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 inIcluded 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

Cairnes's key is as follows: The strip along the international boundary is divided into blocks comprising 10 minutes of latitude and approximately 10 minutes of longitude, which are numbered from north to south with roman numerals. Each main block is subdivided into units approximately one-fourth of a mile square, designated from south to north by the numbers 1 to 46 and from west to east by the letters a to t. The main blocks falling within this area and the method of designating their subdivisions are indicated in Figure 3.

On the assumption that this scheme was used consistently and that no typographic errors are present in the numbers of the fossil collections given in Cairnes's memoir and after making the best possible adjustment of some of the obvious inconsistencies, the following lots of Cairnes's fossils are interpreted by the writer as having been collected from the essentially noncalcareous rocks, ranging in age from Ordovician to Carboniferous.

XX i 25:

Dicranograptus cf. D. ramosus (Hall).

Retiograptus geinitzianus Hall.

Diplograptus foliaceus incisus Lapworth.

Obolus sp.

Ostracode.

Ptychoparia sp.

Isotelus? sp.

Harpes? sp.

The three graptolites listed above were determined by R. Ruedemann to be of Lower Ordovician age, or, more specifically, equivalent to the Normanskill, the upper part of which, however, Ruedemann considers to be of early Black River (Lowville) age, which is commonly classified as Middle Ordovician. The other invertebrates were determined by L. D. Burling.

XXI o 44:

Ostracode.

Monticuliporoid coral.

Atrypa? sp.

Proetus-like trilobite.

The above collection, determined by E. M. Kindle and E. 0. Ulrich, was referred with some hesitation to the Ordovician.

XV a b 35:

Cladopora cf. C. dichotoma Hall.
Phillipsastraea verneuili Milne-Edwards.
Proetus cf. P. macrocephalus Hall.

This collection, determined by E. M. Kindle as Devonian but possibly different from the type Middle Devonian fauna, was obtained, according to the scheme of locations given in Figure 3, about 2%

miles west of the boundary and an equal distance south of the Kandik River, in an area believed to be composed essentially of Lower Cretaceous rocks. If no error in placement has been made, these fossils probably came from essentially noncalcareous Devonian rocks infolded into the Cretaceous.

L. D. Burling, who accompanied the boundary party in 1912, also made several Ordovician collections along the boundary north of the Yukon. These have been studied by R. Ruedemann, and the results of his study are given in the identifications below tabulated:

4636:

Cactograptus n. sp.

Callograptus cf. C. diffusus Hall.

4662:

Lingula n. sp.

Obolus n. sp. Strongly ribbed.

Paterula n. sp.

Caryocaris n. sp. Large form with fringed posterior extremity.
Sponge spicules.

Tetragraptus similis (Hall).

Didymograptus patulus Hall.

Didymograptus cf. D. extensus Hall.

4680:

Caryocaris n. sp.

Tetragraptus similis (Hall). Young specimen.

Didymograptus nitidus Hall.

Didymograptus extensus Hall.

4683:

Lingula n. sp.

Caryocaris n. sp.

Sponge spicules.

Tetragraptus similis (Hall).
Didymograptus nitidus Hall.

Three of the four collections above given include the genus Caryocaris. Ruedemann,82 who discovered this little crustacean among the Burling collection, published a separate note thereon, as follows:

In a collection of lower Ordovician praptolites from the Alaska-Yukon boundary sent to the writer by L. D. Burling, of the Geological Survey of Canada, for identification, a small number of specimens of Caryocaris were noted, one of which retains the abdomen in place. This fact as well as the presence of other characters hitherto unknown have suggested this note, the material having been kindly presented to the New York State Museum by Mr. Burling. The Alaska-Yukon material is not obscured by an imperfect cleavage and leaves no doubt that the posterior margin of the carapace was indeed furnished with a fine comb of uniform bristles or teeth corresponding to the "fringe" observed in certain species of Ceratiocaris.

In 1928 P. S. Smith made a collection of Lower Ordovician graptolites from a bed just above the Upper Cambrian limestone, at the

* Ruedemann, R., Note on Caryocaris salter: New York State Mus. Bull., Nos. 227-228, pp. 97-98, 1919.