388 



ANNUAL BEPOBT SMITHSONIAN INSTITUTION, 1913. 



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 



SOUTH-WCST 



wmmmaim^mmB^^mi^^^ 



FRC-CMABRIAN 



JURASSIC 



CHETACEOUS - EOCENE 



/MIOCENE 



SHANTUttO MTS. 



PRESENT 



Fig. 2.— Diagrams to illustrate Geological Conditions in China at Different 



Periods in its History. 

 The features are neces.sarily much generalized and in part hypothetical. 



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

 required gradient ; and on the other hand, if any part has too steep 

 a declivity, it is gradually worn down to the proper slope. In con- 

 sequence of this law, the parts of China which were slightly bulged 

 above their original levels were reattacked by the branching systems 

 of rivers with renewed. vigor. By carving out the softer rocks, these 



