﻿Microscopic Constitution of certain Cornish Rocks, 95 



crystals, which give brilliant colours with polarized light. Under 

 a high power the ultimate fragments, of which the slate is com- 

 posed, are still very indistinct, but some long slender crystals, 

 probably apatite, are distinctly seen ; there are also some minute 

 crystals of an apparently triclinic mineral, which may be axinite. 

 Rock from Botallack, one hundred and thirty fathoms deep. Sp. 

 gr. = 2*82. — This specimen, which was sent under the name of 

 " killas," has lost all traces of cleavage, and has a cross fracture 

 with polished surfaces resembling those so common in many varie- 

 ties of serpentinous rock*. It has a very dark green colour, 

 with slight traces of a lighter shade of the same tint • and in places 

 minute crystals of pyrites are seen by the aid of a lens. The 

 fragments chosen for analysis were apparently free from these 

 crystals, and afforded the following results : — 



I. II. 



Waterf 11'06 11-12 



Silica ..... 3293 33'03 



Titanic acid . . . trace trace 



Phosphoric acid . . trace trace 



Alumina .... 16-69 16-77 



Ferric oxide . . . 7*17 6-88 



Ferrous oxide . . . 13-67 13-75 



Sulphur .... trace trace 



Lime ..... 5-02 4-78 



Magnesia .... 11*43 11-61 



Potassa -78 -68 



Soda -64 '61 



99-39 99-23 



In this rock, which slightly attracts the magnetic needle, 

 grains of oxide^' of iron are more numerous than in specimens 

 obtained nearer the surface ; the fragments of which it is com- 

 posed are also larger and more clearly defined. It contains but 

 few long crystals of the kind found in sections procured from 

 the surface rock, but is traversed by minute veins of quartz 

 in which transparent acicular crystals, probably of some horn- 

 blendic mineral, are observed. It will be remarked that both the 

 specimens of so-called killas from this mine contain a large 

 amount of magnesia, which is all but entirely wanting in the 

 clay-slates from the neighbourhood of St. Austell. In fact, from 

 its physical constitution, as well as from its chemical compo- 

 sition, the rock from the deeper level may be regarded as an 

 impure serpentine. 



Roofing-slate } Delabole. Sp. gr. = 2'81. — The quarries of De- 

 labole are situated near the town of Camelford, and yield large 



* From some of these polished surfaces thin plates of calcite were 

 detached. 

 t Of which 4*12 was lost in the water-bath. 



