12S .Robinson — Tertiary Peneplain of the Plateau District. 



mostly of Triassic sandstones, from Moencopie Wash southward 

 to the vicinity of Winslow. From there, south and east, the 

 soft shales and marls overlying the sandstones are encountered 

 and the cliffs become much diminished in height. 



The smaller streams of the Grand Canyon District appear 

 to be, without exception, consequent in origin. Davis* has 

 already ascribed such an origin to several on the north side of 

 the Colorado, xlmong others are Peach Spring Wash and 

 Diamond Creek, described by Gilbert in the passage quoted. 

 All the washes that head in the Black and Mogollon mesas 

 and run northeasterly into the Little Colorado are clearly con- 

 sequent. In the vicinity of San Francisco Mountain the 

 courses of Hull Wash, Oak and Sycamore Creeks are governed 

 by faults. A similar example is Bright Angel Creek, as shown 

 on the recently published Bright Angel topographic sheet, and 

 the fault that has marked out its course may be traced south 

 of the canyon by the line of the Grand Canyon Railroad. 



The antecedent origin of the Green and Colorado rivers, 

 advanced by Powell and Dutton, has been generally accepted 

 for many years. It is to be noted, however, that Emmons, f 

 as the result of his work on the Uinta Mountains in 1871, 

 ascribed a superposed origin to the Green River, while Gilbert,;}: 

 in 1876, said that : 



"A large share of the drainage of the Plateaus is not conse- 

 quent. How much is superposed, and how much antecedent 

 remains to be determined. With the solution of the problem 

 are involved the determination of the antiquity and history of 

 the Green and Colorado rivers, and the physical history of the 

 great Tertiary lakes." 



Jefferson, in 1897,§ advanced " an independent and confirm- 

 atory argument for the consequent origin of the Green- 

 Colorado . . . based on the curvature of the river and especially 

 on its meanders. The meanders are surprising in themselves 

 from the steepness of the river and their deep incision ; yet 

 more surprising is the location of strong meander reaches just 

 up stream from structural displacements." Davis, || in the 

 same year, pointed out that the origin of the Green River was 

 still an open question, and after visiting the Grand Canyon 

 District in 1900<[ wrote : 



" That the facts now on record, combined with* such knowl- 

 edge of the region as our party was able to gather, warrant 

 the consideration of at least one hypothesis alternate to the 



*Op. cit. (a), pp. 153-157. 



f Explorations of the 40th Parallel, vol. ii, pp. 194, 205. 



| This Journal, 3d ser., vol. xii, 1876, p. 102. 



§ Science, vol. vi, 1897, p. 293. 



|| Ibid., vol. v, 1897, p. 647. 



If Op. cit. (a), p. 166. 



