GEOLOGICAL STUDY OF SOILS 



69 



iron ; dolomite, a double carbonate of calcium and mag- 

 nesium; serpentine, a silicate of magnesium; and gypsum 

 calcium sulphate. All rocks and minerals are subject to 

 disintegration and change in chemical composition and 

 physical properties. The process of soil formation has 

 resulted in numerous chemical and physical changes. 

 These changes are still taking place, and as a result 

 plant food is constantly being made available. 



CHEMICAL COMPOSITION OF ROCKS n 



77. Value of Geological Study of Soils. Agricul- 

 tural geology is a valuable aid in studying soil prob- 

 lems, but like other sciences it is incapable alone of 

 solving all the problems of soil fertility. Means have 

 not yet been devised for accurately determining the 

 extent of rock disintegration and the rapidity with 

 which it has taken place or the degree to which dis- 



