526 F. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



suddenness as to suggest that it must have had a long anterior period 

 of development, perhaps on some great land-mass that was not then 

 receiving sedimentation, or is still unexplored. Berry ^^ continues : 



"It has been 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 suc- 

 cessive waves of migration. There is considerable evidence in support of this 

 theory, but the unexplored Cretaceous sediments of the great continent of 

 Asia and 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 west- 

 ward, has not received the consideration which it merits as a center of distri- 

 bution, nor have the American tropics received much attention, . . . This 

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

 dicotyledonous floras took place somewhere in the great massed land areas of 

 the Northern Hemisphere," 



Although there are marked differences between many of the more or 

 less widely separated floras that are believed to be in approximately 

 similar stratigraphic position, there are, nevertheless, so many species 

 held in common as to lead to the inevitable conclusion that climatic 

 zones such as now exist did not obtain during Upper Cretaceous time. 

 We find, for instance, that the Atane flora ranged from Greenland along 

 the Atlantic Coastal Plain and Gulf region to Texas with but little 

 change, and the Dakota flora spread with practically no change from 

 Kansas to Argentina. In fact, as Berry says, "they cross the equator 

 unchanged in both the Eastern and Western Hemispheres." 



The inference to be drawn from the above facts i^that during Upper 

 Cretaceous time the temperature conditions were very uniform and 

 equable over the greater part of the earth^s surface, extending from at 

 least 72° N. in Greenland to at least 60° S. in Argentina, with the prob- 

 ability that it was practically world-wide. 



The moisture conditions may next be considered. The study of these 

 floras reveals very little evidence of deciduous habits, which implies an 

 uninterrupted growing season and an abundant, or at least an adequate, 

 supply of moisture, well distributed throughout the year. An example 

 in the existing floras is shown by the warm temperate rain-forests ol 

 many parts of the world. The existence of an abundance of moisture is 

 also further shown by the presence of very considerable deposits of coal, 

 especially in the Eocky Mountain region, which implies wide-spread and 

 long-continued swamp or marsh conditions. 



E. W. Berry : Maryland Geol. Survey, Upper Cretaceous, 1916, p, 311. 



