses of acid magma into the terrestrial crust. This magma, as we have seen, was often markedly liquid, and was impelled, sometimes with what might be called explosive violence, into the irregular cracks of the shattered surrounding rocks or into pre-existing dyke-fissures. Hence long straight dykes of the acid rocks are much less common than short irregular tortuous veins and strings.
Fig. 376.—Plan of portion of the ridge north of Druim an Eidhne, Glen Sligachan, Skye, showing three dykes issuing from a mass of granophyre.
a, gabbros; b, granophyre; I. II. III., three dykes proceeding from the granophyre. The arrows show the direction of dip of the bands of gabbro.
Much difference may be noticed among the granophyre bosses in regard to their giving off a fringe of apophyses. Thus, along the well-exposed boundary of Beinn-an-Dubhaich in Skye, though the edge of the boss is remarkably notched, hardly any veins deserving the name diverge from it. On the other hand, the ridge of Meall Dearg at the head of Glen Sligachan, already referred to, is distinguished by the number and variety of the dykes and veins which proceed from the granophyre and traverse the banded gabbros. As this locality has been elsewhere fully described, I will give here only the leading structural features which it presents.[432]
[432] Professor Judd (Quart. Journ. Geol. Soc. vol. xlix. (1893), p. 175) described the granophyre dykes of this locality as inclusions of Tertiary granite in the gabbro, and cited them in proof of his contention that the acid eruptions of the Western Isles are older than the basic. Their true character was shown by me in a paper published in the Quart. Journ. Geol. Soc. vol. 1. (1894), p. 212.
Fig. 377.—Weathered surface of spherulitic granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size.
Within a horizontal distance of less than 100 yards three well-marked dykes issue from the spherulitic edge of the Meall Dearg granophyre, and run in a south-easterly direction in the handed gabbros ([Fig. 376]). The most northerly of these is traceable in a nearly straight line for 800 feet. The central dyke, which can be followed for 200 feet or more, rises as a band six to ten feet broad between the dark walls of gabbro as represented in [Fig. 379].
These dykes are marked by the most perfectly developed spherulitic and flow-structures (Figs. [375], [377]). Numerous detached portions of other dykes and also irregular veins are to be observed cutting the banded gabbros all over the ridge of Druim an Eidhne for a distance of a mile or more. Many of these exhibit the same exquisitely beautiful spherulitic and flow-structure displayed by the dykes which can actually be traced into the main body of granophyre. The lines of flow conform to every sinuosity in the boundary-walls of gabbro, and sometimes sweep round and enclose blocks of that rock. The example of this structure, given in [Fig. 378], shows how these lines, curving round projections and bending into eddy-like swirls, exhibit the motion of a viscous lava flowing in a cleft between two walls of solid rock. Sometimes the laminæ of flow have been disrupted, and broken portions of them have been carried onward and enveloped in the yet unconsolidated material. Certain portions of this dyke are richly spherulitic, the spherulites varying from the size of small peas up to that of tennis-balls. Occasionally two large spherulites have coalesced into an 8-shaped concretion, and it may be observed in some cases that the spherulites are hollow shells.
Fig. 378.—Plan of pale granophyric dyke, with spherulitic and flow-structure, cutting and enclosing dark gabbro, Druim an Eidhne.
Fig. 379.—Dyke (six to ten feet broad) proceeding from a large body of granophyre and traversing gabbro, from the same locality as Figs. [375] and [377].
A remarkable feature has been recently observed by Mr. Harker among the abundant granophyre dykes and veins which intersect the gabbros and older rocks, along the eastern flanks of the Red Hills of Skye between Broadford and the Sound of Scalpa. Broad dykes of granophyre which traverse the Cambrian limestone of that district might be supposed at first sight to be cut off by the intrusions of gabbro. But closer examination proves that their apparent truncation arises from their suddenly breaking up into a network of small veins where they abut against the basic rock. This structure evidently belongs to the same type as that of the St. Kilda granophyre.
Fig. 380.—Section of intruded veins of various acid rocks above River Clachaig, Mull.
a a, basalt, dolerite, etc.; b b, granophyre.
Compound dykes and sills, where one or more of the injections has consisted of acid material, have been already noticed as intimately associated together in Skye (p. 162). Dykes of this nature are more particularly abundant in Strath, especially along its eastern side. In addition to the examples cited already from that district, I may refer to other two which intersect the Middle Lias shales and limestones in the island of Scalpa. They are both compound dykes, but the more basic marginal bands are not always continuous, having possibly been here and there dissolved by the acid invasion. Though they do not show any distinct spherulitic forms, the presence of flow-structure is indicated by the thin slabs into which the rocks weather parallel to the dyke-walls. The rock in each case is a fine-grained felsitic mass, with bi-pyramidal crystals of quartz. It is observable that where these dykes come directly against the Liassic strata, the latter are more seriously indurated than where they are traversed by the ordinary basic dykes.
In the central mountainous tract of the island of Mull veins of acid material are extraordinarily abundant. They probably proceed from a much larger subterranean body of granophyre than any of the comparatively small bosses of this rock which appear at the present surface of the ground. They show themselves partly at the margins of the visible bosses, but much more profusely in that tract of altered basalt, with intrusive sheets and dykes of basalt, dolerite and gabbro, which lies within the great ring of heights between Loch na Keal and Loch Spelve. In some areas, the amount of injected material appears to equal the mass of more basic rock into which it has been thrust. Pale grey and yellowish porphyries and granophyres, varying from thick dykes down to the merest threads, ramify in an intricate network through the dark rocks of the hills, as shown in the accompanying illustration ([Fig. 380]), which represents a portion of the hillside between Beinn Fhada and the Clachaig River. Such a profusion of veins probably indicates the existence here of some large mass of granophyre or granite, at no great depth beneath the surface.
In Mull, as in the other islands of the Inner Hebrides, two horizons on which protrusions of acid materials have been specially abundant, are the base of the bedded basalts of the plateau and the bottom of the thick sheets of gabbro. Dykes and veins of granophyre, quartz-porphyry, felsite and other allied rocks are sometimes crowded together along these two horizons, though they may be infrequent above or below them.
Illustrations of solitary veins in the midst of unaltered plateau-basalts or in older rocks may be gathered from many parts of the Western Isles. Some remarkable instances are to be seen among the basalts that form the terraced slopes on the north side of Loch Sligachan. Several thick dykes of granophyre run up the declivity, cutting across hundreds of feet of the nearly level basalt-beds. Some of them can be seen on the shore passing under the sea. They trend in a S.S.E. direction towards Glamaig, and they are not improbably apophyses from that huge boss, the nearest edge of which is three-quarters of a mile distant. Another example may be cited from the basalt-outlier of Strathaird, where two veins of felsite, one of them a pale flinty rock showing flow-structure parallel to the walls, may be seen on the west front of Ben Meabost. In this case, the veins are three miles and a half from the granophyre mass of Strath na Creitheach to the north, four miles from that of Beinn an Dubhaich to the north-east, and nearly three miles from that of Coire Uaigneich at the foot of Blath Bheinn.
A special place must be reserved for the pitchstone-veins. Ever since the early explorations of Jameson and Macculloch, the West of Scotland has been noted as one of the chief European districts for these vitreous rocks. From Skye to Arran, and thence to Antrim, many localities have furnished examples of them, but always within the limits of the Tertiary volcanic region. That all of the pitchstones are of Tertiary age cannot, of course, be proved, for some of them are found traversing only Palæozoic rocks, and of these all that can be absolutely affirmed is that they must be younger than the Carboniferous or even the Permian system. But, as most of them are unquestionably parts of the Tertiary volcanic series, they are probably all referable to that series. Not only so, but there is, I think, good reason to place them among its very youngest members. It is a significant fact that they almost always occur either in or close to granophyre or granite bosses, the comparatively late origin of which has now been proved.
Fig. 381.—Pitchstone vein traversing the bedded basalts, Rudh an Tangairt, Eigg.
The first pitchstone observed in Skye was found by Jameson on the flanks of the great granophyre cone of Glamaig. Another rises on the side of the porphyry mass of Glas Bheinn Bheag, in Strath Beg. Several occur at the foot of Beinn na Callich. In Rum, I found a pitchstone vein traversing the western slopes of the wide granophyre boss of Orval. In Eigg, the well-known veins of this rock intersect the plateau-basalts ([Fig. 381]), but they are accompanied, even within the same fissure, with granophyre, and in their near neighbourhood lie the masses of this rock already alluded to.[433] In Antrim, pitchstone and obsidian occur in the midst of the rhyolite. The only marked exceptions to the general rule, with which I am acquainted, are those of the island of Arran. Most of the pitchstone-veins in that district traverse the red sandstones which may be Permian. But none of them are far removed from the great granite boss of the northern half of the island, while large masses of quartz-porphyry, which strikingly resemble some of those of Skye and Mull, lie still nearer to them. It is also worthy of notice that pitchstone-veins rise through the Arran granite boss itself, the probably Tertiary date of which has been already discussed.
[433] For an account of the pitchstone veins of Eigg, see Quart. Journ. Geol. Soc. xxvii. p. 299.
This common association of pitchstone-veins with the Tertiary eruptive bosses of acid rocks can hardly be a mere accidental coincidence. It seems to prove a renewed extravasation of acid material, now in vitreous form, from the same vents that had supplied the granitoid, granophyric, porphyritic and felsitic varieties of earlier protrusions. We must remember that the pitchstone-veins are not mere local glassy parts of the larger bodies of granophyre or granite in which they lie. Their margins are sharply defined; they are indeed in all respects as manifestly intruded, and therefore later masses, as are the basalt-dykes. Their occurrence, therefore, within the acid bosses proves them to be younger than these members of the Tertiary volcanic series. Whether they are also later than the latest basalt-dykes cannot yet be decided, for I have never succeeded in finding an example of the intersection of these two groups of veins and dykes. But, with this possible exception, the pitchstones are the most recent of all the eruptive rocks of Britain.
As a rule, the intrusive pitchstones occur as veins which cannot be traced far, and which vary from a few yards to less than an inch in width. They generally show considerable irregularity in breadth and direction, sometimes sending out strings into the surrounding rock ([Fig. 381]). The outer portions are not infrequently more glassy and obsidian-like than the interior. Occasionally the vitreous character disappears by devitrification, and the rock assumes the texture of a compact felsite or of a spherulitic rock.
Among the later movements of the acid magma account must be taken here of the pale fine-grained veins which have already been referred to as traversing the granophyre bosses. These intrusions, so well seen in the bosses of Skye and St. Kilda, are often so close in texture that they may be called quartz-felsites. Their sharply-defined edges and felsitic character suffice to separate them from what are termed "veins of segregation." In at least one instance, that of Meall Dearg, already cited, a mass of typical granophyre which has developed spherulitic and flow-structures along its margin, and which sends out dykes having the very same structures for a distance of several hundred feet across the banded gabbros, is itself traversed by a dyke of precisely similar character. Here we see that after the intrusion of its apophyses, and after its own consolidation in the upper parts, the granophyric magma that rose into rents in the solidified portion retained the same tendency to produce large spherulites as it had shown at first.
