PHYSIOGRAPHIC CRITERIA OF FAULTING 313 



Ranges,"* expressed f in concise and clear form two physiographic cri- 

 teria for range origin by faulting. He says : 



"In the first place, the structure of the ranges is commonly oblique to their 

 border, so that the faulted margin passes indifferently from one structure to an- 

 other, as in the accompanying figure; if the ranges were the residuals of a long 

 period of undisturbed erosion, such a lack of correlation between border and 

 structure would not be looked for; but if the ranges are limited by faults at one 

 side or both, the indifference of border to structure is natural enough." 



These two statements taken together present fairly what we may call 

 the first physiographic criterion of a " fault-block " range — the principle 

 of the unity of the range as an elevated mass and the persistence of 

 ridge line and range front in the face of discordant and varying struct- 

 ures. No further presentation or explanation of the principle seems 

 called for. It is evident that in mountains of the Appalachian type 

 (ranges of folding, whether modified or not by erosion) no such phe- 

 nomenon can be observed. 



In discussing the bedrock complex, especially under the title " Rela- 

 tion to Range Outline," J the relationship of the structure to the range 

 form of the southern group of the Humboldt mountains was described 

 in some detail. We may sum it up by saying that the range shows a 

 marked continuity of crest and front, the former passing over various 

 structures, while the folds of the bedrock complex are distinctly discor- 

 dant with the front and vary in obliquity to it from to 50 or 55 degrees. 

 As far as this test is concerned, the range corresponds to a faulted block. 



The test of erosion forms. — Again Davis says : § 



"In the second place, the body of each range is usually continuous, although 

 it may be incised by sharp-cut valleys ; if the ranges were the residuals of a period 

 of undisturbed erosion long enough to have permitted the excavation of broad 

 intermont valley-lowlands, each range should be divided into isolated mountain 

 groups by the opening of wide branch-valleys in its mass ; but if the depressions 

 and the ranges are blocked out by recent faulting, the continuity of the ranges is 

 to be expected." 



Reference to the detailed section will show that the broad intermon- 

 tane valleys are from two to three times as wide as the ranges. If these 

 broad basins were cut out by stream corrasion and general erosion there 

 should be, as stated above, broad valley arms extending into or com- 

 pletely through the ranges, and broad, low, flat strike valleys (subsequent 



*Loc. cit. , / 



t Science, n. s., vol. xiv, p. 458, September, 1901. 

 J See page 299. 



I Science, n. s., xiv, 459. See also Gilbert, U. S. Geog. Survey West of the 100th Meridian, vol. iii, 

 p. 41. 



XLI— Bull. Geol. Soc. Am., Vol. 15, 1903 



