> FOSSIL FLORA OF THE SIERRA NEVADA. 
influences have left less irrefragable marks of their activity. The ves- 
tiges of glacial action — moraines, erosions, striated rocks — are seen every- 
where in the valleys of California; while the glacial drift on the eastern 
slope of the United States scarcely passes south of the Ohio River. And 
as the immense plains extending from the Missouri River to the base 
of the Rocky Mountains have evidently been covered by water during 
the prevalence of the terrace epoch, or after the glacial period, this bar- 
rier, and also that of the chain of mountains still more impassable to 
plants than to water, forcibly prevented a western redistribution of the 
species destroyed in California by glacial agency. 
Notwithstanding these destructive influences, the flora of California still. 
preserves a few of the Pliocene types, and these, by their present habitat 
and the apparent modifications of their characters, seem to point to what 
have been the essential causes of the disappearance of the others. For 
instance, Betula equalis, Acer Bolundert, Cercocarpus antiquus, have now repre- 
sentatives which seem to have been gradually dwarfed or modified by 
the influence of the cold, and thus acclimatized gradually to the tempera- . 
ture of the subalpine zone which they now inhabit. Preserved during 
the glacial period in some sheltered nook, they have thus apparently 
wandered gradually to the mountains, following the disappearance of the 
ice. <A few other species have remained with their typical characters and 
their habitat, — Castaneopsis chrysophylla and Cornus Kelloggii, for instance, 
plants of hard texture and of great tenacity of life. According to the 
data kindly furnished by Professor Bolander, these species inhabit now 
near Oakland from an altitude of 1,800 feet to the Sierras, where Cas- 
taneopsis chrysophylla is met with to an altitude of 8,000 feet. Very few, 
if any, arborescent species of the present time have such a vertical 
range of more than five‘thousand feet. Cornus Kelloggi, according to the 
same authority, occupies the base of densely wooded slopes of the Sierras, 
or is found in open places, where there is sufficient terrestrial moisture ; 
even in boggy places of the Yosemite Valley, ascending to 5,000 feet. 
Another species, Cornus ovalis, which was probably very abundant in the 
Pliocene flora, has been about totally destroyed in California. It looks 
like an isolated remnant of a type mostly driven southward at the glacial 
period, and now inhabiting Mexico. The two species of Sequoia — one 
the more predominant, the other the more remarkable, of the flora of 
California — are evidently also remnants of the Pliocene. S. gigantea, which 
