EVIDENCE FROM EROSION 315 



quent in nature, for their positions have been determined directly by 

 the topographic slopes due to faulting, as shown in figure 4. The 

 stream flows in the angle between the fault scarp and the surface of the 

 dropped block. The fact that a friable tuff is encountered under the 

 basalt makes cutting rapid and easy. 



Another prominent feature of the mountain canyons is their real 

 " hanging " character with respect to the intermontane valleys. They 

 debouch at from three to seven hundred feet above the valley, which 

 they reach by a long alluvial cone. This means that throughout the 

 life of the mountain streams corrasion has not been active enough in 

 the valley to cut away the alluvial cones or to allow the mountain 

 streams to cut down to the level of the valley. That there were not 

 original conditions of active corrasion, followed by choking of the streams 

 and raising of their levels, is proved by the fact that they lie in bedrock 

 bare of detritus, except a thin film in or near the present channel. 

 Their channels have in no case been built up from a former lower posi- 

 tion. 



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Figure 4. — Consequent Stream Valley formed by Faulting. 



Evidence from basalt and erosion. — In a previous discussion it was stated 

 that erosion alone could not produce the distribution of the basalt 

 which we actually find. Some further consideration of this may be of 

 value. The west side is the side that has no basalt covering, though 

 several large patches occur at the west base, while the east side is cov- 

 ered from summit to base; but the western valley is only two-thirds 

 the width of the eastern valley. It would seem remarkable on any ero- 

 sion hypothesis that erosion should be so much greater on the side of 

 the smaller valley. On the side of the broader valley the basalt, which, 

 as has been already shown by lithological evidence, is probably but 

 slightly eroded by general agencies, passes down with the same appear- 

 ance and thickness underneath the valley alluvium, and this for several 

 miles along the range, except Avhere cut by outflowing transverse 

 streams. In £>ther words, on the edge of a valley some 75,000 feet wide 

 there has been no visible action referable to the lateral corrasion of a 

 valley stream on the surface layer of the mountains. It should be 

 added that no evidence has been found, either on the mountains or at 

 their base, of a higher layer than the basalt. This evidently means that 



