EAELIER TERTIARY (EOCENE AND OLIGOCENE). 777 



conditions of deposition, but the carbonaceous shale and workable coal beds intercalated with 

 the other rocks serve to distinguish the member. As indicated above, the coal beds of the 

 middle member do not begin abruptly but are presaged ia the lower member by thin beds of 

 coal. At the upper limit of the middle member, however, there is a sharp transition from the 

 productive measures to the barren beds above. * * * 



The upper barren member * * * jg composed, Hke the lower member, of sandstone 

 and shale, with a very little carbonaceous material. Shale predominates in the lower portion 

 and sandstone in the upper portion. 



L 13. PLAINS EAST OF THE BIGHORN MOUNTAINS, WYOMING. 



The Tertiary strata of northern Wyoming were described by Darton^^' under 

 the names De Smet formation and Kingsbury conglomerate and were assigned to 

 the Cretaceous system. Later investigations show that the "De Smet" formation 

 and the Kingsbury conglomerate comprise the Fort Union formation (Eocene). 

 Details of the coal-bearing section of the strata in the Sheridan coal field are given 

 by Ta£f/°^ and in the Buffalo coal field by Gale and Wegemann.^®^ The name 

 "De Smet" has been abandoned by the Survey in favor of Fort Union, and the 

 Kingsbury conglomerate Is now treated as a member of the Fort Union, as it also 

 is of Fort Union age. 



L 13. MILES CITY COAL FIELD, MONTANA. 



The Fort Union formation- in the Miles City coal field is described by Collier 

 and Smith ^" as follows : 



The rocks consist of comparatively fuie material, mainly clay shale and sandstone, iucludiag 

 lignite and thin beds of impure limestone. They are all fresh-water deposits and contain an 

 extensive fossil flora of about 400 species of plants which resemble those of modern times, and 

 a fauna including fishes, fresh-water moUusks, and reptiles. 



The total thickness of Fort Union rocks exposed ia the Miles City field is about 900 feet, 

 which may be increased by data obtained from drill holes at Miles City to a total of 1,400 feet. 

 From a comparison of the section at Miles City with that exposed about 70 mUes to the northeast 

 near Glendive, it is inferred that the base of the Fort Union formation is not very deeply buried 

 and it is probable that some of the drill holes at Miles City have penetrated to underlying 

 Cretaceous rocks. 



The Fort Union rocks exposed are rapidly separated into two members by a marked differ- 

 ence in lithologic character. The lower member, about 500 feet thick, consists mainly of 

 alternating beds of clay shale and sandstone, having a general dark-gray or somber hue. In 

 detail the individual layers of this part of the formation, especially the sandstones, show numer- 

 ous irregularities of deposition. Thin layers of ferruginous Umestone occur at short intervals 

 throughout the section, and concretion-like masses of the same material, more or less lenticular 

 in form, are included in the shales and sandstones. The thicker sandstone beds are usually 

 characterized by more or less irregular cross-bedding. Coal is found at numerous horizons, 

 but the beds are extremely variable in thickness and horizontal extent. 



L 13. SENTINEL BUTTE COAL FIELD, NORTH DAKOTA AND MONTANA. 



The Fort Union formation in the Sentinel Butte field (latitude 47°, longitude 

 104°) resembles the beds as developed in the Miles City field. It is described by 

 Leonard and Smith ^^* as follows: 



The Fort Union formation has a thickness * * * as measured on its outcrop of 900 

 feet. If to this be added 820 feet of lignite-bearing rocks, which are probably to be referred to 



