FOX HILLS AND LOWER MEDICINE BOW 31 



conditions of growth. Equally in harmony is thc ncgative evidence of the absence 

 of the usual cool tempcrate genera and thc scarcity of conifers. 



CORRELATION 



In attempting to cstabUsh age relationships between other rccordcd floras and 

 that of the lower Medicine Bow forraation, thrce basic assumptions are made. 

 These assumptions are: (1) that tlie Sphenodiscus zone of the Fox Hills formation 

 is of essentially the same age whcrever encountered; (2) that the Triceratops dino- 

 saur zone is Kkewise essentially contemporaneous wherever found; and (3) that the 

 Lance, Laramie, and lower Medicine Bow formations are conformable on the 

 Sphenodiscus-hearing Fox Hills formation, and, thercfore, only shghtly younger 

 than Fox Hills age. 



Since none of these assumptions seems, in my opinion, doubtful or unsupported, 

 the age of the lower Medicine Bow flora is not open to serious question. Its 

 occurrcnce in beds lying within 1300 feet conformably above the Sphenodiscus zone 

 of the Fox Hills formation and 2500+ fect below Triceratops-hearing beds clearly 

 estabhshcs the upper and lowcr hmits of its time-range. Whether either or both 

 of these zones are referred to the Upper Cretaceous or the Paleocene is immaterial 

 to the immcdiate problem of correlation. In the following analysis, therefore, the 

 lower Medicinc Bow flora is regarded as a standard flora of known stratigraphic 

 position with which othcr floras may bc advantageously compared or contrasted. 



As indicated in table 4, the Medicine Bow plant species of the present report 

 total 64, of which 14 are either new or represcnted by specimcns insuflficicnt for 

 comparison with previously recorded species. The rcmaining 50 spccies have bcen 

 describcd or reported in othcr floras, which are discussed in the order of their 

 similarity to the Medicinc Bow flora. 



Laramie flora — It is at oncc apparent from table 4 that the Laramie flora ^ 

 of the Denver basin, Colorado, shows the closest relationship to the lower Medicine 

 Bow flora, with 27 species in common. The samc conclusion was reachcd by 

 Knowlton on the basis of smaUer, undescribed plant coUections obtained from 

 scattered locahties in the Medicine Bow formation." This close relationsliip indi- 

 cated by the floras is amply corroborated by several hncs of evidence: (1) Both 

 the Laramie and the Medicine Bow formations occupy the same stratigraphic posi- 

 tion conformably above the Fox Hills formation, which in both sections carries the 

 diagnostic Sphenodiscus; ^ (2) both formations he conformably or with only local 

 disconformity below beds yielding remains of Triceratops; (3) in the Medicine Bow 

 formation the plant remains were obtained from the basal 1300 feet, which is 

 approximately the same thickness of beds from which the Laramie plants wcre 

 obtained; (4) the fresh-water invertebratc fauna recordcd from the Medicine Bow 

 formation is of cssentiaUy the same character as that of the Laramie formation.* 

 In my opinion there is thus sufficient unanimity of stratigraphic, palseontologic, 



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



2 Knowlton, F. H., ibid., 100. 



s Dobbin, C. E., and Roeside, J. B., Jr., U. S. Geol. Survey Prof. Paper 158, 23, 1930. 



* Henderson, J., Geol. Soc. Am., Spec. Pap. No. 3, 32, 1935. 



