ECOLOGICAL SIGNIFICANCE OF EAGLE CREEK FLORA 579 
vegetation cover. Conditions become progressively more arid 
toward the east, with only a sparse occurrence of pines and oaks 
at The Dalles. 
GEOLOGIC RELATIONS OF THE EAGLE CREEK FORMATION 
A recent study of the geology of the Columbia River Gorge by 
Drs. I. A. Williams and J H. Bretz under the auspices of the Oregon 
Bureau of Mines has given us a rather complete knowledge of the 
general geology of this little-known region.t As far back as 1873 
LeConte recorded the presence of fossil leaf impressions in the 
volcanic conglomerate at the base of the basalt series.? In 1895 
Diller secured a small collection of leaves near the mouth of Moffatt 
Creek which has been described by Knowlton,? and four years 
later Gilbert made a larger collection from a talus block near 
Cascade Locks, a collection which has never been described. ‘The 
collections which are the basis of this paper are, however, the first 
which are sufficiently complete to give any conclusive evidence 
regarding the age of the Eagle Creek formation. 
There are but few cases of such an illuminating record of the 
history of a mountain range as has been furnished by the Columbia 
River in its path across the Cascades. Following is the generalized 
section exposed by the Columbia: gravels and river terraces of 
recent origin; Herman Creek lava—andesitic basalt; Satsop 
formation—stream gravels and volcanic ash; Columbia River 
lava—successive flows of basalt; Eagle Creek formation—volcanic 
conglomerate, ash, and tuff. 
The Eagle Creek formation—The Eagle Creek formation is 
exposed along the bottom of the gorge from Warrendale to Viento 
on the Oregon side with a corresponding distribution on the north 
side of the river. It is the oldest formation recognized in the region, 
and is brought to the surface in the axis of the great north-south 
anticline which is the backbone of this portion of the range. The 
thickness of the exposed part of the formation varies from 2,700 
‘Tra A. Williams, Bull. of the Ore. Bureau of Mines and Geol., Vol. II, No. 3, 1916; 
J H. Bretz, unpublished manuscript. 
2 J. LeConte, Am. Jour. Sci., 3d Series, VII, 167-80. 
3 F. H. Knowlton, Twentieth Ann. Rept. U.S. Geol. Surv., pp. 37-64. 
