164 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



during the last chapter of the great denudation must have been over 

 a considerable distance, for each line of cliffs should have been, before 

 the faults were made, worn back further on the west of a flexure than 

 on the east ; while now, in consequence of the faults, the reverse 

 relation has been brought about. The total recession since faulting 

 has therefore probably been decidedly greater than the distance by 

 which the cliffs on the east of a fault-line now stand north of their 

 fellows on the west ; and thus an even greater antiquity for the faults 

 is suggested than has been thus far suspected. 



The Bends of the Grand Canyon. — There are three peculiar features 

 in the course of the Grand canyon : one is the southward bend around 

 the Kaibab, the second is a northward bend toward the mouth of 

 Kanab creek, and the third is a southward bend around the Shivwits 

 plateau. Jefferson has suggested that the first and third of these bends 

 may be consequent on the form of the surface given by the flexures ; 

 but according to the analysis of events here presented, the drainage 

 consequent on the flexures ran to the east and northeast, and the west- 

 ward course of the Colorado was not assumed until after the flexed 

 plateaus had been greatly denuded, and until the denuded surface had 

 been raised on the east by the block faulting. Something later than 

 the initial slopes of the flexed surface should therefore be found to 

 guide the river, if an antecedent or initially consequent origin is not 

 accepted for it. It has seemed to me that certain details in the flexing 

 of the Kaibab and Coconino plateaus may here be appealed to. 



The eastern lobe of the Coconino is a striking feature as seen from 

 the lower plateau on the south (Figure 13) and from the valley of the 

 Little Colorado on the east. The surface of the district at the time of 

 flexure was presumably covered by some of the mesozoic formations. 

 Just before the time of faulting, the Trias may have been pushed back 

 to some such outline as would reveal the Permian in the valleys on the 

 south and west of the Kaibab, and perhaps even some of the Aubrey 

 was laid bare on the highest part of the Kaibab. Up to this time, the 

 drainage hereabouts was eastward down the slopes of the Kaibab and 

 Echo flexures, but many longitudinal subsequent streams must have 

 been developed along the weak lower Triassic and Permian strata west 

 of and underlying the Triassic escarpment that was then retreating 

 eastward towards its present position on the axis of the Echo flexure. 

 Now as the drainage of the interior basins of the northeast was turned 

 sonthwestward by the upheavals associated with the faulting — long 

 after the close of the Eocene — the most available point for its escape 



