russell.] AKTESIAN WELLS. 183 
solid black lava (basalt) is reached. This will probably be at a depth 
of 150 or 200 feet. If such borings are unsuccessful, one of them, near 
the town, should be continued until the sedimentary beds, such as sand, 
gravel, clay, etc., presumably occurring beneath the highest sheet of 
lava, are passed through and the drill enters the next lower lava sheet. 
No one can predict with certainty what results will follow such a 
series of tests as is here recommended, but there is a chance of success. 
Other suggestions in reference to a water supply for Mountain Home 
may be found on pages 171 and 172. 
PROBABILITY OP THE PRESENCE OP WATER BENEATH THE EASTERN PORTION OP THE SNAKE 
RIVER PLAINS. 
The largest track of waterless country in Idaho is comprised in the 
portion of the Snake River Plains lying to the north and west of Snake 
River, above Shoshone Falls. This region, measured roughly, is from 
150 to 170 miles long, and from 30 to 50 miles wide, and within it there is 
but one perennial spring — the one at Big Butte, which discharges about 
one gallon cf water per minute. A portion of the northeastern mar- 
gin of this immense tract, it is hoped, can be watered from storage 
reservoirs on Henry Fork, but by far the greater part is beyond the 
possibility of irrigation from surface streams. In reference to sub- 
surface water but little that is encouraging can be said. 
The portion of the Snake River Plains here referred to is floored 
with lava, which was poured out from many vents, and is still in essen- 
tially horizontal sheets. The rocks are not cut by streams so as to 
form canyons, nor are there other exposures by means of which the 
character of the strata beneath the surface can be judged. One can 
only surmise what the subsurface conditions are from analogy with 
the somewhat deeply dissected western extension of the same forma- 
tions. This analogy and the general principles pertaining to the man- 
ner in which broad basins ma}^ become filled or partially filled with 
alluvial deposits, lacustral sediments, and lava flows favor the assump- 
tion that beneath the broader portion of the Snake River Plain (as, for 
example, between Kimama and Big Butte), there is a succession of lava 
sheets and sedimentary beds. The lava, although in part in broad 
sheets, consists mainly of local flows which overlap and are probably 
relatively thin. Thicknesses of from 50 to 150 feet are to be expected. 
The sedimentary beds, if laid down as alluvial deposits, will be thick- 
est about the border of the basin and will thin out and become finer in 
texture toward their center, and, if deposited in lakes, will be thicker 
in their central part and become thinner and coarser in texture toward 
their margins, there merging with alluvial deposits. From these con- 
siderations it is to be expected that an alternation of pervious and 
impervious beds is present. The rocks, however, have not, so far as 
the surface sheets are concerned, been bent into a basin shape, and 
