XLV. THE ACID ROCKS…

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]

[Footnote 359: Mineralogical Travels, ii. 90.]

[Footnote 360: Western Isles, see the descriptions of Skye, Mull and Rum.]

[Footnote 361: Berger, Trans. Geol Soc. iii. (1816), p. 190; Portlock, Journ. Geol. Soc. Ireland, vol. i. (1834), p. 9.]

[Footnote 362: Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871), pp. 54, 77, 84, 88.]

[Footnote 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.

[Footnote 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.

[Footnote 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.

[Footnote 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]

[Footnote 367: Mr. Teall, Quart. Journ. Geol. Soc. vol. 1. (1894) p. 219. See also his British Petrography, p. 327.]

[Footnote 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]

[Footnote 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]

[Footnote 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.]

[Footnote 371: Tschermak's Min. und Pet. Mittheil. 1878, p. 412.]

[Footnote 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.

[Footnote 373: Western Islands, i. p. 368; see also pp. 488, 575, 578.]

[Footnote 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]

[Footnote 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]

[Footnote 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.]

[Footnote 377: Edin. New Phil. Jour. xl. (1846) p. 84.]

[Footnote 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.]

[Footnote 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.

[Footnote 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.

[Footnote 381: Op. cit. p. 711.]

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]

[Footnote 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]

[Footnote 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).]

[Footnote 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.

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]

[Footnote 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.]

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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]

[Footnote 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.

[Footnote 387: The best account yet published of these varieties in Skye is that by Prof. Zirkel, Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871) p. 88.]

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]

[Footnote 388: Edin. New Phil. Jour. xl. p. 78.]

[Footnote 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]

[Footnote 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.

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]

[Footnote 391: Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 62.]

[Footnote 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.

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]

[Footnote 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.]

[Footnote 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.

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.

[Footnote 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].

[Footnote 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]

[Footnote 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.

[Footnote 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]

[Footnote 399: Quart. Journ. Geol. Soc. vol. lii. (1896) p. 320.]

(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).

[Footnote 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.

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.

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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.

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]

[Footnote 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.

[Footnote 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)."]

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.

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'.

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.

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.

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.).

[Footnote 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).

[Footnote 404: Western Islands, vol. i. p. 487.]

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.

[Footnote 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.

[Footnote 406: Description of the Western Isles, vol. ii. p. 54.]

[Footnote 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.]

[Footnote 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.

[Footnote 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.

[Footnote 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]

[Footnote 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.

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.

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.

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.

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.

"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."

[Footnote 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).

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.

[Footnote 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.]

[Footnote 414: Description of the Western Islands of Scotland, vol. ii. p. 388.]

[Footnote 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.

[Footnote 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.

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).

[Footnote 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.

[Footnote 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]

[Footnote 419: Journ. Geol. Soc. Ireland (1843), p. 113.]

[Footnote 420: Quart. Journ. Geol. Soc. vol. xii. (1856), p. 171; xiv. p. 300; and Journ. Geol. Soc. Ireland (1876), p. 91.]

[Footnote 421: Sheet 71 of the Geol. Surv. Ireland, and accompanying Explanation. These were the work of Mr. W. A. Traill.]

[Footnote 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]

[Footnote 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.

[Footnote 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.

[Footnote 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]

[Footnote 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.

[Footnote 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.]

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]

[Footnote 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.

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.

[Footnote 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.

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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.

[Footnote 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.

"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]

[Footnote 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.

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).

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.

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]

[Footnote 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.]

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.

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.

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.

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.

[Footnote 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.

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