120 



ECONOMIC GEOLOGY 



cross faulting occurred. The ore is found chiefly at the intersec- 

 tion of these two sets of fault planes. 



According to J. E. Spurr, the ores were deposited by magmatic 

 waters ascending vertically along these faults and were precipi- 

 tated by the reaction between the solutions and certain wall rocks, 

 chiefly shales. The mingling of solutions at the intersection of 

 the fissures also played an important role in the formation of the 

 ore bodies. 



According to W. H. Weed, the richer ore at the intersection of 



^r^^si:^ t 2 it e iJ^liii^ii/tf/V-w^ ! ' 



SSp^f^^ 





^-^^aTT'r'en--* -p.-'- '; .'.-'. ;. X ; . 



__5tv-7-^5?'^"-sr^, I ^o*_ 'S.e .'... : 



FIG. 78. Sketch showing structure of Silver Shield lode, northeast face of 

 stope. (After J. M. Boutivell, U. S. Geological Survey.) 



these fault planes was due to secondary deposition, while Spurr 

 finds little evidence of secondary sulphide formation. The ores 

 are peculiarly free from other metals (Fig. 78). 



In Utah and Nevada, argentiferous galenite yields much lead 

 as a by-product (see chapter on the precious metals). In Idaho 

 the bulk of the lead has been from the Coeur d'Alene district 

 in Shoshone County. This district has for many years been 

 one of the leading lead-producing sections of the country. The 



