70 H. D. MACGINITIE 



their present habitats. The northern oceans were still warmer than 

 at present. On the West Coast the 18° isotherm had moved to lati- 

 tude 35° N., 5° to 6° north of the existing location. There is no clear 

 evidence of permanent polar icecaps at this time. The composition 

 of the fossil flora that the writer has been collecting near Valentine, 

 Nebraska, at latitude 43° N., the richest late jVIiocene (or lowest 

 Pliocene) flora yet found in that area, indicates that the minimum 

 temperatures were higher than at present and that the severe out- 

 breaks of polar continental air that characterize the present winter 

 climate had not yet reached their later intensity. It is surprising to 

 find abundant Cedrela and Meliosma in a flora of Barstovian (late 

 Miocene) age in northern Nebraska, surprising, because these genera 

 are now confined to the tropics, although Meliosma grows at eleva- 

 tions of 6,000 feet in the mountains of southern Mexico. A majority 

 of the species, however, would find a congenial habitat in southeast- 

 ern Oklahoma, some 8° or 9° to the south. 



Fr>'e and Leonard (1957) have been able to reconstruct the 

 sequence of late Tertiary climatic changes on the high plains by 

 studying the lithologic characters of the beds and the types of fossil 

 land snails found in them. The picture is one of steadily deteriorating 

 climates with decreasing rainfall, increased seasonal temperature 

 fluctuations, and lowered minimum temperatures. Conditions 

 essentially the same as those of the present were reached by the 

 Upper Pliocene. All except the hardiest and most drouth-resistant 

 trees of the once rich western montane floras had become extinct. 

 The seasonal distribution of rainfall, with the cooling of the border- 

 ing Pacific Ocean, had changed over the area west of the Rockies 

 from adequate rainfall at all seasons to the present Mediterranean 

 type with dry summers. Along the West Coast of the United States, 

 and especially in California, only those plants capable of withstand- 

 ing summer drouth were able to survive. Pollen studies show that 

 the prairies had become well established by the Middle Pliocene. 



One of the climatic consequences of low^-lying continents and 

 warm oceans are truly equable climates, with comparatively small 

 seasonal temperature changes. In the modern world such climates 

 are found at moderate elevations in the tropical mountains, such as 

 the Tierra Templada of southern Mexico and Central America. 

 The earlier floras show conclusively that the modern severe winters 

 with their cold waves were nonexistent as late as the Upper Miocene. 



