131 



recognized already in the London clay, ascends to the Lower Miocene of 

 Germany and Italy. In North America, the same species of Quercus has been 

 found, in numerous and finely-preserved specimens of leaves, by Prof. Jos. 

 Le Conte, under the lava-beds of the Cascade Mountains of Oregon ; by Prof. 

 J. D. Whitney, in clay-beds of the Spanish Mountains of California; and by 

 myself, in more fragmentary leaves in the Lignitic of Golden. Thus we have 

 a series of closely-allied forms of oaks recognized, in the Quader sandstein 

 of Bohemia as PhyUitcs Geiniizianus ; in the Upper Cretaceous of Belgium 

 as Dryophyttum cretaceum; in the Nebraska Cretaceous as Quercus primor- 

 dialis; in the Lower Eocene, or Paleocene of France and Belgium in numer- 

 ous species of the same Dryophytlum; in the Eocene of Europe and in the 

 Lignitic of the United States as Quercus furcinervis. Analogous forms of the 

 same type are traced farther up in the Miocene of Europe and of America, 

 in the Pliocene of California, and at a later epoch in a large number of our 

 present species of oaks. 



What conclusions can be derived from these facts 1 In regard to the 

 flora of the Dakota group, the re-appearance in a subsequent period in the 

 European Tertiary of one of its types does not modify in the least the remark 

 on the disconnection of this flora from the antecedent and next succeeding 

 vegetable groups as far as they are known. This genus, Dryophyllum, seems 

 to have had, since its origin, a large number of representatives, and to have 

 been widely, if not universally, distributed. Its presence, therefore, in suc- 

 cessive formations, as in different local groups of floras of synchronous stages 

 of the Cretaceous, merely denotes an omnipresent, and, at the same time, a 

 persistent type, which, like those of Salix, Sassafras, Platanus, &c., has passed 

 through all the geological floras to that of our time with more or less definite 

 modifications. 



Nevertheless, the flora of Sezane, like that of Gelinden, of the Gyps of Aix, 

 represent, from the Upper Cretaceous to the Upper Eocene, a series of land- 

 formations with vegetable groups, which are absent in the American geology. 

 This fact may furnish an argument against the assertion of the as yet uiiex- 

 plainable disconnection between the Cretaceous flora of Kansas and Nebraska 

 and that of the Lignitic of the Rocky Mountains ; for, in the long period of 

 time manifested by these successive and already diversified groups of plants of 

 the Lower European Eocene, the characters of the floras must have been con- 

 siderably changed, either by introduction of new species or by modifications of 



