Minerals of the Composition Mg Si O z . 395 



fore, grave doubt on a possible isomorphic relation between the 

 two substances, especially when the different zonal develop- 

 ment of the two is also taken into consideration. The investi- 

 gation of this important question is still in progress. 



Polysynthetic twinning after the orthopinacoid occurs almost 

 invariably and is apparently as characteristic of this particular 

 pyroxene as twinning according to the albite law is character- 

 istic of the plagioclase feldspars : between crossed nicols the 

 sections frequently bear a striking resemblance to those of 

 plagioclase. 



A peculiar type of these monoclinic crystals was observed in 

 several of the preparations ; they were developed in flat tabu- 

 lar form after the orthopinacoid, and in consequence of this lay 

 invariably on this face when immersed in one of the refrac- 

 tive liquids. Thus the extinction was of course always paral- 

 lel and might have led to erroneous conclusions in regard to the 

 crystal system and confusion with enstatite, if special pains had 

 not been taken to test their behavior in other positions. The 

 crystals were accordingly imbedded in thickened Canada bal- 

 sam, in which medium they could be slowly turned by mov- 

 ing the cover glass. The thin upturned edges then showed 

 the polysynthetic twinning after 100, and the extinction angles 

 characteristic of this form. In the examination of later prep- 

 arations, a device for turning the crystals in any direction 

 and in liquids of equal refractive index was constructed and 

 proved satisfactory. The immersion method in Canada bal- 

 sam, however, is less complex and usually answers equally 

 w^ell. 



The hardness of the monoclinic pyroxene is about 6 ; the 

 crystals are only slightly attacked by acids. 



The refractive indices were determined by the immersion 

 method of Schroeder van der Kolk : 



a = 1-647 + -003, /? = l-652'±-003, y = 1 '658 ±-003 ; 



the birefringence is not strong — about -01. The extinction 

 angle on the clinopinacoid — remarkably low for a pyroxene — 

 was carefully measured on well-developed crystals obtained 

 from a fusion of the metasilicate in magnesium chloride. 

 These crystals were twinned poly synthetically and the extinc- 

 tion angle was obtained, as in the feldspars, by using the 

 symmetrical extinction angles of adjacent lamellae. An aver- 

 age of 25 readings gave c : c = 21° *8 (with extreme values of 

 19°*5 and 24°'5). Measurements in sodium light were also 

 made with practically the same result. The dispersion of the 

 bisectrices was found to be very slight. The plane of the 

 optic axes lies in the clinopinacoid ; the optical axial angle is 

 very large. The optical character is positive and was ascer- 



