A series of mica schists forms a narrow belt, not over 3 miles wide, near the head of Willow Creek. This belt has an east-and-west direction, and is known to extend not farther east than Tsadaka Creek and to disappear westward beneath the gravel floor of Susitna Valley.

The schists are thoroughly foliated rocks of medium grain, and show in general no variations in appearance. Under the microscope they are found to comprise garnetiferous mica schists and chloritealbite schists. The garnetiferous mica schist is composed largely of quartz, which shows powerful strain shadows, muscovite in long laths, chloritized biotite, orthoclase, and garnet altered almost completely to chlorite. In the albite schist soda feldspar is the dominant constituent. The albite poikilitically incloses various other constituents, chief among which is clinozoisite in long, stout prisms, usually oriented parallel to the schistosity. In addition to the albite abundant quartz, crushed and showing strain shadows, is present in considerable amount, with some chlorite and clouds of magnetite dust. Muscovite is but an accidental constituent, and is only found included in the albite plates. Some of the albite schists, however, contain muscovite as an important constituent. Such a schist, ex

amined microscopically, shows large albites in the form of augen, quartz badly crushed and strained, muscovite of a sericitic nature, and chlorite, and is evidently a schist affected by a second period of dynamic activity.

Some serpentine and pyroxenite are included in the mica-schist series. The pyroxenite presents a pseudoporphyritic aspect under the microscope, due to crushed plates of monoclinic pyroxene lying embedded in a groundmass of serpentine. The pyroxene is noteworthy on account of the fine multiple twinning parallel to the orthopinacoid and of the slicing which has taken place along that plane. The shearing of the pyroxenite is probably to be correlated with the movement which crushed the schists.

The strike of the schistosity varies from N. 45° E. to N. 60° W. The dips average near 40°, but are inclined on both sides of the vertical. Fossils have not been obtained from this formation, nor is their occurrence in it likely. On account of its thoroughly metamorphic character, which considerably exceeds that of any of the terranes presently to be described, this formation is regarded as representing the oldest rocks of the region. Its members show a petrographic similarity to the mica schists of the Yukon-Tanana region, and a possible correlation with the pre-Silurian of that district is suggested.

UNDIFFERENTIATED METAMORPHICS.

Metamorphic rocks, which, because of lack of fossil and other stratigraphic criteria, can not be placed definitely in the stratigraphic column, occur in the lower portion of Talkeetna Valley-in the extreme northwestern part of the region traversed-and in the Knik River region-in the southern portion.

SLATES AND SCHISTS OF THE LOWER TALKEETNA VALLEY.

A series of metamorphic rocks outcropping along the upper course of Susitna River and the headwaters of the Cantwell was named the Susitna slate by Eldridge, who describes the beds as

essentially quartzitic, varying in coarseness of material from an extremely fine homogeneous rock to one of granular structure. In addition to quartz there are occasional orthoclase, plagioclase, biotite, muscovite, scattered grains of iron oxide, and minute fragments of slate, apparently of the same nature as the fine-grained slates of the series itself. The entire series has been extensively sheared and the sand grains crushed, so as to produce the partial schistose or slaty structure that so generally prevails. Whitman Cross, of the Survey, who examined the rock microscopically, found no grains referable to igneous rocks, in spite of the impression from megascopical examination that the rocks were sheared eruptives.

A feature of the formation in localities where great crumpling of the strata has taken place is the presence of large quartz seams, half an inch to 2 feet thick, reticulating the exposed surface of the series with considerable intricacy. Such seams often show mineralization with sulphide of iron.o

The work of the season of 1906 showed that a series of rocks similar to the Susitna slate and schist has a considerable areal distribution along the lower course of Talkeetna River. In that region they show somewhat greater petrographic diversity than where studied by Eldridge. With the slates are associated various microcrystalline schists; that is, schists that are so fine-grained that their constituents are not recognizable by the unaided eye. Some are of aphanitic texture and display only a feeble schistosity. Examined microscopically, they prove to be composed largely of light-green hornblende in slender prismatic development. Quartz, chlorite, epidote, and biotite are the remaining constituents. Other schists, whose foliation is more apparent to the eye, consist of fine-grained aggregates of quartz and biotite, with much included carbonaceous material. In general the metamorphism of these schists has proceeded far enough to obliterate the evidence of their original clastic origin. In the lower Talkeetna River region they are associated with batholithic masses of granitic rock and locally are intimately intruded by apophyses of such masses. Much of their metamorphism may be imputed to this relation. The Susitna rocks have yielded no fossils nor are their stratigraphic relations known, so that their age must remain conjectural. The unmetamorphosed character of the Jurassic shales and sandstones of the Talkeetna Mountains indicates that the Susitna rocks are probably pre-Jurassic. The Susitna slates and schists show some lithologic resemblance to the rocks of the Sunrise "series" of the Turnagain Arm region, but appear for the most part to be somewhat more highly metamorphosed.

The microcrystalline schists are interstratified with the clay slates and, especially those containing notable amounts of hornblende, appear to represent metamorphosed bands of impure dolomites. Similar rocks are not known from the Sunrise "series." On account of these differences, which on the whole appear to outweigh the similarities, the Susitna slate is placed beneath the Sunrise formation in the geologic column.

