GEOLOGICAL STUDY OF SOILS O? 



cal composition, and some disintegrates more readily 

 than others. Gneiss belongs to the granite series, but 

 differs from true granite in containing a large amount 

 of mica. Mica schist contains a larger amount of mica 

 than gneiss. 



72. Zeolites. The zeolites are a large group of sec- 

 ondary or derivative minerals formed from disintegrated 

 rock. They are polysilicates containing alumina and 

 members of the alkali and lime families, and all contain 

 water held in chemical combination. They are partially 

 soluble in dilute hydrochloric acid and belong to that class 

 of compounds which are capable, to a certain extent, of 

 becoming available as plant food. In color, they are white, 

 gray, or red. Zeolites are quite abundant in clay and are 

 an important factor in soil fertility. It is this group of 

 hydrated silicates which takes such an important part in 

 the process of fixation. The zeolites, when disintegrated, 

 particularly by glacial action, form very fertile soils. 



73. Apatite or Phosphate Rock. Apatite is com- 

 posed mainly of phosphate of lime, Ca 3 (PO 4 ) 2 , together 

 with small amounts . of other compounds, as fluorides 

 and chlorides. It is generally of a green or yellow 

 color and is present in many soils, but is of little value 

 as plant food unless associated with organic matter and 

 soluble alkaline salts. 



74. Kaolin is chemically pure clay and is formed by 

 the disintegration of feldspar. When feldspar is de- 



