prindlb.] GEOLOGIC SKETCH. 35 
has undergone many changes in level, which have been recorded on 
the valley sides by benches and deposits of gravel, which indicate the 
former water levels just as mud and driftwood register the level of 
high water in a stream. The simplest case is perhaps that of Sev- 
entymile Creek, where the stream is at present sunk, with nearly ver- 
tical banks, to a depth of about 20 feet within an older valley- a half 
mile or more in width. The floor of the older valley, Avhich is com- 
paratively flat, is formed of the upturned beds of the conglomerate 
and shale. Before the stream commenced to cut its present shallow 
canyon it had deposited gravels widely over the surface of the valley 
floor, where they have been left. The tributaries necessarily had a 
similar development. The clean-cut rock bench near the mouth of 
Flume Creek, 20 feet in height, capped by several feet of gravel, is 
the best example that was observed. The conditions here at a level 
of only 20 feet above the present stream were repeated at various 
levels. The greatest height at which stream gravels occur on the 
Seventymile has not been determined. The most prominent bench is 
500 feet above the water, but no gravels were observed at this level 
(PI. Ill, B). On Mission Creek, about a mile below the mouth of 
Excelsior, there is a bench 15 feet high, capped by about 7 feet of 
gravel, and a short distance above the mouth of Excelsior, on Mission 
Creek, a bluff 70 feet high, capped by 20 feet of stream gravels. 
The Fort3miile and its tributaries are prominently benched. The 
nost extensive of these benches, like that on the Seventymile, is 500 to 
>00 feet above the stream. Gravels were observed near Bonanza Bar 
?00 feet above the stream, and gold in paying quantities Avas reported 
n the high gravels nearly opposite Steele Creek. On Chicken Creek 
)ench gravels are found 275 feet above the creek, and the discovery 
f good values in them is leading to an investigation of similar 
leposits in many places. 
Stream gravels. — The gravels of the present streams have been 
lerived, so far as has been observed, from the material within their 
mmediate drainage areas, either that of the bed rock or of older 
gravels within their borders. Bars were worked extensively in the 
jarly days on the Fortyniile, and at the present time a small amount 
jf this kind of work is being done. 
IGNEOUS ROCKS. 
Igneous rocks occur in great variety in the Yukon-Tanana country, 
'hose of the Fortymile and Birch Creek regions have been described 
l detail by Spurr, a and only a brief reference to the most important 
inds and occurrences will here be undertaken. 
The intrusives are granite, diorite, and gabbro, with many inter- 
«Op. cit., pp. 225-250. 
