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PLATE 10. A, Anticline and syncline in Tahkandit limestone (Permian) along
11. A, Infaulted block of Tahkandit limestone (Permian) in midst
12. Geologic map of the Eagle-Circle district----
GEOLOGY OF THE EAGLE-CIRCLE DISTRICT,
By J. B. MERTIE, Jr.
LOCATION OF THE AREA
The district here designated the Eagle-Circle district lies between meridians 140° 45′ and 144° 15′ west longitude and parallels 64° 30′ and 66° north latitude and comprises a nearly square area of about 10,000 square miles. (See fig. 1.) The Yukon River runs diagonally across the area from the southeast corner to the northwest corner. This area is geographically the connecting link between the Fortymile mining district, to the southeast, and the Circle mining district, to the northwest. It embraces the Seventymile district and the American Creek, Fourth of July Creek, and Woodchopper Creek precincts.
This region has interested prospectors and geologists for many years. Gold placer mining was begun in the Fortymile district in 1887 and in the Circle district in 1894, and mining operations have continued in these two districts up to the present day. In view of their productiveness it was but natural that the attention of prospectors should be directed to the intervening area south of the Yukon. A great deal of prospecting has been done, and in the aggregate a considerable amount of gold has been recovered in the last 25 or 30 years, although no large mining camps have so far been established. The Geological Survey, also, has been greatly interested in this intervening area because of its potential importance to the mining industry and its unique geologic features.
The area considered in the present report may logically be divided into two parts; one, which comprises some 40 per cent of the total, lies northeast of the Yukon, and the other lies southwest of the Yukon. The northeastern part is unmapped, both topographically and geologically, except for the work of the international boundary survey along the international boundary and the observations of
to prospectors and miners the obvious benefits of good topographic and geologic maps and to correlate the facts of mining and geology is concentrated in the environs of mining districts, in order to give Much of the initial work of the Geological Survey in a new region
fifths, because of its potential economic significance, has been mapped almost completely, both topographically and geologically, on the workers who have traversed the Yukon. The southwestern three
reconnaissance scale of 1:250,000.
FIGURE 1.-Index map showing location of area covered by this report
for a better understanding of the processes of mineralization. This procedure, of course, is most desirable; but geologically it is often disappointing, because regions in which intensive mineralization has occurred are likely to be regions of more than ordinarily complex geology. Under such circumstances the geologist undertakes to decipher the geologic record under the least favorable conditions, and instead of working from the simple to the complex he is obliged to work in the reverse order. The area here considered is an excellent example of this condition. The southwestern part is a region composed largely of metamorphic rocks, and although a considerable amount of geologic work has been done there, the total results are somewhat disappointing. The northeastern part contains geologic formations of the same general age as those south of the river, in a relatively unmetamorphosed condition, but it is practically an unknown land so far as the geologist is concerned. The answers to numerous unsettled geologic problems mentioned in this paper will ultimately be found through study of the hitherto neglected territory north of the Yukon.
The earliest geologic notes on this stretch of the Yukon, from Circle to the international boundary, were made in 1888 by McConnell, in the course of an exploratory trip from the Mackenzie River across to the Porcupine, down that stream to Fort Yukon, and up the Yukon to the confluence of the Pelly and Lewes Rivers. Ogilvie, also in 1888, made a traverse up the Tatonduk River and over to the Porcupine, taking some geologic notes upon the rocks in the Tatonduk Valley. In 1889 Russell3 made a rapid trip up the Yukon River from St. Michael to the headwaters. His observations, however, were more concerned with the surficial features of the region than with the hard-rock geology.
Systematic geologic work in this region was first attempted in 1896, when a Geological Survey party under the leadership of J. E. Spurr traversed the Yukon from its headwaters as far downstream as Nulato, visiting on the way the mining camps contiguous to the river. The report embodying the results of this expedition is of great interest in so far as it relates to the history of exploration and early mining activities, and many of the geologic observations also are useful. Three of Spurr's formation names, Birch Creek "series," Rampart "series," and Tahkandit "series," have been redefined to
1 McConnell, R. G., Report on an exploration in the Yukon and Mackenzie Basins, Northwest Territory: Canada Geol. Survey Ann. Rept., new ser., vol. 74, pp. 134–139D, 1890.
2 Ogilvie, William, Exploratory survey of part of the Lewes, Tatonduk, Porcupine, Trout, Peel, and Mackenzie Rivers, Canada Interior Dept., 1890.
Russell, I. C., Notes on the surface geology of Alaska: Geol. Soc. America Bull., vol. 1, pp. 99-156, 1890.
* Spurr, J. E., Geology of the Yukon gold district, Alaska: U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 87-392, 1898.