120 PHYSIOGRAPHY. CLASS II. 



streaked ; that of P r + 1 parallel to the edges of 

 combination with P. 



Lustre vitreous, inclining to resinous. The latter 

 more distinct in the fracture. Colour white, ge- 

 nerally yellowish, approaching to orange-yellow ; 

 sometimes passing into various shades of grey. 

 Streak white. Semi-transparent ... translucent. 



Brittle. Hardness = 3-0 . . . 3-5. Sp. Gr. = 4-301, 

 a white semi-transparent cleavable variety. 



Compound Varieties. Twin-crystals : axis of re- 

 volution perpendicular, face of composition parallel 

 to a face of P + x. The individuals continued 

 beyond it, as in the preceding species. Globular, 

 tuberose, reniform, botryoidal shapes : surface 

 rough, uneven, and drusy ; composition granular, 

 often strongly coherent. Massive : composition 

 either granular, or columnar ; more frequently the 

 latter. Sometimes multifarious composition. 



OBSERVATIONS. 



1. According to BUCHOLZ, the di-prismatic Hal-baryte 

 consists of 



Baryta 79-66. 



Carbonic Acid 20-00. 



Water 0-33. 



Its chemical formula is Ba C 2 , which corresponds to 77'66 

 of baryta, and 22-34 of carbonic acid. Before the blow- 

 pipe it decrepitates slightly, and melts easily into a trans- 

 parent bead, which loses its transparency on cooling. It 

 is soluble with effervescence in dilute nitric or muriatic 

 acid. 



2. It occurs in veins traversing limestone, which rests 

 upon red sandstone, and alternates with sandstone, slaty 

 clay and coal seams, accompanied by prismatic Hal-ba- 



