THE ATMOSPHERE AS A GEOLOGICAL AGENT, 



47 



and radiation, a rock surface becomes hotter in the day and cooler at 

 night beneath a dry atmosphere than beneath a moist one. Aridity 

 therefore favors the disruption of rock by changing temperatures. 



Turning from the conditions of the atmosphere which affect the dis- 

 ruption of rock to the conditions of the rock which influence the same 

 process, several points are to be noted. In the first place, the disrupt- 



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Fig. 29. — Top of Xotch Peak, Bighorn Mountains. Wyo. Shows the thoroughly broken 

 character of the rock on the summit, the absence of soil, vegetation, etc. (Kiimmel.) 







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Fig. 30. — A detail from Fig. 29 showing the size of the rock blocks. (Kiimmel.) 



ing 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 the loose material are coarse, 

 like bowlders, their surfaces are affected like those of larger bodies of 

 rock. The color of rock, its texture and its composition, also influence 

 its range of daily temperature by influencing absorption and conduction. 

 Dark-colored rocks absorb more heat than light-colored ones, and 

 compact rocks are better conductors than porous ones. Great absorp- 



