Siliceous Genus. 127 



lour, whioh is a bluifh or yellowifli red, its 

 tranfparency is often obfcure, except it be 

 held to a ftrong light : it is generally cry- 

 ftalized in various polygon forms, but it is 

 frequently amorphous : its texture is granu- 

 lar ; its fpecific gravity from 3,6 to 4,188. 



It melts per fe, though difficultly, into a 

 black flag. Mineral alkali does not attack it 



fo powerfully as borax or microcofmic fait. 



< 



It retains its colour in a ftrong calcining 

 heat. 



It is often found in fmall grains, imbodied 

 in ftones of the commoner]; kind. 



100 parts of it contain, according to Mr. 

 ) 48,3 of filiceous Earth, 30 of argill, 

 11,6 of calcareous Earth, and 10 of iron. 



Opake garnets, over-loaded with iron, 

 fcarcely give fire with fteel, are fometimes 

 cryftalized and fometimes amorphous, and 

 either red, yelloxvifh, or blackifh, They af- 

 ford about 20 per cent, of iron. Sometimes 

 they contain tin and even lead, but very rare- 

 ly. 2 Bergm. 106. 



SPECIES XV. 



