GENERAL LAWS OF ORIGIN AND MIGRATION. 243 



Tention 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 roots 

 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 i 

 1872. 



f Yet even here the bald cypress {Taxodium dislichum), 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," ISIS. 



^ Lesquereux, " Report on Cretaceous Flora.'' 



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



