34 CONTRIBUTIONS TO PALiBONTOLOGY 



Hills sandstone yielding Sphenodiscus lenticularis,^ and is below or within the zone 

 of abundant Triceratops remains. Further discussion of the stratigraphic and 

 floral problems of the Lance will be presented in later papers. 



Black Buttes flora — With 20 species common to the Medicine Bow formation, 

 the flora from near Black Buttes in southwestern Wyoming is apparently com- 

 parable, though not necessarily exactly contemporaneous. The Black Buttes flora 

 occurs in the so-called "Laramie" formation (Black Buttes coal group),- which 

 in this region is said to lie stratigraphically just above the Lewis shale. Knowlton 

 has given an excellent historical review of the work on this flora.^ Although he at 

 one time considered the flora of true Laramie age and equivalent to that of the 

 Hell Creek member ("Ceratops beds") of the Lance,^ Knowlton in his later years 

 was incHned to regard it as younger than the Laramie, and considered the beds to 

 Ue unconformably on the massive sandstones (Fox Hills?) at the top of the Lewis 

 shale.^ I visited the Black Buttes region during the summer of 1936 and failed to 

 find any evidence of the unconformity which Knowlton had proposed. Until a 

 complete restudy and revision of the stratigraphy, palseontology, and palseobotany 

 of this section is completed, it will be impossible to make a definite age determina- 

 tion of the flora. The high degree of similarity to the floras of the lower Medicine 

 Bow formation cannot, however, be overlooked, and suggests similar age for at 

 least some of the plant-bearing horizons of the Black Buttes group. 



Denver-Dawson floras — There are 17 species in common between the floras of 

 the Denver and associated formations ^ and those of the lower Medicine Bow. 

 This is considered a sufficiently large number of species to indicate that at least a 

 part of the Denver-Dawson beds are of lower Lance-Laramie age. Until a few years 

 ago such a statement might have been considered absurd, since it was supposed that 

 both the Dawson and the Denver (with its basal Arapahoe) were separated from 

 the underljdng Laramie by a great unconformity representing a hiatus of sufficient 

 length to remove 14,000 feet of sedimentary rocks from the mountains to the west 

 of the region.'' Dane and Pierce ^ have recently shown that the Arapahoe-Denver 

 sequence is equivalent in position and presimiably in age to the Dawson, and that 

 the great unconformity is at best of local stream-channel origin. It is, therefore, 

 not unreasonable to expect Laramie species to extend into the Denver or Dawson. 

 Of the 17 lower Medicine Bow species which occur in the Denver-Dawson, only 

 four species in fact do not occur also in the underlying Laramie. 



There are still many apparent differences between the large Denver-Dawson 

 floras and those of the lower Lance, lower Medicine Bow, and Laramie formations. 

 As Dane and Pierce have indicated, "the upper parts of the Dawson and Denver 

 may possibly include beds of Eocene age." ^ More careful coUecting will be neces- 



' Dobbin, C. E., and Reeside, J. B., Jr., U. S. Geol. Survey Prof. Paper 158, 20, 1930. 



2 Schultz, A. R., U. S. Geol. Survey BuU. 702, 22-24, 1920. 



' Knowlton, F. H., U. S. Geol. Survey Prof. Paper 130, 62, 1922. 



' Knowlton, F. H., BuU. Geol. Soc. Am., vol. 8, 136, 143, 156, 1897. 



' Knowlton, F. H., U. S. Geol. Survey Prof. Paper 130, 62-65, 1922. 



« Knowlton, F. H., U. S. Geol. Survey Prof. Paper 155, 1930. 



' Emmons, S. F., Cross, W., and Eldridge, G. H., U. S. Geol. Survey Mon. 27, 209, 1896. 



» Dane, C. H., and Pierce, W. G., BuU. Am. Assoc. Petr. Geol., vol. 20, no. 10, 1936. 



» Dane, C. H., and Pierce, W. G., ibid., 1328. 



