GENERAL LAWS OF ORIGIN AND MIGRATION. 243 



vention of any other formation,* are beds rich in plants 

 of much more modern appearance, and referred by Heer 

 to the Miocene period, a reference, as we have seen, not 

 warranted by comparison with the Tertiary plants of Eu- 

 rope or of America. Still farther north this so-called 

 Miocene assemblage of plants appears in Spitzbergen and 

 Grinnell Land ; but there, owing to the predominance of 

 trees allied to the spruces, it has a decidedly more boreal 

 character than in Greenland, as might be anticipated from 

 its nearer approach to the pole.f 



If now we turn to the Cretaceous and Tertiary floras 

 of western America, as described by Lesquereux, New- 

 berry, and others, we find in the lowest Cretaceous rocks 

 there known those of the Dakota group which may be 

 in the lower part of the Middle Cretaceous, a series of 

 plants J essentially similar to those of the so-called Upper 

 Cretaceous of Greenland. They occur in beds indicating 

 land and fresh-water conditions as prevalent at the time 

 over great areas of the interior of America. But over- 

 lying this plant-bearing formation we have an oceanic 

 limestone (the Niobrara), corresponding in many respects 

 to the European chalk, and extending far north into the 

 British territory,* indicating that the land of the Lower 

 Cretaceous was replaced by a vast Mediterranean Sea, 

 filled with warm water from the equatorial currents, and 

 not invaded by cold waters from the north. This is suc- 

 ceeded by thick Upper Cretaceous deposits of clay and 

 sandstone, with marine remains, though very sparsely 



* Nordenskiold, " Expedition to Greenland," " Geological Magazine," 

 1872. 



f Yet even here the bald cypress (Taxodium distichwn}, or a tree 

 nearly allied to it, is found, though this species is now limited to the 

 Southern States. Fieldcn and De Ranee, " Journal of the Geological So- 

 ciety," 1878. 



J Lesquereux, " Report on Cretaceous Flora." 



* G. M. Dawson, " Report on Forty-ninth Parallel." 



