abstracts: geology 49 



primarily of sedimentary origin, and accordingly they differ from most 

 of the other manganese deposits known in this general region, which 

 are related to metalliferous quartz veins. 



On Wigwam and Cherry creeks irregular lenslike bodies of man- 

 ganese oxides, chiefly psilomelane and subordinately manganite and 

 wad, are found in an ancient marbleized limestone that underlies the 

 Cambrian Flathead quartzite. The general features of these deposits 

 clearly show that the ores were deposited in solution cavities and have 

 also made room for themselves by replacing the limestone. Their com- 

 position and structure suggest that they were deposited by surface 

 waters circulating through joints or other openings in the limestone, 

 probably during the Tertiary period. Under these conditions the only 

 apparent source of the manganese is the limestone itself, which contains 

 small proportions of manganese and iron oxides. 



Iron ore and manganiferous iron ore are exposed about 3 miles south- 

 west of Renova. The localization of the deposits at the top of shales 

 overlying the Flathead quartzite and their close association with an 

 old stuface of erosion suggest that they are portions of an iron-bearing 

 stratum enriched by weathering when the land was being worn down 

 to the level of the old surface and therefore are to be regarded as pri- 

 marily of sedimentary origin. If this is the true explanation, 

 similar ore bodies are to be expected at the same geologic horizons in 

 the neighboring districts. R. W. Stone. 



GEOIyOGY. — The structure and stratigraphy of Gravina and Revillagigedo 

 islands, Alaska. Theodore Chapin. U. S. Geol. Survey Prof. 

 Paper 120--D. Pp. 83-100. 1918. 



The discovery of fossils of Jurassic or Cretaceous age has made 

 possible some changes in the geologic mapping of Gravina Island. 

 This paper is a supplemental report on the region and discusses the dis- 

 tribution, character, structure, stratigraphic relations, age, and correla- 

 tion of rocks ranging from .Devonian to Quaternary. R. W. Stone. 



GEOLOGY. — The Nesson anticline, Williams County, North Dakota. 

 A. J. Collier. U. S. Geol. Survey Bull. 691-G. Pp. 21 1-22 1, 

 with map and section. 1918. 



The Nesson anticline, discovered by a U. S. Geological Siurvey party 

 in 1 91 7, is a fairly well marked arch or dome about 30 miles east of 



