OKIGIN OF THE SOIL 



17 



cold, and gravity, and water influenced by these forces, are 

 the important agents. Variations in temperature have a 

 greater effect in weathering than is commonly supposed. 

 Under the rays of the noonday sun, exposed rocks rapidly 

 rise in temperature and expand; at night they as rapidly 

 cool and contract. This alternate heating and cooling is 



Photograph by II. L. llollister Land Co. 



FIG. 5. A crest in the Rocky Mountains showing rock fragments 

 split off by alternating heat and cold 



frequently sufficient to cause the splitting off of large flakes 

 weighing many pounds, with reports like pistol shots. The 

 different minerals in the rocks have their own rate of expan- 

 sion and contraction, and if these varying movements under 

 changes of temperature do not cause the actual splitting off 

 of particles of rock, they leave minute openings between 

 them into which water may penetrate and begin its work of 

 dissolution. Changes of temperature have little effect upon 



