118 H. S. Washington £ H. E. Merwin — Mineralogy. 



the simple habit which is usually shown by such loose 

 crystals of augite from basaltic rocks. The planes pres- 

 ent are: a(100), b(010), m(110), and 5(111). Some of 

 the crystals are twinned on the front pinacoid (100). 

 The crystals were measured goniometrically, but subse- 

 quent sectioning showed the presence of a surface film, 

 about 0.05 mm. thick, having a much higher refractive 

 index, stronger pleochroism, and other properties, indi- 

 cating a higher ferric iron content than the rest of the 

 crystal, so that the goniometric measurements cannot be 

 of definite value, and they are therefore not given. 



In thin section the color is a very pale gray, almost col- 

 orless, but with a faint tinge of green ; the pleochroism is 

 so slight as to be scarcely noticeable. Extinction angles 

 were measured on two sections parallel to the side pina- 

 coid (010), cut centrally through two crystals that were 

 twinned on a (100). The angle y A c = 47°-48° for red 

 (630 fifi), and 49° for blue (480 /*/*). For the same wave 

 lengths 2V r measured 61°-62° and 2V b = 58°-60°. The 

 birefringence y-<* on these sections was .024; p-a (calcu- 

 lated) is .006. 



Measurements of refractive indices were made on a 

 sample of the powder prepared for analysis. The lowest 

 value found was 1.695, the highest was 1.727; /? was 

 1.704-1.709. Thus, the following indices represent the 

 material used for the chemical analysis ; a = 1.700, ft = 

 1.706, y = 1.724. 



The density was kindly determined by Dr. L. H. 

 Adams, using a pycnometer and thermostat, on the mate- 

 rial used for the analysis. The value obtained was 3.358. 



A chemical analysis was made of material carefully 

 separated by heavy solutions and the electromagnet, that 

 was practically free from the outer film and from glass 

 and other inclusions. The powder was dried at 110°. 

 The results are shown in the table of analyses, with 

 analyses of other augites from basalts for comparison. 

 The analysis does not differ materially from those of 

 other augites from basaltic lavas, but the presence of 

 about one-quarter of one per cent of chromic oxide is 

 of interest. Steiger found 0.18 per cent of Cr 2 3 in the 

 olivine of Mauna Loa, and several analyses of various 

 Hawaiian lavas show that this constituent is present in 

 many of them in readily determinable amounts. 



