Now, if you look closely at this sand, you will see that it is full of
minute crystalline particles, and that, in places where it lies
undisturbed, these hard and jagged grains wedge themselves into the softer
ones and form a coherent crust. It was observed that the wind cannot raise
this crust, and the problem how to manufacture it in the neighbourhood of
the oases was solved by enclosing the near-lying tracts of half-desert
within low mounds crowned by upright palm branches, and forbidding all
access to man and beast. The flying plague heaps itself against the
palisade and submerges it; a new set of branches is then inserted, and so
the structure grows higher and more efficacious every year. The soil
within the enclosures, meanwhile, grows hard; wild shrubs sprout up to
help in the work, and though the crust yields, like thin ice, at the
slightest pressure of the fingers, the end is accomplished.
The protected districts are already assuming a different aspect from the
true desert outside, which shifts with the breeze; apart from their tufts
of vegetation, the soil has become quite dark in colour.
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