The fine felsitic veins that traverse the granophyre of the Red Hills are now being mapped by Mr. Harker during the progress of the Geological Survey. He has not yet obtained evidence of the age of these veins in relation to the latest basic dykes. He has observed that they appear to be on the whole rather less acid than the material of the surrounding bosses, though they were probably all connected with the same underlying acid magma from which the bosses were protruded. A somewhat similar relation has been noticed between older granites and their surrounding dykes, as in Cornwall and Galloway.
CHAPTER XLIX
THE SUBSIDENCES AND DISLOCATIONS OF THE PLATEAUX
There can be no doubt that considerable alterations of level have taken place over the volcanic areas of North-Western Europe since the eruptions that produced the basalt-plateaux, These alterations embrace general and local subsidences, and also dislocations by which considerable displacements of the crust either in a downward or upward direction have been effected.
i. SUBSIDENCES
The mere fact that in many places the lower members of the series of terrestrial lavas have been submerged under the sea may be taken to prove a subsidence since older Tertiary time. Along the west coast of Skye this depression is well shown by the almost entire concealment of the bottom of the plateau under the Atlantic. In the Faroe Isles the subsidence has advanced still further, for not a trace of the underlying platform on which the basalts rest remains above water. In Iceland, too, the complete submergence of the base of the Tertiary volcanic sheets points to a widespread subsidence of that region.
Another strong argument in favour of considerable depression may be derived from a comparison of the submarine topography with that of the tracts above sea-level. It is obvious that the same forms of contour which are conspicuous on the land are prolonged under the Atlantic. If we are correct in regarding the valleys as great lines of subærial erosion, their prolongations as fjords and submarine troughs must be considered as having had a similar origin. We can thus carry down the surface of erosion several hundred feet lower than the line along which it disappears under the waves.
I know no locality where this kind of reasoning is so impressively enforced upon the mind as the west end of the Scuir of Eigg. The old river-bed and its pitchstone terminate abruptly at the top of a great precipice. Assuredly they must once have continued much further westward, as well as the sheets of basalt that form the main part of the cliff. Yet the sea in front of this truncated face of rock rapidly deepens to fully 500 feet in some places. Had any such hollow existed in the volcanic period it would have been filled up by the long-continued outflowings of basalt. Making every allowance for concealed faults and local subsidences, we can only account for this submarine topography by regarding it as having been carved out, together with the topography of the land, at a time when the level of the latter was at least 500 feet higher than it is now.
The subsidence which is thus indicated along the whole of the North-West of Europe probably varied in amount from one region to another. We seem to have traces of such inequalities in the varying inclinations of different segments of the basalt-plateaux. The angles of inclination are almost always gentle, but they differ so much in direction from island to island, and even among the several districts of the same island, as to indicate that certain portions of the volcanic plain have sunk rather more than other portions.
Thus in the Faroe Islands, where the bare cliffs allow the varying angles of inclination to be easily determined, a general gentle dip of the basalts in a south-easterly direction has been noted among the central and northern islands by previous observers. This inclination, however, is replaced among the southern islands by an equally gentle dip towards the north-east. The centre of depression would thus seem to lie somewhere about Sandö and Skuö. The highest angle of inclination which I noticed anywhere was at Myggenaes, where the basalts dip E.S.E. at about 15°.
Among the Western Isles, also, where similar variations in the inclination of the basalt-sheets are observable, it might be possible by careful survey to ascertain the probable position of the areas of maximum depression, and to show to what extent differential movements have affected the originally nearly level volcanic floor. It would doubtless be found that everywhere the dominant movement has been one of subsidence. The vast outpourings of lava would tend to leave the overlying crust unsupported, and to cause it to sink into the cavities thus produced.
Perhaps the most extensive subsidence of this kind, at least that which admits of most satisfactory investigation, because it still remains above sea-level, is displayed by the vast hollow in the Antrim plateau, which embraces the basin of Lough Neagh and the valley of the Lower Bann. This depression measures about 60 miles in length by about 20 in breadth. Its axis follows the N.N.W. trend so characteristic of the volcanic features of Tertiary time. The depression may be said to involve the entire basaltic plateau of Antrim, for with the exception of a few insignificant areas along the borders, especially on the east side between Larne and Cushendall, the whole region slopes inward from its marginal line of escarpments, which reach heights of 1800 feet and upwards, towards the great hollow in its centre (see Map VII.).
Lough Neagh, which occupies the deepest part of this hollow, and covers about one-eighth of the whole area of subsidence, is the largest sheet of fresh water in the British Isles, for it exceeds 150 square miles in extent of surface. Yet, for its size, it is one of the shallowest of our lakes, its average depth being less than 40 feet. Its shallowness, compared with its wide area, marks it out in strong contrast to most of the larger British lakes. Its surface is only 48 feet above the level of the sea.
The origin of Lough Neagh, the theme of various legends, has been seriously discussed by different writers, but most exhaustively by the late E. T. Hardman of the Geological Survey.[434] This author connected the formation of the lake-basin with a series of large faults which are found intersecting the rocks around the basin, and passing under the water in a general north-easterly direction. He showed that these faults have produced serious displacements of the strata, amounting sometimes to as much as 2000 feet, and he believed that it was by the concurrent effect of such dislocations that the depression of Lough Neagh had been caused.
[434] "On the Age and Formation of Lough Neagh," Journ. Roy. Geol. Soc. Ireland, vol. iv. (1875-76), p. 170; also Explanation of Sheet 35 of the Geol. Surv. Ireland (1877), p. 72.
It is possible that these displacements may have contributed to at least the earlier stages in the history of the Antrim subsidence. They have undoubtedly taken place after the outpouring of the basalts, for these rocks are involved in their effects. But in the hollow of the Bann valley north of Lough Neagh the faults which have been detected in the basaltic plateau are few and trifling. The bold and bare escarpments, that so clearly display the relations of the rocks, reveal few traces of any important transverse dislocations. Nor has any proof of large longitudinal faults parallel with the axis of depression been obtained within the area of the Bann valley.
The earliest evidence for the existence of a lake on the site of the present Lough Neagh has been supposed to be furnished by certain fine clays, sands, seams of lignite and clay-ironstone, which have been referred to the Pliocene period. These deposits have been regarded as indicating the accumulation of fine sediment with drift vegetation brought down into a quiet lake by streams entering from the south. Their fresh-water origin was believed to be further corroborated by the occurrence of shells belonging to the lacustrine or fluviatile genus, Unio.[435]
[435] These shells were regarded as forms of Unio by the late W. H. Baily; but Dr. Henry Woodward assigned them to Mytilus. See Prof. Hull's Physical Geology and Geography of Ireland, 2nd edit. p. 101. The shells have been more recently dug out by Mr. Clement Reid, who has found them to be the common Mytilus edulis.
The thickness of this series of strata, their position above sea-level, and their distribution are important parts of the evidence for the geological history of the locality. At one place the deposits are said to have been bored through to a depth of 294 feet, and Mr. Hardman believed them to be not less than 500 feet deep. The same observer found that they certainly reach a height of 120 feet above the sea, and he was of opinion that in some places their height was not less than 140 feet. The deposition of strata to the depth of 300 feet below a level of 120 feet above the sea would, of course, entirely fill up Lough Neagh, and spread over a large tract of low ground around it. The pottery-clays and lignites, however, appear to be confined to the southern half of the lake, from which they rise gently into the low country around.
The distribution of these deposits and their extraordinary variations in altitude, as described by Mr. Hardman, present great difficulties in the attempt to regard them as the sediments of a Pliocene lake. A more recent examination of the ground by Mr. Clement Reid of the Geological Survey has led that able observer to believe that two totally different groups of strata at Lough Neagh have been confounded. He noticed the Mytilus-clay to be a dark blue mass full of derived boulder-clay stones, and yielding Mytilus edulis and seeds of a sedge. This deposit cannot be Pliocene, but must be of Glacial or post-Glacial age, possibly contemporary with the Clyde beds. The junction of this clay with the pipe-clays is not at present seen, but the lithological contrast between the two groups of strata is so strong as to indicate their independence of each other. Mr. Reid found the white, red and mottled pipe-clays with their masses of lignite to present a strong resemblance to the Bagshot group in the Tertiary series. It is possible, as already suggested, that the pipe-clays and lignites may belong to the sedimentary zone that separates the lower and upper basalts of Antrim. At all events they furnish no proof of any Pliocene lake, and may not indicate more than a deeper part of the depression in which the tuffs, lignites and iron-ore were laid down.
The existence of the Mytilus-clay shows that in Glacial or post-Glacial times the valley of the Bann was a strait or fjord into which the sea entered. Thick masses of drift have been laid down all round and over the depression now occupied by Lough Neagh, insomuch that had any older lake existed here in Glacial times, it could hardly have escaped being filled up.
The observer, who from one of the basalt-heights looks down upon the expanse of Lough Neagh and the broad peat-covered plain that continues the level platform of the lake-surface down the valley of the Bann, cannot but be impressed with the size of this wide hollow in the heart of the Antrim plateau, and with the evident continuity of the whole depression from the lake to the sea. If he be a geologist, he will be further struck by the fact that while the Chalk and other older rocks appear from under the basalt-escarpments all round the plateau, at heights of many hundred feet above the sea, the floor of this wide hollow is entirely covered with basalt. Had the depression been merely due to denudation, the rocks that underlie the volcanic series would have been exposed to view. The base of the basalts which, on either side of the depression, is often more than 1000 feet above the sea-level, sinks below that level in the hollow of the Bann and Lough Neagh.
This inequality of position may have been partially brought about by faults like those around Lough Neagh, and may thus have been begun long before the Glacial period. But it appears to me to be mainly due to a wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction from the present coast up the valley of the Bann and the basin of Lough Neagh to beyond Portadown.
We may conceive that after the cessation of the outflows of basalt, the territory overlying the lava-reservoir that had been emptied would tend to subside, partly by ruptures of the crust producing faults and partly by a downward movement of a more general kind. In course of time, these disturbances turned the drainage into the hollow now traversed by the Bann. Denudation would necessarily accompany them, and the surface of the country would be continually eroded and lowered.
Lough Neagh has been carefully sounded by the Admiralty, and its chart affords much suggestive material for the consideration of the geologist.[436] From the soundings there given it has long been known that the lake deepens towards its northern end, and attains a maximum depth of 102 feet. But it is not until we trace on the chart a series of contour-lines for successive depths, as shown by the soundings, that we realize the remarkable form of the lake bottom. We then discover that below a depth of 50 feet a well-defined channel extends for rather more than half the length of the lake. This channel begins to be distinctly perceptible between Kiltagh Point and Langford Lodge. It first runs in a northerly course on the west side of the centre of the Lough, but when it comes into a line with Saltera Castle on the western shore, it wheels round so as to conform to the curve of the Antrim coast-line, which it follows northward until, about two miles from the exit of the lake, its outline ceases to be traceable on the gently shelving bottom. Its total length is thus about 12 miles.
