168 ALASKAN MINERAL RESOURCES IN 1906. 
It is quite possible, however, that the basin of the Kuzitrin may liavJ 
been in part covered by glacial ice at one time. 
The stream and river gravels, which constitute the best-known type] 
vary in character according to their place of occurrence. The devel- 
oped placers are nearly all in the stream and river gravels. So far as 
known they carry, as a rule, only material derived from the basins in 
which they are found. Much of the material is well rounded, but that 
of the pay streaks in the placers is in many places subangular. Quartz* 
usually dominates as a constituent of the pebbles. In some streams! 
such as Kuzitrin River, the gravel bars are made up of iron-stained 
quartz, almost to the exclusion of all other material. Sands and some 
clay occur interbedded with the stream gravels, forming, however, but 
a small percentage of the bulk of the alluvium. In all the smaller 
streams and in parts of the larger ones a bed of clay or sandy clay, in 
which more or less vegetable matter is intermingled, forms the top- 
most layer. This surface bed, which varies in thickness from 2 to 30 
feet and is called by the miners " tundra," appears to be a subaerial 
accumulation, due in part to the decay of vegetable matter and in part 
to the deposition of silts during the rainy season. Though sometimes 
explained as a lake deposit by the miners, its distribution and character 
seem to preclude lacustrine origin. 
The gravels, sands, and clays of the basin lowlands, forming the 
second type of alluvium, are known only where exposed by river ero- 
sion. At such places they consist of material identical in every way 
with thai of the first group except that the material is somewhat finer. 
Some fragmentary evidence furnished by drill records goes to indicate 
thai much of the basin is rilled by clay deposits whose genesis can] 
probably be best explained by lacustrine action. The determination 
of the outline of this old lake and of the cause of its formation must 
await further investigation. It is worthy of mention, however, that 
the surface deposits of gravel and sand in the Kuzitrin basin probably 
as a rule do not exceed 20 or 30 feet in depth. This, however, applies 
only to the basin, for in the constricted part of the valley the gravel 
deposits probably have a much greater depth. 
Bench gravels are of common occurrence in the district. The best] 
known are those along the main Kougarok between Taylor and Windy 
creeks, and these have proved to be auriferous. Similar deposits 
occur on the upper Kougarok, but have been little prospected. These 
bench gravels are of the same character as the alluvium of the present 
streams. The sands and gravels which form the extensive bench at 
the mouth of Quartz Creek and continuing up that stream to Dahl 
Greek are described on page 173. 
Another type of surficial deposit that deserves mention is the ground 
ice, which here occurs more extensively than in the Nome region. 
Along the southern slopes of the valleys it forms in many places almost 
