DAVIS : THE GRAND CANYON OF THE COLORADO. 177 



portiouate development of the several elements, cliff, rock-house, talus- 

 slope, and platform on the valley side, should vary from that of the 

 corresponding elements, cliff, cave-of-the-winds, rock-heap, and reach 

 along the valley bottom, on account of the difference cf behavior 

 between a sluggish stream of waste and a nimble stream of water. In 

 the latter, the vertical element of form is reduced to a small fraction 

 of the horizontal at an early stage of the cycle; while in the former, 

 the vertical dimension remains strong until the stage of maturity is 

 past. 



Graded reaches make the greatest part of even a young river, but 

 graded platforms widen slowly on valley sides and attain no great 

 breadth until late maturity. It is along these graded reaches that the 

 finer waste from the rock-heap, as well as from further up-stream, is 

 steadily carried forward ; comminution of rock waste is active, but 

 erosion of the rock bed is here extremely slow. Rock-heaps under 

 waterfalls are so unimportant topographically that they are seldom in- 

 cluded in geographical descriptions; yet they are really characteristic 

 elements of a young river's course in structures of the kind here con- 

 sidered. Talus slopes on valley sides are of much greater dimensions ; 

 so great indeed that they form a systematically graded surface. Their 

 grade is steep compared to that of the platform because their waste 

 is coarse. These accumulations of coarse waste, both rock-heap and 

 talus, consist chiefly of fragments broken from the cliff above ; the fine 

 waste that is weathered from them is carried forward or down-stream, 

 while the surface on which they rest retreats backward or up-stream. 

 Bock-heap and cave-of-the-winds vary inversely; the sum of their 

 vertical measure together with that of the face of their fall ledge 

 (a constant) makes the height of the fall : talus-slope and rock-house 

 are similarly related. The cave-of-the-winds is larger but less habitable 

 than the rock-house ; but in the wet as well as in the dry cave, air- 

 movement is an important factor of erosion. The cliffs in the two cases 

 are so much alike that they need no further comment. 



The strongest fall-and-cliff-makers endure longest, while the less 

 strong ones are sooner worn back until they disappear under a graded 

 reach or platform (Gilbert, b, p. 100), or retreat into the cave-of-the- 

 winds or the talus-slope under a master cliff. Among the disappearing: 

 falls and cliffs, those which are down-stream from a very resistant ledge 

 •will be the first to vanish, because their more rapid recession will soon 

 push them back under the slowly retreating master ledge ; the canyon 

 walls give many examples of this kind. On the other hand, the dis- 



