bussell.] SNAKE RIVER LAVA. 59 
a completely inclosed basin exists. Other facts in this connection, the 
best method of testing the artesian conditions of this basin, etc., will 
be considered in the second part of this report. 
POSSIBLE EASTWARD EXTENSION OF THE TERTIARY SEDIMENTARY BEDS. 
The wide extent of the exposed portions of the lacustral deposits in 
the western part of the Snake River Plains, and the absence of expo- 
sures of such rocks in their eastern portion, is seemingly to a great 
extent in the nature of an accident. In the region below the mouth of 
Salmon River, lava sheets are less thick and probably less numerous 
than east of that locality. Below Shoshone Falls the Snake has cut 
deeply into the beds beneath the plains, and each tributary stream, 
at least near its mouth, is also in a canyon; but upstream from the 
locality mentioned the plains are much less dissected, and over vast 
areas they are uneroded. The presence of extensive sedimentary beds 
beneath the lava flooring the broad eastern division of the plains can 
not now be determined from direct evidence. What is known of the 
pre-Tertiary topography of the Snake River Basin, however, and the 
extent and thickness of the sedimentary deposits in its western por- 
tion, etc., favor the hypothesis that beneath the surface lava sheets in 
its eastern part extensive beds of clay, sand, etc., do occur. This is 
an important question in reference to the possibility of obtaining water 
in the broad, unbroken portion of the plains, but one that can be 
definitely answered only by drilling. 
SNAKE RIVER LAVA. 
The name Snake River lava is here proposed as a general term by 
which to designate the basaltic rocks that underlie by far the larger 
part of the Snake River Plains, and to a great extent form their actual 
surfaces. It includes, also, the lava streams and associated cinder 
cones, etc., which have descended lateral valleys or adjacent mountain 
slopes, and united with the sheets of similar material extruded from 
numerous craters on the plains. When this extensive formation is 
studied in detail, it will, I judge, be found practicable to separate it 
into several distinct portions, and to correlate some of these with sedi- 
mentary beds containing fossils, and thus determine their precise geo- 
logical age. 
EXTENT AND THICKNESS. 
The extent of the Snake River lava is represented approximately on 
the map (PL I), on which the boundaries of the lava-covered country 
where known are indicated by a broken line, and where merely inferred 
the shading is not definitely margined. The area shaded by single lines 
