NOTES ON THE SILVEK DEPOSITS AT BUTTE, MONTANA 

 TEKEITOEY 

 By R. pea roe, JtrifE., F.G.S. 



These Mines are in granite, and the lodes resemble in many 

 respects the tin deposits of Cornwall. 



In 1864 I read a short paper at the meeting of the Boyal 

 Cornwall Polytechnic Society of Cornwall, the main object of 

 which was to show that many of the so-called lodes of Cornwall 

 were merely bands of stanniferous altered granite, and that this 

 change or metamorphism had probably been effected by the 

 circulation of water through the natural joints of the granite, 

 the water containing in solution the necessary elements to effect 

 this change. 



The change of felspar into chlorite was clearly shewn in some 

 of the specimens accompanying the paper ; well marked pseudo- 

 morphs could be traced, the whole impregnated with oxide of tin 

 and forming a mass of rock, which although generally recognised 

 as lode matter, was in reality no vein matter at all, in the general 

 acceptation of the term. 



The subject to which, I believe, I was the first to draw atten- 

 tion, has more recently been taken up by Dr. Le Neve Foster, 

 who has written a very able paper, which appeared in the trans- 

 actions of the Greological Society of London ; and I am pleased to 

 find that the opinions I held 1 8 years ago, and expressed with a 

 great deal of diffidence, have been confirmed by so good an 

 authority as Dr. Foster. I may add that I have recently sub- 

 mitted a piece of tin rock from Dolcoath Mine, consisting mainly 

 of quartz, mica and chlorite, with one or two scattered crystals 

 of felspar, to the microscopist connected with the geological 

 survey here, Mr. Cross, and he confirms my theory as regards 

 the change of felspar into chlorite. 



The granite in which the Montana Silver Mines are situated, 

 is traversed by dykes of a porphoritic rock resembling our 

 Cornish elvans, but which really consists of Ehyrolite, according 

 to the diagnosis of Mr. Cross — who has studied the rock from 

 prepared microscopic sections. 



