consists mainly of quartzite and quartz-mica schist but includes also certain green schists of igneous origin and beds of crystalline limestone. The Klondike series consists principally of light-colored sericite schist and subordinately of darker-colored chlorite schist, both believed to be of igneous origin. Associated with and constituting, in fact, a subdivision of the Klondike series of McConnell is a group of ancient rocks of gneissic character that are believed to have intruded into his Nasina series and are probably the deepseated equivalents of the sericite schist of the Klondike series. These rocks are known as the Pelly gneiss, a name which, according to McConnell,21 was applied originally to them by Brooks, although the name does not appear in Brooks's report on the White and Tanana River Basins. Another cartographic unit recognized by McConnell 22 is the Moosehide diabase, composed mainly of altered diabase and supposed to be nearly contemporaneous with the Klondike series. Of these units, the Nasina series alone is regarded as correlative with the Birch Creek schist, as redefined in this report.

Subsequently Cairnes, 23 in 1914, introduced the term Yukon group to include "all these older metamorphosed, schistose, and gneissoid rocks of both sedimentary and igneous origin." He further defined the Yukon group as pre-Cambrian in age. This term, therefore, would include not only the Birch Creek schist but all the associated metamorphic rocks of igneous origin here mapped with the schist. No general term, similar to Yukon group, to include all the pre-Cambrian rocks, is at present recognized in the United States Geological Survey nomenclature. Unfortunately, as shown on page 33, Cairnes included in his mapping of the Yukon group certain Paleozoic rocks, some of which are as young as Devonian. This, however, should in no wise detract from the use of the term Yukon group as defined by Cairnes.

24

Cockfield, in the most recent geologic work done in Yukon Territory, has used the general term Yukon group, but has divided the group into four subgroups-the Nasina series, at the base; the Klondike series, overlying the Nasina; the amphibolites, unnamed, overlying the Klondike; and the Pelly gneiss, which invades the Nasina and Klondike series as well as the amphibolites. It seems to the writer rather likely that parts of the Klondike series, of the amphibolites, and of the gneiss may later turn out to be of Paleozoic age.

21 McConnell, R. G., op. cit., footnote p. 13B.

22 Idem, pp. 22B-23B.

23 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, p. 40, 1914.

24 Cockfield, W. E., Sixtymile and Ladue Rivers area, Yukon: Canada Geol. Survey Mem. 123, 1921.

CAMBRIAN OR PRE-CAMBRIAN ROCKS

TINDIR GROUP

DISTRIBUTION

The term Tindir group was applied by Cairnes to groups of similar rocks at four distinct localities, as follows: Along the Porcupine River where it crosses the international boundary; along the international boundary between Fort and Orange Creeks; along the international boundary between Tindir, Cathedral, and Hard Luck Creeks; along the international boundary southeastward from the mouth of Eagle Creek. The first two localities lie outside the area covered by this report and will not be further discussed. The rocks of the fourth locality are here called the slate-quartzite group and for lack of definite paleontologic evidence have been assigned to the group of undifferentiated noncalcareous Paleozoic rocks. (See p. 34.) The rocks of the third locality, though not seen by the writer, come within the area shown on Plate 12 and will now be considered.

As mapped by Cairnes, the rocks of the Tindir group extend from the Cambrian limestone at Jones Ridge northward in a solid block for 5 or 6 miles to Cathedral Creek, thence northward in a narrow zone up a tributary of Cathedral Creek and on to the northwest and north of Mount Slipper. A small outlying mass lies on the north side of Tindir Creek about 4 miles north of Mount Slipper. Because of the proximity of these areas to Tindir Creek, it seems best to regard them as the type locality for this group of rocks. Here certainly is the best available evidence for evaluating the age of the Tindir group.

