THE KURUK-TAGH AND JING-PEN. 29
does it attract it? My answer to this is, that it must of necessity repel the river, — that is on the assumption that the latter is situated sufficiently near to the base of the mountains. The line a a, a„ in fig. 27 and 28 indicates the vertical section of a mountain-slope, with a plain at its foot along which a river flows parallel to the base of the mountain. Matter is removed from the face of the slope by weathering in the same proportion as the detritus-scree at its foot, composed of the accumulated products of the disintegrating process, goes on increasing After a time the outline of the section assumes the shape shown by b 6, b,,. As this process is accompanied by an extension outwards
Fig. 2 7.
of the base of the detritus-scree, which at the same time grows flatter, the bank of the stream lying next the foot of the mountains is necessarily encroached upon. The mountains thus exercise a certain pressure upon the river, and compel it to shift its bed farther away from their base. In the sketch the vertical elevation is of course to a large extent
Fig. z 8.
exaggerated. But in reality, under the circumstances supposed, a transportation of material must take place, and what does take place is exactly what has happened at the southern foot of the Kuruk-tagh. The process is an extremely slow one, because of the rarity of the rainfall; but in this particular region, where the prevailing wind blows from the north-east, the rainfall has an active ally in the wind. During the course of centuries and tens of centuries the products of disintegration, which, after being loosened from the mountain sides by the frost and the action of the atmosphere, are deposited through the agency of the rains, that fall periodically with greater or less violence, cooperate gradually to raise the height of the detritus scree;
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