284 GEOLOGICAL SURVEY OF THE TERRITORIES. 



in some regions are mostly marine, and have as yet afforded too scanty 

 materials to define somewhat clearly the characters of its flora in the 

 numerous subdivisions of the formation. 



The Cretaceous flora of North America, as far as it is known from its 

 representatives in Kansas, Nebraska, Dakota, and Minnesota, has been 

 reviewed in this report and speaks for itself. ' Its characters, as they 

 are known now, will be more expressively compared to those of the 

 Lignitic flora, and the differences more distinctly seen when the Tertiary 

 species are published with figures. From the multiplicity of its types, 

 some of them transient or indefinite, it is now easily understood that 

 the attempt of a comparison of the few first leaves discovered in Ne- 

 braska could but mislead the most competent and careful paleontologist 

 in looking for typical relation in order to determine their age. The 

 records of this Cretaceous floracould not be read, indeed, before they had 

 been written, or when they were exposed by a few scattered Words only. 

 Now the North American Cretaceous plants represent a definite group, 

 which, though susceptible of wide extension by new discoveries, has its 

 essential characters already defined, and is thus available as a point of 

 comparison for paleontological documents, either from this country or 

 from Europe. It is in this point of view especially that the importance 

 of the publication of the fossil plants of this country has to be judged. 

 That the geological age of the Dakota group flora, as loug as its char- 

 acters were unknown, should have been subjectivps to the evidence af- 

 forded by its overlying marine strata, which were, clearly determined 

 by invertebrate animal remains, is a matter of course. But now this 

 flora affords a collateral evidence which by its vegetable types may be 

 used for geological determinations just as legitimately as the fauna. 

 From a subordinate it becomes an assistant. 



I consider that this discussion upon the authority of vegetable paleon- 

 tology in regard to the determination of the age of the disputed strata, 

 Cretaceous Lignitic or Lignitic Tertiary, has been o±^ great value to Amer- 

 ican geological science. It has induced wide and more careful researches, 

 and brought forth a large number of important discoveries which, with- 

 out it, would have probably been indefinitely postponed. No depart- 

 ment of geology should be disregarded or considered as of an inferior 

 concern. All have an equal right as members of a same body. And 

 was it only for the reason that vegetable paleontology has been gen- 

 erally, and is still now, considered by many as of little value as an as- 

 sistant to geological pursuits, I am the more disposed to persist in 

 putting it forward as an authority superior to that of animal pale- 

 ontology for the determination of the age of the strata of land forma- 

 tions. 



The above remarks all tend to the same purpose, and serve as an in- 

 troduction to a more detailed examination of the age of the Lignitic as 

 exposed by the fossil flora. 



To appropriately enter into the subject, we should have a clear under- 

 standing of the now adopted names and limits of the numeroussubdivis- 

 ions or groups of the Tertiary, as marked by European authors. Though 

 it may be that some of these groups are not positively defined, either in 

 their geological relation or in their paleontological characters, they are 

 serviceable for comparison. 



Table of subdivisions of the Tertiary of Europe, according to tlie floras. 



Pliocene. Lower limits not positively fixed: largely developed in 

 Italy. 



