746 H. H. ROBINSON 



stripped and somewhat dissected plain [a, p. 139] Since the uplift by 



which the canyon cycle was introduced, sufficient time has elapsed for an ex- 

 tensive removal of the weaker Permian strata from the plateau surface 



Even the resistant upper Aubrey strata, revealed by the stripping of their 

 Permian cover, early in the canyon cycle, have suffered a significant amount 

 of dissection, as seems to be the case over much of the Kanab Plateau; but 

 the dissection here is not so mature as that by which the higher Kaibab is 

 characterized [a, p. 137]. 



The general conclusions reached by Huntington and Gold- 

 thwait, as the result of a study of the Toquerville District, Utah/ 

 coincided with those of Davis outlined above, but as the work 

 was of considerable detail they were able to present the problem 

 with somewhat greater fulness than had previously been done. 

 In particular they were able to show "that at the end of the inter- 

 fault cycle of erosion (the period of great denudation) the whole 

 country was physiographically mature or even old. Certain 

 regions of soft strata, chiefly near the Colorado River, had 

 been reduced very nearly to base-level forming the Mohave 

 peneplain." 



Evidence has been presented by the writer^ showing that 

 the region about the San Francisco Mountains,^ south of the Grand 

 Canyon, was also reduced to a peneplain which involved not only 

 soft Permian and Triassic strata but also the highly resistant 

 upper Aubrey cherty limestone. It was concluded from a study 

 of the literature that the peneplain most probably covered the 

 entire southern portion of the present Colorado Plateaus and 

 extended, in the Bradshaw Mountains, into the Basin Range 

 country of Arizona. The remnants of the peneplain are sufficiently 

 numerous to make it certain that practically the entire Grand 

 Canyon District was reduced to base-level at the close of the period 



' "The Hurricane Fault in the Toquerville District, Utah," Bull. Mus. Comp. 

 Zoology, Harvard College, XLII, February, 1904. 



^ "The Tertiary Peneplain of the Plateau District, and Adjacent Country, in Ari- 

 zona and New Mexico," Am. Jour. Sci., XXIV (August, 1907), 109-29. 



3 The term "San Francisco Mountains" is here used to designate a group of six 

 large and several hundred small volcanoes and their associated lavas, which covers 

 an area of some 2,000 square miles on the plateau south of the Grand Canyon. The 

 group takes its name from San Francisco Mountain, the largest mountain volcano of 

 the region.. 



