DAVIS : THE GRAND CANYON OF THE COLORADO. 183 



Relation of the Inner and Outer Canyons. — In the third place, the 

 position of the inner canyon along the middle of the esplanade is, as 

 was suggested to me several years ago by Mr. C. H. White (then one of 

 my graduate students, now instructor in mining at Harvard), singularly 

 significant of a single period of erosion. This may be understood by 

 considering the two alternatives in order. If the esplanade repi'esented 

 a mature valley floor, broadened by the lateral swinging of the river 

 during a lower stand of the land, rather than by the rapid wasting of 

 the weak lower Aubrey layers in contrast to the persistence of the 

 strong Red-wall group, then when renewed uplift revived the process of 

 canyon cutting, the river must have occupied an irregular path along 

 the esplanade, and the inner canyon would have been cut at one place 

 near the northern wall of the outer canyon, and at another near the 

 southern wall. This condition is actually illustrated in the valley of 

 the Rhine ; here a narrow inner gorge is incised in the flat floor (espla- 

 nade) of a broad trough which in turn is eroded beneath the bordering 

 uplands ; a structural origin for the trough is inadmissible because the 

 rocks are greatly deformed ; and, moreover, river gravel and silt still 

 cover the floor of the trough. The narrow gorge is intrenched irregu- 

 larly along the trough floor, sometimes turning so far to one side that a 

 continuous descent leads from the high upland directly to the river, while 

 a correspondingly broad mid-level floor remains on the other side. The 

 same relation occurs in the valley of the Moselle, whose young gorge 

 meanders conspicuously from side to side in the floor of the mature 

 trough. The valleys of the Lot and the Dordogne in southwestern 

 France exhibit similar features, except that the younger or inner mem- 

 ber of the composite valleys are here opened wide enough to have scrolls 

 of incipient flood plain on one or the other side of the river, while flood 

 plains are as yet only just begun in the gorges of the Rhine and the 

 Moselle. 



If, on the other hand, both the outer and the inner canyons of the 

 Colorado are the work of a single cycle of erosion, and the esplanade is 

 of structural origin, then it is necessary that the walls of the outer 

 canyon should retreat symmetrically on either side of the inner canyon, 

 and that the inner canyon should bisect the floor of the esplanade thus 

 produced. This case would correspond to that of all one-cycle valleys 

 in horizontal strata, where the symmetry of the benches and slopes on 

 the two walls results from their essentially equal retreat under the 

 weather. Innumerable illustrations of such symmetry may be seen in 

 the side canyons of the Kaibab section, where the Red-wall cliffs are 



