GEOLOGY-EVIDENCES OF GLACIAL ACTION. 
41 
plateau preserves its general horizontality, only broken by subordinate hills and mountains, 
which mark the cracks and fissures formed in the convulsions by which it has been shaken. 
Toward the west the rocky strata, with many fractures, bent down toward the depression of 
the ocean’s bed, by their broken and upturned edges, bad formed the succession of mountain 
peaks which constitute the great breadth of the chain. The nature of the rock exposed at the 
various points which we visited seemed to lead to the same conclusion. The line of fracture is 
marked by a series of volcanic peaks—many of them of great altitude—of which the fires are 
not yet wholly extinguished ; their sides being covered by an immense accumulation of volcanic 
material, of which the greater part seems as fresh as if thrown out but yesterday. The streams 
of lava which have poured down their sides now stand bare, black, and ragged, scarcely a 
lichen even as yet having found a foothold on them. Toward the west the mountain masses 
are composed of highly metamorphosed slates, set on their edges and inclined at every possible 
angle. 
LOCAL GEOLOGY. 
As we ascended the Des Chutes river we soon left behind us the pumice plains and the strati¬ 
fied marls which line its banks, and at an altitude of 4,500 feet entered a region south of the 
Three Sisters, abounding in lakes, mountain meadows green and fresh, and forests of fir and 
pine, of different species from those occupying the plain below. The soil in many places was 
fertile, the scenery as picturesque as can be found in any part of the world. The only rock 
exposed being a dark vesicular trap, or a nearly black compact basalt. 
Ascending to a pass between the snow mountains in the group I have mentioned, we passed 
over a surface covered with comparatively recent volcanic material, heaped up in the greatest 
confusion. This we traced to its origin in a crater, half a mile in diameter, which lies between 
two of these snow mountains, which form portions of its once continuous enclosing walls. The 
southern rim of this crater has an altitude of 6,500 feet; the northern rim being two or three 
hundred feet higher. The eruption, of which we saw such evident traces, seems to have taken 
place from the southern side of the mountain, the crater being here opened by a deep fissure, 
through which a stream flows from the lakes occupying its centre. The walls of the mountain 
which border the crater are formed of black lava or blood red scoria, and immense piles of pumice 
and obsidian, fresh and bare, mark the recent date of its activity. 
EVIDENCES OF GLACIAL ACTION. 
The north wall of this crater is composed of black porphyry, which is very compact, and 
apparently older than most of the trap and lava which we saw in the vicinity. This rock, 
though intensely hard and very homogeneous, everywhere bore the marks of the action of some 
powerful agent. It was planed down to a smooth and even surface, or scored into deep grooves 
or furrows, which were sometimes continuous for rods. These grooves ran down the north¬ 
east slope of the mountain, were confined to the outside of the crater, and seemed to radiate 
from a point over its crater. On a subsequent occasion, descending the mountain toward the 
southwest, we traced these grooves for several miles to a point two thousand feet below the line 
of perpetual snow. On this slope the marks of glacial action were mucli more conspicuous than 
at Crater pass. All the projecting points and ridges of the older trap rock were worn down, 
smoothed off, and cut by deep furrows, which now pointed northeast toward the centre of the 
mountain mass formed by the Three Sisters. 
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