that all the great deserts run in this long, almost unbroken series, beginning with the greater and the smaller Sahara, continuing in the Libyan and Egyptian desert, spreading on through the larger part of Arabia, reappearing to the north as the Syrian desert, and to the east as the desert of Rajputana (the Great Indian Desert of the Anglo-Indian mind), while further east again the long line terminates in the desert of Gobi on the Chinese frontier.

In other parts of the world, deserts are less frequent.  The peculiar combination of circumstances which goes to produce them does not elsewhere occur over any vast area, on so large a scale.  Still, there is one region in western America where the necessary conditions are found to perfection.  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.

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 and no earth-worms, therefore, there can be no soil; and under those circumstances we get what we familiarly know as a desert.