530 F. H. KXOWLTOX EVOLUTION OF GEOLOGIC CLIMATES 



very considerable nimiber of modern plant types were undoubtedly estab- 

 lished by the end of Cretaceous time, there are nevertheless notable 

 differences to be observed in the composition of the floras of the Cre- 

 taceous and Tertiary. Much of the sharpness of this distinction has in 

 the past been lost or obscured by the vicarious placing of the line between 

 Cretaceous and Tertiary. This has been especially true in Xortti 

 America, where the placing of such important plant-bearing units as 

 the Eaton, Dawson, Denver, Lance, etcetera, in the Cretaceous has had 

 unfortunate effects; but now, supported by important diastrophic data, 

 it seems to be pretty generally conceded that they properly belong in 

 the Tertiary. As bearing on the distinctness of Cretaceous and Tertiary- 

 floras, it may be stated that approximately 40 per cent of the genera in 

 existence in the Cretaceous had become extinct at the close of that time. 



Xorth America is fairly rich in Eocene floras, especially in the Gulf 

 region, Eock}^ Mountain region, and in the northern Pacific coast region. 

 The oldest of the plant-bearing units in the Gulf region is that of the 

 Midway formation, but the known forms are so few that they become 

 negligible in the present connection. Above the Midway is a thick series 

 of sediments known as the Wilcox group, which has a flora of 134 genera 

 and over 330 species. The Wilcox flora has been fully elaborated by 

 Berry,^^ who points out that it is '^overwhelmingly that of a strand flora, 

 of which some of the elements indicate that they grew on the sandy 

 beaches, others in muddy tidal flats, others between or behind dunes 

 or beach ridges, and others in estuary bayous or marshes. Xone of the 

 forms can certainly be considered as inland or upland types.*^ 



Berry says further : 



•'It may be noted that all of the Wilcox plants, almost without exception, 

 are plants whose modern representatiA'es inhabit the warmer parts of the 

 earth. There is not a single strictly temperate type in the w^hole assemblage. 

 . . . Compared with recent costal floras, it is at once apparent that its 

 affinities are entirely with those of tropical and subtropical America. . . . 

 It is obvious that the flora could not have existed if the region was ever A'is- 

 ited by frost, and temperatures appear to have been like those found today on 

 the Florida Keys." 



In other words, a subtropical rain forest. 



Of approximately the same age as the Wilcox, or possibly slight iy 

 older, is the flora of the Eaton formation of the Eaton Mesa region of 

 Xew Mexico and Colorado. This flora contains 148 species, over 40 of 

 which are identical or closelv related to those of the Wilcox flora. One 



>» E. W. Berry : Proc. Am. Thilos. Soc. vol. 53, 1914, p. 135 ; U. S. Geol. Survey Prof. 

 Paper 91. 1916. p. 74. 



