proportion of the hydromicas, brittle micas, and chlorite. At several localities, as for instance at the head of Woodchopper Creek and in the headwaters of the Salcha River, the difference in the character of the micas constituted the main basis for the separation of the Birch Creek schist from younger rocks.

Graphitic and calcareous schist and crystalline limestone are found to some extent throughout the Birch Creek mass, but for the most part such rocks are localized in their distribution in such a way as to suggest that they characterize the upper part of the sequence. No large bodies of limestone occur in the Birch Creek schist; the beds or zones range in thickness from a few inches to 100 feet. One of the unsolved problems of these older rocks is the definite identification of the carbonaceous and calcareous members as integral parts of the unit. Even without an exact determination of age, sufficient lithologic differences exist between these rocks and the older pre-Cambrian rocks to afford the basis for a cartographic differentiation whenever more detailed work can be attempted.

Among the igneous rocks associated with the Birch Creek schist granitic and dioritic gneisses, particularly the former, are conspicuous. In general, these rocks appear to invade the Birch Creek schist but like it have undergone sufficient metamorphism to acquire a gneissoid and in places a schistose texture, with the resultant formation of new minerals. Albitization of the feldspars, with the production of calcite, and alteration of the dark minerals to epidote, chlorite, and secondary hornblende or mica are the more common secondary processes. One of the more striking types of these metamorphosed intrusive rocks is augen gneiss, in which feldspar augen as large as 2 inches in diameter have been observed. Plate 3, B, shows a typical exposure of the augen gneiss. Sericite and chlorite schists of uncertain origin are also included with the Birch Creek schist as mapped. Some of these schists are of igneous origin, derived probably in part from lavas and fine-grained intrusive rocks of acidic and intermediate character, and therefore do not properly constitute a part of the Birch Creek schist. Others are undoubtedly derived from argillaceous sedimentary rocks and are closely related to the sedimentary mica schists. These rocks present a difficult problem, for their cartographic differentiation will have to be made on lithologic rather than genetic differences. These schists, like the carbonaceous and calcareous metamorphic rocks, belong in the upper rather than the lower part of the sequence and would have to be included there in any except the most detailed type of geologic mapping.

The associated amphibolite and hornblende schists, produced in large measure by the metamorphism of intermediate, basic, and ultrabasic igneous rocks, form another possible mapping unit. These

rocks in some localities, as at the falls of the Seventymile River, occur with crystalline limestone and quartzite in such a way as to suggest at least the possibility of a sedimentary origin. They are also found at many places, however, in close association with the gneissoid rocks and are locally intruded by those rocks. The origin of all these rocks can not be stated with assurance, but their field relations to the gneisses and to adjacent sedimentary rocks suggest that most of them are recrystallized dike and sill rocks that were related genetically to the ancient granitic magma that produced the gneisses.

STRUCTURE AND THICKNESS

The pre-Cambrian rocks have been subjected to diastrophism during many periods and have unquestionably been intensely deformed in several stages. They therefore probably reflect in their present structure the combined effects of close and open folding, together with thrust and normål faulting, repeated several times and accentuated by proximity to intrusive rocks. Original bedding planes, except in the more massive quartzites and quartzite schists, are quite obliterated, and the present visible structure exhibits a multitude of diverse cleavage planes and of close or even recumbent folds, which give little idea of the original sequence of deposition. It is believed that the Birch Creek schist has a structure almost if not quite as complex as the pre-Ordovician schists of Seward Peninsula, and neither of these groups of rocks is likely to be understood structurally for many years to come.

In addition to the regional metamorphism, portions of the preCambrian rocks have been subjected also to intense contact metamorphism caused by granitic intrusions of at least three eras. The pre-Cambrian (?) granite gneiss represents the earliest of these, the Mesozoic granite batholiths the second, and the Tertiary granitic intrusive rocks the third. It is difficult without detailed work to assign to each of these intrusive periods its proper share in the contact-metamorphic effects now visible, but the Mesozoic granitic rocks. have unquestionably produced a large part of the contact metamorphism.

