Siliceous Genus. 1 1 3 



inward protuberances. The occidental is of 

 various colours, and often veined with quartz 

 orjafper; it is moftly found in fmall pieces 

 covered with a cruft, and often running in. 

 veins through rocks, as flints and petro-filex, 

 to which it is analogous, but more tranfparent, 

 the Earths being better combined and the 

 grain clofer ; its fpecific gravity is 2,64 ; it 

 often forms the ftony matter of petrifactions. 



Agates that prefent arborifations are called 

 mochoes. 



Alkalis, borax and microcofmic fait affect 

 this ftone in the fame manner as they do 

 flints. 



II. VARIETY. 



Opa/ 9 Oculus Mundi) Lapis Mutabilis. 



This ftone is of different colours, white, 

 yellowifh, or greeniih brown ; its peculiar 

 characteriftic is to reflect different colours, 

 according to the different pofitions of the eye ; 

 it is moftly of a fpungy texture, and admits 

 water into it, and generally too foft to ftrike 

 fire with fteel; its fpecific gravity is from 1,7 

 to 2,24. 



The ufual fluxes act on it as on flints. Mr. 

 Bergman has extracted argill from it. 



I Pfeudofal 



