GEOLOGY IGNEOUS ROCKS. 23 
QUATERNARY. 
The changes in elevation with reference to sea level which the Yukon-Tanana coun- 
try has undergone have left at various altitudes benches, sometimes of considerable 
extent, which stand generally in a definite relation to the present drainage lines. An 
accompaniment of benching has been the deposition upon some of the benches of 
gravel deposits, part of which are regarded as of Pleistocene age. Benches are 
prominently developed along Hess Creek and its tributaries, along the Minook, and 
along the tributaries of Baker Creek. The deposits of the high bench of the Minook, 
approximately 500 feet above the present stream, are of interest with reference 
to the occurrence of gold in the tributaries of Minook Creek. The bench gravels of 
the Baker drainage have proved in some places to be of great economic importance. 
The description of these gravels and the deposits of the present streams is given 
elsewhere in this report in the account of the gold placers. 
IGNEOUS ROCKS. 
The eastern part of the Yukon-Tanana country is characterized by an abundance 
of intrusive and extrusive rocks of varied composition. There are several batho- 
lithic masses varying in composition from granite to quartz-diorite, with numer- 
ous apophyses in the country rock. Acid granitic rocks are abundant, and occur 
commonly in small dikes, some of which are comparatively fresh, while others have 
been rendered schistose along with the country rock. These become less abundant 
toward the west and seem to disappear. In the Birch Creek and Fairbanks regions 
the most common rock of igneous origin is biotite-granite. After crossing the 
White Mountains no rocks of igneous origin except greenstones were encountered 
until reaching the immediate vicinity of Rampart. That region has been an area 
of great igneous activity, in which granitic rocks and more basic granular rocks are 
common, occurring as small intrusive masses and narrow dikes, and their extrusive 
representatives are also to be found in small amount. The igneous rocks will not 
be considered in detail, and only such description will be included as seems neces- 
sary for the purposes of this report, They may be grouped roughly as granitic 
rocks with their surface representatives, monzonitic rocks, greenstones, and basalt 
with associated glass. The most characteristically developed of them are the green- 
stones which occur throughout the area. The basaltic rocks so far as observed have 
but a limited distribution. 
GRANITIC ROCKS. 
The granitic rocks are not of common occurrence. The most extensive mass 
observed forms a part of the summit of Wolverine Mountain, where it occupies an 
area about 1,000 feet wide. It is a porphyritic, massive, gray rock composed chiefly 
of quartz, phenocrysts of orthoclase a half inch or more in diameter, considerable 
piagioclase, some biotite and hornblende, and a little pyroxene. The rock is finer 
grained toward the margin. The slates in contact with it have been indurated and 
their fracture surfaces are flecked with the products of metamorphism, chiefly 
andalusite. 
A similar rock occurs west of the mountain near the saddle where the trail passes 
through the ridge to descend toward the Hutlina. This is also a gray porphyritic 
rock, but the porphyritic feldspars sometimes an inch or more in diameter have a 
tabular development. The proportion of pyroxene is greater, there is less quartz, 
and the composition of the rock is transitional to that of the monzonitic rocks. 
Rocks similar in composition to those of Wolverine Mountain occur also in Lynx 
Mountain along with monzonitic rocks, but their outcrops were not observed. 
A siliceous rock occurs west of Minook Creek about 7 miles from Rampart. It is 
a finely banded, grayish-brown rock and has been complexly folded on a small scale, 
