CHINA, MONGOLIA, AND JAPAN. 41 



of the process which is causing the retrogression of Niagara falls, and which pro- 

 bably plays an important part in all valley erosion. 



In intimate connection with this loam-deposit, stands the formation of the 

 numerous isolated lakes met with on the route through the region we are now 

 considering. I have frequently alluded to bars, or low watersheds, formed of the 

 terrace-deposit, and stretching across valleys, causing the drainage to flow in oj)po- 

 site directions. These form the barriers to which almost every lake or pond, that 

 has been mentioned, owed its existence after the retreat of the main body of the 

 great inland sheet of fresh water. 



We have seen that in those broad valleys where the lake-deposit has not been 

 much subjected to erosion, its surface is not horizontal throughout, but rather, 

 adapting itself to the general surface of the ground, or ancient valley, on which it 

 lies, it rises from the centre to high on the sides of the surrounding mountains. 

 Now when the sides of a valley approach each other and form a gorge connecting 

 two broad enlargements of the valley, the terrace-deposit rises from the centres of 

 both these basins, till it fiUs the gorge to about the same height as that at which it 

 stands on the mountain sides around the basins. The height attained by the lake 

 deposit in these narrow places is, in almost every instance, due to the fact that the 

 usual deposit of loam was augmented by the large amount of detritus from the 

 bordering hills. 



As the large inland body of water disappeared and sank to the level of each of 

 these bars, the sheet behind this remained isolated. In some instances the lakes 

 thus formed have found outlets by cutting through their bars, but this was only 

 where they received an important supply of water, derived from an extensive drainage 

 area. In all other cases the barriers have suffered comparatively little from erosion. 



Since their isolation these lakes have diminished in size, till they now possess but 

 a small fraction of the volume necessary to fill their separate basins to a level with 

 the surface of the inclosing bar. 



I now propose to consider briefly the conclusions which the facts observed in this 

 part of northern China seem to warrant. 



The oldest stratified rocks seen throughout this region are highly metamorphosed 

 and appear to belong to two distinct epochs ; the hornblendic and chloritic series of 

 schists representing the older, and the gneiss and granulite series, the younger. 



After the deposition of the older metamorphic strata there seems to have been a 

 disturbance producing folds with a trend between N. and W. Disturbances had 

 also occurred by which the ridge between Nankau and Chatau was elevated and 

 again depressed before the deposition of the great limestone formation, for the beds 

 of this latter rest here immediately on the granite. Northwest of this ridge the 

 limestone would seem to have been deposited in a shallower part of the sea, the 

 character of the Hwaingan beds — which appear to represent the limestone — indi- 

 cating the neighborhood of land. 



After the deposition of the limestone strata these were traversed by the eruptive 

 porphyries of Hiamaling, the debris of which form the chief ingredient of the con- 

 glomerate lying between the limestone and the coal-bearing series of Chaitang. 



The next marked event was the forming of the coal-bearing rocks. 



6 May, 1866. 



