LANCE FLORA OF EASTERN WYOMING 123 



designation but are not specifically determinable. Most of the species are regarded 

 as only distantly related to existing genera. Some of the species are still of very 

 uncertain systematic position ; others have been systematically revised or described 

 in detail on the basis of better and more abundant specimens and of comparisons 

 with modern plants. A quantitative analysis of the plant remains indicates that 

 10 species may be regarded as the dominants of the floral association. 



Studies of the structural characters of the dicotyledonous leaves in the Lance 

 flora indicate lowland, humid, warm temperate conditions of growth, approaching 

 subtropical. This conchision is substantiated by the present distribution of modern 

 correlatives of the Lance species. 



For purposes of correlation the Lance formation at its type locahty is made the 

 basis of a new provincial time term, " Lancian age." This time unit is regarded, on 

 the basis of the stratigraphic position and palseontology of the type Lance formation, 

 as latest Cretaceous, i.e., post-Montanan and pre-Paleocene. 



Analyses of other late Cretaceous floras of the Rocky Mountain region indicate 

 that (1) the Laramie, Medicine Bow, and Colgate-HeU Creek formations are of 

 Lancian age, as is shown also by their stratigraphic position and by other fossil 

 remains; (2) the Arapahoe-lower Denver, lower Dawson, upper Vermejo-lower 

 Raton, and Whitemud-lower Ravenscrag formations and the so-called "Laramie" 

 of southwestern Wyoming are also of Lancian age; (3) the Almond-Ericson, Mesa- 

 verde, lower Vermejo, and Fruitland formations are of late Montanan age. 



Analyses of Paleocene floras indicate that: (1) the floras of the type Fort Union 

 formation or other undoubted Paleocene beds have less than 10 per cent of their 

 species in common with the type Lance flora or other floras of Lancian age; (2) 

 the floras of the type TuUock and Ludlow beds and equivalents are closely related 

 to the Paleocene Fort Union flora and have httle in common with Lancian floras. 



The placing of the Cretaceous-Tertiary boundary below the TuUock, Ludlow, 

 and demonstrable equivalents and above the HeU Creek and Lance is consistent 

 with the recent discovery of Paleocene foraminifera in the CannonbaU beds of the 

 Dakotas. 



