DAVIS: THE GRAND CANYON OF THE COLORADO. 169 



the river is hardly more than a transporting and comminuting agent, 

 that brought Powell's boat journey within the limits of possibility, even 

 while the lingering rapids made it terribly dangerous. The occurrence 

 of rapids on the down-stream side of boulder heaps that have been 

 washed into the main canyon by the flooded streams of side chasms, 

 also indicates a retarded rate of corrasion by the main river. Powell 

 mentions several such obstructions (a, pp. 82, 83, 97) ; Gilbert gives a 

 more explicit account of them and points out their probably temporary 

 character. He says that many of the tributary canyons " are of very 

 rapid fall, and are occasionally traversed by powerful torrents, which 

 sweep down bowlders of great size — in some instances 10 or 15 feet 

 in diameter — and heap them in the main canon in dams, that must 

 often be of great depth. Over each of these the water finds pass- 

 age at the edge opposite the tributary, and descends the lower slope 

 with swift current and broken surface ; and thus arise the great major- 

 ity of the rapids." It is further pointed out that there are times " when 

 each of these dams in turn is removed ... so that, while the dams will 

 recur at the same localities and with the same characters, they can- 

 not be regarded as strictly permanent" (a, p. 71 ; see also Dutton, c, 

 p. 241). 



It may therefore be concluded that although the canyon is still to be 

 deepened, the river as a whole approaches and for considerable parts of 

 its course actually reaches the delicate balance between degradation and 

 aggradation which characterizes a maturely graded stream. After such 

 a condition is reached, further deepening of a valley is possible only with 

 the decrease of the load furnished to the main stream by its side streams, 

 in the later maturity of the region. 



Junction of Trunk and Branch Streams. — It is now to be deter- 

 mined how the side streams join the main river at this relatively early 

 stage in the present cycle of the Colorado. The importance of this 

 question turns on the prevailingly discordant junction of side and main 

 valleys in strongly glaciated mountains, as has recently been pointed 

 out by De Lapparent and Richter for the fiords of Norway, by Gannett 

 for Lake Chelan in the Cascade range, and by Penck for the valleys of 

 the Alps. I have lately prepared a general review of this problem in 

 connection with my own observations on the valley of the Ticino in 

 southern Switzerland and elsewhere in Europe (c, p. 310; d, p. 138). 

 It has been contended hy some that the discordant junction of side and 

 main valleys in the Alps is the normal result of the more rapid deepen- 

 ing of the main valley by its larger stream, even to the point of thus 



VOL. XXXVIII. — no. 4 5 



