chemistry'of soil. 51 



2d. Silicates of alumina and lime with magnesia form 

 hornblende. 



3d. Silicate of magnesia forms serpentine and talc ; and 

 silica almost pure, is quartz. 



61. The iron and manganese in the table (59), are re- 

 garded as accidental mixtures of silicates of these metals. 

 Silicate of soda is often present in place of potash, and this 

 constitutes an extensive variety of the felspar family. 



