72 Elements of Mineralogy. 



hardnefs of chalk, and like that, it marks 

 white : its hardnefs is nearly as that of ftea- 

 tites, and does not feel as fat as common clay 

 does : its fpecific gravity is 1,669 : * ts colour 

 is fnow white. When examined with a mi- 

 crofcope, it is found to confift of finall tran- 

 fparent cryftals ; and by his experiments it 

 plainly appears to be an argill faturated wi'h 

 fixed air. It effervefces with acids, and con- 

 tains a very fmall proportion of calcareous 

 Earth, and lometimes of gypfum, and fome 

 feeble traces of iron. It is found near Halle s. 

 15 Naturforfch. 209. 



SPECIES II. 



Combined but not faturated with Serial Acid, 

 in a loofe or femi-indurated form^ and mixed 

 with fine Quartz or Silex tn various propor- 

 tions, a fmall quantity oj Water, and gene- 

 rally of Iron. 



Clay, Creta of the Italians. 



This fpecies receives a number of different 

 denominations, arifing from its hardnefs, fu- 

 fibility or infuiibility, or the different ufes it 

 is applied to, or relative to its colour, or origin; 

 fuch as ftone clay or lithomarga, fufible or 

 apyrous clay, marl, Fuller's earth, pipe clay, 

 kaolin, brick clay, umber, coloured clays, 

 boles, pouzzolana, terras, tripoli, &c. All 



clays 



