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prominent rocks, trees, bushes, or what not, these give rise to inequalities in the distribution of the sand. A steep talus of grains gathers in the sheltered lee, while a more gently sloping bank gradually rises on the windward side of the obstruction, until this is
F1g. 83. Format1on Of Sand-dunes.
o, obstacle intercepting sand; ?v, windward side; /, lee side; tt, sea-level.
eventually overtopped and buried. (See Fig. 83.) In this way a dune is formed, and continues to increase until it reaches its maximum height, determined by the strength of the wind and the supply of the materials, and probably in some measure also by the
mm ui in tu a.~— F10. 84. Advance Of Sand-dunes,
Illustrated by the burial of a church, and its subsequent reappearance, in the neighbourhood of the Kurisches Haft. (G. Bcrendt.)
size of the sand-grains. As the sand continually travels up the gentler windward slope, and comes to rest on the steeper leeward slope, it follows that a dune itself must constantly, if slowly, move forwards. Thus in time the nucleus that gave origin to such a sand-hill may become again exposed. (See Fig. 84, p. 259.)
Coastal sand-hills, like those of inland regions, are frequently arranged in successive parallel ridges or undulations. These, however, are often interrupted by transverse hollows, and the dunes frequently run into one another irregularly. In other places little or no parallel arrangement can be traced, the hills and hummocks showing a tumultuous and tumbled surface of winding and straggling ridges, of isolated banks and knolls, and confused groups of mounds and hillocks, the hollows amongst which form a perfect labyrinth. Should grasses or other vegetation clothe the dunes, these become fixed, but in the absence of any plantgrowth the surface of the sand-hills is kept in constant motion by the wind.
In the hollows amongst sand-dunes marshes, pools, and lakes now and again appear. In some parts of the Sahara, for example, long straggling basins of groundwater extend between the sand-ridges. Again, the advance of sand-dunes from a coast has often obstructed the natural drainage, and formed swamps and lakes of larger or smaller extent. The lagoons, which in many places are separated from the sea, have frequently been cut off from the outside ocean by the combined action of the waves and the wind in raising up sand-banks and -dunes.
In desert regions the bulk of the sand driven forward by the wind rises to no great height above the surface; its abrading and scouring action is largely confined to the basal portions of the rocks against ■which it is borne. But the finer-grained matter—the powdery dust—is often swept upwards to great heights, and may be transported for hundreds or even thousands of miles from the place of its origin. As might have been expected, however, it is over the region immediately surrounding a desiccated area that the dust chiefly falls. In time such regions become more or less thickly mantled with this dust, which usually yields a fertile soil. After long ages of accumulation the whole surface of the dust-covered tracts becomes greatly modified. Inequalities are smoothed over, and everywhere softly flowing features are produced. As no hard-and-fast line separates an area of wind-erosion from one of dust-accumulation, sand and dust become commingled along the borders of the two regions, or there is a gradual transition from the one kind of material to the other. The fertility of the Nile Valley is rightly attributed to the fine silt and loam of the annual floods, but desertdust has also added its share to the soil of Egypt. Similarly it is believed that dust has played an important part in the formation of the fine porous soils of many other lands. According to Baron Richthofen, the vast loss accumulations of China are true dust-deposits. Loss is a fine-grained, homogeneous calcareous and sandy loam, penetrated vertically by numerous root-like pores and tubes, which have the same effect on the deposits as joints in rocks—they allow the loss to cleave in a vertical direction. When
it is intersected, therefore, by streams and rivers it forms bold bluffs and cliffs. It usually contains landshells, and now and again the bones of land animals. Fresh-water shells rarely occur, while marine organisms are wholly wanting. In Northern China this remarkable accumulation covers vast areas, and attains in places a thickness of 1500 feet or even of 2000 feet. The regions occupied by it have the aspect of extensive plains, which look as if they might be traversed with ease in any direction. They are abundantly intersected, however, by deep valleys and precipitous rock-like gullies and ravines, in the vertical walls of which the natives have excavated their dwelling-places. Richthofen believes that this great deposit has been gradually accumulated by the winds flowing outwards from the desiccated regions of Central Asia. Vast quantities of fine sand and dust are there swept up during storms and scattered far and wide, and in this manner adjoining territories, such as the grassy steppes, are ever and anon receiving increments to their soil. The finely sifted material thus obtained is highly fertile and favours the growth of the grasses, so that every fresh deposit of dust tends to become fixed, and the steppe-formation continues to increase in thickness. It is this continued growth of vegetation, keeping pace, as it were, with the periodical accumulation of soil, which is supposed to produce the porous capillary structure referred to above as the cause of the vertical cleavage of the loss.
Loss occurs in many other countries, but it no-vvhere attains so vast a development as in China. In Europe we meet with it in the valley of the Rhine and in the low grounds traversed by the Danube, -where, although it forms no enormous plains like those of Northern China, it nevertheless mantles the ground so as in some measure to conceal the older features of the land. The extensive sheets of black earth which cover the surface of the great plains of Southern Russia are also a variety of loss. The origin of the European deposits has been much discussed by geologists, but it seems to be now the general opinion that the materials of the loss were, in the first place, introduced into the low grounds chiefly by the flooded rivers and inundations of the Ice Age. Muddy water escaping from the glaciers of the Alps and other mountains, and from the terminal front of the great " inland ice" of Northern Europe, doubtless drowned wide areas, while torrents derived from the melting snows of extraglacial tracts must likewise have swept down large quantities of fine-grained sediment. Thus, long after the periodical inundations of glacial times had diminished in extent and finally ceased, the lower reaches of the great valleys and the broad plains, formerly subject to floods, must have been more or less sheeted with sandy loams. We know now that Tundra- and Steppe-conditions have succeeded in Central Europe. Already towards the close of glacial times a well marked Tundra-fauna had spread south to the Alps and west into France