158 . GRAVEL AND PLACER MINING IN ALASKA. [bull. 263. 
40 feet in depth — are an essential in dredging enterprises. « Thus broad rather than 
narrow, gorge-like valleys must he sought. Bed rock of a soft, decomposed character, 
which can be easily cut into by the dipper or bucket of the dredge, is a necessity, 
since placer gold, by virtue of its gravity, often sinks from 1 to 2 feet into the crevices 
of the rock underlying. It is hardly necessary to state that the amount of gold 
increases in geometrical ratio the lower it lies in a given bed of gravel. Thus old 
rather than young valleys are favorable for dredging. 
Additional reasons of great weight why geologically old valleys should be looked 
for are that the size of bowlders is greatly decreased, gravel becomes by long abrasion 
uniform in size, the angularity of the fragments disappears, and a bed of pebbles, 
round and easily handled, is the result. 
The even distribution of the gold which, as mentioned above, is an invariable 
accompaniment of old and wide valleys, is a point in favor of this sort of mining, 
looked at from the standpoint of a business enterprise. At the same time it is evi- 
dent that the finely divided state in which such gold is found necessitates the 
highest skill in recovering it, For example, at Oroville, although some of the opera- 
tors are saving gold the subdivision of whose particles is almost microscopic, it is 
thought with good reason that a considerable percentage of values is lost in the water 
used for the washing. 
The conditions on Yuba River in California are, from the dredging standpoint, as 
favorable as those at Oroville. 
The conditions above mentioned as important to the success of gold 
dredging should be taken account of as much by the men who con- 
template installing dredges in Alaska, as in any other part of the world. 
While it is true that there are in Alaska broad valle} r s, such as those 
of Yukon River and its larger tributaries, and that they have been 
found to be to a certain extent gold bearing, yet peculiar and formid- 
able conditions serve as a barrier to the exploitation of these streams 
by dredging. A remarkable illustration of this fact came to my notice 
during the last summer. In a small island facing the settlement of 
Eagle, in the interior Yukon country, a shaft sunk through rolled 
stream gravel to a depth of about 40 feet in winter penetrated perpetu- 
ally frozen ground until near the limit of its depth. At this point an 
underflow of water was encountered so strong as to prevent further 
prospecting. It was said that good values in gold, even up to $1 to 
the yard, were found in sinking this shaft. If this be true it would 
imply that considerable areas of the underlying immensely wide 
gravel flats of Yukon River are gold bearing. Under other conditions 
portions of the ground might be found workable by gold dredges. 
Experience has shown that in Alaska the permanent frost which exists 
in much of the river country renders the exploitation of the gravel by 
ordinary mechanical means absolutely impossible. It is the opinion 
of those who have attempted the dredging of frozen gravel in the 
Klondike that, while the devising of a dredge strong enough to dig 
frozen gravel is not impossible, the undertaking has not yet reached 
a Since the above was written the enlarging and strengthening of dredges has resulted in success- 
ful digging (on Yuba River, California) to the depth of 60 feel . 1 1 should be noted, however, that the 
upper 25 feet of the Yuba gravel section consists of loose tailings. 
