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than real, depending to a considerable extent upon differences in color, which are due to weathering and which do not persist laterally along the beds. The section from top to bottom is given herewith:

Section of Calico Bluff formation in south side of Calico Bluff, on Yukon River

below Eagle

[Zones indicated by letters are shown in fig. 5 and pl. 8, B]

Feet

60

95

194

8

215

14

119

Covered. Lower part composed of drab shale, with a few

beds of limestone ---
Slope of gray shale, with a little limestone. Contains in the

upper part two or three beds of sandstone that weathers a

light ocherous yellow, each 3 or 4 feet thick.--
Calcareous shale, light gray or darker gray, weathering to a

light gray. Upper 20 feet covered.----
Massive black limestone, noticeably carbonaceous. Weathers

light gray. Zone e-----
Thin beds of black shale and limestone, both of which

weather light and dark gray. These rocks are fossilif-
erous, but the shale contains invertebrates of genera dif-
ferent from those found in the limestone, indicating an

oscillating condition of sedimentation and of marine life..
A cliff-forming group of beds. Consists of thin-bedded limy

shale that weathers to a chocolate color at the top; a 2-foot
bed of white-weathering limestone in the center; and black

shale at the base_---
Thin, fissile black shale that weathers to a bronze color, ex-

cept in the middle of the sequence, where it appears choc

olate-colored for 10 or 15 feet--
Upper 56 feet is mainly thin-bedded limestone with some

shale, weathering to a light chocolate color. Lower 27 feet
is fissile black shale that weathers blackish brown.

Zone d------
Fissile black shale, weathering brown at base and red and

yellow farther up, givi a general bronze color to the beds.
Alternating thin beds of white-weathering limestone and fissile

black shale. Fossils occur in a thin band at the very top--
Dark-gray fetid limestone that weathers light gray. Fossilif-

erous at the base ---
Dark-gray fissile shale that weathers yellow-brown..
Beds of limestone as much as 3 feet thick, alternating with

shale. Contains about midway of the sequence a 2-foot bed

of fossiliferous limestone----
Dark-gray fetid limestone that weathers light gray. Fossilif-

Zone O-----
Alternating thin beds of dark-gray limestone and thin, fissile

black slate that is probably calcareous. Beds 2 to 8 inches

thick
Black slate, somewhat siliceous. Near the top is a 6-inch

bed of fetid black fossiliferous limestone that weathers

white, Zone b-----
Alternating beds of limestone and shale with some chert. In-
cludes 1 thick zone of black siliceous slate, which is zone

Thickness estimated -

83

48

10

4

15

26

erous.

14

24

39

300

The base of this section, as here given, is about at the end of the point shown in Figure 5 as projecting into the Yukon. (See also pl. 8, B.) This basal line, as previously stated, is an arbitrary one, placed at about the level where, in going down in the section, the limestone and fossiliferous beds cease. This line therefore serves also as the upper limit of the lower Mississippian sequence, already described.

STRUCTURE AND THICKNESS

The structure of the rocks at Calico Bluff is that of an open syncline which plunges gently about N. 30° W. The other two belts of this formation to the northwest, one on the northeast side of the Yukon and one on the southwest side, are interpreted in a broad way as the northeast and southwest limbs of this same syncline. Strike faulting, however, has materially modified the synclinal structure. On the north side of the Yukon, north of Calico Bluff, occurs a rapid alternation of Upper Cambrian, lower Mississippian, upper Mississippian, and Upper Cretaceous rocks, standing practically on end. The lower Mississippian rocks are here repeated three times in the section, and the Upper Cretaceous rocks twice. Similarly, on the southwest side of the Yukon strike faulting is apparent, particularly opposite the mouth of the Tatonduk River, where the areal distribution of the lower and upper Mississippian rocks is exactly the reverse of what would be expected if normal synclinal structure were present. None of the four known upper Mississippian localities northwest of Calico Bluff, therefore, may be expected to contribute much stratigraphic evidence about this formation. The rocks at Calico Bluff, for some unknown reason, constitute a little stratigraphic island of relatively simple structure, surrounded on all sides by beds that are more intricately folded, as well as faulted, and all the structural and stratigraphic information available is concentrated at this one locality.

The northwestward plunge of the syncline is indicated on the south side of Calico Bluff (see fig. 5), where the strike of the rocks is east and the dip is 150–35° N., averaging perhaps 20°. Minor crumpling is evident toward the east side of the bluff, but in general the rocks are little metamorphosed and show perfectly the original bedding planes, with incipient fracture cleavage only in some of the weaker beds.

