364 J. LeConte on the great Lava-flood of the West, 
dences of depression are of the clearest kind. In an admirable 
article published in the Overland Monthly (Nov., 1871), and 
entitled ‘‘ The Willamette Sound,” Mr. Condon traces an old sea- 
and sound-level, some 300 feet above the present sea-level, from 
the ocean, up the Columbia River, around the great valley of 
the Willamette and then up the Columbia again to the Dalles. 
Much of the evidence of this was shown me by Mr. Condon, 
on my recent visit to Oregon. Here there is undoubted evi- 
dence of depression and re-elevation. It is generally admitted 
also, that during the same period, all the flat-lands and valleys 
about the Bay of San Francisco, and the whole valley of the 
Sacramento and San Joaquin Rivers was covered with water— 
the latter forming an immense lake or sound. 
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E. Formation of the cavion of the Columbia River.—This enor- 
mous cafion—nearly 100 miles long, from two to five miles 
wide and 8,000 to 4,000 feet deep at the Cascades, has evidently 
been formed wholly by erosion, since the period of the lava- 
flood, i. e., since the Tertiary period. At the beginning of the 
only outlet to the sea would have been over this gap. In that 
case, the principal part of the erosion was produced by ice dur- 
gon, and A 
region. The dramage of the waters of this immense basin was 
through the Columbia River gap. The erosion, whether by 1¢¢ 
or water or both, during Post-Tertiary times, was certainly 
very great. I believe the distinction hetero the enormou 
Post-Tertiary erosion and the smaller recent erosion is still 
detectable. “Fig. 7 is a diagrammic section across the cafion at 
the Cascades. Between the high basaltic cliffs 2a, the dis- 
tance at the Cascades is at least five miles. This grand cafon, 
through 3,500 feet of lava down to the conglomerate, was, 
