112 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
supplement observations in other similar instances, and thus assist in 
a small way in reaching a proper understanding of the series of similar 
depressions in volcanic regions in various countries. 
OLDER CRATERS AND LAVA FLOWS. 
LAVA CONES. 
The occurrence of low cones with immensely broad bases on the 
Snake River Plains has already been referred to. These elevations 
are composed almost entirely of lava extruded in a molten and highly 
fluid condition, seemingly from a comparatively small vent, and owe 
their height mainly to the cooling and thickening of the lava about 
the place of emergence, so as to form the necessary slope down which 
it could continue to flow. As these mounds or low buttes are distinct 
from cinder cones, I venture to suggest the name "lava cones." An 
example of recent date is furnished by Black Butte, described on a 
previous page. Their characteristic shapes (PI. XVIII) have already ^ 
perhaps, been sufficiently well described not to require repetition. 
They occur usually at wide intervals over the Snake River Plains, 
especially to the east of Shoshone and to the south of Big Butte. 
Owing to their distance apart and the absence of water in the region 
where they occur, only three were visited hj me. The most character- 
istic of these is Notched Butte, 4 miles south of Shoshone. This is a 
conical elevation with gently sloping sides, 250 feet high, with a nearly 
flat top, but broken so as to present notches when seen from a distance. 
The sides are formed of lava sheets in which there are caves due to the 
flowing out of still molten rock from beneath a thick, rigid crust, as 
already explained. The summit is formed of highly scoriaceous lava. 
There is evidence of considerable explosive violence at the top in the 
form of scoria in clot-like masses, mingled with thin sheets of vesic- 
ular lava. Evidently a shallow crater once existed at the summit. 
A connection between cinder and lava cones is thus shown, and the two 
are but extremes of a single process. When explosions predominated 
cinder cones were built, either with or without lava flows; and when 
the lava predominated a lava cone resulted. 
Lava cones are evidently more enduring structures than cinder 
cones, and for this reason perhaps seem to have been the type of vol- 
canoes that furnished most of the old lava sheets now occurring 
beneath the Snake River Plains. 
LAVA SHEETS. 
The fact that the Snake River Plains are underlain by lava sheets 
throughout perhaps eight-tenths of their area is indicated on the map 
forming PL I. The number of sheets can not be even approximately 
determined, but at several localities in Snake River Canyon four have 
