purington.] GEAVEL AND PLACER MINING IN ALASKA. . 157 
dim: DOING. 
INTRODUCTION. 
The opportunities for gold dredging in the far Northwest are not by 
any means numerous. Perhaps no part of the gold-mining industry 
is dependent on so many conditions for its financial success as the 
dredging of auriferous gravel deposits for gold. Some years ago, in 
connection with Mr. J. B. Landfield,® I published a review, setting 
forth the geological conditions which are necessary for the success of 
gold dredging, the consideration of which is frequently neglected by 
those contemplating the installation of dredges. The portions of that 
paper which deal with the present case are here quoted. The prime 
consideration is that conditions of a peculiar character are a necessary 
accompaniment of the values in any locality where it is proposed to 
win gold by dredging. Recent experience has proved that the 
greatest attention must be given to topographic and geologic condi- 
tions in the country in which the enterprise is to be inaugurated, 
otherwise success in the operations is not to be looked for. The 
following explanation of the conditions necessary for dredging is 
therefore pertinent: 
Taking the country about Oroville, Cal., as the best example in the United States, 
let the geological conditions be considered. To the north and east the erosion of a 
vast extent of mining country, whose rocks are penetrated by gold-bearing veins, 
has contributed little by little through geologic ages to the mass of detritus now 
occupying the bed of Feather River. The wearing down of mountains, originally 
very much higher than at present, through a vast amount of time has caused the 
formation of a valley of extraordinary width but of no great depth. The massing of 
stream detritus is also responsible for a decrease in the gradient and a slowing down 
almost to a topographical equilibrium of the formerly swift current of the river. 
The stream, unable under these conditions to cut its way by virtue of the material 
which it carried in suspension, has for a long time been depositing its load, filling its 
wide valleys with sand and gravel, together with the less destructible of the metallic 
particles, and notably gold. Geologically speaking, the rate of deposition of the 
river's material may be said to be on the increase and the current of the water, still 
of considerable velocity, to be greatly lessening in swiftness. Later in its geologic 
cycle Feather River will doubtless assume, on a smaller scale, the present character 
of the Mississippi, forming oxbow curves, cut-offs, and, as it were, losing its way 
among constantly shifting sand bars. 
The accompanying residual gold, as it travels to a greater and greater distance from 
its original source in the veins of the mountains, becomes more and more finely 
divided, even to microscopic dimensions, and increases in purity, and the degree of 
evenness with which the particles are distributed in the gravel becomes a phenom- 
enon of constantly increasing definiteness and importance. 
Such a set of conditions as that obtaining on Feather Iiiver is the exception 
rather than the rule in the western part of the I Fnited States, and even in California. 
The above explanation has been entered into in order to present in some measure 
the reasons why, on the California river referred to, the conditions are not only favor- 
able but eminently suited to gold dredging. Shallow gravels- that is, those less than 
" Purington, C. W., and Landfield, .7. B., jr., The gold-dredging fields of eastern Russia: Eng. Mag., 
vol. 22, 1901, pp. 398-407. 
