ABSTRACTS 



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 this issue. 



GEOLOGY— TAe Yentna District, Alaska. S. R. Capps. Bulletin U. 

 S. Geological Survey No. 534, pp. 72, with maps, sections, and views. 

 1913. 



The geological formations that outcrop are: (1) a thick slate-gray- 

 wacke series of undetermined age, but pre-Tertiary; (2) intrusive gran- 

 ites and diorites, probably of Jurassic age; (3) Eocene clays and sands 

 with some lignite; (4) Tertiary gravels; (5) glacial moraines and out- 

 wash gravels; (6) recent stream deposits. 



The gravel series which overlies the Eocene beds was found to be 

 structurally conformable upon the Eocene, and to antedate by a con- 

 siderable time interval the period of maximum glaciation. These grav- 

 els have hitherto been thought to be Pleistocene. 



The placer gold of Cache Creek and the neighboring creeks is thought 

 to have been derived from quartz veins in the slate-graywacke series. 

 Its present distribution has been largely influenced by glacial erosion, 

 the present placers being found only in those places where ice erosion 

 was feeble, or where post glacial erosion has effected a reconcentration 

 of the glacially scattered gold. On Twin Creek and its tributaries the 

 placer gold is the product of post glacial concentration of gold from 

 the Tertiary gravels. S. R. C. 



GEOLOGY. — Bismarck, N. Dak., folio. A. G. Leonard. Geologic 

 Atlas of the United States, No. 181. U. S. Geological Survey in 

 cooperation with North Dakota Geological Survey. 1912. 

 The formations represented range in age from Cretaceous to Recent. 

 The Fox Hills sandstone (marine), the lowest of the formations, is ex- 

 posed in the lower parts, of the bluffs of Missouri River in the southern 

 part of the area and in some tributary valleys. The base is not ex- 



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