weight.] ECONOMIC GEOLOGY. 19 
streams of ice. Previous to melting, the glaciers were mainly eroding, 
and few, if any, local deposits could have been formed until they 
began to retreat. As they gradually disappeared, the materials which 
they ground off from the bed rock, with other debris furnished by 
tributary streams, have been concentrated in the creek beds by the 
sorting action of flowing water, thus forming valuable placers. 
ECONOMIC GEOLOGY. 
DISTRIBUTION OF AURIFEROUS GRAVELS. 
The geologic map (PL V) shows the distribution of the gold- 
bearing gravels. It will be observed that the deposits actually pro- 
ducing gold are confined to the creeks which crosscut the areas of 
mineralized slates, and that neighboring valleys contain no valuable 
amounts of gold. The auriferous gravels on the Porcupine extend to 
the junction of McKinley Creek and up this tributary for a distance of 
1 mile. Gold is limited to the lower claims on Nugget Creek, and a 
imilar occurrence may be expected on Glacier Creek, though this has 
not been sufficiently prospected to determine the local distribution. 
The gravels of the tributaries entering Klehini River from the north 
are believed to be of no economic importance. 
CLASSIFICATION OF PLACERS. 
There are three types of gravel deposit in the Porcupine basin— 
3reek gravels, side benches, and high benches. The creek gravels fill 
the present channel to various depths and can not be definitely sepa- 
rated from the side benches or gravel banks which rise in places 20 
md 30 feet above the stream. The high gravels occupy portions of 
former channels which in some places have been preserved and in 
others have been cut out by the downward erosion of the streams. 
The first of these side-bench deposits, at No. 2 below Discovery, is 
m extensive gravel bed on the east side, 25 feet above the creek, 
tich gravels occur at high benches 80 feet above the creek on the first 
Jiree claims above Discovery, and a similar deposit has been found 
hi the south bank of McKinley Creek, three-quarters of a mile from 
ts mouth, at an elevation of 200 feet. Of the above types the creek 
•ravels, as those found on Discovery claim and those near the mouth 
)f McKinley Creek, are of greatest value. 
CHARACTER OF GRAVEL. 
The stream and bench gravels consist mainly of a fine wash contain- 
ng worn slabs and fragments of slate, and rounded bowlders often 2 
>r 3 feet in diameter, mostly of diorite, with some greenstone. In the 
ipper portions of the creeks the gravel wash becomes coarser and 
nore angular. There are no materials in the gravels which could not 
lave been derived from the drainage basin in which they occur. 
