PHYSIOGRAPHY OF NORTHWESTERN CHINA. 241 
Another fact which makes difficult the recognition of the T’ang-hién 
surface is the lack of constant relations to features of earlier or later stages. 
It commonly happens in a district, where the development of successive 
physiographic stages has been somewhat uniform, that one may find the 
features of different epochs represented in regular order in the mountain 
slopes as one ascends from canyon to summit. Under such conditions 
the youthful canyon and older valley floor in which the canyon is cut 
may be recognized. There may be other stages of older valley floors, 
and perhaps a higher level which represents the peneplain, the oldest 
surviving topographic feature. Where a complete sequence of features is 
thus preserved, the recognition of any one is aided by its relation to the 
others. The case is similar to that in the study of strata, where we some- 
times identify a geologic formation by its relation to clearly defined strata 
above or below, even though its own characteristics are somewhat obscure. 
But in reference to features of the T’ang-hién stage we are unable to apply 
this method of identification, since the development of the topography 
of the region has been unlike in different districts. Thus in the area in 
which the mature features of the T’ang-hién stage are now elevated, as 
in the Wu-t’ai-shan and Ki-chéu-shan, they are seen to be dissected by 
younger canyons. But in the vicinity of T’ang-hién and in the Loess 
Basins, the surface which we assign to the T’ang-hién stage is not dis- 
sected. It is the latest surface of erosion. There are indeed some excep- 
tions to this general statement; for instance, the valley of the Hang-ho 
above Si-ta-yang is characterized by some youthful aspects, which have 
been attributed in the preceding description to the fact that the stream 
was superimposed upon the low divides of the underlying mature topog- 
raphy; but as a whole it is true that, in the topography of the southeast 
foothills or of the Loess Basins, there are no widely developed features 
younger than those of the T’ang-hién epoch. In these districts, on the 
contrary, instead of being dissected, the features of the T’ang-hién stage 
are covered by the Huang-t’u formation. The areas in which they are 
thus buried are distinguished as areas of aggradation from those of degra- 
dation, in which they are cut by the canyons of a later epoch. 
Though neither altitude above sea nor position in sequence of topo- 
graphic forms can be relied upon as criteria by which to identify the T’ang- 
hién stage, its widely separated occurrences are correlated with confidence 
on the basis of a common physiographic phase, that of advanced maturity. 
Maturity is a phase reached only after erosion has been active during a 
considerable length of time. When the features of advanced maturity 
are stamped upon an extensive region, the epoch of degradation must be 
recognized as one of notable duration, even geologically speaking. Hence 
the correlation of features as belonging to the T’ang-hién stage is not a 
