were positively recognized. The upper limit of the metamorphic Paleozoic rocks along the Yukon is a fault zone, and the base is undetermined, so that it seems utterly useless to hazard even a guess as to the thickness of this part of the sequence. If the limestone band and the slate-quartzite group have about the same degree of complexity, the width across the strike indicates that these two units have thicknesses of about the same order. The limestone is better exposed than the slate-quartzite group and therefore affords a better basis for an estimate of thickness. Though not accurately measured, the total thickness of this limestone certainly can not exceed 3,000 feet, and when allowance is made for the probable duplication of beds due to folded structure, an estimate of half that thickness seems all that is warranted.

The following structural data are given by Cairnes with regard to the Silurian-Cambrian limestone and dolomite, here mapped as undifferentiated Paleozoic limestone:

*

The rocks are all so much folded and faulted that only in a few places could the positions of the different beds within the series be even approximately determined stratigraphically; and unless fossils could be found it was impossible, even in these places, to draw the geological age boundaries, as no distinctive persistent lithological horizon markers could be distinguished. These beds in the northern portion of the belt have an aggregate thickness of 4,000 feet and possibly very much more than this amount, but no section of them was at all closely measured at any one point, it being found very difficult to do so on account of folding and faulting. * * * To the south these beds do not appear to be so thick, but even there they have an aggregate thickness of at least 3,000 feet.

Cairnes's statement with regard to the thickness of the Devonian limestone, also mapped in this report as undifferentiated Paleozoic limestone, is as follows:

These Devonian limestones appear to have an aggregate thickness of from 300 to 500 feet, and wherever a contact was observed with the underlying Silurian beds they overlie these unconformably.

AGE AND CORRELATION

Rocks of very diverse character and age are grouped on the map into two lithologic units, under the general assignment of undifferentiated Paleozoic. The evidence concerning the fauna and age of these rocks may conveniently be presented under three general headings, as follows: Undifferentiated metamorphic Paleozoic rocks along the international boundary and similar and associated rocks south of the Yukon; undifferentiated nonmetamorphic and essentially noncalcareous Paleozoic rocks along the international boundary north of the Yukon; undifferentiated Paleozoic limestone along the international boundary north of the Yukon.

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Metamorphic rocks.-Only the three fossil collections noted below have so far been made from the undifferentiated metamorphic Paleozoic rocks within the area covered by this report.

3AP77. Valley of a headwater tributary of Boundary Creek, 13 miles south of Eagle; from a thin bed of crystalline limestone; collected by L. M. Prindle, 1903. These fossils were identified by G. H. Girty as crinoid columns and showed only that the rocks are of Paleozoic age.

Two other collections, made by the writer during the season of 1925, were found in the lower valley of a stream which empties into the Yukon on the same side of the river and about a mile upstream from Boundary Creek. The exact localities are:

25AMt52 (2059). West slope of Loop Mountain, in Yukon Territory, Canada, 2 miles S. 40° E. of international boundary topographic station 112. 25AMt53 (2060). Boulder in creek, directly down hill from 25AMt52 and about 1.9 miles S. 40° E. of international boundary topographic station 112.

Both these collections consist of crinoid columnals. With regard to them, Edwin Kirk states: "These two lots are apparently of the same age. They are Paleozoic and not earlier than Silurian. They probably are Devonian." These crinoids are especially significant because they occur in rocks previously mapped as a part of the Yukon group. A part of the rock from which collection 25AMt53 (2060) was taken is shown in Plate 4, B.

To the south, however, beyond the area covered by this map but within rocks that are grouped with the undifferentiated Paleozoic rocks, L. M. Prindle made three collections in 1904, 1905, and 1907, all of which offer corroborative evidence of the Paleozoic age of this group of rocks. The numbers and localities of these collections are as follows:

4AP46. 40 miles west of Eagle.

5AP319. Dennison Fork, about 20 miles south of Chicken Creek.

7AP82. Fortymile River, one-fourth mile below mouth of Napoleon Creek. Collection 4AP46 consisted of a coral doubtfully referred to Zaphrentis? sp., which suggested to E. M. Kindle a Devonian or Silurian age. Collection 5AP319 showed some striated plant stems which were referred by F. H. Knowlton to Calamites radiatus and indicated only a Paleozoic age. Collection 7AP82, from a somewhat schistose limestone, was identified by Kindle as crinoid stems of little diagnostic value.

