Both for their evidence and their extent the great ochre stretches of the disk claim attention first. Largely unchangeable, these show essentially the same day after day and from the year’s beginning to its end. In hue they range from sand color to a brick red; some parts of the planet being given to the one tint, some to the other. It is to the latter that the fiery tint of Mars to the naked eye is due. The differences in complexion are local and peculiar, both in place and time. For though saffron best paints the greater part of the light areas, certain localities present at times a red like that of our red sandstone. Hellas is one of these ruddy regions and Aeria another. It is only on occasion that they thus show, and to what to assign their variability is as yet matter of conjecture. Possibly it is owing to Martian meteorologic condition; possibly to something else. But whatever its origin, the change is not so much contradictory of, as supplementary to, the general fact of unalterableness, which is after all the basic trait about them and the keynote to their condition.

Land the ochre regions have generally been taken for, and land they still make good their claim to be considered. For the better they are seen, the greater the ground for the belief. Indeed, they seem to be nothing but ground, or, in other words, deserts. Their color first points them out for such. The pale salmon hue, which best reproduces in drawings the general tint of their surface, is that which our own deserts wear. The Sahara has this look; still more it finds its counterpart in the far aspect of the Painted Desert of northern Arizona. To one standing on the summit of the San Francisco Peaks and gazing off from that isolated height upon this other isolation of aridity, the resemblance of its lambent saffron to the telescopic tints of the Martian globe is strikingly impressive. Far forest and still farther desert are transmuted by distance into mere washes of color, the one robin’s-egg blue, the other roseate ochre, and so bathed, both, in the flood of sunshine from out a cloudless burnished sky that their tints rival those of a fire-opal. None otherwise do the Martian colors stand out upon the disk at the far end of the journey down the telescope’s tube. Even in its mottlings the one expanse recalls the other. To the Painted Desert its predominating tint is given by the new red sandstone of the Trias, the stratum here exposed; and this shows in all its pristine nakedness because of the lack of water to clothe it with any but the sparsest growth. Limestones that crop out beside it are lighter yellow, whitish and steel-gray, and seen near give the terrane the look of a painter’s palette. Seen from far they have rather the tint of sand; and the one effect, like the other, is Martian in look. And as if to assimilate the two planetary appearances the more, the thread of blue-green that attention traces athwart the Painted Desert marks the line of cottonwoods along the banks of the Little Colorado River—deserts both, if look be any guarantee of character, with verdure banding them.

In other ways these earthly deserts offer a parallel to the Martian. No desert on the Earth is absolutely devoid of life of some kind, vegetal and animal. The worst conditioned are not what one is taught in childhood to believe a desert to be—a vast waste of sand, with a camel and a palm thrown in to heighten the sterility. In all Saharas outside of the pages of the school books some vegetation grows, though it is commonly not of a kind to boast of, being rather a succÉs d’estime, as sagebrush, cacti, and the like. But what is of interest here in the connection is its color. For it is commonly of a more ochreish tint than usual, in keeping with its surroundings, a paling out of the green to something more tawny, indicating a relative reduction of the chlorophyll and an increase of the lipochromes in the tissues of the plant, since the one gives the green tint to the leaves, the other the yellow. As this vegetation, poor as it is, has its annual history, it must alter the look of the desert at times and produce precisely those slight variations in tint observable on Mars in like circumstance.

The Arizona desert dates from no further back than early Tertiary times, as the limestone of the Cretaceous there present shows. Water then stretched where desert now is, and the limestone beds were laid down in it. How old the Martian Saharas are we have no means of knowing. But one thing we may predicate about both: a desert is not an original, but an acquired, condition of a planet’s surface, demonstrably so in the case of a planet which has had a sedimentary epoch in its life-history. In the Arizona desert the surface is composed of depositary rocks of Mesozoic times, except where lava streams have flowed down over it since then. The land, then, was once under water, and cannot but have been fertile for some time after it emerged.

