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.