Siliceous Genus. 133 



SPECIES XVIII. 



Siliceous Earth mixed <with 0,3 of its 'weight 

 of Ar gill) 0,17 of mild Calcareous Eartb> 

 0,04 of Magnejiai and 0,5 of Iron. 



Bafaltes, Trapp of the Swedes^ Saxum Dane- 

 morenfe. 



A dark grey or black ftone, generally in- 

 verted with a ferruginous cruft and cryfta- 

 lized in opake triangular or polyangular 

 columns, is called bafaltes ; that which is 

 amorphous, or breaks in large, thick, fquare 

 pieces, is called trapp. Their conftituent 

 principles, and relation to acids and fluxes, 

 are exaftly the fame. The texture of this 

 ilone is either coarfe, rough, and diftinft, or 

 fine and indifcernable. This latter fort is 

 often reddifh ; it is always opake, and moul- 

 ders by expofure to the air ; fome fpecimens 

 give fire with fteel very difficultly, though it 

 is always very compact ; fometimes it is 

 fprinkled over with a few minute fhining par- 

 ticles : its fpecific gravity is 3,000. 



When heated red hot, and quenched in 

 water, it becomes by degrees of a reddifh 

 brown colour : it melts perfe in a ftrong heat 

 into a compaft flag. Borax alfo diflblves it 

 in fufion, but mineral alkali not intirely. 



K 3 According 



