Page:Popular Science Monthly Volume 82.djvu/115

Rh gradient; and on the other hand, if any part has too steep a declivity, it is gradually worn down to the proper slope. In consequence 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 magnificent 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 rapidly 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