DEPOSITS OF TRIBUTARY CHANNELS. 273 



of these points rises in a short distance 700 feet above the bottom of the 

 channel. 



Grades : 



North Bloomfield to Backbone inlet, 3 miles, 152 feet per mile. 



Backbone inlet to Moores flat, 4J miles, 137 feet per mile. 



Moores flat to Snow Point, 21 miles, 87 feet per mile. 



The Derbec Channel — The Derbec shaft is sunk on a very different 

 channel from that of North Bloomfield; it pays for drifting, which the 

 main channel docs not, as a rule, and it carries a great many granite 

 bowlders, which I think are derived from a hidden area under the volcanic 

 flows rather than carried down from the granitic area above Washington. 

 It has been mined for a distanee of 3,500 feet in an easterly direction 

 from the shaft. It represents a tributary to the main channel, and I 

 think it very probable that it connects under the lava with the Belief 

 inlet, its course from there upward is very uncertain, as so much of it 

 has been eroded. A connection with the Omega gravel area and others 

 below that on the brink of the South Yuba canyon seems quite probable, 

 but the problem is complicated by the existence of the deep Centennial 

 channel on the Washington ridge, and more investigations are necessary 

 before a final result can be reached. On the other hand, the Derbec 

 channel undoubtedly joins the main channel at some place between 

 North Bloomfield and the Backbone house. 



Grades : 



Derbec shaft to Relief, 3 miles, 115 feet per mile* 



From the Derbec shaft down to the main channel there is a pretty 

 heavy grade of about 120 or 150 feet to be accounted for, hut the dis- 

 tance, allowing for some curves, might have been nearly one mile. 



The Forest City ('limine!. — The channel between Orleans flat and Forest 

 City is sufficiently known from Mr Pettee's notes. f Above Forest City 

 the Neocene valley is continuous as far as City of Six, overlooking 

 Downieville. From here it is not easy to trace it any further. 



Between Forest City and the Ruby mine there is, however, a break in 

 form of a mass or dike of volcanic rock. At the Ruby nunc there are 

 two channels, one lower connecting with the City of Six, and one upper 

 running toward Forest City. The Bald Mountain Extension is mining 

 a tributary from the northeast. In its former tunnel a basalt dike was 

 found cutting across the andesitic breccia and showing that the eruption 

 of the basalt masses of the Forest hill table mountain took place in tin 

 vicinity. 



* Auriferous Gravels, p. 105. 

 t [bid., p. 4;;:! e< seq. 



XLI— Buix. Of.oi.. Soc. Am., Vol. 4, 1892. 



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