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Circle by way of Chatanika, and this road should be of great benefit to the Circle district, but no steps have yet been taken to connect the Fortymile district with the Tanana Valley.

CLIMATE

The climate of the Yukon-Tanana region as a whole is characterized by long, cold winters and short, relatively warm summers. The extremes of temperature are from 80° below zero in winter to 90° above zero or perhaps higher in summer, with an annual mean temperature of about 24°. At Eagle, according to the United States Weather Bureau,10 there are on the average 56 days during the year when the maximum temperature exceeds 70°, 255 days when the minimum temperature is less than 32°, and 120 days when the minimum temperature is less than zero.

The mean maximum temperature from May 15 to September 15 is about 65° and the mean minimum about 40°; the mean maximum from November 1 to April 1 is about 10° and the mean minimum for the same period about -15°. Commonly, the alluvial deposits are permanently frozen to great depths and thaw only a few feet at the top during the summer. A marked exception to this condition exists along the banks of the larger streams, where circulating ground water has in places thawed the ground for several hundred feet back from the river banks. In winter ice freezes on the lakes and quiet ponds to a depth of 5 feet or more. The permanently frozen ground is believed to be evidence of a previous geological epoch, in part Pleistocene, during which the regional climate was even more frigid than at present. This deep frost may, therefore, be regarded as an inorganic fossil record of a preexisting climatic condition.

The larger streams, such as the Yukon, usually begin to freeze over about the middle of October, and the ice breaks up about the middle of May. The smaller streams freeze earlier in the fall and open later in the spring. In the higher country killing frosts are rare in midsummer but begin in the middle of August and sometimes continue throughout May. In the lower country, as along the Yukon, the season free of frost is somewhat longer.

The average annual precipitation at Eagle, based on observations made over a period of 18 years, is 10.4 inches. The average winter snowfall, based on observations over a period of 12 years, is 51 inches. Without doubt, both rainfall and snowfall are somewhat greater in the mountains away from the Yukon, but the region as a whole is semiarid. On account of the frozen substrata, the circulation of

10 Summary of the climatological data for Alaska, by sections: U. S. Weather Bureau Bull. W, 2d ed., vol. 3, 1926.

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deeper ground water is restricted. A heavy surficial covering of sphagnum moss tends to limit the run-off and to hold much of the precipitation near the surface. This wet ground favors the growth of vegetation, so that the semiarid nature of the region is not at once apparent to the casual observer.

VEGETATION

The common trees are spruce, cottonwood, and white birch, of which spruce is the most plentiful. Black birch and tamarack are also found. Spruce trees a foot or more in diameter may be seen in the bottoms of the larger valleys, but on the ridges and spurs the trees are small, both in height and girth, though in places very densely spaced. Timber line is at about 2,500 feet on most of the ridges, but on limestone ridges and in the heads of the larger valleys it may be several hundred feet higher.

In addition to the trees, a variety of other vegetation thrives. Along the smaller streams grow dwarf willows and alders, which in some places are almost impenetrable, and on many of the spurs, just below and above timber line, the dwarf black birch forms a dense undergrowth. Many smaller plants and shrubs also abound. Partial lists of these plants have been made by different observers, usually in connection with some other work, such as geology, but no comprehensive study of the flora has yet been attempted. Several species of moss are very common, covering the ground in the lower valleys and extending well up on the ridges. Toward and above timber line the lichens become more prevalent. These constitute one of the sources of food for the herds of caribou that roam through the country, particularly in winter, when the brush and other flowering plants are dormant.

Grass for horses is abundant in most of the river valleys and on the lower spurs. On the upper slopes of many spurs a moderate quantity of bunch grass suitable for stock may be found. Along the river bars a horsetail rush grows in great abundance, and locally the pea vine; both of these, particularly the pea vine, are eaten by horses. Stock can therefore find pasturage during June, July, and August.

Many varieties of berries grow wild, of which the blueberry and low-bush cranberry are the most abundant and useful. Red raspberries, red and black currants, high-bush cranberries, and other berries are also found. At Eagle, Circle, and other places along the river gardens are planted, and all the hardy vegetables, including potatoes, turnips, cabbage, lettuce, rhubarb, beets, carrots, and radishes, are grown without difficulty.

