GENERAL CONCLUSIONS. 51 



determinable condition have been referred, either positively or probablv, 

 to existing species of the United States flora, most of them now inhab- 

 iting; a few degrees farther south." 1 



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 ; 

 ami from the data exposed in detail in his Report, Professor Whitney 

 estimates the amount of denudation, during the period since the volcanic 

 ina>s 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, 

 hut 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 ('hulk Bluffs of California have a remarkable affinity to those of the 

 Pliocene of the Mississippi, above referred to by Professor Gray, — Quercm virens and its varieties, for 

 example. The lithological characters of tin- clay-beds, which at Columbus, Kentucky, are overlaid by 

 a thick deposit of agglomerated gravel, arc also the same, so that it mighl not l»- inconsistent to 

 admit synchronism for thofe two formations. 



: Geological Survey of California, by J. 1). Whitney, State Geologist. Geology, Vol. T. pp. 244, 245. 



