18 COAL RESOUKCES OF THE YUKON. [no. 218. 
and sands at the Palisades, which he called the "Palisade conglomer- 
ate." These two occurrences he regards as probably of Miocene or 
Pliocene age. 
Fine silt deposits are widely distributed along the Yukon and many 
of its tributaries. These occur as terraces along the valley walls, and 
the bluffs formed by the undercutting by the river of these terraces 
ate conspicuous features of the landscape of the Yukon. Within the 
limit of glaciation these silts are found mantling the till. The silt has 
a thickness of from 50 to 200 feet or more. 
These beds have been described by various writers, but more espe- 
cially by Spurr/' who named them the Yukon silts. Typically, the ter- 
rane is made of bull-colored silts, often resting on basal gravel beds, 
which may or may not be of the same age. The Yukon silts are for 
the most part of Pleistocene age/' as is shown by both vertebrate and 
invertebrate fossils. The silts are found resting unconformably on all 
of the older formations, except possibty some gravels which may be 
of Pliocene and Miocene age. They are separated by an unconformity 
from the Kenai series. The silts, though entirely unconsolidated, are 
in places thrown up into broad, open folds, and at one locality faulting 
was observed. This deformation usually makes it easy to separate 
them from the recent alluvium deposits, but in some cases the recent 
and older silts are entirely conformable and can not be differentiated. 
It is probable that more detailed studies will show that a part of what 
is now regarded as Yukon silt will be found to belong to one of the 
later Tertiary horizons. Some impure lignite beds have been found 
in these silts. 
The youngest water-laid deposits here to be considered are the 
alluviums of the river and stream valleys. These are often very simi- 
lar to the Pleistocene silts, but are usually of a darker color. They 
are found in the flood plains of the river and form extensive deposits 
in the broad lowland known as the Yukon Flats. In places where 
the Yukon silts have been warped down below the present level of 
the river the alluviums rest conformabl\ r upon them, but usually the 
relation is one of unconformity. 
Some areas of volcanic rocks are found on .the Yukon below the 
mouth of the Koyukuk. These are lavas and tuffs of andesitic and 
basaltic nature. Basaltic or andesitic dikes are not uncommon in the 
pre-Miocene sediments. Spurr c has discussed similar occurrences in 
the Yukon Basin at some length, and suggested the early Miocene or 
late Pliocene age of these extrusives. Local deposits of lignite, 
a Geology of the Yukon gold district: Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, pp. 
200-221. 
t> Spurr, op. cit., p. 199; Brooks, A reconnaissance in the Tanana and White River basins: Twentieth 
Ann. Rept. U. S. Geol. Survey, pt. 7, 1899, p. 475. 
c Geology of the Yukon gold district: Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, pp. 
242-250. 
