VARIABLE GRADES EXPLICABLE BY FAULTING. 297 



of distributed faults of slight throw or equivalent plastic deformation, as 

 held by Mr G. F. Becker* the grades of the Neocene rivers might more 

 easily be reduced to somewhat uniform figures by assuming that along 

 distances showing exceptionally heavy grades a more intense faulting 

 or deformation has taken [dace. The considerable and even steep grades 

 of some longitudinal channels show, however, that even by this means 

 the rivers cannot be reduced to gentle and uniform grades. 



The vertical curve of the present Neocene channel- would appear to 

 offer a criterion by means of which it might be ascertained whether, in 

 addition to the general uplift, the flank of the Sierra has been materially 

 deformed. The grades of two principal forks of the old rivers which in 

 general have a transverse direction have been plotted in plate 9, the 

 distances being taken along the curves of the streams. In the same 

 plate the vertical curve of one ^>l the modern forks has been constructed 

 in order to serve for comparison. The two curves in the upper part of 

 the plate cannot be directly compared, and the difference in the ordi- 

 nates does not directly indicate the amount of recent erosion, for the 

 curve of the Neocene river is somewhat longer than that oi the modern 

 equivalent. It should first be noticed how regular is the curve of the 

 recent river in spite of the fact that the country drained by it is only 

 in the earliest stages of baseleveling. It is, strictly speaking, composed 

 of two curves, the junction of winch nearly coincide- with the lower limit 

 of glaciation. The existence of the upper curve must be referred to the 

 ice-cap protecting the higher part of the mountains from active erosion 

 during a large part of the Pleistocene."* 



If the modern river curve shows such regularity it would be natural 

 to expect that that ^>t the Neocene river, which represents a more ad- 

 vanced stage of base-levelling, should be still more so. But the plotted 

 curve of the Neocene Middle Yuba river does not correspond to the 

 normal curve of erosion. Instead, it appears to be composed of two 

 curves with the convex side upward. I think this convexity, winch 

 cannot be explained by differences in the resistance of the rock-masses 

 over which the river flows, must he due to a deformation nf the surface 

 during the uplift of the Sierra. The most pronounced departure from 

 the normal curve of erosion results from the present steep grades of the 

 Neocene channels near the valley. This is marked in both the profiles 

 given and must. I think, be regarded as indicating a subsidence of the por- 

 tion adjoining the sediment-filled trough of the meat valley relatively 

 to the middle part of the range, or a rise of the latter relatively to the 

 former. Another deformation would appear to have taken place in the 



♦Bull. G-eol. Soc. Am . vol. 2, pp. iJ4 and 73; also idem, vol. 4. p. ■ 

 fSee G. F. Becker: Bull. Geol. Soe. Am., vol. 2. p. 



XLIV— Bull. Geol. Sue. Am., Vol. 4. 1S92. 



