DAVIS: THE GRAND CANYON OF THE COLORADO. 161 
the second chapter of this time, after the later flexures had been formed 
(the inter-flexure-and-fault cycle), the destructive processes which had 
previously denuded only the mountains of the Basin range province 
are believed to have extended to the Grand canyon district also; but 
deposition still continued in the northeast, interrupted only by minor 
unconformities. The northward retreat of the great escarpments by 
which the High plateaus are now bordered on the south, must have 
then begun; the retreat was probably greater on the southwest, where 
the stepping flexures are supposed to have caused the greatest renewals 
of uplift. At the close of the cycle that began with flexing and ended 
with faulting, the Triassic escarpment, for example, may have receded 
to a line that is roughly marked by three points: the lava mesa on 
the Shivwits plateau, the junction of the Great and the Little Colo- 
rados, and the eastern side of the lava fields around Mt. San Francisco ; 
but something of a northward bend must be made in this line as it 
crosses the flat arch of the Kaibab. The amount of denudation accom- 
plished during. this cycle is indeterminable ; yet if any are disposed-to 
limit it to a small measure, their attention should be called to the local 
instances of great erosion in the Cretaceous-Tertiary interval, when the 
Waterpocket flexure was essentially baselevelled. As complete a con- 
sumption of the up-flexed blocks in the southwest may have been accom- 
plished before faulting took place, although no unconformities remain 
there to prove it. But although the denuded southwestern area from 
which the Trias had been stripped may have been reduced to moderate 
relief, it need not have been a lowland with altitude but little above 
sea-level, for its streams discharged into interior continental basins, 
very probably without outlet, on whose floors the Tertiary sediments 
were accumulating. Some of these basins may have held lakes, inter- 
mittently at least ; but the occurrence of mountain ranges for hundreds 
of miles to windward (west and southwest) must have tended to produce 
a dry climate in the lower continental area to leeward ; many of the 
basins may have been nearly or quite dry, gathering (as I have else- 
where pointed out (0)) fluviatile rather than lacustrine deposits at con- 
siderable altitudes above the sea. 
The Effect of the Faults. — The faulting of the region is the next 
important occurrence. The faults have their uplift in nearly all cases 
on the east ; hence the eastern area, from having long served as a seat 
of deposition, became in turn the seat of extensive denudation. At the 
same time, the western area was greatly depressed from its long resi- 
dence at a high mountainous altitude. It is not desired to assert that 
