Fossils. 147 



a greenish cast, a fibrous striated tex- 

 ture, and a silky lustre. Among them we 

 may mention talc, steatite, serpentine, 

 chlorite, asbestos, &c. Pure magnesia 

 does not form with water an adhesive duc- 

 tile mass. It is in the form of a very 

 white spongy powder and perfectly 

 tasteless. 



Stones are formed by the mixtures of 

 the earths together, and of the mixtures of 

 earths with alkalies, and sometimes with 

 acids. Stones bound together by some 

 cement, form rocks. There is also a kind 

 of stones of a peculiar nature produced by 

 the fire of volcanoes. 



The stones in which the acids and al- 

 kalies abound are called saline stones, and 

 the mixtures of the earths with each oth- 

 er form stones properly so called. Of 

 stones properly so called, those in which 

 the siliceous earth abounds and predom- 

 inates are very numerous ; the principal 

 of which we shall briefly notice. 



Siliceous mixtures have sufficient hard- 

 ness to strike fire with steel. Of this de- 

 scription are the precious stones, rock crys- 

 tat, or quartz, felspar, silex, crysopryse^ 

 lapis lazuli, jasper, and schorl* 



