202 EARTH FEATURES AND THEIR MEANING 



point, the wind makes its entrance and removes the interior por- 

 tion so as to leave a hollow shell the characteristic " pocket 

 rock " (Fig. 208) of the desert. The nummulitic limestone of 

 Mokkatan and many of the great hewn blocks of Egyptian lime- 

 stone sound hollow under the tap of the hammer, and when 

 broken, they reveal a shell a few inches only in thickness (Fig. 

 209). 



The brown desert varnish is one of the most characteristic 

 marks of an arid country. It is found in all deserts under much 



the same conditions, and 

 is especially apt to be pres- 

 ent in sandstone. When 

 scratched, the surface of 

 the rock becomes either 

 cherry-red, indicating an- 

 hydrous ferric oxide, or it 

 is yellowish, due to the 

 hydrated iron oxide which 



FIG. 209. Hollow hewn blocks in a wall in the , 



WadiGuerraui (after Walther). We knOTT as iron rust. 



Thus it is seen that the 



sands of deserts, in contrast to those yielded by other processes 

 within humid regions, have a characteristic red color, and this 

 may vary from brownish red upon the one hand to a rich carmine 

 upon the other. 



The mechanical breakdown of the desert rocks. The chemi- 

 cal changes of decomposition within desert rocks are, as we have 

 seen, largely due to the action of concentrated solutions of salts 

 at high temperatures. That there is a certain mechanical rending 

 of these rocks, due to the " freezing " of salts within the capil- 

 lary fissures, has been already mentioned. A further strain 

 effect arises in rocks like granite, which are a mixture of different 

 minerals. Heated to a high temperature during the day and 

 cooled through a considerable range at night, the different minerals 

 alternately expand and contract at different rates and by dif- 

 ferent relative amounts, so that strains are set up, tending to 

 tear them apart. The effect of these strains is thus a surface 

 crumbling of rocks. 



But rock is, as already pointed out, a relatively poor conductor 

 of heat, and hence it is a relatively thin skin only which passes 



