260 = J. LeConte on the great Lava-flood of the West, 
layers are in some cases really separate flows is shown by the 
fact that they are separated by volcanic conglomerate or breccia 
or by partially cemented soil, and are often of different kinds 
of materials. In other cases it seems doubtful whether they 
Simcoe Mountains still farther north, were formed by similar 
flows. The lava of these ridges is entirely different from and 
more trachytic than that of the plains. Whether the lava of 
the ridges is older or newer than that of the plains, 1 am un- 
certain, but they are more probably older. The lava layers of 
the Des Chutes Hills seem to dip slightly toward the west, as 
indicated in fig. 1. It seems most probable that the ridges were 
first formed and disturbed, and then the lava of the plains was 
poured out and spread about their feet, and finally the river 
cut its channel 1000 feet deep along the sinuous line of contact 
between the two. 
B. Relative uge of different kinds of lava.—Without doubt a 
careful study of the 3500 feet of lava which forms the Cascade 
Mountains, and which is exhibited in section at the Columbia 
and Des Chutes Rivers, by one thoroughly versed in the lithology 
of igneous rocks, would afford a splendid opportunity of testing 
the truth of Richthofen’s view, that there is an invariable order 
of succession in the appearance at the surface, by fissure-erup” 
tion, of the different kinds of lava, the order being, according 
to him, propylite, andesite, trachyte, rhyolite, basalt.* I confess, 
however, that my knowledge of the divisions and subdivisiop® 
the species and varieties of voleanic rocks, according t 
Richthofen’s somewhat complex classification, is not sufficient 
to make my rather hasty observations of much value. Such 
* Memoirs of Cal. Acad. of Science, vol. i, pt. 1. 
