LANCE FLORA OF EASTERN WYOMING 105 



tion. This principle is best applied to unfossiliferous rocks occurring above, below, 

 or between rocks containing datable fossil assemblages. It does not imply that 

 dissimilarities of stratigraphic succession preclude the possibiUty of correlation. 



7. Exact correlation of fossil assemblages does not necessarily inchide exact age 

 determination unless one or another of the assemblages has been properly placed in 

 the geologic time scale. Although sometimes forgotten, this principle is obviously 

 fundamental in all problems of correlation, since the geologic time scale is an arbi- 

 trary and empirical classification based in the first instance on superposition rather 

 than on fossil content. There are known instances in which diverse fossil assem- 

 blages have been proved synchronous yet cannot be accurately assigned to a definite 

 position in the time scale because of their isolation from rocks of known age. 



8. Accuracy in paliEontoIogical correlation depends largely on the vaUdity of 

 the identifications of the fossils used. 



Lance flora as a standard — The Lance fiora, herein described, is an exceUent 

 standard flora with which to compare and contrast other late Cretaceous and early 

 Tertiary floras of the Rocky Mountain region. It is composed of specimens which 

 were all coUected from the type locaUty of the Lance formation. It is a compara- 

 tively large flora, comprising 70 species, of which 57 have an outside distribution. 

 It is accurately dated in terms of the geologic time scale (aside from its own floral 

 testimony), since it occurs in beds which have yielded abundant remains of dino- 

 saurs of the Triceratops zone of latest Cretaceous age and which are conformably 

 above the Sphenodiscus zone of true Fox Hills (latest Montanan) age. It also con- 

 tains a sufficient number of index species of short stratigraphic range elsewhere to be 

 reUable for correlation. Finallj^, the stratigraphic succession and the relation of the 

 floral assemblage to the Triceratops zone and to the Sphenodiscus zone in this region 

 are strictly comparable with the conditions observed in several other regions. 



Definition of Lancian age — There does not exist at present a clearly defined 

 temporal term for the latest Cretaceous of the Rocky Mountain region. For the 

 practical purposes of clarity and precision it is here proposed to use Lancian age as a 

 convenient provincial time term, based on the Lance formation at its type locality 

 near Lance Creek, Niobrara County, Wyoming. This time unit is deUmited below 

 by true Fox Hills time (i.e., latest Montanan age, characterized by marine sand- 

 stones comprising the well-defined Sphenodiscus zone) , and delimited above by the 

 beginning of Paleocene time. The terrestrial sediments of Lancian age carry the 

 characteristic mammals ' and dinosaurs - of the Triceratops zone, as well as the plants 

 here described. 



Lanceflora — As is shown in table 3, there are 57 species of the Lance flora which 

 are known elsewhere in the late Cretaceous and to some extent the early Tertiary 

 deposits of North America, chiefly the Rocky Mountain region. In the analysis of 

 the relation of this assemblage to other assemblages, the stratigraphic range of spe- 

 cies elsewhere is determined on the following basis : species restricted to Montanan 

 age are those whose occurrences, where stratigraphic and palaeontologic relations are 



' Wood, H. E., et al., Bull. Geol. Son. Amer., vol. 52, 8, 1941. 

 2 Ruasell, L. S., Proc. Amer. Philos. Soc, vol. 69, 139-141, 1930. 



