146 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



less. On the whole I think we may safely take the mean width 

 of the whole lode at 6 feet for the coppery portions and 10 feet 

 for those yielding tin (of which the leader or actual fissure 

 filling will not average more than one foot) for the whole period 

 since the mines have yielded tin, but occasionally running up to 

 6 feet as in the great bunch recently worked at the 280 in 

 Carn Brea. 



The walls in the great lode are generally fairly distinct, but 

 less so in depth than nearer " grass." The hanging wall is 

 generally better defined than the footwall — especially in the 

 deeper workings. Vughs and cavities were much more common 

 in the lode in the shallower than they are in the deeper workings, 

 but they are still occasionally met with in all the mines on the 

 lode.* 



The contents of the lode have as already stated varied at 

 different depths. In the shallower portions above adit there 

 was much gozzan, consisting of eai-thy brown iron ore with iron 

 pyrites and spongy quartz, and in places earthy black copper ore, 

 with various rare crystallized arseniates and phosphates of iron 

 and copper, also much chalcopyrite and chalcocite. In the 

 eastern part of the mine a good deal of tin was raised from 

 the shallower workings. Some of the earthy brown iron ore 

 was found as far down as the 197 fathom level. The rich parts 

 while yielding copper usually gave it as chalcopyrite with few 

 crystals. Soon after reaching the granite most of the copper 

 gave out and the mine changed to a tin mine. The richest ores 

 of tin are of a bluish colour, not very hard but quite compact, 

 and permeated in all directions by strings of rather light-coloured 

 oxide of tin. Often this bluish rock passes into a dark red 

 ferruginous mass without becoming poorer in tin. Careful micro- 

 scopic examination shows that the blue tin-stone, apart from the 

 tin, consists of quartz, chlorite, and schorl — the latter mostly in 

 minute needles ; and the change from blue to red seems to be a 



*In November, 1814, a cavern was discovered in the main lode at a depth of 

 170 fathoms. It was from 18 to 20 fathoms long, 3 fathoms high, and from 4 to 

 9 fathoms wide. It contained much loose material which had fallen from the 

 sides and communicated by narrow channels with many subsidiary cavities. — See 

 Trans. Roy. Geol. Soc. Corn., Vol. 1. Similar vughs have been found in nearly 

 all the master lodes of the West of England. 



