DEGRADATION. 193 



instrument of corrasion, and again declivity is multiplied as a factor in 

 degradation. 



To this rule that sapping promotes corrasion by adding to the instru- 

 ment of corrasion, there are three curious exceptions, one of which is of 

 great importance. The first is where the cliff produced by deep corrasion, 

 before it has retreated from the bank of the stream, tumbles down so as to 

 choke the stream. In this case the material falling from the cliffs serves to 

 protect the underlying stream bed, and, so far as it ponds the water up stream, 

 it causes it to precipitate its load, and thus deprives it of the instrument of 

 corrasion. But this fallen matter is rapidly attacked by the stream, is 

 soon taken up as load, and used as an instrument of corrasion. 



The second exception is where a lateral stream enters a main one, the 

 lateral stream having so great a declivity as to be able to transport great 

 quantities of coarse material during local flood time, or that flood time 

 which pertains to the secondary stream, but not to the primary. In this 

 case it may sweep along its greatly -inclined bed into the main channel, 

 load derived from sapping, or even from erosion, which cannot be farther 

 transported by the main stream, and this new matter for the time being 

 serves as a protection to the bed of the main stream. In the Colorado 

 River, with very few exceptions, all the falls and rapids which beset its 

 course through the great canons are caused by dams made by side streams 

 having great declivity. A few of the falls are made by dams formed by the 

 sapping of the immediate cliffs of the main river. 



The third exception is found where streams having great declivity run 

 through beds of incoherent sands. Here load is rapidly added to the stream 

 by gravity, and the stream not being confined by water tight rocks, its 

 Abaters penetrate the interstitial spaces of the sands and serve to overcome 

 the friction of slope, and thus assist rock power in transporting the sand 

 into the stream. Here the stream cannot have high banks, and the channel 

 is greatly widened and is engaged in transporting load furnished it from the 

 sides, and can make but little progress in corrasion, for every particle car- 

 ried from the bottom of the stream is rapidly replaced by one from the 

 sides. This condition is finely illustrated in many places along the course 

 of the Virgin River, a tributary of the Colorado. In one place where its 

 13 P G 



