46 



GEOLOGY. 



altitude, and humidity. Other things being equal, the greatest daily 

 ranges of temperature occur in high-temperate latitudes, though to 

 this general statement there are local exceptions, depending on other 

 conditions. High altitudes favor great daily ranges of temperature, 

 so far as the rock surface is concerned (see Figs. 29, 30), for though 



Fig. 28. — Exfoliation on a mountain slope. Mt. Starr-King (Cal.) from the north. 



the rock becomes heated during the sum^iy day, the thinness and dry- 

 ness of the atmosphere allow its 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 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. 

 Isolated peaks, because of their greater exposure, are subject to 

 rather greater daily ranges of temperature than plateaus of the same 

 elevation. 



The daily range of temperature is also influenced by humidity. 

 Because of the effect of water vapor in the atmosphere on insolation 



