The result of such a violent temperature change in the course of a day is a breaking up of the rocks which subsequently and gradually are ground to fine dust by unobstructed winds wherever vegetation does not bind the soil. In this manner the sand deserts are formed. The arid wastes of Asia have lately been vividly described by Sven Hedin. The mountains eroded by the sandstorms resemble dilapidated ruins, standing as monuments of an ancient highland. The sand in East Turkestan is reduced to such a fine powder that it can float in the air for several days after a storm, revealing itself in gorgeous sunsets. Winds sweep the sand into long dunes, which shift in the direction of the blast. It is ferruginous and therefore red or if powderized reddish-yellow. When moistened it assumes a brown to black shade. After rain, the water descends toward the valley, carrying with it the sand in the form of silt. This, through evaporation, is transformed into a plastic black dough, slides like a glacier slowly down the hillsides, and finally comes to rest in some broad hollow which it gradually fills. Such a silt aggregation is called in Persia a “Kevir.” Its surface dries, but the interior remains moist. As evaporation continues it becomes richer in salt so that white crusts of this substance are formed during dry periods. In other districts, as in the basin of the Tarim River, the water occasionally appears in the lowest parts, the so-called “Bayirs” (see Fig. 9), formations similar to the Kevirs, or in salt lakes between the sand dunes. Sand carried by the winds quickly fills these lakes so that they too move in the direction of prevailing winds. They lie with their longest dimensions parallel to each other and at right angle to the course of the Tarim River. The sketch map taken from Hedin’s work shows the Bayirs strung out in line with the lakes somewhat like panels in a tapestry pattern. This reticulation of the landscape is the result of the dune formations. The main dunes with steep western slopes run in the direction N.N.E.-S.S.W. They stand at right angle to the prevailing winds. Nearly perpendicular to their crest lines dunes of smaller height are thrown up by winds in another common direction but less frequent than those which raised the fundamental dunes. This system brings to mind the cloud formations called mackerel sky, clouds rippled in two directions frequently almost at right angle to each other. They owe their peculiarity to two series of wave motions propelled by winds from two different directions in the upper strata of the atmosphere. The cloud patches correspond to the wavecrests on a surging sea. The map of the Bayirs suggests a chessboard with squares somewhat elongated and irregular. [Illustration: Fig. 9. Tarim river with adjacent lakes and Bayirs, drawn by Sven Hedin. (solid arrow) Predominating direction of winds. (dashed arrow) Direction in which the river is shifting. (hollow box) Vegetation. (dotted box) Sand.