54 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251. 
Graphitic schists, containing abundant quartz seams, form a portion 
of the bed rock about the headwaters, and crystalline limestone out- 
crops on the divide at the head of Discovery Fork. Schists similar to 
those above described occur again on American Creek about a mile 
above its junction with Discovery Fork. Below this point to Discov- 
ery Fork and on the latter stream, the bed rock is mostly serpentine, 
frequently cut by basic dikes. 
Though American Creek has been worked for several years and 
considerable coarse gold found upon it, no preparations were made 
to work the gravels on a large scale until 1903, when a hydraulic 
plant was installed at a point about 1 mile above Discovery Fork, 
where there is an ill-defined bench on the west side. The gravels at 
this locality are about 5 feet thick, composed of schist and igneous 
rocks. There are 3 feet of loose gravel called "chicken feed," and 2 
feet of similar material with a large proportion of clay. Gold is 
found throughout this lower deposit, but the best pay is next to bed 
rock, some being also found in the bed rock itself to a depth of 2 feet. 
Barite is a common associate of the gold. 
The slope is a rather steep one, from the edge of which to the water 
of the creek there is a fall of about 15 feet. A flume, 7,200 feet in 
length, with a capacity of 1,200 inches, has been built from the upper 
portion of American Creek, giving a head of 150 feet and furnishing 
water intended to supply two hydraulic elevators. The capacity of the 
plant was expected to be about 1,000 cubic yards of dirt in a day of 
twenty-four hours, and the expense of working about 10 per cent. At 
the time of the writer's visit the water was turned on the ground for 
the first time. The plant is a very complete one for' work and, if the 
water supply can be depended upon, should accomplish good results. 
It is shown in PI. XL" 
Discovery Fork. — On Discovery Fork there are from 4 to 10 feet of 
gravels lying on the serpentine bed rock, covered with as much as 12 
feet of muck. The pay is mostly on bed rock and is rather coarse. 
Values are found over a width of from *S to nearly 30 feet. The creels 
has produced perhaps $20^000. During the season of 1903 two outfits 
were working on it. 7 ' 
SEVENTYMILE AREA. 
Broken Neck Creek. — This creek is about 20 miles northwest of 
Eagle. It is short, entering the Seven tymile from the north just 
above Mogul Creek, which enters from the south. Its valley is 
bounded by precipitous walls several hundred feet high, and where 
a The water supply was found insufficient for the demands of the plant, and during 
1904 modifications of the method were being tried to make a more effective use of the 
available water. 
h During 1004 excellent results were secured by the use of an automatic dump gate. 
