Fig. 327.—Section of two Sills in schistose grits, west end of Beinn na h-Urchrach, Ardnamurchan.
a a, crystalline schists; b, neck of volcanic agglomerate; c, small sill; D, massive sill of Beinn na h-Urchrach; F, sill proceeding from the series forming Ben Hiant and joining that of Beinn na h-Urchrach.
On the south-east side of the mountain where the bedded basalts can be traced close up to the intrusive dolerites, they are found to present the usual dull indurated aspect so characteristic of contact alteration among these rocks. There cannot therefore be any doubt that Ben Hiant never was itself a volcano. Its rocks are characteristically those of subterranean intrusions. They seem to have been injected from a line of fissure or from several such lines, running in a general north-easterly direction, at some late part of the volcanic period. The group of agglomerate necks of older date shows that already the ground underneath had been drilled by a number of distinct volcanic funnels, and discloses a weak part in the terrestrial crust.
iv. FAROE ISLES
In the Faroe Islands the actual base of the volcanic series is nowhere visible. Hence, the great lower platform of intrusive sheets being there concealed, this feature of the basalt-plateaux is less conspicuous than it is in the Inner Hebrides. A number of sills, however, have been noticed by previous observers,[324] and I have observed others on the sides of Stromö, Kalsö, Kunö and other islands. In the lofty precipices of the Haraldsfjord, many of the massive light-coloured prismatic sheets are intrusive, for though they preserve their parallelism with the bedded sheets for considerable distances, they may be seen sometimes to break across these, as is strikingly shown in one of the great corries on the east side of Kunö.
[324] See in particular Prof. James Geikie and Mr. Lomas, in the papers already cited on p. 191.
Fig. 328.—Sill traversing bedded Basalts, cliffs of Stromö, at entrance of Vaagöfjord.
The caves and notches shown at the bottom of the precipice mark the position of the vents represented in Figs. [311], [312], [313], [314].
One of the most remarkable sills in the Faroe Islands is probably that which forms so prominent an object on the western cliffs of Stromö, at the entrance into the Vaagöfjord (Figs. [328], [329]). It is prismatic in structure, and where it runs along the face of the cliffs, parallel to the bedded basalts among which it has been intruded, presents the familiar characters of such sheets. The precipice of which it forms a part is that which rises above the row of volcanic vents already described. But it there begins to ascend the cliffs obliquely across the basalts until it reaches the crest of the great wall of volcanic rock at a height of probably about 1000 feet above the waves. From the crest of the precipice the upward course of the sill is continued into the interior of the island. It pursues its way as a line of bold crag along the ridges of the plateau, gradually ascending till it forms the summit of one of the most prominent hills in the district ([Fig. 329]).
Some further idea of the enormous energy with which the sills were injected may be formed from this example, where the eruptive materials followed neither the line of bedding nor a vertical fissure, but took an oblique course through the plateau-basalts for a vertical distance of probably more than 1500 feet.