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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

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

300

500

M. Black shale and subordinate thin bands of limestone__ 60
L. Black carbonaceous shale_-----

50

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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 b 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.

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Mississippian fossils collected along the Yukon River between Eagle and Circle

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