40 



PHYSIOGRAPHY. 



Mica. 



Fig. 300. 



Fig. 301. 



The inclination of P to the lateral planes is generally 90 ; in a few 

 cases, it is 96<V or a little above. When 90, the planes M do not ap- 

 pear upon these crystals. Color dark greenish or bluish grey. Cleav- 

 age parallel with P Fracture uneven. Sp. gr. = 2-893, of a crystal 

 from Lancaster. Emits an argillaceous odor when moistened. 



2. Before the blow-pipe, several varieties first lose their transparency, 

 and then melt into a scoria, or into a glass, white or colored, or even 

 black. Others are infusible, and they show in general as much differ- 

 ence in this respect, as in their composition. 



