32 



PHYSICAL GEOLOGY 



u ally acquiring heat during the day is still warm when the surface is 

 cool and contracted. The contracted exterior is then too small for 

 the still expanded interior and the surface of the rocks tends to shell 

 off (Fig. 8), forming onion-like, concentric layers, the process being 

 known as exfoliation. It is stated that in certain parts of Africa the 

 rock temperature rises to a height of 137 F. during the day and falls so 

 rapidly at night as to throw off, by contraction, masses as much as 

 200 pounds in weight. Slabs of granite 8 to 10 inches thick and 10 

 feet long are known to have been broken ofF by changes in daily 



temperature. In the 

 western part of the 

 United States, where 

 the climate is too dry 

 to afford much scope 

 for the operation of 

 frost, cliffs are slowly 

 disintegrated by 

 these changes in tem- 

 perature, producing 

 talus slopes of large 

 size. 



The alternate heat- 

 ing and cooling of a 

 rock causes its disin- 

 tegration in still an- 

 other way. When a 

 rock is composed of 

 minerals differing in color and composition, it is especially liable to 

 disintegration by changes in daily temperature. Since dark-colored 

 minerals absorb heat more rapidly than light-colored ones and also 

 radiate it more quickly, rocks containing both expand and contract at 

 different rates, with the result that the grains are gradually loosened 

 until the surface is reduced to sand. The fact that the coefficients 

 of expansion of the various minerals differ widely also aids in the dis- 

 integration of the rock. Igneous rock, 1 composed of minerals of dif- 

 ferent kinds, is therefore more easily disintegrated by this process 

 than rocks made up of one mineral. 



Changes in daily temperature are especially effective in high alti- 



Fig. 8. — Exfoliated granite. (U. S. Geol. Surv.) 



■ Igneofifl rock are tho e which have been formed from molten masses by cooling. 

 page 320 for a disi 11 ion of these rocks. 



See 



