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 palzontological 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 paleophytologists. 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 Cenozoic 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 types 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. Hayden in Nebraska of a fev 
leaves apparently referable to Sassafras, Liriodendron, Platanus, 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 
