54 GEOLOGT OF SW. IDAHO AND SE. OREGON. [bull. 217. 
stream valley until its progress was checked by the rising gradient. 
In places the fresh, black lava is bordered by rims of older rock, 
showing that small canyons in the preexisting surface had been nearly 
filled. Near the eastern limit reached by the lava it ascended the 
small canyon of Cow Creek for about 1 mile, and on cooling formed 
a dam, which now retains the largest and most northern of the Cow 
Creek lakes, as has been described on a previous page. About its 
southeastern margin there are several other lakes. 
This great lava stream or lava sheet came from a small crater, 
termed above Crater No. 4, the bottom of which is approximately 300 
feet across. Seemingly the actual conduit must be of still smaller 
diameter. This effusion of highly liquid lava, with only mild explo- 
sions at the beginning of the discharge, is a typical illustration of the 
manner in which many extensive lava sheets of Idaho, Oregon, and 
Washington were poured out. The lava from Crater No. 4 seems to 
have progressed mainly by flowing beneath a stiffened crust, and in 
this manner was prevented from cooling quickly, thus permitting it to 
advance far and spread widely. Tunnels in the lava were produced 
by the outflow beneath the crust formed on its surface, and in certain 
places, as near the source of the stream, the roofs of such tunnels 
have fallen in, leaving irregular pits with overhanging walls from 50 
to 70 feet deep. 
In many portions of the lava flow pressure ridges and domes like 
those so common in the immense lava sheets about the Cinder Buttes, 
in the older sheets of Snake River lava, and at other places, are well 
displayed. A photograph of one of these domes, situated near the 
northern border of the lava sheet, and midway between its source and 
the most northerly of the Cow Creek lakes, is reproduced on PL XV, A. 
A similar ridge on a lava sheet of somewhat older date, which came 
from the Diamond Craters, is shown on PI. XV, B. That the lava in 
each of these instances was horizontal before bei ag ] >ressed up into dome- 
like ridges, is shown by the corrugation on the surface of the dome, 
to be seen in PI. XV, A, and of the columnar structure in PL XY, B. 
DIAMOND CRATERS. 
A group of recently extinct volcanoes which it is convenient to term 
the "Diamond Craters" is situated in the east-central part of Harney 
County, Oreg. , about 6 miles west of Diamond post-office, The craters, 
about 20 in number, occupy an area of about 5 square miles, and are 
surrounded by at least 30 square miles of rough lava which flowed 
from them. 
The Diamond Craters are situated near the base of the long west 
slope of Stein Mountain, and came into existence after the land over 
which they discharged their lava had been deeply dissected by ero- 
sion. The lava entered the valleys and canj^ons and obstructed the 
drainage so as to cause swamps to form. A significant fact in this 
