russell.] BLACK BUTTE. 67 
mountain springs supply creeks and rivers, and that the molten rock 
flowed down valleys leading to the central depression, where it 
spread out in lake-like sheets, in a similar way to the downward flow 
and expansion of streams of water under analogous conditions is 
well illustrated by a volcano of recent date, and the lava poured out 
from it, situated on Big Wood River, about 20 miles to the north of 
Shoshone. 
This volcano has no name, but in my notes I have designated it as 
Black Butte, on account of the color of the bare lava of which it is com- 
posed. It is situated about 20 miles from the general border of the 
Snake River Plains, and rises in the center of a valley at a locality 
where, previous to its formation, the valley bottom was about 500 feet 
above the level of the neighboring portion of the plains. The volcanic 
mound is only about 300 feet high, with a base measuring, perhaps, 
2 miles in diameter. It is composed of highty scoriaceous lava, which 
flowed away in all directions in thin sheets. The copious outflow of 
molten rock was apparently continuous from the beginning of the 
eruption to its close, and the appearance of thin sheets is due to 
the manner in which cooling and the consequent stiffening of the 
material occurred. While the butte is composed of vesicular lava, 
there is no evidence from the presence of lapilli, cinders, etc., of an 
explosive eruption. There is no crater at the summit, such as explo- 
sive volcanoes build, although a deep, wide gulf at the top of the butte 
at first suggests such a depression. This gulf, however, is of subse- 
quent origin, due to fractures and the subsidence of a large mass of 
material, and connects with similar irregular breaks on the sides of 
the butte, due in part to the falling in of the roofs of lava tunnels. 
The fact that this fresh and uneroded volcanic mound, without soil 
and almost entirely bare of vegetation, is not a cinder cone, but is com- 
posed of lava that hardened about the vent from which it was extruded, 
places it in the same category as the low, flat-topped lava buttes 
referred to on a previous page as being characteristic of certain of the 
older volcanoes on the Snake River Plains. 
Black Butte is, as stated, situated in a valley, and the lava poured out 
by the volcano which formed it had greater freedom to flow down the 
valley than in other directions. Although the conduit leading down 
into the earth was certainly small, a vast volume of highly liquid rock 
was poured out and flowed down the valley of Big Wood River for a 
distance of 35 to 40 miles, and spread out in a sheet from 2 to fully 6 
miles broad. The area covered by this single eruption is about 150 
square miles. The lava is black, with an exceedingly rough surface, 
and can be distinguished at a glance from the older sheets of similar 
material over which it flowed. About 3 miles below its source it passed 
between two lava volcanoes of the same general type as the one from 
which it came, but of much older date, and now covered with soil of 
