Art. VI. — Jmra-Trias Section of Sotttheasterii Idaho 



asid Westerfii WyoMiiias'. 



By A. C. Pealle, M. D. 



In connection with Paleontological Papers ]S"o. 9, pnblislied in this 

 Bulletin (Art. Y, pp. 105-117) by Dr. C. A. White, in which a number of 

 new and specially interesting- species of fossils are described, I have 

 thought it may be both interesting and useful to give briefly a few notes 

 on the section of the Jura-Trias of the region from which these organic 

 remains were obtained. 



The rocks under consideration are exposed in considerable areas in the 

 district assigned me for examination during the field-season of 1877, 

 especially between the Wyoming and Portneuf Eanges. The former is 

 in longitude 110° 48' and the latter in approximate longitude lISC'. 



The region thus indicated is one of complicated folds and great dis- 

 fylacements, and the Jura-Trias rocks, with the conformably underlying 

 Carboniferous, enter largely into the structure of the mountains ; entsire 

 ranges, indeed, being carved from them. 



The Triassic Eed Beds are found on the summit and on the western 

 slopes of the Wyoming Eange, in the former occupying a broad synclinal 

 depression, and in the latter dipping westward into the valley of John 

 Day's Eiver. An immense fault extends along the eastern front of the 

 Wyoming Eange, giving it a monocUnal structure at this i)lace, and 

 when we cross the fold, which forms the western side of the range, we 

 find another similar fault along the eastern side of the Salt Eiver Eange, 

 which is parallel to the Wyoming Eange, and only from eight to ten 

 miles west of it. These faults are several thousand feet in extent. 



In the Salt Eiver Eange, the Jura.-Trias rocks are found entering into 

 the compMcated folds that form its central portions. 



Twenty-five miles farther west, we reach the Blackfoot Basin, in which 

 the Jura-Trias forms a large portion of the surface, the ridges having a 

 comparatively low elevation, and the folds not being so greatly eroded 

 as in the higher mountains. It is in this locality that the interesting- 

 collection of fossils, described by Dr. W^hite, was found, and to which 

 his paper is confined. The following is the general section of the Jura- 

 Trias, beginning at the top : 



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