'632 SANDS OF EGYPT. 



by precipitous cliffs — ^wherever a ravine or other considerable de- 

 pression occurs in the wall of rock, one sees wbat seems a stream 

 of desert sand pouring down, and common observers have hence 

 concluded that the whole vaUej is in danger of being buried 

 under a stratum of infertile soil. The ancient Egyptians appre- 

 hended this, and erected walls, often of unburnt brick, across the 

 outlet of gorges and lateral valleys, to check the flow of the sand- 

 streams. In later ages, these walls have mostly fallen into decay, 

 and no preventive measures against such encroachments are now 

 resorted to. The extent of the mischief to the soil of Egypt, 

 and the future danger from this source, have been much over- 

 rated.* The sand on the borders of the Nile is neither elevated 

 so high by the wind, nor transported by that agency in so great 

 masses, as is popularly supposed ; and of that which is actually 

 lifted or rolled and finally deposited by air-currents, a consider- 

 able proportion is either calcareous, and therefore readily de- 

 composable, or in the state of a very fine dust, and so in neither 

 case injurious to the soil. There are, indeed, both in Africa and 

 in Arabia, considerable tracts of fine, siKcious sand, which may 

 be carried far by high winds ; but these are exceptional cases, and 

 in general the progress of the desert sand is by a rolling motion 

 along the surface.f So little is it lifted, and so inconsiderable is 



* Klein, Physische Oeographie, p. 371. 



f Sand heaps, three and even six hundred feet high, are indeed formed by 

 the wind, but this is effected by driving the particles up an inclined plane, 

 not by lifting them. Bremontier, speaking of the sand-hills on the western 

 coast of France, says : " The particles of sand composing them are not large 

 enough to resist wind of a certain force, nor small enough to be taken up by 

 it, like dust ; they only roU along the surface from which they are detached, 

 and, though moving with great velocity, they rarely rise to a greater height 

 than three or four inches." — Memoire sur les Dunes, Annales des Fonts et 

 CJiaussees, 1833, ler semestre, p. 148. 



Andresen says that a wind, having a velocity of forty feet per second, is 

 strong enough to raise particles of sand as high as the face and eyes of a man, 

 but that, in general, it roUs along the ground, and is scarcely ever thrown 

 more than to the height of a couple of yards from the surface. Even in these 

 cases, it is carried forward by a hopping, not a continuous, motion ; for a very 

 narrow sheet or channel of water stops the drift entirely, all the sand drop- 

 ping into it until it is filled up. 



Blake observes, Pacific Bailroad Report, vol. v., p. 242, that the sand of the 

 Colorado desert does not rise high in the air, but bounds along on the surface 

 or only a few inches above it. 



The character of the motion of sand drifts is well illustrated by an inter 



