and Martin

made a reconnaissance of the Matanuska coal field in 1905. In the preparation of the following report, the work of these earlier investigators has been drawn on, especially that of Martin, who examined the coal resources of the lower Matanuska basin.

The field work of the season of 1906 was in charge of T. G. Gerdine, topographer, as chief of party. Landing was made at Knik, the head of steam navigation on Knik Arm of Cook Inlet, on May 29. Work was begun tiefe on June 1. During June the region on the south side of Kik Arm was mapped; T. G. Gerdine, accompanied by Adolph Knopf as geologist, ascended Knik River to Knik Glacier, and R. H. Sargent, assistant topographer, accompanied by Sidney Paige as geologist, covered the territory south of old Knik. At the end of June Mr. Gerdine, with Mr. Knopf as geologist and a total party of 6 men and 8 horses, started for the interior by way of Matanuska Valley, Hicks Creek, Divide Creek, Caribou Creek, Billy Creek, and Nelchina River. The party returned to Knik by skirting along the western edge of Copper River basin and traveling down Matanuska Valley, arriving at Knik August 31. No great difficulties were encountered, although topographic work was often seriously hampered by rain.

In the meantime R. H. Sargent, with Sidney Paige as geologist and a total party of 6 men and 8 horses, made the circuit through the interior of the Talkeetna Mountains, by way of Chickaloon Creek and Talkeetna River, and returned to Knik along the western flank of these mountains. Some trouble was encountered in fording the numerous streams, which were swollen from continuous heavy rains. During the later part of September the topographers ran a traverse of the eastern shore of Cook Inlet from Point Possession to the mouth of Kasilof River. Tustumena Lake and Kasilof River were also traversed by Mr. Gerdine. Work was closed at Seldovia on September 27. The geologists of the party had meanwhile started overland to Seward on Resurrection Bay. They visited the placer districts of the Turnagain Arm region en route and arrived at Seward September 27.

GEOGRAPHY.

The Talkeetna Mountains occupy the larger part of the region dealt with in this report (Pl. I, pocket). They form an approximately circular mountain mass, bounded on the west and north by the wide valley of Susitna River, on the east by the Copper River basin, and on the south by Matanuska Valley. In general the Talkeetna Mountains merge gradually into the surrounding lowlands, except on the south, where they rise abruptly from the deep depres

a Martin, G. C., A reconnaissance of the Matanuska coal field, Alaska, in 1905: Bull. U. S. Geol. Survey No. 289, 1906.

sion of Matanuska Valley. The interior of this mountain mass is an exceedingly rugged area, and access to it is possible only along the larger stream systems. Few of the peaks exceed 5,000 to 6,000 feet in altitude, but in the central portion some of the highest points reach altitudes of 8,000 to 9,000 feet.

The drainage of the Talkeetna area is rudely radial. The larger portion of it is tributary to Susitna River. In the southeastern portion of the area the headwaters of Susitna, Copper, and Matanuska rivers form a closely interlocking network of streams.

The Susitna is the largest river of the region. This great stream, which has a width of 2 miles near its mouth, rises in the northwestern part of the Copper River basin, and, after flowing westward around the northern flank of the Talkeetna Mountains, turns southward and empties into the northern end of Cook Inlet. The most important eastern tributary of the Susitna in the region under discussion is Talkeetna River, which heads at an altitude of about 5,000 feet in the heart of the Talkeetna Mountains. It flows northwestward for about half its length, then turns abruptly to the southwest and continues to Susitna River, which it enters about 85 miles above its mouth. The Talkeetna is about 75 miles long. Near its mouth it is a swift stream, 300 or 400 feet wide. During high water it can be crossed with a pack train only by swimming the horses and rafting the outfit. Sheep Creek is a tributary of Talkeetna River 14 miles above its mouth. It flows more slowly than the river and is, because of its depth, difficult to cross, so that rafting is necessary.

A considerable number of large streams, which, however, are smaller than the Talkeetna, drain the western flank of the Talkeetna Mountains and empty into Susitna River. They are, in order from north to south, Sunshine Creek, Montana Creek, Kashwitna River, Little Willow Creek, and Willow Creek. Little Susitna River, which is the next stream south of Willow Creek, belongs to the drainage basin of Matanuska River, but empties directly into Cook Inlet. East of the Talkeetna Mountains lies the broad expanse of the Copper River basin, extending to the Wrangell Mountains on the east and bounded by the Alaska Range on the north and the Chugach Mountains on the south. The surface relief of this area is slight and is produced mainly by long, low gravel ridges. Innumerable ponds and lakes are the most striking features of the landscape. The southwestern portion of the basin is drained by Matanuska River, which, with its headwater tributaries, has cut gorges 300 to 400 feet deep in the underlying gravel deposits.

