406 PHYSIOGRAPHY. CLASS n. 



tic needle. Hardness = 5-5 ... 65. Sp. Gr. 

 = 5-251, a crystalline variety from Sweden. 



Compound Varieties. Twin-crystals: 1. Axis 

 of revolution perpendicular, face of composition par- 

 allel to It GO ; the individuals are continued be- 

 yond the face of composition (Altenherg, Saxony). 

 Sometimes two individuals in the same position are 

 joined in a face of li -f- oo, and terminate at this 

 face (Stromboli). 2. Axis of revolution perpendi- 

 cular, face of composition parallel to a i'ace of R, 

 generally observable in the reversed situation of 

 thin films engaged in the mass (Elba). Faces of 

 composition in these directions must not be con- 

 founded with faces of cleavage. 



Globular, reniform, botryoidal, and stalactitic 

 shapes : surface generally smooth, composition more 

 or less thin columnar, sometimes even impalpable ; 

 in this case the lustre becomes imperfect metallic, and 

 the colour red; fracture of impalpable compound va- 

 rieties even, flat conchoidal, or uneven. Compound 

 varieties often join in a second and third composi- 

 tion, which are curved lamellar and granular ; the 

 junction of granular masses produces frequently 

 very smooth faces, while the reniform surface of 

 the curved lamellar compositions is rough, and 

 more difficultly obtained by separating the par- 

 ticles than the first. Massive: composition, 1. Co- 

 lumnar, generally imperfect, thick, and diverging 

 from common centres. 2. Granular, and often 



