collier.] SKETCH OF GEOLOGY. 17 
Cretaceous age. These beds would belong with Spurr's Mission Creek 
group. This name has, however, not been retained in the present 
! stratigraphic classification, because it was found that under it Spurr 
had included beds as widely separated in the geologic column as the 
Eocene and Carboniferous. The Lower Cretaceous beds outcrop for 
about 80 miles along the Yukon. The beds are rather closely folded 
and somewhat faulted. 
On the lower portion of the river, near Nulato, some fresh-water 
eycad-bearing beds have been found which are also probably assignable 
to the Lower Cretaceous. These beds are chiefly calcareous sand- 
stones, with some shales. They differ from the Lower Cretaceous of 
the upper river in that they are apparently of fresh- water origin. 
Their relation to the Upper Cretaceous which outcrops in the vicinity 
has not been determined. 
The Upper Cretaceous is represented near Nulato by sandstones, 
conglomerates, and dark-colored shales, which outcrop along the river 
at intervals for about 100 miles. These have in part yielded a marine 
invertebrate fauna of Upper Cretaceous age, as determined by Stan- 
ton. Fossil plants were found in the same general horizon and were 
assigned by Dr. Knowlton to the Upper Cretaceous. It is evident, 
therefore, that these beds include both marine and fresh-water deposits. 
The fresh-water beds carry coals of commercial importance. The 
Cretaceous beds have undergone considerable deformation, but are not 
metamorphosed, and, in fact, are only slightly indurated. 
The succeeding horizon is made up chiefly of sandstone and con- 
glomerate, with some shale. It is called the Kenai series, and is of 
Upper Eocene age. These beds usually carry abundant plant remains, 
which show them to be of fresh- water origin. The Kenai series has 
been named from its typical occurrence on Kenai Peninsula and has 
been identified in various other parts of Alaska. It occurs in isolated 
areas on the Yukon near the boundary and near Rampart, and is more 
extensively developed near Nulato. On the upper river it unconform- 
ably overlies various horizons below the Upper Cretaceous, but near 
Nulato its relation to the Upper Cretaceous seems to be one of 
conformity. The Kenai beds are, as a rule, little disturbed, but in 
some localities they have suffered considerable deformation. The 
Kenai is the great coal-bearing horizon of Alaska. Its coals are usu- 
ally characterized by the presence of fossil resin, or amber. 
Tertiary horizons younger than the Kenai have as }^et been only 
locally differentiated in the province, but the evidence points to con- 
siderable development of Miocene or Pliocene beds. Spurr a described 
some lignite-bearing sands and clays on Mission Creek near the Yukon, 
under the name "Twelvemile beds," and some cross-bedded gravels 
"Geology of the Yukon gold district: Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, pp. 197 
and 199. 
Bull. 218—03 2 
