116 ALASKAN MINERAL RESOURCES IN 1905. 
in the valley slopes of Fairbanks Creek 600 feet above the valley floor. The gravel is 
rather well rounded and contains bowlders up to 1 foot in diameter. These gravels have 
been somewhat prospected, but, so far as known, without favorable results. 
FORMATION OF PLACERS. 
It appears from a cursory examination that the pay gravels were deposited under con- 
ditions somewhat different from those which now prevail. Though the details can not be 
here discussed, some of the facts bearing on this matter deserve mention. The genera] 
uniformity in altitude of the ridges has been noted. This uniformity is the result of erosion 
when the region stood at a lower level than at present. Its topography then was that of 
an undulating surface dotted with rounded hills and broken by isolated groups of hills and 
ridges of greater prominence. The valleys furthermore were open and of low grade. It 
is probable that the stream deposits were deep and that there was much weathered bed 
rock in the interstream areas, awaiting transportation. Elevation of the region enabled the 
streams to cut the present valleys and thus form the avenues, or sluice boxes, as they might 
be called, through which passed the products of long-continued weathering as well as the 
deposits of the former streams. r riu> bench deposits above described form a remnant of 
these old deposits. 
In the constant, slow, and often interrupted progress of the unsorted coarse and fine 
material down the valleys the particles of gold, because of their high specific weight, tend 
to lag behind the particles of other materials and to find a lower position in the mass or a 
lodgment in the crevices of t he bed rock. They offer a passive resistance to onward motion 
and an active assistance to downward motion in the vertical direction. The accumulating 
deposit of gold is mixed with unsorted material, which was probably, for the most part, not 
originally in association with the gold, but was derived from some source farther up the 
valley. This deposit will closely follow the cutting action of the stream in the bed rock 
and be the first to cover the bare surface of the latter when the opportunity offers. 
Active erosion and an abundance of previously accumulated auriferous material appear 
to be the favorable conditions for the formation of placers. The so-called "wandering'' 
placers which have been noted from Australia," where the pay dirt is often shifted at times 
of melting snows to the claims lower down the valley, appear to represent an early stage 
in the development of placers. With the lessening of the stream's activity, accompanied 
often with the exhaustion of the great part of the auriferous material, the mobility of the 
deposits is diminished. There is then an increasing amount of barren material deposited 
over the pay gravels; there may be an abandonment by the stream of the part of the valley 
in which it has hitherto worked, and the pay streak becomes practically a stationary deposit. 
In the interior of Alaska the pay streak has become not only permanently stationary, but 
also, through the cementing agency of ice, for the most part permanently consolidated. 
Few facts are known regarding the amount, distribution, or circulation of the under- 
ground waters and the consequent extent of the permanently or only temporarily uncon- 
solidated gravels. There are valleys in the Yukon-Tanana country whose deposits are so 
"spotted," as it were, with live water that it is practically impossible to work them by 
drifting. The presence of large amounts of live water in many valleys during the winter 
is shown by the repeated overflows to which streams arc subject and by the unexpected fill- 
ing of prospect holes with water from below. It is possible, therefore, that the extent of 
the unfrozen ground is greater than is generally supposed. The extent of consolidation, 
while dependent primarily on the climate, is probable greatly modified by local conditions. 
The slope of the valley, the character and thickness of the deposits, and the quantity of 
water are factors which together may become of dominating importance, counteracting 
successfully the tendency of the climatic conditions to cause consolidation of deposits to 
great depths. As a result, a part of the deposits of a valley, be they talus or stream gravels, 
where these are not too deep, may retain a capacity for further differentiation. This, 
a Schmeisser, Karl, Die Goldfelder Australians, p. 100. 
