182 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



TRIDYMITE (211) 



Silicon dioxide, Si0 2 Hexagonal. 



CLARK COUNTY 



The hexagonal form of silica, tridymite, is probably a more com- 

 mon mineral of lavas than is usually supposed, since it is usually 

 recognizable only with the aid of a microscope. This mineral was 

 found by Dr. E. S. Larsen to make up a considerable proportion of 

 a number of specimens of rhyolitic lavas collected by H. T. Stearns 

 in the vicinity of Libbys Hot Springs west of Dubois. 



OPAL (212) 



Silicon dioxide, usually with some water. Amorphous. 



Opal ranging from brown to white opaque common opal, through 

 various kinds of semiopal to fine precious opal has been found at a 

 number of localities in Idaho, the principal occurrences being those 

 noted below. 



BOISE COUNTY 



Sandstone cemented by fire opal has been collected from Neocene 

 lake beds on both sides of Moore Creek at Idaho City. 78 



CUSTER COUNTY 



Silica in the form of opal is abundant in miscroscopic grains in 

 the copper silicate ore of the Alder Creek (Mackay) district. It 

 is easily recognized in index solutions by its isotropism and low 

 index of refraction. 77 



KOOTENAI COUNTY 



Specimens, some of large size, of common opal obtained from 

 near Setters, in the Spokane Valley between Spokane and Coeur d' 

 Alene City, have been sent to the National Museum by Henry 

 Fair, of Spokane. The material ranges from translucent to almost 

 transparent and is yellow and greenish yellow to resinous brown in 

 color. There is also some nearly opaque white opal in the lot. 

 Nothing is known regarding the occurrence of this material. 



LATAH COUNTY 



In August, 1890, fine opal was found in digging a well near 

 Whelan, 20 miles from Colfax, Wash., near Moscow, Idaho, and 

 almost on the line between Idaho and Washington. It occurred 

 more or less plentifully, and the last 4 feet of the rock contained 

 cavities filled with precious opal. This opal occurs in a basalt in 

 which most if not all of the feldspar and pyroxene as well as the 

 groundmass seem to be altered. Buildings were erected and the 

 locality named Gem City. The total yield of the mines during the 



« Waldemar Lindgren. U. S. Geol. Survey, 18th Annual Report, pt. 3, p. 665, 1897. 

 « J. B. Umpleby. U. S. Oeol. Survey, Prof. Paper 97, p. 54, 1917. 



