GEOLOGICAL STUDY OF SOILS 55 



are frequently found pieces which are rotten, and, when 

 crushed, closely resemble the prevailing soil of the 

 field. This is particularly true of clay soils where there 

 are fragments of disintegrated feldspar that when 

 crumbled are similar to the soil in which the feldspar 

 was embedded. The process of soil formation is ex- 

 tremely slow and the various agents have been at work 

 for an almost unlimited period. 



Weathering is the joint action upon rocks of the vari- 

 ous atmospheric agencies. Some rocks are more sus- 

 ceptible to it than others, and in different localities even 

 the same kind of rock may vary in the rapidity with 

 which it responds to weathering. 



55. 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 agencies. 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 parti- 

 cles 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 in cohesion of the rock 

 particles. The action of frost also is favorable to soil 

 formation. The freezing of water in rock crevices 



