12 



SOIL PHYSICS AND MANAGEMENT 



or even the coarse soil constituents because it has approached more 

 closely the limit of mechanical division. The others may be broken 

 down into smaller fragments by the agencies of weathering. 



1 



Fia. 4. A more advanced stage of weathering than Fig. 3. (Chamberlain and Salisbury, 

 Courtesy Henry Holt & Co.) 



Fio. 5. "Capitol Rock," Butte, Montana. The different levels are due to varying hard- 

 ness of the rock strata. 



I. PHYSICAL AGENCIES. 



(a) Heat and Cold. In general, substances expand when 

 heated and contract when cooled. This is true of rocks. They are, 

 however, poor conductors of heat and the high temperature of the 



