40 



LARAMIE FLORA OF THE DEN^-ER BASIN. 



As the title indicates, the authors were con- 

 cerned principall}- with the lower Eocene mam- 

 mal-bearinfi formations and only incidentally 

 witii the underlying djnosaur-bearing beds. 

 As this phase of the paper is treated at length 

 under the discussion of the San Juan Basin (pp. 

 G9-7H) it need not be further considered here. 



In May. 1914, after a prolonged discussion 

 of all the lines of evidence then available, it 

 was decided by the United States Geological 

 Survey that, so far at least as its official publi- 

 cations are concerned, the Lance formation 

 is to be classed as "Tertiary (?)" instead of 

 "Cretaceous or Tertiary," as previously, and 

 that the Arapahoe, Denver, Dawson arkose, 

 and Raton formations, with which I believe 

 the Lance formation to be in part synchronous, 

 are to be classed as Eocene instead of Cretaceous. 



One of the first places in wliich this newer 

 interpretation of the position of the Lance 

 formation appears was a paper by Bauer,^^ 

 published in 1914. This paper presented the 

 results of an examination of an area in north- 

 eastern Montana along the north border of the 

 Fort Peck Indian Reservation. The only 

 sedimentary formations present are the Lance 

 and Fort Union. Concerning the line between 

 them Bauer said: 



Owing to the absence of a sharp lithologic boundary 

 between the formations and to the lack of exposures near 

 the critical horizon in this field and also in the northern 

 part of the reservation, the accurate mapping of the Lance- 

 Fort Union boundary is impossible. In drawing the line 

 shown on the map the known outcrops of somber-colored 

 beds are considered to belong to the Lance, and those of 

 yellow beds to the Fort Union. * * * Its [the 

 Lance's] separation from the overlying Fort Union forma- 

 tion is thus based on its stratigraphic position and litho- 

 logic character. In these particulars it agrees with the 

 Lance formation as recognized in other areas in eistern 

 Montana and in North Dakota, where it is further charac- 

 terized by a dinosaur faima which has not been found in 

 the Fort Union. Formerly the Survey considered the 

 evidence of the age of the Lance so conflicting that it was 

 ascribed to the Cretaceous or Tertiary, but recently the 

 close correlation of the Lance flora with that of well- 

 determined Tertiary formations of the Gulf coast, con- 

 sidered together with the mountain-making movements 

 that are supposed to have immediately preceded the de- 

 position of the strata, has led the Survey to assign the 

 formation to the Tertiary (?) system. 



A small coal area on the west side of liig 

 Horn River, Mont., in the angle formed \>\ its 



" Bauor, C. M., Lignite in the Wciiiity of Plcntywood and Seobey, 

 Sheridan County, Mont.: fj. S. Geol. Survey Bull. 541, pp. 293, 315, 

 19M. 



junction with the Yellowstone, was described 

 by Rogers.^- The strata examined are mainly 

 referable to the Lance formation, concerning 

 which Rogers said: 



The Lance formation in this area may be divided into 

 two parts, namely, a coal-bearing member, which com- 

 prises the upper 2.50 feet, and a lower portion about 900 

 feet thick. This distinction is made partly on lithologic 

 grounds and partly because of the slight difference in the 

 fossils found in the two dixisions. 



In tlie same volume with the two papers 

 just mentioned was one by Lloyd ^^ on the 

 lignite field of the Cannonball River region. 

 N. Dak. The four principal sedimentary for- 

 mations recognized were the Fox Hills sand- 

 stone. Lance, Fort Union, and White River 

 (Oligocene). Concerning the Fort Union and 

 Lance formations he said: 



The Fort L'nion formation, of the lower part of the 

 Tertiary system (Eocene), which contains the greater part 

 of the valuable lignite in the Dakotas and eastern Mon- 

 tana, embraces the surface rocks in the western and 

 northwestern parts of the Cannonball River field. Under- 

 neath the Fort Union is a series of beds which are now 

 tentatively classified as probably of early Tertiary age and 

 which have been referred to the Lance formation. The 

 upper 250 or 300 feet of this formation is in the field 

 markedly different in lithologic character from the under- 

 hdng more typical Lance and has been found at numerous 

 places to contain the remains of a marine fauna which 

 has not pre%'iously been known in this part of the strati- 

 graphic section. These beds have been mapped separ- 

 ately and are herein designated the Cannonball maiine 

 member of the Lance formation. The underlying lower 

 part of the Lance is of fresh-water origin and is composed 

 of alternating beds of shale and sandstone which on erosion 

 give rise to the badlands described above. Its thickness 

 is ajjproximately 400 feet. 



The fauna of the Cannonball member was 

 alluded to as follows: "In the Cannonball 

 River field several collections of marine in- 

 vertebrate fossils made from the Cannonball 

 member * * * have been identified by 

 T. W. Stanton as belonging to a modified Fox 

 Hills fauna." 



In 1915 Lloyd and Hares'* described in 

 considerable detail the areal distribution, 

 litlu)logic character, and stratigraphic relations 



" Rogers, G. 8., Geology and coal resources of the area southwest of 

 Custer, Yellowstone and Big Horn counties, Mont.: U. S. Oeol. Survey 

 Bull. .541, pp. 316-32S, 1914. 



" Lloyd, E. R., The Cannonball River lignite field, Morton, Adams, 

 and Ilcllingcr counties, N. Dak.: U. S. Oeol. Survey Bull. 541. pp. 243- 

 2<r2, 1914. 



"' Lluyd, E. R., and Hares, C. J., The Caiuionball marine member of 

 the Lance formation of North and South Dakota and its bearing en the 

 Lance- Laramie problem: Jour. Geology, vol. 23, pp. 523-547, 1915. 



