GEOLOGY OF AGRICULTURE. 56 



2, alumina ; 3, lime ; 4, magnesia ; 5, potassa ; 6, soda ; 

 7, oxide of iron ; 8, oxide of manganese ; 9, water ; 

 10, carbonic acid." It should be observed that every 

 one of these binary compounds are formed out of the 

 fifteen simple elements heretofore described, under the 

 head of chemistry (Table I.): Every one of them is 

 a compound of oxygen with one other element, so 

 that only eleven of the elements enter into their com- 

 position. 



87. Perhaps, for some of my readers, a description 

 of the before-mentioned minerals may be needful. 



Quartz is of various colors, but generally almost 

 white ; and when crystallized, it is transparent, a hard, 

 flinty substance, composed almost wholly of silica, or 

 silicic acid (SiO^), known as flint, flinty stones and sand. 



Felspar exists in connection with quartz and mica 

 in granite, and may be distinguished from either by a 

 glossy fracture when broken, somewhat resembling 

 that of fine earthenware. 



Mica^ the third constituent of granite, is known ex- 

 tensively as isinglass, is of various colors, but more 

 commonly nearly colorless, divisible into thin, flexible 

 plates. 



HornUende is a common mineral, of various colors, 

 occurring sometimes massive, at others in crystals; 

 the crystals are sometimes short, but more generally 

 long and slender, blade-like, sometimes fibrous. 



Carbonate of Lime is a ternary compound, as its 

 name implies ; oxygen and calcium first uniting to 

 form lime (oxide of calcium), and then carbonic acid 

 uniting with lime to form the carbonate. It is knowa- 



