224 WALL'S MANUAL 



f>yj*r^ 



abundantly in many rocks which yield lime to soils 

 when decomposed. It is an element of the slate and 

 trap rocks. 



Feldspar. Feldspar contains a large proportion of 

 clay. It is the chief ingredient of porphysy, the 

 hardest and most enduring of all rocks. It is a 

 white or flesh-colored mineral, and by decomposition 

 furnishes potash and clay to soils ; also, the fine 

 clays for porcclian ware. 



Mica. Mica occurs in semi-transparent plates, 

 which may be split into elastic leaves of almost any 

 degree of thinness. It withstands fire, and is used 

 as a substitute for glass in the doors of stoves. It is 

 frequently called isinglass. Quartz, feldspar and 

 mica compose granite, which underlays all other rock 

 formations. Much of the Rocky Mountains, Andes 

 Alps, Pyrenese and all the highest mountains in the 

 world are granite. Upon many of the "silicates" the 

 car exerts a destructive agency. The carbonic acid 

 of the air slowly unites with their bases, thus break- 

 ing the bond which united their elements, and 

 combining with them. By absorbing into their 

 pores moistures which expands when freezing, they 

 are mechanically crumbled down. These -joint forces 

 are constantly active in disintegrating and wearing 

 down rocks and stones, and reducing them to the 

 condition of s&iL 



ARTIFICIAL SILICATES. 



Although not immediately connected with agricul- 

 ture, yet it may not be uninteresting to mention 

 some of the artificial silicates : 



Glass. The several kinds of glass are composed of 



