354 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



are so definite that a single specimen of a species of Acer or Platanus would 

 suffice to positively identify this group as Miocene, just as a few specimens 

 of Quercus furcinervis proved the Eocene age of the Cascade Mountains of 

 Oregon, whose formation was at first supposed to be post-Tertiary, or of 

 recent origin. 



We have seen also that the fioraof the fourth group has a peculiar facies 

 without distinct relation to that of the third. I regard it as Ujiper Miocene 

 by affinity of some of its types to those of the same formation of Europe. 

 A number of geologists, Dr. Hayden the first of them, have determined the 

 position of the Green River group as above the lignite-bearing beds of the 

 Rocky Mountains. 



I do not consider, however, as definitive, the distinction of the geo- 

 logical divisions as they are established in this work and marked upon the 

 table of distribution. New researches and discoveries may afford sufficient 

 reasons for reversing my conclusions. They are only the expression of 

 the testimony obtained from the determination of the vegetable remains 

 examined until now. I have also acknowledged already that the groups 

 of plants of the American Tertiary compared with those of Europe, repre- 

 senting different geological periods, do not demonstrate positive identification 

 between the formations of both continents. I admit the lower group as 

 Lower Eocene; the second group, which seems intermediate between this 

 and the Carbon, may be Upper Eocene; the relation of the third group is 

 by its plants with the Lower and Middle Miocene of Eui'ope, and that of the 

 fourth with the Upper. These are like the first outlines traced for the prep- 

 aration of a map: they may be erased or modified; the spaces have to be 

 filled as our acquaintance with the Tertiary becomes more intimate. 



Now reviewing the whole question of the age of the Lignitic, I readily 

 admit the fact, established from sufficient evidence by the researches of Profs. 

 C9pe and Meek, that a fossil Cretaceous fauna has left traces of its presence 

 up to tbe very base of the Lower Lignitic measures, and that there the 

 remains of a few invertebrate animals, and those of one Saurian, all of Creta- 

 ceous types, has been found, in connection with plants whose characters have 

 been considered until now in Europe as representing a Tertiary Flora. Is 

 the flora to decide the relation of age of the formation or the Saurian bones 

 of Black Buttes, with the few shells of brackish water, either found there or 

 which may be found hereafter in the same circumstances? The slow upheaval 



