2O Elements of Mineralogy. 



may fay, that any proportion which produces 

 peculiar effects, or is the foundation of fome 

 particular ufe, is notable. When iron is found 

 in any Earth or Stone, in the proportion of 

 14 or 15 per cent, it commonly renders it 

 magnetic, either before or after torrefaction, 

 according to its ftate of phlogiftication ; and 

 if it be found in the proportion of 30 per 

 cent, or more, it gives the compound the de- 

 nomination of an ore. To ftones that con- 

 tain a greater quantity of iron than is effen- 

 tial to th^m in the pureft ftate, I add the de- 

 nomination martial, or ferruginous. 



When fmiple Earths, belonging to different 

 genera^ are mixed or combined with each 

 other, I generally place them under that 

 genus of which the compound contains the 

 largeft proportion, yet not always, for if 

 the compound poffefles the peculiar characters 

 of the component part, which is in a fmaller 

 proportion ; or if it attracts the attention, 

 and is fubiervient to the ufes of mankind, 

 merely on account of the lefs copious ingre- 

 dient, I range it under the genus of that in- 

 gredient. Thus, though common clay contains 

 much more of filiceous than of mere argillace- 

 ous Earth, yet, as it pofieffes fmoothneis, vitci- 

 dity, and foftnefs, in a high degree, it would 

 appear ridiculous to place it under the filice- 

 ous genus, whofe characters are the very re- 

 verfe. For the fame reatbn, I place the 



' precious 



