200 EARTH FEATURES AND THEIR MEANING 



from the tropics to the poles, has the same salts in essentially the 

 same proportions, while in the desert the widest variations are 

 found both in the salts which are present and in their relative 

 quantities. 



Upon the borders of the ocean are found ridges of yellow sand 

 heaped up by the wind, but these ramparts are small in comparison 

 to those which in deserts are found upon the borders (plate 7 A). 



The desert is a land of geographic paradoxes. As Walther has 

 pointed out, we have rain in the desert which does not wet, springs 

 which yield no brooks, rivers without mouths, forests preserved 

 in stone, lakes without outlets, valleys without streams, lake basins 

 without lakes, depressions below the level of the sea yet barren 

 of water, intense weathering with no mantle of disintegrated rock, 

 a decomposition of the rocks from within instead of from without, 

 and valleys which branch sometimes upstream and sometimes 

 down. 



Within the deserts curious mushroom-like remnants of erosion 

 afford a local relief from the searching rays of the desert sun. 

 Pocket-like openings large enough for a hermit's habitation are 

 hollowed out by the wind from the disintegrated rock masses. 

 Amphitheaters open out from little erosion valleys or wadi, and 

 isolated outliers of the mountains stand like sentinels before their 

 massive fronts. 



Because of the general absence of clouds above a desert, no 

 shield such as is common in humid regions is provided against the 

 blinding intensity of the sun's rays. Sun temperatures as high 

 as 180 Fahrenheit have been registered over the deserts of 

 western Africa. Every one is familiar with the fact that a 

 blanket of thick clouds is a prevention of frosts at night, for, with 

 the setting of the sun and the consequent radiation of heat from 

 the earth, these rays are intercepted by the clouds, returned and 

 re-returned in many successive exchanges. Over desert regions 

 the absence of any such blanket of moisture is responsible for the 

 remarkable falls of temperature at sunset. Though shortly before 

 temperatures of 100 Fahrenheit or greater may have been 

 measured, it is not uncommon for water to freeze during the 

 following night. Much the same conditions of sudden tempera- 

 ture change with nightfall are experienced in high mountains when 

 one has ascended above the blanketing clouds. 



