SUN AND WIND IN THE LANDS OF INFREQUENT RAINS =The law of the desert.=—It is well to keep ever in mind that there is no universal law which dominates Nature’s processes in all the sections of her realm. Those changes which, because often observed, are most familiar, may not be of general application, for the reason that the areas habitually occupied by highly civilized races together comprise but a small portion of the earth’s surface. In the dank tropical jungle, upon the vast arid sand plains, and in the cold white spaces near the poles, Nature has instituted peculiar and widely different processes. The fundamental condition of the desert is aridity, and this necessitates an exclusion from it of all save the exceptional rain cloud. Thus deserts are walled in by mountain ranges which serve as barriers to intercept the moisture-bringing clouds. They are in consequence saucer-shaped depressions, often with short mountain ranges rising out of the bottoms, and such rain as falls within the inclosure is largely upon the borders. Of this rainfall none flows out from the desert, for the water is largely returned to the atmosphere through evaporation. The desert history is thus begun in isolation from the sea from which the cloud moisture is derived, a balance being struck between inflow and evaporation. Yet if deserts have no outlets, it is not true that they have no rivers. These are occasionally permanent, often periodic, but generally ephemeral and violent. The characteristic drainage of deserts comes as the immediate result of sudden cloudburst. As a consequence, the desert stream flows from the mountain wall choked with sediment, and entering the depressed basin, is for the most part either sucked down into the floor or evaporated and returned to the atmosphere. The dissolved material which was carried in the water is eventually left in saline deposits, and the great burden of sediment accumulates in thick stratified masses which in magnitude outstrip the largest deltas in the ocean. =The self-registering gauge of past climates.=—From the initiation of the desert in its isolation from the lands tributary to the sea, its history becomes an individual and independent one. An increasing quantity of rainfall will be marked by larger inflow to the basin, and the lakes which form in its lowest depression will, as a consequence, rise and expand over larger areas. A contrary climatic change will bring about a lowering of the lakes and leave behind the marks of former shorelines above the water level (Fig. 205). Deserts are thus in a sense self-registering climatic gauges whose records go back far beyond the historic past. From them it is learned that there have been alternating periods of larger and smaller precipitation, which are referred to as _pluvial_ and _interpluvial_ periods.