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Paleozoic rocks of many types are here grouped together for convenience in mapping; some whose age has been demonstrated or for which separate delineation could be made on the basis of pronounced lithologic differences will be found separately listed elsewhere in this report. This grouping of rocks of diverse age and lithologic character has been rendered necessary for several reasons. First, little detailed work has been done in the Circle and Fortymile quadrangles, and practically nothing but exploratory mapping has been attempted as yet away from the Yukon River. Second, the undifferentiated Paleozoic rocks south of the Yukon have proved particularly difficult to subdivide into groups or formations because they are more than ordinarily metamorphosed and because they contain apparently none of the easily recognized horizon markers, such as the Skajit (Silurian) limestone of northern Alaska or its counterpart as seen in the White Mountains of the Fairbanks quadrangle. Finally, along the international boundary, where all the systems of the Paleozoic appear to be represented, as indicated by numerous fossil collections, Cairnes was unable, in the time available for the boundary survey, to differentiate and map them separately.

The differences in the Paleozoic section north and south of the Yukon in this longitude bring to mind another fact that is only beginning to be appreciated in the geology of interior Alaska-namely, that no one district appears to present a complete sequence of Paleozoic rocks. The Cambrian limestone, for example, is so conspicuous that it could scarcely have been missed, even in reconnaissance work. Yet no limestone corresponding to it has yet been seen south of the Yukon. Again, the Silurian limestone, which forms one of the most conspicuous horizon markers of northern Alaska and is almost as prominent in the Fairbanks quadrangle, is nevertheless not typically developed in the Circle and Fortymile quadrangles nor along the Yukon, although both older and younger formations that adjoin it elsewhere are here present. Still other examples might be cited. It is therefore becoming increasingly apparent that a complete Paleozoic section will be finally obtained only by piecing together the fragmental sections from all of interior Alaska, and the result will be a far greater thickness of rocks than has been formerly supposed.

The principal undifferentiated Paleozoic rocks south of the Yukon here grouped together are quartz-feldspar sandstone, or arkose, metamorphosed to a greater or less degree, quartzite, shale, slate, phyllite, chert and chert conglomerate, a little limestone, and greenstone, including serpentine, some of which has been separately mapped.

The arkose is composed of rounded to subangular grains of quartz and feldspar, in about equal amounts, commonly cemented by a matrix of quartz, feldspar, and argillaceous and ferruginous material. Locally such rocks have been metamorphosed, with the resulting development of sericite and a schistose fabric. Commonly, however, they are inclined to be massive and appear to have withstood the effects of metamorphism about as well as the quartzites. They occur in beds from 1 foot to several feet thick, interbedded with quartzite and slate, but they appear for the most part to be restricted to zones bordering or near the Birch Creek schist and are believed to have been derived in large measure from the sedimentary rocks of that group. By a decrease in the proportion of feldspar and a corresponding increase of quartz, the arkosic rocks grade into feldspathic quartzite.

The quartzites proper occur at several horizons in the Paleozoic and are therefore contemporaneous only in part with the arkosic rocks. Where the lower Paleozoic arkose and quartzite are appreciably metamorphosed, it is difficult to separate them from the Birch Creek schist.

Shale occurs at many places. It is usually drab, dark gray, or black, but the more metamorphosed varieties, such as the slate and phyllite, are inclined to be more conspicuously colored in hues of green, red, and purple. This difference in coloration, however, is not believed to be a function of degree of metamorphism but is due rather to original differences in the composition of the rocks. In other words, the more brightly colored argillaceous rocks, though they happen to be older than the shaly rocks proper and therefore are more metamorphosed, are believed to be also different in original composition.

