256 PHYSIOGRAPHY. CLASS IT. 



2 4 



Pr + co. Fig. 86. 



Measurements of angles : M on P = 93 20', M on x = 93 

 50', M on o = 113 32', 7 1 on Z =121 38'. The re-entering 

 angles produced in Fig. 87. by P and P' measure 186 

 40', those produced by a? and a/, 187 40'. 



Cleavage, __ perfect ; less distinct Pr 4- co ; the cleavage 



parallel to / _jt_?L is sometimes more easily obtained 



2 v 



than either. Fracture imperfect conchoidal, uneven. Sur- 

 face, the prisms parallel the axis streaked parallel to their 



Pr 

 common intersections ; _ generally uneven or a little rough ; 



r (-L 3/ rough, but very even. 



Lustre vitreous, often inclining to pearly upon perfect faces of 



cleavage. Colour white, passing into grey, red, and green. 



Streak white. Transparent, rare and only in small crystals, 



... translucent on the edges. 

 Brittle. Hardness = 6-0. Sp. Gr. = 2-613, small transpa- 



rent crystals from Dauphiny. Limits 2-61. ..2-68. 



Compound Varieties. Twin-crystals. 1. Axis of revolution 

 parallel, face of composition perpendicular to Pr + co. Fig. 87. 

 2. Axis of revolution parallel to the edges of combination be- 

 tween Pr + co and Pr -f co ; face of composition parallel to 

 Pr -f co. The latter composition allows of two cases, and is 

 similar to Fig. 80. and Fig. 81. It occurs frequently along 

 with the preceding one, so that laminae of four individuals al- 

 ternate with each other, composed parallel to Pr 4- co. All 

 these compound and simple varieties have been observed in 

 the varieties from St Gothard and Dauphiny. Massive : com- 

 position granular of various sizes of individuals, sometimes 



compressed parallel to the faces of Pr 4- co or -- T, and then 

 the composition assumes a lamellar appearance. 



