PHYSICAL CONDITIONS. 



135 



most extensive of known fossil floras, an as- 

 semblage of extinct species which sheds con- 

 siderable light on the physical conditions of the 

 marginal lands of Wilcox time. 



Before taking up in detail the evidence of tlie 

 flora I wish to pomt out certain general climatic 

 conditions in the Wilcox area based on cosmic 

 causes and deduced from studies of recent 

 climates. 



As the factors governing atmospheric circula- 

 tion are general and not local, the relatively 

 slight changes in the relation of land to sea in 

 Wilcox time as compared with the present are 



low, more or less landlocked water would have 

 a very appreciable effect in raisuig total tem- 

 peratures and in preventing widely separated 

 extremes. At the same time it would increase 

 the rainfall and increase tlie width of the 

 marginal lands over which this augmented 

 rainfall would be effective. Wliether this 

 would be sufficient to furnish the subtropical 

 conditions that the flora seems to indicate is 

 doul)tfuL 



Speculation regarding the Eocene climate of 

 the world as a whole is perhaps out of place, 

 but it may be said that the sum total of 



99" 97° 96° 93° 91" 89° 87* 85° 83° 81° ?9° 77° 75° 



FiGUBE 9.— Mapshowtag (A-A') the strand line at the beginnins of Wilcox deposition, (B-B') the maximum extent of the Wilcox transgression, 

 and (C-C') the northern limit of the Wilcox flora under existing climatic conditions. 



entirely too small to have caused much modifi- 

 cation of the then existing conditions. Then 

 as now there was a persistent area of high pres- 

 sure over the North Atlantic and an equally 

 persistent area of low pressure over the con- 

 tinent. Consequently the prevailing winds 

 were from the east. Cyclonic disturbances, 

 like those which originate to-day in the Gulf 

 of Mexico or those more Violent and wide- 

 spread storms of the West Indian hurricane 

 type which to-day originate in the Caribbean 

 Sea, would traverse at least a part of the Mis- 

 isssippi embayment. So large an area of shal- 



paleontologic evidence indicates that tlie fa- 

 miliar succession of seasons or of types of vege- 

 tation in passing from the luxuriant Tropics to 

 tlie ice-capjicd poles did not bold in Eocene 

 time. Palcobotanists have long maintained 

 that the climate of the present is essentially a 

 Pleistocene climate of an interglacial character 

 and that for the great bulk of geologic time 

 uniformity and not differentiation has been the 

 rule rather than the exception. Though the 

 older palcobotanists were inclined to over- 

 estimate the conditions of torridity, it remains 

 true that from tlie Lower Cretaceous until 



