74 Elements of Mineralogy. 



Fufible Clays. 



All clays, which, befides argillaceous and 

 iiliceous Earths, contain a mixture of calca- 

 reous or muriatic Earths, or gypfum, or fluor, 

 or felt fpar, or iron, are more or lefs fufible, 

 according to the proportion of thefe foreign 

 ingredients. 



Apyrous Clays. 



Clays that confift of argillaceous and fill- 

 ceous Earths fingly, without any mixture of 

 iron or other metallic fubftance, or at leaft 

 only in an inconfiderable proportion, are in- 

 fufibl 1 . Such is that found near Plombiere. 

 Mem. Par. 1778, p, 433. 



Argillaceous Marl, 



Any clay that contains from 20 to 50 per 

 cent, of mild calcareous Earth, comes under 

 this denomination. It differs prodigioufly in 

 point of hardnefs and colour : the hardeft is 

 called hthomarga^ or ft one marl. The beft 

 for agriculture is the grey or white ; the co-* 

 loured generally contain metallic particles : 

 when it contains 20 or 30 per cent, of calca-- 

 reous Earth it is fufible perje. The coloured 

 as they contain more iron, are ftill 



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