INI'ILLING OF THE VALLEYS 287 



fact, however, there is no such localized arrangement traceable. The 

 detrital slopes have a remarkably even and continuous upper limit next 

 the erosion faces, though they are commonly higher between the torrent 

 mouths than elsewhere. This peculiarity is probably due to the fact 

 that the positions of the mouths of the torrents are evidently liable to 

 frequent changes, and that in the geologic periods during which these val- 

 ley deposits have been accumulating this constant alteration would tend 

 to effect an even distribution of the material brought down by the trib- 

 utary streams. 



The changes in the positions of the torrents are brought about in sev- 

 eral different ways. Thus the process known as the robbing of one 

 drainage basin by another is of very frequent occurrence in this re- 

 gion. Instances of it may be noted in almost any area that is atten- 

 tively examined. Again, where, as in most mountainous districts, the 

 strata have varied inclinations, the channels are led by the attitudes of 

 the beds to change their positions as they work downward. Yet, fur- 

 ther, the heaping up of debris in the center of a detrital fan tends to 

 divert the streams to the margins of the steep delta, so that the range of 

 the deposition is greatly extended. Moreover, the various til tings to 

 which the several areas have been subjected have doubtless served to 

 change the course of these streams and the accumulation of their dis- 

 charge. All these influences and probably others not yet discovered 

 have served to bring about in course of time a tolerably even distribu- 

 tion of the materials brought down b}^ the steep graded streams into 

 the broad valleys. 



Turning now to the main rivers which dram the valleys, let us con- 

 sider what are the conditions of their action during the period when the 

 greater part of the precipitation is in torrential rains. As it is charac- 

 teristic of these rains to be very local, they often being limited to a few 

 square miles of area, the amount of water they send into the rivers is 

 too small to have much effect upon their volume or speed. Moreover, 

 for the reason that the detrital materials of the valley are very open in 

 structure they quickly absorb much of the flood, so that the reinforcement 

 of the main stream is apt to be slight, while the water which comes to it is 

 heavily charged with debris too coarse for the slower movement to bear 

 away. The result of these conditions is that the streams in the center 

 of the broad valleys tend to aggrade their channels, while in normal 

 rivers the contributions of detritus brought to them by torrents come so 

 slowly to their beds that the materials have a chance to decay and break 

 up to a state where the}^ are readily transported in solution, in suspen- 

 sion, or by being rolled along the bottoms. In these abnormal streams 

 the excessive discharge from the torrents cannot be thus removed. As 



XLI— Bull. Geol. Soc. Am., Vol. 12, 1900 



