PRECIOUS METALS 



75 



The vein itself is a true fissure vein about 4 miles long, and 

 several hundred feet wide, branching in the upper portions, and 

 faulted 3000 ft. in the center. The faults gradually die out as the 

 ends of the veins are reached. The lode contains gold and silv r 

 and the chief gangue mineral is quartz. According to Von 

 Richthofen, the ores and gangue minerals were brought up by 

 ascending solutions. Fluorine, chlorine and sulphur were the 

 agents of solution. 



The ore occurs in bonanzas of remarkable richness. One of 

 these bonanzas is said to have furnished $110,000,000 in gold and 

 silver. The ores are marked also by great irregularity. Gold pre- 

 dominates over silver in the ratio of 3 : 2. The mine has yielded 

 nearly $400,000,000 and is still a steady producer (Fig 55). 



FIG. 54. East-west section through the Comstock lode in Nevada showing 

 the position of two of the ore bodies, and of the Sutro tunnel. 



The Pacific Coast Region. This belt extends along the Pacific 

 coast from Lower California northward through California, 

 Oregon, Washington and British Columbia. The California 

 field is the most important of all for it furnishes an annual out- 

 put of approximately $20,000,000. The belt is characterized by 

 quartzose ores and auriferous sulphides. In the more northerly 

 portion of the belt silver occurs with the gold and the auriferous 

 sulphides are without free gold. The region is characterized by 

 many placers which have been derived from the weathering of 

 the upper portions of the quartz veins (Fig. 56). 



The Mother Lode Belt: This belt comprises a large series of 

 quartz veins stretching in a northerly and southerly direction for 



