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-tsuga). 

 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 hendirei, 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 e^Tn 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. Stcrculea, and Liquidamhar 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 fliora by these temperate 

 species. Apparently the climate, while somewhat warmer, was 

 approaching the temperature conditions of the region today. 



The flora of the P^agle 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 



