Under these circumstances, the rocks crumble away very slowly by
mere exposure into small fragments, which the wind knocks off and blows
about the surface, forming sand or dust of them in all convenient
hollows. The frequent currents, produced by the heated air that lies
upon the basking layer of sand, continually keep the surface agitated,
and so blow about the sand and grind one piece against the other till it
becomes ever finer and finer. Thus for the most part the hollows or
valleys of deserts are filled by plains of bare sand, while their higher
portions consist rather of barren, rocky mountains or table-land.
The effect upon whatever animal or vegetable life can manage here and
there to survive under such circumstances is very peculiar. Deserts are
the most exacting of all known environments, and they compel their
inhabitants with profound imperiousness to knuckle under to their
prejudices and preconceptions in ten thousand particulars.
To begin with, all the smaller denizens of the desert--whether
butterflies, beetles, birds, or lizards--must be quite uniformly
isabelline or sand-coloured. This universal determination of the
desert-haunting creatures to fall in with the fashion and to harmonise
with their surroundings adds considerably to the painfully monotonous
effect of desert scenery. A green plant, a blue butterfly, a red and
yellow bird, a black or bronze-coloured beetle or lizard would improve
the artistic aspect of the desert not a little.
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