70 R- W. Chaney 



The more important o£ the two elements of the Miocene flora 

 which are absent or sparsely represented in the living forests of 

 western North America is the deciduous element comprising 

 such genera as Carpinus, Castanea, Fagus, Hicoria, Liquid- 

 amhar, Nyssa, Ostrya, Sassafras, Tilia, and Ulmus. Whereas such 

 genera as Aca-, Aesculus, Alnus, Amelanchier, Betula, Cornus, 

 Corylus, Fraxinus, Juglans, Philadelphus, Platanus, Populus, 

 Prunus, and Salix have survived on the Pacific Coast, retaining 

 their deciduous habit, Ceanothus, Quercus, Rhamnus, and Rhus 

 are predominandy evergreen; and a group of genera including 

 many of the most numerous and wide-ranging forms in the 

 modern forest — Arbutus, Castanopsis, Lithocarpus (Quercus), 

 Myrica, Odostemon, and Umbellularia — are evergreen. A ten- 

 tative suggestion may be made that the source of this element 

 was in the south. The extensive representation of modern spe- 

 cies of Lithocarpus in the low-latitude forests of Asia, and of 

 evergreen members of the Lauraceae in the tropics of both 

 hemispheres, suggests that their ancestors, shifting northward 

 in the Eocene, may have survived in California and Oregon, 

 or have continued their migration in the Miocene. There is 

 some indication from the paleontologic history of the Ericaceae 

 that they also had their origin in the south. In any event, the 

 present abundance of broad-leafed evergreens in low latitudes 

 and their known occurrence there in Cretaceous and Eocene 

 time makes plausible the suggestion of their southern origin. 

 The elimination of a large number of deciduous genera from 

 the flora of western America, with an increased number of 

 broad-leafed evergreens, appears to be related to fundamental 

 changes in climate since the Miocene, involving the concentra- 

 tion of rainfall in the winter months. This Lower Miocene flora 

 as a whole indicates a temperate climate over much of western 



