388 SUPPLEMENTARY INVESTIGATIONS IN THE GRAVEL REGION. 
not been uncovered, but the whole width of gravel may be taken as at least from fifteen to eighteen 
hundred feet. At American Hill the width is nearly the same as at Manzanita Hill. At San 
Juan I estimated the width from bank to bank as not far from one thousand or twelve hundred feet. 
The highest banks worked in this district are at Manzanita and American Hills, where the 
thickness of the gravel amounts to as much as 220 or 240 feet. Of this thickness nearly one half 
is frequently blue in color. A thickness of from fifty to seventy-five feet of blue gravel is quite 
common at these two mines. At Manzanita Hill there are no banks worked at present lower than 
about 125 feet, excepting near the rim or where the surface has been worn down by ravines. 
Mr. Hague’s estimate of 140 feet for the thickness of the gravel below San Juan is a very good 
fair average.* 
The pebbles which are met with in the gravel, or which go to make up its bulk, represent a 
great variety of rocks. At Kate Hayes Flat, the top gravel seemed to be, on the average, finer 
than the corresponding gravel at Smartsville, and to contain fewer independent strata or masses of 
sand and clay. At Manzanita Hill there was a very small percentage of white quartz in the gravel, 
while at San Juan, in the upper strata of red gravel, the pebbles seemed to be almost exclusively 
quartz. At this last-mentioned bank work has been stopped for several years, and the whole face, 
from 125 to 150 feet in height, is of a reddish or brownish color, At the lower end of San Juan 
Hill there is still some gravel left upon the bed-rock, which shows the characters of the ordinary 
blue cement. Chinamen are at work in organized gangs stripping and cleaning this portion of the 
mines. 
The larger boulders, which are met with in connection with the finer gravel, present some fea- 
tures of interest, particularly at American Hill. The bed-rock has been uncovered at this mine for 
a distance of nearly three quarters of a mile, measuring from the upper or northern extremity. On 
the eastern rim large granitic boulders, similar to the bed-rock, are to be seen in abundance, while 
on the western rim, the side towards the slate rock, boulders of quite a different character have 
been found. In the upper part of the mine these boulders were not very frequent, but near the 
present face of the bank there is a remarkable accumulation of them, resting immediately upon or 
lying very near the granite bed-rock. They range in size from three or four feet in diameter up to 
at least twenty. I did not see any very small or very large ones. They lie very close together, 
and occupy a space, — say 500 feet in length by 200 in width and nearly 50 in height. It is 
not impossible that they may be found beyond these limits, when the banks to the south and 
west are washed away. The chief peculiarity of these boulders is their lithological character, 
which is widely different from that of any bed-rock known to exist in the district, or of any 
boulders met with in the other mines. I was told that similar boulders had been seen in the 
ceafion of the Middle Yuba, near American Hill. Seen from a little distance they present a mottled 
or weathered appearance, a dark-colored base being marked with light spots of different sizes and 
shapes, some of them being as much as three or four inches across. The white surfaces are not 
distributed with any regularity, but make up, on the whole, about one half the whole surface of 
the boulders. A broken boulder shows on a fresh surface no obvious signs of being a mixture, but 
a closer inspection brings to light evidences that the rock is a conglomerate of some kind. The 
rock is exceedingly hard, and rings like an anvil when struck with a sledge.t 
The boulders at the eastern end of San Juan Hill comprised some large granitic, with other 
smaller ones, largely quartz or quartzose in character. Possibly one third of what are left lying on 
the bed-rock at this point are of metamorphic rocks not distinctly quartzose. 
The bed-rock tunnels for the mines in this district have been run from the caiion of the Middle 
Yuba, excepting at French Corral, at which place the tailings are emptied into the South Yuba. 
[I have no complete list of the tunnels which have been driven, but will give such data as I have 
in respect to them, and to the methods of working. 
* See ante, p. 202. 
+ This rock appears from microscopic examination of a thin section to be a much altered volcanic material, — 
an amygdaloidal melaphyr tufa, or porodite. 
