180 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



specimens of the pseudomorphous quartz. Typical argentine 

 calcite having the platy structure and pearly luster is also present 

 in a specimen from a tunnel dump on the south side of Long Gulch 

 below the Blaine tunnel. 



B. CRYPTOCRYSTALLINE VARIETIES 

 e. CHALCEDONY 



Chalcedony is essentially the same as agate, except in that agates 

 usually are more or less highly colored by included impurities, most 

 frequently iron oxide. The chalcedony occurs in Idaho occasionally 

 in veins but more frequently in cavities in basic igneous rocks. 



Blaine County. — -Translucent white or bluish chalcedony occurs in 

 the Tertiary basalts in many places in the Hailey Quadrangle and 

 adjacent regions and many of the lava hills are strewn with nodules 

 and fragments of translucent chalcedony, usually white but some- 

 times colored by the presence of included iron oxide. Many of the 

 irregular or rounded lumps have originally filled amygdaloid cavities 

 in lava. Usually the first lining of the cavity has been calcite 

 followed by the chalcedonic silica and the nodules show on their 

 outer surface the gash-like molds of the acute rhombohedral crystals 

 of calcite. Sometimes the masses are solid chalcedony with a con- 

 centrically banded structure, but more frequently they are hollow, 

 sometimes with a botryoidal inner surface, but more frequently with 

 a lining of quartz crystals, usually colorless but occasionally 

 amethystine. 



Custer County. — Chalcedony occurs in the lava areas of Custer 

 County in the same manner as in Blaine County, especially near the 

 North Fork of Lost River. Specimens of the rare zeolite, mordenite, 

 from near Challis have a crust of chalcedony between the mordenite 

 and the inclosing rock and nodules of chalcedony occur in the area. 

 Cherty silica referable to chalcedony occurs in some limestones in the 

 vicinity of Mackay. 



Owyhee County. — Chalcedony occurs in lavas and a dense flinty 

 chalcedonic silica occurs in some of the silver veins as a gangue 

 mineral. A brownish radially fibrous variety occurs in ores of the 

 Oro Fino mine. A specimen from the new ore shoot, De Lamar 

 mine, consists of fine grained gray chalcedonic silica and a specimen 

 from the Summercamp tunnel, De Lamar district, consists of dense 

 very fine grained chalcedonic silica with lusterless fracture with pyrite 

 replacing rhyolite. 



Lemhi County. — Chalcedonic silica forms a gangue material in 

 veins of the Parker Mountain district. 



