226 RESEARCH IN CHINA. 
THE DEVELOPMENT OF STREAMS. 
Before proceeding to discuss the evolution of topographic forms in 
historic sequence for the entire region, I propose to take up the growth 
of the several rivers and to describe each of them in turn, thus obtaining 
an assemblage of facts which can later be ordered in historic sequence. 
The T’ang-ho and the Hang-ho.—These two streams flow in parallel 
courses from the high mountains of northwestern Chi-li to the Great Plain, 
and have in any generalized map the directness and simplicity of conse- 
quent streams developed upon the mountain slope. But in describing 
the features along our route it has been stated that in detail they present 
many facts expressive of a complex history, and a closer analysis shows 
several successive stages of development. The further discussion may 
begin with their lower courses in the Great Plain. 
Referring to the T’ang-hién atlas sheet, F I, the T’ang-ho may be 
seen debouching from the mountains of the Ning-shan basin, and flowing 
between the summits of the submerged hills out upon the Great Plain. 
Its course is aggraded and determined upon the alluvial plain as are the 
courses of a river in its delta. In general character the T’ang-ho presents, 
in fact, an example on a small scale of the rivers which enter the plain, 
including even the great Yellow river. It takes its course down the slope 
of the plain, building up its banks after the manner of overloaded streams 
where the fall is insufficient; but near the foothills, within the area of 
our survey, maintaining a close balance between filling and corrasion, so 
that it spreads out in sandy flats and neither lowers nor raises its channel. 
Its course under these conditions is determined by the general slope of 
the surface in the practically homogeneous material of the Huang-t’u 
formation. Were it, in consequence of an uplift, to sink its present channel 
in the soft alluvium, it would discover a very uneven surface of hard rock 
beneath, and its valley would be characterized by short canyons in the 
hard rocks and open basins where the alluvium was still deeper. 
The description of the T’ang-ho in the preceding paragraph applies 
to it as far up as Si-ta-yang. Above that point the southeastern range 
of the Ning-shan basin is of so continuous and elevated a character as to 
introduce a seemingly new phase of relations. The Hang-ho, for example, 
flows within a rather well-defined canyon across the mountains; the wide 
valley plain and the prevailing cover of the Huang-t’u formation are lack- 
ing, and the condition under which the stream took its course across the 
height is not immediately clear. It is necessary to examine the features 
with reference to their relative age to reach an understanding. 
The Hang-ho flows, as I have said, in a narrow valley which locally 
has the aspect of a canyon. Never very continuously or narrowly shut 
