SUN AND WIND IN LANDS OF INFREQUENT RAINS 203 



through the daily round of wide temperature range. This outer 

 shell when heated is expanded, and so tends to peel off, or ex- 

 foliate, like the outer skin of an onion. The process is therefore 

 described as exfoliation. In all rocks of homogeneous texture the 

 continued action of this process results in convexly spherical sur- 

 faces, the material scaled off in the process remaining as a slope 

 or talus which surrounds the projecting knob (Fig. 210). Naked, 



FIG. 210. Smooth granite domes shaped by exfoliation and surrounded by a rim of 

 talus. Gebel Karsala, Nubian Desert (after Walther). 



these projecting domes rise above the rim of debris at their bases. 

 Not a particle of dust adheres to the fresh rock surface no 

 dirt interferes with its glaring whiteness. Yet close at hand lie 

 masses of debris into which wells may be carried to depths of 

 more than six hundred feet without encountering either solid 

 rock or ground water. The bare walls of granite sometimes mount 

 upwards for thousands of feet into the air, as steep and as inac- 

 cessible as the squared towers of the Tyrolean Dolomites. 



Rock is such a poor conductor of heat that special strains are 

 set up at the margin of sunlight and shade. This localization of 

 the disintegration on the margin of the shaded portions of rock 

 masses is known as shadow weathering (see Fig. 215, p. 206). 



There is, however, still another mechanical disintegrating 

 process characteristic of the desert regions, which is likewise 

 dependent upon the sudden changes of temperature. Rains, 



