Siliceous Genus. 103 



CHAP. VIIL 



Siliceous Genus. 



All the (tones I place under this genus 

 give fire with fteel, except opals, and yel- 

 low carnelians; and none effervefce with 

 acids, except Lapis Lazuli in powder, Bar- 

 fhoerl, martial muriatic fpar, and Turky 

 hone. 



SPECIES I. 

 Quartz, Cryjlal. 



The ftones of this fpecies are in general 

 the pureft of the filiceous genus, though moft 

 contain a flight mixture of other Earths; 

 the moft obvious diftindtion among them, 

 arifes from their tranfparency or opacity. 



SERIES I. 

 Tranfparent >uartz y Cryftal. 



This is either colourlefs and cryftalized irt 

 hexagonal pyramids, and then called moun- 

 tain cryftal, or in various other forms, or 

 amorphous. Its fpecific gravity is from 2,65 

 to 2,7; its texture lamellar and generally 

 ihattery, its appearance glafly ; it cracks and 

 lofes its tranfparency when heated ; it pof- 

 feffes all the other properties of pure filiceous 

 H 4 Earth. 



