280 W. LINDGREN TWO NEOCENE RIVERS OF CALIFORNIA. 



by which any outlet by way of Nevada City and Grass Valley could be 

 effected. The Harmony and Grass Valley channels are several hundred 

 feet above the Quaker hill channel. There were, however, two low gaps 

 in the ridge through which a part of the rhyolitic masses overflowed both 

 down the Grass Valley and the Harmony channels (-see page 269). From 

 Blue Tent, where the deep channel is only 500 feet above the bed of the 

 South Yuba, the direct continuation is given, as indicated by Mr Pettee, by 

 the correspondingly low trough of the Grizzly hill channel on the oppo- 

 site side of the river. Again, from here there is no possible low outlet 

 except by Badger hill, and this way the Neocene river must have taken. 

 Mr Pettee's own notes in regard to the former depth of the gravel :it 

 Spring creek confirm the existence of a deep channel beyond Grizzly 

 hill.* In channels of slight grade such as this one, ups and downs of 

 ten or twenty feet are very common. All these relations will appear 

 much clearer on the geologic map which, it is hoped, soon will be 

 printed. 



If the deep channel between Blue Tent and Quaker hill would pay 

 for drifting it would be a magnificent field for enterprise. Unfortunately, 

 there are some doubts concerning this. The gold of extraordinarily 

 heavy gravel beds is more commonly divided through the whole mass 

 than concentrated on the bed-rock. However, drifting operations have 

 been carried on with profit at You Bet, and it is not improbable that 

 this part of the channel would pay well, at least in some places. It is 

 said that an attempt to drift the deep channel at the Blue Tent outlet 

 was not attended with success. The deposit would have to be opened 

 up by long tunnels from the' South Yuba canyon at Blue Tent, or east 

 of that place. 



Section CO is atypical one through the great Neocene South Yuba 

 valley. On it may be noted the prominent bed-rock point of Banner 

 hill — composed of diabase-breccia — and the andesitic and rhyolitic flows, 

 as well as the upper gravel benches and the central trough or gutter. To 

 one peculiarity of the rhyolitic flows attention should be called : although 

 the flows descended the valley in a northwesterly direction and should 

 present a level surface from east to west, still it is found quite generally 

 that the eastern margin is higher by one or two hundred feet than the 

 western. It is perhaps best not to attach too much importance to tlii-^ 

 as an indication of tilting, for in the first place a part of the flow was 

 drained off through one or two lower gaps toward the west, and in the 

 second place some erosion doubtless degraded the even surface in the in- 

 terval between the rhyolitic and the andesitic flows. 



♦Auriferous Gravels, p. 393. 



