PHYSIOGRAPHY OF NORTHWESTERN CHINA. 257 
by effects of warping, and which flows among some of the most striking 
ranges. It seems, therefore, to furnish the name which, better than any 
other, may serve to designate the time during which the river itself was 
affected and during which the great mountain growths that parallel its 
valley were developed. Accordingly, we name the recent epoch of domi- 
nant mountain growth in northwestern China the Fén-ho epoch. 
The features of the Fén-ho epoch are elevations and canyons. The 
attention is naturally attracted by the great elevations and by the deep 
canyons, but dimension is not one of the characters by which we may 
discriminate the features of the stage. The elevation may be slight and 
the canyon will be correspondingly shallow; or the elevation may be great, 
even as we speak of the greatest mountains, and the canyon will be corre- 
spondingly deep, even among the most profound of gorges. 
Features of the Fén-ho stage are characterized by youth and may also 
be distinguished by their relations to surviving surfaces of earlier epochs. 
The youthful aspect of the canyons is obvious in the field and it may also 
be noted in the topographical maps of the atlas as well as in several illus- 
trations of this volume. 
The vigorous autogenous valley of the upper Sha-ho; the inner canyon 
of the T’ai-shan-ho—both in the Wu-t’ai schists and Sinian limestones; 
the canyons of the Shi-t’ou-ho, Sing-ho, and Hu-t’o-ho in the eastern 
Ki-chéu-shan; the gorge of the Fén-ho between Ling-shi-hién and P’ing- 
yang-fu—these are plainly young valleys which have not yet progressed 
beyond the initial stage of development, that of vertical incision. 
Similarly, the southeastward slope, in which the upper Sha-ho valley 
is growing; the warped surface south of the Loess Basins; the fault-scarps 
of the Ki-chéu-shan, Ho-shan, O-shan, Féng-huang-shan, Hua-shan, and 
Ts’in-ling-shan, and the warped surfaces into which they pass; the upwarp 
across the valley of the Fén-ho between Ling-shi-hién and P’ing-yang-fu, 
and that which caused the westward diversion of the lower Fén-ho— 
these are all steeply inclined surfaces, which, whether they be fault-scarps 
or surfaces due to erosion on warped slopes, are now being attacked by 
autogenous consequent streams, whose growth is still very moderate. 
Considering the declivities, the limited development of ravines implies 
very recent warping and faulting, and the implication is strengthened 
by the apparent persistence of the Huang-t’u deposits on some warped 
surfaces. A qualifying factor, which tends somewhat to lengthen the 
estimate of time since warping and faulting began, is found in the aridity 
of climate, which lessens the effectiveness of corrasion. The value of 
this factor is probably large, and it serves to keep any estimate of moun- 
tain growth within normal limits, which it might otherwise transgress. 
