640 



IDAHO MINING DISTRICTS. 



of the feldspars and ferromagnesian silicates and the immunity of 

 the quartz is noteworthy. 



From the Silver Wreath tunnel, Willow Creek district, Boise 

 County, two rocks were collected, analyses of which are given below. 

 The first is a perfectly fresh granitic rock, the ordinary country rock 

 of the district. The second, occurring only a few feet away, is the 

 same granite altered by the vein-forming agencies, and a comparison 

 between the two will show the character of the process. Both rocks 

 are unaffected by atmospheric agencies. 



Analyses of rocks from Silver Wreath tunnel. 

 [Analyst, George Steiger.] 



The unaltered rock is light gray and coarse grained, the average 

 size of the constituents being 5 mm to 6 mm . With the naked eye, white 

 plagioclase, reddish orthoclase, biotite, titanite, and quartz may be 

 distinguished. Under the microscope the quartz proves to be very 

 abundant and is slightly crushed. The plagioclastic feldspars pre- 

 dominate and occur generally in anhedrons, more rarely in roughly 

 prismatic forms. Symmetric extinctions show a maximum of 10. A 



