destruction must be much slower; the northern body of the plateau will hold a greater elevation than the southern, and the northern face of the great uplift will be more nearly vertical. (Hence it is that history must record the giving over of Tibet to a southern, not a northern power.)
As time goes on this giant mound must grow smaller in every dimension—for Neptune will have it that all the mountains of the earth shall be dragged down to the sea —and he sends up hourly millions of little rain-drop coolies who dig the very rocks away. If the structure of such a great mass be relatively homogeneous, the wear will be less ragged, particularly on the wide top, at considerable distance from the much-disturbed edges. But if there be somewhere a line of soft material, or if the tilt of the surface, however small, shall chance to throw considerable volumes of water along a given line, even of average hardness, then we shall find a great depression—as that of the Blue Nile, whose steep gorge descends five thousand feet below the neighbouring plateau elevation—yet the river-bed is still four thousand feet above sea. And so here in Tibet the long line of the Tsang-Po, or upper Brahmaputra, flowing in a depression which begins many miles away on either side of it, lies also about five thousand feet below the northern (relatively undisturbed) plateau, and about eleven thousand feet above sea-level. It leaves to the south, bordering its east and west course, high lands and great peaks ere the true descending ramp be met.
Such a gash having been formed, the process of denudation is hastened, because the width of level table-land is diminished, and the small surface streams become less sluggish. Wherever the elevation has been so lessened that snow-coverings are removed by summer warmth, there enters another element tending to quick removal
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