48 ANIMAL LIFE IN DESERTS 



Gravel desert, a type which consists apparently 

 of pebbles with very little fine soil to fill up the 

 interstices, has been described by Blatter and Hal- 

 berg in Rajputana, and by Thomas on the edge of 

 the Libyan Desert, north-west of Cairo. The inter- 

 vals between the individual pebbles allow the 

 immediate escape of any rain which falls ; on the 

 other hand, the surface of gravel desert is not 

 liable to be moved by the wind. Gravel desert, 

 as might be expected, supports a very poor flora : 

 Thomas says that " standing in most places it is 

 impossible to see a single plant anywhere around 

 one. ... At a period when the annuals have 

 disappeared I have traversed several miles on 

 patrols, without seeing a single plant." Though 

 he studied his area during and after the spring 

 rains he was only able to identify eight flowering 

 plants in the gravel desert. 



Outcrops of bare rock may occur in deserts of 

 almost any type. Small areas of rock present 

 intense difficulties to colonization by plants or 

 animals : in any cHmate the plants growing on 

 rock surface are rare, because soil is only found in 

 small pockets (Fig. 18), and because nearly all the 

 rain received runs off the rock at once : in deserts 

 the difficulties are intensified. Larger areas of 

 rock are less hostile to flora and fauna because they 

 nearly always include patches of soil and little 

 islands of plants. In most large areas of rock, 

 caves occur, with their speciaUzed fauna and rela- 

 tively stable climate. A reference to the animals 

 which are found in desert caves will be found on 



