ON THE TERTIARY FLORA OF THE NORTH AMERICAN LIGNITIC, 

 CONSIDERED AS EVIDENCE OF THE AGE OF THE FORMATION. 



The purpose of this memoir, as indicated by the heading title, is to 

 present, with more details, the evidence offered by the flora of the Lig- 

 nitic Measures of the West, in regard to the geological age of their for- 

 mation, which 1 cousider as Tertiary. The reasons advanced in favor of 

 this opinion in the two former annual reports of Dr. Hayden, and in a 

 paper in the American Journal of Science and Arts,* are controverted by 

 some geologists who consider the Lignitic as Cretaceous, denying to vege- 

 table paleontology the authority of evidence in a question of this kind. 

 Their arguments may be briefly exposed in the order in which I propose 

 to examine and discuss their importance. 



1st. Fossil plants are rarely found in the geological formations which 

 are mostly marine ; the vegetable remains are generally undetermina- 

 ble fragments of leaves ; they have been, as yet, scarcely studied in this 

 country ; the records of the fossil floras are nearly mere blanks. 



2d. If even the fossil flora of the great Lignitic of the West had been 

 widely studied, it could not afford any reliable evidence, on account of 

 the impossibility of a conclusive comparison of its species. A compari- 

 son of the fossil plants of this continent with those of Europe can prove 

 nothing in regard to identification of geological periods, for the reason 

 that at the same epoch the floras of both continents may have been far 

 different in their character, a necessary result of differences in their 

 atmospheric circumstances of the same period of time. 



3d. Even supposing that the evidence could be admitted, it is put at 

 naught by the presence of cretaceous animal remains in strata within or 

 above the lignitic formations, and animal remains must have priority 

 for the determination of geological groups. 



4th. The strata of the Lignitic conformably overliethe Cretaceous, and 

 the nature of the compounds of these so-called different formations are 

 similar. 



5th. Stratigraphy and animal paleontology have forced the conclu- 

 sion in regard to the cretaceous age of the western Lignitic, and geolo- 

 gists of high standing have, by their opinion, given full authority to 

 this conclusion. 



The text of the argumentation in favor of the cretaceous age of the 

 Lignitic is here exposed in the fairest possible way. It could be said 

 merely that the objections have been already considered and answered, 

 but nobody would be satisfied or enlightened by this assertion. It is, 

 therefore, advisable to reconsider the subject and to expose, instead of 

 contradicting arguments, facts, which, corroborating former assertions, 

 bring some new light upon the controverted question. For, in the two 



* Annual Report of the United States Geological and Geographical Survey, &c, for the 

 explorations of 1872, p. 318, &c. Same Report for explorations of 1873, p. 367. On the 

 age of the lignitic formations of the Rocky Mountains, American Journal of Science and 

 Aits, vol. vii, June, 1874. 



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