﻿94 Mr. J. A. Phillips on the Chemical Composition and 



rock are seen to be chiefly made up of an aggregation of trans- 

 parent angular particles, giving colours with polarized light; 

 disseminated throughout the mass are numerous shades and 

 patches of a greenish colour, which are probably due to chlorite. 

 Under a high power they are found to contain many black grains 

 of magnetite and titaniferous iron, together with fan-like aggre- 

 gations of transparent acicular crystals, which often diverge 

 from a particle of oxide of iron as a centre. 



Besides these radiated combinations, there are other appa- 

 rently similar crystals, which cross each other in all directions ; 

 these, as well as the former, are probably either hornblende or 

 schorl. There are also some broken and rounded fragments of 

 mica and of a semi-translucent mineral, which is not sufficiently 

 opaque for oxide of iron; these last are mechanically embedded 

 in the slate. 



Killasfrom Botallack, Penzance, surface near lode. Sp.gr. = 2'95 . 

 — This rock, which is of a dark greenish-grey colour, sometimes 

 contains minute crystals of iron pyrites, and is very hard ; its 

 cleavage has been to a great extent obliterated by metamorphism ; 

 but it usually divides, when broken, into roughly tabular masses, 

 of which the planes approximate to those of original cleavage. 

 For analysis, a specimen was chosen free from crystals of pyrites 

 and from ferruginous stains. The following were the results 

 obtained ; — 



I. II. 



Water* 3*12 3*13 



Silica ..... 40-27 40-16 



Titanic acid .... -15 *15 



Phosphoric acidf . . '66 -66 



Alumina .... 24-03 2399 



Ferric oxide . . . 4*26 4*16 



Ferrous oxide . . . 11-34 11-20 



Sulphur .... trace trace 



Lime 4'16 4-05 



Magnesia .... 6'46 6'58 



Potassa 1-66 1*68 



Soda 3-54 3-60 



99-65 99-36 



The low power ordinarily employed shows this rock to be 

 composed of an apparently amorphous base, often of a green 

 colour, probably due to chlorite, in which no distinct grains are 

 distinguishable ; in this are porphyritically enclosed numerous 

 elongated transparent crystals and crystalline patches, the latter 

 being frequently fringed at the edges by a growth of acicular 



* Of which -39 was lost in the water-bath. 



t Only one estimation of phosphorie acid was made. 



