MODE I. ALUM ROCK. 247 



STRUCTURE I. MASSIVE ALUM ROCK. 



Texture, granular; on a large scale somewhat Characters, 

 stratified. 



Hardness, gypsic. Fracture, earthy or uneven. 

 Fragments, amorphous, not sharp. 



Weight, carbon ose. 



Lustre, dull. Opake. 



Colour, greyish white, greyish yellow, or yel- 

 lowish white. Bergman found it to contain about 

 43 of sulphur and other volatile matter; 35 argil ; 

 22 silex ; and some iron. 



Mr. Kirwan says, that veins of this kind have 

 been discovered in Bretagne; but when he sup- 

 poses that the Roman alum also runs in veins, he 

 contradicts the ample account of Ferber, no in- 

 accurate observer, who formally and repeatedly 

 informs us that it is extracted from a rock consti- 

 tuting hills, and containing veins of quartz *. 



STRUCTURE II. ALUMINOUS SLATE. 



This substance abounds in many countries, characters, 

 Werner divides it into the common and the 

 glossy. 



* The name Roman alum is now also given to the finest, 

 wherever fabricated. 



