pbindlb.] GEOLOGIC SKETCH. 29 
they underlie the latter. The Fortymile rocks occur 6 miles cast of 
the forks of Birch Creek, where they have a northwesterly strike and 
a northeasterly dip. Farther west the strike swerves round toward 
the north, and this would carry them outside of and dipping away 
from the rocks of the Birch Creek formation. The same format ion 
occurs in the eastern portion of the Fortymile region, where their 
occurrence has been described by Spurr in the report to which refer- 
ence has already been made. 
FORTYMILE FORMATION. 
The formation described and named by Spurr" the Fortymile series 
js most extensively developed in the Fortymile region. A fine ex- 
posure of the rocks composing this formation, however, is shown 
in the ridge referred to above between the forks of South Fork of 
Birch, about 6 miles east of the main forks of Birch Creek, where a 
long, irregular spur about 8 miles in length descends gradually toward 
the northwest between the deep, narrow 7 valleys of the two forks. 
The roughest part of this spur is formed by outcrops of massive crys- 
talline gray limestone, and the schists also contain occasional thin 
beds of limestone. In contrast to the quartzite-schists, these rocks 
vary greatly in character. 
The most common rock is quartz-mica-schist, containing much gar- 
net, staurolite, and biotite. Hornblende-schists occur, sometimes 
closely associated with limestone, and there are some thin beds of 
quartzite-schist. The strike, as already mentioned, is northwest, and 
the dip '20° to 45° to the northeast. The spur terminates abruptly in a 
prominent point 1,500 feet in altitude above the forks and 1J miles 
to the east of them. Just below the forks the stream has cut a nar- 
row canyon in garnet and quartzite schists, which may be transitional 
to the Birch Creek formation. The thin-bedded character of the 
schists, their northeasterly dip, and the spur of garnet-staurolite- 
schist to the southeast are shown in PL V, A. They a*re here cut by 
small granitic dikes. The continuation along the strike of the Birch 
Creek rocks would carry them across the valley of Crooked Creek 
toward the Crazy Mountains. In the other direction they extend to 
the southeast, and to the south of the northern tributary of South 
Fork of Birch form a prominent ridge, which attains a height of 
about L,000 feet, and is furrowed on its southern slopes by the deep 
canyons of minor tributaries. The strike is uniformly to the north- 
west and the dip to the northeast. 
The Fortymile schists attain a strong development in the Forty- 
mile region and have been described in detail by Spurr. They 
include mica, garnet, hornblende, quartzite, and graphite schists. 
" Op. cit., pp. 145-155. 
