310 PHYSIOGRAPHY. CLASS u. 



ticularly in the Scottish varieties. Fracture more 

 or less perfectly small conch oidal, uneven. Sur- 

 face, P oo rough, sometimes faintly striated 

 parallel to the edges of combination with 

 (Pr -f oo) 5 . The vertical prisms always striated, 

 sometimes deeply, parallel to their common edges 

 of combination. The pyramids and horizontal 

 prisms smooth. 



Lustre vitreous. Colour, white, yellow, green, 

 blue. Various, but generally pale shades. Streak 

 white. Transparent ... translucent, sometimes 

 only on the edges. 



Hardness = 8-0. Sp. Gr. = 3-499 of a transpa- 

 rent crystallised variety ; = 3*494, of the colum- 

 nar compositions of Pycnite. 



Compound Varieties. Massive: composition gra- 

 nular, of various sizes of individuals ; faces of com- 

 position rough. There occurs also columnar com- 

 position, the individuals being thin, long and paral- 

 lel, and easily separated, and their faces of compo- 

 sition longitudinally streaked. 



OBSERVATIONS. 



1. However easily the different varieties of prismatic 

 Topaz may be found out and distinguished from the rest of 

 minerals, if we attend to their characteristic properties, yet 

 this has only succeeded after several unsuccessful attempts, 

 and even now it forms in the Wernerian system three dif- 

 ferent species, though the varieties of rhombohedral Eme- 

 rald, and of rhombohedral Tourmaline, which formerly 

 were united with it, have at last been referred to their 

 own peculiar species. Topaz contains crystalline varieties 



