THE SOIL AND WATERCOURSES 49 



page 104. In Fig. 18 I have illustrated a type 

 of rock desert in which strata of hard and soft 

 calcareous rock alternate. On the softer large 

 numbers of spring flowers grow : these valleys are 

 full of caves, both natural and artificial. Typical 

 birds in this type of country in Southern Palestine 

 are Rock Doves, Wheatears {Saxicola lugens and 

 others). Rock Partridge (Alectoris =Caccabis), 

 Ravens (Corvus ruficollis), and Tristram's Grackle 

 {Amydrus tristrami) : among mammals, Porcupines 

 and the Coney or Hyrax (Procavia). 



The influence of watercourses upon desert life 

 is very much greater than might be supposed. 

 Many of them are temporary, fed by snow, or by 

 rain faUing on some distant mountain mass, and run 

 out into the desert plains, where they gradually 

 disappear owing to evaporation and seepage into 

 the soil. The length of such a river varies immensely 

 with the season : at the end of a period of drought 

 the whole bed may be dry (Figs. 19, 20, and 21) ; 

 after a period of heavy rain a torrent may be pro- 

 duced sufficient to flow several hundred miles down 

 a bed which has been dry for years. One of the 

 longest of these dry watercourses is the Wadi 

 Rummah, which rises near Medina in the highlands 

 of Western Arabia, and at one time crossed the 

 peninsula to discharge its waters into the Shatt al 

 Arab near Basra. It is more than a thousand miles 

 long, though its course is now blocked by large 

 areas of sand dunes and water never runs from 

 end to end of it. In some of these torrent-beds 

 bushes and other vegetation grow (Figs. 20 and 21) : 



