Siliceous Genus. io<j 



their fpecific gravity greater ; 3 d * that they 

 are always covered with an ochry cruft ; but 

 they break with conchoidal furfaces, as flints 

 do. Their colour is reddifh, yellow, black, 

 brown, or grey, or beautifully variegated. 

 They probably contain metallic particles. 



SPECIES III. 



Siliceous Earth intimately mixed 'with from 

 to -5- of its weight of Argill, and from T * T 

 to ,-*, of its 'weight of Calcareous. 



Petro-filcx, Chert. 



This ftone differs but little from the former 

 in external appearance, only it is duller and 

 lefs tranfparent ; it is found of all colours, 

 but generally dark blue, or yellowifli grey ; 

 it breaks with conchoidal furfaces like the 

 former, but is generally fofter ; it runs in 

 veins through rocks, and hence derives its 

 name; its fpecific gravity is from 2,59 to 

 2,7 : in fire it whitens and decrepitates like 

 filex, but is generally more fufible, for it 

 commonly melts per fe\ it is not totally dif- 

 folved in the dry way by mineral alkali, but 

 borax and microcofmic fait diflblve it without 

 effervefcence. 



From a reddifh petro-filex ufed in the 

 Count dc Lauragais Porcelain Manufactory, 



and 



