and on the age of the Cascade Mountains. 261 
observations as I made, however, I here present merely to 
draw attention to this important subject. 
As I have already said, the sub-lava conglomerate is com- 
posed wholly of porphyritic pebbles and boulders. I think 
most geologists would call the material of these boulders and 
pebbles porphyry. I have never seen this rock zm situ, but it 
doubtless exists ax situ beneath the lava. Its position agrees 
well with the view of Richthofen as well as of most writers, as 
to the Mesozoic age of porphyry. 
Again, on the north side of the Columbia River gorge, the 
cliffs are in some places composed, in their lower parts, of a 
whitish or grayish ¢rachyte, while their upper parts are composed 
of regular horizontal layers of columnar basalt. King also ob- 
served the same relation in the trachyte and basalt which forms 
the cliff of the Snake River Cafion in Idaho; 300 feet of 
trachyte below and 400 feet of basalt above These facts are 
also in accordance with Richthofen’s view. 
trachytic. These last facts, however, viz: Mt. H achy 
tufa overlying columnar basalt, and the simultaneous ejection 
of basaltic and trachytic lavas by Mts. Adams and Hood, are 
Y no means fatal to Richthofen’s views; since the regular suc- 
cession of different kinds of lava strictly apply only to fissure- 
eruptions, such as those which form the lava-layers of Columbia 
and Des Chutes Rivers, and not to crater.eruptions, such as those 
Which form the lava-streams from Mts. Hood and Adams, still 
tions in relation to each other. 
less to crater and perigee 6 LA : 
There is evidently nothing to hinder the supposition that while 
- Adams may have been, by secondary eruption, ejecting 
materials from a more superficial basaltic region of Cascade 
lavas, Mt. Hood has been simultaneously and similarly ejecting 
materials from a deeper trachytic region.* 
wt covering in Oregon ae 
e a 
ains lying be- 
is whole region 
* In a recent before the California Academy of Science (Proc., vol. 
V, p. 210), Di. B ae sills in the Puebla range, Nevada, trachyte over- 
lies basalt in regular layers, indicating true fissure-eruptio 
