220 THE GEOLOGICAL HISTORY OF PLANTS. 



floating ice, and permitting a temperate flora of 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 in- 

 creased, and the extension of new plains along the older 

 ridges of land was coincident with the deposition of the 

 great Laramie series, and with the origination of its pe- 

 culiar flora, which indicates a mild climate and consider- 

 able 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 continents 

 gradually assuniedT their 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 submergence of land occurred 

 on the eastern continent which must have assimilated its 

 physical conditions to those of the Middle Cretaceous. 

 This great change, affecting materially the flora of Eu- 

 rope, was not equally great in America, which also by the 

 north and south extension of its mountain-chains per- 

 mitted movements of migration not possible in the Old 

 World. From the Eocene downward, the remains of 

 land-animals and plants are found chiefly in lake-basins 

 occupying the existing depressions of the land, though 

 more extensive than those now remaining. 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 Mountains belongs, perma- 

 nently modified and moulded the forms of the continents. 



These statements raise, however, questions as to the 

 precise equivalence in time of similar floras found in dif- 