[436] Lough Neagh surveyed and sounded by Lieut. Thomas Graves, R.N.
There can hardly be any doubt that this channel is a former bed of the River Bann. It occupies exactly the position which that stream would take if the lake were drained, and its depth and breadth correspond to those of the valley-bottom of the present river. If this conclusion be accepted, some important conclusions may be further deduced from it.
1. The presence of a former course of the Bann on the bottom of Lough Neagh proves the lake to be much younger than the Ice Age. The thick boulder-clays and Glacial gravels which so encumber the country around and descend under the lake, would assuredly have filled up the river-channel had it existed at the time of their deposition. The channel has obviously been cut out of these drifts since the Glacial period. When the erosion took place, the present Lough Neagh could not have existed, but the Bann followed a continuous course across the plain which the lake now covers. The river probably maintained its place for a long period, so as to be able to excavate so wide and deep a bed in the drifts, if, indeed, it did not to some extent slowly carve its bed out of the underlying basalts. It must be remembered that sediment is being continually poured into Lough Neagh, and that some of the silt must have accumulated in the submerged river-course, thus lessening its depth and width. That the channel should still be so marked may be used as an argument for the comparatively late date of the subsidence.
2. The submerged river-course is a clear proof of subsidence. The present Lough Neagh cannot be looked upon as a glacial lake formed by rock-erosion or by irregular deposition of drift. Its floor must have been a land surface when the Bann cut out its bed upon it. The whole area has sunk down, the drainage has been arrested, and some 20 miles of the course of the Bann are now under a sheet of shallow water. This subsidence was not brought about by faults. It seems rather to have resulted from a general sinking of the ground. The movement was probably comparatively rapid, otherwise the river-course would hardly have survived so well.
3. These inferences, based upon purely geological considerations, have an interesting bearing upon the allusions to the origin of Lough Neagh contained in some ancient historical documents. Various legends have from an early period been handed down as to the first appearance of this sheet of water. These myths, though differing in details, agree in describing such a sudden or rapid accumulation of water as destroyed human life, in a district which had previously been inhabited by man. The earliest records indicate that the alleged catastrophe took place in the first century of the Christian era.[437] It appears to me not improbable that the tradition,thus preserved in these legends, may have had its basis in the actual disturbance which, on geological grounds, can be shown to have determined the existence of Lough Neagh. Though the event may go back far beyond the first century, there can be no doubt that, in a geological sense, it was one of the most recent topographical changes which the British Isles have undergone.
[437] For versions of the legends, see Dr. Todd's "Irish Version of the Historia Britonum of Nennius," Roy. Hist, and Archæol. Assoc. Ireland; Dr. Reeves' "Ecclesiastical Antiquities of Down," etc., p. 370; Mr. J. O'Beirne Crowe's "Ancient Lake Legends of Ireland," No. 1 in Journ. Roy. Hist. and Archæol. Assoc. Ireland, vol. i. (1870-71), p. 94; Giraldus Cambrensis, vol. v. cap. ix. p. 91—"de lacu magno miram originem habente." Moore's well-known lines embody the popular belief that round towers and other buildings were submerged by the inundation.
Thus the Antrim basalt-plateau, in addition to the high interest of its volcanic history, has the additional claim to our attention that it has preserved, more fully and clearly than any other of the plateaux, the evidence for the latest subterranean movements that followed the long series of volcanic eruptions during Tertiary time. It contains the record of a post-Glacial subsidence that gave birth to the largest lake in Britain.
ii. DISLOCATIONS
Though I have not observed any features among the Tertiary basalt-plateaux of the British Isles that can be compared to the remarkable rifts and subsidences of Iceland, it can be shown that these piles of volcanic material have undoubtedly been fractured, and that portions of them have subsided along the lines of dislocation.
Careful examination of the basalt-escarpments of the Inner Hebrides discloses the existence of numerous faults which, though generally of small displacement, nevertheless completely break the continuity of all the rocks in a precipice of 700 or 1000 feet in height. Not infrequently such dislocations give rise to clefts in the cliffs. Some good illustrations of this feature may be noticed on the north side of the island of Canna, where the precipice has been fissured by a series of dislocations, having a hade towards the west and a throw which may in some cases amount to about 20 or 25 feet. The cumulative effect of this system of faulting, combined with a gentle westerly dip, is to bring down to the sea-level the upper band of conglomerate which further to the east lies at the top of the cliff. Again, the basalt-escarpment on the west side of Skye, from Dunvegan Head to Loch Eynort, is traversed by a series of small faults. On the east side of Skye and in Raasay, a number of faults, some of them having perhaps a throw of several hundred feet, has been mapped by Mr. H. B. Woodward.
The largest dislocation observed by me among the basalt-plateaux of the Inner Hebrides is that already referred to (p. 209), which runs at the back of the Morven outlier, in the west of Argyllshire, from the Sound of Mull by the head of Loch Aline to the mouth of Loch Sunart, along the line of valley that contains the salt-water fjord Loch Teacus and the fresh-water lakes Loch Durinemast and Loch Arienas. While the Cretaceous deposits and the bottom of their overlying basalts rise but little above the sea-level on the south-west side of this line, they are perched as outliers on hill-tops on the north-east side, where they rise to 1300 feet above the sea. The amount of vertical displacement here probably exceeds 1000 feet. The fault runs in a north-westerly direction, and has obviously been the guiding influence in the erosion of the broad and deep valley which marks its course at the surface.
This dislocation is only the largest of a number by which the basalt-plateau has been broken in the district of Morven. Their effects are well shown in the outlier of basalt which caps Ben Iadain, where two parallel faults bring down the lavas against the platform of schists on which they lie (see [Fig. 266]).
Many faults have been traced in the Antrim plateau, and are represented on the Geological Survey Maps. In general they are of comparatively trifling displacement. Occasionally, however, they amount to several hundred feet, as in those already referred to as occurring near Ballycastle and around the southern part of the basin of Lough Neagh.
To what extent the dislocations that traverse the British Tertiary basalts are to be regarded as comparable to those which in Iceland have been referred to subsidence caused by the tapping and outflow of the lower still liquid parts of lava-sheets must be matter for further inquiry. So far as my own observations have yet gone, the faults do not seem explicable by any mere superficial action of the kind supposed. Where they descend through many hundreds of feet of successive sheets of basalt, and dislocate the Secondary formations underneath, they must obviously have been produced by much more general and deep-seated causes.
It is conceivable that, if these dislocations took place during the volcanic period, they broke up the lava-plains into sections, some of which sank down so as to leave a vertical wall at the surface on one side of the rent, or even to form open "gjás," like those of Iceland. But it is noteworthy that the fissures, which have been filled with basalt and now appear as dykes, comparatively seldom show any displacement in the relative levels of their two sides. In Iceland, also, the great lava-emitting fissures seem to be in general free from marked displacements of that kind.
Fig. 382.—Reversed fault on the eastern side of Svinö, Faroe Isles.
The faults in the Inner Hebrides, so far as I have observed, are all normal, and indicate nothing more than gentle subsidence. But among the Faroe Islands I have come upon several instances of reversed faults, which, in spite of the usually gentle inclinations of the basalts, probably point to more vigorous displacement within the terrestrial crust.
On the east side of Svinö a fault with a low hade runs from sea-level up to the top of the cliff, a height of several hundred feet. It has a down-throw of a few yards, but is a reversed fault, as will be seen from Fig. 382. Another similar instance may be noticed on the north-east headland of Sandö, where, however, on the upcast side, the basalts appear as if they had been driven upward, a portion of them having been pushed up into a low arch ([Fig. 383]).
Fig. 383.—Reversed fault on the north-east headland of Sandö, Faroe Isle.
When the Tertiary basalt-plateaux of the Hebrides and the Faroe Isles come to be worked out in detail, many examples of dislocation will doubtless be discovered. We shall then learn more of the amount and effects of the terrestrial disturbances which have affected North-Western Europe since older Tertiary time. In the meantime evidence enough has been adduced to prepare us for proofs of very considerable recent displacements even among regions of crystalline schists, like that which has been disrupted by the Morven faults above alluded to. While the study of the Tertiary volcanic rocks demonstrates the vast general denudation of the country since older Tertiary time, the proofs that these rocks have been faulted acquire a special interest in relation to the origin and evolution of the topography of the region.
CHAPTER L
EFFECTS OF DENUDATION
Among the more impressive lessons which the basalt-plateaux of North-Western Europe teach the geologist, the enormous erosion of the surface of this part of the continental area since older Tertiary time takes a foremost place. He may be ready almost without question to accept the evidence adduced in favour of a vast amount of denudation among such soft and incoherent strata as those of the older Tertiary formations of the south-east of England or the north-west of France. But he is hardly prepared for the proofs which meet him among the north-western isles that such thick masses of solid volcanic rocks have been removed during the same geological interval.
To gain some idea of the amount of this waste we must, in the first place, picture to our minds the extent of ground over which the lavas were poured, and the depth to which they were piled upon it. Though we may never be able to ascertain whether the now isolated basalt-plateaux of Britain were once united into a continuous plain of lava, we can be quite certain that every one of these plateaux was formerly more extensive than it is now, for each of them presents, as its terminal edge, a line of wall formed by the truncated ends of horizontal basalt-sheets. And there seems no improbability in the assumption that the whole of the great hollow from the centre of Antrim up to the Minch was flooded with lavas which flowed from many vents between the hills of ancient crystalline rocks forming the line of the Outer Hebrides on the west, and those of the mainland of Scotland on the east.
It is certain that the depth to which some parts of this long hollow were overflowed with lava exceeded 3000 feet, for more than that depth of rock can be shown to have been in some places removed. The original inequalities of surface were buried under the volcanic materials which were spread out in a vast plain or series of plains, like those that have been deluged by modern eruptions in Iceland. Owing, however, to a general but unequal movement of subsidence, the lava-fields sank down here and there to, perhaps, an extent of several hundred feet, so that the old land-surface on which they began to be poured out now lies in those places below the level of the sea.
I have shown that even during the volcanic period, while the lavas were still flowing from time to time, erosion was in active progress over the surface of the volcanic plain. The records of river-action in Canna and Sanday, and the buried river-channel of the Scuir of Eigg, prove that, while eruptions still continued, rivers descending from the mountains of the Western Highlands carried the detritus of these uplands for many miles across the lava-fields, swept away the loose material of volcanic cones, and cut channels for themselves out of the black rugged floor of basalt.
The erosion thus early begun has probably been carried on continuously ever since. The present streams may be looked upon as practically the same as those which were flowing in the Tertiary period. There may have been slight changes of level, oscillations both upward and downward in the relative positions of land and sea, and shiftings of the water-courses to one side or other; but there seems no reason to doubt that the existing basalt-plateaux, which were built up as terrestrial areas, have remained land-surfaces with little intermission ever since, although their lower portions may have been in large measure submerged.
