6 SOILS 



effective, though often not as noticeable, as weather- 

 ing from other causes. The changes of temperature 

 from summer to winter, and even from the heat of 

 mid-day to evening, are sufficient to tear rocks to 

 pieces. Rocks are made of several or many dif- 

 ferent minerals, each of which expands and con- 

 tracts differently when subjected to heat or cold. 

 The result is that the rocks are cracked and split 

 from being pulled many ways. There are few 

 parts of the world where surface temperatures are 

 uniform for any length of time; hence nearly all 

 surface rocks, even the smallest stones, and espe- 

 cially those in the North, are being slowly pushed 

 and pulled to pieces by alternate expansion and 

 contraction. According to Shaler, a change of 

 temperature of 150 F., which is common in the 

 North between the extremes of summer and winter, 

 makes a granite rock 100 feet in diameter expand 

 one inch. 



In regions having great extremes of temperature 

 daily, particularly in Texas, Montana, Arizona, 

 and other parts of the West where rocks are sparsely 

 protected by vegetation, the splitting of rocks is 

 quite noticeable and is sometimes attended with 

 gun-like reports and cracking sounds loud enough 

 to be heard many rods. Livingstone states that 

 in South Africa blocks of stone weighing 200 pounds 

 are frequently split off during the night by the con- 

 traction due to the rapid fall or temperature. 

 Many people have noticed how pieces are 

 chipped off from the foundation stones of a 

 bunding that has burned. In most parts of eastern 

 United States, where the rocks are more or less 

 protected by vegetation, the cracking of rocks from 

 this cause is less noticeable; but it is certain that 

 all rocks everywhere are being affected more or 



