112 AMERICAN MANURES. 



Phosphorite, Slate and Sandstone. The relative 

 value of these rocks in soil formation can be 

 seen when we examine the nature of their con- 

 stituents. 



Granite belongs to the most ancient family of 

 rocks, which appear to have originally formed 

 the basis of the solid structure of the globe. 

 This mineral derives its name from its marked 

 granular structure, and is a mixture, in variable 

 proportions, of quartz, felspar, and mica. These 

 grains vary considerably in size ; in some varie- 

 ties, the crystals are in uniform small grains, 

 while in other varieties the laminae of mica are 

 some inches across. 



Quartz, which forms the transparent grains 

 in the granite, consists simply of silica (silicic 

 acid). 



Felspar, the dull cream-colored opaque grains 

 in granite, generally contains silica, alumina, 

 potash, soda and lime. 



Mica, so named from the glittering scales 

 which it forms in the rock, is a compound of 

 silica, alumina and potash ; but in some varieties 

 the alumina is displaced by the peroxide or rust 

 of iron, and the potash by magnesia. 



. By the long continued action of the atmos- 

 phere in connection with rain, the granite rock 

 is gradually crumbled down and disintegrated, 

 an effect due to both mechanical and chemical 



