Siliceous Genus. 151 



the quartz remained unaltered; but when all 

 three are powdered and mixed, it melts 

 fomewhat better, yet ftill the quartz may be 

 diftinguifhed by the help of a lens Gerh. 

 Gefch. 51. This well explains why fmall 

 white grains are frequently found in lavas. 

 Mr. Jy Arcefs experiments coincide with this, 

 and alfo thofe of Mr. Sauffurc^ p. 124. 



V. VARIETY. 



Quart %i Fdtfpar^ Shoerl and Mica. 



This is alfo a fort of granite obferved by 

 Mr. Sauffiire in the Alps. 



VI. VARIETY. 

 Jade> Shoerl, and Mica. 



A fort of granite obferved by Mr. Sauffwe. 



VII. VARIETY. 



Quartz, Garnet^ and Mica* Norka t Murkfteln. 



This is either grey or reddifh, and ufed 

 for mill- (tones, the mica is foon wore off, 



VIII. VARIETY, 



Porphyry. 



Under this name I comprehend with Mr. 

 all thofe ftones which in a compact 

 L 4 fiUceous 



