Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1044

914 to go as far as Manacle Point, a promontory, according to the map, formed of the greenstone. This rock is, in the veins, very compact, of a dull greyish green colour, extremely hard and tough, weathering brown; in general appearance it is not unlike the 18-inch dyke described above. Under the microscope it is found to be a finely crystalline rock, closely resembling the aforesaid specimen—the augitic constituent now appearing generally as a pale greyish or brownish rather fibrous mineral, very feebly dichroic, changing with polarized and analyzed light from a pale or golden yellow to a blue or puce-brown tint. The felspathic constituent also only alters from light to dark milky grey; hence I have no doubt that it too originally was a finely crystallized basalt.

To the north of this there is a little serpentine associated with gabbro (schistose in places), in the neighbourhood of Polkerris Point. These have been briefly described by Prof. Sedgwick and Sir H. De la Beche; and I have examined specimens in the Woodwardian and Mr. S. Allport's collections, but have not yet been able to visit the district. I hope, should they prove of sufficient interest, to make them the subject of a future communication.

In conclusion, I must briefly mention two places inland where I have more particulary examined the serpentine:—one, at its first appearance, on the road from Helston to Llandewednack. Here, so far as I know, the junction is concealed by vegetation; but a little stream running through a ravine must be very close to it. On the north side of this is seen altered Devonian rock, apparently shattered and baked. On the south the serpentine rises in some picturesque knolls by the roadside. It is a dark-coloured homogeneous rock (no. 14), resembling that at the south end of the Mullion mass, weathering a brownish grey, and showing, more especially in the weathered surfaces, the banded structure already noticed.

The other instance is on Goomhilly Downs, on the road from Helston to Coverack Cove. In the first serpentine-pit which I passed (about half a mile beyond Sir L. Vyvyan's lodge), that rock is found to be dull and dark in colour (no. 15.) with but few bronzite crystals, homogeneous in aspect, but showing the banded structure remarkably well, so that on weathered surfaces it quite mimics stratification. Another quarry, to the right of the road, about seven miles from Helston, shows a similar structure; but here the serpentine varied much in different parts of the pit, some specimens barely indicating it. Many of the latter were extremely compact in texture, almost conchoidal in fracture, and very beautiful, having a dull-reddish to greenish groundmass, with veins of bright red and of yellowish steatite; the bronzite crystals are small and inconspicuous. The serpentine weathers a brownish grey, often with a very rugged surface.