Matanuska Valley, which lies between the Talkeetna and Chugach mountains, opens out at its head and merges into the flats of the Copper River basin. In its upper course it is about 1 mile wide, but

it gradually widens downstream to 5 to 10 miles, and below Tsadaka Creek flares out to a width of 20 miles. In minor details the floor of the valley is decidedly irregular in relief, consisting of a succession of undulating low ridges and knolls with interspersed kettles.

Matanuska River, the second largest stream of the region, drains into the eastern end of Knik Arm, instead of into Cook Inlet proper. It heads directly against Tazlina River, of the Copper River drainage, from which it is separated by a low, almost imperceptible divide, having an altitude of 3,000 feet. The general course of the Matanuska is S. 70° W., and its length is about 100 miles. The river flows in a trough or valley sunk into the floor of the broader valley previously described. The newer valley is a mile or two wide along the lower stretches of the stream, but in places closes to a narrow box canyon 300 or 400 feet deep. The Matanuska is a swiftly flowing, silt-laden stream, with a current reaching probably 7 miles an hour. The volume of water varies with the season and is highest in June. The average width of the river is about 400 feet.

Caribou, Hicks, Chickaloon, Kings, Granite, Eska, and Tsadaka creeks are the principal streams entering Matanuska River from the north. Caribou Creek, the largest of these streams, drains a considerable area in the Talkeetna Mountains. Chickaloon Creek has a length of about 30 miles. A pass at its head, over which a pack train can easily be taken, connects its valley with that of the Talkeetna. The two valleys thus form a highway through the heart of the Talkeetna Mountains. A large number of streams enter the Matanuska from the south, draining the north flank of the Chugach Mountains, but none attain large size.

Near the head of Knik Arm, uniting irregularly with the various tidal sloughs, the Matanuska is joined by Knik River from the southeast. Knik River heads in Knik Glacier, and its length is only about 25 miles. Several small tributaries enter the Knik from both sides, the largest and most important of which is Metal Creek, emptying into the river from the east near the front of the glacier. Knik Valley is 3 to 4 miles wide, perfectly flat, and covered with gravel, through which protrude occasional hummocks of bed rock. Its course is remarkably straight throughout its entire length and it is walled in by mountains which rise steeply from the valley floor. Knik River possesses no single well-defined channel, but spreads out over its gravel plain in a number of branches, whose positions are likely to vary from year to year. Boats can be towed up the river by hand, although with some difficulty on account of bars and quicksands, to within 5 miles of the glacier.

The rugged group of mountains lying on the south side of Matanuska Valley is a portion of the Chugach Mountains. As seen from Matanuska Valley, they include numerous peaks 7,000 to 10,000 feet

high and present a very inaccessible and forbidding aspect. The Chugach Mountains have a general northwesterly trend from Mount St. Elias, and constitute the high barrier between the Pacific coastal belt and the interior. At the head of Knik Arm they swing southward and continue, though with diminished altitudes, into Kenai Peninsula and Kodiak Island.

GEOLOGY.a

OUTLINE.

The rocks of the area investigated range in character from crystalline schists of probable pre-Silurian age to unconsolidated Quaternary stream and glacial gravels. A great central area of various granitic rocks, of middle Mesozoic age, makes up the mass of the Talkeetna Mountains. (See Pl. II, pocket.) On its southern flank this granite core is in part bounded by a narrow belt of albite and garnetiferous mica schists, which is in turn, on its southern border, bounded by a narrow band of granite similar in type to the central

mass.

On its northwestern margin it is bordered by a series of slates and graywacke-slates. The latter series shows some similarity to the great series of graywackes and slates which occur on the south side of Knik Arm and strike into the Chugach Mountains.

East of Chickaloon Creek marine Jurassic strata attain an extensive development. They are divided into two unconformable series. The lower series is characterized by a volume of andesitic breccias, agglomerates, and amygdaloids of greenstone habit, exceeding 1,000 feet in thickness. These volcanics are conformably overlain by several thousand feet of sandstones, shales, and conglomerates. The younger unconformable series of Jurassic rocks consists of shales, sandstones, and conglomerates, with some interstratified tuff and arkose. The age indicated ranges from the late Middle Jurassic through the Upper Jurassic. Conformably overlying these strata is a Lower Cretaceous limestone, 300 feet thick.

Upper Eocene rocks are represented by the Kenai formation, a series of folded sandstones, shales, arkoses, and conglomerates, carrying thick beds of bituminous coal. These coal beds are limited to the lower basin of Matanuska River.

A series of post-Eocene basaltic lavas and associated pyroclastics overlie the older rocks unconformably. They attain a thickness of 1,000 feet, and form the summit topography of much of the area. An extensive sheet of glacial gravels is the most recent deposit of the region.

For an abstract of this paper see Bull. Geol. Soc. America, vol. 18, pp 325-332.

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