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Page:Popular Science Monthly Volume 24.djvu/259

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247
THE LOESS-DEPOSITS OF NORTHERN CHINA.

connection with the waters compressed below, when, owing to the law of diffusion, the ingredients, being released, mix with the moisture of the little canals, and are there taken from the lowest to the topmost levels, permeating the ground and furnishing nourishment to the plant-roots at the surface. It is on account of this curious action of loess that a copious rainfall is more necessary in Northern China than elsewhere, for with a dearth of rain the capillary communication from above, below, and vice versa, is interrupted, and vegetation loses both its moisture and manure. Drought and famine are consequently synonymous terms here.

As to the origin of loess, Baron von Richthofen's theory is substantially as follows: The uniform composition of this material over extended areas, coupled with the absence of stratification and of marine or fresh-water organic remains, renders impossible the hypothesis that it is a water-deposit. On the other hand, it contains vast quantities of land-shells and the vestiges of animals (mammalia) at every level both in remarkably perfect condition. Concluding, also, that from the conformation of the neighboring mountain-chains and their peculiar weathering, the glacial theory is inadmissible, he advances the supposition that loess is a subaƫrial deposit, and that its fields are the drained analogues of the steppe-basins of Central Asia. They date from a geological era of great dryness, before the existence of the Yellow and other rivers of the northern provinces. As the rocks and hills of the highlands disintegrated, the sand was removed, not by water-courses seaward, but by the high winds ranging over a treeless desert landward, until the dust settled in the grass-covered districts of what is at present China Proper. New vegetation was at once nourished, while its roots were raised by the constantly arriving deposit; the decay of old roots produced the lime-lined canals which impart to this material its peculiar characteristics. Any one who has observed the terrible dust-storms of Northern China, when the air is filled with an impalpable yellow powder, which leaves its coating upon everything, and often extends in a fog-like cloud hundreds of miles to sea, will understand the power of this action during many thousand centuries. This deposition received the shells and bones of innumerable animals, while the dissolved solutions contained in its bulk staid therein, or saturated the water of small lakes. By the sinking of mountain-chains in the south, rain-clouds emptied themselves over this region with much greater frequency, and gradually the system became drained, the erosion working backward from the coast, slowly cutting into one basin after another. "With the sinking of its salts to lower levels, unexampled richness was added to the wonderful topography of this singular formation.

Mr. Pumpelly, while accepting this ingenious theory in place of his own (that of a fresh-water lake deposit), adds that the supply of loess might have been materially increased by the vast mers-de-glace