118 
South 
Grand Canyon 
Shinarump Cl. 
Vermilion C), 
North 
BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
20M 
Figure 3. 
Reduced from Dutton (c). 
North-south section through Kanab plateau; vertical and horizontal scales about the same. 
corded. They err, as a rule, towards too great 
regularity of structural features, as in the view 
of Vermilion cliffs of the Paria (Figure 5). As 
a whole, they are diagrams rather than pictures. 
The Great Denudation, 
Two CycLes or DEenupation. — Some time be- 
fore our excursion I had heard doubts expressed 
by a competent and critical observer as to the 
necessity of postulating pauses in the uplift of 
the region in order to explain the production 
of the existing topography of the Grand canyon 
district. These doubts were based on the close 
association of the general surface of the plateaus 
bordering the canyon on the north and south, and 
of the floor of the esplanade, with certain resistant 
strata ; the first with the upper Aubrey, the second 
with the Red wall, as indicated in Figure 3. Omit- 
ting consideration of the esplanade for the present, 
let us consider the possibility of producing the 
broadly denuded plateau and the sharply en- 
trenched canyon in one cycle of erosion, introduced 
by an essentially continuous uplift, without sig- 
nificant pause during the movement or supplement 
after its close. 
It is here assumed that the normal progress of 
erosion in a single cycle demands the relatively 
rapid deepening of the main valleys by corrasion, 
and the correspondingly early attainment of a 
graded slope along the valley bottoms; a weath- 
ering of the valley walls, slower at first than the 
deepening of the valleys but faster afterwards, yet 
always at a decreasing rate; an associated but 
on the whole a slower series of changes along the 
minor water-courses ; and with the progress of all 
these processes, a gradual advance of rapidly awak- 
ening activities to a phase of maximum develop- 
ment, followed by a much longer phase of relaxation 
and a very gradual attainment of the remote and 
ultimate phase of rest. The assumption of a still- 
