4354 SUPPLEMENTARY INVESTIGATIONS IN THE GRAVEL REGION. 
stratum is about 250 feet in thickness at Bald Mountain. It occurs on both sides of the Downie- 
ville trail, but has been eroded entirely away along the line of the north fork of Oregon Creek. It 
is seen also as massive blufls to the southeast of City of Six. Looking from the junction of the 
two forks of Oregon Creek, the general level of the ground is seen to rise quite rapidly to an 
elevated “flat,” about 350 feet above the bed of the creek, beyond which the Bald Mountain 
bluff rises abruptly like a huge wall of rock. In climbing to the bluff, tufaceous and basaltic 
boulders are observed up to a point near the bluffs base. Above this altitude the tufaceous 
variety is entirely lacking. The rock of the bluff is jointed and fractured in such a way as to be 
easily mistaken for a metamorphic slate. The planes of jointing have a north and south direction, 
with a nearly vertical dip, sometimes to the east and sometimes to the west. The top of the 
mountain is barren of trees, but it is covered with a thick growth of manzanita chaparral, which 
makes progress in any direction, out of a beaten track, very slow and difficult. The precise boun- 
daries of the basaltic cap cannot be given at present ; it probably covers a greater area than I have 
marked upon the map. 
The thickness of the voleanic capping is not constant. The altitude of the highest point of the 
Bald Mountain bluff I made to be 5,570 feet, or over 1,100 feet higher than Ellery’s Forest City 
Hotel. The bed-rock rises very rapidly in a northeasterly direction, as is proven by the workings 
in the Bald Mountain mine, but there can be no doubt that, where thickest, there is as much as 
900 feet of lava above the old river channel. To the west and southwest, and possibly also to the 
east, the thickness of the lava stratum diminishes. Between Forest City and Alleghany the thick- 
ness is nearly 600 teet ; between Chips’s Flat and Minnesota, about 250 feet ; and near American 
Hill, between 600 and 700 feet. 
The bed-rock of this portion of the ridge is in part slate, and in part serpentine or of a serpen- 
tinous character. I saw good exposures of slate at the Mountain House ; at Minnesota; at Rock 
Creek, on the Downieville trail ; on the northern slope of Kanaka Creek Caiion, above Alleghany ; 
at the Crescent Tunnel; near Cornish Ranch ; and at American Hill. The serpentine belt crosses 
the ridge in a northwesterly direction, and is exposed to view at a great. many points between 
Minnesota and Goodyear’s Bar. This same belt, or one nearly parallel, can also be traced to the 
north of the North Yuba, certainly as far as Plumas County. A westerly line of junction between 
slate and serpentine passes within a few rods of the Mountain House, and an easterly line can be 
traced near Minnesota and Chips’s Flat. Whether the whole of the intervening belt is serpentine, 
or whether there are alternating strata of slate and serpentine, I cannot say with certainty. There 
are several reasons for supposing the latter to be the true arrangement. For example, at Minne- 
sota there are several quartz veins, having nearly the same strike, which were described to me as 
contact deposits between slate and serpentine. I did not take the time to verify the statements. 
At Chips’s Flat the line of junction between serpentine and slate has a northwesterly course, 
N. 50° W. (magnetic). A little to the west of the serpentine, the slate bed-rock changes abruptly 
from a nearly white to a black variety, the plane of junction having a dip of 80° to the southwest. 
Serpentinous and slaty bed-rock both occur under the gravel at the Bald Mountain and the North 
Fork drift mines at Forest City. The slate rock, in particular, is quite different in appearance from 
that exposed to view outside of the mines. For a depth of fifteen or twenty feet below the gravel 
the slate is almost always of a soft and clayey consistency, though still showing the planes of 
stratification or of cleavage. Below this depth the rock is again firm and hard. There are frequent 
changes in color and in texture. The frequency of the occurrence of “ rotten bed-rock” under the 
gravel suggests the inquiry whether the waters which percolate the gravels may not have a greater 
disintegrating power than the surface waters do. Or, is the absence of clayey deposits, away from 
the gravel, to be explained solely by the fact that the products of disintegration are removed as 
rapidly as they are formed ? 
Hydraulic mining has not been carried on very extensively upon this portion of the ridge. Small 
banks have been washed away, as at Minnesota, Chips’s Flat, Forest City, and City of Six, where 
the erosion of the modern creeks has exposed the gravel to view upon the sides of the cafions, and 
