MINERALOGY* 3* 



SECT. XXV: 



E. Calcareous Earths blended with an argilla- 

 ceous earth, Terra c ale are a argilla in time 

 mixta. Marie, Marga. 



1. When crude, it makes an effervefcence 

 with acids, but 



2. Not after having been burnt ; by which 

 operation it is obferved to harden, in pro- 

 portion as the clay exceeds the calcareous 

 iubftance. 



3. It eafily melts by itfelf into a glafs, and 

 even when it is mixed with the moft re- 

 fractory clay. 



4. It is of great ufe in promoting the 

 growth of vegetables, fince the clay tem- 

 pers the drying quality of the calcareous 

 earth. 



5. When burnt in a calcining heat, it rea- 

 dily attracts water : and, expofed to the 

 air, in time, it falls into a powder. 



The varieties of this kind, worthy to 

 be taken notice of, depend on the differ- 

 ent quantities of each of their component 

 parts, and on the quality of the clay. I 

 fhall, for inftance, fpecify the following 

 examples. 



SECT. XXVI. 



1. Loofe and compact, Marga friabilis. 

 This dlffolves in water like common clay. 



r» r t'neir parts, fubjedl to the fame varieties as the other fpecie* 

 of lime-frone : and it is to be obferved, that a volatile a!cali 

 is commonly fuppofed to have entered into the composition of 

 the foetid ftones, though it has never yet been difcovered by 

 any experiment. 



a. Reddifh 



