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.