756 H. H. ROBINSON 



but, on the other hand, it certainly appears to have been much 

 longer, taking the various factors into account, than the time 

 required for the canyon cutting. 



The .evidence, therefore, indicates that the post-peneplain 

 cycle of erosion should be clearly marked off from the preceding 

 peneplain cycle and from the succeeding canyon cycle. The his- 

 tory of the region, as previously given, must be amended, conse- 

 quently, by saying that after the close of the peneplain cycle of 

 erosion, and the widespread eruption of basalt, another period 

 of erosion began during which unprotected Permian and Triassic 

 strata up to a thickness of i,ooo feet were removed from an area 

 embracing the greater part of the Grand Canyon District. Where 

 these strata were protected by the basalt their removal was incom- 

 plete and remnants now form the mesas and buttes, such as Mount 

 Trumbull and Red Butte, which are found at various localities. 

 It is also to be inferred from the widespread removal of these strata 

 that the high cliffs bounding the region on the north and east 

 experienced a further retreat. Davis says: ". . . .in the case 

 of the Vermillion cliffs at Pipe Spring the retreat is likely, it seems 

 to me, to have been several miles at least" (5, p. 37). After the 

 removal of the Permian and Triassic strata erosion proceeded still 

 farther and developed a mature topography of low relief in the 

 underlying and resistant upper Aubrey cherty limestone and to a 

 less extent in other formations. The revival of the forces of erosion 

 is supposed to have been brought about by a slight elevation of the 

 region above its stand at the close of the peneplain cycle. It is 

 considered that the uplift was associated with a period of faulting, 

 which has thus far not been specifically recognized. The move- 

 ments of different periods have so often taken place along the same 

 line of displacement in this region that detailed study is necessary 

 in order to discriminate between them.^ 



'^ In a paper entitled "A Geological Excursion in the Grand Canyon District" 

 (Proc. Boston Soc. Nat. Hist., May, 1909) and published since the above was written, 

 Dr. D. W. Johnson describes a third period of faulting on the Hurricane displacement 

 in the vicinity of Toquerville, Utah. It occurred between the faulting of the plateau 

 cycle — the first faulting of Huntington and Goldthwait — and that which introduced 

 the canyon cycle of erosion, and amounted to about 1,000 feet. This appears to be 

 the faulting which is here supposed to have immediately preceded the post-peneplain 

 cycle of erosion. 



