GEOLOGICAL WORK OF THE ATMOSPHERE 107 



of rock, scores and even hundreds of pounds in weight, 1 are some- 

 times detached in this way, and started on their downward course, 

 but small pieces of rock are of course much more commonly 

 broken off than large ones. 



The sharp peaks which mark the summits of most high mountain 

 ranges (Fig. 65) are largely developed by the process here outlined. 

 The altitude at which the serrate topography appears varies with 

 the latitude, being as a rule, higher in low latitudes and lower in 

 high. But even in the same latitude it varies notably with the 

 isolation of the mountains and the humidity of the climate. Thus 

 within the United States the sharply serrate summits (Fig. 67) 

 appear in Washington and Oregon at altitudes of 6,000 to 10,000 



Fig. 67. Serrate moutain tops. Cascade Range, Washington. (Photo. 



by Tolman.) 



feet, while in the isolated Wichita range of Oklahoma, much farther 

 south in a drier climate, the same sort of topography is developed 

 at altitudes of 2,500 to 3,000 feet. 



Even in low latitudes and moist climates the effects of temper- 

 ature changes are often seen. For example, thin beds of lime- 

 stone at the bottoms of quarries are sometimes so expanded by the 

 heat of the sun as to arch up and break. 



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 be- 



1 Livingstone has reported that the temperature of rock surfaces in 

 Africa sometimes reaches 137 Fahr. during the day, and cools sufficiently 

 at night to split off blocks of 200 Ibs. weight. 



