54 
POPULAR SCIENCE REVIEW. 
from their fauna are considered by Dr. Cope to bridge over 
what is generally considered one of the greatest gaps in geolo- 
gical chronology. The Dakota group, corresponding to the 
Upper Cretaceous of Europe, like the beds of Aix-la-Chapelle 
and the Aachenian sands of Belgium and others of Bohemia, are 
immediately superposed on the Palaeozoic rocks, thus showing a 
great physical break, or lapse of unrepresented time, during 
which in other areas thick marine sediments were accumu- 
lating. 
The Dakota group has been recognised to have a vast range 
along the flanks of the various mountain-ranges from north to 
south, extending from North Texas to the northern limits of 
the State of Minnesota, is presumed to extend into British 
America, and as the fossil leaves of the Upper Cretaceous of 
Greenland represent some genera and perhaps species of the 
Dakota group, it would seem that this formation lias been con- 
tinuous from the Gulf of Mexico. to the Arctic lands, Green- 
land, &c., over 35° of latitude. The fossils have been chiefly 
found on the plains in the eastern portions of Kansas and 
Nebraska. 
About 130 species of plants have been determined, including 
six ferns, nine coniferae (Sequoia, Ac.), one cycad, three mono- 
cotyledons ; the remainder are dicotyledons, and represent 
species referable to the genera Liquidambar, Populus , 
Salix , Betula , Myrica , Celtis , Quercus , Ficus , Plntanus , 
Laurus , Sassafras , Ginnamomum , of the Apetalese; Dio- 
spiros of the Gamopetaleae ; Arolia , Magnolia , Lirioden- 
dron , Negundo or Acer, Paliurus , Rhus or Juglans. and 
Prunus of the Polypetaleae ; or, merely considering the affini- 
ties to our present flora, seventeen genera are those to which 
belong the species of our trees and shrubs which have the more 
general and the widest range of distribution. Indeed, all our 
essential or arborescent types are there, except those which are 
marked by serrate or doubly serrate leaves. This general 
facies of the leaves of the Dakota group, viz. integrity of 
borders and coriaceous consistence of leaves, is peculiar. 
The borders, if not perfectly entire, are merely undulate or 
obtusely lobed ; to this there is scarcely an exception. This 
mode of division of the borders of leaves is very rare in species 
of our present times, except perhaps in some species of poplars.* 
Considered as a whole, most of the types of the Dakota group, 
related to those of our present flora, represent a moderate 
climate, like the one prevailing now between 30° and 45° of 
North latitude. Professor Heer has the same opinion in regard 
to the climate of the upper Cretaceous epoch of Greenland, 
as indicated by its flora.f 
* See Lesquereux, u The Cretaceous Flora/’ p. 128. 
t Ibid. p. 39. 
