nearly so, but certain alternate layers weather red in places. The chert beds also in places become shaly or even friable when exposed to the atmosphere and decrepitate somewhat readily.

Cairnes also describes a "shale group," which he believed to be of Carboniferous age but mapped with his Ordovician-Carboniferous sequence. With regard to the lithology of these rocks, he states:

The shale group within the mapped boundary belt here being considered is developed at a number of points between Jones Ridge and Yukon River and wherever identified overlies the members of the Devono-Ordovician shale-chert group. Possibly the most extensive individual development of these Carboniferous shale beds occurs just to the north of Hard Luck Creek, where they compose the greater part of a low northeasterly trending ridge to the east of the boundary line and to the south and east of Jones Ridge. These beds were also identified on McCann Hill and at other points but become rapidly much less prominent to the south of Hard Luck Creek.

On the geological map to accompany this memoir these Carboniferous shale beds have been mapped with the members of the Devono-Ordovician shalechert group, as except where fossils were obtainable it was in many places difficult or impossible in the field to distinguish certain members of these rock groups, as both contain beds that are lithologically practically identical.

This shale group consists dominantly of shales but includes also clays, cherts, calcareous sandstones, and thinly bedded limestones. The shales and clays are prevailingly soft and friable and range in color from light gray to black, but dark bluish-gray beds are possibly the most extensively developed. The cherts are dominantly dark gray to black in color but constitute a much smaller portion of this formation than the shale members. They occur prevailingly in beds having a thickness of less than 1 inch, but chert strata were noted which are as much as 3 inches or more in thickness. Calcareous sandstones and arenaceous clays are also developed in places and are typically grayish to brownish in color. The limestones generally occur in beds less than 12 inches thick and range in color from light gray to brown. This shale section has thus quite a decidedly striped or ribboned appearance, due to the frequently alternating shale, limestone, and sandstone beds.

The band of limestone south of Eagle Creek, which lies between the undifferentiated metamorphic Paleozoic rocks and the slatequartzite group referred by Cairnes to the Cambrian or pre-Cambrian, is composed of a number of heavy plates of limestone separated from one another by greater thicknesses of thin-bedded limestone. The general appearance of this limestone belt, when viewed from a distance, would lead to the belief that the limestone is a white crystalline variety, but close inspection shows it to be in large measure a dark-gray noncrystalline thin-bedded variety. Even the thicker plates show well-developed bedding planes and are by no means typically massive limestone. One peculiar phase of this limestone belt is the presence of beds of limestone conglomerate and breccia. The fragments consist usually of pieces of noncrystalline limestone, of varying shades of light gray, commonly well rounded but in part subangular, embedded in a matrix of dark-gray

north of Zage and in me north wie of Mason Creek mi dere s

Arrie found that the indiferentiated Imestice found at the beg a a zarnuation of this inestone, Khoongh I is more metimurphosed

The two reat names of limestore. biomate, ni somated rocks long the international boundary from the Think Biter to Hard Luck Creek and between Cathedral and Eurain Creess, which were mapped by Calmes as ranging in are from Camtean to Derician. a nere mapped as a part of the miferentiated mimmons Paleozouz sequence. With regard to the Cambrian-Simman part of this sequence. Calmes7s the following linkage dana:

These meka are prevailing vice to Cric my in mice bar peensional beds weer laying a try to nearly back ir even 1 pink je zažist a renice. Nearly everywhere, however in weathered miss the £ferent members have the penilar mayish to binsiy much appeazove daterarist : of time stones. The mcka are dominantly crywalne, and a places beds of partienlarly beautiful mare o vich prevalfdy ne a sve su pare white through racions iades of gr17. senasional redfish beds bang, however, orted in places.

In texture these Izest:te-dcmite rieks vary from firm. lense doion tes to carwly crystalline almest prize Smest toes, They are use suneteristicily emerhat massive in creams, die hely to the degree of metamorphisma which they have mifered; but where the bedding plates an isoumible the wrata are dominantly from 1 to 6 feer in thickness, athen mach thinzer beds from 1 to 6 inches thick are locally characteristic of the seres. Beds of 1.mestone Caring an coille strictate also occur on the south of Tanik River and elsewhere, the oc ide grains being generaly about opetenth of an inch Lameter.

