18 GEOGRAPHY AND GEOLOGY OF RAMPART REGION. 
SEDIMENTARY AND METAMORPHOSED FORMATIONS. 
PRE-DEVONIAN. 
The most characteristic rocks in the eastern portion of the Yukon-Tanana country 
are gneisses, quartzite-schists, quartz-mica-schists, garnetiferous and hornblende 
schists, and crystalline limestone. In the Fortymile region the last three types 
predominate and have been called a by Spurr collectively the Fortymile formation; in 
the Birch Creek region and in the recently developed Fairbanks region the quartzite- 
schists and mica-schists, with occasional thin interbedded highly crystalline lime- 
stones, constitute the most common rocks and have been called by the same author 
the Birch Creek formation. They are considered by him older than the Fortymile 
schists. The age is not known, and for the purposes of this report they are both 
referred to the pre-Devonian. 
The western limit of the large areas of these highly metamorphosed schists is in 
the ridge limiting on the northwest the valley of Chatanika River. The ridge is 
formed of the typical schists with some schistose limestone and graphitic schists, 
and like the rocks in the Fairbanks region the dip is generally low, a common atti- 
tude and one indicating not necessarily simplicity of structure but oftentimes a 
folding so extreme that the rocks have been ^folded upon themselves until the dips 
are often nearly horizontal. The first indication of a change in lithologic character 
was found in the occurrence a few miles nearer the White Mountains of a feldspathic 
schist, and then about 3 miles farther to the northwest overlooking Beaver Valley 
outcrops of a rather coarse-grained, greenish, impure, feldspathic quartzite showing 
no schistosity and dipping steeply to the west. The northwestern limit of the old 
schists is apparently somewhere in the intervening densely spruce-clad areas. 
No more schists were observed until reaching the Rampart region, where on 
Minook Creek a small area of garnetiferous quartz-mica-schist with associated mar- 
bles occurs. This is apparently best developed on Ruby Creek, where the garnets 
of the schist accumulate rapidly in the sluice boxes. This formation of schist and 
limestone crosses the Minook at Hopkins Bridge with a northeast strike and a low 
dip to the southeast. Eagle Rock, about 1 mile below the bridge, is a bluff, 100 feet 
or more in height, composed of highly crystalline, banded limestone. There are 
minor folds and a general low dip to the north. Although the area is small and the 
marbles resemble somewhat other crystalline limestones in the vicinity they are so 
much more highly metamorphosed than any of the other rocks and are so closely 
associated with garnetiferous schists which are not known to occur elsewhere in the 
region that both the limestone and the schists stand in contrast to the adjacent 
rocks. The contact relations were not observed, but these rocks are believed to be 
unconformable with and older than the adjacent rocks to the north and south. 
Their resemblance to some of the schists and marbles of the Fortymile region is a 
striking one. They apparently strike off northeast under younger rocks. 
No rocks were observed in the areas between these two occurrences of schist which 
suggest a correlation. Further investigation may reveal the horizontal equivalency 
of some of the highly metamorphosed schists with less metamorphosed rocks of 
known age, but all that it seems possible to do at present is to place them by them- 
selves and call them pre-Devonian. 
DEVONIAN. 
Between the two areas of schists above described, the one the westward extension 
of the schists of the Fairbanks region, the other a small area the limits of which 
are not yet known, are the rocks most of which are regarded provisionally as Devo- 
nian. The bulk of the formation, as already mentioned, is made up of cherts, slates 
a Spurr, J. E., and Goodrich, H. B., Geology of the Yukon gold district, Alaska: Eighteenth Ann. 
Kept. U. S. Geol. Survey, pt. 3, 1898, pp. 140-155. 
