34 FAIRBANKS AND RAMPART QUADRANGLES. 
is rare, the sinking usually being so gradual as to permit the removal 
of mining apparatus. In such cases a parting often takes place be- 
tween the gravels and the overlying muck leaving the latter as a roof. 
The solidly frozen gravels are practically impermeable to the surface 
waters and to any underground water that may be present and the 
underground mining operations are comparatively dry. Unfrozen 
areas are encountered at many places, and where they occur in the 
deeper ground the presence of " live water " adds to the expense of 
mining. In other places, notably near the heads of some creeks 
where the gravels are shallow, unblanketed by muck, and well 
drained, the greater part of the ground thaws during the summer. 
Bench gravels are not common in the Fairbanks region. A deposit 
of gravels composed essentially of quartz-mica schist, carbonaceous 
schist, and vein quartz has, however, been found on the northern val- 
ley slope 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 aj)pears from a cursory examination that the pay gravels were 
deposited under conditions somewhat different from those which now 
prevail. Though the details can not be discussed here, some of the 
facts bearing on this matter deserve mention. The general uni- 
formity in the altitude of the ridges has been noted. This uniformity 
is the result of the erosion of a surface which formerly stood at a 
lower level than at present. Its topography was then undulating, 
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 the interstream areas bore much weathered bed rock, await- 
ing 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. 
The bench deposits above described form a remnant of these old 
deposits. 
In the constant, slow, and often interrupted progress of the un- 
sorted 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 lodg- 
ment in the crevices of the bed rock. They offer a passive resistance to 
onward motion and an active assistance to vertical downward motion. 
The accumulating deposit of gold is mixed with unsorted material 
which, for the most part, was probably not originally associated with 