In the existing valleys, fjords and sea-straits by which these plateaux have been so deeply and abundantly trenched, we may recognize some of the drainage-lines traced out by the rivers which flowed across the volcanic plains. The results achieved by this prolonged denudation are of the most stupendous kind. The original lava-floor has been cut down into a fragmentary tableland. Hundreds of feet of solid rock have been removed from its general surface. Outliers of it may be seen scattered over the mountains of Morven, whence they look into the heart of the Highlands. Others cap the hills of Rum, where they face the open Atlantic. Several miles from the main body of the plateau in Skye, a solitary remnant, perched on the highest summit of Raasay, bears eloquent witness that the basaltic tableland once stretched far to the east of its present limits.
Two lines of observation and of argument may be followed in the effort to demonstrate how great the denudation has been since older Tertiary time. In the first place, there is the evidence of the level or nearly level sheets of basalt that form the plateaux, and, in the second place, there is the testimony of the dykes, sills and bosses by which these lavas have been disrupted.
1. The study of the denudation of the Tertiary volcanic rocks of North-Western Europe is most satisfactorily begun by an attempt to measure the minimum amount of waste which in certain places the basalt-plateaux can be proved to have undergone. For the purposes of this study, the stratification of the lavas and their nearly horizontal, or at least very slightly disturbed, position afford exceptional facilities. Amorphous rocks, such as granites and gabbros, or even foliated masses like the old gneisses and schists, may have been enormously denuded. Their mere presence at the existing surface may be taken as proof of such waste, yet they furnish in themselves no criterion by which the amount of removed material may be estimated.
But in the case of the basalt-plateaux, as in that of horizontal sedimentary formations, the successive lines of superposition of the component beds of the whole stratigraphical series supply admirable datum-lines which, on the one hand, vividly impress the imagination by the demonstration which they afford of the reality and magnitude of the denudation, and, on the other hand, furnish a measure by which the minimum amount of this denudation may be actually computed.
Availing ourselves of this kind of evidence it is easy to show that valleys many miles long, several miles broad, and from crest to bottom several thousand feet deep, have been excavated out of the basalt-plateaux since the close of the volcanic period. And if this conclusion can be demonstrated for these plateaux, it must obviously apply equally to the rest of the country. We thus obtain a most important contribution to the investigation of the origin and relative age of the present topographical features of the surface of the land.
Let me give a few illustrations of the nature of the investigation and of the results to which it leads. Throughout the Western and Faroe Islands the level bars of basalt present their truncated ends in the great escarpment-cliffs which wind mile after mile along their picturesque coasts. Where they front the open sea, it is obviously impossible to say how much further seaward they once extended. But where they retire in fjords or sea-lochs, and sweep inland into glens, it is easy to measure the distance from the bottom of the eroded hollow to its bounding watersheds, and to estimate the amount of material that has been worn out of it. The only uncertainty in this computation arises from our inability to determine to what extent movements of subsidence may have come into play to aid in the disappearance of the basalts. Where the bottom of the lavas can be seen at the same level on either side of an inlet, with no evidence of faulting, or where a definite horizon in the volcanic series can be traced round the head of a glen or sea-loch, the influence of underground movements may be eliminated. The evidence of vast denudation is always visible, the proofs of subsidence are much less frequently observable.
The island of Mull supplies many striking examples of the enormous waste of the basalt-plateau. The Sound of Mull, for instance, has been eroded out of the volcanic series for a distance of 20 miles, with a mean breadth of about two miles. From the deepest part of this fjord to the summit of the Mull plateau is a vertical height of 3600 feet. The whole of this vast excavation has taken place since older Tertiary time. On the opposite side of Mull the hollow of Loch Scridain has been eroded to a mean depth of at least 1200 feet below the average level of the surrounding plateau, with a breadth of rather more than a mile.
The scattered islands which lie to the west of Mull tell the same tale. They are all outliers of the same basalt-plateau, and have not only been greatly lowered by the removal of their upper lavas, but have been separated by the erosion of long and deep hollows between them. Thus from the summit of the Gribon cliffs in Mull to the deepest part of the sea-floor between that precipice and the Treshnish Isles a vertical depth of at least 2000 feet of rock has been removed since the basalts ceased to be erupted.
I have referred to the impressive evidence of denudation displayed on the west side of the island of Eigg. The vertical distance from the summit of the Eigg plateau to the bottom of the submarine valley between this island and Rum is about 1500 feet, but as that summit lies below the original surface of the lava-field, the depth of rock which has been removed must exceed 1500 feet. We thus learn that since the close of the volcanic period the hollow between the islands of Eigg and Rum has been eroded to this great depth.
Still more striking is the evidence of enormous waste presented by the Faroe Islands. The cliffs there are loftier and barer, and the fjords have been cut more deeply and precipitously out of the basalt-plateau. I shall never forget the first impression made on my mind when the dense curtain of mist within which I had approached the southern end of the archipelago rapidly cleared away, and the sunlit slopes and precipices of Suderö, the two Dimons, Skuö and Sandö, rose out of a deep blue sea. Each island showed its prolongation of the same long level lines of rock-terrace. The eye at once seized on these features as the dominant element in the geology and the topography, for they revealed at a glance the true structure of the islands, and gave a measure of the amount and irregularity of the erosion of the original basalt-plateau. And this first impression of stupendous degradation only deepened as one advanced further north into the more mountainous group of islands. Probably nowhere else in Europe is the potency of denudation as a factor in the evolution of topographical features so marvellously and instructively displayed as among the north-eastern members of the Faroe group.
Availing ourselves of the datum-lines supplied by the nearly level bars of basalt, we easily perceive that in many parts of the Faroe Isles the amount of volcanic material left behind, stupendous though it be, is less than the amount which has been removed. Thus the island of Kalsö is merely a long narrow ridge separating two broad valleys which are now occupied by fjords. The material carved out of these valleys would make several islands as large as Kalsö. Again, the lofty precipice of Myling Head, 2260 feet high, built up of bedded basalts from the summit to below sea-level, faces the north-western Atlantic, and the sea rapidly deepens in front of it to the surface of the submarine ridge 200 to 300 feet below. The truncated ends of the vast pile of basalt-sheets which form that loftiest sea-wall of Europe bears testimony to the colossal denudation which has swept away all of the volcanic plateau that once extended further towards the west.
Nevertheless, enormous as has been the waste of this plateau of the Faroe Islands, we may still trace some of its terrestrial features that date back probably to the volcanic period. Even more distinctly, perhaps, than among the Western Isles of Scotland, we may recognize the position of the original valleys, and trace some of the main drainage lines of the area when it formed a wide and continuous tract of land.
A line of watershed can be followed in a south-westerly direction from the east side of Viderö, across Borö to the centre of Osterö, and thence by the Sund across Stromö and Vaagö. From this line the fjords and valleys diverge towards the north-west and south-east. There can hardly be any doubt that on the whole this line corresponds with the general trend of the water-parting at the time when the Tertiary streams were flowing over the still continuous volcanic plain. Considerable depression of the whole region has since then sent the sea up the lower and wider valleys, converting them into fjords, and isolating their intervening ridges into islands.
The topography of the Faroe Islands seems to me eminently deserving of careful study in the light of its geological origin. There is assuredly no other region in Europe where the interesting problems presented by this subject could be studied so easily, where the geological structure is throughout so simple, where the combined influences of the atmosphere and of the sea could be so admirably worked out and distinguished, and where the imagination, kindled to enthusiasm by the contemplation of noble scenery, could be so constantly and imperiously controlled by the accurate observation of ascertainable fact.
2. Impressive and easily comprehended as are the proofs of denudation supplied by the basalts of the plateaux, they are perhaps to a geological eye less overwhelming than those furnished by the eruptive rocks which have been injected into these plateaux. In the case of at least the basic intrusions, we may reasonably infer that they assumed their present position under a greater or less depth of overlying rock which has since been removed. When, therefore, they are found at or above the summits of the plateaux, they demonstrate that a vast amount of material has been removed from these summits.
The argument from the position of the dykes has already been enforced. It is absolutely certain that valleys several thousand feet deep must have been excavated since these dykes were erupted, for had such valleys existed at the time when the dykes were injected across their site, the molten rock, instead of ascending to the tops of the surrounding mountains, would obviously have rushed forth over the valley-bottoms. I have shown that this reasoning applies not merely to the volcanic districts, but to the whole surface of the country within the region of dykes. Thus the uplands of Southern Scotland, and wide areas in the Southern and Western Highlands, can be proved to have had glens cut out of their mass to a depth of hundreds of feet since the Tertiary volcanic period.
Not less convincing is the evidence afforded by the great eruptive masses of gabbro. We have seen that these complex accumulations of sills, dykes, and bosses include rocks so coarse in grain as to show that they must have consolidated at some considerable depth, but that they now appear in hill-groups 2000 to 3000 feet in height, the whole of the original basaltic cover having been stripped off from them. But these gabbro hills have been in turn traversed up to the very crests by later basalt-dykes, which thus supply additional proof that the erosion here has been stupendous.
The granophyre bosses tell the same tale. Though, like the domite Puys of Auvergne, they may still retain, in their conical forms, indications of the original shapes which their component material assumed at the time of its protrusion, we may be confident that their existing surfaces have been reached after the removal of much rock which once lay above them. This inference is confirmed by the fact that these eruptive bosses have been invaded by a younger system of dykes. The black ribs of basalt which may be traced along their pale declivities, which cross the glens that have been eroded in them and which mount up to their very crests, prove that since the latest manifestations of volcanic energy in the West of Scotland, extensive changes in the topography of the land have been effected by the operation of the subærial agents of degradation.
So much for what can be demonstrated. But how much more may, with the highest probability, be inferred! The original limits of the plateaux are unknown. The waves of the wide Atlantic now roll over many a square league of the old lava-plains, and wide tracts of the islands and the mainland from which the basalt has been entirely stripped, or where it remains only in scattered outliers, were once deeply buried under piles of lava-sheets. It would probably be no exaggeration to affirm that over the British area, as well as over the Faroe Isles, the amount of Tertiary volcanic rock that now remains, large as it is, falls short in amount of what has been removed. The geologist who has made himself familiar with the effects of denudation in other Tertiary volcanic districts, such as Central France, Saxony and Bohemia, will be prepared for almost any conceivable amount of erosion among the far older volcanic series of the north-west of Europe.
To the student of the origin of the existing topography of the land there is a profound interest in the demonstration which these volcanic rocks supply of the vast changes which the terrestrial surface has undergone within a period geologically so recent as older Tertiary time. When, on the one hand, he finds himself more and more restricted in his demands for time by the confident assertions of the physicist that all the phenomena of geological history must have been comprised within a few millions of years, and when, on the other hand, he watches the seemingly feeble and tardy operations of the forces of denudation and sedimentation which have played the chief parts in that history, he may well be excused if sometimes he is apt to despair of ever reconciling the facts which he observes with the physical deductions that are somewhat dogmatically brought forward in opposition to his interpretation of them. He may feel sure that his facts cannot be gainsaid, and he may be unable to find any other way of comprehending them save by the admission that they necessitate a liberal allowance of time. Yet he may not feel himself to be in a position to offer any valid objections to the arguments from physical considerations that would so seriously abridge the length of time which geology requires.
