1(38 SNAKE EIVER PLAINS OF IDAHO. [bull. 199. 
such ii guide is absent, pits should be sunk at various horizons, begin- 
ning where the water is known to be present and extending up the 
canyon walls until the outcrop of the supplying layer is discovered. 
In some instances it will be more economical to begin at the top of a 
talus slope, above a spring, and clear away the stones in a descending 
direction, thus avoiding the danger of working at the bottom of a 
loose mass of debris. 
When the precise horizon at which tunnels should be excavated is 
learned, it will serve to indicate where other similar excavations in the 
same neighborhood should be made, but such deduction can not be 
carried to a distance, for the reason that the lava sheets exhibit great 
and frequently abrupt changes in thickness. As previously explained, 
while the surface of a lava sheet may be horizontal, or essentially so, 
for many miles, its bottom may be markedly irregular. Another 
suggestion which may be of value is that the water escaping into a 
canyon from its walls Hows over the surface of an impervious stratum. 
If, then, a bed of clay or compact lapilli (which in Snake River Can- 
yon is usually dull yellowish, with black glass-like grains) is encoun- 
tered in excavating, it is evidence that water, if present, will occur at 
the summit of the bed. 
It need scarcely be mentioned that where fissure springs occur low 
in a canyon, and even in the central part of a flat-bottomed valley, as 
they do at times, the escape of the water being perhaps through fis- 
sures of small depth, vertical instead of horizontal wells must be exca- 
vated or drilled in order to increase the How. Some assistance in 
reference to choosing a location for a horizontal well, method of devel- 
opment, etc., may be had by consulting a previous report a relating to 
the Columbia River lava, and associated formations. 
FISSURE SPRINGS. 
Fissure springs are not abundant in the portion of southern Idaho 
examined by me, and. so far as can be judged from the report of 
others, are of rare occurrence throughout the entire extent of the 
Snake River Plains. The Boise Hot Springs, situated on a fault 4^ 
miles east of Boise, are of small volume, and have a temperature, as 
stated by Lindgren in the Boise folio, b varying from 120° F. to near 
the boiling point. The belt of faulting on which these springs are 
located, extends for several miles in a southeast-northwest direction, 
and near it and about midway between the springs and Boise artesian 
wells, described later, yield a large volume of water with a temperature 
ranging from 75° to 170° F. 
About 8 miles northeast of Mountain Home, a mile south of the 
a I. C. Russell, Geology and water resources of Nez Perce County, Idaho, Water-Supply and Irriga- 
gation Paper No. 54 U. S. Geol. Survey, 1891. 
b Geologic Atlas U. S., folio 45. 
