178 J. LeConte on the great Lava-flood of the West. 
of slate or granite hills in the vicinity, but only of porphyry 
or porphyritic lava boulders; nevertheless, I think diligent 
search will yet find the evidences of slate and granite beneath 
the lava* If the question is again asked, What then is the 
age of the Cascade Range and to what system does it belong ? 
IT answer: Like most mountain ranges, it was not formed at 
once. It aban first commenced as a continuation of the 
Sierra Range, and at the same time, viz: the end of the Juras- 
sic; and this part belongs to the Sierra system. But its great 
bulk was formed at the end of the Miocene coincidently with 
the Coast Range; and this part may be said to belong, so far as 
age ws concerned, to the Coast Range system. 
IV. Theory of - Ae of the lava- aoa etre of the forma- 
of the Cascade Mount 
er the hae of the lava-flood, eo along the crests 
of the Cascade Hanae, are scattered a number of extinct vol- 
canic cones. From the Des utes plains is obtained a mag- 
nificent view of this range, from the Three Sisters on the south 
to Mt. Rainier on the north, a distance of over 200 miles. 
cones; many ot maller ones probably-exist. Are these. 
then, the Beate Rb which flowed the great lava-flood? 
Has all this immens f fluid matter been ejected b 
It seems to me incredible: the effect is out of all proportion to 
the cause. It is only necessary to state the proposition in 
order to see its absurdity. Besides, the lava of the Columbia 
and Des Chutes Rivers is nearly all perfectly solid, entirely free 
from those vesicles so universal in lavas ejected by steam from 
craters. Only in the uppermost parts was there found any evi- 
ence in the structure of the lava of true volcanic action—any 
a8 ee already, in a previous paper,t maintained that the 
interior contraction of the earth determines irresistible horizon- 
* New speaks of the cliffs of the Columbia River gorge (P. R. R. Surv.. 
VI; Geology, p. 57) i ies of lava and slate. I have never pone Seb 
put lava. Just pee the Cascades, on both sides of the river, are seen slaty-look 
ing mountains. I myself, at one time, thought they were slate. They sob in fact 
phonolite. In the John Day Valley, indeed, I saw genuine slate hills wi quartz 
veins, exposed bey 2 the erosion of the lava-flood; but these belong net to the 
Cascade Mountain system. 
not the 
Journal, vol. iv, p. 345. 
