USEFUL METM.* 145 



c will precipitate copper on the same mim-rals, pn-hnitc, datolitc, 

 etc.. ini which it occurs in the mine, as Fernekes has shown. 



(J) The traps contain combustible gases, as R. T. Chamberlain 

 has shown. 



(K) Certain beds are abnormally rich in copper for many 

 miles. 



(L) Copper often replaces chlorite, and in the Calumet <fe Hecla, 

 pebbles chlorite replaces felsite, and the copper the chlorite. 



(M) Copper may even replace vein quartz. 



(N) Copper is formed generally after those minerals which are 

 the products of alteration and contain lime, and before those sec- 

 ondary minerals which are the products of alteration and contain 

 soda and potash. 



(O) Therefore at the time the copper formed the mine water 

 might have lost lime but could not have lost sodium. The rock 

 might have lost both. 



(P) The Calumet & Hecla lode averages less rich (very rich 

 in spots) near the surface, attains its greatest richness at a cer- 

 tain depth, say about 2,000 ft., and then gradually decreases in 

 richness. 



(Q) The silver occurs more abundantly in the upper levels. 



In producing the copper solution and guiding it in its circula- 

 tion Lane considers the following factors: 



(A) The waters were originally contained in the lava. 



(B) That which early filled it, whether it was buried on land 

 or beneath seas, may have included condensed volcanic vapors 

 containing copper chloride, as in Stromboli, or in the evaporation 

 of desert pools. 



(C) The absorption of water in the hydration of the rocks. 



(D) The absorption of water in the cooling of the formation 

 (water in cooling shrinks more than rock). 



(E) Faults in the formation facilitating the intermingling of 

 solutions of different compositions. 



(F) Erosion of the formation and concentration of the copper 

 contained either in pools on the land surface or in the water which 

 found its way down into the rocks, while the deposition of the 

 Keweenawan as a land formation was going on. 



(G) The ordinary circulation of the water entering at the 

 higher parts and emerging in springs. 



(3) The Cordilleran Section. (See Fig. 85.) Butte, Montana, 

 is the most important mining camp in this district. In fact it is 

 \ 10 



