THE SOIL AND WATERCOURSES 47 



is desert, not because of its climate, but because 

 of the physical characters of its soil. 



The fact that smooth, hard deserts of silt or 

 clay or loess support a very small fauna is partly 

 due to the small number of plants occurring, but 

 also to the fact that natural shelter from sun, 

 drought, wind, and light is difficult to obtain. 

 Even species which are capable of excavating 

 burrows for themselves are liable to be drowned 

 out by every storm of rain. 



Very large areas of the Great Palsearctic Desert 

 are composed of mixed stone and earth. This 

 type of desert consists of varying proportions of 

 stones, large and small, lying on and in clay or loess 

 or fine earth of other kinds (Fig. 26). This mixed soil 

 carries a relatively rich flora, if cHmatic conditions 

 are not imfavourable. The finer element in the 

 soil supports the flowers which it would naturally 

 support if it occurred alone, and the presence of 

 stones which hinder evaporation and by that means 

 tend to conserve soil moisture, enables other plants 

 to exist. In fact the mixed type of soil results in 

 a rather varied flora, and this in turn increases 

 the fauna. Apart altogether from their depen- 

 dence on the plants, animals are more numerous 

 in stony deserts because they find shelter from the 

 cHmate under the stones. This point will be dis- 

 cussed later (pages 100-101 ). Stony desert, and semi- 

 desert, is very widely distributed in the Palsearctic 

 region, for instance in the Northern Sahara, the 

 south of Sinai and Palestine, and large areas in 

 Persia, Afghanistan, and Baluchistan. 



