July 21, 1911 J 



SCIENCE 



91 



on the whole, has been continuous and the 

 erosion of the canyon has been accomplished 

 in a single cycle. 



Another fact which eliminates the necessity 

 of considering that the esplanade — ^Eedwall 

 bench- — ^of the Toroweap and the Tonto bench 

 of the Kaibab section represent a common 

 base-level of erosion and a halt in the uplift 

 of the region is that these two benches occur 

 in the same locality one above the other in 

 undisturbed condition and separated vertically 

 by over 1,000 feet. Evidently they can not 

 indicate one base-level of erosion and it is 

 necessary to suppose, as in the previous case, 

 that they represent two base-levels, each coin- 

 ciding with the summit of a resistant forma- 

 tion, or that the benches are structural sur- 

 faces exposed by the removal of soft overlying 

 beds. The latter explanation, taken in con- 

 nection with other lines of evidence, is to be 

 considered the correct one. 



In the foregoing paragraphs it has been as- 

 sumed that Dutton considered the Tonto 

 bench as equivalent to the esplanade farther 

 west and as indicating a base-level of erosion. 

 It should be said that Dutton does not spe- 

 cifically make this correlation. He states that 

 the esplanade represents a base-level of erosion 

 (p. 121) and in speaking of the first stage of 

 the canyon cutting says that " during this 

 paroxysm of upheaval the outer chasm of the 

 Grand Canyon was cut; the river corrading 

 down to the level of the esplanade in the 

 Kanab and Uinkaret divisions but helow that 

 horizon in the Eaihab" (p. 226). In speak- 

 ing of the second stage he says that " the 

 narrow inner gorge of the Toroweap was 

 swiftly cut, and is in this respect a type of 

 the lower deeps of the entire canyon " (p. 

 22Y). The only lower horizon in the Kaibab 

 which has a development corresponding to the 

 esplanade of the Kanab and Uinkaret sections 

 is the Tonto bench. And the only part of the 

 lower depths of the Kaibab which is com- 

 parable with the inner gorge of the Toroweap 

 is the granite gorge. The Tonto bench lies 

 immediately above the granite gorge in the 

 Kaibab, as does the esplanade above the inner 

 gorge of the Toroweap. It has seemed to the 



writer that the logical interpretation of these 

 statements permitted the correlation of the 

 Tonto bench with the esplanade; but this may 

 be taking too great a liberty with the written 

 word. 



A minor topographic detail, of definite im- 

 port, however, which argues against the Tonto 

 bench being a base-level of erosion and indi- 

 cating a halt in the uplift of the region, is 

 seen just west of the mouth of Bright Angel 

 Creek. At this locality the Tonto formation 

 has been faulted to the extent of about 400 

 feet, and the faulting is apparently of pre- 

 Carboniferous date and in any case older than 

 the cutting of the canyon. (The displace- 

 ment here referred to is not the Bright Angel 

 fault.) The point is that a bench has been 

 perfectly developed at the same horizon in the 

 sandstone, judging from the eroded remnants, 

 on both sides of the fault, notwithstanding 

 the difference in the elevation of the strata in 

 the two blocks. It is evidently unduly com- 

 plicating matters to suppose that the lower 

 bench, for instance, represents a base-level of 

 erosion and that the upper one originated in 

 some other way, for this is calling in two 

 processes to explain exactly similar things. 

 The true explanation is clearly to be found in 

 the single process of stripping and conse- 

 quently the Tonto bench in general, like the 

 older benches in the Carboniferous forma- 

 tions, does not indicate a pause in the uplift 

 of the region, but is simply a stripped struc- 

 tural surface exposed by the removal of the 

 softer overlying beds. 



The writer has little doubt that when this 

 problem is studied in more detail many other 

 facts will come to light supporting the general 

 conclusion of Davis "that while many par- 

 tial cycles of erosion may have preceded the 

 long pause during which the broad denuda- 

 tion of the plateaus was completed, only a 

 single uplift and a single down-cutting are 

 recorded in the canyon." 



H. H. Robinson 



THE H^'MAN FACE 



It seems surprising how little we are in- 

 fluenced by the scientific method in the ex- 



