By the feet of passing animals or by wheels of vehicles, the loess is crushed, and a portion is lifted and carried away by the wind. Thus in the course of time roadways sink deep into the mass as steep-walled cañons (Fig. 217). A portion of the now structureless clay remaining upon the roadway is at the time of the rains transformed into a thick mud which makes traveling all but impossible, though before its structure has been destroyed the loess is perfectly drained to the bottom of its deposits. The particles which compose the loess are sharply angular quartz fragments, so fine that all but a few grains can be rubbed into the pores of the skin. Fine scales of mica, such as are easily lifted by the wind, are disseminated uniformly throughout the mass. The only inclosures which are arranged in layers consist of irregularly shaped concretions of clay. These show a striking resemblance to ginger roots and are called by the Chinese “stone ginger”, though they are elsewhere more generally known by their German name of _Loessmännchen_, or loess dolls. These concretions are so disposed in the loess that their longer axes are vertical, and they were evidently separated from the mass and not deposited with it. CHAPTER XVI THE FEATURES IN DESERT LANDSCAPES [Illustration: FIG. 218.—Diagrams to illustrate the effects of obstructions of different types in arresting wind-driven sand. _a_, An unyielding obstruction which permits the wind to pass through it; _b_, a flexible and perforated obstruction; _c_, an unyielding closed barrier (after Schulze).] =The wandering dunes.=—Over the broad expanse of the desert, sand and dust, and occasionally gypsum from the saline deposits, are ever migrating with the wind; on quiet days in the eddying “sand devils”, but especially during the terrifying sand storms such as in the windy season darken the air of northern China and southern Manchuria. This drift of the sand is halted only when an obstruction is encountered—a projecting rock, a bush, or a bunch of grass, or again the buildings of a city or a town. The manner in which the sand is arrested by obstacles of different kinds is of great interest and importance, and is utilized in raising defenses against its encroachments. If the obstacle is unyielding but allows some of the wind to pass through it, no eddies are produced and the sand is deposited both to windward and to leeward of the obstruction to form a fairly symmetrical mound (Fig. 218 _a_). An obstruction which yields to the wind causes the sand to deposit in a mound which is largely to leeward of the obstruction (Fig. 218 _b_). A solid wall, on the other hand, by inducing eddies, is at first protected from the sand and mounds deposit both to windward and to leeward (Fig. 218 _c_ and Fig. 219).