464 Kozu and Washington — Augite from Stromboli. 



streaks of lava adherent to the surface, but we saw none 

 of the olivine grains mentioned by Bergeat. 



As noted bv Bergeat, the forms are very simple. Only 

 the planes a(100), fr(010), 3 w(110), and .s (Til) were 

 observed. In habit they are elongated in the direction of 

 the vertical axis and, with this modification, are repre- 

 sented by figs. 16, 17, 18 on page 354 of Dana's System. 

 Twinning according to a (100) is common, and inter-pene- 

 tration cruciform twins, some of them complex and made 

 up of several individuals, are frequent. These have 

 been studied by H. L. Barvir, 4 who recognizes five twin- 

 ning laws besides that just mentioned. Though this 

 was not studied by us, it would seem that some of the 

 complex individuals are merely fortuitous intergrowths. 



The regular cleavage is fairly perfect and the fracture 

 is conchoidal, both giving bright surfaces. In thin splin- 

 ters and cleavage flakes the color is a light olive-green. 

 Pleochroism is scarcely discernible. 



The specific gravity is 3-243 at 19°, as determined on 

 selected fragments used for the analysis, with the 

 pyknometer of Johnston and Adams, 5 which is capable of 

 very accurate results. 



. Optical Chaeactebs. 



The three principal refractive indices were determined 

 with Klein's total-reflectometer. The mean values of 

 the observed critical angles for sodium light are as 

 follows : 



6a =10° 23-5' (9/3= 70° 58-7' 6 y = 73° 3-9' 



The images produced by total reflection were not dis- 

 tinct, owing to hour-glass and zonal structures. How- 

 ever, trials made at different parts of the crystal on 

 different dates showed that the deviations of the indices, 

 caused by the crystal structure mentioned above, fall 

 within the limit of the fourth decimal place. Hence the 

 following values, obtained by computation from the 

 angles given, were taken as the means of the principal 

 refractive indices of the mineral. 



a = 1-693 /3 = 1-699 y = 1719 

 y _ a = 0-026 y — = 0-020 — = 0-006 



3 This is usually very narrow. 



4 Of. abstract in Zs. Kryst., 39, p. 398, 1904. 



5 Johnston and Adams, J. Am. Chom. Soc. 34, p. 566, 1912. 