The highest beds have been eroded at Calico Bluff, and it is therefore not possible to obtain a complete stratigraphic section. The thickness of beds from the arbitrary basal line to the top of the bluff at its south end is believed to be about 1,270 feet. The thickness from the base of zone b to the top, as measured instrumentally, is 968 feet.

In this connection it is of interest to examine the section made by Brooks,62 who first studied in detail the rocks exposed at Calico Bluff and gave the following section:

Calico Bluff section

[As given by Brooks)

Feet
0. Gray to blackish shale, with few bands of limestone---- 300
N. Gray limestone in 6-inch to 12-inch bands, interbedded
with thin bands of black slate------

500
M. Black shale and subordinate thin bands of limestone-- 60
L. Black carbonaceous shale---

50 K. Gray chert and black shale--

6 J. Dark-gray limestone full of Productus, etc..

9 I. Dark carbonaceous shale---

20 H. Gray cherty limestone -

25 G. Soft fissile black shale..

55 F. Gray chert.---

8
E. Fissile black carbonaceous shale_

150
D. Black carbonaceous shale, with slaty cleavage developed
in most beds

100
C. Black carbonaceous shales, with some thin black chert
bands interbedded.

40
B. Black cherts in 4-inch to 6-inch bands and interbedded
black shale.-----

55
A. Black carbonaceous fissile shale---

150 From an inspection of this section it is fairly certain that zones L and G correspond, respectively, to zones b and a of Figure 5. Brooks's section, from the base of his zone L to the top of the bluff, is given as 910 feet, and when it is remembered that this measurement was made without instruments, it shows a remarkable similarity to the instrumentally measured section of the same sequence from the base of zone 6 to the top, which, as measured by the writer, is 968 feet.

AGE AND CORRELATION

Fossils are very plentiful at Calico Bluff, as well as at the other four upper Mississippian localities, and nearly every geologist who has gone down the Yukon, from the time of Collier's trip in 1902 to the present, has made collections at these localities. Some of the fossiliferous zones at Calico Bluff are indicated in the section above given, but others are doubtless present though not seen by the writer, for it was noticed that some of the fossiliferous zones had a rather slight lateral extent along the bedding planes, and any one section would therefore necessarily miss some zones. The writer, like his

& Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska : Geol. Soc. America Bull., vol. 19, p. 288, 1908.

predecessors, collected additional fossil material from this unique locality at Calico Bluff. It has seemed best, however, to assemble and correlate all the collections, and the result is shown in the following table, which may be said to represent the intermittent collecting of seven geologists over a period of 24 years. A few of the earlier paleontologic determinations were made by Charles Schuchert, now of Yale University, but the great mass of determinative work has been done by G. H. Girty, of the United States Geological Survey. Girty himself made a trip down the Yukon in 1918 and collected some of the best of the material tabulated below. The localities are arranged roughly in the chronologic order of the collections. Although some of the locality descriptions are inadequate and some are slightly in error, they are published as given by the original collectors, with the exception that the terms right bank and left bank, as used in some of the locality descriptions, have been altered to north, east, west, or south bank, as the case may be. Interpretations of the locality made by the present writer are indicated in parentheses. Zones indicated by letters are those shown in Figure 5.

Cystodictya sp. Ptilodictyl sp Coeloconus sp. Polypora sp. Rhombopora sp. Pinnatopora sp; Cystodictya cf. C. lineata. Fenestella sp. Hemitrypa sp. Anisotrypa? sp Anisotrypa sp. Stenopora? sp. Stenopora sp. Leioclema sp. Cystodictya cf. C. pustulosa. Spirorbis sp Batostomella sp. Fistulipora sp. Fistulipora? sp. Stenopora cf. S. ramosa. Fistulipora cf. Serpula? sp. Stenopora ci. S. carbonaria. Echinocrinus sp. Rhipidomella cl. R. permiana. Auivpora? sp. Stromatopora? sp. Cladochonus sp. Michelinia sp. Favosites? sp. Coral.--Amplexus? sp Lithostrotion? sp. Dibunophyllum? spZaphrentis? sp. Zaphrentis sp. Campophyllum sp. Michelinia cf. M. princetonensis. Rhombopora cf. R. lepidodendroides Triplophyllum? sp. Triplophyllum sp. Cyathaxonia sp. Hadrophyllum? sp

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Mississippian fossils collected along the Yukon River between Eagle and Circle

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