TIN STOCKWORKS IN CORNWALL. 



657 



mass of tourmaline schist 6 or 8 inches wide between two tinny 

 branches. 



Carrigan, about 2 miles E.N.E. of Koche, on the northern edge of 

 the great Hensbarrow boss of granite, affords the finest display of 

 Greisen in the county of Cornwall. Mr. J. H. Collins alluded to it 

 in a paper read before the Eoyal Cornwall Geological Society last 

 year, but only very briefly ; and I believe that some extracts from 

 notes that I made two years ago will not be without interest. 



The mass of Greisen wrought at Carrigan Quarry (Crogan Hock, 

 Ordnance Map, Sheet 30) is known for a width of 50 yards, and a 

 length of 100 yards and a depth of 60 feet. On the S.E. side it is 



Fig. 3. — Section at Carrigan Mine. 



S.E. N.W. 



A A A. A- A A.. 



Kcdl-e'sS, 5" IticTi, to ifoob* 

 O X 2. 3 4 5 £l<eet 



A A. Leaders. 



C. Flucan. 



bounded by a large clay vein ov flucan, C (fig. 3), and on the N. it 

 disappears under the alluvium of the neighbouring valley. The rock 

 is a mixture of quartz and mica with a good deal of schorl, some gil- 

 bertite, and a little iron-pyrites, fluor, and cassiterite. The mass is 

 traversed by a number of so-called leaders A A, which are quartz- 

 veins with tin-ore, schorl, gilbertite, and clay, dipping 85° S. and 

 running E. 7° N. Very often they are an inch or two inches wide, 

 and from 1 foot to 6 feet apart. Occasionally the leader adheres to 

 the enclosing rock (country) by one side only, and has a clay vein 

 on the other. On washing the clay, broken crystals of cassiterite 

 are generally found, proving, I think, that since the deposition of 

 the tin-ore in the fissures there has been a movement of the walls. 

 "When tin was at a higher price this mass of Greisen and tin- veins 

 was quarried and stamped on a large scale ; in fact, 27,500 tons of 

 rock were stamped, yielding 64 tons of tin-ore, or 5-2 lbs. of tin 

 (black tin) per ton, say \ per cent. It was expected that the wholly 

 virgin ground would produce 8 lbs. of tin-ore per ton. As the rock is 

 very much harder than the killas of Mulberry, for instance, it cannot 

 be profitably worked at present prices. In my opinion this mass of 

 Greisen is merely altered granite. When we can observe (and in 

 no places more clearly than at St. Michael's Mount and Cligga) the 



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