In composition these beds more from limestrões 1 dikim nas but appear to be all dominantly more or less magnesan. They are frequenty tested in the feld with cord acid, and only rarely was a member of this grap fand that efferered freely, but nevertheless nearly all were more or less attacked. It wild has seem that these rocks are prevally transitical in composition between pare Limestones and the dolomites, either of these forms being of somewhat exceptional conTTEECE. The more dimitir beds are prevailingly harder and fner-textured than the lines ces and are bom panty white light gray in color none of the very dark cciers ocurring such as characterize the Imestones in places. Further, the mites in places, as on Mount Mariow and elembers in Oville Moins are more or less pircus and contain Enterom. castles, vilch are genera ly quite small but rance in size from spms! Trhes in diameter. These to “e containing meks

* Calmes, D D. sp. cit., pp. 58-61.

to be very rough on weathered surfaces. The cavities are dominantly lined with well-defined crystals, mainly of quartz and calcite, and are considered to indicate rather conclusively that the dolomites are of secondary origin and are derived from limestones, the amount of pore space representing the decrease in volume during the replacement process. Also, as fossils were very rarely, if ever, found in the dolomites and are quite plentiful in places in the adjoining limestones, this would seem to indicate that some change had occurred in the dolomite beds since originally deposited which destroyed any contained organic forms.

In a few places grayish, yellowish, to nearly black shales are intercalated with these limestone-dolomite beds, and at one point, on the western side of Mount Slipper, over 200 feet of thinly bedded shales occur, with dolomites above and below them. Shales, however, are of very minor importance quantitively in this Silurian-Cambrian terrane.

The entire series is prevailingly siliceous, and toward the south the beds contain a great amount of translucent to semitranslucent chalcedonic quartz or chert, which in places considerably exceeds the limestones and dolomites in amount. This chert has in places been deposited largely along the bedding planes of the containing rocks in seams ranging from microscopic up to 8 or 10 inches in thickness and thus gives the rocks in general a decidedly banded appearance. When somewhat regularly deposited along the bedding planes in this way the chert has in places the appearance of being contemporaneous with the containing beds, but when more closely examined it may be seen to intersect the strata; in fact, seams or masses of chert occur cutting the limestone and dolomite beds at all angles, and the smaller seams are frequently distinctly traceable back to larger seams or irregular bodies.

The Devonian part of this limestone sequence is described as follows:

The Devonian limestones resemble very closely the limestone beds of the Silurian-Cambrian group, and except where fossils can be found it is difficult or impossible in many places to distinguish these rocks from the older limestones. They are, however, as a rule somewhat more homogeneous and darker in appearance, being typically dark bluish gray in color. They are also in most places characteristically coarsely crystalline, and when broken they generally emit a strong oily odor, which was seldom noted in connection with the underlying formations. In places a heavy bed or series of beds of white to light-gray sugar-grained quartzite occurs at the base of this limestone series, as in the vicinity of Tindir Creek, but this quartzite appears to be only locally developed.

Another belt of limestone of unknown age begins in the hills west of Nation at Spring Creek and extends in a northwesterly direction 8 or 10 miles, as far as Glenn Creek. This limestone shows several variations. Some of it is dark gray to black and noncrystalline, some is light gray and finely crystalline, and some beds of silicified cherty oolite and also beds of limestone breccia were seen. The dip appears to be dominantly northward, but as the nearest good exposures of other rocks are some miles away, this apparent structure has little significance. As no fossils were found the age of the limestone is indeterminate. It resembles perhaps more nearly than any other rock the plate of limestone that crops out on the north side of the

Yukon about 2 miles below Calico Bluff, which is believed to be of Upper Cambrian age.

A similar belt of limestone crops out on Woodchopper Creek about 3 miles from the Yukon. This limestone also has a general northwesterly trend, but its extent along the strike northwest and southeast of Woodchopper Creek is not known. It is also varied in aspect. Some of it is black, noncrystalline, and carbonaceous and resembles the limestone south of Eagle Creek, and some of it resembles more closely the limestone west of Nation and that below Calico Bluff.

