4 g 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 Magnoliacca 7 , 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. Liriodcndron 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 scarceky different by the character of its leaves from those 

 of the living Tulip-tree. There is also an Asitrim known by its leaves in 

 the Miocene of Carbon, and another by its fruits in the Eocene of the 

 Mississippi. The Meni&permacece have, in the American Cretaceous, leaves 

 of characters quite similar to those of Menispermiim Canadense and Coccu- 

 lus CaroUnus. To represent the Nymphacece, there arc two species of Nelvm- 

 binm in the Eocene of Colorado. The Anacardiaccw have a Zanthoxylum 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 Vitacece, Cretaceous by different 

 leaves described under the generic name of Ampehphyllum, appear more 



