DESCRIPTIVE GEOLOGY. 25 
QUATERNARY. 
The changes in elevation with reference to sea level which the 
Yukon-Tanana country has undergone have left benches at various 
altitudes, some of them of considerable extent, which stand generally 
in a definite relation to the present drainage lines. Gravel was de- 
posited on some of the benches during their formation. Part of this 
gravel is regarded as of Pleistocene age. 
Such deposits have been found in the Fairbanks region in the val- 
ley of Fairbanks Creek, and in the Rampart region 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, approxi- 
mately 500 feet above the present stream, are of interest with refer- 
ence 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. 
Silts also have accumulated in great quantities in the larger valleys 
throughout the interior of Alaska, and as gravels have been repeat- 
edly deposited on successively lower benches, so, too, silts have been 
deposited at different levels down to that of the present flood plains. 
These were probably laid down under lacustrine conditions and the 
interaction of lacustrine and fluviatile conditions, and the age of depo- 
sition dates from the Pleistocene, or earlier, to the present time. 
IGNEOUS ROCKS. 
In the geologic history of the Yukon-Tanana region igneous rocks 
have played a very important part, and though not so widely dis- 
tributed in this western part of the area they form nevertheless an 
essential part of the bed rock. 
The igneous rocks include material that in a more or less molten 
condition has penetrated the other rocks in various forms at various 
times and at various depths below the contemporary surface and also 
material that has been poured out upon the surface as lava flows or 
has been ejected as fragmental material. The rocks formed by the 
consolidation of the molten material have characteristics so indica- 
tive of their origin that miners generally distinguish with little diffi- 
culty the igneous from the sedimentary rocks with which they arc 
associated, or the igneous material present in the gravels from the 
sedimentary material with which it is mixed. 
The differences in the composition of the igneous rocks are nol so 
readily observed. Coarse-grained, fine-grained, and glassy rocks en- 
tirely different in appearance may result from the same material 
solidifying under different conditions, and furthermore, the igneous 
