THE EOCKY MOUNTAIN BEGION OF CANADA. 21 



both contiuonts ; but a ciroiimpolar belt of laud seems to have been maintained, protecting 

 the Atlantic and Pacific basins from floating- ice, and permitting a temperate flora of great 

 richness to prevail far to the north, and especially along the southern margins and exten- 

 sions 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 coincident 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 assumed its present form, 

 and the vegetation became still more modern in aspect. In that period of the Eocene, 

 lioweA'er, in which the great uunimulitic 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 

 Europe, was not equally gi-eit 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 downward, 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 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, permanently 

 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 different latitudes. However equable the climate, there must 

 have been some appreciable difference in proceeding from north to south. If, therefore, 

 as seems in every way probable, the new species of plants originated on the Arctic land 

 and spread themselves southward, this latter process would occur most naturally in times 

 of gradual refrigeration or of the access of a more extreme climate, that is in times of the 

 elevation of land in the temperate latitudes, or conversely, of local depression of land in the 

 Arctic, leading to invasions of northern ice. Hence the times of the prevalence of particular 

 types of plants in the far north would precede those of their extension to the south, 

 and a flora found fossil in Greenland might be supposed to be somewhat older than a 

 similar flora when found farther south. It would seem, however, that the time required 

 for the extension of a new flora to its extreme geographical limit, is so small in comparison 

 with the duration of an entire geological period, that practically, this diflereuce is of little 

 moment, or at least does not amount to antedating the Arctic flora of a particular type by 

 a whole period, but only by a fraction of such period. 



It does not appear that, during the whole of the Cretaceous and Eocene periods, there 

 is any evidence of such refrigeration as seriously to interfere with the flora, but perhaps 

 the times of most considerable warmth are those of the Dunvegan group in the Middle 

 Cretaceous and those of the later Laramie and oldest Eocene. 



It would appear, that no cause for the mild temperature of the Cretaceous needs to be 

 invoked, other than those mittations of land and water which the geological deposits them* 



