36 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251. 
mediate types and altered derivatives. The extrusives, so far as 
known, are diabases and basalts. 
GRANITE. 
The granites vary greatly in composition and may be roughly 
divided into three varieties — alaskite, biotite-granite, and hornblende- 
granite. The alaskite is composed almost entirely of quartz and feld- 
spar; it occurs generally as small unaltered dikes and sills in the 
schists. The relation of this rock to quartz veins has been described 
in detail by Spurr in the chapter to which reference has already been 
made. The biotite-granite has a darker color, due to the presence 
of a considerable proportion of biotite, and occurs sometimes in large 
areas. The largest mass observed is west of Charley River, where it 
was traversed for about 10 miles. The rock forms a bare ridge 
which attains an altitude of about 4,000 feet. It is an evenly granu- 
lar, medium-grained biotite-granite, with pegmatitic areas and a 
Jocal development of a gneissoid structure. A similar rock occurs 
commonly in the Fortymile region, and a fine porphyritic phase of it 
occurs in the Birch Creek region, where it is found on the gold- 
producing creeks. Some of the gneisses have probably been formed 
by metamorphism from granitic rocks which were perhaps older than 
the schists, and others, similar in character, from later intrusions 
which have been metamorphosed along with the rocks in which they 
are contained. There are some areas of hornblende-granite. 
DIORITE. 
Diorite occurs in limited quantity, and many rocks intermediate 
in composition between it and granite are found. These rocks resem- 
ble hornblende-granite, but contain much plagioclase and are more 
closely related to quartz-diorite. A rather fine-grained porphyritic 
variety of a rock, which is closely related to quartz-diorite and 
may be called a granodiorite-porphyry, occurs on Chicken Creek and 
is composed of corroded quartz phenocrysts, abundant plagioclase,- 
orthoclase, much hornblende and biotite, considerable titanite, andj 
apatite. 
A large mass of a medium-grained fresh rock, varying in composi-j 
ti'on from granodiorite to quartz-diorite, forms the sharp gray ridgej 
of Glacier Mountain, 20 miles west from Eagle, and covers a consider-j 
able area to the westward. Some of this rock contains abundant' i 
quartz, plagioclase, biotite, and hornblende. Other varieties contain 
considerable orthoclase. There is a fine, sharp, isolated peak, about t 
0,000 feet in height, on the westernmost fork of Charley River, com- 
posed of this same beautiful rock. This locality is bounded on the 
north by the large area of biotite-granite already described and on the 
west by schists. Like the granites, this rock also has its gneissoid 
a During 1904 many detached areas of extrusives were observed southwest of the Forty- 
mile quadrangle. 
