184 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
placed 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 Bodfish already referred to (Dutton, 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 (Dutton, ¢, 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 cafion, 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. 
Relation of the Esplanade to the Toroweap Fault. — Finally, it may be 
noted that the early date given on page 146 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 
