OBIGIN AND DEVELOPMENT OF 0EE-DEP0SIT8. 131 



Thus, we have seen that the average produce of the granite 

 stockworks proper has been about 8 lbs. to the ton of stuff 

 removed. Only a very rough approximation can be made to the 

 mass value of the Balleswidden stuff, but it certainly seems to 

 have been much richer than this. If we estimate the ground 

 removed at 300 fathoms long, 4 fathoms wide, and 40 fathoms 

 deep; and few would be disposed to reckon anything like so 

 much, who have seen the workings and studied the plans ; then 

 the ground removed, allowing for that left for pillars, could not 

 exceed 500,000 tons, and this would require to yield about 50 lbs. 

 of tin to the ton to account for the 12,000 tons sold. The current 

 statements as to the riches of Beam and Bunny would certainly 

 indicate a somewhat similar state of concentration for the tin of 

 these mines. 



Of course this greater concentration is to some extent 

 apparent only, since the greater expense of working under- 

 ground, and at a considerable depth, would compel the miners to 

 leave much of the poorer ground below, and untouched, which if 

 raised and treated would materially lower the average produce. 

 But after making due allowance for this consideration, it yet 

 appears that when the main- joints in a stockwork are so well- 

 defined as to appear more or less like distinct lodes, an enrichment 

 is the result, in other words the riches bear some direct relation, 

 speaking generally, to the "strength" of the strings, veins, or 

 lodes. 



The total ground excavated for this class of stockworks can 

 hardly be less in all than four times that worked out at Balles- 

 widden, and the tin thus obtained must have been from 40,000 

 to 50,000 tons . 



5 — TIN STOCKWOEKS IN ELVAK. 



Many elvans have been worked as tin stockworks to a small 

 extent, as for instance Polgooth, near St. Austell ; Terras, in St. 

 Stephens ; Budnick and Wheal Coates, in St. Agnes ; Castle-an- 

 dinas and Belowda, near Roche ; Poldory, in Grwennap ; Wheal 

 Unity Wood, and Bissoe Bridge.* Very rich tin-ores were also 

 obtained by under-ground mining at the Wherry Mine near 

 Penzance, in an elvan course 18 feet wide. "On a close inspection 



* Henwood, Trans. Eoy. Geol. Soc, Corn., v, 82, 87. 



