THE ATMOSPHERE AS A GEOLOGICAL AGENT. 49 



In view of these considerations the breaking of rock by changes 

 of temperature shoukl be greatest on the bare slopes of isolated eleva- 

 tions of crystalline rock, where the temperature conditions of temper- 

 ate latitudes prevail, and where the atmosphere is relatively free from 

 moisture. All these conditions are not often found in one place, but 

 the disrupting effects of changing temperatures are best seen where 

 several of them are associated (Figs. 29, 30, and 31). 



The importance of this method of rock-breaking has rarely been 

 appreciated except by those who have worked in high and dry regions. 

 Climbers of high mountains know that almost every high peak is covered 

 with broken rock to such an extent as to make its ascent dangerous 

 to the uninitiated. High serrate peaks, especially of crystalline rock^ 

 are, as a rule, literally crumbling to pieces (Fig. 31). The piles of 

 talus which lie at the bases of steep mountain slopes are often 

 hundreds of feet in height, and their materials are often in large 

 part the result of the process here under discussion. In mountain re- 

 gions where atmospheric conditions fa^'or sudden changes of tempera- 

 ture, the sharp reports of the disruption of rock masses are often heard. 

 Masses of rock, scores and even hundreds of pounds in weight, are fre- 

 quently thus detached and started on their do^mward course.^ Small 

 pieces of rock are of course much more commonly broken off than large 

 ones. The disruption of rock by changes of temperature is not usually 

 the result of a single change of temperature, but rather of many 

 successive expansions and contractions. 



The sharp needle-like peaks which mark the summits of most high 

 mountain ranges (Fig. 32) are largely developed by the process here 

 outhned. 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 with, the aridity of the climate. 

 Thus within the United States the sharply serrate summits appear in 

 some places, as in Washington and Oregon, at altitudes of 6000 to 10,000 

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

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

 developed at altitudes of 2500 to 3000 feet. 



^ Livingstone has reported that the temperature of rock surfaces in Africa some- 

 times reaches 137° Fahr. during the day, and cools sufficiently at night to split off 

 blocks of 200 lbs. weight. 



