ORDER VII. RHOMBOHEDRAL TOUJiMALIXE. 351 



Cleavage. R, P -f GD, difficult. Fracture imper- 

 fect conchoidal, uneven. Surface, the prisms 

 deeply striated parallel to the axis ; the rest of 

 the faces generally smooth, and of pretty much 

 the same physical quality. 



Lustre vitreous. Colour, brown, green, blue, red, 

 white, frequently black ; generally dark, and 

 scarcely ever bright. Streak white. Transpa- 

 rent ... almost opake, agreeably to die colour. 

 Less transparent if viewed in a direction paral- 

 lel to the axis, than perpendicular to it, and ge- 

 nerally different colours in these directions. 

 Hardness = 7-0 ... 7-5. Sp. Gr. = 3-076, of a 

 deep pistachio-green ringstone. 

 Compound Varieties. Massive : composition sel- 

 dom granular, of various sizes of individuals; ge- 

 nerally columnar, of various sizes of individuals, 

 often very thin, straight, and parallel or divergent ; 

 sometimes again aggregated into larger granular or 

 wedge-shaped masses ; faces of composition smooth 

 and longitudinally streaked. 



OBSERVATIONS. 



1. Tourmaline and Schorl, which have been formerly dis- 

 tinguished as two particular species, differ in nothing but 

 their colours and transparency. The varieties of green, 

 blue, red, brown or white colour, in general, such as are not 

 perfectly black, though of occasionally very dark tints, and 

 not absolutely opake, form Tourmaline, while the perfectly 

 black and opake ones are comprised within the name of 

 Schorl. Yet even among the latter varieties there are some 

 which are translucent in thin splinters. 



