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3,000 feet. This, it seems to the writer, is a minimum estimate and if subject at all to reconsideration should be increased rather than decreased. In view of the absence of detailed work done in this area it seems hardly worth while to hazard any other figure. It should be noted, however, that this estimate does not include the small area of rocks in the head of the Charley River to which Prindle assigned a thickness of 1,000 feet. From Prindle's figures, therefore, 4,000 feet may be given as a minimum thickness for the sequence of rocks here described as Upper Cretaceous and Eocene.
The proper age assignment for this group of rocks is a moot question. Martin,' who has made a comparative study of the Mesozoic rocks of Alaska, is inclined to favor the assignment of all these rocks to the Upper Cretaceous. Prindle,2 on the other hand, depending upon numerous determinations of plant fossils by F. H. Knowlton, has described these rocks, with the exception of the small area at the head of the Charley River, as Eocene. Perhaps the answer to the problem is the same as in the Ruby-Kuskokwim region, farther down the Yukon, where the writer collected Upper Cretaceous (?) invertebrates and Eocene (?) plants from the same slab of rock. The fossil remains may not be sufficiently diagnostic for reliable stratigraphic correlation; but it may also be that the Eocene in this region was not sharply separated from the Upper Cretaceous by the beginning of crustal movements and accompanying changes in conditions of sedimentation. Such a gradual transition from the Mesozoic to the Cenozoic might easily result in an equivocal intermingling of fossils.
In all, 20 collections of fossil plants have been made from this sequence of rocks, and many more could be gathered, for at certain localities the rocks are full of plant remains. Eight of these collections, which were found along the Yukon, are believed at present to be of Upper Cretaceous age. The other 12 collections, which were found at some distance south of the Yukon, have been assigned to the Eocene, but a number of genera and species are common to both groups. Arthur Hollick has recently studied these collections, and the results of his work when published may throw additional light upon this problem. Under the circumstances, and especially in view of the fact that the names of some of the species and even the genera
1 Martin, G. C., The Mesozoic stratigraphy of Alaska: U. S. Geol. Survey Bull. 776, pp. 387-390, 1926.
'Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska: U. S. Geol. Survey Bull. 538, pp. 30-31, 1913.
'Mertie, J. B., Jr., and Harrington, G. L., The Ruby-Kuskokwim region, Alaska: U. S. Geol. Survey Bull. 754, p. 40, 1924.
may later be changed, the writer feels that no good purpose will be served by publishing a mixed fossil list. As a matter of record, however, the localities from which these collections were made are listed here.
Upper Cretaceous (?) collections
2973. Yukon River, west bank 2 miles below Seventymile River. Collector, A. J. Collier.
2971. Yukon River, west bank 7 miles above Nation River. Collector, A. J. Collier. (This collection may have come from the Nation River formation.) 3243. Yukon River, west bank about 3 miles below Seventymile River. Collector, Arthur Hollick.
Kindle 11h. Yukon River 11⁄2 miles above Seventymile River. Collector, E. M. Kindle. (It is partly upon the evidence afforded by this collection and partly upon the lithology that the writer has referred the rocks at this locality to the Upper Cretaceous, rather than to the Nation River formation, to which Blackwelder suggested that they be referred.)
Kindle 20. Coal mine, Washington Creek, 16 miles from Yukon River. Coldector, E. M. Kindle.
7404. Yukon River, south bank at mouth of gulch 11⁄2 miles below Seventymile River. Collector, G. C. Martin.
7408. Yukon River, south bank 2 miles below mouth of Seventymile River. Collector, G. C. Martin.
6815. Yukon River, south bank at mouth of draw 11⁄2 miles below Seventymile River. Collector, G. C. Martin.
Eocene (?) collections
Spurr 3. Yukon River below Mission Creek. Collector, J. E. Spurr.
Collier 27. Coal Creek about 12 miles from Yukon River. Collector, A. J. Collier.
Collier 40. American Creek 100 yards below crossing of Eagle-Valdez trail. Collector, A. J. Collier.
3AP330. Wolf Creek, tributary of Seventymile River. Collector, L. M. Prindle.
3AP336. Branch of Wolf Creek, tributary of Seventymile River. Collector, L. M. Prindle.
3AP348, 3AP349, 3AP350. Bryant Creek, tributary of Seventymile River. Collector, L. M. Prindle.
3AP355. Mogul Creek, tributary of Seventymile River. Collector, L. M. Prindle.
3AP432. Mission Creek 2 miles above junction with Excelsior Creek. Collector, L. M. Prindle.
Atwood 10. Seventymile River half a mile below mouth of Mogul Creek. Collector, W. W. Atwood.
Atwood 11. Bryant Creek 3 miles above its mouth. Collector, W. W. Atwood.
Three other collections, which belong with these, were found in the Fortymile district, south of the area covered by this report; the first was determined as Cretaceous (?) in age; the other two as Eocene.
5AP178. Liberty Creek, tributary of O'Brien Creek. Collector, L. M. Prindle. 3AP2242. McDowell claim, Chicken Creek. Collector, L. M. Prindle. 3AP251. Chicken Creek. Collector, L. M. Prindle.
Unconsolidated sediments ranging in age from Pleistocene to Recent are present in all the stream valleys of this area, and the physiographic history of their deposition constitutes a geomorphologic problem of great magnitude, which has not yet been attacked in this area.
The Alaska Range, together with southern and southeastern Alaska, was intensely glaciated during Pleistocene time and probably during early Recent time. In fact, some of this territory has not yet really emerged from the glacial epoch, being still covered with great ice caps. The Brooks Range, which crosses northern Alaska, was also glaciated in Pleistocene time, though not nearly so severely or extensively as the Alaska Range, and it is now nearly free from glaciers. The great stretch of country lying between these two ranges, however, has not been glaciated except in certain groups of high mountains, where local valley glaciation of the alpine type has occurred.