In these circumstances it is some satisfaction to be provided with definite measurements of the amount of geological change which has been effected within a limited and relatively recent period of time. This change has resulted from the operation of the same agents by which it is still being carried on. No break in the history can be detected. There is not the least reason to suppose that the agents of denudation and sedimentation have, during the period in question, differed in their rate of working. Their activity at the present time is probably neither greater nor less than it was then. If, therefore, during so recent an interval such a stupendous amount of material has been worn away from the surface of the land and deposited on the sea-floor as the Tertiary volcanic rocks demonstrate, the geologist may surely contemplate without misgiving the lapse of time required for the completion of older geological revolutions. He may oppose to the arguments of the physicist the measurements and computations which he himself makes from data which are at least as reliable as the postulates whereon these arguments are based. The rate at which denudation and sedimentation are now taking place has been measured with tolerable accuracy, and a fair average for it has been obtained. Whatever may be maintained as to this rate in early geological ages, there can be no serious opposition to its being taken as fairly constant since older Tertiary time. We are thus provided with data for estimating the minimum amount of time that can have elapsed since the volcanic plateaux began to be denuded. But as no relic remains of the original upper surface of those plateaux, and as we are consequently ignorant of how much rock has been removed from their highest surviving outliers, it is obvious that such estimates are more likely to err in understating than overstating the amount of time required.
It would be beyond the scope of the present volume to enter fully into the measurements and calculations required for the adequate treatment of this subject. I will merely illustrate my argument by again taking a few data from the plateau of Mull. The original height of this plateau is shown by the outlier of Ben More to have been at least 3200 feet. If to this figure we add the portion of the basalt-group submerged under the sea the height will probably be increased by several hundred feet. But let us take 3000 feet as a moderate computation for the average thickness of the volcanic series here at the close of the plateau-period. Until a number of sections have been carefully plotted from the Ordnance Maps, in order to ascertain with approximate accuracy the average height of the present surface of the Mull basaltic plateau, making due allowance for the vast erosion of the Sound of Mull and the numerous glens and sea-lochs that traverse the island, any estimate which may be offered as to this average must be merely provisional. If, in the meantime, we suppose the present mean level of the plateau to be 1000 feet above the sea, the difference between this amount and the assumed original height will be 2000 feet. If, further, we take the present average rate of degradation of the Mull plateau to be 1/6000 of a foot in a year, which has been shown to be probably a fair estimate, then the time required for the lowering of the Mull plateau from its original to its present average level amounts to twelve millions of years. Yet this period, vast though it be, does not carry us back even as far as the beginning of Tertiary time.
In concluding this lengthened discussion of the Tertiary volcanic history of Britain, I may, perhaps, usefully add a brief summary of the leading features of the long record.
The region within which volcanic activity displayed itself during older Tertiary time in the British Isles, if our estimate of its area is restricted to those parts of the country where igneous rocks, probably of that age, now appear at the surface, embraces the North of England and of Ireland, the southern half and the west coast of Scotland—a total area of more than 40,000 square miles. Over that extensive region volcanic phenomena were displayed during an enormously protracted interval of geological time. The earliest beginnings of disturbance may possibly have started in the Eocene, and the final manifestations may not have ceased until the Miocene period. So prolonged was the duration of the eruptions, that enormous topographical changes from denudation, and probably also considerable variation in the fauna and flora, alike of land and sea, may have been effected.
Owing to some cause which has not yet in this relation been investigated, but which is probably referable to secular terrestrial contraction, the volcanic region underwent elevation, while, at the same time, a vast subterranean lake or sea of molten rock existed underneath it. Enormous horizontal tension thus arose, and at last the stretched terrestrial crust gave way. A system of approximately parallel fissures opened in it, having a general direction towards north-west. The rapid and simultaneous production of such a gigantic series of rents must have given rise to earthquakes of enormous magnitude and destructive force. The great majority of the fractures, doubtless, did not reach to the surface of the ground, though probably not a few did so. Such was the potency of this development of terrestrial energy, that the fissures ran through the most varied kinds of rocks and the most complicated geological structures, crossing even earlier lines of powerful dislocation, and yet retaining their direction and parallelism for sometimes 50 or 100 miles.
Into the fissures thus formed the molten magma from underneath was forced for many hundreds or even thousands of feet above the surface of the subterranean lava-reservoir. Solidifying between the fissure walls, it formed the crowd of basic dykes that stand out as the most widespread and distinctive feature of the volcanic region.
Where the fissures reached the surface or near to it, the molten rock would seek relief by egress in streams of lava. This probably occurred in many places from which subsequent denudation has removed all vestige of superficial volcanic manifestations. But, in the great range of basalt-plateaux, from Antrim northwards through the chain of the Inner Hebrides, there are still left abundant remains of the surface-outflows. Like the modern lavas of Iceland, the molten material probably flowed out sometimes from the open fissures, sometimes from vents formed along the chasms. After the convulsions ceased which produced the earliest dykes, the communication that had been established between the magma-reservoir underneath and the air above would be maintained, and repeated eruptions might take place, either from the original fissures and vents or from others afterwards opened by the volcanic energy.
As in the modern eruptions of Iceland, new fissures are successively opened through the older lava-sheets, so in the Tertiary volcanic areas, renewed ruptures of the earth's crust allowed later dykes to be formed. The basalt-plateaux are traversed by such dykes, even up to their highest sheets. It is impossible to say how often the process of dyke-making may have been repeated. Not improbably it recurred again and again during the building of the basalt-plateaux, and we know that it was renewed even after the protrusion of the granophyre bosses which mark one of the latest phases of volcanism in the region.
For a protracted geological period, with long intervals of quiescence, various basic lavas (basalts, dolerites, etc.), with occasionally some of intermediate composition (andesites, trachytes), and perhaps in Antrim acid rhyolites, flowed out from fissures and vents until they had filled up the hollows of the great valley, which then stretched from the south of Antrim northwards between the west coast of Scotland and the chain of the Outer Hebrides. In some places the accumulated pile of these ejections even now exceeds 3000 feet in thickness, but we cannot tell how much material has been bared away from its top by denudation. The volcanic discharges consisted mostly of lava, fragmentary materials being comparatively insignificant in amount and local in origin, though layers of fine tuff and basalt-breccias occur in all the plateaux. None of the erupted materials thicken towards any centres that might be taken to mark volcanoes of the type of Vesuvius or Etna. On the contrary, the persistent flatness and uniformity of the volcanic series, and the thinning out of the separate beds in different directions, show that the lavas issued from many points all over the region. The positions of some of the actual vents can still be ascertained. They are now filled sometimes with dolerite, sometimes with coarse agglomerate.
The surface over which the lava flowed seems to have been mainly terrestrial. Here and there, between the successive sheets of basalt, the leaves, stems, and fruit of land-plants, sometimes in most perfect preservation, may be observed, together with the remains of insects and fresh-water fish. Distinct relics of old river-channels can be recognized which have been buried under streams of lava. Among the deposits left by these streams the uppermost layers are commonly dark with decayed vegetation, while layers of coal are found here and there between the basalts.
As the pile of erupted materials gradually thickened, and the subterranean energy possibly grew feebler, the ascending magma was forced between the layers of sedimentary strata underneath the basalts, or between these strata and the overlying volcanic series, or along any other plane of weakness in the terrestrial crust. In this way arose the multitudinous sills or intrusive sheets.
When the great volcanic plateaux had been built up to a thickness of several thousand feet, another remarkable episode in the history occurred. At certain points large bodies of coarsely crystalline basic rocks were pushed into and through the plateaux-basalts, upraising them in dome-shaped elevations, and ultimately solidifying as dolerites, gabbros, troctolites, picrites, etc. There is reason to believe that the points of extravasation of these materials were mainly determined by the positions of the larger or more closely clustered vents of the plateau-period, where points of weakness consequently existed in the terrestrial crust. Rising as huge bosses through such weak places, the gabbros and associated rocks raised up the overlying bedded basalts, and forced themselves between them, forming thus a fringe of finer-grained intrusive sills and veins around the central banded and amorphous masses of more coarsely crystalline material. Whether, in any of these vast domes of upheaval, the summit was disrupted, so as to allow the basic intrusion to flow out as lava at the surface, cannot now be told, owing to enormous subsequent denudation.
The next chapter in the chronicle shows us that probably long after the eruption of the gabbros, when possibly all outward symptom of volcanic action had ceased, a renewed outbreak of subterranean activity gave rise to the protrusion of another and wholly different class of materials. This time the rocks were of a markedly acid type. They included varieties that range from obsidians, pitchstones, flinty felsites and rhyolites, through porphyries and granophyres, into compounds which cannot be classed under any other name than granite. These masses likewise availed themselves of older vents in the plateaux, and broke through them. They now form huge conical hills, which, in their outer aspect, and even to some extent in their inner structure, recall the trachytic puys of Auvergne. But the granophyres not only ascended through the basalt-plateaux and the gabbro-bosses; they sent into these rocks a network of veins, pushed their way in huge sheets or sills between the strata below, and actually incorporated a considerable proportion of the basic materials into their own substance. Around the bosses of gabbro and granophyre, the bedded basalts have undergone considerable contact-metamorphism.
The gabbro and granophyre bosses of the Inner Hebrides demonstrate with singular force how unreliable petrographical characters are as a test of the relative age of rocks. No one, looking at hand-specimens of these rocks, or even studying them in the field, would at first suspect them to be of Tertiary date. They closely resemble rocks of similar kinds in Palæozoic and even Archæan formations. Yet, of their late appearance in geological time, there cannot be any possibility of doubt.
After the uprise of the granophyre, and the injection of the network of felsitic veins, there came once more a period of terrestrial convulsion, like that of the earliest basic dykes, but of less intensity. Again, the crust of the earth over the volcanic region was pushed upward and rent open by another system of parallel fissures. Again, from a reservoir or basin of basic lava underneath, molten rock was forced upwards into the rents, and thus another system of basic dykes was formed. These dykes are found crossing those of earlier date, and rising through the other volcanic rocks. They traverse the plateau-basalts from bottom to top; they climb to the summits of the gabbro mountains, and they even pursue their undeviating course over the huge domes of granophyre. No proof has yet been found that from any of these dykes there was a superficial outflow of lava. But so great has been the subsequent denudation of the areas, that such outflows might quite well have taken place, and have subsequently been destroyed.
