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



Water I h yg rometric * 55 



Water \ combined 1-14 



Silica 49-66 



Phosphoric anhydride -25 



Alumina 20-85 



Ferric oxide 2-07 



„ persulphide trace 



Ferrous oxide 4-99 



Manganous oxide trace 



Lime 14-19 



Magnesia 2*7? 



Potassa 1-44 



Soda 2-05 



99-96 



Thin sections of this rock are seen to consist of a granular trans- 

 parent nearly colourless ground-mass, containing quartz, in which are 

 enclosed numerous imperfect yellowish crystals, which seldom exceed 

 0*5 millim. in diameter, and are often disposed in irregular bands ; 

 these alternate with stripes of a mineral resembling sericite. 



The guttate bodies exhibiting these rounded outlines, and which 

 are not unlike distorted globules of gum-arabic, are probably to a 

 large extent imperfectly crystallized garnets, although they some- 

 times afford colours in polarized light. These minute crystals, how- 

 ever, are intimately mixed with the double-refracting ground-mass, 

 portions of which have often become enclosed in them ; and it is con- 

 sequently difficult to determine to what extent the colours referred 

 to may be due to this circumstance. In chemical composition this 

 rock does not materially differ from the altered dolerites. 



About three quarters of a mile from this point, on a farm called 

 Trenewth, a little north-east of the town of Camelford, is a hackly 

 bluish-green slaty rock, traversed by numerous thin veins of milky 

 quartz. A specimen of this rock, which is quarried, under the name 

 of "blue elvan," as a road-material, when subjected to analysis, 

 afforded the following results (sp. gr. =2-97) : — 



Water I h 7g rometric * 30 



\ combined -60 



Silica 48-23 



Phosphoric anhydride -37 



Alumina 19-35 



Ferric oxide 3-57 



,, persulphide trace 



Ferrous oxide 8-38 



Manganous oxide trace 



Lime 8-04 



Magnesia , . , 4»14 



Potassa 1-21 



Soda 5-84 



100-03 



