GENERAL CONCLUSIONS. 39 



Miocene formations of Oregon, and a few also from California ; they are 

 reserved for a later publication. The relation of these plants is, however, 

 casually considered in this memoir. 



In 1873 I delivered to Professor Whitney a preliminary report on these 

 plants, with descriptions of the species, remarking, as a conclusion, that 

 the flora of the auriferous gravel of California had a predominance of 

 species either identical or closely allied to some of the present North 

 American flora, but had still some representatives of Miocene types, 

 which imprinted on it a character of antiquity more marked than is 

 generally expected in the vegetation of a Pliocene period. I therefore 

 considered this group of plants as referable to the oldest Pliocene, or to 

 a formation intermediate between the Miocene and the Pliocene. 



These conclusions were neither positive nor definitive, for we had then 

 for comparison, outside of the plants of our time preserved in the her- 

 bariums, merely palceontological works on the Miocene species of Europe, 

 and from this it was irrational to draw conclusions on the characters or 

 the relations, either antecedent or subsequent, of a flora so closely allied 

 to that of the present epoch of North America, whose types, especially 

 for the arborescent species, are far different from those of Europe. 



Now the circumstances are greatly changed in this country, and have 

 become far more favorable to the studies of the palaeophytologists. The 

 collections of specimens have been enriched in a remarkable degree by 

 the discoveries of later years, and what has been published until now 

 on the vegetable remains of the Mesozoic and Camozoic formations of this 

 continent may be used with a degree of reliance for the determination 

 of the geological age of some deposits, or at least for defining the rela- 

 tion of the groups of plants pertaining to them. 



The Cretaceous flora of the Dakota group deserves first to be mentioned, 

 not merely on account of its precedence in the order of the discoveries, 

 but especially on account of the remarkable characters of its dicotyledo- 

 nous leaves, which already represent some t^ypes reproduced in species 

 living at our time, and, as may be reliably inferred, in those of the inter- 

 mediate formations. Our first acquaintance with those plants is derived 

 from the discovery made by Dr. F. V. Harden in Nebraska of a few 

 leaves apparently referable to Sassafras, Liriodendron, PZatanns, etc., and 

 from the discussions on their characters and their true relation, as recorded 

 in the American Journal of Sciences and Arts of 1859, especially. This 



