ORIGIN AND DEVELOPMENT OP ORE-DEPOSITS. 161 



of his pupils, after a number of excursions to Carn Marth and 

 other mining localities, wrote as follows : — 



" The general character of the granite of the hill (Carn 

 Marth) was 'primary,' or composed simply of quartz, felspar, 

 and mica, but where a vein of quartz or veins of other characters 

 were discovered, the felspar for 2 or 3 inches on each side was of 

 a pinkish colour, and it sometimes contained chlorite ; where 

 the vein was larger the granite was altered for a still greater 

 space on each side of it, apparently as if by the influence of the 

 vein." 



' ' This alteration of the granite was more plainly shewn at 

 West Wheal Damsel. Here we found all the granite near the 

 lode to be very much altered, more or less according to its 



proximity the change being greatest near the lode ; the 



felspar was sometimes entirely absent, chlorite occupying its 

 place, sometimes only partially ; and that broken at a still greater 

 distance from the lode was merely of a flesh colour. In all our 

 excursions we have noticed this alteration of the granite by the 

 influence of lodes and veins, and in some cases a still further 

 change from these influences was observed. At a burrow at old 

 Wheal Jewell there are stones of granite to be found where the 

 felspar has been replaced by tin and schorl (as at Dolcoath, &c.) 

 In a cross-cut at East Wheal Damsel, at a distance of five or 

 six fathoms from the lode, the granite contained cuprite." 



A similar change occurred at the junction of the lode and 

 elvan at the United Mines.* Mr. Pearce further observes that 

 primary and secondary granite were not formed at different times 

 or under different circumstances, but that the original granite 

 was merely changed " by the influence of the lodes which pass 

 through or near it, these various alterations being effected by 

 the decomposition of the mineral constituents of the lodes and 

 the solvent action of water.f 



6. — It is sometimes said that very hard ground is unfavour- 

 able. In one sense this is always so, since it increases the cost of 

 the underground operations, but in this sense very soft ground 



*Ibid, Notes of Excursions, read September 8, 1864, 

 fibid, Report for 1863, 



