ANCIENT CHANNELS FILLED BY PLEISTOCENE DEPOSITS. 285 



Diamond Springs in a northwesterly direction toward Pilot Hill is indeed 

 proved by the small remaining deposit of gravel and rhyolitic tuff found 

 at the low, broad gap of Granite hill in such a position that it must in- 

 dicate the lowest point of the channel at this place. The town of Pilot 

 Hill occupies a similar position in a broad gap, on each side of which 

 hills of older rocks rise to a height of several hundred feet. Besides 

 some Pleistocene angular gravel, there is found at this place a small rem- 

 n; nt of another deposit with large, very well rounded bowlders, such as 

 can lie formed only by a stream of some magnitude. The course from 

 here on is largely hypothetical, but it may be assumed as probable that 

 the river continued in a westerly direction for some miles, joining the 

 Neocene North fork coming down from the vicinity of Forest hill. From 

 here on the two rivers, united, probably flowed in a southwesterly direc- 

 tion to the Neocene gravel masses exposed and mined near the plains 

 between Rocklin and Folsom, at the Lee or Chabot drift mine. From 

 this point the ancient river is hidden under the later accumulations of the 

 plains. The Neocene surface in this vicinity appears to have formed a 

 gently undulating country with little relief as compared with the lowest 

 part of the Yuba river. This is explained by the occurrence of a large 

 massif of easily eroded and crumbling granitic rock, over which the old 

 river here made its way. That the gravels at the Lee mine are pre-vol- 

 canic cannot be absolutely asserted, but they are certainly of Neocene 

 age. 



If the gravel at the Lee mine represents the lowest point known of the 

 Neocene river, it follows that it was here only about 100 feet higher in 

 elevation than the present river in a corresponding position is now. 



On the northern side of the sloping breccia table of " Bowlder ridge," 

 extending from Auburn to Lincoln, there is another Neocene depression, 

 the gravels of which have been drifted in places; but this cannot repre- 

 sent the lower course of the Neocene American, for a low granitic ridge 

 separates it from the basin in which the continuation of the upper courses 

 of this stream must be sought. On the other hand, the large accumu- 

 lations of gravel near Folsom are distinctly post-volcanic and were ac- 

 cumulated by the river in early Pleistocene time. That the Neocene 

 river followed the course of the present stream from Folsom up appears 

 very improbable and is not supported by any geologic evidence. 



( ! rades : 



Lee mine to Pilot Hill, about 12 miles, 80 feet to the mile. 

 Pilot Hill to Granite hill, 11 miles, 32 feet to the mile. 

 Granite hill to Diamond Springs, miles, 21 feet to the mile. 

 Diamond Springs to Newtown. — After a detailed examination of the 

 gravel region in the vicinity of Placerville, Goodyear arrived at the con- 



