No one familiar with the well-marked distinctions between the lavas of the plateaux and the sills which traverse them can hesitate in which series to place the rocks of Ben Hiant. Since, however, these rocks have been claimed by Professor Judd as the superficial lava-currents of a volcano which broke out after the time of the plateau-basalts, like the Scuir of Eigg, some further details in regard to the geological structure of the district, which would otherwise be superfluous, may here be given.
The number of sills and dykes in Ardnamurchan is astonishingly great. There must be hundreds of them visible, and perhaps as many more concealed under superficial coverings. They are well exposed on the shore traversing the Jurassic strata and the schists. The sills become especially large and abundant in the direction of Ben Hiant, which has evidently been the principal centre from which their materials were injected. The rocks composing these sills are quite similar to those of Ben Hiant, save that, as they occur in thinner sheets than in that mountain, they do not attain the same coarseness of texture which the more massive beds there display. They generally possess fine-grained chilled selvages along their upper and under surfaces.
Fig. 326.—Ground-plan of Sills at Ben Hiant, Ardnamurchan.
a a, crystalline schists; b b, necks of volcanic agglomerate; c c, numerous thin sills; D, massive sill of Beinn na h-Urchrach; E, north side of Ben Hiant; F, sill proceeding from the series forming Ben Hiant and joining that of Beinn na h-Urchrach. The arrows mark the dip.
These abundant sills may be traced up into the mass of Ben Hiant from which they have issued, and of the individual sheets of which they are a continuation. One of the most striking and easily-followed examples of this connection is to be seen on the north side of the mountain. A thick sheet in the middle of Ben Hiant descends from among its contiguous sheets and, as a prominent rib, runs down the scree-slope into the valley below, where it forms a prominent feature. Crossing the streamlet in the middle of the valley, where a section has been cut through its upper surface, it gradually bends round towards the north-east, mounts the side of Beinn na h-Urchrach until it reaches the crest of the ridge and joins the other sills of which this eminence is built up. The route of this band of rock will be understood from the annexed ground-plan ([Fig. 326]).
That this prolongation of one of the thick beds of Ben Hiant is in no respect a superficial lava-stream but a true sill, is proved not only by its escarpment and dip-slope, but by its actually passing under and indurating the schistose grits, as may be seen in the stream-section. Again Beinn na h-Urchrach, which is mapped by Professor Judd as a northern expansion of Ben Hiant, is likewise not a lava but a true sill. Not only does it dip northwards at an angle of about 20°, having the schists immediately below its crest on the one side and descending with a long dip-slope on the other, but dwindling down rapidly from a thickness of 100 or 200 feet in the centre to no more than a few feet in a south-westerly direction, it there passes under schistose grits like those on which it lies. The strata that adhere to its upper surface are as usual indurated.
A section drawn across this attenuated development of the Beinn na h-Urchrach sill and that from Ben Hiant shows the structure represented in the accompanying diagram ([Fig. 327]), which simply gives the facts as exposed on the ground. The lower sill is that which issues from the main body of Ben Hiant, massive at first but diminishing in thickness as it recedes from its source.
Again, among the sheets which descend from the northern face of the summit of Ben Hiant and strike into the Jurassic outlier below, intensely indurated shale may be seen lying between two of the dolerites, which are unquestionably sills that have been injected into the Jurassic series.
The ridge of Ben Hiant is thus found to consist of a thick and complex series of sills, some of which are not traceable beyond the side of the mountain, while others can be followed outwards among the surrounding rocks. The specially marked dyke-like sills diverge from the main mass and run for some distance north-eastward, one of them, fully a mile long, descending among the schists into the valley and ascending into the basalt-plateau on the opposite side.[323]
[323] The sills of Ben Hiant descend on the south-west side into the sea, and can be examined along the slopes and the beach, where Professor Judd has mapped a continuous platform of agglomerate. The broad hollow between that mountain and Beinn na h-Urchrach, over which he has spread his "augite-andesite lavas," appears to be underlain mainly by the crystalline schists through which sills from Ben Hiant have been injected. The northern eminence, which he has united with Ben Hiant, is entirely separate and, as above shown, is an obvious sill.