40 



HOW EOCKS AND CLIMATE AFFECT SOILS 



A Rock is an Aggregate of Mineral Particles. — On examining 

 rocks closely we find them composed of mineral particles massed 

 or cemented together.^ In some rocks, as sandstone, the particles 

 are mostly of the same kind. In others they are of different kinds. 

 In a granite, for example, the dissimilar particles may be easily 

 distinguished by differences in hardness, color and crystal form. 

 They are called "rock-forming minerals." 



Rock-f onning Minerals of Many Kinds. — We shall not attempt 

 to study all the many kinds of minerals of which rocks are com- 

 posed, but only the more common and important ones as are given 

 in the following table: 



The Common and Important Rock Minerals 



Minerals 



Feldspars 



Quartz 



Micas 



Hornblende, angite, etc. 



Calcite 



Dolomite 



Apatite 



Hematite 



Pyrite 



Gypsum 



>aU others. 



Relative 

 abundance 

 in crust of 

 earth, % 



Elements of which they are 

 composed (p. 30) 



Si, Al, Na, Ca, K, 0. 



Si, 0. 



Si, Al, Na, Ca, Fe, Mg, K, 0. 



Si, Al, Na, Ca, Fe, Mg, 0. 



Ca, 0, C. 



Ca, Mg, 0, C. 



Ca, P, 0, Fl. 



Fe, 0. 



Fe, S. 



Ca, S, O. 



A study of the chemical composition of these rock minerals 

 will make clear to us the source of the mineral elements necessary 

 for plant growth. 



No nitrogen is found in any of these minerals. 



Quartz particles are the grains of which sandstones are 

 mostly formed. 



Why Some Soils Are Deficient in Some of the Important 

 Mineral Elements. — ^A sandy soil composed of quartz sand, or 

 derived from pure sandstone would naturally contain only a very 

 small amount of the important mineral elements. Why? 



Soils composed of mineral particles derived from rocks contain- 

 ing such minerals as feldspars, hornblende, micas, apatite, etc., 

 are usually well supplied with the mineral plant-food elements. 



The fact that peat soils do not contain any appreciable amount 



Lava or glassy rocks are exceptions. 



