6 SOIL PHYSICS AND MANAGEMENT 



nearly all soils giving the characteristic iron color, the former im- 

 parting a yellowish or brownish yellow color while the latter gives 

 a decidedly reddish color. Varying proportions of these mixed 

 together give many shades of red, brown and yellow. 



12. Magnetite (Fe : ,0 4 ) or (FeO. Fe 2 3 ). Magnetic iron ore 

 or magnetite exists in nearly all igneous rocks -in small quantities 

 but in some in sufficient amounts to form a very important soil con- 

 stituent. It does not decompose very readily, but remains as black 

 magnetic particles in the soil. Black sands of some parts of North 

 Carolina and some of the alluvial soils of California contain this 

 mineral. It may be easily recognized by its magnetic properties. 

 Like quartz sand it is inert and soils formed largely of this mineral 

 would be very poor. 



ROCKS 



Rocks are masses of minerals or mineral aggregates and are 

 divided into three classes, igneous, those formed through the agency 

 of heat, aqueous, those formed through the agency of water, and 

 metamorpliic, where igneous or aqueous rocks are changed through 

 one or both of these agencies into different forms, but having prac- 

 tically the same chemical composition. 



1. Igneous rocks are divided into two groups, first, intru- 

 sive or plutonic, those formed at considerable depth in the earth's 

 crust where they cooled with sufficient slowness to crystallize, and 

 later exposed through erosion; second, eruptive or volcanic, those 

 thrown out on the surface of the earth through volcanic agencies. 

 Both classes of igneous rocks are formed by the fusing and mixing 

 of rocks, such as limestones, shales and sandstones, due to the heat 

 developed in the folding of the earth's crust in its adjustment to 

 the shrinking interior. After the adjustment takes place, this 

 molten mass gradually cools, the minerals crystallize, forming the 

 group of crystalline rocks. In this folding, if a fracture should 

 occur extending to the surface of the earth, much of this molten 

 mass may be forced out on the surface and constitute the volcanic 

 rocks. This may cool rapidly and solidify into a glassy or semi-crys- 

 talline condition. In some cases the violence of the explosion that 

 frequently accompanies volcanoes throws immense masses of this 

 material into the air, which falls in the form of ash in the vicinity 

 of the volcano, but sometimes as dust is carried over large areas of 

 the earth's surface by air currents. The igneous rocks are divided 

 into several groups according to their mineral composition. 



