LOCATION, CHARACTER, AND FAUNA OF THE SECTIONS 265 



pertinent in connection with other sections in Wyoming and Colorado 

 soon to be considered. The dark shale between the sandstones is appar- 

 ently of marine origin, but no fossils have been reported from it in the 

 Black Hills region. 



Continuing the comparison of sections to the northwest, the next sec- 

 tion diagrammatically summarized on the chart (number 14) is the Soap 

 Creek section, in southern Montana, at the east base of the Bighorn 

 Mountains. It was described by W. T. Thorn, Jr., and G. F. Moulton in 

 a recent press bulletin of the United States Geological Survey. The prin- 

 cipal key horizons for our comparison are the Niobrara and the Mowry. 

 The Dakota, as such, is not recognizable. Is it represented in the 550 

 feet of Thermopolis shale beneath the Mowry or is it in the top of the 

 Cloverly, which when originally described was supposed to include the 

 equivalent of the Dakota ? 



Near the Little Rocky Mountains in northeastern Montana the section 

 recently studied by A. J. Collier (number 15 of chart) is similar to the 

 Soap Creek section, in that the Dakota is not recognizable and the inter- 

 val between the Mowry shale and the varicolored continental deposits here 

 called Kootenai is occupied by 500 feet of dark shale corresponding to 

 the Thermopolis. 



The recognition of the Dakota sandstone, as such, at any localities far- 

 ther west and north is not believed to be justified. 



WYOMING AND COLORADO SECTIONS 



Turning southward again, let us review several representative sections 

 east of the Rocky Mountain front in Wyoming and Colorado. The first 

 one in central Wyoming (number 12 of the chart), between Lander and 

 Casper, is generalized from the descriptions of Woodruff 12 and Hares. 13 

 Here the Niobrara is recognizable by its fauna, and the Mowry is con- 

 spicuous as a lithologic and topographic feature. Beneath the Mowry, 

 and separated from it by 150 to 300 feet of Thermopolis shale, is a sand- 

 stone, varying in thickness from to 60 feet, which has yielded a char- 

 acteristic Dakota flora, as determined by Knowlton, and the sandstone 

 was therefore identified as the Dakota. Beneath it there are dark shales, 

 with some thin sandstones, having a thickness of 100 to 350 feet and 

 resting on a conglomerate of varying thickness up to 60 feet, which in 

 turn overlies the Morrison. In the dark shale beneath the Dakota of this 

 section I found some marine pelecypods, and stated, as quoted in Wood- 

 ruff's report, that "from its stratigraphic position and its general char- 



12 U. S. Geol. Survey, Bull. 452, 1911, pp. 16-22. 



13 U. S. Geol. Survey, Bull. 041, 1917, pi. 23. 



