Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1031

Rh the lower part of which bifurcates; and just beyond this is an included fragment of schist of singular form. The dyke in the mass has a slightly serpentinous aspect. It is a finely granular, almost compact, very dark grey rock, faintly variegated with minute white specks. Under the microscope its appearance is as follows:—There is a tolerably clear homogeneous-looking base containing a large number of small prismatic crystals and folia of greenish hornblende with fairly marked dichroism, and a good many small grains of magnetite. With the two Nicols the base exhibits the microcrystalline pseudomorph after felspar, and the hornblende shows brilliant colours. A little has a rather fibrous structure, and on rotating the stage be- haves as an orthorhombic mineral; it is strongly dichroic, showing a brown tint (? anthophyllite); there is, however, not enough to enable one to be certain about it. A vein is filled by asbestiform mineral, possibly a variety of chrysotile.

Proceeding along the shore we pass in quick succession some other narrow dykes (three, I think) of very similar appearance. The last but one (2 to 3 feet thick) cuts through a vein of coarse gabbro about 10 inches thick, which shows again in one or two places in the face of the cliff. A little to the north it is apparently cut by another dyke; but after carefully examining the latter, especially where it is exposed on the shore, I believe it to be only an included fragment of a peculiar compact variety of the schist, highly altered. About 30 yards to the north is another included mass of the schist, standing upright in the serpentine cliff so as to look wonderfully like a dyke.

Just where the sandy beach of Kennack Cove commences is another large mass of included schist, occasionally resembling vein granite, which may be traced some way inland. Two or three intrusive tongues of highly decomposed serpentine may be seen in this mass.

Kennack Cove is a sandy tract at the embouchure of two flattish valleys, divided by a low headland of serpentine. In this is a small dyke about a foot (or rather less) thick, closely resembling those described above, but perhaps even more compact.

Crossing the second stretch of sand we come again to cliffs of dark serpentine, and find almost at the first point a dyke, generally from 4 to 5 feet thick, which bifurcates above. The appearance of this rock is very similar to those already described. Under the microscope it is found to consist of longish, rather irregularly outlined, plagioclase crystals, and a quantity of aggregated grains or imperfectly shaped small crystals of hornblende, green-coloured, strongly dichroic, and showing bright colours with polarized light. There are also some irregular grains of magnetite and a few needles of apatite.

A short distance further along the shore a mass of rock, forming a group of low reefs, bears, at first sight, a close resemblance to a granite vein. This is heightened by the extraordinary way in which, in one place, it inserts itself in thin tongues into the adjoining rock, which is a crumbling, dull, greenish to reddish substance, not