Davis — Notes on the Colorado Canyon District. 253 



diagram by Gilbert, fig. 52, in Walcott's article, above referred 

 to). Close to the base of the Unkar is a sheet of basalt, 

 described as a contemporaneous flow by Walcott (1. c, 516- 

 5J8), but to our view seeming rather to be an intrusive sill, 

 inasmuch as it broke from one layer to another, instead of 

 lying conformably between the under and overlying beds. 



The floor of the Paleozoic series is somewhat less regular 

 than that of the Unkar. A few miles down the canyon there is 

 a mound of schists that rises rather higher than the basal 

 Tonto sandstone, and the harder layers of the lower Unkar 

 also interrupt the Tonto for a mile or more up stream from 

 the apex of the wedge on the northern side of the canyon. It 

 is notable that the deformed schists beneath the Unkar series 

 descend by a continuous and very steep slope to the river; 

 hence [their whole mass must be about equally resistant ; but 

 west of thejapex of the wedge and for a number of miles down 



Fig. 1. Diagram to illustrate the section and profile of the canyon in the Kaibab 



stream, the schists show a distinct bench just beneath the 

 Tonto before steepening to their precipitous- lower slope ; 

 hence here the upper part of the schists must be weaker than 

 the lower. It occurred to me that this weakness may be 

 ascribed to a pre-Tonto subseriai weathering of the schists. If 

 so, the gently undulating sub-Tonto floor may be, with renewed 

 confidence, regarded as an ancient lowland of subaerial denuda- 

 tion, as described by Powell, only slightly modified by marine 

 erosion previous to Tonto deposition ; while the sub- Unkar 

 floor may have been planed down by the sea to smooth and 

 firm rock from some unknown but probably moderate measure 

 of relief, before the Unkar series was laid down on it. 



The erosion of the plateaus. — The retreating cliffs of Per- 

 mian, Triassic and other formations, on the north and east of 

 the Grand Canyon district, have been taken to prove extensive 

 erosion over the plateaus in which the canyon has been cut. 

 Dutton concludes that this erosion was accomplished during a 



