Maryland Geological Survey 311 



Conclusion' 



The time has not yet arrived for a satisfactory discussion of the place 

 of origin or the subsequent migrations of the great dicotyledonous flora 

 that with seeming suddenness makes its appearance almost coincident 

 with the dawn of the Upper Cretaceous. It may be pointed out that this 

 apparent sudden predominance is probably based on a relatively long ante- 

 cedent evolution in areas remote from regions of sedimentation. 



It has been commonly assumed, and it is certainly the most attractive 

 hypothesis, that the origin of the dicotyledons was in high latitudes from 

 which region they spread southward over the continents of the northern 

 hemisphere in successive waves of migration. There is considerable evi- 

 dence in support of this theory, but the unexplored Cretaceous sediments 

 of the great continent of Asia and of most of the lands in the southern 

 hemisphere invalidates too hasty generalizations. The land mass of Asia 

 with free land communication during Middle Cretaceous time to the 

 northward, southward, eastward, and westward, has not received the con- 

 sideration which it merits as a center of radiation, nor have the American 

 tropics received much attention, although the writer's studies show the 

 latter region to have unquestionably occupied a very important place in 

 any discussions of the early Tertiary history of dicotyledonous floras. 

 This one conclusion seems warranted, that the origin and initial radiation 

 of dicotyledonous floras took place somewhere in the great and massed 

 land areas of the northern hemisphere. 



The Upper Cretaceous floras show a great modernization as compared 

 with those of the older Mesozoic. The essentially Jurassic flora of the 

 Lower Cretaceous with its wealth of conifers, cycads and ferns is replaced 

 with a forest of mixed conifers and dicotyledons, the ferns occupy a sub- 

 ordinate position and the cycads are rapidly waning. All the species are 

 extinct, in fact scarcely any survive into the Eocene. Many of the genera, 

 particularly among the conifers, die out before the close of the period, 

 and a large number of the dicotyledons are generalized and primitive 

 types. 



The physical conditions which these Upper Cretaceous floras indicate is 

 also one that must of necessity be discussed with caution, since so little 



