Siliceous Genus. 137 



II. VARIETY* 



Compact Lavas. 



Thefe have undergone the ad or more per- 

 fect degree of fufion, yet they are not entirely 

 dcftitute of cavities which contain finer cry- 

 ftals, or pieces more perfectly vitrified ; their 

 colour is black or brown ; their fracture is ftill 

 obfcure, and not glafly, as the ftones them- 

 felves are opake : if not cracked, they give 

 a clear found when ftruck. 



The proportion and quality of their con- 

 ftituent parts are the fame as in the foregoing 

 variety. The ufual fluxes attack them with 

 difficulty, and microcofmic fait has fcarce any 

 power over them. 



III. VARIETY, 

 Vitreous Lavas. 



Thefe have beert more completely melted, 

 and form vitrifaftions of different colours^ 

 generally black or afh-coloured, rarely blue 

 or greenifh, 



A fpecimen of this glafs, examined by- 

 Mr. Bergman, afforded 49 per cent, of filex f 



35 



