GENERAL CONCLUSIONS. 5l 
determinable condition have been referred, either positively or probably, 
to existing species of the United States flora, most of them now inhab- 
iting a few degrees farther south.” ? 
Professor Heer also, in his Flora of Alaska, admits that the essential 
types of the North American vegetation of our time are far more distinct 
there than they are in the Miocene of Europe. This, therefore, invalidates 
the old hypothesis of the migration of vegetable Miocene species from 
Europe to America, a supposition which was warranted at the time by 
the relation of our present Northeastern flora with that of the European 
Tertiary. 
What is known of the disturbances which have followed the Pliocene 
epoch in California is sufficient to explain the destruction of its flora. 
Professor J. D. Whitney says of the auriferous deposits of Tuolumne 
County, from which were obtained a large number of the specimens 
described here, that the Table Mountain covering them has been formed 
by a flow of lava which filled the valley after running forty miles down 
the slopes of the Sierra, and forming a continuous ridge elevated more 
than two thousand feet. The lava covers detrital beds of gravelly mate- 
rials which in the centre of the valley are fully two hundred feet thick ; 
and from the data exposed in detail in his Report, Professor Whitney 
estimates the amount of denudation, during the period since the volcanic 
mass took its present position, at three or four thousand feet of perpen- 
dicular depth. And yet this was done during the most recent geological 
epoch, and these surprising changes have not been peculiar to this region, 
but the whole slope of the Sierras through the gold region has been 
the scene of similar volcanic overflows and subsequent remodellings of 
the surface into a new system of relief and depressions.” 
This tells the whole story, and clearly accounts for the disappearance 
of a number of vegetable Pliocene types in California during the recent 
geological epochs by marine submersion, the all-destroying glacial agency, 
and volcanic cataclysms of long duration; and contrariwise it explains their 
preservation on the eastern part of the continent, where the destructive 
1 Some of the species of the Chalk Bluffs of California have a remarkable affinity to those of the 
Pliocene of the Mississippi, above referred to by Professor Gray, — Quercus virens and its varieties, for 
example. The lithological characters of the clay-beds, which at Columbus, Kentucky, are overlaid by 
a thick deposit of agglomerated gravel, are also the same, so that it might not be inconsistent to 
admit synchronism for those two formations. 
2 Geological Survey of California, by J. D. Whitney, State Geologist. Geology, Vol. I. pp. 244, 245. 
