104 



GEOLOGY 



latitudes. (2) High altitudes favor great daily ranges of temper- 

 ature, so far as the rock surface is concerned, for though the rock 

 becomes heated during the sunny day, the thinness and dryness 

 of the atmosphere allow the heat to radiate rapidly at night. Here, 

 too, the daily range of temperature is likely to bring the wedge- 

 work of ice into play. Since the south side of a mountain (in the 



Fig. 64. Erosional forms characteristic of dry regions where erosion by the 

 wind is effective. Fissure Canyon, on the north slopes of the La Sal 

 Mountains, Utah. The rock is Permian. (Cross, U. S. Geol. Surv.) 



northern hemisphere) is heated more than the north, it is subject 

 to the greater daily range of temperature, and the rock on this side 

 suffers the greater disruption. Similarly, rock surfaces on which 

 the sun shines daily are subject to greater disruption than those 

 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 



