18 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251, 
river. On the southern side the ridges often persist to the Tanana, 
or become gradually lower toward its valley or those of its tribu- 
taries. The valleys, too, are more open than those on the Yukon side. 
DRAINAGE. 
There is no well-defined divide between the two rivers, and the 
drainage is carried away by their interlocking tributaries, which 
often pursue most irregular courses. The most important tributa- 
ries of the Yukon within this area are the Fortymile, Mission, Sev-; 
entymile, Charley, Birch, Preacher, Beaver, Hess, and Minook ; those 
of the Tanana are the Volkmar, Goodpaster, Salcha, Chena, Tolo- 
vana, and Baker. 
FORTYMILE REGION. 
General character. — The greater portion of the Fortymile region,. 1 
about 2,100 square miles, is included in the Fortymile quadrangle* 
which was mapped by Mr. E. C. Barnard in 1898, and shown on PL. 
VII of this report. 
In this region the mountains are of two types — ridge and dome. 1 
Glacier Mountain, 15 miles west from Eagle, which is one of thL 
most prominent examples of the ridge type, is partly shown in PL, 
II, B. It is a gray, sharp-peaked ridge, with a northeast-southwest! 
trend, an altitude of about 6,000 feet, and a length of about 10 miles..! 
It forms a strong contrast to the surrounding countrv. From its* 
Hanks can be seen several domes which attain heights up to 5,000 oii 
more feet. Fortymile Dome, about 15 miles south of the Yukon}- J 
along the boundary, is one of the most conspicuous examples. Be|| 
tween these types of prominences extends the undulating plateau-like f 
areas interrupted by the valleys of many streams. 
The Yukon River at Eagle is about 810 feet above sea level, and its'-l 
tributaries have consequently a fall of something like 2,000 feet froiriij 
their sources in the plateau country to the level of the main river. J 
As the Yukon has been cutting for itself a narrow valley in the Yukorfiffl 
Plateau, so the many tributaries have been deepening their channel^! 
until the prevalent type is that of the canyon. The most import a nM 
streams are the Fortymile, Mission, and Seventy mile. 
Fortymile system. — Fortymile Creek, which drains about two- j 
thirds of the area, is formed by the union of North and South forks at ! 
a point about 45 miles southwest from Eagle and 25 miles west of the* 
boundary and flows in a meandering easterly direction to the Yukon' 
through a narrow valley (PL III, A), the bottom of which, wheri 
the forks join, is about 1,000 feet above sea level. From the higher I 
elevations a few miles back long spurs descend toward the stream 1 
with gradual slopes, which at an elevation of about 500 feet above 1 
