184 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOCxY. 



1 symmetrically with respect to the narrow gorges in the Tonto 

 sandstones below and between them ; the detailed map of part of the 

 canyon in the Kaibab by Bodlish already referred to (Button, c, Plate 

 XLIL), gives good illustration of many examples of this kind. 



Now returning to the facts, there seems to be little doubt that the 

 inner canyon bisects the esplanade. Its medial position is very striking 

 in the majestic view that we had of both these features eastward from 

 Vulcan's throne ; a view made famous by Holmes's wonderfully effective 

 drawing (Button, c, Atlas, sheet VI.). All the published maps and 

 sections of the district exhibit this arrangement in the most systematic 

 manner through the Kanab and Uinkaret plateaus, and as well in the 

 lateral canyons of Cataract and Kanab creeks as in the main canyon. 

 Even the first accounts of the canyon made mention of the medial 

 course of the inner chasm. Ives, descending from the plateau south of 

 the Kanab section by a branch canyon, reached a floor (a branch of the 

 esplanade) which, when seen from the enclosing cliffs, looked smooth, 

 but which was really covered with hills thirty or forty feet high ; " along 

 the centre we were surprised to find an inner canon, a kind of under cel- 

 lar " (p. 107). Powell's first mention of the esplanade, as seen from the 

 cliffs near the border of the Kaibab and Kanab plateaus, is as follows : 

 " The walls seem to rise very abruptly, for two thousand five hundred or 

 three thousand feet, and then there is a gently sloping terrace, on each 

 side, for two or three miles, and again we find cliffs, one thousand five 

 hundred or two thousand feet high. From the brink of these the pla- 

 teau stretches back to the north and south, for a long distance. . . . 

 The effect of the terrace is to give the appearance of a narrow winding 

 valley, with high walls on either side, and a deep, dark, meandering 

 gorge down its middle" (a, p. 92 ; also p. 196). It should be noted, 

 however, that the topographical maps show the Shivwits section of the 

 canyon to have walls of less symmetrical form than prevails further 

 east ; but not having seen this part of the canyon, I shall not venture 

 to discuss its complications. 



R( lotion of the Esplanade to the Torotveap Fault. — Finally, it may be 

 noted that the early date given on page 14G for the Toroweap-Sevier 

 fault is not at all inconsistent with the discontinuity of the structural 

 esplanade where it crosses this fault-line ; for if the floor of the esplanade 

 is of structural control, it would be opened at whatever level the upper 

 surface of the Red-wall group occupied at the time of the erosion of the 

 canyon. If, on the other hand, the esplanade had been controlled by a 

 former baselevel, its discontinuity at the Toroweap would demand a 



