6 Canadian Record of Science. 



north. Toward the end of the Jurassic and beginning of 

 the Cretaceous, the land of the Northern Hemisphere was 

 assuming greater dimensions, and the climate probably be- 

 coming a little less uniform. Before the close of the Lower 

 Cretaceous period the dicotyledonous flora seems to have 

 been introduced, under geographical conditions which per- 

 mitted a warm-temperate climate to extend as far north as 

 Greenland. 



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 limestones over the sur- 

 face 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 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 

 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 consider- 

 able variety of station in mountaiu, plain and swamp, .as 

 well as in great sheets of shallow and weedy fresh water. 



In the Eocene and Miocene periods, the continent gradu- 

 ally assumed 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 submergence of land occurred on the Eastern 

 Continent which must have assimilated its physical condi- 

 tions to those of the Middle Cretaceous. This great change, 

 affecting materially the flora of Europe, was not equally 

 great in America, which also by the north and south exten- 

 sion of its mountain chains permitted movements of migra- 



