GEOLOGICAL WORK OF THE ATMOSPHERE 105 



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

 composition also influence its range of daily temperature, by in- 

 fluencing absorption and conduction. In general, crystalline 

 rocks are more subject to disruption by this means than sedimen- 

 tary rocks, partly because they are more compact, and partly 



Fig. 65. A mountain top, showing a common condition of the rock in the 



mountain peaks. 



because they are made up of aggregates of crystals of different 

 minerals which, under changes of temperature, expand and con- 

 tract at different rates, while the common sedimentary rocks are 

 made up largely of numerous particles of one mineral. 



The disrupting effects of changes of temperature are slight or 

 nil where the solid rock is protected by soil, clay, sand, gravel, 

 snow, or other incoherent material. If the constituent parts of 



