ORDER I. 1>RISMATOIDAL (iY PSUM-HALOIDE. 59 



their common intersections. The faces of 



4 pj. g P 



I and + commonly rounded, 



2 2 * 



which gives rise to the well known lenticular 

 shapes, if in combinations like 3, the faces of 

 P + x and Pr + oo disappear. 



Lustre vitreous. Pr + oo possesses a pearly lustre, 

 more or less distinct, both upon faces of cleavage 

 and faces of crystallisation. 



Colour, generally white, sometimes inclining and 

 passing into smalt-blue, flesh-red, ochre-yellow, 

 honey-yellow, and several shades of grey. Im- 

 pure varieties assume dark-grey, brick-red and 

 brownish-red tinges. Streak white. Transpa- 

 rent ... translucent. 



Sectile. Thin laminae are flexible in the direction 

 of those edges which arise from the intersection 



of Pr + oo, with - *^p?, and *_. 



Hardness = 1*5 ... 2-0. The lowest degrees upon 

 Pr -j- x, the highest degrees in the direction of 

 p GO, in which the crystals very often are 

 rounded. Sp. Gr. = 2-310, a perfectly trans- 

 parent crystal from Oxford. 



Compound Varieties. Twin-crystals. 1. Axis of 

 revolution perpendicular, face of composition pa- 

 rallel to Pr -f oo. (Hence forms like the Chaux sul- 

 Jutee prominule of HAUY). 2. Axis of revolution 

 perpendicular to Pr -f oo ; face of composition pa- 

 rallel to Pr -h x. 3. Axis of revolution per- 