GRAYWACKES AND SLATES OF THE KNIK RIVER REGION.

DISTRIBUTION AND CHARACTER.

The rocks on the south side of Knik Arm and in the valley of Knik River consist largely of graywackes, slates, and phyllites, with

◄ Eldridge, G. H., Reconnaissance in Susitna basin and adjacent territory, Alaska, in 1898: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, 1900, p. 15.

some greenstones, rhyolites, and tuffs. The general strike of the series (N. 60° E.) tends to carry them eastward approximately parallel to the course of Matanuska River. Their distribution to the south is indicated by the fact that the moraine of the great glacier at the head of Knik Valley is composed exclusively of graywacke and slate. As the same rocks are reported by Mendenhall from along the shores at Port Wells on Prince William Sound, it is probable that this section across the Chugach Mountains consists largely of graywackes and slates.

The graywackes are highly indurated rocks, whose most conspicuous feature to the unaided eye is the unusual abundance of small angular slate fragments. They are of dark bluish-gray color and at some places are not easily distinguishable from basic volcanics. Under the microscope the graywackes prove to be of rather heterogeneous derivation. Angular fragments of quartz, containing capillary rutile, fragments of feldspar, plagioclase in part, usually altered and occasionally distinctly rounded, hornblende, augite, and fragments of slate comprise the main constituents. A little muscovite, epidote, pyrite, magnetite, and zircon are present as accessories. The binding material is a feebly polarizing aggregate flecked with chlorite. Other thin sections of the graywackes show, in addition to the slate particles, fragments of quartzite, of micropegmatite, and of a chloritized volcanic, probably an andesite. The graywackes are roughly schistose, and pass by increasing fineness of grain into graywacke slates and into highly cleaved clay slates. The slates locally assume a schistose foliation, become glossy, and pass into phyllites. At Knik Glacier they are affected by slip-strain cleavage, and are acutely crumpled and contorted. The interbedded graywacke, however, has been rendered only roughly schistose. Quartz stringers are very abundant in the slates and graywackes of the Sunrise "series," and at places form an intricate network of small veins, averaging about an inch in thickness. Mineralization of the quartz is not visible to the eye. In the Sunrise district, however, free gold is found at some places in these stringers.

Near the mouth of Knik River a great volume of greenstone tuffs, estimated as probably 1,500 feet thick, was found associated with the graywackes. They are dense, fine-grained rocks of green color, conspicuous on account of the numerous angular fragments of black slate which they inclose. The microscope shows that the rock is largely composed of opaque angular fragments of a greenish-yellow color. In places such fragments contain phenocrysts of highly altered feldspar and porphyritic augite in fine idiomorphic development. The remaining constituents of the tuff comprise fragments of brown hornblende, augite, graywacke, intersertal basalt (much altered), and

quartz diorite. Chlorite and calcite form a scanty binding material, and a little leucoxene occurs as an accessory mineral.

The massive greenstones found in the Sunrise "series" are, as a rule, nonporphyritic rocks of aphanitic texture. Secondary minerals, such as light-green hornblende, chlorite, and epidote, are strongly developed, and the feldspars are represented by aggregates of decomposition products. The former structure is thus often obscured, but remnants of ophitic or closely allied textures are sufficiently well preserved to leave no doubt as to the original character of the rock.

Rhyolites and their tuffs are associated with slates at a locality half way up Knik River. The acidic volcanics are of felsitic habit and include flow-banded types and porphyritic phases with inconspicuous quartz phenocrysts. A rude schistosity has been impressed on them, and in consequence they show a faint unctuous appearance on their foliation planes. In thin section a strongly marked eutaxitic structure is displayed, which is further emphasized by the fact that certain flow bands show a more advanced devitrification than others. Porphyritic constituents are not numerous. Of these, quartzes are most abundant. They exhibit in places some magmatic corrosion, but are usually broken and dispersed. A few orthoclase phenocrysts are present, crushed and largely converted to sericite. Epidote, replacing original femic phenocrysts, is a rare constituent. The groundmass is microcryptocrystalline and very feebly polarizing. Occasionally certain flow bands are noteworthy on account of the abundance of very minute flakes of sericite.

The tuffs accompanying the rhyolites, like the greenstone tuffs, are remarkable for the abundance of small, angular slate fragments which they include. Microscopic study indicates that the rhyolite tuffs grade into graywackes. A transitional type is represented by a rock in which glassy quartzes and white feldspars are studded in porphyritic fashion in a dense greenish matrix. In thin section about half of the slide is composed of numerous fragments of microcryptocrystalline volcanics showing fluxion phenomena. They are sparingly porphyritic, and in one section were found to contain dihexahedral quartz. These, as it were, constitute a groundmass for angular fragments of quartz and plagioclase. The microclites commonly referred to rutile are unusually well shown in the quartz fragments, as is also a little zircon. The feldspar occasionally shows rounded edges, and is usually in an advanced state of alteration. Some partially chloritized augite and a little hornblende are also present. Slate particles are numerous, and show molding around the other constituents in consequence of the shearing to which the rock has been subjected. Chloritic matter forms the binding material of the rock.

The general petrography of the rocks of the Knik region proves that the graywackes were derived largely from the destruction of a