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of the group was modified in the area northeast of Eagle. So far as known, these rocks are all of fresh-water origin and represent conditions of sedimentation not unlike those that existed at the time the Nation River formation was laid down. In fact, these rocks are so similar lithologically to the rocks of the Nation River formation that it is difficult to differentiate between them at the localities where the two formations adjoin.

Where seen along the Yukon, the Upper Cretaceous and Eocene rocks consist of impure greenish-gray to almost black sandstone, graywacke, sandy shale, and beds ranging from grit to coarse conglomerate. A short distance above the international boundary, in the hills north of the Yukon, the conglomerate consists of pebbles from a quarter of an inch to 3 inches in diameter, in a brownish matrix. The pebbles here were mainly of vein quartz and chert, with some quartzite, quartzite schist, graphitic phyllite, and decomposed granitic or dioritic material. This is about the average character of the conglomerate. At some places, however, the conglomerate is coarser, and boulders as large as 3 feet in diameter have been observed. At most localities all these rocks are loosely consolidated and therefore by weathering form on top of the ridges gravel deposits that simulate high bench gravel. At places, however, as for example in the valley of the Seventymile River, these rocks have been more than ordinarily metamorphosed and occur as hard, well-indurated sandstone and conglomerate.

A small area of coarse conglomerate and sandstone is shown at the head of the Charley River. These rocks consist of gray more or less carbonaceous sandstone, interbedded with very coarse conglomerate containing boulders as much as 6 feet in diameter. Like most of the strata of this series, they carry plant remains.

The sequence of rocks in this series, as interpreted by Prindle," comprises shale and sandstone with beds of lignite at the base of the formation, overlain by sandstone, shale, and conglomerate, with thick beds of conglomerate at or near the top.

The upper or conglomeratic beds appear to be the proximate though not the ultimate source of a part of the gold in the placers of the Seventymile River and of Fourth of July, Coal, Woodchopper, and other creeks along the south side of the Yukon between Eagle and Circle. This statement, which was made originally by the writer 96 in 1923, in discussing the origin of the gold deposits of interior Alaska, has not been clearly understood by some of the mining men of this district, who apparently have been led to believe

Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska: U. S. Geol. Survey Bull. 538, pp. 32-34, 1913.

9 Mertie, J. B., jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regions: U. S. Geol. Survey Bull. 739, p. 164, 1923.

that all the creeks which cut this formation ought therefore to have placer gold in the present stream valleys, derived from these conglomerates by recent erosion and stream sedimentation. This belief, of course, is incorrect. As examples may be cited Fourth of July and Crowley Creeks, south of Nation; the former contains workable gold placers, but the latter contains little or no gold in its stream gravel.

Two considerations must be borne in mind-first, no reason exists for believing that gold is universally distributed in those conglomerates, for here as elsewhere only certain streams or even certain parts of some particular stream draining from a mineralized area contain auriferous gravel; second, these rocks have been highly folded at many places, particularly in this zone south of the Yukon, with the result that even if the conglomerate beds had originally carried equally distributed deposits of gold, gold would no longer be equally distributed areally. The gold in this mineralized belt from Eagle to Circle came originally from mineralized rocks at the border of the granitic rocks and also, in part, from the granitic rocks themselves within the drainage basins of the streams, and it has continued to accumulate as stream placers ever since these mineralized rocks were exposed to surficial erosion. The indurated conglomerates are but one stage in this process. Gold placers may therefore exist in any of these streams within whose drainage basins occur bodies of mineralized rock, but the chances for the formation of gold placers may be said to be appreciably enhanced in the areas where these conglomerates now occur, for in such areas both the original mineralized rocks and the fossil placers of the conglomerate are available as sources of gold.

STRUCTURE AND THICKNESS

No studies have yet been made that are sufficiently detailed to warrant a structural section across this belt of rocks. The rocks are known, however, to be everywhere folded and at some localities intensely deformed. The structure of the Upper Cretaceous and Eocene rocks south of the Yukon is believed in general to be much more complex than that of the Lower Cretaceous rocks north of the Yukon, notwithstanding the fact that the Lower Cretaceous rocks are older and probably were subjected to deformational processes before the Upper Cretaceous and Eocene rocks were laid down. This apparent anomaly is due in reality to the closer proximity of the Upper Cretaceous rocks to the seat of volcanism farther south. The same conditions were observed in the Paleozoic rocks, those south of the Yukon being much more metamorphosed than rocks of similar age north of the Yukon. This interpretation of the structure of the Upper Cretaceous rocks involves axiomatically the idea that vol

canism such as produced the great Mesozoic batholith to the south and led to the deformation of the late Paleozoic and Triassic rocks was renewed in Tertiary time. Evidence of Tertiary volcanism in southwestern Alaska has already been presented by the writer; 97 and although no stratigraphic evidence of invasion of Cretaceous rocks by granitic bodies has been recorded in the Yukon-Tanana region, the evidence from southwestern Alaska, based on the types of metallization, has been extrapolated by the writer 9s into the Yukon-Tanana region to prove that Tertiary intrusions of granitic magmas occurred in that region, particularly in the Tolovana and Rampart districts. Hence the idea of a period of deformation in Tertiary as well as in Mesozoic time is not at variance with present interpretations.

The evidence of the intense deformation of the Upper Cretaceous and Eocene rocks is cogent at numerous localities, particularly along the Seventymile River, where these rocks lie nearest to the volcanic rocks to the south. On Barney Creek, a tributary of the Seventymile, the conglomerate beds stand nearly vertical at places, and even farther north, in the valley of Fourth of July Creek, they are nearly vertical at one exposure. Along the south side of the Seventymile River below the falls, where these rocks strike about N. 55° W., they dip consistently south at angles from 30° to 70° and are highly indurated. They consist here largely of sandstone and conglomerate, with some lignitic beds, but farther up Crooked Creek from the Seventymile the bedrock changes by degrees to soft shale and sandstone. This sequence indicates, if Prindle's interpretation of the order of deposition is correct, that these beds along the Seventymile River, though highly deformed, are at least right side up, the conglomerate resting normally upon the stratigraphically underlying shale and sandstone.

The beds of heavy conglomerate, however, are apparently duplicated by folding and crop out repeatedly in the area between the Seventymile and Yukon Rivers. This duplication of beds by folding and perhaps also by faulting unquestionably explains the great distance across the strike of these rocks from the falls of the Seventymile to the Yukon. It is therefore entirely possible that no great thickness of rocks exists in this belt, even though the greatest distance across the strike is nearly 15 miles. On the other hand, all of the sequence may not be present in that portion of the belt where the distance across the strike is the least. Prindle 99 estimated the thickness of the sequence between the Seventymile and Yukon Rivers at

7 Mertie, J. B., jr., and Harrington, G. L., The Ruby-Kuskokwim region, Alaska: U. s. Geol. Survey Bull. 754, pp. 71-72, 1924.

98 Mertie, J. B., Jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regions: U. S. Geol. Survey Bull. 739, pp. 156-158, 1923.

Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska: U. $. Geol. Survey Bull. 538, p. 33, 1913.

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.

AGE

The proper age assignment for this group of rocks is a moot question. Martin,1 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, 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. Collector, 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.

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