In dry countries, especially in the large tracts of Central Asia and of Africa, the air is often so thick with a fine yellow dust that the sun's light struggles through it as through a London fog. The dust settles on everything, and after many centuries a deposit, which may be hundreds of feet deep, is thus accumulated on the surface of the land. Some of the ancient cities of the old world, Nineveh and Babylon for example, after being long abandoned by man, have gradually been buried under the fine soil which the wind blew over them. Even in England the Roman town of Silchester, not far from Reading, after falling into decay when its inhabitants left it, has been buried under the accumulations of two thousand years, and its walls and floors now lie underground and have to be carefully unearthed in order to lay them bare. But we need not seek these exceptional cases in order to perceive what the wind is doing with sand and the fine dust of the earth's uppermost layers. At many places round the coast are sand-dunes. On sandy shores, exposed to the winds that blow off the sea, the sand is dried and carried away from the beach, gathering into long mounds or ridges which run parallel to the coast-line. These ridges are often fifty or sixty feet, sometimes even more than 250 feet high, with deep troughs and irregular hollows between them, and they sometimes form a strip several miles broad bordering the sea. These sand-hills creep farther inland, till their progress is stopped by the fields or woods they encounter, or till, by seeds finding a root, vegetation springs up on them and they harden and consolidate under the influence of their own vegetation and move inland no farther. But in many parts of Western Europe and Eastern America the dunes are marching inland at the rate of twenty feet a year. Off the coast of Friesland and North Germany the danger has grown so threatening that scientific attention has been given to the problem; and the German scientific men have employed ingenious devices of planting wind-stakes--something like the wooden breakwaters that are to be found along every seaside beach, but arranged at different angles,--of forcing the sand-dune to heap itself up so as to form an obstruction to further arrivals; or of sowing those plants in the sand that will bind its particles together, in order to preserve the land from further invasion. [Illustration: _Stereo Copyright, Underwood & U._ _London and New York_ A Curious Rock greatly revered by the Natives This is the Dance Rock of the Walpi Indians of Arizona. Its curious shape is the result of weathering.]