ORIGIN AND DEVELOPMENT OP OBE-DEPOSITS. 375 



mostly situated in the slaty rocks (killas) near to the granite, 

 hut occasionally in the granite itself. In the former situations, 

 as in the great siliceous lodes of Devon Grreat Consols and 

 Grawton in the Tamar valley, it is in the whole more closely 

 associated with copper than with tin, although both metals are 

 generally present. In the Camborne district it is perhaps more 

 closely associated with tin than with copper, and sometimes 

 copper is altogether absent. On the whole it is probable that 

 arsenic in the veins of the West of England is less abundant than 

 tin, though much more abundant than wolfram. As a rock- 

 constituent it is like wolfram, very rare. 



Mispickel contains about 46 per cent, of arsenic combined 

 with nearly 35 per cent of iron, and about 19 percent of sulphur. 



Copper. The number of minerals containing copper is very 

 great, but practically they are all confined to the veins and rock- 

 joints ilear veins. The rich gray ores of the St. Just mines, of 

 the Godolphin mine in Breage, of the Camborne and Eedruth 

 mines (whence the richest is sometimes called Eedruthite), 

 and of some of the mines near St. Austell; the rich red and 

 black oxides and blue and green carbonates of the Caradon 

 district ; as well as the rarer arseniates, phosphates, and uranates, 

 and the native copper of many of the coppery gozzans ; all seem 

 to have been derived by chemical and electrical agencies from 

 the double sulphide of iron and copper known as chalcopyrite, 

 which contains when purest nearly equal proportions of copper, 

 iron, and sulphur. 



The great cindery courses of ore at the Gwennap mines, and 

 the still larger siliceous ore-masses at Devon Great Consols, some- 

 times formerly as much as 40 feet wide, strike the imagination 

 with their brilliant appearance, but unlike the best tin-veins 

 they have for the most part become impoverished in depth, so 

 that copper mining, which began in comparatively recent times, 

 has now sunk to very small dimensions ; and has been for the 

 greater part of its history far inferior in importance to tin 

 mining. 



The best copper deposits have always been in veins near to 

 but not actually in the granite. Some copper has also been 

 found in veins at considerable distances from the granite, quite 



