PHYSIOGRAPHY OF NORTHWESTERN CHINA. 255 
time. We conceive that the valleys of the Fén-ho, Hu-t’o-ho, and other 
streams of northern Shan-si and Chi-li were covered by flood-plain deposits 
during a gradual subsidence, and the hills were buried as they sank till 
they were partially or wholly submerged in the Huang-t’u formation, as 
they are now in the typical district about T’ang-hién. 
If the preceding inductions are correct, eastern China was margined in 
Hin-chéu time by a plain, consisting of the confluent flood-plains of the 
streams flowing from the interior of the continent and bringing volumi- 
nous loads of loess and alluvial sand. These flood-plains extended north- 
eastward over the present site of the northern Loess Basins. To what 
extent they may have inclosed mountainous islands or promontories such 
as Shan-tung now presents, we are unable to determine with accuracy; 
but it is probable that such islands existed with moderate relief, though 
perhaps of considerable extent. There are areas, such as the summits of 
the Wu-t’ai-shan, which have not been deeply eroded since the T’ang-hién 
epoch or an earlier time, and which are covered with residual soil. We 
may regard this soil as a remnant of that mantle of saplite which cloaked 
the surface during the Pei-t’ai and T’ang-hién epochs. 
As the process of subsidence and aggradation proceeded, and the 
relatively shallow valleys filled till their flood-plains became confluent, 
conditions became peculiarly favorable to rearrangement of streams. 
The method of readjustment has been sketched in describing the Hang-ho. 
The effects are clearly recognizable in the growth of rivers, whose south- 
eastern courses are at right angles to the southwestern courses of the 
older systems. The Hang-ho, the Sha-ho below Féu-p’ing, the Hu-t’o-ho 
below Tung-yii, the Sing-ho, and the Shi-t’ou-ho (atlas sheets F I, E I, 
and C I, and Plate IL) are streams which are believed to have developed 
their present courses in consequence of aggradation. The cause of their 
growth is thought to have been a slight southeastward tilting, sufficient 
to determine the slope of the confluent plains in the direction in which 
the streams now flow. The rivers are regarded as consequent upon that 
slope. With reference to buried ridges, which they discovered as they 
sank their channels, they are superimposed streams—the Hang-ho and 
possibly the Sha-ho exhibit such features in the Ning-shan district— 
and with reference to the later mountain growths of the Foén-ho epoch 
they are antecedent. 
It is possible that tilting and development of consequent streams 
antedated the aggradation of the surface, and that the growth of the 
present southeasterly courses should be assigned to the close of the T’ang- 
hién epoch. The topography of the mature surface was no doubt then 
favorable, and the suggestion finds support in the fact that such valleys 
as that of the Sing-ho appear to have been cut out with a southeast course 
