136 bulletin: museum of comparative zoology. 



in time removed from the new-made spur. Then the process is reversed. 

 Several examples that seemed to be explainable in this way were noted 

 on the Permian scarps east of Hurricane ledge ; they are shown in 

 diagrammatic fashion in Figure 9. 



The Stripped Plateaus. — The details presented in the four preced- 

 ing sections concerning mature valleys, landslides, migration of divides, 

 and bare scarps, all support Dutton's conclusion that the plateau surface 

 north and south of the Grand canyon was " planed down to a com- 

 paratively smooth surface " (c, p. 119) before the erosion of the canyon 

 was begun ; yet they can hardly be regarded as giving altogether inde- 

 pendent demonstration of the conclusion, for some of them at least 

 might all have arisen from some minor uplift of recent date. It is only 

 when all of the elements of the problem are considered together, as 

 Button often remarks, that a consistent explanation of the history of 

 the region can be inferred ; and then its erosion in two cycles seems 

 essential. It is not desired to assert that two simple uplifts, separated 

 by a long interval of perfect rest, constitute the diastrophic history of 

 the region ; but that a long period of relative rest and persistent denu- 

 dation with respect to a comparatively constant baselevel occurred be- 

 tween earlier and later periods of uplift and dislocation. Each period of 

 movement may have been of a considerable duration, complicated by 

 pauses and intermittent displacements. Some of the earlier movements 

 may have been separated by long pauses, of which no record now re- 

 mains ; but nothing less than a long period of relative quiescence before 

 the last period of uplift seems sufficient to account for the great denuda- 

 tion of the plateaus and the narrowness of the rapidly widening canyon, 

 along with the correlated details above described. The date of the faults, 

 by which the plateau district is divided into great blocks, with respect to 

 the earlier and later periods of denudation will be considered later. 



The original uplift of the region, by which deposition was stopped and 

 denudation begun, has been with good reason assigned by Dutton to 

 Eocene time (a, p. 21, c, p. 217). The same author places the " period of 

 quiescence " in " late Miocene or early Pliocene time" (c, pp. 77, 221) ; 

 but in the absence of local fossiliferous deposits, by means of which the 

 greater and lesser denudations can be strictly correlated with the geo- 

 logical time scale, I shall for the present use the terms, plateau cycle 

 and canyon cycle, to name the greater and lesser periods in which 

 the work of erosion has been performed ; and so far as the events in 

 the history of the district can be arranged in order, they will be thus 

 dated. 



