THE GEOLOGIC HISTORY OF CHINA 



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

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

 quence of this law, the parts of China which were slightly bulged above 

 their original level were re-attacked by the branching systems of rivers 

 with renewed vigor. By carving out the softer rocks, these have made 

 deep valleys with intervening mountain ranges. Some of the larger 

 rivers, such as the Yang-tze-kiang, maintained their courses in spite of 

 the slow uplifts directly athwart their courses. A result is the magnifi- 

 cent series of gorges along the central Yang-tze where the great river 

 has sawed its way through a slowly rising mass of hard complexly 

 folded rocks. 



On the other hand, the broad areas which were depressed not only 

 below the general level of stream action, but below sea-level, were rap- 

 idly filled with sand, loam and clay washed down out of the adjacent 

 mountains by the streams. The process of filling the depressions is 

 the exact complement of the process of etching out the highlands. No 

 doubt the rivers have been able in large measure to keep pace with the 

 sinking movement of the ground, so that great rivers like the Huang-ho 

 may have maintained perfectly graded courses across the region of 

 depression from the mountains to the sea. While thus engaged in 

 building up its channel, the river in time of flood frequently breaks 

 through its low banks, shifts its channel, and then begins to fill up a 



Fig. 13. Mountain Slopes in Northwestern China, terraced to prevent 



the erosion of the loess. 



