Siliceous Genus. 139 



SPECIES XXI. 



Siliceous Eartb mixed 'with o, 1 1 or o, 1 7 of 

 its 'weight of Magnefia, and a little of 

 Calcareous Earth. 



Pumice-ftone y Pierre Ponce, Bimftein. 



This feems rather a volcanic ejection than 

 a volcanic product; its colour is grey, white, 

 reddifh brown, or black : it is hard, rough, 

 and porous, and confifls of {lender fibres pa- 

 rallel to each other ; very light, for it fwims 

 on water, and difficultly gives fire with fteel. 

 It feems to have originally been an afbeftos 

 decompofed by the action of fire. 



100 parts of it contain, according to Mr. 

 Bergman, from 6 to 15 of magnefia, with a 

 very fmall proportion of calcareous Earth, 

 the remainder filex. 



SPECIES XXII. 



Siliceous Earth mixed 'with lefs than its own 

 weight of Magnejia and Iron. 



Martial Muriatic Spar. 



Mr. Monnct, a Mineralogift of considerable 

 note, lately dilcovered this ftone at St. Ma- 

 rie aux Mines, and called ilfifoKtes} but as a 



very 



