CHANNEL OF THE ANCIENT MIDDLE YUBA. 275 



western side the bed-rock is very high, sometimes forming the crest of the 

 divide between the North and Middle Yuba. On the other hand, the 

 lava bed-rock contact runs very low on the slope of the canyon side, while 

 it rises to considerable elevations on the opposite, southeastern side of 

 the river. This relation is clearly illustrated in section D D (plate 7). For 

 a long distance above American hill there is no indication that the chan- 

 nel passes out from under the lava into the eroded canyon. Supposing 

 a fairly uniform grade from Milton to American lull, it cannot emerge 

 until about nine miles from Milton, and very possibly less. At any rate, 

 the old channel for many miles above American hill would appear to 

 offer an excellent field for drifting operations. It is easily accessible 

 from the canyon of the Middle Yuba by moderately long tunnels, which, 

 for instance, at the section D D would probably have to lie placed at an 

 elevation of 5,000 feet. It is quite likely that the gravel in this channel 

 would pay for drifting, especially as it would have received the debris 

 from a part of the quartz mines south and southwest of Sierra City. An 

 attempt to open up this channel was made at the Savage tunnel, about 

 four miles above American hill, but it was abandoned long before com- 

 pletion. 



Attention should he called to the deep valley through which the section 

 shows the channel to have flowed. Those accustomed to profiles of equal 

 horizontal and vertical scale will readily recognize the abrupt slopes of 

 its sides. It is not probable that Pinoli peak lias suffered any large 

 degradation since the Neocene times, as andesitic Hows cover it on the 

 eastern side almost to the top. 



High bed-rock continues on the divide north of the Middle Yuba up to 

 Milton, while the lava runs far down on the northern canyon slope. The 

 detailed investigations in this region have not yet been completed, and 

 just where the channel leaves the lava How and follows the eroded course 

 of the present river is not quite certain. Neither to the north nor to the 

 south is there, however, as far as I am aware, any possible outlet by 

 which it could have turned from this course until Milton is reached. 



At Milton, however, there are both north and south of the Middle 

 Yuba places with sufficiently low bed-rock to allow the Neocene river to 

 deviate from its course parallel to the river. The first is about a mile 

 southeast of Milton and forms the distinct outlet of the subsequently 

 described Milton-Meadow lake channel; its elevation is not far from 

 5,950 feet. The second is northeast of Milton, where a gap appears to 

 exist, with rapidly rising bed-rock on both sides. The approximate ele- 

 vation of this gap, which I do not know from personal inspection, has 

 been determined by Mr Pettee to be 5,938 feet.- It would seem to rep- 



* Auriferous Gravels, p. 112. 