But we are not left to inference in the matter, however good that inference may be. A little to the south of the Painted Desert, in the midst of the barren plateau of northern Arizona, of which the former makes a part, stand the remains of a petrified forest. Huge chalcedony trunks of trees, so savingly transmuted into stone that their genus is still decipherable, lie scattered here over the barren ground in waste profusion, one of them still spanning a caÑon just as it fell in that, to it, destructive day of a far prehistoric past. The rock stratum on which their remains lie is of Triassic and Cretaceous times and the petrifications show that in the Cretaceous a stately forest overspread the land. In those days at least the spot was fertile where now sparse sagebrush and cacti find a living hard. Not here alone where the blocks are so conspicuous as to invite their carrying away is a former flourishing growth of vegetation attested, but over large adjoining areas of desert search has brought the like past tenancy to light. Fragments of what once were trees have been picked up in the Little Colorado basin and in the neighborhood of Ash Fork, on both sides, that is, of the present forest crown that covers the higher part of the plateau from which rise the San Francisco Peaks. In the blue distance the mountains look down verdure-clad upon a now encircling waste, but one which in earlier eras was as pine-bearing as they. Their lofty oasis is all that is now left of a once fertile country; the retreat of the trees up the slopes in consequence of a diminishing rainfall, until a rise of two thousand feet from what once was timber-land is necessary to reach the tree-line of today, being typical of desert lands, and testifying to greater aqueous affluence in the past. In the same manner streams descend from the cedar-clad range of the Lebanon to lose themselves in the Arabian desert just without the doors of Damascus; and Palestine has desiccated within history times. Palestine, a land once flowing with milk and honey, can hardly flow poor water now, and furnishes another straw to mark the ebbing of the water supply.

This making of deserts is not a sporadic, accidental, or local matter, although local causes have abetted or hindered it. On the contrary, it is an inevitable result of planetary evolution, a phase of that evolution which follows from what has been said in Chapter XII on the abandonment of a planet by its water. Deserts are simply another sign of the same process. The very aging which began by depriving a body of its seas takes from it later its forest and its grass. A growing scarcity of water is bound to depauperate the one, as it depletes the other. We have positive proof of the action in our own deserts. For these bear testimony, in places at least, to not having always been so, but to have gradually become so within relatively recent times. But we have more general proof of the action from the position occupied on the earth’s surface by its deserts.

The significant fact about the desert-making so stealthily going on is that only certain zones of the earth’s surface are affected. Those belting the two tropics of Cancer and Capricorn, for several degrees on either side of them, most exhibit the phenomenon. Such positioning of the deserts is not due to chance. Directly, of course, desertism is due to dearth of rain. This in turn depends on the character and condition of the winds. If a wind laden with moisture travel into a colder region of the globe, its moisture is precipitated in rain and we have a fertile country; if it voyage into a warmer clime it takes up what little moisture may be there already and a desert is the result.

Now our system of winds is such as to produce a fall of rain for the different latitudes, as tabulated by Supan, thus:—

Zones II and VI, the zones of minimum rainfall, are also those in which the deserts occur. The northern one traverses southern California, Arizona, New Mexico, the Sahara, Arabia, and the Desert of Gobi; the southern, Peru, the South African veldt, and central Australia. The belts are wavy bands which by their form betray both a general underlying trend to drought at these parallels and also the effect of local topography in the matter.

From being distributed thus in belts, it is evident that the deserts are general globe phenomena, and from their being found only in the zones of least rainfall, that the earth has itself entered, though not far as yet, upon the desert stage of its history. Once begun, the desert areas must perforce spread as water becomes scarce, invading and occupying territory in proportion as the rainfall there grows small.

Now the axial tilt of Mars is almost exactly the same as that of our Earth, the latest determinations from the ensemble of measures giving 24° for it. Here, then, we have initial conditions reproducing those of the earth. But from the smaller size of the planet that body would age the earlier, since it would lose its internal heat the more rapidly, just as a small stone cools sooner than a larger one. On general principles, therefore, it should now be more advanced in its planetary career. In consequence, desertism should have overtaken more of its surface than has yet happened on earth, and instead of narrow belts of sterility we should expect to find there Saharas of relatively vast extent.

Now, such a state of things is precisely what the telescope reveals. The ochre tracts occupy nine tenths of the northern hemisphere and a third of the southern. Three fifths, therefore, of the whole surface of the planet is a desert.

Of cosmic as well as of particular import is the correlation thus made evident between the physical principles that effect the aging of a planet and the aspect Mars presents. Experimental corroboration of those laws is thus afforded, while, reversely, confidence in their applicability is increased. With continued observation the planet appears more desiccate as improved conditions bring it nearer. Dry land as it was thought to be proves even drier, something which lacks water for the ordinary necessities of a living world.

The picture the planet offers to us is thus arid beyond present analogue on Earth. Pitiless as our deserts are, they are but faint forecasts of the state of things existent on Mars at the present time. Only those who as travelers have had experience of our own Saharas can adequately picture what Mars is like and what so waterless a condition means. Only such can understand what is implied in having the local and avoidable thus extended into the unescapable and the world-wide; and what a terrible significance for everything Martian lies in that single word: desert.