Chert and chert conglomerate constitute a considerable part of the Paleozoic sequence at several localities in this area. Chert and silicified rocks approaching chert in composition are probably present in formations of different ages within the Paleozoic. Much of the chert and chert conglomerate, however, is of Devonian and Mississippian age, and where possible such rocks have been separately mapped. The rock here designated chert conglomerate is, so far as known at present, unique in that it is found only among the Paleozoic rocks. It is not just a conglomerate composed of chert pebbles but is a conglomerate made up of rounded to angular chert fragments in a chert matrix. It is so well consolidated that the rock commonly breaks across the chert pebbles. The chert conglomerate, as well as much of the chert, presents a peculiar problem in stratigraphic genesis, which is considered at greater length on pages 90-92 of this report.

No large bodies of limestone are included in the major grouping of undifferentiated noncalcareous Paleozoic rocks. Only a few small lenses of limestone and some larger bodies of calcareous shale were seen by Prindle and the writer in 1911. Such rocks occur usually in thin beds, varying in color from blue-gray to black, and show the results of metamorphism by their closely folded and locally recrystallized condition.

Basic and ultrabasic rocks, commonly designated greenstone and serpentine, are found at numerous localities in this region. Like the cherts, but to a greater degree, they are distributed throughout the Paleozoic sequence and may not in general be mapped as a lithologic unit without assembling rocks of very diverse age. Along with these basic igneous rocks are found more or less shale and chert, commonly in shades of light and dark green, not unlike the greenstones themselves, and probably of contemporaneous origin. Notwithstanding the undesirability of assembling together rocks of different ages, an attempt has been made to map these greenstones separately, so far as possible, with the hope that they may some time be further subdivided according to relative age. The undifferentiated greenstones are described further on pages 148-150. The prominent bluff on the Yukon just below Eagle, at the mouth of Mission Creek, is composed of undifferentiated greenstone. (See pl. 7, 4.)

At the northeast side of the band in the Yukon, below Calico Bluff, and just east of the band of Upper Cambrian limestone that crops out along the river bank, the rocks are black, yellow-brown, and green shales, with numerous beds of hard black quartzite that alters to a rusty-colored rock. These rocks crop out for 200 feet or more along the beach, and fragmental material along the beach for some distance farther east indicates their presence higher up the hillside. These shales and quartzites, which dip southward, appear to underlie the Upper Cambrian limestone and to overlie the Middle Cambrian limestone farther up the hillside, but the obvious faulting in this particular area makes it hazardous to classify them definitely as part of the Cambrian sequence, though they appear to belong to that system.

A more conspicuous group of undifferentiated Paleozoic rocks occurs along the southwest bank of the Yukon, cropping out intermittently from a point just below Fourth of July Creek downstream to Glenn Creek. Just below Fourth of July Creek a mass that extends out into the river a short distance and projects above the water is known locally as the "Rock of Ages." About 4 miles below Fourth of July Creek the same rocks form a more extensive reef that projects a considerable distance into the river. This group consists essentially of thin-bedded black to light-gray dolomite, usually

half an inch to 5 inches thick, interbedded with argillite, slate, and a few beds of limestone conglomerate. Much of the dolomite shows very fine alternating black and gray laminae, at some places as many as 50 to the inch. Some of the laminae are at an angle to the upper and lower surfaces of the beds and strongly suggest cross-bedding. All these beds are silicified to a greater or less degree, and some of the silicified argillite and slate is perhaps better designated chert. The fragmental limestone beds are of two types. The more common type consists of rounded limestone pebbles in a limestone matrix; some of these rocks show little or no silicification. The other type is an oolitic limestone in which the oolites are largely silicified to little balls of chert. Just above Glenn Creek this group of dolomitic rocks contains some altered intrusive rocks that were probably originally of basaltic or diabasic character but that now consist of secondary minerals, such as calcite and chloritic products, and are classifiable generically as greenstones. The rocks along the Yukon at the boundary, mapped by Cairnes 29 as the Yukon group and assigned to pre-Cambrian time, are in this report included with the undifferentiated Paleozoic rocks. Cairnes describes the Yukon group in general as composed of schistose amphibolites and quartzite and mica schists, with a few beds of limestone. He further states that these rocks are much folded, faulted, and distorted and are so highly metamorphosed in places that it is difficult or impossible to determine their origin or original character.

