270 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



SHOSHONE COUNTY 



Pegmatites consisting mainly of white or light colored feldspar, 

 probably orthoclase, have been reported from the vicinity of Marble 

 Creek in southern Shoshone County. 



WASHINGTON COUNTY 



Broad cleavages of fine fresh pink orthoclase have been examined. 

 These were said to come from pegmatites on Council Mountain in 

 Washington County. It is reported that a man in Boise was buying 

 a limited quantity of this material at $5 a pound for use in a "secret 

 chemical process." It is typical potash feldspar which is worth 

 approximately $5 a ton on the eastern market. 



ORTHOCLASE, variety ADULARIA or VALENCIANITE 



A form of orthoclasse occurring in white or colorless crystals 

 probably deposited from solution rather than by crystallization from 

 fusion has been called adularia and such orthoclase occurring as a 

 vein mineral associated with silver ores has received the additional 

 varietal name valencianite from the locality of Valenciana, in 

 Guanjuato, Mexico. This was supposed to be distinct from ortho- 

 clase because the crystals differed, apparently, in angle. The 

 occurrence of this unusual feldspar in Idaho was first noted by Lind- 

 gren in the silver mines of the Silver City district in Owyhee County, 

 where it is abundant in fine large crystals, in several mines. It has 

 since been noted by Umpleby in microscopic grains in similar precious- 

 metal veins in Custer and Lemhi Counties. 



CUSTER COUNTY 



Valencianite (adularia) occurs in Custer County as a common 

 though not abundant constituent of the gangue of the late Tertiary 

 gold-silver veins. It is usually of microscopic size and occurs either 

 intergrown with the quartz or* included in it. These veins, which are 

 confined to the Yankee Fork district, have been mined for gold and 

 silver and are fissure veins inclosed in Miocene tuffs and lavas. 



Adjacent to the veins the wall rocks are intensely altered, hydro- 

 thermally, by silicification or sericitization, or both, for a distance of 

 50 or 100 feet and in some cases 1,000 feet or more from the deposits. 

 The silicification is most intense next to the veins and gives way to 

 sericitization at a distance of 50 to 100 feet. The unaltered ore of 

 the veins is a fine grained quartz, locally accompanied by a con- 

 spicuous amount of calcite and a little chalcedony, opal, and adularia. 

 Of the metallic minerals pyrite alone is abundant and uniformly 

 present. Tetrahedrite is not uncommon and chalcopyrite, galena, 

 and, rarely, enargite occur. Widespread and important in the richer 

 ores are blue-black submetallic bands and blotches in which pyrite, 



