[301] It may here be remarked that there is evidence of great differences in the level of the base of the Jurassic series and the bottom of the volcanic plateau in this district. On the south and west sides of Ben Hiant the Jurassic conglomerates may be seen lying on the edges of the crystalline schists only a little above high-water mark, while on the north side, the schists, with their overlying unconformable cake of limestones, rise several hundred feet above sea-level. The surface on which the basalts were poured out was probably very uneven, but there may also have been some considerable displacements of these basalts either before or during the injection of the dolerite sills of Ben Hiant.
An interesting feature at this locality is the peculiar grouping of some of the large dykes in the area around the agglomerate. They run in the direction of the vent, and one or other of them may represent the fissure or fissures on which the volcanic orifice was blown open to the surface. Another notable element in the geological structure of the ground is the vast amount of intrusive material, both in dykes and sheets, which has been erupted. The intrusive sheets of Ben Hiant form the most prominent eminence in this part of Ardnamurchan. Reserving them for description in the following Chapter ([p. 318]), I will only remark here that they partly overlie the agglomerate, and are therefore, to some extent at least, younger than the vent. They belong to that late stage in the history of the basalt-plateaux when the molten material, no longer getting ready egress to the surface, forced its way among the rocks about the base of the bedded basalts, and more especially on the sites of older vents, which were doubtless weak places, where it could more easily find relief.
The large neck now described is only one of a group scattered around it in the ground to the north. Two of these may be seen rising through a detached area of Jurassic limestones and shales at the northern base of Ben Hiant. A third, almost obliterated by the intrusive sheets, may be traced at the western end of that mountain above Coiremhuilinn. Two others rising through the schists on either side of Beinn na h-Urchrach, have been much invaded by the sills of that eminence ([Fig. 326]). It is doubtless owing to the extensive denudation of the basalt-plateau, and the consequent uncovering of the rocks underneath it, that this series of vents has been laid bare.[302]
[302] Professor Judd has united these scattered vents into a continuous platform of volcanic agglomerates, which he represents as underlying the supposed lavas of Ben Hiant. Since the publication of his map and description, I have re-examined the ground without being able to discover any trace of this platform. All the visible agglomerates are separate necks, their actual walls being sometimes exposed, as in the neck immediately north of the base of Ben Hiant, where the limestone in contact is marmorised, though twelve yards of it is an ordinary dull blue rock.
By far the largest mass of agglomerate in any of the Tertiary volcanic areas of Britain is that which occurs on the north side of the main valley of Strath, in Skye.[303] Unfortunately, it has been so seriously invaded by the eruptive rocks of the Red Hills, that its original dimensions and its relations to the surrounding rocks, especially to the bedded basalts, are much obscured (see [Fig. 348]). It can be followed continuously from the lower end of Loch Kilchrist along the southern slopes of Beinn Dearg Bheag round to the western roots of Beinn Dearg Mhor—a distance of more than two miles in a straight line, and from Kilbride to the flank of Beinn na Caillich above Coire-chat-achan—a direct distance of two miles and a quarter. A similar rock, possibly a portion of the same mass, appears in Creagan Dubha, on the north side of the Red Hills. If the whole of this agglomerate forms part of one originally continuous mass, it must have been upwards of two miles in diameter. There may, however, have been two or three closely adjacent vents. The Beinn na Caillich patch, for example, appears to belong to a different area, and that of Creagan Dubha is also probably distinct. But there seems no reason to doubt that the mass which forms Cnoc nam Fitheach, and all the long declivity on the southern flank of Beinn Dearg Bheag, occupies part of the site of a single volcano. Owing to the absence of sufficient sections, it is hardly possible to determine how much of this fragmentary material should be assigned to the actual chimney. The diameter of the whole mass is almost two miles. But possibly a considerable proportion of this accumulation belongs to the external cone which gathered round the vent, so that the eruptive pipe might thus be of much smaller dimensions than the superficial area of the agglomerate. The subsequent invasion of so much granophyre, not only that of the Red Hills, but that of numerous smaller intrusions, has indurated the agglomerate and made the investigation of its structure somewhat unsatisfactory.
[303] This extensive mass was not separated from the "syenite" of the Red Hills by Macculloch. Von Oeynhausen and Von Dechen noticed it as a conglomerate with quartz pebbles, but did not realise its volcanic nature (Karsten's Archiv, i. p. 90). In my map of Strath (Quart. Jour. Geol. Soc. xiv. plate i.) I distinguished it from the rock of the Red Hills, but no name for it appears in the legend of the map, nor is it referred to in the text. Its character as a true volcanic agglomerate was recognised by Professor Judd, op. cit. p. 255. See postea, pp. 384 et seq.
It might be supposed that the mere existence of intrusive bosses and veins rather furnishes an argument in favour of considering the visible agglomerate to belong to a deeper-seated part of the erupted material than the external cone. But, as will be afterwards shown, there is some reason to regard the present conical or dome-shaped outlines of the granophyre hills as not far from their original forms, and to believe that, like the trachytic Puys of Auvergne, they were much more superficial than plutonic eruptions. A study of the cinder cones of Central France shows that even these superficial accumulations have been invaded not only by bosses but by dykes.[304]
[304] The existence of a small dyke of andesite on the northern rim of the well-known crater of the Puy Parion has already been noticed.
The agglomerate of the great Strath vent is a coarse tumultuous assemblage of blocks and bombs, imbedded in the usual dull, dirty-green matrix. Among the stones, grit and sandstone, together with scoriaceous, vesicular and amygdaloidal basalts are specially abundant; also pieces of various quartz-porphyries and granophyres, among which a black felsite like that of Mull may often be recognised. In some places, large masses of altered limestone and quartzite (Cambrian) are included; in others, pieces of yellow sandstone and dark shale (Jurassic), or of the bedded lavas. Some of these masses may be 100 yards or more in length. Occasionally a breccia, mainly made up of acid materials—granophyre or granite,—has been noticed by Mr. Harker along the north side of the Red Hills, which he thinks may rather be of the nature of a crush-breccia than a part of the true agglomerate.
The agglomerate of this district is wholly without stratification or structure of any kind. On the north-west side of Loch Kilchrist, indeed, it weathers into large tabular forms, the parallel surfaces of which dip to south-west; but this is probably due only to jointing. Here and there, dykes of basalt cut the rock in a general north-westerly direction, but their number is remarkably small when compared with the prodigious quantity of them in the limestone at the bottom and opposite side of the valley, some of which may possibly mark the fissure on which the vent was placed. More abundant and extensive are the masses of granophyre that rise particularly along the outer margin of the agglomerate near Loch Kilchrist. These may be connected with the great boss that forms the Red Hills, of which further details will be given in Chapter xlvi.[305]