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Creek. The total thickness, as estimated by the writer, is therefore probably about 4,500 feet, but if the cherty rocks at the north end of these exposures belong in part to a later horizon, the thickness may be as little as 3,500 feet.
AGE AND CORRELATION
A considerable Silurian fauna has been collected along the boundary from rocks which are not separately mapped, and this fauna has heen listed in the section on the undifferentiated Paleozoic rocks.
In addition, five collections are tabulated below, of which three, Nos. 847, 173, and 174, came from the rocks near Takoma Creek. The other two, Nos. 848 and 849, were collected years ago and appear from the labels now with them to have come from the limestone on the northeast bank of the Yukon, north of the north end of Calico Bluff, which is now classified as Upper Cambrian. It is possible that this material may not originally have been in place, and it is also possible that the fossils may have become separated from their originally correct labels. As collections 848 and 849 are regarded by Kirk as Silurian, however, they are here included with the Silurian fauna.
Fossils from Silurian rocks along Yukon River between Eagle and Circle
847. 12 miles below Woodchopper Creek, south bank of Yukon River. Col. lector, E. M. Kindle.
848. 2 miles below Calico Bluff, Yukon River, in ravine below bend. Collector, E. M. Kindle.
849. 2 miles below Calico Bluff, east bank of Yukon River. Collector, E. M. Kindle.
173 and 174. About 3 miles below Thanksgiving Creek, southwest bank of Yukon River. Collector, Eliot Blackwelder.
Collection 847 evidently came from the limestone horizon just south of Takoma Creek and is doubtless the collection referred to by Brooks. This collection was regarded by Kindle as indecisive but indicative possibly of the Devonian. It has recently been redetermined by Edwin Kirk, who refers it questionably to the Silurian.
47 Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska : Geol. Soc. America Bull., vol. 19, p. 279, 1908.
Collection 173 was referred definitely to the Silurian. Collection 174 was referred questionably to the Devonian but is here included with the Silurian.
It is characteristic of many of the rather poor collections from this general horizon, both here and elsewhere in Alaska, that the determinations given are invariably “Devonian or Silurian,” “Silurian or Devonian," "questionably Devonian,” or “ questionably Silurian."
" This uncertainty, of course, is due in part to the paucity and poor quality of much of the material collected, but there seems to be a horizon in interior Alaska which can not readily be assigned definitely either to the Devonian or the Silurian. This is all the more difficult to understand, inasmuch as the Lower Devonian appears to be absent in Alaska. Therefore, when fossils are determined as “Silurian or Devonian" or "Devonian or Silurian," one must place them, if further assignment is attempted, either in the lower part of the Middle Devonian or the upper part of the Silurian. It can not be said that any definite evidence is available along the Yukon to favor either assignment, but from the general experience of the writer in interior and northern Alaska it seems advisable at present to favor a Silurian age for these questionable fossils. The sequence of beds on the south bank of the Yukon below Takoma Creek is therefore here referred to the upper Silurian (?). The Silurian rocks along the boundary include beds of both upper and middle Silurian age.
The Silurian system is widespread in Alaska. In interior Alaska it is especially well developed in the White Mountains, north of Fairbanks, where middle Silurian fossils have been collected from a heavy limestone formation by Prindle, 48 Blackwelder, and the writer. The same horizon is represented along the international boundary between the Yukon and Nation Rivers by the middle Silurian fossils collected by Cairnes, but this horizon has not yet been recognized as such along the Yukon between the boundary and Circle.
Schrader,50 in 1901, described a great Silurian limestone in northern Alaska which he called the Skajit limestone; this formation has subsequently been traced eastward by the writer 51 from its type locality on the John River into the Chandalar Basin and westward
* Prindle, L. M., A geologic reconnaissance of the Fairbanks quadrangle, Alaska : V. S. Geol. Survey Bull. 525, pp. 39-45, 1913.
4 Blackwelder, Eliot, unpublished manuscript.
50 Schrader, F. C., A reconnaissance in northern Alaska : U. S. Geol. Survey Prof. Paper 20, pp. 56-58, 1904.
51 Mertie, J. B., jr., Geology and gold placers of the Chandalar district, Alaska : U. S. Geol. Survey Bull. 773, pp. 229-233, 1925; Geology and geography of the ChandalarSheenjek region, Alaska : U. S. Geol. Surrey Bull. (in preparation).
A. CAMBRIAN LIMESTONE AND ASSOCIATED ROCKS ON THE NORTH BANK OF
THE YUKON RIVER JUST NORTH OF CALICO BLUFF Looking upstream. Shows thin plate of Upper Cambrian limestone dipping toward the river
in center and at left. Middle Cambrian limestone along top of ridge at right. Photograph by Eliot Blackwelder.