44 GROVE KARL GILBERT— DAVIS IUM0,E ' v<£xxi; 



plateau blocks shows the Grand Wash Cliffs, by which the plateau province is limited on the 

 west, to have retreated moderately from the great fault at their base, the Hurricane and Toro- 

 weap scarps that separate the three western blocks are drawn directly on the line of their faults, 

 thus confirming the impression given by the passages just quoted, that the topographic features 

 of these faults result from displacement little modified by erosion, although this is by no means 

 true. It is not until later mention is made of the vast denudation by which the southern plateaus 

 have been stripped of the higher strata which form the clift terraces in the north (81), that the 

 original topographic expression of the fractures and flexures may be inferred to have been 

 greatly modified by the deep and wide-spread erosion which has worn the present surface 

 thousands of feet below the original surface; and even this inference is not well assured because 

 the faulting is not explicitly stated to have preceded the widespread erosion. 



However, the topographic features associated with certain faults — for example, the long 

 flexed fault of the Sevier Valley which was seen at intervals for a distance of 225 miles without 

 reaching either end of it — is represented by cross sections in which a considerable amount of 

 post-faulting erosion is indicated, so that the reader is not left altogether uninformed as to a sig- 

 nificant measure of change in the surface forms of the faulted structures. Indeed, although it 

 is unguardedly said on one page that "the Kaibab fold throws the [Triassic] belt twenty-five 

 miles to the north" (176), it is briefly and more truthfully explained on an earlier page that 

 the northward shift of certain cliffs on the east side of a fault as compared to the west side is 

 "not due to any horizontal displacement along the line of fault, but merely to the fact that the 

 eastern portions, being lifted higher than the western, became subject to different conditions 

 of denudation" (51) ; but this explanation is so incomplete, not to say obscure, that it has to be 

 worked out by the reader. On the whole one must conclude, and with some surprise, that the 

 physiographic treatment of fractures — a subject in which Gilbert has usually been regarded as 

 a leader and a master — is not always illuminating. 



The physiographic treatment of flexures is in certain respects more satisfactory than that 

 of fractures. The structure of some of the flexures is described clearly and quantitatively; 

 thus along the margins of the Kaibab Plateau, the highest of the blocks in the plateau province, 

 the heavy Carboniferous limestone is said to have been flexed on a curve of from 2 to 3 miles 

 radius; the massive Triassic sandstone, 1,000 feet thick, is said to be bent on the fine of the 

 long Sevier Valley fault through an arc of 15° or 20°; and in the Paria flexure farther east, the 

 same heavy sandstone is described as "seamed throughout, as though it had been crushed and 

 reunited, like the bars of ice in Professor Tyndall's celebrated experiments on regelation" (56). 

 There is no correspondingly clear statement in the first report of the degradation that the 

 flexures have suffered, although it may be inferred from certain generalized sections (fig. 26, 

 p. 51; fig. 29, p. 53). 



The second report briefly describes a monoclinal flexure in the southern part of the plateau 

 province, trending northwest and producing a throw of 1,500 to 2,000 feet to the southwest in 

 Cretaceous and Triassic strata; and the relation of form to structure is represented in a section 

 from which it appears that the flexure had been reduced to small relief before a basalt flow was 

 poured over it, for a remnant of the flow fortunately survives in a mesa that unconformably 

 covers the eroded edges of the upturned strata on the line of maximum bending. Yet even 

 here emphasis is given in the text to "the antiquity of the eruption," which "is measured by 

 the general degradation of the country of more than 500 feet," with the resulting isolation of the 

 basalt mesa; and little is said of the much greater prebasalt degradation, for although this is 

 well represented in the section, the text merely states that "the fold is older than the basalt" 

 (557). A somewhat fuller account is given of the Nutria flexure, with a throw of from 

 3,500 to 4,000 feet, along the southwestern side of the Zuni uplift. The great erosion that the 

 flexure has suffered is well represented in several sections, and is clearly set forth in the text in 

 connection with the denudation of the Zuni dome, as will be shown below. It was a matter of 

 deep regret to all the American members of the Transcontinental Excursion of 1912, when their 

 special train, while making the final eastward turn of its long circuit, stopped in the gap of the 

 vertical Triassic sandstones that follow the axis of this superb flexure not far east of Gallup on 



