academy or sciences] .WHEELER SURVEY 33 



clean-cut fractures which, when traced to the natural section afforded, by the east-west course 

 of the Grand Canyon, are found to have vertical planes; but other blocks were seen to be sep- 

 arated by monoclinal flexures, which, as they sometimes become fractures when followed along 

 their length, were closely associated with faults; and concerning this land of displacement it 

 was said in a special article: 



The monoclinal fold, barely recognized by geologists heretofore, but here the predominant structure, is 

 one of the simplest elements of corrugation, and the fruit of its careful study cannot fail to be of great import 

 to the student of dynamical geology. 5 



Inasmuch as certain flexures, when followed along their length, become fractures, the 

 familiar term, fault, was used with "a somewhat more extended meaning than the one ordi- 

 narily given to it," so as to include both kinds of displacement, and no reason was seen why, 

 "regarding the phenomena as the results of a slow-acting force, we may not suppose that in 

 depth, as well as longitudinally, the relation and alternation of fractures and flexures will depend 

 on the nature and condition of the beds affected" (56). 



As to the proximate cause of these displacements, Gilbert was most specific. Although 

 he showed that volcanic eruptions did not as a rule heave up the strata through which their 

 lavas rise to the surface, he was nevertheless convinced that forces of vertical upheaval, not of 

 horizontal compression, were responsible for most of the displacements detected in the nearly 

 horizontal fault blocks of the plateau province, as well as in the more strongly tilted fault blocks 

 of the basin ranges. It was recognized that the vertical displacements of the huge plateau 

 blocks may have been in some cases accompanied by small horizontal movements, resulting in 

 a slight diminution of the breadth of the region, ''but it is impossible ... to suppose that 

 the vertical movements have been caused by lateral pressure applied to the strata in which 

 they are manifested. Whatever the place and mode of the remote cause, the immediate acts 

 vertically and from some position beneath the strata we are to examine" (56). The relation 

 of the displacements to the rigidity of the displaced strata is studied. It is at first noted that 

 "stupendous blocks of rock, ten, twenty, or even thifty miles in diameter, and of unknown 

 depth, have changed their relations to other blocks, with which they were once continuous, 

 and have themselves remained rigid, all evidence of movement being at the common boundaries 

 of the dissociated blocks" (558). It is then suggested as a corollary that " the meaning of these 

 movements of the earth, in vast but limited masses, is, that rigidity is an important factor in 

 the determination of the superficial manifestations of subterranean movements." In the case 

 of flexures, "the fact that, at points of differential movement . . . the rocks were not 

 fractured but were flexed, proves that changes were of secular slowness, and the rigidity that 

 resists secular applications of force . . . demands for its interpretation that we shall grant 

 to the rigid masses a depth commensurate with their superficial dimensions, and suppose that 

 the forces which move them are situated still deeper" (559). 



UPHEAVAL OF THE ZUNI DOME 



The fine example of diastrophism seen in the Zuni Mountain dome close to the western 

 border of New Mexico — later studied by Dutton and now familiar to many travelers on the 

 Santa Fe Railway which crosses its northern margin at the continental divide — attracted 

 GUbert's particular attention, and was explained as due to the " upward transfer of subterranean 

 material" which did not penetrate its cover. The domed structure involves a deep body of 

 fundamental Archean rocks with a heavy cover of Carboniferous, Triassic, and Cretaceous strata, 

 now partly removed, as further stated below; so that while the residual mountain mass measures 

 only 45 by 20 mfles, with a local height of 3,000 feet, the original dome would measure 70 by 

 35 miles, with a height of 6,000 feet. The reconstructed dome is contrasted with the volcanic 

 cone of Mount Taylor, 30 or 40 mfles to the east; in that mass, " the rising rock passed through 

 the superstrata, and, piling itself on the surface, built a mountain of its own substance; in the 

 other it moved a comparatively short distance, but lifted all above it, and built a mountain by 



c 



• Proc. Amer. Assoc. Adv. Sci. for 1874, 1875, Ft. II, 35. 



