If the mountains furnish water enough the lowest part of each plain contains either a salt lake or else a "dry lake," known as a playa. Such a lake at most times presents the appearance of an absolutely flat expense of smooth clay, but after one of the occasional rains it is transformed into a lake in which the water may be only a few inches deep.
In the well-populated parts of the world vegetation is so abundant that it hides the soil. In deserts, however, the vegetation is so scanty that the traveler is constantly reminded of the rock and soil. What little vegetation he sees, however, is peculiarly interesting because of the strange forms which it has acquired in its attempt to meet the conditions of drought.
Sandy Deserts.—Although sandy deserts do not occupy any larger area than those of bare rock, gravel, and clay, they present much the most interesting appearance, largely because of the presence of dunes of every size. That is why the most familiar pictures of deserts show sandy dunes. In the Takla Makan desert of western China millions of sand dunes of many tones of yellow, brown, and pink, look like the waves of a huge dry sea 600 miles long. Only the boldest explorers dare launch their caravans on such a waterless sea. The wind piles the dunes up to a height of 500 feet and thus causes them to be an almost impassable barrier. Even when the dunes are small the feet of men and animals sink into the unstable sand and slip and slide so that progress is extremely slow. Often it is impossible to climb the steep leeward slope of a dune, although the gentler wind ward slope may be comparatively easy. When violent winds blow the sharp sand with cutting force into the traveler's face there is nothing to do but turn one's back to the wind and try to escape suffo cation. The camel has become so well adapted to this condition of desert life that he is able to close his nostrils and only open them at long intervals for a quick breath.
How Loess is Formed and Used.—When the wind blows over the desert it not only heaps the particles of sand into dunes, but carries away the finer dust and deposits it in the form of loess. From the Takla Makan and Gobi deserts the dust is sometimes blown in such quantities that 60 or 100 miles to the southeast it makes the air so hazy that the sun is hidden even at noon. Beyond the limits of
the deserts it falls as a fine yellow powder. It even sifts into the tightly closed houses and makes it difficult to write by coating the paper and clogging the pen. In northwestern China this desert dust has accumulated in some places to a depth of scores of hundreds of feet over an area larger than France. It is very fertile provided it is well supplied with water. Where there is no vegetation to hold it, however, it is so light that it is quickly blown away. Along the roads in the loess country the feet of horses stir up the dust and then it is blown away so that the roads become deep trenches. In spite of its lightness the loess does not easily crumble. Its thick, fine grained masses can be cut like cheese and it sticks together so well that houses can be excavated in it. Near the borders of the deserts in northwestern China many peasants live in such houses dug in the walls of the sunken streets.
Why Desert Lakes are Salt.—Aside from the dunes and the vege tation one of the most striking features of the desert is the lakes. Occasionally they are beautiful, but oftener they have flat muddy shores crusted with white crystals and looking something like tide flats and smelling far worse. Desert lakes generally have no out lets and hence are salty. This is because both in deserts and else where every stream contains a little dissolved salt, although ordinarily this cannot be detected except by chemical analysis. In deserts, however, evaporation removes the water without removing the salt, which in ordinary lakes escapes through the outlet. Hence all the salt that is brought in by the streams remains in the lake, and finally forms a strong brine so heavy that the bather finds himself lifted from the bottom when he walks out as far as his armpits. Woe be tide him, in the Dead Sea, for example, if he gets the stinging brine into his eyes or nose. The Dead Sea is well named for there is prac tically no life in it. Many great salt lakes have dried up, leaving their salt in solid layers, scores or hundreds of feet thick. In Central Asia such salt deposits cover thousands of square miles.