Whether these basic dykes were the last manifestation of volcanic energy in our region cannot yet be decidedly affirmed. So far as the evidence at present goes, they are possibly older than another series of acid veins and dykes (pitchstone, felsite, and granophyre), which are found at many points from Antrim to the far end of the Inner Hebrides. These protrusions traverse every other member of the volcanic series, except some of the youngest basic dykes, and do not appear to be themselves cut by any.
Since the close of the volcanic period considerable disturbance of the basalt-plateaux has taken place. The whole volcanic region has subsided, some districts having sunk more than others. In Britain the most striking evidence of such depression is supplied by the basin of Lough Neagh. But throughout the Inner Hebrides much of the lower portion of the terrestrial lava-plateaux is now below sea-level. In the Faroe Islands and in Iceland the subsidence has been still more marked. Dislocations, also, sometimes amounting to more than a thousand feet of displacement, have occurred among the volcanic masses. The bedded basalts, originally on the whole nearly flat, have thus been broken up into large blocks of country wherein the sheets are now inclined in various directions.
One of the most important lessons taught by the Tertiary volcanic series of the north-west of Europe is the extent of the denudation of the land since the close of the volcanic period. The horizontal or gently inclined layers of bedding among the basalts afford datum-lines from which the minimum amount of material removed may be measured. As a reasonable estimate it may be inferred that in the case of the Mull plateau, for example, the average amount by which its surface has been lowered since the close of the volcanic period cannot be less than 2000 feet. If the rate of lowering of the land-surface in western Europe by subærial denudation be taken as 1/6000 of a foot in a year, then the lapse of time required for the degradation of the Mull plateau must amount to about twelve millions of years. Some such interval has therefore elapsed since the last Tertiary volcanoes became extinct.
CHAPTER LI
SUMMARY AND GENERAL DEDUCTIONS
The foregoing chapters comprise a connected narrative of the history of volcanic action in the area of the British Isles during the vast succession of ages from the early Archæan dawn down to the latest eruptions of Tertiary time. In this final chapter I propose to present a brief summary of the facts of largest import and widest interest which this protracted history has placed before us, together with a statement of deductions which may be drawn from them regarding the nature and progress of volcanism in the evolution of the globe.
1. Among the broad features which soonest arrest attention in such a survey is the geographical position of the theatre of this volcanic activity. In the distribution of volcanoes at the present time we are familiar with their tendency to range themselves along continental borders or in oceanic islands. The volcanic energy so conspicuous in the geological history of Britain has shown itself along the western or Atlantic margin of the European continent. When the eruptions have not been actually on the land itself, they have taken place within the shallow tracts near the land, where the lavas and tuffs have been interstratified with sediments derived from the adjacent coasts.
Moreover the volcanic rocks in Britain are ranged along the greatest length of the group of islands, in a general north and south line, from the south of Devonshire to the far Shetlands. It is on the western side of the country that they occur. East of a line drawn from Berwick by Leicester to Exeter, although the geological formations, ranging from the Carboniferous Limestone to the latest Pleistocene deposits, are there abundantly exposed to view, they include no contemporaneous volcanic rocks.
2. A second and still more remarkable feature in the geological history of Western Europe is the persistence of volcanic activity along the site of the British Isles. Evidence has been brought forward in these volumes that from the primeval time vaguely termed Archæan, onward to that of the older Tertiary clays and sands of the south-east of England—that is to say, through by far the largest part of geological history, as chronicled in the stratified crust of the globe—this long strip of territory continued to be intermittently a theatre of volcanic action. Every great division of Palæozoic time was marked by volcanic eruptions, sometimes over tracts hundreds of square miles in area and on a colossal scale. After a long period of quiescence during the Mesozoic ages, the renewed outbreak of volcanic energy in older Tertiary time, so marked over the western half of Europe, reached its maximum of development along the Atlantic border, from the north of England and Ireland through the chain of the Inner Hebrides to the Faroe Islands, Iceland and Greenland.
3. Not only has there been a remarkable persistence of volcanic activity over the comparatively limited area of the British Isles, viewed as a whole, but if we examine the different parts of this area we perceive that many of them, of relatively restricted extent, have been the sites of a recrudescence of volcanic action, again and again, through a vast succession of geological periods. While the whole region has been in different quarters and at different times affected, there have been districts where the volcanic fires have been rekindled after long intervals of quiescence, the new vents being opened among or near to the sites of earlier volcanoes. In the south-west of England, for example, the Middle Devonian tuffs and diabases were succeeded in the Carboniferous period by the eruptions of the Culm-measures, and in the very same tracts came last of all the lavas and tuffs of the Permian conglomerates. Still more astonishing is the record of volcanic energy in the south of Scotland, where, within a space of not many hundred square miles, there are the chronicles of the Arenig, Llandeilo and Bala eruptions of the Southern Uplands, the huge piles of lavas and tuffs of the Lower Old Red Sandstone, the long succession of the plateaux and then of the puys of the Carboniferous period, the groups of tuff-cones of the Permian period, and, lastly, the numerous dykes connected with the Tertiary volcanoes.
While some portions of the region have been specially liable to exhibitions of volcanic action, others have continuously escaped. Some of these "horsts," or stationary and unaffected blocks of country, have been surrounded by or have risen close to the borders of this volcanic district, yet have maintained their immunity through a long series of ages. Thus the Central Highlands of Scotland, though they were flanked on the south and south-west by the active volcanoes of the Old Red Sandstone, and again on the south by those of Carboniferous time, had no vents opened on their surface after the metamorphism of their schists. Still more striking perhaps is the immunity of the Southern Uplands. Though they were in large measure surrounded by the volcanoes of the Lower Old Red Sandstone, then by those of the Calciferous Sandstones and Carboniferous Limestone, and though they looked down on the Permian eruptions of Ayrshire and Nithsdale, which spread streams of lava and showers of ash along their flanks, these hills formed a solid block that seems to have resisted perforation by the volcanic funnels. Again, the tracts covered with Carboniferous Limestone in England and Ireland almost entirely escaped from invasion by volcanic eruptions.
We thus learn that even within comparatively restricted regions some portions of the terrestrial crust have been areas of weakness, liable to serve again and again as lines of escape for volcanic energy, while close to them other portions of greater solidity have been persistently left intact.
4. The sites of volcanic vents in all the geological systems wherein they occur in Britain have not usually been determined by any obvious structure in the rocks now visible. They comparatively seldom depend on ascertainable lines of fault, even when faults, probably already existent, occur in their near neighbourhood. This independence, to which, however, there are occasional marked exceptions, comes out more particularly in the coal-fields pierced by vents, for mining operations have there revealed the positions of many more faults than can be traced at the surface. If the sites of the vents have been fixed by dislocations or lines of weakness in the terrestrial crust, these must generally lie below the formations now visible at the surface.
There is one striking connection between the sites of the vents and ancient topographical features to which frequent reference has been made in the foregoing chapters. All through the long volcanic history, as far back as such features can be traced, we see that orifices of discharge for the erupted materials have been opened along low grounds and valleys rather than on ridges and hills. The great central hollow of the Scottish midlands was a depression even as long ago as the time of the Lower Old Red Sandstone, and though it has probably been several times since then filled up, and more or less completely effaced, its ancient features have been partially revealed by extensive denudation. This vast depression, 40 miles broad, between the Highland mountains on the one side and the Southern Uplands on the other, was the chief centre of volcanic activity in western Europe during the latter half of Palæozoic time. The vents of the Old Red Sandstone, Carboniferous and Permian series are scattered all over it, but few or none of them are to be found on the high grounds that bound it. Again, in Tertiary time, the great outpouring of lava took place in the hollow that lay between the ridge of the Outer Hebrides and the mainland of Scotland. This wide and long tract of low ground was buried under upwards of 3000 feet of lava and tuff, but these materials were erupted from fissures and vents within its own border and not from the mountains on either side.
But perhaps the most conspicuous example of any in which the vents keep to the valleys is that supplied by the Permian necks of Nithsdale and the neighbouring glens. These depressions are as old as Permian, and even as Carboniferous time, but they appear to be entirely hollows of erosion; at least they have yielded no evidence that their direction has been determined by lines of fault. The chain of vents can be followed from the lowlands of Ayrshire up to the base of the Southern Uplands, down the wide valley cut by the Nith in these hills and up some of the tributary valleys, and though the volcanoes continued for some time in vigorous eruption, not a trace of any contemporary vent has yet been met with on the surrounding hills.
While the position of volcanic vents in lines of valley may be generally due to guiding lines of fissure in the crust underneath, either within or below the rocks visible at the surface, there may sometimes be conditions in which other dominant causes come into play. The curious coincidence between variations in the upper limit of dykes and inequalities in the configuration of the overlying ground, suggest that where the subterranean magma has ascended to within a comparatively short distance from the surface, a difference of a few hundreds or thousands of feet in the depth of overlying rock, such as the difference of height between the bottom of a valley and the tops of the adjacent hills, may determine the path of escape for the magma through the least thickness of overarching roof.
5. Volcanic phenomena cannot be regarded as a mere isolated and incidental feature in the physics of the globe. During the short time within which man has been observing the operations of existing volcanoes, he has hardly yet had sufficient opportunity of watching how far they can be correlated with other terrestrial movements. Nor, when he endeavours to trace some such connection among the records of the geological past, has he yet collected materials enough to furnish a sufficiently broad and firm basis of comparison. One formidable obstacle is presented by the difficulty in determining chronological equivalents in separated groups of rock. Geologists have tried to discover whether the volcanoes of some particular period or region were in any way connected with such geological changes as extensive plication, dislocations of the crust, or elevation of mountain-chains. In regard to the volcanic history of Britain, various possible relations of this kind obviously suggest themselves. Thus the division of geological time comprised within the Lower Silurian period was undoubtedly an interval of considerable terrestrial disturbance in western Europe. The unconformabilities and overlaps in the series of formations belonging to that period, the frequent conglomerates, the great and often rapid changes in the thickness and lithological characters of the strata, all point to instability of land-surface and sea-floor. During these oscillations a prolonged and widespread series of volcanic eruptions took place. The volcanic manifestations began in Cambrian time and continued in intermittent activity till towards the close of the deposition of the Lower Silurian formations. It is certainly a significant fact that the Upper Silurian deposits, in their lithological characters, present a strong contrast to those that preceded them. They point, on the whole, to quiet sedimentation, during an interval of comparative calm in the terrestrial crust. With this evidence of tranquillity there is, over almost the whole of the British Isles, an entire absence of any trace of renewed volcanic activity. With the exception of the Dingle lavas and tuffs, in the extreme west of Ireland, not a single undoubted instance is yet known of an Upper Silurian volcano.
After the deposition of the Upper Silurian rocks an interval of great terrestrial disturbance ensued, and these rocks over a large part of Britain were intensely plicated and crushed. The movements, continued into the period of the Lower Old Red Sandstone, were, in their later stages, accompanied or, at least, followed by the vast outpourings of lava which now cover so much of the tracts of Old Red Sandstone in Scotland and Ireland.[438]
[438] Trans. Geol. Soc. Edin. vol. ii. part iii. (1874).
