356 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



When the crystals are examined in polarized light lying on nib 

 &(010) face most of them show faint lines dividing the crystal into 

 four quarters, by lines of apparent twinning, and the extinction of 

 adjacent sectors is inclined 11-12° to each other and 6° to the dividing 

 line which is parallel to the long edges of the crystals. Such crystals 

 show the optic normal in convergent light. When the crystals are 

 crushed they yield elongate fragments of ragged termination, showing 

 that there is no cleavage parallel to the end face of the crystals 

 oriented upright. Most of the fragments lie on the 6(010) cleavage 

 and give high birefringence colors and show the emergence of the 

 optic normal. These are ragged in outline showing that the o(100) 

 cleavage is very imperfect. Other elongate fragments which rest 

 upon a (100) show the obtuse bisectrix normal to the gram. All 

 fragments show negative elongation, so that the mineral is optically 

 negative, 2V small. The optical orientation is X = c, Y = &, Z = a. 

 The refractive indices, which seem to be variable, average about: 

 a= 1.490, j8*= 1.503, 7 = 1.507. Many of the fragments, especially 

 those lying on (100) show a lamellar structure and rather uneven 

 extinction, suggesting albite twinning but very irregular and confused. 



Mr. Hewett regards the stilbite in this mine as having been intro- 

 duced later than the ores. 



CUSTER COUNTY 



D. F. Hewett, of the United States Geological Survey, reports 

 finding stilbite at Galena Summit on the north (Salmon River) side 

 within 1,000 feet of the summit, in slabs up to 2 inches thick, in 

 Tertiary andesite. 



KOOTENAI COUNTY 



Stilbite occurs with prehnite and other zeolites coating a fracture 

 in a diabasic rock on a specimen from Post Falls, Idaho, submitted 

 for examination by Henry Fair, of Spokane. The stilbite forms 

 small colorless to slightly yellowish crystals lining cavities. These 

 are rectangular and entirely like those described above from Adams 

 County. They have the habit of typical epidesmine. When lying 

 on the 6(010) cleavage they show another prominent cleavage 

 parallel to a(100). The mineral is optically negative, with X = c, 

 Y = 6, and Z = a. The b pinacoidal sections show the emergence of 

 the optic normal and extinction variably inclined from 0° to 11°. 

 The refractive indices measured are a =1.500, 0=1.505, 7=1.507, 

 all ±0.003. 



The exact locality for the specimen is not stated, but it is said to 

 be from a claim belonging to a man who recently caused some local 

 interest by pretending to find rubies and diamonds at the prospect 

 from which the zeolites came. 



