South Mountain Mining District, Idaho. 121 



aetinolite, garnet, epidote, and ilvaite, carrying in places 

 pyrrhotite, chalcopyrite, sphalerite, and galena. "I did 

 not pay any attention to the paragenesis of the minerals, 

 but, if my memory serves me well, I should say they are 

 all distributed in the irregular manner characteristic of 

 contact deposits. The ilvaite is in bunches scattered 

 through the masses." 



Description of Ilvaite. — The specimen received from 

 Mr. Hershey consists of a large aggregate of imperfect 

 black prisms of ilvaite associated with white calcite and 

 granular aggregates composed of small dodecahedral 

 crystals of brownish-red garnet. The ilvaite only 

 approximates prismatic form and no good faces occur. 

 The specimens received from Mr. Ellis show far better 

 crystals. The first of these consists of a mass of trans- 

 lucent, glassy, white quartz, completely filled with grains 

 and crystals of ilvaite. These are commonly prismatic 

 and deformed by mutual interference, but some of them, 

 when removed from the enclosing quartz, are suitable for 

 measurement on the reflecting goniometer. Only the 

 prismatic zone is represented, these embedded crystals 

 never, so far as observed, being terminated. The most 

 common habit is like the prism zones of figs. 1 to 4, 

 although all of the forms observed in this zone on the 

 terminated crystals were found on the embedded crys- 

 tals. The crystals are often deeply striated and rounded, 

 apparently by oscillation between the prisms m(110) and 

 s (120). They greatly resemble embedded black tourma- 

 line and a casual observer would probably mistake them 

 for that mineral. The forms observed on one of the best 

 of these crystals were identified as: a (100), 6(010), 

 w(110), M210), s(120), d(140). 



The measurements obtained on one of these embedded 

 crystals are compared with the angles given by Dana, as 

 follows : 



Calculated 

 Observed. (a = 0.6665, D). 



m:m"' 66°38' 67°46' Av. 67°12' 67°22' 

 h:h'" 37°28' 38°00' 37°44' 36°52' 



s:s' 72°42' 73°10' 72°56 / 73°45' 



This agreement with the calculated angles is as close 

 as was obtained upon any of the crystals measured. 

 That there should be a discrepancy is not at all surpris- 



