14 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
The Owyhee Mountains, in southwestern Idaho and the adjacent 
portion of Oregon, consist of a broad dome-shaped mass, composed of 
sheets of igneous rock and thick deposits of Tertiary lake and stream 
sediments, but as indicated by the debris in the stream beds leading 
away from the central part of the extensive uplift, quartzite and 
granitic rocks are present in its higher portion. The great dome, 
from a cursory examination of a part of its bordering slopes, has been 
deeply dissected by streams and given a rough surface. 
Similar to the Owyhee Mountains in their general features are the 
mountains of the northern portion of Malheur and Harney counties, 
but too little is known of that region to permit one to speak with con- 
fidence, even of the larger features in the structure. It is evident, 
however, that great erosion has taken place, and that all the second- 
ary features, or those which give diversity and picturesqueness to 
the scenery, are due to the weathering of the uplifted rocks and the 
erosive work of streams. 
Mahogany Mountain, in the east-central portion of Malheur County, 
Oreg., is a long, narrow, sharp-crested ridge, trending northeast and 
southwest, and presenting a bold escarpment to the northwest and a 
much more gentle, but still steep slope to the southeast. The rocks 
of which it is composed are to a considerable extent, at least, rhyolitic 
tuffs, which occur in well-stratified beds and are resistant to the 
agencies of erosion. Beneath the beds of tuff there are probably soft 
Tertiary lacustral and stream deposits, and the entire series is inclined 
downward to the southeast at angles varying from perhaps 10° to 15°. 
In short, this uplift has the characteristics of the Basin ranges of Utah 
and Nevada, and, as appears from the evidence in hand, may be con- 
sidered as a ridge produced by the upheaval and tilting of the rocks on 
one side of a break; that is, the mountain is due to the tilting toward 
the southeast of an elevated fault block. Erosion has roughened the 
slopes of the mountain, but not enough to conceal the leading features 
due to uplift. It is clothed with picturesque groves of mountain 
mahogany, Avhence its name. 
The most prominent mountain in eastern Oregon, and by far the 
most conspicuous of all the elevations of a region embracing many 
thousands of square miles in Idaho, Oregon, and Nevada, is Stein 
Mountain, in the southeastern portion of Harney County. This con- 
spicuous uplift rises to a height of about 9,000 feet above the sea, 
by aneroid measure, and about 5,000 feet above the desert plain iu 
Alvord Valley at its east base. It is a long, narrow range, trending 
in general about northeast and southwest, but its crest line is some- 
what curved. On the east, in its highest and most characteristic 
portion, it presents a remarkably bold escarpment, and on the west 
it slopes gradually down to the valley in which Harney and Malheur 
lakes are situated. The crest line of the uplift is near its eastern 
border, the descent on the east being precipitous and seemingly 
