54 FOSSIL FLORA OF THE SIERRA NEVADA. 
balanced by species of Betula, Fagus, Uinus, ete., whose range of distribu- 
tion goes much farther north, and scarcely descends below the 30th parallel. 
Hence, a climate like that of the gulf shores, the zone of the Live Oak, 
is about the same as that represented by the fossil plants described from 
Nevada County. 
As a conclusion to these remarks, the essential points of information 
derived from the examination of the groups of planis of the Chalk Bluffs 
of Nevada and Tuolumne Counties, California, may be briefly recalled as 
follows : — 
1. This flora is, up to this time, limited to fifty species. These are re- 
lated by some identical or closely allied forms to the Miocene, and still 
more intimately by others to the present flora of the North American 
continent. 
2. The North American facies is traced by some species to the Mio- 
cene, the Eocene, even the Cretaceous of the Western Territories. Hence 
it is not possible to persist in considering the essential types of the pres- 
ent North American flora as derived by migration from Europe or from 
Asia, either during the prevalence of the Miocene or after it. This flora 
is connatural and autochthonic. 
3. The relation of the Pliocene plants of Nevada and Tuolumne Coun- 
ties is with the flora of the Atlantic slope, and not with that of Cali- 
fornia at the present time. This fact is explained by the influence of 
glacial action during the prevalence of the ice period, and is even clearly 
exposed by the distribution of the few Pliocene species remaining in the 
flora of the Pacific coast. The modification of the characters of the pres- 
ent flora of California have, therefore, to be looked for in climatic or 
other phenomena subsequent to the glacial period. This remarkable fact, 
so clearly demonstrated by nature, may serve as an exemplification of 
the causes of the disconnection of some of the other groups of our geo- 
logical floras. | 
4. This small group of Pliocene fossil plants from California denotes 
the importance of the study of the North American Pliocene in relation 
to that of the characters and of the distribution of the present flora of 
the continent. Professor A. Gray, as seen above, has already alluded to 
the probable evidence which might hereafter be obtained bearing on the 