In proportion as the volcanic energy was vigorous, widespread and long-continued, we may expect it to have been connected with important terrestrial movements affecting extensive regions of the earth. The Tertiary volcanic history seems to afford a remarkable instance of this connection. A wide area of the European continent is dotted over with old centres of volcanic activity which were in eruption at successive epochs throughout the Tertiary period. Of all these centres the most important was that of the north-western basalt-plateaux, where floods of lava were discharged over many thousand square miles from Ireland to Greenland. The geological date of these outpourings probably coincides with the last great orographic movements that gave to the mountain-chains of Europe their latest elevation and dimensions.
But without entering into what must be for the present a field of speculation, we can be assured of one important fact in the connection of ancient volcanoes with movements of the terrestrial crust. A study of the records of volcanic action in Britain proves beyond dispute that the volcanoes of past time have been active on areas of the earth's surface that were sinking and not rising. We usually associate volcanic action with elevation rather than subsidence, and there are certainly abundant proofs of such elevation around active or recently extinct volcanoes. Many of the active vents of the present time, like Vesuvius and Etna, began with submarine eruptions and have been gradually upraised into land. It may be, however, that such uprise is merely a temporary incident, and that if we could survey the whole geological period of which human history chronicles so small a part, we might find that subsidence, and not upheaval, is ultimately the rule over volcanic areas.
Be this as it may, there can be no question that with the one solitary exception of the Tertiary volcanoes, which were terrestrial and not submarine, all the British vents were carried down and eventually buried under aqueous sediments. Even the Tertiary lava-fields have in many places sunk down below sea-level since their eruptions ceased.
That there are any Palæozoic volcanic rocks now visible at the surface is obviously due to subsequent movements not immediately connected with their original conditions of eruption, and to gigantic denudation. The amount of subsidence which followed on a volcanic episode was sometimes enormous, even within the same geological period, as one may see by observing the prodigious piles of sedimentary material heaped over the lavas and tuffs of Arenig time, or over those of the Lower Old Red Sandstone. I do not wish to maintain that the downward movement was necessarily a consequence of volcanic ejections, for we know that it took place over tracts remote from centres of eruption. But I have sometimes asked myself whether it was not possibly increased as a sequel to vigorous volcanic action; whether, for instance, the great depth of the Palæozoic sedimentary rocks in some regions, as compared with their feeble development in others, may not have been due to an acceleration of subsidence consequent upon volcanic action.
6. A review of the geological history of Britain cannot but impress the geologist with a conviction of the essential uniformity of volcanism in its manifestations since the early beginnings of geological time. The composition and structure of the materials erupted from the interior have remained with but little change. The manner in which these materials have been discharged has likewise persisted from the remotest periods. The three modern types of Vesuvian cones, puys and fissure-eruptions can be seen to have played their parts in the past as they do to-day.
Among the earliest igneous masses of which the relative geological date can be fixed are the dykes which form so striking a system among the Archæan rocks of the north-west, and show how far back the modern type of volcanic fissures and dykes can be traced. No relic, indeed, has survived of any lavas that may have flowed out from these ancient fissures, but so far as regards underground structure, the type is essentially the same as that of the Tertiary and modern Icelandic lava-fields.
The early Palæozoic volcanoes formed cones of lava and tuff comparable to those of such vents as Vesuvius and Etna. In the Lake District the pile of material ejected during Lower Silurian time was at least 8000 or 9000 feet thick. In the Old Red Sandstone basins of Central Scotland there were more than one mass of lavas and tuffs thicker than those of Vesuvius.
The puys of the later half of Palæozoic time closely resembled their Tertiary successors in Central France, the Eifel, and the Phlegræan Fields.
Nor, as regards extent and vigour, did the eruptions of the geological past differ in any important respect from those of the present time. There is assuredly no evidence that volcanic energy has gradually waned since the dawn of geological history. The latest eruptions of North-Western Europe, forming the Tertiary basalt-plateaux, far exceeded in area, and possibly also in bulk of material discharged, all the eruptions that had preceded them in the geological record.
7. Nevertheless, while the Tertiary eruptions showed no diminution of vigour, it is undoubtedly true that the volcanic energy has not manifested itself in a uniform way since the beginning of geological time. There have been periods of maximum activity followed by others of lessened force. Thus if we take a broad view of the general features of volcanic action during the Palæozoic ages in Britain, we see clear evidence of a gradual diminution in its vigour. The widespread outpourings of lava and tuff in the Silurian period in England, Wales, Scotland and Ireland were succeeded by the somewhat diminished, though still important, eruptions of the Lower Old Red Sandstone basins. The latter were followed by the still lessened outflows of the Carboniferous plateaux, which in turn were succeeded by the yet feebler and more localized eruptions of the Carboniferous puys, the whole prolonged volcanic succession ending in the small scattered vents of the Permian period. There were of course oscillations of relative energy during this history, some of the maxima and minima being of considerable moment. But though progress towards extinction was not regular and uniform, it was a dominant feature of the phenomena.
8. The Permian volcanoes were the last of the long Palæozoic series, and, so far as we yet know, the whole of the Mesozoic periods within the area of Britain were absolutely unbroken by a single volcanic eruption. The chronological value of this enormous interval of quiescence may, perhaps, never be ascertainable, but the interval must assuredly cover a large part of geological time. It was an era of geological calm, during which the Triassic, Jurassic and Cretaceous formations were slowly accumulated over the larger part of Europe. The stratigraphical quietude was not indeed unbroken. The widespread subsidence of the sea-bottom was interrupted here and there by important upheavals, and considerable geographical changes were in process of time accomplished. But, save in one or two widely separated areas of Europe, there were no active volcanoes over the whole continent.[439] Here again the scarcity or absence of intercalated volcanic rocks is in harmony with the general stratigraphy of the formations.
[439] The Triassic eruptions of Predazzo and Monzoni were important, and traces of others are said to occur in the Cretaceous system in Portugal and Silesia.
9. After the prodigious interval represented by the whole of the Mesozoic and the earlier part of the Tertiary formations, a time of disturbance arose once more, and the great basalt-floods of the north-west were poured forth. Evidence has been adduced in the foregoing chapters that this latest volcanic period was one of vast duration; that it was marked by long intervals of quiescence, and by repeated renewals of volcanic energy. Yet over the area of Britain the whole of its manifestations were probably comprised within the earlier (Oligocene and perhaps early Miocene) part of older Tertiary time. Since its eruptions ceased, another interval of profound quiescence has succeeded, which still continues. But this interval is almost certainly of less duration than that which elapsed between the Palæozoic and Tertiary outbursts. In other words, remote as the date of these Tertiary volcanoes appears to be from our own day, it comes much nearer to us than did the era of the last Permian eruptions to the earliest of the Tertiary series.
10. By the dissection which prolonged denudation has effected among the old volcanic centres of Britain, materials are supplied for studying the sequence of events from the beginning to the end of a volcanic period. These events have generally followed the same tolerably well-defined order.
In the case of fissure-eruptions, rents formed in the crust of the earth and communicating with the surface have allowed lava to rise and flow out above ground, either from the lips of the fissures or from vents opened along the lines of chasm. The thousands of parallel dykes in Britain remain as evidence of this mode of the ascent of the molten magma. Lines of large cones of the Vesuvian type may be presumed to have risen along guiding fissures in the terrestrial crust.
But it is evident from a study of the British examples that the existence of a fissure in the visible part of the crust is not always necessary for the production of a volcanic vent. In hundreds of instances, communication from the internal magma to the surface was effected by successive explosions, which finally blew out an orifice at the surface with no visible relation to any fissures or dykes. Of course, beneath the formations that now form the surface, and through which the necks rise, there may be lines of fault or weakness in older rocks which we cannot see. But, in what can be actually examined, vents have commonly been drilled through rocks independently of faults.
The discharge of explosive vapours was sometimes the first and only effort of volcanic energy. Generally, however, fragmentary volcanic materials were ejected, or, if the eruption was more vigorous, lava was poured out. In a vast number of cases, especially in the later ages of Palæozoic time, only ashes were projected, and cones of tuff were formed. In the earlier ages, on the other hand, there was a much larger proportion of lava expelled. Towards the close of a volcanic period, the vents were gradually choked up with the fragmentary materials that were ejected from and fell back into them. Occasionally, during the process of extinction, an explosion might still occur and clear the chimney, so as to allow of the uprise of a column of molten rock which solidified there; or the sides of the crater, as well as of the cavernous funnel underneath, fell in and filled up the passage. Heated vapours sometimes continued to ascend through the debris in the vent, and to produce on it a marked metamorphism.
There seems to have been commonly a contraction and subsidence of the materials in the vents, with a consequent dragging down or sagging of the rocks immediately outside, which are thus made to plunge steeply towards the necks.
When the vents were plugged up by the consolidation of fragmentary matter or the uprise of lava in them, the final efforts of the volcanoes led to the intrusion of sills and dykes, not only into the rocks beneath the volcanic sheets, but also, in many instances, into at least the older parts of the sheets themselves. These subterranean manifestations of volcanic action may be recognized in almost every district. They vary greatly in the degree to which they are developed. Sometimes, as in the Cader Idris, Arenig and Snowdon regions, they attain considerable importance, alike as regards the number and thickness of the sheets. In other cases, they are exhibited on so small a scale that they might be overlooked, as in the tract of Carboniferous puy-eruptions in the north of Ayrshire. But they are so generally present as to form a remarkably characteristic feature of the volcanic activity of each geological period from the earliest time to the latest. The basic sheets in the Dalradian series of Scotland display early and colossal examples. All through the successive eruptive periods of Palæozoic time, sills are found as accompaniments of superficial ejections.
The Tertiary basalt-plateaux supply numerous and gigantic examples of intruded sheets. Tertiary cones of Vesuvian type are not found in Britain, but where on the continent they have been sufficiently laid open by denudation, they present sometimes an astonishing series of sills. As a striking illustration of this structure reference may be made to the sheets of trachyte that have been injected between and have marmorized the Cretaceous strata on which Monte Venda stands, among the Euganean Hills.[440]
[440] G. vom Rath, Zeitsch. Deutsch. Geol. Gesellsch., xvi. (1864), p. 461. E. Suess, Sitzungsber. k. Akad. Wien., lxxi. (1875), p. 7; Antlitz der Erde, vol. i. p. 193. E. Reyer, Die Euganeen, 1877. This volcano is further referred to, postea, p. 477.
It is obvious that the time of intrusion of the sills cannot be precisely determined. They were not likely to be injected at an epoch when the volcanic magma could find ready egress to the surface. That they did not arise before such egress was obtained may be inferred from their petrographical characters, which are usually those of the later and not of the earlier outflows of the magma; and from the fact that they not only lie among the rocks below the volcanic series, but intersect the lower parts of that series, sometimes even the higher parts. We may therefore, with every probability, regard the sills as among the closing phases of a volcanic period.
