174 J. A. PHILLIPS ON THE SO-CALLED 



borne Dressing Floors, a wide band of bluish-grey .greenstone, which 

 has a distinctly slaty structure, crops out some thirty or forty feet 

 above the surface. 



An analysis, in duplicate, of this rock, afforded the following 

 results (sp.gr. =3-03):— 



I. II. 



Water { combined -60 -65 



Silica 48-30 48-41 



Phosphoric anhydride trace trace 



Alumina 17-04 17-02 



Ferric oxide 2-73 2-68 



„ persulphide traces traces 



Ferrous oxide 9-50 9-41 



Manganous oxide trace trace 



Lime , 13-30 13-15 



Magnesia 6-18 6-20 



Potassa -30 -30 



Soda 2-01 2-13 



100-10 100-07 



Sections examined under a low power are seen to consist of 

 flaky hornblende and green or brown microlites, thickly dissemi- 

 nated through a transparent base, in which sooty magnetite and 

 cloudy viridite are somewhat abundant ; under a moderately high 

 power the latter is observed to closely resemble certain varieties of 

 chlorite. Some of the sections prepared from this rock expose irre- 

 gular patches of transparent base, into which imperfect hornblendic 

 crystals project from the surrounding mass, as observed in the case 

 of hornblendic slates from the neighbourhood of Penzance. This 

 rock sometimes also encloses small imperfect crystals, which are 

 probably garnets ; and a general tendency to a banded arrangement 

 of its constituent minerals is observable. 



A little west of the Camborne station the same mass of green- 

 stone has been laid open by the railway-cutting; here the rock is 

 similar to that last described, but a little darker in colour, and 

 perhaps somewhat closer in texture. It is, however, traversed both 

 by bands and veins of a granular brown substance, which has occa- 

 sionally a pinkish or greenish tint. As in the case of specimens 

 obtained from the same mass of rock two miles further west, the 

 minerals distributed through the base exhibit a tendency to arrange 

 themselves in nearly parallel bands. 



A microscopical examination of numerous sections from this loca- 

 lity shows no considerable difference between this rock and that from 

 Cam Camborne : the granular brown substance before referred to 

 as occurring in specimens from this cutting consists principally of 

 small imperfect garnets, with which is probably associated a little 

 axinite. One of the bands of greenstone here laid open, which in 

 hand specimens does not appear to differ materially from the others, 



