Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1024

894 marked foliated structure, the felspar and the pyroxenic constituent being to a great extent separated, and the latter running in seams roughly parallel to the longer sides of the vein, except towards the top, where it tends to become parallel to the upper surface. This structure is more conspicuous near the lower surface. The felspar, at any rate on the exterior, is a dull yellowish white; the other constituent, as is very common here, is chiefly made up of minute crystals of hornblende, which, as will presently be explained, have almost entirely replaced the diallage. Some six feet above the end of this vein a tongue of serpentine, about 1$1⁄2$ foot wide, is exposed in the schist.

The bay is bounded by a small headland; and the shore is strewn with fallen blocks of schist and coarse gabbro. I will first describe their general relations, then discuss their lithological character.

The gabbro and hornblende schist are here mixed up in the most extraordinary way; the gabbro has penetrated again and again through the latter, crumpling up pieces of it in places so much that it is difficult to believe they come from a sedimentary rock. The best example of this intricate intrusion can be seen from a narrow track just above the headland. Here some of the veins of gabbro are only an inch or two wide and about a quarter thick; they thin away to mere strings, but remain rather coarsely crystalline to the last. This seems to indicate that the whole mass of the rock was at a pretty uniform high temperature at time of the intrusion. The so-called granite vein is a grey bed of a highly altered rock: a crystalline granular compound of quartz, felspar, and a little mica or hornblende. At the base it is most difficult to distinguish from vein granite. Still, after several very long and careful examinations of this part of the coast, I am quite convinced that it is merely a case of extreme alteration, and that there is no granite here. Below this is another intruded mass of gabbro, terminating in a broad broken tongue, the root of which rests upon a prominence of serpentine. Hence both these rocks are here intrusive in the schist. On the northern side of the headland we have a large mass of gabbro intrusive in and enclosing blocks of hornblende schist—and three masses of serpentine, one of large size.

The hornblende schist is here rather variable in character, having much less hornblende than in the ordinary black variety, and a considerable quantity of felspar and quartz. The gabbro usually consists of an opaque white or pale cream-coloured mineral of rather granular fracture, and of diallage which is often replaced wholly, or to a great extent, by a green mineral, something like chlorite. The former under the microscope appears irregular in outline, partly semitransparent, partly occupied by more or less opaque dotted aggregates of dark grey dust. Indications of cleavage-planes may be sometimes traced in this, shewn by fine parallel more transparent lines. With crossed prisms this mineral is, as might be expected, almost, or quite, dark, the clearer portion appearing as an aggregate of minute crystalline granules and microliths of ill-defined form showing faint colours, with occasional small irregular interspaces of