Tin; MINING DI8TEICTB OP CORNWALL. 345 



could be deprived of its mica and felspar by this reagent, and that 

 its quartz crystals became rounded, just as in the elvans. The 

 Cornish elvans and the minerals associated with them, all contained 

 traces of fluorine ; he had found that a very small percentage of 

 fluorine could effect great alteration in crystalline rocks, giving rise 

 to a peculiar structure and the formation of a certain class of mine- 

 rals, e.g. tourmaline, fluor-spar, lepidolite, &c, in short the usual 

 minerals accompanying elvans and tin-lodes. 



Mr. J. A. Phillips, in reply, stated that some of the minerals 

 were no doubt brought up by superheated steam. He added, that 

 if a vein cuts through stratified rock, the horses always keep iu the 

 line of their original stratification, and inquired how this condition 

 of things could be brought about if the deposition of the vein were 

 due solely to the action of heat. 



