HANNIBAL: A PLIOCENE FLORA FROM CALIFORNIA 341 
All the species live today in the Coast Ranges of California 
north of San Francisco Bay. Indeed one familiar with the plants 
of that region would immediately identify this flora with that 
which inhabits the valleys of Sonoma, Mendocino, and Humboldt 
counties a few miles from the ocean in the zone of overlap of the 
redwood belt (Transitional zone) and chaparral belt (Upper Sono- 
ran zone). That it does not show the extreme humid conditions 
of the open coast is evident from the presence of such species as 
Alnus rhombifolia, and species of Cercocarpus and Arctostaphylos. 
On the other hand .Sequoia delineates it as at least partially fog 
belt in aspect. 
EVIDENCE OF A COLD EPOCH 
This flora agrees very well with the evidence of the aquatic 
mollusca and points to perceptibly colder conditions in central 
California during Pliocene time, but makes it certain that this 
cold facies was due not to elevation, but to actual migration of 
isotherms. Such a condition could not have been a local phe- 
nomenon but was probably widespread on the Pacific Coast. 
This offers an explanation of the isolated occurrence of numer- 
ous arctic and boreal plants on the tops of high mountains far 
south of their normal distribution. The Santa Clara period was 
succeeded by the Sierran epoch, when the entire coast seems to 
have been carried by a great orogenic uplift many hundreds of 
feet above its present elevation. Very little is known of the 
climatic conditions which existed in California at that time. The 
rapid erosion prevented the formation of extensive lakes, and 
later depressions have carried all the marine deposits of the period 
even deeper beneath the ocean. This page, torn from the other- 
wise complete geologic record of the late Cenozoic of the coast, 
probably corresponds at least partially to the Glacial epoch else- 
where. Hence it is commonly assumed to have been colder and 
more humid than the present. It seems probable that the northern 
plants continued to exist south of their present southern limits 
during the early Quaternary, as a result of the cold epoch, the 
high elevation, the effects of which would be similar, or both, and 
were finally isolated toward the close of the Sierran epoch in the 
middle Quaternary. 