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Along the ridge south of Fortymile the writer noted greenstone schist, quartzite schist, massive quartzite, graphitic schist, and several varieties of green schist, which correspond closely to the Yukon group as described. Such rocks continue downstream from Fortymile to the boundary, where they appear to be more consistently greenish and to contain a larger proportion of metamorphosed basic igneous rocks. In the river bluffs along the north side of the Yukon below Ogilvie's station are seen sheared, banded, and massive greenstones and some green and yellow rocks of sedimentary origin, within which. are included several bands of marble, one of them 5 feet or more thick. Rocks of the same general character occur in the hills on the opposite side of the river, and in the interbanded marbles of this sequence, a short distance back from the Yukon, Paleozoic fossils were discovered in 1925. In rocks of the same type 13 miles south of Eagle, in the headwaters of Boundary Creek, Prindle in 1903 found Paleozoic fossils. These rocks, therefore, have in this report been included as a part of the undifferentiated Paleozoic sequence. This is not to be interpreted in any way as a reclassification of the age of the Yukon group as defined by Cairnes. It is merely a withCairnes, D. D., op. cit., pp. 39-44.

drawal of a certain portion of the rocks previously mapped as a part of the Yukon group, with a new assignment of them to the Paleozoic instead of to the pre-Cambrian.

To the north of these undifferentiated Paleozoic rocks, but separated from them by an overlapping band of Cretaceous and Eocene rocks, is a belt of limestone, which in turn is adjoined to the north by a group of rocks mapped by Cairnes as a part of his Tindir group, of Cambrian or pre-Cambrian age. This group of rocks consists essentially of drab shale and slate with some relatively thin beds of quartzite and conglomerate. Northwest of the mouth of Eagle Creek these rocks are well exposed along the north bank of the Yukon in a greatly disturbed fault zone and are there seen to contain also greenstone, probably of intrusive character. Except in this faulted zone the rocks of this formation are not so greatly metamorphosed as the undifferentiated Paleozoic rocks along the banks of the Yukon farther upstream, but this fact alone can not be regarded as decisive evidence of Paleozoic age. For reasons given later these rocks are not believed to be of Cambrian or pre-Cambrian age, as stated by Cairnes, and they are, therefore, here included as a part of the undifferentiated noncalcareous Paleozoic rocks. Farther north along the international boundary, in several irregular areas from McCann Hill to Hard Luck Creek, Cairnes 30 maps a so-called "shale-chert " group, in age which ranges in from Ordovician to Carboniferous. These rocks are mapped in this report as undifferentiated noncalcareous Paleozoic. With regard to their lithology, Cairnes makes the following statement:

This series consists dominantly or entirely of shales and cherts, which are prevailingly closely and finely interbedded. The cherts in places become really cherty shales or shaly cherts and occur in most places in beds ranging in thickness from 1 to 6 inches. Locally, however, they are more thinly bedded, and occasionally, on the other hand, they are in strata as much as 12 inches thick. They are also generally dark gray to black in color. The shales are also typically thinly bedded and in most places are soft and friable and gray to black or bluish black in color, the darker beds being in places decidedly calcareous in character. Occasional red shales also occur, however, locally intercalated with the darker strata, but these do not appear to be very persistent, or at least the color is not. Hard gray quartzitic shales are in addition somewhat extensively developed in places. These quartzitic beds contain locally sufficient iron to produce upon oxidation a bright-red to yellow coloration on weathered surfaces, but only rarely are these rocks red on a fresh fracture. These reddish beds decompose readily to form a red or yellowish sand or mud, which is a very noticeable feature of many of the hillsides on which vegetation is lacking.

Along Hard Luck Creek, where these shale-chert beds are well exposed, they have a red and black and in places even a decided ribboned appearance. The chert bands are generally from one-half to 2 inches in thickness and are finely interbedded with the shales. All the beds of the entire section are black or

30 Cairnes, D. D., op. cit., pp. 81-44.

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