50 SOILS AND FERTILIZERS 



49. Action of Heat and Cold. — The cooling of the 

 earth's surface, followed by a contraction in volume, 

 resulted in the formation of fissures which exposed a 

 larger area to the action of other agents. The unequal 

 cooling of the rock caused a partial separation of the 

 different minerals, resulting in the formation of smaller 

 rock particles from larger rock masses. This is well 

 illustrated by the familiar splitting and crumbling of 

 many stones when heated. The action of frost upon 

 rock is also favorable to soil formation. The freezing; 

 of water in rock crevices results in breaking up the 

 rock masses, forming smaller bodies. The force ex- 

 erted bv water when it freezes is sufficient to detach 

 large rocks. 



50. Action of Water. — Water acts upon soils both 

 chemically and physically. In its physical action, 

 water has been the most important agent that has 

 taken a part in soil formation. The surface of rocks has 

 been worn away by moving water and in many cases deep 

 ravines and canons have been formed ; the pulverized 

 rock, being carried along by the water and deposited 

 under favorable conditions, forms alluvial, soil. This 

 is illustrated in the workings of large rivers where the 

 pulverized rock masses are deposited along the river 

 and at its mouth. A large portion of the soil in 

 valleys and river bottoms has been deposited by water. 

 The action of water is not alone confined to the forma- 

 tion of soils along water courses, but is equally impor- 



