182 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



masses of the West of England. The study of these belongs to 

 contact metamorphism generally, and has no particular relation 

 to the phenomena of ore-deposits. The same may be said of the 

 crystalline concretionary patches in the granite already referred 

 to, which consist of "an abnormal arrangement of the crystalline 

 minerals of the granite itself, found by microscopic examination 

 to be practically of the same composition, but usually containing 

 more mica."* 



The most noticeable sporadic crystallizations on a large scale 

 (macro-porphyritic) to be seen in the West of England are, no 

 doubt, the "horse-tooth" crystals of orthoclase, and the cross- 

 macles of the same mineral in the granite of Tol-Pedn-Penwith, 

 and elsewhere. The separately developed crystals of orthoclase 

 in the elvans are often very striking. But next to orthoclase, 

 black tourmaline or schorl is the most striking mineral thus 

 developed, and there is good reason, too, to regard it as a 

 secondary mineral in all cases, i.e. it has been formed after the 

 original consolidation of the rock. 



Schorl occurs almost invariably in acicular crystals, and 

 often with radiate arrangement ; the occurrence of independent 

 prisms more than -^ of an inch in thickness being rare, although 

 Bovey Tracey, Stenna Gwyn, St. Agnes, and Tremearne may be 

 mentioned as localities for such larger prisms. 



As was pointed out long ago by Delabeche, schorl seems to 

 be limited to the neighbourhood of the granite junctions, whether 

 it occurs in the granite or in the killas. It is abundant under 

 the following conditions. 



1. As schorl-rock, which appears to be merely an altered 



granite, and is, I think, always near killas — as at 

 Eoche Eock, St. Mewan Beacon, &c. It is sometimes 

 notably stanniferous, as at Eock Hill near St. Austell. 

 Other modifications of granite containing schorl are the 

 varieties known as Luxullyanite and Trowlesworthite. 



2. As tourmaline schist, which is merely an altered killas, 

 as at the Gwennap Mines, Wheal Vor, the south side of 

 Carclaze pit, &c. 



* Phillips Q.J.G.S., xxxxi, p. 1, and xxxviii, p. 216. 



