202 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
Of the grade of the supposed main channel, on this divide, there are few 
detailed measurements. The difference between the height of the bed-rock 
at Snow Point and at French Corral is stated by Mr. Hague at 2,500 feet, 
which would give a grade of about 100 feet per mile on the average. It is 
said that the height of the bed-rock at Woolsey’s Flat, as compared with 
that at North Bloomfield, indicates that if, as generally supposed, the chan- 
nel crosses under the lava so as to be continuous between those points, its 
grade must be very steep; as much as 180 feet to the mile. Between Griz- 
zly Hill and Badger Hill the bed-rock has but a very slight inclination, or 
only nine feet to the mile. Between San Juan and French Corral the descent 
is 500 feet, giving a grade of between eighty and eighty-five feet per mile. 
The width of the channel and the form and character of the rim-rock in 
the region between the South and Middle Yuba rivers are as yet but very 
imperfectly known. Enough has been revealed by the various shafts, tun- 
nels, and other excavations made by the different companies at work here, 
either in their ordinary mining operations or for prospecting purposes, to 
show the existence of an astonishing amount of gravel, which cannot be 
washed away, even with the utmost possible activity, until many years have 
elapsed. 
From San Juan to French Corral, the channel is comparatively regular in 
its form, and its width is estimated by Mr. Hague at something over 1,000 
feet. Above this it is much more irregular, and, on the whole, much wider, 
being in places, as at Columbia Hill, fully a mile and a half wide. In fact, 
this channel far surpasses in its dimensions the “big channel” at Forest Hill, 
besides having the advantage of being much less extensively covered by the 
volcanic formations. 
In the element of depth, also, the gravel between the South and Middle 
Yuba shows itself pre-eminent. In the lower portion of the channel below 
San Juan, the depth of the gravel is not much over 150 feet; * but above 
that place this figure is greatly exceeded. The ground of the North Bloom- 
field Company is said to have been extensively prospected by shafts, next 
to the bed-rock, revealing a total depth of gravel amounting to as much as 
300 feet, and in places (according to Mr. Skidmore) of fully 600 feet. At 
Columbia Hill the depth of the gravel is also stated as being from 300 to 
600 feet. 
In its general character the gravel of this region resembles that already 
* Mr. Hague calls the depth of the gravel below San Juan 140 feet, probably intended as an average. 
