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 deuuded 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, aud 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 i*anges 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 (b)) 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 hi"h mountainous altitude. It is not desired to assert that 



