98 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
those of granite are exceedingly rare. ‘There is in the bank at this claim a great deal of opal, 
almost all of which is green, a part of it being of a bright emerald color. The boulders in the bank 
are almost entirely undecomposed, and the opal occurs in the interstices between them. 
At Blacksmith Flat, on the south side of the ridge between Long Canon and the Middle Fork 
of the Middle Fork of the American River, the maximum thickness of the gravel exposed in the 
hydraulic banks is fifteen feet. The gravel is here and there overlain by an irregular stratum of 
sand ranging from five to ten feet thick. Above this come several hundred feet in thickness of 
volcanic materials forming the crest of the ridge. The gravel is smoothly rounded and made up 
of a great variety of metamorphic rocks, with a large admixture of granite boulders, some of which 
are eight or ten tons in weight. For a distance of two or three miles above Blacksmith Flat the 
ditch, which runs 300 feet above the bed-rock, is cut through gravel, and there are indications at 
other localities that the quantity of gravel in this ridge between Long Caiion and the Middle Fork 
of the Middle Fork of the American River is very considerable, although at the same time the 
capping of volcanic materials along the central portion of the ridge is very heavy. 
At Castle Hill, near Georgetown, the gravel varies, in the channel, from one or two inches to two 
or three feet in thickness, and contains some pretty well washed quartz pebbles, with many frag- 
ments of bed-rock. It is immediately overlain with voleanic cement, which is generally grayish in 
color. This volcanic capping is probably 125 feet deep on the erest of the ridge; and it contains 
many large boulders, which are equally plentiful towards the top or the bottom of the mass. 
At Centerville, some ten or twelve miles below Georgetown, the material called gravel consists 
mostly of angular fragments of the bed-rock, of all sizes. Among the great variety of dioritic, 
hornblendie, and porphyritic rocks of which this gravel is made up, there are a few of granite, and 
these are well rounded. Numerous quartz boulders also occur, many of which are very large, 
some even weighing from eight to fifteen tons. The greater portion of these boulders, whether 
large or small, are but little rounded by water, although some of them are thoroughly so. One 
large boulder of compact white quartz found here yielded $8,000 in gold, and others have also 
proved valuable. The bed-rock at this place is chiefly slate, but there are dioritic rocks farther to 
the southwest, in the sides of Pilot Hill. The bed-rock slopes to the East and the West; and on 
the eastern slope there is a certain area covered with a few feet in depth of well-washed gravel, 
which is very firmly cemented together and very hard. This gravel is overlain by the brecciated 
mass ten to twenty feet thick, which is the ordinary “gravel” of the district, and which is in all 
probability only a local deposit, perhaps accumulated from the slopes of Pilot Hill. 
The general character of Buffalo Hill, near Georgetown, is much like that of Mameluke Hill; the 
crest of the ridge runs for half a mile, or more, in a direction N. 18° W. (magnetic), between West 
and Illinois cafions. It is capped with volcanic bouldery cement, in places a good deal decom- 
posed, and with a maximum depth of from seventy-five to eighty feet. There appears to be, in 
general, but little gravel on the bed-rock, beneath the cement; and, although numerous shafts 
have been sunk here, the results do not seem to have been pecuniarily satisfactory. 
At Tipton Hill, two and a half miles a little west of south from Kentucky Flat, the gravel 
ranges from four to six feet in thickness, and is almost entirely made up of quartz, in fragments of 
moderate size, not much rounded. This gravel is covered with the ordinary bouldery cement, and 
the maximum height of the banks is about thirty-five feet. 
At the Excelsior Claim, near Placerville,* the maximum height of the bank is 170 feet, of 
which the lower sixty or seventy feet are “ pay gravel” consisting of quartz, metamorphic rocks, and 
sand ; the upper hundred feet is made up of well-rounded and water-worn volcanic gravel. This 
is said also to contain a little fine gold. The line of demarcation between the pay gravel and the 
volcanic capping is pretty well defined, the former being yellow, and the latter bluish-gray. There 
are a few metamorphic pebbles in the volcanic beds; but no volcanic materials in the pay gravel. 
About twenty acres of ground have been washed off here, with an average thickness of something 
over fifty feet of “pay gravel.” Along the northern side of Hangtown Hill, just south of Placer- 
* For the position of the localities and the claims about Placerville, see diagram, Plate C. 
