The high snow-clad peaks of the Rocky Mountains on the
one side check and condense all the moisture that comes from the
Atlantic; the Sierra Nevada and the Wahsatch range on the other, running
parallel with them to the west, check and condense all the moisture that
comes from the Pacific coast. In between these two great lines lies the
dry and almost rainless district known to the ambitious western mind as
the Great American Desert, enclosing in its midst that slowly
evaporating inland sea, the Great Salt Lake, a last relic of some
extinct chain of mighty waters once comparable to Superior, Erie, and
Ontario. In Mexico, again, where the twin ranges draw closer together,
desert conditions once more supervene. But it is in central Australia
that the causes which lead to the desert state are, perhaps on the
whole, best exemplified. There, ranges of high mountains extend almost
all round the coasts, and so completely intercept the rainfall which
ought to fertilise the great central plain that the rivers are almost
all short and local, and one thirsty waste spreads for miles and miles
together over the whole unexplored interior of the continent.
But why are deserts rocky and sandy? Why aren't they covered, like the
rest of the world, with earth, soil, mould, or dust? One can see plainly
enough why there should be little or no vegetation where no rain falls,
but one can't see quite so easily why there should be only sand and rock
instead of arid clay-field.
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