ECONOMIC GEOLOGY. 35 
the gold but was derived from some source farther up the valley. This 
deposit closely follows the cutting action of the stream and is the first 
to cover the bare surface of the bed rock when opportunity is ottered. 
Active erosion and an abundance of previously accumulated aurif- 
erous material appear to be favorable to the formation of placer-. 
The so-called "wandering" placers which have been noted in Aus- 
tralia," where the pay dirt is shifted at times of melting snows in 
claims lower down the valley, appear to represent an early stage in 
the development of placers. With the lessening of the stream's activ- 
ity, accompanied often by the exhaustion of the great part of the 
auriferous material, the mobility of the deposits is diminished. An 
increasing amount of barren material is then deposited over the pay 
gravels, the stream may abandon the part of the valley in which it has 
hitherto worked and the pay streak may become practically a station- 
ary 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 cir- 
culation of the underground waters and the consequent extent of 
the permanently or only temporarily unconsolidated gravels. In the 
Yukon-Tanana country there are valleys whose deposits are so 
"spotted," as it were, with live water that it is practically impossi- 
ble to work them by drifting. The presence of large amounts of live 
Avater in many valleys during the winter is shown by the repeated 
overflows to which streams are, subject and by the unexpected filling 
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 probably 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, whether talus 
or stream gravels, where not too deep, may be further differentiated. 
This differentiation, under the mobility imparted by the contained 
water and by the stream action to which they may be subjected,, may 
bring about the gradual accumulation of the gold on or near the bed 
rock. In the Fairbanks region this process would be most active in 
the shallow deposits which are generally confined to the headwaters 
of the valleys. Although the quantity of weathered material now 
at hand is not so large as formerly, when the product of long-con- 
tinued weathering had accumulated, and although the proportion 
" Schmeisser, Karl, Die Goldfelder A.ustraliens, p 100. 
