abstracts: petrology 239 



the Triassic (?) are rocks of Permian (?) and Pennsylvanian age, which 

 do not outcrop in the field but are exposed on the slope of Wind River 

 Mountains. They are entered by some of the oil wells and are believed 

 to be the source of most, if not all, of the oil. 



The Triassic (?) is represented by the Chugwater formation, the 

 Jurassic by the Sundance and Morrison formations, and the Upper 

 Cretaceous by the Dakota sandstone, Mancos shale, and the Mesaverde 

 formation. An unconformity separates the Wind River formation 

 (Eocene) from the Upper Cretaceous rocks. The formations, including 

 from the Chugwater to the top of the Upper Cretaceous, aggregates 

 nearly 9500 feet in thickness. 



The structural province in which this field lies comprises very broad 

 synclinal basins bordered by relatively high narrow mountain ranges 

 with small steep-sided, slightly elevated anticlines that are exposed about 

 the edges of the basins in a zone between the basins proper and the moun- 

 tains. If the strata in the interior of the basins are folded, the folds 

 are concealed beneath beds which were deposited subsequent to the 

 major structural movements in the province. 



The Salt Creek oil field lies in the northeast corner of Natrona County, 

 Wyoming, and in the drainage basin of Powder River. Topographically 

 the region includes broad flat-topped interstream areas, separated by 

 broad flat valleys. Scarps representing the outcropping edges of hard 

 strata are plentiful. The geologic sequence includes about 6,800 feet 

 of Upper Cretaceous strata, divided into 13 formations. In addition 

 to these, there are the Morrison formation of Jurassic age (?), the Lance 

 formation, which is Tertiary or Cretaceous, and the Fort Union, which 

 is Eocene. 



A broad anticlinal arch, more or less faulted, crosses the field in a 

 northwest-southeast direction, the limits of which dip at angles of from 

 8° to 29°. The steepest dips are found on the west side of the arch. 

 Within this anticline are two domes. To the west of this arch is a 

 second anticline of similar character. The oil occurs at or near the 

 crest of the anticline. A. H. Brooks. 



PETROLOGY. — Geology of the pegmatites and associated rocks of Maine, 

 including feldspar, quartz, mica, and gem deposits. Edson S. Bas- 

 tin. Bulletin U. S. Geological Survey No. 445. Pp. 152. Illus- 

 trated. 1911. 

 The commercially important pegmatite deposits are principally 

 in the southern and western parts of Maine. Most of the pegmatites 

 are distinctly intrusive into the surrounding schists and granites. They 



