28 FORTYM1LE. 
in the Birch Creek region and southward to the Tanana. The rock 
varies from a massive quartzite to quartz-mica-schist; a quartzite- 
schist is most common. At the one extreme is a pure, light-colored, 
massive quartzite, composed almost entirely of fine, rounded or inter 1 
locking quartz grains, with occasional grains of bluish quartz which 
attain a diameter of a millimeter or more. In other localities the 
rock breaks more easily along planes which show glistening flakes of 
mica on their surfaces. The quartz grains are still round, and the 
development of mica is practically the only change which has taken 
place ; then the grains become elongated, the proportion of mica, both 
muscovite and biotite, increases, and finally, in some localities, the 
rock has become a quartz-mica-schist. Graphitic phases are com- 
mon, and metamorphic minerals, especially garnet, are frequently 
developed during this process. 
The rocks have not only undergone changes in composition, but 
have been folded upon themselves so closely that the structure is 
apparently simple. Examination, how T ever, discloses minor folds 
with limbs so slightly divergent as to appear like separate beds of 
strata in nearly parallel positions; furthermore, the folds are often 
horizontal, and the area resembles one of slightly folded rocks. Most 
instructive examples of these closely appressed folds were observed 
along the trail from Circle to Fairbanks, the structure being appar- 
ently a characteristic one. Determinations of the thickness of these 
rocks become, therefore, very uncertain. The strike of the axial 
planes of the folds, where observable, is about east and west, and their 
dips have a maximum of about 45° ; the most important system of 
joints strikes northwest. 
In the section from the Birch Creek to the Fairbanks region, a dis- 
tance of nearly 150 miles, the rocks show a great uniformity in com- 
position. A small outcrop of closely folded calcareous beds was 
observed at the mouth of Mastodon Creek, in the Birch Creek region, 
and a small quantity of limestone is interbedded with the quartzite- 
schist about 16 miles northeast of Fairbanks Creek. The strike at this 
locality is N. 80° E., and the dip of the structure 30° to the south. 
Occasional feldspathic schists, which are possibly altered igneous 
rocks, were observed interbedded with the others. Intrusives occur in 
the Birch Creek region and in the Fairbanks region, but none were 
observed in the intervening area. The rocks contain numerous small 
quartz seams, and locally have been more or less mineralized. Pyrite 
is often seen in the schists, and pieces of schist have been found con- 
taining gold-bearing quartz. The gold in the placers of the Birch 
Creek and Fairbanks regions has been derived from these rocks. 
The contact of these rocks with the Fortymile formation was not 
observed, but from the dominant lines of structure it would seem that 
