Worn by its impact upon neighboring sand grains while in transport, but much more as it is thrown against the ground or hard rock surfaces, the wind-driven or _eolian_ sand is at last worn into smoothly rounded granules which approach the form of a sphere. Compared to the surface which sea sand acquires by attrition, this shaping process is much the more efficient, since in the water the beach sand is buoyed up and is more effectively cushioned against its neighboring grains. The grains of beach sand when examined under a microscope are found to be much more irregular in form and usually display the original fracture surfaces only in part abraded. [Illustration: FIG. 216.—Cliffs in loess 200 feet in height which exhibit the characteristic vertical jointing (after von Richtofen).] =The dust carried out of the desert.=—When, standing upon the mountain wall that surrounds a desert, the traveler gazes out to windward over the great depression, his field of view is generally obscured by the yellow haze of the dust clouds moving across the margins. Upon the mountain flanks and extending far outside the borders, this cloud of dust settles as a shrouding mantle of impalpable yellow powder, which is known as _loess_. These deposits are continually deepening, and have sometimes accumulated until they are hundreds or even thousands of feet in thickness. Before reaching its final resting place the dust of this deposit may have settled many times, and has certainly been in part redistributed by the streams near the desert margin. In it are the ingredients which are necessary for the nourishment of plants, and it constitutes the most important of natural soils. Continually fed by new deposits from the desert, and refertilized from below by a natural process so soon as the upper layers become impoverished, it requires no artificial fertilization. Without artificial aids the loess of northern China has been tilled for thousands of years without any signs of exhaustion. [Illustration: FIG. 217.—A cañon in loess worn by traffic and wind. A highway in northern China (after von Richtofen).] Though easily pulverized between the fingers, loess is none the less characterized by a perfect vertical jointing and stands on vertical faces as does the solid rock (Fig. 216), but it is absolutely devoid of layers or bedding. Its capacity of standing in vertical cliffs the loess owes to a never failing content of lime carbonate which acts as a cement, and to a peculiar porous structure caused by capillary canals that run vertically through the mass, branching like rootlets and lined with carbonate of lime. This texture once destroyed, loess resolves itself into a common sticky clay.