SAND DUNES AND PLAINS. 
463 
cultivable soil of the valley is, on the whole, trenching upon 
the domain of the desert, not retreating before 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, unless 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 
protection against the ravages of the sea ; but, in general, the 
vicinity, and especially the shifting of bodies of this material, 
are destructive to human industry, and hence, in civilized 
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 erec¬ 
tions 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 aban¬ 
doned without a struggle, and surrendered to perpetual deso¬ 
lation.* 
Sand Dunes and Sand Plains. 
Two forms of sand deposit are specially important in Eu¬ 
ropean and American geography. The one is that of dime or 
shifting hillock upon the coast, the other that of barren plain 
in the interior. The coast dunes are composed of sand washed 
* In parts of the Algerian desert, some efforts are made to retard the 
advance of sand dunes which threaten to overwhelm villages. “ At Debila,” 
says Laurent, “ the lower parts of the lofty dunes are planted with palms, 
* * * hut they are constantly menaced with burial by the sands. The 
only remedy employed by the natives consists in little dry walls of crystal¬ 
lized 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.”— 
Memoires sur le Sahara , p. 14. 
