118 ALASKAN MINERAL RESOURCES IN 1905. 
acidic igneous rocks observed in the Fairbanks region are intrusive porphyritic biotite- 
granites. The acidic dikes so common in the Fortymile region are absent, and the gold of 
the placers has probably been derived from small quart/- seams in the schists. 
It is often a subject of surprise to the miners that when gold is abundant in the placers it 
should be found so rarely in the bed rock. It might be said that if gold were commonly 
encountered in the bed rock the proportion of it in the placers, considering the amount of 
bed rock that has been removed, ought to be much greater. There is the possibility also 
thai the veins in the country rock, which contributed the material for the first deposits of 
the valleys, were richer in gold than those now exposed. Be that as it may, it is certain that 
through long-continued weathering and soiling of the rock material a concentration of the 
! ,i\ ier indestructible 1 contents, including the gold, takes place, yielding auriferous detrital 
deposits which are made richer in gold than the parent rock. 
PLACERS. 
FAIRBANKS CREEK. 
Fairbanks Creek is about 10 miles long and flows in an easterly direction to Fish Creek, a 
tributary of Little Chena River. The floor of its valley is 200 to 300 feet broad, but widens 
rapidly about 3 miles from the month. The productive area of Fairbanks Creek comprises 
about 4 miles of the valley, shirting from a point about 2 m ; !es below the source. The 
gravels usually vary from 25 to 10 feet in thickness, but in the lower part of the valley are 
much thicker. The pay streak ranges from i to 8 feel in thickness, averaging about 5£ feet, 
while in some places 2 to 3 feet additional of bed rock are mined. The pay streak ranges 
from 40 to 200 feet in width and in the upper part of the valley lies close to the present 
stream bed, but below it diverges toward the north valley slope. As it has not been traced 
through the lower part of the valley, it i^ uncertain whether it continues as a well-defined 
pay streak or becomes disseminated or distributed over a considerable area. 
Fairbanks ('reek has been a good producer, but the gold does not appear to be evenly dis- 
tributed, and while much good ground i> left, there should be careful prospecting to deter- 
mine values before the introduction of expensive machinery. Several mining plants have 
been installed, some of which are liiieb equipped. The production of the past year has 
probably exceeded $1,000,000. 
PEDRO, GILMORE, AM) GOLDSTREAM CREEKS. 
Pedro Valley is similar in its general character to Fairbanks Valley. The productive area 
includes about 2 miles of the stream between the mouth of Twin Creek and the point where 
it is joined by Gilmore Creek. The deposits have a thickness ranging from 8 to 30 or more 
feet, and in some places values have been found through as much as 8 feet of gravel and 4 
feet of bed rock, while in others they are confined almost exclusively to the bed rock. The 
distribution of the values has been found to be irregular. 
Gilmore Creek, a tributary to Pedro Creek from the east, about 5 miles long, forks 3 miles 
above the- mouth. The northern fork is called Tom Creek and the eastern fork retains 
the name Gilmore. The grade of Gilmore ('reek below its junction with Tom Creek is about 
UK) Feet to the mile, while above it averages about 300 feet to the mile. Some development 
work has been done in the upper part of the valley. The deposits in the Gilmore Valley are 
similar to those of the other valleys described, and range in thickness from a few feet in the 
upper part of the valley to about 60 feet near the mouth. Values have been found, but 
insufficient work has been done to determine their extent. The gold lies mostly on the sur- 
face of the bed rock or within it. Noteworthy features of the o< currence here are that the 
gold is reported to assay $19.25 to the ounce, the highest in the region, and that native bis- 
muth is sometimes found intergrown with the gold. 
Goldstream Creek is the continuation of Pedro Creek below Gilmore Creek. Here the 
valley widens and flattens and I he deposits deepen, so that near the upper end there is a 
thickness of 30 to 40. feet of gravel. Though work has been done at intervals along se\ erftl 
