314 GEOLOGICAL SUEVEY OF THE TEREITORIES. 



of disputed ground. It even appears that, in some case at least, fossil 

 animals vouch for their Cretaceous relation.* It is the same case with 

 the fossil plants described from Mississippi and Tennessee.t They were 

 at first considered, from geological evidence, as Cretaceous, and have 

 been definitelj' admitted as of the Eocene age.t The relation of the 

 species examined here from specimens of Dr. Hayden, and which are 

 placed in the Eocene section of the table, have the same general charac- 

 ter as those of the Mississippi, and are evidently of the same age. 

 Admitting, therefore, the third section of the table for plants of the 

 Eocene, the species of the second section must be referred to the Lower 

 Miocene. They differ by their general facies from those of the third 

 section, and the strata with which they are connected have been recog- 

 nized as of a higher geological horizon. The first division of the table, 

 marked Middle Miocene, is by its flora indefinite, its species, as already 

 remarked, having relation to all the stages of the Tertiary. 



It will not be possible to know anything of the characters of the flora 

 of the different stages of our North American Tertiary except after pro- 

 longed and careful researches; for in trying to ascertain the species of 

 plants pertaining to a peculiar division of an epoch, the results appear 

 to be the same for the Tertiary as for the Carboniferous formations. 

 Local groups are generally well limited; their characters are at first 

 considered as resulting from difference of age. But more extended 

 researches show identical vegetable forms at other i^laces of evidently 

 different horizons, forcing the conclusion that most generally', at least, 

 geographical distribution is the essential cause of the diversity of vege- 

 table groups in the same formations. 



§ 5. Typical Analogy of the present Flora of North Aivierica 

 WITH Tertiary and Cretaceous Species. 



I have already alluded in a general way§ to this fact: that the essen- 

 tial types of our actual flora are marked in the Cretaceous, and have 

 come to us after passing, without notable changes, through the Ter- 

 tiary formations of our continent. Before any species of our Tertiary 

 had been recognized and described, the general facies of the European 

 Miocene had been compared to that of the present North American 

 flora, and from the remarkable analogy of the vegetation of both epochs, 

 the conclusion bad been driven, that the present flora of ours had received 

 its essential representatives from species migrated from the European 

 Tertiary. If the assertion brought forth in the beginning of this chapter 

 is right, the contrary conclusion is true ; that is, the Tertiary flora of 

 Europe is essentially a compound of American types, and our Cretaceous 

 flora is the ancestor as well of our present flora as of that of the Tertiary 

 of Europe. It is worth while, therefore, to briefly consider the essential 

 proofs of this assertion, reserving for a future report a detailed exposi- 

 tion, which may be rendered more conclusive by the collecting of new 

 materials.il 



* Dr. F. V. Hayden in letters, 



t Species of Fossil Plants fiom the Tertiary of Mississippi, in Trans. Am. Phil. Soc, 

 Tol. XIII, pp. 4-2G and 427. 



t The discussion on the age of these strata is clearly and thoroughly exposed in 

 Danas's Manual of Geology, pp. 509-511. See also F. V. Hayden's Annual Report for 

 1S70, p. 363. 



'^ American Journal of Science and Arts, vol. XL VI, p. 104. 



II The word tj/pe is here used in its more general sense,' as a figure of something to come, 

 without considering antecedence. In that way it is more acceptable than the word 

 race, which rather offers an idea of derivation. 



