18 THE PORCUPINE PLACER DISTRICT, ALASKA. [bull. 236. 
The quartz veins are not very abundant, and as a rule are short and 
small, often merely stringers parallel with the structure of the slates. 
A few which cut directly across the formation carry galena and sphal- 
erite, with a small amount of chalcopyrite, and, though quite narrow, 
often persist for considerable distances. Calcite veins, which are 
more numerous than those of quartz, are usualty a foot or more in 
width, and are often weathered to a light-brown color on the surface, 
while of a bluish color and fine granular structure when freshly 
broken. They often carrj- cubes of pyrite, which occasionally meas- 
ure an inch across. From veins of this nature up McKinley Creek 
some native gold has been reported. 
Besides the small veins a quartz ledge 100 feet wide outcrops at an 
elevation of 2,000 feet on the ridge south of Porcupine. Although 
apparently quite barren, a small sample from this gave an assay value 
of $5.28 in gold. A similar ledge occurs across the Klehini at 1,500 
feet elevation, on the ridge west of Bowlder Creek. About 2 miles 
below Porcupine is a third mineralized deposit rich in sulphides, 
with calcite as gangue mineral, but a sample taken here gave an assay 
value of only 41 cents. These different ledges have all been located and 
relocated, and on some assessment work has been done, but none have 
been developed, owing to transportation difficulties. 
GLACIATION. 
Evidence of general glaciation was found over the greater par 
of the area explored. Observations show that the entire district 
was formerly covered by a field of ice similar to that now existing to 
the north of Muir Glacier, with only the highest, most rugged peaks 
protruding. The greatest altitude at which the action of ice was 
observed was 1,500 feet, on Granite Mountain just west of Bowlder 
Creek. 
The main dissection of the country is attributed to water erosion 
previous to the Glacial epoch, but many of the topographic features, 
such as rounded ridges and wide U-shaped valleys, are attributable to 
the action of ice. Of especial interest are the high gravel-filled chan- 
nels on the mountain slopes several hundred feet above the present 
creeks. These channels have probably been formed by a field of 
moving ice, which cut troughs into the bed rock along the trend of| 
the softer interbedded slate strata. During the retreat of the ice from 
the valleys gravels were deposited in these channels, which, owing to 
later erosion of the creek bottoms, have been left at high elevations 
on the valley slopes. At about 200 feet elevation above the present 
creek, on the south bank of the McKinley, a deposit in one of these 
channels is being hydraulicked. This is crossed by the present creek 
canyon, exposing a good section of the gravel bed (PI. VI, B). 
The concentration of gold in the placers has no doubt been going on] 
in the different streams ever since their lower valleys were vacated by 
