66 SOILS AND FERTILIZERS 



mainly insoluble in acids, and do not as a rule form 

 very fertile soils. 



70. Mica. Mica is quite complex in composition, 

 is an abundant mineral, and is composed of silica, iron, 

 alumina, manganese, calcium, magnesium, and potas- 

 sium. Mica is a polysilicate. The color may be white, 

 brown, black, or bluish green, owing either to the ab- 

 sence of iron, or to its presence in various amounts. 

 The chief physical characteristic of the members of this 

 group is the ease with which they are split into thin 

 layers. It is to be observed that the mica group con- 

 tains all the elements of both feldspar and hornblende. 

 Mica is quite resistant to chemical change. 



Soils formed from thoroughly disintegrated mica are 

 usually fertile, owing to the variety of essential elements 

 present. 



71. Granite is composed of quartz, feldspar, and 

 mica. It is a very hard rock and slow to disintegrate. 

 The different shades of granite depend upon the pro- 

 portion in which the various minerals are present. 

 Inasmuch as it contains so many minerals, it usually 

 follows that granite soil is fertile ; although when not com- 

 pletely disintegrated or when disintegrated and leached, 

 it is unproductive. Pure powdered granite before un- 

 dergoing disintegration furnishes but little plant food. 

 After weathering, the plant food gradually becomes 

 available. Granite varies in both physical and chemi- 



