INFILLING OF THE VALLEYS 283 



tent formed in lakes. There are, however, various possible explanations 

 of the infilling action which need to be examined. In approaching the 

 inquiry it is well to see clearly that the main question is as to the ways 

 in which the detritus swept from the fields of erosion in the mountain 

 ranges has come to be retained in the valleys near its sources, or, in 

 other words, how the disturbance in the normal balance between the 

 agents which serve to break up rock materials and start them down- 

 ward and those which provide for their distant transportation has been 

 disturbed. It is evident that while these valleys were in process of ex- 

 cavation the rivers were competent to carry away all the debris that 

 came to them, and that for a long time they have been unable to effect 

 this work. The question is as to what has brought about this change. 



There are several diverse influences which may serve to diminish the 

 capacity of streams for carrying detritus to the sea. These may be 

 stated as follows : First, the surface over which they flow may be so far 

 lowered that the grades from the base line upward may be too slight to 

 give the rivers the speed required for carrying the amount of debris that 

 is brought down to them by the torrents ; second, by a diff'erential move- 

 ment of the lands the valleys may be so tilted that their outlets are dis- 

 turbed and their exit grades diminished ; third, by change in the adjust- 

 ments of the rainfall and of the atmospheric decay of the rocks, which 

 goes far to determine its erosive value, the amount of material brought 

 down from the heights into the valleys may much exceed that which 

 the streams can bear away to the lower country or to the sea, and the 

 state of this eroded material may be such that it is not readily moved 

 by the rivers. 



As for the first of the working hypotheses above suggested, it may be 

 said that the general subsidence of the Cordilleras, or possibly a rise of 

 the sealevel, or both movements combined, has probably brought the 

 bed-rock floors of these valleys nearer to the ocean level than they were 

 when the process of excavation was completed. Yet these troughs 

 remain, in most instances, so high and their grades are so steep, as com- 

 pared with others which have not become occluded, that we can not 

 regard any hypothesis dependent on a change of grade as in itself suf- 

 ficient to account for the facts. As for the second suggestion, it may 

 again be said that it demands a complicated series of movements in the 

 way of warping. It will hereafter be noted that such local movements 

 due to isostacy have probably taken place, and that their efi'ect has 

 been to bring the bottoms of many of the valleys to a lower level than 

 they occupied when they were in process of excavation ; but these move- 

 ments are a consequence of the infilling and not the cause of that ])ro- 

 cess. Though they helped to extend the work after it was otherwise 