LITHOLOGY AND STRUCTURE

As the Tindir group in its type locality has not been seen by the writer, the description given by Cairnes, 25 referring to the Tindir, Cathedral, and Hard Luck Creek areas, is quoted here:

The Tindir section exposed still farther south along Tindir Creek and between Ettrain and Hard Luck Creeks particularly resembles that observed along Porcupine River, the members including mainly dolomites, limestones, quartzites, slates, shales, and greenstones. Here, however, the greenstones are developed to a much greater extent than to the north of Orange Creek and along the Porcupine, and the quartzites instead of being dominantly white to grayish include more greenish, reddish, and dark-colored members, even quite black quartzites being prominent in some places.

The dolomites weather characteristically rough and reddish to brownish as elsewhere and are generally bedded, the strata ranging in places from a fraction of an inch to a foot or more in thickness. They also include numerous bands of chert 1 to 2 inches thick. Intercalated with these dolomites also are

Cairnes, D. D., op. cit., pp. 52–53.

occasional grayish limestone beds and also some of black slate. These dolomites appear to be at least 700 feet in thickness.

Mount Slipper is capped by Devono-Cambrian limestones and dolomites, which are underlain by the members of the Tindir group. Thus around the western and southern faces in particular of this mountain a splendid section of a part of the Tindir rocks is exhibited. There these beds include mainly dark to black calcareous shales, limestones, and quartzites, all invaded by greenstones which occur both as dikes and sills. The quartzites are dominantly thinly bedded and nearly black but weather in places to a dark reddish or reddishbrown color. The limestones are prevailingly also thinly bedded and dark to nearly black in color and grade into very soft, thinly bedded, friable black calcareous shales, the beds of the upper 500 feet at least of the section being very calcareous. The Tindir beds exposed here on Mount Slipper evidently constitute the upper portion of the Tindir group in this locality and very closely resemble the shale member comprising the upper 1,000 feet in the section measured along Porcupine River.

A typical member of the Tindir beds south of Tindir Creek also is a finely laminated rock consisting of alternating white and black bands, there being on an average about 20 laminae to the inch. The light bands consist dominantly of quartz and the dark bands of argillaceous shaly material.

Certain shales and quartzites also exhibit considerable hematite, and in places portions of these beds contain up to 30 per cent or even possibly 40 per cent metallic iron.

The greenstones are dominantly diabases and occur in sills, dikes, and irregular intrusive masses and in places constitute a considerable portion of the entire formation. The sills are in places as much as 100 feet and the dikes 200 feet in thickness. Since these intrusions were rarely noted intruding the overlying Devono-Cambrian limestones and dolomites, it is concluded that they are dominantly at least older than these rocks.

The Tindir rocks here, in the vicinity of Tindir Creek and between Ettrain and Hard Luck Creeks, as farther north, are characteristically much indurated, folded, and contorted, and are also brightly and varicolored, black and shades of red, gray, and yellow being conspicuous. A single side hill in places may exhibit reddish to brownish dolomites, yellow to black quartzites, gray limestone beds, gray, red, or black shales or slates, and black to bright-red iron ore containing beds all ribboned and intersected by irregular brownish-weathering greenstone dikes and sills. The hills on which these rocks outcrop are dominantly lofty and irregularly distributed and are characterized by long, sharp, steep-sided ridges, with smooth slopes covered with a fine talus. The bright and contrasted colors which they exhibit also constitute one of the most striking pictorial features of the landscape of the district.

AGE AND CORRELATION

The assignment of the Tindir group by Cairnes to the Lower Cambrian or pre-Cambrian was made for the following reasons: The Tindir group underlies the Devonian-Cambrian limestones and dolomites between the Black River and Fort Creek (north of the area here considered); the Tindir group underlies the Devonian-Cambrian limestones at Mount Slipper, north of Cathedral Creek; along the north side of Jones Ridge the Tindir beds were observed to underlie

unconformably a limestone-dolomite series in which Cambrian fossils were found.

It is not believed by the writer that a continuous Devonian-Cambrian limestone sequence exists along the boundary. Unquestionably Devonian, Silurian, Ordovician, and Cambrian fossils have been collected from massive limestone and doubtless also at many places where it is impracticable to map separately the different limestone series. But all the stratigraphic data from the Paleozoic rocks of Alaska indicate that such limestone series are separated from one another by thick groups of essentially noncalcareous rocks and also by one or more unconformities. The great unconformity between the upper Silurian and the Middle Devonian rocks, as seen elsewhere in central and northern Alaska, is an example of such discontinuities of sedimentation. Hence the statement that the Tindir group lies beneath a Devonian-Cambrian limestone sequence implies so much doubt with regard to the areal distribution and interrelations of the several limestones within that range of age that such a statement fails to be convincing. The first two reasons above given are therefore discounted at the outset.

