YUKON PLACER FIELDS. 117 
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 areas of 
shallow deposits, generally confined to the headwaters of the valleys. Although now there 
is not as large a quantity of weathered material at hand as formerly, when the product of 
long-continued weathering had accumulated, and although the proportion of gold may be 
different now from what it was formerly, nevertheless it is reasonable to suppose, and the 
occurrence of gold near the headwaters renders such a supposition entirely justifiable, that 
the deposition of auriferous material is there in progress. At the present time the streams 
come in closest relation, in the vertical section, to the bed rock near the heads of the valleys, 
and there, if anywhere within the valleys, downward cutting of the bed rock is in progress. 
The lower parts of the valleys have been areas of abundant deposition. Near the heads 
deposition closely follows cutting and there the deeply buried, more or less permanently 
frozen pay streaks of the lower valleys merge into the deposits within the zone of the pres- 
ent streams' activities. 
The gravels in the valleys of the Fairbanks region are composed of materials derived 
from the bed rock in which the valleys have been cut, and were deposited through stream 
action, uninfluenced by any glaciation, yet under conditions somewhat different than those 
of the present. The position of the pay streak in a valley marks the position of an earlier 
drainage as well as that part of the cross section of the valley which was probably the 
deepest. 
The successive stages of development may have been somewhat as follows: (1) Elevation 
of the region, the surface being laden with much unassorted weathered material and older 
stream deposits; (2) a period of active erosion by the streams, during which there was little 
opportunity for the formation of permanent deposits; (3) a penod of deposition, when the 
streams were nearly down to grade and when the pay streaks were, for the most part, laid 
down with their clay content, which may have been derived in pat from the abundant clay 
of the weathered material and in part directly from the bed rock; and (4) a period of stream 
shifting, valley widening, and further deposition, with the gradual development of the 
unsymmetrical type of valley of the present day. This unsymmetrical shape — one side 
steep and the other a more or less gradual slope — is a characteristic feature of many valleys 
in Alaska and results probably from several causes. It suffices here to mention only one 
of these, often observed by miners, that the sunny side of a valley is subject to more rapid 
wear than the shady side, which remains locked in frost for a much greater part of the open 
season. In the course of time there results an accumulation of waste which forces the 
stream toward the opposite slope of the valley. 
The greater mobility of the material was due probably in purt to the greater activity of 
the streams, which were at that time just becoming graded; i.i part to the more abundant 
precipitation, as is suggested by the much greater extension of the glaciers of the Alaska 
Range; and in part, perhaps, to a higher average temperature, though it would seem that, 
with the other factors present, no essential difference in the temperature would be required. 
Whatever the conditions of formation — and these are only imperfectly known — the dominant 
facts of economic importance are that in general but one pay streak has been laid down ; t hat 
this is next to bed rock beneath a considerable thickness of other deposits, and that its forma- 
tion is, for the most part, a closed incident. 
SOURCE OF THE GOLD. 
The origin of the gold in the placers, while not definitely determined, is suggested by the 
character of the gold itself and by its association. Most of that found near the heads of the 
creeks is rough and practically unworn; much of it is flat, as if derived from small seams; 
most of the coarse pieces are intimately intergrown with quartz and are often flat like the 
small fragments of thin quartz seams which are common in the schists. That mineraliza- 
tion has not been confined to gold is shown by the occurrence of native bismuth intergrown 
with gold, of veins of stibnite, and of the cassiterite often found in the gravels. The mosl 
