228 RESEARCH IN CHINA. 
streams that flowed southwestward. These ancient streams were instru- 
mental in the development of the mature topography. It is probable 
that they maintained their courses during the accumulation of the Huang- 
t’u formation, and aided in distributing it. If we suppose that the accu- 
mulation of the Huang-t’u went on until the old rock divides—which were, 
in consequence of their age, very low—were buried, we should then have 
a region of more or less isolated hills, such as we now find between T’ang- 
hién and Si-ta-yang, but extending further northwestward, at least into 
the Ning-shan basin. Were this surface tilted downward toward the 
southeast, the growth of gullies in the Huang-t’u formation would take 
place in a northwest direction, across the lines of ancient drainage, yet 
within the transverse valleys of the old topography. The old drainage 
would be diverted, and such a stream as the Hang-ho would be the result. 
If, in consequence of the tilt, some part of the bed-rock surface were suffi- 
ciently raised, it would be denuded of the Huang-t’u formation and the 
old surface more or less clearly exposed. The valley of the Hang-ho 
would be in part characterized by features of the mature valleys, within 
which it had developed, and in part by youthful features carved from the 
ridges, upon which it had been superimposed. On the other hand, that 
portion of the old surface which, in consequence of tilt, sunk lower would 
become the area of aggradation and deeper burial, such as we recognize 
as the outer margin of the foothills. 
If the preceding interpretation be correct, we find between T’ang- 
hién and Ning-shan evidences of the following stages of physiographic 
development: 
n: A stage in which the relief of the region had reached advanced 
maturity with the development of wide valleys and more or less isolated 
hills and groups of hills. The streams of this stage flowed southwestward, 
and two of them are recognized in the valleys northeast and southwest 
of Ning-shan. 
n+1: The mature topography of the stage n was buried beneath 
the Huang-t’u formation to a depth sufficient to cover the rock divide 
between Si-ta-yang and Ning-shan. 
n+2: The surface was tilted southeastward, with the result that 
the rivers of the stage n+ 1 were cut into sections in consequence of 
the growth of streams on the sloping surface of the Huang-t’u. In some 
instances these sections appear to have retained enough of their headwaters 
to continue as parts of the present drainage, constituting a northwest- 
southeast element of the streams. Such is the T’ang-ho between Nan- 
t’ang-mei and Si-ta-yang. In other instances diversion has been at such 
intervals as to isolate a part of the old valley and leave it practically 
