PALEONTOLOGICAL GEOLOGY 
67 
What is needed in place of the two paleogeographical maps re¬ 
produced in the Stanton report is one, or several of them, to ex¬ 
press a reasonable hypothesis of the course of retreat of the sea 
as the land barrier rose and apparently cut off entirely the restrict¬ 
ed northern ocean from the Mexican Gulf. 
Cross. 
floral Continuity in Lance and Union Sections. The problem 
of establishing the divisional line between the Mesozoic and Cen- 
ozoic formations of the Rocky Mountain province is one of the 
geological storm centers of years. The question can only be 
settled when all available sources of evidence are evaluated and 
harmonized. Drawing the line at the top of the Union beds of the 
Black Hills region profoundly affects other areas and other equal¬ 
ly important problems, the true significance of which is either 
underestimated or completely overlooked. 
The flora of the Lance formation is unmistakably a Union - 
group of plants. It ranges through the entire vertical extent of 
the older terrane. Some of the most characteristic Union species 
occur within a few feet of the base of the section. Union beds 
are commonly regarded as Eocene in age. They yield a very 
large variety of forms, approximately 500 species. The flora of 
the lance terrane is also a rich one, comprising about 125 species, 
eighty per cent of' which are found also in the Union strata. 
Of the entire Lance-Union flora 15 species only are reported 
from the Cretacic rocks. 
Sedimentation appears virtually uninterrupted through the 
Lance and Union successions. From the floral data available 
it is utterly impossible to draw a line satisfactorily separating 
the two formations. It is sometimes assumed that there is here 
a continuous and unbroken sequence of deposits extending from 
the Pierre and Fox formations to the top of the Union section 
and that reported erosion planes, which occur between several of 
the formations are due to nothing more than changes from marine 
to brackish-water and fresh-water conditions, or to irregularities 
characteristic of epirotic deposits, the local breaks not representing 
a loss of geological time of any notable historical value. 
The plants certainly do not sustain this contention. They strong¬ 
ly point to a very considerable hiatus between the acknowledged 
