Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1035

Rh unaltered olivine; but I think I have detected a few pseudomorphs. This rock, then, is a basalt only slightly altered.

The gabbro veins continue to the eastern side of the cove. One, which shows a marked schistose structure, contains, as usual, many long strips of serpentine. Close by it is another gabbro vein, only a few inches thick, but quite unique in character. It consists mainly of felspar, a whitish to bluish-grey labradorite, with crystals often from an inch to an inch and a half long, in excellent preservation. In the interstices of the felspar are aggregated minute crystals of a dull green mineral. The latter under the microscope proves to be actinolite. The aggregated clusters of crystals are very beautiful objects with polarizer and analyzer; and the felspar crystals (which are a little decomposed) contain, especially near their edges, many small acicular crystals of the same mineral.

Close to this vein is a small dyke (6 to 12 inches wide) of a very slaggy-looking, compact, dark rock, which, I have no doubt, is an old basalt. Some of the serpentine near this weathers to a dull green colour, and contains distinct crystalline grains of magnetite. Beyond this point is about a mile of coast, coloured as serpentine in the map, which I have had to pass over almost unvisited; and the steep cliffs will, I think, render detailed examination no easy task.

(b) Coverack Cove.—We then come to Coverack Cove, one of the most interesting localities on the coast. Without a regular survey and large-scale map it is not possible to give very precise details; but the following general description may render clearer those which I can furnish.

The cove terminates in a widish valley. On the right bank is a headland of serpentine, on the left the great gabbro mass which rises from the sea to the upland of Crousa Down.

The village stands on a low cliff (a raised beach, traces of which are also conspicuous along the edge of the gabbro mass) on the right bank of the cove. Beneath it is a rocky foreshore.

The principal rock beneath the village is serpentine; but this is broken up by a network of dykes and intrusive veins. As will be seen from what follows, we have here the following association of rocks in order of age:—

(1) Serpentine (no. 12).

(2) A gabbro, generally of a dull reddish green colour, which I shall refer to as the Older Gabbro.

(3) A gabbro of more normal aspect, which I shall call the Newer Gabbro.

(4) Some dark trap dykes, similar in general appearance to those described at Kennack and Lankidden.

At the eastern end of the village is a small harbour bounded by a pier, which starts from a little headland. I commenced my examination on the further side of this. The rock here is serpentine, much jointed and cracked, and often rather decomposed. This is also found on the other side of the harbour, where at the base of a high wall a vein of gabbro is exposed, about 4 feet thick and with a