1873.] DREW TTPPER-INDTTS BASIN. 469 



The problem that now presents itself is to account for the streams 

 and rivers being at one time denuders, at another accumulators, and 

 at a third denuders again, — in other words, to discover why these 

 streams shall at one time lower their beds, cutting down through 

 the rock, at another raise them by depositing alluvium and rising 

 upon it, and at last again lower their beds by cutting down through 

 that alluvium. 



In any river, taking it in the round of a year, there is sure to 

 be, at all parts of its length, a downward movement of material. 

 If at any particular spot there is a greater amount of material 

 brought from above than is carried off down, accumulation occurs 

 there ; it need not be that more is carried down past that spot, 

 but at all events more arrives than is carried away. Vice versa, if 

 at any particular point in the river's course the amount of mate- 

 rial carried away is greater than that supplied from above, then 

 there is denudation or lowefing of the river-bed ; in this case the 

 supply from above is not necessarily, nor probably, cut off, but it is 

 less than what is carried away. 



In the diagram (fig. 13), let us take a, b, c, 'd, e and / to be 

 portions of the course of a stream, in each of which the action may 



Fig. 13. — Diagram illustrating river-action in the deposition and 

 denudation of alluvium. 



5 



be considered equable for that length ; and let the part marked m 

 represent the sources of the river in the mountains, where loosening 

 and transportation of material takes place, but not the formation of 

 alluvium ; then let us for a first case suppose that alluvium was 

 accumulating in the spaces a to /; it must follow that the material 

 moved through a is less than the material supplied to a ; but the 

 material supplied to a equals the material moved through b ; there- 

 fore the material moved through a is less than the material moved 

 through b. For the same reason the material moved through b is 

 less than that moved through c, and so on. Lastly, the material 

 moved through / is less than that supplied to / ; or, in other words, 

 the material moved through / is less than that which is supplied to 

 it from the region of waste. That is to say, there is a greater 

 amount of material moved as one goes up stream, and the amount 

 supplied from the rocks is greater than that which is moved through 

 the alluvial district. 



On the contrary supposition, which corresponds to the last state of 

 things that has occurred, of the stream deepening its channel, cutting 

 down through its alluvium, the material moved through a is greater 

 than that supplied to a ; but the material supplied to a equals that 



