﻿Microscopic Constitution of certain Cornish Rocks. 93 



For the purpose of analys 

 as far as possible free from si 

 lowing results were obtained 



fragments of a pure grey colour, 

 stains of iron, were chosen. The fol- 



Water* . . 

 Silica . . . 

 Titanic acid . 

 Phosphoric acid 

 Alumina 

 Ferric oxide 

 Ferrous oxide 

 Lime . . 

 Magnesia 



Soda 



I. 



II. 



6-16 



6-16 



53-40 



53-20 



trace 



trace 



trace 



trace 



21-39 



22-06 



6-25 



5-78 



4-28 



4-28 



trace 



trace 



•75 



•75 



2-90 



2-94 



4-34 



4-05 



99-47 99-22 



No sections of this rock were made. 



Killas from Dolcoath Minef, two-hundred- and-fif teen-fathom 

 level. Sp. gr. =2*71. — This celebrated mine, which is situated 

 near the town of Camborne, is worked in killas at a short dis- 

 tance from its junction with the granite. It yields ores of both 

 tin and copper. 



The specimen of rock, from which a portion was broken for 

 analysis, is exceedingly hard, and the cleavage imperfect, the 

 prevailing colour being dark grey. When freshly broken the 

 surfaces of fracture exhibit numerous minute, but brilliant, in- 

 distinct crystals, probably of some hornblendic mineral. Its 

 composition was found to be as follows :— 



Water { . . 

 Silica 



Titanic acid 

 Alumina 

 Ferric oxide 

 Ferrous oxide 

 Lime . 

 Magnesia 

 Potassa . 

 Soda . . . 



99-91 99-94 



When examined by the aid of a low power, sections of this 



* Of which "35 was lost in the water-bath. 



t I am- indebted to the kindness of Mr. W. J. Henwood, F.R.S., for 

 procuring various specimens of rock from Dolcoath, Botallack, and Dela- 

 bole. He obtained the first through Mr. Josiah Thomas, Manager of the 

 mine ; the second were supplied by Mr. Stephen Harvey James, and those 

 from Delabole by Mr. J. Hockaday, Director of the works. 



I. 



II. 



1-14 



1-16 



67-34 



67-29 



•13 



•13 



20-94 



20-75 



2-68 



2-99 



1-66 



1-66 



2-10 



1-95 



trace 



trace 



•58 



•61 



3-34 



3-40 