While all this glaciation was taking place, both north and south of the Yukon Valley, climatic conditions of a peculiar type must have existed in the interior of Alaska. The mean annual temperature of this region, bordered on the north, east, and south by ice caps, must have been much lower than at present, and it is probable also that the annual precipitation was even less than at present. Rivers like the Tanana, which headed in large part in a glaciated area, handled and reworked great volumes of outwash material from the glaciers, which was subsequently redeposited to build up great alluvial plains like that in the present Tanana Valley, north of the Alaska Range. Streams that headed in essentially nonglaciated areas, however, like those between the Yukon and Tanana Rivers, went through a physiographic cycle which is as yet only imperfectly understood. Their deeply buried preglacial stream gravel, where uncovered by mining operations, is seen to be covered with great deposits of black muck, composed largely of silt and peaty material, with some beds of sand and layers of gravel. These are the sediments which were deposited during this physiographic cycle, and it was during the time of their deposition that numerous forms of preglacial life, such as the bison and mastodon, became extinct in this region, while other more adaptable animals and plants were modified to conform with the new environment. Where small areas were subjected to local glaciation, the emerging alpine glaciers built up a few morainal deposits. Two such morainal deposits in the southwest corner of the area covered by this report have been shown separately on the accompanying geologic map. All the other alluvial deposits, of both Pleistocene
and Recent age, have been grouped together as a single cartographic unit.
As the glaciers began to retreat, climatic conditions began to change and to approach more nearly those of the present time. Stream erosion and sedimentation of the normal type were renewed; and the formation of alluvial deposits of silt, sand, and gravel which began. then has continued to the present time. Evidence exists at numerous localities that this era was ushered in by a lowering of the base-level of the master stream of this region, which produced many changes in the disposition of preexisting drainage channels. Many interesting problems, including stream superposition, reversals of stream flow, "inlaid" gravel deposited on preexisting deposits of muck, terracing, and a multitude of kindred phenomena, are here represented and should sometime be studied in their physiographic and geomorphologic aspects.
Five mappable units of igneous rocks are shown on Plate 12. These are undifferentiated Paleozoic greenstone; Middle Devonian greenstone interbedded with sedimentary rocks; lower Mississippian greenstone interbedded with sedimentary rocks (Circle volcanics); Mesozoic and Tertiary granite, diorite, and related rocks; and Tertiary lava flows, mainly rhyolite, but including some dacite.
The pre-Cambrian igneous rocks associated with the Birch Creek schist are described in connection with that formation on pages 15-16 and are not separated from the Birch Creek on the map.
The Middle Devonian and Mississippian volcanic formations, although essentially basaltic, have been described on pages 77 and 85 in connection with the sedimentary rocks, with which they logically belong because of their bedded character. So far as the other igneous rocks are concerned, no additional work was done by the writer during the season of 1925. The petrology of these eruptive rocks was described in considerable detail by the writer in 1911. Several hundred thin sections were studied and described at that time, and all that seems necessary for the present report is a brief summary of that work.
UNDIFFERENTIATED GREENSTONE OF PALEOZOIC AGE
The principal areas mapped as undifferentiated greenstone are the Mount Sorenson massif, at the head of the Seventymile River, some
Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska, with a chapter on the igneous rocks by J. B. Mertie, jr.: U. S. Geol. Survey Bull. 538, pp. 37-48, 1913.
scattered areas farther east, and the greenstone that forms the bluff at Eagle and extends northwestward up the north side of American Creek. Certain small areas along the international boundary, mapped by Cairnes, are also included in this grouping.
The greenstone at Mount Sorenson is an ultrabasic rock of peridotitic character, which weathers reddish brown. In relatively unweathered specimens it is seen to be essentially a dunite consisting entirely of olivine, much of which is altered in part or wholly to serpentine. This mass of serpentine is undoubtedly intrusive. Similar serpentine is exposed on American Creek, forming the bedrock from the forks downstream for a mile or two. This serpentine evidently forms a narrow belt trending northeast, but it is included in the undifferentiated Paleozoic rocks because its extent in the direction of maximum elongation has not been determined. Other small areas of serpentine as well as greenstone of original basaltic character occur at a number of localities along the Seventymile between the Mount Sorenson massif and Mission Creek. The greenstone that forms the bold bluff at Eagle and continues northwestward up the north side of Mission Creek is essentially a basaltic greenstone, with which are interbedded flow breccia and tuff, also of greenstone habit, as well as more or less quartzite and crystalline limestone. This body of greenstone is mainly of effusive origin, though it may also contain some intrusive rock.
Greenstones are known at many horizons in the Paleozoic and prePaleozoic sequence of Alaska, and most of these rocks resemble one another greatly. Theoretically, the present grouping of undifferentiated greenstone includes all the Paleozoic basic and ultrabasic rocks of greenstone habit. The serpentine, however, which forms a large part of these rocks, is known to intrude the Silurian and Middle Devonian rocks of this region but has not been seen anywhere in rocks of later age. This does not definitely establish its age, for it may have been intruded at depths below the surface too great to reach the later Paleozoic rocks. But a stratigraphic discontinuity and perhaps angular unconformity of considerable magnitude is believed to separate the Middle Devonian rocks of this area from the overlying Carboniferous sequence, and with the idea in mind that such rock deformation is usually accompanied by volcanism, it seems possible that these ultrabasic rocks are Upper Devonian. The greenstone at Eagle may also be of Devonian age, but if so, it is more likely to be correlative with the Woodchopper volcanics and therefore to be late Middle Devonian. No data are avail