Garnetiferous schists are common near the contact with the great granitic batholith that stretches from Glacier Mountain westward tothe Salcha River, and the Mesozoic granite itself at places shows the effects of its intrusion by a primary gneissoid fabric. At the heads of Woodchopper and Coal Creeks, near the contact with the Mesozoic granitic rocks, Prindle 12 and the writer in 1911 noted also a stauro

Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska: U. S.. Geol. Survey Bull. 538, p. 24, 1913.

litic garnetiferous schist, in which crystals of staurolite as much as half an inch in length were especially abundant. Mica, particularly biotite, is also prominent in the schist near its contact with the granite. It is apparent that the schists have received by injection a considerable amount of material from the granitic intrusions, for the schists near the contacts are locally feldspathic and have even in some places themselves been changed into augen gneiss much like the older metamorphosed granitic rocks. The schistosity has also been accentuated by contact metamorphism. These contactmetamorphic effects, however, are sporadic rather than universal and can not therefore be said to characterize the schist-granite contacts as a whole. As the Mesozoic intrusions must have affected the Paleozoic as well as the pre-Paleozoic rocks, it is altogether likely that contact metamorphism has been mistaken for regional metamorphism at some localities, resulting in the mapping of altered Paleozoic rocks as pre-Cambrian.

Veins of quartz in greater or less number are of course present in nearly all the geologic formations of this region, but the Birch Creek schist, because it is the oldest terrane, contains more vein quartz than any of the younger formations. The quartz is diverse in character, owing to differences in age and mode of formation. Much of it is a white vitreous quartz that ranges from tiny seams to veins several feet thick, but some, particularly in the smaller seams, is almost colorless and transparent. Little of the porous quartz with crystal outlines that is so characteristic of the gold lodes of the Fairbanks district has been found. It was the presence of so much quartz that led mining men as well as the earlier geologic workers in this region to believe that areas of Birch Creek schist were the most favorable localities for prospecting. Locally, to be sure, the quartz is mineralized with sulphides, chiefly pyrite, and also gold, but most of the quartz in the Birch Creek schist is barren. As it is now known that the granitic rocks are the ultimate sources of the gold, the Birch Creek schist can no longer be regarded as the mother lode, except in so far as it has been directly mineralized.

As the Birch Creek schist is the oldest terrane recognized in this region, its base has not been seen, and at present its upper limit is also indeterminate. These facts, together with the intensely complicated structure, make it impossible to hazard any exact estimate of the thickness. It suffices to state that at least several thousand and perhaps many thousand feet of strata are represented in this complex of metamorphic rocks.

AGE AND CORRELATION

The Birch Creek schist is here classified as pre-Cambrian in age. The oldest fossils found in this region by Spurr in 1896 were of

Devonian age, and as the Birch Creek schist antedates the rocks in which those fossils were found it was designated originally preDevonian. Subsequently, in 1909, Prindle 13 discovered Middle(?) Ordovician fossils in the White Mountains, in a limestone that is younger than the Birch Creek schist. A great thickness of rocks, however, lies between the known Ordovician beds and the Birch Creek schist, and Prindle suggested at that time the probability that the Birch Creek was pre-Cambrian. Blackwelder collected Lower Ordovician fossils from the White Mountain district in 1915, further strengthening this conviction. Meanwhile Cairnes, 14 in 1911 and 1912, had found Upper Cambrian fossils along the international boundary, and during the summer of 1925 and again in 1928 the writer collected Middle and Upper Cambrian fossils along the Yukon below Eagle. The Birch Creek schist is therefore now definitely known to be older than Middle Cambrian, and as two other formations that are not a part of the Birch Creek schist appear to lie between it and the Middle Cambrian beds, assignment of the Birch Creek schist to the pre-Cambrian seems now to be fully justified.

Some of the igneous rocks associated with the Birch Creek schist, notably the gneissoid rocks of granitic affinity, are definitely intrusive in character and may or may not have been formed prior to the Cambrian period. Evidence that at least some of the gneisses are of Paleozoic age was discovered by the writer 15 in the Chandalar district of northern Alaska in 1923. In that region the Paleozoic rocks up to and including the Silurian are metamorphosed to a greater or less degree, and in them the intrusive gneiss was found within a few feet of Silurian (?) fossils. This gneiss is believed to be of late Silurian or early Devonian age. Similarly, a chlorite schist believed to have been originally a lava flow of Silurian age was found adjoining the great middle Silurian limestone of this region. Doubtless, rocks of similar character are included with the Birch Creek schist as mapped and will some day have to be separated from it.

