2o6 The American Geologist. October, i8S9 
stroyed or forced to migrate by fresh deposits of lava. These 
migrations are very noticeable in central Washington where 
fossils assigned to the Miocene period are abundant. These 
fossils are distinctively akin to Tertiary species, but as much so 
to modern. They are confined to migratory types of life; the 
marked and significant absence of non-migratory types, and 
the abundance of those types capable of so escaping, together 
with the equal similarity of the fossils so far found to modern 
species, cause the writer to doubt the correctness of assign- 
ing this group of beds to the Miocene period.* The same 
class of evidence which would assign them to the Miocene 
would also assign them to a much more recent period. 
Traces of volcanic heat yet remain, as is evidenced by the 
existence at numerous localities throughout the area of hot 
springs, fumaroles and heated earth. In the Yellowstone park- 
are several thousand vents for hot water, gases, and boiling 
mud. 
The vents from which these vast inundations escaped are 
not known. Although numerous extinct craters exist in the 
area, the slopes of the lava plain are such as to preclude the 
' view that these were the main sources. The main outflow must 
have been from vast rifts now concealed, or at least undiscov- 
ered. Irregular and unequal movements of the crust have oc- 
curred since the outflow, so that the present slopes do not al- 
ways coincide with those of the flowing lava. 
The character of the lava varies in successive layers and in 
different localities. In some instances it is so soft that it can 
be cut with a knife, and in others is a compact, brittle, and col- 
umnar basalt. In some instances this lava sheet directly over- 
lies Tertiary formations. 
In its more elevated portions, particularly in the western 
part, evidences of late local glaciation occur. To the north and 
west glacial action of enormous extent and energy is noted. 
To the .southwest in the Sierras, glacial action is also of great 
extent. Southerly glacial action is more scattered and "local," 
as are the irregular lava outbreaks extending through the ad- 
joining states into Mexico. 
*Bulletin No io8, U. S. Geol. Survey. 
