3J2 UNITED STATES GEOLOGICAL SUKVEY— TERTIARY FLORA. 



assistance of Count Saporta, who, iVoiii the examination ol' tlie plat(>s, and 

 1)V the comparison of onr species with European specimens, advised some 

 modifications, wliich 1 readily accepted, and wliich, as seen in the descrip- 

 tive part, have been credited to this celebrated paleontologist. This explains, 

 for some of our species, a synonymy whicli may seem at first to be regretted; 

 but for an American monograph, which, as the first on this subject, may be 

 hereafter used as a basis for the comparative .study of the flora of the 

 Lignitic, the precision of the work, as far as it was oljtainable in any ])os- 

 sible way, had to be alone considered. And, certainly, the determination of 

 our American species of fossil plants confirmed by Saporta, who is now the 

 leading paleo-botanist of France and the successor of Brongniart, gives to tliis 

 memoir a reliability which could not have been secured without the means 

 furnished for comparative study by the immense collections of living and 

 fossil plants of Europe. 



Another objection is likely to be made against the mixed references 

 of the fossil plants to the geological formations which they represent. The 

 divisions could have been rendered more striking indeed by describing, in 

 separate monographs, the species represented in each of the groups. I was 

 at first disposed to prepare the work in this way; but, as will be seen by 

 the examination of the distribution of the plants from the list given be- 

 yond, the relation of age of some of these groups is not positively ascer- 

 tained; and I believe that the comparison of species made from the general 

 exposition of the whole flora will serve to elucidate the question concerning 

 those uncertain references. Indeed, as the specimens were sent to me in 

 successive lots or series, which sometimes were not clearly defined in regard 

 to their place of origin, I needed myself such a kind of general comparison 

 of species, whose geological age was merely presumed, in order to fix my 

 opinion about their real aflinities. And also, as every year, at different times 

 of the year, 1 have been, and am still, in receipt of specimens from distant 

 localities, it became evident, from the first, that I should have to describe, 

 too late for the arrangement of the plates, a number of species wliich could 

 not be distributed according to the plan proposed. This is tlic case, for 

 example, for the five last plates, made from specimens from Point of Rocks, 

 Lower Eocene, from the Carbon, and from the Upper Green River group, 

 that of the Parks. To obviate an apparent confusion, which may give, at 

 first sight, in the examination of the ])lates, .some uncertainty in regard 



