.80 PHYSIOGRAPHY. CLASS ir. 



Cleavage, Pr 1 and (Pr -f o^) 3 4 with greater 

 facility and perfection Pr -f- oc. Fracture, con- 

 choidal, uneven. Surface, generally smooth, 

 P 1, and (Pr I) 3 rough. (Pr + oc) 5 and 

 Pr -f- oo often a little uneven, Pr uneven. The 

 curvature of the faces parallel to the axis very 

 often produces acicular crystals, variously ag- 

 gregated. 



Lustre vitreous, inclining to resinous upon faces of 

 fracture. Colour white, prevalent ; sometimes 

 passing into grey, yellow, mountain-green and 

 violet-blue. Some of the green colours probably 

 owing to foreign admixtures. Streak greyish- 

 white. Transparent, translucent. 



Brittle. Hardness = 3-5... 4-0. Sp. Gr. = 2-931, 

 the transparent crystals from Bohemia. 



Compound Varieties. Twin-crystals. 1. Axis 

 of revolution perpendicular, face of composition 

 parallel to one of the faces of (?r + oo) 5 , Fig. 39. ; 

 2. Axis of revolution parallel, face of composition per- 

 pendicular upon one of the lateral edges of (?r + n) 3 . 

 These two kinds of regular composition are, the 

 one as it were the complement of the other, since 

 twin-crystals, in which the substance of the indivi- 

 duals is continued beyond the face of composition, 

 as is very frequent in the present species, may be 

 explained upon either supposition, at least if we 

 pay no attention to the internal structure of the 

 crystals. The composition is very often repeated 