As the lavas and tuffs of each volcanic period are intercalated among the successive geological formations, a definite beginning and end to the period are stratigraphically fixed. We see exactly where in the sedimentary series the first showers of ashes fell, and where the last mingled with the ordinary sand and mud of the sea-door. The same record shows that the volcanic accumulations were finally washed down, that they subsided with the rest of the ground around them, and that usually they were buried under overlying conformable sedimentary deposits. Thus cones of ashes and lava which may have been several thousand feet high completely disappeared.
10. A consideration of the distribution of the volcanic rocks in time shows not only how singularly uniform the course of volcanic activity has been, but that there is no evidence of the cessation of any of the broader petrographical types during geological history. Quite as much variety may be observed among the erupted materials of Tertiary time in Britain as among those of the early ages, when the earth was younger and its volcanic vigour might be supposed to have been greater and more varied than it is now. The table on the following page will make these features at once apparent. From this table it will be seen that while some of the acid rocks have not always been extruded, the basic masses have played their part in every volcanic period.
11. A study of the volcanic products of a long series of eruptions within the same geographical region may be expected to throw light on the changes that take place during the course of ages in the character of the internal molten magma. In a former chapter ([vol. i. p. 27]) reference was made to the subject of volcanic cycles and to the sequence, observed in various widely separated parts of the world, among the materials erupted from below. Allusion was likewise made in a later chapter ([vol. i. p. 90]) to the remarkable differences in texture and composition noticeable within some large bodies of eruptive material, and to the evidence which these differences furnish of a segregation or differentiation among the constituents of an eruptive mass after it has been injected into its position within the crust of the earth.
Table of the Periods of Volcanic Action in the British Isles and
of the Chronological Distribution of the Volcanic Products.
| Granites, Granophyres, etc. | Felsites, Rhyolites, etc. | Dacite, "Pitchstone" of Eigg. | Trachytes. | Andesites (Porphyrites). | Gabbros. | Dolerites, Basalts (Diabases). | Picrites and highly basic lavas. | Tuffs, acid or basic. | ||
| Older Tertiary | ||||||||||
| (Plateaux, dykes, necks, bosses, sills) | * | * | * | * | * | * | * | * | * | |
| Mesozoic | ||||||||||
| No volcanic rocks. | ||||||||||
| Permian | ... | * | ... | ... | * | ... | * | * | * | |
| Carboniferous | ? | |||||||||
| Puy type | ... | * | ... | ... | * | ... | * | * | * | |
| Plateau type | ... | * | ... | * | * | ... | * | * | * | |
| { | Devonian | ... | ... | ... | ... | ... | ... | * | ... | * |
| Old Red Sandstone | ||||||||||
| Upper | ... | ... | ... | ... | ... | ... | * | ... | * | |
| Lower | * | * | ... | * | * | ... | * | ... | * | |
| Silurian | ||||||||||
| Upper | ... | * | ... | ... | ... | ... | ... | ... | * | |
| Lower, Bala | * | * | ... | * | * | * | * | * | * | |
| " Arenig | * | * | ... | * | * | * | * | ... | * | |
| Cambrian | ... | * | ... | ... | * | ... | * | ... | * | |
| Uriconian | ... | * | ... | ... | ... | ... | * | ... | * | |
| Dalradian | ... | ... | ... | ... | ... | ... | * | ... | ? | |
| Torridonian | ||||||||||
| Lewisian | * | ... | ... | ... | ... | ... | * | * | ... | |
From the history of volcanic action in the British Isles it is clear that differentiation is effected under three distinct conditions.
In the first place, a notable difference may be occasionally observed between two adjacent parts of the same mass of lava which has flowed out at the surface. Thus, in the Carboniferous picrite of Blackburn, there has been a separation of the heavy basic constituents, which have in great part settled down into the lower part of the sheet, while the lighter felspar has mainly come to the top. In this case the gradual transition from top to bottom suggests that the separation occurred after the lava had reached the surface and taken the form of a stream or sheet.
In the second place, segregation has taken place in the magma within the terrestrial crust after intrusion, for it is frequently observable in large bosses and sometimes in sills, the basic elements having tended to mass themselves towards the margins of the rock, leaving more acid material in the centre. The cases of Garabol Hill among the Dalradian schists of Scotland, of Carrock Fell among the Silurian strata of the Lake District, and of the Cramond picrite among the Carboniferous formations of Midlothian, with others that might be cited from various other regions and geological formations in Britain, prove to what a considerable extent a separation of ingredients may take place in a boss, and even sometimes in a comparatively thin sill before the molten mass consolidates.
In the third place, there is good evidence that already before the magma is either intruded or extruded, and while it still lies within the internal reservoir, it may not possess a general uniformity of composition, but may have become more or less heterogeneous. In regard to intrusive rocks, the extraordinarily banded gabbros of the Tertiary series of Skye obviously proceeded from a magma in which the molten material consisted in some parts mainly of felspar, and in others mainly of the ferro-magnesian minerals and iron-ores. Streams from these differently constituted parts of the magma were simultaneously or successively injected as sills into the older portions of the volcanic series, while, as the process of differentiation within the magma proceeded, still more felspathic liquid was left behind, to be thrust into cracks in the sills previously consolidated.
Moreover, the banded basalts of the Tertiary plateaux show that this heterogeneity was not confined to internal intrusions, but maintained its place even when the molten material was ejected to the surface. The differentiation indeed is not so striking there as among the sills of gabbro; but its presence, even in a less degree, proves that the separation of constituent minerals was not due to any general cooling of an erupted body of igneous rock, but was already developed in the reservoir from which the molten material was propelled to the surface.
Attention has been called to the remarkable similarity of structure between these banded intrusive rocks and some of the ancient gneisses. The resemblance is so close that we may with every probability infer that the gneisses acquired their characteristic banding as intrusive masses of igneous rocks, discharged from heterogeneous magmas, like that which supplied the gabbros of the Cuillin Hills. And as these gneisses belong to pre-Cambrian formations, we are thus led to the interesting result that the tendency to develop heterogeneity was already as characteristic of the magma-basins of the earliest geological time as it has been of those of later periods.
The evidence of differentiation presented by superficial lavas, and by intrusive sills and bosses, acquires great interest when considered in connection with the changes which are seen to have occurred in the character of the materials erupted during the course of a definite volcanic period. An attentive examination of the volcanic products of the various ages, so fully recorded in the geological structure of the British Isles, shows that a recognizable sequence in the nature of the materials erupted during a single volcanic period can be traced from the earliest to the latest times, and that, in spite of occasional departures, the normal order remains broadly uniform.
With the important exception of the Snowdonian region and possibly others, we find that the earlier eruptions of each period were generally most basic, and that the later intrusions were most acid. Thus the diabase-lavas and tuffs at the base of the Cambrian series of St. David's are pierced by quartz-porphyry veins. The andesites of the Lower Old Red Sandstone were succeeded by bosses, sills, and dykes of granite, felsite, and lamprophyre. The eruptions of the Carboniferous plateaux began with extremely basic lavas, and ended with trachytes, felsites, and quartz-porphyries. The basalts of the great lava-fields of the Tertiary period are pierced by masses of granophyre and even granite.
There has evidently been, on the whole, a progressive diminution in the quantity of bases and a corresponding increase in the proportion of acid in the lavas erupted during the lapse of one volcanic period. This sequence is so well marked and so common that it cannot be merely accidental. The acid and basic rocks, occurring as they do at each volcanic centre in the same relation to each other, are obviously parts of one connected series of eruptions. We seem to see in this sequence an indication of what was taking place within the subterranean magma. There was first an extensive separation of the more basic constituents, such as the ferro-magnesian minerals and ores, and the lavas which came off at that time were heavy and basic basalts, and even picrites. The removal of these elements left the magma more acid, and such rocks as andesites were poured out, until at last the deeper intrusive sills, dykes and bosses became thoroughly acid rocks, such as felsite, quartz-porphyry and granite, while if any superficial outflow took place it was such a rock as dacite.
In the case of the Tertiary volcanic series there is evidence that after the acid protrusions a final uprise of basic material occurred. No satisfactory proof of any similar return to basic eruptions has been detected among the Palæozoic formations. But it is possible that some of the basic sills and dykes, the precise age of which cannot be fixed, may really mark such a reversion, even in the earlier volcanic periods.
Some illustrative examples of volcanic cycles from other countries were cited in Chapter iii. To these I may add another instance which presents a close analogy to some of the phenomena characteristic of the British examples of Palæozoic as well as of Tertiary age. Monte Venda in the Euganean Hills, already alluded to (p. 474), may be cited as an interesting specimen of an older Tertiary volcano, which has been so dissected by denudation as to show not only the succession of its superficial discharges, but the position and order of its subterranean intrusions. The volcanic eruptions of this neighbourhood, judging from the area which they still cover and the height they reach, may have piled up a mountain rivalling or surpassing Etna in dimensions. In Monte Venda the lowest visible igneous rocks are sills of oligoclase-trachyte that have been thrust between and have highly altered Cretaceous (Tithonian) limestones. Other intrusive sheets of trachyte follow in the overlying Cretaceous strata (Neocomian and Scaglia). It is not until the older Tertiary formations are reached that undoubted tuffs and lavas occur, indicative of truly interstratified volcanic materials. These formations, consisting of nummulitic limestones and other strata together with fossiliferous tuffs, show that the volcano began as a submarine vent. It discharged dark basic dolerites and tuffs. The highest lava, however, crowning the summit of the mountain is a trachyte. There appears to have been a rapid decrease of the bases in the magma, for the later lavas were rhyolites, accompanied with rhyolitic tuffs of Oligocene age, and followed in the end by the black vitreous trachyte of Monte Sieva.
12. From the evidence detailed in these volumes, it appears that the sequence from basic to acid discharges was on the whole characteristic of each eruptive period. It is obvious, however, that as the protrusions of successive periods took place within the same limited geographical area, the internal magma during the interval between two such periods must in some way have been renewed as regards its constitution, for when, after long quiescence, eruptions began once more, basic lavas appeared first and were eventually followed by acid kinds. This cycle of transformation is admirably exhibited in Central Scotland, where the andesites of the Old Red Sandstone with their felsite sills are followed by the limburgites, picrites and other highly basic lavas at the bottom of the Carboniferous plateaux, succeeded in turn by the andesites, trachytes and acid sills of that series. When the puy eruptions ensued, the magma had once more become decidedly basic.
That the true explanation of these alterations is of a complex order may be inferred from the exceptions which occur to the general rule. I have alluded to the Snowdon region, where the acid rhyolites are followed by more basic andesites, and where the sills are also more basic than the superficial lavas. In the Arenig and Cader Idris country the sills are likewise more basic than the bedded lavas. Among the Carboniferous puys of the basin of the Firth of Forth, the sills are not sensibly more acid than many of the superficial basalts, and they even include such rocks as picrite. Possibly in this last-named region we see an arrested sequence, the volcanic protrusions having from some cause ceased before the general uprise of the more acid magma.