ACTION OF THE ATMOSPHEEE 161 



tiiring of the masses from the parent ledge is sometimes attended 

 with gunJike reports snffieiently loud to be heard at a consid- 

 erable distance. H. von Streeruwitz states^ that the rocks of the 

 Trans Pecos (Texas) region undergo a very rapid disintegration 

 from diurnal temperature variations, which here amount to 

 from 60° to 75° Fahr. '^I frequently observed on the heights of 

 the Quitman Mountains a peculiar crackling noise and occasion- 

 ally loud reports, . . . and careful research revealed the fact 

 that the crackling was caused by the gradual disintegration and 

 separation of scales from the surface of the rock, and the loud 

 reports by crackling and splitting of huge boulders." The 

 scales thus split off vary in thickness from one-half to four 

 inches, and their superficial area from a few square inches to 

 many feet. This form of disintegration is necessarily confined 

 to slopes unprotected by vegetation, and is the more pronounced 

 the greater the diurnal variations. 



In Arabia Petrea, according to Marsh/ ''when a wind pow- 

 erful enough to scour down below the ordinary surface of the 

 desert and lay bare a fresh bed of stones is followed by a sudden 

 burst of sunshine, the dark agate pebbles are often cracked and 

 broken by the heat." According to Livingstone, the rock tem- 

 peratures in certain parts of Africa, on the immediate surface, 

 rise during the day as high as 137° F. and at night fall so rap- 

 idly as to throw off by their contraction sharp, angular masses 

 in sizes up to 200 pounds' weight. Stanley, in his reports, 

 is inclined to lay considerable stress on the effects of cold rains 

 upon the heated rock surfaces, though it is doubtful if this is 

 as powerful an agent as his descriptions would give us to under- 

 stand. (See further under action of water.) Throughout the 

 desert regions of lower California, as observed by the writer, 

 the granitic and basic eruptive rocks subject to very little 

 rainfall, and hence almost completely bare of vegetation, under 

 the blistering heat of the desert sun have weathered down into 

 dome-shaped masses, their debris in the form of angular bits of 

 gravel being strewn over the plain. Particles of this gravel, 

 when compared with those which are a product of chemical 

 agencies, are found to differ in that each, however friable, is a 

 complex molecule of quartz, feldspar and mica or other mineral 

 that may have composed the rock from which it was derived. 



1 4tli Ann. Eep. Geol. Survey of Texas, 1892, p. 144. 

 2 The Earth, as modified by Human Action, p. 552. 

 12 



