ART. 3. 



BLACK SANDS FROM IDAHO SHANNON". 



25 



:> 



Boise Basin. This mineral coincides roughly in distribution with 

 the rose-pink or purple variety of garnet referred to pyrope. In 

 several Snake River sands augite is the most abundant ingredient. 

 Those from Wapi and Minidoka especially contain- 

 ing 6 per cent or more of the mineral. The augite- 

 bearing sands frequently contain more or less olivine 

 in clear yellow crystals. 



The augite occurs in irregular grains and imperfect 

 crystals which vary from emerald green through 

 various shades of pistachio and olive green in color. 

 They are very similar in color to some of the titanite 

 occurring in the sand from Bear Creek, Camas 

 County, but differ in form and luster. The crystals 

 are commonly prismatic in form and are etched and 

 corroded so that, although bright and glassy, very 

 few of them have faces which yield measurable sig- 

 nals. There is no cleavage visible and the glassy 

 green grains and crystals were at first thought to be 

 olivine. Careful search, however, revealed crystals 

 which could be measured and these gave the angles 

 of augite. The few measurable crystals found gave 

 fairly good signals in the prism zone but the terminal 

 faces are invariably etched and dull. One crystal gave a 

 " schimmer " reading on a terminal face which indicated approxi- 

 mately the negative pyramid s(Ill). The angles measured on this 

 crystal which identify it as pyroxene are given in the following table : 



Angles of augite crystal from Minidoka (fig. 21). 



F I G. 23. RUTILE 



CEy.STAL FROM 

 CBNTBHVILLK. 



faint 



Reflection. 



Very good . . 



Good 



Very good. . 



Poor 



Good 



...do 



Very poor. . 

 ...do 



Several of the coarser unscreened sands from Minidoka, which 

 were rich in augite, contained small pebbles, the great majority of 

 which were seen under the binocular to consist of a cellular gray 

 rock containing glassy-green grains in a light gray ground. The 



