PHYSIOGRAPHY OF NORTHWESTERN CHINA. 239 
The relief of the T’ang-hién stage—that is, the height of the hills above 
the adjacent valleys—varies from 300 feet, 90 meters, to possibly 1,000 
feet, 300 meters, with a general average of 500 feet, 150 meters, or less. 
The forms of the surface are usually rounded and smoothly modulated. 
In the distribution and extent of the valleys and in the insignificant alti- 
tude of the divides the topographic aspects closely resemble the features 
of Shan-tung. The wide valleys are the result of erosion, their continuity 
across divides is the result of head water competition, and the isolation 
of the numerous low hills has followed from the extensive development 
of every line of drainage. 
Areas having the characteristics of the T’ang-hién stage were recog- 
nized throughout our route across Chi-li and Shan-si. In the neighborhood 
of T’ang-hién, where the surface rises from beneath the great level, plain 
of eastern China, it appears first in isolated island-like hills, and may be 
recognized more extensively in the flat valley plain and the adjacent hills 
northeast of Si-ta-yang, and in the Ning-shan basin. The elevation of 
the valleys of the T’ang-hién stage is here about 400 feet, 120 meters, 
above sea, but as the surface sinks beneath the alluvium of the plain 
and the hills greatly diminish and are buried eastward, it is probable 
that it soon passes below sea-level. It is possible that features of the bad 
lands along the Sha-ho east of Féu-p’ing-hién to some extent represent 
it, but no definite recognition of any specific form is possible in that deeply 
denuded region. The characteristic forms are also wanting on the upper 
Sha-ho. It was not until we reached the divide above Lung-ts’tian-kuan 
that we were again able to see, in the profiles of the Wu-t’ai-shan, the 
outlines of the surviving features of mature aspect which might be assigned 
to the T’ang-hién epoch. These features have been referred to in previous 
descriptions and are illustrated in Plate XXX, where they are designated 
by the letter “b.”’ 
After recognizing the T’ang-hién stage in the Wu-t’ai-shan, we lost 
sight of its representative features in descending to the Loess Basins, and 
did not again find them until we studied the region about Wu-t’ai-hién. 
In the mountains of Paleozoic limestone southeast of Wu-t’ai-hién, the 
broad uplands clearly present the surface of mature erosion, which we 
assign to the T’ang-hién, and this mature surface was traced from the 
summits of the limestone mountains northwestward into the valley at 
Wu-t’ai-hién, and was recognized as prevailing in the hills all about tht 
Loess Basins. It there has the characteristics which it exhibits near 
T’ang-hién, the hills being sculptured from rocks of similar weakness and 
being to a great extent buried in the prevailing Huang-t’u formation. 
The aspects of the Shi-ling pass and of the old river valley in the 
vicinity of Huang-t’u-chai are typical of the T’ang-hién stage. The moun- 