ANIMAL LIFE

The larger animals include caribou, moose, bear, and sheep. Many thousands of caribou live in this region and form an important source of food for the white and native population. The caribou assemble in large bands in August and begin to migrate across country. One of the impressive sights of the region is the spectacle of such a passing band, whose transit sometimes takes several days. Moose also are fairly plentiful, but their numbers would be counted in hundreds rather than thousands. Bears also are fairly numerous, the black bear being more common. In the higher hills, however, the great brown grizzly bear, with the light-colored back, is seen. Sheep are found only in the highest mountain ranges and are relatively

scarce.

The fur-bearing animals include chiefly fox, lynx, marten, muskrat, squirrel, weasel, beaver, mink, land otter, wolf, and of late years coyote. Other animals, such as porcupines, rabbits, tree and ground squirrels, and mice, are also found.

The native game birds are ptarmigan and grouse, but in summer ducks, geese, and other waterfowl inhabit the country. Other birds include the loon, tern, gulls, owl, hawks, kingfisher, raven, swallows, sparrows, junco, thrushes, warblers, waxwing, jay, and shrike.

Grayling are found in nearly all the streams and trout in a few. Salmon run up the larger streams, and other fish, such as whitefish, pike, pickerel, and lake trout, are also present in the lakes and rivers.

DESCRIPTIVE GEOLOGY

PRE-CAMBRIAN ROCKS

BIRCH CREEK SCHIST

DISTRIBUTION

Proterozoic or pre-Cambrian rocks have so far been recognized definitely only in the southern half of the area covered by this report. The distribution of these rocks as shown on the accompanying geologic map (pl. 12) should be regarded only as a provisional mapping that may be materially changed by later work. One reason for this uncertainty is the presence near by of metamorphosed lower Paleozoic rocks, which in reconnaissance work are difficult to distinguish from the pre-Cambrian rocks. It is possible and even probable that the band of rocks mapped as Birch Creek schist around the Glacier Mountain massif, as well as the bands on the Seventymile River and on the Middle Fork of the Fortymile River, may later be found to be Paleozoic rocks that have suffered contact metamorphism as a

result of their proximity to the great mass of granitic rocks which they border. The rocks on the South Fork of Birch Creek and thence westward, however, are almost surely of pre-Cambrian age.

LITHOLOGY

The name Birch Creek schist, as used in this report, is a designation for all the definitely pre-Cambrian sedimentary of rocks of this region. The term was introduced originally by Spurr 11 in 1898 as Birch Creek "series," to characterize the oldest rocks of sedimentary origin in the Birch Creek and Fortymile districts, although Spurr stated that "there are also found, although rather sparingly, schists of igneous origin, being dikes which have intruded into the sedimentary series previous to the shearing." As the name is used at present, however, the igneous rocks, although not separated from the sedimentary rocks on the map, are not considered a part of the Birch Creek schist. It is expected that the mass will eventually be subdivided, when more detailed work is done. Even at present the more distinct lithologic units are recognized, but no attempt has yet been made to delimit these units on a geologic map. It is possible that the Tindir group may prove to be of pre-Cambrian age, and in that event the Birch Creek schist would represent only the basal part of the pre-Cambrian sequence.

Most of the rocks of the Birch Creek schist are either schistose or gneissoid. They include quartzite, quartzite schist, quartz-mica schist, mica schist, graphitic schist, crystalline limestone, and calcareous schist. The associated metamorphic igneous rocks include granitic and dioritic gneiss, amphibolite, hornblende schist, and a certain proportion of sericite and chlorite schists. Nearly all these rocks are recrystallized, but in some of them traces of the original sedimentary or igneous fabric can still be seen.

Among the schistose rocks quartzite schist is perhaps the most common, followed closely by quartz-mica schist, quartzite, and mica schist. The quartzite schist and quartzite occur for the most part in beds 1 foot to several feet thick and weather into blocky talus piles. Plate 3, A, shows a typical exposure of quartzite in the Birch Creek schist. In many places these quartz-rich rocks are covered by a black lichen, which gives the outcrops a dark, forbidding appearance from a distance. Both muscovite and biotite occur with the quartz in these rocks, but the biotite is perhaps the more common. presence of these micas affords one criterion for distinguishing these pre-Cambrian rocks from the metamorphosed lower Paleozoic rocks, for the latter, even where schistose, are likely to contain a larger

The

11 Spurr, J. E., The geology of the Yukon gold district: U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 140-145, 1898.

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