712 STUDIES FOR STUDENTS 



covered with blocks of stone which have been separated from 

 the mass beneath by just this process.^ 



It is natural that this form of disintegration should be most 

 pronounced in massive, close-grained rocks. In regions of great 

 extremes of daily temperature, the rupturing of these masses 

 from the parent ledge is frequently attended by gun-like reports 

 sufficiently loud to be heard at a considerable distance. H. von 

 Streeruwitz states^ that the rocks of the Trans Pecos (Texas) 

 region undergo a very rapid disintegration from diurnal temper- 

 ture variations, which here amount to from 6o° to 75° F. He 

 says, "I frequently observed in summer, as well as in winter 

 time, on the heights of the Quitman Mountains, a peculiar crack- 

 ling noise, and occasionally 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 of crackling and splitting of 

 huge bowlders." The scales thus split off, he says, vary in 

 thickness from one-half to four inches, and their superficial area 

 from a few square inches to many feet. This form of disinte- 

 gration is necessarily confined to slopes unprotected by vegeta- 

 tion, and is the more pronounced the greater the diurnal vegeta- 

 tion. Dr. Livingston reports that in certain parts of Africa the 

 rock temperatures on the immediate surface rise during the day 

 as high as 137° F., and at night fall so ra})idly as to throw off 

 by their contraction sharp angular masses in sizes up to 200 

 pounds weight. Throughout the desert regions of Lower Cali- 

 fornia, as observed by the writer, the granitic and basic eruptive 



' The rifting action of heat upon granitic masses is said to have been made a 

 matter of quarry in India. It is stated {Nature, January 17, 1895,) that a wood fire 

 built upon the surface of the granite ledge, and pushed slowly forward, causes the 

 stone to rift out in sheets six inches or so in thickness and of almost any desired 

 superficial area. Slabs 60 x 40 feet in area have been thus obtained varying not 

 more than half an inch from a uniform thickness throughout. In one instance men- 

 tioned the surface passed over by the line of fires was 460 feet, setting free an area of 

 stone of 740 square feet of an average thickness of five inches. This stone was 

 undoubtedly one of remarkably easy rift, but the case will nevertheless serve our 

 present purposes of illustration. 



^Fourth Ann. Rep. Geol. Survey of Texas, 1892, p. 144. 



