CHAPTER VI 

 THE BASIN RANGES IN THE WHEELER REPORTS 



THE PROBLEM OF THE BASIN RANGES 



The review of various features and problems of the plateau province, concerning which 

 Gilbert's results have been practically undisputed, being now concluded, his discussion of the 

 basin ranges, which led to a long controversy, niay be taken up. Two phases of this discussion 

 deserve attention. One, of general scientific interest, is the share that Gilbert contributed 

 to the conclusions later reached regarding the origin of the ranges; the other, of more personal 

 interest, concerns the first statement of his contribution, as found in the Wheeler report. 

 As to the final conclusions, certain general statements are made in an essay, upon which Gilbert 

 was engaged at the time of his death; and as these should be borne in mind while their first 

 outline and later evolution are considered in this and following sections, a brief summary of 

 them may be here presented, as follows: The region of the basin ranges was deformed by com- 

 pression about the close of Jurassic time; the mountains then formed were greatly eroded and 

 for the most part degraded to small relief in Cretaceous and Tertiary time; the worn-down 

 region of deformed rocks was then broken into great blocks by numerous, subparallel, generally 

 north-south faults; and the fault blocks of deformed and degraded rocks were upheaved and 

 diversely tilted by uplifting forces without significant lateral compression and probably with 

 lateral extension. The higher parts of the upheaved blocks, now more or less eroded, con- 

 stitute the basin ranges of to-day; the relatively depressed intervening areas constitute the 

 intermont troughs or "valleys," which are more or less heavily aggraded with detritus from the 

 ranges. Complications due to the extrusion of volcanic rocks are not here considered, but will 

 be briefly referred to in a later section. In a few words, the mountains of post-Jurassic com- 

 pression have been essentially obliterated, and the existing ranges are relatively modern fault- 

 block fragments of the earlier, worn-down mountain region, upheaved by vertical or exten- 

 sional forces and as a rule imperfectly degraded. 



Gilbert's original theory was much more simple: The region was broken into great north- 

 south blocks on vertical fissures about the close of Jurassic time; the blocks were uneven, up- 

 heaved and warped into monoclinal or moderately bent structures; and the higher blocks 

 were gradually carved into the existing ranges, while the lower blocks were buried under the 

 detritus eroded from the higher ones. But for certain members of the basin ranges in Utah 

 and Nevada another simple theory had already been proposed by the geologists of the Fortieth 

 Parallel survey; namely, that the existing mountains are the unconsumed remnants of much 

 greater mountains that were produced by post-Jurassic compressional folding, the intermont 

 depressions being regarded as chiefly the work of later erosion. Thus explained, the basin 

 ranges would be classed as monogenetic, because only a single period of deformation was con- 

 cerned in their production; but under Gilbert's original explanation also, as well as under a 

 later combination of both theories, the existing mountains are monogenetic, because only a 

 single though possibly a prolonged period of deformation was concerned in their production 

 as visible topographic features. It is not the existing mountains, but the deformed rock struc- 

 tures within the mountains that are potygenetic in Dana's sense of that term. Gilbert's theory 

 as first proposed and as later modified will now be analyzed. 



PHYSIOGRAPHIC PRINCIPLES 



It has already been pointed out that many of the more novel truths learned by Gilbert in 

 the plateau province were physiographic in being concerned largely with the production and 

 description of existing surface forms, rather than geologic in being concerned largely with the 

 discussion of underground structures and the conditions of their production. The same may 

 be said and with even greater emphasis regarding his work on the basin ranges; for the chief 



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