YUKON PLACER FIELDS. 115 
above them. The thickness of the pay gravels varies from a few inches to a dozen or more 
feet, an average which is rather uniformly maintained over large areas. The under surface 
of the gravels not only rests upon the bed rock, but where the latter is blocky is found within 
it to a depth of from 1 to 3 feet or more. The width of the pay gravels varies in different 
creeks and in different parts of the same creek, but in most cases makes up only a small 
part of the width of the valley. Pay streaks 30 feet or less to 450 feet and, in one instance, 
800 feet wide have been reported. The average width of gravels carrying values sufficient 
to pay for working under present conditions is probably about 150 to 200 feet and this, like 
the thickness, is fairly constant. The pay streaks in the valley floors often bear no uniform 
relation to the present stream beds. 
While vertically but one pay streak has thus far been found, the great width of some of 
the valleys and the flatness of the underlying bed rock render possible the presence of more 
than one pay streak in the horizontal direction. In some places the presence of a second 
pay streak has been suspected and prospecting was under way last season to determine 
definitely whether such is the case. The gold occurs evenly distributed throughout the 
pay gravels, mostly near the bed rock, or occasionally chiefly within the bed rock. 
The great bulk of the gold is composed of flatfish pieces of various sizes up to one-fourth 
inch in diameter and of granular pieces, some of which are very minute. The proportion of 
very fine gold, however, is apparently small, and there is but little flaky gold. Nuggets 
form an inconsiderable part of the clean-ups; those worth a few dollars are common, how- 
ever, and a few have been found of considerable value. Some of the largest were worth 
approximately $145, $160, $190, $233, and $529. The nuggets often contain quartz. 
Most of the gold found near the heads of the creeks is angular. Downstream there is in 
general a gradual decrease in the average size of the pieces and an increase in the amount 
of wear they have sustained. Nuggets, too, are less common in the lower parts of the 
valleys. In some cases the coarse and fine gold occur together, while in others the coarse 
gold is found mostly on one side of the pay streak. At occasional localities there appears 
to be an abrupt change from gravels carrying a large percentage of coarse gold to those 
immediately below on the same stream whose gold contents are chiefly tine. The values in 
the pay gravels which are now being exploited range from about 2 cents to 20 cents or more 
to the pan and there is a large part of the ground which will average about 8 cents to the 
pan, or about $10 to the cubic yard, or $2 to the square foot of bed rock. Some of this 
ground will average $3 to $3.50 to the square foot and some carries even better values. 
Assay values were, reported ranging from $16.16 to $18.25 of gold per ounce and the gold 
from one locality was said to assay as high as $19.25. 
The minerals most commonly associated with the gold, aside from the quartz, with which 
it is often intergrown, are garnet, rutile, and black sand. The proportion of the black sand 
is small and it is composed mostly of magnetite. Cassiterite is rather commonly found and 
there is some stibnite. Bismuth occurs in close association with the gold in a nugget 
which was found by the miners on Gilmore Creek, and was presented by them to the Sur- 
vey party, and tested by Mr. Schaller in the Survey laboratory. 
The frozen deposits are tough, in distinction from the muck. The gravels can not be 
broken with a pick and are with difficulty rent by explosives. A sudden caving in of the 
ground undermined in drifting is rare, the sinking usually being so gradual as to permit the 
removal of mining apparatus. In such cases a parting often takes place bet ween tin 1 gravels 
and the overlying muck, leaving the latter as a roof. The solidly frozen gravels are pracl i- 
cally impermeable to the surface waters and to any underground water that may be presenl 
and the underground mining operations are comparatively dry. Unfrozen areas are often 
encountered, and where they occur in the deeper ground the presence of "live water" adds 
to the expense of mining. In other places, notably near the heads of some creeks where t he 
gravels are shallow, unblanketed by muck, and well drained, the greatest part of the ground 
becomes thawed during the summer time. 
Bench gravels are not common in the Fairbanks region. A deposit of gravels composed 
essentially of quartz-mica-schist, graphitic schist, and vein quartz has, however, been found 
