THE SOIL AND WATERCOURSES 43 



ceasingly with his basket ; he also dusts his vege- 

 tables with a small brush to free them from drifted 

 sand. Figs. 10 and 12 show clearly the contrast 

 between the flourishing palm which has tapped the 

 subsoil water, and the barren dunes which surround 

 it. The palm-leaves inserted in the sand in Fig. 12 

 are intended to check wind-driven sand: there is 

 no undergrowth because the water supply is so 

 deep. 



The Nefud, or areas of sand dune in Arabia, are 

 relatively fertile and support so much indigenous 

 vegetation that they are recognized winter and 

 spring pastures. Camels grazing in the Nefud in 

 spring are able to dispense with drinking for long 

 periods. Doughty records that the inhabitants 

 planted a dune at Boreida with tamarisks, and that 

 after one year the trees were established and no 

 longer in need of artificial watering. 



Sand deserts of various types may be distin- 

 guished : bare, shifting dunes, more stable dunes 

 covered with plants, and undulating plains. In 

 the desert west of the Nile, and in many other 

 places, areas of loose sand alternate with outcrops 

 of rock, and the rock is often etched and carved 

 with fantastic shapes by the wind-driven sand 

 grains (Fig. 11). Sand desert of one kind or 

 another is said by Cana to occupy 700,000 square 

 miles of the Sahara, an area equal to about one- 

 fifth of the whole ; according to Augieras, dunes 

 up to 200 metres in height exist in the Western 

 Sahara : no other area of equal extent is known. 

 The Takla Makan, in Eastern Turkestan, is a sandy 



