472 J. A. PHILLIPS ON THE SO-CALLED " GREENSTONES " 



This stone is generally somewhat fine-grained, very dark in 

 colour, and without visible crystals of felspar ; in places, however, 

 near the centre of the mass, there are patches of a lighter colour, 

 in which distinct felspar crystals, 3 millims. in length, can be distin- 

 guished. 



For the purposes of analysis fragments of the darker and more 

 close-grained rock were selected, since the lighter- coloured variety 

 containing visible felspar can only be regarded as of exceptional 

 occurrence. An analysis, in duplicate, afforded the following results 

 (sp.gr. =2-97):- 



I. II. 



WatGr fnygrometric ..'. -27 '30 



warer \ combined -71 -67 



Silica 50-42 50-41 



Phosphoric anhydride -74 -83 



Alumina 19-10 18-91 



Ferric oxide 2-50 2*44 



„ persulphide (FeS 2 ) -43 -42 



Ferrous oxide 11-03 11-11 



Manganous oxide *40 -42 



Lime 5-81 5-90 



Magnesia 1-80 1*72 



Potassa 1-67 1-60 



Soda 5-13 5-22 



Lithia -13 -11 



100-14 100-06 



The above analysis does not materially diifer from those of the 

 augitic rocks from the neighbourhood of Penzance, the chief points 

 of variation being that there is slightly more silica and decidedly 

 less lime and magnesia than is present in the rocks on the shores of 

 Mount's Bay. 



The principal difference between the above results and those 

 obtained by the analysis of more highly altered specimens from the 

 same locality, consists chiefly in the presence of a slightly larger 

 amount of silica and lime, together with an appreciable amount of 

 magnesia, while the percentage of ferric oxide is considerably less. 

 Microscopical examination shows that the larger proportion of lime 

 and magnesia is accounted for by the fact of the fresher rock con- 

 taining both unaltered augite and numerous hornblendic pseudo- 

 morphs after that mineral. The larger quantity of ferric oxide 

 contained in the rock analyzed in 1870, as compared with that 

 found in specimens more recently examined, is explained by the 

 presence in the former of numerous large and partially peroxidized 

 crystals of magnetite, while in the latter that mineral is more 

 sparingly disseminated, and is in an unaltered state. Lithia was 

 not sought for in the earlier analysis ; and consequently, if present, 

 this body was estimated with the other alkalies. 



Thin sections of the dark close-grained rock of which an analysis 

 is given above, when examined under the microscope, are seen to be 



