914 T. G. BONNET ON THE SERPENTINE AND 



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 dnll greyish green colour, extremely hard and tough, weather- 

 ing 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. 



Some Inland Sections. 



In conclusion, I must briefly mention two places inland where I 

 have more particular^ 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 bo dull and dark in colour (no. 15.) with but few bronzitc 

 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; buC here the ser- 

 pentine 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. 

 Tho serpentine weathers a brownish grey, often with a very rugged 

 surface. 



