SANDS OF EGYPT. de 
proportion is either calcareous, and, therefore, readily decom- 
posable, or in the state of a very fine dust, and so, in neither 
vase, injurious to the soil. There are, indeed, both in Africa 
and in Arabia, considerable tracts of fine, silicious sand, which 
may be carried far by high winds, but these are exceptional ca- 
ses, and in general the progress of the desert sand is by a rolling 
motion along the surface.* So little is it lifted, and so incon- 
* Sand heaps, three and evensix 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. Brémontier, 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 roll 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.” — Mémoire sur les Dunes, Annales des Ponts et Chaus- 
sées, 1833, ler sémestre, 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 rolls 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 
dropping into it until it is filled up. 
Blake observes, Pacific Railroad 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- 
esting fact not much noticed hitherto by travellers in the East. In situa- 
tions where the sand is driven through depressions in rock-beds, or over de- 
posits of silicious pebbles, the surface of the stone is worn and smoothed 
much more effectually than it could be by running water, and I have picked 
up, in such localities, rounded, irregularly broken fragments of agate, which 
had received from the attrition of the sand as fine a polish as could be given 
them by the wheel of the lapidary. 
Very interesting observations, by Blake, on the polishing of hard stones by 
drifting sand will be found in the Pacific Railroad Report, vol. v., pp. 92, 
230, 2351. 
The grinding and polishing power of sand has lately received a new and most 
ingenious application in America. Jets of sand, and even of small particles 
of softer substances, thrown with a certain force, are found capable of cutting 
the hardest minerals and metals. A block of corundum, some inches thick, 
has been bored through in a few minutes by this process, and it promises to 
be highly useful in glass-cutting and other similar operations, 
