248 DOMAIN III. ARGILLACEOUS. 



Texture, schistose. 



Hardness, gypsic. Fracture, sometimes straight, 

 sometimes waved. Fragments, laminar. 



Weight, from carbonose to granitose. 



Lustre, glimmering ; the glossy kind glistening. 

 Opake. 



Colour, greyish or bluish black. It is the black 

 slate celebrated by the vulgar for its medical pro- 

 perties. 



Aspect 1. Common. This is generally used in 

 the British manufactories of alum. The finest 

 specimens are from an old coal-mine near Glas- 

 gow, in Scotland. 



Alum slate, from the vicinity of Glasgow. 



The same, with some small appearances of the 

 alum. 



The same, more expanded or decomposed, with 

 beautiful fibres of alum, like amianthus. 



The bituminous shale of Kirwan*, though he 

 ranks it with alum slate, seems to belong to a dif- 

 ferent mode. Pyrites sometimes decompose to 

 alum, vitriolic acid being formed by the oxydised 

 sulphur, which, by exposure to air and moisture, 

 slowly re-acts on the argil, and forms alum. 



w 



* II. 19. 



