536 SAND DUNES AND PLAINS. 



cessive tyrannies of Egypt to keep up some of the canals and 

 other arrangements for irrigation, was not felt with respect to 

 the advancement of the sands ; for their progress was so slow 

 as hardly to be perceptible in the com-se of a single reign, and 

 long experience has shown that, from the natural effect of the 

 inundations, the cultivable soil of the valley is, on the whole, 

 trenching upon the domain of the desert, not retreating be- 

 fore it. 



The oases of the Libyan, as well as of many Asiatic deserts, 

 have no such safeguards. The sands are fast encroaching upon 

 them, and threaten soon to engulf them, imless man shall resort 

 to artesian wells and plantations, or to some other efficient means 

 of checking the advance of this formidable enemy, in time to 

 save these islands of the waste from final destruction. 



Accumulations of sand are, in certain cases, beneficial as a pro- 

 tection against the ravages of the sea ; but, in general, the vicin- 

 ity, and especially the shifting of bodies of this material, are 

 destructive to human industry, and hence, in civiHzed countries, 

 measures are taken to prevent its spread. This, however, can be 

 done only where the population is large and enlightened, and the 

 value of the soil, or of the artificial erections and improvements 

 upon it, is considerable. Hence in the deserts of Africa and of 

 Asia, and the inhabited lands which border on them, no pains 

 are usually taken to check the drifts, and when once the fields, 

 the houses, the springs, or the canals of irrigation are covered or 

 choked, the district is abandoned without a struggle, and sur- 

 rendered to perpetual desolation.* 



SandrDunes amd Sa/nd-Plains. 

 Two forms of sand deposit are specially important in Eu- 

 ropean and American geography. The one is that of dune or 



* In parts of the Algerian desert, some efforts are made to retard the ad- 

 vance of sand-dunes which threaten to overwhelm villages. "At Debila," 

 says Laurent, "the lower parts of the lofty dunes are planted with palms, 

 .... but they are constantly menaced with burial by the sands. The only 

 remedy employed by the natives consists in little dry walls of crystallized 

 gypsum, built on the crests of the dunes, together with hedges of dead palm- 

 leaves. These defensive measures are aided by incessant labor ; for every 

 day the people take up in baskets the sand blown over to them the night 

 before and carry it back to the other side of the dune," — Memoirea aur It 

 8dha/ra, p. 14. 



