SOIL-FOBMING PROCESSES 17 



usually mineral aggregates, and these minerals vary 

 in their coefficients of expansion. With every change 

 of temperature, differential stresses are set up which 

 ultimately must produce a considerable effect. When 

 the separate minerals expand they expand differently, 

 and when they contract they never again assume quite 

 their former relationships to one another. Thus crevices, 

 cracks, and rifts are created in rocks, especially those of 

 heterogeneous mineral composition. The expansion coef- 

 ficient of granite is .0000048 of an inch to a foot for every 

 degree Fahrenheit, while that of marble is about .0000056 

 of an inch. This seems to be very slight, but it must be 

 remembered that under natural conditions large surface 

 areas of rock are concerned. A sheet of granite 100 feet 

 long will expand one-half an inch with a change of 75*^ 

 Fahrenheit, which is not an uncommon variation of 

 temperature in arid regions or high altitudes. This 

 leads to chipping, flaking, and exfoliation. The rock 

 fragments may range from microscopic sizes to large 

 blocks, which are often split off with great violence. 



14. Frost. — Great as is the action of a simple change 

 of temperature, its effects become many fold more ap- 

 parent when water is present. We then have the action 

 of frost. The cracks and crevices made by heat and cold 

 will in a humid region become filled with water. This 

 moisttire, on freezmg, exerts a very great force. The 

 expansive power of water passing from the liquid to the 

 solid state is equal to about 150 tons to a square foot, 

 which is equivalent to the weight of a column of rock 

 about a third of a mile in height. Moreover, most rocks 

 contain a certain amount of water in themselves. This 

 water is recognized in excavation operations as quarry 

 water. The passage of the quarry water to a solid state 

 c 



