176 BULLETIN: museum of comparative zoology. 



under the Tonto are weaker in their upper than in their lower portion. 

 It occurred to me that this might be the result of deep weathering of 

 a pre-Tonto peneplain, before it was buried by the palseozoic series, and 

 that the incursion of the Tonto sea over the peneplain was too rapid to 

 allow the waves to abrade anything more than the superficial soils ; 

 thus leaving the weakened rocks to reveal their weakness in the canyon 

 profile to-day. The sub-Unkar floor, on the other hand, seems to have 

 been smoothly worn down to firm rock by marine abrasion before it was 

 covered by Unkar sediments; but whether the advancing Unkar sea 

 destroyed great mountains of schists or merely planed off the soils and 

 weathered rocks of a pre-Unkar peneplain is an unsolved question. 



Correlation of Water Streams and Waste Streams. — The pa- 

 laeozoic walls of the canyon in the Kaibab exhibit a combination of cliffs, 

 slopes, and platforms in great variety, appropriate to the horizontal 

 structures on which they are eroded. The widening of the canyon here 

 has not yet produced broad benches or platforms, although a systematic 

 beginning of their development is repeatedly observed ; but in the 

 Kanab, Uinkaret, and Shivwits sections of the canyon the upper sur- 

 face of the Red-wall group has been denuded to form a broad platform, 

 called the esplanade by Dutton, to which fuller reference will be made 

 below. Returning to the Kaibab, a pleasing comparison may be made 

 between the path of a running stream of water along the valley bottom 

 and that of a creeping stream of waste down the valley side ; the 

 simplest examples for comparison being in horizontal strata such as are 

 so magnificently displayed in the canyon walls. In both cases, graded 

 slopes are first established in discontinuous stretches on the less re- 

 sistant strata, while cliffs remain ungraded on the outcrops of the 

 stronger strata. The close analogy of the two cases is brought to mind 

 by comparing a plunging fall of water, leaping down the nearly vertical 

 face of a resistant stratum in the valley bottom, with the discontinuous 

 fall of rock waste over the cliffed outcrop of a resistant stratum on 

 the valley side. The evenly graded reach of the water stream on the 

 weaker strata above the fall corresponds with the flat platform in the 

 valley walls above the cliff; and the " cave-of-the-winds " in the weak 

 strata just behind the waterfall corresponds with the hollow or " rock- 

 house " beneath the overhanging base of the cliff; the scanty heap of 

 rock fragments beneath the waterfall corresponds with the sheet of 

 rock waste (with the coarsest fragments lowest down) that cloaks the 

 under slope of weak strata from the rock-house down to the back or 

 inner margin of the next platform below. It is natural that the pro- 





