" GREENSTONES " OF WESTERN CORNWALL. 175 



is seen, in thin sections, to consist principally of an aggregation of 

 rounded grains of quartz, having an average diameter of jfo inch. 

 These are closely cemented together by a mineral of a greenish 

 colour ; and among them are small fragments of hornblende. 



At South Roskear the greenstone is coarser and more slaty in 

 texture than at Camborne ; magnetite occurs more plentifully, but 

 has been to some extent replaced by a siliceous material ; no garnets 

 were observed in this rock. 



The specimens obtained from East Pool are rather fine-grained, 

 contain a less amount of hornblende, and are much mottled by the 

 presence of the brown crystalline mineral before described. Eighteen 

 sections of rocks from this district were cut and examined. 



Newlyn East. — At North Huel Rose, in this parish, there is a 

 band of greyish greenstone, which is not laid down upon the Map 

 of the Geological Survey. This rock has a very compact structure, 

 almost resembling that of chert, and although exceedingly hard and 

 tough, exhibits distinct evidence of a disposition to break into plates. 

 An analysis, in duplicate, afforded the following results (sp. gr. 

 3-02):— 



I. II. 



\ COL 



Silica 46-57 46-70 



Phosphoric anhydride *63 -59 



Alumina 19-50 19-74 



Ferric oxide 1-84 1-81 



„ persulphide traces traces 



Ferrous oxide 12-80 12-81 



Manganous oxide trace trace 



Lime 9-91 9-89 



Magnesia 3-93 3-84 



Potassa '51 *53 



Soda 3-50 3-40 



Lithia trace trace 



Water \ "-/S rometric * 24 * 20 



combined -64 -52 



100-07 100-03 

 Under the microscope it is seen to consist of a transparent base, 

 enclosing matted crystals of hornblende, together with magnetite 

 and viridite. Between crossed prisms the base presents the ap- 

 pearance of a coloured mosaic, indicating its granular constitution. 

 The hornblende is disseminated, principally in the form of undulating 

 filamentary and approximately parallel bands, between which the 

 base is more or less charged with a flocculent green mineral ; a 

 nebulous greyish substance is also usually present, and angular 

 patches of magnetite are somewhat numerous. Four sections of 

 this rock were examined. 



St. Stephens. — At Terrace, about a mile and a half west of St.- 



Stephens church, there is a large blue elvan, which has for many 



years been quarried as a material for repairing the parish roads~ 



This greenstone is not shown on the Geological Map, but is several 



Q.J.G.S. No. 126. n 



