Now let us see how these winds might leave traces in the geological record. When soil is exposed to the sun its surface becomes dust, and the wind carries it off. Even where turf protects the surface, bare places may always be found whence this covering has been removed. Rabbits and moles bring up the earth to the surface; the earthworms sometimes bring as much as ten tons of earth to the surface of a single acre of turf in the course of a year. The earthworms bring up only the finest particles of mould; and these, of course, are the very particles readily converted into dust and borne away by the wind if they are not washed away by rain. In tropical countries the white ant conveys a prodigious amount of fine earth up into the open air, building walls sometimes sixty feet high. Although, therefore, the layer of vegetable soil which covers the land appears to be a permanent protection, it does not really prevent a large amount of material from being removed even from grassy ground. The wind carries this fine dust far and wide over the land, and over the sea as well. After the eruption of the island of Krakatoa in 1883, the dust which was the product of that mighty explosion was carried round the world, and even in England we saw the dust particles furnishing extraordinary colours in sunset skies.
Stereo Copyright, Underwood & U.
London and New York
The Garden of the Gods, Colorado
These peaks exhibit the gradual wearing away of hard rocks by the action of rain and wind.
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.
Stereo Copyright, Underwood & U.
London and New York