50 



GEOLOGY. 



Even in low latitudes and moist climates the effects of tempera tm-e 

 changes are often seen. Thin beds of limestone at the bottom of quarries 

 are sometimes so expanded by the heat of the sun as to arch up and 

 break.^ In desert and arid regions,^ whatever the altitude, the effects 

 of temperature changes are often striking. 



Fig. 32. — Serrate peaks of granitic rock in Black Hills. (Darton, U. S. Geol. Surv.) 



The disruption of rock by changes of temperature is one phase of 

 weathering. It tends to the formation of a mantle of rock waste, which, 

 were it not removed, would soon completely cover the solid rock beneath 

 and protect it from further disruption by heating and cooling; but 

 the loose material thus produced becomes an easy prey to running 

 water, so that the work of the atmosphere prepares the way for that 

 of other eroding agencies. 



//. Evaporation and Precipitation. 



Perhaps the most important work of the atmosphere as a dynamic 

 agent lies in its function as the medium for the circulation and distribu- 



» Buckley. Surv. of Wis., Bull. IV, pp. 19, 20. 



' For an excellent discussion of erosion in dry regions see Walther's Die Denudation 

 in die Wiiste. 



