no 



THE POPULAR SCIENCE MONTHLY 



Fig. 12. Soil Reservoirs on a Hillside in the Loess Country. 



covered northern India and part of Thibet. In that sea were deposited 

 the thick beds of limestone which are now found in some of the west- 

 ern mountain ridges. 



Again in the Miocene period, the forces of distortion within the 

 earth accumulated to such strength that they were able to repeat the 

 mashing and folding, but this time the area affected lay farther to the 

 west and south. At the same time, or perhaps earlier, the eastern part 

 of China was cracked in various directions ; and the intervening blocks, 

 settling somewhat unevenly upon their bases, left a group of escarp- 

 ments and depressions comparable to those now to be found in western 

 Nevada and southern Oregon. As before, the work of erosion and the 

 leveling of the surface was at once accelerated, so that even before the 

 deformation had spent itself the blocks were deeply scarred. It is un- 

 certain how far this period of erosion succeeded in reducing China to 

 base-level. The consummation may have been prevented by gentle 

 warpings of the surface, rising very slowly here and sinking there. 

 When compared with the great breadth of the areas affected, these 

 changes of level seem very slight, but they are nevertheless sufficient to 

 cause great changes in the aspect of the country. 



It is one of the basal principles of physiography that streams tend 

 to produce in their channels an almost uniform slope from their head- 

 waters to the sea. If any part of the channel is so flat that the stream is 

 too sluggish to carry sediment, it is built up until it reaches the re- 