The Yukon-Tanana region is the type locality of the Birch Creek schist, for it is in this region that these rocks are best exposed and in this region only that their pre-Cambrian age has so far been demonstrated. There has been a tendency to correlate schistose rocks seen elsewhere in Alaska and Yukon Territory with the Birch Creek schist, merely on the ground that such rocks are greatly metamorphosed. Many such metamorphic rocks, however, are probably of Paleozoic age and have been altered to their present condi

13 Prindle, L. M., A geologic reconnaissance of the Fairbanks quadrangle, Alaska: U. S. Geol. Survey Bull. 525, pp. 38-39, 1913.

14 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, pp. 63-65, 1914.

15 Mertie, J. B., jr., Geology and gold placers of the Chandalar district: U. S. Geol. Survey Bull. 773, pp. 243-244, 1925.

tion either by intense local dynamic disturbances or by contact metamorphism. Age assignment on degree of metamorphism alone, without contributory evidence, leads directly to such fallacies. One of the most striking examples of relative differences in degree of metamorphism may be seen along the Tanana River near Hot Springs, where slightly metamorphosed pre-Silurian rocks may be seen on the east side of Baker Creek, and semischistose Lower Cretaceous rocks on the west side of the same creek. Contact metamorphism has been the cause of this anomaly.

Various names have been proposed to designate the metamorphic rocks of Alaska and Yukon Territory. Spurr 18 in 1896 proposed the name 66 Birch Creek series " for the oldest rocks and the name "Fortymile series " for an assemblage of metamorphic rocks which he considered to overlie the Birch Creek schist. This dual nomenclature, however, has not survived, and the term "Fortymile series " has been abandoned, the rocks comprising that so-called "series "being now assigned in part to the Birch Creek schist and in part to the Paleozoic.

Brooks,17 in 1898, used the name "Nasina series " to describe the schistose rocks of the lower White River and the term "Tanana schist" for the schistose rocks of the upper Tanana and differentiated a group of granitic gneisses which he believed to be older than either of these. In 1899 Brooks 18 used the term "Kotlo series " to designate the metamorphic rocks of the White River Basin and apparently included under this name both the "Nasina series" and the "Tanana schist." He indicated, however, that the "Kotlo series " probably included metamorphosed lower Paleozoic as well as preCambrian rocks. None of these three names have persisted in the United States Geological Survey nomenclature, but the beds to which they were applied include in part at least the Birch Creek schist, though they may be in part younger.

In Yukon Territory adjacent to Alaska McConnell 19 has divided the metamorphic rocks into two main groups-a lower and older series, called by him originally the Indian River series and later redesignated the Nasina series, a name earlier applied by Brooks 20 to other rocks in the lower valley of the White River; and an upper series called the Klondike series. The Nasina series of McConnell

18 Spurr, J. E., The geology of the Yukon gold district, Alaska: U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 140-155, 1898.

17 Brooks, A. H., A reconnaissance in the White and Tanana River Basins, Alaska, in 1898: U. S. Geol. Survey Twentieth Ann. Rept., pt. 7, pp. 460-470, 1900.

18 Brooks, A. H., A reconnaissance from Pyramid Harbor to Eagle City, Alaska, including a description of the copper deposits of the upper White and Tanana Rivers: U. S. Geol. Survey Twenty-first Ann. Rept., pt. 2, pp. 357-358, 1900.

McConnell, R. G., Report on the Klondike gold fields: Canada Geol. Survey Ann. Rept., vol. 14, pp. 10B-23B, 1905.

20 Brooks. A. H., A reconnaissance in the White and Tanaa River Basins, Alaska, in 1898: U. S. Geol. Survey Twentieth Ann. Rept., pt. 7, pp. 465-467, 1900.