60 SNAKE RIVER PLAINS OF IDAHO. [bull.199. 
on the map represents the region where lava sheets, and, to a minor 
extent, beds of scoriae and lapilli, occur at the surface, or are present 
beneath a thin covering of soil. In certain localities in the western 
portion of the Snake River Plains lava sheets are more or less com- 
pletely covered by sedimentary deposits, and hence are not indicated 
on the map, which represents only the formations on the surface. The 
change is gradual from where the lava is the most prominent surface 
feature in southeastern Idaho to where, at the west, sedimentary de- 
posits greatly predominate, but it was found impracticable to repre- 
sent this on the accompanying map. 
The estimated area covered by the Snake River lava is in the 
neighborhood of 20,000 square miles. So far as is now definitely 
known, there is but one lava held in North America of greater extent, 
namely, the Columbia River lava, the estimated area of which is about 
200,000 square miles. In Snake River Canyon, below Shoshone Falls, 
nearly 700 feet of lava in horizontal sheets are exposed, but whether 
this is the maximum thickness or not can not be told. As a rule, the 
various sheets of lava are relatively thin, averaging perhaps 50 to 80 
feet, and widely extended. That many independent outflows of lava 
have occurred is easily seen, but in the walls of Snake River Canyon, 
where the best sections are exposed, it is difficult to determine their 
number, unless lacustral deposits, beds of lapilli, etc., occur between 
them. Near the mouth of Rattlesnake Creek four separate sheets are 
present, and others not represented at that locality occur but a few 
miles awa} T . In no single vertical section along Snake River could 
more than four sheets be definitely recognized; but this number will 
no doubt be increased when an opportunity is afforded for detailed 
examination. 
While the various sheets of lava are in general essentially horizontal 
and in the position in which they were spread out, there are interest- 
ing exceptions to this rule. Over an area beginning near The Thou- 
sand Springs and extending westward the lava sheets have been gently 
folded, and near Salmon Falls there is a suggestion that the lowest j 
sheet of lava exposed was gently folded before the thick bed of clay 
and the lava sheets above it were spread out. These deformations, 
however, are so gentle and extend over such large areas that much 
more extended and accurate data than could be obtained during a 
reconnaissance are needed for their study. 
RELATION TO THE COLUMBIA RIVER LAVA. 
The lava occurring on the Snake River Plains is so similar in nearly 
all its features to the still more extensive formation known as the 
Columbia River lava, through which Snake River has cut a magnificent j 
canyon in western Idaho, near Lewiston, and from there to its junction 
