abstracts: geology 565 



GEOLOGY. — Revision of the Beckwith and Bear River formations of 

 southeastern Idaho. G. R. Mansfield and P. V. Roundy. U. S. 

 Geological Survey Professional Paper 98-G. Pp. 9, with illus- 

 trations. 1916. 

 It has been found necessary to apply new names to strata hitherto 

 referred to the Beckwith and Bear River formations or to portions of 

 the Laramie as mapped by the Hayden Survey. The formations dis- 

 cussed extend from the northeastern part of the Montpelier quadrangle 

 northward through the eastern part of the Wayan quadrangle and 

 thence northward an undetermined distance, possibly including a con- 

 siderable part of the Caribou Range. They include about 17,000 feet 

 of strata, unless there are unrecognized repetitions by folding or fault- 

 ing. The strata in this area that were formerly called the Bear River 

 are here assigned to the Wayan formation, of Cretaceous, possibly 

 Lower Cretaceous, age, and the so-called Beckwith is divided into seven 

 formations, of which the lower two are marine formations of Jurassic 

 age, and the remaining five are nonmarine formations assigned to the 

 Ganett group, of Cretaceous (?) age. This paper gives a statement of 

 the stratigraphic problems involved and a description of the formations. 



R. W. S. 



GEOLOGY.— The Yukon-Koyukuk region, Alaska. H. M. Eakin. 

 U. S. Geological Survey Bulletin 631. Pp. 85, with maps, sec- 

 tions, and view. 1916. 



The Yukon-Koyukuk region lies in central Alaska and is drained by 

 the two rivers from which it takes its name. Most of its relief is low, 

 but locally there are northeasterly trending mountain ranges (5000- 

 6000 feet). The predominant type of topography consists of rolling, 

 maturely dissected uplands of moderate relief broken by extensive low- 

 lands. The timber is chiefly spruce and birch, and timber line is at an 

 altitude of about 2000 feet. Meadows of luxuriant growths of grass 

 break the timbered areas. 



The bed rock consists of metamorphic sediments (Paleozoic or older), 

 greenstones (Post Devonian?), and more extensive areas of conglom- 

 erates, sandstones and shales probably chiefly of Upper Cretaceous 

 age. Two epochs of granitic intrusion are recognized. The older 

 granites cut the metamorphic series and are more or less altered while 

 the j^ounger granite is intruded into the Cretaceous and is in most 

 places entirely massive. There is some auriferous mineralization along 

 the margins of the younger intrusive masses. The Quaternary is repre- 



