formations grade into each other. Field conditions indicate that the limestone-argillite and the Middle Cambrian limestone also grade into each other, but their contact is not so well exposed, and the evidence is therefore somewhat weaker. In other words, if any stratigraphic break should be postulated in this sequence of three formations it should more reasonably be assumed to exist at the base of the Middle Cambrian limestone. However, as no fossils have been collected in the two lower formations, and no definite structural data have been collected by the writer that will serve to prove or disprove the existence of any discontinuity of sedimentation in these three formations, they can merely be represented on the geologic map as a succession of beds of which the Middle Cambrian limestone is the top and the red beds the basal part.

These structural uncertainties and the lack of fossils make it difficult to assign definite age labels in the explanation that accompanies the map. If a continuous sedimentary sequence exists, all three of these formations are probably of Cambrian age; but if, as thought by Cairnes, an unconformity exists at the base of the Middle Cambrian limestone, it is easy to conceive that the two lower formations are of pre-Cambrian age. This uncertainty has been registered on the map in the designation "Middle Cambrian or older" for the limestone-argillite formation, and by the designation "Lower (?) Cambrian or pre-Cambrian" for the red beds. The interrogation point in the expression "Lower (?) Cambrian or pre-Cambrian" is intended to indicate the possibility that the red beds may be of Middle Cambrian age.

So far as correlation with other rocks is concerned little can be said, for Cambrian rocks have not been recognized in Alaska except along the international boundary. No rocks that are lithologically exactly similar in interior Alaska have been described, irrespective of geologic age. The nearest lithologic counterpart consists of certain red slates and phyllites and associated arkosic rocks that constitute a part of the Tatalina group, in the Fairbanks quadrangle. The Tatalina group lies stratigraphically between rocks of Middle Ordovician and pre-Cambrian age, and therefore this correlation. seems a possible interpretation.

In Cairnes's description of the lithology of the Tindir group, which he assigned to the Lower Cambrian or pre-Cambrian (see extract, p. 22), he mentions the high hematitic content of some of the Tindir shales and quartzites. This is most suggestive and leads to the inference that these red beds will ultimately be found to be a part of the Tindir sequence. The geographic position of the Tindir group, which lies along the strike and 8 miles to the north of the red beds. here described, may be taken as corroborative evidence of this correlation.

UNDIFFERENTIATED PALEOZOIC ROCKS

DISTRIBUTION

The undifferentiated Paleozoic rocks have been divided for cartographic purposes into two groups, of which one includes all the dominantly noncalcareous rocks and the other includes essentially the calcareous and dolomitic rocks.

The dominantly noncalcareous undifferentiated Paleozoic rocks are found chiefly in the valley of Slate Creek, a tributary of the North Fork of Fortymile River; in the headwaters of the Charley and Salcha Rivers; in a zone extending from the head of the Seventymile River to the southeast corner of this district; in a bifurcated belt that extends eastward from Thanksgiving Creek to Washington Creek and northwestward into the Crazy Mountains, south of Circle; along the international boundary in an irregular zone that extends from the valley of the Tatonduk River northward to Hard Luck Creek. Some smaller areas included with this group of rocks are a small belt of slate-quartzite rocks that crops out along the north bank of the Yukon above Eagle and extends southeastward up the Yukon Valley; certain areas on the north side of the Yukon below Calico Bluff; and a belt extending along the south bank of the Yukom from Fourth of July Creek downstream intermittently to Glenn Creek. The group that extends southeastward from the head of the Seventymile River continues up the Yukon River beyond the limits of the area shown on the accompanying map in the direction of Fortymile and comprises some of the rocks included by Cairnes 28 as part of his pre-Cambrian or Yukon group. The reassignment of these rocks from pre-Cambrian to undifferentiated Paleozoic is made on the basis of their lithology and of certain fossils found in them.

The undifferentiated calcareous and dolomitic rocks of the Paleozoic, because of their striking lithologic differences from the other undifferentiated Paleozoic rocks, are shown by a separate pattern on the accompanying geologic map. They occur chiefly as follows: A limestone belt crops out along the international boundary midway between Eagle Creek and the Yukon and continues southeastward up the Yukon Valley and northwestward from Eagle up the north side of Mission Creek; two great masses of limestone and dolomite are found along the international boundary, one cropping out in the valley of the Tatonduk River and extending northward to Hard Luck Creek, the other extending from Cathedral Creek northward almost to Ettrain Creek; a belt of limestone lies northwest of Nation, extending from Spring Creek northwestward across Bull Creek to Logan Creek; a small wedge of limestone crosses Woodchopper Creek about 3 miles from its mouth.

Cairnes, D. D., op. cit., map 140A.

LITHOLOGY

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 dif ferent 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, A.)

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