ORIGIN AND DEVELOPMKNT OF ORE-DEPOSITS. 305 



contain tin. The " St. Martin " and " Screed " lodes at Polgooth 

 Mine are certainly heaved by the " great elvan," as already 

 stated, t and it is not unlikely that some of the tin lodes of the 

 parish of St. Just are of similar antiquity. Here, too, should, 

 I think, be placed the stanniferous deposit (Stockwork) of Wheal 

 Vyvyan in Constantine, which is thus described by Mr. 

 Henwood. 



' ' It has the character of an enormous granitic lode, bearing 

 20-30*' S. of W., and dipping 35-50^ N., is from 5 to 10 fathoms 

 wide, and in some places even more. Its composition differs 

 little from the country near it, except that, perhaps, it contains a 

 rather smaller proportion of mica ; they have both a porphyritic 

 structure with bufO and flesh-coloured crystals of felspar. The 

 whole substance of the lode is thinly interspersed with tin- ore, 

 copper pyrites, and also with spots of iron pyrites, and here and 

 there a little vitreous copper-ore. These metalliferous minerals 

 are, however, chiefly disposed in small veins and strings, which 

 most commonly have the direction of the lode, and generally a 

 northern inclination ; the joints in the rock seem to coincide 

 with the veins both in bearing and dip, and where there are 

 crevices in them their faces are commonly coated with fine 

 crystals of tin-ore. 



There are also veins of granite in the lode which dip S., 

 and sometimes cut off the small strings of tin and copper-ores, 

 throwing them upwards. 



The whole of this remarkable deposit resembles those of 

 Balswiden, GUgger, Carcla%e, and the Bunny in granite, and of 

 Wheal Music and Polherrow in slate, except that in neither of 

 the others do the ores of tin and copper occur together.*" 



Here, too, may be placed the lode of West Yor in Breage, 

 where the elvan, according to Mr. Argall (Rep. M.A. of C. & D.), 

 on coming ** into contact with the lode, destroyed it, and it could 



*Itis not, indeed, improbable that there are two sets of tin-lodes existing at 

 Polgooth, one older than the other, and both older than the great elvan. Accord- 

 ing to the plan given by Mr. J. Hawkins [Trans. Geol. Soc. Cornwall, 1, 152.^ 

 Vanvean lode is heaved by the elvan and itself heaves Screeds and St. Martin's 

 lodes. 



t Henwood, Trans. Roy. Geol. Soc, V, pp. 72, 73.) 



