64 G. r. BECKER — STRUCTURE OF THE SIERRA NEVADA. 



The faulting studied in this paper has certainly increased the western 

 slope of the range, for all the east walls of the fissures have risen relatively 

 to the western walls. It must also have promoted erosion something as a 

 westward tilting of the range would do. The influence of this distributed 

 displacement on erosion must have been greatest where the fractional dis- 

 placement was greatest, probably a little west of the present crest of the 

 range. It must also have produced the greatest effect immediately after the 

 faulting occurred. Now the faulting seems to have been contemporaneous with 

 the eruptions, and to have extended throughout Pliocene times. Hence one 

 would expect to find evidences of great erosion during the Pliocene, and 

 particularly towards the crest of the range from which the surface material 

 loosened by the fissures would be swept down to low^er altitudes. One might 

 plausibly infer that coarse gravels resting on relatively high slopes would 

 result from such conditions. Now, independently of the fissure system, it is 

 known that the Pliocene was a period of great erosion near the crest, and 

 that enormous quantities of coarse gravel accumulated on the western flank 

 of the range. Thus the faulting, and the accompanying increase of slope 

 are entirely consistent with other observations, but aflTord no explanation of 

 the more recent deepening of the channels. In my opinion the eflfects of the 

 faulting on the erosion of the range were exhausted during the Pliocene, nor 

 does it seem to me that canon erosion precisely like that observed would be 

 produced by such faulting as forms the subject of this paper or by a tilting 

 of the range as a single block. Uniform tilting would uniformly increase 

 the grade of the streams flowing westward, and, as in the case of faulting, 

 these would probably take up all the material they could transport high up 

 towards the crest. This does not correspond to the phenomena to be ex- 

 plained. 



While the modern rivers have eroded below the Pliocene channels, the 

 deepening of the channels has taken place to a considerable degree only 

 within certain limits of altitude. The plains of the great valley are as high 

 or perhaps higher relatively to the mass of the range than at the end of the 

 Tertiary, and the old and new channels substantially coincide in level when 

 they reach the plains. In the higher mountains, too, the old and new water- 

 courses seem coincident. Thus on the south fork of the Stanislaus, near the 

 locality called Donnell's flat on the state map, columnar lava 40 feet thick 

 occurs on the river bank and therefore close to the lowest portion of the val- 

 ley. The top of the lava is covered with granite erratics, while the columns 

 extend down to the level of the water as it stood in August last. This lava 

 is thus of Pliocene age and the river, which is now only a few feet in depth, 

 cannot have eroded more than two or three feet since that time. Several 

 other similar cases occur within distances of ten miles from this point. In 

 general in the glaciated region there is abundant evidence that the preglacial 



