46 FOSSIL FLORA OF THE SIERRA NEVADA, 
Pliocene flora of California, and in the present flora of the Atlantic slope 
of this continent, but not at all in that of the- Pacific. 
This remarkable dislocation of the flora of the Pliocene from that of 
California may be explained in two ways: either by modifications in the 
physical circumstances of the Pacific slope of the United States after 
the Pliocene epoch, or by the old hypothesis of a case of spontaneous 
production of new vegetable types, which were supposed to be generated 
for every new geological formation. 
To set aside this last hypothesis, we have only to refer briefly to the 
essential characters of the ancient floras of North America from the ap- 
pearance of the dicotyledonous plants in the Cretaceous, and to see 
if the essential types of the Atlantic flora and of the Pliocene of Cali- 
fornia are there already distinctly recognized. To do this I will merely 
consider the more marked groups of arborescent vegetables in the order 
in which they are described in Gray’s “Botany of the Northern United 
States.” 
Beginning with the Magnoliacee, this family of plants is positively Cre- 
taceous. Species of Magnolia first described from the Dakota group of 
Nebraska and Kansas (also from the Cretaceous of Moletin, Germany) 
are found, more and more related to those of the present time, in the 
Eocene Lignitic of the Mississippi and that of the Rocky Mountains, 
especially of New Mexico; in the Miocene of Carbon and in the Pliocene 
of California, where the specific forms become apparently identical with 
some of those known now and described by Gray. Liriodendron is one 
of the best defined genera of the same Cretaceous formation, the Dakota 
group, where its numerous leaves have been referred to three species, 
one of them scarcely different by the character of its leaves from those 
of the living Tulip-tree. There is also an Asinina known by its leaves in 
the Miocene of Carbon, and another by its fruits in the Eocene of the 
Mississippi. The Menispermacee have, in the American Cretaceous, leaves 
of characters quite similar to those of Menispermum Canadense and Coccu- 
lus Carolinus. 'To represent the Nymphacee, there are two species of Nelum- 
bum in the Eocene of Colorado. The Anacardiaceee have a Zanthozylum and 
a number of species of Rhus in the Pliocene of California, and still more 
of a similar type in the Upper Miocene of Colorado. This last order 
seems to be of recent origin, while the Vilacew, Cretaceous by different 
leaves described under the generic name of Ampelophyllum, appear more 
