国立情報学研究所 - ディジタル・シルクロード・プロジェクト
『東洋文庫所蔵』貴重書デジタルアーカイブ

a continuous zone of loess lands covered with grass. From the time when the
domestication of the horse and ox made possible the movements of organized
bodies of men, these grassy plains have been used as an easy road for the migration
of whole populations and for the devastating irruptions of barbarian hordes.

To turn again to the more peaceful side of the subject, southern Russia,
Hungary, Austria, and the valley of the Rhine enjoy great profit from this fertile
soil; and this is true to an even greater extent in the United States over the loess-
covered areas of Iowa, Missouri, and Dakota, where it compensates for the lighter
rainfall.

All previous geologists had considered it to be a sedimentary deposit, laid
down in still water, and this was the view also of the writer after seeing it in some
of the connected basin-like depressions of northern China and Mongolia in 1863.
But later Richthofen, in his extensive exploration of China, found it mantling
mountains several thousand feet high and containing there only remains of land
animals and land snails. He found also that waters eroding this mantle deposited
it in basins and on flood-plains, where it then often contained pond and river
shells. As a result of his studies he found that the loess had been brought by winds.
In developing his theory Richthofen showed that certain widely existing conditions
were necessary to the formation of this product, so important to man. These
conditions he explained in connection with the broad and fundamental theory,
mentioned above, of the effect of climatic conditions in the "central" parts of
continents as contrasted with the coastal or "peripheral" zones. All water is
brought by winds from the oceans, and, generally speaking, by far the greater
part of the moisture that is not precipitated in the coastal regions is condensed
on the mountain ranges bordering the interior. The streams that flow from these
mountains toward the central area sustain vegetation only where they can be
spread over the land, leaving practically the whole vast area in a more or less arid
state, excepting where more or less precipitation occurs over a zone near the
mountains.

It was in the action of the winds on the unprotected surface of these plains
that Richthofen found the source of loess. Wind not only takes up dust, but
it wears away slowly and surely all soft, unprotected rocks, and, following lines
of weakness, it removes thick strata of harder texture, leaving masses carved into
fantastic forms, towers, and pinnacles, often hundreds of feet high, to prove the
gigantic scale on which it can work with the aid of time. The products of this
action may be classed as dust, sand, and still coarser material. The wind itself
makes this classification. Separating the sand and dust from the coarser detritus,
it carries the dust afar, while the sand is moved slowly forward, piling up into
wave-like dunes, which progress by drifting away from the windward slope to
accumulate on the lee side. As long as the dust is borne over a barren surface,
it is ever at the mercy of the wind; but in many places near the borders of the arid
region, as in northern China and southern Mongolia, there is sufficient precipitation
to permit the ground to be covered with grasses adapted to such a climate. Once
caught in this grass it remains to nourish the growth that is to continue the process