ECONOMIC GEOLOGY. 31 
The region, though dependent on the outside for the greatest part 
of its supplies, is in the matter of lumber and fuel mostly independ- 
ent. The spruce timber along the sloughs of the Tanana and the 
lower parts of the valleys of its largest tributaries is of good quality 
and much of it exceeds 2 feet in diameter at the butt. The small 
spruce and birch, so abundant on the hillsides, furnish a supply of 
fuel which has not up to the present time been heavily taxed, but 
which is becoming year by year a more important item in the cost of 
production. 
BED ROCK. 
The bed rock is predominantly schist, closely folded and striking 
in general northeast and southwest. The main structural planes vary 
from nearly horizontal to nearly vertical. The alternating beds of 
blocky quartzite schist and schists with a large proportion of mica, 
give rise to a bed-rock surface of varying influence on the distribu- 
tion of the gold. The gold sinks along the structural planes of the 
blocky bed rock to a depth of several feet in some places, but the 
compact clayey surface of the softer beds is practically impermeable 
to the gold. Quartz veins, some of them being 2 feet or more thick, 
are common in the schists, but are not so abundant that quartz 
becomes a conspicuous constituent of the gravels. 
Granitic intrusions are present at several localities in the main 
ridge. They form the bed rock at the upper ends of some of the 
valleys and have furnished material for a small proportion of the 
gravels in many of the valleys. Greenstone schist forms a large part 
of the ridge west of Cleary Creek, and this rock is generally conspicu- 
ous by the quantity of garnets it contains. A small amount of black 
basaltic rock outcrops at the point of the ridge that forms the north- 
ern limit of the valley of Fairbanks Creek. 
ALLUVIAL DEPOSITS. 
The loose material formed by the weathering of the bed rock 
varies in character according to the amount of transportation it has 
undergone. Outcrops of bed rock in the Fairbanks region are con- 
fined mostly to the summits of the ridges and to the steeper slopes of 
the valleys, while on the gentler slopes and in the bottoms of the 
valleys the bed-rock surface is covered with a mantle of material 
ranging in thickness from a few feet to over 300 feet. This mantle 
is composed partly of heterogeneous talus, which is continually 
working down the sides of the valleys, and partly of the material in 
the valley floors, which has been worked over so many time- by run- 
ning water that it has a fairly uniform structure throughout. All of 
these deposits are, for the most part, frozen throughout the year. 
As the streams generally flow close to one side of their valleys the 
deposits are mostly on but one side. Their upper surf ace slopes grad 