At Jones Ridge, however, a different condition appears to prevail. There only Cambrian fossils were found in the limestone along the south flanks of the Tindir group, and Cairnes saw the Cambrian limestone resting unconformably upon the Tindir. Also two recorded strikes and dips on Jones Ridge within the Cambrian limestone area show the limestone striking in a general northerly to northeasterly direction and dipping east to southeast, and this attitude of the beds, if continued to the Cambrian-Tindir contact, would undoubtedly place the Cambrian stratigraphically above the Tindir. Hence it seems reasonably sure that the rocks mapped as Tindir group north of Jones Ridge in fact underlie the Cambrian limestone of Jones Ridge. Cairnes's Cambrian fossils, however, have been determined as essentially Upper Cambrian in age, with one lot questionably Middle Cambrian. But if, as seems likely, the Middle Cambrian is definitely represented at Jones Ridge, and Middle Cambrian limestone rests unconformably upon the Tindir group, it seems probable that the Tindir group is, as stated by Cairnes, Lower Cambrian or older. Cairnes, evidently believing that this unconformity represents a discontinuity of major magnitude, was inclined to stress the possible pre-Cambrian age of all or at least a part of the Tindir group. This interpretation, though by no means proved, is a most suggestive one and is fraught with far-reaching consequences in connection with early Paleozoic and pre-Cambrian correlations. Cairnes suggests, for example, the correlation of the Tindir group with the Tatalina group, of the Fairbanks district, and this corre

lation, if made with the lower part of the Tatalina group, seems indeed to be a very logical one, for the Tatalina includes rocks of Lower Ordovician and probably of Cambrian and pre-Cambrian age. If the Tindir group should finally be proved to be pre-Cambrian in age, it would naturally be correlated with some part of the Algonkian system; and if the Tatalina group should also be found to include rocks of that age, the Birch Creek schist, which has heretofore been regarded merely as undifferentiated pre-Cambrian, would probably come to be regarded as Archean. Much, therefore, hangs upon a definite age determination of the Tindir group in its type locality, and future geologic work along the boundary should have this for one of its most important objectives.

RED BEDS

DISTRIBUTION

So far as known, the red beds, which are here regarded as underlying the Middle Cambrian, have been seen only at one locality, in the valley of the Tatonduk River about 3 miles downstream from. the international boundary. They crop out on both sides of the valley, ending sharply to the east against Silurian and later Paleozoicrocks; to the north or northeast, however, along the strike, they appear to continue for a number of miles into the heart of the Ogilvie Mountains.

LITHOLOGY

These rocks may be described collectively as red beds. As seen in. the bluffs along the Tatonduk River they consist of red sandstone and sandy shale, with layers of almost pure red hematite as well as layers of hematitic breccia. The fragmental material of the breccia consists of poorly assorted subangular to angular pebbles and cobbles that range in diameter from a few inches to a foot. The red shale is well indurated in the outcrops along the valley walls, but it disintegrates rapidly on exposure to the weather and is seen on the gravel. bars downstream at many places as lumps or mounds of red clay or mud. On the slopes along the south side of the Tatonduk River it was found that the red beds contained a large proportion of conglomerate. At an altitude of 1,400 feet above the valley floor on this south side of the valley a massive bed of red conglomerate perhaps 75. feet thick was seen continuing up the spur from the creek. It consists of angular pebbles of quartz, chert, slate, and phyllite. This conglomerate is followed by red sandstone and slate, which in turn are succeeded at 1,600 feet above the creek by another bed of red conglomerate about 100 feet thick, composed of subangular to well-rounded pebbles, cobbles, and boulders, the largest of which are 2 feet in diameter.