16 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
vents, and spread out in the previously formed basin, making, in truth, 
lakes of molten rock. Besides these two processes of upbuilding, that 
is, sedimentation in lakes and the outpouring of lava which spread 
widely, there was a third, namely, the washing of debris from the 
uplands and its deposition in alluvial cover and widely extended sheets 
of sand, gravel, and silt in the valleys. In addition, there are wide- 
spread seolian deposits. The volcanic eruption continued after the 
lakes were either filled or drained, so that by far the larger portion 
of the Snake River Plains is directly underlain by sheets of basalt. 
The last of the extensive volcanic discharges happened in very recent 
times, and the process of stream deposition still continues. 
The leading characteristics of the Snake River Plains at the time of 
their formation still remain, but the sheets of lacustral and stream- 
deposited sediments, and the equally well-stratified la} T ers of hardened 
lava, although for the most part still horizontal, have in places, more 
particularly in southwestern Idaho, been gently flexed and in certain 
instances, near the bases of the bordering mountains, broken and 
faulted. The principal changes in the surface features of the plains 
since their floors of lava were formed are due to erosion. These 
changes in general, however, have not been sufficiently pronounced to 
greatly alter the character of the broad, flat bottoms of the valleys, and 
to-day, especially when viewed in the strong side light of early morn- 
ing, or in the greatly lengthened purple shadows of evening', the plains 
seem to be absolutely level, and from many points of view of ocean- 
like extent. 
The statement that the broad, lava-covered area on each side of 
Snake River throughout its course across southern Idaho is a plain is 
strictly true, so far as its prevailing and most impressive features are 
concerned, but it is a dissected plain. In places its surface is mildly 
uneven, and in other places excessively rough, owing to the character 
of the naked lava of which it is in part composed. It is ms.rked also, 
as the practiced eye readily discerns, by many volcanic cones, and by 
broad, low elevations, formed of lava which was poured out from 
widely separated vents and spread in various directions. In the cen- 
tral portion of the northeastern extension of the plains, midway 
between Snake and Big Lost rivers, there are three prominent buttes, 
two of which, Big and East buttes, are ancient rhvolitic volcanoes 
which rise as islands through the surrounding 1 basalt. 
The channels cut by streams in the Snake River lava and the eleva- 
tions that rise above its surface are in realit} T all minor features of the 
topograph} 1 - and do not detract from the impression, which is strength- 
ened by familiarity, that the country bordering Snake River is essen- 
tially one vast plain from near the eastern border of Idaho to beyond 
the Oregon boundary. 
The northern and southern borders of the Snake River Plains, or 
