398 PHYSIOGRAPHY. CLAM II. 



Fundamental form. R =85*5^. Vol. I. Fig. 7. 

 R. G. 



a =. J 5-6. 



Simple forms. R co (a) ; | R 2 = 127 4(X ; 

 R 1 ( C ) = 115 8' ; R (JR) ; R + 1 (d) 

 = 68 42' ; P + I (b) - 128 1', 122* 28'. 



Char, of Comb. Hemi-rhombohedral with parallel 

 faces. Zi = 91 2(X. 



Combinations. 1. R oo. R. t^4 Fig. 141. 142. 



2. R QD. fR 2. R 1. R. ?I. 



3. R x. R 1. R. ^-\ R+l. Fig. 144. 



Irregular forms, grains. 



Cleavage, perfect parallel to R oo, less distinct 

 R, not always observable. Fracture conchoidal. 

 Surface, R 1 streaked parallel to its edges of 

 combination with R CD ; the other faces gene- 

 rally more rough than smooth, and all of them 

 alike. 



Lustre imperfect metallic. Colour dark iron-black. 

 Streak black. Opake. 



Brittle. Hardness = 5-0 ... 5*5. Sp. Gr. = 4-661. 



Compound Varieties. Twin-crystals : axis of re- 

 volution perpendicular, face of composition parallel 

 to R GO, Fig. 143. The compositions of this 

 kind, as hitherto observed, are not quite regularly 

 formed, but consist generally of several alternating 

 laminae. The situation of the individuals is, how- 

 ever, recognizable from the direction of their faces. 



