58 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251. 
SUMMARY. 
The gold-producing localities of the Fortymile region are divided 
geographically into three rather widety separated areas, the Forty- 
mile area, the Eagle area, and the Seventymile area. 
Through most of the Fortymile area tlie bed rock belongs to the 
formation known as the Fortymile formation. It is composed of 
various schists with interbedded crystalline limestone. All have been 
closely folded, highly metamorphosed, and intruded by many kinds 
of igneous rocks. Some of these were intruded so early in the history 
of the region as to have also undergone metamorphism; others are 
comparatively fresh. Small quartz veins are numerous, and locally 
the rocks have become mineralized and are a source of gold. The 
occurrence on Chicken Creek is different. The bed rock is mostly 
olivine-basalt and a formation of sandstone, coal, and shale which 
probably belongs to the Kenai. The walls of the valley are olivine- 
basalt, granular and porphyritic igneous rocks of types ranging from 
hornblende-granite to quartz-diorite, and greenstones of the Ram- 
part formation. As the rocks differ from those of the other localities, 
so, too, there is a difference in the character of the gold, which is gen- 
erally granular, of a dark color, and suggests a different origin. At 
hvo localities gold has been found in place. One of these is out- 
side the valley of Chicken Creek, but in a dioritic rock which is found 
abundantly within the valley; another is in a shale of undetermined 
age which may possibly belong to the Rampart formation. The 
placer gold on Chicken Creek may have been derived mostly, if not 
entirely, from these two sources. 
In the Eagle area the occurrence has not been traced to a source. 
The bed rock of American Creek near the headwaters is schist and 
greenstone. In the lower part of its course the creek flows mostly 
through greenstones and serpentine. 
In the Seventymile area also the origin is indefinite. The creeks 
from the north flow through conglomerate of the Kenai forma- 
tion. No case is known where gold has been found in place in the 
conglomerate, but it has been found in a wash containing heavy 
quartz ite bowlders which occurs locally, overlying the conglomerate. 
No such bowlders were found in the conglomerate, and the wash may 
possibly have been derived from rather distant sources. 
The most interesting recent developments in the Fortymile region 
are the discovery of gold in place on the Chicken, the working of the 
high-bench gravels between the Chicken and the Lost Chicken, the 
draining of North Fork by an artificial cut-off at the Kink prepara- 
tory to working the ground on an extensive scale, and the installation 
of a hydraulic plant on American Creek. The best results so far have 