In addition to such fragmentary paleontologic evidence, however, considerable is known of the age of this group of rocks from other fossil collections made in this same district and near-by districts, and from formations which, though belonging to the same group, have been in part differentiated into mappable units. The other fossil lists will be presented under the appropriate headings. The

age of certain other parts of the undifferentiated Paleozoic assemblage that are unfossiliferous either here or elsewhere has been determined within fairly narrow limits from correlation with similar rocks in near-by districts where their stratigraphic relations to contiguous formations are better known. The feldspathic sandstone or quartzitic arkose, for example, which appears to adjoin the Birch Creek schist, is probably equivalent to part of the Tatalina group in the Fairbanks quadrangle, where such rocks have been differentiated into a separate unit. The Tatalina group, which lies beneath Middle Ordovician rocks and above the Birch Creek schist, includes not only the arkosic and quartzitic rocks above described but also many red, green, and purple slates, and it doubtless represents that part of the Lower Ordovician and Cambrian sequence which is present south of the Yukon, and it may extend down into the pre-Cambrian. It does not, to be sure, include a Cambrian limestone, but possibly its multicolored slates are in part at least the metamorphosed equivalents of the two formations that underlie the Middle Cambrian limestone in the valley of the Tatonduk River. It is therefore not too much to state that the base of this undifferentiated group of Paleozoic rocks may be as old as Cambrian.

The thin-bedded dolomites between Fourth of July and Glenn Creeks are particularly hard to place stratigraphically. The beds dip both southwest and northeast, showing reversals due to folding, but apparently the dominant dip is southwestward. They lie northeast of a belt of undifferentiated limestone of unknown age that crops out in the hills southwest of the Yukon. Farther northeast are Lower Cretaceous and Carboniferous rocks. The structure and surrounding stratigraphy therefore yield little information regarding their age. Lithologically, these rocks resemble more than any other the thin-bedded argillite and limestone that underlie the Middle Cambrian limestone of the Yukon and Tatonduk Rivers. They are included as undifferentiated Paleozoic rocks because the information at present available does not seem to warrant a closer age assignment. The age of the slate-quartzite group just north of the Yukon at the boundary can not be stated definitely, inasmuch as no fossils were found in it or in the adjoining limestone. Even the stratigraphic sequence is doubtful, as previously pointed out. But as fossils not older than Silurian occur in the Paleozoic metamorphic rocks just to the south, and the rocks adjoining to the north are regarded as Middle Devonian, the apparent interpretation is that the age of the slate-quartzite group is somewhere between Silurian and Devonian. However, in an overturned sequence such as is believed to exist here the apparent interpretation is not necessarily the true one, and this suggestion is made with no dogmatic insistence of its absolute truth.

Metamorphosed basic igneous rocks of several different ages collectively designated greenstone exist in the Yukon-Tanana region. Fossils collected from the volcanic tuffs and other sedimentary beds associated with the lavas indicate the presence of a Middle Ordovician, a Middle Devonian, and a Mississippian sequence of basaltic rocks of greenstone habit. Another group of greenstones of ultrabasic character, which for reasons elsewhere stated in this report are considered to be of Devonian age, are also present. Each of these four Paleozoic greenstone formations has certain distinguishing characteristics that make possible a reasonable guess regarding the age in areas where the fossils are absent. From such data the greenstones of necessity included in these undifferentiated Paleozoic rocks are believed to be mainly of Devonian age. Likewise, some of the shales and thin beds of limestone are more likely to be of Devonian or late Silurian age than otherwise.

Chert occurs in varying amounts throughout the Paleozoic rocks, and two formations containing a considerable proportion of chert, of late Middle Devonian and lower (?) Mississippian age, have been separately mapped. The chert conglomerate, above described, may belong in either of these two formations, but the weight of present evidence favors its assignment to the lower Mississippian.

Upper Mississippian and Permian rocks constitute well-known geologic units along the Yukon and elsewhere in interior Alaska, but they are so extremely fossiliferous that it is inconceivable that their fossil content could be obliterated by metamorphism of the degree present in this undifferentiated group. It is therefore believed that practically none such rocks are included in this group.

It may be said, therefore, that the group of undifferentiated metamorphic Paleozoic rocks along the international boundary and south of the Yukon includes formations ranging in age from Cambrian to Devonian. It should again be emphasized, however, that the sequence included in this grouping represents only a part, and possibly only a small part, of the Cambrian-Devonian sequence.

Nonmetamorphic noncalcareous rocks.-The presentation of the evidence concerning the fauna and age of the essentially noncalcareous nonmetamorphic rocks along the international boundary north of the Yukon which were described and mapped by Cairnes is difficult because Cairnes published his faunal lists without giving exact locations; it is therefore impossible from his memoir alone to correlate the fossils with the geologic mapping. From Dr. E. M. Kindle, at Ottawa, however, the writer obtained a key to the locations of Cairnes's fossils, which, although obviously in error with regard to the placement of certain particular lots of fossils, gives the localities of most of them within a quarter of a mile.

FIGURE 3.-Index to localities of fossils collected along the international boundary by the Canada Geological Survey, as listed by Cairnes. A, Key to numbering of main blocks; B, sample block showing method of indicating subdivisions