Siliceous Genus: 143 



So alfo that of Bro<wnedge> in Stafford/hire* 

 which is fo perfect a fand-ftone that its fpe- 

 cific gravity is only 2,397; an< * t ' ie w hitnli 

 grey freeftone from Uttoxeter. All thefe con- 

 tain a little of iron. 



Stones of this fpecies are ufed for whetting 

 tools, and for filtring water, and in fome 

 countries as flates. The principal diftinclion 

 among them arifes from the finenefs or coarfe- 

 nefs of their grain and texture. Their colours 

 are various, receiving different tinges from 

 iron. 



SPECIES XXVI. 



Siliceous Sand confolidated by femipblog ifticatect 

 Calx of Iron. 



This ftone does not fall into fand when 

 powdered, and in this refpedt differs from 

 the foregoing. It is generally of a brown or 

 black colour, but grows reddifh or yellowifh, 

 and moulders by expofure to the air j its fpe- 

 cific gravity is from 2,8 to 3,6 ; it gives fire 

 with fteel, and does not effervefce with acids, 

 unlefs it contains teftaceous particles, as it 

 frequently does : it is even often covered with 

 fliells. 



The agglutinating power of folutions of 

 iron has been fhewn in an ingenious paper in 



the 



