ON THE " GEEENSTONEs" OF CENTEAL AND EASTEEN COENWALL. 471 



31. On the so-called " Geeenstones" of Centeal and Easteen 



Coenwall. By J. Aethtje Phillips, Esq., E.G.S. (Head 



April 3, 1878.) 



[Plates XX.-XXIL] 



On a former occasion I had the honour of laying before this 

 Society the results of a series of microscopical and chemical exa- 

 minations of the principal " Greenstones " of the western districts of 

 Cornwall *. In the present paper it is my intention to describe the 

 cognate rocks of the central and eastern portions of that county, 

 commencing near the point where I then left off, and gradually 

 proceeding eastward. 



Centeal Southeen Disteict. — A mile west of St. Austell, and a 

 quarter of a mile north of the turnpike-road leading from that town 

 to Truro, is a quarry from which stone has long been obtained for 

 road-making. This quarry, which adjoius the Sanctuary fields 

 belonging to the glebe of the parish of St. Mewan, yields a hard 

 blackish-green rock, traversed by numerous cross joints, of which 

 the surfaces are covered by an unctuous brownish-red material, 

 containing a large proportion of hydrated ferric oxide. 



The band furnishing this stone courses in an easterly direction 

 from the farm of Quoit to the quarry above mentioned, and is shown 

 on the Geological Map as extending thence in a south-easterly 

 direction to the sea-cliff near Duporth. Its width in the Sanc- 

 tuary quarry is about thirty yards ; near the centre of the band 

 the rock is distinctly crystalline, but becomes gradually less so 

 on either side, until finally, assuming a slaty structure, it merges 

 into the enclosing killas. 



This rock was analyzed and microscopically examined by the 

 author in the year 1870 (Phil. Mag. xli. p. 97, 1871), at which time the 

 portion of the baud laid open was much fissured, and had been sub- 

 jected to such extensive alteration that it was difficult to obtain 

 fresh representative specimens. Thin sections, however, were found 

 to consist of much-altered felspar (a portion of which was triclinic), 

 hornblendic belonites, viridite, numerous large crystals of magnetite 

 (many of which were partially converted into hydrated peroxide of 

 iron), and hexagonal prisms of apatite — the whole being more or less 

 obscured by a flocculent greenish-grey product of alteration. 



On a more recent visit to this quarry (January 1877), it was found 

 that the rock which was then being raised was not only more 

 decidedly crystalline, but also considerably fresher than that 

 examined seven years previously, and then supposed to be an altered 

 diorite. It likewise closely resembled some varieties of dolerite 

 from the neighbourhood of Penzance; and a new series of sections 

 was consequently prepared, and an analysis made of the rock in 

 its less-modified state. 



* " On the so-called ' Greenstones' of Western Cornwall," by J. Arthur 

 Phillips (Quart. Journ. Greol. Soc. vol. xxxii. pp. 155-178). 



