Ao^ or scumcas] WHEELER SURVEY 57 



sequent erosion has completely destroyed the corrugated surface, and nothing now remains of 

 the upheaved folds but the long, narrow ridges which precisely reveal the attitude of the most 

 resistant strata, while the valleys and lowlands are worn down on the weaker intervening strata. 

 It is only after understanding this threefold sequence of considerations that the full significance 

 of Gilbert's next sentence can be measured; for he goes on to say: "It is impossible, by any 

 hypothetical denudation, to formulate the Basin Ranges as remnants of a system of anticlinal 

 and synclinal folds"; that is, a system of folds not traversed by dominant north-south faults. 

 "The simple monoclinal may indeed be explained as the side of an anticlinal, by the harsh 

 assumption that the remaining parts have been removed below the level of the adjacent valley 

 [intermont trough], but the explanation will not apply to the compound monoclinal" (41). 



Unfortunately, these brief assertions were not supplemented by analytical exposition, and 

 no justification was offered for calling the explanation of the simple monoclinal ranges as rem- 

 nants of broad anticlinals — an explanation that had been tacitly accepted as satisfactory by 

 some of Gilbert's contemporaries — a "harsh assumption." Other similarly brief assertions 

 immediately follow. The erosion is not "conceivable that would carve Worthington moun- 

 tain . . . from an anticlinal" (41), for this is a 15-mile range consisting of a narrow segment of 

 nearly horizontal strata which outcrop in the steep slopes that limit it on both sides; and con- 

 cerning the same remarkable range, it had already been noted, when its singular structure and 

 form were described, that no erosion could be conceived of "that should have left this thin 

 segment as the remnant of an inclined table or of a fold" (37). If this idea had been elaborated 

 so far as to describe the form of a mountain that could have been produced by the erosion of 

 an extensive table or fold, and if such a form had then been contrasted with that of the observed 

 range, the argument might have been better understood. 



Other terse statements are found a page later: "Ridge [range] lines are more persistent 

 than structures. In the same continuous ridge [range] are monoclinals with opposed dip"; 

 that is, warped monoclinals; and other ranges are both "monoclinal and anticlinal" (42). 

 The same comment may be made here as above; a whole volume of meaning is packed into 

 these too short sentences, which even the illustrative examples next adduced did not suffice to 

 make fully intelligible to geologists whose physiographic eyes had not been opened. In illus- 

 tration of the important generalization that the ranges do not trend with the structure of their 

 rocks, the section of Spring Mountain Range at Ivanpah in southern Nevada is selected : 



The mountain there shows an axis of granite, flanked on each side by limestone, but the trend of the 

 anticlinal is oblique to that of the range and it quickly runs out, the granite giving place at the north to the 

 eastern mass of limestone, which rises and, as an eastward dipping monoclinal, there constitutes the entire range, 

 while the western limestone mass becomes, in the same manner, supreme at the south (42). 



The meaning here is evidently that, if this range were the eroded residual of an un faulted 

 anticlinal fold, the granitic axis and the limestones on its two flanks ought to continue indefi- 

 nitely along the strike of the fold; but that as " there is not on a grand scale that close dependence 

 of form on durability that must maintain were the great features of the country carved by denud- 

 ing agents" (41), some other explanation for the ranges than nonfaulted folding and sequential 

 erosion must be sought for. 



THE BASIN RANGES AS UPHEAVED AND WARPED FAULT BLOCKS 



The idea that the basin ranges are the eroded remnants of great anticlinal and synclinal 

 folds was therefore dismissed; and after a concise generalization of the leading features of the 

 ranges, Gilbert's own views are announced. 



To begin with the simplest generalization, the ranges are a system; not indeed formed at the same time, but 

 exhibiting certain common characters, over a great area. They are parallel; they recur with some regu- 

 larity of interval; they are of moderate dimensions. 



Then comes the new theory: 



The ridges [ranges] of the system occupy loci of upheaval and are not mere residua of denudation; the 

 valleys of the system [the intermont troughs] are not valleys of erosion, but mere intervals between lines of maxi- 

 mum uplift (41). The movements of the strata by which the ridges [ranges] have been produced have been in 



