OBDER v. RHOMBOHEDBAL TALC-MICA. 199 



Cleavage, R oo, highly perfect, and easily ob- 

 tained, passing in less perfectly formed varieties 

 into slaty structure. Traces of P + oo. Frac- 

 ture scarcely observable, uneven. Surface P 

 and P -f oo horizontally streaked, the other 

 faces, particularly R oo, smooth. 



Lustre pearly, often inclining to metallic upon 

 R QD ; the other faces, if they are smooth 

 enough, present a kind of lustre between vitre- 

 ous and adamantine. Colour various shades of 

 grey, generally passing into green, brown, and 

 black, also into white and red (particularly 

 peachblossom-red). Superficial tinges of pinch- 

 beck-brown. Streak white, grey. Transparent, 

 imperfectly... translucent on the edges. It is 

 less transparent in the direction of the axis 

 than perpendicular to it. There is also a dif- 

 ference of colours observable in these directions, 

 for instance, oil -green in the first, and liver- 

 brown in all the others. 



Sectile. Thin laminae are elastic. Hardness = 2-0 

 ...2-5. The acute edges of the laminae, however, 

 will sometimes scratch glass. Sp. Gr. = 2-949, 

 a greenish-black variety in large individuals. 



Compound Varieties. Globular forms, both 

 imbedded and implanted : surface of the latter 

 rough ; composition columnar, sometimes joining in 

 a second curved lamellar composition. Massive: 

 composition granular of various sizes of individuals ; 

 or also imperfect columnar, faces of composition 

 irregularly streaked and rough. 



