34 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
the walls seem to be almost vertical precipices. The main difficulty 
in crossing- the plains is the scarcity of water. As in most flat regions 
where stock raising is a leading industry riding is largely practiced. 
BIG, MIDDLE, AND EAST BUTTES. 
An account of the Snake River Plains, howeA^er brief, would not be 
complete unless it directed special attention to three conspicuous moun- 
tain-like elevations that break their monotony. These elevations, known 
in their order from southwest to northeast as Big, Middle, and East 
buttes, are situated in the central part of the vast lava-covered region 
of southeastern Idaho, about 25 miles northwest of Blackfoot. Their 
location is indicated on the map forming PL I. Owing to the appar- 
ently perfect flatness of the surrounding plain the}' are prominent 
objects in the landscape when seen from distances of 100 or more miles, 
and are no doubt familiar to many persons who have passed over 
either of the two divisions of the Oregon Short Line Railroad which 
diverge at Pocatello, one going northward and the other westward. 
Owing to the isolation of the buttes and the absence of water in sum- 
mer anywhere in their vicinity, with the exception of a small spring at 
the northeast base of Big Butte, they are seldom visited. For these 
and other reasons it seems appropriate to close this introductory chap] 
ter with an account of these three most interesting and instructive 
elevations. 
Two of the buttes, namely, Big and East buttes, are rhyolitic vol- 
canic cones of ancient date, which are completely surrounded by Snake 
River lava and furnish admirable examples of steptoes, as such islands 
in a once molten sea of lava have been termed. Middle Butte differs 
from its companions, being an upraised block of stratified basalt. 
The elevation of Big Butte above the surrounding- plain, as deter- 
mined by aneroid measurements, is 2,350 feet, and by estimate the 
height of Middle Butte is 400 and that of East Butte about 700 feet. 
Big Butte, as may be judged from PL V, rises as a deeply sculptured 
mountain precipitously from the nearly flat plain surrounding its 
base, and terminates in two ridges about a mile apart, with a deeper 
depression between, which is apparently a remnant of a crater. At its 
base the mountain is 5 or 6 miles in diameter, and is somewhat elon- 
gated in a northwest-southeast direction. To reach its summit, no 
matter how carefully the trip may be planned, true mountain climbing 
is necessary, and the steepness of the slopes will be forcibly impressed 
on the climber's mind before he reaches the top. From whatever 
direction the mountain is ascended the abruptness with which it rises 
from the plain, without gradual approaches or foothills, becomes 
impressive. In the gulches and deeply cut ravines there are alluvial 
fans which extend out onto the surrounding plain for 2 or 3 miles, 
