314 G. D. LOUDERBACR — STRUCTURE OF THE HUMBOLDT REGION 



valleys) within the ranges themselves. No approximations to such val- 

 leys are found. The range is, as already described, a unit, with a sharply 

 defined marginal line, which runs in flowing curves ; no ridge arms pass 

 out into the valley, no valley lobes run into the range. The second 

 physiographic test, then, holds for this range. 



But there are other sculptural features which give confirmatory evi- 

 dence of the general fault origin and also add details to the history. 



Evidence from stream valleys. — If the intermontane valleys had been 

 formed by erosion or had been for any considerable time secondary base 

 levels for the streams flowing from the mountains, these streams would 

 have followed the usual fluvial-physiographic cycle, and while still 

 vigorously cutting downward along their upper stretches and producing 

 sharp V-shaped canyons, at their mouths or junctions with the valleys, 

 they would broaden their channels and produce, at least on a small 

 scale, a flood or laterally cut plain, while the canyon would open into 

 an obtuse V. In this connection the west slope (presumably the fault- 

 scarp side), which, where the range is highest, is made up entirely of the 

 bedrock complex, is the more interesting. We find here the conditions 

 just the reverse of these described. The waterways, which usually run 

 at right angles to the mountain front, are generally narrow and gorge- 

 like near their mouths. Along the middle slopes of the mountains, how- 

 ever, they open out and show a tendency to adjust themselves to lines of 

 strike.* These gullies are, of course, everywhere cut in bedrock. 



This open and partially subsequent character of the streamways of 

 the middle and upper mountain slopes and the gorge-like and unad- 

 justed character along the lower slopes may be explained by the fault 

 hypothesis if we consider the movement to have taken place gradually 

 or in stages. The streams along the lower slopes are, then, flowing over 

 more recently exposed ground, exerting all their energies in downward 

 cutting, while the atmospheric and other agencies of general erosion have 

 not had time to widen the canyons. The upper stretches have been 

 longer exposed, and, although the streams are now cutting downward, 

 the agencies of general erosion have had a longer time to act and subse- 

 quent tendencies have been developed, perhaps at some period of com- 

 parative rest. It is hard to see how any folding or erosion hypothesis 

 can explain these drainage characters. 



On the east slope, as might be expected, very little information can be 

 obtained on this point. The trunk drainage lines are in general perpen- 

 dicular to the uplift, but longitudinal valleys are common. While these 

 are strike valleys as regards the volcanic series, they are truly conse- 



*This is described more fully under " geomorphic relationships," p. 298. 



