86 PHYSIOGRAPHY. 



CLASS II. 



times observable in a strong light. Curved faces 

 of cleavage commonly originate in composition. 

 Fracture perfect conchoidal, difficult to be ob- 

 tained. 



Surface generally even, only certain rhombohedrons 

 and pyramids curved. R oo commonly rough ; 

 II 1 striated parallel to its own inclined dia- 

 gonals, or to the edges of combination with R ; 

 R -f oo smooth. P and P -f oo streaked paral- 

 lel to their edges of combination with R ; so are 

 likewise the scalene six-sided pyramids belonging 

 to R, and above all (P) 5 . 



Lustre vitreous. The lustre of P oo, particu- 

 larly as a face of composition inclining to pearly. 

 Colour, white, prevalent. Also different shades of 

 grey, red, green, yellow ; all of them pale. Dark 

 brown and black colours, owing to foreign ad- 

 mixtures. Streak greyish- white. Transpa- 

 rent ... translucent. Double refraction very 

 considerable, and easily observed. 



Brittle. Hardness 3-0. Sp. Gr. = 2-721, a 

 transparent crystal of the form var. 13. 



Compound Varieties. Twin-crystals. 1. Axis 

 of revolution perpendicular, face of composition pa- 

 rallel to R oo, as in Fig. 129. and 135.*, parallel 

 toR 1, as in Fig. 130. and Fig. 133., parallel to 

 R, as in Fig. 132., or parallel to R + oo, as in Fig. 



This figure represents a crystal from Alstonmoor, in 

 the collection of Mr ALLAN, in which the substance of the 

 "individuals is continued beyond the face of composition. H. 



