298 W. LINDGREN — TWO NEOCENE RIVERS OF CALIFORNIA. 



upper part of the first profile, while that of the second profile seems 

 more like the oormal curve of erosion. 

 It is evident that if, besides the deformation, a general increase in the 



slope has taken place the curves do not represent that deformation quite 

 correctly, for a diminishing of the slope would affect the grades of the 

 different sections differently according to their angle with the trend of 

 the range. On recalculating the grades for a general decrease in the 

 slope of 50 or 60 feet to the mile it is found, however, that the peculiar 

 convex forms of the curves remain as before or even are slightly more 

 accentuated. 



Although the present steep grades of the old Neocene channel can thus 

 be shown to have resulted to a considerable extent from elevation and 

 deformation, it does not follow that the Neocene river system had very 

 slight grades throughout. On the contrary, I believe that a careful study 

 of the Neocene topography, as shown in valley slopes and cross-sections, 

 which hardly can have been influenced by subsequent deformation and 

 certainly have not been faulted to any notable extent, will lead to the 

 conclusion that the Sierra Nevada, before the accumulation of gravels 

 began, was a mountain range greatly worn down by erosion, but not 

 reduced to a baselevel of erosion. It cannot even on the whole be 

 regarded as a peneplain, above which isolated and more resistant hills 

 projected ; the declivities and irregularities of the old surface are too 

 considerable for that, nor are the projecting hills invariably composed of 

 the hardest rock-masses. 



Conclusions. 



The observations recorded in this paper appear to prove conclusively 

 that the Sierra Nevada in Neocene times, in the watersheds of the Yuba 

 and American rivers, formed a mountain range as distinct as that of 

 today, and that its first summit in general coincided with the corre- 

 sponding modern divide. They further appear to prove that the grades 

 of the remaining Neocene gravel channels are to a certain extent deter- 

 mined by the directions in which the}' flowed in such a way as to strongly 

 suggest that the slope of the Sierra Nevada has been considerably in- 

 creased since the time when the Neocene ante-volcanic rivers flowed 

 over its surface. It finally appears probable from a study of the grade 

 curves of the remaining channels that the surface of the Sierra Nevada 

 has been deformed during this uplift, and that the most noticeable defor- 

 mation has been caused by a subsidence of the portion adjoining the 

 great valley relatively to the middle part of the range. 



