26 



GEOLOGIC WORK OF ATMOSPHERE 



much shielded by clouds. (3) The daily range of temperature is 

 also influenced by humidity, a rock surface becoming hotter by 

 day and cooler by night beneath a dry atmosphere than beneath 

 a moist one. Aridity, therefore, favors the disruption of rock by 

 changing temperatures. The color of rock, its texture, and its 

 composition also influence its range of daily temperature by in- 

 fluencing absorption and conduction. The disrupting effects of 

 changes of temperature are slight or nil where solid rock is pro- 

 tected by soil, clay, sand, gravel, snow, or other incoherent material. 

 In view of these considerations, the breaking of rock by changes 

 of temperature should be greatest on the bare slopes of isolated 

 elevations of rock, where the atmosphere is dry. All these 

 conditions are not often found in one place, but the disrupting 



Fig. 17. A mountain top, illustrating a common condition of the rock in 

 mountain peaks. 



