74 BRITISH EOCENE FLORA. 



the closely-allied English Lower Eocene and Greenland Middle Eocene floras, and they 

 possess species in common, not only with these but with the warmer Middle Eocene flora 

 of the South of England, and therefore must be assigned on plant evidence to an age inter- 

 mediate between these periods. They must, in fact, be relics either of the passage of the 

 Lower Eocene flora north, or of the descent of the Miocene flora south, or else the 

 permanent Middle Eocene flora of those latitudes. Against the former supposition we 

 have the occurrence of the flora in many localities in both Ireland and Scotland, 

 rendering it unlikely that they all were deposited in so limited a period as that in which 

 the change in temperature can be proved to have taken place, even if the mixture of 

 Middle Eocene forms from the south among them were not conclusive. Against the 

 second supposition we have the appearance of immense antiquity presented by the basalts, 

 and the dissimilarity of these floras to those of the European Miocenes. It must be 

 remembered that there is no stratigraphical or other evidence whatever, except the floras, 

 as to the ages of the Scotch, Irish, Icelandic, or Greenlandic plant deposits, though the 

 colossal denudation of the basaltic formations in which they are enclosed, had led many 

 independent observers to assign to them a far greater antiquity than the plant evidence 

 has hitherto seemed to warrant. 



Onoclea hebraidica occurs fossil at Mull, at Greenland, and in the Fort Union group 

 of Dakotah, and it still exists unchanged as Onoclea sensibilis in the United States and 

 on the temperate east coast of Asia. It might therefore be found among any Tertiary or 

 more recent assemblages of fossil plants in countries in which it still lives without 

 afi'ording a clue to their age, and it would therefore be useless to endeavour to make its 

 presence a test of the real age of the Fort Union group. Professor Newberry, however, 

 made its occurrence in the American Tertiaries the basis of speculation which it now 

 seems desirable to challenge, as the more recently ascertained distribution may be equally 

 well read in a difierent sense. He considered that since this and other species, at that 

 time couimon to the Old and New Worlds, have disappeared from Europe, while they 

 continue to flourish in America, it would follow that these were American types which 

 had colonised Europe by migration, and that when their connection with the mother 

 country was severed they were overpowered and exterminated by the present flora of 

 Europe, which Prof. Gray has shown to be mainly of north Asiatic origin. The occur- 

 rence of Onoclea sensibilis on the Island of Midi, while it has not been found in the 

 Tertiary beds of other parts of Europe, indicated he believed not only an Ameiican 

 connection during the Miocene period, but an American origin for that species. We 

 need not here follow Prof. Newberry further, as the remainder of his essay is based on 

 the community of American and European Miocene genera, a question not directly 

 connected with the present subject, but it is only necessary to point out that if the 

 Onoclea occurs in older rocks in Europe than in America, as well as in the intervening 

 Greenland, the evidence, so far as it goes, favours the theory of its migration north from 

 Europe to Greenland during the warmer Eocene, and south from Greenland to America 



