38 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
cones are known to exist. Not including the Cinder Butte, or East 
Butte, about twenty craters can be counted on the broad plain, and still 
others occur which can not be readily recognized from a distance. 
Evidently a very large portion of the lava occurring as a surface 
covering on the Snake River Plains came from small and inconspicuous 
craters, many of which have escaped burial by later eruptions and 
still exist as elevations. 
After climbing Big Butte I rode across the plain, in part occupied 
by recent lava, but mostly a grass and sagebrush covered region, to 
Middle and East buttes, but owing to lack of time did not climb either 
of them. Middle Butte, as already stated, is composed of stratified 
basalt, and does not present evidence of being a cinder or lava cone, 
and does not mark a site from which either scoria or liquid lava was 
extruded. About in a line between Middle and East buttes, however, 
and overgrown with juniper trees, is a low elevation of scoriaceous 
lava, which appears to be a remnant of a basaltic crater. 
East Butte has preserved its original shape much more completely 
than Big Butte, probably for the reason that it presents much less 
surface on which water might collect into streams, and also because 
of its less height. It rises precipitously from the surrounding basalt 
on all sides (PI. Y. A) and is free from conspicuous talus slopes and 
alluvial fans. The rock is a white rlryolite, and what is clearly a 
remnant of a crater still exists at the summit. 
Middle and East buttes are covered with an open forest of small 
juniper trees, which extends far over the plain to the east. To the 
west of them there is a broad, rolling prairie. No water is to be had 
except in winter, when an occasional depression may perhaps be filled 
by the melting snow. Except for the absence of water, the region is 
as fair and inviting as the most favored portions of the prairie States. 
The chief lesson learned by a visit to the "Three Buttes" is that 
two of them are ancient rhyolitic volcanoes, entirely surrounded by 
the Snake River lava; and that on the encompassing plain there are 
many extinct volcanoes which poured out basaltic lava. 
GENERAL GEOLOGY. 
Previous investigations. — Southern Idaho is to a great extent an 
untrodden held to geologists. The most detailed survey that has been 
made within its extensive area embraces a region of about 864 square 
miles in the vicinity of Boise, which has been described and mapped, 
by Waldemar Lindgren in what is known as the Boise folio, published 
by the U. S. Geological Surve}^/' This folio furnishes an admirable 
beginning for the study of the geology of the region surrounding the 
aThis folio measures 18i by 21i inches, contains 7 pages of descriptive text, 4 full-page maps, sec- 
tions, etc., and is sold by the Survey for 25 cents per copy. 
