44 ANIMAL LIFE IN DESERTS 



desert of considerable size ; Sven Hedin states that 

 he travelled for several days in it without seeing 

 any sign of vegetable life. 



Deserts of clay, fine river silt or loess, present 

 contrasts to sand deserts in nearly every particular. 

 The soil is extremely fine in texture. The surface 

 is generally flat (Figs. 13 and 14) or nearly so, 

 smooth, and hard ; rain which falls upon it tends 

 to run off rapidly either into watercourses or into 

 depressions from which it rapidly evaporates. Fur- 

 ther, these finely grained soils have a high capacity 

 for holding water ; therefore that portion of the 

 rain which sinks into them does not penetrate to 

 a great depth, but is held in the superficial layers 

 of the soil and rapidly lost by evaporation. For 

 these two reasons soils of this type waste a large 

 proportion of the rainfall which they receive. For 

 a similar reason they are wasteful of any ground 

 water with which they may be endowed : as the 

 particles of the soil are very fine and the interstices 

 between them minute, the force of capiUary attrac- 

 tion exerted by them is great : ground water is 

 therefore carried up towards the surface and made 

 available for shallow-rooted plants ; but large 

 quantities of it are evaporated. 



The ground water brings up with it mineral 

 salts, which are left in the supei-ficial layers of the 

 soil as it evaporates. The rain water, which runs 

 into a shallow depression and then evaporates, tends 

 to concentrate these salts in patches. As most of 

 these deserts are flat they are devoid of good sur- 

 face drainage, and as their soil is so fine in texture 



