Cretaceous Florus of Canada. 957 
In the Cenomanian, we find the Northern Hemisphere tenanted 
with dicotyledonous trees closely allied to those of modern times, 
though still indicating a climate much warmer than that which at 
present prevails. In this age, extensive but gradual submergence 
of land is indicated by the prevalence of chalk and marine lime- 
stones over the surface of both continents; but a circumpolar belt 
of land seems to have been maintained, protecting the Atlantic and 
Pacific basins from floating ice, and permitting a temperate flora ot 
great richness to prevail far to the north, and especially along the 
southern margins and extensions of the circumpolar land. These 
seem to have been the physical conditions which terminated the 
existence of the old Mesozoic Flora and introduced that of the 
Middle Cretaceous. 
As time advanced, the quantity of land gradually increased, and 
the extension of new plains along the older ridges of land was coinci- 
dent with the deposition of the great Laramie series, and with the 
origination of its peculiar flora, which indicates a mild climate and 
considerable variety of station in mountain, plain and swamp, as 
well as in great sheets of shallow and weedy fresh water. 
In the Eocene and Miocene periods the continent gradually as- 
sumed its present form, and the vegetation became still more 
modern in aspect. In that period of the Eocene, however, in 
which the great nummulitic limestones were deposited, a submer- 
gence of land occurred on the Eastern continent which must have 
assimilated the physical conditions to thdse of the Cenomanian. 
This great change, affecting materially the flora of Europe, was 
not proportionately great in America, which also by the north and 
South extension of its mountain chains permitted movements of 
migration not possible in the Old World. From the Eocene 
downwards, the remains of land animals and plants are found only 
In lake basins occupying the existing depressions of the land, though 
more extensive than those now existing. It must also be borne 
In mind, that the great foldings and fractures of the crust of the 
earth which occurred at the close of the Eocene, and to which the 
final elevation of such ranges as the Alps and the Rocky Moun- 
tains belongs, permanently modified and moulded the forms of the 
Continents, 
These statements raise, however, questions as to the precise 
