ad 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.

Well, the answer is that without vegetation there is no such thing as
soil on earth anywhere. The top layer of the land in all ordinary and
well-behaved countries is composed entirely of vegetable mould, the
decaying remains of innumerable generations of weeds and grasses. Earth
to earth is the rule of nature. Soil, in fact, consists entirely of dead
leaves. And where there are no leaves to die and decay, there can be no
mould or soil to speak of. Darwin showed, indeed, in his last great
book, that we owe the whole earthy covering of our hills and plains
almost entirely to the perennial exertions of that friend of the
farmers, the harmless, necessary earthworm. Year after year the silent
worker is busy every night pulling down leaves through his tunnelled
burrow into his underground nest, and there converting them by means of
his castings into the black mould which produces, in the end, for lordly
man, all his cultivable fields and pasture-lands and meadows. Where
there are no leaves a