30 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251. 
Crystalline limestone occurs in beds from a few inches to 50 feet or 
more in thickness. The rocks arc closely folded and have a variable 
strike and dip, with a general structure nearly east and west. Con- 
torted strata of hornblende-schist are shown in PL V. B. These 
rocks, like those of the Birch Creek region, contain many quartz 
veins. Locally the rocks and the veins have been mineralized and 
become the source of gold. 
In the Fortymile region they have been intruded most complexly 
by igneous rocks. Some of these have undergone metamorphism 
along with the schists and have become an intimate part of them, 
while others are fresh and vary in size from large intrusive masses 
down to thin sheets and narrow dikes. The accompanying photo- 
graph (PL VI, A) shows a typical outcrop of hornblende-schist with 
a granitic sheet C> inches in thickness parallel to the schistositv. In 
prospecting the rocks of the Fortymile formation the marble beds 
have often been taken for veins, and much labor has been lost. They 
are often in a nearly vertical position on account of the close folding 
which the rocks have undergone, but an examination will show that 
they form an integral portion of the country rock, and can be traced 
sometimes for an indefinite distance. 
These schists are prominently developed on all of the gold-produc- 
ing creeks of the Fortymile area, with the exception of Chicken Creek. 
They occur to the southwest of the Fortymile quadrangle around the 
head of the Buckskin, along Confederate Creek, and on North Fork 
of Fortymile Creek. They were observed also far to the west, along 
Charley River. 
In the northern portion of the Fortymile quadrangle the area! 
boundaries are not clearly determinated. Hornblende-schists and 
schistose limestones occur at the falls on the Seventymile, and are pro- 
visionally correlated with the Fortymile rocks. Quartzite-schists, 
similar to schists of the Fortymile, dip away to the east and west 
from the northern termination of the intrusive mass of Glacier I 
Mountain. It seems best at present to draw the boundary between 1 
the quartzite-schists, which are often graphitic, and the limestone, I 
which at several localities occurs to the north of them and is included 
provisionally in the Rampart formation. This limits the northern 
extension of these rocks to a line running northwesterly from the 
headwaters of American Creek. The boundary between this and the 
succeeding formation is probably very irregular. 
The Birch Creek and Fortymile formations and all the other 
metamorphic schists were included by Brooks a under the name of j 
. . . 
Kotlo series, and in the geologic map (PL IV) no attempt has been 
made to differentiate the various members. The age of these rocks 
"Brooks. Alfred II.. A reconnaissance from Pyramid Harbor to M-igle City, Alaska: 
Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 2, 1900, pp. 357-358. 
