590 RALPH W. CHANEY 
that the upland conditions were such as to support an oak forest. 
Today the uplands, in the western part of the gorge at least, are 
occupied by the more mesophytic Douglas spruce (Pseudo-isuga). 
Comparing the upland plant associations of the Eagle Creek epoch 
and the present, we are justified in concluding that the climate was 
more arid then than it is now. Looking to the mesophytic valley 
association of the Eagle Creek flora, the presence in it of such 
moisture-requiring forms as Acer bendirei, Ulmus speciosa, and 
others indicates that in the depressions the air was moist. This 
moisture was no doubt contributed in large part by the streams 
occupying the valleys, but the very presence of so mesophytic an 
association indicates that even semiaridity did not exist anywhere 
in the region. Rather the moisture conditions were like those at 
present existing in the eastern part of the Great Plains, where the 
mesophytes are restricted to the stream borders along the valleys 
because the soil there is moist. Such at least is a reasonable con- 
jecture. The cause of the greater aridity in Eocene times than at 
present is not known. Presumably a mountain range to the west 
may have cut off the moisture-bearing winds, thus reducing the 
amount of rainfall, though there is no direct evidence for supposing 
that such a range existed. 
Concerning the temperature, the presence of such tropical or 
subtropical forms as Smilax, Sterculea, and Liquidambar suggests 
that the climate was warmer than that in the region today. The 
dominance of such temperate types as Quercus, Acer, Ulmus, and 
others puts the evidence in favor of a climate which was cooling, 
with the resultant invasion of the tropical flora by these temperate 
species. Apparently the climate, while somewhat warmer, was 
approaching the temperature conditions of the region today. 
The flora of the Eagle Creek formation gives valuable sugges- 
tions as to the length of time involved in the epoch. From a 
purely physical standpoint, the great thicknesses of ash, tuff, and 
conglomerate might have been piled up in a comparatively short 
time, perhaps measured in scores of years rather than in units of a 
larger denomination. Turning, however, to the evidence of the 
plants, it may readily be shown that the time required is much 
greater. ‘The number of years necessary for the development of a 
