176 J. LeConte on the great Lava-flood of the West. 
had cut down into the underlying conglomerate. Then the 
commenced to work back as perpendicular falls, until they 
reached their, present position. The cascades of the Columbia 
River, as well as the perpendicular falls at the head of the deep 
side-caiions of the tributaries, have all been formed and their places 
determined by the cutting of these streams through the harder 
basalt into the softer underlying conglomerate. 
Ill. Age of the Cascade Range. 
There has been hitherto much doubt as to the age of the 
Cascade Range. The Coast and Sierra Ranges, so distinct in 
middle California, become in northern California and southern 
Oregon blended and undistinguishable. In northern Oregon 
and in Washington, the two ranges become again quite distinct, 
the eastern and larger one being here called the Cascade Range. 
Orographic evidence therefore seem to indicate that the Cas- 
cades are a continuation of the Sierras and formed at the same 
time. But Whitney has shown * on paleontological evidence 
that, while the Sierras were formed at the end of the Jurassic, 
the Cascades in northern California have certainly been muc: 
disturbed and to some degree at least elevated, since the Cre- 
taceous. Farther north, the stratified rocks of the Cascade 
Range are so deeply buried beneath the lava-flood, that 1t has 
been impossible hitherto to find stratigraphical or paleonto- 
logical evidence of the age of the range. This evidence is, I 
believe, now for the first time furnished by my examination of 
the sections made by the Columbia River and its tributaries at 
the Cascades. The sub-lava conglomerate, with its leaf-bed, fur- 
nishes the means of determining the probable age of the lava-flood, 
and, therefore, of the great mass of the Cascade Range. 
y first conclusions on this subject, in 1871, were, I now 
believe, erroneous. The fact that the lava flows of California, 
on the southern skirts of the great Cascade lava-flood, and some 
astern 
skirts, are underlaid by the latest Pliocene ; and the additional 
fact of the great resemblance of the sub-lava conglomerate of 
the Columbia River to the cemented sub-lava drift-gravels of 
California, led me, very naturally, but perhaps too hastily, to 
conclude that the great lava-flood took place and the Cascade 
Range was built up during the Post-Tertiary period; and I 
stated this as my view in my paper, “On the great features of 
the earth surface”+ I now acknowledge my error. ihe 
paleontological evidence is the only certain one, and this 1s 
unmistakeably in favor of a greater age. I sent the box of 
leaves and petrified wood gathered from the leaf-bearing stra- 
* See Geol. Surv. of Cal., vol. 1, p. 320 and seq. 
+ This Journal, vol. iv, p. 470. 
