GENERAL DISTRIBUTION OF THE GRAVELS. 307 
Indeed, it appears almost certain that during the gravel epoch, the ultimate 
connection of this whole region between the Middle Yuba and the South 
Feather with the Great Valley was by way of what is now the divide be- 
tween the Middle and South Yubas; this being the case, it is not so difficult 
to understand how it is that the gravel deposits on this divide are so exten- 
sive and continuous. At the same time there seem to be other and probably 
subordinate channels coming in from the northeast, in accordance with the 
direction of the lava-spurs on the divides. It is naturally the inclination of 
the miners to seek for channels in this direction, as being in conformity with 
the general condition of things in the Sierra, and as promising new fields for 
discovery and profitable labor ; and in the extreme northeastern portion of 
the district under consideration the gravels have indeed been traced to a 
great elevation in the range, namely, as high as 6,000 feet, — almost at the 
very summit, indeed. (See Plates R and T.) It will be noticed that, in leav- 
ing the gravels of the divide between the Middle and South Yuba behind, 
and following the general northerly direction of the channels, we have been 
rising to higher levels, while of course receding from the base of the Sierra. 
The low bed-rock at La Porte is over 4,800 feet above the sea-level ; and to 
the south of Slate Creek, in the extreme southwest corner of the district 
shown on Plate R, the bed-rock under the gravels nowhere descends as low 
as 4,000 feet. 
The thickness of the gravels also diminishes gradually as we recede from 
the Middle Yuba in a northerly direction. Still, there are localities where 
the amount of detrital material is very considerable, even far to the north 
and high up in the range; as, for instance, on the property of the Niagara 
Company, at the head of the North Fork of Slate Creek, where a total thick- 
ness of gravel, clay, and pipe-clay equal to 300 feet is reported, the gravel 
itself, however, making less than a hundred of this. 
There are some deposits of gravel near Onion Valley, the stream heading 
there being a tributary of the Middle Fork of the Feather, the South Fork 
of this stream being much less important than the Middle Fork. Indeed, the 
former heads near Gibsonville and to the southwest of Onion Valley, while 
the latter rises much higher in the range, or, indeed, entirely to the east of 
it, and runs in a northwesterly direction for many miles before breaking 
across it, to unite with the North Fork, whose course is equally abnormal 
as compared with that of all the other rivers to the south. The Onion 
Valley gravels appear to be of but little importance, and not connected in 
