46 SOILS AND FERTILIZERS 



was embedded. The process of soil formation is a slow 

 one and the various agents have been at work for an 

 almost unlimited period. 



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 un- 

 equal cooling of the rocks caused a partial separation 

 of the different minerals of which the rocks were com- 

 posed, resulting in the formation of smaller rock par- 

 ticles from the larger rock masses. This is well 

 illustrated by the familiar splitting and crumbling of 

 stones when heated. Shaler estimates that a variation 

 of 150 F. will make a difference of i inch in the 

 length of a granite ledge 100 feet long. As a result of 

 changes in temperature there is a lessening of the 

 cohesion of the rock particles. The action of frost 

 also is favorable to soil formation. The freezing of 

 water in rock crevices results in breaking up the lock 

 masses, forming smaller bodies. The force exerted by 

 water when it freezes is sufficient to rend large rocks. 



50. Physical Action of Water. Water acts upon 

 soils both chemically and physically. It is the most 

 important agent that has taken a part in soil forma- 

 tion. The surface of rocks has been worn away by 

 moving water and in many cases deep ravines and 

 canons have been formed. This is called erosion. 

 The pulverized rock, being carried along by the water 

 and deposited under favorable conditions, forms 

 alluvial soil. This physical action of water is illus- 



