58 GROVE KARL GILBERT— DAVIS [U ™ oia \vo?xxt 



chief part vertical along planes of fracture, and have not involved great horizontal compression. . . . The 

 forces which have been concerned in the upheaval of the Basin ranges have manifested themselves at the surface 

 as simple agents of uplift, acting in vertical, or nearly vertical planes (42). 



In little more than these phases was a new theory of mountain making first announced. 



Additional statements are given under "General considerations," on a later page, after 

 a description of the moderately dislocated, north-south fault blocks in the plateau province. 

 It is there again announced that the forces which produced the basin ranges acted "by nearly 

 vertical upheaval; and that they were deep-seated," and it is then added that, as forces of the 

 same kind displaced the plateau blocks, "a single short step brings us to the important con- 

 clusion that the forces were identical (except in time and distribution) ; that the whole phenomena 

 belong to one great system of mountain formation, of which the ranges exemplify advanced, 

 and the plateau faults the initial, stages." It is therefore "impossible to overestimate the 

 advantages of this field for the study of what may be called the embryology of mountain 

 building. In it can be found differentiated the simplest initiatory phenomena, not obscured, 

 but rather exposed by denudation" — that is, in the little-disturbed plateau blocks, the structure 

 and displacements of which are so well brought to light by the transecting canyons — " and the 

 process can bo followed from step to step, until the complicated results of successive defor- 

 mations and erosions" — that is, in the basin ranges — " baffle analysis" (60, 61). The analogy of 

 the northern plateau blocks to the ranges is especially indicated a few pages later: 



A portion of the valleys of the Plateau country, and especially those of the upper Sevier, are, like the 

 troughs of the Range region, structural, and lie between the monoclinal ridges produced by the system of 

 faults (67). 



Returning now to the earlier page, there soon follows an illuminating comparison which 

 throws a much-needed fight on the mechanism of Gdbert's theory: 



In the Appalachians corrugation has been produced commonly by folding, exceptionally by faulting; in 

 the Basin Ranges, commonly by faulting, exceptionally by flexure. The regular alternation of curved syn- 

 clinals and anticlinals [in the former] is contrasted with rigid bodies of inclined strata, bounded by parallel 

 faults [in the latter]. The former demand the assumption of great horizontal diminution of the space covered 

 by the disturbed strata, and suggest lateral pressure as the immediate force concerned; the latter involve little 

 horizontal diminution, and suggest the application of vertical pressure from below (61). 



The author recognizes that much is yet to be learned about the basin ranges and that it 

 would therefore be premature "to attempt a reconciliation of these antithetical phenomena," 

 but he — • 



cannot forbear a brief suggestion before leaving the subject. It is, that in the case of the Appalachians the pri- 

 mary phenomena are superficial; and in that of the Basin Ranges they are deep-seated, the superficial being 

 secondary; that such a force as has crowded together the strata of the Appalachians . . . has acted in the 

 Ranges on some of the earth's crust beneath the immediate surface; and the upper strata, by continually adapting 

 themselves, under gravity, to the inequalities of the lower, have assumed the forms we see. Such a hypothesis 

 . . supposes that a ridge, created below, and slowly upheaving the superposed strata, would find them at one 

 point coherent and flexible, and there produce an anticlinal; at another hard and rigid, and there uplift a frac- 

 tured monoclinal (62). 



This explanation was regarded as according particularly well "with the persistence of ridges 

 where structures are changed" (62) ; it plainly credits the vertical forces with deforming as well 

 as upheaving the fault-block strata. 



Not only was a mechanical cause thus outlined for the upheavals, but indication was also 

 given of their effect in deforming a preexistent surface and producing a new surface, the defining 

 of which is the object of the second member in the threefold sequence of physiographic treatment. 

 This was done very explicitly in a chapter contributed by Gilbert to Wheeler's progress report 

 for 1872, in which a generalized cross section of the region "discounting denudation," represents 

 a number of diversely deformed fault blocks, bounded by nearly vertical fault planes and un- 

 evenly upheaved. 1 A more philosophical presentation of the same topic is given in the main 

 report in the following sentence: 



1 Engineer Department, United States Army. Progress report upon geographical and geological explorations west of the one hundredth 

 meridian, in 1872. . . . Washington, 1874, p. 50. 



