ACADEMY OF SCIENCES] WHEELER SURVEY 61 



sic, deformation of the region affected only the ancient sedimentary strata; that a long period 

 of erosion then followed by which the deformed sedimentary strata were reduced to the moder- 

 ate relief that they show beneath the lavas which eventually sealed them over; and that the 

 monoclinal tilting, by which the basin-range structure was produced, was so recent as to be 

 later than the lava outpourings. 



Had the geological history of these two ranges been thus worked out, it would probably 

 have been found similar to that deciphered by Louderback 30 years later for the west Humboldt 

 Range in northern Nevada; but unfortunately, the inferences above made, simple as they now 

 seem, were not made in 1873, perhaps for the reason that Gilbert saw the Gila Range imper- 

 fectly, and that he saw only the western lava-sheeted slope of the Mimbres Range; the occur- 

 rence of Paleozoic rocks along the eastern base of this range was reported to him by other mem- 

 bers of the survey. On a later page, the Natanes Plateau, a gently inclined mass capped with 

 sanidin-dolerite and lying in the upper basin of the Gila, is provisionally "regarded as a mem- 

 ber of the Basin Range system"; it has a southwest-facing escarpment about 1,500 feet in 

 height and 25 miles in length, "unquestionably due to a fault ... of not less than 2,000 feet 

 throw, . . . which has occurred since the eruption of the sanidin-dolerite. " This is the example 

 already cited, in the scarp of which, "instead of the scalloped figure, made up of convex curves, 

 that results when erosion controls, we have a straight line, interrupted only by angular embay- 

 ments, where it is intersected by water-ways"; and it is the only example in which these char- 

 acteristics of a slightly dissected fault scarp, in contrast to a retreating escarpment of erosion, 

 are specifically stated (528). It thus appears that the results gained during the third field 

 season, as presented in the second report, do not materially modify the conclusions announced 

 in the first report; although, had it been possible to take full advantage of the structure that 

 appears to characterize the Mimbres Range, important modifications of those conclusions 

 might have been presented. 



PHYSIOGRAPHY AND GEOLOGY IN THE BASIN-RANGE THEORY 



The physiographic competency of Gilbert's basin-range theory, as stated chiefly in his 

 first report, may be tested, as it has been in part already, by confronting its statement con- 

 cerning the origin of the visible mountain forms with the requirements of the threefold sequence 

 of physiographic treatment. It thus appears, first, that the preupheaval form for most of the 

 region was, by implication, the smooth uppermost surface of its heavy sedimentary series; 

 second, that the new forms introduced by deforming upheavals, denudation being "discounted" 

 for the moment, would have been a system of huge north-south fault blocks, with inclined or 

 warped upper surfaces, some standing higher, some lower; all the higher ones having nearly 

 vertical fault-scarp sides; or, more truthfully, that the upheavals being regarded as slow and 

 prolonged, so that much erosion went on during their progress, the warped upper surface and 

 the scarped sides of the blocks would be continuously dissected as they were slowly raised; 

 and third, that as to post-upheaval erosion so much has been accomplished, as just intimated, 

 that the "details of relief show the inequality of erosion due to unequal resistance," although 

 a great erosional task still remains to be completed. The leading feature of the theory, namely, 

 the limitation of the ranges by faults on one side at least, was thus reasonably connected with 

 the leading fact of their present form, namely, the discordance of their marginal lines with their 

 structures. It is true that a demonstrated identification of the theoretical fault lines with the 

 visible marginal lines was not systematically presented in the published discussion, but there 

 can be no question that it was understood. 



On the other hand, the above extracts disclose a striking unlikeness between the geological 

 elements of the basin-range theory in its original form and the form that it later assumed, 

 largely through supplementary suggestions proposed by others and accepted by Gilbert. In 

 the original theory, not only was the monoclinal tilting of the range strata thought to be caused 

 by the vertical forces of upheaval, but also the anticlines and synclines which occur as subsidiary 

 features in some of the ranges were believed to have been produced at the same time and by the 

 same upheaving forces, or rather by gravity in combination with the upheaving forces. This 



