26 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251. 
westward from the Canadian boundary probably to the Salcha Val- 
ley. These gneisses, provisionally assigned to the Archean, are 
Hanked both north and south by the metamorphic sediments, which 
include the Birch Creek schists and Fortymile formation. These 
later occupy a broad zone, which is probably continuous from the 
Fortymile basin to the valley of Beaver Creek, interrupted only by 
smaller areas occupied by intrnsives. or Devonian or Tertiary forma- 
tions. The Devonian (Rampart formation) finds an irregular dis- 
tribution in small patches in the eastern portion of the country; west 
of the Beaver Valley it attains an extensive development. Some 
Mississippian, Permian, and Cretaceous rocks outcrop along the 
Yukon between Eagle and Circle, and Carboniferous rocks have been 
found west of the Beaver Valley, but rocks of these ages have not been 
recognized elsewhere in the district, though it is quite possible that 
detailed surveys may discover them. 
If this map has any significance to the placer miner, it lies in the 
distribution of the metamorphic sediments which nearly everywhere 
seem to be the source of the placer gold. It is possible that the igne- 
ous rocks of the Rampart formation may also be gold bearing, but in 
the placer districts studied by the writer the source of the gold seemed 
in most instances to lie in the metamorphic sediments. This fact 
leads to the conclusion that the most promising fields for prospecting 
will be the areas occupied by these metamorphic rocks. Furthermore, 
the prospector should pay special attention to localities in which igne- 
ous intrnsives are plentiful, for the auriferous veins seem to be largely, 
if not entirely, confined to such associations. The economic impor- 
tance of the Tertiary lies in its being the coal-bearing horizon of the 
district. 
SEDIMENTARY AND METAMORPHIC FORMATIONS. 
There are many unsolved problems regarding the subdivisions and 
stratigraphic succession of the rocks occurring in different regions 
of the Yukon-Tanana country. Gneisses, quartzite-schists, and 
schists of various kinds with interbedded crystalline limestones, have 
been considered as the oldest rocks and their relations have been 
studied by a number of observers. Gneisses and schists, possibly 
forming a basal complex, are most extensively developed in the 
southeastern portion of the country toward the Tanana, the quartzite- 
schists in the Birch Creek and Fairbanks regions, and schists with 
interbedded limestones in the valley of the Fortymile. In both the] 
Birch Creek and Fortymile regions, however, the latter types occui 
in close relation, associated with local areas of gneiss. The problem 
of separation is further complicated by the close folding which th( 
schists have undergone. These old rocks will be considered here h 
