16 GEOLOGY OF SW, IDAHO AND SE. OEEGON. [bull, 217. 
miles along the east base of the highest portion of Stein Mountain, 
there are fine exposures of sedimentary rock, mostly shale, beneath 
the basalt composing the main mass of the mountain. These sedi- 
ments have not, as yet, yielded fossils, but are probably of early Ter- 
tiary age. Their exposed thickness is over 1,000 feet. On the west 
side of Stein Mountain, in the vicinity of the east borders of Malheur 
and Harney lakes, there are, again, good exposures of sedimentary 
rocks and of tuft, but these are younger than the Stein Mountain basalt. 
A detailed study of this region can not fail to furnish most instruc- 
tive results as to the succession of rocks which underlie a great por- 
tion of eastern Oregon and the neighboring parts ot Idaho and Nevada, 
as well as typical illustrations of the manner in which the earth's 
crust throughout the same region have been bent and broken and 
otherwise deformed. 
Traversing the immensely thick succession of basaltic sheets, in the 
east face of Stein Mountain, there is a number of nearly vertical basaltic 
dikes, ranging in thickness from about 15 to 60 feet. These dikes 
are horizontally columnar in their central portion and are well exposed 
in several instances for distances of a thousand feet or more in the 
walls of nearly vertical cliffs. These dikes, as is well known, are due 
to the filling of fissures with molten rock injected from below. The 
slow cooling of the intruded magma resulted in producing joints 
which define the sid<>s of the usually six-sided, horizontal columns, 
which forms such a conspicuous iVal are of the parts of the dikes now 
exposed. The horizontal columns appear not unlike the steps of 
great stairways in the walls of the cliffs traversed by them, and a 
person climbing the mountain finds it convenient to utilize them as 
stepping stones, since in some instances the} 7 furnish the easiest means 
of ascent. 
Since the upheaval of Stein Mountain it has undergone a large 
amount of erosion, and deep canyons have been excavated in it, 
especially on its western side. From the head of Donner and 
Blitzen River northward to Kieger Creek, a distance of about 25 
miles, each of the principal westward-flowing streams, six in number, 
is situated in a deep canyon of its own making. The boldest and 
most characteristic of these excavations is the one occupied by Kieger 
Creek. This creek has its ultimate source at the crest line of the 
uplift, which, it will be remembed, is on the east margin of the tilted 
block forming the mountain, and flows westward in a steep-sided 
canyon that for a distance of about 8 miles is over 2,000 feet deep. 
The walls of this canyon, like all the other profound trenches cut in 
the west slope of the mountain, have been excavated in gently dipping 
beds of basalt* None of the westward-flowing streams, however, so 
far as known, has cut through the basalt so as to expose the rocks on 
which it rests. The canyon of Kieger Creek exhibits well-marked 
contrasts on its two sides. The north wall is a continuous and 
