If deserts have a fault (which their present biographer is far from admitting), that fault may doubtless be found in the fact that their scenery as a rule tends to be just a trifle monotonous. Though fine in themselves, they lack variety. To be sure, very few of the deserts of real life possess that absolute flatness, sandiness and sameness, which characterises the familiar desert of the poet and of the annual exhibitions—a desert all level yellow expanse, most bilious in its colouring, and relieved by but four allowable academy properties, a palm-tree, a camel, a sphinx, and a pyramid. For foreground, throw in a sheikh in appropriate drapery; for background, a sky-line and a bleaching skeleton; stir and mix, and your picture is finished. Most practical deserts one comes across in travelling, however, are a great deal less simple and theatrical than that; rock preponderates over sand in their composition, and inequalities of surface are often the rule rather than the exception. There is reason to believe, indeed, that the artistic conception of the common or Burlington House desert has been unduly influenced for evil by the accessibility and the poetic adjuncts of the Egyptian sand-waste, which, being situated in a great alluvial river valley is really flat, and, being the most familiar, has therefore distorted to its own shape the mental picture of all its kind elsewhere. But most deserts of actual nature are not all flat, nor all sandy; they present a considerable diversity and variety of surface, and their rocks are often unpleasantly obtrusive to the tender feet of the pedestrian traveller.
A desert, in fact, is only a place where the weather is always and uniformly fine. The sand is there merely as what the logicians call, in their cheerful way, 'a separable accident'; the essential of a desert, as such, is the absence of vegetation, due to drought. The barometer in those happy, too happy, regions, always stands at Set Fair. At least, it would, if barometers commonly grew in the desert, where, however, in the present condition of science, they are rarely found. It is this dryness of the air, and this alone, that makes a desert; all the rest, like the camels, the sphinx, the skeleton, and the pyramid, is only thrown in to complete the picture.
Now the first question that occurs to the inquiring mind—which is but a graceful periphrasis for the present writer—when it comes to examine in detail the peculiarities of deserts is just this: Why are there places on the earth's surface on which rain never falls? What makes it so uncommonly dry in Sahara when it's so unpleasantly wet and so unnecessarily foggy in this realm of England? And the obvious answer is, of course, that deserts exist only in those parts of the world where the run of mountain ranges, prevalent winds, and ocean currents conspire to render the average rainfall as small as possible. But, strangely enough, there is a large irregular belt of the great eastern continent where these peculiar conditions occur in an almost unbroken line for thousands of miles together, from the west coast of Africa to the borders of China: and it is in this belt that all the best known deserts of the world are actually situated. In one place it is the Atlas and the Kong mountains (now don't pretend, as David Copperfield's aunt would have said, you don't know the Kong mountains); at another place it is the Arabian coast range, Lebanon, and the Beluchi hills; at a third, it is the Himalayas and the Chinese heights that intercept and precipitate all the moisture from the clouds. But, from whatever variety of local causes it may arise, the fact still remains the same, 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.
The normal course of events where new land rises above the sea is something like this, as oceanic isles have sufficiently demonstrated. The rock when it first emerges from the water rises bare and rugged like a sea-cliff; no living thing, animal or vegetable, is harboured anywhere on its naked surface. In time, however, as rain falls upon its jutting peaks and barren pinnacles, disintegration sets in, or, to speak plainer English, the rock crumbles; and soon streams wash down tiny deposits of sand and mud thus produced into the valleys and hollows of the upheaved area. At the same time lichens begin to spring in yellow patches upon the bare face of the rock, and feathery ferns, whose spores have been wafted by the wind, or carried by the waves, or borne on the feet of unconscious birds, sprout here and there from the clefts and crannies. These, as they die and decay, in turn form a thin layer of vegetable mould, the first beginning of a local soil, in which the trusty earthworm (imported in the egg on driftwood or floating weeds) straightway sets to work to burrow, and which he rapidly increases by his constant labour. On the soil thus deposited, flowering plants and trees can soon root themselves, as fast as seeds, nuts or fruits are wafted to the island by various accidents from surrounding countries. The new land thrown up by the great eruption of Krakatoa has in this way already clothed itself from head to foot with a luxuriant sheet of ferns, mosses, and other vegetation.
First soil, then plant and animal life, are thus in the last resort wholly dependent for their existence on the amount of rainfall. But in deserts, where rain seldom or never falls (except by accident) the first term in this series is altogether wanting. There can be no rivers, brooks or streams to wash down beds of alluvial deposit from the mountains to the valleys. Denudation (the term, though rather awful, is not an improper one) must therefore take a different turn. Practically speaking, there is no water action; the work is all done by sun and wind. 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. But no; the animals will hear nothing of such gaudy hues; with Quaker uniformity they will clothe themselves in dove-colour; they will all wear a sandy pepper-and-salt with as great unanimity as the ladies of the Court (on receipt of orders) wear Court mourning for the late lamented King of the Tongataboo Islands.