68 H. D. MACGINITIE 



across the United States in the region of the present high plains and 

 the prairies. While the relationship of the Oligocene floras is mark- 

 edly Asiatic, the early Eocene floras of the West Coast contained 

 many species that are related to plants now living in the upland 

 floras of Mexico and Central America. This indicates a rather free 

 interchange of floristic elements north and south at that time. 



The later Tertiary was characterized by repeated flurries of 

 mountain growth, in the Cordilleran region and westward, which 

 finally culminated in the great uplifts of the late Pliocene and the 

 Pleistocene. This orogeny resulted in an increasing complexity of 

 climatic barriers and in the local differentiation of faunas and 

 floras. According to the coral faunas studied by Durham, the 18° 

 isotherm, by lower Miocene time, had moved down the West Coast 

 to approximately the latitude of northern California. The climate of 

 the western states in the Lower Miocene, though more genial than 

 at present, shows none of the subtropical aspects of the earlier 

 Tertiary (except along the coast at the south). The flora in Oregon 

 and Washington and over the northern Great Basin was essentially 

 a mixed deciduous-conifer forest of warm-temperate aspect, with 

 many species of deciduous trees having modern counterparts in the 

 Appalachian region and in eastern China. There is evidence of 

 abundant summer rainfall over the area which at present has little 

 or no summer rainfall. The climatic barrier to the south and in the 

 plains area that was initiated in the Eocene effectively prevented 

 the migration of forest species east-west and also north-south until 

 well into the Miocene. There appears to have been little or no ad- 

 mixture of forest species between the Appalachians and the Rocky 

 Mountains in the interval between the Lower Eocene and the Lower 

 Miocene. The Upper Miocene floras of the Columbia Plateau area 

 contain many species of oaks, elms, beeches, maples, and the like, 

 that are closely related to living species in the eastern states, indi- 

 cating a dispersal path between the two areas, probably through 

 southern Canada. As the climate cooled in the Upper Tertiary, the 

 vegetation of the region between the Mississippi and the Rocky 

 Mountains, which was at first a subtropical scrub, was slowly re- 

 placed by the modern herbaceous prairie vegetation. 



Evolution of the herbs was comparatively rapid after the close of 

 the Oligocene. Pollen of the sunflower family (Compositae) first 

 becomes noticeable in the deposits of the Middle Miocene and such 



