Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1041

Rh probably some of the microliths in it are the pseudomorphs already mentioned. As a type of the other variety, I take a specimen from a dyke about four inches wide (just opposite the north end of a row of white cottages), which cuts through two rather thin veins of newer gabbro to a slight hummock of serpentine (I believe, no. 3 in my description above). A hand specimen might very easily be taken for a dull-coloured serpentine, were it not for its greater hardness and its behaviour under the hammer. Microscopic examination shows it to be an altered basalt, infiltrated here and there with serpentinous veins, full of minute crystals of plagioclase (or probably a pseudomorph after it), occasionally about 0⋅02 inch long. Here and there occur pale brownish films replacing the ordinary dusky granulated matrix. Examination with polarized light shows that actinolite is present here; and minute bright specks over the whole slide suggest the presence of the same mineral. The grains of magnetite (perhaps also of ilmenite) are much decomposed. Here, then, is a basalt, probably a magma basalt, greatly altered, but still not in any true sense of the word converted into serpentine. Another specimen, broken in 1873 from a very similar dyke, shows a still greater change, the felspar crystals being still just discernible, but the actinolite more characteristic and perfectly formed.

Near the western end of the village, in the little open glen which comes down to the cove, and about 100 yards from the sea, is a quarry in serpentine. This (no. 13) is rather a dull dark variety of an aluminous aspect, and is much cracked and jointed. This is probably due to some small gabbro veins (coarsely crystalline and much decomposed) which have been exposed in opening the quarry. On the shore, east of a small row of houses, a mass of the newer gabbro is exposed, very schistose in the upper part. Gabbro veins may be seen on the hill-side behind the village.

Proceeding onwards, we observe that the shore, where free from sand, consists of serpentine with intrusive veins of the newer gabbro. These become more numerous as we approach the main mass of gabbro, which is reached about a quarter of a mile from the end of the village. In plan this is an irregular oval, a little more than four miles long and two wide, which forms the elevated upland of of Crousa Down, some 300 feet above the sea. The line of the raised beach is well marked by a low cliff, and in one place by a projecting rocky plateau of considerable extent a short distance above sea-level. The rock at the southern edge of the massif varies greatly in texture, being sometimes moderately coarse (often, owing to the prevailing greyish-blue tint, seeming from a short distance quite compact), sometimes very coarse, with crystals of felspar and diallage nearly an inch across. In the latter the felspar generally weathers to a dull white; the diallage is more or less metallic, commonly with a greenish tinge. Some of the more compact varieties also weather to a decided mottled white and dull green, like the gabbro of Mont Colon and the Matterhorn (Pennine Alps); but others (for what reason I cannot say) assume a slightly rusty tinge on their surface as the felspar weathers out, leaving the diallage crystals