60 GROVE KARL GILBERT— DAVIS lMEil0m \v™xxt 



concerning "loci of upheaval"'; for while the intennont valleys were taken to represent aggraded 

 belts of relative depression, and hence the work of deposition rather than of erosion, it was 

 clearly announced that "within the ranges there are indeed eroded valleys, and the details 

 of relief show the inequality of erosion due to unequal resistance of the rocks" (41). More- 

 over, it is later added that "since the close of the Jurassic period" the upheaved masses "have 

 been subjected to the unceasing play of atmospheric eroding agents, wearing away their sum- 

 mits, furrowing their flanks, and conveying their substance to the intervening valleys" (63). 

 The aggradational forms there produced have already been noted. 



Evidence of the great volume of post-faulting erosion is further given by references to 

 the vast amount of the detritus accumulated in the arid intermont troughs. "The principal 

 deserts of the region are relatively depressed regions, marked by excessive accumulations of 

 detritus, which have so filled the valleys [the troughs marking the less uplifted fault blocks] 

 as to connect them in a continuous plain, beneath which the minor ranges are completely 

 buried, and through which the peaks of the more lofty jut as islands" (65). Again, in describ- 

 ing the lower-lying portion of the range region that is occupied by the desert of Great Salt 

 Lake, it is noted that "the lowest part of the included depression has been filled with a sea 

 of detritus, until some of its ranges are completely submerged and others protrude only insular 

 buttes to mark where they are sunk. If these hidden mountains rise as high above their bases 

 as do their neighbors on the rim of the basin, . . . the desert sediments . . . may have a 

 maximum thickness of 5,000 or 6,000 feet" (65, 66). Yet great as the volume of eroded mate- 

 rial thus seems to be, it was held that "erosion, which began in the Ranges and the Plateaus, 

 as they were successively exposed to the atmosphere . . . has accomphshed only a small 

 fraction of its task" (187). 



BASIN RANGES IN THE SECOND WHEELER REPORT 



All the foregoing discussion of Gilbert's basin-range theory is based upon the report of his 

 first two seasons of field work, 1871, 1872; for although the report is dated July, 1874, its main 

 conclusions had been earlier formulated, as appears from the pages which he contributed to 

 Wheeler's repoit of progress for 1872, already cited. Gilbert's second report is concisely 

 phrased and does not generalize for the region as a whole. So far as the basin ranges are con- 

 cerned, it considers only a few far southeastern examples, most of which have northwest-south- 

 east trends. " The usual structure is monoclinal, demonstrably due to faulting in the Chiri- 

 cahui and Pinal ranges, and presumably so in all the others" (517) ; and the faulting is of later 

 date than the comparatively modern lavas of which certain monoclines are largely composed. 

 Of the two ranges just mentioned, the first lies in the southeastern corner of Arizona; the sec- 

 ond is farther northwest and is traversed at its middle by Salt River, in a notch, the present 

 site of the famous Roosevelt Dam. Another example is the Gila Range, north of the first 

 above named and east of the second; quartzite was seen at its southwestern base, but its greater 

 mass is made of — 



trachytes and trachyte conglomerates, unevenly bedded and surmounted by basalt. These are all exhibited 

 in section in the southwestern face, which is steep, while the opposite face is constituted by the upper lavas, 

 which, dipping in that direction, are continuous to the Bonito river, five miles away. The range is at this 

 point a monoclinal mass of bedded lavas, whose eruption took place before the dislocation which produced the 

 ridge, and the same structure probably continues to the northwest for fifteen miles (514). 



The Mimbres Range, in southwestern New Mexico, more specifically described than most 

 of the others, also affords significant structural elements; it consists largely of heavy lava beds, 

 the uppermost of which is seen in a continuous sheet, wonderfully uniform in texture and habit, 

 for 60 miles along the western slope, while Paleozoic rocks outcrop in the eastern slope. The 

 whole mass was regarded as a monoclinal uplift with a fault along the eastern base, where the 

 Paleozoic rocks are disclosed (519). If the Paleozoic rocks in this and in the Gila Range are 

 more steeply deformed than the moderately inclined lava sheets, as is the case elsewhere in 

 that district, and if the contact between the Paleozoic and the lavas is unconformable, as is 

 elsewhere said to be the case, it might have been inferred that the first, presumably late Juras- 



