Cenozoic Floras Around the Northern Pacific 63 



tion may readily be made that all the more common plant re- 

 mains in a fossil deposit are of lowland types, from trees adjacent 

 to the site of deposition. The actual record is, therefore, of these 

 plants which grew in lowlands, where the accumulation of their 

 leaves and other structures is made possible by their inclusion in 

 sedimentary deposits. In the Tertiary of North America there 

 are only occasional remains, such as wind-borne seeds and leaves 

 of exceptional durability, which give material evidence of the 

 nature of upland vegetation. 



It seems obvious that in any Tertiary situation the trees of the 

 upper slopes and ridges had a more xerophytic aspect than that 

 of the flood plain, lake borders, and coastal lagoons, the vegeta- 

 tion of which has been preserved in the fossil record. Just as the 

 redwood forest of today has a border phase dominated by Doug- 

 las fir, live oak, and madrone, and as the rain forest of the tropics 

 grades up into an association characterized by oaks and other 

 temperate trees, so the redwood forest of the Miocene and the 

 rain forest of the Eocene may be assumed to have had border 

 phases conditioned by less humid climate the temperate range 

 of which was more extreme. The nature of these border phases 

 can rarely be direcdy determined from an adjacent fossil record, 

 since only exceptionally has the upland vegetation been pre- 

 served in sedimentary deposits. An indirect but effective method 

 of determining the nature of the vegetation of the uplands may 

 be followed by examining the contemporary floras of higher 

 latitudes. In the same manner as the high altitude border phase 

 of modern cUmax forests shows a parallel development in higher 

 latitudes near sea level, the upland vegetation bordering a Ter- 

 tiary climax in middle latitudes may be indicated by the nature 

 of the fossil record to the north, at lowland situations suited to 

 its deposition and preservation. 



