PHYSIOGRAPHY OF NORTHWESTERN CHINA. Zay, 
in by walls, it nevertheless is bounded by occasional bluffs, which rise 
almost precipitously for 100 feet, 30 meters, or more. These give it a 
somewhat youthful aspect and would lead one to expect, if the relief of the 
district had had a uniform history, that the ravines among the adjacent 
mountains would be narrow V-shaped valleys. But this is not the case. 
Among the hills the valleys are comparatively broad, and they are so 
continuous, both along and across the range, as to divide it, even within 
the limited area of our observation, into many separate groups of heights. 
The aspect of the range indeed reminds one of the mature features of 
Shan-tung, or suggests a mountain mass like that just north of T’ang-hién, 
the only difference being that the one is exposed to its base and the other 
is largely buried. 
Since the canyon of the Hang-ho is young and the valleys of the 
mountain district are relatively mature, we are shut out from any explana- 
tion which involves the assumption that the river is older. For example, 
we may not assume that the Hang-ho had its present course on a pene- 
plain above the mountain summits, and has sunk its canyon after the 
fashion of an antecedent river, during the upwarp of the mountain range. 
Were that the case the valleys among the mountains would exhibit the 
features of youth which characterize the Hang-ho. Nor does it seem 
probable that the Hang-ho, as a tributary of the T’ang-ho at Si-ta-yang, 
has grown by headwater erosion from the proportions of a small gully 
not more than a couple of miles long, across the range, and so diverted 
whatever drainage formerly existed in the Ning-shan basin. The differ- 
ences of elevation on the two sides of the range, and the watershed which 
we might assign to the incipient Hang-ho, are both too limited to afford 
foundation for such a hypothesis. 
To understand the course of the Hang-ho above Si-ta-yang, let us 
consider the course of the rivers below that point, and the peculiar fea- 
tures of the buried topography. It is evident that, beneath the plain, 
there is a very mature topographic surface which emerges from under 
the Huang-t’u formation along the southeastern side of the Ning-shan 
district. The mountains bounding that district are of that mature stage 
of development. Before the accumulation of the Huang-t’u formation 
they had reached a degree of valley erosion resulting in the wide expansion 
of the valley plains and the isolation of the hills. With the accumulation 
of the Huang-t’u formation, the hills were to a greater or less extent buried, 
and the streams, in whatever direction they flowed, acquired aggraded 
channels on a surface above the bed-rock. In describing the region, 
reference was made to the valley plains between T’ang-hién and Nan- 
t’ang-mei, and in the Ning-shan basin, which represent the courses of 
