Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1030

900 curiously like vein granite; but I am convinced all is metamorphic rock. The serpentine near the junction is, as usual, much decomposed; a line of breccia either of schist or altered serpentine extends from the top of the first to the second. The northern end of the third piece all but joins the fourth; and the beds in it are absolutely squeezed together by the pressure which they have undergone. Then we pass another small fragment, and after about a hundred yards come to two or three more, all highly altered. Another, chiefly on the shore itself, occurs after about fifty yards; and they are common for the next hundred yards or so. A fragment occurs just at the angle where the cliff turns inland towards the serpentine-works in Caerleon Cove. Following this for a short distance we come to a remarkable "greenstone" dyke at the foot of the low cliff. It is from 4 to 5 feet wide; the sides for about 6 to 10 inches are very dark and compact, and so platy in structure as to be almost undistinguishable from some specimens of the hornblende schists. This structure is lost rather suddenly; and the rock assumes the ordinary aspect of an igneous rock, consisting of a finely crystalline mixture of white felspar and dark hornblende, with porphyritic crystals of the former as much as $1⁄4$ inch long.

I have had a section made of the heart of the dyke; and my friend Mr. Allport has kindly lent me two of his own cutting, one being from the outside. The former shows that the rock consists of a much decomposed plagioclase felspar in long narrow crystals. The interstices are occupied by a pale-coloured (generally greenish) hornblende of rather fibrous or filmy aspect. There is also some magnetite. The aspect of this rock is so like a typical dolerite, and so unlike a diorite, that I suspect the hornblende to be a secondary product, as in the gabbro. The other section is totally different, and closely resembles the hornblende schist above described. I examined the rock at the time to see whether a piece of schist had been caught up; but if it was, I failed to detect it.

Some of the quarries belonging to the Poltesco works are up the valley inland; but these I have not been able to visit. From Caerleon Cove a steep ascent leads us up again to the main plateau.

Hornblende schist may be seen on the shore of the next little cove, and on its left bank, above the path, intrusive gabbro, coarse, but showing a rather foliated structure, may be seen on the rough grassy slope. I observed three exposures; near to the furthest there appears also to be a little schist, the rock generally on each side of Caerleon Cove being serpentine.

This rock, on the beach beyond the next headland, includes many large fragments of quartzose rock, some of which very closely resemble granite veins. After careful examination, however, I am of opinion that they are only bands in the schist. The more quartzose and harder layers have been forced among the softer, so as to mimic intrusion.

Beyond this, in cliffs of a dark serpentine, is a greenstone dyke,