66 H. D. MACGINITIE 



beginnings of climatic trends that were later accelerated. The flora 

 represented by the fossil leaves and fruits was a streamside and 

 lakeside flora of warm-temperate or subtropical aspect. The abun- 

 dance of small and coriaceous leaves and leaflets shows that the 

 vegetation on the open divides and higher ground around the lake 

 basins was somewhat like our modern subtropical scrub (Chancy, 

 1944). The pollen flora of the Green River gives us a picture of 

 temperate forests, with oaks and conifers, occupying the surrounding 

 uplands. Subhumid or even arid local conditions are indicated by 

 pollen of the desert shrub Ephedra. We also find in the Green River 

 flora a few representatives of the Arcto-Tertiary flora, the vanguard 

 of a dispersal, down the moderate elevations of the Rocky Mountain 

 axis, that was later to result in a complete replacement of the Lower 

 Tertiary forests. The negative evidence of the Green River flora is 

 significant. There is no pollen of grasses or other herbs except that 

 of a few primitive aquatic forms. There is no pollen of the sunflower 

 family or sagebrush or greasewood and the like. The modern her- 

 baceous vegetation or desert shrub vegetation evidently did not 

 exist in that area during the Middle Eocene (Wodehouse, 1933, 

 pp. 518-522). 



A general view of the Eocene forests of the United States shows 

 us essentially tropical floras along the Gulf Coast, slowly merging 

 into warm-temperate floras at the north. There appears to have been 

 a slow shift to the north of these warmer floras, culminating in the 

 late Eocene (Chaney, 1947, p. 143). The most tropical Tertiary 

 flora of the West Coast is at Goshen, Oregon. All the Upper Eocene 

 floras at Comstock, Oregon, at Steel's Crossing, Washington, and 

 at LaPorte, California, have a definitely tropical aspect. 



When the time boundary between the latest Eocene and the 

 Lower Oligocene is crossed, we find a marked climatic change, and 

 we can look down the vista of cooling and drying climates that 

 finally culminated in the glaciation of the Pleistocene. There was an 

 irregular, but, in the long run, a continuous change over the whole 

 world toward cooler and also, in general, drier climates. This trend 

 is clearly shown by the many fossil floras of the Upper Tertiary 

 that are scattered over the western states, from the high plains to 

 the Pacific Coast, and by the fossil floras of Europe and Asia. In a 

 recent study of fossil pollen from localities in southwestern Russia, 

 Bogelepov (1955, p. 988) has shown the same sequence of forest 