STRUCTURE AND THICKNESS

Structural observations on the undifferentiated Paleozoic rocks were made by the writer chiefly in the rock bluffs along the Yukon in the vicinity of the international boundary. The rocks in this zone, like most of the other undifferentiated Paleozoic rocks, lie northeast of a great mass of granitic rocks which are believed to have determined the degree of metamorphism and in large measure the dominant structure of the adjacent rocks. Although the observations are admittedly fragmental, they are nevertheless believed to represent conditions rather typical of the structure of all the undifferentiated metamorphic Paleozoic rocks lying northeast of the great Mesozoic batholith.

These rocks as exposed along the river and in the near-by hills north and south of the river are, as previously stated, closely folded and cleaved and show all the typical indications of intense compression induced by lateral thrusting. Except in some of the more massively resistant greenstones and in the easily recrystallized limestones, flow cleavage is commonly present. Nevertheless, bedding also can be discerned at many places, particularly in the thin beds of quartzite, and where such bedding was recognized close folding is also prevalent. Where the thrusting movements were localized, as along fault and shear zones, closely appressed and even recumbent folds are clearly visible.

Numerous observations of the attitude of the cleavage planes and of the axial planes, both of the drag folds in the less resistant rocks and of the more diagnostic appressed folds of competent beds, were made from Fortymile downstream to Eagle. Bedding planes also were recorded where visible. It can not be stated that any absolute uniformity exists in these observations. Nevertheless about 80 per cent of the cleavage planes and axial planes of the folds dip south at angles of 15° to 65°, with an average of about 30°. The strike of the dominant structural feature, which here is the cleavage, is about N. 70° W. The bedding planes, where visible, appear to dip both to the southwest and to the northeast, but it is worthy of note

that the dip is prevailingly southward along the side of the river, where the rocks are most metamorphosed. In other words, cleavage and bedding in the metamorphic rocks are prevailingly parallel. These data suggest to the writer that the rocks have been thrust northeastward and overturned by enormous tangential forces acting from the southwest, and their degree of metamorphism is explained as a concurrent effect of this prodigious thrusting. The cause of the forces applied is believed to be a great batholith of Mesozoic granitic rocks that lies to the southwest.

At the boundary and just to the east are three groups of rocks, all mapped as a part of the undifferentiated Paleozoic sequence, that present a difficult stratigraphic problem; these are the metamorphic Paleozoic rocks along the river, the slate-quartzite group, and the band of limestone. Fossils found in the Paleozoic metamorphic rocks give some idea of the age of those rocks, but no fossils have yet been found in the other two formations, and the relative ages of the three must be deduced, if at all, from available structural data. The metamorphic rocks at this locality were regarded by Cairnes as preCambrian and were included as part of his Yukon group; and the slate-quartzite group, which he believed to be of Cambrian or preCambrian age, he correlated with his Tindir group, exposed farther north along the boundary; the limestone band was believed to be Cambrian. Cairnes's sequence, then, was metamorphic rocks at the base, overlain by the slate-quartzite group, overlain in turn by the belt of limestone.

The following data on the geologic structure may shed some additional light on this correlation. All three of these formations appear to be parallel to one another, trending about N. 55° W. The rocks of the Paleozoic metamorphic group are schistose, but where the bedding is visible or may be inferred the bedding planes also dip prevailingly southwestward. The angle of dip of the cleavage and bedding planes here, as farther upstream, averages about 30°.

Just below the mouth of Eagle Creek a well-developed fault zone may be seen exposed in the bluff along the north bank of the Yukon. This is not a clean-cult fault but rather a zone of dislocation along which adjustments consequent upon great thrusting movements took place. Doubtless numerous similar fault zones are present in this area, but it is doubtful if any will be discovered where the rocks are so well exposed and where the structural interpretation is so apparent. The rocks involved in this fault zone are the Paleozoic metamorphic rocks, which at this locality are greenstone and greenstone schist, and the rocks of the slate-quartzite belt. The strike of this fault zone is about N. 55° W., and the attitude of the axial planes of numerous drag folds shows that the thrusting came from the south