ch. in FIXATION OF UNSTABLE SOILS 289 



various species of Aerva ; succulent and prostrate herbs, 

 such as Limeum viscosum, Zygophyllum spp. ; other 

 herbs of the same natural order, such as Tribulus or 

 Fagonia; sedges, such as Cyperus cruentus; and various 

 grasses, such as several species of Aristida, Andropogon, 

 Cenchrus, Pennisetum, Tetrapogon, and Aelurops ; 

 while on the edges of depressions, especially edges of 

 streams or rivers where the sand pours into them, Acacia 

 arabica, A. albida, and the Egyptian halfa grass, 

 Desmostachya cynosuroides, can be made to spring up. 



To prevent the sand from encroaching on fresh laud 

 thorn fences can be set up, as described before. In the 

 Northern Sahara the natives keep the encroaching sand 

 from their cultivation by diverting it by means of low 

 mat fences, made of woven grass or palm leaves, which 

 are set obliquely to the wind and thus deflect the 

 current. In Upper Egypt and Northern Sudan, the 

 flooding of the land and subsequent cultivation works 

 off the top layer of sand, but this requires an abundant 

 supply of water during part of the year at least. 



As regards some of the grasses mentioned above, it 

 is, I think, desirable to state that, in countries where 

 camels are commonly used, the Spinifex grass, although 

 a good sand-binder, is objectionable on account of its 

 globose spinescent seed spike, which is very apt to give 

 camels sore feet. The Cenchrus catharticus, although 

 a terrible nuisance to passers-by, owing to its hooked 

 awns, which catch in the clothing and are apt to dis- 

 tribute themselves over all parts of the body and to give 

 unpleasant sores, is a favourite fodder of the camel ; 

 indeed, in time of scarcity, the natives eat the seed. 



It is evident that, while an effort is being made to 

 fix shifting sands, all traffic or grazing over the portions 

 taken in hand should be excluded. 



Fig. 91 shows drift-sand pouring over the banks of 

 the Nile in the Province of Dongola, and covering up 

 the strip of cultivable land above water-level. 



Like many inland-dunes, unstable slopes are largely 

 the result of deforestation or removal of other vegetable 



u 



