ACADEMY OF SCIENCES] WHEELER SURVEY 43 



a marked advance from the doubts expressed as to widespread denudation in the field notes of 

 1872, cited above; for the huge cliff-edged terraces came later to be well understood as sub- 

 ordinate incidents in the vast erosion that the plateau province has suffered : 



Of the immensity of the denudation that has reduced the Plateaus to their present condition, we have 

 unmistakable, and at the same time unexpected evidence, in the existence of insular masses of strata, remote 

 from the mesas [terraces?] of which they once formed part. The most important of these are found . . . [in the 

 Uinkaret mountains not far north of the western part of the Colorado canyon, and again to the south of the 

 canyon, farther east], and consist of limited tables of Triassic rocks, resting on the broad Carboniferous floor 

 [of the lowest terrace], and surviving the general destruction in virtue of protecting mantles of lava (SI). 



It is interesting to note that in Gilbert's reprints of his reports, an interleaved statement 

 supplants this quoted passage, as follows: 



Professor Powell discovered in the Uinkaret Mountains an island of Triassic strata, from which the corre- 

 sponding cliffs have retreated twenty-five or thirty miles; and has surmised with much plausibility that Red 

 Butte, south of the Grand Canon and fifty miles west of the nearest point of the Triassic escarpment, is similarly 

 constituted. 



This is probably one of the "sentences that were suppressed in the manuscript"; it was 

 thus restored by Gilbert to prevent the impression that he "disregarded through ignorance or 

 discourtesy the work of other geologists." He never did that; yet scrupulously honest as he 

 was in acknowledging his own indebtedness, he exacted no such acknowledgment from his 

 associates with regard to the overflowing abundance of helpful suggestions that he gave them 

 all through his generous life; not even if they, after a longer or shorter interval of semi- 

 conscious assimilation, sometimes gave forth his ideas as their own. 



TOPOGRAPHY OF FRACTURES AND FLEXURES 



The unlike forms of retreating escarpments due to long-continued denudation and of fault 

 scarps due to recent fracturing are described only in very general terms in the first report, 

 and are but briefly mentioned in the second report 'in describing a specific feature, the scarp 

 of the lava-capped Natanes Plateau beyond the southernmost part of the plateau province. 



There, instead of the scalloped figure, made up of convex curves, that results when erosion controls [the 

 form of lava-capped mesas], we have a straight line, interrupted only by angular embayments, where it is inter- 

 sected by waterways; and the steepest cliffs, instead of overhanging the points of most rapid present erosion, 

 are along the rectilinear front, which faces a broad, streamless valley. This character maintains for twenty 

 miles, and is unquestionably due to a fault — a fault of not less than 2000 feet throw (528). 



This example will be referred to again in connection with the basin ranges, as the Natanes 

 Plateau is assigned to that province. The quotation suffices to show that Gilbert recognized 

 the contrast between young fault scarps and far-retreated escarpments as clearly as Powell 

 did; but the statement, clear as it is, loses much of its value by being associated with a special 

 locality of moderate dimensions, instead of being placed under the account of the northern 

 plateaus. 



The great fractures and flexures of the plateau province, which were regarded with good 

 reason in the first report as the unlike but possible contemporaneous effects of similar vertical 

 forces, are briefly described as to their displacements and surface forms (48-57), but unfor- 

 tunately the descriptions are so phrased as to give the reader little understanding of the immense 

 denudation that the displaced structures have suffered. It is indeed too often implied in the 

 report on the first and second seasons of field work that the topographic features now seen along 

 the lines of these flexed or fractured faults are largely the result of the fracturing or flexuring. 

 For example, a summary regarding the plateaus states that they are "subdivided by longitu- 

 dinal — north and south — cliffs, produced by faults" (57); and in an account of the long Echo 

 Cliffs flexure of the Triassic sandstones, which comes up from the south and crosses the Colorado 

 at Lees Ferry, it is noted that "on both sides of the river the fold produces conspicuous topo- 

 graphic features" (51) ; but as a matter of fact the topographic features are the product of an 

 enormous denudation of the flexed strata. Moreover, although a generalized section of the 

 20154°— 26— 10 



