64 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
frequently 8 to 10 miles or more in diameter at the base, and perhaps 
only 200 or 300 feet high. In fact, their bases merge so gradually 
with the surrounding plain that no eye can recognize where the ascend- 
ing surface actually begins. The sides of the elevations, although 
gentle throughout, increase in slope as the center is approached, and 
rise more perceptibly to meet the usually flat-topped summit portion. 
The profile of one of these broad, gently sloping elevations is shown 
on PL XVIII. The most noticeable portion of the surface descent 
from the central flat area seldom has an inclination over 10°, while 
the long slopes farther away are so gentle that the angle they make 
with a horizontal line can scarcely be measured with an ordinary 
clinometer. These broad, low elevations are composed of black lava 
all the way from their indefinite margin to the flat area forming the 
summit, which is usually composed of highly scoriaceous, with at 
times minor quantities of fragmental material (lapilli, etc.). The 
fragmental material of the nature here considered is absent from the 
older elevations, or easily passes unnoticed, owing to the effects of 
weathering and the growth of grass, sagebrush, etc. 
The history of these elevations, as recorded in their form and in the 
nature of the material composing them, is briefly as follows: They 
represent volcanic vents, from which highly liquid lava in vast quan- 
tities flowed away in all directions, and on their outer borders came to 
rest and hardened in a horizontal position. When the vents opened 
on a plain, the requisite slope for the outward flow of the lava was 
obtained by the thickening of the lava sheet itself, and it is this 
increase in thickness that produced the low elevation still remaining. 
These outweilings did not occur along the general course of a Assure, 
but from local vents. These were not accompanied by explosions, or 
the explosions were of a mild character, and but little fragmental 
material was produced. J udging from the sequence of events recorded 
in the Cinder Buttes, described below, the volcanoes of the type here | 
considered probably built cinder cones during the earlier stages of 
their eruptions, which were curried away and perhaps buried by the 
subsequent effusion of lava. As stated above, the summits of the low 
lava volcanoes are characteristically flat when seen in profile. When 
visited, the flat portion of the fresher cones is found to be composed 
of highly scoriaceous lava, and may contain a depression. About 15 i 
miles north of Pocatello, a flat-topped butte of the nature just 
described has a small lake in the bowl at the summit during the rainy i 
season. It is probable that man}^ of the similar elevations once had 
craters, but in most instances their rims have been broken and the 
depression within has been filled. It fact, a succession of examples 
illustrating this process might easily be obtained. 
The number of lava craters of the nature just described has not 
been determined, as they are widely distributed over the lava plains. 
