The Succession and Distribution of Cenozoic Floras 

 Around the Northern Pacific Basin 



By Ralph W. Chaney 



THE VEGETATION o£ wcstem North America in the Ceno- 

 zoic era has been made known by the studies of Lesque- 

 reux, Newberry, and Dawson, followed by a more 

 modern group, among whom Knowlton was the outstanding 

 contributor. Until recently, however, the floras of this most re- 

 cent major unit of geologic time, occupying perhaps the latest 

 hundred million years of the earth's history, have not been criti- 

 cally studied on the continent of Asia. With the investigations 

 of Kryshtofovich and others, it becomes possible to compare 

 Tertiary floras on both sides of the Pacific Basin, and to deter- 

 mine not only their sequence in geologic time, but also their 

 probable routes of migration and their modern geographic situa- 

 tion. The purpose of this paper is to summarize the evidence of 

 the succession and distribution of the Cenozoic floras around the 

 northern part of the Pacific Basin, and to oudine the general 

 principles which guide the interpretation of this evidence. 



The underlying basis for the interpretation of the distribution 

 of Cenozoic floras, both in time and in space, is the assumption 

 of a trend from mild, humid climate at the early part of the era 

 to cool, semiarid climate toward its close. This does not imply 

 that there were not minor changes in reverse direction in the 

 Cenozoic, nor that the change toward cooling and drying was 

 of uniform rate. A series of fossil floras resulting from such a 

 climatic trend is an example of what Clements'* has termed a 



* Superior figures refer to items In the bibliography at the end of this essay. 



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