CHAPTER VII. 



PLANTS FROM THE TERTIARY TO THE MODERN PERIOD. 



It may be well to begin this chapter with a sketch of 

 the general physical and geological conditions of the pe- 

 riod which was characterised by the advent and culmina- 

 tion of the dicotyledonous trees. 



In the Jurassic and earliest Cretaceous periods the 

 prevalence, over the whole of the northern hemisphere 

 and for a long time, of a monotonous assemblage of gym- 

 nospermous and acrogenous plants, implies a uniform 

 and mild climate, and facility for intercommunication in 

 the 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 

 becoming a little less uniform. Before the close of the 

 Lower Cretaceous period the dicotyledonous flora seems 

 to have been introduced, under geographical conditions 

 which permitted a warm temperate climate to extend as 

 far north as Greenland. 



In the Cenomanian or Middle Cretaceous age 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 sub- 

 mergence of land is indicated by the prevalence of chalk 

 and marine limestones over the surface of both conti- 

 nents; but a circumpolar belt seems to have been main- 

 tained, protecting the Atlantic and Pacific basins from 



